Photoluminescence of CVD grown CdS epilayers on CdTe substrates

Photoluminescence of CVD grown CdS epilayers on CdTe substrates

Journal of Crystal Growth 118 (1992) 304—308 North-Holland I ~ CRYSTAL GROWTH Photoluminescence of CVD grown CdS epilayers on CdTe substrates N. L...

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Journal of Crystal Growth 118 (1992) 304—308 North-Holland

I

~

CRYSTAL GROWTH

Photoluminescence of CVD grown CdS epilayers on CdTe substrates N. Lovergine, R. Cingolani, A.M. Mancini Dipartimento di Scienza dei Materiali, Uniiersità di Lecce, Via per Arnesano, 1-73100 Lecce, Italy

and M. Ferrara Dipartimento di Fisica, Unitersith di Ban, Via G. Amendola 173, 1-70126 Ban, Italy Received 1 October 1991; manuscript received in final form 5 December 1991

Photoluminescence (PL) measurements of high structural quality CdS epitaxial layers grown on {111}A CdTe substrates by chemical vapour deposition (CVD) are reported. Low temperature PL spectra show intense excitonic emissions due to both free and bound exciton recombinations, together with their longitudinal optical phonon replicas. At high excitation intensity a strong biexciton emission dominates the spectra. Also, the presence of a weak donor—acceptor pair recombination band around 520 nm suggests that the impurity content in the present samples is not high. A broad emission band peaked at about 590.0 nm is identified as due to excitons bound to isoelectronic substitutional Te atoms on sulfur lattice sites. The dependence of this emission on the growth temperature and epilayer thickness, clearly indicates that Te atoms diffuse from the underlying CdTe substrates into the CdS layers at the relatively high temperatures (570—700°C) used for the CVD growth, giving rise to the autodoping of the CdS epitaxial layers. This is the first time that such Te diffusion is evidenced.

1. Introduction The growth of Cd5 epitaxial layers attracts a continuous interest, mainly due to the value of the CdS energy gap, i.e. 2.42 eV at 300 K [1], which makes it suitable as transparent semiconducting material in the fabrication of photovoltaic solar cells. Epitaxial growth of CdS on CdTe substrates is a relevant topic, for the n-Cd5/pCdTe heterojunction has theoretical efficiency of 19.7% under AMO (air mass zero) sunlight spectral conditions [2]. However, such heterostructure raises the problem of matching the hexagonal (wurtzite) CdS lattice with the cubic (zincblende) one of CdTe. Until now, only three CdTe substrate orientations have proved successful in growing Cd5 epitaxial layers, namely {110) [31, {111}A [41and (221} [51,although in the latter case it is not known whether the exact orientation 0022-0248/92/$05.00 © 1992



was (221)A or {221}B. For growth on the (111}A surface, the well-known epitaxial relationship (0001)CdS ~(111)A-CdTe holds, resulting in a 9.74% mismatch for the in-plane lattice parameters. Despite such a high value of the lattice mismatch, the growth of high crystalline quality hexagonal CdS epilayers on {1 11}A CdTe by chemical vapour deposition (CVD) has been recently demonstrated by some of us [61by channeling Rutherford backscattering spectrometry analysis. In this paper we investigate their photoluminescence (FL) properties. The spontaneous emission of CdS has been studied as a function of temperature and excitation intensity. This allows one to identify most emission lines in the spectra and to estimate the optical and structural quality of the present layers. Also, the diffusion of Te atoms into the CdS epilayer have been evidenced through the tern-

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grown CdS epilayers on CdTe substrates

perature and intensity dependences of the Te related extrinsic emission in the FL spectra.

