Evaluation of epitaxial n-GaAs for nuclear radiation detection

NUCLEAR

INSTRUMENTS

AND

METHODS

94

463-476;

(1971)

(~ N O R T H - H O L L A N D

PUBLISHINGCO.

EVALUATION OF EPITAXIAL n-GaAs F O R N U C L E A R R A D I A T I O N DETECTION J. E. E B E R H A R D T , R. D. R Y A N and A. J. T A V E N D A L E

Australian Atomic Energy Cammission, Research Establishment, Lucas Heights, N.S.W. Australia 2232 Received 22 M a r c h 1971 T h e high purity of n - G a A s grown by the liquid phase epitaxial process m a k e s it suitable for the evaluation of this c o m p o u n d s e m i c o n d u c t o r in nuclear radiation detection. The wide band gap a n d high atomic n u m b e r m a k e it possible to operate surface barrier diodes at r o o m temperature as high resolution detectors. The resolutions (fwhm) at 300 K have been f o u n d to be 21 keV for 5.49 MeV c~-particles and 2.5 keV for 140 keV ?,-rays. T h e best resolutions observed were 640 eV (130 K) for 59.54 keV v-rays, 1.3 keV (200 K) for 84 keV conversion electrons a n d 16 keV (205 K) for 8.785 MeV ~-particles. T h e thickness of the epitaxial region was ~ 6 0 / z m with contact diameter up to 3 ram. T h e energy per electron hole pair for c~-particles is 4.27 eV + 0.05 at 300 K with a linear variation with b a n d gap of 2.7 over the t e m p e r a t u r e range 87 K to 340 K. T h e F a n o factor for ?,-rays is _< 0.18 __ 0.04 ( T ~ , 125 K). Variations of capacitance a n d pulse height with detector bias, as well as etch patterns, indicated a

discontinuity in the impurity density profile in the region of the interface between the substrate and the epilaxial layer. Consideration of the theory o f excess carrier recombination in direct band gap semiconductors s h o w s that the charge collection efficiency is not expected to be significantly reduced by direct radiative recombination, even in the case o f fission fragments. This is confirmed by the experimental observations that ~'~2Cf fission fragment energy deficits of 14.4 a n d 6.3 M e V are not m u c h larger t h a n have been observed with Si detectors. T h e advantages of r o o m temperature operation open up a wide field o f application for G a A s detectors in both the physical and biomedical fields. F o r their full exploitation it will be necessary: (a) to grow thicker layers of high purity G a A s by the epitaxial or s o m e other process; (b) to overcome the problem of signal a t t e n u a t i o n at the epitaxial-substrate interface; (c) to develop a m o r e rugged form of front contact and of detector encapsulation.

1. Introduction The operation of high resolution n-GaAs a-particle and 7-ray radiation detectors has been reported earlier ~). Further results have been obtained and in this paper a more detailed description of the detectors is presented. The lack of resolution observed in other work with GaAs can be attributed to the presence of a high trap density in the melt-grown material used. This limitation does not apply to high purity epitaxial GaAs. Surface barrier diodes made from this material can be operated at room temperature as high resolution spectrometers lbr a-particles, conversion electrons, fission fragments and y-rays. The useful y-ray particle energy ranges are limited because only relatively thin epitaxial layers are presently available. The piezoelectric effect present in GaAs is responsible for an increased sensitivity to vibration generated noise.

detection of charged particles and low energy 7-rays. High resolution performance can only be realised by cooling to reduce the bulk leakage current. The increased efficiency of Ge ( Z = 32) makes it suitable for higher energy 7-rays but low WG makes cooled operation ( T < 100 K) essential. The larger volume Si(Li) and Ge(Li) detectors require reduced temperature for storage as well as operation, to prevent Li diffusion. The first elemental semiconductor material used as a detector, diamondS), has a very wide band gap but a low Z. Resolution is limited by the lack of purity of the available material.

l.l.

LIMITATIONS OF ELEMENTAL SEMICONDUCTORS

The elemental semiconductors Si and Ge have well established uses in high resolution nuclear particle detection 2-4) but owing to their band gaps (WG) and atomic numbers (Z) (table 1), they must be operated at low temperature and their efficiency for ?-radiation is relatively low (compared, for example, with Na[ scintillation detectors). For limited resolution work it is possible to operate Si diffused and surface barrier p-n junction detectors at room temperature. However, low Z and small depletion depths restrict them to the

1.2. COMPOUND SEMICONDUCTORS Compound semiconductors which have wide band gaps and high Z have been tried as nuclear radiation detectors but their performance has been markedly inferior to that of Si and Ge. The performance of all semiconductor detectors is determined by the crystal perfection of the material used and the highly developed technology of Ge and Si is gradually being matched by that of GaAs which is used in high efficiency Gunn effect devicesS). GaP and InP are other I I I - V compounds which have been grown with good crystal qualities6-7). The II-VI compound CdTe has attracted considerable attention 4) because of its high Z~,v= 50, but the material so far used does not have the degree of crystal perfection of the best epitaxial GaAs. CdTe and GaAs have 463

464

J. E. E B E R H A R D T et al. TABLE 1 Properties of detector semiconductors at 300 K. Parameter C

Z Wa (eV) e (eV) F /tn (cm 2 V-I S--t) /to (cm 2 V -1 s 1) Bandgap Structure References

6 5.47 17 1800 1600

3, 5

Elements Si

14 1. l 15 3.76" < 0.10" 1350 480 Indirect Diamond 4

Ge

32 0.665 2.96* < 0.08* 3900 1900

4

Compounds GaAs CdTe

31, 33 48, 52 1.43 1.44 4.27 4.65 < 0.18 8600 1000 400 100 Direct Zincblende 5, this work 4

* Measured at 77 K.

similar semiconducting properties and they the Gunn effect, have direct band gaps, the crystal structure and, in contrast to C, Si non-zero piezoelectric constant (see section

both show zincblende and Ge, a 7).

