Photoluminescence and electroluminescence studies of hot-pressed polycrystalline mixed ZnS-ZnSe powders

Journal of Luminescence 15 (1977) 87—99 © North-Holland Publishing Company

PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE STUDIES OF HOT-PRESSED POLYCRYSTALLINE MIXED ZnS—ZnSe POWDERS Norman M.P. LOW and David I. KENNEDY Research and Development Division, Bowmar Canada Limited, Ottawa, Ontario, Canada Received 20 April 1976

An investigation has been conducted to study the photoluminescence and electroluminescence of polycrystailine mixed ZnS—ZnSe powders. A hot-pressing process is developed to compress the powders into good quality and high bulk density substrates which are suitable for fabrication of light-emitting devices. A series of devices based on the metal-semiconductor device structure have been prepared and these devices emit light varying from yellow-orange to green-blue. The room temperature quantum efficiency of 5—i04 photons/electron the devices green-blue region lower is found in the the reported i0 range whichemitting is aboutin anthe order of magnitude than maximum efficiency values for the blue-emitting devices based on ZnS and GaN single crystals. The hot-pressed ZnS—ZnSe mixtures are found to be solid solution and exhibit similar luminescent characteristics as those of the zinc sulpho-selenide single crystal materials. A spectral shift with compositional changes is observed in both photoluminescence and electroluminescence.

1. Introduction Studies of photoluminescence and electroluminescence in Il—VI compound semiconductors are quite extensive in recent years. Various workers [1—91have reported the fabrication of solid-state light-emitting devices from both single crystal and polycrystalline ZnSe materials. These devices are reported to have emission characteristics in the 5800—6000 A region of the visible spectrum. But the development of high band gap semiconductor materials for blue electroluminescent devices has been very slow. This is due to the complication in either the crystal growth conditions such as in SiC, or in the difficulty of achieving p-type conductivity such as in ZnS and GaN. Devices fabricated from ZnS materials at present are mostly those luminescent cells based on polycrystalline films which are doped with various impurities [10—13].Recently, there is some interest in zinc sulfo-selenide (ZnS~Se 1~) single crystal materials, since devices fabricated from these crystals can emit light in the deep green to blue (4800—5400 A) region of the visible spectrum. Several investi87

88

N.M.P. Low, D.I. Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

gations have been reported in the literature [14 --171. However, there appears no information available in the literature concerning the study of luminescent characteris-

tics and the possibility of device fabriaction from similar ZnS---ZnSe materials based on polycrystalline form. An investigation was conducted in our laboratory to explore this possibility and polycrystalline mixed ZnSe and ZnS powders were used as starting sources. The mixed powders were compressed into high bulk density substrates suitable for device fabrication by a hot-pressing process. A series of devices emitting light varying from yellow-orange to green-blue have been fabricated from the hot-pressed polycrystalline mixed ZnS—ZnSe powders. These devices were fabricated by the use of a metal-semiconductor (MS) device structure. This paper describes some experimental results about the photoluminescence and electroluminescence characteristics observed in our investigation.

2. Experimental The schematic drawing of the hot-pressing assembly unit is shown in fig. 1. The graphite die-plunger unit was supported by a pair of steel rods and end-plates. They were then enclosed in a quartz tube and were placed on the platform of a CARVER hydraulic press. The assembly was then heated by a LEPEL RF generator. The temperature was measured with a pair of Pt—Pt + 13% Rh thermocouple. High purity

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N.M.P. Low, DI. Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

89

(99.999%) ZnS and ZnSe powders obtained from Harshaw Chemicals Co. were used. The particle size of the ZnSe powders are mostly in the 20—60 pm range with a small portion in the 2—5 pm range; whereas the ZnS powders consist of mostly submicron size grains. The appropriate portions of the ZnSe and ZnS powders were thoroughly premixed with an agate mortar and pestle prior to the hot-pressing steps. About 3 grams of the mixed powders were then loaded into a graphite die assembly and hotpressed at 1050°Cand 10200 psi pressure for 30 mm phere. Several compositions in the (ZnS)~(ZnSe)

