Austin, JC

Hughes, WC

Swanson, ML

Carolina Digital Repository

Indium-Hg vacancy interactions in Hg 1 ,,Cd,Te measured by perturbed angular correlation W. C. Hughes, M. L. Swanson, and J. C. Austin Department of Physics & Astronomy, CB No. 32.55, University of North Carolina, Chapel Hit/, North Carolina 2 7599-3255 (Received 15 April 1991; accepted for publication 28 May 1991) The interaction of mercury vacancies with dopant indium atoms in Hg0,7&d0.t,Te was studied using the perturbed ‘y”/ angular correlation (PAC) technique+ Two dominant PAC signals, characterized by quadrupole interaction strengths YQ~ = 83 MHz and yoL = 91 MHz and asymmetry parameters v1 = r/2 = 0.08, were observed and attributed to one or more In-Vu, complexes. The complexes appeared after annealing doped samples at T>350 “C in vacuum and quenching. The fraction of In atoms associated with vacancies was increased further by annealing at 80 “C for > 10 h. The In-vacancy complexes vanished on annealing in a Hg-saturated atmosphere. The II-VI narrow band-gap semiconductor Hgi _ .C!d,Te (x = 0.2) is technologically important for its use as an infrared detector in the 8-12 pm wavelength range. In its as-grown state, (Hg,Cd)Te is often p type due to the large concentration of mercury vacancies. When these vacancies are removed by a low-temperature anneal in Hg-saturated conditions, the material becomes n type because of unknown residual impurities.’ Indium is a common donor dopant in (Hg,Cd)Te. Im- plantation of In produces an n-type layer and causes a large amount of damage which inhibits carrier mobility.2 The junction depth is dominated by a tail in the carrier concentration that does not coincide with the implanted region.3 If the In is diffused in under vacuum, Hg vacancies are introduced. The interaction between In atoms and Hg vacancies has been inferred from electrical measurements,4 but In-vacancy complexes have not been identified. Using the perturbed yy angular correlation (PAC) technique, we have made the first atomic-scale measure- ments of the trapping of Hg vacancies by In atoms in (Hg,Cd)Te. PAC has been used in the past to measure defect structure in metals,’ and in semiconductors such as Si6*7 and CdTe8s9 by measuring the hyperfine interaction between the nuclear moment of a probe atom and the local electric field gradient (EFG) caused by the defect struc- ture. This method allows us to measure the short-range interaction between the isotope “*In and point defects in (Hg,Cd)Te. For this experiment, bulk, randomly oriented single- crystal samples of Hg, _ ,Cd,Te (n = 0.21) were provided by H. F. Schaake of Texas Instruments. The samples were grown by melting, quenching, annealing at 660 “C!, at 550 “C in vacuum, and finally at 270 “C in Hg vapor. The first anneal was done for grain growth, the second to re- duce the dislocation concentration, and the third to remove native Hg vacancies and to convert to n type. These sam- ples were cleaned with a 0.5% Br/methanol solution to remove surface impurities and native oxide. They were then immediately sealed in cleaned evacuated quartz tubes in which a solution of about 10 FCi (10” nuclei) of ‘*‘InCl~:HCl had been dried. The samples were then an- nealed at 350 “C for 6 h or 400 “C for 30 min to diffuse in the In. The samples were further prepared by various an- neals in vacuum or Hg atmosphere followed by water quenches or slow cools. One sample was crushed to permit polycrystalline PAC analysis. The PAC experiment consists of measuring the time- dependent anisotropy between the gamma ray, yl, which populates the I = 5/2 state in “‘Cd (the isotope to which ‘“In decays by electron capture) and a second gamma ray, y2, emitted by the transition to the ground state. This is done by measuring the number of coincidence counts be- tween NaI detectors at 90” to each other using a standard slow-fast coincidence technique. The number of coinci- dence counts 1(@,t) of y1 and y2 observed at detectors separated by an angle 0 is directly related to the angular correlation function W( @,t) by5 i(@,t) =loe-“‘W[O,t). (1) Here, r is the lifetime of the intermediate state (120 ns) and I+‘(@,t) is defined by W(@,t) = 1 + &G(t)P2(cos O), (2) where AZ2 is the anisotropy factor determined by the iso- tope and the detector geometry and P2( cos 0) is the Leg- endre polynomial, The time-dependent part is ~~~~=fo + Tfk nIio&n coskn~m%otl~ (3) where vk is the asymmetry parameter of the /c” electric field gradient (EFG), fk is the fraction of In probe atoms in the kth EFG, ok0 is the fundamental precession fre- quency associated with the kth EFG, g, is a parameter that depends on the asymmetry, and Sk, are coefficients depen- dent upon the orientation and asymmetry of the EFG. The EFG is characterized by the parameters q, representing the asymmetry of the EFG, and uQ”eQV,/h, the quadrupole interaction strength. Nere, V,, is the magnitude of the prin- cipal component of the