**3. Doping in ZnO**

ZnO has a strong potential for various short-wavelength optoelectronic device applications. To realize these applications, the reliable techniques for fabricating high quality n-type and p-type ZnO need to be established. Undoped ZnO exhibits n-type conduction due to the in‐ trinsic defects, such as the Zinc interstitial (Zni ) and oxygen vacancy (VO). It is easy to obtain the high quality n-type ZnO material by doping group-III elements. However, it is a major challenge to dope ZnO to produce p-type semiconductor due to self-compensation from na‐ tive donor defects and/or hydrogen incorporation. To achieve p-type ZnO, various elements (N, P, As, Sb and Li) have been tried experimentally as p-type dopants with various techni‐ ques, such as pulse laser deposition, magnetron sputtering, chemical vapor deposition (CVD), molecular-beam epitaxy, hybrid beam deposition (HBD), metal organic chemical va‐ por deposition (MOCVD) and thermal oxidation of Zn3N2 [2, 3].

#### **3.1. n-type ZnO**

A number of researchers investigated the electrical and optical properties of n-type ZnO mate‐ rials by doping III elements, such as Al, Ga and In, which can easily substitute Zn ions [1-3].

Kim et al reported the high electron concentration and mobility in AZO films grown on sap‐ phire by magnetron sputtering [18]. AZO films exhibited the electron concentrations and mobilities were of the order of 1018cm3 and less than 8cm2 /Vs, respectively, however, when annealed at 9000 C, the films showed remarkably improved carrier concentrations and mobi‐ lities, e.g., about 1020 cm3 and 45 – 65 cm2 /Vs, respectively. Other researchers also reported the improved electrical properties in Al-doped zinc oxide by thermal treatment [19].

Bhosle et al investigated the electrical properties of transparent Ga-doped ZnO films pre‐ pared by PLD [20]. Temperature dependent resistivity measurements for the films showed a metal-semiconductor transition, which was rationalized by localization of degenerate electrons. The lowest value of resistivity 1.4×10−4 Ωcm was found at 5% Ga. Yamada et al reported the low resistivity Ga-doped ZnO films prepared on glass by ion plat‐ ing with direct current arc discharge [21]. The ZnO:Ga film with a thickness of 98nm, exhibited a resistivity of 2.4×10−4 Ω cm, a carrier concentration of 1.1×1021cm−3 and a Hall mobility of 23.5cm2 /Vs. Liang et al reported the Ga-doped ZnO films prepared on glass by magnetron sputtering and found that a carrier concentration exhibited only a slight change with the thickness variations [22].

Wang et al studied the properties of In-doped ZnO crystal by the hydrothermal technique [23]. The indium-doped ZnO crystals have a resistivity lower than 0.015Ωcm with a free car‐ rier concentration (mostly due to indium donors) of 1.09×1019/cm3 at room temperature. Quang et al reported the In-doped ZnO films grown by hydrothermal [24]. The films had a mobility of 4.18-20.9cm2 /Vs and a concentration of 6.7×1018 - 3.2×1019/cm3 . The In-doped ZnO films with a carrier concentration of 3.22×1020/cm3 were grown by sol–gel method [25].

VII elements such as F and Cl are also used as n-type dopants in ZnO, which substi‐ tuted oxygen ions. Cao et al reported F-doped ZnO grown by PLD with a minimum resistivity of 4.83×10-4 Ωcm, with a carrier concentration of 5.43×1020cm-3 and a mobili‐ ty of 23.8cm2 / Vs [26]. Chikoidze et al grew Cl-doped ZnO films by MOCVD with a resistivity of 3.6×10-3 Ω cm [27].

### **3.2. p-type ZnO**

anisotropic strain state in heteroepitaxial QW and the reduction of structural defects and the improvement of surface morphology were correlated with a strong enhancement of the photo‐ luminescence properties. Su et al investigated the optical properties of ZnO/ZnMgO single quantum well (SQW) prepared by plasma-assisted molecular beam epitaxy [17]. The photolu‐ minescence peak of the SQW shifted from 3.31 to 3.37eV as the well layer thickness was de‐ creased from 6 to 2nm (Figure5). ZnO/MgZnO superlattices were also fabricated by laser molecular-beam epitaxy and the excitonic stimulated emission up to 373K was observed in the superlattices. The emission energy could be tuned between 3.2 and 3.4eV, depending on the

ZnO has a strong potential for various short-wavelength optoelectronic device applications. To realize these applications, the reliable techniques for fabricating high quality n-type and p-type ZnO need to be established. Undoped ZnO exhibits n-type conduction due to the in‐

the high quality n-type ZnO material by doping group-III elements. However, it is a major challenge to dope ZnO to produce p-type semiconductor due to self-compensation from na‐ tive donor defects and/or hydrogen incorporation. To achieve p-type ZnO, various elements (N, P, As, Sb and Li) have been tried experimentally as p-type dopants with various techni‐ ques, such as pulse laser deposition, magnetron sputtering, chemical vapor deposition (CVD), molecular-beam epitaxy, hybrid beam deposition (HBD), metal organic chemical va‐

A number of researchers investigated the electrical and optical properties of n-type ZnO mate‐ rials by doping III elements, such as Al, Ga and In, which can easily substitute Zn ions [1-3].

Kim et al reported the high electron concentration and mobility in AZO films grown on sap‐ phire by magnetron sputtering [18]. AZO films exhibited the electron concentrations and

Bhosle et al investigated the electrical properties of transparent Ga-doped ZnO films pre‐ pared by PLD [20]. Temperature dependent resistivity measurements for the films showed a metal-semiconductor transition, which was rationalized by localization of degenerate electrons. The lowest value of resistivity 1.4×10−4 Ωcm was found at 5% Ga. Yamada et al reported the low resistivity Ga-doped ZnO films prepared on glass by ion plat‐ ing with direct current arc discharge [21]. The ZnO:Ga film with a thickness of 98nm, exhibited a resistivity of 2.4×10−4 Ω cm, a carrier concentration of 1.1×1021cm−3 and a

the improved electrical properties in Al-doped zinc oxide by thermal treatment [19].

and less than 8cm2

C, the films showed remarkably improved carrier concentrations and mobi‐

/Vs. Liang et al reported the Ga-doped ZnO films prepared on

) and oxygen vacancy (VO). It is easy to obtain

/Vs, respectively, however, when

/Vs, respectively. Other researchers also reported

well thickness and/or the Mg content in the barrier layers.

trinsic defects, such as the Zinc interstitial (Zni

mobilities were of the order of 1018cm3

por deposition (MOCVD) and thermal oxidation of Zn3N2 [2, 3].

and 45 – 65 cm2

**3. Doping in ZnO**

30 Optoelectronics - Advanced Materials and Devices

**3.1. n-type ZnO**

annealed at 9000

lities, e.g., about 1020 cm3

Hall mobility of 23.5cm2

To realize ZnO-based LEDs, the most important issue is the fabrication of high quality ptype ZnO. However, undoped ZnO exhibits n-type conduction and the reliable p-type dop‐ ing of the materials remains a major challenge because of the self-compensation from native donor defects (Vo and Zni ) and/or hydrogen incorporation. Considerable efforts have been made to obtain p-type ZnO by doping different elements (N, P, As, Sb, Li, Na and K) with various techniques [2, 3]. Here, we present the typical results of p-type ZnO materials.

Among all potential p-type dopants for ZnO, N is considered the most promising dopant due to similar ionic radius compared with oxygen. It substitutes O sites in ZnO structure, resulting in the shallow acceptors. N2, NO, N2O, NH3 and Zn3N2 are acted as N sources de‐ pended on growth techniques [2, 3]. Liu et al reported p-type ZnO:N films grown on c-sap‐ phire by plasma-assisted molecular beam epitaxy [28]. The anomalous Raman mode at 275cm-1 was confirmed to be related to substitution of N for O site (NO) in ZnO. The films exhibited a hole concentration of 2.21×1016cm-3 and a mobility of 1.33cm2 /Vs. Zeng et al in‐ vestigated p-type ZnO films prepared on a-plane (11–20) sapphire by MOCVD [29]. The op‐ timized result was achieved at the temperature of 400°C with a resistivity of 1.72Ωcm, a Hall mobility of 1.59cm2 /Vs, and a hole concentration of 2.29×1018cm−3. Wang et al prepared ptype ZnO films by oxidation of Zn3N2 films grown by direct current magnetron sputtering [30]. For oxidation temperature between 350 and 5000 C, p-type ZnO:N films were achieved, with a hole concentration of 5.78×1017cm-3 at 5000 C. Kumar et al reported on the growth of ptype N,Ga-codoped ZnO films prepared by sputtering ZnO:Ga2O3 target in N2O ambient [31]. The film deposited on sapphire at 5500 C exhibited p-type conduction with a hole con‐ centration of 3.9×1017 cm−3.

Beside N, other group V elements (P, As and Sb) are also used to be acceptor dopants to ob‐ tain p-type ZnO. However, first-principle calculations show that XO (PO, AsO and SbO) are deep acceptors and have high acceptor-ionization energies, owing to their large ionic radii as compared to O, which make it impossible for XO to dop ZnO efficiently p-type [32]. We could not contribute the p-type behavious in X doped ZnO to XO, simply. Recently, for the large-size-mismatched impurities in ZnO, Limpijumnong et al proposed XZn−2VZn acceptor model [33]. In the model, X substitute Zn sites, forming a donor, then it induces two Zn va‐ cancy acceptors as a complex form XZn−2VZn. The ionization energy of AsZn - 2VZn complex was calculated to be 0.15eV (0.16eV for SbZn − 2VZn ).

with a hole concentration of 5.4×1018cm-3 [43]. p-type ZnO:Na films were fabricated on Si substrates by PLD [44]. The hole concentration ranged from 1.5×1018 to 1.1×1019 cm −3. Wu et al grew K-doped p-type ZnO films on (0001) Al2O3 substrates by radio frequen‐

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 33

ZnO has attracted considerable attention because of its promising applications in UV LEDs and laser diodes. The fabrication of high-quality p-type ZnO remains great challenge. Many researchers reported on the heterojunction LEDs with n-type ZnO grown on p-type materi‐

als of Si, GaN and conducting oxides, as summarized in Table1.

cy magnetron sputtering [45].