1.3. EARL|ER WORK ON COMPOUND SEMICONDUCTORS Harding et alfl) and Northrop 3'9) reported the first results on semi-insulating GaAs bulk conductivity counters. The pulse response to s-particles and to y-rays was limited by polarisation and poor charge collection, while the field which could be applied was limited by the occurrence of low frequency oscillations. The latter problem was not encountered with the surface barrier diodes used in later work. Kobayashi and Takayanagi 1°) detected 5.15 MeV s-particles with a resolution of 420 keV fwhm in a diode with a depletion depth of over 60/~m. The electron mobility (~n) of 4400 cm 2 V - L s- 1 at 300 K measured in the n-OaAs used indicates that this melt grown material probability had a high level of compensation. Akutagawa et al. 11) found no evidence of photopeaks in 1 m m thick semi-insulating OaAs; no further reports on GaAs detectors have been found. Work has been concentrated on CdTe which did show photopeaks 11) and Arkad'eva et al. 12) observed a resolution of 7.5 keV at 122 keV with an n-CdTe detector having a sensitive depth of 30 ~m. The growth from the vapour phase of more homogeneous crystals of high purity ( N D + N A ~ 1015 cm -3, where ND,NA are the donor and acceptor impurity densities) enabled Zanio et al.t3'14) to fabricate CdTe detectors over 1 m m thick which have a room temperature 7-ray resolution of 28 keV at 122 keV and clear photopeaks at 662 keV. Bell et al. 15) obtained useful results with CdTe prepared by the travelling heater method. The peak broadening in these de-

tectors is attributed primarily to the loss of charge carriers to traps. The results reported in this paper show that high resolution is also possible in GaAs if its purity is sufficient to keep the level of trapping very low. Before presenting these results it is necessary to discuss the characteristics of the material and the method of detector fabrication. 2. Material characteristics

With GaAs at present restricted to the surface barrier diode junction structure, the detection of y-rays requires a wide depletion region at a reasonably low bias voltage which is only practical in material with a low level of net impurity concentration [ ( N o - N A ) < 1014 cm-3]. A high charge collection efficiency (low trap density) is also necessary for high resolution and can only be realised in material with a low total (electrically active) impurity concentration (No+NA) and high stoichiometry. These conditions, which have been realised in Si and Ge, have not been obtained in GaAs with the conventional melt-growth methods. The requirements of high efficiency Gunn effect devices which also can only be met by GaAs with a low (ND+ NA) have stimulated a search for other methods of growing high purity material. 2.1. EPITAXIALGROWTH (LPE OR VPE) At present epitaxial growth from the liquid or vapour phase (LPE or VPE) is the only technique which can provide such low impurity levels 16-18). The starting materials, Ga, As, AsC13 and H2 must be of high purity. The single layer thickness is at present limited to near 200/~m for both LPE and VPE. The low distribution coefficient of most impurities between the solid and liquid phases means that in LPE most

E P I T A X I A L n-GaAs FOR N U C L E A R R A D I A T I O N DETECTION

impurities remain in the gallium liquid. When not deliberately doped, the epitaxial layers are found to be n-type, with carrier concentration as low as 1013 cm -3. Electron mobility, which at low temperatures is mainly limited by ionized impurity scattering, is a very sensitive indicator of total impurity level over the range 10 t3 to 1015 cm -3. For ( N A / N D ) < 0 . 5 the mobility at 77 K is greater than 105 cm 2 V - I s - i up to carrier concentrations of 1015 cm -3 iv), Practical devices, such as nuclear detectors and Gunn effect structures, usually require a low resistance ohmic contact to the epitaxial layer which is conveniently made through an n + substrate. However, Hall effect mobility and resistivity measurements to characterise the material are more usually made through contacts to the top surface of epitaxial layers deposited on semiinsulating substrates. The characteristics of layers grown on n + substrates and used in the work reported here are assumed to be the same as those of control layers grown at the same time on adjacent semiinsulating substrates~9). 2.2. EPITAXIAL-SUBSTRATEINTERFACE(ES[) The measured values of n and Pn apply to the upper part of the epitaxial layer but measurements show that n = N o - N A may vary significantly through the nominally uniform layer, with the possibility of a large discontinuity as the junction with the substrate is approached19-26). Such discontinuities have been observed in both LPE and VPE samples and can also occur under alloyed ohmic contacts made to the top surface of the epitaxial layer/l). Recent'results 25) ob-

E I $

Fig. 1. Edge on view of a GaAs liquid phase epitaxial layer and substrate wafer after cleavage and using the etch of Abrahams and Buiocchi2S), showing the epitaxial layer (E), interface region (D width ~ 10/~m, and substrate (S).

465

tained with a high resolution scanning electron microscope on samples grown by VPE have been interpreted as showing a very thin anomalous interface region (0.2-2 pro), with a net acceptor concentration in the range 10 TMto 1016 c m - 3 . With the more limited spatial resolution of other methods such as Schottky diode capacitance variation 2°) and X-ray diffraction topography23), the apparent interface thickness has been put at 2-4/~m. Copper is one contaminant which could introduce acceptor sites, during VPE24). Generation of compensating acceptors through the loss of arsenic by diffusion from the substrate surface during pregrowth heating leads to high resistivity interface layers in LPE grown GaAs23). It is shown in section 3 that this epitaxial-substrate interface (ESI) is present in the detectors reported on in this paper and that it degraded their performance (sections 4 and 5). 3. Detector fabrication The base material 27) came from two different wafers of n-type GaAs grown by liquid phase epitaxy on ( 1 0 0 ) oriented substrates which were tin doped to 10 t7 carriers cm -3. Epitaxial layers were intially ~ 95 and 65 pm thick but during processing were reduced to 80-60 ILm. The electrical properties of simultaneously grown control layers were 33 ohm-cm, 2 × 1013 carriers cm -3 and mobilities of 8200 and 118000 c m 2 V - 1 s - I at 300 K and 77 K respectively19). The net impurity concentration as measured in this work from diode capacitance appeared to be closer to 6 × l0 ~a cm -a. Microscope examination of a cross-section of the material after fresh cleaving and etching with a resistivity dependent etch 25'2s) (see fig. 1) revealed an anomalous ESI layer. Full area ohmic contacts to substrate sections 3 × 3 mm 2 were made by first lapping lightly with a slurry of no. 600 grade SiC powder and water on glass, thoroughly cleansing in water and then wetting with a thin film of G a - I n eutectic alloy. An alloying cycle was made by raising the dice temperature (in air) to 475 K over 5 rain and then cooling to room temperature in 10 rain. During this operation the dice were mounted with the epitaxial surface down on cleaved mica. Excess G a - l n was removed from the substrate by wiping carefully so as not to contaminate the epitaxial surface, and the substrate was then masked with Apiezon wax. The dice were etched for 10 sec in an etchant of 3H NO3 : 2 H 2 0 : 1H F to remove any damaged material from the epitaxial layer surface, followed by rinsing in demineralised water. The dice were then rinsed in methanol and the wax dissolved in xylene

