Trace analysis in cadmium telluride by heavy ion induced x-ray emission and by SIMS

NUCLEAR

INSTRUMENTS

AND M E T H O D S

168 ( 1 9 8 0 ) 3 6 7 - 3 7 1 ;

(~) N O R T H - H O L L A N D

PUBLISHING

CO.

TRACE ANALYSIS IN CADMIUM TELLURIDE BY HEAVY ION INDUCED X-RAY EMISSION AND BY SIMS C. SCHARAGER, R. STUCK, P. SIFFERT Physique et Applications des Semi-Conducteurs J. CAILLERET, CH. HEITZ, G. LAGARDE and D. TENORIO Laboratoire de Chimie Nuch;aire, Centre de Recherches Nuclkaires associ~ ~ I'Universit~ Louis Pasteur, Strasbourg, France

The possibilities of using both selective heavy ion induced X-ray emission and secondary ion mass spectroscopy (SIMS), for the identification of impurities present at low concentrations in cadmium telluride are examined. The relative concentrations of the impurities along CdTe crystals have been determined by exciting the X-ray emission of the elements in several slices with Ar and Kr ions and by comparing the relative characteristic X-ray emission yields. As a consequence of the quasimolecular inner shell ionization mechanism in heavy ion-atom collisions, Ar and Kr ions allow a strong excitation of the main impurities seen by SIMS, namely Si, CI and Ge, As, with only a minor contribution of Cd and Te. From the changes of the concentrations of the various impurities along the crystal, informations about segregation coefficients and compensation can be obtained.

1. Introduction High purity cadmium telluride is an optimal material as a nuclear gamma-ray detector which can work at room temperature and as photovoltaic converter for solar energy~). Several properties of this material depend on the nature and the concert-

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tration of trace elements present in crystals. These impurities originate from the starting products or are introduced intentionally by doping, or are due to contamination during the processing. As the concentrations are of the order of 1 ppm or less, high sensitive analytical procedures are needed in

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Fig. 1. Schematic correlation diagram for Ar-Si and Kr-As collisions. Straight lines between the electron levels of the separated and the united atoms indicate the electron promotion during the collision. Vertical arrows indicate the filling of vacancies and the consecutive X-ray emission. V. S P U T T E R P R O F I L I N G AND SIMS

368

c. SCHARAGER et al.

order to identify and control the distribution of the dopants and impurities in the crystals. Furthermore, due to the matrix corhposition, interference effects from cadmium and tellurium have to be avoided, or at least limited to a minimal level. This can be achieved by SIMS or by selective heavy ion induced X-ray analysis. The first of these techniques is strongly affected by the presence of impurities: the ion production rate for a given element depends on the presence of foreign atoms in the lattice or at the surface even at low concentrations. For example the enhancement of secondary ion yields by oxygen is well known2). Concerning CdTe it has been shown 3'4) that surface layers are formed following the etching of the crystals. These layers influence the ion yields of Cd and Te. Furthermore, the sputter rates of these elements are probably different leading to different Cd/Te intensity ratios at the surface and in the bulk for clean crystals. Therefore, SIMS will yield ambiguous quantitative results, despite its high sensitivity and another analytical technique is useful for the understanding and interpretation of SIMS measurements. Heavy ion induced X-ray emission via quasimolecular inner shell ionization mechanism during heavy ion-atom collisions, allows both a high sensitivity for selected elements and the possibility of determining their relative concentrations in several samplesS). This goal is reached by a convenient choice of the nature and of the energy of the bombarding ion. The

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choice of Ar and Kr ions to induce selectively the X-ray emission of elements in the AI-C1 and in the Z n - B r regions is justified by the schematical electron promotion diagrams for Ar-Si and K r - A s collisions (fig. 1) and for collisions of the same ions with the major bulk elements Cd and Te (figs. 2, 3). The diagrams are based on the theoretical description of inner shell ionization in heavy ion collisions from Fano and Lichten 6) and Barat and Lichten7). They allow the following predictions: - in the case of Ar bombardment a strong excitation of the K X-rays of Si (or of the neighbouring elements) is expected, whereas no K X-rays from Cd or Te will be produced; only a contribution from CdL lines due to the Ar 1 s - C d 2p level matching effect is expected. - In the case of Kr bombardment the K levels of As and of the neighbouring elements are strongly excited together with the L levels of Cd and Te. The intensity of the X-rays originating from the last can be reduced by the use of absorbers.

