P.K. Larsen

Morphological and structural characteristics of homoepitaxial GaN grown by metalorganic chemical vapour deposition (MOCVD)

MOCVD-grown GaN on the N-polar surface of GaN substrates has been found to exhibit gross hexagonal pyramidal features (typically 10}50 lm in size depending on layer thickness). The evolution of the pyramidal defects is dominated by the growth rate of an emergent core of inversion domain (typically 100 nm in size). The inversion domains nucleate at a thin band of oxygen containing amorphous material (2}5 nm in thickness), being remnant contamination from the mechano-chemical polishing technique used to prepare the substrates prior to growth. Apart from pyramidal hillocks, the #at-topped hillocks are also formed. The arguments are presented on the association between these features and the core dislocations, which constitute the source of the growth steps. Improvement in the substrate polishing procedures allowed the e!ective elimination of these surface hillocks. ( 1999 Elsevier Science B.V. All rights reserved.

Influence of radius of curvature on the lateral etch rate of the weight induced epitaxial lift-off process

The ‘Weight Induced Epitaxial Lift-Off’ (WI-ELO) process is used to free single crystalline films from the GaAs substrates on which they have been deposited by etching a sacrificial AlAs release layer. The lateral etch rate Ve of this process is influenced by the weight induced radius of curvature R of the film. Bulk-etch experiments of AlxGa1−x As layers were conducted to compare an unhampered etching process with WI-ELO. It is found that standard WI-ELO etching demonstrates etch rates exceeding the bulk etch rate. Further experiments have shown that the WI-ELO etch rate is not constant in time, but consists of a slow initial etch rate Ve,i, followed by the faster nominal etch rate Ve,n. The latter part is influenced by the applied radius of curvature R via Ve,n=3.1+293R−1.2 with R in mm and Ve,n in mm h−1. This result implies an etch rate consisting of a constant plus a radius-induced part, resulting in both a qualitative and quantitative discrepancy with established theory. The explanations could be the different reaction kinetics and the occurrence of stress or strain in the film.

Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001) GaN substrates. In contrast to broad PL emission in exact (0001) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.

GaN growth on single-crystal diamond substrates by metalorganic chemical vapour deposition and hydride vapour deposition

Abstract In this study a thick hexagonal GaN layer has been grown on a (110) single crystalline diamond substrate utilising two different deposition techniques. Using an AlN nucleation layer, metal–organic chemical vapour deposition (MOCVD) has been used to deposit an initial GaN layer on a (110) single crystal diamond substrate. The layer consists of closely packed GaN grains with a thickness of approximately 2.5 μm and with different orientations with respect to the substrate. Low temperature photoluminescence indicates a poor optical quality of the layer due to poor structural properties and/or a high incorporation of impurities. This layer was used as a template in a hydride vapour phase epitaxy (HVPE) growth experiment. As a result of this, the GaN grain size has increased enormously and the layer consists of large, hexagonal shaped pillars with a diameter of approximately 50 μm and a height of more than 100 μm protruding from a polycrystalline background having a more uniform thickness. PL spectra of this film show a strongly increased intensity of the exciton related emissions when compared to the MOCVD deposited film. X-Ray diffraction analyses revealed that the dominant orientation of the GaN crystallites perpendicular to the substrate changed from [001] for the thin MOCVD film to [112] for the HVPE layer.

Strain-accelerated HF etching of AlAs for epitaxial lift-off

Epitaxial lift-off (ELO) is a process which allows for the separation of a single crystalline III/V thin film or device from the substrate it was deposited on. This process is based on the selective etching of an intermediate AlAs release layer in an aqueous HF solution. The lateral etch rate of the AlAs release layer through a narrow crevice in the weight-induced epitaxial lift-off (WI-ELO) process is much larger than observed for unobstructed planar AlAs layers. It is possible that this increase in etch rate is caused by the tensile strain induced upon the AlAs layer in the WI-ELO setup. In order to verify this assumption, planar AlAs layers, subjected to a controlled curvature, were etched in HF solutions and their etch duration was measured. The applied curvature reduced the already present compressive strain due to lattice mismatch. For large applied bending radii no change in etch rate was observed, because the induced bending is smaller than the already present bending due to the lattice mismatch. Further bending induces a total compressive strain from −0.126% to −0.11%, resulting in an etch rate variation from 0.054 up to 0.066 mm h−1. Measurements on AlAs layers experiencing a tensile strain of +0.286% showed much higher etch rates of 0.134 mm h−1. The present results obtained on etching experiments in the lateral plane are extrapolated to the perpendicular direction so that a combination with the data from previous work becomes feasible. This results in a better microscopic picture of the etch front in the WI-ELO process. It is found that the force exerted by the weight can be projected on an area, limited by the sample width and a depth of approximately 6 µm.