**Table 1.** Structure and emission of n-ZnO based LEDs.

**4. ZnO-based LEDs**

**4.1. Heterojunction LEDs**

*4.1.1. n-ZnO heterojunction LEDs*

Xiu et al reported p-type P-doped ZnO films grown by MBE using a GaP effusion cell as a phosphorus dopant source [34]. PL spectra clearly indicated the existence of competitions between *D* <sup>0</sup> *X* and *A* <sup>0</sup> *X* for the phosphorus-doped ZnO films. The films exhibited a carrier concentration of 6.0×10<sup>18</sup> cm−3, Hall mobility of 1.5 cm<sup>2</sup> /Vs, and resistivity of 0.7Ωcm. Kim et al achieved p-type ZnO:P films on a sapphire substrate using phosphorus doping and a thermal annealing process [35]. As-grown n-type ZnO:P prepared by radio-frequency sput‐ tering were converted to *p*-ZnO:P by an rapid thermal annealing process under a N<sup>2</sup> ambi‐ ent. The films had a hole concentration of 1.0×1017 – 1.7×1019/cm3 , a mobility of 0.53 – 3.51cm<sup>2</sup> /Vs and a resistivity of 0.59 – 4.4 Ωcm. Pan et al prepared p-type ZnO:P films on the insulating quartz with a hole concentration of 1.84×1018 cm−3 by MOCVD [36]. Vaithianathan et al grew p-type ZnO:P films on Al2O3(0001) by PLD [37]. The films exhibited a hole con‐ centration of 5.1×10<sup>14</sup> −1.5×1017cm−3, a hole mobility of 2.38 − 39.3 cm<sup>2</sup> / V s, and a resistivity of 17 − 330 Ωcm.

Ryu et al investigated the electrical properties of As-doped ZnO films on O–ZnO substrates by hybrid beam deposition [38]. The electrical behavior of ZnO:As films changed from in‐ trinsic *n*-type to highly conductive *p*-type with increased As dopant concentration. They achieved p-type ZnO:As films with a hole concentration of 4×1017 cm-3 and a mobility of 35cm<sup>2</sup> /Vs. Vaithianathan et al reported As-doped p-type ZnO films using a Zn3As2 /ZnO tar‐ get by PLD [39]. As-grown ZnO:As showed n-type conductivity, however, ZnO:As films af‐ ter annealed at 200°C in N2 ambient for 2 min exhibited *p*-type conductivity with the hole concentrations varied between 2.48×1017 and 1.18×1018cm−3. Kang et al grew ZnO films on GaAs by sputtering and annealed at 500°C in an oxygen gas pressure of 40 mTorr for 20 min. After annealing, ZnO film on GaAs showed *p*-type conductivity with a hole concentra‐ tion of 9.684×1019cm−3, a mobility of 25.37cm2 /Vs, and a resistivity of 2.54 ×10−3 Ωcm. The ac‐ ceptor binding energy was calculated to be 0.1445eV, which was in good agreement with the ionization energy of AsZn - 2VZn acceptor complex (0.15eV) [40].

Guo et al reported p-type ZnO:Sb films grown by PLD [41]. The films showed a resistivity of 4.2 – 60Ωcm, a Hall mobility of 0.5 – 7.7cm2 /V s, and a hole concentration of 1.9–2.2×1017cm−3. In the (HR) TEM images of p-type ZnO:Sb, they observed a high density of threading dislo‐ cations originating from the film/substrate interface and a large number of partial disloca‐ tion loops associated with small stacking faults. Xiu et al fabricated p-type ZnO:Sb films grown on n-Si (100) by MBE [42]. The film had a concentration of 1.7×10<sup>18</sup> cm−3, and a high mobility of 20.0cm2 /Vs and a low resistivity of 0.2Ωcm. The acceptor energy level of the Sb dopant was about 0.2eV above the valence band, which was agreement with the ionization energy of SbZn − 2VZn (0.16eV).

Some researchers prepared p-type ZnO using Group I elements (Li, Na and K) as accept‐ or dopants. Yi et al fabricated p-type ZnO:Li films grown on quartz substrate by PLD with a hole concentration of 5.4×1018cm-3 [43]. p-type ZnO:Na films were fabricated on Si substrates by PLD [44]. The hole concentration ranged from 1.5×1018 to 1.1×1019 cm −3. Wu et al grew K-doped p-type ZnO films on (0001) Al2O3 substrates by radio frequen‐ cy magnetron sputtering [45].


**Table 1.** Structure and emission of n-ZnO based LEDs.

### **4. ZnO-based LEDs**

could not contribute the p-type behavious in X doped ZnO to XO, simply. Recently, for the large-size-mismatched impurities in ZnO, Limpijumnong et al proposed XZn−2VZn acceptor model [33]. In the model, X substitute Zn sites, forming a donor, then it induces two Zn va‐ cancy acceptors as a complex form XZn−2VZn. The ionization energy of AsZn - 2VZn complex

Xiu et al reported p-type P-doped ZnO films grown by MBE using a GaP effusion cell as a phosphorus dopant source [34]. PL spectra clearly indicated the existence of competitions between *D* <sup>0</sup> *X* and *A* <sup>0</sup> *X* for the phosphorus-doped ZnO films. The films exhibited a carrier

al achieved p-type ZnO:P films on a sapphire substrate using phosphorus doping and a thermal annealing process [35]. As-grown n-type ZnO:P prepared by radio-frequency sput‐ tering were converted to *p*-ZnO:P by an rapid thermal annealing process under a N<sup>2</sup> ambi‐

insulating quartz with a hole concentration of 1.84×1018 cm−3 by MOCVD [36]. Vaithianathan et al grew p-type ZnO:P films on Al2O3(0001) by PLD [37]. The films exhibited a hole con‐ centration of 5.1×10<sup>14</sup> −1.5×1017cm−3, a hole mobility of 2.38 − 39.3 cm<sup>2</sup> / V s, and a resistivity

Ryu et al investigated the electrical properties of As-doped ZnO films on O–ZnO substrates by hybrid beam deposition [38]. The electrical behavior of ZnO:As films changed from in‐ trinsic *n*-type to highly conductive *p*-type with increased As dopant concentration. They achieved p-type ZnO:As films with a hole concentration of 4×1017 cm-3 and a mobility of

ceptor binding energy was calculated to be 0.1445eV, which was in good agreement with the

Guo et al reported p-type ZnO:Sb films grown by PLD [41]. The films showed a resistivity of

In the (HR) TEM images of p-type ZnO:Sb, they observed a high density of threading dislo‐ cations originating from the film/substrate interface and a large number of partial disloca‐ tion loops associated with small stacking faults. Xiu et al fabricated p-type ZnO:Sb films grown on n-Si (100) by MBE [42]. The film had a concentration of 1.7×10<sup>18</sup> cm−3, and a high

dopant was about 0.2eV above the valence band, which was agreement with the ionization

Some researchers prepared p-type ZnO using Group I elements (Li, Na and K) as accept‐ or dopants. Yi et al fabricated p-type ZnO:Li films grown on quartz substrate by PLD

/Vs and a low resistivity of 0.2Ωcm. The acceptor energy level of the Sb

/Vs. Vaithianathan et al reported As-doped p-type ZnO films using a Zn3As2 /ZnO tar‐ get by PLD [39]. As-grown ZnO:As showed n-type conductivity, however, ZnO:As films af‐ ter annealed at 200°C in N2 ambient for 2 min exhibited *p*-type conductivity with the hole concentrations varied between 2.48×1017 and 1.18×1018cm−3. Kang et al grew ZnO films on GaAs by sputtering and annealed at 500°C in an oxygen gas pressure of 40 mTorr for 20 min. After annealing, ZnO film on GaAs showed *p*-type conductivity with a hole concentra‐

/Vs and a resistivity of 0.59 – 4.4 Ωcm. Pan et al prepared p-type ZnO:P films on the

/Vs, and resistivity of 0.7Ωcm. Kim et

/Vs, and a resistivity of 2.54 ×10−3 Ωcm. The ac‐

/V s, and a hole concentration of 1.9–2.2×1017cm−3.

, a mobility of 0.53 –

was calculated to be 0.15eV (0.16eV for SbZn − 2VZn ).

32 Optoelectronics - Advanced Materials and Devices

concentration of 6.0×10<sup>18</sup> cm−3, Hall mobility of 1.5 cm<sup>2</sup>

tion of 9.684×1019cm−3, a mobility of 25.37cm2

4.2 – 60Ωcm, a Hall mobility of 0.5 – 7.7cm2

ionization energy of AsZn - 2VZn acceptor complex (0.15eV) [40].

