Preparation And Characterization Of ZnO: Ga Thin Films On Epi-GaN Substrate

Wide band gap transparent conductive oxide films have features of high transmittance in the visible region, high reflectivity in the infrared region, and excellent electrical conductivity. GaN material has characteristics of wide band gap, high electronic saturated drifting velocity, and high thermal conductivity. Both of them have been widely used in photoelectric device fields. At present, GaN-based light-emitting diodes (LEDs) have been industrialization and have been widely used in applications. One method to improve the efficiency of GaN-based LEDs is using low resistance transparent conductive films to improve the luminous efficiency. Nowadays, the mainly used transparent conductive film in LED market is indium doped tin oxide (ITO). But, indium (In) is a shortage resource and has strong radiation which can cause recession of device performance.ZnO is an abundant resource and can keep stability under high temperature, so it is good for being applied in production. ZnO and GaN have approximate band gaps (EgZno=3.37eV, EgGaN=3.40eV) and well matched lattices (ZnO:ao=3.25OA, co=5.207A; GaN:a0=3.189A, C0=5.186A), reducing potential barrier effect and defects caused by interface stress, respectively. So, it is easy to obtain high quality ZnO films by using GaN substrates. Based on preparation of high quality ZnO single crystal films, appropriate doping can improve the electricity performance, so that ZnO can be made as good transparent electrode materials for GaN-LEDs, and also can be used for photoelectric devices based on ZnO. In brief, the research topic about ZnO films prepared on GaN substrates has a strong application potential. Under such background, the preparation and characterization of ZnO:Ga transparent conductive films on epi-GaN/a-Al2O3(0001) substrates are investigated in this article.The major research works and results are as follows:1. High quality ZnO single crystal epitaxial films were successfully prepared by MOCVD. Ultra high purity Zn(C2H5)2, O2 and N2 were used as the metallorganic (MO) source, oxidant and carrier gas, respectively. Analysis results show that films deposited on epi-GaN/a-Al2O3(0001) substrates have the best crystal quality, compared with films deposited on a-Al2O3(0001) and 7059-glass. A group of ZnO films were deposited on epi-GaN/a-Al2O3(0001) substrates at 600℃, with reactor pressure changing from 10 Torr to 40 Torr Analysis indicates that the films deposited at 20 Torr have the best crystal quality, highest mobility and good optical properties.2. By using epi-GaN/a-Al2O3(0001) substrates, a group of ZnO films were prepared at 20 Torr reactor pressure, with the substrate temperature changing from 500℃to 650℃. Analysis indicates that the films deposited at 600℃are single crystal epitaxial films with wurtzite structure. The epitaxial relationships with GaN (0001) substrates are ZnO(0001)|| GaN(0001), ZnO[1120]|| GaN[1120] and ZnO[1010]|| GaN[1010]. Among all prepared films, the films deposited at 600℃have the highest mobility, relatively low carrier concentration and resistivity. The average transmittance in the visible range for the prepared samples is higher than the transmittance for epi-GaN/a-Al2Os substrates. The prepared fims have increased the transmittance of samples.3. The ZnO:Ga films were deposited on 600℃by MOCVD, with the reactor pressure of 20 Torr. High purity Zn(C2H5)2 and Ga(CH3)3 were used as Zn and Ga metallorganic sources. Meanwhile, N2 and O2 were used as carrier gas and oxidant, respectively. Analysis indicates that ZnO:Ga (4%) films deposited at epi-GaN/a-Al2O3(0001) substrates have better crystalline quality and less optical band gaps than ZnO:Ga (4%) films deposited at a-Al2O3(0001) and 7059-glass. ZnO:Ga films with various concentration of gallium were fabricated on epi-GaN/a-Al2O3(0001) substrates. The structural, electrical and optical properties of samples were investigated in detail. All samples have the wurtzite structures of ZnO and have a preferred orientation along c-axis. As doping concentration increases, the prepared ZnO:Ga films change from single crystalline to polycrystalline, and the surface morphology and root-mean-square varies. The films with Ga concentration 4～6% have good electrical and optical properties. In this concentration scope, the films have resistivity of 1.24×10-2～1.26×10-2Ω·cm, Hall mobility of 11.7～23.2 cm2V-1s-1, and carrier concentration of 2.18×1019～4.22×1019cm-3, by measured at room temperature. The transmittance in the visible range for the samples is over 80%.