Correlations between mercuric iodide photoluminescence spectra and nuclear detector performance

Nuclear Instruments and Methods in Physics Research A317 (1992) 194-201 North-Holland

orrelations be e mercuric iodide photoluminescence spectra uc ear detector performance X.J. Ba® and T.E. Schlesinger

Department of Electrical and Coinnliter Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

James and S.J . Han~ey

Advanced Materials Research Dirision, Sandia National Laboratories, Livermore, CA 94550, USA

A.Y. Cheng, V. Gerrish and C. Ortale

EG&G Energy Measurements, Inc., Goleta, CA 93116, USA Received 30 December 1991

Low temperature photoluminescence spectroscopy was performed on a variety of HgI 2 samples and also on graded Hg:2 nuclear detectors . Correlations were found between features it the photoluminescence spectra and a crystal's ability to produce high-quality detectors . The intensity of a broad emission band centered at 6200 A (designated as band 3) is weaker in crystals that yield high-quality detectors . Therefore, the defects responsible for this emission band are undesirable in the fabrication of HgI 2 nuclear detectors. The measurements also revealed that stronger emission in the exciton region (designated as band 1) is associated with crystals which produce high-quality detectors, indicating that a high degree of structural perfection is important for Hgl, detector applications. These Correlations, together with earlier results from studies of processing-induced defects, lead to suggestions regarding improvement of the manufacturing yield of high-quality Hgl, detectors .

1. Introduction Increased interest in mercuric iodide (HgI2 ) in recent years stems from the ability to fabricate X-ray and gamma-ray detectors from this material that have relatively high stopping power and can be operated at room temperature [1-3] . These characteristics provide considerable advantages over conventional semiconductor nuclear detectors such as lithium-drifted silicon (Si(Li)), lithium-drifted germanium (Ge(Li)) or highpurity germanium (HPGe) detectors . The stopping power is related to the efficiency of the interaction between the detector material and the radiation to be detected . For a wide range of photon energies (less than about 1 MeV), the main contribution to the interaction is the photoelectric effect, which is proportional approximately to the fifth power of the atomic number [4] . Thus, the atomic numbers of HgI 2 (80 and 53), compared with those of Si (14) and Ge (32), increase considerably the probability of an incoming photon to interact with the detector material . The

operating temperatures of Si or Ge detectors are determined by the amount of dark current and the need to preserve the lithium profile. The latter requirement is much more stringent and inconvenient since the detectors have to be kept cool at all times after fabrication [5] . In most applications, the dark currents of Si and Ge detectors are intolerable at room temperature due to their intrinsic conductivities, which are determined by the energy band gaps of the materials . HgI 2 provides another advantage since its band gap at room temperature is 2.12 eV [6], as compared to 1 .12 eV for Si and 0.67 eV for Ge. Under dark conditions, a resistivity of 10'3 fl cm can be easily achieved for undoped HgI 2 [2]. One major concern in the commercialization of HgI 2 detectors is the manufacturing yield of high-quality devices, which has been limited to about 20% [7] . This yield is sometimes limited by the poor quality of the as-grown crystals. This is illustrated by the fact that among crystals that were grown under similar conditions, some crystals consistently produce low-quality

0168-9002/92/$05 .00 n 1992 - Elsevier Science Publishers B.V . All rights reserved

