ZnSe MQWs

JOURNAL OF

LUMINESCENC ELSEVIER

Journal

of Luminescence

72-74

(1997) 869-870

Circularly polarized exciton emission and fine structure of the luminescence band in Znl - ,Cd,Se/ZnSe MQWs A. Reznitsky”, A. Kornievsky, S. Permogorov, L. Tenishev, S. Verbin, A.Yu. Kaminski, S. Ivanov, S. Sorokin Russian Academy of Science, A.F. I&e

Ph.vsical-Technical

Institute, 194 021 St.Petersburg,

Russian Federation

Abstract Circular polarization of exciton photoluminescence (PL) in MBE grown ZnCdSe/ZnSe MQWs at T = 2 K has been studied in Faraday configuration up to 7 T. We have found that in strong magnetic fields the PL band shape is sensitive to polarization sign of circularly polarized exciting light. On this basis, we were able to decompose the PL band into two components of approximately equal intensity and width (FWHM = 12 meV) but with the maxima separated by 7 meV. In accordance with different field dependence of polarization degree for the two bands, the high-energy component has been assigned to emission of excitons localized by compositional and/or well-width fluctuations (LE), while the other component was attributed to excitons bound to neutral donors (D’x). Electron and hole g-factors as well as the relaxation times and parameters of energy-spin diagram for LE have been estimated. Keywords:

Exciton emission; ZnCdSe/ZnSe

MQWs; Spin structure

We have studied the PL spectra of laser diode structures with active region formed by 5 Zn,.8Cd,.zSe QWs (d,/d,, = 716 nm). Very bright QW PL is strongly circularly polarized even at

B = 0 at circular excitation both above and below the band gap of ZnSe. Spectral dependence of Pcirc (Fig. 1) indicates the complex nature of the PL band. We have decomposed it into two bands using two spectra of total emission intensity I,,, = (I+ + I_) taken at different polarization signs of excitation light in a strong magnetic field. Similar doublet structure was also observed in Ref. [l], in which the band A has been assigned to LE emission

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while the band B to emission of D’x. Spin level diagrams in magnetic field for LE and D”x states are shown as insets of Fig. 1. As it can be seen from Fig. 1, I,,, for D”x is not affected by the sign ( + or - ) of optically populated state, whereas for LE optical pumping of + 1 or - 1 can result in different I,,, if electron and hole spin relaxation times r: and rt differ significantly and the following nonequality for LE radiative lifetime rr,d holds: r rad

z

min(ri, rf) 6 max(rt, r,“).

This interpretation of A and B bands is confirmed by the field dependence of magneto-circular polarization of PL (MCPL). MCPL reflects the difference in thermal population of spin levels split by a magnetic field [2]. The two bands have quite different field dependence of Pcirc (Fig. 2(a)). For

870

A. Rrznitsb,

et ul. 111 Journal of’Luminescence

h E

2.53

2.54

2.55

Energy,

2.56

A CD L al

2.57

eV

Fig. 1. Decomposition of MQW PL spectrum (solid line) into two bands (dotted lines). The symbols indicate the spectrum of circular polarization degree pcirc = (I+ - ]_)/(I+ + I_). Insets show the energy levels for D’x and LE states in Faraday configuration of magnetic field. Dotted, thick and thin arrows show spin relaxation of hole and electron and optical allowed transitions, respectively.

A-band pCi,,has nonmonotonous dependence on B. We have attributed this behavior to anticrossing of exciton spin sublevels in the magnetic field. By fitting the experimental dependence of pcirc(B) (curve 2 in Fig. 2(a)) with the model of Ref. [2], we have estimated the relaxation times for spin sublevels (numbers in brackets) and obtained LE spin diagram (Fig. 2(b)) with the parameters indicated in the caption. The estimated relaxation times of LE as expected satisfy the relation

For B-band Pcirc is proportional to B with the effective g-factor geff = 0.18 + 0.02. True g-factor of the unpaired hole in D”x complex is given by where r, and ~~ are spin YD”X = geff(l + rJ2ro), relaxation and total lifetimes of D’x, respectively.

:

Magnetic

field,

T

Fig. 2. (a) Field dependence of MCPL signals at two spectral points indicated by arrows 1 and 2 in Fig. 1. Solid lines show model fit of experimental points. Values of T:, ~,h and nonradiative lifetime rnr (in units of T& are indicated in brackets. (b) Energy diagram of LE spin sublevels with (solid) and without (dots) anticrossing effects. Parameters of model [Z] used for fitting are
Using for a hole in D’x-center TJZ~ = 15 (as for LE), we obtain the estimation for D”x g-factor 1.5 f 0.2. gD”x = This work was partially supported by INTAS Foundation (grants 94-324 and 94-481).

References [I]

S. Permogorov Semiconductors, [2] E.L. Ivchenko (1995) 768.

et al., in: Proc. 23rd Int. Conf. on Physics of Berlin, (1996) to be published. and A.Yu. Kaminskii, Phys. Sol. State 37