雾化学气相外延法异质外延Ga_(2)O_(3)薄膜的生长特性

Growth Characterization of Heterogeneous Epitaxial Gallium Oxide Films by Mist Chemical Vapor Deposition

氧化镓各晶相的应用广泛,亚稳相只能通过外延的手段获得。选择合适的生长方法和提高异质外延水平仍是学术界一直探讨的问题。采用雾化学气相沉积(Mist-CVD)法在蓝宝石衬底上异质外延生长了氧化镓薄膜,通过优化工艺条件实现了物相调控,系统研究了外延膜的结晶质量、组分和表面形貌,以及生长速率变化规律。同时,探究了Mist-CVD生长系统中,Ga_(2)O_(3)外延膜的晶体成核及晶相控制机理。在原有α-Ga_(2)O_(3)生长工艺的基础上,制备了α/ε混相薄膜、ε-Ga_(2)O_(3)纯相薄膜,呈三维岛状生长模式。在ε-Ga_(2)O_(3)纯相薄膜的基础上,随着氧气流量的增加,结晶质量先稳定后下降,生长速率先增加后降低,最高达到3μm/h。通过退火,获得了β-Ga_(2)O_(3)纯相薄膜,表面粗糙度由10.80 nm降低至5.42 nm。在利用Mist-CVD法进行不同晶相外延调控及其生长机理研究方面具有一定借鉴意义。

Introduction Gallium oxide crystal phase is widely used,and the metastable phase can be only obtained by means of epitaxy.Selecting suitable growth methods and improving the heteroepitaxy level are still the problems that the academic circles have been discussing.The mist chemical vapour deposition(Mist-CVD)method does not require a vacuum system,which is safe,economical,simple and controllable,demonstrating a potential application for metastable phase growth of gallium oxide.The level of heterogeneous epitaxy of various crystalline phases of gallium oxide still needs to be further improved.For instance,there is still a problem of mixing phases during the growth process,which leads to a difficulty of regulating the crystalline phases.In terms of the nucleation mechanism,there is a lack of process-specific research and explanatory notes.In this paper,the most suitable growth intervals of Ga_(2)O_(3) crystalline phases on sapphire substrates were explored in a vertical mist chemical vapour deposition epitaxy furnace.The relationships among Ga_(2)O_(3) crystal phase modulation,crystalline quality,growth rate and growth process parameters,as well as the optimisation of surface morphology by annealing were investigated respectively.The nucleation mechanism of the films and the precursor droplet motion law of Mist-CVD were analyzed,and the experimental results were discussed.Methods Heterogeneous epitaxial Ga_(2)O_(3) films were prepared by the Mist-CVD method in a vertical mist chemical vapour deposition epitaxy furnace with a 2-inch commercial sapphire substrate.Gallium acetylacetonate(Ga(acac)_3)aqueous solution was used as a precursor for Ga and O at 0.05 mol/L,and 1.5% HCl was used as a co-solvent.High-purity Ar(99.999%)was used as a carrier gas with Ar flow rate of 2250 mL/min and growth pressure of 101.325 kPa,respectively,and O_(2) was used as a dilution gas.The growth parameters were adjusted in the growth temperature range from 600℃ to 750℃ and O_(2) flow rate from 225 mL/min to 600 mL/min for 1 h.A model D8-ADVANCE X-ray diffractometer(XRD)was used to determine the crystal structure.The elemental composition and content of the epitaxial layer surfaces were determined using a model ESCALAB-250 X-ray photoelectron spectrometer(XPS).In addition,the cross-sectional image and thickness of the Ga_(2)O_(3) epitaxial layer were characterized by a model S-4800 scanning electron microscope(SEM).The AFM images and root-mean-square(RMS)surface roughness of Ga_(2)O_(3) epitaxial layers were analyzed by a model ICON-type atomic force microscope(AFM).A LAMBDA-950 UV-Vis spectrophotometer(UV-Vis)was used to characterize the transmittance of the samples and calculate the forbidden band width.Results and discussion Based on the XRD analysis on the crystal structure of the epitaxial films of gallium oxide at 600-750℃,there exist α and ε mixed phases at 600-700℃,and the film diffraction peaks are weak with no obvious selective orientation.This is due to the higher lattice matching of the α phase to the c-plane sapphire substrate,which is more easily induced to grow on the sapphire substrate at low temperatures and stabilised under lattice mismatch strain.The ε-pure phase is grown at 750℃ with a selective orientation.The crystalline phase of gallium oxide at higher temperatures is not explored,and pure phase films of β-Ga_(2)O_(3)are expected to be obtained at higher growth temperatures due to the limitations of the current epitaxial furnace equipment with a maximum set temperature of 750℃.ε-Ga_(2)O_(3) pure phase is generated at different oxygen flow rates.However,the half-height width of the diffraction peaks of the films appears at a lower level of 0.141°and 0.150°for oxygen flow rates at 225-250 mL/min.When the oxygen flow rate exceeds400 mL/min,the half-height widths of the samples become larger,i.e.,0.178°,0.162°and 0.162°,respectively,and the crystalline quality of the films decreases,but the overall crystalline quality of ε-Ga_(2)O_(3) is better.Based on the analysis on the cross-section of the film thickness by SEM,the growth rate of the film firstly increases and then decreases with the increase of the oxygen flow rate,reaching a growth rate of 3μm/h at 250 mL/min and 400 mL/min.A higher growth rate can be obtained at 250-400 mL/min.For theε-Ga_(2)O_(3) samples grown at 750℃,the surface of the ε-Ga_(2)O_(3) sample shows a cluster with coarser particles and a cloudy morphological features,and its surface roughness is 10.8 nm.From the 3D stereogram,the surface morphology shows multiple bulge-like or hill-like bumps,and the height of the surface bumps is between 50 nm and 100 nm,and its overall height is larger than the grain size of the film,indicating that the epitaxial film grows in accordance with the three-dimensional island-like growth mode.The β-Ga_(2)O_(3) sample after annealing at 900℃ is similarly characterized by AFM.The surface roughness of the thin film of β-Ga_(2)O_(3)samples decreases as RMS=5.42 nm,and the grain size becomes larger,ranging from 50 nm to 80 nm.At high temperatures,the height of the surface bumps is between 5 nm and 30 nm,which is smaller than the grain size,showing that the film densities increase at high temperatures.Conclusions Pure phase films of ε-Ga_(2)O_(3) were obtained at 750℃ in a three-dimensional island growth pattern with a high surface roughness of 10.8 nm at low temperatures.The growth rate of the film of ε-Ga_(2)O_(3) at an oxygen flow rate of 225 mL/min was 1μm/h with a half-height width of 0.141°.The growth rate of the film increased to 3μm/h with increasing the oxygen flow rate,and the film was still in the pure phase of ε-Ga_(2)O_(3).However,the crystalline quality of ε-Ga_(2)O_(3) was not improved,while rather deteriorated due to the high oxygen flow rate.β-Ga_(2)O_(3) pure-phase thin films were obtained after annealing at 900℃,and the surface morphology was further optimized with a reduced roughness of only 5.42 nm.The pure crystalline phases of Ga_(2)O_(3) with a considerable thickness could be prepared by a low-cost mist chemical vapor deposition method in a Mist-CVD equipment.