wide-bandgap compound semiconductors for x- or gamma-ray detectors

ISSN 1063�7397, Russian Microelectronics, 2011, Vol. 40, No. 8, pp. 543–552. © Pleiades Publishing, Ltd., 2011.Original Russian Text © V.M. Zaletin, V.P. Varvaritsa, 2010, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Materialy Elektronnoi Tekhniki, 2010, No. 3, pp. 4–13.

543

INTRODUCTION

By its nature, the semiconductor detector ofnuclear radiation places enormous demands on thematerial employed. The highest purity and structuralquality can be achieved with silicon or germanium;accordingly, these were the first to be used for the pur�pose. Indeed, they brought a dramatic change tonuclear spectrometry, monitoring, and screening, andcontinue to occupy a dominant position in the area.

On the other hand, the two materials, and silicon inparticular, are not well suited for radiation detection inindustrial or field environments. They show modeststopping power and radiation hardness; furthermore,detectors based on any of them require cooling (somealso do so when not operated). Efforts are thereforebeing made to find alternative semiconductors thatwould be suitable for wider areas of application andwould operate at room temperature. Specifically, theyshould meet the following requirements [1–6]:

(1) The energy gap Eg should exceed 1.4 eV in orderto ensure a sufficiently low carrier density in the bulkmaterial at room temperature.

(2) The effective atomic number Z should exceed30 in order to provide adequate absorption of X� andgamma�rays.

(3) The energy ε of electron–hole pair productionshould be close to the energy gap.

(4) For gamma�ray detectors, Eγ/ε � whereEγ is the energy of an incident photon; n0 is the equi�librium carrier density, cm–3; and V is the sensitive vol�ume of the detector, cm3. This means that room�tem�

n0V,

perature fluctuations in the total number of equilib�rium carriers should be far less in magnitude than thetotal number of photogenerated carriers.

(5) (μτ)e, h > 10–5 cm2/V ⋅ s, where μe, h and τe, h arethe carrier mobility and lifetime, respectively, the sub�scripts e and h standing for electrons and holes. Theinequality is designed to ensure full charge collectionand to prevent polarization.

These are stringent specifications; in fact, theyconflict with each other in some cases and are there�fore impossible to meet by any material. So we have tobe prepared to make compromises. Requirement (1)suggests using binary or pseudobinary compounds;otherwise, departures from stoichiometry are likely.

Table 1 lists candidate compound semiconductorsfor room�temperature detection of X� or gamma�rays.AlSb is probably the most tempting to try, with its largeeffective atomic number (Z = 13, 51), an energy gap aswide as 1.68 eV, and carrier mobilities a few times ashigh as those of other compound semiconductors. Infact, efforts to grow AlSb crystals were made in theSoviet Union (at the Giredmet research institute inMoscow and the High�Purity Metal Works in Svetlo�vodsk) and elsewhere, including Skylab [7]. None ofthem were successful due to the great affinity of alumi�num for oxygen and the high volatility of antimony.

Below, we present a brief status report on theresearch into wide�bandgap compound semiconduc�tors for X� and gamma�ray detectors in Russia.

Wide�Bandgap Compound Semiconductorsfor X� or Gamma�Ray Detectors

V. M. Zaletina and V. P. Varvaritsab

aDubna International University of Nature, Society, and Man, Dubna, Moscow oblast, RussiabAnalitnauchtsentr Limited Liability Company, Moscow, Russia

Abstract—Although Ge and Si are currently the major semiconductor materials for nuclear�radiation detec�tors used in high�resolution nuclear spectroscopy, and will remain so in the foreseeable future, their limita�tions that hamper their use in field and industrial environments have given the impetus for research into alter�native semiconductors that would be suitable for wider areas of application and would operate at room tem�perature. Requirements are formulated for semiconductors in which to make room�temperature detectors ofX� or gamma�rays. A brief overview is given of the work in Russia on such detectors using a wide�bandgapcompound semiconductor, namely, CdTe, GaAs, HgI2, or TlBr. The standard of semiconductor�materialstechnology is shown to be a key factor in developing this type of detector.

Keywords: gamma�rays, detector, crystal, semiconductor, resolution, photons, compounds, efficiency,energy, X�rays.

