SEMICONDUCTOR NUCLEAR RADIATION DETECTORSl

By A. J. T A VENDALE A ustralian Atomic Energy Commission Research Establishment

Lucas Heights, N.S. W., Australia

INTRODUCTION

Since the appearance in this series some five years ago of the compre­hensive review paper by Miller, Gibson & Donovan (1) on semiconductor particle detectors, considerable progress has been made in the development of devices having improved energy resolution and detection efficiency, par­ticularly with the advent of the new Iithium-ion-drifted germanium 'Y-ray spectrometers.

By comparison, however, relatively slow progress has been made towards a better understanding of problems concerning charge production and col­lection mechanisms in semiconductor detectors, especially in relation to base-material quality factors such as crystalline dislocation density, impurity content, and minority carrier lifetime. In addition, the absence of reliable commercial sources of high-quality silicon and germanium, so necessary for the production of high-performance detectors, has proved a significant factor in retarding detector development.

It is intended in this article to review the present state of detector tech­nology and performance rather than to discuss the many successful applica­tions of these devices, and the reader is therefore referred to a number of re­cent review papers and conference reports concerning the uses of detectors in experimental nuclear physics (2-9), chemistry (10), medicine (11. 12), and space science (13). Similarly, discussion of developments in associated electronic instrumentation such as low-noise amplifier systems and pulse height analysers is excluded, but pertinent publications in this field are indi­cated (14, 15). Several conference proceedings (16-20), reviews (21-25), books (26, 27), and bibliographies (28, 29) are available on the subject of detectors.

CLASSIFICATION AND FABRICATION OF SEMICONDUCTOR DETECTORS

CLASSIFICATION OF SEMICONDUCTOR JUNCTION DETECTORS

p-njunctions.-Semiconductor detectors may be classified into essentially two types of diode structures. p-n and p-i-n as shown in Figure 1. Applica­tion of a reverse bias to the p-n junction diode (or more strictly n+-p in the example shown) extracts electrons and holes from the vicinity of the junction to produce a so-called depletion region of very high resistivity (:> 106 ohm).

1 The literature survey for this review was concluded in February 1967. 73

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74 PHOSPHORUS-DIFFUSED

n CONTACT <-!,.u THICK)

CHARGED PARTICLES � + _ +

+ -TO .... --,...--41 -+ +-

PREAMPLIFIER +

RL

+v I

TAVENDALE

LlTHIUM-OIF"F"USEO n ,CONTACT (�500 fA' THICK

O) RIFT C - OMPENSATED

DEPLETION LAYER \ LAYER

ELECTRONS

+V

K�·� I I

tE'

�

ELECTRIC 1 J

FIELD '

, -w- • w

FIG. 1. Basic p-n and p-i-n semiconductor junction detector structures. Left: Phos­phorus-diffused p-n junction particle detector. For p-type silicon, W""O.3y'pV

microns where p is the resistivity (ohm-em); junction capacitance C"" AjWX 104 pF where A is the contact area (cm2). Right: Lithium-ion drift p-i-n gamma detector. W"'''y'2pVnt, where Vn is the drift bias and t is the drift time. �1.4AjWX104 pF for germanium.

If the resistivity of the p-type material is much greater than that of the n contact (phosphorus diffused, ",1 p, thick), then the width of the depletion layer in a silicon diode is W ",-,0.3 y'p V microns, where p is the p-type resis­tivity (ohm-em) and V the bias (volts). The junction capacitance is simply c= 104 kA/47rW"-'104 AjW pF where k is the dielectric constant for silicon and A is the contact area (cm2). These relations are available in nomograph form (30). Thus, using 1000 ohm-em p-type silieon of 1 cm2 area, a depletion­layer depth of 100 p, and a junction capacitance of 100 pF are obtained at

100 V bias. Such a device may be used as a thin-entry-window detector of a par ticles, {J particles, and protons having maximum energies of 15 MeV, 200 keV, and 4 MeV. The electrons and holes generated by ionization within the depletion layer drift to the contacts under an average field of 5 X 103

V fcm in ",1 nsee, to form a charge pulse, whose amplitude in the absence of trapping or recombination of carriers is linearly related to particle energy_ The average energies � requi red to produce electron-hole pairs in silicon and germanium are 3.5 and 2.9 eV respectively, compared with ",30 eV per ion pair in a gas counter, and ",1000 eV per photoelectron at the photocathode of a scintillation counter.

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 75 Surface-barrier junction detectors may be formed by depositing a thin

layer (",100 A) of gold on lHype silicon or germanium. The p contact of such a junction is produced by a combination of ionized states of surface chemical impurities, the presence of oxygen and the metal forming the con­ducting contact. In practice, depletion layers of up to 3 mm arc obtainable in surface-barrier detectors using currently available high-resistivity (10,000 ohm-cm) n-type silicon (the intrinsic resistivity of silicon is about 200,000 ohm-cm).

p-i-n junctions.�In applications such as high-energy particle or ')'-ray spectroscopy it is usually desirable to have depletion-layer thicknesses much larger than is practical using p-n junctions. Following the technique of lithium-ion drift compensation introduced by Pell (31), layers ",10 mm or so of material with nearly intrinsic resistivity are possible. These "i" regions deplete at moderately low voltages (",100 V). The method, in essence, con­sists of a first-stage diffusion of lithium (a donor) to a depth of several hun­dred microns in comparatively low-resistivity p-type silicon or germanium (e.g. 100-1000 ohm-cm silicon or 1-20 ohm-cm germanium). A reverse bias is applied to the n+-p junction so formed, and lithium iUIls .drift into the p-type base material under the action of the depletion-layer electric field. The lithium donors charge-compensate the acceptor impurities to give almost intrinsic resistivity material, the i layer.

FABRICATION OF SEMICONDUCTOR DETECTORS

p-n junction detectors.�Methods for the systematic mass production of phosphorus-diffused n+ -p silicon detectors have recently been developed by Hansen & Goulding (32). They have found that Si02-Si donor interface states produced during oxide passivation of such diodes are a significant source of surface leakage current. Control of these states may be effected, however, by a prediffusion of compensating gallium.