3.51cm<sup>2</sup>

35cm<sup>2</sup>

of 17 − 330 Ωcm.

mobility of 20.0cm2

energy of SbZn − 2VZn (0.16eV).

ent. The films had a hole concentration of 1.0×1017 – 1.7×1019/cm3

#### **4.1. Heterojunction LEDs**

#### *4.1.1. n-ZnO heterojunction LEDs*

ZnO has attracted considerable attention because of its promising applications in UV LEDs and laser diodes. The fabrication of high-quality p-type ZnO remains great challenge. Many researchers reported on the heterojunction LEDs with n-type ZnO grown on p-type materi‐ als of Si, GaN and conducting oxides, as summarized in Table1.

Chang et al reported the MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode [46]. They grew 1-μm-thick undoped GaN buffer layer on Al2O3 and a 500-nm-thick Mgdoped p-GaN layer by MOCVD, and grew 300-nm-thick n-ZnO by MBE on p-GaN layer. They observed a broad yellowish green emission peaked at around 570nm. The EL emission was attributed to the electron injection from n-ZnO to p-GaN. Hwang et al fabricated an n-ZnO:Ga/p-GaN:Mg heterostructure on Al2O3 substrate [51]. Undoped ZnO (buffer layer) and n-ZnO films doped with about 1% Ga were grown at 75W radio frequency (RF) power in 100% O2 atmosphere, at 8000 C and 7000 C, respectively. I-V characteristics exhibited the typical rectifying behavior and the EL emissions from the n-ZnO:Ga/p-GaN:Mg heterostruc‐ ture at room temperature show peaks at 430nm, 440nm and 480nm along with a broad band of yellow light.

Chichibu et al fabricated p-CuGaS2/n-ZnO:Al heterojunction LEDs by metal-organic vapor phase epitaxy (p-CuGaS2) and helicon-wave-excited-plasma (HWEP) sputtering method (n-ZnO:Al) [63]. The EL spectra exhibited emission peaks and bands between 1.6 and 2.5eV. Ohta et al reported on p-SrCu2O2/n-ZnO heterojunction LEDs [56]. The I-V curve of the de‐ vice exhibited nonlinear characteristics where the turn-on voltage was approximately 1.5V. An UV emission band centered at 382nm was observed at room temperature when a for‐

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 35

**Figure 7.** UV emission spectra of the *p*-SrCu2O2 /*n*-ZnO *p* – *n* junction LED for several currents. electric currents were

Ye et al reported the distinct visible electroluminescence at room temperature from *n*-ZnO/*p*-Si heterojunction [59]. A high-quality ZnO layers were fabricated by metal organ‐ ic chemical vapor deposition technique on *p*-type Si (111) substrate at 650°C. Before grew ZnO layer, a thin ZnO buffer layer (~25nm) was deposited to relieve the strains due to large lattice mismatch between Si and ZnO and to avoid the oxidation of Si sur‐ face. The EL peak energy coincided well with the deep-level photoluminescence of ZnO, indicating that the EL emission was originated from the radiative recombination via

To improve the emission of n-ZnO-based heterojunction LEDs, dounble and triple hetero‐ junction LEDs were fabricated. Osinsky et al reported MgZnO/ZnO/AlGaN/GaN triple het‐ erostructures light-emitting diodes [50]. I-V curves of the device showed a rectifying characteristics with a turn-on voltage of ~3.2 V. Strong optical emission was observed at ~390nm. Mares et al prepared a hybrid n-MgZnO/CdZnO/p-GaN LEDs with a Cd0.12Zn0.88O quantum-well [49]. Under forward bias, visible electroluminescence was observed at room temperature in the EL spectrum of the device. The EL red shifted from 3.32 to 3.15eV as the

(a) 10mA, (b) 11mA, (c) 14mA, and (d) 15mA, respectively. From Ref. [56].

deep-level defects in *n*-ZnO layers.

ward bias voltage greater than 3V was applied to the device, as shown in Figure7.

Lee et al investigated the origin of emission of the annealed n-ZnO/p-GaN heterostructure LED [52]. They fabricated n-ZnO/p-GaN heterojunction LED on Al2O3 substrates by MOCVD (GaN layer) and RF sputtering (ZnO layer). After fabrication, ZnO films were an‐ nealed in a thermal furnace in air and nitrogen ambient at 800°C for 30–120min. For the LED annealed in N2, room-temperature EL in the blue region with peak wavelength 400nm was observed, and for the LED annealed in N2, a broad band from 400 to 700nm was detected in the EL emission spectrum, as shown in Figure6. Alivov et al reported the n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6H-SiC substrates [61]. *n*-type ZnO layer with a thickness of 1μm was deposited on *p*-Al0.12Ga0.88N and I-V curve of the devices showed a rec‐ tifying diode-like behavior with threshold voltage ~3.2 V, a high reverse breakdown voltage of 30V and a small reverse leakage current of about 10-7 A. Under forward bias, UV EL with a peak emission near 389nm (~3.19eV) and a full-width at half-maximum (FWHM) of 26nm was observed in the EL spectrum of the device. The emission was stable at temperatures up to 500K and was attributed to the recombination of the carriers within the ZnO. They also observed 430nm electroluminescence from ZnO/GaN heterojunction LEDs [62]. Yu et al re‐ ported ZnO/GaN heterostructure LEDs with a donor–acceptor pair emission band at 3.270eV [58]. In the EL spectrum of the device, two emission peaks, a strong emission peak (384.0nm), together with a weak emission (365.4nm) feature on the higher-energy side.

**Figure 6.** EL spectra of n-ZnO/p-GaN heterosturcture LED annealed (a) in N2 and (b) in air ambient. Inset: pictures of Light emission. From Ref. [52].

Chichibu et al fabricated p-CuGaS2/n-ZnO:Al heterojunction LEDs by metal-organic vapor phase epitaxy (p-CuGaS2) and helicon-wave-excited-plasma (HWEP) sputtering method (n-ZnO:Al) [63]. The EL spectra exhibited emission peaks and bands between 1.6 and 2.5eV. Ohta et al reported on p-SrCu2O2/n-ZnO heterojunction LEDs [56]. The I-V curve of the de‐ vice exhibited nonlinear characteristics where the turn-on voltage was approximately 1.5V. An UV emission band centered at 382nm was observed at room temperature when a for‐ ward bias voltage greater than 3V was applied to the device, as shown in Figure7.

Chang et al reported the MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode [46]. They grew 1-μm-thick undoped GaN buffer layer on Al2O3 and a 500-nm-thick Mgdoped p-GaN layer by MOCVD, and grew 300-nm-thick n-ZnO by MBE on p-GaN layer. They observed a broad yellowish green emission peaked at around 570nm. The EL emission was attributed to the electron injection from n-ZnO to p-GaN. Hwang et al fabricated an n-ZnO:Ga/p-GaN:Mg heterostructure on Al2O3 substrate [51]. Undoped ZnO (buffer layer) and n-ZnO films doped with about 1% Ga were grown at 75W radio frequency (RF) power

typical rectifying behavior and the EL emissions from the n-ZnO:Ga/p-GaN:Mg heterostruc‐ ture at room temperature show peaks at 430nm, 440nm and 480nm along with a broad band

Lee et al investigated the origin of emission of the annealed n-ZnO/p-GaN heterostructure LED [52]. They fabricated n-ZnO/p-GaN heterojunction LED on Al2O3 substrates by MOCVD (GaN layer) and RF sputtering (ZnO layer). After fabrication, ZnO films were an‐ nealed in a thermal furnace in air and nitrogen ambient at 800°C for 30–120min. For the LED annealed in N2, room-temperature EL in the blue region with peak wavelength 400nm was observed, and for the LED annealed in N2, a broad band from 400 to 700nm was detected in the EL emission spectrum, as shown in Figure6. Alivov et al reported the n-ZnO/p-AlGaN heterojunction light-emitting diodes on 6H-SiC substrates [61]. *n*-type ZnO layer with a thickness of 1μm was deposited on *p*-Al0.12Ga0.88N and I-V curve of the devices showed a rec‐ tifying diode-like behavior with threshold voltage ~3.2 V, a high reverse breakdown voltage of 30V and a small reverse leakage current of about 10-7 A. Under forward bias, UV EL with a peak emission near 389nm (~3.19eV) and a full-width at half-maximum (FWHM) of 26nm was observed in the EL spectrum of the device. The emission was stable at temperatures up to 500K and was attributed to the recombination of the carriers within the ZnO. They also observed 430nm electroluminescence from ZnO/GaN heterojunction LEDs [62]. Yu et al re‐ ported ZnO/GaN heterostructure LEDs with a donor–acceptor pair emission band at 3.270eV [58]. In the EL spectrum of the device, two emission peaks, a strong emission peak (384.0nm), together with a weak emission (365.4nm) feature on the higher-energy side.

**Figure 6.** EL spectra of n-ZnO/p-GaN heterosturcture LED annealed (a) in N2 and (b) in air ambient. Inset: pictures of

C, respectively. I-V characteristics exhibited the

C and 7000

in 100% O2 atmosphere, at 8000

34 Optoelectronics - Advanced Materials and Devices

of yellow light.

Light emission. From Ref. [52].

**Figure 7.** UV emission spectra of the *p*-SrCu2O2 /*n*-ZnO *p* – *n* junction LED for several currents. electric currents were (a) 10mA, (b) 11mA, (c) 14mA, and (d) 15mA, respectively. From Ref. [56].

Ye et al reported the distinct visible electroluminescence at room temperature from *n*-ZnO/*p*-Si heterojunction [59]. A high-quality ZnO layers were fabricated by metal organ‐ ic chemical vapor deposition technique on *p*-type Si (111) substrate at 650°C. Before grew ZnO layer, a thin ZnO buffer layer (~25nm) was deposited to relieve the strains due to large lattice mismatch between Si and ZnO and to avoid the oxidation of Si sur‐ face. The EL peak energy coincided well with the deep-level photoluminescence of ZnO, indicating that the EL emission was originated from the radiative recombination via deep-level defects in *n*-ZnO layers.

