| Wide band gap oxide semiconductor material has excellent optical and electrical properties, so it has wide application prospect in light-emitting diodes, semiconductor laser, thin film solar cell, transparent thin film transistor, and gas detector etc. β-Ga2O3film is a kind of multifunctional transparent oxide semiconductor material with excellent thermal, chemical and physical stability, with excellent transmission property in the visible region, which can be used in fields of transparent optoelectronic devices, gas sensors, solar cells and ultraviolet laser, and so forth. However, the β-Ga2O3, films prepared by traditional techniques such as magnetron sputtering and electron beam evaporation preparation are amorphous, microcrystalline or polycrystalline. The crystal quality of the sample is poor, with many defects inside the film, which bring down the luminescent efficiencyand the doping efficiency of gallium oxide films. Thus these β-Ga2O3films cannot meet the requirements of high-quality semiconductor optoelectronic devices.At present, there are few reports on the epitaxial growth of β-Ga2O3films, the β-Ga2O3epitaxial films show better structure of uniformity with excellent optical performance and physically and chemically stability. As a result, they can be used for fabricating high performance semiconductor devices. In this paper, epitaxial β-Ga2O3films with different orientations have been prepared on MgO, MgAl2O4and MgAl6O10substrates by metal organic chemical vapor deposition. The structural, optical and electrical properties were investigated in detail. On this basis, the n-type doped β-Ga2O3, epitaxial films were deposited, the electrical properties of the β-Ga2O3, films can be improved. So the research of this paper is of great importance in scientific significance and practical application value.The key research work and results of this paper are as follows:1. β-Ga2O3, films were deposited on MgO (110) and MgO (111) substrates at temperatures of550,600,650and700℃, the high purity Ga(CH3)3, O2and N2were used as Ga source, oxidant and carrier gas, respectively. (1) The structural analysis of the films grown on MgO (110) substrate indicated that the film prepared at500℃was amorphous or microcrystalline. The β-Ga2O3films deposited at600.650and700℃were grown along single β-Ga2O3(102) orientation. β-Ga2O3films deposited at650℃had the best crystallinity. The microstructure of the samples prepared at650℃was investigated by multifunctional XRD and HRTEM. A schematic diagram was proposed to clarify the growth mechanism of the two-fold rotation domain structure in the film. The in-plane orientation relationship between the film and substrate is β-Ga2O3[201]||MgO [110] and β-Ga2O3[010]||MgO [001]. The lattice mismatch is about0.5%along the β-Ga2O3[201] orientation and about7.7%along the β-Ga2O3[010] orientation. The average transmittance of the samples in the visible wavelength range exceeded80%.(2) The RBS analysis of the films grown on MgO (111) substrate revealed that the samples contained the elements of Ga, O and Mg. The thickness of the films was estimated from the RBS results, and the growth rate of the films was affected by the substrate temperature. From the XRD analyses, thesamples deposited at650℃had the best crystallinity, and the out-plane relationship between the film and substrate is β-Ga2O3(201)||MgO (111). The schematic diagram was proposedto explain the six domains rotated by60°inside the film. The in-plane orientation relationship between the film and substrate is β-Ga2O3[010]||MgO<110>. The optical band gap of the films deposited at550,600,650and700℃were4.94,4.90,4.79and4.86eV, respectively. The average transmittance of the films in the visible wavelength range exceeded88%.2.β-Ga2O3films were deposited on MgAl6O10(100) substrates at temperatures of550,600,650and700℃.β-Ga2O3films grown at550and600℃were amorphous or microcrystalline. As the substrate temperature increased to650℃,β-Ga2O3film was grown along a single orientation which isparallel to β-Ga2O3(100) plane and the crystallinity was the best.β-Ga2O3film prepared at700℃was polycrystalline, and the film had a preferred orientation of β-Ga2O3(100). For the film deposited at650℃, the in-plane orientation relationship between the film and substrate is β-Ga2O3[001]||<011>. A schematic diagram was proposed to illustrate the four-fold domain structures rotated by90°inside the film. In the HRTEM measurements, the results of EDS showed that the AI element in the substrate would diffuse into the gallium oxide film during the deposition of the film, and a transition region similar to buffer layer could be formed, the region would perform the function of stress releasing, which may improve the crystalline quality of the film. As the wavelength of the incident light increased from300nm to800nm, the refractive index of the samples decreased rapidly at the beginning, and then was placid after about600nm. The average transmittance of the film grown at650℃in the visible range exceeded78%and the optical band gap of the film was about4.86eV.Photoluminescence (PL) spectra measured at room temperature revealed that all the films exhibited intense ultraviolet (UV)-green emission from300to650nm.A minor deep UV emission around275nm was observed for the sample prepared at700℃.The intensity of the emission increased markedly when measured at low temperature and the location of the peaks had a blue shift for the