Study of voltage tunable asymmetric quantum well structure for infrared detection

The performance of GaAs/AlGaAs asymmetric quantum well infrared detectors exhibiting inter subband absorption has been studied. The asymmetric quantum well structure considered for the present study consists of two regions, a step quantum well with a 100Å Al.18Ga.82As step and 80Å GaAs well, followed by a double barrier tunneling structure with 50Å Al.4Ga.56As barriers and a 40Å Al .2Ga.8As well. The quantum well structure has three bound to bound energy levels, one below the step (E1) and the other two above the step (E2 & E3). Transitions from E 1→E2 and E1→E3 are employed for dual band detection. The energy difference E 1→E2 and E1→E3 changes with the applied voltage due to stark effect. The absorption coefficient calculated at zero bias reaches the peak at 11μm and 17μm corresponding to the transitions from E1→E3 and E 1→E2 respectively. However with increasing applied bias the absorption coefficient peaks for smaller wavelengths. Fig.2 shows the variation of absorption coefficient with respect to wavelength at different bias voltage. Resonant tunneling occurs at 20mV and 80mV for E 1→E3 and E1→E2 transitions corresponding to 10μm and 14μm respectively. Photo generated carriers then tunnel through the double barrier structure. Thus the photocurrent is generated by inter subband absorption followed by resonant tunneling phenomena. The responsivity reaches a maximum of 0.37A/W at 20mV corresponding to 10μm and 0.48A/W at 80mV corresponding to 14μm. The calculated detectivties are 1.066×106 (cm√Hz/W) at 20mV and 1.319×10 6 (cm√Hz/W) at 80mV.