R. S. Kern

REAL-TIME ASSESSMENT OF OVERLAYER REMOVAL ON GAN, ALN, AND ALGAN SURFACES USING SPECTROSCOPIC ELLIPSOMETRY

Spectroscopic ellipsometry was used to assess the preparation of smooth and abrupt GaN, AlN, and AlGaN surfaces by wet chemical treatments in real time. About 20–50 A of overlayer typically can be removed from air‐exposed samples.

Epitaxial growth of AlN by plasma-assisted, gas-source molecular beam epitaxy

Monocrystalline AlN(0001) films with few defects were deposited on vicinal α(6H)–SiC(0001) wafers via plasma-assisted, gas-source molecular beam epitaxy within the temperature range of 1050–1200 °C. The Al was thermally evaporated from an effusion cell. An electron cyclotron resonance plasma source was used to produce activated nitrogen species. Growth on vicinal Si(100) at 900–1050 °C resulted in smooth, highly oriented AlN(0001) films.

Aluminum nitride/silicon carbide multilayer heterostructure produced by plasma‐assisted, gas‐source molecular beam epitaxy

Pseudomorphic structures containing β(3C)‐SiC and 2H‐AlN have been grown on vicinal α(6H)‐SiC(0001) at 1050 °C by plasma‐assisted, gas‐source molecular beam epitaxy. Reflection‐high energy electron diffraction and cross‐sectional high‐resolution transmission electron microscopy showed all layers to be monocrystalline. The AlN layers were uniform in thickness. Defects in these layers initiated at steps on the 6H‐SiC film. The 3C‐SiC layers contained a high density of stacking faults and microtwins caused primarily by the interfacial stresses generated by the mismatch in lattice parameters between AlN and β‐SiC coupled with the very low stacking fault energy of SiC. This is the first report of the deposition of single crystal SiC/AlN/SiC thin film heterostructures on any substrate as well as the first report of the epitaxial growth of single crystal layers of binary materials with three different crystal structures.

Layer-by-layer growth of SiC at low temperatures

Abstract A novel reactor for layer-by-layer deposition of compound semiconductors has been designed and commissioned for the deposition of SiC. The substrates rested on a heated, rotating platform. They encountered individual fluxes of Si2H6 and C2H4 and subsequently paused beneath a hot filament. The filament was used to encourage the surface reaction between silicon adatoms and carbon precursors. Heteroepitaxial films were grown between 850 and 980 °C on Si(100) substrates oriented 3° off-axis toward 〈011〉. They were analyzed for composition, crystallinity, growth per cycle, and morphology using depth profiling Auger spectroscopy, reflection high-energy electron diffraction, ellipsometry and transmission electron microscopy. Growth, as measured by ellipsometry and transmission electron microscopy, corresponded to approximately one monolayer per cycle. Monocrystalline films were achieved. Initial growth and characterization results of representative films are presented and discussed.

Solid solutions of AlN and SiC grown by plasma-assisted, gas-source molecular beam epitaxy

Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC), the only intermediate phases in their respective binary systems, have been grown at 1050 [degree]C on [alpha](6H)--SiC (0001) substrates cut 3[minus]4[degree] off-axis toward [11[bar 2]0] using plasma-assisted, gas-source molecular beam epitaxy. A film having the approximate composition of (AlN)[sub 0.3](SiC)[sub 0.7], as determined by Auger spectrometry, was selected for additional study and is the focus of this note. High resolution transmission electron microscopy (HRTEM) revealed that the film was monocrystalline with the wurtzite (2H) crystal structure.

DEPOSITION AND DOPING OF SILICON CARBIDE BY GAS-SOURCE MOLECULAR BEAM EPITAXY

Thin films of silicon carbide (SiC) have been deposited at 1400–1450 °C on vicinal and on-axis 6H-SiC(0001) substrates by gas-source molecular beam epitaxy using the SiH4-C2H4-H2 gas system. Polytype control (6H- or 3C-SiC) was established by utilizing substrates of particular orientations. Residual, unintentionally incorporated nitrogen impurity levels were affected by changing the SiH4/C2H4 gas flow ratio, in agreement with the “site-competition epitaxy” model. In situ doping was achieved by intentional introduction of nitrogen and aluminum into the growing crystal.Thin films of silicon carbide (SiC) have been deposited at 1400–1450 °C on vicinal and on-axis 6H-SiC(0001) substrates by gas-source molecular beam epitaxy using the SiH4-C2H4-H2 gas system. Polytype control (6H- or 3C-SiC) was established by utilizing substrates of particular orientations. Residual, unintentionally incorporated nitrogen impurity levels were affected by changing the SiH4/C2H4 gas flow ratio, in agreement with the “site-competition epitaxy” model. In situ doping was achieved by intentional introduction of nitrogen and aluminum into the growing crystal.

