Segregation of impurities and defects in Hg0.8Cd0.2Te by laser annealing

Thin Solid Films, 241 (1994) 151-154

151

Segregation of impurities and defects in Hgo.sCdo.2Te by laser annealing R. Ciach and M. Faryna Institute of Metallurgy, Polish Academy of Sciences, Reymonta 25, 30-059 Cracow (Poland)

M. Ku~ma, M. Pociask and E. Sheregii Pedagogical University, Institute of Physics, Rejtana 16A, 35-310 Rzeszdw (Poland)

Abstract

A method using lasers of segregation of impurities or interstitial mercury atoms (IMA) in solid phase Hgo.sCdo.2Te (MCT) is presented. A theoretical model for this process is also proposed. The equation for the diffusion of impurities or IMA was completed by a term describing the influence of phonon flux on diffusion processes. Computer simulations of the laser annealing process reveal the possibility of obtaining a sharp maximum of Hg concentration for appropriately chosen parameters of laser pulse. This was verified experimentally with MCT specimens annealed by using a YAG:Nd 3+ laser.

I. Introduction

Laser annealing of semiconductors is carried out mainly by laser pulse radiation with a pulse length ranging from picoseconds to nanoseconds [ 1, 2]. However, for some applications of laser annealing, as is shown below, long pulses are more effective. This is because the range of phenomena connected with the diffusion of atoms or defects in semiconductors have relatively large velocities and characteristic times, owing among other things to mass transport. In the present paper we demonstrate that one can produce a large local change in concentration of impurities or intrinsic defects in a crystal by long pulses (in fact only by long pulses) of laser radiation.

2. Computer simulation of diffusion processes of impurities on laser annealing

Diffusion processes are described by the standard diffusion equation. However, there is a large increase in the diffusion parameters, such as the diffusion coefficient, length of diffusion etc. [3, 4], in the case of laser annealing. This is caused mainly by the following factors: large temperature gradients, which produce the flux of photons ("phonon wind") taking impurity atoms away, and a field of thermoelastic deformations [5, 61. These factors are taken into account in the diffusion equation by the additional atom flux [5, 6]:

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where D = Do e x p ( - E a / k B T ) , Ea is the activation energy of diffusion, Cv is the specific heat, (rf/rfi) is the mean value of relaxation time for scattering of phonons on atoms of the crystal and on impurity atoms respectively, N is the impurity concentration, V T is the temperature gradient, G is Young's modulus, v is Poisson's coefficient, "r is the coefficient of thermal expansion, f2~ and Qo are elementary volumes of atoms of the crystal net and impurities respectively. In eqn. (1) we have assumed that ~- is constant in the whole volume of the crystal. The first term in eqn. (1) arises from the force of the photon wind and the second from thermoelastic forces. These forces act effectively on atoms of foreign impurities, the atomic diameters of which differ considerably from the diameter of the net atoms. We assume that g21 ~ f2o and (zf/zn) ~ 1. This is the correct approximation for the ease when the net atom becomes an interstitial impurity atom and the number of atoms in the elementary cell is large. Both these conditions are sufficiently fulfilled for interstitial mercury atoms in Hg~ _xCdxTe. Thus the diffusion equation containing the additional atom flux arising from the action of the phonon wind has the form

-~

~ CvVr

(2)

