Indian Journal of Pure & Appl ied Physics Vol. 1,7. Octoher I'YN. pp. 774-777

Evaluation of indium phosphide surface barrier detector uSIng gamma rays

P Jayavel & J Kumar

Crystal Growth Centre. Anna University. Chennai 600025

and

5 Ghosh. 0 K Avasthi , A Jhingan. P Sugathan & K Asokan

Nucl e~lr Sc ience Centre. Aruna AS ~lf Ali M'l rg. P B IOS02. New Delhi 67

Received 2R July 1999 : accepted 10 Septemher 1999

Liquid EIlC~lpsulated Cznchralski (LEC) grown Fe doped indium phosphide (IIlP) surface harrier detector of thickness 1, SO

Pill has been f,lhricated allli elect ri ca l characteri sti cs have been analysed. T he detector has heen eva luated at room temperature llsing 57Co ( 122 keY) and 1:

17Cs ((,62 keY) gamma sources and obtained energy resolution of20 keV and 11 2 keY at FWHM.

1 Intmduction In the past few years, there has been a grow in g

interest in the development of compou nd semi conductor based surface barri er detectors mainl y GaAs and Inpl. Ea rl y bulk devices suffe red from poor charge co llect ion and burst noise, and whil e later dev ices fabricated usin g I iqu id or chemi ca l phase epi tax ial tec hn iques proved to be good detectors. I n most cases the detection efficiency was poor due to the difficulties involved in producing thi ck layers with low doping concentrati ons. Presently , room temperature radi at ion detectors are being fabri­cated on Liquid Encapsul ated Czochralsk i (LEC) grown undoped semi-insul ati ng (ST) GaAs and semi-insu lating (h on doped) fnP si ngle crysta ls I., . InP has undergone an ex tensive researc h as a material for YLSf app li cations a .~ we ll as the base substrate for severa l exotic electronic dev ices. In P is more radiation res istant than GaAs and Si C. It is promising candidate for space applications es pec iall y solar ce ll s. The performances of such devices arc strongl y influenced by t he presence of nat i ve defects. Many locali zed defect leve ls are introd uced from inten­ti onal eloping in addition to the non-in tenti onal impuri ­ti es or lattice defects . However, defect centres are usuall y present in the compo und semiconductor whi ch reduce the charge co ll ec ti on effici ency (CCE) of the detectors. It is possible to alter the distribution. density and occupancy of traps by va ryi ng the growth condi t ions and post- growth annealing of the crystal. It is eas ier to obta in low dislocation density crysta ls wit h low leve l impurit y doping . SI- In P crysta ls are realized by doping

with iron, which forms a deep acceptor~ . The deep level defect EL2 has ionizati on energy near the middle of the bandgap and it pins the fermi level near the mid-gap resulting in hi gh res ist ivity.

InP is a direct band-gap se miconductor with a band gap of 1.35 e V. It can be used as a detector at room temperature. Olschner e f 01.' have reported that iron doping, in principle, can cause the churg carri er trap­ping in InP:Fe. Increas ing Fe concentration res ults in

substantially lower electron lifeti me (~'U. Schottky contact radi ation detectors from bulk InP have been suggested as low energy ga mma rays and neutri no de­tectors5J

,. Due to the radiati on hardness. the SI-InP sur­face barrier detector may be suitabl e candidate for the next generati on of experiments in hi gh energy phys ics . We have investigated on the perfo rmance of SJ-InP detector with low energy gamma rays at room tempera­ture.

2. Expel"imental Details

2.1 Growth of SI-lnP single crystal

InP si ngle crystal used in this study was grown at the Crystal Growth Centre, Chennai using a high-pressure puller (Cl3S I) and empl oy ing the Liquid Encapsu lated Czoc hralski (LEC) tech nique. The relevant and opti­mized growth parameters are given in Table I. Polycrys­talline InP with the ca lcul ated amount of iron has been as the starting material for the growth of Sf_Tnp7X . The S I-I nP single crysta l is al so achieved with proper control of sto ichi ometry of the starting material and with the use

l'

JA YA VEL el ai.: SURFACE BARRIER DETECTOR 775

of pBN crucibles which eliminates introduction of cru­cibl e related impuritiesx

.