DOI: 10.1134/S1063739711080208

MATERIALS SCIENCEAND TECHNOLOGY: SEMICONDUCTORS

544

RUSSIAN MICROELECTRONICS Vol. 40 No. 8 2011

ZALETIN, VARVARITSA

Tabl

e 1.

Ele

ctri

cal p

rope

rtie

s of

com

poun

d se

mic

ondu

ctor

s co

nsid

ered

for

room

�tem

pera

ture

det

ecti

on o

f X�

or g

amm

a�ra

ys

Com

poun

d

ZD

ensi

ty,

g/cm

3E

g, e

Vρ

, Ω c

mP

air�

prod

uc�

tion

en

ergy

, eV

Tm

elt, °C

μ, c

m2 /(

V s

)τ,

sμτ,

cm

2 /V

Typ

eF

orm

ula

eh

eh

eh

AIV

BIV

4HS

iC14

, 03.

213.

26>

106

7,8

2827

1000

115

5 ×

10–

710

–7

4 ×

10–

48 ×

10–

5

AII

I BV

InP

49, 1

54.

721.

3410

64,

210

5547

0015

010

–6

10–

95 ×

10–

6<

1 ×

10–

5

GaA

s31

, 33

5.33

1.43

107

4,2

1238

1000

050

010

–8

10–

910

–4

5 ×

10–

7

GaP

31, 1

54.

132.

2610

97,

017

9030

015

010

–8

10–

910

–5

10–

7

GaN

31, 7

6.15

3.4

1011

10,2

>25

00~

1000

4010

–7

10–

910

–4

10–

8

AlS

b13

, 51

4.2

1.65

108

4,71

1060

1200

850

10–

710

–9

1.2 ×

10–

48.

5 ×

10–

7

AII

BIV

CdT

e48

, 52

5.85

1.5

109

4,43

1105

1050

803 ×

10–

62 ×

10–

63 ×

10–

32 ×

10–

4

CdZ

nTe

48, 3

0,

525.

781.

4810

104,

6410

92–

1295

1000

120

3 ×

10–

61 ×

10–

64 ×

10–

31.

2 ×

10–

4

CdS

e48

, 34

5.81

1.72

109

5,5

1239

650

501 ×

10–

610

–9

6.3 ×

10–

5–

ZnT

e30

, 52

5.68

2.25

1010

6,34

1240

3000

110

4 ×

10–

6–

––

Me–

Hal

Hgl

280

, 53

6.2

2.13

1013

4,2

252

120

510

–6

10–

71 ×

10–

54 ×

10–

7

Pbl

282

, 53

6.4

2.32

1013

4,9

408

82

10–

710

–8

3 ×

10–

73 ×

10–

8

TIB

r81

, 35

7.56

2.68

1011

6,5

460

504

2.5 ×

10–

63.

7 ×

10–

54 ×

10–

52 ×

10–

6


WIDE�BANDGAP COMPOUND SEMICONDUCTORS 545

II–VI COMPOUND SEMICONDUCTORS

Cadmium telluride, a II–VI compound semicon�ductor, is widely considered to be the first wide�band�gap material used in room�temperature detectors of X�and gamma�rays [8, 9]. These semiconductors aremade up of a group�IIb metal (Zn or Cd) combinedwith a group�VIa cation (S, Se, or Te); the latter ele�ment tends to crystallize into a hexagonal or cubicstructure. Pseudobinary compounds are also possible,such as Cd1 – xMnxTe and Cd1 – xZnxTe. The II–VI com�pounds show a greater tendency to form ionic bonds ascompared with the III–V compounds, which resultsfrom a larger difference in electron affinity between theconstituent elements. This type of semiconductor ischaracterized by a wide range of energy gaps, extendingfrom 0.15 eV for HgTe to 4.4 eV for MgS.

A team led by O.A. Matveev at the Ioffe Physico�Technical Institute of the Russian Academy of Sciencesreported the growth of CdTe crystals from the melt withadded chlorine by a moving�heater process [8, 9]. Bothn� and p�type semiconductors were produced withresistivity as high as 108–109 Ω cm. The researcherswere thus able to make detectors up to 2 mm thick, witha sensitive area of up to 10 mm2, which showed room�temperature energy resolutions of 3–4, 5–7, and~25 keV for 59.6�, 122�, and 662�keV gamma�ray pho�tons, respectively.

