H. Heinke

X-ray diffraction analysis of the defect structure in epitaxial GaN

High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in (001)-oriented GaN epitaxial layers. For this, (00l) and (hkl) Bragg reflections with h or k nonzero were studied, the latter one measured in skew symmetric diffraction geometry. The defect analysis was applied to a variety of GaN layers grown by molecular-beam epitaxy under very different conditions. The outcome is a fundamental correlation between the densities of edge- and screw-type dislocations.

Strain relaxation in AlGaN under tensile plane stress

Relaxation of tensile strain in AlxGa1−xN layers of different compositions epitaxially grown on GaN/sapphire is investigated. Extended crack channels along 〈211¯0〉 directions are formed if the aluminum content exceeds a critical value, which decreases with increasing layer thickness. This process is found to limit the average strain energy density to a maximum value of 4 J/m2. By calculating the stress distribution between cracks and the strain energy release rate for crack propagation, the relaxed strain as measured by x-ray diffraction is correlated to the crack density, and the onsets of crack channeling and layer decohesion are fitted to a fracture toughness of 9 J/m2. Moreover, the crack opening at the surface is found to linearly increase with the stress. Annealing of samples above the growth temperature introduces additional tensile stress due to the mismatch in thermal expansion coefficients between the layer and substrate. This stress is shown to relieve not only by the formation of additional cr...

Relaxation and mosaicity profiles in epitaxial layers studied by high resolution X-ray diffraction

Abstract A novel nondestructive method correlating the strain gradient in partially relaxed epilayers with a profile of crystalline mosaicity is demonstrated by using high resolution X-ray diffraction. By reciprocal space mapping, a distribution of the scattered intensity near reciprocal lattice points was found, which is characteristic for partially relaxed layers. This distribution is described by a relaxation-mosaicity triangle which is modified in a characteristic way for different reflection orders. The increase in mosaicity with progressive relaxation is quantified from diffraction profiles recorded along selected directions in reciprocal space. The determination of the depth profile of strain implies also the knowledge of the depth gradient of the mosaicity. The method is demonstrated for a partially relaxed ZnSe layer grown by MBE on (001) GaAs.

Growth of MgTe and Cd1−xMgxTe thin films by molecular beam epitaxy

Abstract We report on the growth of the compound semiconductor MgTe as well as the ternary alloy Cd1−xMgxTe by molecular beam epitaxy. This is to our knowledge the first time that this material has been grown by any epitaxial technique. Bulk MgTe, which is hygroscopic, has a band gap of 3.0 eV and crystallizes usually in the wurtzite structure. Pseudomorphic films were grown on zincblende CdTe substrates for a MgTe thickness below a critical layer thickness of approximately 500 nm. In addition, Cd1-xMgxTe epilayers were grown with a Mg concentration between 0 and 68%, which corresponds to a band gap between 1.5 and 2.5 eV at room temperature. The crystalline quality of the layers is comparable to CdTe thin films as long as they are fully strained. The lattice constant of zincblende MgTe is slightly smaller than that of CdTe, and the lattice mismatch is as low as 0.7%. In addition highly n-type CdMgTe layers were fabricated by bromine doping. The tunability of the band gap as well as the rather good lattice match with CdTe makes the material interesting for optoelectronic device applications for the entire visible range.

INCORPORATION OF INDIUM DURING MOLECULAR BEAM EPITAXY OF INGAN

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

Analysis of the Defect Structure of Epitaxial GaN

The type and density of threading dislocations in GaN epitaxial layers grown on c-plane sapphire have been analyzed by using nondestructive high resolution X-ray diffraction. The highly distorted GaN layers were described as mosaic crystals characterized by a mean tilt and twist angle between the mosaic blocks which are correlated with the densities of screw and edge type threading dislocations, respectively. Triple axis rocking curves of (00l) reflections for varying l-indices were used to determine the tilt angle, while the twist was extrapolated from ω-scans for (hkl) Bragg reflections with h or k nonzero, measured in skew symmetric diffraction geometry. This defect analysis was applied to selected GaN layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD) revealing clear differences between both sample types.