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Fig. 2. Schematic correlation diagram for Ar-Cd, Te collisions. Dashed lines correspond to the Ar ls-Cd 2p level matching effect.

2.1. MATERIALS High resistivity cadmium telluride is prepared in a graphite-coated quartz tube by reaction of equal amounts of ultrapure (6N) Cd and Te. Three ingots

369

T R A C E A N A L Y S I S IN C A D M I U M T E L L U R I D E

with series number 1321, 1327 and 1336 have been used for this work. The first two ingots have been purified by zone melting at ll00°C (20 passes at 2.8 m m h -~) whereas no further purification has been performed on the last one. The crystal growth has been performed by the Travelling Heater Method (THM)3). The thickness of the melted zone was 0.85 cm and the temperature 800°C. Doping of the crystals occurs during the same sequence by addition of defined quantities of the dopants to the Te zone. Ingots 1321 and 1336 have been doped with CI by addition of 0.67 mg of CdC12. Ingot 1327 has been doped by addition of 10 mg of high purity Ge. For analysis, the ingots are cut in slices 3 mm thick, which are lapped with grain 600 carborundum and etched in a methylbromide solution. The etching is quenched with pure methanol. 2.2. ANALYSES 2.2.1. Secondary ions mass spectroscopy (SIMS) For SIMS analysis a commercial apparatus is used. The measurements are performed on the positive ions sputtered off the surface of the samples by a 3 keV Ar + beam. The scanning time of each mass is 3 s. The current density of the Ar ÷ beam is kept below 1/~m.cm 2 in order to sputter off a negligible thickness of material during the measurement. Positive charge accumulation on the semi-insulating sample is avoided by simultaneous low energy electron bombardment. The residual pressure in the analysis chamber is approximately 10 -9 tOrT. In order to be free of surface contamination a preliminary sputtering of 6 h is used to ensure that the bulk material is really investigated. 2.2.2. Heavy ion induced X-rays The 0.8 MeV Ar + ions and 5.2 MeV Kr 2+ ions used in this work are delivered by the 4 MV CRN Van de Graaff accelerator. The samples are disposed perpendicular to the beam axis and the Xrays are detected in a Si(Li) counter at an angle of 135 ° with respect to the beam direction. The Si(Li) detector having an energy resolution ranging from 160 to 250eV in the region of interest (1.25-15keV) is applied directly to the vacuum system through a 100/~m Be window. In the case of Kr bombardment a 20/~m A1 absorber has been disposed ~n front of the detector in order to reduce the intensities of low energy X-rays (L radiations from Kr, Cd and Te). In order to avoid pulse pile-

up, the counting rate has been kept below 400 pulses s-~ by limiting the beam intensity. The intensities of the various X-ray lines have been computed with a program elaborated by Schuhg). Since the goal of these measurements is the study of the evolution of the relative concentration of impurities from slice to slice along the crystals no absorber corrections have been performed. In order to examine the materials' stoichiometry K X-rays of Cd and Te elements have been induced by a 3 MeV proton bombardment. 3.

Results

and

discussion

A typical mass spectrum obtained by SIMS is shown in fig. 4. It shows the lines of trace elements originating obviously from the chamber (Fe and Cr). X-ray spectra from Ar ÷ and Kr 2+ ion bombardment are presented in figs. 5 and 6. The detected elements are indicated on the spectra. The main features of the spectra are the following: - Concerning the target bulk, overlapping L lines from Cd and Te. - Concerning the projectiles, A r - K line appearing as a hump on the low energy side of the C d - L z line in fig. 5, and K r - L line at the low energy end of the spectrum in fig. 6. - Concerning trace elements, peaks corresponding to impurities or dopants from which the most important are those of Si in the Ar induced spectrum, and As in the Kr induced spectrum.

