A Study of Semiconductor Quantum Structures by Microwave Modulated Photolumenescence
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ABSTRACT Mixed types I - type II multiple quantum wells (QW) structures consist of alternating narrow- and wide GaAs wells, separated by AlAs barriers. Transfer of electrons from the narrow- to the wide well results in the formation of a two-dimensional electron gas (2DEG) in the wide wells and a hole gas (2DHG) in the narrow ones. The present study investigated the effect of these gases on the various photoluminescence (PL) bands. The study utilized two modulations techniques: double beam PL and microwave modulated PL (MMPL), offering high-resolution spectroscopy, control of the 2DEG density and effective temperature. The results showed that the formation of a low density 2DEG in the wide wells cause the formation of trions. However, a large density of excess electrons makes mutual collisions with other photo-generated species, causing the dissociation of the trions and excitons. In addition, electrons transfer through the barrier gives rise to barrier-well indirect recombination emission. INTRODUCTION The optical properties of quantum well (QW) structures, containing a two-dimensional electron gas (2DEG) and a two-dimensional hole gas (2DHG), have a scientific and technological
importance
[1,2,3].
Primarily, these properties
have been examined
in a
modulation-doped quantum wells (QW), where the electron density is controlled by applying a bias voltage [4]. Alternatively, the influence of 2DEG has been studied in mixed type I - type II QW's (MTQW) structure (vide infra), controlling the electron-density (ne) by optical pumping [5,6,7,8]. The present study utilized MTQW's structure that is drawn schematically in figure 1. This structure consists of a sequence of alternating narrow- and wide GaAs wells, separated by AlAs barriers [1]. Furthermore, it is designed in such a way that the lowest F state of the GaAs narrow well (NW) is higher in energy than the lowest X state of the adjacent AlAs layer, leading to type H1alignment. However, the last X state is energetically higher than the F state of the wide GaAs well (WW), creating type I alignment. Then, photocreated electron-hole pairs become spatially separated by rapid F-X-F electron transfer from the NW to the WW. It is already known that the initial F-X transfer occurs on a sub-picosecond time scale, while the subsequent X-F electron transfer takes about 30 picosecond [2]. Conversely, the hole tunneling through the barrier occurs in the millisecond range, thus leading to a temporary accumulation of the 2DEG in the WW. The present study describes our attempts to follow the interactions of the 2DEG and 2DHG with other photogenerated species. The results revealed the existence of the following events: (a) attachment of an excess electron to an existing exciton in the WW, to form a negatively charged specie, named a trion [5,6]; (b) direct recombination of an ensemble of free electrons with an existing hole within the WW, forming a plasma-like emission band [9]; (c) further collision of an excess electron with an existing exciton or trion within the WW, resulting
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