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Nucler Instruments nd Methods in Physics Reserch A322 (1992) 487-492 North-Hollnd ~~1]2511'~TMP2_z"_t_f Cl~ A poumus2~ fs PMUAIIG Section A ougls S.

1 Nucler Instruments nd Methods in Physics Reserch A322 (1992) North-Hollnd ~~1]2511'~TMP2_z"_t_f Cl~ A poumus2~ fs PMUAIIG Section A ougls S. s room temperture rdition detectors cgregor ', Glenn F. Knoll ', Yosef Eisen ' nd Richrd Brke " Deprtment of Nucler Engineering, The University of Michign, Ann Arbor, MI , USA Soreq Nucler Reserch Center, Isrel Atomic Energy Commission, Yrne 70600, Isrel Rdition Protection Group, Los Almos Ntionl Lbortoq, Los Almos, NM 87545, USA Bulk GAs, wide bnd gp semiconductor, shows potentil s room temperture rdition detector. Schottky diode detectors were fbricted from LEC bulk GAs crystls. The bsic construction of these diodes employed the use of Ti/Au Schottky contct nd Au/Ge/Ni lloyed ohmic contct. Pulse height chrcteristics of these diodes indicte ctive regions of more thn 109 jim. Pulse height spectr were recorded from lph prticle irrdition of the Schottky contct surfce resulting in best energy resolution of 2.5% t 5.5 MeV. Low energy gmm rys mesured under room temperture operting conditions resulted in photopeks with 37,17c FWHM t 60 kev. 1. Introduction In order for semiconductor mteril to be useful s rdition detector t room temperture, its bndgp vlue must be wide enough to inhibit excessive lekge current due to therml crrier genertion. As prcticl mtter, this requirement trnsltes into minimum bndgp energy of ev if reltively thick detectors re to be fbricted [1,2]. Gllium rsenide (GAs) is wide bndgp semiconductor (1-42 ev t 25'C) tht hs received ttention s cndidte mteril for rdition detectors [3-5]. Its bndgp is sufficiently lrge to llow for roon-. temperture opertion of bulk detectors nd its verl.gc tomic number is equivlent to tht of germnium, the most widely pplied semiconductor mteril in gmm ry spectroscopy. The investigtion of GAs s detector mteril dtes bck to the lte 1960s nd erly 1970s [3,4]. At tht time, epitxil growth ws the only mens vilble to produce high qulity GAs lyers of sufficient thickness, generlly limited to less thn 100 gm. Since then, high qulity semi-insulting GAs mteril grown by the liquid encpsulted Czochrlski (LEC) method hs become widely vilble from number of commercil suppliers. Compenstion of bckground impurities hs improved such tht quoted resistivities (> 107 fl cm) nd crrier concentrtions (< 10'/cm ; ) pproch theoreticl intrinsic vlues. Compenstion of high resistivity LEC GAs mteril is ccomplished by blnce between residul crbon shllow cceptor impurities with the ntive defect deep donor EL2 [6,7]. In this work, we report mesured results from detec- toes fbricted from commercilly vilble high purity semi-insulting LEC bulk GAs crystls. 2. Device fbriction Bulk LEC GAs wters were diced into 5.08 mm X 5.08 mm pieces nd polished to thicknesses of 45, 100 nd 250 jim. Previous results indicted the need for Surfce Pssivtion to reduce lekge Currents [81, thus ws plsm deposited onto ech piece to provide Si3N4 necessry pssivtion. Photolithogrphy ws used to define 4.32 mm dimeter circulr regions which were then used to etch through the S'3N4 to the bre GAs. A series of lternting Au, Ge, nd Ni lyers were vcuum deposited onto the GAs surfces with weight rtios of 88%, 12%, nd 5%, respectively. The contcts were nneled t 405'C to form ohmic contct to the GAs substrtes. Photolithogrphy ws then used to define nd protect res on the front surfces 4.32 mm in dimeter. The front side of the devices were ion implnted with protons t dose of 10 16/cm 2 to decrese crrier lifetimes contributing to lekge on the outer edges of the substrte. Afterwrds, Si 3N4 ws deposited on the front surfces, ptterned, nd etched s previously described. Ech piece ws bonded into BN (boron nitride) collr with high resistivity potting epoxy. These BN collrs were subsequently bonded into brss connector rings. A lyer of Ti/Au ws vcuum deposited onto the bre GAs surfces to form Schottky contct to the GAs substrtes nd electricl cont,-,-t to the front connector rings. A finl Au lyer ws (ieposited over the ohmic contct detector /92/$ ENeWer Science Publishers B.V. All rights reserved Ill. OTHER MATERIALS

D.S. 1~9Wrcy~or et l. / Bcc/k GctAs RT rdition dctoctors fces to form electricl contct to the bck connector rit'`~s. A cross section digrm of completed device cn be sun in fig. 1. ~o r6 àce le fing chrcterüst6cs The C-l :, 1-I ', pulse height, nd spectroscopic chrcteràstics `vere observed for ech device.

