Professor Robert Taylor

Abstract:

We have studied the microscopic surface morphology of AlN which was revealed by using an atomic force microscopy (AFM). AlN was sputtered on Si (111) and Si (100) substrates for 30 and 60 minutes. AlN was observed to crystallize as cubic-AlN at an annealing temperature of 900 °C and an annealing time of 60 minutes. We present a model to explain this cubic AlN bonding configuration. GaN was grown on AlN/Si(111) and AlN/Si(100) substrates by using a hydride vapor phase epitaxy technique. A terrace with saw-tooth-shaped formations was observed on the GaN surface and could be explained in terms of the lattice mismatch and the gas diffusion rate.

Abstract:

The dynamics of carriers in GaN epilayers is investigated using femtosecond pump-probe spectroscopy. After the residual chirp on the continuum probe is removed, the normalized difference spectra (NDS) for different probe energies are synchronized, recovering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, which shows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electron-LO phonon interaction. Employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, a measure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons. © 2005 Elsevier B.V. All rights reserved.

Abstract:

Time-resolved photoluminescence measurements are performed in a In 0.02Ga 0.98N/In 0.16Ga 0.84N multiple-quantum well structure in order to investigate the spontaneous emission mechanism. The radiative and the non-radiative recombination times are deduced from the photoluminescence decay times using the temperature-dependent internal quantum efficiency measured from the temperaturedependent normalized photoluminescence intensity. We found that the radiative recombination time has a linear temperature-dependence for T ≤ 100 K, consistent with the two-dimensional nature of excitons. This suggests that at low temperatures 2D excitons are weakly localized at the potential minima in the quantum well.

Abstract:

We present an investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved microphotoluminescence measurements, supported by three-dimensional multiband k·p computational modeling. We observe that with increasing optical excitation the carrier lifetime decreases and emission energy blueshifts. This originates from the screening of built-in piezo- and pyroelectric fields in the quantum discs by photogenerated free carriers. Due to nonresonant tunneling of carriers from the smaller disk to the larger disk, free-carrier screening is enhanced in the larger disk. Computational modeling was in good agreement with the experimental results. ©2006 American Institute of Physics.

Abstract:

We have studied the effect of annealing AlN buffer layers on the properties of subsequently grown GaN layers. The AlN buffer layer was deposited on a Si(111) substrate by using RF sputtering, and different samples were then annealed at temperatures of 700°C, 800°C, and 900°C. Thick GaN was grown using a hydride vapor phase epitaxy (HVPE) system for 1 hour at 1050°C with the resultant thickness being 150 μm. The morphologies of the AlN and the GaN layers were observed by using both atomic force microscopy (AFM) and scanning electron microscopy (SEM). The surface roughness and grain size of the AlN buffer layer was increased by raising the annealing temperature; this was accompanied by an improvement in the two-dimensional lateral growth of the GaN layer. X-Ray diffraction (XRD) patterns showed the typical results expected for GaN (0002) and (0004) faces, revealing a highly preferred orientation of the GaN(0001) surface. However, a residual compressive stress was observed between the GaN and the Si substrate, independent of the annealing treatment of the buffer layer.