This Small Business Innovative Research (SBIR) Phase I project seeks to research the metal-organic chemical vapor deposition (MOCVD) epitaxial growth and explore new passivation techniques for very low-noise aluminum gallium antimonide (AlGaSb) avalanche photodiodes (APDs). At one particular composition (~6% Al), the AlGaSb bandgap and valence spin-orbit splitting energy become equal, resonantly enhancing the hole ionization rate. Several groups have reported high (7 to 20) hole-to-electron ionization ratios in AlGaSb. Other semiconductors with 1.0-1.7 micron response lack high hole/electron or electron/hole ionization ratios InGaAs~2, e~1.5, InGaAsP~3) crucial for low noise APDs. For example, the AlGaSb APD excess noise factor should be 2, versus 7.5 for Ge APDs at a gain of 10. Previous AlGaSb APDs were made 10-20 years ago by liquid phase epitaxy (LPE), which had quality and uniformity (e.g. 20% thickness variation) issues, and different optimal compositions were reported. MOCVD with high purity sources will be used to grow more uniform (<3% doping variation, <1% for thickness and composition), higher quality epilayers that can now be better characterized. This should enable the exact composition and epi structure to best use the fairly sharp resonant enhancement.
Available APDs in the 1.0 to 1.7 micron range are noisy compared with PIN/trans-impedance amplifiers. The proposed low-noise AlGaSb APDs would allow a single component to replace most of the PIN/trans-impedance amplifier front end, and significantly extend the design space in which optical communications and laser radar systems are constrained.
