Jürgen Albert

Radiative recombination via intrinsic defects in CuxGaySe2

A detailed analysis of the radiative recombination processes in CuxGaySe2 epitaxial layers is presented aiming at an investigation of the intrinsic defect levels as a function of chemical composition. CuxGaySe2 is grown by metalorganic vapor phase epitaxy to allow a precise control of composition. Temperature and excitation intensity dependent photoluminescence is used to identify different recombination mechanisms and to determine the ionization energies of the defect levels involved. Defect-correlated optical transitions in Cu-rich epilayers are described in a recombination model consisting of two acceptor and one donor levels showing ionization energies of (60±10) meV, (100±10) meV, and (12±5) meV, respectively. The identification of a shallow compensating donor in CuxGaySe2 and the assignment of the 100 meV state to an acceptor are the most important new aspects in this model. Photoluminescence properties of layers showing Ga-rich compositions are discussed in a model of highly doped and highly compen...

Excitonic Photoluminescence from CuGaSe2 Single Crystals and Epitaxial Layers: Temperature Dependence of the Band Gap Energy

Excitonic line spectra from CuGaSe2 single crystals and epitaxial layers are investigated as a function of temperature. Near band edge luminescence from free and bound excitons is observed at 10 K. The identification of both, free exciton ground and first excited state allows to determine the free exciton binding energy, which is found to be (13±2) meV. The bound exciton line is attributed to the recombination of an exciton bound to a neutral acceptor (A0, X). The widely discussed phenomenon of an anomalous temperature dependence of the band gap energies in different chalcopyrite-type I-III-VI2 compounds is reconsidered for CuGaSe2 on the basis of temperature dependent photoluminescence studies. No anomalous behaviour of the band gap energy as a function of temperature is found in CuGaSe2.

Band offsets at the ZnSe/CuGaSe2(001) heterointerface

The formation of the ZnSe/CuGaSe2 heterointerface was studied by x-ray photoelectron spectroscopy (XPS). ZnSe was sequentially grown on CuGaSe2(001) epilayers. In situ photoemission spectra of the Ga 3d and Zn 3d core levels as well as XPS valence bands were acquired after each deposition step. The valence-band offset is determined to be ΔEV=0.6±0.1 eV. As a consequence, a nearly symmetric “type-I” band alignment for the ZnSe/CuGaSe2 heterojunction with a conduction-band offset of ΔEC=0.4±0.1 eV is found. Concerning the band alignment ZnSe can, therefore, be expected to be a suitable buffer material for CuGaSe2-based thin-film solar cells.

Stability of surfaces in the chalcopyrite system

The stable surfaces in chalcopyrites are the polar {112} surfaces. We present an electron microscopy study of epitaxial films of different compositions. It is shown that for both CuGaSe2 and CuInSe2 the {001} surfaces form {112} facets. With increasing Cu excess the faceting is suppressed. This indicates a lower surface energy of the {001} surface than the energy of the {112} surface in the Cu-rich regime, but the {001} surface is higher in energy than the {112} surface in the Cu-poor regime.

Defects in CuGaSe2 thin films grown by MOCVD

Abstract Optical and electrical properties of CuGaSe2/GaAs(001) heteroepitaxial layers as a function of chemical composition are presented. The photoluminescence (PL) spectra observed for Cu-rich epitaxial layers are interpreted in a model consisting of one donor and two acceptor levels with ionization energies of 80, 50 and 10 meV, respectively. The radiative recombination of Ga-rich samples is dominated by broad donor-acceptor-pair bands that shift to lower energies with decreasing Cu/Ga-ratio. The peak energies of these broad emissions are strongly excitation power-dependent showing a blueshift of up to 17 meV per decade. The observed PL properties of Ga-rich samples are discussed in terms of strong compensation as supported by Hall measurements. Hall mobilities of above 250 cm2/V s have been found for near stoichiometric, slightly Ga-rich epitaxial layers. The electrical properties of CuGaSe2/GaAs(001) grown under Cu-rich conditions are found to be influenced by a CuxSe secondary phase as expected from the Cu-Ga-Se phase diagram.

Epitaxially grown single grain boundaries in chalcopyrites

To allow a specific investigation of grain boundaries with a given orientation we have grown epitaxial grain boundaries of CuGaSe2 by metal organic vapour phase epitaxy (MOVPE). The epitaxy on either side of the grain boundary and the Σ3 character of the grain boundary are shown by electron back-scattering diffraction (EBSD) scans. Scanning electron microscopy (SEM) micrographs show a dense grain boundary. Transmission electron microscopy (TEM) micrographs prove that the grain boundary in the film is the direct continuation of the grain boundary in the substrate. High-resolution TEM (HRTEM) shows that the grain boundary in the film is a twin as well, and thus a Σ3 boundary. This also justifies the use of a classification scheme that is derived for the cubic system for the tetragonal chalcopyrites. Thus by using a Σ3 grain boundary in the cubic GaAs substrate as a template, a Σ3 grain boundary is obtained in the tetragonal CuGaSe2 film.

Structural and Optical Characterization of Cu x Ga y Se 2 Thin Films under Excitation with Above and Below Band Gap Laser Light

CuxGaySe2 MOCVD and PVD grown films were structurally and optically characterized by Raman, Micro-Raman and photoluminescence spectroscopy. Defect related photoluminescence excitation with wavelengths varying across the material band gap reveals: a) in Cu-rich CuGaSe2 films, three band edge splitting due to the spin-orbit interaction and the crystal field, and donoracceptor pair recombination between a shallow donor and two different acceptor levels, and b) in Ga-rich CuGaSe2 films, donor-acceptor pair transitions between quasi-continua of donor and acceptor levels related to potential fluctuations. Raman spectra of CuxGaySe2 films, excited by laser light near and below the material band gap, show intense modes at 197cm -1 , 187cm -1 , and 277cm -1 , which can be used as indicators of crystallinity and Ga-content of the films. Polarization- and angular- dependent micro-Raman spectra of MOCVD CuGaSe2 indicate that CuxSey-crystallites, dispersed on the surface of Cu-rich films, are grown oriented with their caxis perpendicular to the film surface.

Production scale MOCVD growth of II–VI semiconductors

Abstract The growth of II–VI semiconductors is performed in MOCVD systems (AIX 200 and AIX 200/4) with a capacity of one 2 in and one 4 in wafer or equivalent, respectively, on 2 in substrates. The growth of ZnSe, ZnSSe and ZnMgSSe has been carried out with several different precursor materials. The growth rate and compositon homogeneities across a 2 in wafer for ZnSe and ZnSSe, with and without rotation of the susceptor, were investigated. The growth rate homogeneity for ZnSe was calculated to be ±2%. The fabrication technology can be easily transferred to multiwafer reactors for the production of devices. Homogeneities in the range of ±1% are expected, due to the unique design with double rotation of the substrates.