ZnO-Based Light-Emitting Diodes

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].


Introduction
In the past decade, light-emitting diodes (LEDs) based on wideband gap semiconductor have attracted considerable attention due to its potential optoelectronic applications in illumination, 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 candidate 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 optoelectronic and electronic devices trig the interest in ZnO-based devices [2][3][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 heterojunction LEDs using n-ZnO deposited on p-type layers of GaN, AlGaN, conducting oxides, 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 prepared 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. The inset has logarithmic scale in current with F and R denoting forward and reverse bias conditions, respectively. (c), Electroluminescence spectrum from the p-i-n junction (blue) and photoluminescence (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 currents 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 interface between n-ZnO and p-GaN, as shown in Figure2, which makes ZnO as a strong candidates 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. Although 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. 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.

Band gap engineering in ZnO
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 constants, electron affinity for heterostructure device fabrication [2,3].
Band-gap engineering in ZnO can be achieved by alloying with MgO, CdO or BeO. The energy band gap Eg(x) of ternary semiconductor A x Zn 1-x O (A = Mg, Cd or Be) can be calculated by the following equation: where b is the bowing parameter and E AO and E ZnO are the band-gap energies of compounds AO and ZnO, respectively. While adding Mg or Be to ZnO results in an increase in band gap, and adding Cd leads to a decrease in band gap [3,8].
Both MgO and CdO have the rock-salt structure, which is not the same as the ZnO wurtzite structure. When Mg and Cd contents in ZnO are high, phase separation may be detected, while BeO and ZnO share the same wurtzite structure and phase separation is not observed in BeZnO [2,8]. Ryu [12]. The strong reflectance peaks at room temperature were detected from 3.42eV (x=0.05) to 4.62eV (x = 0.61) from ZnMgO layers at room temperature. PL spectra at room temperature were also observed for energies up to 4.06eV (x = 0.44). Wassner et al studied the optical and structural properties of MgZnO films with Mg contents between x = 0 and x = 0.37 grown on sapphire by plasma assisted molecular beam epitaxy using a MgO/ ZnMgO buffer layer [13]. In their experiments, the a-lattice parameter was independent from the Mg concentration, whereas the c-lattice parameter decreases from 5.20Å for x = 0 to 5.17Å for x = 0.37, indicating pseudomorphic growth. The peak position of the band edge luminescence blue shifted up to 4.11eV for x = 0.37.
Makino et al investigated the structure and optical properties of Cd x Zn 1-x O films grown on sapphire (0001) and ScAlMgO 4 substrates by PLD [14].  Using MgZnO as barrier layers, Chauveau et al prepared the nonpolar a-plane (Zn,Mg)O/ZnO quantum wells (QWs) grown by molecular beam epitaxy on r plane sapphire and a plane ZnO substrates [16]. They observed the excitonic transitions were strongly blue-shifted due to the 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 photoluminescence properties. Su et al investigated the optical properties of ZnO/ZnMgO single quantum well (SQW) prepared by plasma-assisted molecular beam epitaxy [17]. The photoluminescence peak of the SQW shifted from 3.31 to 3.37eV as the well layer thickness was decreased 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 well thickness and/or the Mg content in the barrier layers.

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 intrinsic defects, such as the Zinc interstitial (Zn i ) and oxygen vacancy (V O ). 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 native 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 techniques, such as pulse laser deposition, magnetron sputtering, chemical vapor deposition (CVD), molecular-beam epitaxy, hybrid beam deposition (HBD), metal organic chemical vapor deposition (MOCVD) and thermal oxidation of Zn 3 N 2 [2,3].

n-type ZnO
A number of researchers investigated the electrical and optical properties of n-type ZnO materials by doping III elements, such as Al, Ga and In, which can easily substitute Zn ions [1][2][3].
Kim et al reported the high electron concentration and mobility in AZO films grown on sapphire by magnetron sputtering [18]. AZO films exhibited the electron concentrations and mobilities were of the order of 10 18 cm 3 and less than 8cm 2 /Vs, respectively, however, when annealed at 900 0 C, the films showed remarkably improved carrier concentrations and mobilities, e.g., about 10 20 cm 3 and 45 -65 cm 2 /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 prepared 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 plating with direct current arc discharge [21].  [24]. The films had a mobility of 4.18-20.9cm 2 /Vs and a concentration of 6.7×10 18 -3.2×10 19 /cm 3 . The In-doped ZnO films with a carrier concentration of 3.22×10 20 /cm 3 were grown by sol-gel method [25].
VII elements such as F and Cl are also used as n-type dopants in ZnO, which substituted 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 5.43×10 20 cm -3 and a mobility of 23.8cm 2 / Vs [26]. Chikoidze et al grew Cl-doped ZnO films by MOCVD with a resistivity of 3.6×10 -3 Ω cm [27].

