Exciton Trions in II-VI Heterostructures

Abstract

Optical spectra associated with transitions that create or annihilate charged excitons X -or X+ can be observed in quantum well heterostructures containing an electron gas or a hole gas, respectively. A review is given of properties of trion states in CdTe quantum wells in zero field and of the magnetic field-dependence of the circular polarization and oscillator strength of the trion optical resonance. The possibility that disorder is needed to stabilise trion states in concentrated 2D electron or hole systems is discussed.PACS numbers: 73.20. Dx, 78.66.Ηf, 71.35.Ji, 78.20.Ls In 1958, M.A. Lampert [1] extended the analogy between the exciton Χ in a semiconductor and the hydrogen atom H in free space by postulating the existence of two types of charged exciton species or "trions" . The first is X -(two electrons bound to one hole). For mh » me , this is a close analogue of the H -ion where two electrons with antiparallel spins move in a very diffuse orbital around a proton. The second species is X+ (two holes and one electron). For mh » me , this is analogous to the Η2 molecular ion where one electron moves in a molecular orbital around two protons (Lampert suggested the nomenclature X2 , but "X+" has become more usual).Both kinds of trion have only a very small binding energy and their observation in bulk semiconductors has proved very elusive. However, the calculated binding energy of trions is enhanced by an order of magnitude in 2 dimensions [2]. Moreover, an excess of electrons (or holes) can readily be produced in a quantum well heterostructure by modulation doping, favoring the formation of X - (or X+) in the well.The first clear identification of the trion X -was made in 1993, in the optical spectra of modulation-doped CdTe quantum wells containing 10 10 to 10 11 electrons/cm2 [3]. Trions X- [4] and X+ [5] were then identified in GaAs (99)

Graded Quantum Well Structures Made of Diluted Magnetic Semiconductors

“…Such behavior was actually observed by us [41]. Elements f similar behavior was also observed by Cox et al [33], however, the electron concentration in their sample being much higher than our upper range limit f 2DΕG concentrations, the X -line could not be observed by them at Β = 0 but only above 4.5 T. Therefore, the full complexity of the changes f polarization behavior were not observed in Ref. [33].…”

“…In the PL spectrum two peaks are seen: the one at higher energy is due to excitons localized by the well width fluctuations and the lower energetic peak is due to recombination f negatively charged excitons X - [32] (for a review f X -complexes see Ref. [33]). With an increase in n (spectra from the region with n = n0) a strong X -line appears in the reflectivity and the X -emission dominates the corresponding PL spectrum.…”

Positively versus negatively charged excitons: A high magnetic field study of CdTe/Cd1−xMgxTe quantum wells

“…The negatively charged species T − are studied in detail in various material systems, such as GaAs/(Al,Ga)As, [3][4][5] CdTe-based, 6,7 and ZnSe-based [8][9][10] structures. Magnetooptical techniques play a key role in these investigations and allow one to obtain comprehensive information about the energy and the spin structure of the T − complex.…”

Binding energy of charged excitons in ZnSe-based quantum wells

“…Since, positively-and negatively charged excitons have been studied experimentally in III-V heterostructures based on GaAs and in II-VI quantum well structures based on CdTe, (Cd,Mn)Te, ZnSe and (Zn,Mn)Se (see e.g. [3,4,5,6] and references therein).…”