Topographic methods of studying defects in Hg1−xCdxTe crystals

In/bared Phw. Vol. 30, No.

I, pp. 61-70,

1990

ooze-0891po $3.00 + 0.00 Pergallon Press plc

Printed in Great Britam

TOPOGRAPHIC

METHODS OF STUDYING Hg, _,C&Te CRYSTALS

DEFECTS

IN

Yu FUJU, Xv SANBAO and ZHANG SHUMING Shanghai Institute of Technical Physics, Academia Sinica, 420 Zhong Shan Bei Yi Road, Shan~ai,

China

(Received 7 June 1989> AIrstract-In this paper, three topographic methods of studying defects in Hg, _ ,Cd,Te crystals are described: the back-reflection method with a “white” beam, the scanning-reflection method with monochromatic radiation, and the glancing-incidence method with a “white” beam. Unique results obtained by these methods are shown: topographs as a referable criterion of various defects are laid down and explanation of the topographs is carried out theoretically. A set of topographs with reversed contrasts is made and the microorientation differences among the mosaic blocks are determined. Topographs of the intensive strain field in the core region of ingots prepared by the quench-recrystallization method are made and the mechanism of crystal growth is interpreted according to these topographs. In addition, typical topographs made by the three methods respectively, are shown.

1 t INTRODUCTION

Hg, _,rCd,Te crystals are among the most important semiconductor materials used for infrared detectors. Crystals prepared by the method of a travelling heater, by LEP, quench-recrystallization, as well as other methods usually have various defects as follows: precipitate, dislocation, slip band, subgrain boundary, intensive strain field, small angle grain-boundary, large angle grain-boundary, twin crystal etc.(‘) X-ray topography is a method of determining the microstructure of Hg, _,Cd,Te material, and its advantage is that it is nondestructive and has a high sensitivity to lattice distortion in the crystal. Investigation of the shape, size and distribution of these defects in a crystal can be carried out by X-ray to~graphy. Thus the mechanism of various defects can be studied and also a way to reduce and eliminate the defects can be found. The technique of crystal growth can be improved as well. In order to increase the yield of an infrared detector, especially large area detectors and multielement array detectors, a perfect crystal has to be prepared. However the physical and chemical processes for the preparation of the ternary alloy Hg, _,Cd,Te are very complex, thus it is difficult to grow a crystal with uniform composition and perfect structure. The impurities are easily absorbed owing to higher energy in the region of the defect, thus the electrical property of the device containing defects frequently deteriorates. Therefore investigation of the relation between crystal defects and device technology by X-ray topology is very important. X-ray topography can also be used in choosing wafers for device technology. In this paper some unique results for the topography of Hg, _ .C&Te crystals are shown and the relationship between defects and the technology of crystal growth is also discussed. 2.

EXPERIMENTAL

METHODS

The back-reflection method with “white” beam, the scanning-reflection method with monochromatic radiation, and the glancing-incidence method with “white” beam are used for the determination of defects in Hg, _ ,Cd,Te crystals. In practice the three methods are modifications of the Berg-Barrett and Laue methods, and are described below. (1) Back-rejIection method with white beamC2j A schematic of the back-reflection method with white beam is shown in Fig, 1. If a “white” X-ray beam impinges upon the specimen, part of the beam, which wavelength satisfies the Bragg condition for a set of crystallographic plane, can be diffracted, then the diffraction image is recorded on film. 61

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Sour

Film

Fig. 1. Schematic of back-reflection method with “white” beam.

When the X-ray beam passes through a region with some defects, the diffraction intensity is enhanced and the image of the defect is overlapped onto the background diffracted by a perfect region. By designing an appropriate optic system, and the selection of lower high-voltage (for target MO, 15 kV x 20 mA) in order to reduce background fluorescence,‘3’ a clear topograph can be obtained. (2) Scanning-rejlection

method with monochromatic radiation”.44’

A schematic of the scanning-reflection method with monochromatic radiation is shown in Fig. 2. A definite Bragg reflection occurs after adjusting the system gradually for a collimated monochromatic X-ray beam and using an appropriate crystallographic plane. For the reason of geometry the planes (333), (440), etc are chosen for CuKa, radiation. A topograph may be obtained over a wide area of the specimen if the specimen and the film are moved synchronously and reciprocally in a direction parallel to the specimen surface ot in a direction perpendicular to the incidence beam. A high resolution topograph with a one to one correspondence with the specimen surface may be obtained if the film is placed as close as possible to the crystal and as perpendicular as possible

Scanning direction

t perpendicular to incident beam

u

n

3rd slit

Scanning direction paralleled to specimen surface

u

1

u

4th slit

R

Detector

Fig. 2. Schematic of scanning-reflection method with monochromatic radiation.

