Nuclear Instruments and Methods in Physics Research A253 (1987) 423-426 North-Holland, Amsterdam

423

RECENT PROGRESS IN MATERIAL CHARACTERIZATION OF a-H& CRYSTALS FOR X- AND y-RAY DETECTORS

M. PIECHOTKA and E. KALDIS

Laboratorium fiir Festkijrperphysik, ETH HGnggerberg, CH-8093 Ziirich, Switzerland

o-H& is presently the most promising material for room temperature X- and y-ray detectors. A certain drawback, however, is the low mobility of holes which is attributed to defects in the crystals. We have investigated organic impurities and nonstoichiometry as possible sources of defects. Mass spectrometric investigation of a molecular beam formed by evaporating HgI, crystals shows clearly the existence of nonstoichiometry. Both excess of Hg or excess of I are possible. After thermal treatment, pure nonstoichiometric samples give stoichiometric crystals. However, if hydrocarbons are also dissolved in the lattice the nonstoichiometry becomes fixed. Furthermore, the evaporation measurements show a change in the state of the surface of o-HgI, crystals at 67°C. Our investigations up to now indicate the existence of a surface reconstruction at this temperature. As this temperature is appreciably lower than the crystal growth temperature by sublimation (about 110°C) this effect should be responsible for the high concentration of defects found up to now near the surface of the as-grown crystals.

1. Introduction 2. Organic impurities and nonstoichiometry of HgI,

Mercuric iodide is now accepted as the best material for high energy detectors operating at room tempera- ture [l]. Its wide band gap (2.13 eV at room tempera- ture) results in a very low intrinsic carrier concentra- tion, which can be estimated to be 20cm-3 (corres- ponding to a resistivity greater than lOi 0 cm). The resistivity of currently available HgI, crystals lies in the range of 10’“-1014 s2 cm [2] which corresponds to an extrinsic carrier concentration of 106-10’ crnm3 (electrons are assumed since their mobility exceeds that of holes by a factor of 20). It has been shown that HgI, detectors can operate in a very wide energy range: from 526 eV (X-ray of oxygen [3]) to 1.33 MeV (y-ray of 6oCo [4]). The energy resolution of HgI, spectrometers is being continuously improved, espe- cially at the low energy limit where the best reported value (fwhm = 14.5 eV for Al K X-rays [5]) is compar- able to that of commerically available Si(Li) spec- trometers. Nevertheless, HgI, devices suffer from some problems which include sensitivity to mechanical damage (lattice distortions), lack of purity as well as nonstoichiometry of the crystals. There are also sever- al surface effects (traps, structural and chemical de- fects) with little or no evidence available as to their origin. In the present paper we briefly discuss progress in material characterization of HgI, crystals based on our mass spectrometric studies [6,7] as well as on the recently published literature data.

Nonstoichiometry of HgI, crystals has been investi- gated by several laboratories in 1981 [8-lo]. Their work (except ref. [lo]) based on wet chemical analysis used at the limit of accuracy gave somewhat con- tradictory results. The lattice constant and density measurements [lo] indicated indirectly nonstoich- iometry on both sides of the stoichiometric point. Our recent mass spectrometric measurements prove direct- ly that both excess of iodine as well as excess of mercury is possible in cu-HgI, crystals [6].

Organic impurities have been revealed in mercuric iodide by spark source mass spectrography (SSMS) in 1983 [13-151. SSMS studies pointed to iodine as the source of hydrocarbons in HgI, [13]. The mass spec- trometric investigations of evaporation of mercuric iodide confirm the presence of organic impurities [6]. In addition, it has been shown that they can be introduced not only by iodine (starting material for synthesis) but also by contact of mercuric iodide with organic vapors. This can happen either on purpose, like in polymer assisted growth of HgI, platelets for X-ray detectors [16], or unintentionally in vacuum systems used for the evacuation of ampoules for cryst- al growth. We have measured large signals of organic impurities both in HgI, platelets as well as in HgI, samples, which have been sublimed in the presence of vapors of oil for rotary pumps [6].