Junction detectors may be made by the implantation of phosphorus or boron ions of energies ",100-500 keY into silicon. Martin, Harrison & King (33) have produced ion-implanted a-particle detectors with windows as thin as 0.1 J1.. The ion implantation technique lends itself to the production of mul­tielement arrays of miniature detectors on single slices of crystal using a pro­grammed, scanning beam of a Van de Graaff accelerator. Dearnaley (25) has suggested that ion implantation may prove useful in increasing the efficiency of lithium-drifted germanium planar detectors by stacking ion-implanted thin-window units. He warns that the high-temperature annealments (5000 C in the case of silicon junctions) necessary to reduce contact resistance after implantation, may reduce carrier lifetime below the limits acceptable for high-resolution spectroscopy.

Lithium-drifted silicon p-i-n detectors.-Following the successful applica­tion of the lithium-ion drift technique by Mayer, Baily & Dunlap (34) and others (35, 36) to the production of Si(U) detectors with compensated layers of several millimeters thickness, further advances have been made in the de-

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76 TAVENDALE

sign of such detectors, especially in the development of devices with thin entry windows. Dearnaley & Lewis (37) have used the technique of forming a p-type gold surface-barrier contact opposite the lithium-diffused contact, while Williams, Wilburn & Harvey (38) have shown that thin ('"'-'1 p.) boron­diffused, p-contact windows which are relatively stable, show no "overdrift" effects, and are much more rugged, may be employed.

A useful study on the origins of surface leakage currents in Si(Li) de­tectors and their control by geometric means has been made by Llacer (39). He has demonstrated, using potential probe techniques, that owing to the presence of impurity ions on the surface of the i layer, considerable distortion of the surface fields in cylindrical, planar-contact detectors can occur, which results in a reduction in the maximum applicable operating bias. It is found, however, that in some special diode configurations, such surface effects can be reduced.

Lithium-drifted germanium p-i-n detectors.-The techniques involved in the fabrication of Ge(Li) detectors (40) are in principle similar to those for Si(Li) devices. Because the smaller forbidden band gap in germanium gives rise to large thermally generated currents, drifting of germanium must be carried out at much lower temperatures «600 C) than drifting of silicon ( < 1500 C). Ohmic heat is usually extracted during drift by conduction to a cooled plate, temperature-stabilized electronically (41-43), or by immersion in such low-boiling liquids as chloroform (44), freon (45), or propane (46), utilizing nucleate boiling for heat transfer, as was originally done by Miller, Pate & Wagner (36) for Si(Li) diodes. The latter method enables power dissi­pations of up to 250 W to be maintained stably in large Ge(Li) diodes. with drift depths of � 10 mm usually being obtained in four weeks (44) .

To increase the sensitivity of Ge(Li) diodes for 'Y rays, Tavendale (44) and MaIm, Tavendale & Fowler (47) have fabricated coaxial diodes by radially drifting lithium into horizontal zone-refined crystals (trapezoidal cross section) to produce the "single-open-ended" configuration shown in Figure 2a. Detectors with sensitive volumes up to 57 cm3 have been fabri­cated in this way (45, 47-50). An advantage of this configuration is that con­stant surface leakage current is maintained with increasing length, and there­fore increasing volume. A drawback is the presence of the residual inactive p core of scattering material usually having a nonuniform cross section along its length, as observed by MaIm & Fowler (45). Such nonuniformity leads to difficulties in the calculation of detector efficiencies. In addition, axial variations in the charge collection field, together with low-field regions at the closed end, give rise to an u ndesirable time response characteristic as meas­ured by Graham, MacKenzie & Ewan (51). MaIm (SO) finds that improved timing can be obtained either by removing the closed end or, better still, by drifting "double-open-ended" cylindrical, coaxial sections of germanium. Levy (52) has reported the construction of a cylindrical, coaxial detector with the p core removed, as shown in Figure 2b, to reduce internal scattering effects.

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 77 ,...-----, .... >----- LITHIUM-DIFFUSED

\ n CONTACT \ 0:��I--+"'\"""'--- p-CORE CONTACT ---.,<...... \

, , , , , , I \'

, ........ _1 I I I I I REGION ----t--I- I I I

" �

/ .......... ------.-�,..,. (d)

LITHIUM-DIFFUSED n CONTACT

,...-J.----,---, , , , /I

�A C '? , ,

J-----'---'

)

GOLD-PLATED P CONTACT

LITHIUM-DIFFUSED n CONTACT

l REGION ---"

(C)

(b)

" \ , ,', , , ,

I , ,

'4 I

/

FIG. 2. Configurations for lithium-drifted p-i-n germanium detectors: (a) and (b) are coaxial types and (c) is planar.

The production of large-volume planar Ge(Li) detectors with depletion widths of up to 16 mm has been reported by Mann, Janarek & Helenberg (53) using successive diffusions and drifting operations from opposite faces of a planar section. A thin p-type gold-plated contact replaces one of the lithium contacts after drifting is completed. They also demonstrated that a-c drifting of double lithium-diffused diodes was possible, as shown in Figure 2c. The a-c technique has also been used by Jamini (54) in the produc­tion of Ge(Li) diodes. Various schemes have been reported for the fabrica­tion of low-capacitance (�1 pF), large-volume detectors (55). Thin-window Ge(Li) detectors with aluminium alloy or gold film contacts suitable for par­ticle spectroscopy have also been described (44, 56--58). The use of guard­ring structures for low-noise operation at high bias levels (up to 4000 V) has been demonstrated (59, 60).

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78 TAVENDALE

Studies of the effects of various chemical preparations on Ge(Li) detector leakage currents at 77° K have been made by Davies & Webb (61) and Armantrout (62). I t is apparent from this work that surface current levels are extremely sensitive to chemical treatment and storage ambients, so en­capsulation of detectors is advantageous (63, 64). Vacuum cryostat housings for detectors, designed for simplicity and minimal maintenance, have been reported by several workers (65-69) .

I n many laboratories the yield of useful Ge(Li) detectors during fabrica­tion has been found to be extremely variable, and problems such as rapid loss of lithium at the diffused contact by precipitation, reduced drift rate, and high leakage currents are frequently encountered. In most instances these effects are related to imperfections or impurities such as oxygen in the germanium. Studies have been made on the effects of various types of accep­tor impurities (70) and oyxgen (70, 71) on the drift rate of lithium in germa­nium used for detector fabrication. Fox (71) found, by measuring precipitation rates, that for some material the diffusion coefficient for lithium was reduced nearly three decades by the presence of oxygen, and that such material was nondriftable. He concluded that oxygen concentrations must be <: 1014 cm-3• Armantrout (72) has found that it is possible to correlate lithium ion mobility and minority carrier mobility in germanium and has observed a re­duction in carrier mobility in nondriftable material at 7JO K.

