W. G. Perry

GaN thin films deposited via organometallic vapor phase epitaxy on α(6H)–SiC(0001) using high‐temperature monocrystalline AlN buffer layers

Monocrystalline GaN(0001) thin films, void of oriented domain structures and associated low‐angle grain boundaries, have been grown via organometallic vapor phase epitaxy (OMVPE) on high‐temperature monocrystalline AlN(0001) buffer layers predeposited on vicinal α(6H)–SiC(0001) wafers using TEG, TEA, and ammonia in a cold wall, vertical, pancake‐style reactor. The surface morphology was smooth, and the PL spectrum showed strong near‐band‐edge emission with a full width at half‐maximum (FWHM) value of 4 meV. The dislocation density within the first 0.5 μm was ≊1×109 cm−2; it decreased substantially with increasing film thickness. Controlled n‐type Si doping of GaN has been achieved for net carrier concentrations ranging from ∼1×1017 to 1×1020 cm−3. Double‐crystal XRC measurements indicated a FWHM value of 66 arcsec for the GaN(0004) reflection.

Raman analysis of the configurational disorder in AlxGa1−xN films

Raman analysis of the E2 mode of AlxGa1−xN in the composition range 0⩽x⩽1 is presented. The line shape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the line shape. The model calculations also indicate the lack of a long-range order in the chemical vapor deposition alloys. These results were confirmed by x-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x≅0.5 indicative of a random disordered alloy system.

Undoped and doped GaN thin films deposited on high-temperature monocrystalline AlN buffer layers on vicinal and on-axis α (6H)–SiC(0001) substrates via organometallic vapor phase epitaxy

Monocrystalline GaN(0001) thin films have been grown at 950 °C on high-temperature, ≈ 100 nm thick, monocrystalline AlN(0001) buffer layers predeposited at 1100 °C on α (6H)−SiC(0001) Si substrates via OMVPE in a cold-wall, vertical, pancake-style reactor. These films were free of low-angle grain boundaries and the associated oriented domain microstructure. The PL spectra of the GaN films deposited on both vicinal and on-axis substrates revealed strong bound excitonic emission with a FWHM value of 4 meV. The near band-edge emission from films on the vicinal substrates was shifted slightly to a lower energy, indicative of films containing residual tensile stresses. A peak attributed to free excitonic emission was also clearly observed in the on-axis spectrum. Undoped films were too resistive for accurate Hall-effect measurements. Controlled n -type, Si-doping in GaN was achieved for net carrier concentrations ranging from approximately 1 × 10 17 cm −3 to 1 × 10 20 cm −3 . Mg-doped, p -type GaN was achieved with n A −n D ≈ 3 × 10 17 cm −3 , ρ ≈ 7 Ω · cm, and μ ≈ 3 cm 2 /V · s. Double-crystal x-ray rocking curve measurements for simultaneously deposited 1.4 μ m GaN films revealed FWHM values of 58 and 151 arcsec for deposition on on-axis and off-axis 6H−SiC(0001) Si substrates, respectively. The corresponding FWHM values for the AlN buffer layers were approximately 200 and 400 arcsec, respectively.

Optical studies of GaN and GaN/AlGaN heterostructures on SiC substrates

We present the results of spectroscopic studies on GaN based epitaxial materials on SiC substrates by metalorganic chemical vapor deposition. A variety of techniques has been used to study the optical properties of GaN epilayers and GaN/AlGaN heterostructures. Sharp spectral structures associated with the intrinsic free excitons were observed by photoluminescence and reflectance measurements from GaN based materials grown on SiC substrates. The residual strain was found to have a strong influence in determining the energies of exciton transitions. Picosecond relaxation studies of exciton decay dynamics suggest that an AlGaN cladding layer with a small mole fraction of AlN can be relatively effective in enhancing the radiative recombination rate for excitons by reducing the density of dislocations and suppressing surface recombination velocity in the GaN active layer for the GaN/AlGaN heterostructure samples.

Intrinsic exciton transitions in GaN

Intrinsic excitonic transitions in GaN have been studied using a variety of spectroscopic measurements. Sharp spectral structures associated with intrinsic free excitons could be observed in photoluminescence, reflection, and absorption spectra. The energy positions of excitonic transitions in GaN epitaxial layers were found to be influenced by the residual strain resulting from lattice-parameter and thermal-expansion mismatches between the epilayers and the substrates. The values of the four principal deformation potentials of wurtzite GaN were derived by using the strain tensor components determined by x-ray measurements. The observation of spectral features involving the emission of LO phonons in absorption and photoluminescence excitation spectra at energies above exciton resonances indicate that a phonon-assisted indirect excitation process, which simultaneously generates a free exciton and a LO phonon, is a very significant and efficient process in GaN. The lifetime of the free excitons is found to ...

