GaAIAsSb/GaSb Avalanche Photodetectors

AbstractThe growth and fabrication of GaAIAsSb/GaSb avalanche photodiodes is discussed. The leakage current, gain, and uniformity of heterojunction, ion-implanted and Schottky-barrier structures are examined. In addition, the effect of the valence band spin orbit splitting upon avalanche noise is briefly discussed. It is shown that the prevalent problem of these devices is a high surface leakage current.IntroductionAn increasing demand for more reliable and higher performance optical sources and detectors is occuring with the increasing role that fiber-optic systems have in communica­ tion networks. In particular, for long distance systems, the low loss and dispersion of optical fibers in the 1.2 - 1.6 ym wavelength region place stringent requirements upon device design and materials of light emitters and detectors.1 "10In the area of photodetectors, operation in the avalanche mode has an obvious advantage compared to a pin detector, of obtaining more electrons out from a fixed number of photons. Unfortunately, owing to the statistical distribution of the impact ionization process, there is an additional noise source when operating a device in the avalanche mode.For an avalanche photodiode operating in such a mode, the signal-to-noise ratio can to written as(qnPM/hv)22 qB [(Id + qnP/hv) M2F Ig] amplifier noisewhere, n is the quantum efficiency of the diode, P is the received optical power, M is the gain of the diode, hv is the photon energy, B is the bandwidth of the amplifier, 1^ is the bulk dark current, F is the excess noise factor, and Ig is the surface dark current. The operation of the APD will be heavily affected by the excess noise factor, F. As pointed out by Mclntyre,11 F depends on the ratio of the ionization rate of holes, 3, and that of the electrons, a. When a»3 (or 3»a) , the excess noise factor approaches unity if elec­ trons (or holes) are purely excited to start the impact ionization process. On the other hand, if a = 3, the excess noise factor equals M. In germanium, a « 3; therefore, the noise term goes up with M3 , while the signal term only goes up with M2 . At high enough gain when the total noise of the diode overcomes the preamplifier noise, the signal-to- noise ratio will decrease.Obviously, the ionization coefficients a and 3, the dark current and multiplication determine the signal to noise ratio. These parameters vary with the device structure as well as the type of material from which the devices are constructed.For long wavelength avalanche photodiodes, two III-V semiconductor material systems are presently of most interest; InGaAsP/InP and GaAIAsSb/GaSb. Adjustment of the alloy compo­ sition permits variations in energy gap while maintaining the same lattice parameter. With this degree of freedom the 1.2 - 1.55 ym wavelength region can be readily spanned with these materials. Of particular interest in this paper is the GaAlAsSb alloy system. The materials preparation, performance of various structures as well as the effect of spin orbit splitting in the valence band will be examined.Materials Preparation and GrowthThe GaAIAsSb/GaSb structures of the present work are grown by LPE. 12" 14 Growth temper­ atures of 500-550°C were employed to increase the arsenic solubility in the gallium melt. In some of the Be-implanted structures, a minor amount of In was added to the Ga melt in an attempt to increase the arsenic solubility. No measurements have been taken thus far to verify the expected effect.