The most important result of our studies, however,

016%9002/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

III. NONSTANDARD MATERIALS

424 M. Piechotka, E. Kaldis I Material characterization of a-H@, crystals

c

Fig. 1. Change in (I/Hg)’ ratio in the ion beam of two mercury-rich HgI, samples. A - HgI, synthesised from ele- ments under excess of mercury, B - HgI, single crystalline platelets grown in presence of organic additives (this sample has been kindly supplied by Dr. K. Conder, Warsaw Techni- cal University, Poland). Dotted line - temperature rise

period, solid line - constant temperature period.

is the interplay between organic impurities and non- stoichiometry of HgI, crystals. The excess component (either Hg or I) can be removed by subliming non- stoichiometric mercuric iodide in a semi-open system (i.e. having a small effusion hole) provided that there are no detectable hydrocarbons in the sample. If hydrocarbons are present the excess component can- not be separated from HgI, by sublimation. The non- stoichiometry becomes fixed and does not change till the end of sublimation. Fig. 1 shows the change with time of the (I/Hg)’ ratio in the ion beam for two different HgI, samples during their total evaporation in a Knudsen cell (semi-open system). Both samples

were mercury-rich. Mercury-rich crystals are recog-

nised by the (I/Hg)+ ratio smaller than 1.0. (I/Hg)+ = 1.0 is characteristic for stoichiometrlc HgI, in our mass spectrometer [6]. Sample A, in which no hydrocarbons have been detected, lost the excess mercury in the course of evaporation (step on curve A in fig. 1 after 5 h). On the other hand, sample B (HgI, platelets containing organic impurities) maintained a (I/ Hg) + value of 0.8 over the entire constant temperature period of evaporation (flat part of curve B in fig. 1). Fixed iodine excess in the presence of hydrocarbons has been also observed [6].

We consider now briefly the possible consequences of these findings in terms of the electrical properties of HgI, crystals and their detector performance. Fig. 2 shows a comparison of the concentration ranges for extrinsic carriers, traps, nonstoichiometric components and organic impurities in HgI, detector crystals as reported in the literature [8-15,171. Disparity in or- ders of magnitude between the electrical (carrier and traps) and chemical (nonstoichiometry and impurities) parameters of the crystals is clearly seen. This dispari- ty means that the vast majority of the chemical defects in HgI, crystals are not electrically active, i.e. they contribute neither to carriers nor to traps. However, the beneficial role of iodine doping and the disadvan- tageous role of mercury doping in HgI, detector per- formance have been indeed experimentally proved [2,18]. Iodine doping increases the mobility of holes, whereas mercury doping increases the mobility of electrons. Our mass spectrometric results show that HgI, platelets are contaminated with organic im- purities and they are mercury-rich at the same time. As mercury excess increases the mobility of electrons this can explain why HgI, platelets are very good

Concentration , cm3

10” 106 ,to 13 ld” 15 10: : lo’% ld9 10 20 10” 10”

t

1 Concentration of traps 1

1 Excess of iodine !

1 Excess of mercury 1

+l 4 x 102’ Hgl*

molecules/cm3

c ----------- Organic impurities ---_-- ----

c

-----_----- Inorganic impurities ____-----_-

Fig. 2. Variation range of some electrical and chemical parameters of a-HgI, crystals.

M. Piechotka, E. Kaldis I Material characterization of a-HgIZ crystals 425

X-ray detectors [16]. Indeed, in X-ray detectors only electrons are collected. For the case of HgI, platelets we can also conclude that organic impurities do not influence directly their electrical properties. However, as we have shown, they do affect the nonstoichiometry of mercuric iodide. Thus, for high energy applications, where both hole and electrons are collected, hyd- rocarbon-free stoichiometric crystals are still required for the best detectors.

Another reasonable explanation for the existing dis- parity in concentration levels of the chemical defects on one hand and charge carriers and traps on the other, is that a single (or few) impurity (-ties) interact with the nonstoichiometric defects and it is the formed complex which is electrically active. Such interaction has been recently reported in II-VI semiconductors [19]. The opposite case can, of course, not be ex- cluded: impurities, especially hydrocarbons may com- pensate the vast majority of the nonstoichiometric defects making them electrically inactive. Anyway, unless hydrocarbon-free mercuric iodide is available, no final explanation can be put forward.

35-150°C [7]. It showed, unexpectedly, a 42% change of evaporation enthalpy at 67°C. Since measurements by differential scanning calorimetry revealed no bulk heat effects at this temperature, we conclude that a change in state of the crystal surfce takes place which influences the vapor pressure of HgI, but does not affect its bulk thermal properties. This possible surface reconstruction - which is marked as “surface phase” in fig. 3 -could be the source of the high concentration of defects reported on the surface of the vapor grown HgI, crystals [21]. As the crystals are grown at a temperature above lOo”C, they must undergo this surface reconstruction during cooling to room tem- perature. It is not clear at the moment how far impurities or nonstoichiometry (e.g. for solution grown crystals) can affect the temperature of the surface reconstruction.

