Research On The Growth Of Ga₂O₃ And β-(Al_xGa_1-x)₂O₃ Films By MOCVD

As a representative of wide band gap semiconductor,gallium oxide material attracts more and more researchers because of its excellent electrical and photoelectric properties.The larger band gap makes it has great potential in the application of optoelectronic devices and power electronic devices.Ga₂O₃ can form(Al_xGa_1-x)₂O₃ films with aluminum atoms,by further widening the band gap,not only can be used for deep ultraviolet detection,but also can be applied in the study of heterojunction,is expected to achieve two-dimensional electron gas（2DEG）generation,and promote the development of new type high electron mobility transistor（HEMT）.β-Ga₂O₃ has attracted the most attention as the most stable crystal structure of gallium oxide materials,and the metastable phaseε-Ga₂O₃ has attracted the attention of researchers because of its ferroelectric properties.In this paper,we mainly focus onβ-Ga₂O₃ andε-Ga₂O₃,and the growth of Ga₂O₃ andβ-(Al_xGa_1-x)₂O₃ films was carried out by metal-organic chemical vapor deposition（MOCVD）technology.The results of this paper are as follows:1.The effects of growth parameters onβ-Ga₂O₃ film were investigated.β-Ga₂O₃ film was grown on c-plane sapphire substrate,and 800℃was found to be the suitable temperature forβ-Ga₂O₃ film growth,the increase of O₂ flow rate was conducive to the improvement of crystal quality,but too high O₂ flow rate would cause adverse effects on the growth rate and surface morphology.The growth rate ofβ-Ga₂O₃film was decreased obviously with the increase of growth pressure.β-Ga₂O₃ films along the（-201）crystal plane family were prepared on a-plane sapphire and epi-Ga N/Sapphire substrates.The growth ofβ-Ga₂O₃ films by pulsed MOCVD technology was studied.The growth rate ofβ-Ga₂O₃ films grown by pulse growth method was slow,but the surface morphology ofβ-Ga₂O₃ films was more uniform,and the migration of atoms on the substrate surface was improved effectively,which laid the foundation for the two-step growth method on homogeneous substrate.2.The growth ofβ-Ga₂O₃ films was studied on（-201）homogeneous substrate,and the surface morphology ofβ-Ga₂O₃film was improved by combining continuous growth method and pulsed growth method.By optimizing the growth parameters,the surface RMS roughness was reduced to<0.5 nm at the growth pressure of 37 Torr,and the grain protrusions and grooves on the surface were greatly inhibited.In-assisted growth ofβ-Ga₂O₃films was carried out on（-201）and（100）homogeneous substrates.In order to reduce the amount of In delivered to the reaction chamber,the TMIn organic source was delivered to the reaction chamber in the form of gas pulse,avoiding excessive In atoms to form alloy oxides withβ-Ga₂O₃,while other pipelines maintained continuous access.The effect of TMIn pulse interval time on the growth ofβ-Ga₂O₃ films was investigated.It was found that In played a positive role in theβ-Ga₂O₃ films grown on（-201）and（100）substrates.Theβ-Ga₂O₃ film on（-201）substrate effectively reduced the surface protrusions and grooves,and the two-dimensional growth ofβ-Ga₂O₃film was realized.Theβ-Ga₂O₃ film on（100）substrate showed a trend of transforming from three-dimensional growth to two-dimensional growth,and a step-like morphology was obtained.It was found that the auxiliary role of In can be exerted most effectively by respectively using 0.1 min and 0.2 min as the access time and interval time of TMIn pulse.3.The growth of Sn-dopedβ-Ga₂O₃ films on（-201）homogeneous substrate was investigated,and the carrier concentration ofβ-Ga₂O₃ film without Sn doping was of the magnitude of10¹⁶ cm^-3,and the highest electron mobility measured was 142.67 cm²/（V·s）.The Sn-dopedβ-Ga₂O₃ film grown at the Sn organic source flow rate of 10 sccm had a carrier concentration of the magnitude of 10¹⁷ cm^-3,but the electron mobility decreased rapidly to<10 cm²/（V·s）,and SIMS measurement result indicated that the most of Sn atoms in the film were in an inactive state.By reducing the Sn source flow rate to 2 sccm,the electron mobility increased to 22.10 cm²/（V·s）,and the carrier concentration increased from 2.31×10¹⁷ cm^-3 to 5.07×10¹⁷ cm^-3,indicating that the activation rate of Sn atoms in theβ-Ga₂O₃film was effectively improved.The electrical properties ofβ-Ga₂O₃ films were further improved by decreasing the temperature of Sn organic source.The carrier concentration and electron mobility of Sn-dopedβ-Ga₂O₃ film grown at-15°C were 2.41×10¹⁸ cm^-3 and 51.50 cm²/（V·s）,respectively.The surface morphology of Sn-dopedβ-Ga₂O₃film was very uniform,and the surface RMS roughness was reduced to<0.7 nm.4.The growth ofβ-(Al_xGa_1-x)₂O₃ films was studied.Using TMAl as Al source,the XRD diffraction peaks ofβ-(Al_xGa_1-x)₂O₃ films grown on c-plane sapphire substrate gradually shifted to the larger diffraction angles with the increase of TMAl flow rate,and Al atoms replaced Ga atoms in the lattice ofβ-Ga₂O₃,resulting in the decrease of crystal plane spacing.The crystalline structure ofβ-(Al_xGa_1-x)₂O₃ film was distorted,and the intensity and full width at half maximum（FWHM）of XRD diffraction peak weakened and broadened respectively.Compared withβ-Ga₂O₃ films,the surface RMS roughness and band gap ofβ-(Al_xGa_1-x)₂O₃ films increased,but the surface RMS roughness ofβ-(Al_xGa_1-x)₂O₃ films was slightly decreased due to higher TMAl flow rate.It was found that the Al component inβ-(Al_xGa_1-x)₂O₃ films grown at lower growth temperature was larger,and the XRD diffraction peak shifted to a larger diffraction angle.Theβ-(Al_xGa_1-x)₂O₃ film grown at 500℃were thermally annealed at N₂ and O₂ respectively,and the（-201）and（-603）XRD diffraction peaks were enhanced,indicating that the crystallinity ofβ-(Al_xGa_1-x)₂O₃ film improved.β-(Al_xGa_1-x)₂O₃ film was grown on（100）β-Ga₂O₃ substrate at 900°C,and the band gap of theβ-(Al_xGa_1-x)₂O₃ film was calculated to be 5.80 e V by fitting the O 1s energy level peak.5.Pure-phaseε-Ga₂O₃ film was prepared on c-plane sapphire substrate by decreasing the growth temperature to 400°C,and the variation of TEGa flow rate and growth pressure would lead to the form ofβ-Ga₂O₃ in the film,and the phase transition ofε-Ga₂O₃ film was realized by high-temperature thermal annealing under a mixed atmosphere of N₂ and O₂.ε-Ga₂O₃ films were prepared on a/r-plane sapphire and epi-Ga N/Sapphire substrates.The rocking curves of HRXRD found that theε-Ga₂O₃ film on a-plane sapphire substrate had a FWHM of 1.01°,while theε-Ga₂O₃ film on epi-Ga N/Sapphire substrate had a FWHM of0.65°,indicating that the crystallization quality ofε-Ga₂O₃ film grown on epi-Ga N/Sapphire substrate was better than that on a-plane sapphire substrate.In conclusion,the growth of Ga₂O₃ andβ-(Al_xGa_1-x)₂O₃ films was comprehensively studied and analyzed,and the research results obtained in this paper played an important guiding role in the subsequent film growth.