X.J. Bao et al. / Hg12 PL spectra and nuclear detector performance detectors [8]. In addition, many parameters during crystal growth have to be well controlled to produce crystals suitable for detector fabrication [9,10]. The yield is also affected by the processing procedures, which can introduce defects that lower the quality of the detectors. This is demonstrated by the wide range of performance for detectors made from the same crystal and also by the fact that poor detectors can be refabricated into good detectors [11]. Extensive research has been conducted along both lines to investigate causes of the degradation of the detector quality. Correlations between detector quality and stoichiometry, structural imperfections in the crystal, impurity contamination, processing procedures, or contact materials have been actively pursued. Randtke et al. [12] have shown that HgI2 samples with etch pit counts of 103 to 104 cri, -Z have acceptable detector performance, while h-;her dislocation concentrations should be avoided. Schieber et al. [13] observed that nonhomogeneity can degrade the energy resolution of the detector. Using neutron and X-ray diffraction measurements, Yelon et al. [14] concluded that poorer structural perfection and larger width of the X-ray rocking curve result in detectors with lower energy resolution . Beinglass et al. [10,15] reported that iodine treatment of the starting materials for crystal growth has resulted in better crystal quality, while a large excess of iodine during growth failed to produce good crystals . Doping experiments during crystal growth performed by Whited et al. [16] showed that the mobility-lifetime product for electrons increases slightly with mercury doping but decreases by two orders of magnitude with iodine doping, and that the mobility-lifetime product for holes decreases by two orders of magnitude in mercury-doped crystals. Stoichiometry analysis and detector performance evaluation by Tajine et al. [17] found that crystals most nearly stoichiometric give the best performance. Chemical etching experiment with KI aqueous solutions by Levi et al. [18] showed that etching with a KI solution of approximately 20% by weight at about 0°C and a subsequent exposure to ambient atmosphere for roughly 24 h before contact deposition may reduce the surface recombination velocity and thus improve the quality of HgI2 detectors. Chemical etching with KI solution has been standardized at EG&G for cutting and polishing during detector fabrications ; however, Nicolau et al. [19] reported that HN03 solution worked better than KI solution in terms of producing high quality detectors. Disagreement remains regarding the best choice of contact material for detector applications [19]. In this work, we have employed low temperature photoluminescence (PL) spectroscopy to study the correlations between features in the PL spectra and the ability of HgI 2 crystals to produce high-quality nuclear detectors . With these results and observations from

195

Table l Samples used in this work Set Sample 1 N1-7-1

2

3 4

5

6

7

8 9

Color

GPA

Sample

Color GPA

0

3 .6

S8-21-1

3

2 .0

N7-8-1

0

3 .4

S8-21-2

3

2.0

N7-8-2

0

3 .4

S8-21-3

3

2.0

N13-13-1

0

3.0

N7-12-2

8

1 .7

N13-13-2

0

3.0

N4-16-1 :C

0

3.3

V2-8-1 :C

1

2.0

V2-8-2:C

1

2.0

S8-32-1 :C

3

2 .0

V8-4-1 :A]

0

3.0

N11 -8-1 :A1

0

2 .0

V8 - 4-2 :A1

0

3.0

N11 -8-2 :A1

0

2 .0

V8-4- 1 : 1 n

0

3.0

N2-' 5-8:ln

0

1 .7

V8-4-2 :b

0

3 .0

N2-15-9:1 n

0

1 .7

N245-101n

0

1 .7

V8-4-1 :Pd

0

3 .0

N2-15-1 :Pd

0

1 .7

V8-4--2 :Pd

0

3 .0

N2-15-2:Pd

0

1 .7

V8-4-3 :Pd

0

3 .0

N2-15-3 :Pd

0

1 .7

V8-4-4 :Pd

0

3 .0

N2-15-4:Pd

0

1 .7

V8-4-5 :Pd

0

3 .0

V8-4-6:Pd

0

3 .0

V2-9-1 :Au

0

2.8

N5-10-1 :Au

5

1 .6

V2-9-2:Au

0

2.8

N5-10-2 :Au

5

1 .6

V2-9-3:Au

0

2.8

N2-18-1 :11 VC 0

3.0

IN7-12-1 :93°C

8

1 .7

N2-18-2 :134°C 0

3.0

N7-12-2 :142°C 8

1 .7

N2-18-3 :150°C 0

3.0

V10-1-1 :60°C

0

1 .8

N4-16-4:Ni

0

3 .3

V2-8-1 :Ni

1

2.0

N4-16-5 :Ni

0

3 .3

N4-16-1 :Ag

0

3 .3

S8-21-1 :Ag

3

2 .0

N4-16-2 :Ag

0

3 .3

S8-21-2:Ag

3

2 .0

N4-16-3 :Ag

0

3 .3

S8-21-3:Ag

3

2 .0

previous studies of processing-induced defects [20], we will discuss areas where improvements might be made to increase the manufacturing yield of HgI2 detectors. 2. Experimental techniques All HgI2 samples used in this work were obtained from single crystals grown at EG&G Energy Measurements Inc. by vapor phase growth. Slices with dimension of about 1.0 x 1.0 x 0.1 cm were cut along the crystallographic C plane by sawing with a thread dipped in an aqueous KI solution. The slices were next polished in a 20% KI solution, followed by a 30 s etch in a 10% KI solution and a rinse in deionized water for several minutes to remove the mechanical damage from sawing . An array of samples was used and they are tabulated in table 1. In the designation of each sample, the first letter and the first two numbers represent the crystal, from which the sample was obtained. Also listed in table 1 are the color number and grade point

