Journal of Crystal Growth 128 (1993) 1040-1046 j . . . . . . . . C R Y S T A L North -Holland G R O W T H

Recent advances in bulk crystal growth from the vapour: the case of a - H g I 2

M. Piechotka , G. Wetze l , A. Rossberg , H.-J. Schwer, M. Z h a and E. Kaldis

Laboratorium fiir Festkgrperphysik ETH-H6nggerberg, CH-8093 Zurich, Switzerland

A brief review is given of recent progress in growing bulk crystals of mercuric iodide from the vapour phase. Two topics in particular are discussed in more detail: (a) kinetics of the growth; (b) perfection of the crystals.

1. Introduction

A great deal of work has been carried out on the bulk vapour growth of many compounds ( I I - VI and others) which has been summarized in the past [1-3]. The fundamental aspects of vapour growth will be reviewed elsewhere [4].

With the maturing of MBE, MOCVD and other film growth techniques, the point has been reached where the quality of thin films is limited by the perfection of the substrate. Melt growth is commonly used to produce bulk crystals of the substrate materials. However, many materials melt incongruently, decompose or undergo struc- tural phase transitions at lower temperatures. Al- ternative methods are, therefore, needed. Vapour growth seems to be one of the most promising techniques. An important question which has not been intensively addressed in the past, is the mechanism of vapour growth of large single crys- tals. The assumption has been made that one can investigate the mechanism of growth of small crystals and then scale up to large dimensions. Unfortunately, as we discuss in the following sec- tions, this is not the case, as it has been clearly shown by the recent progress in HgI 2 and the I I -VI compounds.

A commonly discussed drawback of vapour growth (and also of solution growth) is the low growth rate, which lies in the range of 10 6 c m / s (0.9 ram/day) . However, at a constant linear growth rate, while the size of a crystal increases

linearly with time the crystal mass and volume increases as the third power of time. Thus, in long duration experiments, when adequate crystal volume has been acquired, the mass growth per day can readily increase from the range of several g / d a y to the 50-100 g / d a y region. This means that crystals of weight = 500 g grow very fast (see fig. 23 of ref. [3]). In solution growth, in spite of the lower growth rates (typically 10 -7 cm/s ) crys- tals of several kg are grown in a few months with conventional techniques. Mass transport, thermal and solutal convection can be controlled by stir- ring. In vapour growth, the control of the process seems to be more difficult because heat and mass transfer through the vapour phase is more diffi- cult to control than through a liquid. This seems to be the major problem to be solved in order to achieve large good quality crystals from the vapour.

Here, we present our recent work on HgI 2 as a 'case study. Although this study is still in progress, several characteristics of the growth of large sin- gle crystals are becoming evident. HgI 2 crystals are grown presently not for substrates but for use as large wafers for detectors and arrays.

2. Material properties of HgI 2

The a-modification of HgI 2 is red (bandgap 2.13 eV) and crystallizes in a tetragonal layer structure with space group P42/nmc. The struc-

0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

M. Piechotka et al. / Recent advances in bulk crystal growth from the vapour: a-Hgl 2 1041

tural unit is built from a mercury atom tetrahe- drally coordinated with iodine. The (001) sheets are formed with corner linked, slightly distorted HgI 4 tetrahedra containing three cubic layers of atoms (iodine/mercury/iodine). In this way, an iodine double layer is formed between adjacent sheets, so that only weak van der Waals forces exist between them. This structural anisotropy results in a strong anisotropy of the physical properties. Thus, the thermal conductivity along the c-axis (vertically to the Van der Waals layers) is 0.1 W / m - K , which is four times lower than that perpendicular to that axis [5]. At the same time it is 200 times lower than the corresponding value for silicon near its melting point (20 W / m - K [6]). This unusually low thermal conductivity has a great impact on the crystal growth of this material.

