436 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 4, APRIL 1981

If a significant fraction of the current is carried by the low energy electrons and holes produced by these processes, the fractionf(e), and hence the quantum efficiency will drop.

IV. CONCLUSIONS We conclude that the saturation in the quantum efficiency

could be a fundamental limitation related to the band structures of the host materials. The magnitude of this limitation can be estimated from Fig. 2 in which the maximum obtainable effi- ciency is a factor of 50 lower than the extrapolated efficiency.

ACKNOWLEDGMENT The author is indebted to J. W. Allen for helpful discussions.

REFERENCES [ 11 A. W. Livingstone and J. W. Allen, “Impact ionisation of deep im-

purities in ZnSe,” J. Phys. C: Solid-state Phys., vol. 6, pp. 3491-

3500. W. A. Thornton, “Electroluminescent thin films,” J. App l . Phys.,

A. Vecht, N. J. Werring, and P. J. F. Smith “High efficiency electro-

1968. luminescence in ZnS (Mn, Cu),” J. Phys. D., vol. 1, pp. 134-136,

R. Nitsche, “The growth of single crystals of binary and ternary chalcogenides by chemical transport reactions,” J. Phys. Chem. Solids, vol. 17, pp. 163-165, 1960. J. W. Allen, “Electroluminescence in reverse-biassed Schottky di- odes,” J. Lum., vol. 7, pp. 222-240, 1973. G. A. Baraff, “Distribution functions and ionisation rates for hot electrons in semiconductors,”Phys. Rev., vol. 128, no. 6,pp. 2507- 2517,1962. P. Eckelt, “Energy band structures of cubic ZnS, ZnSe, ZnTe, and Cd Te (KKR Method),” Phys. Stat. Sol., vol. 23, pp. 307-312, 1967. J. P. Walter and M. E. Cohen, “Calculation of reflectivity, modu- lated reflectivity, and band structure of GaAs, Gap, ZnSe, and ZnS,”Phys. Rev., vol. 183, pp. 763-772, 1969. G. W. Ludwig and M. Aven, “Gun effect in ZnSe,” J. Appl . Phys.,

V O ~ . 30, pp. 123-124, 1959.

V O ~ . 38, pp. 5326-5331, 1967.

Photoluminescence of ZnS,Se,, Epilayers and Single Crystals

AXEL HEIME, WILHELM SENSKE, HELMUT TEWS, AND HARALD MATTHES

Absfract-Photoluminescence properties of ZnS,Sel -, (0 < x < 1) epilayers on GaP substrates and of corresponding ZnS,Sel-, single crystals are examined. The samples have been grown by vapor phase iodine transport. The blue emission bands of the single crystals are quenched in the epilayers. Comparison of the luminescence properties of ZnSe crystals grown by sublimation without iodine clearly shows that iodine creates deep recombination levels of high concentration. By simple sublimation without transporting agent and subsequent anneal- ing in Ga and Za deep emission bands are suppressed in favor of blue edge emission. Differences in the spectra obtained by the use of source materials from two manufacturers are assigned to unidentified back- ground impurities.

A I. INTRODUCTION

MONG the applicable semiconductors for the realization of blue LED’s the 11-IV pseudobinary compound

Manuscript received September 10, 1980; revised December 4, 1980. This work was supported% by the German Federal Ministry of Research and Development under Contract 403-7291-NT 08570.

A. Heime and H. Matthes were with AEG-TELEFUNKEN Forschungs- institut, Goldsteinstrde 235, D-6000 Frankfurt 71, Federal Republic of Germany. They are now with AEG-TELEFUNKEN, Geschiifts- bereich Halbleiter, Abt. Opto-Elemente, D-7100 Heilbronn, Federal Republic of Germany.

W. Senske is with AEG-TELEFUNKEN Forschungsinstitut, Gold- steinstrafie 235, D-6000 Frankfurt 71, Federal Republic of Germany.

H. Tews is with CNRS, 92190 Meudon, France.

ZnS,Se, -, appears to have favorable properties: The two bi- nary compounds ZnS and ZnSe are mixable over the whole compositional range 0 < x < 1. Accordingly, the achievement of every wavelength from the UV to the blue region of the spectrum may be expected. Furthermore, these compounds have the advantage of being direct semiconductors. Extensive investigations concerning growth, doping, and annealing condi- tions have been performed [ l ] -[6]. The luminescence proper- ties of a variety of impurities of the ZnS,Sel -, system are well established [l] -[6]. Some authors have reportedefficient blue photoluminescence (PL) at room temperature for n-type ZnS,Se, -, single crystals [7] -[9]. However, the growth of ZnS,Se, -, single crystals is actually too expensive for the production of blue LEDs. An alternative method.is the epi- taxial deposition of ZnS,Se, -, on commercially available substrates such as Si, Ge, GaAs, and GaP [lo], [ 1 11.