2 D.S. 1~9Wrcy~or et l. / Bcc/k GctAs RT rdition dctoctors fces to form electricl contct to the bck connector rit'`~s. A cross section digrm of completed device cn be sun in fig. 1. ~o r6 àce le fing chrcterüst6cs The C-l :, 1-I ', pulse height, nd spectroscopic chrcteràstics `vere observed for ech device. 1-V s ements indicted reverse bis brekdown volt- _~"= ~ in f xcess of 6i) V for 4~ p.m thick detectors, nd in excess of 1-IU ~~ for the 1E)î1 p,m thick detectors. It ws obser~ ed tht in ll cses of reverse bis, the reverse current r:ched mximum vlue nd begn to decrc:sc: vith higher ` oltges. Reverse currents from the d~ i~~ ~ng~:d in vlue from?7 to >OEI na. Tho d~uc- ~~~ dentonstrt~:d rectif~ ing beh`wior s sho`vn in C.,. FI~j~`~~v or, the idolàt` fctors of the dàodes s merured from the f9~~,~rd current o` zro high, sugg~ting tht the current `vs dug: to mechnisms other thn plzrnir~nic et~issicn. Tho turn-on voltge: for forwrd bis diodes v` ~sbs~:t~ ed to increse `~ ith incresing detec cr thickness. Initil ~-1 ' mesurements of the devices `ti er tken vi h mo-dultin, frec~uencd of 1 ~Iz resulting in no pprent chnge: in cpcitnce e~ ith incresing revers bis. t~enerll`~, the disply of constnt cpcitnce ; ith ~hn`ing bis voltgo indictes full d~:pletion. o`~ever, C, -i-' mesurements of high resistivity mterils cn render erroneous results [9]. It is believed tht the: FL2 de::p level donors in the mteril hve lifetitnc longer thn or comprble to the period of the t~c~duhtion fredu~:nc~~ nd conclusions s to the bchvic~r of the EIr? d~:~:p donors nd their effect on cpcitnce could not be mde [9,10]. Conseduently, conclusions s to the electric field distribution of these detectors could not be. drwn through simple C'-V techniques. bs~:n, tions of the pulse height dependence on bis v~olt~e `vere conducted to enhnce understnding of the device chrct ;.rtsttcs. CiAs detectors 43 wm thick nd le)> tm thick were irrdited by n -'~~Am Pottin~ EP y Au Ewportion Prof mm Fig. 1. l~ulh GAs detector device cross section. E v e~ -2 x S -1G -~ G 5 10 Applied Voltge (volts) Fig.?. 1-V chrcteristics of 43 p.m thick GAs Schottky hrrier detector. The forwrd current increses much slower thn tht expected for n idel diode. Reverse nd fonvrd currents re represented with different scles. spectroscopic lph prticle source under vcuum from both the front (Schottky) contct nd bck (ohmic) contct. The rnge of 5.; IVIeV lph prticle in GAs is pproximtely 21 p,m [I1]. Crrier genertion is gretest ner the end of the lph prticle trck length llowing for the bulk of the electron-hole pirs to be creted ner the middle of the 43 p.m thick detector when irrdited from either side. The verge pulse height produced by the lph prticles ws mesured s function of bis voltge. For both thicknesses, the pulse height ws observed to increse rpidly with incresing reverse bis when irrdited from the Schottky contct side. The pulse height sturted t mximum vlue in ll cses of front side irrdition, thus indicting extension of the detector ctive region beyond the lph prticle rnge (figs. 3 nd 4). The voltge t which sturtion occurred ws similr for ll thicknesses of detectors tested. Detector resolution improved with incresing bis voltge, indicting improved chrge collection with incresing bis. The energy resolution v,~s observed to rech sturted vlue when the pulse height curve reched its 95~Ic vlue (fig. 5). It is believed tht the improving resolution is from incresed chrge collection resultnt from the detector ctive region extending beyond the lph prticle rnge. Assuming tht 4.2 ev re necessry to crete n electron-hole pir in C~As [3,4], preliminry mesurements indicte ner complete chrge collection for the 45 p,m thick detectors s compred to the pulse height spectrum observed from silicon surfce brrier detector. The ncrn;liz~d pulse height chrcteristics of the CpAs detectors differ only slightly, indicting tht the electric field distribution ner the front contcts of both detectors re similr (fig. 6). In the hypotheticl cse in which semicon- 0 -, c. n c 3..

