| A new kind of far infrared detector based on intersubband transitions in semiconductor quantum wells has been made. Detection in both in-plane photoconductive and gate photovoltage modes has been demonstrated in two different quantum well structures at temperatures well above 4K. These mechanisms may allow future iterations of these detectors to fly on satelites without the complication, weight, and cost of liquid cryogens. In photoconductive detection, electrons absorb Terahertz light in intersubband transitions, raising the electron temperature. This change is registered, through the temperature-dependent electron mobility, as a change in the in-plane resistance of the device. Photoconductive detection was observed for a double quantum well structure at temperatures from 5K to over 75K, peaking at 20K. The temperature dependence is in good qualitative agreement with a bolometric model. In gate photovoltage detection, Terahertz light raises electrons from the ground to the excited subband, displacing the electron gas from its unilluminated equilibrium position. This net displacement changes the capacitance between the front gate metalization and the sheet of charge in the quantum well. Also, the rise in electron temperature causes a change in the chemical potential. Both effects contribute to the gate photovoltage. A combined circuit model with a single relaxation time explains features of the data that cannot be explained by either effect alone. Using this circuit model, the relaxation time has been extracted from simultaneous gate photovoltage and absorption measurements on a single wide quantum well with the charge density and electric field varied independently. Two issues limit the application of this model, however. The model nonlinear equation diverges without a solution at low charge densities, and the calculated electron temperatures are unreasonably large. The model equation solutions match the data by the small difference of two larger terms. The mathematical non-robustness of this situation may make the model sensitive to small changes in the input parameters, possibly even to within the experimental error of the parameters. Nonetheless, these measurements indicate that both the polarization and chemical potential effects should be considered for future quantum well gate photovoltage devices and for engineering the thermal design of future Terahertz detectors. |