2. Experimental procedure (0001) oriented hexagonal CdS layers have been grown on {111)A-CdTe by CVD using hydrogen as transport agent from the source to the deposition region [7]. 5N purity nominally stoichiometric CdS from Cerac Inc. has been used as starting material. The (111) oriented CdTe substrates, supplied by Cominco Inc., were lapped and polished with diamond paste to a mirror finish. The (11i}A face of CdTe substrates was identified by selective etching [8] in 1 HF: 1 HNU3: 1 CH3COOH. A light etch of the substrates by 1% bromine—methanol solution immediately before the introduction in the growth chamber was followed by an in situ thermal etch at 600°Cfor 15 mm in a H2/N2 (2:1) flow. The Cd5 epitaxial layers were grown in a 300 SCCM total hydrogen flow at deposition temperature ranging between 570 and 700°C,the source ternperatures being 100°Chigher. These conditions gave growth rates in the range 0.05—0.5 ~tm/min. 5everal samples have been grown for the present FL measurements, having total epilayer thicknesses between 4.6 and 27.6 ~tm. FL excitation measurements have been carried out in the 10—300 K temperature range by using the third harmonic emission (356.2 nm, 3.48 eV) of a Nd-YAG laser having a repetition frequency of 10 Hz and a 15 ns pulse duration. The linear absorption coefficient of Cd5 crystal at this excitation wavelength is about iO~cmt [9], resulting in a penetration depth of light of about 0.1 ~rn. The excitation power density has been varied 2 by to neutral-density filters from about 10 mW/cm about 200 kW/cm2. The detection system, fully computer controlled, consisted of a 0.6 m monochromator equipped with a photomultiplier tube and a boxcar Integrator. The monochromator entrance slit used for the present measurements had a width of 100 ~tm.

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3. Results A typical low temperature FL emission spectrum of our Cd5 epilayers is shown in fig. 1. Three distinct recombination channels can be identified: (i) the excitonic emission between 485.0 nm and 502.0 nm, (ii) a donor—acceptor pair recombination band around 520 nm and (iii) a broad Te-related emission band peaked around 590.0 nm. In fig. 2 the edge emission of a typical Cd5 epilayer is reported for two different temperatures and at low excitation intensity. The spectra exhibit three main features around 486.0 nm (FE), 487.3 nm (‘2) and 489.4 nm (‘i) together with their respective longitudinal optical (LU) phonon replicas. The higher energy contribution is identified as due to the 19 free exciton recombination [10], with two LU satellite peaks (FE-LU, FE2LU). The 1~and 12 lines can be readily assigned to impurity bound exciton emissions, the exciton binding energies on the impurity sites being 6.3 and 17.2 meV, respectively, for the 487.3 nrn (12) —

_________

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11 a

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Tn

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540 nm)

WAVELENGTH

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506

I

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WAVELENGTH (nm) 531

568

599

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Fig. 1. The typical 10 K PL spectrum of a CdS epilayer. The excitonic emissions with the two main bound exciton I~and 12 lines, the DA pair recombination bands the Tes bound cxciton emission are indicated with their relative intensities. The . . insert in the figure shows the observed blue-shift of the DA pair bands for two excitation intensities, i.e. (a) 3.2 kW/cm2 and (b) 2.6 kW/cm2.

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[11]. The biexciton molecule binding energy of CdS is reported to be 5.7 meV [111,a value which

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-~ .4-.

the 12 and the FE lines at high excitation intensities. CdS 520 epilayers an interband transitionOur around nm as show reported in the FL spectra

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FE - LU 10 FE 2L0

I-u = LU

50K

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10K

LU I

415

~-~_Y\~

x-LO

I

481

499

WAVELENGTH (nm) Fig. 2. The main features of the excitonic emission of the CdS epilayers are shown for two different temperatures and for a low excitation intensity. The peak labels used in this figure are explained in the text.

excitation of fig.showing 1. Such intensity, as shown exhibits inposition one the main insertion peak of at lines compares about third LU 512.8 well phonon nmaemission with together marked replicas, the blue-shift energy with theits splitting first, with second increasing between of these and fig. 1. Such behaviour allows us to identify this emission as due to donor—acceptor (DA) pair recombination, in accordance with what reported in the [121. value of their hydrogenic Dueliterature to the large radii, extending over several tens of lattice penods, excitons are a sensitive probe of the long range crystal periodicity. Our CdS epilayers have