However, crystals made by the process suffer from amosaic structure and deep levels. The former disadvan�tage makes it very difficult to develop detectors with alarger sensitive area. The deep levels, which appear to berelated to Cd vacancies, give rise to polarization.

Better results have been achieved with the Cd1 – xZnxTecompounds [10–14]. Note that their energy gap andrelated properties can be controlled by varying x, forthis is known to affect the lattice spacing. The rela�tionship of energy gap to lattice spacing is illustrated inFig. 1.

It was found that x = 0.1 and x ~ 0.7 are optimum val�ues for detectors operated at –30°C and room tempera�ture, respectively. Experiments showed that doping CdTecrystals with Zn during their growth results in a higherdefect energy of formation, thus reducing defect densityand enhancing the mechanical strength of the lattice. Ingeneral, Cd1 – xZnxTe crystals exhibit smaller amounts ofmosaic structure and polarization compared with CdTecrystals grown under similar conditions.

The Giredmet Joint�Stock Company (Moscow) emp�loys a modified Bridgman process to grow Cd1 – xZnxTeand CdTe crystals, which allows a degree of controlover crystal properties by varying the Cd vapor pres�sure during growth, annealing, and cooling [13]. Thetechnique yields crystals with a resistivity in the range5 × 108 to 1010 Ω cm and a μτ product in the range(0.5–1) × 10–3 or 10–6 to 10–5 cm2/V for electrons orholes, respectively. They can be employed in discretedetectors with dimensions ranging from 3 × 3 × 3 to 5 ×5 × 2.5 mm3 for spectrometric applications; the upper

limit is placed by microscopic inhomogeneities causedby component segregation during crystallization, atypical feature of II–VI compound semiconductors.Larger detectors are suitable for use in dosimeters andradiometers.

The Petersburg Nuclear Physics Institute of theRussian Academy of Sciences (Gatchina, Leningradoblast) has developed and uses a process technology tofabricate X� and gamma�ray detectors in CdTe orCd1 – xZnxTe on a par with the best in the world; theyinclude PIN detectors having a dark current almost1/100 that of metal–semiconductor–metal structures.However, both the PIN detectors and the accompany�ing preamplifier have to be cooled to –35°C, througha thermoelectric cooling element [12, 14].

Figure 2 compares 241Am spectra obtained at the Peters�burg Nuclear Physics Institute by means of two Cd1 – xZnxTedetectors made by Giredmet or ev PRODUCTS. Theformer detector is among the best, having x = 0.05.The latter is based on a PIN structure with x = 1. Thespectrometric performance of the detectors was evalu�ated based on their widths at half their peaks at 13.9and 59.6 keV. The respective energy resolutions werefound to be 0.8 and 1.33 keV for the Giredmet detec�tor. Those for the ev PRODUCTS detector were foundto be 0.92 and 1.31 keV. The specimens were compa�rable in parameter values.

All the CdTe detectors presented so far have thedisadvantage of requiring a certain amount of cooling(albeit, small) and suffering from polarization, whichlimits their lifetime.

A technique of CdZnTe crystal growth that appearsto preclude polarization is being developed byYu.M. Ivanov at the Shubnikov Institute of Crystallo�graphy [15, 16]. Based on ideas of Obreimov andShubnikov, the approach enables one to make single

5

4

3

2

10.54 0.660.58 0.62

MgS

MgSe

ZnSe

ZnTe

CdSe

CdTe

MnTe

Cd1 – xZnxTe

MgTe

x = 0.7

x = 0.1

ZnS

Lattice spacing, nm

En

ergy

gap

, eV

Fig. 1. Energy gap as dependent on lattice spacing for II–VI compound semiconductors [6].

546


ZALETIN, VARVARITSA

crystals up to 110 mm in diameter and up to 50 mm inheight with a resistivity in excess of 1010 Ω cm, havingan insignificant mosaic structure. In recent tests at theIoffe Physico�Technical Institute, crystals grown inthis way displayed high values of μτ for both the elec�trons and the holes and considerable uniformity oftransport properties over the bulk material, and werefound to be free from polarization.

III–V COMPOUND SEMICONDUCTORS

Figure 3 depicts the relationship of the energy gapto lattice spacing for III–V compound semiconduc�tors, the solid curves representing the probability ofternary compounds being formed by the binary ones.It indicates an insignificant change in lattice spacingfor GaAs–AlAs systems (GaAs–AlAs lattice mis�match 0.127%), a favorable condition for solid solu�tions of Al in GaAs over a wide range of solubility.