Growth of SiC and III–V nitride thin films via gas-source molecular beam epitaxy and their characterization

Abstract Silicon carbide (SiC) and aluminum nitride (AlN) thin films have been grown on 6HSiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050°C. Step flow, step bunching and the deposition of 6HSiC occurred at the outset of the exposure of the (1 × 1) vicinal substrate surface to C 2 H 4 Si 2 H 6 gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3CSiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 × 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth. Essentially atomically flat monocrystalline AlN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the Si C bilayer steps with the AlN bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (10 2 Ω · cm) and Mg-doped, p-type (0.3 Ω · cm) monocrystalline GaN films having a thickness of 0.4–0.5 μm have also been grown via the same technique on AlN buffer layers without post-processing annealing.

Gas-source molecular beam epitaxy of III–V nitrides

Abstract Amorphous, hexagonal and cubic phases of BN were grown via ion beam assisted deposition on Si(1 0 0) substrates. Gas-source molecular beam epitaxy of the III–V nitrides is reviewed. Sapphire(0 0 0 1) is the most commonly employed substrate with 6H-SiC(0 0 0 1), ZnO(1 1 1) and Si(1 1 1) also being used primarily for the growth of wurtzite GaN(0 0 0 1) in tandem with previously deposited GaN(0 0 0 1) or AlN(0 0 0 1) buffer layers. Silicon(0 0 1), GaAs(0 0 1), GaP(0 0 1) and 3C-SiC(0 0 1) have been employed for growth of cubic (zincblende) β-GaN(0 0 1). The precursor materials are evaporated metals and reactive N species produced either via ECR or RF plasma decomposition of N2 or from ammonia. However, point defect damage from the plasma-derived species has resulted in a steady increase in the number of investigators now using ammonia. The growth temperatures for wurtzite GaN have increased from 650 ± 50°C to 800 ± 50°C to enhance the surface mobility of the reactants and, in turn, the efficiency of decomposition of ammonia and the microstructure and the growth rate of the films. Doping has been achieved primarily with Si (donor) and Mg (acceptor); the latter has been activated without post-growth annealing. Simple heterostructures, a p-n junction LED and a modulation-doped field-effect transistor have been achieved using GSMBE-grown material.

Homoepitaxial SiC growth by molecular beam epitaxy

The homoepitaxial growth of SiC thin films by solid- and gas-source molecular beam epitaxy is reviewed and discussed. Our recent results regarding the homoepitaxial growth of single crystal 3C-SiC(111) and 6H-SiC(0001) thin films are also presented. The 3C-SiC(111) films were grown on both vicinal and on-axis 6H-SiC(0001) substrates at temperatures between 1000 and 1500 °C using SiH 4 and C 2 H 4 . They contained double positioning boundaries and stacking faults and the surface morphology and growth rate depended strongly on temperature. Films of 6H-SiC(0001) with low defect densities were deposited at high growth rates on vicinal 6H-SiC(0001) substrates by adding H2 to the reactant mixture at temperatures between 1350 and 1500°C. At temperatures below 1350°C, only the cubic phase was formed. A kinetic analysis of the SiC deposition process is also presented. The SiC films were resistive with an n-type character and a lower N concentration than the p-type CVD-grown epilayers of the substrate. Undoped 6H-SiC films with the lowest atomic nitrogen and electron concentration had a mobility of 434 cm 2 V -1 s -1 , the highest room temperature value ever reported for this polytype. Both the 6H-SiC(0001) and the 3C-SiC(111) epilayers were controllably doped using a NH 3 /H 2 mixture (for lighly n-doped films), pure N2 (for heavily n-doped SiC epilayers) and Al evaporated from a standard effusion cell (for p-type doping).

Aluminum nitride-silicon carbide solid solutions grown by plasma-assisted, gas-source molecular beam epitaxy

Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC) have been grown at 900{endash}1300thinsp{degree}C on vicinal {alpha}(6H)-SiC(0001) substrates by plasma-assisted, gas-source molecular beam epitaxy. Under specific processing conditions, films of (AlN){sub x}(SiC){sub 1{minus}x} with 0.2{le}x{le}0.8, as determined by Auger electron spectrometry (AES), were deposited. Reflection high-energy electron diffraction (RHEED) was used to determine the crystalline quality, surface character, and epilayer polytype. Analysis of the resulting surfaces was also performed by scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) revealed that monocrystalline films with x{ge}0.25 had the wurtzite (2H) crystal structure; however, films with x{lt}0.25 had the zincblende (3C) crystal structure. {copyright} {ital 1998 Materials Research Society.}