ENERGY (eV) 2.58

2.56

2.54

2.52

2.50

488

492

496

2.48

and 489.4 nrn (Ii) lines. Their spectral positions respectively with the well-known exciton bound to a neutral donor and exciton bound to a neutral acceptor emissions, already reported in the literaand the values of the binding energies coincide ture [9,10]. At the lower energy side of the I~line a shoulder appears (X), suggesting the presence of an additional weaker bound exciton line at about 490.0 nm; in fig. 2 the positions of this

peak, with proportional inalmost emission cated K reduces bydisappears arrows. and theof‘2together Increasing itsline due LOintensity, tophonon bound the atemperature while replica exciton theare dissociaI~line to indi50 tions, resulting in a more intense free exciton crease of its LO phonon replicas. Fig. 3 shows the CdS edge emission as a function of the excitation intensity. With increasing intensities all the emissions saturate but the 12 line. For pump power higher than 20 kW/cm2, the ‘2 line broadens and shifts towards longer wavelengths. This behaviour resembles the spectral evolution of the well-known M-band due to biexciton recombination in CdS single crystals

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WAVELENGTH (nm)

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Fig. 3. The dependence of the near band edge emission of CdS on the excitation intensity. The value of excitation intensity for each spectrum is indicated in percentage of I~= 200 kW/cm2. The so-called M-band dominating the spectrum at the higher intensities is due to biexciton recombination.

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PL of CVD grown CdS epilayers on CdTe substrates

invariably shown a relatively intense free exciton peak, together with its first and second LO phonon replicas. Also, an intense biexciton molecule emission has been detected under high excitation intensity conditions. This is a confirmation of the high structural quality of the present epilayers. The presence of a weak DA pair recombination band in the spectra, even at low excitation intensities, together with the observed rapid saturation of the bound exciton emissions, suggests that the impurity concentration in the present samples is not high. However, the identification of the chemical nature of these impurities on the basis of the present excitonic emissions is not straightforward; available literature data [9,10,13] for CdS report many donor and acceptor related bound exciton lines in a narrow wavelength interval around the 487.3 and 489.4 nm lines observed in our samples.. Let us now turn to the discussion of the extrinsic emission in the transparency region of the crystal. The PL spectra of the CdS epilayers show an additional broad band at longer wavelengths, having its maximum at about 590.0 nm (2.10 eV). This band coincides well with the emission from excitons bound on substitutional tellurium atoms on sulfur lattice sites (Te5) as reported by Hopfield et al. [14]. Te5 defects introduce a deep level about 0.218 eV above the CdS valence band; this level acts as a hole trap, the binding energy of an electron to the hole captured on the Te5 trap being 5 meV [15]. However, the additional Te related band at 730.0 nm reported by Cuthbert and Thomas [16] for heavily doped CdS crystals was never seen in our samples. This second band is ascribed to recombination at pairs of Te atoms on nearest-neighbour sulfur sites for CdS3. crystals containing more appreciation then 1019 Teofatoms/cm A qualitative the Te content of our CdS layers can be made by taking the ratio of the emission intensity of the ‘Te band to that of the ‘2 line; the present measurements have shown that the 12 line intensity is fairly constant for all the examined samples suggesting that the amount of associated donor impurities in the CdS layers does not change. Hence, comparing the emission intensities of the ‘Te and ‘2 excitonic recombina-

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217

2.10

2.03

197

1.91

610

630

650

10 1

05 —

2 01 550

570

590

WAVELENGTH (nm) Fig. 4. The ratio of ‘Te emission intensity to that of the 12 line is shown as a function of the emission wavelength for three different CdS samples, their growth temperature and thickness being respectively: (1) 700°C,4.6 ~m, (2) 700°C,27.6 ~.em and (3) 600°C 4 6 ~em

