Subpicosecond Luminescence Study of Capture and Intersubband Relaxation in Quantum Wells

Abstract

We have studied the mechanisms of capture and intersubband relaxation in quantum wells by time resolved luminescence with subpicosecond resolution. The quantum mechanical capture time does not show strong oscillations due to the competing influence of different capture mechanisms. Intersubband relaxation is measured with times of the order of 1 ps, for well widths of the order of 100 Å.The behaviour of quantum well based devices, and particularly of quantum well lasers or quantum well amplifiers, relies on the two dimensional character of the density of states. However, in most of these devices, active electrons and holes are injected by external contacts into the barrier layers. Therefore, correct operation of such devices rely on the precise mechanisms of capture and intersubband scattering. Time resolved techniques, and particularly luminescence, provide very interesting tools to get direct information on the times involved in such processes.In quantum wells, capture mechanisms as well as intersubband scattering have not yet been fully characterized. Both of these should show a well defined variation as a function of the well thickness. In the case of capture, deep oscillations have been predicted when the well thickness is varied, as a result of the profound changes in the overlap of the wavefunctions of the states confined in the well and of those above the barriers 1. In the case of intersubband transitions, variation of the q vector of the emitted phonon changes the relaxation rate by one order of magnitude when the thickness of the well goes from 200 Å to 60Å 2. Furthermore, very large differences should be observed as soon as it is not any more possible to emit one LO phonon.Luminescence allows quite easily to monitor the capture process as the barrier luminescence basically disappears as fast as the carriers are captured into the well. We show here that the capture time does not show the expected oscillations and we shall try to explain why.Intersubband scattering is more difficult to monitor because of the necessary presence of holes to observe luminescence. We will show how the use of coupled quantum wells allows to overcome this difficulty. Strong differences are observed depending wether one optical phonon can be emitted or not.KeywordsOptical PhononCapture ProcessBarrier ThicknessCapture TimeQuantum Mechanical DescriptionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.