© 1994-- Elsevier Sequoia. All rights reserved

R. Ciach et al./ Segregation of impurities in HgoxCdo.2Te

152

One can notice from eqn. (2) that the direction of diffusion of atoms caused by the phonon wind depends on the sign of the second term on the right-hand side of eqn. (2) and therefore depends on the sign of the derivative of the temperature gradient. From this it follows that we have an inversion of sign in the case where there exists an extremum of the temperature gradient, which provides an influx of atoms or defects from two sides to the point of the temperature gradient extremum. Solutions of the thermoconductivity equation and eqn. (2) are found using a net method [7]. For this purpose these equations were used in the subtraction form. Our considerations were restricted to the onedimensional case, because the condition for onedimensionality is fulfilled here: the diameter of the homogeneous laser beam is much greater than the thickness of the sample. The set of 30 linear equations for 30 spatial layers (we assumed this number of layers) should be solved for each time step. Calculations were performed for the first 30 layers (the width of each layer is 1 ~tm) of an Hg~_xCdxTe (x = 0.2) crystal with thickness 1 = 1 ram. A neodymium laser (2 = 1.06 ~tm) was chosen. In semiconductors of Hg~_ x Cdx Te type, a prominent role is played by intrinsic defects such as vacancies of mercury atoms, which are acceptors, and interstitial mercury atoms (IMA) which are donors. At high temperature, the main model of diffusion of IMA in Hgl_ x Cdx Te is a vacancy mechanism, which allows the chemical diffusion of mercury. This diffusion process of IMA determines the concentration of both donors and acceptors and brings about a change in chemical composition of the material [8]. For this reason the aim of the calculations was to determine the concentration distribution of

IMA. At the same time, laser annealing generates additional defects (vacancies and IMA). For long pulses, this generation is of the thermal type. This increase in the concentration of defects is included in the thermal dependence of the diffusion coefficient D ( T ) = D o e x p ( - E ~ / k B T ) , where E a is the activation energy for the creation of vacancy. The initial concentration of IMA was taken as N ° = 1 x 102~ m 3 and diffusion parameters as D o = 6 x 10 -6 m 2 S - l , E a = 0.96 eV [9]. The thermoconductivity equation was solved under the following boundary and initial conditions: To = 300 K; the irradiated surface is thermally isolated from the environment; there is a linear gradient of temperature between the 30th layer and the other side of the sample. For the boundary conditions of the diffusion equation the concentration of mercury in air was taken as N o = 0. That determines the negative diffusion, i.e. mercury atoms are evaporated from the irradiated surface (the first layer). It was also assumed that N31 = N 3 0 . In addition, the following parameters, describing the thermal properties of the sample, were used for the calculations: thermoconductivity 2 = 5 W m 1 K 1, temperature conductivity a = ,t/Cvp = 8.3 x 1 0 - 6 m 2 s 1, intrinsic heat C v = 8 0 J k g IK-~, mass density p = 7.63 x 103kgm -3 [10]. The power of the laser beam was taken so that the maximum temperature in the sample was 1050K [10] (the melting temperature of Hgo.8 Cd0.2 Te). The results of calculations of time-spatial distribution of the temperature, temperature gradient and concentration of IMA in H g l _ x C d x T e for two laser pulses, 1 laS and 250 gs, are presented in Figs. 1 and 2. It can be seen from Fig. 1 that visible changes in the IMA concentration are not observed for a pulse of 1 las,

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Fig. 2. (a) Time-spatial distribution of the temperature T(x, t) after laser irradiation with pulse length of t 2 = 250 gs and absorbed power density w2 = 3.08 x 10]3 W m-3 and for absorption coefficient cc (b) Time-spatial distribution of the temperature gradient V T(x, t) (parameters of annealing as in Fig. 2(a)). (c) Time-spatial distribution of the concentration N(x, t) without the action of pbonon wind (parameters of annealing as in Fig. 2(a)). (d) Time spatial distribution of the concentration N(x, t) (parameters of annealing as in Fig. 2(a), initial concentration No).

except for the total evaporation of IMA from the surface layer. However, for the longer pulses (250 gs) the area with a large temperature gradient is increased, inducing a non-homogeneous distribution of IMA (Fig. 2(b)) and causing a sharp maximum in the IMA concentration with a value two times greater than the initial value (Fig. 2(c)). The maximum is at a depth of 2 pm from the surface and this position is in accordance with the position of the extremum of VT (Fig. 2(b)). Comparing these results with the results in Fig. 2(d), where the concentration distribution was calculated without the last term in eqn. (2), we can state that this non-homogeneous concentration is caused only by phonons. The important feature of this method of segregation of impurities is the additive rule. This means that multiple pulse repetition of laser irradiation of a sample also causes an increase in the maximum of distribution of impurities. This situation is presented in Fig. 3(b).