To improve the emission of n-ZnO-based heterojunction LEDs, dounble and triple hetero‐ junction LEDs were fabricated. Osinsky et al reported MgZnO/ZnO/AlGaN/GaN triple het‐ erostructures light-emitting diodes [50]. I-V curves of the device showed a rectifying characteristics with a turn-on voltage of ~3.2 V. Strong optical emission was observed at ~390nm. Mares et al prepared a hybrid n-MgZnO/CdZnO/p-GaN LEDs with a Cd0.12Zn0.88O quantum-well [49]. Under forward bias, visible electroluminescence was observed at room temperature in the EL spectrum of the device. The EL red shifted from 3.32 to 3.15eV as the forward current was increased from 20 to 40mA. Zhang et al investigated the effects of the crystalline and thickness of AlN layer on the electroluminescent performance of n-ZnO/AlN/p-GaN [54]. They found that the better crystalline quality of AlN barrier layer may facilitate the improvement of EL performance of the device. For the thinner AlN layer, it was not enough to cover the whole surface of GaN, while in the thicker AlN layer, many of electrons were captured and nonradiatively recombined via the deep donors, indicating that AlN barrier layer played an important role on the performance of the device. In their experiments, AlN layer at the growth temperature of 7000 C with an optimized thickness of around 10nm improved EL performance of the devices.

Xu et al reported ordered ZnO nanowire array blue/near-UV LEDs [64]. The devices were fabricated by a conjunction of low temperature wet chemical methods and elec‐ tron beam lithography. ZnO nanowire arrays were grown on Mg-doped p-type GaN films. *I* – *V* curves of the devices exhibited the typical rectifying behavior. Under for‐ ward bias, each single nanowire was a light emitter. The EL spectra of the devices were shown in Figure8. It can be seen that the contour of the EL spectrum does not change much with the biased voltage in the range of 4-10V and the dominant emis‐ sion peak is slightly blue shifted in the range of 400nm– 420nm. By Gaussian deconvo‐ lution of the emission spectrum, the blue/near-UV emission is attributed particularly to three distinct electron-hole recombination processes. The LEDs give an external quan‐ tum efficiency of 2.5%, displaying great potential applications in high resolution electron‐

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 37

**Figure 8.** EL spectrum as a function of the forward biased voltage. Inset shows by Gaussian deconvolution analysis the blue/near-UV emission could be decomposed into three distinct bands that correspond to three different optoelec‐

Zhang et al fabricated high-brightness blue-light-emitting diode using a ZnO-nanowire ar‐ ray grown on p-GaN thin film [69]. The EL spectrum of the device showed broad emission peaks from UV (370nm) to blue. When the forward bias increased from 10 to 35V, the emis‐ sion peak was significantly enhanced and the main emission peak shifted from 440 to 400nm when the forward bias was increased (Figure9), indicating that the modification of external voltage to the band profile in the depletion region. Lupan et al observed UV emission at 397nm with a low forward-voltage emission threshold of 4.4V and a high brightness above

Alvi et al investigated the n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes, systematically [67]. In their experiments, ZnO nanostructures were grown on p-GaN substrates using a low temperature aqueous

C) forming p – n heterojunctions. The EL spectrum of ZnO‐

ic display, optical interconnect, and high density data storage.

tronic processes. From Ref. [64].

5– 6V in ZnO-nanowire/p-GaN LEDs [75].

chemical growth method (<1000

#### *4.1.2. Heterojunction LEDs with ZnO nanomaterials*

The optical devices using ZnO nanomterials have attracted considerable attention due to their promising optical properties, such as enriched radiative recombination of carriers. Var‐ ious ZnO nanomaterials have been grown by different methods. Based on the growth of ntype ZnO nanomaterials, some researcher reported the heterojunction LEDs with ZnO nanomaterials, as summarized in Table2. Here, we only present the typical results on heter‐ ojunction LEDs with ZnO nanomaterials.


**Table 2.** Structure and emission of heterojunction LEDs with ZnO nanostructures.

Xu et al reported ordered ZnO nanowire array blue/near-UV LEDs [64]. The devices were fabricated by a conjunction of low temperature wet chemical methods and elec‐ tron beam lithography. ZnO nanowire arrays were grown on Mg-doped p-type GaN films. *I* – *V* curves of the devices exhibited the typical rectifying behavior. Under for‐ ward bias, each single nanowire was a light emitter. The EL spectra of the devices were shown in Figure8. It can be seen that the contour of the EL spectrum does not change much with the biased voltage in the range of 4-10V and the dominant emis‐ sion peak is slightly blue shifted in the range of 400nm– 420nm. By Gaussian deconvo‐ lution of the emission spectrum, the blue/near-UV emission is attributed particularly to three distinct electron-hole recombination processes. The LEDs give an external quan‐ tum efficiency of 2.5%, displaying great potential applications in high resolution electron‐ ic display, optical interconnect, and high density data storage.

forward current was increased from 20 to 40mA. Zhang et al investigated the effects of the crystalline and thickness of AlN layer on the electroluminescent performance of n-ZnO/AlN/p-GaN [54]. They found that the better crystalline quality of AlN barrier layer may facilitate the improvement of EL performance of the device. For the thinner AlN layer, it was not enough to cover the whole surface of GaN, while in the thicker AlN layer, many of electrons were captured and nonradiatively recombined via the deep donors, indicating that AlN barrier layer played an important role on the performance of the device. In their

The optical devices using ZnO nanomterials have attracted considerable attention due to their promising optical properties, such as enriched radiative recombination of carriers. Var‐ ious ZnO nanomaterials have been grown by different methods. Based on the growth of ntype ZnO nanomaterials, some researcher reported the heterojunction LEDs with ZnO nanomaterials, as summarized in Table2. Here, we only present the typical results on heter‐

C with an optimized thickness of

experiments, AlN layer at the growth temperature of 7000

**Table 2.** Structure and emission of heterojunction LEDs with ZnO nanostructures.

around 10nm improved EL performance of the devices.

*4.1.2. Heterojunction LEDs with ZnO nanomaterials*

36 Optoelectronics - Advanced Materials and Devices

ojunction LEDs with ZnO nanomaterials.

**Figure 8.** EL spectrum as a function of the forward biased voltage. Inset shows by Gaussian deconvolution analysis the blue/near-UV emission could be decomposed into three distinct bands that correspond to three different optoelec‐ tronic processes. From Ref. [64].

Zhang et al fabricated high-brightness blue-light-emitting diode using a ZnO-nanowire ar‐ ray grown on p-GaN thin film [69]. The EL spectrum of the device showed broad emission peaks from UV (370nm) to blue. When the forward bias increased from 10 to 35V, the emis‐ sion peak was significantly enhanced and the main emission peak shifted from 440 to 400nm when the forward bias was increased (Figure9), indicating that the modification of external voltage to the band profile in the depletion region. Lupan et al observed UV emission at 397nm with a low forward-voltage emission threshold of 4.4V and a high brightness above 5– 6V in ZnO-nanowire/p-GaN LEDs [75].

Alvi et al investigated the n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN white-light-emitting diodes, systematically [67]. In their experiments, ZnO nanostructures were grown on p-GaN substrates using a low temperature aqueous chemical growth method (<1000 C) forming p – n heterojunctions. The EL spectrum of ZnO‐ nanowall LED exhibited three peaks centered at 420nm (violet emission), 450nm (violet– blue) and broad peak covering from 480 to 700nm (green, yellow, orange and red emis‐ sions). For ZnO nanoflowers LEDs, the emission peaks centered at 400nm (violet emission), 450nm (violet–blue) and a broad peak covering EL emissions from 480 to 700nm (green, yel‐ low, orange and red emissions) were detected. ZnO nanorods and nanotubes LEDs showed the same EL spectra, and EL peaks centered at 400nm (violet emission), 450nm (violet–blue) and 540nm (green emission) were observed. For ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN LEDs, the color rendering indices (CRI) were 95, 93, 87 and 88, and the correlated color temperatures were 6518, 5471, 4807 and 4801K, respectively.

rods (Figure11). The color rendering index and correlated color temperature of the white

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 39

**Figure 10.** Schematic illustration of ZnO NRs/PFO hybrid device on PEDOT:PSS coated flexible plastic. From Ref. [65].

**Figure 11.** Room temperature EL spectrum and Gaussian fitting of the PFO/ZnO hybrid white LED. Inset: a photo‐

graph of white light emission from flexible device folded at a large angle during operation. From Ref. [65].

**Figure 12.** Typical color coordinates characteristics of the ZnO – PFO hybrid white LEDs. From Ref. [65].

LEDs were calculated to be 68 and 5800K, respectively (Figure12).

**Figure 9.** The electroluminescence spectrum of the (n-ZnO NWs)/(p-GaN film) LED device under various forward bias voltages (10, 15, 20, 25, 30, 35V), showing broad emission peaks from UV to blue and blue shift with increasing of bias voltage. From Ref. [69].

Xi et al fabricated heterojunction NiO/ZnO LEDs using low temperature solution-based growth method [77]. The devices exhibited room-temperature electroluminescence, and the steady increase of the UV-to-visible emission ratio was obtained for increased bias voltage, which was good agreement with some of the reported behavior of ZnO LEDs.

