Resonant photoluminescence measurements in As- and P-doped ZnTe epilayers

PHYSICA 1

Physica B 185 (1993) 169-173 North-Holland

Resonant photoluminescence P-doped ZnTe epilayers

measurements

H.P. Wagner, S. Lankes, K. Wolf, M. W&-z, T. Reisinger, H. Stanzl and W. Gebhardt Universitiit

Regensburg,

Institut fiir Festkiirperphysik,

Regensburg,

in As- and A. Naumovl,

W. Kuhn,

Germany

The near-gap photoluminescence (PL) of MOVPE ZnTe epilayers grown on (00 1) GaAs substrates has been extensively studied using resonant-excitation methods. Heteroepitaxial ZnTe layers show a small biaxial strain which splits excitonic transitions. We obtained the Luttinger parameter y1 = 3.8 from transition of the split free exciton 1s and 2s levels. Shallow acceptor states were observed in free standing As- and P-doped ZnTe layers and the Luttinger parameters ‘yz= 0.72 and -ys= 1.3 were derived. The magnetic field splitting of the lS,,, acceptor ground state unambiguously identifies the acceptor bound exciton line Z1, as double light-hole state (A:,, X,). Furthermore the magnetic parameters K* = -0.27 and qA = -0.015 were determined.

1. Introduction

ZnTe with a direct gap energy E,, of 2.286 eV at room temperature represents a promising candidate for optoelectronic devices in the visible green region. In addition, ZnTe has received attention since selective acceptor doping produces low resistive p-type material that could yield p-n junctions in heterostructures of superlattice as ZnTe-ZnSe [l] and ZnS-ZnTe [2]. For future applications, however, the exact identification of the near band luminescence and a detailed knowledge of the characteristic parameters in thin ZnTe epilayers is absolutely necessary. Therefore we extensively studied the near gap photoluminescence with a tunable dye laser to investigate shallow donors or acceptors with two-electron (TETs) or two-hole transitions (THTs) and selective pair luminescence (SPL) for donor-acceptor pairs [3-51. Reflection and

Correspondence to: H.P. Wagner, Institut fiir Festkiirperphysik, UniversitHt Regensburg, Universiftsstrasse 31, D8400 Regensburg, Germany. 1Permanent address: A.F. Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia.

0921-4526/93/$06.00 0

absorption spectra in the region of the fundamental band gap E, provide important information on strain effects [6,7] which are produced both by lattice mismatch and by different thermal expansion of substrate and epilayer. Excitation spectroscopy (ES) studies and investigations in an applied magnetic field corroborate the given assignments of luminescent lines.

2. Experimental All ZnTe layers used in these experiments were grown in our laboratory on (0 0 1) GaAs substrates by MOVPE. We used diethylzinc (DEZn) and diisopropyltelluride (DiPTe) as precursors which were diluted in H, gas. The growth rate was 1 *m/h at an optimized growth temperature of 350°C. More details of the MOVPE growth and doping with gallium, iodine, arsenic and phosphorus are described in [8,9]. The ZnTe layers show good optical quality which was routinely tested by Nomarski microscopy, reflectivity and photoluminescence measurements. The description of the experimental setup was described in previous publications [4-61.

1993 - Elsevier Science Publishers B.V. All rights reserved

H. P. Wagner et al. I Resonant photoluminescence

170

3. Results

and discussion

The position of the excitonic transition is very sensitive to ingrown strain or to applied stress which is in general present in heteroepitaxial layers. Thus the Is and 2s free exciton transition X in ZnTe grown on GaAs is split into a heavy X, (mj = t t)- and a light X, (mj = +$)-hole component. This splitting can be seen in the PL spectrum of fig. 1 of an undoped 2 p.rn thick MOVPE ZnTe layer on GaAs at 2 K excited with the 457.9nm (2.7067 eV) Ar-ion line. The exciton lines II0 and I,, mark acceptor-bound transitions where the acceptor a is identified as As which diffused from the substrate into the layer during growth. The acceptor c is unknown up to the present. An Z, line which designates the donor-bound exciton luminescence cannot be observed directly since it is covered by the strong X, luminescence. Resonant excitation near the X, line however makes the (Do, X) transition deEnergy 2.40