In this paper we present PL studies of ZnS,Se, -, epilayers in the whole compositional range grown by iodine transport on GaP substrates, The influence of the GaP substrate on the PL properties of the epilayers was analyzed by comparing to PL spectra of ZnS,Se, -, single crystals grown under the same conditions. To investigate the influence of the iodine trans- porting agent on the creation of deep recombination levels a detailed luminescence analysis of ZnSe crystals was performed.

0018-9383/8l/0400-0436$00.75 0 1981 IEEE

HEIME e t al.: PL OF ZnSxSe1-, EPILAYERS AND SINGLE CRYSTALS 431

For comparison some crystals were grown by iodine transport and others by sublimation without a transporting agent. Fur- thermore, source materials of two manufacturers were used. Subsequently the samples were annealed with gallium and zinc. The main features of the different PL spectra from the various samples are discussed.

11. EXPERIMENTAL DETAILS ZnS,Se, -x (0 < x < 1) single crystals and corresponding

epilayers were grown on Si, Ge, GaAs, and GaP substrates. The best results were obtained with Gap: well adherent, crack- free ZnS,Se, -, layers were deposited on (1 11) B-GaP wafers [12], Before growing the pseudobinary compounds ZnSe (MERCK-SUPRAPUR) and ZnS (MRC-MARZ) powders as source materials were mixed and heated for 15 min at 400°C under high vacuum (- torr) in the growth ampoule.

The crystals were grown by subliming the source material , from one end (900-950°C) to the opposite end (850°C) of

the ampoule (volume 60-70 cm'). For sublimation growth with the iodine transporting agent the ampoule was filled with 1-2 mg/cm3 iodine vapor by heating a subsidiary ampoule filled with iodine crystallites. The sublimation process was repeated several times to ensure homogeneous growth.

The epilayers with a thickness of about 30 pm were prepared under the same conditions stated above. Here the crushed synthesized ZnSSe crystals or the mixed binary powders were used as source materials. Further details of the growth process and the epilayer properties are described in [ 121.

Post annealing of ZnSSe in liquid gallium is known to reduce deep radiative recombination levels [ 11, [ 31. Annealing with zinc is necessary to ensure low resistivity of the material [ 11, [3]. In the present case several ZnSe crystals were ann-ealed for 24 h at 600°C in liquid gallium and subsequently for about 70 h at 600°C in zinc vapor. The wafer thickness was less than 1 mm. After annealing the wafers were etched in hot NaOH. Some samples grown by iodine transport were annealed in gallium up to 7 days,

The different samples were analyzed with high resolution PL spectroscopy at a temperature of 1.6 K to get detailed infor- mation about the influence of the various growth and annealing procedures. The samples were excited by an Ar-ion laser (ex- citation line 3.4 eV) with an output of less than 100 mW. A photon counting system permitted a sensitive detection of very low signal levels.

Some ZnSe crystals were additionally excited by a dye-laser (excitation line 2.75 eV) to ensure bulk excitation of the sam- ples. No deviations in the spectra obtained with the different lasers were observed.

111. RESULTS AND DISCUSSION The PL spectra of ZnS,Se, -x epilayers on GaP substrates

and of corresponding single crystals are plotted in Fig. 1. All samples have. been prepared by iodine transport. Two super- imposed broad bands (energy maxima indicated by arrows) from deep level recombination dominate the spectra in the whole compositional range 0 < x < 1. The ZnSe single crystal (right section) exhibits Cu-green (2.3 eV) and SA (2.02 eV) emission whereas the ZnSe epilayer on GaP (left section) shows Cu-red

Zn S, Sel-, epilayer single crystal

0- L ' 2.0 " 2 2 2.4 2 6 1 2.8 ' , '

photon energy (evl - Fig. 1. PL spectra at 1.6 K of ZnS,Sel -x epilayers on GaP and of cor-

responding ZnS,Sel -x single crystals. Both varieties of samples were grown by vapor phase iodine transport.

(1.98 eV) and some SA (2.08 eV) emission [13]. With in- creasing ZnS content the bands shift to higher quantum energies according to the increasing band gap energies [6]. The relative luminescence intensities of the two bands de- pend on the concentration of the two impurity types and vary for different samples with the same composition x . The higher energy emission bands (analogous to ZnSe Cu-green) of the ZnSSe single crystals are quenched in thin films. The lower energy bands of the single crystals (SA emission) are nearly identical with the higher energy bands of the epilayers. The ZnSSe epilayers exhibit additional bands at lower transition energies (probably analogous to ZnSe Cu-red). According to [I31 these differences may be understood by a different amount of Cu contamination comparing single crystals and epilayers. The small superimposed intensity maximum at about 2.2 eV ( x = 0.8 and 1) is attributed to spurious luminescence from the GaP substrate.