X.J. Bao et al. / Hgl~, PL spectra and nuclear detector performance

19 6

ayerage (GPA) for each crystal . The color number is a number between 1 and 10 that indicates the redness of the visually observed color of the vapor during crystal growth . This number corresponds to the relative concentration of iodine in the vapor during crystal growth, where a larger value indicates a higher iodine content in the vapor . GPA for each crystal was obtained as follows . Approximately ten to twenty detectors were fabricated from each crystal and their performances were evaluated in terms of their response to 662 keV radiation from a 11'Cs source. A grade of A, B, C or D was assigned to each detector according to its energy resolution and peak-to-valley ratio, where grade A corresponds to the highest quality and grade D the lowest. GPA is determined by assigning weighting factors of 4, 3, 2 and 1 to grades A, B, C and D, respectively and then taking the weighted average. A total of 52 samples obtained from 14 different crystals were used in these studies . These samples were divided into two groups . In the first group (the left column in table 1), the samples all have GPAs equal or greater than 2.8. In the second group (the right column in table 1), the samples all have GPAs equal or smaller than 2.0. In each group, there are nine sets of samples, which have been treated differently before the PL measurements. The first set includes bare crystals . These are as-grown crystals that have never undergone any treatment except KI etching . The PL spectra were taken after the samples had been stored in a dessicator for at least one day. The sample sets designated as two through six, eight and nine include samples that were contacted with a semitransparent contact layer of carbon, aluminum, indium, palladium, gold, nickel or silver, respectively. However, the PL data reported in this

paper were all taken from regions of the sample surface away from the contact . Sample set seven contains bare crystals that had been surface heated. These samples were heated for a very short period of time (less than about 30 s) in a vacuum of about 5 x 10' Torr by a hot tungsten filament. The temperature was monitored by a thermocouple placed near the sample. The maximum temperatures attained for this set of samples are shown in table 1 . All the contacted and surface heated samples were etched with a KI aqueous solution just prior to the contact deposition or surface heating, and the PL measurements were performed a few days after the KI treatment . PL measurements were performed at 4 .2 K with an argon laser as the excitation source . The laser was operated at 20 mW and tuned to the 488 nm line. The laser beam was chopped at a frequency of 750 Hz. The spot size of the laser excitation on the samples was about 1 mm in diameter. Luminescence was dispersed by a Spex 1404 double pass spectrometer and detected by a Hamamatsu R943-02 photomultiplier which has an S-20 response . 0The spectrometer has a spectral resolution of 1.0 A for all the measurements. The current output of the photomultiplier was amplified by a Keithley 427 current amplifier and then an EG&G 5207 lock-in amplifier . Since the excitation source has a photon energy of 2.54 eV, which is above the band gap of HgI, at 4.2 K (2.37 eV), the PL was generated from a region with a depth less than a few microns from the surface . At least three spectra were taken for each sample from different spots. We have also taken PL spectra from three working detectors graded B, C and D. These measurements were made with an S-1 photomultiplier, which did not

,v

Z) crV1 .0

G7 ~

S

0 0 0

u PO

r

P2 5290

lk_/_~

x 0.1 5350

1

5410

5410

6730

8050

Wavelength (A) Fig . l. 4.2 K photoluminescence spectrum from a Hg1 2 bare crystal showing (a) the well structured band 1 (from 5290 to 5410 A) and (b) the broad emission of band 2 (5595 A), band 3(6200 A) and band 4 (7550 A).