Nuclear compensated p-i-n detectors.-The production of high-resistivity silicon utilizing the reaction of thermal neutrons

• • 2.6h SI80(n,y) SI81-..... p81 + ". (a = 0.11 ± 0.01 barn) by which phosphorus atoms compensate boron-doped silicon has been car­ried out by Messier, Le Coroller & Flores (73). Damage by neutron-excited silicon atom recoils was annealed at 5000 C to give useful detectors with de­pletion depths up to 2 mm.

Maslova et al. (74) have shown that small germanium p-i-n detectors can be made successfully by the production of compensating vacancy defects in n-type germanium under irradiation at 77° K with a C060 'Y-ray source. Such detectors show good resolution for small depletion depths and are claimed to be stable on storage at room temperature.

Surface-barrier silicon detectors.-The method introduced earlier by Borkowski & Fox (75) for the construction of epoxy-stabilized, gold surface­barrier silicon detectors of large area has become a generally accepted process in most laboratories. Klema (76), with refinements to the process, has made detectors capable of operation up to 1800 V bias at room temperature, giving depletion layers of several millimetres.

Studies of the formation of rectifying surface contacts using various met­als on n-type silicon have been made by Siffert, Cache & Laustriat (77) and Siffert & Coche (78) . The presence of oxygen was found necessary for barrier formation, rectification being absent if the contact was held in vacuo after

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 79 vapour deposition. It is considered that the negatively charged oxygen ions diffuse to the Si02-Si interface to produce a p-type inversion layer. The Mott-Schottky barrier generated by the high work function of the gold rela­tive to the silicon possibly assists in the fixation of oxygen ions at the inter­face. Contacts of metals with high work functions (Au, Ag, Cr, Pt, Sn, or Sb) formed low-leakage diodes quickly, but even a manganese contact (low work function) , initially a poor rectifier, showed high rectification after one year. Walter & Boshart (79), as a result of their investigation of both gold and mercury surface-barrier diodes, rank the variables affecting detector characteristics in order of importance as 1) chemical surface treatment (which largely determines the nature of the surface inversion layer) , 2) adsorbed gases, and 3) work function. They also suggest that the porosity of the evaporated metal contact may play an important role in barrier formation.

Transmission (.1E/.1x) detectors.-The discrimination properties of the semiconductor E ·.1E/.1x particle identifier system, as developed by Wegner (80, 81) , improve with the use of very thin transmission detectors of uniform thickness. The use of p-type silicon with phosphorus-diffused and gold sur­face-barrier noninjecting contacts has been described by Inskeep & Edison (82) and Madden & Gibson (83). The latter workers have used oxide surface passivation and planar etching techniques to produce highly uniform devices with variations of < 1.2 f.L for 40-f.L-thick detectors of �1.8 cm2 area. It has recently been found that owing to the phenomenon of particle channelling in semiconductors, which can lead to anomalously low ionization loss rates in transmission detectors, certain restrictions must be applied on the orienta­tion of contacts relative to the crystal axes and planes in such detectors (see later).

RESOLUTION AND DETECTION EFFICIENCY OF SEMICONDUCTOR DETECTORS

OPTIMIZING OPERATING CONDITIONS

Goulding (23) has analysed in detail the effects of such factors as the statistics of ionization processes in the detector, leakage current, and capaci­tance, together with amplifier noise, on the resolution of the semiconductor spectrometer system. For high-resolution operation, detectors should have capacitances < 10 pF with leakage currents '< 10-9 A, especially in the case of (3- or 'Y-ray spectrometry, since here resolution is not limited by other intrin­sic factors such as pulse height defects which occur in heavy-ion detectors. Operation of detectors at low temperatures is necessary in order to reduce thermally generated bulk leakage currents to the required level, while ap­plication of high bias, within limits of increasing surface-leakage currents, ensures that pulse rise times and trapping effects are minimal. Ge(Li) de­tectors usually function without significant charge trapping at liquid nitrogen temperatures (770 K) to give leakage currents �1O-1o-1O-9 A at �1000 V bias. Recent measurements by Sakai & MaIm (84) have shown that optimum

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80 TAVENDALE

operating temperatures are in the range 20-300 K. However, Si(Li) detectors often exhibit serious polarization in these temperature regions, and optimum operating temperatures are usualIy in the range ",120-150° K (85) .

PHOTON SPECTROMETERS Energy resolution of Ge(Li) "I-ray detectors.-Since the efficiencies of the

various photon absorption processes increase with atomic number, it is ad­vantageous to use germanium instead of silicon semiconductor diodes as "I-ray detectors. For example, the linear photoelectric absorption coefficient for 100-keV 'Y rays in germanium is 2 . 1 cm-l, or about 40 times that for silicon (0.05 cm-l) . The lithium drift technique was first applied to germanium by Freck & Wakefield (86) who reported a resolution of 21 keY (full peak width at half peak height, FWHM) for 661-keV "I rays from a Ge(Li) detector of 0.2 cma sensitive volume (W = 1.5 mm) . The resolution was limited by amplifier noise. Webb & Williams (87) further showed that, with detectors up to 0.25 emS volume (W;;5 mm) , resolutions of 7 keY were obtainable at the same "I-ray energy. This work was followed by the fabrication by Taven­dale (88) of devices with significantly larger volume (",2 emS) suitable for high-energy "I-ray detection. Resolutions of 3 . 1 and 6.0 keY were measured at 122- and 1333-keV ")'-ray energies respectively. The general performance of these detectors has been reviewed by Ewan & TavendaJe (89) .

The development of low-noise, field-effect transistor (FET) amplifiers has recently enabled linewidths of 1 .84 and 1 .86 keY to be obtained for C060 "I rays at 1173 and 1333 keY energy, respectively (90). Linewidths of 750 eV for 103-keV Gd15s'Y rays and 2.26 keY for 1836-keV y88 "I rays have reportedly been attained with a small detector (2 cm3 volume) (15).