With GaAs–InAs and GaAs–GaSb systems, bycontrast, incorporating In or Sb into GaAs to formGa1 – xInxAs or GaAs1 – ySby, respectively, involves aconsiderable change in lattice spacing, making stable

compounds impossible, whatever the solubility of Inor Sb in GaAs.

The early GaAs detectors were made from a melt�grown bulk crystal, showing poor resolution and severenoise due to the high content of defects and impurities[17]. Detector�grade GaAs materials were first pro�duced in the form of epitaxial films [18, 19].

In the Soviet Union, GaAs spectrometric detectorswere first created at the Academy’s Institute of Semi�conductor Physics in Novosibirsk, in cooperation withthe Riga�based Research and Development Institutefor Radioisotope Apparatus (now known as Baltic Sci�entific Instruments). The devices were built around ann�type epitaxial film with a carrier density of about1013 cm–3 and a carrier mobility in excess of 7000 cm2

V–1 s–1 at room temperature, grown from a Ga–AsCl3–H2 vapor phase [19]. The detectors were imple�mented as a surface�barrier diode with a junction inthe form of an ni epitaxial layer on an n+ substrate, thebarrier made by vacuum deposition of gold. The sensi�tive region was 50 to 200 μm thick.

The detectors showed efficiencies of 16, 3, and0.2% at photon energies of 14, 59.6, and 122 keV,respectively. With gamma�rays, the best energy resolu�tions, 1.7 and 1.9 keV, were found for 59.6� and 122�keVlines, respectively (57Co). The instrument spectra werecharacterized by evident escape peaks, whose heightwas 15–18% of that for the main peak. The escapepeaks at 9.2 and 10.5 keV (corresponding to the K�ab�sorption edges of gallium and arsenic) were shiftedtoward the lower energies; they can seriously increasethe background in X�ray radiometric analysis.

X�ray radiometric tests found the detectors to beuseful within the range 5–50 keV, which was dictatedby their thickness and area (≤200 μm, ≤6 mm2).

6144

4096

2048

0165 1189293 421 549 677 805 933 1061

11 20 28 36 45 53 62 70 78

(b)

Channel

Pu

lse

cou

nt

Photon energy, keV

384

256

128

0(a)

1 513 1025 1537 2049 2561 3073 358513 27 41 56 70 85 99

Fig. 2. Comparison of 241Am spectra obtained withCd1 – xZnxTe detectors made by (a) Giredmet or (b)ev PRODUCTS, the latter based on a PIN structure [13].The operating voltage is (a) 170 or (b) 1000 V. The operat�ing temperature is –37°C.

2.5

2.0

1.5

1.0

0.5

00.54 0.640.56 0.58 0.60 0.62

GaPAlP

AlAs

GaAs–lnAsGaAs–GaSb

GaAs

lnP

AlSb

GaSb

lnAs

GaAs–AlAs

Lattice spacing, nm

En

ergy

gap

, eV

Fig. 3. Energy gap as dependent on lattice spacing for III–V compound semiconductors.



Currently, research aimed at creating GaAs posi�tion�sensitive detectors of single photons is being con�ducted at the Federal Research Institute of Semicon�ductor Devices in Tomsk, Tomsk State University, andthe branch of the Kotel’nikov Institute of Radio Engi�neering and Electronics in Fryazino, Moscow oblast[20–24]. Their potential applications include imagingsystems with high spatial resolution and ultralow�dosemedical technologies.

In these studies, semiinsulating GaAs specimens inepitaxial or bulk form are produced by compensationthrough high�temperature diffusion of a donor impu�rity with Cr atoms. (Both liquid� and vapor�phase epi�taxy are employed.) The material allows one to makedetector structures with resistivity as high as 109 Ω cm.Other advantages are as follows [20, 21]:

(1) The electron lifetime is an order�of�magnitudelonger than that in commercial counterparts.

(2) Detectors can be made with a linear current–voltage characteristic.

(3) The electric field is uniform and extends fromcathode to anode.

Figure 4 represents the configuration of a position�sensitive detector with a strip pitch of 50 or 100 μm,made from a GaAs(Cr) wafer 76 mm across. Tests con�ducted at the European Organization for NuclearResearch evaluated its spatial resolution at about14 μm and its resistance to ionizing charged particlesat 2 × 1014 particles cm2 [21].

