Observation of coupled LO phonon-intersubband plasmon modes in GaSb/InAs quantum wells by resonant Raman scattering

Abstract

Two new peaks are observed in resonant Raman scattering from GaSb/lnAs quantum wells grown on (001) GaAs by MBE. These two lines are assigned to the coupled LO phonon-intersubband plasmon modes originating from the lnAs wells. The lower-frequency branch of the coupled system (L-mode) lies between the LO and the TO frequencies of InAs, and the line intensity depends strongly on the two-dimensional carrier concentration and the laser excitation energies (resonant near the E, gap of InAs). The L, line'is weak and relatively broad. Its frequency increases with increasing carrier concentration in the lnAs wells and is not observed when the carrier concentration is low.

Intersubband Raman spectroscopy of two-dimensional electron gases in GaSb/InAs quantum wells

“…Figure 1 shows the polarized (z(x x )z) and the depolarized (z(x y )z) spectra for a single-well sample (IC411) excited with the 514 nm laser line. This sample was studied previously in [10] and has a thinner cap layer (100 Å) than the other single-well samples investigated. A thin cap is particularly favourable for the observation of the weak intersubband plasmon modes because the GaSb surface donors generate a higher density of carriers in the InAs well and a thick GaSb cap layer can severely attenuate the laser excitation (for example, for the 514 nm line, the optical depth given by 1/2α is ∼ 100 Å [26]).…”

“…Generally the 514 nm line of the Ar + laser was used. Although this line is somewhat shifted away from the peak of the E 1 resonance of InAs [10], the high sensitivity of the Renishaw instrument [24] makes it possible to observe electronic Raman scattering. A few Raman spectra excited with 488 nm laser line (closer to the E 1 gap of InAs) were recorded with a T64000 Jobin-Yvon spectrometer [25].…”

“…However, for InAs-based quantum well systems, the easily accessible bandgaps in resonant Raman scattering are the E 1 and E 1 + 1 gaps. Only coupled LO phonon-intersubband plasmon modes which may couple to the light via the electro-optic, deformation potential and Fröhlich mechanisms can be observed [10][11][12][13].…”

“…Since our first observation of coupled LO phononintersubband plasmon modes in GaSb/InAs quantum wells [10], two other groups have reported the observation of these coupled modes in the similar AlSb/InAs system [11,12]. These measurements were restricted to a few single quantum well samples having a limited range of parameters such as carrier concentrations and well widths.…”

“…Both single and double quantum wells with a variety of well widths are studied. Two different Raman systems are employed, both having a large improvement in sensitivity over our previous system [10]. The coupled LO phononintersubband plasmon modes, L 23+ and L 13+ , associated with higher-energy intersubband transitions in the InAs well are detected for the first time.…”