Typical spectral responses of a large-volume (23 emS) "single-open­ended" coaxial Ge(Li) detector to C057 and N a24 "I rays (49) are shown in Figures 3 and 4. Figure 3 shows that the pulser and full-energy peaks have almost the same linewidth value which in turn is almost that expected from the calculation of system noise on the basis of detector capacitance and amplifier noise slope. At higher energies, as shown in Figure 4, amplifier gain and biased-amplifier threshold drift combine to broaden the pulser peak to 4.5 keY (FWHM). Carrier trapping, as indicated by the low-energy tail on the full-energy peak, combines with instrument drift to limit the resolution to 8.S keY at 2754 keV'Y-ray energy. MaIm (91) has measured the resolution of a parallel array of four coaxial Ge(Li) detectors totalling 150 emS active volume (335 pF capacitance) at 12.6 keY on the full-energy peak of 2514-keV 'Y rays. The linewidth from two detectors in parallel (82 cm3 volume) at C060 energies indicated that the individual detector pulse height responses differed by <0.03 per cent.

Efficiency of Ge(Li) "I-ray detectors.-The intrinsic full-energy peak effi­ciency factors (percentage fraction of "I rays incident on a detector which con­tribute to the full-energy spectral response peak) for single and stacked Ge (Li) detectors with volumes ranging from ,...,1-54 cms have been reported

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10'

T-77·K. CAP',22pF. VOL.' 23cm3 BIAS =1250V. IL'3xIQ-10A 122 keY PULSER

! �

fr � 1-47keV-!j_ 1·35keV-.ij-

/. Ii [! 136r Ii' ! l � /. ! \ .-n, ....... 1 .49 keV /.\ . . I . I 1\ ·1 1 TI l .

! L il :w. L II "',�. '.' ". !'.1 '. 10 �--�----�----�----�----�----�----�--�

o 100 200 300 400 500 600 700 800

CHANNEL No.

FIG. 3. Response of a coaxial lithium-drifted germanium detector to 'Y rays from a Cc>"7 source [Tavendale (49)].

I ffi 103 (l. <J)

�

T -noK. CAP.' 22pF. VOL' 23cm3

BIAS'1250V. IL' 3xIO-10A

1368 keY

�

I DOUBLE ESCAPE 'lI'-2754

_ __ 1732 keY 2754 keY

5·6keV � ! � � SINGLE ESCAPE 'lI'-2754 t'

-5.5 keY 2243 keY

� � 83 kev--+�---

� i . L....". t 'f '- ��� � ·1 ��" "fA It I • • .. 11 I·

CHANNEL' No.

\ ·r Jl -V'

FIG. 4. Response of a coaxial lithium-drifted germanium detector to 'Y rays from a Na24 source (Tavendale (49)].

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82 TAVENDALE

(45, 49, 52,89, 92, 93) . At 1-MeV energy, efficiencies are approximately 0.3, 1.0, and 8 per cent for detectors of 0.88, 4.0, and 54 cm3 active volume respec­tively; the latter detector (3.3-cubic inch volume) has an efficiency compara­ble to that of a 1.5 X 1 inch N aI scintillator (45). The larger Ge(Li) detectors show a considerable improvement in the peak-to-Compton total ratio that is due to a more effective reabsorption of scattered 'Y rays.

Si(Li) X-ray detectors.-Silicon detectors are particularly useful as spec­trometers in the X-ray region, :( 50 keY. Detectors with depletion depths ",2-3 mm and 1 cm2 area are easy to prepare and operate under high-resolu­tion conditions ( ",,1 keY) in conjunction with cooled FET preamplifiers. Elad & Nakamura (94) have reported a resolution of 700 eV for 6.4-keV Fe67 X rays from a small Si(Li) detector.

CHARGED-PARTICLE SPECTROMETERS /X- and {3-particle detectors.-The reported resolutions for /X-particle de­

tectors are tending to a limit of ""to keY, being a combination of statistical ionization losses associated with plasma trapping of carriers, window effects, and inelastic nuclear scattering within the depletion layer. The resolution of 15 keV from a diffused-junction silicon guard-ring detector for Am241 5.47-MeV Cl particles reported by Goulding (95) is typical of a good device, al­though a resolution of 1 1 keY for a surface-barrier detector at room tempera­ture has been reported (9). Continued improvement in resolution perform­ance, reported for silicon surface-barrier {3-particle detectors, is mainly due to the use of low-noise FET preamplifiers. Linewidths of 2.0 keV (96) for Ba137 625-keV conversion electrons and 1.5 keY (85) for 89.4-keV conversion elec­trons from a U237 source have been measured.

High-energy proton detectors.-Pehl, Landis & Goulding (57) have recently examined the responses of germanium and silicon lithium-drifted detectors to 29 and 40 MeV protons (the latter having a range equivalent to 4.8 mm in germanium) . Linewidths of 28 (0.01 per cent) and 44 keV respectively were measured for a Ge(Li) detector of dimensions 1 Xl XO.6 em thickness with a gold surface-barrier window. The silicon device gave a comparable perform­

ance (35 keY resolution for 29-MeV protons) but at approximately half the stopping power. Cyclotron beam energy spread limited the resolution attain­able at low energies. Bertrand et al. (97) have reported a resolution of 159 keV for 59-MeV protons incident "edge-on" to a Ge(Li) detector. Window and beam straggling largely contributed to the observed linewidth.

Heavy-ion detectors.-The resolution of silicon detectors for heavy ions is ",,1 MeV (98) and is due mainly to the statistics of ionization produced by screened atomic collisions near the end of the particle range, which are rela­tively inefficient ionizing events and in turn lead to the so-called pulse height defect of value ",,10 MeV. Moak, Dabbs & Walker (99) have shown that it is possible to obtain linewidths of ",,0.1-0.3 MeV for Ar and I ions which have been channelled in a silicon detector by selectively orienting particle entry along the [1 10) crystal axis. Walter (98) has investigated the behaviour of several silicon surface-barrier detectors in which charge multiplication for

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 83

fission fragments was noted. The effect, which is a source of nonlinearity, can be eliminated by appropriate chemical preparative techniques. It is possibly due to the tunnelling of electrons through the silicon dioxide layer at the bar­rier contact near the dense positive-hole space charge following particle ionization.

