Newly demonstrated 320 x 256 focal plane array uses quantum-dot-based detectors
For applications including night vision, missile tracking, and environmental monitoring, there is interest in developing mid-wavelength infrared (MWIR, 3–5μm) and long-wavelength (LWIR, 8–12μm) focal plane arrays (FPAs).1–3 Presently, many high performance MWIR and LWIR detectors are based on mercury cadmium telluride (MCT). Due to a dramatic change in the bandgap as a function of material composition, obtaining large-area homogeneous materials suitable for LWIR FPAs is a major challenge. In contrast, mature materials-growth technologies for III–V semiconductors can provide very-accurate control of compositions and homogeneity. There is, therefore, interest in developing IR photodetectors using III–V materials.
One of the most successful III–V semiconductor LWIR detectors is the quantum-well infrared photodetector (QWIP),4 which employs the intersubband or the subband-to-continuum transitions in quantum wells. The major drawback of n-type QWIPs is that they cannot detect normally-incident light due to the restriction of selection rules for the optical transition. (A grating or an opto-coupler must be integrated with the QWIP to get light into the device, which makes fabrication more complex, and hence more costly). In contrast, the intersubband optical transitions in quantum dots (QDs) do not have this restriction due to three-dimensional quantum confinement. Theoretically, quantum-dot infrared photodetectors (QDIPs) and DWELL detectors offer several advantages over QWIPs, including lower dark current (hence higher-temperature operation), higher responsivity, normal incidence detection, and improved radiation hardness.5–8
We report the first long-wave infrared quantum dot FPA, based on a voltage-tunable InAs/InGaAs/GaAs DWELL structure. Here, multiple layers of InAs quantum dots are placed in In0.15Ga0.85As wells, which in turn are placed in a GaAs matrix. These detectors have lower dark current, demonstrate good control over the operating wavelength, and have enhanced quantum confined Stark effect (QCSE).7 The photocurrent spectra of the detectors reveal two peaks originating from the same active region: 5.5μm for lower biases and at 8–10μm for higher biases. Using calibrated blackbody measurements on single pixels, specific detectivities (D*) were estimated to be 7.1 × 1010cmHz1/2W (Vb=1.0V) and 2.6 × 1010cmHz1/2/W (Vb = 2.6V) at 78K for the MWIR and LWIR band, respectively.
Bias-dependent spectral response curves for a pixel with a 100μm-diameter aperture, measured using a Nicolet 870 Fourier-transform infrared spectrometer (FTIR), are shown in Figure 1. The spectra contain peaks centered in the MWIR, the LWIR regime, and the VLWIR regime. Subsequently, a 320×256 QDIP FPA array was fabricated using the DWELL material and was hybridized with an Indigo 9705 read out integrated circuit (ROIC). After the deposition of a nitride planarization layer, the substrate was removed and indium was evaporated onto the metal contacts of each pixel. The array was then bonded to the ROIC using flip-chip (indium bump) technology. After bonding, backfilling was used for mechanical stability. One important point to note is that the thickness of the epitaxial layer was not optimized to maximize the Fabry-Pérot cavity resonance. Due to the thin epitaxial layer, the cavity-resonance cut off was estimated to be 6μm, which lead to a reduced response in the LWIR regime.
Thermal imaging was undertaken at an estimated FPA temperature of 80K, using different optical filters between 3–5μm, and 8–12μm. Figure 2 shows an image of one of the scientists testing an FPA, fabricated using a different QDIP wafer with a similar design.
It should be noted that this FPA has not been optimized. Mechanical thinning, coupled with a lack of anti-reflection coating, has caused optical losses such as scattering and reflection. Moreover, the thin cavity has reduced performance in the LWIR regime. In addition, the ROIC was not optimized for the given QDIP device. Nevertheless, the excellent imagery obtained from these devices marks a promising beginning of the development of QD FPAs.
The authors would like to acknowledge support from AFRL Contract F29601-01-C-0156, and National Reconnaisance Organization and NSF ECS Grants 0428756/ 0401154.