Defects in Hg, _ ,Cd,Te crystals

Fig. 3. Schematic of glancing-incidence

63

method with “white” beam.

to the diffraction beam. Alternatively the film may be adjusted parallel to the specimen surface and developed at one side of the film to avoid overlapping the image. (3) Glancing-incidence method with white beamf7’ A schematic of the glancing-incidence method with “white” beam is shown in Fig. 3. Its imaging process is similar to the back-reflection method with “white” beam. The incident beam impinges upon the specimen surface at a small angle (usually 2” to 5”), so that the background fluorescence will be reduced and a large area of the specimen can be illuminated. A topograph with very high resolution may be made if a fine focus X-ray source is used.

3. IMAGING

PRINCIPLE

OF CRYSTAL

DEFECTS

The structure perfection of a ternary compound Hg, _,Cd,Te crystal is not nearly of Ge, Si even of binary compounds InSb, GaAs, thus the imaging principle of its defects can be explained with the aid of the kinematical theory of diffraction. (8,9)The incident beam with a divergence of 2-3 minutes may be treated as a spherical wave. For a perfect crystal, as shown in Fig. 4(a), only a small part of the incident spherical wave, the wave vector of which is very close to K,,satisfies the Bragg condition (solid lines):

K'--K=G,, where K is the incident wave vector, K'is the diffraction wave vector, and G,, is the reciprocal vector. In addition, the primary extinction exists in this situation. For these reasons, the X-ray intensity diffracted by a perfect crystal is weaker, and a grey image is recorded on film. However, for a distorted crystal, as shown in Fig. 4(b), more parts of the incident spherical wave satisfy the Bragg condition, and the primary extinction is broken. Thus the X-ray intensity diffracted by distorted crystal is greater, and a black image is recorded on film. If the crystal is distorted too heavily, the Bragg condition will not be satisfied, therefore the diffraction beam disappears, and a white image is recorded on film.

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Incident spherical

WaVe

Perfective crystal

(b)

(a)

Fig. 4. Diffraction illustrations of perfect crystal and distorted crystal.

(1) Imaging principle of substructure

As shown in Fig. 5(a) the orientation differences between the mosaic blocks, which form the so called polygonization substructure, are usually from several seconds to a few minutes. Since the divergence of the incident beam is about 2-3 min, the substructure will be imaged on the film if the orientation differences between the mosaic blocks is less than the divergence of the incident beam. Adjacent subgrains formed various contrasts: a black contrast represents the diffractions strengthened, a grey contrast represents the diffractions on dynamics, and a white contrast represents no diffraction which is attributed to where the orientation differences between mosaic blocks are larger than the divergence of incident beam, or the mosaic block is not in the Bragg position. (2) Imaging principle of precipitate An area of distortion is found around a precipitate, for the precipitate compresses its surrounding lattice, see Fig. 5(b). Therefore an enhanced diffraction occurs at this area in the divergence of the incident beam. Generally the precipitate has an independent orientation or different structure to the matrix, in which case no diffraction is produced. (3) Imaging principle of bending crystal face The X-ray beam diffracted by a curving crystal face as shown in Fig. 5(c) is convergent or divergent depending on the curvature of the crystal-concave or convex respectively. In the case of monochromatic topography, the diffraction image corresponding to a limited set of curving crystal faces appears as a band, and the contrast of the band changes gradually. A white image was formed if the radius of curvature of the crystal is so large that it is over the divergence of the incident beam. Diffraction beam Incident beam

Incident beam Diffraction

Mosaic blocks

(a)

Precipitate

crystal

lb)

(cl

Fig. 5. Imaging principle of several different defects.

