| In recent years, intense interest has been paid to transparent oxide semiconductors (TOSs) owing to their potential applications in light-emitting diodes, laser diodes, ultraviolet (UV) detectors, transparent thin-film transistors, flat panel displays and thin-film solar cells. Both In2O3 and Ga2O3 are good TOSs with direct band gap energy of 3.6 and 4.9 eV, respectively. In2O3 is a very important n-type transparent semiconductor and has been widely used in many fields such as solar cells and flat-panel displays owing to its excellent optoelectrical properties. The optical bandgap of the conventional transparent oxide semiconductors such as ITO films is usually smaller than 3.7eV. A need for materials which are transparent in the UV region has emerged with the development of transparent optoelectronics and optoelectronic devices. Ga2O3 with the monoclinic structure (β-Ga2O3) is a promising candidate for deep-ultraviolet (UV) transparent conductive oxides. But Ga2O3 as a deep-UV transparent conductive material presents some difficulties due to its over high band gap energy. Hence, it is necessary to investigate novel bandgap tunable TOSs materials. (Ga2-2xIn2xO3) is considered to be an alloy of In2O3 and Ga2O3. The band gap of (Ga2-2xIn2xO3) could be tuned from 3.6eV to 4.9eV by controlling the composition of the films, and this make (Ga2-2xIn2xO3) a promising UV photoelectric material. Until now, there is still lack of systematical investigation for properties of (Ga2-2xIn2xO3) (0.1≤x≤0.9) films. To prepare Ga2-2xIn2xO3 films and investigate the growth mechanism, film composition, crystal structure, phase transition and optoelectrical properties of the films will lift this material to higher potential in the field of transparent and UV optoelectronic devices. Under such background, the preparation and characterization of Ga2-2xIn2xO3 films are investigated in this article.The content of this article consists of three parts. In the first part, the Ga2O3 films were deposited by MOCVD. The structural and optical properties of the films were investigated. In the second part, high quality In2O3 films were prepared by MOCVD. The structural, optical and electrical properties of the films were investigated in detail, and the PL phenomena of band-to-band transition in In2O3 films prepared onα-Al2O3 were first observed. In the third part, Ga2-2xIn2xO3 (0.1≤x≤0.9) and Sn doped Ga1.4In0.6O3 films were prepared by MOCVD. The structural, optical and electrical properties of the films were investigated systematically.The major research work and results of the first part are as follows:The Ga2O3 films were successfully prepared onα-Al2O3 (0001) substrates at 600℃by MOCVD. Ultra high purity Ga(CH3)3, N2 and O2 were used as the organometallic source, carrier gas, and oxidant, respectively. The film deposition method was described in this part and the annealing effect on the structural and optical properties of deposited films were investigated. XRD and AFM results indicated that the deposited film was amorphous and transformed to polycrystalline structure ofβ-Ga2O3 film after annealing. The increase of the annealing temperature could enhance the preferred orientation, enlarge the grain size and improve the crystallization of the film. The optical transmittance spectra showed that the average transmittance in the visible range for the as-deposited and annealed films was over 95%. The transmittance of the annealed film in the near UV region was obviously improved and the absorption edge shifted to shorter wavelength. The optical gaps for the as-grown and annealed samples were 4.71 eV and 4.89eV, respectively.The major research work and results of the second part are as follows:1. High-quality In2O3 epitaxial films were successfully prepared onα-Al2O3 (0001) substrates by MOCVD for the first time. Ultra high purity In(CH3)3, N2 and O2 were used as the organometallic source, carrier gas, and oxidant, respectively. The structural, optical and electrical properties of the films as a function of substrate temperature were investigated, and the epitaxial growth mechanism of single crystalline In2O3 films was analyzed. The XRD results indicated that the prepared samples are pure In2O3 films of body-centered cubic (bcc) structure with a single orientation of (111) direction. The sample prepared at 650℃substrate temperature has the best crystalline quality. HRXRD and HRTEM results indicated that the sample was single crystalline film having the bcc structure of pure In2O3 with a clear epitaxial relationship of In2O3 (111)//A12O3 (0001) with In2O3 [(?)0]//Al2O3[11(?)0]. The fullwidth at half maximum of the to-rocking curve of (222) reflection is only 0.14°, indicating a high quality crystalline structure. The optical transmittance spectra showed that the average transmittance in the visible range for all films was over 90%. The optical band gap for the 650℃prepared sample was 3.66eV. The