EFG and Q is the nuclear quadru- ple moment. For an axially symmetric EFG, the interac- tion strength is related to the fundamental precession frequency by w. = 37rvd10, and g,:g2:g3 is 1~2~3. A ratio R(t) of the counts i( @,t) for detectors at 180” and 90” gives the time-dependent part of the angular cor- relation: 938 Appl. Phys. Lett. 59 (8), 19 August 1991 0003-6951/91/330938-03$02.00 0 1991 American Institute of Physics 938 R(t) =&G(t). (4) This experimentally measured R(t) can be fitted using a least-squares routine to determine the following informa- tion: (i) the fraction fk of In probe atoms having an EFG associated with a specific defect complex, (ii) the quadru- pole interaction frequency vQ associated with the EFG, (iii) the asymmetry parameter r) of the EFG, and (iv) the orientation of the EFG with respect to the axes of a single crystal. For samples that were doped with “‘In by annealing in evacuated quartz capsules at 350 “C for 30 min and cooling slowly in the furnace, no well defined EFGs were observed. The R(r) spectrum decayed slowly with time, indicating that a large fraction of the In atoms were in sites associated with small EFGs. After diffusion, the samples were an- nealed successively in vacuum for 30 min at 80, 200, 300, 350, and 400 “C and the capsules were quenched in water at 283 K. A PAC spectrum was taken after each of the anneals. As shown in Fig. 1 (a), the anneal at 400 “C pro- duced two signals, corresponding to interaction frequencies vQl = 83 MHz and Ye = 91 MHz, both with asymmetry parameter 77 = 0.08. Similar signals were observed after annealing at 350 “C for 6 h in vacuum and water quench- ing. These signals are attributed to well defined defect com- plexes containing the “‘In probe atoms. The fraction of probe atoms associated with these de- fect complexes was increased by further annealing at 80 “C for > 10 h and water quenching [Fig. l(b)]. The Fourier transform of this spectrum, shown in Fig. 1 (c), indicates the presence of two distinct EFGs. As seen from Eq. (3), every EFG has three frequencies associated with it. The presence of two peaks in Fig. 1 (c) near 160 MHz suggests that there are two nearly equal fundamental frequencies in the vicinity of 85 MHz. These frequencies are indicated by an asymmetric broadening of the Fourier transform peak near 85 MHz and by the separation of the first harmonics at 160 and 170 MHz. The enhancement of the PAC signals caused by annealing at 80 “C in Fig. 1 (b) shows that the vacancies can be rearranged about the In probe atoms without escaping from the vicinity of the In at this tem- perature. This is consistent with previous measurements which have shown that annealing at this temperature is insufficient to remove radiation damage.3*10 The effect of annealing atmosphere on the defect con- centration is shown in Fig. 2. The top panel shows the PAC spectrum obtained after a 6 h vacuum anneal at 350 “C (water quenched) followed by a 12 h anneal at 80 “C (water quenched). This spectrum is similar to that in Fig. 1 (b). After annealing the same sample in a saturated Hg atmosphere for 6 h at 350 “C and water quenching the PAC signals were not seen (bottom panel). Since this treatment reduces the Hg vacancy concentration, this result implies that the signals are due to In-Vn, complexes. The PAC signals were suppressed by annealing in vac- uum at 150 “C for 6 h or at 200 “C for 30 min and quench- ing. Stefanis3 has annealed In-implanted (Hg,Cd)Te for 1 h in vacuum and observed a change in the conductivity from the as-implanted n type top type after anneals at 200 -0. I 2 !Y -0.05 0 0 loo 200 300 400 Time 01s) -0. I & a -0.05 0 100 Tir%?ns) 300 400 !.2 .I “!“‘.I’ ‘I”.‘3” t I (c)j 0.8 2 0.6 0.4 0.2 n 0 50 100 150 2w 250 300 o(MHd FIG. 1. PAC spectra of “‘In in Hg,,,Cd,,,,Te at 293 K. (a) Annealed at 400 “C for 30 min in vacuum and water quenched. (b) Subsequently annealed at 80 “C for 10 h in vacuum and water quenched. The solid lines represent least-squares fits of the data. (c) Fourier transform of R(t) spectrum shown in (b). and 250 “C. Higher temperature anneals were shown to give n-type material which is presumed to be due to the activation of implanted impurities. Similar treatment of Xe-implanted materials show the same switch to p type, but no return to n type on higher anneals as with the In-implanted samples.3 These data along with the present measurements show that the destruction of Hg vacancy clusters in (Hg,Cd)Te occurs in the temperature range between 150 and 200 “C. A series of measurements was done to determine the influence of measuring temperature on the defect fractions and interaction frequencies. A sample was annealed at 350 “C for 6 h and water quenched, crushed to powder to create a polycrystal, then annealed at 80 “C for 12 h to obtain the maximum defect fractions. The powder sample allows the use of known polycrystalline Skn values [Eq. (3)] to deter- mine the defect fractions