Short Notes

phys. stat. sol. (a) - 60, KlO5 (1980)

Subject classification: 1.3; 12.2; 22.4.4 Solid State Physics Laboratory, Delhi Compositional Characterization of the Hgl _,C_dxTe Alloy System

B.B. SHARMA, S.K. MEHTA, and V.V. AGASHE

1)

BY

Introduction for making infrared detectors over a wide range of wavelengths /l/. A major difficulty in obtaining good quality material is associated with the large scale HgLCd segregation present in this system /l/. A normally cooled ingot is in- homogeneous on a macroscopic scale showing large scale Hg/Cd segregation. A long time annealing at temperatures close to the solidus point is necessary for

homogenization. A check of the compositional uniformity is therefore an essential and important step in the material development programme. The two techniques normally employed for this purpose a re electron microprobe analysis (EMPA) and density measurement. The latter, of course, is incapable of giving a point to point analysis. During the course of our work we found that X-ray microradio- graphy is a very convenient and sufficiently accurate technique for revealing’ seg- regation in this system. It has even certain advantages over the well established

EMPA technique. Apart from this, it has been found that the microhardness number is also very sensitive to any variation in composition and its measure- ment at different places can give a good estimation of compositional k i formi ty These techniques alongwith some results obtained are discussed below.

Hgl _,CdxTe (0 x 5 1 ) has proved to be a very versatile material

Material preparation The alloy compositions corresponding to x = 0, 0.2, 0.4, 0.6, and 1.0 were synthesised from spectroscopically pure elements in

thick walled quartz ampoules sealed under a vacuum of heating rate was adopted to eliminate any possibility of explosion due to high

Hg pressure.

Torr. A very slow

The composition corresponding to x = 0.8 could not be synthesised because

the ampoule with this composition always used to explode at temperatures close

1 ) Lucknow Road, Delhi 11 0007, India. 8 physica (a)

See Short Note by B. B. SHARMA et al.

Fig. 2. X-ray rnicroradiogrephs of HgCdTe samples recorded in projection mode a t D magnifi- cation of 2 and optically enlarged x 6 (total magnification 12) a) before annealing and b) after annealing for 2 weeks at 660 "C. The darker regions in a) correspond to excess of Hg. The black dots, seen in the upper part of a) show some fine droplets of pure Hg

See Short Note by C. HAMANN et al.

a b C

Fig. 1. Micrograph of a holographic grating, recorded on GDP of 3,4-dimethyl-l,2,6,-thiadiazol. a) Y = 260 mm-l, b) v = 600 mm-', c) Y = 1000 mm-l

K106 physica status solidi (a) 60

Fig. 1. Variation of microhardness with composition r:ri 8 &

50

40

3 'e 30

to the liquidus point. The samples used for micro- hardness measurements were prepared from quenched ingots of different compositions. The ampoules carrying

P - the melt a t high temperatures were quickly removed e

from the furnace and allowed to cool down to mom temperature. This was done to minimize segregation

' ''0 02 OL 06 08 I0 X-

which would have caused large variation in the microhardness number from places on the same sample.

microhardness measurements /2, 3/. The samples were lapped and polished to a mir ror finish and finally etched in a 5% Br-methanol solution. All the measure- ments were taken under a weight of 20 p and at five different places randomly chosen on each sample. The values did not differ by more than 10% from the mean value for each sample. Fig. 1 shows a plot of the microhardness number versus composition. A peak in the curve is typical for an alloy system /3/.

The technique is based on differential absorption of

Microhardness testing A PMT-3 (SU) microhardness tester was used for

Microradiography X-rays by the different elements present in the sample. It is discussed in detail in several standard books /4, 5/. In the present work a Rigaku Denki Microfocus X-ray generator was used. All the microradiographs were recorded on Kodak Dental X-ray films in projection mode with a magnification x2.