Klason et al reported the EL spectra obtained from ZnO nanodots/p-Si heterojunction LEDs [71]. The asymmetric EL emission peaked at around 600nm was observed and the emission from the devices having buffer layer were a bit blue shifted when compared to samples with‐ out the buffer layer. The buffer layer increased both the stability and efficiency of the devices.

Bano et al reported the ZnO-organic hybrid white LEDs grown on flexible plastic using low temperature aqueous chemical method [65]. Figure10 shows the structure of ZnO-organic hybrid white LEDs, schematically. I-V curve of the device exhibited the typical rectifying be‐ haviors. The EL spectrum displayed a broad emission band covering the whole visible re‐ gion and hence provided white light. The white light emission was the superposition of a violet line (448nm), blue line (469nm) from the PFO combined with green emissions (503 and 541nm), and a red emission (620nm) due to deep level defect emissions in ZnO nano‐ rods (Figure11). The color rendering index and correlated color temperature of the white LEDs were calculated to be 68 and 5800K, respectively (Figure12).

nanowall LED exhibited three peaks centered at 420nm (violet emission), 450nm (violet– blue) and broad peak covering from 480 to 700nm (green, yellow, orange and red emis‐ sions). For ZnO nanoflowers LEDs, the emission peaks centered at 400nm (violet emission), 450nm (violet–blue) and a broad peak covering EL emissions from 480 to 700nm (green, yel‐ low, orange and red emissions) were detected. ZnO nanorods and nanotubes LEDs showed the same EL spectra, and EL peaks centered at 400nm (violet emission), 450nm (violet–blue) and 540nm (green emission) were observed. For ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN LEDs, the color rendering indices (CRI) were 95, 93, 87 and 88, and the correlated color temperatures were 6518, 5471, 4807 and 4801K, respectively.

**Figure 9.** The electroluminescence spectrum of the (n-ZnO NWs)/(p-GaN film) LED device under various forward bias voltages (10, 15, 20, 25, 30, 35V), showing broad emission peaks from UV to blue and blue shift with increasing of bias

Xi et al fabricated heterojunction NiO/ZnO LEDs using low temperature solution-based growth method [77]. The devices exhibited room-temperature electroluminescence, and the steady increase of the UV-to-visible emission ratio was obtained for increased bias voltage,

Klason et al reported the EL spectra obtained from ZnO nanodots/p-Si heterojunction LEDs [71]. The asymmetric EL emission peaked at around 600nm was observed and the emission from the devices having buffer layer were a bit blue shifted when compared to samples with‐ out the buffer layer. The buffer layer increased both the stability and efficiency of the devices.

Bano et al reported the ZnO-organic hybrid white LEDs grown on flexible plastic using low temperature aqueous chemical method [65]. Figure10 shows the structure of ZnO-organic hybrid white LEDs, schematically. I-V curve of the device exhibited the typical rectifying be‐ haviors. The EL spectrum displayed a broad emission band covering the whole visible re‐ gion and hence provided white light. The white light emission was the superposition of a violet line (448nm), blue line (469nm) from the PFO combined with green emissions (503 and 541nm), and a red emission (620nm) due to deep level defect emissions in ZnO nano‐

which was good agreement with some of the reported behavior of ZnO LEDs.

voltage. From Ref. [69].

38 Optoelectronics - Advanced Materials and Devices

**Figure 10.** Schematic illustration of ZnO NRs/PFO hybrid device on PEDOT:PSS coated flexible plastic. From Ref. [65].

**Figure 11.** Room temperature EL spectrum and Gaussian fitting of the PFO/ZnO hybrid white LED. Inset: a photo‐ graph of white light emission from flexible device folded at a large angle during operation. From Ref. [65].

**Figure 12.** Typical color coordinates characteristics of the ZnO – PFO hybrid white LEDs. From Ref. [65].

#### *4.1.3. p-ZnO heterojunction LEDs*

Currently, the fabrication of p-type ZnO materials has made remarkable progress and some researchers attempted to prepare p-type ZnO based hereojunction LEDs by various meth‐ ods, as summarized in Table3.

doped ZnO film(50nm) was grown at low temperature on *n*-Si(100) substrate as a buffer layer, followed by p-type ZnO:Sb layer (370nm) at a higher temperature by MBE. After the growth, thermal activation of Sb dopant was carried out in *in situ* in vacuum at 800°C for 30 min. The I-V curves of the heterojunction LEDs displayed a typical rectifying behavior with higher leakage current at both higher temperatures and higher biases, which may origin from the band alignment of wide-band-gap *p*-ZnO and narrow-band-gap *n*-Si. Figure13 shows the EL spectra obtained at different temperatures for an injection current of 110mA. Four emission peaks at 381, 485, 612 and 671nm were detected from the spectra at 9 K. The peak at 381nm was the near-band edge emission and the other peaks were attributed to in‐ trinsic defects in ZnO. A small UV peak at 396nm was also observed in the EL spectra at 9K, which was related to Zn vacancies. With increasing temperature in the range from 9 to 300K, both the small UV peak and the near band edge emission redshifted and became a single peak at higher temperatures. The intensity of emissions decreased throughout the spectra with increasing temperature, which was due to the increase in nonradiative recombinations

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 41

**Figure 14.** *I*-*V* curve of the device, showing rectifying characteristics.Top inset shows the linear *I*-*V* of *n*-contacts (red line) and *p*-contacts (blue line), respectively. Bottom inset shows device structure of the sample: SbZnO/CdZnO/

Li et al reported on the blue electroluminescence from ZnO based heterojunction LEDs with CdZnO active layers [80]. *p*-ZnO:Sb/*i*-CdZnO/*n*-ZnO was grown on *n*-type Si substrates by plasma-assisted molecular-beam epitaxy. Figure14 shows the typical rectifying characteris‐ tics of the heterojunction LEDs, and the top inset of Figure14 reveals the I-V curves of Au/Ni and Au/Ti metal contacts on p-type ZnO:Sb and n-type Si, respectively, indicating that good ohmic contacts were formed on both electrodes. The PL spectra showed ZnO near band edge (NBE) emission of p-type ZnO:Sb at 378nm and NBE emission of CdZnO active layer at 445nm. Figure15 shows EL spectra of a hereojunction under different injection currents. The blue EL emissions at around 459nm were observed and the emission intensity increased

at higher temperatures.

GaZnO/Si. From Ref. [80].


**Table 3.** Structure and emission of p-type ZnO based heterojunction LEDs

**Figure 13.** Temperature dependent EL spectra obtained at an injection current of 110 mA. EL from LED is obtained at 9, 50, 100, 200, and 300 K. From Ref. [79].

Mandalapu et al reported ultraviolet emission from Sb-doped *p*-type ZnO based heterojunc‐ tion LEDs fabricated by growing p-type ZnO:Sb films on n-type Si substrates [79]. Thin un‐ doped ZnO film(50nm) was grown at low temperature on *n*-Si(100) substrate as a buffer layer, followed by p-type ZnO:Sb layer (370nm) at a higher temperature by MBE. After the growth, thermal activation of Sb dopant was carried out in *in situ* in vacuum at 800°C for 30 min. The I-V curves of the heterojunction LEDs displayed a typical rectifying behavior with higher leakage current at both higher temperatures and higher biases, which may origin from the band alignment of wide-band-gap *p*-ZnO and narrow-band-gap *n*-Si. Figure13 shows the EL spectra obtained at different temperatures for an injection current of 110mA. Four emission peaks at 381, 485, 612 and 671nm were detected from the spectra at 9 K. The peak at 381nm was the near-band edge emission and the other peaks were attributed to in‐ trinsic defects in ZnO. A small UV peak at 396nm was also observed in the EL spectra at 9K, which was related to Zn vacancies. With increasing temperature in the range from 9 to 300K, both the small UV peak and the near band edge emission redshifted and became a single peak at higher temperatures. The intensity of emissions decreased throughout the spectra with increasing temperature, which was due to the increase in nonradiative recombinations at higher temperatures.

*4.1.3. p-ZnO heterojunction LEDs*

40 Optoelectronics - Advanced Materials and Devices

ods, as summarized in Table3.

9, 50, 100, 200, and 300 K. From Ref. [79].

**Table 3.** Structure and emission of p-type ZnO based heterojunction LEDs

Currently, the fabrication of p-type ZnO materials has made remarkable progress and some researchers attempted to prepare p-type ZnO based hereojunction LEDs by various meth‐

**Figure 13.** Temperature dependent EL spectra obtained at an injection current of 110 mA. EL from LED is obtained at

Mandalapu et al reported ultraviolet emission from Sb-doped *p*-type ZnO based heterojunc‐ tion LEDs fabricated by growing p-type ZnO:Sb films on n-type Si substrates [79]. Thin un‐

**Figure 14.** *I*-*V* curve of the device, showing rectifying characteristics.Top inset shows the linear *I*-*V* of *n*-contacts (red line) and *p*-contacts (blue line), respectively. Bottom inset shows device structure of the sample: SbZnO/CdZnO/ GaZnO/Si. From Ref. [80].

Li et al reported on the blue electroluminescence from ZnO based heterojunction LEDs with CdZnO active layers [80]. *p*-ZnO:Sb/*i*-CdZnO/*n*-ZnO was grown on *n*-type Si substrates by plasma-assisted molecular-beam epitaxy. Figure14 shows the typical rectifying characteris‐ tics of the heterojunction LEDs, and the top inset of Figure14 reveals the I-V curves of Au/Ni and Au/Ti metal contacts on p-type ZnO:Sb and n-type Si, respectively, indicating that good ohmic contacts were formed on both electrodes. The PL spectra showed ZnO near band edge (NBE) emission of p-type ZnO:Sb at 378nm and NBE emission of CdZnO active layer at 445nm. Figure15 shows EL spectra of a hereojunction under different injection currents. The blue EL emissions at around 459nm were observed and the emission intensity increased with the increase of the injection current. A comparison of PL and EL spectra of the device shows that the EL emission of the device origined from the radiative recombinations in CdZnO active layers in the film.