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V. S P U T T E R P R O F I L I N G AND SIMS

370

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Fig. 5. ×-ray spectrum from a 0.8 MeV Ar bombarded CdTe sample (ingot 1321).

For ingot 1321 the ]3°Te/1]4Cd ratio obtained from SIMS and of the proton induced K X-ray intensity ratio show a significant difference as a function of slice position in the ingot as shown in fig. 7. While the X-ray intensity ratio remains constant along the crystal, revealing a constant macro composition, the mass intensity ratio varies showing the same trends as the resistivity presented on the same figure. This effect is probably related to the variation of the ion production ratio of Cd and Te along the ingot. Indeed, the evolution of the resistivity along the crystal is probably due to an impurity, which not only changes the free carrier concentration but also modifies the ion production rate4). For comparison of the Si and CI concentrations in the different slices the corresponding X-ray intensities are compared to the intensity of the Cd L X-ray line. Indeed, SIMS measurements cannot be used: the observation of Cr and Fe in the spectra suggests that Si may also partly originate from the chamber material (stainless steel). Furthermore, poor statistics due to the short time of measurements leads to large fluctuations on the

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T R A C E A N A L Y S I S IN C A D M I U M T E L L U R I D E

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28Si/114Cd mass intensity ratio. The Si/Cd X-ray intensity ratios for the melted zone purified ingot 1321 and the non purified ingot 1336 are shown in fig. 8. Significant differences appear. In the purified sample, the concentration of Si is an order of magnitude higher in the first slices than in the non purified sample. Then the concentration drops to comparable values in the two crystals. This suggests that during zone melting Si diffuses from the quartz

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tube and is concentrated in the first slices of the ingot. This phenomenon indicates a segregation coefficient for Si higher than 1. The doping procedure with C1 allows a direct determination of the segregation coefficient of this element. Using the C1/Cd X-ray intensity ratio in fig. 9 a value of 0.01 is calculated from classical zone melting laws~°). This result is by a factor 2 higher than the value given by Zanio~1). Search for Ge in ingot 1327 has been unsuccessful. Whenever Ge does appear (fig. 6) it does so only as a light shoulder on the low energy side of a surprisingly high As peak. At this state of the experiment the origin of As cannot be explained unambiguously mainly because this element is not observed for all slices and because no regular trends of the As/Cd X-ray ratio along the ingot are observed. 4. Conclusion The results show that SIMS and selective heavy ion induced X-ray analysis are complementary methods for trace analysis and profiling. SIMS gives a direct indication of impurities which are present. However, in these compounds errors are possible by this technique and another evaluation procedure is required. The latter can be selective X-ray excitation by heavy ions which allows better statistics and is not affected by the presence of foreign atoms. Furthermore, the incident ion and its energy can be chosen as a function of the impurity under analysis in order to obtain a maximum sensitivity, especially when compared to conventional light projectile excitation.

References

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1) p. siffert, Nucl. Instr. and Meth. 150 (1978) 1. 2) p. Williams and C. A. Evans, Surface Sci. 78 (1978) 324. 3) M. H. Patterson and R. H. Williams, J. Phys. D : Appl. Phys. 11 (1978) L83. 4) M. Hage-Ali, R. Stuck, A.N. Saxena and P. Siffert, Appl. Phys. 19 (1979) 25. 5) C. Heitz, M. Kwadow and D. Tenorio, Nucl. Instr. and Meth. 149 (1978) 483. 6) U. Fano and W. Lichten, Phys. Rev. Lett. 14 (1965) 627. 7) M. Barat and W. Lichten, Phys. Rev. A6 (1972) 211. 8) R. O. Bell, N. Hemmat and F. Wald, Phys. Stat. Sol. 1 (1970) 375. 9) H. W. Schuh, Diplomarbeit 1975, Universit~it K61n. 10) B. R. Pamplin, in Crystal growth Int. Ser. Monogr. Sol. State Sci., Vol. 6 (Oxford Univ. Press; London, 1975) p. 117, 11) K. Zanio, J. Electron. Mater. 3 (1974) 327.

V. S P U T T E R P R O F I L I N G AND SIMS