To suppress the surface leakage current of InAs/GaSb Type-II superlattice (T2SL) infrared photodetectors, atomic layer deposited (ALD)-Al<sub>2</sub>O<sub>3</sub> passivation effects have been investigated. By using the ALD-Al<sub>2</sub>O<sub>3</sub> passivation layers, surface leakage current was more effectively suppressed than by using chemical vapor deposited-SiO<sub>2</sub> passivation layers. The deposition temperature of ALD Al<sub>2</sub>O<sub>3</sub> played an important role in minimizing the surface leakage current of T2SL infrared photodetectors. We found that the dark current density of mid-wavelength (MW) p-i-n structures was limited by their bulk components with Al<sub>2</sub>O<sub>3</sub> passivation layers deposited at or below 200 °C, while the dark current density increased with the surface leakage when the layers were deposited at 260 °C. From the capacitance–voltage analysis, it was found that the deposition at 260 °C led to a large interface trap density at the Al<sub>2</sub>O<sub>3</sub>/GaSb interface. The results of X-ray photoelectron (XPS) spectroscopy show that the spectra of Sb<sub>2</sub>O<sub>3</sub> decreases while that of Ga<sub>2</sub>O<sub>3 </sub>increases when the deposition temperature increases from 200 to 260 °C. This indicates that the reaction of Sb<sub>2</sub>O<sub>3</sub> with GaSb is thermally enhanced. Based on these results, we conclude that Ga<sub>2</sub>O<sub>3</sub> and/or elemental Sb may lead to an additional leakage path. Hence, suppression of the thermal decomposition of Sb-related oxides during Al<sub>2</sub>O<sub>3</sub> deposition is required to obtain good passivation effects.
A novel quantum-well infrared photodetector (QWIP) with peak responsivity in the mid-wavelength (MW) range was characterized, and the performance of a focal-plane array (FPA) based on the MW-QWIP was investigated. InGaAs/AlGaAs quantum wells were used for the QWIP, resulting in a peak absorption wavelength in the range of 4.5~5.0 μm. The doping concentration and In composition of the well layers were varied to improve the photocurrent of the QWIP. The relationship between the noise of the QWIP and the number of multi quantum well (MQW) layers was also investigated, and the optical gain g was estimated. The noise-equivalent temperature differences (NETDs) of QWIPs with various numbers of MQW layers were calculated, and the optimum number of MQW layers was evaluated. It was found that controlling the In composition of the wells was very effective for improving the photocurrent. As a result, a MW-QWIP FPA with a NETD of 21 mK at an operation temperature of 80 K, an integration time of 16 ms, and F2.0 optics was fabricated.
We investigated the mechanism of the photocurrent transmission in mid-wavelength quantum-well infrared photodetectors that were made using InGaAs/AlGaAs quantum wells so that their peak absorption would be at a wavelength near 5 μm. Analyzing the bias-voltage dependence of the photocurrent for the samples with different well layer thicknesses, we found that the photocurrent transmission could be accounted for by taking into account the tunneling process via the triangular barrier, the effect of the intrinsic electric field due to the unintentional impurities, and the effect of the drift velocity.
We have developed a nondestructive evaluation method for HgCdTe. We focused on laser beam induced current (LBIC) which features a high specific resolution and nondestructive evaluation. The LBIC technique shows the electrically active regions in HgCdTe wafer as an image. We have considered the measurement temperature versus the LBIC signal. The LBIC technique at room temperature (300 K) can be used to evaluate non-uniformities in carrier concentrations in HgCdTe more sensitively. Using etch pit studies and secondary ion mass spectroscopy (SIMS), we have identified that non-uniformities of carrier concentration in the HgCdTe wafer arise from metal impurities around dislocation clusters. This nondestructive technique is useful for screening HgCdTe wafers before fabricating devices.
We developed a technique for growing Hg<SUB>0.7</SUB>Cd<SUB>0.3</SUB>Te on CdTe/sapphire. Using metal organic chemical vapor deposition (MOCVD), we grew CdTe on sapphire substrates. We use a combination of isothermal vapor phase epitaxy (ISOVPE) followed by liquid phase epitaxy (LPE) to grow Hg<SUB>0.7</SUB>Cd<SUB>0.3</SUB>Te on them. The ISOVPE converts the surface of the CdTe layer to Hg<SUB>0.7</SUB>Cd<SUB>0.3</SUB>Te, which decreases the lattice mismatch between CdTe and HgCdTe and decreases the surface defects on HgCdTe that originate from lattice mismatching. After growth, we annealed the wafer in a Hg atmosphere to control the carrier concentration. We used the performance of photovoltaic detectors to examine the wafer quality. A typical diode with a 5-micrometers cutoff wavelength had a responsivity (Re) value or 2.87 A/W. The diffusion current limits R<SUB>0</SUB>A down to 120 K, and the generation- recombination (g-r) current limits R<SUB>0</SUB>A at 77 K.