Resonance capture of holes in modulation-doped n-AlGaAs/GaAs quantum well structures

Capture of excess carriers from widegap layers into a quantum well plays a decisive role in operation of many devices based on semiconductor quantum well structures, such as lasers and photodetectors. The mechanism of capture of excess current carriers into a rectangular quantum well was studied theoretically in (1-3). Quantummechanical calculations showed that probability of capture is a nonmonotonic function of the well width, while, within the classical model, the probability grows monotonically with the well width. The nonmonotonic character is related to the occur� rence of the socalled virtual bound states in the con� tinuum at the well widths at which the highest quan� tum level coincides with the barrier height. Under these conditions, overlap of the carrier wave function at the band edge in a barrier with the wave function of the bound state in the well grows, which leads to an increase in probability of capture of a carrier into the well. For the rectangular quantum wells, the depen� dence of probability of capture on the well width is an oscillating function with a constant period that con� tains resonance maxima at the well widths corre� sponding to the occurrence of the next quantum level (1). According to the results of the calculations, cap� ture rates in the maximum and the minimum can dif� fer by orders of magnitude; holes are captured several times faster than electrons (2). The predicted effect was confirmed experimentally with the use of subpicosecond equipment in (4-6), where the times of capture of electrons into the AlGaAs/GaAs quantum wells of different thicknesses were measured by the luminescence method. The mea� surements verified a resonance character of capture: capture times of electrons in the maximum and the minimum of the oscillations differed by 1.5-2 orders of magnitude, which is in good agreement with the theory. Such a sharp increase in efficiency of capture of excess carriers should lead to a noticeable growth in the intensity of photoluminescence (PL). However, in the first experimental studies of the dependence of PL intensity on quantum well width, no growth of inten� sity at resonance capture was observed (7, 8). The experime ntal observation of this effect creates much difficulty. The main problem is that a halfwidth of the maxima is small, so to reveal them, a large number of structures with a small pitch of variation in quantum well width are required. It is important that the struc� tures possessing the same optical quality and the pro� cesses of nonradiative recombination did not affect the dependence of PL intensity on the quantum well width.