J. Aderhold

Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap

(a) Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia (b) Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany (c) Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan (d) Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus (e) LfI, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany

InN thin films grown by metalorganic molecular beam epitaxy on sapphire substrates

In this priority communication it will be shown that InN thin films can be successfully grown using the metalorganic molecular beam epitaxy (MOMBE) method. For the first time the proper choice of growth conditions allows to obtain good quality InN thin films with a charge carrier concentration as low as 8.8×1018 cm−3.

Point defects in gamma-irradiated n-GaN

Radiation-induced point defects and their annealing in silicon-doped n-GaN have been investigated by means of Hall effect measurements and Raman spectroscopy. Correlated compensation effects due to simultaneous introduction of donor and acceptor centres are observed in irradiated n-GaN. The defect production rate is dependent on the dopant concentration. This means that the model of all native defects immobile at room temperature is not true. The behaviour of radiation-induced defects upon heating is complicated, exhibiting two prominent stages of reverse annealing. The presence of radiation defects is still observable after annealing to T = 750 °C.

Strain, morphological, and growth-mode changes in AlGaN single layers at high AlN mole fraction

We report on morphological and residual-strain characteristics of high-AlN-mole-fraction N-polar AlxGa1−xN epilayers on sapphire. Nominally relaxed, thick single-alloy layers in the compositional range 0.4<x<1 were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and characterized structurally and optically. High-resolution X-ray diffraction using an extension of the Bond method was employed to examine residual strains while film morphologies were examined directly with the use of atomic force microscopy. Under nominally identical PA-MBE growth conditions apart from Al flux, a continuous change in growth characteristics as a function of Al flux (alloy composition) is observed. For one particular value of Al flux (corresponding to x≅0.65), a maximum in growth rate and minimum in surface roughness are found whereas at a somewhat greater flux value (corresponding to x=0.86) a minimum in stress (lateral and vertical) is obtained. The observed growth-mode phenomenology suggests an approach for improving PA-MBE growth of high-AlN-mole-fraction layers of certain AlGaN/GaN structures such as the distributed Bragg reflector. Finally, optical transmission experiments lead to a bandgap bowing parameter in the large-x region of AlxGa1−xN of b≅0.75.

Smooth GaN surfaces by photoinduced electro-chemical etching

We have etched n-GaN grown both by plasma assisted MBE and MOCVD in 0.5 M KOH under HeCd-laser illumination (110 mW cm -2 ) and controlled the photocurrent by an external voltage source. The reproducible etch depths were linear with respect to the charge. The examination of the dissolution potential revealed strong dependence on illumination and current. Thereby, regimes to obtain smoothly etched surfaces were found at low dissolution potentials. Demonstration of the possibilities in photoelectrochemical (PEC) etching are given.

Current Controlled Photoelectrochemical Etching of Gan Leaving Smooth Surfaces

We have etched GaN grown by plasma source MBE in aqueous solutions of KOH in an electrochemical cell under HeCd laser illumination and additional current control.The etch rate was dramatically enhanced up to 8 μm/h by an applied current density of 6.4 mAcm -2 . Photocurrent control leads to etched GaN surfaces exhibiting mirror-like appearance with uniform interference color. According to mechanical profilometry, they have a roughness of less than 3.5 nm after etching of several hundred nanometers, which is comparable to the roughness prior to etching. This etching process allows in situ control via photocurrent and induced yellow luminescence.

Band Gap of Hexagonal InN and InGaN Alloys

We present results of photoluminescence studies of the band gap of non-intentionally doped single-crystalline hexagona InN layers and In-rich InxGa1-xN alloy layers (0.36 < x < 1). The band gap of InN is found to be close to 0.7 eV. This is much smaller than the values of 1.8 eV to 2.1 eV cited in the current literature. A bowing parameter of b ≈ 2.5 eV allows one to reconcile our and the literature data for the band gap values of InxGa1-xN alloys in the entire composition region.

Raman spectroscopy of disorder effects in AlxGa1−xN solid solutions

Abstract The results of Raman spectroscopic studies of the disorder effects in hexagonal Al x Ga 1− x N epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E 2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A 1 (TO) polar phonon mode for x or (1− x )≪1 and the large broadening for x ≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of Al x Ga 1− x N epitaxial layers and its inhomogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line frequencies.

HRTEM study of AlxGa1−xN/AlN DBR mirrors

Abstract In the present work semiconductor quarter wavelength distributed Bragg reflector (DBR) mirrors have been studied by high resolution transmission electron microscopy (HRTEM). The mirrors have been fabricated monolithically by plasma assisted molecular beam epitaxy (MBE) on sapphire (0001) substrates. The samples are conformed of a large number of Al x Ga 1− x N/AlN layers with 5.5 and 20.5 periods, both with different aluminium concentration. The samples have been designed utilising spectroscopic ellipsometry (SE) dispersion spectra of previously fabricated single layers. The aim of this work was to determine the distortion of lattice parameters of Al x Ga 1− x N/AlN epilayers, since this is important for the later production of vertical cavity surface emitting lasers (VCSELs). Distortions of half periods layers were determined from HRTEM techniques and are compared with the distortion determination using an equilibrium theory and high resolution X-ray diffraction (XRD) measurements.

Structural characterization of high temperature AlN intermediate layer in GaN grown by molecular beam epitaxy

Transmission electron microscopy has been used to study the structural quality of GaN grown on sapphire by plasma assisted molecular beam epitaxy using high temperature AlN intermediate layers with different thicknesses. The introduction of an AlN intermediate layer with an optimum thickness is observed to minimize the density of dislocations reaching the overgrown GaN surface. In this sample, the measured threading dislocation density reaching the surface of 1 x 10 10 cm -2 resulted to be seven times lower than that of a reference sample, without any AlN interlayer. The bending at the GaN/AlN interface and following interactions between dislocations have been observed in cross-sectional transmission electron micrographs. This fact explains the decrease of dislocation density reaching the GaN surface.