Photoconductive Properties Of A GaAs-A1GaAs Quantumwell Infrared Photodetector
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PHOTOCONDUCTIVE PROPERTIES OF A GaAs-A1GaAs QUANTUM-
WELL INFRARED PHOTODETECTOR TOSHIYUKI UEDA, KOJI SHINOHARA, and YOSHIHIRO MIYAMOTO Fijitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya,
Atsugi, 243-01
Japan
ABSTRACT We investigated the conduction mechanism and performance of GaAs-A1GaAs quantum-well infrared photodetectors and proposed a conduction model consisting of Poole-Frenkel-like emission from wells and carrier relaxation wells. The model agrees with the measured photocurrent and dark current. The detector's noise current is given by a simple shot noise formula. Analysis has shown that the photocurrent and the noise current are inversely proportional to the barrier width, while the dark current is inversely proportional to the square of the barrier width.
INTRODUCTION Recently, a GaAs-AlGaAs quantum-well infrared photodetector was proposed, which is sensitive to long-wavelength infrared radiation [1]. The response peak wavelength of this detector corresponds to the intersubband transition within a quantum well, and can be changed by adjusting the quantum well's structure. In addition to this, because of well-established fabrication technology, this quantum-well detector is one of the most promising candidates for large-scale infrared area imagers. The quantum-well infrared photodetectors were thought to be a kind of photoconductor. However, measured photocurrent and dark current conduction were quite different from those of conventional photoconductors. In this paper, a carrier transport model is proposed. This carrier transport model predicts the measured photocurrent and dark current and their dependence on the detector structure. By deriving current formula, we gathered insights into the detector and its performance.
EXPERIMENTS Two types of quantum-well infrared photodetectors were fabricated. The quantum wells were grown by metalorganic chemical vapor deposition (MOCVD). Both quantum wells consist of fifty periods of Mat. Res. Soc. Symp. Proc. Vol. 228. 1.1992 Materials Research Society
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4 nm-wide quantum wells. Sample A has 60 nm-wide barriers, and Sample B has 90 nm-wide barriers. The wells are made of n-type GaAs doped to a concentration of 2x10 18 cm- 3 . The barriers are made of undoped AlGaAs with an Al content of 0.34. The well width and barrier height were designed using the envelope-function approximation [2] to have a response peak at 7.1 gtm . The quantum wells are sandwiched by doped GaAs layers to form ohmic contacts. Photodetectors having a 200 by 1950 gim mesa structure were fabricated using conventional wet etching. The photo-sensitive area was 200 by 1450 jim. The dark current was measured using a cold shield at 77 K and the photocurrent was measured using a 500-K black body with incident angle of 12 degrees.
RESULTS AND DISCUSSION Figure 1 shows the measured photocurrent while figure 2 shows the dark current. This conduction properties differ from those of conventional photoconductors. To explain this difference, a carrier transport model is proposed. We proposed the fo
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