Russian researchers have achieved unequivocalsuccesses in developing pixel and strip detectors frommodified bulk material [20, 21]. Note that onlyGaAs(Cr) is suitable for making such detectorsbecause it allows one to dispense with Schottky barri�ers. Position�sensitive detectors were also built arounda PIN structure made by liquid� or vapor�phase epit�axy, the intrinsic layer being semiinsulating GaAs(Cr).The performance of the PIN structures was found tobe independent of the Cr�doping technique (diffusionor in situ doping). Processes were developed to fabri�cate detectors with an exceptionally low dark current,involving chemical treatment and oxidation of thewafer surface to a depth of 0.2–0.3 μm.

Anode

Cathode

Position�sensitive detector

X�rays

(V–Au)

GaAs : Cr

n�GaAs

(V–Au)

X�r

ays

Fig. 4. GaAs position�sensitive detector [20, 21].

548


ZALETIN, VARVARITSA

MERCURY DIIODIDE

The main advantages of HgI2 over CdTe and GaAsare as follows:

(1) The large atomic numbers of Hg and I (80 and51) make it possible to achieve high levels of efficiency.

(2) Its energy gap is 2.13 eV wide.(3) Its resistivity is as high as 1012–1013 Ω cm.Yet the application of HgI2 (and related materials)

in solid�state electronics is still in its infancy. Thecompound has a complicated crystallographic struc�ture, with a number of varieties, and is subject to vander Waals’ forces as well as ionic and covalent bonds.When heated from room temperature to 123°C, mer�cury diiodide passes from tetragonal to orthorhombicsystem, changing its color from red to yellow. Thistransformation limits the choice of growth processes.

Efforts to develop HgI2 detectors were reported bythe Institute of Semiconductor Physics in Novosibirsk[25, 26]. The growth involved sublimation and peri�odic variation of temperature for the source or thecrystal to produce an alternating pattern of growth andevaporation, which suppressed spontaneous nucle�ation. Single crystals up to 3 cm3 in volume were thusgrown from vapor and then cleaved along (001) planesinto wafers with an area of 20–100 mm2 and a thick�ness of 0.2–1.0 mm.

The detectors were made in the form of a metal–mercury diiodide–metal structure, the metal beingpalladium.

Their estimated efficiency exceeded those of CdTeand GaAs detectors over an energy range as wide as 5–

5000 keV [25]. Its deterioration over the lower energieswas caused by the absorption in the metal and the exitberyllium window 50 μm thick.

Figure 5 shows the energy resolution calculated asa function of gamma�ray photon energy for differentnoise�equivalent energies of the circuitry, togetherwith the best experimental data points. The dashedline represents the energy resolution that depends onelectron–hole pair production only.

Figure 6 displays instrument spectra obtained witha HgI2 detector for specific radionuclides. Their char�acteristic features include asymmetry of the totalabsorption peak and modest contrast as measured bythe peak�to�background ratio. The former is related todifferent conditions of charge collection for the elec�trons and the holes and to the pattern of charge loss inthe sensitive region. The inadequate contrast shouldbe linked to other considerations, as evidenced by thefact that the total�absorption peak, at 59.54 keV, has atail extending to 40 keV but no farther when the elec�trons and the holes differ from each other in mobilityand lifetime by a factor of 25. It is important not to for�get the adverse effect of charge collection outside thecontacts, where the field is much weaker and the driftof photogenerated carriers is much slower. This makesfor longer charge collection and hence for greater loss,particularly when detecting gamma�ray photons arebelow 30 keV. Thus, the smaller the area not coveredby the contacts, the higher the contrast ratio. Distor�tion caused by the differentiating and the integratingcircuits is another adverse factor.

The contrast ratio of HgI2 detectors varies between10 and 100 for photon energies of 3–150 keV.