2.2 Oetector fah"i catiol1

The grown bu lk crys tals were cut in to wafers of 500

pm thick ness. The wafers were then lapped and polished through chemo-mec hanica l process using bromine­

methanol solution to a thickness of 350 I-1m . The pol­

ished substrate has been cleaved into 10 mm x 10 mm size samples. The samples were cleaned using warm acetone, tricholoroethylene and methanol. Au/Ge/Ni al­loyed ohmic contact of th ickness 2000A was realized 0 11

the back side of the sample and annea led at 723 K under argon (hi gh purity) atmosphere for 5 minutes. Au Schot­tky contact of area 19.625 mm} and thickness IsonA was deposited on the polished side of the sample. Before the meta lli zation process, the samples were etched in HF:H}Q in the ratio of I: 1 for 60 s to remove any native ox ide present on the surface. The metal I ization process has been carried out usi ng an e-beam evaporation system

under a vacuulll of 2 x 10-(' torr. The schematic diagram of the detector is shown in Fig. I.

Hall measurements were performed at room tempera­ture using a Van der Pauw confi guration . The charac­teri stic parameters of the LEC grown fnP single crystal are given in Table 2. Current - voltage (I-v) measure­ments ha ve been ca rried out usi ng an automated 617 Keithl ey e lectrometer. Capacitance - vo ltage (C-V) measurements ha ve been carried out at I MHz frequency on the fabricated detector at room temperature using an automated Boonton 7200 capacitance meter. The tes ting of the detector usin g gamma ray sources of 'i

7Co and J:\7Cs has been carried out at Nuclear Science Centre, New Delhi .

Schottky Contact Au

.:- '. M."

350 l-lm

Fe doped InP

Au/GelNi Ohmic Contact

Fi g. I - Schematic diagram orthe SI- lnP detector

Tab le I - Opti mized parameters for the growth of InP single crystals

Growth parameters Range of variation Optimised

Weight or polycrys- 1200-75()gm talline InP

Seeel orientation < IO(b .. < 111>

Seed rotation rate 4-10 rpm

Crucibl e rotation 4- 15 rpm rate

8 20 , water content 100-300 ppm

8 20, thi ckness 10-20 mm

Argon counter pres- 40-60 atm su re

Pull ing rate 4-10 mm/hr

X()() gill

< III >

8 rpm

l) rpm

100 ppm

12 Illm

40 atm

7mm/h r

Table 2 - Characteristics 0'· the LEC grown SI-ln P crystal s

Parameter

Diameter (mm)

Orientation

Dopant

Wafer thickness (~m)

Resi sti vity (n-cm)

Mobility (cm2 V.sec)

Carrier Concentration (Clll- ')

Results and Discussion

Vallie

50.X

100

Fe

350

2.06 x 107

2XOO

3.1 Bias characteristics of the detector

The characterist ics of a detector are given by ther-

mionic emission equation

i=/, l exp(~ l- 1 11 k/l T

... ( 1 )

, ( - q<p 1 J, =A ** T -exp k/J; ... (2)

<p =~ In k T (A **T } 1 Ii q / ,

... (3)

where A ** is the effective Richard son constant

(A cm - 2K-2), T is the absolute temperature (K), 11 is the

ideal ity factor, kJl is the So ltzman constant (J K- I), <P 1l

is the barrier height (eV), i s is the saturalion cu rrenl

density (A cm- 2) , V is the app lied vo ltage and i is the

current density (A clll-2) .

776 INDIAN J PURE APPL PHYS, VOL 37, OCTOBER 1 999

Fig. 2 shows the room temperature /- V characteristics

of 350 jlm SI-InP radiation detector and the evaluated

barrier height (<PH) and saturation current density (j,) of

the detector are 0.84 eY and 5 .24 x I cr') Ncm2

respec­tively . The evaluated ideal ity factor (n) value of the detector being greater than one which indicates that there is a contribution of the recombinat ion current, although the thermionic process is the main transport mechanism. The reverse leakage current of the detector is the most sens it i ve parameter of the incident radiation

and it is found to be 2.7 jlA. The h igh leakage current i s due to the presence of h igh density of trapping centres . The measured capacitance of the detector is 1 3 . 5 pF. It is observed that there is no change in the capacitance with increasing bias. This shows that the detector i s independent of the appl ied bias, indicati ng that the de­tector is fu l ly depleted. The /- V characteristics of th is detector are consi stent with the results reported in the l i terature2 .

