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.
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 investigated the behavior of the dark current (Id) in quantum well infrared photodetectors (QWIPs) in which the barrier layers were selectively doped instead of the well layers. Because the selective doping bends the conduction band (CB) edge in the portion of the barrier near the interface, the mechanism by which carriers in the wells can be emitted over the barriers, i.e. thermal emission and tunneling through this portion of the barrier, could be emphasized. We first confirmed that selectively doping the barrier layers clearly affects the Id-V characteristics. Then, by evaluating the activation energy obtained from the temperature dependence of Id, we found that the Poole-Frenkel emission (PFE) mechanism and the thermal-assisted tunneling (TAT)-like mechanism are dominant in the lower bias and higher bias regions, respectively.