p-type ZnO
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 doping of the materials remains a major challenge because of the self-compensation from native donor defects (Vo and Zn i ) 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. N 2 , NO, N 2 O, NH 3 and Zn 3 N 2 are acted as N sources depended on growth techniques [2,3]. Liu et al reported p-type ZnO:N films grown on c-sapphire 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 (N O ) in ZnO. The films exhibited a hole concentration of 2.21×10 16 cm -3 and a mobility of 1.33cm 2 /Vs. Zeng et al investigated p-type ZnO films prepared on a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) sapphire by MOCVD [29]. The optimized result was achieved at the temperature of 400°C with a resistivity of 1.72Ωcm, a Hall mobility of 1.59cm 2 /Vs, and a hole concentration of 2.29×10 18 cm −3 . Wang et al prepared ptype ZnO films by oxidation of Zn 3 N 2 films grown by direct current magnetron sputtering [30]. For oxidation temperature between 350 and 500 0 C, p-type ZnO:N films were achieved, with a hole concentration of 5.78×10 17 cm -3 at 500 0 C. Kumar et al reported on the growth of ptype N,Ga-codoped ZnO films prepared by sputtering ZnO:Ga 2 O 3 target in N 2 O ambient [31]. The film deposited on sapphire at 550 0 C exhibited p-type conduction with a hole concentration of 3.9×10 17 cm −3 .
Beside N, other group V elements (P, As and Sb) are also used to be acceptor dopants to obtain p-type ZnO.  [33]. In the model, X substitute Zn sites, forming a donor, then it induces two Zn vacancy acceptors as a complex form X Zn −2V Zn . The ionization energy of As Zn -2V Zn complex was calculated to be 0.15eV (0.16eV for Sb Zn − 2V Zn ).
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 0 X and A 0 X for the phosphorus-doped ZnO films. The films exhibited a carrier concentration of 6.0×10 18 cm −3 , Hall mobility of 1.5 cm 2 /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 sputtering were converted to p-ZnO:P by an rapid thermal annealing process under a N 2 ambient. The films had a hole concentration of 1.0×10 17 [38]. The electrical behavior of ZnO:As films changed from intrinsic n-type to highly conductive p-type with increased As dopant concentration. They achieved p-type ZnO:As films with a hole concentration of 4×10 17 cm -3 and a mobility of 35cm 2 /Vs. Vaithianathan et al reported As-doped p-type ZnO films using a Zn 3 As 2 /ZnO target by PLD [39]. As-grown ZnO:As showed n-type conductivity, however, ZnO:As films after annealed at 200°C in N 2 ambient for 2 min exhibited p-type conductivity with the hole concentrations varied between 2.48×10 17 and 1.18×10 18 cm −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 concentration of 9.684×10 19 cm −3 , a mobility of 25.37cm 2 /Vs, and a resistivity of 2.54 ×10 −3 Ωcm. The acceptor binding energy was calculated to be 0.1445eV, which was in good agreement with the ionization energy of As Zn -2V Zn acceptor complex (0.15eV) [40].
In the (HR) TEM images of p-type ZnO:Sb, they observed a high density of threading dislocations originating from the film/substrate interface and a large number of partial dislocation 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 18 cm −3 , and a high mobility of 20.0cm 2 /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 Sb Zn − 2V Zn (0.16eV).
Some researchers prepared p-type ZnO using Group I elements (Li, Na and K) as acceptor dopants. Yi et al fabricated p-type ZnO:Li films grown on quartz substrate by PLD