Although there is no systematic investigation of the crystal growth phenomena of crystals with layer structures, one can expect that absorption of impurities will take place in the Van der Waals planes. As we have shown in the past [7,8], a high concentration of hydrocarbons is present in com- mercial HgI 2 and special purification methods are necessary. However, there is, as yet, no direct evidence for intercalation. Structure and bonding in HgI 2 lead to easy plastic deformation of these crystals under mechanical stress [9]. In fact it has been estimated that a crystal of HgI 2 a few cm 3 in volume could be plastically deformed at the growth temperature under its own weight [10]. This mechanical sensitivity of HgI 2 is not unique. It appears also in other important electronic ma- terials like Hgl_xCdxTe [11] or organic optoelec- tronic materials. This makes cutting, polishing and handling difficult.

As in many other layer structures, HgI 2 has a series of structural modifications leading to phase transitions. The yellow/3-modification is stable at T > 130°C till the melting point and belongs to the orthorhombic system. In the orange modifica- tion, the structural unit is formed from Hg4Ilo tetrahedra composed from four corner linked a- HgI 2 tetrahedra (HgI4). A more detailed discus- sion of these and other metastable structural modifications is given in an earlier review to- gether with many other materials aspects of HgI 2

[12]. The existence of these modifications is a serious problem for the nucleation and growth.

Small but important deviations from stoi- chiometry are possible in HgI2, both in the Hg- and I-rich range. Incorporation of organic and inorganic impurities occurs very easily and strongly influences mass transport and crystal growth. The problems of nonstoichiometry and impurities have been reviewed in the past [7,8]. Presently, we synthesize the material by purifica- tion of the vapours of the elements followed by a reaction in the vapour phase. Subsequent treat- ments are necessary for adjusting stoichiometry and for the final purification. This purified mate- rial does not leave any solid residue upon evapo- ration in contrast to mercuric iodide starting ma- terial prepared by other techniques.

The importance of ot-HgI 2 as a material for room temperature semiconductor detectors for X- and y-rays is based on the large band gap and the large absorption cross section for X- and y-ray quanta. Both are much larger than the values for Si and Ge, which are the main com- mercial detector materials at present. These de- tectors need liquid nitrogen cooling because the kT energy at ambient temperature is comparable to their band gap. On the other hand, HgI 2 detectors may be efficiently used at room temper- ature and their energy resolution approaches that of Si commercial detectors [13]. Their drawback is, however, a low mobility of holes, which is attributed to traps formed by various defects. In fact, most of the material properties mentioned above may cause such defects. Thus, the goal of the HgI 2 research is to grow not only large but as perfect and as homogeneous crystals as possible.

There are many applications of HgI 2 detectors in various fields [13,14]; three of them are men- tioned here: (a) elemental analysis of the EDAX type, handheld/portable equipment with appli- cations in pollution detection, metal recycling, mineral prospecting, etc., (b) nuclear medical ap- plications, and (c) array imaging techniques (y-ray cameras, etc). In addition, these detectors are important for radiation monitoring in all nuclear installations, in nuclear physics and astrophysics.

In addition to these interesting applications, this compound has appreciable advantages as a

1042 M. Piechotka et aL / Recent advances in bulk crystal growth from the vapour: ~x-Hgl 2

model substance for vapour growth. Due to the rather low temperature of sublimation ( T < 120°C), the use of transparent furnaces is possible allowing in situ observation of nucleation and growth. Another advantage is the early experi- mental evidence that vapour growth of a-HgI 2 can lead to large crystals [15,16]. Both arguments make this compound a model substance for the study of the growth of large single crystals from the vapour phase.

3. Growth of large crystals: the rate determining step

5 E o 4

~ 3 g ~ 2

c -

O o o

I H 0 t n I I I I I I

1 2 3 AT, K

Fig. 2. Growth rate as a function of the apparent undercool- ing at the initial stage of each of the steps of fig. 1. Change of the apparent undercooling does not influence the growth rate.

After Isshiki et al. [19].

