**3. ZnO/SiO2/Si UV photodiodes**

#### **3.1. Ultrathin SiO2 films**

Many the various types of photodiodes which include homojunction, heterojunction and met‐ al-semiconductor-metal (MSM) photodiodes much attention has been paid in recent years to metal-oxide-semiconductor (MOS) structures [30-33]. An ultrathin silicon dioxide (SiO2) films has been the most commonly used material for diffusion barriers and insulating layers for various applications in MOS devices due to its properties such as low defect density, high thermal stability, high resistivity, high electric insulating performance, high reliability, and reasonable dielectric constant [34,35]. In general, an ultrathin SiO2 films (≤ 1 nm) was formed on the silicon substrate that the silicon/SiO2 interface becomes crucial for good transistor be‐ havior. Several fabrication methods have been employed for the formed of ultrathin SiO2 films, such as rapid thermal oxidation (RTO) [36], oxidation with excited molecules and ions [37,38], plasma oxidation [39,40], photo-oxidation [34,41], ozone oxidation [43], metal-promot‐ ed oxidation [44], anodic oxidation [45,46] and nitric acid (HNO3) vapor oxidation [47,48] etc. When a reverse bias is applied to a MOS photodiode, the energy bands in the semiconduc‐ tor bend and a potential well is formed between the oxide and the semiconductor. Electronhole pairs generated near the junction by incident light will be stored in the potential well, and current transport occurs through the oxide layer via tunneling.

**Figure 5.** a) Energy-band diagram of a reverse-biased n-ZnO/p-Si structure. (b) Spectral responsivity curves obtained

Additionally, we found that an intermediate SiO2 ultrathin film can improve the quantum ef‐ ficiency and the responsivity by decreasing the surface state density and increase the tunnel‐ ing photocurrent [49-51]. Figure 6 (a) shows a schematic cross-section of the complete structure. The inset in this figure shows a schematic cross-sectional TEM image of nanostruc‐ ture *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si substrate. The ZnO film had an anomalous nano‐ scale columnar structure with a diameter of 50-80 nm. The ultrathin oxide layer between the ZnO film and the Si substrate had a thickness of approximately 26 Å, as estimated from the

**Figure 6.** a) Schematic cross-section of the complete structure. (b) Cross sectional TEM image of *p*-ZnO/SiO2 ultrathin

Figures 7(a) and 7(b) present a schematic band diagram to elucidate the current components.

+ *JSn* + *JTn* (2)

Si-Based ZnO Ultraviolet Photodiodes http://dx.doi.org/10.5772/48825 201

+ *JTp*

Based on Figure 7(a), the dark current can be described as [30,53]

*Jdark* = *JSp*

under the reverse biases [52].

interlayer/*n*-Si structure [49,51].

TEM image in Fig. 6(b) taking the area A in Fig. 6(a).

Recently, Chen et al. [49-51] reported the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si substrate struc‐ ture photodiodes. An ultrathin SiO2 film as interlayer was formed on a (111)-oriented sili‐ con substrate by heating the substrate in wet oxygen ambient at 650 C for 10 min to improve the performance of ZnO/Si photodiodes by inserting a SiO2 ultrathin interlayer.

#### **3.2. ZnO/SiO2/Si UV photodiodes**

In 2003, Jeong et al. [52] presents *n*-ZnO/*p*-Si photodiodes through use of a SiO2 ultrathin ox‐ ide interlayer that unintentionally doped n-ZnO thin films were deposited on p-type Si sub‐ strates by RF magnetron sputtering. A schematic cross-section of the complete structure is shown in Figure 5 (a). The *n*/*p* heterojunction has a thin SiO2 layer about 3 nm at the *n*-ZnO/*p*-Si interface and hence the photoelectrons may face a transport barrier. The result indicates that *n*-ZnO/*p*-Si photodiodes could detect UV photons in the depleted *n*-ZnO and simultaneous‐ ly detect visible photons in the depleted *p*-Si. Figure 5 (b) presents the spectral responsivity curves obtained from the *n*-ZnO/*p*-Si photodiode. The responsivity of a photodiode for visi‐ ble light was as high as ~0.26 A/W at 5 V and 0.4 A/W at 30 V. The UV-driven responsivity spectra are quite different, showing a noticeable increase with voltage. Higher responsivity is found for more energetic UV photons from the photodiode. For the 310 nm UV photons, the *n*-ZnO/*p*-Si photodiode shows responsivity of 0.09 A/W at 5 V and 0.5 A/W at 30 V. Howev‐ er, they show relatively weak response near 380 nm, which is the band gap of ZnO.