The contrast in any radiograph can be improved by a careful choice of ra- diation. Fortunately, a commonly used radiation in X-ray diffraction work - MoK, - is also the most suited one for microradiographic analysis of'HgCdTe samples. This wavelength can excite the L-spectra of Hg and is therefore strongly absorbed by it. Consequently there is a large difference in the absorption coefficients of Hg and Cd for this wavelength. This situation is very favourable for studying the Hg/Cd segregation. It may be mentioned that a similar situation exists for another alloy system, Pbl -xSnxTe, which is also very important as an intrinsic semiconductor material with variable band gap like Hgl ,xCdxTe. Here the M o k radiation excites the L-spectra of Pb which, therefore, has an anomalously hlgh absorption coefficient for this wavelength.

The intensity of the X-ray beam transmitted through the sample is given by

the usual absorption equation

Short Notes Kl07

where I transmitted intensity, I. incident intensity, px linear absorption co- efficient, and t specimen thickness traversed by the beam. px is a function of the composition parameter x and can be easily calculated according to the stan-

dard procedure /4, 51. For MoK, it is given by 2)

* 687 (1 - 0.74 X) . (2) PX

Differentiating (1) and substituting for px from (2) we get

(3) dI - IJ 508 t dx . I

From (3) it can be seen that for a 150 p thick slice a variation of 0.01 in x will cause a variation of about 7.5% in the transmitted intensity. Similar cal-

culations can be made for Pbl - xSnxTe and the corresponding figure in this case is about 8%.

A difference of 7 to 8% in the transmitted intensity produces a clearly visible contrast in the microradiographs and therefore even a visual inspection of the radiograph can reveal a variation of 0.01 in x. For routine examination of samples it proved to be a very handy technique. The large scale segregation present in an unannealed sample can be clearly seen in Fig. 2a (see on the photo pages before the Short Notes part). The other picture (Fig. 2b) shows the effect of annealing for two weeks - the segregation is almostcompletely absent. A res- olution of 50 pm can be achieved on a routine basis by using a 50x50 pm focal spot size of the X-ray source and recording microradiographs in projection mode. A resolution of the order of 0.5 grn is possible by using high resolution plates and recording microradiographs in contact mode rather than in the projection mode.

2

One of the most important advantages of X-ray microradiography over electron microprobe analysis (EMPA) is that the former is capable of examining substantial volume of the material (because of the great penetrating power of X-rays), the latter, on the other hand, provides only a surface analysis. Another

2 advantage is that a large area (1 cm o r more) of the sample can be examined at

2) The alloy density (ex) used in arriving a t (2) is given by = 8..076 - X - 2 . 2 2 6 ~ / I / .

Kl08

a time. In EMPA an area = 200x200 p m is usually scanned at a time. A disad-

vantage of the microradiographic technique is that the test sample has to be

thinned down to $* 150 pm to get good microradiographs in reasonable time.

The microradiographs shown in Fig. 2a and b were exposed for about 2 h at 25 kV and 1 mA ratings. Higher voltages reduced the contrast substantially.

The EMPA is, perhaps, more suited for quantitative determination of the com-

position parameter x.

physica status solidi (a) 60 2

References /1/ D. LONG and J. L. SCHMIT, in: Semiconductors and Semimetals, Vol. 5,

Infrared Detectors, Ed. R. K. WILLARDSON and A.C. BEER, Academic

Press, New York 1970 (p. 175). /2/ D. TABOR, Rev. Phys. Techn. - 1, 145 (1970). /3/ V. N. GLAZOV and V . N. VIGDOROVICH, Microhardness of Metals and

Semiconductors, Consultants Bureau, New York 197l.

/4/ E .F . KAELBLE (Ed.), Handbook of X-Rays, Mc Graw-Hill, New York 1967

(Part 6).

/5/ V. A. PHILLIPS, Modern Metallographic Techniques and Their Applications,

Wiley-Interscience, New York 197l (p. 373).

(Received April 25, 1980)