466

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before further washing in methanol, water and drying in air. Gold contacts ( ~ 200 ,~ thick and 1.5 or 2.0 m m diameter) were next evaporated onto the epitaxial layer through mica masks. The diodes were finally mounted by clamping the dice in aluminium holders with a spring steel wire pressing a small piece of gold foil against the Au-contact. A pad of indium foil was placed between the n + substrate contact and the base of the holder. Four satisfactory detectors were made from one wafer and one other from a second wafer. Devices made in the same way from a third high resistivity wafer were unsatisfactory.

295 K, and in vacuo to 77 K. Capacitance measurements were made at a frequency of 130 kHz. Fig. 2 shows leakage and capacitance characteristics for a detector (in vacuo). Differences occur in the form of the capacitance characteristics and full depletion values at 295 K and 77 K (see discussion). Fig. 3 gives the inverse square of capacitance versus bias for GaAs no. 5 at 295 K. The linear relation with bias up to 90 V indicates that the first part ( ~ 75%) of the epitaxial layer has a constant impurity concentration. The nonlinearity from 90 V to depletion at 160 V is due to changes in the charged impurity and carrier concentrations 25) over the remaining 25% of the layer, that is near the ESI. For some detectors a capacitance variation over times of the order of 10 sec was seen immediately after a step increase in reverse bias. Such changes indicate the presence of impurities with deep ionisation levels in the b u l k 29) or at the surface3°). iOs

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Detector leakage currents were slightly affected by ambient conditions, a nitrogen atmosphere giving the lowest value. They also displayed a time dependent increase following an increase in bias, again indicating deep level impurity interactions. The rapid rise in leakage current with bias beyond depletion of the epitaxial layer is to be noted. 5. Spectral response

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5.1. ALPHA PARTICLES AND CONVERSION ELECTRONS

The conversion electrons from a 1°9Cd source at 62.0 a n d 84.0 keV were observed, the resolution being

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better than 1.3 keV fwhm. Checks with higher energy ~3VCs and /°VBi sources showed no total absorption peaks, the ranges of these electrons being greater than the epitaxial layer thickness. Tn an earlier letter 1) an e-particle resolution o f 30

keV was reported at 130 K on the 5.47 MeV 2 4 1 A m

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RADIATION

467

DETECTION

A characteristic of all detectors, evident with c(particles and prays, was the monotonic increase in pulse height with increasing bias, leading in turn to the appearance of multi-peaked spectra for bias levels just less than full depletion. At bias values greater than those required for full depletion, the peaks become single-valued again, with saturation in pulse heights occurring. Fig. 5 shows these variations for 241Am source c(-particles. These changes in response are related to an attenuation effect originating in the high resistivity ESI region (see section 8.2 for discussion). 5.2. FIssIoN FRAGMENTS The response of a GaAs detector at room temperature t o 252Cf source fission fragments is given in fig. 6. An energy calibration was made using a pulser normalised on the 6.1 MeV z-particle line from 252Cf. 5

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CHANNEL NO.

Fig. 7. R e s p o n s e o f a G a A s d e t e c t o r to ~4~Am s o u r c e ; , - r a y s a t 122 K ( e n e r g i e s in k e V ) .

The observed heavy and light mass peak energies are at 65 and 97.5 MeV respectively, indicating corresponding energy deficits of about 14.4 and 6.3 MeV (EHAv=79.5 MeV; ELAv=103.8 MeV)31). These values are similar to those in silicon surface barrier detectors where approximately 5 MeV of the deficit results From nuclear collisions and the remainder from electron-hole recombination32). This indicates that even in the highly ionized plasma of a fission fragment track, electron-hole recombination in the depletion layer of a detector of a direct band gap material such as GaAs does not seriously lower the charge collection efficiency (see section 8.3). The high energy tail beyond the light mass peak (channel no. 110 and higher, fig. 6) is probably due to internal amplification from charge tunnelling at the Au-contact, as has been reported in silicon detectors33).

468

J.E. E B E R H A R D T et al.

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The detectors were thin and only low energy photons produced photopeaks. The best resolution was obtained at about 130 K (640 eV fwhm on the 59.54 keV 7-ray of 241Am) (fig. 7). The linearity of this spectrum is better than ½ percent.