Significant advances in growing HgI2 crystals weremade by the mid�1980s. They set the stage for thedevelopment of various prototypes at the Institute ofSemiconductor Physics in Novosibirsk and at Novosi�birsk State University, with the collaboration ofexperts from research institutes outside the Academy.Designed for ambient�temperature operation, thedevices included an optoelectronic�feedback spectro�metric module, an on�board spectrometer for spaceresearch, a borehole gamma radiometer, an X�rayspectrometric module for scanning electron micro�scopes, a portable sensor of heavy metals in workings,and an intracavitary radiation counter to measure thebody level of radioactive contamination. They weremostly employed in the laboratories of various institu�tions in the academy and outside, including the Ver�nadsky Institute of Geochemistry and AnalyticalChemistry, the All�Union Research Institute ofExploratory Geophysics, and the Orion Research�and�Production Association. Some of them were alsoused in the field.

Finally, we note that HgI2 was probably the onlywide�bandgap semiconductor material whose growthfor the purposes of radiation detection was attemptedin space [28].

101

100

10–1

100 103101 102

4

3

2

1

En

ergy

res

olu

tio

n,

keV

Eγ, keV

Fig. 5. HgI2 detector performance: energy resolution vs.gamma�ray photon energy for different noise�equivalentenergies of the circuitry: (1) 0, (2) 200, (3) 500, and (4)1000 eV. The solid curves are obtained by calculation. Thedashed line represents the energy resolution that dependson electron–hole pair production only [27]. The closedcircles represent the best experimental data points.



THALLIUM BROMIDE

TlBr has an energy gap as wide as 2.68 eV and con�siderable stopping power due to the large atomic num�bers of its elements (81 and 35) and its high density(7.56 g/cm3). These properties suggest its use in ambi�ent�temperature detectors of low� or intermediate�energy gamma�rays.

Investigations into possible ways of making TlBrdetectors for X�rays or the stated range of gamma�rayswere conducted at the Giredmet Joint�Stock Com�

pany in Moscow and the Institute in Physical andTechnical Problems in Dubna, Moscow oblast,funded by the International Science and TechnologyCenter under project no. 2728 [29–31].

TlBr crystals were grown from the melt by theBridgman–Stockbarger technique optimized withrespect to temperature and time. A reproducible pro�cess was thus developed for making crystals with adiameter of 25–30 mm, a height of 100–120 mm, anda resistivity of about 1011 Ω cm. Figure 7 displays sam�ple ingots and fragments of ingots prepared for charac�terization or processing.

Detectors were built around a metal–TlBr–metalstructure, considering that the semiconductor is char�acterized by relatively small values of carrier lifetimeand mobility. To ensure adequate charge collection, adetector 300 μm thick should be operated under anapplied voltage greater than 300 V, which provides anelectric field of 104 V/cm in the sensitive region. Thedark current should be less than 10 nA if the detectoris to be operated at room temperature.

1000

600

200

0 32040 80 120 160 200 240 280

(c)14.4 keV

1.5 keV

51.3 keV (Kα, Hg)

122 keV

1.5 keV

4.5 keV

133 keV

57Co

Channel

600

400

200

0 40 80 120 160 200 240 280

(b)24Am 59.6 keV

1.2 keV2.25 keV

20.8 keV

26.4 keV31 keV (Kα, I)

13.9 keV17.8 keV

2200

1600

1000

0 80 160 240 320 400

(a)109Cd

200

22.16 keV

1.47 keV

0.96 keV

N

Fig. 6. HgI2 detector performance: instrument spectra

taken at 25°C for specific radionuclides: (a) 109Cd,(b) 241Am, and (c) 57Co [26].

(a)

(b)

Fig. 7. (a) TlBr ingots grown by the Bridgman–Stock�barger technique and (b) fragments of TlBr ingots preparedfor characterization or processing.

550


ZALETIN, VARVARITSA

Spectrometric performance was evaluated on spec�imens with a TlBr thickness of 0.3–0.8 mm and sensi�tive areas in the range 3–12 mm2. The gamma�raysources were 55Fe (5.9 keV), 241Am (13.9 and59.6 keV), 109Cd (22.1 keV), 57Co (122 keV), and 137Cs(662 keV). A pulsed�feedback preamplifier was used inmeasurements involving a Peltier cooling element.The instrument spectra obtained are shown in Fig. 8.The best energy resolutions were found to be 0.54, 1.3,1.8, 2.1, and 7.1 keV for the photon energies 5.9, 22.1,59.6, 122, and 662 keV, respectively [31].