The growth rate of ce-HgI 2 single crystals de- creases with time. This unexpected observation [17] has been later made also for another mercury compound with low thermal conductivity, HgzCI 2 [18]. Although such effects can often be at- tributed to impurities, the reproducibility under various experimental conditions and setups indi- cated that the behaviour has a more fundamental origin. This was supported later by a systematic investigation of the crystal size as a function of time [19]. The apparent supersaturation was step- wise increased or decreased in 0.5°C increments. The lateral and vertical dimensions of the crystal were measured photographically with 0.1 mm res- olution. A stepped curve was obtained, with the evaporation and growth steps overlapping (fig. 1). However, the initial (high) growth rate after each

25

~ 2 0

N~15

~ 1 0

O 5 • 1 . 1 1.6 2.1 2.6 3.1 3.6 AT

i i I I i i i i i i

0 200 400 600 800 1000 Time, hours

Fig. 1. Dependence of the crystal size of HgI 2 on the growth time. In each step the apparent undercooling has been kept constant. Note the overlap of the growth and evaporation

curves (arrows). After Isshiki et al. [19].

increase of the apparent undercooling proved to be independent on the preceding changes (fig. 2). This implies that the actual supersaturation re- mains the same in spite of the stepwise increase of the apparent supersaturation. A mechanism of annihilation of the driving force must, therefore, exist, which increases the temperature of the growing interface during growth. A detailed dis- cussion has been given in ref. [19]. Assuming that the thermal resistance of the crystal is responsible for this phenomenon and only the heat of con- densation has to be dissipated through the crys- tal, one can calculate the experimental growth curve [19] only up to the onset of saturation. An important clue came from the fact that for crys- tals a few cm in size, an apparent undercooling of several degrees is still necessary to keep the crys- tal at zero growth/evaporat ion rate. This shows that, in addition to the heat of condensation, another heat flux is permanently supplied to and dissipated through the crystal. Consequently, when the growth rate of a HgI 2 crystal ap- proaches zero under constant apparent under- cooling, the habit of the crystal tends to a rounded form which follows the isotherms in the gas phase instead of approaching the equilibrium form (facetted polyhedron [20]).

The additional heat flux is supplied to the crystal via radiation. Using a spherically symmet- ric model combining conductive and radiative heat transport, mass transport and interface kinetics, Chernov et al. [21] showed that both the decrease in the growth rate and the rounding of the polyg-

M. Piechotka et al. / Recent aduances in bulk crystal growth from the uapour: e~-Hgl 2 1043

onal habit can be explained. Parallel to this inves- tigation, numerical modeling [22] of the thermal field inside our growth furnaces showed that, taking into account thermal conductivity only, both in the vapour as well as in the solid phase, no conditions can be simulated under which the crystal stops growing. On the other hand, taking into account also the radiative heat transfer to the crystal, the observed cessation of growth at constant undercooling can be simulated with in- creasing crystal dimensions. This unexpected re- sult (the growth proceeds at temperature as low as 115°C) leads also to the conclusion that the geometrical arrangement and the temperature of the heating zones surrounding the growing crystal play an important role in the kinetics of the growth and influence the final habit of large a-HgI 2 crystals.

In conclusion it can be said that, similar to the observations in melts, the thermal field in the growth ampoule determines the shape of large crystals growing from the vapour. As the rounded shape introduces defects, the form of the isotherms becomes very critical for large crystals.

From the practical point of view, a strong decrease of growth rate with time can be corn-

pensated by a continuous increase of the appar- ent undercooling, either by decreasing the tem- perature of the cooling finger [23] or increasing the temperature of the source. However, the tem- perature control is usually not better than + 0.1°C and the temperature changes made in 0.1°C steps lead to the formation of striations, as it is dis- cussed in the next section. The recently released temperature controllers Eurotherm series 900 al- low steps as small as 0.01°C and are currently under tests in our laboratory.

Ideal control of the growth process could be achieved if the temperature could be measured and controlled with an accuracy of 0.01°C directly at the growing crystal interface. Unfortunately, due to the spectral characteristics of mercuric iodide, this is not possible with existing IR detec- tors [24]. Presently, only the pedestal temperature on which the crystal is growing can be controlled [23].