NEUTRON DETECTORS

Silicon detectors using converter foils of Blo, Li6, hydrogenous materials as proton radiators, or threshold reaction materials have been described by many workers [see in particular Dearnaley & Northrop (26)]. Typical of this class of spectrometer is that described by Love & Murray (100) using a thin sandwich foil of Li6F (",,150 p.g/cm2 thickness) between two silicon detectors to detect the reaction products from Li6 (n,a)T. The spectrometer is suitable for neutrons in the range 0.6-15 MeV with a resolution of ",300 keY, being limited mainly by source absorption. Efficiency is ",10-3 for thermal and 10-6 for 2-MeV neutrons. It should be noted that Si28(n,p)AI28 and Si28 (n,a) Mg25 reactions in the silicon detectors produce a background in this type of spectrometer.

Recently, use has been made of the fast-neutron threshold reactions in­duced in silicon for neutron spectroscopy, as suggested earlier by Marazzan, Merzari & Tonolini (101). Miller & Kavanagh (102) have measured neutron energies in the range 7.2-16.4 MeV with a precision of 5-11 keY. The resolu­tion of the neutron peaks was always worse than 125 keY (compared with 30 keY a-particle resolution for the detector) as a result of pulse height defects associated with the Mg25 and Al28 recoils.

CHARGE PRODUCTION AND COLLECTION IN DETECTORS

ENERGY PER ELECTRON-HOLE PAIR A fundamental parameter which in part determines the energy resolution

obtainable from semiconductor detectors is e, the average energy required to liberate an electron-hole (e-h) pair. It is of special interest to know the con­stancy of IE with particle energy and species, i.e. the linearity of energy reo sponse of the detector. Early values for silicon detectors (103) indicated little dependence of IE on particle species except in the case of heavy ions or fission fragments where the pulse height defect due to the production of low-ionizing recoil silicon atoms leads to high values. More recent measurements in silicon detectors (104-107) have shown significantly higher values of e for electrons than for alpha particles [at 3000 K, IE = 3. 79 ± 0.01 eV for 365-keV electrons compared with 3.61 ± 0.01 eV for 5.48-MeV a particles (104)]. Measurements of IE in germanium for electrons have bcen taken with ')'-ray sources, and values about an average of 2.9 eV at 77° K have been reported (89, 106, 108). The excellent linearity of energy response of Ge(Li) 'Y·ray detectors, i.e. con­stancy of IE with energy, is illustrated by the measurements of Berg & Kashy (109) who found the response to be linear to better than ± 0.03 per cent in the range 662 to 2614 keY, and better than ± 0.1 per cent up to 6-MeV ')'-ray

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84 TAVENDALE

energies. Values of E for germanium and silicon are in reasonable agreement with the theory proposed by Shockley (1 10, 1 1 1 ) .

I t is obvious from some of the discrepancies in the values of € measured by various workers that careful interpretation of the results is essential, especi­ally when constructional differences between detectors can affect the pulse height response at low temperatures. For example, Dodge et al. ( 1 12, 113) contest the E=5.22 ±0.02 eV figure obtained by Emery & Rabson (106) for {3 particles in silicon at 20° K and report an experimental value of € = 3.6 eV over the range 4.2-12° K using a silicon surface-barrier detector, and only a slight variation in € over the range 6-77° K, which was explainable on the basis of the temperature dependence of Eg, the forbidden band gap. These workers observed no obvious maxima or minima in experimental E values such as were seen earlier in detectors which were not fully depleted from n contact to p contact (1 14, 1 15).

THE F ANO FACTOR FOR SEMICONDUCTOR. DETECTORS

The statistical fluctuations in the number of e-h pairs generated during the loss of particle energy through the interdependent processes of ionization and phonon production determine the ultimate, intrinsic energy resolution of semiconductor spectrometers. Following the work of Fano (1 16) on the sta­tistics of ionization yields in gases, the Fano factor F has similarly been de­fined for semiconductor detectors as

(N-N)'=F'N where N is the average number E/E of e-h pairs produced, and (N-N)2 is the variance. Theoretical treatments of the statistics of ionization in semicon­ductor detectors have been given by van Roosbroeck (1 17) and Alkhasov, Vorob'ev & Komar (1 18).

Experimentally, the Fano factor is measured by subtraction in quadra­ture of system noise effects, as given by the standard amplitude deviation of artificially injected pulses of charge, from the observed particle pulse peak deviation. Instrumental contributions to line broadening such as amplifier drift, excessive count rates and charge trapping, recombination, and collec­tion time effects in the detector, all tend to give an experimental value of F which is too high.

Currently, minimum measured F values of 0 . 15 and 0.10 are being ob­tained for germanium (15, 1 19) and silicon (96, 120) and these are in reason­able agreement with some theoretical estimates (1 18) . Significantly higher F values for germanium measured by some other workers (89, 108) are not easily explained. It may well be that factors such as crystal impurities and imperfections in some way play a more significant role in the energy loss proc-

. esses than is thought at present.

RESPONSE TIME OF SEMICONDUCTOR DETECTORS

In certain applications such as fast coincidence detection of particles, the response time capability of semiconductor detectors is of prime interest (12 1).

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 85

I t is possible to make use of the response time dependence on track orienta­tion as a means for particle discrimination in detectors (122) .

A fundamental limitation on the response time of semiconductor detec­tors is that of carrier velocity saturation, a phenomenon which occurs at high electric field strength as a result of the heating of carriers with respect to the crystal lattice. The subject of hot-carrier mobility in semiconductors has re­ceived considerable theoretical and experimental attention [see e.g. Paige (123) and Smith (124)].

The measured saturation velocity of carriers in Ge(Li) detectors at 77° K is �107 cm/sec (10 nsec/mm) for fields ,....,1000 V /cm (56, 59, 97) . Thus the collection times for electron-hole carriers may range from �1 nsec for the shallowest p-n junction detectors to not less than 50 nsec for a 10-mm deple­tion-layer Ge(Li) p-i-n detector, for which the carriers are assumed to be generated equidistant from p and n contacts.