Defects in Hg, _ ,Cd,Te crystals

65

The above-mentioned defects are often interlaced in Hg,_,Cd,Te crystals, consequently the various contrasts are usually revealed on each topograph, most of the contrasts can be expounded and studied according to these image principles. 4. EXPERIMENTAL

RESULTS

Using the above-mentioned three topographic methods, we have obtained some research results. (1) Referable criterion of topographs of various defects are laid down after inducing and classifying many topographs, then processes of explaining topographs are reduced. (2) A set of mosaic block topographs where the contrasts are reversed are made, and the ~croo~entation differences between the mosiac blocks are determined according to the reverse of contrast. (3) The topographs of intensive strain field in the core region of some ingots prepared by the quench-r~~s~llization method are made, and the mechanism of crystal growth is interpreted according to these topographs. The three results will be explained separately below in detail.

The crystallization mechanism of Hg, _,Cd,Te is very complex, for example, for most compositions x the solidification line is widely separate from the liquefaction line on the phase diagram near the melting point. (ia)During growth of the crystal, the high vapour pressure of Hg, severe segregation of composition, heat shift, and mechanical vibration cause’ the complexities of shape, size and distribution of various defects in a crystal. As a result the X-ray topographs present diverse patterns. In practice, it is very inconvenient and impossible to explain each topograph in detail, if the topograph is used as a means of measuring and choosing wafers for technological process. For simplicity we have explored one possible path, i.e. “objective classification method” to end the stalemate. After carefully analyzing, identifying and comparing, six typical topographs, as shown in Fig. 6, have been found and can be used as a criterion for analyzing other topographs.

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Fig. 6. Typicat mod& of several different defects. (a) Twin crystals. (b) Small an&e boundary. (c) Dispersive grains. (d) Intensive substructures. (e) Grainy form. (f) Subgrain boundaries. INF M/I--E

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Fig. 7. X-ray topographic patterns of a wafer. (a) Topograph of the back-reflection method with “white” beam of the wafer. (b) Topographic pattern of the right part of the same wafer. (d)-(f) Topographic patterns are made rotating around [l IO] axis every 3 minutes from the primary position (c). (b)+f) CuKa, (440) reflect topographic patterns.

These are topographs of (a) twin crystals; (b) small angle boundaries, where the orientation difference between the two grains is about 2”; (c) diffusion grains, where the total orientation difference between the diffusion grains is about 10”; (d) intensive substructures, where the total orientation difference between the mosaic grains is about 3”-5”; (e) grainy form, this resembled speckles which have shown microorientation differences between the micrograins; (f) subgrain boundaries, this is shown in Fig. 7. Consequently it is possible to quickly and correctly explain most topographs by comparison with the criterion.

(2) Measurement

of size and microorientation

dlrerence

of the subgrains

It is worth noting the size and microorientation difference of the subgrains which can be measured with the aid of reverse contrast on a set of topographs. Figure 7 shows topographs of a wafer. Where (a) is a topograph of a “white” beam back reflection of the wafer, and a small angle boundary at about 40 minutes was presented on the topograph; (b) a topograph of the left part of the same wafer; (c) a topograph of the right part; (d), (e), (f) topograbhs made rotating around the [l lo] axis every 3 minutes from the primary position (c). What merits attention is that not only can the mosaic block and subgrain’boundaries be clearly seen, but also the reverse contrast, i.e. the black regions on one topograph changed to white regions on the others. From the topographs sizes of the mosaic blocks have been measured from 100 pm to a few millimeters. The microorientation differences between the subgrains, in the order of 3-6 minutes, have also been acquired in proportion to the angle of rotation.

Defects in Hg, _,Cd,Te

crystals

67

(3) Study of mechanism of crystal growth

method, firstly, a During preparation of Hg, _x Cd,Te crystals by the quench-recrystallization molten charge is quenched into cold flowing nitrogen gas, to be quickly solidified in order to overcome composition segregation. After quenching, as often happens, a dendrite, intensive stress and strain are caused in a quenched charge. Secondly, the quenched charge is recrystallized at a suitable temperature for a long time to eliminate the dendrite, relax and eliminate the stress and strain and unify the composition of the crystal it is also to obtain a crystal ingot with uniform composition and perfect structure. However disappointingly, diverse defects generally emerge in the crystal ingot due to the complexity of physical and chemical reactions during the preparation period. It seems likely that the technology of crystal growth can be guided studying ‘formative regularity of these defects by X-ray topography. Taking the wafers, the surfaces of which are the cross-sections at the head, the middle, and the tail parts of the ingots respectively, making the topographs, and then inducing and analysing many topographs corresponding to the wafers. As has been shown by analysis: (1) along the length of an ingot the structure perfection at the head part is better at the tail part; (2) the transverse of an ingot shows the structure perfection at the core region to be poorer than at the periphery region, besides the region that is contact with the quartz ampoule, as shown in Fig. 8. A “core structure” exists in the topographs of some wafers. The term “core structure” here means that there is a strain field of radiation type at the center of the ingot. This is attributed to quick quenching of the molten charge in cold nitrogen gas, so the periphery molten charge near the quartz ampoule solidifies first, and the molten charge at the core region finally solidifies due to heat release. This results in intensive stress and strain is concentrated at the core region of the quenched charge (of course there is also the dendrite etc).