more accurately.5 The PAC spec- trum taken at 17 K shows two EFGs with interaction fre- quencies vQl = 87 MHz and ye2 = 94 MHz and asymmetries of n = 0.08. Other spectra were taken at var- ious temperatures between 17 and 295 K. The temperature 939 Appl. Phys. Lett., Vol. 59, No. 8, 19 August 1991 Hughes, Swanson, and Austin 939 -0.2 -0.05 0 loo 200 300 400 Time (ns) FIG. 2. Effect of annealing atmosphere on defect complexes. The top panel shows a PAC spectrum taken at 293 K from a sample vacuum annealed at 350 “C for 6 h and water quenched, then vacuum annealed at 80 “C for 12 h. The bottom panel shows a spectrum from the same sample subsequently annealed at 350 “C for 6 h in a Hg overpressure. The solid lines represent Ieast-squares fits of the data. dependence of the defect fractions fk and the characteristic frequencies vQ are shown in Fig. 3. These data show no observable dependence of the defect fractions on tempera- ture and a small increase in the characteristic frequencies with decreasing temperature. The stability of these defect complexes at room temperature indicates a considerable binding energy ( 20.4 eV). The determination of the actual structure of the com- plexes is complicated by the fact that there are few theo- retical models to allow accurate calculation of EFGs caused by specific defect structures. As mentioned above, the observed EFGs are related to the concentration of I-Ig vacancies present in the sample which implies that they are due to In-VHs complexes. The presence of two separate frequencies can be explained in one of two ways. First, the two EFGs could be related to different complexes or to different charge states of a single complex, Second, the two EFGs could be caused by different local concentrations of Hg and Cd atoms near the In probe atom. If the 12 next nearest neighbor sites to the substitu- tional In impurity are occupied by n-Hg atoms and ( 12-n) Cd atoms, the value of n about a given In atom should be a statistical distribution function of the total Hg to Cd ratio (4: 1 for x = 0.2 ) . Using this assumption, the fraction of In atoms having n-Hg next nearest neighbors can be calcu- lated. This calculation gives four values of n with fractions greater than 10%. This does not seem to be consistent with the observation of only two EFGs. However, recent Ra- man” and NMR12 experiments have shown that there is some local order, i.e., there are preferred numbers of Hg 95 ^ e * l . * s . $ 90- i-3 + + 85 - + + r + . . LOO 150 200 Temperature (K} FIG. 3, Temperature dependence of the quadrupole interaction frequen- cies Y* and defect fractions obtained by least-squares fitting of PAC spec- tra. The crosses represent values for vv, and the solid dots represent values for Vet versus Cd atoms around a single Te site. Therefore, the two frequencies observed may be due to certain preferred co- ordinations. Further experiments will be done to examine these possibilities. The present experiments provide strong evidence for the existence of stable In-YHp pairs in Hgo,79Cdo,2~Te. These are the first atomic scale measurements of such de- fects in this material; similar defects have been observed by PAC in CdTe*+” and (Cd,Zn)Te. t3 This identification of In-vacancy pairs will permit many atomic scale processes to be studied such as vacancy migration, defect clustering, and impurity diffusion. This work was partially supported by a grant from the U.S. Army Research Office. ‘C. L. Jones, M. J. T. Quelch, P. Capper, and J. J. Gosney, J. Appl. Phys. 53, 9080 f 1982). ‘S. Margalit, Y. Nemirovsky. and I. Rotstein, J. Appl. Phys. 50, 6386 (1979). “G. L. Stefanis, J. Vat. Sci. Technol. A 3, 171 (1985). 4H, R. Vydyanath, J. Electrochem. Sot. 128, 2619 (1981). ‘Th. Wichert and E. Recknagel, in Topics in Current Physics Vol. 40: Microscopic Methods in Metals. edited by U. Gonser (Springer, Berlin, 1986), p. 3t7. “Th. Wichert and M. L. Swanson, J. Appl. Phys. 66, 3026 ( 1989). ‘“Th. Wichert, M. Deicher, G. Grubel, R. Keller, N. Schultz, and H. Skudlik, Appl. Phys. A 48, 59 (1989). sR. Kalish, M. Deicher, and G. Schatz, J. Appt. Phys. 53, 4793 (1982). 9D. Wegner and E. A. Meyer, J. Phys. 1, 5403 ( 1989). “G. Bahir, T. Bernstein, and R. Kalish, Radiat. Eff. 4f& 247 (1980). “A. Compaan, R. C. Bowman, Jr.. and D. E. Cooper, Appl. Phys. Lett. 56, 1055 (1990). 12D Zamir, K. Beshah, P. Becla, P. A. Wolff, R. G. Griffin, D. Zax, S. Vega, and N. Yellin, J. Vat. Sci. Technol. A 6, 2612 (1988). “J C Austin, M. L. Swanson, W. C. Hughes, and S. S. Choi, Mater, Res. . . Sot. Symp. Proc. 209, 469 ( 1991). 940 Appl. Phys. Lett., Vol. 59, No. 8, 19 August 1991 Hughes, Swanson, and Austin 940 

Indium‐Hg vacancy interactions in Hg1− x Cd x Te measured by perturbed angular correlation

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Indium‐Hg vacancy interactions in Hg1− x Cd x Te measured by perturbed angular correlation

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