3. Experimental results With the aim of experimental verification of the computer simulated results, the distribution of Hg concentration in Hgx Cdl_ x Te specimens subjected to laser treatment was investigated under the above mentioned conditions (2 = 1.06 ~tm, z = 250 gs, energy density of the beam 0.82 J c m - 2 ) . Annealing was done with one laser pulse as well as with three successive pulses. The specimens were n type with an electron concentration of 1.5 × 1015m -3. Before irradiation they were etched chemically. With the aim of obtaining sufficient resolution of the measurements a wedge with an angle of 2 °, was made on the surface of the specimen. The distribution of the Hg concentration was determined by micro X-ray analysis using a IXA-50A JEOL spectrometer by scanning an electron beam along the wedge surface. The initial distribution of the Hg concentration is shown by triangles in Fig. 3(a). The circles show the

154

R. Ciach et al. I Segregation o f impurities in Hgo. 8 Cdo.2 Te

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d i s t r i b u t i o n after laser t r e a t m e n t with three pulses. A n nealing with one pulse did n o t cause a n y n o t i c e a b l e change in the H g d i s t r i b u t i o n whereas three successive pulses caused an increase in the c o n c e n t r a t i o n at a d e p t h o f 0.2 lam.

4. Conclusions

The m e t h o d o f s i m u l a t i o n o f laser a n n e a l i n g used by us has led to some u n e x p e c t e d results o w i n g to the influx o f H g to the near-surface layer, i.e. to the maxim u m p o i n t o f the t e m p e r a t u r e gradient. Nevertheless, this result was c o n f i r m e d b y o u r experiments. A disa g r e e m e n t in the l o c a t i o n o f the m a x i m u m c o n c e n t r a tion o f H g o b t a i n e d b y c o m p u t e r s i m u l a t i o n a n d f r o m e x p e r i m e n t a l results m a y be due to the two following reasons: (1) p a r t o f the near-surface layer is r e m o v e d d u r i n g chemical etching; (2) the role o f n o n - l i n e a r effects was n o t t a k e n into a c c o u n t in the t h e r m o c o n d u c tivity a n d diffusion equations. E x c e p t for this, we obt a i n e d g o o d q u a l i t a t i v e a g r e e m e n t between theoretical a n d e x p e r i m e n t a l results p r e s e n t e d here a n d in ref. 11.

References

1 J. M. Poate and J. W. Mayer (eds.), Laser Annealing o f Semiconductors, Academic Press, New York, 1982. 2 M. von Allmen (ed.), Laser Beam Interactions with Materials, Springer Series in Materials Science Vol. 2, Springer, Berlin, 1987. 3 S. Damquard, M. Oron, J. W. Petersen et al., Phys. Status Solidi A, 59 (1) (1980) 63-67. 4 S. G. Kijak, W. Krechun and A. A. Manenkov i dr., Fiz. Tekh. Poluprovodikov, 23 (3) (1989) 421-424. 5 V. P. Voronkov, G. A. Gurczenok., Fiz. Tekh. Poluprovodnikov, 24 (10) (1990) 1831-1834. 6 W. B. Fiks, Przewodnictwo Jonowe w Metalach i P6tprzewodnikach, PWN, Warsaw, 1971 (in Polish). 7 N. G. Kondrashov and N. V. Pavlukevich, K rasciotu temperaturnych polej metodom setok. Metody Opredelenia Temperaturnoj Provodimosti, In A. V. Lykov, (ed.), Nauka i Tekhnika, Minsk, 1967, pp. 208-217 (in Russian). 8 F. A. Zaitov, A. W. Gorshkov and G. M. Shaliapina, Fiz. Tver. Tela, 20 (6) (1978) 1601-1605. 9 V. I. Ivanov-Omskii, N. N. Berchenko and A. I. Elizarov, Phys. Status Solidi, 103 (1987) 11. 10 J. Piotrowski and A. Rogalski, P61przewodnikowe Detektory Podczerwieni, WNT, Warsaw, 1978 (in Polish). 11 Kuzma, E. Sheregii, I. Virt and C. Abeynayake, Acta Phys. Pol. A, 80 (3) (1991) 475-479.