Application of the polarized photoluminescence method to the characterization of superlattice corrugations

Abstract

Unique polarization properties of the exciton luminescence in semiconductor superlattices (SL) are the subject of a great current interest. Polarization effects in SLs may be caused by different mechanisms, such as formation of twodimensional subbands, exciton localization, existence ofthe microreliefat heterointerfaces, etc. [1-3]. We have investigated the polarization properties ofthe photoluxriinescence (PL) spectra of GaAs-AlAs superlatlices in a wide range ofquantum well and barrier widths. The sample parameters, i.e. well width d, the barrier width d, the number ofperiods N, the type of SL, and the corresponding degrees oflinear polarization are listed in table 1. In both direct-gap SLs-I (d <4 n.tn) and indirect-gap SLs-II (d <4 nm) the polarization of PL lines is caused by the splitting of heavy and light hole bands and the anisotropy ofheavy holes, but this effect alone cannot explain the polarization ofthe luminescence irradiated in the direction normal to the surface of investigated structure (i.e. at small detection angles). For SLs-I we have observed rather large linear polarization PL=(I-I)/(I.,+IS), where I and I are the intensifies of exciton luminescence polarized in the plane ofdetection and normal to it. In this case polanzation may achieve values ofabout 20%. In SLs-II with narrow wells photoluminescence is caused by indirect exciton transitions between X-electrons of AlAs and r -heavy holes of GaAs. It is known that such excitons are localized at interface inhomogeneities [4]. The value of P1 for these SLs is lower than for the SLs-I (L=5-1° %). We have studied the dependence ofpolarization degree on the angle of PL detection for SL—II with the well width 3.4 run and barrier width 4.0 nm for two cases of crystallographic directions ([1 10] and [1 1 0]). The parameters of such samples were close to the case when direct —indirect crossover is observed. As a consequence of this fact two lines were observed in PL spectrum, one of which is associated with indirect recombination of X-electrons and another with direct recombination of F-electrons. r-Iine is almost unpolarized (L'1)' while X-line is characterized by rather large polarization degree (up to 10%), which decreases with the increase ofdetection angle and changes its sign (for small detection angles) with the 90°-turn of the sample around the axis normal to its surface. This anomalous behavior requires an adequate theoretical explanation. From our point ofview such explanation may be given if the existence of heterointerface corrugations in superlattice is taken into account. At the same time the fact ofsmall broadening of observed PL line (substantially lower than possible line splitting due to monolayer fluctuations in quantum well width) proves a strong correlation between such corrugations on both sides ofeach quantum well, i.e. the shape of corrugations in heterointerfaces in superlattice is determined by the shape of corrugations on the surface ofthe substrate.