**1. Introduction**

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24 Optoelectronics - Advanced Materials and Devices

In the past decade, light-emitting diodes (LEDs) based on wideband gap semiconductor have attracted considerable attention due to its potential optoelectronic applications in illu‐ mination, mobile appliances, automotive and displays [1]. Among the available wide band gap semiconductors, zinc oxide, with a large direct band gap of 3.37eV, is a promising can‐ didate because of characteristic features such as a large exciton binding energy of 60meV, and the realization of band gap engineering to create barrier layers and quantum wells with little lattice mismatch. ZnO crystallizes in the wurtzite structure, the same as GaN, but, in contrast, large ZnO single crystal can be fabricated [2]. Furthermore, ZnO is inexpensive, chemically stable, easy to prepare and etch, and nontoxic, which also make the fabrication of ZnO-based optical devices an attractive prospect. The commercial success of GaN-based op‐ toelectronic and electronic devices trig the interest in ZnO-based devices [2-4].

Recently, the fabrication of *p*-type ZnO has made great progress by mono-doping group V elements (N, P, As, and Sb) and co-doping III–V elements with various technologies, such as ion implantation, pulsed laser deposition (PLD), molecular beam epitaxy (MBE) [2,3]. A number of researchers have reported the development of homojunction ZnO LEDs and het‐ erojunction LEDs using *n*-ZnO deposited on *p*-type layers of GaN, AlGaN, conducting ox‐ ides, or *p*-ZnO deposited on a *n*-type layer of GaN [1,3].

Figure1a shows the schematic structure of a typical ZnO homostructural p–i–n junction pre‐ pared by Tsukaza et al [5]. The I-V curve of the device displayed the good rectification with a threshold voltage of about 7V (Figure1b). The electroluminescence (EL) spectrum from the p–i–n junction (blue) and photoluminescence (PL) spectrum of a p-type ZnO film at 300K were shown in Figure1c, which indicated that ZnO was a potential material for making short-wavelength optoelectronic devices, such as LEDs for display, solid-state illumination and photodetector.

© 2013 Fan et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Fan et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

White-light electroluminescence from n-ZnO)/p-GaN heterojunction LED was reported [6]. The spectrum range from 400 to 700nm is caused by the carrier recombination at the inter‐ face between n-ZnO and p-GaN, as shown in Figure2, which makes ZnO as a strong candi‐

ZnO-Based Light-Emitting Diodes http://dx.doi.org/10.5772/51181 27

Currently, ZnO-based LEDs are leaping from lab to factory. A dozen or so companies are developing ultraviolet and white LEDs for market. The coloured ZnO-based LEDs have been produced by Start-up company MOXtronics, which shows its full-colour potential. Al‐ though the efficiency of these LEDs is not high, improvements are rapid and the emitters have the potential to outperform their GaN rivals. Figure3 shows some EL images of ZnO-

In this paper, based on the introduction of the band-gap engineering and doping in ZnO, we discuss the ZnO-based LEDs, comprehensively. We first discuss the band-gap engineering in ZnO, which is a very important technique to design ZnO-based LEDs. We then present the p- and n-types doping in ZnO. High quality n-type and/or p-type ZnO are necessary to prepare ZnO-based LEDs. Finally, we review the ZnO-based LEDs. In this part, we discuss homojunction ZnO LEDs and heterojunctions LEDs using *n*-ZnO deposited on *p*-type layers (GaN, AlGaN, conducting oxides, et al ) or *p*-ZnO deposited on a *n*-type layer (GaN, Si, et

Band gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys. It is well known that tailoring of the energy band gap in semiconductors by band-gap engineering is important to create barrier layers and quantum wells with matching material properties, such as lattice con‐

stants, electron affinity for heterostructure device fabrication [2, 3].

dates for solid-state light.

**Figure 3.** Some EL images of ZnO-based LEDs. From Ref. [7].

**2. Band gap engineering in ZnO**

al), comprehensively.

based LEDs.

**Figure 1.** ZnO homostructural p–i–n junction shows rectifying current–voltage characteristics and electrolumines‐ cence (EL) in forward bias at room-temperature. (a), The structure of a typical p–i–n junction LED. (b), Current–voltage characteristics of a p–i–n junction. The inset has logarithmic scale in current with F and R denoting forward and re‐ verse bias conditions, respectively. (c), Electroluminescence spectrum from the p–i–n junction (blue) and photolumi‐ nescence (PL) spectrum of a p-type ZnO film measured at 300 K. The p–i–n junction was operated by feeding in a direct current of 20 mA. From Ref.[5].

**Figure 2.** Room-temperature EL spectra of the n-ZnO/p-GaN heterojunction LED measured at various dc injection cur‐ rents from 1 to 15mA at reverse breakdown biases. (Inset) EL image of the LED in a bright room. From Ref. [6].

White-light electroluminescence from n-ZnO)/p-GaN heterojunction LED was reported [6]. The spectrum range from 400 to 700nm is caused by the carrier recombination at the inter‐ face between n-ZnO and p-GaN, as shown in Figure2, which makes ZnO as a strong candi‐ dates for solid-state light.

Currently, ZnO-based LEDs are leaping from lab to factory. A dozen or so companies are developing ultraviolet and white LEDs for market. The coloured ZnO-based LEDs have been produced by Start-up company MOXtronics, which shows its full-colour potential. Al‐ though the efficiency of these LEDs is not high, improvements are rapid and the emitters have the potential to outperform their GaN rivals. Figure3 shows some EL images of ZnObased LEDs.

**Figure 3.** Some EL images of ZnO-based LEDs. From Ref. [7].

**Figure 1.** ZnO homostructural p–i–n junction shows rectifying current–voltage characteristics and electrolumines‐ cence (EL) in forward bias at room-temperature. (a), The structure of a typical p–i–n junction LED. (b), Current–voltage characteristics of a p–i–n junction. The inset has logarithmic scale in current with F and R denoting forward and re‐ verse bias conditions, respectively. (c), Electroluminescence spectrum from the p–i–n junction (blue) and photolumi‐ nescence (PL) spectrum of a p-type ZnO film measured at 300 K. The p–i–n junction was operated by feeding in a

**Figure 2.** Room-temperature EL spectra of the n-ZnO/p-GaN heterojunction LED measured at various dc injection cur‐ rents from 1 to 15mA at reverse breakdown biases. (Inset) EL image of the LED in a bright room. From Ref. [6].

direct current of 20 mA. From Ref.[5].

26 Optoelectronics - Advanced Materials and Devices

In this paper, based on the introduction of the band-gap engineering and doping in ZnO, we discuss the ZnO-based LEDs, comprehensively. We first discuss the band-gap engineering in ZnO, which is a very important technique to design ZnO-based LEDs. We then present the p- and n-types doping in ZnO. High quality n-type and/or p-type ZnO are necessary to prepare ZnO-based LEDs. Finally, we review the ZnO-based LEDs. In this part, we discuss homojunction ZnO LEDs and heterojunctions LEDs using *n*-ZnO deposited on *p*-type layers (GaN, AlGaN, conducting oxides, et al ) or *p*-ZnO deposited on a *n*-type layer (GaN, Si, et al), comprehensively.
