GaSb epilayers

JOURNALUF

LUMINESCENCE ELSEVIER

Journal of Luminescence 6O&61 (1994) 758 791

Time-resolved and CW optical studies of MBE-grown ZnTe/GaSb epilayers M.J. McNamee~~*, R.A. TayIor~’,P.A. Snowa, W. Hayesa, D.E. Ashenford”, B. Lunnh 0 stord L ‘no ersit i’ Departmi’nt of Phi sti s. C larendon / ahorator . Parts Road. 0 sford OX I ~P L . 1. K 7R k 1. K Department of Enqim ertnq i)e,stqn and ~rlunu/acture, L nit cr50 v of Hull, Cotttnqham Road Xorth llutnherstde Ht 6

Abstract

We report optical studies of MBE-gro~nZnTe GaSb. Main impurities and free exciton resonances are determined, and polariton phonon scattering is observed. We also report time-resolved photoluminescence studies. yielding an estimate of the LO phonon lifetime in ZnTe.

I. Introduction In epitaxial ZnTe, the complex nature of the optical properties of what is essentially bulk material has led to a variety of assignments of the prevalent impurities and residual strain, these having been more clearly defined in the last year [1,2]. However, the extent to which the polariton nature of excitations is important in such samples has been assumed rather than investigated. We report a variety of optical measurements on [1 00]-grown ZnTe GaSb epilayers and discuss the influence of both strain and polariton effects on the measured data.

2. CW optical measurements 2.1. FL, FLE and reflectance

The samples were grown in the [1 00] direction by molecular beam epitaxy (MBE). Three samples

*

Corresponding author

have been studied in detail, referred to as samples 284,279 and 309. with thicknesses 0.33. 1.7 and 2.2 tIm, respectively. CW excitation was provided using an Ar ion laser or Coumarin 7 dye laser, and signal was detected using a triple grating spectrometer with a CCD detector. For reflectivity measurements, a tungsten filament light source was used, spectrally dispersed after incidence. Fig. 1(a) shows photoluminescence (PL). PL cx citation (PLE), and reflectance (RI measurements of sample 279 at 4.2 K over a limited energy range embracing free exciton emission. The PL of sample 309 is very similar, except for a slight shift (
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sample shown, this energy is at 2386.9 meV (see equal to the is PLE 2s of free exciton fordiffithe Fig. 1(a)). The sample 284separation; has proved cult to interpret, but for very thin samples it is not clear whether intrinsic features will appear as dips or peaks. Fig. 1(a) also shows the reflectivity of sample 279, in which it is easy to recognise two resonances due —~

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Energy in meV Fig. I. (a) PL, PLE and reflectivity (R) spectrafor sample 279 at 4.2 K. The dashed line is a fit to the reflectivity, displaced vertically for clarity. Arrows indicate estimated free exciton positions, determined as described in the text. (b) Acoustic phonon Stokes shift from 1 LO line as a function of I LO phonon position as the laser energy is varied. Data is shown for samples 279 and 284, the triangles corresponding to LA [111] phonons and the circles to TA [1001 or [1101 phonons. The solid lines indicate fits as described in the text,

2377 meV is attributed to intrinsic recombination. Analysis of the strain dependence of the spectral position of the X acceptor lines following Zhang et al. [2] indicates the presence of light and heavy hole excitons (LHX and HHX) at 2376.9 and 2380.0 meV, respectively, in sample 279 (solid arrows in Fig. 1(a)). Fig. 1(a) also shows the PLE spectrum for the X acceptor principal bound exciton line (2358.9meV in this sample). A sharp dip is seen coincident with a dip in the PL line shape, and this is taken to represent the HHX resonance (we note the short penetration depth of 2000 A at the HHX [5] compared to the sample thickness). In all samples studied, the peaks in the PLE vary somewhat in position and intensity according to the detection conditions, while the main dip remains constant. The samples show increased signal when

to 120A thediameter which LHX compares and HHX. Wewith have themodelled freeFig. exciton the Bohr of 115 Awell (dashed line in 1(a)). For sample 279, HHX and LHX resonances are predicted at the positions indicated by dashed arrows in Fig. 1(a). However, ignoring spatial dispersion but retaining an exciton-free surface layer gives very similar results, except for a different broadening value (but see Section 2.2). The reflectivity of sample 284 is difficult to interpret without a more sophisticated model, due to the presence of interference fringes above and below the resonances. In all respects our samples compare well with the best ZnTe epitaxial layers reported in the literature, but as with previous studies, the thicker samples (279 and 309) prove easier to interpret. The relaxation of the different layers during growth leads to different strain states at 4.2 K, with samples 279 and 309 in biaxial tension and sample 284 in biaxial compression. For a thermally induced strain of 0.12%, we estimate that samples 284 and 279 had in-plane strains of 0.16% and 0.07%, respectively, at the growth temperature (pseudo-morphic growth would have resulted in a strain of about 0.20%). These estimates compare favourably with room temperature X-ray measurements. —