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All the room temperature spectra were obtained using a simple charge sensitive preamplifier (fig. 8). The detector, preamplifier and bias battery were contained in a light tight box which had a beryllium window for X-rays. The output pulse from this preamplifier was negative-going 2/~s, R C - R C shaped and could be Fed directly into the low level input of a Nuclear Data 181 pulse height analyser. Some spectra obtained with this system are 5VCo (122 keV 7-ray, 2.6 keV fwhm) fig. 9, 24lAm(59.54 keV, 2.5 keV fwhm) fig. 10, iodine fluorescent X-rays (K, 28.5 keV, 2.5 keV fwhm) fig. 11, and 99mTc (140 keV 7-ray, 2.5 keV fwhm, not shown). The 24~Am spectrum of fig. 10 was obtained with a thin 25/~Ci source and so, with due allowance For the 0.010 inch thick beryllium window, indicated the variation of photopeak efficiency with energy. The extra peaks in the iodine spectrum (fig. 11) were produced by the 241Am excitation source, as was the continuum below the iodine peaks. The system resolution improved by about 5 percent in the three days Following installation of the detector in the sealed box,

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E P I T A X I A L n-GaAs FOR N U C L E A R R A D I A T I O N D E T E C T I O N -, Bias

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and has remained stable over the subsequent five months, The operating bias of 84 V was arbitrary. The detector had been operated previously at elevated temperatures with a different preamplifier system. At 323 K a peak width of 9.7 keV fwhm was obtained for the 59,54 keV 24~Am y-ray and the X-ray escape peak was still resolved. When the detector temperature was raised to 373 K the increased leakage current worsened the resolution to 20 keV fwhm.

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When radiation is absorbed in a semiconductor detector a constant mean energy per electron-hole pair is observed. This is an intrinsic constant related to the band gap of the material, but independent (to better than 1 percent accuracy) of the type of radiation. Accurate measurements in Si and Ge over a wide temperature range have shown that ~ follows the inverse variation of WG with T. An alpha source (Z4~Am, 5.486 MeV) was used with the system of fig, 12 to measure ~ in the GaAs detectors. The test capacitor (0.430 pF) was made of a I

0.51 Vitramon porcelain capacitor in a stainless steel tube and was stable to one part in a thousand over a period of months at room temperature as measured by a GR 1620-A capacitance measuring assembly. With a loop gain of at least 4000 the charge-sensitive preamplifier had an effective input capacitance of at least 88000 pF after ~ 1 its (the open loop risetime). The detector capacitance was only a few pF, so the attenuation through the 0.01 /~F coupling capacitor was no greater than 0.999. The precision pulse generator had a decay time constant of 600 l~s and so the error produced by its decay was less than I percent with the 5 l~s shaping used. This means that the major system error probably lay in the accuracy with which an observer could adjust the pulser amplitude to overlap the pulse amplitude. This was done with a Tektronix voltage comparator (Z unit) and was repeatable to better than 1 percent.

.

6. Average energy required for electron-hole pair creation (e)

46

469

~00

TEMPERATURE (°K)

Fig. 13. Plot o f energy per electron-hole pair for ~-particles 0:~) vs temperature on GaAs detectors.

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K

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Fig. 13, which was obtained using two detectors, shows that e~ is a linear function of temperattire from 87 K to 340 K. In each case, maximum (breakdown limited) bias operation was used, the bias being always greater than that corresponding to the multi-peaking response region. GaAs no. 5 could not be operated above 130 K because of surface breakdown, GaAs no. 13 broke down at low bias voltages ( < 150 V) when operated below 150 K. As can be seen in fig. 13 their results are in agreement. Using the band gap vs temperattlre figures of S t u r g e 34) the restllts were plotted as eo vs WG (fig, 14). A straight line with a slope of 2.7 fits the points. Using another preamplifier system the variation of s~ with bias voltage was measured using an 24~Am source. The detector signal amplitude (fig. 15) had not

470

J.E. EBERHARDT

et al.

where AEfwhm is the full width at half maximum for a spectral line of energy E. For Si and Ge the reported ~' 4 6 values of F have decreased markedly as the quality of detectors has been improved. i--Spectra for Fano factor estimation were obtained 4.5 Ig ~E with an opto-electronic preamplifier system 36) (fig. 16). The fet was not removed from its header but was ~ 4"4 operated with the top of its case lapped off to permit optical coupling to the dice. Since the detector was biased via its substrate contact and the signal taken ~-- 4"3 from the surface barrier contact, the detector leakage current flowed towards the let gate. The leakage of the 4"2 l I I I I I -1 detector, together with the light induced leakage of the 0 50 IOO 150 200 250 300 fet was extracted by a reverse biased hot-carrier diode REVERSE BIAS (V) which was at the temperature of the cold finger. FeedFig. 15. Plot of relative pulse a m p l i t u d e for a-particles vs bias back and test signals were coupled through 0.1 pF for a G a A s detector. capacitors with boron nitride dielectric. The preamsaturated at 225 V bias. Near 150 V bias in the 290 K plifier output pulses were shaped by a 20 /~s active curve there was an anomaly associated with the deple- filter amplifier. The detector bias was critical, the optimum resolution of the ESI region. The accuracy of the system was checked by meation occurring around 100 V for detector no. 5. At suring e, with a silicon surface barrier detector. The lower bias voltages the apparent energy per electronresult obtained was e= = 3.76 eV at 86 K in agreement hole pair (attenuation) increased as did the detector with Pehl et al.3S). Any charge loss will increase the capacitance. At higher bias voltage the depletion layer apparent e~ and our agreement with PehI's value for Si approached the ESI and secondary peaks were seen. may have been fortuitous since our Si and GaAs de- This effect appears to be caused by a variation in the tectors did not show bias saturation. There could have signal attenuation factor across the detector as the been a systematic error in our e~ measurements for depletion layer unevenly reaches the ESI. When still GaAs which made our value too high. Material with higher bias is employed, the multiple peaks merge and less interface attenuation is necessary to obtain a more the line width improves. The apparent variation ofe with bias and temperature reliable value of ~. raised a problem in the calculation of the effective 7. Fano factor (F) Fano factor of the detector. As explained in the appenThe high resolution ;;-ray spectra reported in section dix the correct value of e to use at reduced bias voltages is 4.45 eV/pair at 130 K and not the apparent value. 5.3 made it possible to calculate an upper limit for the Fano factor 4) which is also an intrinsic material conTable 2 contains the results of twelve estimations; six consecutive runs for 241Am (59.54 keV) and six stant determining the ultimate detector resolution, for 57Co (122.05 keV). The 24lAm runs yield F < AEfwhm= 2.35 (FEE)+, (1) 0.18 + 0.042, while the earlier results for SVCo with a different amplifier system have a mean of 0.246__+0.052. + Bios, The logarithmic plot of the 59.54 keV line in fig. 17 Detector ~ q shows a low energy tail which contributes significantly Detector ,- "1 Leakaqe [-.....~,;~ ...... [ to peak broadening. This may be due to carrier FEZ I trapping, incomplete photo-electron absorption, or a Le~Qkoge [ . 3 ~ Test -- Out combination of these effects. Improvements in GaAs ,c0 ( / detectors may well show F < 0.1. pF ILe°k°qe_J_ [ " / The 5VCo results were obtained with a 5 x 109 (~ m -1-0"11 / Hot corrier~ \ | p F ~ feedback resistor instead of opto-electronic feedback, diode '-" | FeedbQck (Cooled) 10 l(s active shaping was used and the system noise was - Bias higher. When opto-electronic feedback was used with a hot-carrier diode current return, the input leakage Fig. 16. Block d i a g r a m of the opto-electronic preamplifier system currents were very closely balanced and some difficulty used for determination of F a n o factor.