in a following argon atmos-

1 —x system have been prepared with x values ranging from 0.25 to 0.98. In order to achieve useful n-type conductivity, polished thin sections of the hot-pressed substrates were then annealed in a Zn + Al (5%) alloy melt at 950°Cfor 120 h; The Zn—Al alloy and the sample were placed in a graphite container which in turn was sealed in an evacuated quartz ampoule and were heated in an electric furnace. After annealing treatment, about onehalf mil of the hot-pressed substrate was removed by a light polishing step in Al203 abrasive. This was intended to remove any carbon impurity adhered to the hotpressed substrate surface. In—Ga alloy contacts were subsequently deposited on both sides of the hot-pressed substrate and chips about the size 0.05” X 0.05” X 0.015” were prepared and mounted on transitor headers. This fabrication procedure results in the formation of a [Au—InGa—(ZnS—ZnSe)---InGa] device configuration. For photoluminescence studies, a 200 W mercury lamp (3650 A) and a pulsedargon ion laser (5 120 A) were used to excite the characteristic emission bands. Samples containing a greater proportion of ZnSe were excited with the argon ion laser whereas samples containing a greater proportion of ZnS were excited with the

Hg lamp. Current-voltage characteristics and the electroluminescent properties of the fabricated devices were evaluated using standard LED measuring techniques. The microstructure of the hot-pressed substrates was examined using a Cambridge S-4 scanning electron microscope and the structural properties of the hot-pressed mixtures were analysed

by X-ray powder diffraction technique.

3. Results All hot-pressed substrates in the (ZnS)~(ZnSe)1 —x system were found to be highly dense compacted platelets exhibiting a bulk density close to 95% of the theoretical value. Polished thin sections of the hot-pressed substrates were found

semi-transparent and non-brittle. Fig. 2 shows a scanning electron micrograph of the microstructure of a typical hot-pressed platelet which has a nominal composition of (ZnS)025 (ZnSe)075. The apparent particle segregation shown in the SEM micrograph is possibly related to the different sizes of the starting powders of ZnSe and ZnS. X-ray diffraction analysis showed that the hot-pressed substrates consist of a homegenous cubic phase material, and the cubic structure has interplanar spacing values between the two-end-members in the (ZnS)~(ZnSe)1 —x system. Table 1 lists the d-spacing values for the ZnS, ZnSe and some hot-pressed mixtures. The re-

N.M.P. Low, DI. Kennedy / Studies of poivcrystalline mixed ZnS—ZnSe powders

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Table 1 Interplanar spacings of cubic ZnS, ZnSe, and hot-pressed samples in the ZnS-ZnSe system (hKI)

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N.M.P. Low, D.L Kennedy

/ Studies

of polycrystalline mixed ZnS—ZnSe powders

91

suits are indicative that a solid solution of the two compounds has been formed by the hot-pressing process. As thin polished sections were subjected to the prolonged period of annealing treatment in Zn—Al alloy melt, the microstructure of the annealed sample showed some grain growth but the grain boundary orientation remained essentially unchanged as compared with the unannealed samples. Some unique photolummnescence characteristics have been observed in the hotpressed and unannealed substrates of the ZnS—ZnSe mixtures. Under the excitation of a pulsed argon ion laser (5120 A) all samples gave a broad band emission spectrum in the 5000—7000 A region with a half band-width of about 1000 A, and the emission peaks were found to shift as the sample compositions were changed. Hotpressed samples containing a greater proportion of ZnSe gave an emission peak close to the emission peak of hot-pressed ZnSe as that observed in a previous investigation [91.On the other hand, when hot-pressed samples were excited by the Hg lamp (3650 A), only ZnS rich samples exhibited photoluminescence in the 4000—7000 A region. The emission spectra were broad and a spectral shift with compositions was also apparent. Fig. 3 shows a series of emission spectra under both argon ion laser and Hg lamp excitation. The spectral shift with compositions is evident. It was found that a linear relationship exists between the emission peaks and the compositions of the hot-pressed mixtures, as shown in fig. 4. However, this