The epilayers were analyzed by electron microprobe, The focused electron beam (15 kV, 5 X A, 1-pm diameter) was scanned along a cleavage plane from the epilayer surface to the bottom of the GaP substrate. From the emitted X-ray intensities (Fig. 2) a Ga and P content of about 1-2 percent in the ZnSSe epilayers is estimated.

The PL spectra of ZnSe crystals grown from powdered ZnSe source material (obtained from MERCK) are plotted in Fig. 3 ,

438 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28, NO. 4, APRIL 1981

l _ l l - p l e

(S,Se ) substrate cross section

beam position Fig. 2. Normalized X-ray intensities of Ga and P from electron micro-

probe analysis along a cleavage face of a ZnSSe epilayer on GaP substrate.

c wovelength lnrnl c wavelength Inrnl 700 650 600 550 500 450

Phonons i,

7W 650 600 550 500 L50 I

photon energy lev1 -+ photon energy lev1 4

lb,l, wlth iodlne Ibzl, wlth lodlne M d Ga/Zn annealing

Fig. 3. PL spectra at 1.6 K of ZnSe crystals grown from powdered ZnSe source material (MERCK). ( a , ) Sublimation without trans- porting agent. ( b l ) Sublimation by vapor phase iodine transport. (u2 ) Sample (01) after subsequent Ga and Zn annealing. ( b 2 ) Sample ( b l ) after subsequent Ga and Zn annealing.

The sample showing spectrum ( a l ) was grown without trans- porting agent. After annealing this sample with Ga and Zn spectrum ( a 2 ) was obtained. Spectrum (b , ) was emitted from ZnSe crystals grown from the same source material by the use of the transporting agent iodine. Ga and Zn annealing of this sample led to spectrum (b2) . Near band gap emission in the blue wavelength range dominates the spectra (a l ) and ( a 2 ) . In detail both spectra consist of the I2 line (exciton bound to neutral donor) and the Il deep line (exciton bound to neu- tral acceptor) with phonon replicas [4], [5]. Additionally, donor-acceptor pair bands (DAP) and the If9J' line (exciton bound to Li or Na acceptors) appear in spectrum ( a z ) [5], [14]. The deep level emission intensity at energies below 2.6 eV is fairly low. In contrast, the edge emission is reduced drasti- cally when the crystal is grown by iodine transport (spectrum

c wavelength Inrn) t waveienath inrnl

700 650 600 550 500 450 7W 650 600 550 500 450 r I ,

\i * ,J1 I\, ,/11

E " 18 2 0 ' 2 2 ' 2 L ' 2'6 " 2 8 (ail, without lodlne lazI, GaIZn anneoilng, wlthout Nodme

photon energy lev1 --c photon energy lev) -+ lbll. with lodlne Ib21, wlth iodine and GalZn anneallng

Fig. 4. PL spectra at 1.6 K of ZnSe crystals grown from polycrystalline source material (VENTRON). ( a l ) - (b2) the same growth and an- nealing conditions as the corresponding samples of Fig. 3.

(b,)). The deep level emission bands dominate now. Here the low intensity edge emission consists of the I,,, IFJ', and I1 deep lines with phonon replicas as well as DAP bands. After Ga-Zn annealing (spectrum (b,)) the Iz and I l deep lines are canceled. However, the deep level emission is not at all reduced.

The same procedure as with the MERCK material has been performed with polycrystalline source material (from VENTRON). The corresponding luminescence spectra are plotted in Fig. 4. The edge emission dominates again for ZnSe grown by sublimation without iodine (spectrum (al)). As in Fig. 3 in the spectrum of iodine transported material (b , ) the deep level emission is increased considerably relative to the edge emission. Again DAP bands are introduced. The Ga-Zn annealing is more effective for this polycrystalline source material: The low intensity deep level recombination bands disappear (compare a, and a 2 ) or they are no longer superior relative to the near band gap emission (compare b l and b 2 ) .