films grown at700℃.It was concluded by analysis that the major emission originated from recombination of the electron of the donors formed by oxygen vacancy and Ga2+to the holes of the acceptors formed by gallium vacancy and gallium-oxygen vacancy pairs. The minor emission can be attributed to the recombination of the electron of the donors to the holes in valance band edge.3. The β-Ga2O3films had been prepared on MgAl2O4(100) substrates at different temperatures (550-700℃). The film grown at550℃wasmicrocrystalline. As the substrate temperature increased to600and650℃,β-Ga2O3films were grown along a single orientation which is perpendicular to β-Ga2O3(100) plane. The structure of the film became polycrystalline for the film grown at700℃. Thesamples deposited at650℃had the best crystallinity and the epitaxial relationship was β-Ga2O3(100)||MgAl2O4(100) with β-Ga2O3[001]||MgAl2O4<011>, a schematic diagram was proposed to explain the domain structure in the film layer. The lattice mismatch is about0.15%along the β-Ga2O3[001] orientation and about6.0%along the β-Ga2O3[010] orientation.The average transmittance for the films in the visible range was over80%. The band gaps of the samples deposited at550,600,650and700℃are about4.96,4.87,4.84and4.90eV, respectively.An UV-green PL from about350nm to600 nm was observed at room temperature and the emissions can be divided into four peaks at around379nm,416nm,457nm and513nm. The emissions can beattributed to the electron transition between the donor level and the acceptor level formed by the defects of the film.4. On the basis of the study above, we grew Ga2O3:Sn films on different substrates. The high purity Ga(CH3)3, Sn(C2Hs)4, O2and N2were used as Ga source, Sn source, oxidant and carrier gas, respectively.(1) The Ga2O3:Sn films were deposited on MgO (100) substrates at650℃, the concentration of Sn was set to be5%,10%,15%and20%. The gas follow direction of the MO sources in the reaction chamber was paralleled to the substrate. All the samples were annealed in the air for1hour at800℃. The as-deposited samples showed amorphous or microcrystalline structure. After the annealing treatment, the deposited samples were β-Ga2O3films with a (601) single orientation. The electrical properties analyses revealed thatthe resistivity decreased by almost six orders of magnitude compared with the undoped Ga2O3film, with a minimum value of1.2><104Ω·cm obtained at15%(atomic ratio)of Sn-doped sample. The XPS spectrum obtained for β-Ga203films with15%Sn doped after annealing showed that the Sn/Ga atomic ratio is evaluated to be about12.6%. The average transmittances of the samples in the visible range were80%,87%,86%and84%, respectively.The bang gap energy decreases from4.80eV to4.58eV as the Sn content increased from5%to20%.A narrow and strong violet PL from390to430nm was observed at room temperature and this emission came from therecombination of an electron on a donor and a hole on an acceptor.(2) The Ga2O3:Sn films were deposited on MgO (110) substrates at700℃, the concentration of Sn was set to be1%,3%,5%,8%,10%,11%and12%. The gas follow direction of the MO sources in the reaction chamber was perpendicular to the substrate. From the XRD results, the β-Ga2O3:Sn films with Sn contents from1%to11%showed a major peak located at about30°corresponding to β-Ga2O3,(400). The full width at half maximum of the β-Ga2O3(400) peaks increased as the Sn contents increased, which revealed the degradation of the crystalline quality of the films. The10%Sn-doped film exhibited the best electrical conductive properties. The lowest resistivity for the β-Ga2O3:Sn film was about5.21×10-2Ω·cm, reducing by over ten orders of magnitudethan un-doped film. At room temperature, the carrier concentration and mobility of the10%Sn doped films were3.71×1019cm-3and3.35cm2V-1s-1, respectively. The mechanism of doping and conductivity was studied. Micro-structural analysis revealed that the films deposited with10%Sn content had a clear out-plane relationship of β-Ga2O3(100)||MgO (110) and in-plane relationship of β-Ga2O3(201)||MgO (111). The average transmittance of the samples in the visible range exceeded87%and the optical, band gaps of the films varied from4.12to4.80eV corresponding to different Sn concentrations.(3) The Ga2O3:Sn films were deposited on MgAl2O4(100) substrates at700℃, the concentration of Sn was set to be1%,3%,5%,8%,10%and12%. The gas follow direction of the MO sources in the reaction chamber was perpendicular to the substrate. When the contents of Sn were low, the films were polycrystalline, and had a preferred orientation of β-Ga2O3,(400). As the Sn composition increased, the crystalline quality of the films decreased. The lowest resistivity for the β-Ga2O3:Sn film was obtained at the10%Sn doped sample, and the value was about3.1×10-2Ω·cm, the carrier concentration was about2.4×1020cm-3and the mobility was about0.74cm2V-1s-1, the mechanism of the doping and conductivity was investigated. The actual Sn content of the10%Sn doped film was about13%, the results were consistent with the15%(The actual Sn content was about12.6%) Sn doped β-Ga2O3.Sn film grown on MgO (100) substrate. These meant that the β-Ga2O3film with the actual Sn content13%had the best conductive properties. The average transmittance of the10%Sn doped β-Ga2O3:Sn film in the visible range was over80%and the optical band gap was about4.14eV. |
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