**3. ZnO/SiO2/Si UV photodiodes**

200 Photodiodes - From Fundamentals to Applications

**3.2. ZnO/SiO2/Si UV photodiodes**

Many the various types of photodiodes which include homojunction, heterojunction and met‐ al-semiconductor-metal (MSM) photodiodes much attention has been paid in recent years to metal-oxide-semiconductor (MOS) structures [30-33]. An ultrathin silicon dioxide (SiO2) films has been the most commonly used material for diffusion barriers and insulating layers for various applications in MOS devices due to its properties such as low defect density, high thermal stability, high resistivity, high electric insulating performance, high reliability, and reasonable dielectric constant [34,35]. In general, an ultrathin SiO2 films (≤ 1 nm) was formed on the silicon substrate that the silicon/SiO2 interface becomes crucial for good transistor be‐ havior. Several fabrication methods have been employed for the formed of ultrathin SiO2 films, such as rapid thermal oxidation (RTO) [36], oxidation with excited molecules and ions [37,38], plasma oxidation [39,40], photo-oxidation [34,41], ozone oxidation [43], metal-promot‐ ed oxidation [44], anodic oxidation [45,46] and nitric acid (HNO3) vapor oxidation [47,48] etc. When a reverse bias is applied to a MOS photodiode, the energy bands in the semiconduc‐ tor bend and a potential well is formed between the oxide and the semiconductor. Electronhole pairs generated near the junction by incident light will be stored in the potential well,

Recently, Chen et al. [49-51] reported the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si substrate struc‐ ture photodiodes. An ultrathin SiO2 film as interlayer was formed on a (111)-oriented sili‐ con substrate by heating the substrate in wet oxygen ambient at 650 C for 10 min to improve

In 2003, Jeong et al. [52] presents *n*-ZnO/*p*-Si photodiodes through use of a SiO2 ultrathin ox‐ ide interlayer that unintentionally doped n-ZnO thin films were deposited on p-type Si sub‐ strates by RF magnetron sputtering. A schematic cross-section of the complete structure is shown in Figure 5 (a). The *n*/*p* heterojunction has a thin SiO2 layer about 3 nm at the *n*-ZnO/*p*-Si interface and hence the photoelectrons may face a transport barrier. The result indicates that *n*-ZnO/*p*-Si photodiodes could detect UV photons in the depleted *n*-ZnO and simultaneous‐ ly detect visible photons in the depleted *p*-Si. Figure 5 (b) presents the spectral responsivity curves obtained from the *n*-ZnO/*p*-Si photodiode. The responsivity of a photodiode for visi‐ ble light was as high as ~0.26 A/W at 5 V and 0.4 A/W at 30 V. The UV-driven responsivity spectra are quite different, showing a noticeable increase with voltage. Higher responsivity is found for more energetic UV photons from the photodiode. For the 310 nm UV photons, the *n*-ZnO/*p*-Si photodiode shows responsivity of 0.09 A/W at 5 V and 0.5 A/W at 30 V. Howev‐

the performance of ZnO/Si photodiodes by inserting a SiO2 ultrathin interlayer.

er, they show relatively weak response near 380 nm, which is the band gap of ZnO.

and current transport occurs through the oxide layer via tunneling.

**3.1. Ultrathin SiO2 films**

**Figure 5.** a) Energy-band diagram of a reverse-biased n-ZnO/p-Si structure. (b) Spectral responsivity curves obtained under the reverse biases [52].

