Solid solutions in the dilute magnetic semiconductor MnxZn1−xS

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CRYSTAL

GROWTH ELSEVIER

Journal of Crystal Growth 173 (1997) 222 225

Letter to the Editors

Solid solutions in the dilute magnetic semiconductor MnxZnl-xS Victor J. Garcia a'*'l, J. Mauro Bricefio-Valero a, Leonardo Martinez a, Andres Mora a, S. Adan Lopez-Rivera a, Witold Giriat b aDepartamento de Fisica, Facultad de Ciencias, Universidad de los Andes, M#ida 5101, Venezuela b Centro de Fisica, Instituto Venezolano de Investigaciones Cientificas (IVIC), Caracas, Venezuela Received 12 January 1996; accepted 19 September 1996

Abstract

The diluted magnetic semiconductor system MnxZnl-xS was studied using scanning electron microscopy, energydispersive X-ray spectrometry and X-ray diffraction. The samples were grown using the chemical transport technique with nominal Mn concentration (x) of 0.01, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5. Samples with Mn concentration greater than 0.4 presented two different phases while samples with concentration of 0.15 and 0.20 showed chemical inhomogeneities.

Diluted magnetic semiconductors (DMS) are materials formed by partially replacing the cation with a transition metal atom into a host lattice of nonmagnetic semiconductor. These materials show a composition band gap and lattice dependence typical of ternary alloys [1]. Most of the effort in this field has been focused on Mn-based DMS materials, as summarized in several review articles [2-7]. The presence of Mn 2 + leads to spectacular * Corresponding author. Fax: + 1 515 294 1214; e-mail: [email protected]. 1 Present address: iowa State University, Materials Science and Engineering, 3053 Gilman Hall, Ames, Iowa 50011-3110, USA.

magnetic effects due to their strong interaction with free or impurity-bound carriers. Thus, phenomena like magnetic-phase transitions, giant Zeeman split ting and Faraday rotations are common in DMS. The MnxZnl_xS system has been studied using different modes of operation of a scanning electron microscope (Hitachi S-2500) coupled with an energy-dispersive X-ray (EDX) spectrometer for chemical analysis. The purpose of this study was to identify the composition which leads to chemical inhomogeneities and to detect the presence of phases for samples grown with nominal Mn concentration (x) of 0.01, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5. Crystals of MnxZnl xS have been prepared by different techniques [1, 8,9]. The

0022-0248/97/$17.00 Copyright ,~) 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 7 9 3-2

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K ~ Garcla et al. / Journal o f C~stal Growth 173 (1997) 222 225

chemical transport is frequently used to prepare these crystals because of the high melting point involved in the preparation of these materials; and, this was the method employed in the present work. The starting materials were the binaries ZnS and MnS. In order to obtain quality samples, highpurity elements were obtained. To achieve this, sulfur and zinc were triple-distilled and manganese was purified by sublimation. Once the elements were purified, several grams of ZnS and MnS were synthesized as described in Ref. [1]. Then, 3 g of (ZnS)x and (MnS)l-x in stoichiometric quantities were mortar-powdered to fine grain size, and then placed in a clean-etched quartz tube with 10 x 10 .3 g/cm 3 of iodine as a transport agent, The ampoule was placed in a furnace with a temperature gradient of 20°C (1000 980':C) after it was vacuum sealed. The end of the tube containing the material was in the high temperature region. The crystals were expected to grow in the zone of low temperature [9]. From previous experience we have found that this gradient gives the best result when the internal radius of quartz tube is of 15 mm and 12 15 cm of length. After a week of growth, crystals of different sizes with flat and smooth faces were obtained. For this work we prepared samples with nominal Mn concentration (x) of 0.01, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4 and 0.5. The surface topography and the elemental composition of the MnxZnl_~S system was studied using a scanning electron microscope (Hitachi S2500) coupled with an energy-dispersive X-ray (EDX) spectrometer for chemical analysis. In order to obtain clean and atomic flat surfaces the samples were cleaved before they were taken to the scanning electron microscope (SEM) chamber. The lattice parameter of the compounds were determined by X-ray diffraction. The microchemical composition was found by an EDX coupled with a computer-base multichannel analyzer (MCA), (DELTA III analyzer with quantex software from Kevex, USA). For the EDX analysis the K~-lines for S, Mn, and Zn and an accelerating voltage of 15 kV were used. The samples were tilted 35 ° for a better collection of X-ray signal. A standardless EDX analysis was made with relative errors of the order of _+ 5% and detection limits of 0.3 wt% [10], where the k-ratios are based

on theoretical standards (ZAF corrections via M A G I C V [11]). By entering the number of sulfur atoms with its respective valence number into another routine of the quantex software, it was possible to calculate the percentage of the cations present in each sample. The specimens were examined with secondary and backscattered electrons detector. The use of the backscattered detector allowed discrimination between different chemical compositions. Several points, minimum of four, were analyzed with the electron beam in order to study the chemical homogeneity of the crystals [10]. The qualitative analysis of the EDX spectra shows that the only elements present in the samples were Mn, Zn and S, as expected, and the quantitative analysis gave the experimental concentration values shown in Table 1. The experimental concentraion values, when compared with the nominal values, were within the experimental error for all samples except for x = 0.15, 0.20 and 0.50. Let us say a few words about the sample with x = 0.5. The two values obtained were below nominal concentration. When this sample was analyzed with the backscattered electron detector, two chemically different phases were observed as shown in Table 1 Elemental concentrations for the MnxZn~ .,S system Nominal Mn concentration

