Klaus Ploog

GaAs Substrate Preparation for Oval-Defect Elimination during MBE Growth

We have developed a new method for GaAs substrate preparation which significantly reduces the formation of oval defects during MBE growth of selectively doped n-AlxGa1-xAs/GaAs heterostructures. The method simply requires treatment in H2SO4 after mechano-chemical polishing in NaOCl solution and generation of a protective surface oxide during In soldering. Routinely a density of oval defects of less than 200 cm-2 is achieved for 2 µm thick heterostructures. The efficiency of the new preparation procedure is demonstrated by 2DEG mobilities in excess of 106 cm2/(Vs) at 6 K obtained with a spacer width as narrow as 18 nm.

Effect of excitons in AlGaAs/GaAs superlattice solar cells

The effect of excitonic absorption on solar cell efficiency has been investigated using solar cells with AlGaAs/GaAs superlattice active regions. Numerical calculations reveal that excitonic absorption considerably enhances the overall absorption of bulk GaAs. Excitonic absorption shows strong and sharp peaks at the absorption edge and in the energy region above the band gap. Absorption enhancement is also achieved in the AlGaAs/GaAs superlattice. The measured quantum efficiency spectra of AlGaAs/GaAs solar cells are found to be quite similar to the calculated absorption spectra considering the excitonic effect. The miniband structures of the superlattice and the electric field of the p–i–n junction enhance the dissociation of excitons and the extraction of separated carriers. These results suggest that the enhanced absorption by excitons can increase the quantum efficiency of solar cells.

Growth of CuGaSe2 Layers on Closely Lattice-Matched GaAs Substrates by Migration-Enhanced Epitaxy

CuGaSe2 single-crystal films are grown on the As-stabilized (2×4) surface of (001) GaAs by migration-enhanced epitaxy (MEE), where Cu+Ga and Se are alternately deposited. The growth process is monitored by refraction high-energy electron diffraction (RHEED) in the [110] azimuth. Under the Cu-enriched growth condition, a deformed 4-fold pattern is observed in both Cu+Ga and Se deposition periods. The deformed 4-fold pattern is found to be related to the segregation of Cu2Se on the CuGaSe2 surface as confirmed by the results of X-ray diffraction (XRD) measurement. By reducing the beam equivalent pressure of Cu (Cu-BEP), clear 4-fold patterns appear in both Cu+Ga and Se deposition periods instead of deformed 4-fold patterns. Further reduction of Cu-BEP results in clear 4- and 2-fold patterns for Cu+Ga and Se deposition periods. Under these growth conditions, Cu2Se-segregation-free CGS growth is achieved. Thus, the CuGaSe2 single-crystal layers without Cu2Se-segregation are successfully grown on GaAs(001) substrates by optimizing the Cu-BEP.