Nakahara et al fabricated heterostructure LEDs by growing N-doped p-tye MgxZn1-xO layer by MBE on Zn face ZnO crystal using NO and NH3 as N source [81]. The structures of the LEDs were shown in Figures16(a) and (b). The I-V curves of the LEDs exhibited a rectifying property [Figure16(c)]. LEDs A and B (NO as N source) had a turn-on voltage of approxi‐ mately 3V, LEDs C and D (NH3 as N source) showed a turn-on voltage of 10V. In the EL spectra of LEDs A and B, a sharp EL peak at the near band edge (λ = 380 – 400nm) was seen, indicating an effective blocking of electrons by the wide band gap Mg0.1Zn0.9O:N layers and efficient exciton recombination in the *n*-type ZnO layers [Figure16(d)]. The EL of LED C was

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 43

**Figure 17.** a) EL spectra of the LEDs B and D operated with various current densities. (b) Integrated EL intensity for the spectra shown in (a) (LEDs B and D) as a function of applied current density. The data for LEDs A and C are also shown. The integration was conducted in a wavelength range from 350 to 450 nm. (c) EL spectra for bare and with a green phosphor coating for LED D at an operation current of 40 mA. Inset is a picture taken under standard laboratory illu‐

The EL peak energies of the LEDs were slightly lower than the PL peak energy of ZnO, which were due to the self-absorption of the ZnO emission in the thick ZnO substrate and the heating effect during the operation [Figure17(a)]. The outpower of the LEDs ranged from 0.1 to 70μW at the maximum attainable operation current (typically 30–40mA)[Figure17(b)]. Interestingly, when LED was coated with a 0.1-mm-thick epoxy resin containing 5wt % (BaEu) (MgMn)Al10O17 green phosphor, a part of ultraviolet NBE in the EL spectrum of the LED was converted into green, indicating that the UV emission may excite many existing phosphors de‐

mination. Emission from the phosphor can be clearly seen as indicated by an arrow. From Ref. [81].

veloped for fluorescent tube and enable better color rendering [Figure17(c)].

much more intense by a factor of 800 than those of LEDs A and B.

**Figure 15.** Room temperature EL characteristics of the LEDs under different injection currents. From Ref. [80].

**Figure 16.** a) and (b) depict cross-sectional schematics for LED A and those of B–D, respectively. The device was 500μm in diameter and the back side of the ZnO substrates was bonded to a metal plate with In. The top electrode in (a) was formed as Au(200nm)/Ni(10nm) with a diamete of 250 μm. That of (b) consists of a semitransparent electrode of Au (4nm)/Ni (2nm) with a diameter of 500μm and a contact pad of Au (500nm)/Ni (2nm) with a diameter of 350μm. (c) The rectifying *I*-*V* curve measured for LEDs A, B, C, and D. (d) EL spectra from LEDs A–C. All spectra were measured at room temperature. Forward bias operation conditions are also shown. From Ref. [81].

Nakahara et al fabricated heterostructure LEDs by growing N-doped p-tye MgxZn1-xO layer by MBE on Zn face ZnO crystal using NO and NH3 as N source [81]. The structures of the LEDs were shown in Figures16(a) and (b). The I-V curves of the LEDs exhibited a rectifying property [Figure16(c)]. LEDs A and B (NO as N source) had a turn-on voltage of approxi‐ mately 3V, LEDs C and D (NH3 as N source) showed a turn-on voltage of 10V. In the EL spectra of LEDs A and B, a sharp EL peak at the near band edge (λ = 380 – 400nm) was seen, indicating an effective blocking of electrons by the wide band gap Mg0.1Zn0.9O:N layers and efficient exciton recombination in the *n*-type ZnO layers [Figure16(d)]. The EL of LED C was much more intense by a factor of 800 than those of LEDs A and B.

with the increase of the injection current. A comparison of PL and EL spectra of the device shows that the EL emission of the device origined from the radiative recombinations in

**Figure 15.** Room temperature EL characteristics of the LEDs under different injection currents. From Ref. [80].

**Figure 16.** a) and (b) depict cross-sectional schematics for LED A and those of B–D, respectively. The device was 500μm in diameter and the back side of the ZnO substrates was bonded to a metal plate with In. The top electrode in (a) was formed as Au(200nm)/Ni(10nm) with a diamete of 250 μm. That of (b) consists of a semitransparent electrode of Au (4nm)/Ni (2nm) with a diameter of 500μm and a contact pad of Au (500nm)/Ni (2nm) with a diameter of 350μm. (c) The rectifying *I*-*V* curve measured for LEDs A, B, C, and D. (d) EL spectra from LEDs A–C. All spectra were

measured at room temperature. Forward bias operation conditions are also shown. From Ref. [81].

CdZnO active layers in the film.

42 Optoelectronics - Advanced Materials and Devices

**Figure 17.** a) EL spectra of the LEDs B and D operated with various current densities. (b) Integrated EL intensity for the spectra shown in (a) (LEDs B and D) as a function of applied current density. The data for LEDs A and C are also shown. The integration was conducted in a wavelength range from 350 to 450 nm. (c) EL spectra for bare and with a green phosphor coating for LED D at an operation current of 40 mA. Inset is a picture taken under standard laboratory illu‐ mination. Emission from the phosphor can be clearly seen as indicated by an arrow. From Ref. [81].

The EL peak energies of the LEDs were slightly lower than the PL peak energy of ZnO, which were due to the self-absorption of the ZnO emission in the thick ZnO substrate and the heating effect during the operation [Figure17(a)]. The outpower of the LEDs ranged from 0.1 to 70μW at the maximum attainable operation current (typically 30–40mA)[Figure17(b)]. Interestingly, when LED was coated with a 0.1-mm-thick epoxy resin containing 5wt % (BaEu) (MgMn)Al10O17 green phosphor, a part of ultraviolet NBE in the EL spectrum of the LED was converted into green, indicating that the UV emission may excite many existing phosphors de‐ veloped for fluorescent tube and enable better color rendering [Figure17(c)].


*4.1.4. Homojunction LEDs*

junction LEDs.

[Figure1(c)].

eration of LEDs.

estimated to be only 1nW at drive current of 100mA.

Based on the fabrication of p-type ZnO materials, some researchers reported on ZnO-homo‐ junction LEDs. Table4 is a survey of structure, method and emission peak of ZnO-homo‐

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 45

Tsukaza et al fabricated ZnO p-i-n homojunction LEDs by laser MBE using N as acceptor dopant [5]. The structure of the device was shown in Figure1(a). To realize an automatically flat interface, the homojunction structure was grown in layer-by layer. The I-V curve of the LED exhibited a rectifying behavior with a threshold voltage of 7V [Figure1(b)]. The thresh‐ old voltage was higher than the bandgap of ZnO (3.3eV), which was mainly attributed to the high resistivity of the p-type ZnO layer. The EL spectrum of the homojunction LED showed luminescence from violet to green regions with multi-reflection interference fringes. Com‐ pared PL spectrum of p-type ZnO and EL spectrum obtained from the LED, the higher ener‐ gy side peak around 430nm in the EL spectrum matches well with the PL spectrum

Chu et al reported on homojunction UV LEDs based on *p*-type Sb-doped ZnO/*n*-type Gadoped ZnO films. ZnO homojunction was grown on n-type Si(100) substrates by MBE [90]. A thin MgO/ZnO buffer layer was deposited to reduce the lattice mismatch between Si and ZnO, then, the two layer structured Sb-doped *p*-type ZnO (420nm)/Ga-doped *n*-type ZnO (420nm) homojunction was grown on this MgO/ZnO buffer. After the growth, *in situ* ther‐ mal annealing was performed to active Sb dopant in ZnO:Sb films. The I-V curve p-ZnO:Sb/n-ZnO:Ga homojunction exhibited rectifying characteristics with a threshold voltage of 6V. Figure18 shows room temperature EL of the homojunction LEDs under differ‐ ent injection currents. The NBE emission at ~383 nm was observed, the other peaks around 490 and 605nm related to intrinsic defects were also detected. With the increase of the injec‐ tion current, the NBE emission redshifted from 383.3nm (30mA) to 390.9nm (100mA), which was induced by the band gap variations due to the increased heating effects during the op‐

Similarly, Kong et al fabricated Sb-doped *p*-type ZnO/Ga-doped *n*-type ZnO homojunction on Si (100) substrate [91]. *I*-*V* and *C*-*V* curves presented typical electrical properties of a di‐ ode. The EL emission of the homojunction LEDs demonstrated dominant UV emissions. A NBE emission at 3.2eV started to appear when the current is 60mA. When the injection cur‐ rent increases from 60 to 100mA. The intensity of the UV emission increased and the emis‐ sion peak slightly redshifted from 385 to 393nm. The output power of this LED was

Wei et al prepared ZnO homojunction LEDs on *c*-plane Al2O3 substrates by plasma-assisted MBE using a gas mixture of N2 and O2 as the *p*-type dopant [89]. At low temperature (100K), the I-V curve of the LEDs exhibited a typical rectifying behavior with a threshold voltage of 4.0V at forward bias and a low leak current at reversed bias voltage. The LED keeps a good rectifying characteristic even as increasing temperature up to 300K. A emission at at 2.83eV started to appear when the current was 2.42mA. The emission peak blue shifted from 2.83 to 2.95eV as the injection current changed from 2.42 to 3.31mA, indicating that the EL emission