4. Crystal perfection

We have recently discussed [23] the growth rate and morphology of large mercuric iodide

C H A N ~ STRIATIONS

CLEAR f AREA ~ ~_''~:V ~ / ~

to ~'~ U I B E A M S P O T

(b)

< t . -

.(2_ (b rr

0 m- t - O {:7)

IT Hg

(110) Diane

>

[001]

ne

Fig. 3. (a) Schematic presentation of the main defect areas of the central wafer cut from the crystal of figs. 3 and 7 in ref. [23]. Area A contains channels and has (110) orientation. The rest of the crystal (areas B and C) has (001) orientation. (b) Epitactic relationship between area A and the rest of the crystal. The striations area C shows the faceted habit of the crystal for linear growth rates < 1.1x 10 -6 cm/s. The locations of the beam spots for the y-ray rocking curve measurements are also shown. For

more information refer to the text.

1044 M. Piechotka et aL / Recent advances in bulk crystal growth from the vapour." a-Hgl 2

crystals (500 g). Due to the strong thermal anisotropy, crystals with the c-axis parallel (Van der Waals planes perpendicular) to the pedestal are growing with higher perfection and with larger vertical dimensions (better heat dissipation). Those with the c-axis perpendicular to the pedestal are much more flat and less perfect. The former crystals exhibit large facets with shiny rounded parts. The latter have the shape of a rounded curved flat disc. In order to estimate the critical growth rate at which defects are intro- duced in the crystals, the apparent undercooling has been increased up to increments of 0.4°C/day. At this high rate, morphological instabilities ap- peared on one of the flat faces leading to the formation of two conically shaped elevations and the growth rate of this face increased strongly. The as grown crystal has been solution sawn perpendicularly to that face, i.e. parallel to the (001) crystallographic plane, and mapping of the defects has been carried out in an attempt to correlate the formation of these defects with the history of growth. Fig. 3a shows schematically the resulting central wafer and the main areas con- taining defects (denoted A and C). Striations are present in the lower center part of the crystal (C). Two groups of channels start near the location where the conical defects appeared (A).

Striations in vapour grown a-HgI 2 crystals were observed for the first time by Schieber et al. [16,25] and were attributed to the temperature oscillation method, TOM [25], which was used at that time. This was also the reason why this growth method has been abandoned. However, striations were found even in crystals grown un- der non oscillating temperature conditions and were attributed to appreciable fluctuations of temperature (several degrees) [25]. In our growth apparatus, thermal fluctuations were not larger than 0.1°C. Another source of thermal changes, however, was the stepwise decrease of tempera- ture to increase the apparent undercooling and keep the effective undercooling constant (cf. sec- tion 3). These steps were varied from 0.1 to 0.4°C. As we have shown recently with precise measure- ments of the mass transport rate of mercuric iodide as a function of undercooling [26], a steep increase of the transport rate (two orders of

3 0 0 0

i CLEARAREAI STRIATIONS !1 CHANNELS

2000 i! D 8 0.024 ,~ :

1000 i l 0.028 ] ,

0 -- -~-- ; - " ~:;;~-+"-~'~ . . . . . . . - . 2 0 .2 .4

R O C K I N G A N G L E , deg

Fig. 4. 7-ray rocking curves for the (200) reflection in areas B, C and A of fig. 1. Both in the clear area B as well as within the striations (C) very sharp peaks have been found (FWHM 0.028 ° and 0.024 ° , respectively; these values approach the resolution of the spectrometer at ILL, Grenoble). No signal has been detected within the channels due to different orien-

tation of area A (see text).

magnitude) takes place in the range of A T< 0.5°C. Therefore, even small temperature changes result in very large changes of the growth rate. This seems to be the reason for the striations in HgI 2. We conclude, therefore, that a tempera- ture stability of 0.01°C is necessary to avoid stria- tions. Preliminary growth runs with the high reso- lution temperature controllers seem to confirm this conclusion.