There are many reported measurements of the response time characteris­tics of silicon particle detectors (125-129). Meyer & Langmann (126) have made a detailed analysis and experimental study of silicon surface-barrier de­tector response to a particles. Using proposed models for the equivalent cir­cuit of the detectors and taking into account the variation of J.I. with E as given by Shockley theory (130) , they obtained reasonable agreement with measured results. Similarly, Falk, Tove & Madakbas (125) and Fabri & Svelto (127) have analysed the effect of nonconstant carrier mobility on pulse rise time. The importance of reducing the integrating effect of any un­depleted base material in high-resistivity, n-type surface-barrier silicon de­tectors is emphasised in this work.

A limiting factor on the response time of heavy-ion detectors is the delay incurred (�1 nsec) during the ambipolar diffusion of e-h carriers from the plasma of ionization to the collection field (26). Meyer (128) has found for silicon surface-barrier fission detectors that the plasma time follows the sim­ple exponential function of the field, Tp ac kE-n, where 0.7 < n < 1.3 and k is a constant dependent on particle type. Fission-fragment pulse rise times were 1.4 nsec compared with 0.8 nsec for a particles (63 per cent level).

The timing characteristics of coaxial and planar Ge(Li) ')'-ray detectors have been reported by a number of workers (50, 51 , 121, 131 , 132) . Graham, MacKenzie & Ewan (51) found by scanning a closed-end, trapezoidal-cross­sectioned Ge(Li) coaxial detector with a collimated ')'-ray beam of 51 1-keV ')' rays that timing resolutions (FWHM) increased from 8 nsec at the open end to ",35 nsec at the closed end because of weak collection fields in this latter region. This leads to an overall skew-shaped timing distribution curve. MaIm (50) has shown that removal of the closed end gives an improved timing characteristic and that the skewness can be further reduced by lithium-drifting "double-open-ended" coaxial cylindrical detectors having uniform collecting fields, as shown in Figure 5. A resolution of <7 nsec (FWHM) was measured for a lO-mm-deep planar Ge(Li) detector operated at 1000 V. In all this work, the best timing resolution was obtained by using leading-edge discrimination with thresholds set at a few tens of keY energy.

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10

I I I

�+ ;

��� :�0'i, I I� I

I I I I I I

I I I

I I I

o.�r __ -L __ � __ �-LI�I �I __ -L�. 25

::lzo � I� � 10 :3 5 i 0 I ��3! x

OIDOE CROSS SECTION

o 2 3 4 5 6 7 DISTANCE FRDM THE OPEN END IN em

gJ 530 J �� alo � f-aZo

f-Z I .. I I J:<= ,. I e:: I I �::> ...J� ...Jx ir� 0

VI l>J a: 2 f-w =:; i= z UJ 0 0

DIODE CRDSS SECTION

0 2 3 4 5 6 CENTIMETRES

+ ORIGINAL SINGLE OPEN ENDED OIOOE, SENSITIVE VOLUME. 45cm3 CAPACITANCE· 94 pF

• DOUBLE OPEN ENDED DIODE AFTER CUTTING CAm.CITANCE -S9pF

o SINGLE OPEN ENDED DIODE AFlER· CunlNG CAPACITANCE· 36pF

X SINGLE OPEN ENDED DlOOE AFTER CUTTING WITH AN ADDITIONAL 22 pF CAiW:ITANCE CAPACITANCE· &8 pF

A

8

C

I

A UNORIFTED p-TYP[ GERMANIUM

L/- [J<IFTED CDNPENSAT ED REGION

LI-RICH n· SURFAC£ LAYER MATERIAL LOST IN CUTTING, LAPPING, AND POUSHING

SECTION X-X

UNDRIFTED p-TYPE GERMANIUM

Li-DRIFTED COMPENSATED REGION

Li-RICH n SURFACE LAYER

C

2Bcm DIA. ® I FRONT VIEW

FIG. 5. Leading-edge timing responses of (top) trapezoidal coaxial Ge(Li) detector before and after cutting into two sections, and (bottom) circular coaxial detector [Maim (50)].

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SEMICON DUCTOR NUCLEAR RADIATION DETECTORS 87

CHARGE TRAPPING IN SEMICONDUCTOR DETECTORS

Trapping of electron-hole charge carriers by impurity or defect centres during the conduction pulse in semiconductor detectors leads to a degrada­tion in energy resolution due to the statistical spread in the number of car­riers trapped. The trapping effect is usually dependent on particle track orien­tation within the sensitive volume, and is recognizable by the appearance of low-energy tailing on spectral peaks (44). Trapping either may be short term (",1 J.lsec) or may last for several tens of microseconds, before the charge is regenerated to the conduction or valence bands, depending on the nature of the trapping centres and their location in the forbidden energy band gap. Trapped carriers can be completely lost to the conduction pulse by recom­bination.

Coleman & Swartzendruber ( 133), in a series of measurements, have found effective carrier lifetimes of 1-10 J.lsec in gold surface-barrier Si(Li) de­tectors for ex particles. They point out that the minority carrier lifetime value 7, as obtained by the usual photoconductive-decay method in semiconductors, bears no resemblance to that found in a detector, since the former involves loss of carrier charges by recombination and not by trapping. It was con­sidered that shallow traps might account for an observed decrease in 7 with tem perature.

Recently, Dearnaley (25) and Day, Dearnaley & Palms (134) have de­veloped a theory for the usual situation in detectors in which the carrier mean free path X is greater than the depletion depth W of a detector. Their results show that, assuming equal mean free paths for both holes and electrons, a resolution better than 0.5 per cent requires that X must be > 10 W. This illus­trates the rigid requirement of high carrier lifetime in the germanium high­resolution 'Y-ray spectrometers described earlier.

CHANNELLING OF CHARGED PARTICLES IN TRANSMISSION (IlEIllx) DETECTORS

Anomalous, long-range transmission effects of heavy ions in monocrystal­line silicon have been studied by Davies, Ball & Brown (135). These are ex­plainable ( 136) in terms of low-angle, correlated Coulomb scattering leading to a focussing effect between crystal planes and along the relatively open channels of the main crystal axes.

