F. Hitzel

Composition dependence of polarization fields in GaInN/GaN quantum wells

We report an experimental determination of the internal polarization field in GaInN/GaN quantum wells, due to piezoelectric and spontaneous polarization, utilizing the quantum confined Stark effect, with fields as large as 3.1 MV/cm at 22% In. From its dependence on quantum well composition and strain, we find that the total field in GaInN is a linear combination of polarization charges from GaN and InN. The piezoelectric constants d31 for GaN and InN derived from our data are 1.05±0.05 pm/V and 3.7±0.5 pm/V, in fair agreement with theoretical data.

Anti-localization suppresses non-radiative recombination in GaInN/GaN quantum wells

The light emission efficiency of (AlGaIn)N heterostructures and light-emitting diodes is exceptionally high, despite the high density of threading dislocations generally found in such structures. It has become common to attribute the high efficiency to compositional fluctuations or even phase separation in the active GaInN quantum well region. The resulting localization of charge carriers is thought to keep them from recombining non-radiatively at the defects. Here, we show that carriers are mobile at room temperature rather than localized and that under suitable growth conditions hexagonal V-shaped pits decorating the defects exhibit narrow sidewall quantum wells with an effective bandgap significantly larger than that of the regular c-plane quantum wells. Thereby nature provides a unique, hitherto unrecognized mechanism generating a potential landscape which effectively screens the defects themselves by providing an energy barrier around every defect. Thus even for mobile charge carriers non-radiative recombination is effectively suppressed leading to the unexpectedly high emission efficiency.

Influence of low-temperature interlayers on strain and defect density of epitaxial GaN layers

The effect of GaN and InxGa1xN interlayers on the strain state and defect density of GaN buffer layers is studied. The layers are grown by low pressure MOVCD on c-plane sapphire substrates and were investigated by in situ reflectometry, AFM, photoluminescence, high-resolution X-ray diffraction. In addition, etch pit densities are determined by wet chemical etching in hot phosphoric acid. We find that InxGa1xN interlayers slightly decrease the defect density and reduce the strain in the GaN buffer layers. r 2002 Elsevier Science B.V. All rights reserved.

Correlation of Defects and Local Bandgap Variations in GaInN/GaN/AlGaN LEDs

Currently even the best available GaN device structures have an extremely high defect density. Their surprisingly small influence on the carrier recombination rates and brightness of LEDs is still not really understood. We use a scanning nearfield optical microscope (SNOM) to obtain local photoluminescence and electroluminescence spectra of GaN/GaInN/AlGaN LED structures with a lateral resolution better than 200 nm. The optical information is compared with defect positions made visible for atomic force microscopy by wet chemical etching. Peak wavelength maps obtained from the SNOM data show spatial regions of different emission wavelength with typical dimensions of 200-500 nm, associated with the defect structure. We observed no evidence for small scale fluctuations of emission wavelength as expected in case of phase separation of InGaN.

High-resolution near-field spectroscopy investigation of GaN laterally overgrown structures on SiC

We investigated epitaxial lateral overgrown GaN structures with different wing tilt using a spectroscopic scanning near-field optical microscope (spectroscopic SNOM), which takes a complete optical spectrum at each point of a sample surface. From these measurements, we obtain information about strain at different points of the surface, and comparing emission intensity between regions of lateral growth and vertical growth, we directly see the efficiency of defect density reduction. For the high wing-tilt sample, an increased defect density at the window–wing interface could be identified.

Revealing the Defect Structure in Laterally Overgrown GaN Stripes Utilizing Photoelectrochemical Etching Techniques

Photoelectrochemical wet etching (PEC) in KOH solutions and subsequent scanning electron microscopy (SEM) was applied to investigate the defect structure in epitaxially lateral overgrown (ELOG) GaN. We have developed a cross-sectional PEC technique to easily reveal threading dislocations (TDs) at a spatial resolution of about 20 nm. The rich defect structure in the laterally overgrown regions will be discussed.