Additionally, we found that an intermediate SiO2 ultrathin film can improve the quantum ef‐ ficiency and the responsivity by decreasing the surface state density and increase the tunnel‐ ing photocurrent [49-51]. Figure 6 (a) shows a schematic cross-section of the complete structure. The inset in this figure shows a schematic cross-sectional TEM image of nanostruc‐ ture *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si substrate. The ZnO film had an anomalous nano‐ scale columnar structure with a diameter of 50-80 nm. The ultrathin oxide layer between the ZnO film and the Si substrate had a thickness of approximately 26 Å, as estimated from the TEM image in Fig. 6(b) taking the area A in Fig. 6(a).

**Figure 6.** a) Schematic cross-section of the complete structure. (b) Cross sectional TEM image of *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure [49,51].

Figures 7(a) and 7(b) present a schematic band diagram to elucidate the current components. Based on Figure 7(a), the dark current can be described as [30,53]

$$J\_{dark} = J\_{Sp} + J\_{Tp} + J\_{Sn} + J\_{Tn} \tag{2}$$

where *J Sp* is the hole current through surface states, *J Tp* is the hole tunneling current, *J Sn* is the electron current tunneled through surface states, and *J Tn* is the electron current through surface states. As shown in Figure 7(b), the photocurrent mechanisms can be written as

$$f\_{I\text{ light}} = f\_{Tu} + f\_{Dn} + f\_{Ln} + f\_{Tp} + f\_{Dp} + f\_{Lp} \tag{3}$$

tively. As shown in Figure 8 (b), the use of an intermediate oxide film resulted in a greater *R* in the UV/visible/IR region than was measured for the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si struc‐ ture photodiodes. This result suggests that the intermediate oxide ultrathin film passivates surface states and increases the tunneling photocurrent, thus improving both *QE* and *R*.

Si-Based ZnO Ultraviolet Photodiodes http://dx.doi.org/10.5772/48825 203

**Figure 8.** *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si and *p*-ZnO/*n*-Si structure photodiodes. (a) Dark and illuminated (λ = 530

**Figure 9.** shows the cross-section of the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure completed configuration in a

In recent years, diluted magnetic semiconductors (DMSs) are attracted much great scientific interest because of their unique spintronics properties with potential technological applica‐ tions. Consequently, the high Curie temperature ferromagnetism of ZnO and related materi‐

strong magnetic field [50].

nm) (I-V) characteristics (b) Responsivity as a function of wavelength at a bias of -1 V [49].

**4. ZnO/SiO2/Si UV photodiodes in a strong magnetic field**

where *J <sup>T</sup>* is the tunneling current, *J <sup>D</sup>* is the current in the depletion region, and *J <sup>L</sup>* is the photo-generated current. The subscripts *n* and *p* indicate electron and hole, respectively.

**Figure 7.** Schematic energy-band diagram of the p-oxide-n tunnel diode system under (a) dark and (b) illuminated conditions [49].

Figure 8(a) plots the responsivity as a function of (*I-V*) characteristics of photodiodes that were measured in the dark and under illumination (λ = 530 nm) with a 250 W xenon arc lamp at reverse biases from 0 to 1 V. At a reverse bias of 1 V, for the *p*-ZnO/*n*-Si structure, the photocurrent was ~3.9×10-7 A and the dark current was ~8.87×10-9 A. For the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure, the photocurrent and the dark current were ~4.99×10-5 A and ~4.98×10-10 A, respectively. It can be noted that the photocurrent-to-dark-current con‐ trast ratios improved from two orders of magnitude to five orders of magnitude. Evidently, the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure improves the photocurrent-to-dark-cur‐ rent contrast ratio by passivating the surface states and enhancing the tunneling current, as shown in Figures 8(a) and 8(b).