Dearnaley ( 137) suggested that such channelling may affect the perform­ance of thin IlEI Ilx detectors, and demonstrated the anisotropic energy loss rate for 2.1-MeV protons incident on a silicon surface-barrier detector 37 J.I thick. The detector wafer was cut normal to the axis of crystal growth, the [ 111] direction, and was backed in line with the particle beam by a thick, total-absorption or E counter. Rotation of the IlEIllx counter about the [1 11] direction as axis, inclined 35.2° to the beam, produced pronounced high­energy tails on the proton peaks in the E counter at intervals of 120° corres­ponding to anomalously low energy loss along the [1 10], [101], and [011 ] crystal axes of the transmission counter. Beam angle dispersion was 1°. Simi-

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88

10.000 8000 6000 4000 3000 2000

1000 800

...J 600 w z z ex 400 :r u 300 � 8 200 u

100 80 60 40 30

20

10 430

TAVENDALE

4.40

�" 4.9-MeV PROTONS \ " \ "

35.1).1 Si CRYSTAL , " , ' " ,', ...... {Iii} PLANE

':---.......... /-, '\ ..... _/

\\

,

\ \ \ \ \ \ , \ \ , \ , . I

{liD) PLANE \ I \ I

\ \ I I . I , . I , \ \ I

\ I I I I I I

I I I I I I I I I

4.50 460 4.70 ENERGY (MeV)

480

FIG. 6. Energy spectrum of 4.90-MeV protons transmitted through a 3S JJ. silicon crystal with incidence direction parallel to the {111} plane, {110) plane and with in­cidence in a "no-symmetry" or "normal" direction [Gibson (142)].

lar experiments have been carried out by other workers (138-142). Figure 6 shows the energy spectrum reported by Gibson (142) of protons after trans­mission through a silicon f).E/ f).x detector, illustrating the effect in the { 111} and {110} planar directions. Madden & Gibson (138) have found that up to 15 per cent of 5-MeV a particles in a beam of divergence 10° may show an­omalous energy loss for some crystal orientations, while 50 per cent may be affected at higher collimation.

To redU(;:e response anomalies due to channelling in the application of f).E/f).x detectors, Dearnaley (137) has proposed that wafers should be cut at 10-20° from the {111} plane in order to present to the beam a direction of least crystallographic symmetry.

Interrelated with the phenomenon of channelling of charged particles in !:lE/!:lx counters are anomalies in their range, charge production, scattering, radiation damage effects, and nuclear reaction probabilities, all affecting the performance of the detector (142). In fact, for channelled 1+ ions the value of E is found to be the same as for light particles (99) and the pulse height de­fect is absent, i.e. the probability of inelastic scattering collisions with lattice atoms is reduced for focussed ions. It has been suggested (142) that this ab-

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 89

sence of pulse height defect could be used to improve the linearity of re­sponse between differing particle species and to lessen radiation damage effects in fission detectors, within the limits of the high degree of beam colli­mation necessary for channelling.

RADIATION DAMAGE EFFECTS

The mechanisms of radiation damage in semiconductor detectors have been discussed in detail by Dearnaley & Northrop (26) and Goulding (23) who give threshold irradiation limits at which the performance of silicon detectors is affected.

Unfortunately, there is little available experimental information concern­ing radiation damage effects in Ge(Li) 'Y-ray spectrometers, and it appears that no systematic investigation has been made to determine the threshold limits for operation in various radiation environments. However, it has been reported (8) that in the course of operation near an accelerator beam, de­tectable effects were observed at a threshold of ",108 heavy particles/cm2 (mixed radiations) absorbed in the detector. Subsequently, operation at higher bias and further drift treatments permitted detector operation up to ,...., 1012 particles/cm2• Ge(Li) detectors were also damaged by fast-neutron doses of 1012 neutrons/cm2• It may reasonably be expected that for 'Y rays of energy ,....,1 MeV, performance will be affected by ",1016'Y rays/cm2 dosage, but that little damage will occur for'Y rays with energies <: 300 keY.

DETECTORS FOR SPECIAL APPLICATIONS

POSITION -SENSITIVE DETECTORS

Mosaic arrays and linear, position-sensitive "triode" detectors of high spatial resolution are useful in such applications as particle polarization measurements in nuclear reactions and also as focal-plane detectors in mag­netic spectrometers. Multielement arrays of rectangular-shaped surface­barrier detectors can be constructed on single slices of n-type (143, 144) or lithium-drifted silicon (145) .

A two-dimensional position indication may be obtained in Cartesian co­ordinate form by evaporation of mutually orthogonal surface-barrier strip contacts of gold and aluminium on opposite faces of a thin, n-type silicon disk. Hofker et al. (146) have made such a "checkerboard" counter contain­ing 88 sensitive detecting elements with an angular resolution of 10 at a source-to-detector distance of 8 em.

A three-contact, or "triode" linear position-sensitive detector capable of excellent spatial resolution has been developed by Norbeck & Carlson (147) and others ( 148, 149) . Figure 7 shows a triode position-sensitive detector schematically. The device is essentially a long, narrow, rectangular surface­barrier diode with two back contacts and is operated near depletion. A full­energy signal E appears at the gold electrode while the current division in the thin, high-resistance sheet of undepleted material produces the position­sensitive signal X/eX + Y) at the ungrou nded back contact, as shown.

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90 TAVEN DALE

- HV

GOLD

�----------- x - - - - -- -----�- - - - Y----�

ALUMINUM ALUMINUM

FIG. 7. Derivation of signals from a position-sensitive detector

[Ludwig, Gibson & Hood ( 148)].

ENERGY SIGNAL

(E)

POSITION SIGNAL

( X ' X + y)E

Ludwig, Gibson & Hood ( 148) , using web silicon, have obtained better than 1 per cent position resolution for 5.5-MeV a particles in counters with di­mensions 2-3.5 cm X 2.4 mm.

INTERNAL-GAIN DETECTORS

Detectors having internal gain are desirable in such applications as mul­tiple detector arrays, where an economy of amplifiers would result, and in the detection of low-energy radiations such as X rays which may produce signals below the noise level of the following amplifier. Attempts to obtain internal gain using a transistor structure, as described by Williams & Webb (150) , only produced detectors with signal-to-noise ratios and gains comparable to a separate detector and amplifier system. Recent work by Ruth and co­workers (151-154) , who used diffused p+-n junction diodes with specially contoured surfaces to reduce surface currents, has shown that gain factors of several hundreds can be obtained using the process of high-field avalanche multiplication ( ISS, 156) .