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spectral shift was found to be virtually absent when the hot-pressed substrates were after annealing treatment in the Al—-Zn alloy melt. Almost all hot-pressed and annealed samples gave a broad band emission with a peak centred around 6000 A and a half-band-width of about 1000 A. All devices fabricated from the hot-pressed mixtures and based on the [Au—InGa—(ZnS—ZnSe)—lnGa] device structure exhibited a diode characteristic and emitted visible light. The voltage required to induce light emission was found to vary depending on the composition in the (ZnS)~(ZnSe)1 --x system. Diodes fabricated from ZnSe rich material required about 8 V while diodes fabricated from ZnS rich material required as high as 80 V to produce a 5mA current to drive the diodes to achieve sufficient light emission. The [Au—InGa—-(ZnS—ZnSe)—lnGaJ device structure was observed to give electroluminescence in both forward and reverse directions. However, the emission intensity in the reverse direction, that is with the Au wire biased negatively with respect to the bulk semiconductor, was observed to be considerably stronger than in the forward direction. Devices fabricated from ZnS rich samples gave a visual response in the blue region of the visible spectrum; whereas devices fabricated from ZnSe rich samples gave a visual response in the orange-yellow region of the visible spectrum. All diodes exhibited a broad band emission in the electroluminescence and the half band-width of the emission spectrum was about 900 A. Fig. 5 shows

N.M.?. Low, DI, Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders 0

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some typical electroluminescence spectra of a series of diodes fabricated from different compositions. It is evident from these spectra that diodes with composition of (ZnS)025 (ZnSe)075 gave an emission peak centred around 5830 A; while diodes with composition of (ZnS)090 (ZnSe)010 gave an emission peak centred around 5270 A. Again a spectral shift with compositions was evident. In a previous investigation of polycrystalhine ZnSe and ZnS [9] we found that diodes fabricated from hot-pressed polycrystalline ZnSe and ZnS gave electroluminescence with peaks centred at 5950 A and at 51 50 A, respectively. These electroluminescence results and those of the hot-pressed ZnS—ZnSe mixtures exhibited a linear dependence of emission wavelength on compositions in the (ZnS)~(ZnSe)1 —x system as shown in fig. 6. The luminescent characteristics of our devices fabricated from the hot-pressed polycrystalline ZnS—ZnSe mixtures have been found to be consistent with the resuits recently reported by Ozsan and Woods [151and Robinson and Kun [161. The former workers used a ZnS06Se04 single crystal and observed room temperature device emission with a maximum at 5450 A (2.27 eV); whereas the latter workers used a ZnS045Se0~5single crystal containing iodine impurity and observed device emission at about 5900 A (2.1 eV). The results reported by these workers are also marked in fig. 6. However, it must be pointed out that the voltages required to induce the devices to produce light emission between our devices and those of other workers are quite different. Ozsan and Woods have reported that a bias of 11 V is required to operate their device; while Robinson and Kun reported that only 1.4 V is required to activate their devices for light emission. In our investigation we found that diodes fabricated from the hot-pressed polycrystallmne material with a nominal composition of (ZnS)050 (ZnSe)0~0required about 30 V in the reverse direction to produce sufficient light emission in order to be viewed in a semi-shaded area.

N.M.P. Low, DI. Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

94

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The room temperature quantum efficiency of several best diodes emitting in the green-blue region was determined by the integrating sphere technique and was found in the l0~—l0~ photons/electron range in the reverse direction. These device efficiency values were found to be lower than the value of 10~ for the green electroluminescence in single crystal ZnS06Se04 reported by Ozsan and Woods [15] and the value of 10—2 for the yellow luminescence in single crystal ZnS045Se055 reported by Robinson and Kun [16]. However, the device efficiency of our green-blue diodes were only about one order of magnitude lower than the highest external quantum efficiency values (5 X l0~ and 2 X l0~) for the blue-light-emitting ZnS diodes reported by Katayama et al. [18] and for the blue-light-emitting GaN diodes reported by Pankov [19], respectively. Fig. 7 shows an example of the log plot of the emission intensity with diode current for two typical diodes fabricated from the hot-pressed ZnS—-ZnSe mixtures and one similar diode fabricated from hot-pressed polycrystalline ZnSe powders. The emission intensity of all three diodes was observed to increase linearly with the diode current over the range from i0~ to 10—2 A. The emission intensity appeared to increase slightly as the content of ZnS in the diode composition was increased. This was probably related to the spectral sensitivity of the photomultiplier detector. The overall device characteristics of our devices fabricated from hot-pressed ZnS—ZnSe mixtures appeared to be quite similar to those reported by Ozsan and Woods [15] based on ZnS06Se04 single crystal and a [Au---ZnS06Se04---ln]device structure.

