Infrared avalanche diodes are key components in diverse applications such as eye-safe burst illumination imaging systems and quantum cryptography systems operating at telecommunications fiber wavelengths. HgCdTe is a mature infrared detector material tunable over all infrared wavelengths longer than ~850nm. HgCdTe has fundamental properties conducive to producing excellent detectors with low noise gain. The huge asymmetry between the conduction and valence bands in HgCdTe is a necessary starting point for producing impact ionization with low excess noise factor. Other factors in the band structure are also favorable. The low bandgap necessitates at least multi-stage thermoelectric cooling. Mesa diode structures with electron initiated multiplication have been designed for gains of up to around 100 at temperatures at or above 80K. Backside illuminated, flip-chip, test diode arrays have been fabricated by MOVPE using a process identical to that required for producing large imaging arrays. Test diode results have been obtained with the following parameters characterized, dark current vs. voltage and temperature, gain vs. voltage, and spectral response as a function of wavelength and bias. The effect of changing active region cadmium composition and active region doping is presented along with an assessment of some of the trade-offs between dark leakage current, gain, operating voltage and temperature of operation.
We have demonstrated the successful growth of mercury cadmium telluride (MCT) infrared detector material on silicon substrates. Growth on silicon increases the maximum achievable array size, reduces manufacturing costs, and paves the way for infrared detector growth directly on multiplexing circuits. In addition, the thermal match with multiplexing circuits eliminates the requirement for complex thinning procedures. Since the crystal lattice of MCT is not matched to that of silicon, an intermediate buffer layer is required. We have developed a buffer layer technique that is compatible with MCT grown by Metal Organic Vapour Phase Epitaxy (MOVPE). Long-wavelength heterostructure device designs were grown using this technique. Test devices and 128x128 focal plane arrays were fabricated by wet etching mesa structures and passivating the mesa side-walls with a thin layer of CdTe. An indium flip-chip technique was used to form interconnects between the detector material and test or multiplexing circuit. At 77K, 50x50μm test devices with a 10.2μm cut off wavelength have been measured with R0A~1x103Ohm cm2 at zero bias and R.A~1x104Ohm cm2 at 0.1V reverse bias. Arrays from this material have been demonstrated with operabilities up to 99.7%.
Recent advances in the growth of cadmium mercury telluride (Hg1-xCdxTe or MCT) by metal organic vapor phase epitaxy (MOVPE) allow the fabrication of advanced device structures where both the alloy composition x and the doping concentration can be accurately controlled throughout the epitaxial layer. For p-type doping using arsenic, the acceptor concentration can be varied from 5 X 1015 cm-3 to 4 X 1017 cm-3 and for n-type doping using iodine, the donor concentration can be varied from 1 X 1015 cm-3 to 2 X 1017 cm-3. A number of diode arrays have been fabricated in this material and their properties assessed at 77 K, 195 K and 295 K. It has been found that the diffusion currents are at least ten times lower than in homojunctions. In addition, the devices exhibit negative resistance at temperatures above 190 K due to auger suppression. The successful demonstration of auger suppression in these structures has greatly improved the diode leakage currents at room temperature and will enable the development of new devices such as a room temperature laser detector.