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2.2. Acoustic phonon dispersion Excitation of the samples with a laser energy around 1 LO phonon above the free exciton resonances (spectrum not shown) reveals acoustic phonon Stokes sidebands to the 1 LO line, as seen in melt-grown ZnTe by Oka and Cardona [6] and explained in terms of two-phonon polariton

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Raman scattering. Initial scattering from the photon-like branch to the eAciton-like branch occurs by LO phonon emission, and scattering back to the photon-like branch occurs by emission of an acoustic phonon. The acoustic phonons involved in the suggested piezoelectric interaction are LA[l II]. TA[l 00] and TALl 10] (the latter two unresolved from each other), and their energies show disperston as the laser energy is changed. as shown in Fig. 1(b) for samples 279 and 284. A simple model, in which a single exciton branch is characterised hy a transverse exciton energy and isotropic effective mass (a simplification of the real situation in which strain leads to anisotropic hole masses and admixture of valence band states), has been used to fit the data. Our model can be justified for exciton states away from the exciton photon interaction region and assuming the LHX HHX interaction to be averaged over all directions. The data can then be used to indicate the lowest exciton transition in the samples (dotted arrow for sample 279 in Fig. 1(a)) and to give an averaged exciton mass for the process: 0.83 + 0.04 electron masses (me) for both phonon sets in sample 279, and 0.90 + 0.l5m~for both phonon sets in sample 284. These values compare well with the translational exciton mass of around 0.8 m~in bulk ZnTe. Similar dispersive Raman lines have been observed in a material for which a polariton description is not relevant (Cu20). However, despite the fact that the participating exciton-like states appear to lie on the lower exciton branch, the sidebands disappear when the I LO line moves below the higher exciton resonance (almost coincident with the expected start of the upper branch). If the cxciton photon interaction was smeared out (no polaritons), the sidebands should persist as much as 2 or 3 meV lower, merging with the I LO line. Our data is consistent with a significant exciton photon interaction and suggests that a polariton approach to the optical properties is relevant, despite the relatively large line widths observed in the FL (p0 lariton effects are not obser~edabove a critical broadening [7], calculated to be 0.9 meV in ZnTe). Epitaxial layers may provide the degree of control required to clarify the relative importance of such important factors as impurity scattering and surface electric fields in PL line shapes.

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3. Time-resolved measurements We have also performed tinie-resolved PL measurements on these samples, using carrier dens ities above the Molt transition (i.e. polariton effects can be neglected). Experiments were performed using a CW Ar ion laser pumping a Ti-sapphire laser (on loan from Spectra-Physics UK). the I ps output pulses of which were then frequency-doubled using a crystal of beta barium borate. Signal was disper sed spectrally using a 0.25 m subtractive double monochromator and temporally using a synchronously scanning streak camera. The time resolution of the system was 2Ops. The PL was seen to extend to high energies at early times, indicating a hot effective carrier ternperature. This temperature was estimated from the data using the luminescence at later times to represent the cold line shape (assuming an effective carrier temperature of 10K), yielding the effective carrier temperature as a function of time (Fig. 2). Given the large Fröhlich coupling in II VI systems, the dominant cooling mechanism for non-equilibrium

Mi. McNamee

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Journal of Luminescence 60&61 (/994) 788 791

carriers is the emission of LO phonons. The emission time is very fast (— 30 fs) and so a large, nonequilibrium population of LO phonons is built up very quickly leading to a phonon bottlenecking process [8]. The effective cooling time for hot carriers is then determined by the phonon lifetime. When the data is compared with a model of this process (valid in Fig. 2 for times greater than 20 ps), a satisfactory fIt is found using an LO phonon lifetime of 8 ±1 ps. Analysis of similar effects in GaAs have led to estimates for the LO phonon lifetime in good agreement with time-resolved Raman data [8].

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References [1] G. Kudlek and J. Gutowski, J. Lumin. 52 (1992) 55. [2] Y. Zhang, B.J. Skromme and F.S. Turco-Sandroff, Phys. Rev. B 46 (1992) 3872. [3] H. Venghaus and P.J. Dean, Phys. Rev. B 21(1980)1596. [4] N. Magnea, J.L. Pautrat, Le Si Dang, R. Romestain and P.J. Dean, Solid State Commun. 47 (1983) 703. [5] B. Langen, H. Leiderer, W. Limmer, W. Gebhardt, M. Ruff and U. Rdssler, J. Crystal Growth 101 (1990) 718. [6] Y. Oka and M. Cardona, Solid State Commun. 30(1979)447. [7] M. Matsushita, J. Wicksted and HZ. Cummins, Phys. Rev. B 29 (1984) 3362. [8] iF. Ryan, R.A. Taylor, A J. Turberfield and J.M Warlock, Surf. Sd. 170 (1986) 511.