Large improvements are reported in the sensitivity of optically addressed multiple quantum well spatial light modulators. In prior work with these materials the quantum well region has been made semi-insulating. It is shown that this is unnecessary and in fact detrimental to performance. By placing layers containing high trap concentrations at the ends of the structure and leaving the active quantum well layers intrinsic the speed of the device at a given illumination is improved by more than four times, diffraction efficiency is enhanced and spatial resolution is almost the same. © 1995 American Institute of Physics.Recent demonstrations of perpendicular geometry GaAs/ AlGaAs multiple quantum well ͑MQW͒ structures as optically addressed spatial light modulators ͑OASLM͒ have shown that these devices can offer high speed 1 and high resolution. 2 These devices use multiple quantum wells that have been made semi-insulating by ion implantation 2 or chromium doping. 1 An ac electric field is applied perpendicular to the plane of the wells at a rate faster than the dark screening time of the material. The surface of the quantum well is then exposed to a write beam that creates photocarriers which screen the applied field in the illuminated regions. The result is an internal electric field which is modulated in the same pattern as the write beam. The optical properties of the quantum well are then changed through the quantum confined Stark effect, and readout with a beam that is nearly resonant with the heavy hole exciton.All MQW OASLMs to date have had deep traps distributed throughout the MQW region. These ''bulk'' traps have been thought necessary to decrease the dark conductivity of the MQW layers so that screening is largely due to photogenerated, and not background, carriers. In addition, the bulk traps were included to restrict the lateral diffusion of carriers which degrades OASLM resolution. Both these motivations are valid for parallel field geometry MQW OASLMs where photogenerated carriers drift and are trapped within the quantum well layers. 3 This thinking may not, however, apply to the perpendicular geometry for two reasons: First, materials grown by molecular beam epitaxy ͑MBE͒, can have an unintentional doping on the order of 10 14 carriers/cm 3 . This doping concentration is sufficient to screen only about 10% of the high fields applied to these materials in the perpendicular geometry ͑ϳ50 kV/cm͒. Any further screening must be by photocarriers or thermally generated carriers, of which there are few due to the large band gap of GaAs. Thus, background carrier screening may not be a problem in good quality materials.Second, recent work also suggests that bulk traps may not be necessary to maintain OASLM resolution. Theoretical calculations indicate that the screening carriers accumulate in an extremely thin ͑5 nm͒ region at the semiconductorinsulator interface. 4 In addition, experimental measurements of MQW OASLMs indicate that the transit time of the carriers across the quantum well region is ϳ300 ps, 5...

We have investigated different metal contacts (Cr/Au, Ti/Pt/Au, and Au) on p-type GaSb, grown by the molecular beam epitaxy. For Au contacts, specific contact resistivities in the range of 1.4×10−8–7.8×10−8 Ω cm2 have been obtained. These are the lowest values ever reported for p-type GaSb. A simple procedure for surface preparation is also reported.

We report improved performance in semi-insulating GaAs/AlGaAs quantum well based spatial light modulators grown by molecular beam epitaxy. The optically addressed modulator reported here are of a new design and have significantly higher spatial resolution than previously reported devices. Strong diffraction efficiencies are described for spatial periods as fine as 2.6 μm at framing rate as high as 600 kHz. Two modulators are characterized, one with x=0.1 and the other with x=0.3 for the AlxGa1−xAs quantum barriers of the superlattices.

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This letter deals with resonant photorefractive devices fabricated from multiquantum wells of GaAs/Al0.3Ga0.7As and operated in a quantum-confined Stark effect geometry. Details of the processing are presented. Epitaxial lift-off was used to remove the active device from the substrate. Low-temperature Al0.3Ga.07As was used as an insulator to form metal-insulator-semiconductor structures on both sides of the multiquantum wells. Proton implant damage was used to improve the fringe visibility. Photorefractive wave mixing with a diffraction efficiency of ∼0.03% was demonstrated. The incorporation of a nitride layer between the top electrode and the low-temperature AlGaAs increased the efficiency to 0.5%. The improvement is attributed to a reduction in the conduction of carriers across the low-temperature layer into the electrode.