X.J. Bao et al. / Hgl, PL spectra and nuclear detector performance

allow us to fully resolve the exciton region. The spectra were taken from spots away from the contacts . Four spectra were taken for each detector. 3. Results and discussions Fig. 1 is a typical 4.21 K PL spectrum obtained from a bare HgI, crystal freshly etched with a 10% (by weight) KI aqueous solution . The spectrum is divided into the near band gap region, which is labeled as band 1, and the long wavelength region, which includes several commonly observed broad emissions labeled as bands 02, 3 and 4. A well resolved band 1 (from 5290 to 5410 A) consists of at least 26 narrow emission lines, which have been reported previously [21]. The peaks labeled P0, P1 and P1' in fig. 1 have been carefully studied by Akopyan et al. [22], who have attributed these peaks as due to longitudinal exciton, transverse exciton and spin-forbidden free exciton transitions . P2 and P3 have been generally assumed to be due to bound excitons [23-26] . In a previous publication [20], we have tentatively assigned P3 as due to excitons bound to mercury vacancies . Merz et al. [25] have observed a correlation between P2 and band 2, and the defects related to the latter emission were reasoned to result from iodine deficiency in the crystal . The rest of the peaks in the band 1 region are most likely phonon replicas of the above emissions as discussed previously [21,23,25]. Band 2 has been observed in almost every0 PL spectrum of HgI, and is centered at about 5595 A. Bands03 and 4, which are centered at about 6200 and 7550 A, respectively, do not appear in every PL spectrum of HgI, and are usually at least one order of magnitude weaker than the intensities of band 2 or band l . There are several other broad emission bands in the long wavelength region observed in doped HgI, crystals, some of which have been associated with specific impurities such as copper [27], silver [28], and possibly indium and tin [29], and they will not be discussed in this paper. Fig. 2 shows the average intensities of band 3 for the nine sets of samples . The two lines link samples with GPA equal to or greater than 2.8 and samples with GPA equal to or less than 2.0, respectively . The error bar for each point represents the standard deviation of the mean. Clearly, the absolute intensity of band 3 is weaker for samples with large GPAs. The only exception is the ninth set which contains samples contacted with silver . Since silver has been found to react with HgI,, introduce a broad emission band, diffuse rapidly both along the surface and through the bulk [28], it will not be considered further, except when discussing the possible origin of band 3. Also it can be noted that the standard deviations of the means arc

197

80 70

M M ~C

0 ;-'A. wL

40

a

20

sa

1 Crystal 2 Carbon 3 Aluminum 4 Indium 5 Palladium 6 Gold 7 Surface heated crystal 8 Nickel 9 Silver

30

10

Samples Fig. 2. Average intensities of band 3 for two kinds of samples (GPA >_ 2.8 and GPA < 2.0) from nine different sample sets . The error bars are the standard deviations of the means .

fairly large. This is typical of HgI 2 samples which generally show lâ :ge &giees of variations from sample to sample. However, since the data represents a large number of spectra taken from many different samples, we believe that the trend shown in this figure is convincing . Another observation that should be noted in this figure is the relatively large band 3 emissions in samples contacted with nickel and silver, and also in samples that have undergone surface heating. No obvious correlations can be found between the intensities of band 2 and GPAs, of the samples. On the other hand, the intensities of P3 are typically greater for samples with larger GPAs with the rxceptio., of sample sets two and nine. The observed trend for P3 is less convincing than that for band 3. Some of the variations may be due to various degrees of aging of the surfaces where the PL measurements were taken . It has been found that P3 is particularly sensitive to aging, and that a change in the intensity of P3 on freshly KI etched HgI, samples may occur within a period of one day [30]. In addition, the degree of aging on the intensity of P3 was found to vary from sample to sample and sometimes from spot to spot on the same sample [31). Unless the sample has been exposed directly to air for a fairly long period of time (longer than several weeks), the main effect of aging over a period of a few days for samples stored in a dessicator is that the intensity of P3 is decreased considerably, very often by as much as one order of magnitude [20,30]. However, other emissions such as P2 and band 2 remain unchanged within the uncertainty of the measurements. The band 2 to P2 ratio was found to be unaffected by KI etch, aging and vacuum exposure of HgI, samples [20]. A plot of band 2 to P2 ratios is shown in fig . 3. It appears that this ratio is smaller for samples with

198

X.J. Bao et al. / Hgl, PL spectra and nuclear detector performance

0.8 0.6

1 Crystal 2 Carbon -3 Aluminum 4 Indium 'i 5 Palladium 6Gold ~7 Surface heated crystal ~8 Nickel

5

-~- GPA>2.8 (out) -'~-

GPA<2.0 (out)