Somewhat surprising is the formation of a sys- tem of apparently empty macrochannels within the area of conical disturbance, all of them ori- ented in the same direction. Characterization of the crystal slice by means of y-ray rocking curves revealed a very high degree of structural perfec- tion in both optically clear areas (B in fig. 3a) as well as those containing striations (C). Fig. 4 shows the corresponding rocking curves with very sharp single peaks: FWHM (full width at half maximum) approaches the resolution of the spec- trometer (Institut Max yon Laue-Paul Langevin, Grenoble). Thus, striations in HgI 2 do not dis- turb the local structure of the (100) planes. On the other hand, no signal could be detected in the channel area, indicating either a very bad struc- tural quality or another orientation with respect to the (001) plane of the slice. Indeed, X-ray Laue diffractograms showed that in contrast to the rest of the crystal, the area A is parallel to

M. Piechotka et al. / Recent advances in bulk crystal growth from the vapour: a-Hgl 2 1045

the (110) plane and the channels are oriented parallel to the c-axis. A certain epitactic relation- ship exists along the (110) plane between these two crystal regions. The iodine sublattice remains coherent, whereas the coordination of every sec- ond mercury atom is decreased to two instead of four (fig. 3b). This would correspond to a reduc- tion to monovalent mercury like in HgzI2, a well known defect and trap in HgI 2.

In a recent publication of the EG & G / E M laboratory [27] channels of inclusions were found parallel to the a-axis. Considering the model of the a-HgTe2 structure [8], one can see that chan- nels exist in this direction, which might be filled with impurities. In fact, in agreement with the optical micrographs [27], two systems of channels exist perpendicular to each other, due to the tetragonal structure (a = b). On the other hand, the formation of empty macrochannels, as in the case of our crystals, is more difficult to under- stand at present.

5. Conclusions

(1) Large single crystals from the vapour phase may be used for substrate wafers, to increase in certain cases the quality of epitaxial thin films.

(2) A characteristic feature of the vapour growth of HgI 2 is the saturation of growth rate with time.

(3) The high AT still existing at the growth/ evaporation transition as well as other experi- mental observations lead to the conclusion that the above effect is due to the low thermal con- ductivity of HgI 2. This increases the temperature of the growth interface, decreases supersatura- tion and the growth rate.

(4) Spherical model calculations and numeri- cal modeling show that this effect is due to the radiative heat transfer to the crystal.

(5) Striations are formed in HgI 2 single crys- tals due to temperature variations of at least 0.1°C and more. This is due to the very steep dependence of the mass transport rate on under- cooling in the range of A T < 0.5°C.

(6) y-ray rocking curves show that large parts of our crystals grown with undercooling incre-

ments < 0.3°C/day have a high degree of perfec- tion reaching the resolution of the spectrometer (ILL, Grenoble). Similar resolution is achieved in parts containing striations indicating that stria- tions in HgI 2 do not change the structure of the (100) crystallographic planes.

(7) Apparently empty macrochannels are pre- sent in some parts of HgI 2 crystals grown with undercooling increments > 0.3°C/day. The mechanism of their formation has to be further investigated.

References

[1] E. Kaldis, Principles of the vapour growth of single crystals, in: Crystal Growth, Theory and Techniques, Vol. 1, Ed. C.H.L. Goodman (Plenum, New York, 1974).

[2] E. Sch/Anherr, The growth of large crystals from the vapour phase, in: Crystal Growth, Properties and Appli- cations, Vol. 2 (Springer, Berlin, 1980).

[3] E. Kaldis, R. Cadoret and E. Sch6nherr, Crystal growth from the vapour phase, in: Fluid and Materials Science in Space, Ed. H.U. Walter (Springer, Berlin, 1987).

[4] E. Kaldis and M. Piechotka, Vapour growth, in: Hand- book of Crystal Growth, Vol. If, Ed. D.T.J. Hurle (North-Holland, Amsterdam, 1993).