**Table 4.** Structure and emission of ZnO based homoojunction LEDs.

Park et al reported on the growth and device properties of *p*-ZnO/(InGaN/GaN) multiquan‐ tum well (MQW)/*n*-GaN heterojunction LEDs [82]. A GaN buffer layer (30nm) was deposit‐ ed on a sapphire substrate. After high temperature annealing of the buffer layer, undoped GaN(5μm), n-type GaN:Si(2μm) and InGaN/GaN MQW were grown by MOCVD, then, ptype ZnO:Sb layer was deposited on InGaN/GaN MQW. Finally, to active p-type dopant, a rapid thermal annealing was performed in an N2 ambient for 1min. The emission peak at 468nm was observed at room temperature, and the emission intensity of the LEDs increased as injection current increased, indicating that *p*-ZnO:Sb layer acted as a hole supplying layer in the hybrid LEDs. The emission peak red shifted as injection current increased due to the decrease in strain-induced piezoelectric field in the InGaN well by Sb-doped *p*-ZnO and Joule heating. Similarly, Hwang et al prepared p-ZnO:P/n-GaN heterostructure LEDs [84]. The PL spectra of the *p*-ZnO and *n*-GaN films exhibited the emission peaks at 365nm and 385nm, corresponding to NBE emissions of n-type GaN and p-type ZnO, respectively. Un‐ der forward bias, an EL emission at 409nm at room temperature were observed, which was attributed to the band gap of *p*-ZnO:P grown on *n*-GaN.

### *4.1.4. Homojunction LEDs*

**Table 4.** Structure and emission of ZnO based homoojunction LEDs.

44 Optoelectronics - Advanced Materials and Devices

attributed to the band gap of *p*-ZnO:P grown on *n*-GaN.

Park et al reported on the growth and device properties of *p*-ZnO/(InGaN/GaN) multiquan‐ tum well (MQW)/*n*-GaN heterojunction LEDs [82]. A GaN buffer layer (30nm) was deposit‐ ed on a sapphire substrate. After high temperature annealing of the buffer layer, undoped GaN(5μm), n-type GaN:Si(2μm) and InGaN/GaN MQW were grown by MOCVD, then, ptype ZnO:Sb layer was deposited on InGaN/GaN MQW. Finally, to active p-type dopant, a rapid thermal annealing was performed in an N2 ambient for 1min. The emission peak at 468nm was observed at room temperature, and the emission intensity of the LEDs increased as injection current increased, indicating that *p*-ZnO:Sb layer acted as a hole supplying layer in the hybrid LEDs. The emission peak red shifted as injection current increased due to the decrease in strain-induced piezoelectric field in the InGaN well by Sb-doped *p*-ZnO and Joule heating. Similarly, Hwang et al prepared p-ZnO:P/n-GaN heterostructure LEDs [84]. The PL spectra of the *p*-ZnO and *n*-GaN films exhibited the emission peaks at 365nm and 385nm, corresponding to NBE emissions of n-type GaN and p-type ZnO, respectively. Un‐ der forward bias, an EL emission at 409nm at room temperature were observed, which was

Based on the fabrication of p-type ZnO materials, some researchers reported on ZnO-homo‐ junction LEDs. Table4 is a survey of structure, method and emission peak of ZnO-homo‐ junction LEDs.

Tsukaza et al fabricated ZnO p-i-n homojunction LEDs by laser MBE using N as acceptor dopant [5]. The structure of the device was shown in Figure1(a). To realize an automatically flat interface, the homojunction structure was grown in layer-by layer. The I-V curve of the LED exhibited a rectifying behavior with a threshold voltage of 7V [Figure1(b)]. The thresh‐ old voltage was higher than the bandgap of ZnO (3.3eV), which was mainly attributed to the high resistivity of the p-type ZnO layer. The EL spectrum of the homojunction LED showed luminescence from violet to green regions with multi-reflection interference fringes. Com‐ pared PL spectrum of p-type ZnO and EL spectrum obtained from the LED, the higher ener‐ gy side peak around 430nm in the EL spectrum matches well with the PL spectrum [Figure1(c)].

Chu et al reported on homojunction UV LEDs based on *p*-type Sb-doped ZnO/*n*-type Gadoped ZnO films. ZnO homojunction was grown on n-type Si(100) substrates by MBE [90]. A thin MgO/ZnO buffer layer was deposited to reduce the lattice mismatch between Si and ZnO, then, the two layer structured Sb-doped *p*-type ZnO (420nm)/Ga-doped *n*-type ZnO (420nm) homojunction was grown on this MgO/ZnO buffer. After the growth, *in situ* ther‐ mal annealing was performed to active Sb dopant in ZnO:Sb films. The I-V curve p-ZnO:Sb/n-ZnO:Ga homojunction exhibited rectifying characteristics with a threshold voltage of 6V. Figure18 shows room temperature EL of the homojunction LEDs under differ‐ ent injection currents. The NBE emission at ~383 nm was observed, the other peaks around 490 and 605nm related to intrinsic defects were also detected. With the increase of the injec‐ tion current, the NBE emission redshifted from 383.3nm (30mA) to 390.9nm (100mA), which was induced by the band gap variations due to the increased heating effects during the op‐ eration of LEDs.

Similarly, Kong et al fabricated Sb-doped *p*-type ZnO/Ga-doped *n*-type ZnO homojunction on Si (100) substrate [91]. *I*-*V* and *C*-*V* curves presented typical electrical properties of a di‐ ode. The EL emission of the homojunction LEDs demonstrated dominant UV emissions. A NBE emission at 3.2eV started to appear when the current is 60mA. When the injection cur‐ rent increases from 60 to 100mA. The intensity of the UV emission increased and the emis‐ sion peak slightly redshifted from 385 to 393nm. The output power of this LED was estimated to be only 1nW at drive current of 100mA.

Wei et al prepared ZnO homojunction LEDs on *c*-plane Al2O3 substrates by plasma-assisted MBE using a gas mixture of N2 and O2 as the *p*-type dopant [89]. At low temperature (100K), the I-V curve of the LEDs exhibited a typical rectifying behavior with a threshold voltage of 4.0V at forward bias and a low leak current at reversed bias voltage. The LED keeps a good rectifying characteristic even as increasing temperature up to 300K. A emission at at 2.83eV started to appear when the current was 2.42mA. The emission peak blue shifted from 2.83 to 2.95eV as the injection current changed from 2.42 to 3.31mA, indicating that the EL emission originated from the donor-acceptor pair recombination in the *p*-type ZnO layer. The LED can even emit intensive EL in the blue-violet region at the temperature of 350K.

Ryu et al reported on ZnO-based UV LEDs fabricated by the hybrid beam deposition [100]. The LEDs employed a BeZnO/ZnO active layer between *n*-type and *p*-type ZnO and Be0.3Zn0.7O layers, as shown in Figure19. The active layer is composed of seven quantum wells for which undoped Be0.2Zn0.8O (7nm) and ZnO (4nm) form barrier and well layers, respectively. The *p*type ZnO and BeZnO layers were prepared with As as the acceptor dopant and n-type ZnO and BeZnO layers were formed with Ga as the dopant. The I-V curve of the LEDs exhibited a typical rectifying behavior with a high threshold voltage and a low reverse bias current.

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 47

Figure20 shows the EL spectra of the LEDs at room temperature. The peaks centered 388nm (bound exciton — BE) and 550nm (green band — GB) were the dominant features at low forward currents (≤20mA), which were attributed the impurity (donor or acceptor)-bound exciton emission and donor-acceptor pair recombination. As the current injection levels was above 20mA, the peak at 363nm becomes the prominent spectral feature and the peaks at 388 and 550nm have become saturated. The peak at 363nm could be assigned to band-toband recombination, such as from localized-exciton peaks in the active layer of the QWs.

**Figure 20.** EL spectrum measured at room temperature in continuous current mode of a *p*-*n* junction ZnO-based LED having a BeZnO active layer. The primary spectral emission peak is located near 363 nm and arises from localized-exci‐ ton emissions in the QWs. The secondary peak centered near 388 nm is from impurity-bound exciton emissions in

Lim et al fabricated UV LEDs based on ZnO p-n homojunction [98]. A Ga-doped ZnO layer (1.5μm) were grown on a c-Al2O3 substrates at 900°C by sputtering a ZnO target mixed with 1 wt% Ga2O3. A p-type ZnO layer (0.4μm) was grown in situ on the n-type ZnO layer at 900°C by sputtering a ZnO target mixed with 1 wt% P2O5. A rapid thermal annealing proc‐ ess was performed to the LEDs for 5min at 800°C in a nitrogen atmosphere in order to acti‐ vate the p-type ZnO layers. The I-V curve of the device showed clear rectification with a threshold voltage of 3.2V, which was good agreement with the ZnO bandgap energy (3.37eV). The EL spectra of the ZnO homojunction LED is shown in Figure21. A NBE emis‐ sion at 380nm and broad deep-level emissions at approximately 640nm were observed. The

ZnO. From Ref. [100].

**Figure 18.** Room temperature EL spectra at different injection current from 30 to 100mA. From Ref.[90].

**Figure 19.** Schematic illustration of the structure of the ZnO-based UV LED devices that employ a BeZnO/ZnO active layer comprised of MQWs. From Ref. [100].

