Characterization of barrier effects in superlattice LWIR detectors

Abstract

Improved LWIR sensors are needed for defense applications. We report an advance in sensor technology based on diodes in type-II strained layer superlattice structures built in the InAs/GaSb/AlSb materials system. A key feature of the devices is a pair of complementary barriers, namely, an electron barrier and a hole barrier formed at different depths in the growth sequence. The structure is known as CBIRD. This work is a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, with design and growth being performed at JPL, and processing and testing at RVS. We have analyzed the current-voltage characteristics as functions of temperature and junction area, and have measured the spectral response and quantum efficiency as functions of bias voltage. From the temperature dependence of the dark current in a typical case, we infer that the effective barrier height is 0.175 eV. This indicates that dark current is limited by the barriers rather than diffusion or GR mechanisms occurring within the absorber region where the bandgap is 0.13 eV. The barriers prove to be very effective in suppressing the dark current. In the case of a detector having a cutoff wavelength of 9.24 μm, we find R0A > 105 ohm cm2 at 78 K, as compared with about 100 ohm cm2 for an InAs/GaSb homojunction of the same cutoff. For good photo response, the device must be biased to typically -200 or -250 mV. In this condition we find the internal quantum efficiency to be greater than 50%, while the RA remains above 104 ohm cm2. Thus, the device shows both high RA and good quantum efficiency at the same operating bias. We have also measured the capacitance of the CBIRD device as functions of bias and frequency to help characterize the behavior of the barriers. A 256×256 focal plane array was fabricated with this structure which showed at 78K a responsivity operability of more than 99%.