[5] A. Burger, S.H. Morgan, E. Silberman, D. Nason and A.Y. Cheng, Proc. 7th Intern. Workshop on Room Tem- perature X- and y-Ray Semiconductor Detectors and Associated Electronics, Ravello, Sept. 1991 [Nucl. Instr. Methods A 322 (1992) 427].

[6] R.N. Thomas, H.M. Hobgood, P.S. Ravishankar and T.T. Braggins, J. Crystal Growth 99 (1990) 643.

[7] M. Piechotka and E. Kaldis, Nucl. Instr. Methods A 283 (1989) 111.

[8] M. Piechotka and E. Kaldis, Monatsh. Chem. 118 (1987) 137.

[9] T.W. James and F. Milstein, J. Mater. Sci. 18 (1983) 3249.

[10] L. van den Berg and W.F. Schnepple, in: Materials Processing in the Reduced Gravity Environment of Space, Ed. G.E. Rindone (North-Holland, Amsterdam, 1982) p. 439.

[11] C. Parfeniuk, F. Weinberg, I.V. Samarasekera, C. Schve- zov and L. Li, J. Crystal Growth 119 (1992) 261.

[12] M. Piechotka and E. Kaldis, Study of the Technical Requirements for Vapour Growth Experiments in Micro- gravity Environment, ESA Contract No. 5943/84/F/FL.

[13] J.S. Iwanczyk, Nucl. Instr. Methods A 283 (1989) 208. [14] B.E. Patt, A.G. Beyerle, R.C. Dolin and C. Ortale, Nucl.

Instr. Methods A 283 (1989) 215. [15] H. Scholz, Acta Electron. 17 (1974) 69. [16] M. Schieber, I. Beinglass, G. Dishon and A. Holzer, in:

1046 M. Piechotka et al. / Recent advances in bulk crystal growth from the vapour." a-HgI 2

1976 Crystal Growth and Materials, Eds. E. Kaldis and H.J. Scheel (North-Holland, Amsterdam, 1977) p.280.

[17] T. Kobayashi, J.T. Muheim, P. Waegli and E. Kaldis, J. Electrochem. Soc. 130 (1983) 1183.

[18] N. Singh, R.H. Hopkins, R. Mazelsky and M. Gottlieb, J. Crystal Growth 83 (1987) 334.

[19] (a) M. Isshiki, M. Piechotka and E. Kaldis, J. Crystal Growth 102 (1990) 344; (b) M. Isshiki, M. Piechotka and E. Kaldis, in: Proc. 7th European Symp. on Materials and Fluid Sciences in Microgravity, Oxford, September 1989 [ESA SP-295 (1989)13].

[20] V.M. Zaletin, N.V. Lyakh and N.V. Ragozina, Crystal Res. Technol. 20 (1985) 307.

[21] A.A. Chernov, E. Kaldis, M. Piechotka and M. Zha, J. Crystal Growth 125 (1992) 627.

[22] A. Roux, A. Feodosyev and B. Roux, in: Proc. 8th Euro- pean Syrup. on Materials and Fluid Sciences in Micro- gravity, Brussels, April 1992, in press.

[23] M. Zha, M. Piechotka and E. Kaldis, J. Crystal Growth 115 (1991) 43.

[24] CARRAR-MATRA-ETHZ, High Resolution Diagnos- tics, Final Technical Report, ESA No 162/1301/89/ JSG/SD-06.

[25] M. Schieber, W.F. Schnepple and L. van den Berg, J. Crystal Growth 33 (1976) 125.

[26] M. Pieehotka, E. Kaldis, P. Behrmann and H. Stanna, in: Proc. 7th European Symp. on Materials and Fluid Sci- ences in Microgravity, Oxford, September 1989 [ESA SP-295 (1989) 587].

[27] K. James, V. Gerrish, E. Gross and J. Markakis, in: Proc. 7th Intern. Workshop on Room Temperature X- and y-Ray Semiconductor Detectors and Associated Elec- tronics, Ravello, Sept. 1991 [Nucl. Instr. Methods A 322 (1992) 390].