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Fig. 8. Topographic patterns of the intensive strain field in the core of the ingot prepared by quench and rccrystallimtion.

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Fig. 9. Topograph of the back-reflection method with “white” beam of more perfect Hg, _,Cd,Te crystal.

Although work on the process of recrystallization has continued for quite a long time, the stress and strain still remain at the core region of the ingot. This indicates that the condition of recrystallization is not good enough, for example either the duration of recrystallization is not long enough or the temperature of recrystallization is not high enough, and maybe the condition of quench is unsuitable etc. In Fig. 9 (see Section 4) the contrast of the topograph is homogeneous. It is enough to prove that the conditions of quenching and recrystallization are appropriate, so not only the dendrite is eliminated but also the stress and strain are relaxed, and a more perfect crystal is prepared. Without doubt, the three research results are necessary for dissecting the mechanism of crystal growth.

Fig. 10. Topograph of the scanning-reflection method with monochromatic radiation of more perfect Hg, _,Cd,Te crystal D8223-15 CuKa, (004) reflection.

Defects in Hg, _ .Cd,Te crystals

Fig. 11. Topograph of the glancing incidence method with “white” beam of more per&t crystal.

Hg, _,Cd, Te

1 Relation between topography and device technology

There are some examples, for a wafer, if the contrast of the topograph is homogeneous, the yileld of the infrared detector is often higher. However if the contrast of the topograph is ai We of dis,persive speckles, which represents many defects existing in the wafer, the yield of the infrai red de1tector is generally lower. This is suggested because there is higher energy in the region of a defr::ct, an impurity is easily absorbed there, so that the electrical property of device becomes 1Norse.

Fig. 12. Topograph of the back-reflection method with “white” beam of a (001) wafer ofdislocation-free silicon.

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Typical topographs made by the three topographic methods are shown in Figs 9-12. It is clear from the topographs that the contrast of each topograph is more homogeneous, this shows the crystal structure is more perfect. 5. CONCLUSIONS Summing up the above analysis and discussions, it is obvious that X-ray topography is an effective method for determining and studying Hg, _,Cd,Te crystal quality nondestructively. It may be considered an important technique for not only studying the mechanism of crystal growth but for choosing the wafer to device technology as well. Acknowledgements-The authors would like to thank Professor Tang Dingyuang and Professor Xu Shunsheng for their encouragement. The authors would also like to thank Assistant Professor Wu Wenhai, Jiang Xiaolong, Tong Feiming, Mao Peifen and Yu Zhenzhong for their helpful consultation. Special thanks are also given to Mr He J;hua,Zhao Huirong, Chen Xinyu and Miss Ding Suzhen for their assistance. The authors are further indebted to Assistant Professor Shen Jie. Mr Liu Jiming, Zhan Xiaoping and Li Han-dong for providing the Hg, _ ,Cd,Te specimens.

REFERENCES 1. Yu Fuju, WC&I 9, 508 (1980). (In Chinese). 2. L. N. Swink, Metallurg. Trpns. 1, 629 (1970). 3. S. S. Xu, X-Ray Metallurgy, p. 5. Shanghai Publishing Company of Science and Technique, Shanghai (1964). (In Chinese). 4. K. L. Bye, J. Mater. Sci. 14, 619 (1979). 5. U. Mirsky, J. Electron. Mater. 9, 933 (1980). 6. R. G. Rosemeier, J. Vat. Sci. Technol. Al, 1656 (1983). 7. A. B. Hmelo, Mater. Letf. 2, 6 (1983). 8. B. K. Tanner, X-Ray Dlflacfion Topography, p. 24. Pergamon Press, Oxford (1976). 9. A. Authier, Modern Diffraction and Imaging Techniques in Material Science, p. 3. North Holland, Amsterdam (1970). IO. J. C. Brice, J. Crysf. Growth 75, 395 (1986).