3.2 Gamma ray detection

Radiation detectors are operated under reverse bias si nce electron-hole pairs are created in the active region when these are absorbed by the radiation quanta. In the present invest igation, SJ-InP detector has been tested using,7 Co and I:I7Cs gamma source of energy 1 22 and 662 keY respectively at room temperature. Pulse height spectra were collected using a charge sensit ive pre-am­pl ifier (ORTEC 1 42 A) fol lowed by a shaper ampl ifier (ORTEC 57 1 ), and an analog to d igital converter (ADC) coupled to computer. Fig. 3 shows the pu lse height spectra of the S J-JnP particle detector irradiated using S7CO ( 1 22 keY) and I :17Cs (662 keY) gamma rays on the Schottkv contact bv aoolvin!! a oosit ive volta!!e

.... c:: (U I... L. :::> u

-6 10

-7 10

T • 300 K -<>-350 f'm

Voltage (V) Pig. 2 - Current-vol tage characteristics of the InP detector

to the ohmic contact. The energy resol ution of 20 and 1 1 2 keY at FWHM have been obtained respecti vely for S7CO and I 37CS sources . The primary source for the relat ively lower energy resolution in InP may be due to the incomplete charge col lection, trapping, presence of i mperfections originating from crystal growth process and intentional ly introduced impurities. The perform­ance of the Schottky contact semiconductor radiation detectors in h ighly sensit ive to charged i mpurities and defects in the semiconductor material8 . The lower en­ergy resol ution is also dependent upon the non-equi l ib­r ium carrier l i fet ime of the detector. The energy resolution could thus be improved by i ncreasing the

mobi lity (jl) or the mean trapping t ime (1). Thi s would increase the mean d istance traveled by the charge so that, ideal ly, both the electrons and the holes cou ld cross the fu l l thickness of the detector. Detectors based on SI-InP wafers suffer from losing some of the charge signal generated by the incident radiation due to carrier trapping by intrinsic deep level defects.

Lund et af. (, have reported that CCE depends upon the Fe dopant concentration. We have also observed the CCE degrades with i ncreasing the bias of the SI-InP detector and the concentration of doo in!! iron. The Fe

r: OJ o u

(a )

2 25 350 �25

( b l

-OJ

III c: OJ 0 U

300

SOO 575 650 725 800 825

Channel number

�oo 500 700

Channel number Fig. 3 - Pulse height spectra of the I n P surface barrier cietector , . 57 gamma rays on the schollky contact usmg: (a) ' Co ( 1 22kcV ) and (h) I :17Cs(662keV)

.".. I

JA Y A VEL et ([ /. : SURFACE BARRIER DETECTOR 777

dopant concentrat ion in this detec tor is I .OS x 10 1(, cm-]

The CCE is furth e r reduced because of the effec ts due

to incompl e te pene tration of the e lectric field at low bias

levels. The Sc hottky barri e r leakage current is found to

be responsibl e for the variation o f the e lect ri ca lly act ive

deep centres and it therefore influence the CCE.

4 Conclusion LEC grow n Fe doped TnP surface barrier de tector of

thickness 350 ~.lIn has been fabricated and eva luated

lIsing gamma sources. The e lectrical characteristics of

the detector have been discussed. The hi gh reverse leak­

age current indicates the presence of high density of

ITappi ng centres . Gamma ene rgy resolutions of 20 ke V

for :i7Co and I 12 ke V for I:I7Cs at FWHM have been

obtained . The re lati ve ly poor energy resolution of the

detector may be due to the hi gh reverse leakage current

and incomplete charge co ll ection of the detector. At­

tempts are be ing made to improve the energy resolution

of InP detector by proper selection of Fe concentration ,

nove l sy nthes is and annealing processes.

Acknowledgements Th,e authors thank AICTE for the finan c ial support.

One of the authors (Pl) gratefu lly ack now ledges the

Council of Scientific and Industri a l Research, New

Delhi, for the award of Senior Research Fell ow. The

authors also thank Dr A K Sinha, Nuclear Sc ience

Centre for his he lp and usefu l discussions.

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