N.M.?. Low, D.I. Kennedy

/ Studies of polycrystalline

mixed ZnS—ZnSe powders

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4. Discussion

The overall features in our study of the room temperature photoluminescence and electroluminescence of the hot-pressed ZnS—ZnSe mixtures are evident that the luminescence properties of the hot-pressed materials and devices arise from a zinc sulfo-selenide crystal structure. The absence of individual photoluminescence peaks corresponding to either pure ZnSe or pure ZnS under both excitation sources

96

N.M.?. Low, D.L Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

and the consistent results of X-ray diffraction analysis definitely indicate that a solid solution in the (ZnS)x (ZnSe)i x system has been formed from the ZnS- ZnSe mixtures by the hot-pressing process. Since ZnSe and ZnS are isostructure and have cubic symmetry with a space group F43M, the formation of a zinc solfo-selenide crystal lattice requires only the inter-atomic diffusion between the Se and S species from the lattices of the two end members. The hot-pressing process is capable of inducing a localised inter-diffusion of the group VI components. Furthermore, because of the small particle size of both powders, there exists a large surface area in the grain boundaries. It is therefore not unreasonable to envision an inter-atomic diffusion process taking place across the grain boundaries resulting in the formation of a zinc ---sulfo—selenide lattice. This postulate is consistent with the general accepted theory that the hot-pressing process is essentially a diffusion controlled mechanism [20]. Several models have been proposed to account for photoluminescence in undoped ZnSe and ZnS single crystals. For undoped ZnSe single crystals, Stringfellow and Bube [211 have explained the emission peak in the vicinity of 6400 A in the undoped ZnSe single crystal as resulting from a recombination of electrons, either in the conduction band or in a shallow level, with a hole captured at charge state of the Cu impurity substituting for Zn in the ZnSe lattice. The Cu ions appear as intrinsic impurity ions in the ZnSe material. On the other hand, hida [22] has considered a similar energy position in the emission spectrum of ZnSe to be due to a donor--acceptor pair recombination involving subsidiary association of impurities and native defects. For ZnS single crystals, Curie and Prener [231 have interpreted the emission peak at 4600 A as self-activated luminescence involving the cooperation of native defects and residual impurities and the emission peak at 5150 A as a recombination process involving residual Cu impurities, substituting for Zn in the ZnS lattice in combination with a sulphur vacancy, i.e. a CU~n_Vsrecombination. All these models seem adequately to explain the mechanisms of the luminescence emission at a specific wavelength in the visible spectrum under photo-excitation. However, these models seem inadequate to account for the mechanism of photoluminescence observed in the hot-pressed materials in the (ZOS)x (ZnSe) 1 —x system. It is quite possible that more than one mechanism is operative simultaneously. The intrinsic impurities in the starting ZnS and ZnSe powders are not known since no mass spectroscopic analysis are performed on these powders before use. However, it is expected that compositional changes will not alter the impurity species but its concentration may have varied as the amounts of the two powders are changed. In view of this, the spectral shift of the photoluminescence spectra could not be ascribed to the changes of impurity centres in the hot-pressed materials. All hot-pressed and unannealed samples give essentially a single peak and broad band emission at 300 K. It appears possible that this luminescence arises from a band-toband recombination. This assumption is not entirely unreasonable in view of the fact that the hot-pressed ZnS—SnSe mixtures are of a solid solution between the two end members. The formation of a solid solution in the (ZnS)x (ZnSe)i~ system