5160

2.39

2.30

(eV) 2.36

2.37

5200

Wavelength

5240

2.35

5280

(A)

Fig. 1. Photoluminescence (PL) spectrum and excitation spectra (ESl, ES2) of a 2 pm thick MOVPE ZnTe on GaAs layer at 2 K. The arrows indicate the detection energy for the excitation spectroscopy.

measurements

in ZnTe epilayers

tectable [5]. The excitation spectra ES1 and ES2 of the same layer were recorded at 2 K with a tunable dye-laser operating with Coumarin 510. The detection was carried out at the energetic position of the bound exciton lines I,, (ESl) and I,, (ES2). Both spectra show the split free excitons. In spectrum ESl, an excited bound exciton state I;, at 2.371 eV (5227 A) can also be seen. In previous works [6], it was shown that the observed strain is dilatational and mainly caused by different thermal expansion of substrate and layer. The misfit strain is a compressive strain and practically completely relaxed above a critical thickness of about 30 A in ZnTei GaAs layers [lo]. From the split exciton luminescence and the knowledge of the hydrostatic deformation potential a = -5.5 eV which we determined with a diamond anvil cell [6], the axial deformation potential b = - 1.4 -+ 0.2 eV was derived. With the energy difference between the lsand 2s-free exciton luminescence E,, - E,, = 9.6 t 0.1 meV, we find the Rydberg energy R, = 12.5 meV and the binding energy E,, = -12.9 -+ 0.1 meV using the hydrogen model, where we considered small cubic corrections [5,14]. Using the static dielectric constant &St= 9.4 which was obtained by TET studies in iodine donor doped samples [5], the reduced exciton mass is px = O.O81m,. The free effective electron mass was derived from the cyclotron-energy ho, = E++, E 2p_1 of the split n = 2 iodine donor TETs. The transitions were clearly observed at an applied magnetic field up to 5 T. We determined the effective electron mass rn: = 0.117~ which is in good agreement with the value observed by Dean et al. [16]. With lIpx = (l/m:) + (y*lm,), we found the Luttinger parameter y1 = 3.8. It has already been mentioned that thermal strain in a ZnTe layer on GaAs causes a splitting of the exciton and leaves the light-hole exciton at lower energies. We assigned the I,,- and the I,,-line to a recombination luminescence of a bound light hole exciton. This interpretation is based on experimental observations in PL spectra where the I,,- and I,,-line shows the same strain dependence as the X, luminescence in ZnTe layers grown under different growth condi-

H. P. Wagner et al. I Resonant photoluminescence

tions and with varying inplane strain [5]. A theoretical analysis of the PL intensity of light X, and heavy hole X, excitons with a two oscillator model provides an exciton-polariton temperature of T,0l = 30 K. In this model, a boltzmann distribution was taken into account. Obviously, the trapping probability of hot heavy hole excitons is too small to produce acceptor bound heavy hole excitons. This suggestion is also supported by the results of the excitation spectroscopy. In spectrum ES2 of fig. 1, the intensity of the heavy hole 1s exciton X,, is almost three times higher than the X, peak which corresponds to the oscillator strength ratio although the I,, line is related to a light hole exciton. Obviously the heavy hole exciton X,, makes a fast transition into a light hole exciton X, which can be bound. A strong X, signal is also observed in spectrum ES1 in fig. 1. In the energetic range of X,, however, higher excited bound exciton states of I,, must be considered which in addition contribute to the X, peak. Thus the X, signal is not dominant and the effect of the very fast relaxation is not as clearly seen as in spectrum ES2. In SPL and THT measurements from undoped as well as As- and P-doped ZnTe/GaAs layers, a doublet structure of the strain split lS,,,- and 2S,,,-acceptor states is observed. Therefore we more accurately assigned the I,,-line to a recombination of a light hole exciton bound at a light hole neutral acceptor (A:,, X,). We confirmed this interpretation by applying a magnetic field up to 6T which removes the remaining acceptor state degeneracy. The linear splitting of the THT IS& and 2ST, heavy hole o-line gives in Faraday configuration the g-factors g,,, = 0.60 [5] using g: = -0.47 [ll]. Th e respective a-splitting of the light hole 2S Z, state is too small to give a reliable value at fields up to 6 T. Magnetic field measurements of the I,, luminescence in Voigt configuration with fields up to B = 15 T applied along the [0 1 O]-axes exhibit a splitting of the 7r- and no splitting of the a-component [4,5]. Likewise in Faraday configuration with [0 0 11 applied magnetic field, no splitting of the I,, line was observed. These results are consistent with the behaviour of the 2s Z’,-state in the THT spectra and confirm our