Optical investigation of confinement and strain effects in CdTe/(CdMg)Te quantum wells

We present optical investigations on CdTe/(CdMg)Te single quantum wells (QWs) and demonstrate the high structural quality of the pseudomorphic grown QWs structure which shows high photoluminescence efficiency up to room temperature. Due to the large band‐gap difference between CdTe and Cd0.51Mg0.49Te of more than 0.8 eV remarkable strong confinement effects are observable. A strong enhancement of the exciton binding energies is found by decreasing well width. In the 50‐A‐wide QW the binding energy is more than two times larger compared with that of bulk CdTe. In addition, a strong functional dependence of the localization energy of donor bound excitons on the well thickness is found. A valence‐band offset of 30% in the strain‐free limit is determined from the energy difference between heavy‐ and light‐hole excitons which is consistent with the strong blue shift of exciton energies by decreasing well widths and the observed effective electron‐hole confinement.

Wide gap Cd1−xMgxTe: molecular beam epitaxial growth and characterization

Abstract We have grown the ternary alloy Cd 1- x Mg x Te as well as Cd 1- x Mg x Te/CdTe quantum well structures by molecular beam epitaxy - to our knowledge for the first time. Cd 1- x Mg x Te exhibits some very interesting features: The band gap has been determined as a function of Mg concentration, and a band gap of 3.0 eV was found for zincblende MgTe at room temperature. Cd 1- x Mg x Te thin films with Mg concentrations of up to 0.75 were fabricated, which corresponds to a band gap of 2.8 eV at low temperatures. Therefore, the whole visible band gap range (at room temperature) can be covered with Mg concentrations between 0.30 (red) and 0.75 (blue). Bulk MgTe crystallizes in the wurtzite structure, but zincblende MgTe could be grown on (100) oriented CdTe substrates up to a layer thickness of approximately 500 nm. The lattice mismatch between zincblende MgTe and CdTe was found to be as small as 1.0%. The growth of cubic MgTe could be followed by reflection high energy electron diffraction (RHEED) oscillations. In general, excellent structural quality could be reached, which is demonstrated by the FWHM of 22 arc sec for a Cd 1- x Mg x Te thin film with 0.44 Mg concentration on a Cd 1- x Zn x Te nearly lattice matched substrate. The Poisson number of Cd 1- x Mg x Te has been determined by X-ray diffraction as a function of Mg concentration. Cd 1- x Mg x Te/CdTe single quantum well structures have been fabricated with a large confinement energy of up to 0.8 eV. The photoluminescence spectra show exciton lines with very narrow linewidths. We are able to observe excited exciton states, and from the energetic difference between 1s and 2s heavy hole exciton lines we deduce exciton binding energies. Very bright luminescence could be seen even at room temperature, which is an indication of a large exciton binding energy and an effective radiative recombination.

Thermal expansion of bulk and homoepitaxial GaN

The thermal behavior of Mg-doped and intentionally undoped bulk crystals and homoepitaxial GaN was investigated in a wide temperature range from 12 to 600 K. With high-resolution x-ray diffraction, both lattice parameters a and c were determined and the thermal expansion coefficients were calculated. Within the experimental accuracy, mean values were extracted for the temperature ranges 12–100, 100–250, and 250–600 K. These values are essential, especially, for the interpretation of measurements of other GaN properties performed at low temperatures.

p‐type doping of CdTe with a nitrogen plasma source

We report here on the growth and the characterization of p‐type CdTe grown by molecular beam epitaxy on (001) Cd0.96Zn0.04Te substrates. Nitrogen has been used as a dopant, which is activated in an electron cyclotron resonance plasma source. The carrier concentration was determined using a C/V profiler. Nitrogen has been successfully incorporated substitutionally and hole densities up to 2.6×1017 cm−3 have been achieved. In addition we present data from x‐ray diffraction and photoluminescence, which demonstrate the effect of self‐compensation on the nitrogen‐doped CdTe layers.