Figure 8 shows schematically the p+-n junction detector of the type used by Huth et al. Surfaces are bevelled to an angle of ",100 in order to confine the multiplication region within the j unction contacts. Base material is usually �50-100 ohm-em n-type silicon, and bias levels of ",1500-2300 V are necessary to attain multiplying fields of ",250 kV fcm. Pulses of 2-3 V ampli­tude are generated by a particles and for minimum-ionizing {3 particles it is possible to obtain sufficient gain to generate pulses equivalent to 5-MeV en­ergy. Aluminium fluorescence X rays ( 1 .49 keY) have also been amplified to levels equivalent to 400-750 keY, sufficient to drive a tunnel diode trigger

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 9 1

circuit (158). Pulse rise times i n such detectors are < 5 nsec. A disadvantage of these avalanche devices is that their response shows a dependence on par­ticle orientation, since the multiplying field is confined to a narrow zone at the p+ contact.

It is of particular interest to note that some avalanche detectors show an improvement in their intrinsic signal-to-noise level with increasing multipli­cation factor, that is the leakage-current noise is apparently amplified to a lesser degree than the incident radiation signal.

Haitz & Smits ( 159) have analysed theoretically the noise produced in detectors operating in the avalanche mode. Such noise arises from two sources, one from the inherent statistical spatial fluctuations in the avalanche breakdown voltage due to local variations in donor or acceptor impurity densities, and the other from the statistical nature of the multiplication proc­ess itself. They find, for example, that on combining both these noise sources and taking a noise level of 1 .8 keY (FWHM, silicon equivalent) for the follow­ing amplifier, a detector with an internal gain factor of four times improves the energy resolution of the system up to a maximum primary radiation energy of 80 keY.

JUNCTION DETECTORS FROM COMPOUND SEMICONDUCTORS

Interest in junction detectors from compound semiconductors has cen­tred mainly on the high-Z materials for possible application as efficient 'Y-ray spectrometers, particularly for operation at room tempeatures. Several com­pounds with wide hand gaps, for example GaAs, CdTe, and GaSh, have re-

hv ---

I I 200 - I-o-· ---·+-MICRONS

+

� 1- 25 M I CRONS

FIG. 8. Outline of a contoured-surface, silicon avalanche p+-n junction detector [Huth ( 157)].

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ceived attention, but to date the preparation of useful detectors has been prevented by low carrier lifetime.

Lifetimes of 10-9 - 10-8 sec were measured by Kobayashi & Takayanagi ( 160) in a GaAs gold surface-barrier a-particle detector which gave a line­width of 420 keY for S-MeV a particles. No oscillatory behaviour was ob­served such as has been noted in earlier bulk conduction counters of GaAs (26) .

An extensive development programme for the growth of pure CdTe crystals and the performance of junction detectors has been reported by Baily ( 161) and Mayer (24) . Carrier lifetimes varied from one ingot to an­other and were in the range 10-L6 X 10-7 sec with resistivities in either of two ranges, 500--1000 or 10L107 ohm-cm (semi-insulating) . Linewidths for a par­ticles were ",100 keY for small-area (6 mm2) detectors. Arkad'eva et al. ( 162) have detected CS137 'Y rays with signal-to-noise ratios of 15-20 in CdTe diodes. Unsuccessful attempts to make lithium-ion-drifted diodes from GaSb have been reported by Gibbons & Owen ( 163) .

Canepa e t al. ( 164) have made small SiC p-n junction detectors which are operable up to 7000 C because of the large band gap of the material (2 .3 eV for cubic SiC) . These devices have been used as fission detectors and show a much higher tolerance to radiation damage ( ",100 times) than do silicon de­tectors. High-temperature in-pile operation is an obvious application for

these detectors.

PROSPECTIVE DEVELOPMENTS

Future advances in the performance capability of semiconductor detec­tors will depend largely on improvements in the quality control of crystal perfection and purity. This is particularly desirable in large germanium crys­tals for 'Y-ray spectroscopy. Much silicon and germanium, while suitable for transistor manufacture, has proved unsatisfactory for lithium drifting of large volumes owing to high oxygen content, regions of high dislocation density, and low carrier lifetime at liquid nitrogen temperatures. The con­tinuing demand by experimentalists for more efficient photon detectors wiIl no doubt stimulate the technology of growing large ( ",500 cmS) high-quality crystals of germanium. Whether or not such crystals can be grown to the degree of purity « 1010 impurities per cm3) at which lithium-ion drifting is no longer required, remains to be seen. The efforts made in some laboratories to develop high-Z compound semiconductor detectors have so far been un­rewarding. There seems to be no immediate prospect of such devices com­peting with germanium spectrometers, because of difficulties in the prepara­tion of materials with long carrier lifetimes.

As emphasised above, it is only the advent of low-noise, stable amplifiers and pulse height analyzers that has made possible the high-resolution per­formance of semiconductor detectors. Measurements of the spectral response of silicon detectors to X rays in the lO-keV region have indicated that in­trinsic linewidths ",100 eV may be expected, as compared with the amplifier-

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SEMICONDUCTOR NUCLEAR RADIATION DETECTORS 93 limited values of ",,500 eV observed at present. Thus there remains the need for continued development of lower-noise amplifiers associated with such detectors.

LITERATURE CITED 1. Miller, G. L., Gibson, W. M., Dono­

van, P. F., Ann. Rev. Nucl. Sci., 12, 189 (1962)

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3. Shirley, D. A. . Nucleonics, 23, No. 3, 62 (1965)

4. Hollander, J. M., Perlman, I., Science, 154, 84 (1966)

5. Kennett, T. J., Phys. Today, 19, No. 7, 86 (1966)

6. Hollander, J. M . , Nucl. Instr. Methods, 4J, No. 1 , 65 (1966) ; Lawrence Radiation Lab. Rept. UCRL-16307 (1965)

7. Donovan, P. F., IEEE Trans. Nucl. Sci .• NS-1 3, No. 3, 21 (1966)

8. Lithium-Drifted Germanium Detectors (lAEA, STI/PUB/132, Vienna, 1966). Proceedings of a Panel Meeting, June 6-10, 1966

9. Gunnerson, E. M., Rept. Progr. Phys. (1967) (To be published)

10. Sayre, E. V., IEEE Trans. Nucl. Sci., NS-13, No. 3, 18 (1966)

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