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We have investigated the time scale of the excitation of electrons leading to a transition from the quantum Hall ͑QH͒ state to the dissipative state in the two-dimensional electron systems of GaAs/AlGaAs heterostructures. The measurements were performed by applying nanosecond electric pulses to the current contacts of QH devices having various electron mobilities (1ϫ10 5 р H р9ϫ10 5 cm 2 /V s) at integer filling factors ϭ2, 4, and 6. The breakdown of the quantum Hall effect ͑QHE͒ occurs when the applied pulses exceed a critical pulse width t p c that is a function of pulse amplitude, magnetic field, and electron mobility (0.4рt p c р18 ns). By applying a simple drift model, we obtain an estimate for the critical drift lengths, which vary from about 1 to 25 m. No general differences of the response to short electric pulses have been found between Hall bar and Corbino devices. We interpret the critical times as drifting times between inelastic scattering events, and the critical lengths as related to the inelastic scattering lengths ᐉ in of electrons causing the QHE breakdown. Characteristic dependences of the critical drift times and lengths on the amplitude, filling factor, and the mobility have been observed and can be attributed to impurity-assisted inter-Landau-level tunneling.

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.

No abstract

Currently, triple‐junction solar cells realized from III–V semiconductor compounds hold the solar energy conversion efficiency world record. To improve the efficiency significantly, it is necessary to increase the number of junctions and to involve a sub‐cell with an absorber layer in the band gap range of 1 eV. For the realization of a stacked four‐junction device with optimised band gaps, we have grown InGaAsP/InGaAs tandem cells lattice matched to InP substrates, and investigated properties of the absorber bulk material. Time‐resolved photoluminescence of the low band gap In0.53Ga0.47As absorber embedded between InP barriers was measured. The InGaAs/GaAsSb tunnel diode structure used in the tandem has been processed into a separate device and I –V curves were measured. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)