0

w cv

0

0.4

4

2

0.2 0.0®

1

2

3

4

5

6

7

8

9

10

Samples Fig. 3. Average band 2 to P2 ratios for two kinds of samples (GPA >_ 2.8 and GPA < 2.0) from nine different sample sets. The error bars are the standard deviations of the means. larger GPAs, with the exception of sample set eight . Since we have found that the intensity of band 2 is not related to the GPAs of the samples, this figure shows that the intensity of P2 is greater for samples with larger GPAs. There are two advantages to using this ratio over the absolute intensity of P2, P3 or the ratio between band 2 and P3. First, when a ratio is taken, most of the uncertainties due to optical alignment are eliminated. Second, this ratio is only weakly affected by the process of aging and thus less sensitive to the surface condition . Results of PL measurements from graded detectors are summarized in figs. 4 and 5 . Since the spectra were taken with a photomultiplier with an S-1 response, 300 ~a~-

a

"c Z Cç

Band 1(5290-5410A) Band2(5410-5690A)

200

loo

0

Detector Grade Fig . 4. Integrated intensities of band 1 and band 2 for three working detectors graded B, C and D. The intensities of band 1 and band 2 were obtained by integrating from 5290 to 5410 A and from 5410 to 5690 A, respectively. The error bars are the standard deviations of the means .

0

Detector Grade Fig. 5. Band 2 to band 1 ratios for three working detectors graded B, C and D. The intensities of band 1 and band 2 were obtained by integrating from 5290 to 5410 A and from 5410 to 5690 A, respectively . The error bars are the standard deviation of the means.

which is much less sensitive in the spectral region of interest, a lower resolution for the spectrometer had to be used and the band 1 region was not as well resolved . We have integrated the intensity over the band 1 region from 5290 to 05410 A and over band 2 region from 5410 to 5690 A. The integrated intensities of band 1 and band 2 are shown in fig . 4. As can be noted in this figure, the integrated intensity of band 1 increases with the detector quality, while the integrated intensity of band 2 does not follow a monotonic trend . These observations agree with the measurements on bare crystals and the contacted samples discussed earlier in this paper. Since only three detectors were used in this experiment, the ratio between these two bands is less vulnerable to uncertainties in the measurements (see fig . 5). Clearly, band 2 to band 1 ratio decreases as the quality increases for the detectors studied . In the rest of this section, we discuss the results shown in figs. 1-5, behaviors of bands 1, 2, 3 and 4 observed in PL spectra of HgI2, and present several suggestions to improve the yield of high-quality detectors. Merz et al. [25] observed that the intensity of band 2 increases with the number of sublimations used to purify the HgI 2 powder prior to the crystal growth . Band 2 was therefore attributed to iodine deficiency in the crystal since it was believed that repeated sublir:lations reduce the iodine content in the starting material due to the higher vapor pressure of iodine compared with mercury . Merz et al . further correlated the performance of good detectors with a weak band 2 and small P2 to P3 ratio. In this work, from a large number of PL

X.J. Bao et al. / Hg12 PL spectra and nuclear detector performance

spectra, we do not observe any apparent correlation between the intensity of band 2 and the GPA of a crystal . One possible explanation is that samples used in our experiments were obtained from crystals that were grown from starting materials that have undergone the same number of sublimations (four). It can therefore be argued that a crystal's ability to produce high-quality detectors is not sensitive to the ccncentration of the defect responsible for band 2 for starting materials that have undergone four repeated sublimations. Note that at least four sublimations are typically used to purify powders suitable to grow crystals for detector fabrication, even though with each sublimation step, the intensity of band 2 increases considerably . The physical identity and origin of the defect responsible for band 3 is not known. However, we have observed many behaviors of this band in PL spectra that may help to explain this broad emission and may also be useful in trying to improve processing or crystal growth for the fabrication of Hg12 devices . The observations related to band 3 will be summarized as follows, and some interpretation will also be given. 1) Band 3 is often modified after KI etching . It was observed that some samples that do not have a band 3 emission in their PL spectra show band 3 after a KI etch, and some other samples that have a band 3 emission show a reduction in its intensity after KI etch. In the case where band 3 is enhanced by KI etch, there are two possibilities, the first being that band 3 was introduced by KI, and the second being that KI etch simply removes a surface layer which has a relatively low concentration of defect responsible for band 3, thereby exposing a new surface layer with a higher concentration of this defect . We believe both processes can take place depending upon the particular samples used. The evidence can be obtained from vacuum exposure experiments . In those experiments, the freshly KI etched samples were placed in a vacuum system at 5 x 10 -6 Torr for about 30 min. It was found that this vacuum exposure effectively removed a surface layer congruently from the Hg12 sample [20]. At the same time, band 3 disappeared from the PL spectra of some samples while it remained in others . It was therefore reasoned that in the first kind of samples the defect responsible for band 3 was introduced by KI solution and contaminated only a shallow surface region which can subsequently be removed by vacuum exposure, but in the second kind of samples, band 3 remains since the particular contaminant responsible for band 3 was distributed throughout the bulk of the sample . It is possible that the impurities responsible for band 3 are contained in the I common to both KI and Hg 12 starting material. The assumption that band 3 may be introduced by KI etch was further supported by other PL measure-

