Dual-band infrared photodetectors with a modified pBp design have been demonstrated. The modified pBp structure consisted of a p-type InAs/GaSb superlattice for long-wavelength (LW) detection and a p-type InAs/GaSb/AlSb based superlattice for mid-wavelength (MW) detection, which were separated by a hole barrier consisting of an InAs/AlSb superlattice. Our pBp device showed that dual-band detection was possible by changing the bias polarity of the applied voltage. By using an InAs/GaSb/AlSb based superlattice as an MW absorber for a pBp photodetector, a 100 % cutoff wavelength was blue-shifted from 8 μm to 7 μm compared with a conventional InAs/GaSb superlattice, while maintaining the same 50 % cutoff wavelength of around 6.4 μm. Quantum efficiency per period of the modified MW absorber was comparable with that of a conventional MW absorber. These results indicate that our modified pBp structure is expected to be a promising candidate for dual-band infrared photodetectors.
To suppress the surface leakage current of InAs/GaSb Type-II superlattice (T2SL) infrared photodetectors, atomic layer deposited (ALD)-Al2O3 passivation effects have been investigated. By using the ALD-Al2O3 passivation layers, surface leakage current was more effectively suppressed than by using chemical vapor deposited-SiO2 passivation layers. The deposition temperature of ALD Al2O3 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 Al2O3 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 Al2O3/GaSb interface. The results of X-ray photoelectron (XPS) spectroscopy show that the spectra of Sb2O3 decreases while that of Ga2O3 increases when the deposition temperature increases from 200 to 260 °C. This indicates that the reaction of Sb2O3 with GaSb is thermally enhanced. Based on these results, we conclude that Ga2O3 and/or elemental Sb may lead to an additional leakage path. Hence, suppression of the thermal decomposition of Sb-related oxides during Al2O3 deposition is required to obtain good passivation effects.
In the development of InAs/GaSb Type-II superlattice (T2SL) infrared photodetectors, the surface leakage current at
the mesa sidewall must be suppressed. To achieve this requirement, both the surface treatment and the passivation layer
are key technologies. As a starting point to design these processes, we investigated the GaSb oxide in terms of its growth
and thermal stability. We found that the formation of GaSb oxide was very different from those of GaAs. Both Ga and
Sb are oxidized at the surface of GaSb. In contrast, only Ga is oxidized and As is barely oxidized in the case of GaAs.
Interestingly, the GaSb oxide can be formed even in DI water, which results in a very thick oxide film over 40 nm after
120 minutes. To examine the thermal stability, the GaSb native oxide was annealed in a vacuum and analyzed by XPS
and Raman spectroscopy. These analyses suggest that SbOx in the GaSb native oxide will be reduced to metallic Sb
above 300°C. To directly evaluate the effect of oxide instability on the device performance, a T2SL p-i-n photodetector
was fabricated that has a cutoff wavelength of about 4 μm at 80 K. As a result, the surface leakage component was
increased by the post annealing at 325°C. On the basis of these results, it is possible to speculate that a part of GaSb
oxide on the sidewall surface will be reduced to metallic Sb, which acts as an origin of additional leakage current path.
We present a numerical and experimental study of the generation of harmonic mode locking in a silica toroid microcavity. We use a generalized mean-field Lugiato-Lefever equation and solve it with the split-step Fourier method. We found that a stable harmonic mode-locking regime can be accessed when we reduce the input power after strong pumping even if we do not carefully adjust the wavelength detuning. This is due to the bistable nature of the nonlinear cavity system. The experiment agrees well with the numerical analysis, where we obtain a low-noise Kerr comb spectrum with a narrow longitudinal mode spacing by gradually reducing the input pump power after strong pumping. This finding clarifies the procedure for generating harmonic mode locking in such high-Q microcavity systems.
Recently, quantum dot infrared photodetectors (QDIP) have been intensively investigated because they can be
fabricated by conventional matured GaAs processing. QDIP can detect normal incident light in contrast to quantum well
infrared photodetectors (QWIP) which need optical grating or reflector. Also QDIPs operate at higher temperature,
taking advantage of their lower dark current theoretically than that of QWIPs.
In this report, we describe our effort to realize long-wavelength infrared (LWIR) QDIP infrared focal plane array
(IRFPA), which uses molecular beam epitaxially grown self-assembled quantum dot (SAQD) multilayers. We have
successfully "engineered" the transition levels of SAQDs to LWIR (8-12 μm) energy region, where relatively lower
quantum levels were pushed up near the conduction band edge of AlGaAs intermediate layers. In addition, these SAQD
multilayers bring QDIP responsivity enhancement due to their higher dot density.
We applied this structure to 256×256 pixel LWIR QDIP IRFPA. As a result, we realized the response peak
wavelength of 10 μm and noise equivalent temperature difference of our newly developed QDIP was 87 mK at 80 K, 120
Hz frame rate with f/2.5 optics. We obtained the excellent quality of IR image and confirmed our QDIP's high sensitivity
and high speed operation.