N.M.P. Low, D.L Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

97

would very likely cause a change in the band gap energy between the band gap energy of ZnS (3.7 eV) and of ZnSe (2.7 eV). However, this assumption is yet to be confirmed by a band structure investigation. The hot-pressing process involves the rapid application and subsequent sudden cessation of the combined high temperature and extreme hydrostatic pressure on the lattice of the ZnS—ZnSe grains. These sudden changes are possibly influencing the equilibrium of the inter-atomic diffusion process between the Se and S species and the final stoichiometry of the reacted solids. If both cation and anion vacancies coexist in the neighbouring sites of the lattice, vacancy pairs such as [Vzn ~~‘Se,S] may possibly be formed in the lattice, It seems not unreasonable to hypothesize that these native defects in association with the unidentified intrinsic impurities may also contribute to the mechanism giving rise to a luminescence process. However, the exact nature of the recombination process involving these vacancy pair complexes is yet to be understood. Two assumptions can be made. First, a complex is formed by the pairing of a single ionized vacancy pair (Vz~_V~e~) with an ionized impurity center (Ak) to form a [(Vzn_V 5e5)_A~] neutral defect. The luminescence would then be a consequence of an electron transition. Second, the vacancy pair is doubly

ionized and the defect is an ionized acceptor [(V~n_V~,~e)_A~].

In this case,

luminescence would be a donor—acceptor recombination between the unidentified impurity and the acceptor defect. The broad band emission at 6300 A in Al-doped ZnSe has been interpreted as a self-activated luminescence arising from a complex involving a Zn vacancy and an Al~ impurity [23]. This complex is formed at high temperature during the annealing process and consists of a zinc vacancy in which one or two electrons may be trapped and of an Al impurity located at the next neighbour sites in the ZnSe lattice. In the hot-pressed ZnS—ZnSe mixtures the emission peak is located around 6000 A. It is not unreasonable to interpret that this emission is also arising from a complex of zinc vacancy in associated with an Al impurity. The absence of a pronounced spectral shift of the hot-pressed ZnS—ZnSe mixtures with compositional changes after prolonged annealing treatment in the Zn—Al alloy melt is not quite understood at present. The following reasons are tentatively proposed to account for its absence. The zinc vacancy (V~n)which existed in the solid solution lattice has reached an equilibrium state between the lattice and the alloy melt at the annealing temperature, and the pairing of the zinc vacancy with the doped Al impurity became dominantly preferable. This dominant process of defect formation thus gives rise to the main emission peak under photo-excitation, although the band gap energy of the hot-pressed solid solutions is varying with compositional changes. Unfortunately, no low temperature photoluminescence measurements are conducted to verify the details of these spectral characteristics. Based on their studies of n-type ZnSe single crystals, Allen et al. [4,7] have discussed that in the Schottky type diodes under a reverse bias, two mechanisms can give rise to electroluminescence in the n-type ZnSe material. The first mechanism is primarily applicable to materials with luminescent impurity added. Tl1e second me-

98

NM.P. Low, D.L Kennedy / Studies of pol-t’crystalline mixed ZnS—ZnSe powders

chanism is primarily applicable to materials free from deliberately added luminescent impurities. In the first mechanism, electrons tunnel from the metal contact into the semiconductor and are accelerated in the high field of the depletion layer until they acquire sufficient energy to impact-ionize the luminescent centres. Light is subsequently emitted when electrons recombine with the luminescent centres. In the second mechanism, light is emitted resulting from transitions of electrons between different valleys in the conduction bands. In the present study, the [Au—InGa--(ZnS + ZnSe)— InGa] device structure is essentially of a Schottky type. However, the experimental data appear to be insufficient to confirm one of those two mechanisms as a possible mechanism responsible for light emission in the hot-pressed ZnS—ZnSe solid solutions. Although the hot-pressed substrates are deliberately diffused with A1~impurity by a prolonged annealing treatment, the electroluminescence characteristics of the fabricated devices do not seem to exhibit the influence by these impurity centres. On the other hand, the electroluminescence of the devices appears to be more characteristic of the host material. The hot-pressed and unannealed substrate materials are supposedly free from deliberately added luminescent impurities. In view of this, it tempts to ascribe that the transition of electrons between different valleys in the conduction band is a possible mechanism to give rise to the electroluminescence in the hot-pressed ZnS—ZnSe devices.

