We demonstrate the use of bulk Al 0.04 Ga 0.96 Sb and GaSb/AlSb superlattice as the gain material in a separate absorption/multiplication avalanche photodiode with sensitivity up to 1.74 m. Both gain schemes were implemented in a molecular-beam epitaxy grown structure with a selectively doped InAs/AlSb superlattice as the n-type layer. Hole impact ionization enhancement was observed in Al 0.04 Ga 0.96 Sb by using a two wavelength injection scheme. The superlattice gain layer device exhibited multiplication factors in excess of 300, and surface limited dark current at a level comparable to InGaAs/InAlAs devices of similar design. The superlattice gain layer was found to be more promising than its bulk counterpart due to its inherent lower dark current.

Due to its large band gap, AlSb is often used as a barrier in antimonide heterostructure devices. However, its transport characteristics are not totally clear. We have employed ballistic electron emission microscopy ͑BEEM͒ to directly probe AlSb barriers as well as more complicated structures such as selectively doped n-type InAs/AlSb superlattices. The aforementioned structures were grown by molecular beam epitaxy on GaSb substrates. A 100 Å InAs or 50 Å GaSb capping layer was used to prevent surface oxidation from ex situ processing. Different substrate and capping layer combinations were explored to suppress background current and maximize transport of BEEM current. The samples were finished with a sputter deposited 100 Å metal layer so that the final BEEM structure was of the form of a metal/capping layer/semiconductor. Of note is that we have found that hole current contributed significantly to BEEM noise due to type II band alignment in the antimonide system. BEEM data revealed that the electron barrier height of Al/AlSb centered around 1.17 eV, which was attributed to transport through the conduction band minimum near the AlSb X point. Variation in the BEEM threshold indicated unevenness at the Al/AlSb interface. The metal on semiconductor barrier height was too low for the superlattice to allow consistent probing by BEEM spectroscopy. However, the superlattice BEEM signal was elevated above the background noise after repeated stressing of the metal surface. A BEEM threshold of 0.8 eV was observed for the Au/24 Å period superlattice system after the stress treatment.

We have employed ballistic electron emission microscopy ͑BEEM͒ to study the energy positions in the conduction band of Al x Ga 1Ϫx As. Epilayers of undoped Al x Ga 1Ϫx As were grown by molecular beam epitaxy on conductive GaAs substrates. The Al composition x took on values of 0, 0.11, 0.19, 0.25, 0.50, 0.80 and 1 so that the material was examined in both the direct and indirect band gap regime. The Al x Ga 1Ϫx As layer thickness was varied from 100 to 500 Å to ensure probing of bulk energy levels. Different capping layers and surface treatments were explored to prevent surface oxidation and examine Fermi level pinning at the cap layer/Al x Ga 1Ϫx As interface. All samples were metallized ex situ with a 100 Å Au layer so that the final BEEM structure is of the form Au/capping layer/Al x Ga 1Ϫx As/bulk GaAs. Notably we have measured the Schottky barrier height for Au on Al x Ga 1Ϫx As. We have also probed the higher lying band edges such as the X point at low Al concentrations and the L point at high Al concentrations. Variations of these critical energy positions with Al composition x were mapped out in detail and compared with findings from other studies. Local variations of these energy positions were also examined and found to be on the order of 30-50 meV. The results of this study suggest that BEEM can provide accurate positions for multiple energy levels in a single semiconductor structure.

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GaSb/A1Sb/InAs is an attractive system for making low noise avalanche photodetectors (APD) due to possible resonant enhancement of hole impact ionization in AlGai_Sb and potential enhancement of electron impact ionization in GaSb/A1Sb superlattices. We have employed molecular beam epitaxy (MBE) to fabricate device structures so that these effects could be further explored. The devices were grown on GaSb substrates and incorporated a pn+ one sided abrupt junction. The p multiplication region consisted of either bulk Al004Ga96Sb or 10 periods of alternating, 300 A thick GaSb and A1Sb layers. A short period, selectively doped InAs/AlSb superlattice was used as the n+ layer. Dark current suppression in these devices was found to be largely dependent on the InAs/A1Sb superlattice configuration and the resulting band offset at the pn+ heterojunction. Notably, for devices with a 0.6 tm Aloo4Gao.96Sb multiplication region and an optimized InAs/A1Sb superlattice, an avalanche break down voltage of 13 V was observed. The dark current density for this device was 6 A/cm2 at a multiplication factor of 10.Devices with GaSb/A1Sb superlattice multiplication regions exhibited a higher breakdown voltage (18.5 -V) and a lower dark current density (0.4 A/cm2) at comparable gain. Impact ionization rates in Al004Ga96Sb were studied by using 781 nm and 1645 nm laser light. The results were consistent with enhancement of hole impact ionization in Al0 04Ga0 96Sb.