In the following a tentative interpretation of the spectral characteristics of Figs. 3 and 4 is given. During iodine transport large iodine concentrations (approximately 300-500 ppm) are incorporated into the ZnSe lattice. Iodine is known to act as a donor [2] , [ 131. Because of the strong self compensation be- havior of the material acceptors are induced during the growth. Lithium or sodium atoms on lattice sites are known to act as shallow acceptors [2] , [ 151, [ 161. Both may be incorporated during crystal growth as spurious contaminations from the growth ampoule leading to DAP bands in all iodine transported samples (Figs. 3 and 4, (bl) and (b2)) . The I F y lines in these spectra confirm the presence of Li and Na in the crystals; The SA defect is assigned to a complex from a donor impurity (e.g., iodine) and a deep acceptor [2], [13]. Therefore, an incorporation of considerable concentrations of deep acceptor impurities explains the strong SA emission for all samples grown by iodine transport (Figs. 1, 3 ( b l ) , and 4 (bl)). The observed cancellation (Fig. 4 ( a 2 ) ) or partial reduction (Fig. 4

HEIME e t al.: PL OF ZnS,Sel_, EPILAYERS AND SINGLE CRYSTALS 439

(b,)) of the deep emission bands by annealing the samples in Ga and Zn may be due to impurity gettering effects or partially filling of Zn vacancies. We cannot elucidate from our experi- ments which mechanism is responsible for the reported obser- vation. ZnSe crystals grown from powdered MERCK-source material are quite insensitive to Ga-Zn annealing (Fig. 3), a strong indication for high impurity concentration in this starting material. Accordingly recent publications indicate the original punty of these crystals to play a major role concerning the luminescence characteristics of these compounds [E] , [ 161.

The luminescence spectra of all samples have also been re- corded at temperatures of 80 K and 300 K. At 80 K intense blue edge emission has been observed only in the case of the sample corresponding to Fig. 4(a2). At 300 K the edge emis- sion of all spectra is quenched in favor of deep level emission.

IV. SUMMARY Our PL studies have demonstrated that deep level recombi-

nation predominates for iodine transported ZnSSe epilayers on GaP as well as for ZnSSe single crystals in the whole composi- tional range. This behavior is attributed to a large concentra- tion of complexes of iodine donors and acceptors of unknown nature in accordance with former data for ZnSe [4], [13]. Therefore, iodine transport is not an adequate method for the preparation of ZnSSe epilayers with high blue luminescence efficiency. Additionally this transport method leads to an in- corporation of Ga and P from the substrate into the epilayer.

The spectroscopic results are different using powdered or polycrystalline ZnSe source materials from two manufacturers probably due to different impurity concentration. Post an- nealing with Ga and Zn partially reduces deep emission bands but also affects the near bandedge luminescence.

ACKNOWLEDGMENT The authors thank H. Preier for helpful discussions and are

grateful to H. J. Queisser (Max-Planck-Institut fur Festkorper- forschung, Stuttgart) for providing the possibility to perform the luminescence measurements. The technical assistance of C. P. Reeves in the sample preparation is appreciated.

REFERENCES J. C. Bouley, P. Blanconnier, Ph. Ged. P. Henoc, and J. P. Noblanc, J. Appl. Pkys., vol. 46, p. 3549, 1975. Y. S. Park and B. K. Shin, in Topics in Applied Physics J. I. Pankove, Ed. Berlin, Heidelberg, Germany: Springer, 1977, V O ~ . 17, PP. 133-170. M. Yamaguchi, A. Yamamoto, and M. Kondo, J. Appl. Pkys., vol. 48, p. 5237, 1977. 3. L. M&z, H. Kukimoto, K. Nassau, and J. W. Shiever, Pkys. Rev. B., vol. 6, p. 545, 1972. J. L. Merz, K. Nassau, and J. W. Shiever, Phys. Rev. B., vol. 8, p. 1444, 1973. W. Lehmann, J. Electrochem. Soc., vol. 113, p. 449, 1966; -, ibid., vol. 113, p. 788, 1966. A. C. Papadopoulo, A. M. Jean-Louis, and J. Charil, J. Cryst. Growth, vol. 44, p. 587, 1978. S. Fujita, H. Mimoto, and T. Noguchi, J. AppJ. Phys., vol. 50, p. 1079, 1979. T. Ido, M. Kato, A. Yoshida, and M. Ieda, J. Pkys. D., Appl. Pkys., vol. 11, p. L5, 1978. N. Matsuda and I. Akasaki, J. Cryst. Growrh, vol. 45, p. 192, 1978. T. Yao, Y. Makita, and S. Maekawa, Appl. Phys. Lett., vol. 35, p. 79, 1979. H. Matthes, A. Heime, and C. P. Reeves, German Federal Ministry of Research and Development (BMFT) Research Rep. NT 0857 0, 1979. G. Jones and J. Woods, J. Lum., vol. 9, p. 389, 1974. P. J. Dean and J. L. Merz, Pkys. Rev., vol. 178, p. 1310, 1969. K. Kosai, B. J. Fitzpatrick, H. G. Grimrneiss, R. N. Bhargava, and G. F. Neumark, Appl. Phys. Lett., vol. 35, p. 194, 1979. R. N. Bhargava, R. J. Seymour, B. J. Fitzpatrick, and S. P. Herko, Pkys. Rev. B., vol. 20, p. 2407, 1979.