o.lll

EPITAXIAL

n-GaAs FOR N U C L E A R R A D I A T I O N D E T E C T I O N

TABLE 2 Fano factor results. Temp. (K)

fwhm (eV) Line Pulser

the associated double peaking, which is the main problem in the use of all the epitaxial detectors described, cannot be explained by the presence of trapping. In section 8.1 it is shown, using experimental results, that trapping sufficient to account for variations in the alpha pulse spectrum is incomptabile with the high resolution of the 7-ray spectra obtained with the same detector. In section 8.2 a model of ES[ capacitative attenuation is developed which fits the experimental facts. ESI effects were not thoroughly investigated since epitaxial GaAs without a significant ESI has been produced recently 19'2s) and should be used in future detectors. The implications of excess carrier lifetime in GaAs on its operation in nuclear-radiation detectors are discussed in section 8.3. Possible future developments in materials and applications are covered in the last two sections.

F

a. e~Am, 59.54 keV, 100 V bias, optoelectronic preamplifier, counting interval

8min 116 134 122 116 122 130

736 753 680 770 642 731

493 485 475 545 488 488

0.198 0.225 0.138 0.198 0.116 0.20

b. sTCo, 122.05 keV, 100 V bias, counting interval 8 min 140 140 140 140 129 132

1070 1105 1150 1215 1270 1178

770 775 790 840 764 726

0.180 0.215 0.226 0.250 0.327 0.278

8.1. TRAPPING EFFECTS ON RESOLUTION

was experienced with the let operating-point stability and hence gain stability. A digital gain stabilizer was used on the pulser peak for the 241Am spectra and the counting time for each run was restricted to eight minutes. The liquid nitrogen had to be removed from the cold finger during counting runs to prevent the spectra being degraded by considerable piezoelectric noise• 8. Discussion

The variation in pulse height with bias voltage and

Three gross approximations are made in this estimation: the ~ particle range is zero, the electric field is constant across the depletion layer and the electron and hole trapping lengths are equal. Although these approximations improve the expected resolution it is still much greater than the experimental results. The mean range of 5 MeV alpha particles in gallium arsenide is about 20/~m. The detector thickness is up to 60 /~m. However, as a first approximation let the alpha particles all interact at the surface of the detector. The holes will not be trapped and the electrons will have to traverse the depleted thickness d with a probability e-d/;'- of reaching the anode ().,, is the electron trapping length). The charge collection efficiency (r/) is given by

,1 = (;,,/,O(1 -e-~/~"). e4'Am

"6

5954

TEMP.

: 122°K

BIAS

= IOOV

• ,••

• •

•

.

~

-

O 642

z

10 '¸

•

•

°

•

o

IO 0

I

I

I

I0

20

30

471

~. 40

CHANNEL

Fig. 17. Full-energy peak response to '-'HAm source 59.54 keV ;,-rays from a G a A s detector no. 5 (energies in keV).

(2)

Now suppose the variation in apparent e is caused by trapping. At 132 K this variation is from 4.98 at 10O V to 4.45 at 200 V bias. The charge collection efficiency is then (4.45/4.98) or 0.9. Substituting r/= 0.9 in (2) leads to d/2,, ~ 0.32 for this model• Now consider the effect of irradiating the detector with 59•54 keV 7-rays. Since the depleted thickness is small, the interactions will occur approximately uniformly over the active volume. This is the case described by Day et al. s7) when the best resolution occurred with equal 2,/d and 2p/d. The assumption that all2,, = d/2p = 0.32 leads to an expected percentage resolution (100 o/J/) of about 3 percent, i.e. an fwhm of about 7 percent or 4.2 keV

472

J.E. EBERHARDT Ci

Fig. 18. Equivalent circuit model for a surface-barrier, epitaxial GaAs detector with an abnormal ESI layer.

for the 59.54 keV 24'Am 7-ray. The observed resolution was < 700 eV, and is therefore not compatible with carrier trapping of this order. 8.2. THE INTERFACE ATTENUATION MODEL

The equivalent circuit of a detector containing a high resistivity ESI may be approximated by that shown in fig. 18 where, for bias values less than that required for full depletion, the detector is effectively coupled to the preamplifier input via the capacitative component, C~ of the interface boundary. The fraction of the charge generated in the detector which is transferred to the preamplifier input is given by Ci(Cd+Ci) -1, if R~ is large. Since Ca varies as the reciprocal of the half power of bias, then for small values of bias (Cd >> Ci) the fraction of charge transferred is given by the approximate ratio CJCd, increasing towards unity with bias when Cd ~ C~. Eventually, at the highest bias levels, the depletion layer will rapidly incorporate the high resistivity ES[ to give complete charge transfer. This behaviour is exhibited by the e-particle pulse height versus bias characteristics shown in fig. 15. An estimate of the thickness of the ES[ region can be obtained from the relation given above if the relative Au- CONTACT /