measurements

in ZnTe epilayers

171

explanation of the I,,-line as recombination luminescence of a light hole exciton bound to a light hole acceptor (A:,, X,). A Gauss-curve analysis of the split I,,-line yields a g,,, = 1.205. The g-factors of the bound mi = k f and mi = 2 3 holes can be expressed in terms of the magnetic Luttinger parameters K~ and qA by gC3,2ns = -2(‘% + : qA) and g(l/z)ls = -2(‘$ + 5q,) [121. Thus the derived parameters K* = -0.27 2 0.4 and qA = -0.015 + 0.01 are in good agreement with those obtained from spin flip Raman measurements [11,13]. In order to obtain the Luttinger parameters y2 and x, the excited acceptor states were investigated with SPL and THT in As [4] and recently in P-doped ZnTe layers which were lifted from the substrate by chemical etching to obtain strain free samples. Figure 2 shows SPL spectra of a P-doped free-standing ZnTe layer. The spectra are recorded at 2 K in a wavelength range from 5217 A to 5229 A in 1 A steps and plotted versus the difference between dye-laser and luminescence energy. The obtained energy differences between acceptor ground state and excited states which are in good agreement with the values found in doped bulk ZnTe [17,18] are given in

40

45

50

55

60

Excitation energy - Luminescence energy (mew Fig. 2. SPL spectra of a P-doped free-standing MOVPE ZnTe layer recorded at 2 K. The excited acceptor states are assigned in the notation of Baldereschi and Lipari [15].

172

H.P. Wagner et al. I Resonant photoluminescence

Table 1 Energy differences (in meV). Transttton AS P

l&

-+

between

As- and P-acceptor

ground

state

in ZnTe epilayers

and excited

states

2P,,,

2%

2PT/&)

2PS,Z(r,)

3%,

53.2 39.7

58.1 44.6

61.6 47.5

65.0 so. 1

66.6 54.4

table 1. The values correspond to long donoracceptor pair distances where overlap corrections can be neglected. An averaged effective hole mass m,ly, has been derived from the exciton binding energy which yields an acceptor Rydberg energy R, = 40.5 meV. The As and P acceptor binding energies were experimentally determined by free-to-bound transition to E,, = -76.5 2 1 meV and E, = -62? 1 meV. From these values and the transition energies to Pstates, we could derive the parameters p = (67, + 47,) /5y, and 6 = (x - r,)ln by a fit to the theory of Baldereschi and Lipari [15]. Note that P-states do not contain a central cell correction. With S = 0.15, a linear interpolation yields CL= 0.57. The respective Luttinger parameters are calculated as y1 = 3.8, yz = 0.72 and x = 1.3. A comparison with parameters found by other authors is collected elsewhere [4].

4. Summary The consideration of mechanical strain fields which are present in heteroepitaxial grown ZnTe layers and a comparison of the measurements carried out on free-standing layers yield a consistent interpretation of the PL spectra. Furthermore, the investigations allow the determination of characteristic constants of ZnTe as follows: the effective electron mass rn: = O.l17m,; the static dielectric constant E,~ = 9.4; the axial deformation potential b = -1.4 eV, the Luttinger parameters 7, = 3.8, y2 = 0.72, y3 = 1.3; K~ = The I,,-line was un-0.27 and qA = -0.015. ambiguously identified as a light hole exciton bound to a light hole As acceptor ground state (A:,> X,1.

measurements

Acknowledgements The application of magnetic fields above B = 6 T was possible by using a split coil magnet at Section Festkorperphysik TU Berlin. The help and hospitability of G. Kudlek and Professor J. Gutowski is kindly acknowledged. This work was supported by Deutsche Forschungs Gemeinschaft and Bundesministerium fur Forschung und Technology. A. Naumov thanks the Alexander von Humbold Foundation for support.

References

[II

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[lo]

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in ZnTe epilayers

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