A. Sohmer

Absorption of InGaN Single Quantum Wells Determined by Photothermal Deflection Spectroscopy

Photothermal deflection spectroscopy (PDS) is used to study the absorption of GaN/InGaN/GaN double heterostructures in the energy range from 0.6 to 3.8 eV. The heterostructures containing 1–77-nm-thick InGaN single quantum wells were deposited by chemical vapour deposition from organometallic precursors. They are measured to investigate the absorption coefficient, bandgap, indium concentration and fluctuation of the quantum wells. A bandgap increase of hexagonal InxGa1-xN (x≈0.14) of 60 meV is observed with decreasing well thicknesses from 15 to 1 nm. The distribution of In-concentration of the InxGa1-xN layers was estimated from the slope of the absorption coefficient versus photon energy for energies below the bandgap and found to be Gaussian with a full width of half maximum of Δx=0.03.

Diffusion length of photoexcited carriers in GaN

When additional carriers are introduced in a material with a non uniform concentration, they tend to diffuse on a scale given by their diffusion length. This parameter can be measured by different methods. Depending on the conditions, different values can be found as the recombination mechanisms differ. In this paper, we present the situation in GaN with various experiments including the photocarrier grating method, photoluminescence and the spectral response in photoconductors. We show that the diffusion length varies from 0.1 μm to a few μm depending on experimental conditions. The interpretation is given based on the diffusion equations and on the analysis of the recombinations.

In incorporation efficiency and composition fluctuations in MOVPE grown GaInN/GaN hetero structures and quantum wells

Abstract GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. In order to increase the In incorporation efficiency, lower growth temperatures of around 700–800°C are needed. We have optimized the metalorganic vapor-phase epitaxial growth of GaInN by decreasing the H2/N2 ratio in the gas-phase and increasing the growth rate. However, the deposited films showed strong indications for compositional fluctuations. Besides a large miscibility gap predicted for GaInN, the mismatch induced strain for GaN may play a major role in these growth problems.

GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H 2 -partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.

MOVPE of GaInN heterostructures and quantum wells

Abstract GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. This requires a detailed understanding of the growth procedure and the material properties. We have grown GaInN heterostructures and quantum wells by low-pressure metalorganic vapour-phase epitaxy. They have been further analysed by X-ray diffraction, optical spectroscopy and atomic force microscopy. Our results indicate a strong difference in GaInN quality depending on the composition and thickness of the grown layers and the growth temperature. Possible reasons for these problems are shortly discussed. Besides thermodynamic limitations predicted for GaInN, the mismatch induced strain to GaN may play a major role for these growth problems.

Metalorganic vapor phase epitaxial growth of GaInN/GaN hetero structures and quantum wells

GaInN/GaN heterostructures and quantum wells have been grown by low pressure metalorganic vapor phase epitaxy on sapphire using an AIN nucleation layer. We found a significant In incorporation only for growth temperatures of 700°C, although still very high In/Ga ratios in the gas phase had to be adjusted. The In content could be increased by reducing the H2/N2 flow ratio in the main carrier gas. GaInN layers typically show two lines in low temperature photoluminescence which are identified as excitonic-like (high energy peak) and impurity-related-like (low energy) by time-resolved spectroscopy. Quantum wells with a thickness between 8 and 0.5 nm showed only one emission line. The peak of the thinnest wells shows excitonic-like behaviour, whereas we found a smooth transition to an impurity-related-like type with increasing thickness. By scanning transmission electron microscopy studies we found indications for composition fluctuations in these thicker quantum wells which may cause localization effects for the excitons and thus be responsible for the observed optical spectra.

Evidence for quantum-dot-like states in GaInN/GaN quantum wells?

Abstract Localization of excitons in potential wells due to fluctuations in the local In content in GaInN/GaN quantum wells is now widely believed to lead to quantum-dot-like states. This is even thought to be of major importance for the operation of GaInN-based laser diodes. Using photoluminescence excitation spectroscopy we show that there is indeed a strong localization of excitons in such quantum wells at low temperature. On the other hand, a comparison of room temperature absorption spectra with optical gain spectra indicates just a “normal” exciton-to-plasma transition and no relevance of localization for laser operation.

Optical pumping in nitride cavities with etched mirror facets

We present the fabrication of mirror facets by chemically assisted ion beam etching. Optical pumping experiments are performed to characterize the facets. The reflection coefficient is measured and its value confirms the high quality of the etched mirrors. Optical resonators are then realized in a double heterostructure composed of InGaN and GaN. Stimulated and laser emissions are investigated in this heterostructure and we show that population inversion can occur in both materials.

Cathodoluminescence study of crystalline quality of (Al, In, Ga)N heterostructures

Abstract Wurtzite InGaN/GaN and AlGaN/GaN heterostructures grown by metal organic vapor phase epitaxy were studied using cathodoluminescence (CL) combined with secondary electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The surface morphology of samples containing InGaN layers is dominated by three types of defects: mesa-like hexagonal structures, hexagonal pyramids and micropipes. At the positions of pyramids the whole epilayer is thicker than at defect free positions, while at the positions of micropipes the whole epilayer is much thinner. The luminescence efficiency as well as the emission wavelength are influenced by these defects. In SL structures an increasing SL period thickness in the growth direction was observed. Panchromatic CL images show intensity inhomogeneity in both InGaN/GaN and AlGaN/GaN heterostructure, which are related to local variations of the interface quality. In AlGaN/GaN SQW structures a broad deep-level luminescence band at around 543 nm was observed, which is generally absent in InGaN/GaN heterostructures. This deep-level emission is strongly enhanced in defect positions.

Investigations of selectively grown GaN/InGaN epitaxial layers

Abstract We have studied GaN/InGaN heterostructures grown by selective area low pressure metalorganic vapor phase epitaxy (LP-MOVPE). A GaN layer already grown on the c-face of sapphire has been used as substrate, partly masked by SiO 2 . In a second epitaxial step a GaN/InGaN single heterostructure and GaN/InGaN/GaN double heterostructures were grown on the unmasked rectangular fields. We obtained good selectivity for GaN and for InGaN. A larger growth rate as compared to planar epitaxy and strong growth enhancement at the edges was observed. Spatially resolved measurements of the luminescence show an increase in indium incorporation of about 80% at the edges. Besides the larger indium offering at the edges, this is due to an enhanced growth rate. Very smooth facets are obtained. The influence of pressure on the surface morphology and growth enhancement was investigated.