4. Conclusions

3. Phase instability of cu-HgI,

Our work has shown that a large concentration of impurities (especially hydrocarbons), nonstoichiomet- ry and structure instability are the sources of defects in (Y-HgI, crystals.

Red cr-HgI, used for detectors is thermodynamical- Although hydrocarbons themselves do not seem to ly stable below 130°C. However, several other phases be electrically active in HgI,, they stabilize its non- have been already discovered within the stability re- stoichiometry. Therefore, stoichiometric crystals for gion of the a-phase (fig. 3, more details will be given high energy detectors should be grown from a hyd- in ref. [20]) and the phase relationships need further rocarbon-free HgI, starting material. investigations. In the course of our mass spectrometric There is experimental evidence that structural studies we have measured also the vapor pressure changes take place on the crystal surface during post- curve of mercuric iodide in the temperature range growth cooling to room temperature.

Fig.

ORANGE PHASE RED PHASE ci

SURFACE PHASE

YELLOW PHASE

B

UNDERCOOLED YELLOW PHASE

/ /. / / /d/ ,/ / / / / /j

R-r SO 70 130

OC

3. Various phases of mercuric iodide which have been observed within the stability range of the red cr-HgI, modification used for detectors.

III. NONSTANDARD MATERIALS

426 M. Piechotka, E. Kaldis I Material characterization of cr-Hgl, crystals

References [lo] I.F. Nicolau and G. Rolland, ibid. 16 (1981) 759.

[ll] A. Burger, M. Roth and M. Schieber, J. Crystal Growth

56 (1982) 526.

[l] M. Schieber, 5th Int. Workshop on Mercuric Iodide [12] A. Tadjine, D. Gosselin, J.M. Koebel and P. Siffert,

Nuclear Radiation Detectors, Jerusalem (1982) Nucl. Nucl. Instr. and Meth. 213 (1983) 77.

Instr. and Meth. 213 (1983) 1. [13] T. Kobayashi, J.T. Muheim, P. Waegli and E. Kaldis, J.

[2] M. Gospodinov, Krist. Techn. 15 (1980) 263. Electrochem. Sot. 130 (1983) 1183.

[3] J.S. Iwanczyk, A.J. Dabrowski and G.C. Huth, Appl. [14] J.T. Muheim, T. Kobayashi and E. Kaldis, Nucl. Instr.

Phys. Lett. 46 (1985) 606. and Meth. 213 (1983) 39.

[4] M. Schieber, I. Beinglass, G. Dishon, A. Holzer and G.

Yaron, Nucl. Instr. and Meth. 150 (1978) 71.

[5] J.S. Iwanczyk, A.J. Dabrowski, G.C. Huth and W.

Drummond, Adv. X-Ray Analysis 27 (1981) 405.

[6} M. Piechotka and E. Kaldis, J. Electrochem. Sot. 133

(1986) 200.

[15] I.F. Nicolau, ibid. 213 (1983) 13.

[16] P. Faile, A.J. Dabrowski, G.C. Huth and J.S. Iwanczyk,

J. Crystal Growth 50 (1980) 752.

[17] T. Mohammed-Brahim, A. Friant and J. Mellet, Phys.

Stat. Solidi (a) 79 (1983) 71.

[7] M. Piechotka and E. Kaldis, J. Less Common Met. 115

(1986) 315.

[18] R.C. Whited and L. van den Berg, IEEE Trans. Nucl.

Sci. NS-24 (1977) 165.

[S] G. Dishon, M. Schieber, L. Ben-Dor and L. Halitz,

Mat. Res. Bull. 16 (1981) 565.

[9] M.C. DeLong and F. Rosenberger, ibid. 16 (1981)

1445.

[19] R.N. Bhagrava, J. Crystal Growth 59 (1982) 15.

[20] M. Piechotka and E. Kaldis, Material Aspects of LY-

HgI, for X- and y-Ray Detectors, to be published.

[21] R. Lynn, EG&G Santa Barbara Operations, private

communication to E. Kaldis (March 1984).