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In free space optical communication, photodetectors serve not only as communications receivers but also as position sensitive detectors (PSDs) for pointing, tracking, and stabilization. Typically, two separate detectors are utilized to perform these tasks, but recent advances in the fabrication and development of large-area, low-noise avalanche photodiode (APD) arrays have enabled these devices to be used both as PSDs and as communications receivers. This combined functionality allows for more flexibility and simplicity in optical system design without sacrificing the sensitivity and bandwidth performance of smaller, single-element data receivers. This work presents the development of APD arrays rated for bandwidths beyond 1 GHz with measured carrier ionization ratios of approximately 0.2 at moderate APD gains. We discuss the fabrication and characterization of three types of APD arrays along with their performance as high-speed photodetectors.

A high density of electrically active interface states has hindered the development of InP-based insulated-gate technology. The poor electrical quality of native oxides has forced researchers to develop novel methods of surface passivation 1-3 and new techniques to deposit dielectric films without damaging the underlying substrate. [4][5][6] We have previously demonstrated that the electronic properties of metal-insulator-semiconductor (MIS) devices on InP can be improved markedly by the chemical bath deposition of CdS on the substrate prior to dielectric deposition. 7 More recently, we have fabricated MIS capacitors with near-ideal capacitance-voltage (C-V) response on InP(100) by modifying the CdS deposition process to include a pretreatment in an aqueous thiourea/ammonia solution. 8 The CdS passivation process, including pretreatment, has been shown to remove InP native oxides and prevent subsequent reoxidation of the InP upon exposure to air. While this passivation technique has proven effective on InP, as well as InGaAs, the high ammonia concentration and consequent high pH may lead to chemical incompatibilities when applied to aluminumcontaining compounds such as InAlAs, AlGaAs, and InAlGaAs that are prevalent in contemporary optoelectronic devices. In this study we varied the concentrations of ammonia and cadmium sulfate in our CdS depositions. X-ray photoelectron spectroscopy (XPS) was used to examine the thermal stability of the S-passivated InP surface and CdS thin films. Film microstructure was studied by reflection high-energy electron-diffraction (RHEED) and atomic force microscopy (AFM). MIS capacitors were fabricated from CdS-passivated InP. Quasi-static and 1 MHz C-V measurements were used to determine the density of electrically active states in the SiO 2 /CdS/InP interface region. Experimental Sample preparation.-All substrates were undoped (n Ϸ 5 ϫ 10 15 cm Ϫ3 ) InP(100) manufactured by Sumitomo. Stock solutions of 14.5 M ammonia, 0.2 M thiourea, and 0.1 M cadmium sulfate were prepared from VLSI grade ammonium hydroxide, 99% thiourea [CS(NH 2 ) 2 ], and 99.999% cadmium sulfate (3CdSO 4 и8H 2 O), respectively. Thiourea/ammonia pretreatment is performed by taking n-InP substrates directly from the manufacturer's box and immersing them in a 0.033 M thiourea and 12.1 M ammonia solution. The reaction beaker is placed in an 85ЊC water bath, which is used for temperature stability. Timing begins when the solution is placed in the water bath, not when it reaches equilibrium temperature. After 15 min in the 85ЊC bath, the samples are transferred directly to a room-temperature solution of thiourea and ammonia to which the appropriate amount of cadmium sulfate is then added. This CdS deposition solution is then heated in the water bath. The deposition solution and water bath reach an equilibrium temperature within 2 min. Deposition was followed by rinsing in deionized water and then drying with nitrogen.Deposition solutions with the concentrations used are listed in Table I; the pH values were measured wi...

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