Mn(x)

Zn(1 - x)

S

0.01 0.03 0.05 0.10 0.15

0.009 0.028 0.054 0.095 0.312 0.105~ 0.346(1)b 0.157(2)"'b 0.183(1)c 0.204(2)c 0.151(3)~'c 0.234(4)~ 0.295 0.393 0.378 0.181

0,990 0.971 0.945 0.905 0.687 0.895 0.653 0.843 0.817 0.796 0.849 0.766 0.709 0.607 0.628 0.819

1 1 1 1 1 1 1 1 1 1 1 l 1 1 1 I

0.20

0.30 0.40 0.50 (dark region) (bright region)

~The most frequently measured Mn concentration values. bFor identification of these point see Fig. 3a. CFor identification of these point see Fig. 3b.

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Mn Concentration (x) Fig. 1. Secondary electron (upper image) and backscattered (bottom image) micrograph for sample with Mn nominal concentration of 0.50.

Fig. 1. For the sake of comparison, the image corresponding to the same area obtained with secondary electron is also shown in Fig. 1. The existence of these two phases are supported by the results from X-ray diffraction, shown in Fig. 2, where the experimental lattice parameters are given for all the samples. Vegard's law is satisfied by the values of the lattice parameters of all the samples, except for the one corresponding to the crystal with 0.5 nominal concentration. N o w we turn our attention to the samples with nominal concentration of 0.15 and 0.20. For these two samples, the most frequent Mn concentration value found with EDX was 0.105 and 0.151, respectively. These values were found in regions within the cleaved surfaces and are indicated with points 2 in Fig. 3a, and 3 in Fig. 3b for the case of nominal concentration of 0.20. Regions rich in Mn were located on the edges of the crystals, suggesting the precipitation of a second phase. The sample with

Fig. 2. The lattice parameter a (A.) of MnxZn~-xS solid solutions as a function of Mn concentration. The data fit the equation a = 3.823 + 0.184x.

nominal concentration of 0.20 also has some small regions where the Mn concentration was close to the nominal value points 1, 2 and 4 in Fig. 3b. There are two things to notice regarding the different experimental values obtained for the samples with nominal concentration of 0.15 and 0.20. First, the EDX results of the samples grown with nominal concentrations of 0.15 and 0.20 show that the actual concentrations of these samples are 0.105 and 0.151. Second, the few Mn-rich regions found in these samples might be caused by the precipitation of a second phase. The small amounts of this second phase cannot be detected by X-ray diffraction because this technique determines the lattice parameter averaged over all anion and cation sites. We believe that the precipitation of this second phase is due to the smooth transition from zinc blende to wurtzite structure. This behavior has been observed for the MnxZnl-xS system with concentration values within these ranges [1].

I4,Z Garcia et al. / Journal of Crystal Growth 173 H997) 222 225

(a)

225

(b)

Fig. 3. SEM micrograph for the sample with x = 0.2, indicating the points where the EDX data were taken. The EDX values for the areas indicated with numbers are given in Table 1.

T h i s letter s h o w s t h a t t h e s o l u b i l i t y limit of (MnS)x in ( Z n S ) t - x is r e a c h e d for v a l u e s of x bet w e e n 0.40 a n d 0.50. T h e different c o n c e n t r a t i o n v a l u e s o b s e r v e d for s a m p l e s w i t h n o m i n a l c o n c e n t r a t i o n of 0.15 a n d 0.20 m i g h t be e x p l a i n e d by the stress p r o d u c e d in the c r y s t a l lattice d u e to the c o e x i s t e n c e o f the zinc b l e n d e a n d w u r t z i t e struct u r e at these t w o c o n c e n t r a t i o n s . O u r results are c o n s i s t e n t w i t h the p h a s e d i a g r a m r e p o r t e d by S o m b u t h a w e e et al. [12].

References [l] W. Giriat and J.K. Furdyna, in: Semiconductors and Semimetals, Vol. 25, Eds. J.K. Furdyna and J. Kossut (Academic Press, Boston, San Diego, 1988) ch. 1, p. 1. [2] R.R. Galazka, Mater. Sci. Forum 128 (1995) 371.

[-3] N. Samarth and J.K. Furdyna, Proc. IEEE 76 (1990) 990. [4] O. Goede and W. Heimbrodt, Phys. Status Solidi 146 (1988) 11. [-5] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [6] S.A. Lopez-Rivera, L. Martinez, J.M. Bricefio-Valero, M. Moreno, F. Medina and W. Giriat, Proc. 8th Ternary and Multinary Compounds, Eds. S.I. Radautsan and G. Schwab (Kishinev, USSR, 1990). [7] C. Benecke, W. Busse and H.E. Gumlich, Proc. 4th Int. Conf. on II IV Compounds Berlin (1989). [8] A. Pajaczkowska, Progr. Crystal Growth Characterization 289 (1978). [9] R. Nitsche, H.U. B61sterli and M. Lichtensteiger, J. Phys. Chem. Solids 21 (1961) 199. [10] J.1. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, A.D. Romig Jr. and E. Lifshin, Scanning Electron Microscopy and X-ray Microanalysis (Plenum, New York, 1992). [11] J.W. Colby, Quantex-ray Instruction Manual (Kevex, Foster City, CA, 1980). [12] C. Sombuthawee, S.B. Bonsall and F.A. Hummel, J. Solid State Chem. 25 (1978) 391.