3 ..5. A~1cGreyor et l. / ßulk (;ft n RT rujitiun detecturr 4K9 Front/ vcuum Front) 1.81 mg/cm^2 A1 -~-Front/ 3.41 mg/cm^2 Al -o -Bck/ vcuum -o- Hcks l.bl mg/cm^z ~i ~ Bck/ 3.91 mg/cm^2 A' ~0 5~ ~~ 7 ~ App ed doltge (dolts) Fig.? l~l ;sured pulse height s function of bis voltge from 4, p.m thick bulk GAs detector. A spectroscopic grde = ~ tam lph prticle source ws used for the study. The source nd detector were in vcuum chmber nd the pulse height recorded for conditions of no ttenution, ttenution with 1.81 gm/cm -' AI, nd ttenution with 3.41 gm/cm =.~I. Pulse heights were mesured from the detectors s irrdited from the front (Schottky) contct nd gin from the bck (ohmic) contct. ducting mteril is intrinsic or well compensted, such s in zone refined or compensted CFe, the expected result would be n electric field distribution cross the device with grdul slope. 1'he electric field strength could then be estimted by simply dividing the pplied voltge by the detector thickness. In such cse, the required voltge for the thicker detector should be 2.4 times greter thn the thin detector to chieve the sme electric field distribution, nd consequently, Fig. 5. Detector resolution s function of reverse bis voltge for 105 m thick GAs detector. similr pulse height curve. i'he fct t t the pulse height obseved is nerly independent of the detector thickness suggests n electric field behvior ore typiclly found in doped semiconductor teril rther thn intrinsic or compensted mteril. Linder bck side irrdition conditions, very low pulse height ws mesured tht slowly incresed with bis voltge. V~ith further pplied voltge, threshold voltge for drmtic increse in pulse height ws observed indicting the point t which the ctive region reched the end of the lph prticle rnge (figs. 3 nd 4). In ll cses of bck side irrdition, fter the onset of pulse height increse continuum of pulse heights ppered up to mximum vlue. 1'his continuum ws FronU vcuum Bck/ vcuum Applied V Bgs (Volts) F'ig. 4. 1Vlesured pulse height s function of bis voltge from 105 Wm thick bulk GAs detector. A spectroscopic grde -41 Am lph prticle source ws used nd the source nd detector were under vcuum. The pulse height response ws recorded for conditions in which the front nd bûck of the diodes were irrdited seprtely Applied V I e lts) Fig. 6. hdormlized pulse height curves compring the output signl demonstrted by 43 p.m thick detector nd 10~ ~.m thick detector when the Schottky contct ws irrdited with lph prticles under vcuum. The similr chrcteristics indi cte similr electric field distributions ner the Schottky contct for both devices. 1HH. GTIHEIZ litiateh~iais