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 materials 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 Al 2 O 3 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 Al 2 O 3 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% O 2 atmosphere, at 800 0 C and 700 0 C, respectively. I-V characteristics exhibited the typical rectifying behavior and the EL emissions from the n-ZnO:Ga/p-GaN:Mg heterostructure at room temperature show peaks at 430nm, 440nm and 480nm along with a broad band of yellow light.
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 Al 2 O 3 substrates by MOCVD (GaN layer) and RF sputtering (ZnO layer). After fabrication, ZnO films were annealed in a thermal furnace in air and nitrogen ambient at 800°C for 30-120min. For the LED annealed in N 2 , room-temperature EL in the blue region with peak wavelength 400nm was observed, and for the LED annealed in N 2 , 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-Al 0.12 Ga 0.88 N and I-V curve of the devices showed a rectifying 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 reported 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.

Optoelectronics -Advanced Materials and Devices
Chichibu et al fabricated p-CuGaS 2 /n-ZnO:Al heterojunction LEDs by metal-organic vapor phase epitaxy (p-CuGaS 2 ) 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-SrCu 2 O 2 /n-ZnO heterojunction LEDs [56]. The I-V curve of the device 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 forward bias voltage greater than 3V was applied to the device, as shown in Figure7. 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 organic 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 surface. 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-  [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 700 0 C with an optimized thickness of around 10nm improved EL performance of the devices.

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. Various 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 heterojunction LEDs with ZnO nanomaterials.

Optoelectronics -Advanced Materials and 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 electron 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 forward 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 emission peak is slightly blue shifted in the range of 400nm-420nm. By Gaussian deconvolution of the emission spectrum, the blue/near-UV emission is attributed particularly to three distinct electron-hole recombination processes. The LEDs give an external quantum efficiency of 2.5%, displaying great potential applications in high resolution electronic display, optical interconnect, and high density data storage. Zhang et al fabricated high-brightness blue-light-emitting diode using a ZnO-nanowire array 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 emission 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 (<100 0 C) forming p -n heterojunctions. The EL spectrum of ZnO-nanowall LED exhibited three peaks centered at 420nm (violet emission), 450nm (violetblue) and broad peak covering from 480 to 700nm (green, yellow, orange and red emissions). 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, yellow, 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. 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 without 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].

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 methods, as summarized in Table3.  Mandalapu et al reported ultraviolet emission from Sb-doped p-type ZnO based heterojunction 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 intrinsic 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.     Park et al reported on the growth and device properties of p-ZnO/(InGaN/GaN) multiquantum well (MQW)/n-GaN heterojunction LEDs [82]. A GaN buffer layer (30nm) was deposited 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 N 2 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. Under 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.

Homojunction LEDs
Based on the fabrication of p-type ZnO materials, some researchers reported on ZnO-homojunction LEDs. Table4 is a survey of structure, method and emission peak of ZnO-homojunction 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 threshold 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. 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.    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 1×10 14 cm −2 perpendicular to the aligned rods. The implanted ZnO rods were annealed at 900°C for 1 h with an O 2 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 currents. Strong UV emission was observed from both devices, corresponding to the NBE emission 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.

Conclusion and outlook
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 A x Zn 1-x O can be continuously modulated from 0.9eV (CdO) to 10.6eV (BeO) by changing the A concentration. As a n-type semiconductor, high quality n-type ZnO materials can be obtained by doping doping III elements (Al, Ga and In). Although the fabrication of p-type ZnO remain great challenges due to the self-compensation, p-type ZnO have been prepared by doping different elements (N, P, As, Sb, Li, Na and K) with various techniques. ZnO based heterojunction and homojunction LEDs have been achieved, which makes ZnO as a strong candidates for solid-state light. Although the efficiency of ZnO-based LEDs is not high, improvements are rapid and the emitters have the potential to outperform their GaN rivals.
ZnO-based LEDs show great promise for the future, however, there are some severe issues that are in need of further investigation to transition ZnO-based LEDs to commercial use from the current stage. One problem is that the usable, reproducible p-type ZnO are not easy to fabricate, although some researchers have been successful. Another is the achievement of high quality p-n junction based ZnO. The p-n junction with good threshold and breakdown voltages is necessary for the LEDs. In addition, diode-like behavior and light emission have been observed, however, the mechanism of the properties remain unclear.

Acknowledgements
The