Helen M. Dauplaise

High-performance InGaAs-InP APDs on GaAs

Epitaxial layer transfer was used to fabricate In/sub 0.53/Ga/sub 0.47/As-InP avalanche photodiodes on GaAs substrates. The photodiodes displayed a gain-bandwidth product of 80 GHz, dark current of 10.20 nA at 90% of the breakdown voltage, and responsivity of 5.9 A/W at 2 V below breakdown. Performance is comparable to standard devices fabricated on InP substrates, suggesting the transfer process does not degrade material quality.

Gas Cluster Ion Beam Processing of GaSb and InSb Surfaces

Gas Cluster Ion Beam (GCIB) processing has recently emerged as a novel surface smoothing technique to improve the finish of chemical-mechanical polished (CMP) GaSb (100) and InSb (111) wafers. This technique is capable of improving the smoothness CMP surfaces and simultaneously producing a thin desorbable oxide layer for molecular beam epitaxial growth. By implementing recipes with specific gas mixtures, cluster energy sequences, and doses, an engineered oxide can be produced. Using GaSb wafers with a high quality CMP finish, we have demonstrated surface smoothing of GaSb by reducing the average roughness from 2.8A to 1.7A using a dual energy CF 4 /O 2 -GCIB process with a total charge fluence of 4×10 15 ions/cm 2 . For the first time, a GCIB grown oxide layer that is comprised of mostly gallium oxides which desorbed at 530°C in our molecular beam epitaxy system is reported, after which GaSb/AlGaSb epilayers have been successfully grown. Using InSb, we successfully demonstrated substrate smoothing by reducing the average roughness from 2.5A to 1.6A using a triple energy O 2 -GCIB process with a charge fluence 9×10 15 ions/cm 2 . In order to further demonstrate the ability of GCIB to smooth InSb surfaces, sharp ∼900nm high tips have been formed on a poorly mechanically polished InSb (111)A wafer and subsequently reduced to a height of ∼100nm, an improvement by a factor of eight, using a triple energy SF 6 /O 2 -GCIB process with a total charge fluence of 6×10 16 ions/cm 3 .

Novel focal plane array integration technology for use in eye-safe imaging ladar receivers

A novel integration technology for the fabrication of active, or passive, focal plane array imagers has been developed. The integration scheme is based on the transfer of epitaxial layers to a surrogate substrate without critical alignment. Once the epitaxial layer is successfully transferred to the surrogate substrate, photodetector isolation, passivation, and fabrication are completed. To demonstrate the potential of the process, 320 x 256 arrays of InGaAs mesas were successfully transferred onto commercially-available focal plane array readout integrated circuits. Pitch and pixel resolution are limited by the available standard photolithography. InGaAs mesas transferred to silicon wafers with a pitch as small as 10 microns have been demonstrated. The process was optimized for the fabrication of high-performance vertical Schottky photodiodes. Dark-currents below 5 nA were observed with 44 mm diameter photodiodes. Responsivities of 0.55 A/W were obtained with a 1 micron InGaAs absorber. The new integration process can be used to easily achieve photodiodes with bandwidths higher than 20 GHz, without the use of an air-bridge.

Optical and Dielectric Properties of Eu- and Y-Polytantalate Thin Films

Due to their highly efficient photo-luminescent (PL) characteristics, the physical properties of rare-earth polytantalates, RETa7O19 (RE=Eu and Y) were further studied. AFM, SEM, HRTEM, x-ray reflectometry, spectroscopic ellipsometry and standard dielectric testing were used to determine film thickness, roughness, index of refraction, band-gap, dielectric constant, leakage current and breakdown field for as-deposited (amorphous) and post-annealed (crystalline) films. Structural and morphological properties of the Film/SiO2/Si interfaces were also examined.

Preparation and Patterning of GaSb Surfaces with Br-IBAE for Antimonide Based Molecular Beam Epitaxy

Bromine Ion Beam Assisted Etching (Br-IBAE) is shown to be useful in removing GaSb wafer chemical mechanical polish (CMP) surface and subsurface damage; creating microstructure patterns in GaSb surfaces through stencil, photoresist, and oxides masks; and stabilizing the as-etched GaSb surface with a thin, easily thermally desorbed oxide layer. Thus, the Br-IBAE technique is well suited as a GaSb surface final-polish technique in overgrowth applications that require “epi-ready” GaSb (100) surfaces for molecular beam epitaxy (MBE) as well as applications such as quantum wires and dots that require high-quality GaSb/AlInGaSb MBE overgrowth over patterned GaSb (100) surfaces.

Chemical and electrical analysis of CdS interlayers on InP and related materials

Cadmium sulfide (CdS) layers were deposited from aqueous solutions of thiourea, cadmium sulfate, and ammonia on (100) InP, InGaAs, and InAlAs. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and atomic force microscopy (AFM) were used to investigate the structural and chemical nature of the deposited CdS layer and the CdS/semiconductor interface. XPS showed that the deposition process effectively removes existing native oxides on InP and InAlAs before CdS growth occurs. Capacitance-voltage measurements of metal-insulator- semiconductor (MIS) capacitors were used to investigate the interface- state density of samples with and without CdS films between InP and a deposited insulator. CdS interlayers were found to reduce both the hysteresis and the interface-state density of the MIS capacitors. Applications of CdS interlayers for various photonic devices will be discussed.