Optical spin injection in CuGaSe2∕GaAs films

Abstract

We have investigated polarization-resolved photoluminescence in epitaxially grown CuGaSe 2 / GaAs͑001͒ films. Spin-polarized excitons are optically excited both below and above the characteristic crystal field splitting of the chalcopyrite. At low temperatures, a large exciton spin polarization of 35% is measured under resonant pumping but this is reduced by an order of magnitude and reverses its sign for nonresonant excitation. The measurements suggest that optical pumping within a small energy window just above the band gap results in the preferential generation of light holes and electrons that exhibit a long spin relaxation time, comparable to the recombination time in CuGaSe 2 . © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2233684͔There is considerable interest in the spin degree of freedom of carriers in the emerging field of spintronics but the realization of practical spintronic devices depends on efficient injection, transport, and detection of spin-polarized carriers. Thus, it is of interest to study materials where a high spin polarization can be realized. CuGaSe 2 , a member of the chalcopyrite family, is a promising candidate. It is a direct band gap semiconductor ͑band gap energy E g = 1.731 eV at T =10 K͒ with applications in optoelectronics technology, for example, red-light-emitting diodes, light detectors, and solar cell devices. In this letter we report on polarization-resolved photoluminescence experiments on GuGaSe 2 films grown on GaAs substrates to evaluate the potential of this material for spintronic applications.In optical pumping experiments on bulk zinc-blende materials ͓Fig. 1͑a͔͒, the radiative selection rules allow a population of spin-polarized electrons and holes to be excited by circularly polarized light. Due to the degeneracy of the heavy hole ͑hh͒ and light hole ͑lh͒ bands at the center of the Brillouin zone and the 3:1 relative strength of the hh/lh transitions, the maximum optical polarization that can be emitted, P circ , is limited to 25%, corresponding to a maximum spin injected polarization P spin of 50%.1 In strained GaAs films, an internal crystal field is generated that lifts the degeneracy of the valence band so that in principle a 100% spin-polarized electron population can be photogenerated. Strain can be induced by growing on lattice mismatched substrates but this reduces the crystal quality and limits the achievable film thickness. Thick films usually exhibit a high dislocation density and poor optical properties.By contrast most chalcopyrites ͑I-III-VI 2 -compounds͒ and pnictides 2 have an internal crystal field splitting ⌬cf, which results in an energy splitting of the lh and hh bands at the center of the Brillouin zone. Lifting of the valence band degeneracy allows, in principle, the optical injection of 100% spin-polarized carriers, corresponding to emission of 100% circular polarized light 1 ͑P spin = P circ ͒. Figure 1͑b͒ shows the schematic band structure of one widely investigated member of the chalcopyrite family, CuGaSe 2 . This material exhib...

Growth of isolated and embedded Cu-containing chalcopyrite clusters and nanocrystals by dry processing

“…1 Standard methods for the growth of chalcopyrite thin films, such as physical evaporation, chemical transport, or rapid thermal processing, result in closed and laterally homogeneous layers, allowing rapid and easy further processing steps for the fabrication of solar cells. 2 However, chalcopyrites have unique properties, such as high absorption coefficients, complex valence band structures, and forgiving defect chemistries, which make them highly interesting, not only in the field of large-area photovoltaics but also in branches of microelectronics including spintronics, 3 lasers, 4 light-emitting diodes, 5 magnetic semiconductors, 6 and nonlinear optical devices, 7 among others. In all these fields and partly in contrast to the requirements of conventional photovoltaics mass production, patterning and lateral structuring at reduced dimensions are commonplace.…”