Interplay of the Trion Singlet and Triplet State Transitions in Magnetooptical and Time-Resolved Investigation of ZnSe/Zn(S,Se) Single Quantum Wells

Abstract

Negatively charged trions are investigated for ZnSe single quantum wells embedded in ternary barriers. Polarization and excitation dependent photoluminescence measurements were performed in magnetic fields up to 11.8 T. From these measurements we are able to develop an energy-term scheme for the spin-singlet and the spin-triplet state of the trion.Introduction If a neutral exciton X binds a second electron (hole), a negatively (positively) charged exciton X À (X þ ) is formed which is called trion. It was first identified in CdTe/(Cd,Zn)Te quantum wells (QWs) in 1993 by Kheng et al., since then intense investigations mainly on GaAs-and CdTe-based QWs were performed (e.g. [1-3]).Since X À consists of one hole and two identical electrons Pauli's exclusion principle tells us that the total wavefunction must be antisymmetric, which can be factorized into an antisymmetrical (symmetrical) spin part and a (anti)symmetrical spatial function. The antisymmetric spin function obtained for antiparallel electron spins results, together with the hole spin, in the twofold degenerate singlet state X À s whereas parallel electron spins yield the sixfold triplet state X À t . The degeneracy can be lifted by applying a magnetic field (Zeemann splitting). While the X À s state is bound at any applied field, the X À t is unbound at low magnetic fields and becomes stabilized beyond a particular B field value [3,4].Several groups performed experiments to clarify the term scheme of X À t [4][5][6]. In ZnSe based quantum wells, X À t state transitions have been investigated by few groups including ourselves [7,8], however, signatures of the triplet state transition X À t were only reported by [7][8][9]. An X À t term scheme based on magnetooptics has not been proposed up to now and is the object of this paper.

Excitons in Wide-Gap Semiconductors: Coherence, Dynamics, and Lasing

“…Up to now, an increasing manifold of studies has been presented (see, e.g., the contributions to the first conference devoted to trions and organized by Zimmermann in 2001 [11]. In ZnSe-based structures, rather few studies have been conducted so far [12][13][14][15] although the trion binding energy is somewhat increased compared to CdTe or III-V structures.…”

“…We have shown that in particular in ZnSe/Zn(S,Se) QW structures with nominally undoped or lightly Cl doped barriers X --emission can be observed allowing not only for the singlet X s --but also for the triplet state X t --with a total momentum of 5/2 [15,16]. Both transitions clearly gain intensity for increasing magnetic field, with X t --being stable for fields larger than 8 T only.…”

“…Recently, trionic gain and lasing has been claimed to be observed in the low-energy wing of the trion luminescence in d-doped ZnSe QWs at low temperatures (2 K) [15]. Gain values up to 10 4 cm --1 were reported.…”