Plamen Paskov

Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar a -plane GaN layers grown on r -plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resol ...

Photoluminescence up-conversion in InAs/GaAs self-assembled quantum dots

We report up-converted photoluminescence in a structure with InAs quantum dots embedded in GaAs. An efficient emission from the GaAs barrier is observed with resonant excitation of both the dots and the wetting layer. The intensity of the up-converted luminescence is found to increase superlinearly with the excitation density. The results suggest that the observed effect is due to a two-step two-photon absorption process involving quantum dot states.

Stress and wafer bending of a-plane GaN layers on r-plane sapphire substrates

The stress and wafer bending of (11 2- 0) a -plane GaN layers of different thicknesses grown on (1 1- 02) r -plane sapphire substrates by hydride vapor phase epitaxy were studied by high-resolution x-ray diffraction and photoluminescence and photoreflectance spectroscopies. The layers are found to be under compression in the growth plane and under tension in the growth direction. The elastic and thermal anisotropies of the GaN and the sapphire crystal result in an in-plane stress and a wafer curvature, both of which are different in the two in-plane directions parallel and perpendicular to the GaN c axis. The GaN unit cell is no longer hexagonal but orthorhombic. The stress relaxes with increasing GaN layer thickness while the curvature of the wafer increases. Different stress relief mechanisms are considered, and the stresses in the layer and the curvature of the wafer are calculated using standard models on wafer bending. The results suggest that the wafer bending is the dominant stress relief mechanism. In addition, the redshift of the near-band-edge photoluminescence and the free exciton photoreflectance peaks with increasing layer thickness is correlated with the strain data determined by x-ray diffraction.

Lattice parameters of GaN layers grown on a-plane sapphire: Effect of in-plane strain anisotropy

We have studied GaN films grown on a-plane sapphire by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. The in-plane lattice parameter was determined from sets of equivalent interplanar distances measured for six different directions in order to examine the effect of strain anisotropy. It is found that, in both types of films, the obtained six values of the in-plane lattice parameter can be grouped around two values. The strain anisotropy is estimated to have different value in the films grown by the two techniques and possible explanations are suggested.

Deformation potentials of the E1 (TO) and E2 modes of InN

The deformation potentials of the E1(TO) and E2 modes of InN are determined by combining infrared spectroscopic ellipsometry, Raman scattering, and x-ray diffraction measurements, and using a reported value of the mode Gruneisen parameter. The deformation potentials are obtained for two sets of stiffness constants. Strain-free values of the InN E1(TO) mode of 477.9 cm−1 and of the E2 mode of 491.1 cm−1 have been determined.

Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer

The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.

Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates

We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase ep ...

Deformation potentials of the E1(TO) mode in AlN

The deformation potentials of the E-1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of th ...

Optical characterization of III-nitrides

Abstract Recent developments in material properties of GaN and related heterostructure combinations are reviewed, with emphasis on optical data. We discuss recent polarized photoluminescence (PL) data on the free excitons in GaN, obtained from thick HVPE grown layers. The exchange splitting constant is found to be about 0.6 meV, a more accurate value than previous suggestions. The PL signatures of shallow donors and acceptors, i.e. the bound excitons, are discussed and tentatively identified. Intrinsic point defects are discussed in terms of stability and experimental signatures. Quantum well structures in the InGaN/GaN and GaN/AlGaN systems are briefly discussed, with emphasis on localization of carriers and excitons.

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.