tion channels allows one to show qualitatively the variation of the Te concentration relative to the 12 line donor impurity concentration in the CdS layers. This ratio depends on the growth temperature and on the thickness of the CdS epilayers. The ‘Te/’2 intensity ratio shows a decrease of about one order of magnitude when the growth temperature is lowered from 700 to 600°C, as reported in fig. 4 for samples #1 and #3, both of them having a nominal thickness of 4.6 ~tm. About the same decrease can be seen by comparing samples #1 and #2, grown at 700°Cand having thicknesses of 4.6 and 27.6 ~.rm,respectively. This is a clear indication that the presence of Te in the epitaxial layers is due to the diffusion of Te atoms from the CdTe substrate into the CdS. To our knowledge, there are no coefficient data available in the literature on the diffusion of Te in CdS crystals. In this respect, it should be pointed out that the Te-related excitonic emission, a!though fairly weak, has been observed even for relatively thick samples, as reported in fig. 4 for sample #2. This suggests a relatively great amount of Te in depth in these layers. However, more studies are necessary to evaluate the concentration of Te and its in-depth distribution in the CdS layers.

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4. Conclusions We have reported on the PL properties of CdS epitaxial layers grown by CVD on {111)A CdTe substrates. Low temperature PL spectra show exciton related emissions and weak DA pair recombination bands. Both free and bound excitons together with their LU phonon replicas have been identified in the near band edge emission. At high excitation intensities, all these lines saturate and a strong biexciton emission dominates the spectra. The presence in all the examined samples of a relatively intense free exciton line confirms the high crystalline quality of the CdS epilayers. Also, the presence of weak DA pair recombination bands, together with the rapid saturation of the bound exciton emissions at high excitation intensities, suggests that the amount of impurities in these samples is not high. The presence of Te in the layer as substitutional impurity on sulfur lattice sites is clearly demonstrated by the observation of a Te bound exciton emission. FL measurements have shown that the diffusion of Te atoms from the CdTe substrate into the CdS epilayer is responsible for this doping effect, which has been reported for the first time in this paper.

Acknowledgements The authors are greatly indebted to Professor L. Vasanelli for stimulating discussions. The technical assistance of Ms. A.R. Dc Bartolomeo and Mr. L.A. Wee during the growth of the CdS

epilayers is also acknowledged. This work has been partially supported by the Ministero dell’Università e della Ricerca Scientifica e Teenologica (MURST) of Italy.

References [1] Handbook of Chemistry and Physics, 65th ed., Ed. R.C. Weast (CRC Press, Boca Raton, FL 1984—85).

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AL. Farenbruch, V. Vasilchenko, F. Buch, K. Mitchell and R.H. Bube, AppI. Phys. Letters 25 (1974) 605. 131 Igarashi, J. AppI. Phys. 42 (1971) 4035. [410. K. Yamaguchi, H. Matsumoto, N. Nakayama and S. lkegami, Japan. J. AppI. Phys. 15 (1976) 1575. [5] G.R. Awan, A.W. Brinkman, G.J. Russell and J. Woods, J. Crystal Growth 85 (1987) 477. [6] G. Leo, A.V. Drigo, N. Lovergine and AM. Mancini, J. AppI. Phys. 70 (1991) 2041. [7] AM. Mancini, N. Lovergine, C. Dc Blasi and L. Vasanelli, Nuovo Cimento 1OD (1988) 57.

[81K. Durose, PhD Thesis, University of Durham (1986). 191 Landolt—Bdrnstein, Numerical Data and Functional Relationships in Science and Technology, New Series, Vol. 17b, Semiconductors: Physics of Group Il—VI and 1—Vil Compounds, Semimagnetic Semiconductors, Eds. 0. Madelung, M. Schulz and H. Weiss (Springer, Berlin, 1982). [10] D.G. Thomas and J.J. Hopfield, Phys. Rev. 128 (1962) 2135. [11] C. Klingshirm and H. Hang, Phys. Rept. 70(1981) 315. [12] K. Colbow, Phys. Rev. 141 (1966) 742. [13] D.C. Reynolds and C.W. Litton, Phys. Rev. 132 (1963) 1023. [141 J.J. Hopfield, D.G. Thomas and R.T. Linch, Phys. Rev. Letters 17 (1966) 312. T. Fukushima and S. Shionoya, Japan. J. AppI. Phys. 15 (1976) 813 1161 J.D. Cuthbert and D.G. Thomas, J. AppI. Phys. 39 (1968) 1573.

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