Influence of Collective Effects on the Linewidth of Intersubband Resonance

Abstract

We have measured the intersubband resonances of an InAs͞AlSb quantum well with two occupied subbands from cryogenic temperatures to well above room temperature. The higher energy mode is very robust with increasing temperature; the lower energy mode, however, broadens above 200 K. We explain the results in terms of Landau damping and argue generally that the collective nature of the intersubband resonance is crucial for an understanding of the scattering mechanisms that determine the intersubband resonance linewidth. [S0031-9007(98)05471-4] PACS numbers: 73.20.Dx, 73.20.Mf, 78.66.Fd Intersubband resonance (ISR) is a fundamental excitation of a low-dimensional semiconductor system. In a single-particle picture, the resonance corresponds to the transition between two quantized states. However, ISR is not a single-particle process [1,2]. Instead, ISR is a collective phenomenon better described as a plasmon, or charge-density excitation. The most obvious consequence of the collective effects is a shift of the ISR away from the energy separating the single-particle states. This shift tends to be only a small proportion of the resonance energy as the direct electron-electron interaction (depolarization field) and the exchange-correlation interaction (exciton effects) cause blueshifts and redshifts, respectively, and tend to cancel [3,4].Recently, the collective effects have been dramatically revealed by studying systems with a broad single-particle density of states. Nevertheless, for large densities the ISR is a single, narrow line. For instance, in a system with a highly nonparabolic energy dispersion, the single-particle transition energy is a strong function of wave vector k, being smaller at the Fermi wave vector k f than at k 0. The single-particle spectrum is then broad, yet the ISR is a sharp peak [5][6][7]. The collective effects condense all the available oscillator strength into a single mode. A similar effect also occurs in weakly coupled quantum wells which have a broad and complicated single-particle spectrum yet a narrow ISR can be observed [8,9]. In the nonlinear regime where ISR is excited with a very intense source, optical pumping of carriers induces a redshift as the collective effects weaken [10].These experiments show very convincingly that ISR is indeed a collective phenomenon. It has been argued that in the best samples the resonance is homogeneously broadened [11,12], in which case one can pose the question: What are the scattering mechanisms which destroy the coherence of the plasma oscillation? Surprisingly perhaps, a microscopic theory to answer this question does not seem to exist. In fact, the ISR linewidth is usually described with the single-particle scattering rates [11,12], ignoring the collective nature of the resonance. Calculations of the ISR width at low temperature in Si͞SiO 2 from charged ion scattering [13] suggest that this approach is likely to be misleading. Generally, it appears to be unclear how the prevalent single-particle picture of electron scattering, particularly w...

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