5. Conclusion The present investigation has demonstrated that good quality and high bulk density polycrystalhine powder compacts of the lI—yE compound semiconductors can be prepared by a hot-pressing process and the prepared substrates are suitable for fabrication of light-emitting devices. The hot-pressing process has also been shown to be capable of forming a solid solution from the mixed powders in the (ZflS)x (ZnSe) 1 —x system. The hot-pressed ZnS—ZnSe solid solutions exhibit some unique photoluminescence characteristics in which a spectral shift with compositional changes is evident. The fabricated devices from the hot-pressed ZnS—ZnSe solid solutions give light emissions varying from orange-yellow to green-blue depending on the composition of the devices. These observations are consistent with the speculation proposed by Aven and Devine [14] who have suggested that electroluminescence in Zn(S, Se) crystals could cover the complete yellow through blue range of the visible spectrum. The room temperature quantum efficiency of the devices fabricated from hot-pressed polycrystalline ZnS—ZnSe powders is low (10~—l0~photons/electrons), but the ability of preparing devices with green-blue visual response employing polycrystalline materials as starting sources is commendable, it may perhaps be possible to improve both material quality and device efficiency such that the fabricated devices can be utilized for practical applications.

N.M.P. Low, DI. Kennedy / Studies of polycrystalline mixed ZnS—ZnSe powders

99

Acknowledgement This work was carried out as part of a research program jointly supported by the U.S. Air Force under contract F 336l5-72-C-l466 and the Defence Industrial Research Program administered by the Canadian Defence Research Board under grant

XE204. References [i[ A.G. Fischer, Phys. Letters 12(1964) 313. [21 Y.S. Park, C.R. Geesner and B.K. Shin, App!. Phys. Letters 21(1972) 567. [31 A.W. Livingstone and J.W. Allen, Appl. Phys. Letters 20 (1972) 207. [41 J.W. Allen, A.W. Livingstone and K. Turvey, Solid State Electron. 15 (1972) 1363. [5] A.W. Livingstone, K. Turvey and J.W. Allen, Solid State Electron. 16 (1973) 351. [6] K. Ikeda, K. Achida and Y. Hamakawa, J. Phys. Chem. Solids 34 (1973) 1985. [71 K. Turvey and J.W. Allen,J. Phys. C: Solid State Phys. 6(1973)2887.

[8] Hideo Watanabe, Takao Chikamaura and Masanoba Wada, Japan, J. App!. Phys. 13 (1974) 357.

[91 N.M.P. Low and L.H.W. Bradfield, Technical Report AFML-TR-74-246 USAF Contract No. F 33615-72-C-1466, February 1975.

[10] A. Vecht, N.J. Werring, R. Ellis and P.J.F. Smith, Brit. J. App!. Phys. (J. Phys. D) Ser. 2, 2 (1969) 953.

[11] N.J. Werring, A. Vecht and J.W. Fletcher, Brit. J. App!. Phys. (J. Phys. D) Ser. 2, 2 (1969) 509. [12] M.S. Wait and A. Vecht, App!. Phys. Letters 19 (1971) 471.

[13] Royoichi Yamamoto and Hiroshi Yamazol, J. App!. Phys.44 (1973) 3191. [14] M. Aven and J.Z. Devine, J. Luminescence 7 (1973) 195. [15]

M.E. Ozsan and J. Woods, App!. Phys. Letters 25 (1974) 489.

[16] R.J. Robinson and Z.K. Kun, App!. Phys. Letters 27(1975) 74. [171 Z.K. Kun and R.J. Robinson, Paper presented at the International Conference in Luminescence, Tokyo, Japan 1975, Paper F7. [18] H. Katayama, S. Oda and H. Kukimoto, AppI. Phys. Letters 27 (1975) 697. [19] J.I. Pankov, Phys. Rev. Letters 34(1975) 809. [20] T. Vasios and R.M. Spriggs, J. Am. Ceram. Soc. 46 (1963) 493. [21] GB. Stringfellow and R.l-l. Bube, Phys. Rev. 171 (1968) 903. [22] S. Iida,J. Phys. Soc. Japan 26(1969) 1140. [231 D. Curie and J.S. Prenet, in: Physics and chemistry of Il—VI compounds, eds. M. Aven and J.S. Prener (North-Holland, Amsterdam, 1967).