199

ments on samples contacted with semitransparent contacts. In these spectra, it is quite common to find the intensity of band 3 in spectra taken from underneath the contact to be considerably less than that in spectra taken from outside the contact . The differences are much more than can be accounted for by the loss of excitation power and luminescence due to reflection and absorption by the contacts. The most likely explanation is that during the contact deposition, which was performed in a vacuum either by sputtering or thermal evaporation, the contacted region was exposed to the vacuum while the other regions were masked off by a shadow mask. The masked off region had much less material loss and retained the band 3 related defects introduced by KI etch while the material loss from the exposed surface has effectively removed the contaminated surface layer. It seems that if KI solution is to be continually used as the main etchant in the fabrication of Hg12 detectors, it is necessary to understand the impurities present and to purify the KI. In the case where KI etch reduces band 3, it is possible that the sample was contaminated during handling as will be further discussed below. 2) In aging experiments where the samples are stored in a dessicator for a period of days after KI etch, the intensity of band 3 is modified after aging. In the first case where the intensity of band 3 is increased after aging, the defects responsible for band 3 may be incorporated while being stored or during handling. In the second case, the intensity of band 3 is reduced and often to a level beyond our detection limit. In addition, it should be noted that all the samples used in our KI etching experiment in 1) were obtained from thread sawing with the thread dipped in a KI solution. The fact that many of the as-received samples do not have band 3 also suggests the reduction of band 3 due to aging. There are two possible explanations if the defects responsible for band 3 are actually impurities. First, the disappearance of the defects may be in the form of volatile iodine compounds, since it is believed that an aged surface is iodine deficient due to iodine loss [20,30] . Second, the defects may have diffused into the bulk from the surface region after they are introduced by KI etch into the surface region . In either way, the result is a decrease in the concentration of the defects responsible for band 3, which is beneficial for device fabrication . This may explain the results observed by Levi et al. [1$], where a one-day waiting period was found to produce higher quality detectors . It seems that extreme care should be taken to avoid contamination of HgI, crystals during storage and handling . Further, a waiting period of one day may also be helpful in reducing the concentration of defects responsible for band 3.

200

XJ. Bao et al. / Hgl, PL spectra and nuclear detector performance

Table 2 Specified concentration of some impurities in semiconductor grade and reagent nitric acid (in ppm) used in this work. HM stands for heavy metal HNO ; Cl HM SO, As PO4 Cu Fe Ni Semiconductor grade 0.08 0.1 0.5 0.005 0.2 0.02 0.1 0.02 Reagent 0.1 0.2 0.8 0.004 0.2 1 0.2 1.0 3) In another etching experiment, nitric acid (HNO;) solution was used. It was found consistently that reagent HNO3 introduces band 3 in the sample while semiconductor grade HNO3 does not . Since the main difference in these two kinds of HNO 3 is the concentration of impurities, especially metallic ions such as copper and nickel (see table 2), it seems that band 3 is related to impurities . Furthermore, it is known that purification by sublimation reduces the intensity of band 3 considerably with each sublimation step and that after sublimations, there are usually dark residuals left behind. Since semiconductor grade HNO3 is readily available and has low concentration of impurities, some of which may be responsible for band 3, it should be considered as a potential etchant . In cases where KI etch is desirable because of its rapid etching rate, it may be helpful to follow the KI etch with a HNO3 etch. 4) The intensity of band 3 is especially strong in samples contacted with silver or nickel, both in the contacted region and in regions away from the contact . Band 3 is also strong in all the samples that have undergone surface heating . The relationships between band 3 and nickel, silver, or surface heating are being investigated further . In addition, from our measurements on other contact materials [27-29,31,32] and the literature [25], several impurities can be excluded since they do not cause any enhancement of band 3 emission. They include carbon, palladium, indium, tin, germanium, iron, platinum, tungsten, aluminum, chromium, tantalum, copper, magnesium, boron, gold, antimony, lead, and tellurium . The correlations between the intensity of band 3 and the GPA of a crystal and the observations of the behavior of band 3 summarized in 1) to 4) have important implications in the fabrication of HgI, detectors . Firstly, it seems that the presence of the defects responsible for band 3 is detrimental to detector fabrication, and careful control of this defect will improve the manufacturing yield. Secondly, the defects associated with band 3 can be incorporated both during growth and subsequent device processing. For crystal growth, further purification of the starting material should be pursued to get rid of the impurities associated with band 3. Studies to conclusively identify the impurity