1 UNDEPLETED:i: i::):: iLi ::i !.!;i~::.:~;::i, ~; i>::.::J I i::.: ::i:: :):.'::i :: :i / °+'- ~S6CS~.'AiE/////

L-LAYER

(A) Au- CONTACT7 ¢

UNDEPLETED (~//i

! l t| t

~JS-Y.iS, T.RA.TE////~/

(~ 60-80~m)

(B) Fig. 19. Schematicof two depletion modes in a surface-barrier, epitaxial OaAs detector, containing an abnormal ESI layer of varying thickness, (A) low bias, (B) high bias.

et al.

value of pulse height and capacitance of the depleted region in the epitaxial layer ("normal material") is known for a particular bias. For GaAs detector no. 5, this thickness is estimated to be 15/xm. If the ES[ region varies in thickness across the substrate (fig. 19), complete charge transfer will occur first near the region of maximum thickness (fig. 19A), with incomplete transfer for the remainder of the detector giving a double peaking in pulse height, as has been observed in detectors near full depletion. If the spreading resistance of the undepleted layer of "normal" epitaxial material is high (thin layer) then a spread in pulse heights can be expected between the limits for full depletion and incomplete depletion. This type of spectral shape is observed for both e-particles (fig. 5) and 7-rays (fig. 20). The presence of the ESI would explain the rapid decrease in detector capacitance near full depletion Io

i

r

I

N ' o 8~x

4 I-

I

°:U Z00

b

i

i

(?3'NpX-RAYS)

J /

I~

i

Y5054

GaAs ~ 5

g~ 6F Np'L~'76~I[ I ~

~'l

241Arn SOURCE

TEMP=130°K BIAS=120V

~

~ II EXTRA

Jl

-

PULS ~+ I0

II

PEAK~ I ~490

%

300

,

700

800

L

1000

CHANNEL NO.

Fig, 20. Response of a surface-barrier, cpitaxial GaAs detector operating at near full depletion and showing multiple peaking of '-'41Am source ;,-rays (energies in keV).

bias. The observed change in the form of the capacitance versus bias characteristic and reduction in measured depletion capacitance seen at 77 K is possibly due to deep level impurities (donors) freezing out at low temperatures, both in the epitaxial and substrate layers. In addition, optimum energy resolution was found to be above 77 K ( ~ 125 K) indicating some degree of trapping at lowest temperatures. The effect of freeze-out may be to change the effective values of the detector equivalent circuit capacitances, which in turn change the shape of the pulse height versus bias characteristic as seen for e-particles in fig. 15. The very rapid rise seen in detector leakage currents beyond depletion bias may have its origin in the nature of the interface boundary. Schiller 23) using X-ray topographical techniques has recently observed interface regions with high dislocation densities. These may be

EPITAXIAL

n-GaAs

FOR N U C L E A R R A D I A T I O N D E T E C T I O N

the source of the excess current. Alternatively, inclusions of gallium have been found to occur at the interface 38) and could be considered as a source of injection current. It is obvious that development of thicker, higher purity LPE layers of GaAs for radiation detectors will require a detailed examination of the origin and nature of interfacial boundaries and layers. 8.3. CARRIER LIFETIME In section 2 it was pointed out that electron mobility at low temperatures is a good index of the total (electrically active) impurity concentration and of stoichiometry and thus of the suitability of a GaAs crystal for use in a high resolution detector. With high purity Si and Ge the minority carrier lifetime (r) in neutral regions has been a common index of crystal quality, and values as high as 10-3s have been measured. The corresponding values measured in GaAs and CdTe (n > 1015 cm -3) have so far been near 10-7s. These low values are consistent with z ~ rd~, the lifetime due to direct radiative recombination, which has a high probability Bo~ in direct band gap materials. This consideration has been one reason given for not expecting high resolution from GaAs detectors. However, Weisberg and Goldstein's ag) examination of recombination theory has shown that this expectation was not justified, at least for ~-rays. Firstly the theory presented by Hall 4°) shows that at 300 K in high purity GaAs (n < 10 l~ cm-3), rd~ should increase to near 10-%, (not yet observed experimentally), since Bar -- 1.7 x 10- ~0 cm 3 s - i , 2"dr ~

1/Bd~n.

(3)

Secondly the applied field keeps the steady state carrier density very low in the detector depletion region so that the effective value of r is much larger than that applying to a neutral region. In addition the carrier transit time ( ~ 10 ns per ram) is generally much smaller than the effective lifetime. These considerations show that direct radiative recombination should not have a significant effect on charge collection efficiency, so long as the local charge density generated by the nuclear radiation remains small and the holes and electrons are rapidly ( ~ 10 -1 ~s) separated by the field. These conditions certainly hold for X-rays, ~-rays and conversion electrons and so are consistent with the experimentally observed high resolution reported in section 5. With the more heavily ionizing ~-particles and fission fragments, the local charge densities (n~,nr) are high and fast initial separation of the electrons and holes

473

by the applied field is immediately limited by the local plasma space charge field41). Charge separation then proceeds at a reduced rate as a result of plasma erosion by both carrier diffusion and the action of the field on the surface regions of the plasma, so that after a period /'p the plasma is dispersed and carriers again move rapidly under the influence of the applied fielda). The experimental observations and solid state theory do not lead to firm conclusions about carrier recombination under plasma conditions and so some speculation becomes necessary. It might at first be thought that considerable direct radiative recombination would occur in the plasma interval and that this would lead to loss of charge. Such recombination will occur if tp is of the same order as rdr- However, it is difficult either to measure or to calculate t o, particularly for the practical case of the plasma track parallel to the applied field (ref. 41, fig. 2F). The calculation of tp and rdr is also limited by uncertainty as to the initial values of the track radius and n~ or nf. The calculations of Tove and Seibt 41) for the simple cylindrical model in an indirect band gap semiconductor show that ifn~ = 1016 cm -3 then tp falls below 10-1% at the practical field values above 104 V cm -1 . This is almost four orders of magnitude less than "g'dras calculated from eq. (3) so recombination would be negligible. At the other extreme of putting nf = 1023 cm -3, tp would be many orders of magnitude larger than rat which would then take on the lower limiting value of the spontaneous lifetime ( ~ 10-1°s) which applies when both the conduction and valence bands are degenerate (n,p > 1019 cm-3)42). In this case direct recombination would in fact become a dominant process in the plasma and must be taken into account in the calculation of tp for a direct band gap semiconductor. Dumke 43) has pointed out that radiative recombination does not in fact limit the excess carrier lifetime in direct band gap semiconductors since these have a high absorption coefficient K ~ 103 cm -~ for the resulting recombination radiation34). The recombination of an electron-hole pair in the plasma is balanced by the absorption of the resulting photon outside the plasma with the generation of another electron-hole pair. The photon energy is not degraded, except by free carrier absorption 44) which has a low probability, and so the net effect of radiative recombination is simply to add a third process of plasma erosion. I f this argument is accepted then it follows that in direct band gap semiconductors an upper limit of the order of 10-1% is set to t o. This indicates that GaAs may have an advantage over silicon as a fission fragment detector in experi-