On the whole, the investigations provided somesupport for the idea of building ambient�temperatureTlBr detectors suitable for gamma�ray spectrometryover a wide energy range. However, its implementa�tions have so far shown poor stability, inadequate chip�to�chip reproducibility, and other deficiencies thatpreclude their adoption.

Table 2 lists parameter values and current status ofdomestic detectors made in TlBr or another wide�bandgap compound semiconductor.

CONCLUSIONS

Russian laboratories have examined, with a varyingdegree of success, almost any wide�bandgap com�

pound semiconductor showing promise as a materialfor room�temperature detectors of X� or gamma�rays.

The most impressive results have so far beenachieved for GaAs. Discrete, strip, and pixel detectorsof X�rays emerged from collaborative work done at theFederal Research Institute of Semiconductor Devicesin Tomsk and the Siberian Physical�Technical Insti�tute, a research division of Tomsk State University.They represent a product of in�depth basic researchand modern process technologies.

For gamma�rays, however, domestic GaAs detec�tors are still nonexistent, which might be explained bythe insufficiently large atomic numbers of Ga and Asand by challenges facing their manufacture.

As regards CdTe and detectors based on the mate�rial, a large amount of research has been conducted atthe Ioffe Physico�Technical Institute, the Giredmetresearch institute, the Petersburg Nuclear PhysicsInstitute, and the Shubnikov Institute of Crystallogra�phy. Those efforts, however, have yet to yield commer�cial devices, even though a process has been developedat the Petersburg Nuclear Physics Institute for makingspectrometric CdTe detectors (based on a p–n�junc�tion) for X� and gamma�rays from imported material.In fact, such material could be produced in Russia.

55Fe (a)

5.9 keV

540 keV

22.1 keV

1.3 keV

109Cd (b) 241Am (c)

59.6 keV

1.41 keV

57Co 122 keV

2.1 keV

(d) 137Cs (e)

662 keV

7.1 keV

Co

un

t ra

te

Eγ, keV

Co

un

t ra

te

Fig. 8. TlBr detector performance: instrument spectra taken over the photon�energy range 5–662 keV for specific radionuclides:(a) 55Fe, (b) 109Cd, (c) 241Am, (d) 57Co, and (e) 137Cs [31].

Eγ, keVEγ, keV



TlBr has proven suitable for room�temperaturedetection of X�rays, as well as for making gamma�raydetectors. The Giredmet Joint�Stock Company hadsome success in growing TlBr crystals, but launchingthe commercial production of TlBr detectors remainsa challenge to be met.

As a final point, we note that further progress inmaterials technology for room�temperature semicon�ductor detectors appears to depend on advances in twoareas. One of them is the refinement of conventionalprocesses of crystal growth in terms of feedstock purity,process conditions, etc. The other is concerned withheterostructure technology including quantum wells.

REFERENCES

1. Armantrount, G., Swierkowski, S., Sheroman, J., andYee, J., What Can Be Expected from High�Semicon�ductor, IEEE Trans. Nucl. Sci., 1977, vol. 24, no. 1,pp. 121–125.

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Table 2. Parameter values and current status of domestic detectors made in a wide�bandgap compound semiconductor

Material (Top, °C)

Atomic numbers Eγ, eV

Photon�energy range

Dimensions Energy resolution, keVCurrent status

S, mm2 H, mm 5.9 keV 59.6 keV 662 keV

Si,(–50°C)

14 1.12 3–40 25 1–3 0.15 – – Commercial production (in Zeleno�grad)

Ge/HPGe (–17°C)

32 0.76 10–3000 30 5–10 0.2 0.5 1.4 Production from imported HPGe

CdTe(–30°C)

48, 52 1.5 6–800 20 1–2 0.35 0.8 1.8 Small�scale commercial production from imported material; proof�of�concept production from domestic materialCdZnTe

(20°C)48, 30, 52

1.48 6–1000 30 2–5 0.3 0.8 10

GaAs (25°C)

31, 33 1.43 5–60 6 0.2 0.4 1.3 – Full�fledged domestic process tech�nology

HgI2 (25°C)

80, 51 2.13 3–1000 20 1.0 0.3 1.5 12.0 Nonexistent

TlBr (5°C) 81, 35 2.68 5–1000 12 1–2 0.5 1.8 8.0 Device process technology is not finalized, though crystal�growth pro�cess meets international standard

552


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wide-bandgap compound semiconductors for x- or gamma-ray detectors

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