Sun et al reported on the ultraviolet electroluminescence from ZnO homojunction with *n*-ZnO/*p*-ZnO:As/GaAs structure [93]. To obtain p-type ZnO film, firstly, a 400nm ZnO layer was deposited on *p*-GaAs (100) substrate at 450°C by MOCVD, followed by a thermal an‐ nealing at 550°C in 20Pa oxygen ambient for 30min to enhance the diffusion of arsenic and activate the acceptor impurities. After the thermal annealing, a 300nm undoped *n*-type ZnO layer was grown on the ZnO:As film at 350°C. The ZnO homojunction had a rectifying be‐ havior with a turn-on voltage of about 4V and a reverse breakdown voltage of higher than 6V. Under forward injection current of 30mA, the EL emission exhibited two independent bands centered at 3.2 and 2.5eV, which could be assigned to the NBE emission and deeplevel emission, respectively.

Ryu et al reported on ZnO-based UV LEDs fabricated by the hybrid beam deposition [100]. The LEDs employed a BeZnO/ZnO active layer between *n*-type and *p*-type ZnO and Be0.3Zn0.7O layers, as shown in Figure19. The active layer is composed of seven quantum wells for which undoped Be0.2Zn0.8O (7nm) and ZnO (4nm) form barrier and well layers, respectively. The *p*type ZnO and BeZnO layers were prepared with As as the acceptor dopant and n-type ZnO and BeZnO layers were formed with Ga as the dopant. The I-V curve of the LEDs exhibited a typical rectifying behavior with a high threshold voltage and a low reverse bias current.

originated from the donor-acceptor pair recombination in the *p*-type ZnO layer. The LED

can even emit intensive EL in the blue-violet region at the temperature of 350K.

**Figure 18.** Room temperature EL spectra at different injection current from 30 to 100mA. From Ref.[90].

**Figure 19.** Schematic illustration of the structure of the ZnO-based UV LED devices that employ a BeZnO/ZnO active

Sun et al reported on the ultraviolet electroluminescence from ZnO homojunction with *n*-ZnO/*p*-ZnO:As/GaAs structure [93]. To obtain p-type ZnO film, firstly, a 400nm ZnO layer was deposited on *p*-GaAs (100) substrate at 450°C by MOCVD, followed by a thermal an‐ nealing at 550°C in 20Pa oxygen ambient for 30min to enhance the diffusion of arsenic and activate the acceptor impurities. After the thermal annealing, a 300nm undoped *n*-type ZnO layer was grown on the ZnO:As film at 350°C. The ZnO homojunction had a rectifying be‐ havior with a turn-on voltage of about 4V and a reverse breakdown voltage of higher than 6V. Under forward injection current of 30mA, the EL emission exhibited two independent bands centered at 3.2 and 2.5eV, which could be assigned to the NBE emission and deep-

layer comprised of MQWs. From Ref. [100].

46 Optoelectronics - Advanced Materials and Devices

level emission, respectively.

Figure20 shows the EL spectra of the LEDs at room temperature. The peaks centered 388nm (bound exciton — BE) and 550nm (green band — GB) were the dominant features at low forward currents (≤20mA), which were attributed the impurity (donor or acceptor)-bound exciton emission and donor-acceptor pair recombination. As the current injection levels was above 20mA, the peak at 363nm becomes the prominent spectral feature and the peaks at 388 and 550nm have become saturated. The peak at 363nm could be assigned to band-toband recombination, such as from localized-exciton peaks in the active layer of the QWs.

**Figure 20.** EL spectrum measured at room temperature in continuous current mode of a *p*-*n* junction ZnO-based LED having a BeZnO active layer. The primary spectral emission peak is located near 363 nm and arises from localized-exci‐ ton emissions in the QWs. The secondary peak centered near 388 nm is from impurity-bound exciton emissions in ZnO. From Ref. [100].

Lim et al fabricated UV LEDs based on ZnO p-n homojunction [98]. A Ga-doped ZnO layer (1.5μm) were grown on a c-Al2O3 substrates at 900°C by sputtering a ZnO target mixed with 1 wt% Ga2O3. A p-type ZnO layer (0.4μm) was grown in situ on the n-type ZnO layer at 900°C by sputtering a ZnO target mixed with 1 wt% P2O5. A rapid thermal annealing proc‐ ess was performed to the LEDs for 5min at 800°C in a nitrogen atmosphere in order to acti‐ vate the p-type ZnO layers. The I-V curve of the device showed clear rectification with a threshold voltage of 3.2V, which was good agreement with the ZnO bandgap energy (3.37eV). The EL spectra of the ZnO homojunction LED is shown in Figure21. A NBE emis‐ sion at 380nm and broad deep-level emissions at approximately 640nm were observed. The EL spectra of the LED matched well with the PL spectrum of the p-type ZnO film, indicating that the recombination of electrons and holes occurred mostly in p-type ZnO layer.

rine-doped tin oxide (FTO) coated sapphire *c*-plane substrates by a vapor phase transport

and 100keV (device II) at a dosage of 1×1014 cm−2 perpendicular to the aligned rods. The im‐ planted ZnO rods were annealed at 900°C for 1 h with an O<sup>2</sup> flow rate of 100 SCCM at 1Torr to active p-type dopant. Figure22 exhibits the typical *I*-*V* characteristics of different regions in a single vertically aligned *p*-ZnO:P/*n*-ZnO rod. The rectifying behavior with a threshold voltage of 0.8V was observed for *p*-*n* junction. The near-linear relationship was also detected

Figure24 displays the EL spectra of ZnO rod homojunction LEDs at various injection cur‐ rents. Strong UV emission was observed from both devices, corresponding to the NBE emis‐ sion of ZnO. The UV light output intensities increased linearly as injection current was above a threshold current (Figure23 insets). In addition, device I shows a relatively weak and broad emission band in the visible range, indicating a low density of deep-level defects, and the broad emission consisted of one green emission(~510nm) and one nearinfrared peak (~800nm) became stronger for device II. Similarly, Yang et al fabricated ZnO nanorod p-n homojunction LEDs with As implantation [96]. The EL spectrum of the device exhibited a

strong UV band centered at ~380 nm and a weak broad red band peaking at ~630nm.

**Figure 23.** EL spectra of (a) device I and (b) device II under various injection currents, respectively. The insets show the UV light output intensities as a function of forward injection current. The photographs of corresponding light emis‐ sions collected from both (c) and (e) front side (Au anode) and (d) and (f) back side single-sided polished sapphire

With a large direct band gap of 3.37eV and a large exciton binding energy of 60meV, ZnO has attracted much attention for its application in optoelectronics applications, such as LEDs, photodetector and laser diodes. In the paper, based on the introduction of the bandgap engineering and doping in ZnO, we presented a comprehensive review of ZnO-based LEDs. Band-gap engineering in ZnO can be achieved by alloying with MgO, CdO or BeO. Theoretically, the energy band gap of AxZn1-xO can be continuously modulated from 0.9eV

substrate in these two devices under the same bias voltage of 25 V. From Ref. [97].

**5. Conclusion and outlook**

ions with 50keV (deviceI)

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 49

method. After the growth, ZnO rod arrays were implanted with P+

for *p*-*p*, *n*-*n*, and *n*-FTO curves, indicating an Ohmic behavior.

**Figure 21.** EL spectra of p–n homojunction ZnO LED operated at forward currents of 20 and 40 mA; PL spectrum of ptype ZnO obtained at room temperature. From Ref. [98].

**Figure 22.** *I*-*V* characteristics of *p*-*p*, *p*-*n*, *n*-*n*, and *n*-FTO regions in a single ZnO rod. The upper left and lower right insets show the schematic diagram and a SEM image of probing by Zyvex nanomanipulator, respectively. From Ref. [97].

Sun et al reported on UV emission from a ZnO rod homojunction LED [97]. Vertically aligned ZnO rods (Diameters: 200–500nm; Length:3.5μm) were uniformly grown on fluo‐ rine-doped tin oxide (FTO) coated sapphire *c*-plane substrates by a vapor phase transport method. After the growth, ZnO rod arrays were implanted with P+ ions with 50keV (deviceI) and 100keV (device II) at a dosage of 1×1014 cm−2 perpendicular to the aligned rods. The im‐ planted ZnO rods were annealed at 900°C for 1 h with an O<sup>2</sup> flow rate of 100 SCCM at 1Torr to active p-type dopant. Figure22 exhibits the typical *I*-*V* characteristics of different regions in a single vertically aligned *p*-ZnO:P/*n*-ZnO rod. The rectifying behavior with a threshold voltage of 0.8V was observed for *p*-*n* junction. The near-linear relationship was also detected for *p*-*p*, *n*-*n*, and *n*-FTO curves, indicating an Ohmic behavior.

Figure24 displays the EL spectra of ZnO rod homojunction LEDs at various injection cur‐ rents. Strong UV emission was observed from both devices, corresponding to the NBE emis‐ sion of ZnO. The UV light output intensities increased linearly as injection current was above a threshold current (Figure23 insets). In addition, device I shows a relatively weak and broad emission band in the visible range, indicating a low density of deep-level defects, and the broad emission consisted of one green emission(~510nm) and one nearinfrared peak (~800nm) became stronger for device II. Similarly, Yang et al fabricated ZnO nanorod p-n homojunction LEDs with As implantation [96]. The EL spectrum of the device exhibited a strong UV band centered at ~380 nm and a weak broad red band peaking at ~630nm.

**Figure 23.** EL spectra of (a) device I and (b) device II under various injection currents, respectively. The insets show the UV light output intensities as a function of forward injection current. The photographs of corresponding light emis‐ sions collected from both (c) and (e) front side (Au anode) and (d) and (f) back side single-sided polished sapphire substrate in these two devices under the same bias voltage of 25 V. From Ref. [97].
