G. Steude

Optical investigations of AlGaN on GaN epitaxial films

We investigated coherently strained AlxGa1−xN/GaN heterostructures (0<x<0.22) grown by metalorganic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodoluminescence experiments. The energetic positions of the free A exciton as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localization. The broadening of the luminescence linewidth in the alloys can be described by statistical disorder of a random alloy.

Photoluminescence Investigations of AlGaN on GaN Epitaxial Films

AlGaN on GaN epitaxial films with Al contents between 6% and 76% were investigated by stationary photoluminescence experiments which allows to determine the dependence of the energy gap on alloy composition. The observed increase of the luminescence linewidth as a function of the Al molar fraction can be explained by alloy broadening. The localization energy of the bound exciton increases considerably and reaches a value of 53 meV for 61% Al. It could imply that the donor binding energy would markedly deviate from its effective mass value, an unexpected result if the residual donor is Si.

Electrical and optical properties of p-SiC/n-GaN heterostructures

Abstract We report on the optical, electrical and structural properties of GaN films heteroepitaxially grown by low pressure chemical vapor deposition on 6H-SiC substrates. We employed photoluminescence (PL), Hall effect measurements, scanning tunneling microscopy (STM) and X-ray analysis to determine the quality of our films. Heterojunction diodes were fabricated on p-type SiC and characterized by temperature dependent current–voltage and capacitance–voltage techniques. The results are interpreted within the thermionic emission model and the barrier found is attributed to the conduction band offset between 6H-SiC and wurtzite GaN. The diodes show electroluminescence of the donor-acceptor pair recombination type of 6H-SiC at room temperature. By analysis of the injection behavior we can interpret our data, determining the high valence band offset between 6H-SiC and α -GaN to 0.67 eV. This high valence band offset favors applications for hetero-bipolar transistors (HBT).

Low-pressure chemical vapor deposition of GaN epitaxial films

GaN films were grown epitaxially on sapphire and 6H-SiC substrates by low-pressure chemical vapor deposition (LPCVD) using gallium(III) chloride and ammonia as gallium and nitrogen precursor, respectively. The properties of these samples were examined by X-ray diffraction, Hall effect measurements, secondary-ion mass spectroscopy (SIMS) and photoluminescence (PL). All GaN layers exhibit high free carrier concentrations between n = 2 x 10 18 and 5 x 10 19 cm - 3 caused by unintentional n-type doping. We provide evidence that this high unintentionally doping is linked to the oxygen content in the films. The correlation between the optical properties with respect to position and line shape of the band-edge luminescence and the electrical properties. i.e. the free carrier concentration, is discussed.

Optical Investigations of AlGaN on GaN Epitaxial Films

We have investigated Al x Ga l-x N/GaN heterostructures (0 x Ga l-x N films induce additional compressive strain on the underlying GaN film. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localisation. The broadening of the luminescence line width in the alloy can be described by statistical disorder of a random alloy.

Transport Investigations on High Purity MOVPE Grown GaAs

We studied high purity GaAs grown by optically detected cyclotron resonance (ODCR) using microwave frequencies at 36 and 140 GHz. The samples were grown by the metalorganic vapor phase epitaxy (MOVPE) using nitrogen as a carrier gas. The ODCR linewidth which is a measure of the mobility of the sample is dominated by neutral impurity scattering at low temperatures (<10 K) and acoustic deformation potential scattering at higher temperatures (10 to 40 K). At 2 K we obtain a mobility of about 3 x 10 5 cm 2 /Vs, one of the best values for MOVPE grown GaAs. Upon reduction of the photoexcitation power, i.e. reducing the photo-neutralisation of impurities, ionized impurity scattering gives an additional contribution. At high microwave powers the ODCR properties change remarkably, and hot electron relaxation involving longitudinal optical phonon processes is observed.

Growth of GaN and AlGaN by high temperature vapor phase epitaxy

We investigated the influence of the growth temperature on high temperature vapor phase epitaxy of GaN. An almost direct proportionality between the growth rate and the Ga vapor pressure is observed. At optimum conditions growth rates as high as 210 /spl mu/m/h (T=1150/spl deg/c) are achieved. The maximum growth rate is believed to be limited by the supply of ammonia and the starting composition of GaN. Under optimum GaN growth conditions AlGaN layers were grown starting from previously alloyed Al-Ga as well as from co-evaporation of Ga and Al. Adding Al leads to a significant reduction of growth rate and increases the homogeneity of the layers. However, in almost all cases phase separation is found. Besides the binary GaN and AlN phases an intermediate AlGaN phase appears.