N. V. Edwards

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.

Variation of GaN valence bands with biaxial stress and quantification of residual stress

Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.

Trends in residual stress for GaN/AlN/6H–SiC heterostructures

We discuss trends in residual stress as a function of film thickness, growth temperature, and substrate orientation for GaN/AlN/6H–SiC heterostructures. Films are mostly compressive for samples less than about 0.7 μm thick, are tensile up to about 2 μm, then abruptly become less tensile with stress values near 1 kbar thereafter. We interpret this as a successive relief of lattice mismatch and thermal stresses culminating in a catastrophic relief by unknown mechanisms at moderate thicknesses. These data indicate that relaxation processes in these heterostructures are not as well understood as previously supposed.

Optical properties of 4H-SiC

The optical band gap energy and the dielectric functions of n-type 4H-SiC have been investigated experimentally by transmission spectroscopy and spectroscopic ellipsometry and theoretically by an a ...

Ordinary and extraordinary dielectric functions of 4H– and 6H–SiC from 3.5 to 9.0 eV

We report ordinary (??c axis) and extraordinary (??c axis) dielectric function data of 4H- and 6H-SiC from 3.5 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are also compared to recently reported ab initio calculations. Critical point energies were found using real and reciprocal space analysis.

Analysis of optical spectra by Fourier methods

Abstract Results of a systematic investigation of reciprocal-space analysis, applied to optical spectroscopy, are reported. Filtering procedures for removing baseline and noise artifacts are more effective in reciprocal- than in real-space. Also, correlations among parameters are reduced and the functional dependence of real-space lineshapes need be known a priori only in very general terms. We apply reciprocal-space analysis to achieve accurate values of critical point energies for the E 1 and E 1 +Δ 1 transitions of GaAs from ellipsometric spectra and to locate critical point energies in low-temperature reflectance data of GaN. We show that data may easily be corrected for systematic artifacts such as the monochromator slit width, an optimum slit width can be defined, and the degree of improvement needed to achieve a particular level of performance can be predicted.

Time-resolved spectroscopy of strained GaN/AlN/6H–SiC heterostructures grown by metalorganic chemical vapor deposition

Temperature-dependent time-resolved photoluminescence measurements were performed on GaN film/AlN buffer/6H–SiC substrate heterostructures grown by metalorganic chemical vapor deposition. The overlying GaN layers were under tension, as estimated from the free A exciton (FEA) position. The recombination lifetimes were determined for the FEA and for the neutral-donor-bound exciton (D0X). We observed that the recombination lifetime for the FEA has the same value of 40–50 ps in all the layers, whereas the recombination time for the D0X varies for different samples. We observed that the recombination lifetimes for D0X have a clear dependence on the position of FEA, i.e., the recombination lifetime increases with decreasing strain in the layers. We discuss the results in term of the hole states involved in the donor-bound exciton recombination.

Three‐terminal bias induced dual wavelength semiconductor light emitter

A three‐terminal version of a light emitting device, a bias induced color‐tunable emitter (BICE), is reported. The three‐terminal BICE is a multiwavelength semiconductor light emitter in which the emission wavelength is determined by an applied voltage, while the emission intensity is modulated by an applied current (no optical pumping employed). Device operation is based on the bias dependent injection and collection of electrons and holes in quantum wells. In this letter, three‐terminal dual wavelength BICE functionality is demonstrated. The device exhibits coaxial optical output, a large separation of emitted wavelengths (≊570 A), low operational voltages and currents, and high emission contrast ratios of 159:1 and 1:76 at 77 K.

Spectroscopic ellipsometry and low-temperature reflectance : complementary analysis of GaN thin films

Abstract We report spectroscopic ellipsometry (SE) and low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8 to 3.5 kbar) thus far. SE allows us to assess the preparation of smooth and abrupt GaN surfaces by chemical treatments in real time, and, coupled with the reflectance data, the E d n /d E contribution to dispersion, which is important for laser action. The reflectance data explicitly show the non-linear behavior of the B-A and C-A splittings vs. the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal space analysis, allowing us to obtain band parameters such as Δ SO =17.0±1 meV and Δ CF =9.8±1 meV with increased confidence.

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.