We have studied the optical properties of a short-period superlattice composed of 20.4-A GaAs 0 and 14.7-A AlAs layers. The superlattice behaves as an indirect-gap material. A slow and nonexponential decay of the luminescence can be interpreted as the emission from the A indirect excitons localized at the GaAs/A1As interfaces. The temperature dependence of the exciton decay time can be explained in terms of a transition by phonon-assisted tunneling, followed by a nonradiative transition.Recent advances in crystal growth techniques, such as the molecular-beam-epitaxy (MBE) method, have made it possible to produce semiconductor crystals consisting of alternating layers of two difterent semiconductors, i.e. , superlattice structures. These superlattices are expected to form a new device and have been extensively studied, especially the GaAs/AlAs system. A number of experiments on GaAs/A1As superlattices of large layer thickness have been carried out and analyzed successfully by means of the Kronig-Penney model. ' There have been a few works on ultrathin layered GaAs/A1As superlattices. However, considerable confusion exists in the interpretation of the electronic structure of these superlattices, especially of the short-period superlattices with nearly equal GaAs and A1As layer thickness and with periods ranging from -10 to -60 A. Some groups have claimed that these short-period superlattices behave as indirect-gap materials, ' while others have concluded that these materials are direct gap.In order to clarify the electronic structure of short-period superlattices, we have studied the op-0 tical properties of a superlattice composed of 20.4-A GaAs and 14.7-A A1As layers. Special attention is given to the dynamics of photoexcited carriers. The experimental results indicate that our sample behaves as an indirect-gap material and that the emission near the band edge is due to localized indirect excitons.The GaAs/A1As sample used in this study consisted of 250 periods of 20.4-A GaAs/14. 7-A A1As grown by MBE at 550'C on a (100) semi-insulating CxaAs substrate. The Al content in the sample was 42%, which is near the direct-indirect crossover value of -40% in the Al Ga~As alloy system. ' In order to make absorption measurements, the GaAs substrate was removed by preferential etching over a region of 1&1 mm, leaving only the MBE-grown film. The cw photoluminescence and excitation measurements were made by a DCM dye laser pumped by an argon-ion laser. For time-resolved spectra, the sample was excited by a cavity-dumped mode-locked cw dye (DCM) laser synchronously pumped by a mode-locked argon-ion laser. The transmitted light and luminescence were monitored by a double monochromator equipped with a cooled photomultiplier. Lifetime measurements were made by the time-correlated singlephoton counting technique. The sample was immersed in liquid helium. At elevated temperatures, it was exposed to an atmosphere of helium gas.The absorption spectrum at 4.2 K is presented in Fig. 1. The absorption coefficient a(fico) was obtained...
Density-functional theory calculations are performed to investigate the effects of surface modifications and nanosheet thickness on the electronic and magnetic properties of gallium nitride (GaN) nanosheets (NSs). Unlike the bare GaN NSs terminating with polar surfaces, the systems with hydrogenated Ga (H-GaN), fluorinated Ga (F-GaN), and chlorinated Ga (Cl-GaN) preserve their initial wurtzite structures and exhibit ferromagnetic states. The abovementioned three different decorations on Ga atoms are energetically more favorable for thicker GaN NSs. Moreover, as the thickness increases, H-GaN and F-GaN NSs undergo semiconductor to metal and half-metal to metal transition, respectively, while Cl-GaN NSs remain completely metallic. The predicted diverse and tunable electronic and magnetic properties highlight the potential of GaN NSs for novel electronic and spintronic nanodevices.
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