Hall measurements indicated that the lowest resistivity and the highest Hall mobility for the obtained films were 5.10×10-3Ω·cm and 35.2 cm2·v-1·s-1, respectively. The carrier concentration varied between 1.61×1019 cm-3 and 4.38×1019 cm-3.The PL spectra of the In2O3 film deposited at 650℃on sapphire (0001) were measured. An intense and sharp UV PL peak near 337 nm was observed at room temperature for the first time. While measured at low temperature, there were no obvious change for the structure of this UV PL peak, but the peak position shifted towards higher energy. The origin of the UV PL peak near 337 nm was ascribed to the electron transition from the conduction band to the valence band.The major research work and results of the third part are as follows:1. The Ga2-2xln2xO3(0.1≤x≤0.9) films were prepared onα-Al2O3 (0001) substrates at 550℃by MOCVD for the first time. High purity Ga(CH3)3 and In(CH3)3 were used as the organometallic sources. The crystal structures, structural phase transitions, optical and electrical properties as well as compositions of these films were investigated in detail. In addition, the annealing effect on the structural and optical properties of deposited films was discussed. The XRD results indicated that, as the In content decreases, the structure of the films changed from bcc to amorphous, and finally to monoclinic structure ofβ-Ga2O3. After annealing at 700℃, the crystalline quality of all the films was improved and the preferred orientation for the Ga-rich film changed obviously. The compositions of Ga2-2xIn2xO3 films calculated from the XPS and RBS spectra were consist with the experiment enactment values. The optical transmittance spectra showed that the average transmittance in the visible range for all prepared films was over 85%. The Eg of the Ga2-2xIn2xO3 films could be tuned from 3.76 eV to 4.43eV by controlling the Ga content. The transmittance of the films in the near UV region was obviously improved and the band gap energy increased after annealing at 700℃. The resistivity of Ga2-2xIn2xO3 films increased monotonously from 3.414×10-3Ω·cm to 6.71×104Ω·cm and increased further after the annealing treatment.2. The Ga2-2xIn2xO3(0.1≤x≤0.9) films were prepared onα-Al2O3 (0001) substrates at 700℃by MOCVD. High purity Ga(CH3)3 and In(CH3)3 were used as the organometallic sources. The structural, optical and PL properties as well as compositions of these films were investigated in detail. XRD and HRTEM results indicated that, as the In content increases, the films changed from monoclinic structure ofβ-Ga2O3 to bcc structure of polycrystalline In2O3, and finally to single crystalline In2O3 films with a single orientation of (111) direction. The optical transmittance spectra showed that the average transmittance in the visible range for all prepared films was over 90%, and the band gap of the Ga2-2xIn2xO3 films was tuned from 3.72 eV to 4.58eV by controlling the Ga content.The PL spectra of the Ga2-2xIn2xO3 films were measured at room temperature. For the higher In content (x≥0.5), an UV PL peak near 338nm (3.67eV) was observed. As decreasing the In content, the relative intensity of the UV emission decreased. And for the lower In content (x<0.5), only a sharp UV PL peak located at 332nm (3.73eV) was observed, and the relative intensity of the peak increased with decreasing the In content. For x=0.3, while measured at low temperature, the peak position shifted towards higher energy. The UV PL mechanisms of Ga2-2xIn2xO3 thin films were discussed. The origin of the UV PL peak near 338 nm was ascribed to the electron transition from the conduction band to the valence band. The peak near 332nm was originated fromβ-Ga2O3 grains, and the PL mechanisms can be attributed to the electron transition from the conduction band to the acceptor level.3. Sn-doped Ga1.4In0.6O3 films were prepared onα-Al2O3 (0001) substrates at 550℃by MOCVD for the first time. High purity Ga(CH3)3, In(CH3)3 and Sn(C2H5)4 were used as the organometallic sources. The structural, optical and electrical properties of the films dependent on doping level were investigated in detail. The structural and morphological analysis revealed that, when the Sn-doping level exceeded 3%, the films changed from amorphous structure to polycrystalline ofβ-Ga2O3. The grain size of the films increased obviously on elevating the Sn concentration, which indicated an improvement of crystallization. The optical transmittance spectra and Hall measurements indicated that the optical band gap, resistivity, carrier concentration and Hall mobility for the films changed with the doping level. The 5% Sn-doped Ga1.4In0.6O3 film exhibited the best optical and electrical properties with average transmittance over 85% in the visible range and optical band gap of 3.85 eV. The resistivity and Hall mobility for the 5% Sn-doped film were 4.9×10-3Ω·cm and 21.4 cm2·v-1·s-1, respectively.
|
PDF Full Text Request