474

J.E. EBERHARDT et al.

ments where fast timing is required. In the above discussion the usual assumption was made that the carriers generated by the nuclear radiation are rapidly(10-12s) thermalised and in GaAs are located at the zone centre, so normal direct radiative recombination could occur without phonon emission or absorption. In fact the energy m o m e n t u m distribution of carriers below the ionization threshold may extend over wide regions of the Brillouin zone, including the upper conduction band ( 1 0 0 ) valleys. The high field ( > 104 V cm -1) in the depletion region could also excite electrons into these valleys. Such a carrier energy m o m e n t u m distribution would make the probability of direct recombination negligibly small and so give a long excess carrier lifetime. As Zulliger and Aitken 45) have pointed out for Ge it would also have implications for the lower limit to Fano factor. The general conclusions from this discussion are that the intrinsic charge collection efficiency in direct band gap semiconductors is effectively 100 percent for lightly ionizing particles and probably very close to 100 percent even for heavily ionizing particles. These conclusions are consistent with the experimental observations reported in section 5. 8.4. MATERIALS DEVELOPMENT USING EPITAXIAL METHODS

If GaAs is to be fully exploited in the nuclear field, large volume detectors must be developed. The necessary increases in area, thickness and purity present a formidable challenge to the crystal grower. While the average thickness of GaAs layers grown by LPE is less than 100 pm, samples up to 4 0 0 p m thick have been produced38). Growth can be very rapid and in principle it should be possible to grow single layers up to 1 m m thick. Greater thicknesses could be obtained by multiple LPE cycles, but it would be necessary to prevent the formation of high resistivity interfaces between the epitaxial layers. The travelling heater method (THM)~5), which has much in common with LPE, may also be as suitable for the growth of large volume high purity GaAs as it is for CdTe. Unfortunately no measurements of Pn for CdTe grown by this method have been reported. The low carrier concentrations make Hall measurements difficultY5). It is possible that the T H M CdTe suffers from an anomalously low /t, which can be attributed to inhomogeneities in the form of impurity clusters46-av). If GaAs is grown by T H M it should be checked for this type of inhomogeneity by plotting p, vs T. Its presence is certain to reduce the charge

collection efficiency. The results for GaAs that we have presented give encouragement for the study of other compound semiconductors the most important being CdTe. They also suggest that the intrinsic characteristics of a material are best determined by the study of thin detectors. For example, CdTe is not readily grown by LPE or VPE, but thin layers may be prepared from larger crystals. It would be of interest to compare the resolution of thin detectors made from the T H M and VPD materials with that reported by Arkad'eva et al. 12) for CdTe prepared by vertical zone melting. The spectra obtained with thicker detectors show reduced charge collection efficiency (lower resolution) which is attributed to trapping13-15). The accurate measurement of E vs temperature, of the Fano factor, and of the effects of carrier lifetimes in a larger number of semiconductors should contribute to the better understanding of detector processes. 8.5. APPLICATIONS Two obvious fields of application of GaAs detectors are medical and nuclear physics. Because the presently available GaAs is thin it is likely that its initial application will be in the medical field48). For in vivo use with beta emitters like 32p, a GaAs detector would offer a much lower leakage current than a Si detector of the same volume, and a greater efficiency. Small particles of 239pu in wounds49) could be located using a small probe detector which could utilise the 17 keV X-ray emission of 239pu with the detector at body temperature. The photon absorption efficiency of a very small GaAs detector would, of course, be much greater than that of a similar Si detector. For external use GaAs detectors could detect abnormal iodine uptake by the thyroid glands using X-ray fluorescence of 125[ emissionsSO). Larger area detectors could be used in rectilinear scanners or Anger cameras with 99mTm (140 keV y-rays) for example. The improved energy resolution compared with NaI(TI) scintillators would partly compensate for the reduced volume and would allow discrimination against 7-rays scattered in the patient or collimator. Ge(Li) detectors have been evaluated in these roles s°) but the cooling requirements are cumbersome. A presently realisable application in nuclear physics would be alpha detection at elevated temperatures. The mass of the cooling and vacuum systems associated with Ge(Li) detectors often introduces unwanted (n,y) reactions. Large GaAs detectors could be operated in low mass, light-tight enclosures. Aerospace applications include y-ray fuel gauging using 85Kr in aircraft

EPITAXIAL

n-GaAs FOR N U C L E A R R A D I A T I O N D E T E C T I O N

a n d space vehicles, a role for which CdTe detectors are being developed ~a - ~.~), a n d 7-ray spectrometry. H e w k a et al. 5~) have d e v e l o p e d silicon amplifying detectors for medical uses with beta particles and low energy p h o t o n s . A m p l i f y i n g detectors have, at present, a m u c h lower threshold energy and m a y be considered c o m p l e m e n t a r y to the G a A s detectors used here, but the amplifying devices require ~ 1000 V bias for o p e r a t i o n while the present G a A s detectors are o p e r a b l e at 24 V bias ( ~ 4.3 keV fwhm, Z4tAm 59.54 keV, r o o m temperature). The lower bias requirement means that thinner catheters and less insulation m a y be used for in vivo probes52).