Figure 8(b) plots the as a function of wavelength for both a *p*-ZnO/*n*-Si and a *p*-ZnO/SiO2 ultra‐ thin interlayer/*n*-Si photodiode, measured throughout this work at a reverse bias of 1 V. The photodiode responsivities can be divided into three regions of around wavelengths of 400 nm, 530 nm, and 850 nm, denoted A, B, and C. Region A, at a wavelength of around 400 nm, corre‐ sponds to excitonic absorption in the ZnO film [54,55]. Region B, which is defined as the wave‐ length range from about 400 nm to 700 nm, corresponds to band-to-deep level absorption in the ZnO film [26]. Region C (wavelengths between 700 nm to 1000 nm) corresponds to band edge absorption in the Si substrate. According equation (1), for the *p*-ZnO/*n*-Si structure, in re‐ gion A, B, and C, the responsivity (*R*) and quantum efficiency (*QE*) were 0.147, 0.204, 0.206 A/W and 45.57, 47.73, 30.05 %, respectively. However, for the *p*-ZnO/SiO2 ultrathin interlayer/ *n*-Si photodiode, the *R* and *QE* were 0.225, 0.252, 0.297 A/W and 69.75, 58.96, 43.33 %, respec‐ tively. As shown in Figure 8 (b), the use of an intermediate oxide film resulted in a greater *R* in the UV/visible/IR region than was measured for the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si struc‐ ture photodiodes. This result suggests that the intermediate oxide ultrathin film passivates surface states and increases the tunneling photocurrent, thus improving both *QE* and *R*.

where *J Sp* is the hole current through surface states, *J Tp* is the hole tunneling current, *J Sn* is the electron current tunneled through surface states, and *J Tn* is the electron current through surface states. As shown in Figure 7(b), the photocurrent mechanisms can be written as

where *J <sup>T</sup>* is the tunneling current, *J <sup>D</sup>* is the current in the depletion region, and *J <sup>L</sup>* is the photo-generated current. The subscripts *n* and *p* indicate electron and hole, respectively.

**Figure 7.** Schematic energy-band diagram of the p-oxide-n tunnel diode system under (a) dark and (b) illuminated

Figure 8(a) plots the responsivity as a function of (*I-V*) characteristics of photodiodes that were measured in the dark and under illumination (λ = 530 nm) with a 250 W xenon arc lamp at reverse biases from 0 to 1 V. At a reverse bias of 1 V, for the *p*-ZnO/*n*-Si structure, the photocurrent was ~3.9×10-7 A and the dark current was ~8.87×10-9 A. For the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure, the photocurrent and the dark current were ~4.99×10-5 A and ~4.98×10-10 A, respectively. It can be noted that the photocurrent-to-dark-current con‐ trast ratios improved from two orders of magnitude to five orders of magnitude. Evidently, the *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si structure improves the photocurrent-to-dark-cur‐ rent contrast ratio by passivating the surface states and enhancing the tunneling current, as

Figure 8(b) plots the as a function of wavelength for both a *p*-ZnO/*n*-Si and a *p*-ZnO/SiO2 ultra‐ thin interlayer/*n*-Si photodiode, measured throughout this work at a reverse bias of 1 V. The photodiode responsivities can be divided into three regions of around wavelengths of 400 nm, 530 nm, and 850 nm, denoted A, B, and C. Region A, at a wavelength of around 400 nm, corre‐ sponds to excitonic absorption in the ZnO film [54,55]. Region B, which is defined as the wave‐ length range from about 400 nm to 700 nm, corresponds to band-to-deep level absorption in the ZnO film [26]. Region C (wavelengths between 700 nm to 1000 nm) corresponds to band edge absorption in the Si substrate. According equation (1), for the *p*-ZnO/*n*-Si structure, in re‐ gion A, B, and C, the responsivity (*R*) and quantum efficiency (*QE*) were 0.147, 0.204, 0.206 A/W and 45.57, 47.73, 30.05 %, respectively. However, for the *p*-ZnO/SiO2 ultrathin interlayer/ *n*-Si photodiode, the *R* and *QE* were 0.225, 0.252, 0.297 A/W and 69.75, 58.96, 43.33 %, respec‐

conditions [49].

shown in Figures 8(a) and 8(b).

202 Photodiodes - From Fundamentals to Applications

+ *JDp*

+ *J Lp* (3)

*Jlight* = *JTn* + *JDn* + *J Ln* + *JTp*

**Figure 8.** *p*-ZnO/SiO2 ultrathin interlayer/*n*-Si and *p*-ZnO/*n*-Si structure photodiodes. (a) Dark and illuminated (λ = 530 nm) (I-V) characteristics (b) Responsivity as a function of wavelength at a bias of -1 V [49].