4 4% D.S. AléGregor et eel. / Bulk- GAs RTrdition detectors observed for short rnge of voltges, fter which poor resolution full energy pek formed. In ll cses when the source ws unttenuted in vcuum, the sturted verge pulse height voltge ws less thn the pulse height voltge observed from front side irrdition under similr conditions by pproximtely 8-101i, depending on detector thickness. It is believed tht the nture by which the ohmic contcts re mde results in incresed sttisticl fluctutions in energy deposition from bck side irrdition, however, clcultions of energy ttenution in the contct do not completell ccount for the lower sturted pulse height obser%ed. The emerld of the pulse height distribution continued to increse towrds sturted mximum vlue with incresing bis, thus indicting improvement in chrge collection. Resolution degrdtion ws observed to be norse in 100 gm thick detectors thn in 45 gm thick detectors. It is believed tht poor hole collection due to trpping in the mteril is the cuse of inferior chrge collection its compred to front side irrdition ` bleb trnsltes into pulse height reduction nd resolution degrdtion. Aluminum ttenutors were used to reduce the lph prticle energy nd the pulse response ws observed from 43 gm thick detector. The rnge of the lph prticles in GAs decreses %%ith lph energy. Thus, the %ollge required to observe the onset of the %oh--~;e plteu should decrese with ttenutor thickness %%hen the detectors re irrdited from the front, nd the required threshold voltge for pulse height increse should increse \vith utc -nutor thickness when irrdited from the bck. From fig. 3. the unttenuted lph prticle pulse height spectrum reched the plteu voltge t pproximtely 57 V in 43 Rm thick detector. With 1.81 mg/cm- Al ttenutor between the source nd detector, the plteu begins t 51 V. Similrly, the plteu voltge is 45 V when the lph prticles re ttenuted by 3.41 mg/cm- of Al. Bck side irrdition of the detector resulted in the onset of pulse height increse t 49 V for unttenuted prticles, 54 V for prticles ttenuted with 1.81 mg/cm - Al, nd 57 V with 3.41 mg/cm'- Al. Of interest is the fct tht boundries t which pulse height plteus nd thresholds occur do not mtch simplistic depiction pproximtion solutions. The dependence of ctive region width with pplied voltge displys behvior similr to tht expected from nonuniformly doped semiconductor. The reson for such behvior my be consequence of unionized EL2 donors under the condition of zero bis becoming ionized s voltge is pplied. Applied reverse voltge cn ionize EL.? donor sites until the totl concentrtion is fully ionized, which my trnslte to rpid decrese in electric field strength ner the Schottky contct. Further discussion is provided in the ppendix. Noise levels of the GAs detectors mesured t room temperture rnged from 40 to 80 kev. Front side irrdition under vcuum nd t room temperture resulted in best energy resolution of 2.5c,'c FWHM for 5.5 MeV lph prticles (fig. 7). Room temperture mesurements of 60 kev gmm rys from n -'"Am source resulted in best energy resoiution of 37i FWHM (fig. 8). Room temperture mesurements of 122 kev gmm rys from 57CO gmm ry source reveled photopek tht suffered poor resolution. Mesurements with gmm rys of higher energies resulted in the ppernce of continuum with no -observble full energy pek Energy {Mew Fig. 7. Room temperture mesurement of the differentil pulse height spectrum exhibited by 105 [Lm thick bulk GAs detector irrdited while under -vcuum with 5.5 MeV lph prticles from 2-"Am. The detector ic.,oiution ws mesured to be 2.5'_r t FWHM Energy (kev) Fig. 8. Room temperture mesurement of the differentil pulse height spectrum exhibited by 250 p,m thick bulk GAs detector irrdited with 60 k-cv P,mm rys from 24 'Am. The detector resolution ws mesured to be 22 kev (371 ) t FWHM.

D. S. McGregor et l. / Bulk GAs RT rdition detectors 491 4. Conclusion A preliminry investigtion of LEC bulk GAs hs indicted its promising use s room temperture operted rdition detector.