475

with e - - 4 . 4 5 e V / p a i r to determine the effective F a n o factor. It might be t h o u g h t that e = 4.98 e V / p a i r should be used, but this w o u l d not be correct because this value is not referred to the system input and also does not allow for the F a n o factor being related to the variance in the mean n u m b e r o f pairs p r o d u c e d in the detector. It is thus i n d e p e n d e n t o f any noiseless a t t e n u a t i o n in the signal path. In a previous letter ~) e = 4.98 eV/pair was used but this was not a correct i n t e r p r e t a t i o n o f the results.

References x) j. E. Eberhardt, R. D. Ryan and A. J. Tavendale, Appl. Phys.

This w o r k w o u l d not have been possible but for the generosity o f Dr. H. G. B. Hicks o f S t a n d a r d Telec o m m u n i c a t i o n L a b o r a t o r i e s , H a r l o w in supplying the G a A s . The a u t h o r s wish to t h a n k Dr. J. K. Parry and Dr. E. M. L a w s o n for their interest, s u p p o r t and critical reading o f the manuscript.

Appendix SIGNAL ATTENUATION AND FANO FACTOR CALCULATION

The F a n o factor o f a detector m a y be calculated from eq. (I). N o w E is defined by e = E / J (eV/pair), with E the i n p u t energy and J the m e a n n u m b e r o f pairs p r o d u c e d . A s indicated in sections 6 and 8.1 there is evidently some signal a t t e n u a t i o n whose m a g n i t u d e is the ratio o f the a p p a r e n t e's at high bias to that at w o r k i n g bias, 0.9 at 100 V bias (130 K). The pulse g e n e r a t o r is normalised to the o u t p u t pulses o f the detector a n d so its energy scale will be set low by a factor o f 0.9. This is equivalent to connecting the pulser input a h e a d o f the a t t e n u a t i o n a n d setting it up a s s u m i n g an e of 4.45 eV/pair. The detector pulse and the test pulse are thus both referred to a p o i n t ahead o f the a t t e n u a t o r if specified in terms o f energy. The preamplifier noise m a y be referred to the i n p u t o f the system using m u l t i p l i c a t i o n by 1/0.9. This is taken care o f by using the observed pulser width in the eV fwhm f r o m the s p e c t r u m concerned. The c a l i b r a t i o n o f this spectrum is o f course in terms o f the energies o f a pair o f spectral lines a n d is in no way influenced by the signal attenuation. If the a t t e n u a tion is a s s u m e d to be noiseless then b o t h the d e t e c t o r a n d the test pulses have experienced the same processing. Thus the correct m e t h o d o f d e t e r m i n i n g AEfwhm is to p e r f o r m a simple q u a d r a t i c s u b t r a c t i o n o f the spectral and pulser line widths in eV f w h m as observed directly in the spectra. (It is true o f course that the preamplifier resolution is really better by a factor o f 0.9 than is observed.) This AEfwhm can then be used

Letters 17 (1970) 427. 2) A. J. Tavendale, Ann. Rev. Nucl. Sci. 17 (1967) 73. a) G. Dearnaley and D. C. Northrop, Semiconductor counters fi)r nuclear radiations (E. and F. N. Spon, London, I st ed. 1963, 2nd ed. 1966). 1) G. Bertolini and A. Coche, Semiconductor detectors (NorthHolland PuN. Co., Amsterdam, 1968) p. 445. ~) S. M. Sze, Physics of semiconductor devices (Wiley, New York, 1969). G) D. Diguet, Solid-State Electron. 13 (1970) 37. 7) R. C. Clarke, B. D. Joyce and W. H. E. Wilgoss, Solid State Comm. 8 (1970) 1125. ~) W. R. Harding, C. Hilsum, M. E. Moncaster, D. C. Northrop and O. Simpson, Nature 187 (1960) 405. ~J) J. W. Mayer, Nucleonics 20 (1962) 60. 10) T. Kobayashi and S. Takayanagi, Nucl. Instr. and Meth. 44 (1966") 145. ~l) W. Akutagawa, K. Zanio and J. W. Mayer, Nucl. Instr. and Meth. 55 (1967) 383. 12) E. N. Arkad'eva, L. V. Maslova, O. A. Matveev, S. M. Ryvkin and Y. V. Rud, IEEE Trans. Nucl. Sci. NS-15, no. 3 (19681 258. aa") K. Zanio, J. Neeland and H. Montano, IEEE Trans. Nucl. Sci. NS-17, no. 3 (19701 287. 14) K. Zanio, Nucl. Instr. and Meth. 83 (1970) 288. 1~) R. O. Bell, N. Hemmat and F. Wald, IEEE Trans. Nucl. Sci. NS-I7, no. 3 (19701 241. ~6) H. G. B. Hicks and D. F. Manley, Solid State Comm. 7 (19691 1463. 17) C. M. Wolfe and G. E. Stillman, 3rd Int. Syrup. GaAs (Aachen, 1970). is) C. H. Gooch (ed.), Gallium arsenide lasers (Wiley, New York, 1969) ch. 4. 1~) H. G. B. Hicks, private communications. e~J)J. A. Copeland, Trans. IEEE Electron Devices ED-16 (1969) 449. el) J. S. Harris, Y. Nannichi and G. L. Pearson, J. Appl. Phys. 40 (19691 4575. ee) E. Munoz, W. L. Snyder and J. L. Moll, Appl. Phys. Letters 16 (1970) 262. e:31C. Schiller, Solid-State Electron. 13 (19701 1163. e~) T. J. Reid and L. B. Robinson, Proc. 2nd Int. Syrup. GaAs, (Dallas, Oct. 19681 p. 59. e.~) T. G. Blocker, R. H. Cox and T. E. Hasty, Solid-State Comm. 8 (1970) 1313.

476

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