will be especially important for this purpose . The processing steps used to fabricate detectors also need some modifications, especially those involving chemical etching, contact deposition, aging, handling and storage . For chemical etching, KI needs to be purified or other purer etchants should be used either to replace or follow the KI etching . It is also possible to avoid chemical etching completely, especially for some growth techniques where slices of desired dimension are grown instead of crystals of large sizes from which slices are obtained [33,34] . For contact deposition, exposure of the sample to vacuum at room temperature may be helpful but excessive surface heating should be avoided . The choice of contact material should also be made with care regarding the properties of the contact such as purity and ease of deposition by thermal evaporation. For the effect of aging, especially for working detectors, encapsulation is necessary to prevent contamination by defects responsible for band 3 in addition to other passivation purposes. For handling and storage, tools that are used to handle the material should be carefully chosen and a clean environment should be maintained to avoid contamination . The data set for the absolute intensity of band 4 emission was more limited, and we did not look for correlations between this band and GPAs of HgI ., crystals. However, several observations should be mentioned . First, in many cases, band 4 follows band 3, such as in KI etch. Second, the intensity of band 4 was enhanced by the deposition of copper contacts [27]. Thirdly, band 4 has a very similar temperature dependence as band 3 in that an intensity maximum appears at the temperature of about 50 K [30]. For the band 1 emission, it appears that good crystals tend to have stronger P2 and P3, and thus well resolved band 1 region . Stronger P2 and P3 can be a result of two different mechanisms. A higher concentration of the defect that are related to these bound excitons lines may increase the photoluminescence intensity of P2 and P3; alternatively, a high degree of structural perfection of the crystal in general may also increase the luminescence intensity since the carrier lifetime will be increased . Since the samples used in these experiments were all grown under very similar conditions, and well resolved exciton photoluminescence is usually associated with high structure perfection, the latter is more likely . It can then be inferred that the ability of the crystal to produce high-quality detectors is related to the crystal's high degree of structural perfection . 4, Conclusions We have observed correlations between a Hgi 2 crystal's ability to produce high-quality detectors and

X.J. Bao et al. / Hglz PL spectra and nuclear detector performance

features in the PL spectra . Specifically, strong emission of band 3 was found in crystals that produce low-quality detectors . Strong band i emission, represented mainly by P2 and P3, is desirable . Since the intensity of band 2 does not seem to be related to the quality of the crystal in terms of detector production, and that P3 varies a great deal depending on the degree of aging of the surface, it was found that the band 2 to P2 intensity ratio was the best indicator of the quality of the crystal . We have concluded that the structural perfections of the crystal and the control of contamination of the impurities responsible for band 3 are both very important in producing high-quality detectors . Given the behavior of band 3 observed from a variety of measurements, we have made suggestions to reduce the contamination by impurities responsible for band 3. Acknowledgements We would like to acknowledge US Department of Energy for financial support. One of us ME.S.) would also like to acknowledge the support of National Science Foundation through a Presidential Young Investigator award. References [1] W.R. Willig, Nucl. Instr. and Meth. 96 (1971) 615. [2] H.L. Malm, IEEE Trans . Nucl. Sci . NS-19 (1972) 263. [3] G.A. Armantrout, S.P. Swierkowski, J.W. Sherhman and J.H. Yee, IEEE Trans . Nucl. Sci . NS-24 (1977) 121 . [4] W.E. Burcham, Nuclear Physics : An Introduction (Longmans, London, 1963) p. 186 . [5] H. Hill, The Art of Electronics (Cambridge University, London, 1980) p. 610 . [6] R.H. Bube, Phys. Rev . 106 (1957) 703 . [7] N .L. Skinner, C. Ortale, M.M. Schieber and L. van den Berg, Nucl. Instr. and Meth. A283 (1989) 119. [8] R.B. James, private communication . [9] M. Schieber, W.F. Schnepple and L. vai den Berg, J. Cryst . Growth 33 (1976) 125 . [10] 1. Beinglass, G. Dishon, A. Holzer and M. Schieber, J. Cryst . Growth 42 (1977) 166 .