5 D. S. McGregor et l. / Bulk GAs RT rdition detectors Conclusion A preliminry investigtion of LEC bulk GAs hs indicted its promising use s room temperture operted rdition detector. Detectors of 100 gm thick hve demonstrted cceptble resolution for chrged prticle spectroscopy. Thicker devices would provide very ttrctive room temperture X-ry detector provided tht hole collection efficiency is improved. Mteril with lower EL2 concentrtion will be utilized in future genertion devices to reduce possible problems with deep donor ioniztion. Future Schottky bsed detectors will hve epitxil or implnted bck side surfces to reduce ohmic contct resistnce nd devices designed primrily for photon detection will incorporte highly doped p nd n regions in order to fbricte p-i-n device structures. Acknowledgements The uthors wish to express their grtitude to Dvid Wechter, Aln Gibbs, nd Dvid Brown of Los Almos Ntionl Lbortory, Victor P. Swenson nd Willim N. Gibler of Texs A& M University, nd Ronld Rojeski, Jim McCrtney, Jeff Fournier, nd Jim Kulmn of the University of Michign for technicl ssistnce in the fbriction nd testing of the forementioned devices. Proton implnttion ws performed by the Michign Ion Bem Lbortory t the University of Michign. We lso express grtitude towrds our collegues Dr. Jck Est nd Dr. Fred L. Terry, Jr. of the University of Michign for their vluble collbortion with this project. This project hs been funded in prt by Los Almos Ntionl Lbortory subcontrct 9-XG -K Appendb_ EL2 donors re typiclly present in LEC undoped GAs t n pproximte concentrtion of 10 16/cm3 nd pper in the bndgp t n energy of 0.81 ev below the conduction bnd edge [7,121. Assuming tht the Fermi level of the undoped GAs to be ner midgp, the result is incomplete ioniztion of the EL2 donors t room temperture. Using Fermi-Dirc sttistics in which the Fermi level of the undoped GAs is pproximtely ner midgp nd the EL2 energy is 0.81 ev below the conduction bnd edge [121, the totl ioniztion of EL2 sites is on the order of 10 14/cm;, which blnces well with informtion from the supplier tht the residul crbon cceptor concentrtion is on the order of 10' 4/cm3. Under the condition of pplied reverse bis, the conduction bnd is moved up t the Schottky contct nd the difference in energy between the Fermi level nd the energy of the EL2 sites chnges resulting in more ioniztion of the deep donors. From Poisson's eqution we find: ü 2V OE q é)x- where x - A1+ Eti d I - N- - nef), Nd = Nd I ti ' r t t: d E,'i ti r The difference between the Fermi energy nd the donor energy chnges with bis, nd is therefore lso function of the voltge. The solution cn be solved numericlly to give n electric field distribution tht hs two significntly different regions. From the Schottky contct to distnce in the mteril t which complete ioniztion of the EL2 sites is relized, the electric field reduces with constnt slope equivlent to qnd /E5. Beyond the point of full ioniztion, the electric field distribution decys less rpidly to zero. Electron-hole pirs creted by ionizing prticles entering the high electric field region ner the Schottky contct experience, much higher field strength thn if creted ner the ohmic contct. The result is higher collection efficiency ner the Schottky contct for holes. Since electrons rech mximum velocity in GAs ner n electric field strength of 3 x 103 V/cm, the lower field strength ner the ohmic contct poses less of problem for electron collection. The low electric field present ner the bck side contct llows for some electron collection nd my ccount for the smll pulses observed from bck side irrdition t low bis voltges. However, the very low field strength ner the ohmic contct trnsltes to very_ inefficient hole collection for e-h pirs produced in tht region. The resulting inefficient chrge collection would trnslte to poor energy resolution nd lower pulse height. As the electric field is incresed under higher voltges, crrier collection improves llowing for electrons nd holes to be collected more efficiently. eferences [11 S.P. Swierowski nd G.A. Armntrout, IEEE Trns. Nucl. Sci. NS-22 (Feb. 1975) 205. [21 Guy A. Armntrout, S.P. Swierkowski, J.W. Sherohmn nd J.H. Yee. IEEE Trns. Nucl. Sci. NS-24 (Feb. 1977) 121. [31 T. Kobyshi nd S. Tkyngi, Nucl. Insu. nd Meth ) 145. [41 J.E. Eberhrdt, R.D. Ryn nd A.J. Tvendle. Nucl. Insu. nd Meth. 94 (1971) OTHER MATERIALS

4?2 D. S. McGregor et l. / Bulk G,4s RT rdition detectors 1,51 R. Bertin. S. D'Auri, C. Del Pp, F. Fiori, B. Lisowski, [9] D. Look, Electricl Chrcteriztion of GAs Mteril V. O'She, P.G. Pelfer, K.

6 4?2 D. S. McGregor et l. / Bulk G,4s RT rdition detectors 1,51 R. Bertin. S. D'Auri, C. Del Pp, F. Fiori, B. Lisowski, [9] D. Look, Electricl Chrcteriztion of GAs Mteril V. O'She, P.G. Pelfer, K. Smith nd A. Zichichi, Nucl. nd Devices (Wiley, New York, l9h9). Insir. nd Meth. A294 (1990) 211. [10] M. Shur, GAs Devices nd Circuits (Plenum, New York, 161 G.M. Mrtin, J.P. Frges, G. Jcob nd J.P. Hllis, J. 1987). Appl. Phys. 51 (1980) [11] J.F. Ziegler nd J.P. Biersck, TRIM-90, Version ,1 R.K. Willrdson nd A.C. Beer, Semiconductors nd (1990). Semimetls, vol. 20 (Acdemic Press, Orlndo, 1984). [12] Y. Kitgwr, N. Noto, T. Tkhshi nd T. Tkenk, 181 D.S. McGregor, R.D. White nd M.H. Weichold, TEES Appi. Phys. Lett. 48 (1986) Tech. Rept. Ser. (1985).

Performance evaluation of displacement ventilation system combined with a novel evaporative cooled ceiling for a typical office in the city of Beirut

Performance evaluation of displacement ventilation system combined with a novel evaporative cooled ceiling for a typical office in the city of Beirut Avilble online t www.sciencedirect.com ScienceDirect Energy Procedi 75 (2015 ) 1728 1733 The 7 th Interntionl Conference on Applied Energy ICAE2015 Performnce evlution of displcement ventiltion system