Cathodoluminescence studies of the deep level emission bands of AlxGa1−xN films deposited on 6H–SiC(0001)

A comprehensive study of the sub band-edge emission bands of AlxGa1−xN (0.06⩽x⩽1) thin films deposited on vicinal and on-axis 6H–SiC(0001) substrates is presented. At 4.2 K strong band-edge emission, ascribed to donor-bound excitons, shallow donor-shallow-acceptor pair emission, and a deep emission band associated with the “yellow” band of GaN, were observed via cathodoluminescence. The energy shift of the shallow donor-shallow-acceptor pair band with respect to the peak of the donor-bound excitons peak exhibited a less than one-to-one correspondence with increasing Al mole fraction due the increasing localization of either the shallow donor and/or shallow acceptor. The yellow band was observed for all compositions and exhibited a similar energy shift with respect to both the donor-bound excitons and the shallow donor-shallow-acceptor pair bands as the Al mole fraction increased, except for a brief decrease at x≈0.5. This decrease was attributed to a donor oxygen level which entered the band gap at approx...

Biaxial strain in AlxGa1-xN/GaN layers deposited on 6H-SiC

Abstract Al x Ga 1− x N/GaN multilayers ( x ≤0.12) deposited on AlN(0001) buffer layer/6H-SiC(0001) substrate combinations have been investigated using high resolution X-ray diffraction. Rocking curves ( ω ) showed that the Al x Ga 1− x N layers in the main contained a reduced dislocation density relative to the underlying GaN layers. The line widths of the radial scans (2 θ − ω ) of the Al x Ga 1− x N layers increased as the Al mole fraction increased due to alloy broadening. The in-plane lattice constant ( a ) of the Al x Ga 1− x N layer for each sample was equivalent to that of the GaN layer, indicating the layers were coherently strained. The strain in the AlGaN layer was tensile for each sample. This was determined using high-resolution reciprocal space maps of the (015) and (024) planes which revealed that the Al x Ga 1− x N and GaN lattice points had the same value of the in-plane components of the reciprocal lattice vector ( S ‖ ). For higher Al mole fractions, the samples were severely cracked, indicating the strain in the Al x Ga 1− x N layers exceeded critical values.

Stimulated emission in GaN thin films in the temperature range of 300-700 K

We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the uniq...

Growth of AlN, GaN and AlxGa1 − xN thin films on vicinal and on-axis 6HSiC(0001) substrates

Abstract Monocrystalline GaN(0001) thin films were grown at 950 °Con AlN(0001) buffer layers previously deposited at 1100 °C on α(6H)-SiC(0001) si substrates via metallorganic chemical vapor deposition (MOCVD). Films of Al x Ga 1 − x N (0 ≤ x ≤ 1) were grown directly on the same SiC surface at 1100 °C. X-ray rocking curves for the GaN(0004) reflection for 1.4 μm films revealed FWHM values of 58 and 151 arc sec for materials grown on on-axis and offaxis substrates, respectively. Cathodoluminescence exhibited strong near band-edge emission for all materials. Controlled n- type Si-doping in GaN and Al x Ga 1 − x N (for x ≤ 0.4) was achieved with net carrier concentrations ranging from approximately 2 × 10 17 cm − 3 to 2 × 10 19 (Al x Ga 1 − x N) or to 1 × 10 20 (GaN) cm − 3 . Mg-doped, p-type GaN and Al x Ga 1 − x N (for x ≤ 0.13) was achieved with n A − n D ≈ 3 × 10 17 cm − 3 .

Growth, doping and characterization of AlxGa1 − xN thin film alloys on 6H-SiC(0001) substrates

Abstract Thin films of Al x Ga 1 − x N (0.05≤ x ≤0.96) having smooth surfaces were deposited directly on both vicinal and on-axis 6H-SiC(0001) substrates. Cross-sectional transmission electron microscopy of Al 0.13 Ga 0.87 N revealed stacking faults near the SiC/nitride alloy interface and numerous threading dislocations. Energy dispersive analysis, Auger electron spectroscopy (AES) and Rutherford backscattering were used to determine the compositions. These were paired with their respective cathodoluminescence (CL) near band-edge emission energies. A negative bowing parameter was determined. The CL emission energies were similar to the bandgap energies obtained by spectroscopic ellipsometry. Field emission AES of the initial growth of Al 0.2 Ga 0.8 N revealed an Al-rich layer near the interface. N-type (Si) doping was achieved for Al x Ga 1 − x N for 0.12≤ x ≤0.42.