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[11] A. Holzer and M. Schieber, IEEE Trans. Nucl. Sci . NS-27 (1980) 266 . [12] P.T. Randtke and C. Ortale, IEEE Trans. Nucl. Sci . NS-24 (1977) 129. [13] M. Schieber, I. Beinglass, C. Dishon, A. Holzer and G. Yaron, IEEE Trans. Nucl. Sci . NS-25 (1978) 644. [14] W.B. Yelon, R.W. Alkire, M.M. Schieber, L. van den Berg, S.E. Rasmussen, H. Christensen and J.R. Schneider, J. Appl. Phys. 52 (1981) 4604. [15] I. Beinglass, G. Dishon, A. Holzer, S. Ofer and M. Schieber, Appl. Phys. Lett. 30 (1977) 611. [16] R.C. Whited and L. van den Berg, IEEE Trans. Nucl. Sci. NS-24 (197",': 165. [17] A. Tadjine, D. Gosselin, J.M. Koebel and P. Siffert, Nucl. Instr. and Meth. 213 (1983) 77. [18] A. Levi, A. Burger, J. Nissenbaum, M. Schieber and Z. Burshtein, Nucl. Instr. and Meth. 213 (1983) 35. [191 T.E. Felter, R.H. Stulen, W.F. Schnepple, C. Ortale and L. van den Berg, Nucl. Instr . and Meth. A283 (1989) 195. [20] X.J. Bao, T.E. Schlesinger, R.B. James, R.H. Stulen, C. Ortale and A.Y. Cheng, J. Appl. Phys. 68 (1990) 86. [21] X.J. Bao, T.E. Schlesinger, R.B . James, C. Ortale and L. van den Berg, J. Appl. Phys. 68 (1990) 2951. [22] I. Akopyan, B. Novikov, S. Permogorov, A. Selkim and V. Travnikov, Phys. Status Solidi B 70 (1975) 353. [23] B.V. Novikov and M.M. Pimonenko, Sov. Phys. Semicond. 4 (1971) 1785. [24] B.V. Novikov and M.M. Pimonenko, Sov . Phys. Semicond. 6 (1972) 671 . [25] J.L. Merz, Z.L . Wu, L. van den Berg and W.F. Schnepple, Nucl. Instr. and Meth. 213 (1983) 51. [26] I.K. Akopyan, B.V. Bondarenko, B.A. Kazennov and B.V. Novikov, Sov. Phys. Solid State 29 (1987) 238. [27] X.J. Bao, T.E. Schlesinger, R.B . James, R.H. Stulen, C. Ortale and L. van den Berg, J. Appl. Phys. 67 (1990) 7265. [28] R.B. James, X.J. Bao and T.E. Schlesinger, to be published. [29] R.B. James, X.J. Bao, T.E. Schlesinger, C. Ortale and A.Y. Cheng, J. Appl. Phys. 67 (1990) 2571 . [30] X.J. Bao, T.E. Schlesinger and R.B. James, unpublished . [31] X.J. Bao, T.E. Schlesinger, R.B. James, G.L. Gentry, A.Y. Cheng and C. Ortale, J. Appl. Phys. 69 (1991) 4247. [32] X.J. Bao, T.E. Schlesinger, R.B. James, A.Y. Cheng and C. Ortale, Mater. Res . Soc. Symp . Proc. 163 (1990) 1027. [33] S.P. Faile, A.J. Dabrowski, G.C. Huth and J.S. Iwanczyk, J. Cryst. Growth 50 (1980) 752 . [34] M.R. Squillante, S. Lis, T. Hazlett and G. Entine, Mater. Res. Soc. Symp. Proc. 16 (1983) 199.