Interest in radiation detectors has supplied some of the impetus for improving the electronic properties of CVD diamond. In the present discussion, we will restrict our attention to polycrystalhne CVD material. We will focus on the evolution of these materials over the past decade and the correlation of detector performance with other properties of the material

Kania, D.R.

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.. ’::,’ ..UCRLJC-1272SS Pt 2PREPRINTDiamond Radiation DetectorsH. CVD Diamond Developmentfor Radiation DetectorsDon R KaniaTIds paper was prepered for submittal to theProceeding of the Intemationel Summer School of Physics“Enrico Fermi”Bol~ ItalyJuly 3@Auguet ~ 1996Thiaiaapreprint of ●papar intended fiwpubkation in●journal orprocaedi~ Since ‘changer may be made before publicatio~ thk preprint m made available with theunderatandmg that it will not be cited or reproduced without the parmidon of theDISCLAIMERThis document was prepared as an account of work sponsored by an agency ofthe United States Government.  Neither the United States Government nor theUniversity of California nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute orimply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California.  The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or the University of California, and shall not be used for advertisingor product endorsement purposes.VA-PA 135-9Diamond radiation detectorsII. – CVD diamond development for radiation detectorsD. R. KANIAAdvanced Micmtechnology PmgmnL Lwnwnce Lbrnwm NationaJ LabmtoyL4krm.orq CA 9.45511. - IntroductionInterest in radiation detectors has supplied some of the impetus for improving theelectronic properties of CVD diamond. In the present d~cussion, we will restrict ourattention to polycrystalline CVD material. We will focus on the evolution of thesematerials over the past decade and the correlation of detector performance with otherproperties of the material.2. - Colkction distance measurementsTwo techniques are used to measure the detector figure-of-merit d. One istransient photoconductivity (PC) and the other is charge collection measurementsusing WSr or other penetrating, high-energy radiation. PC measurements employ asample with a gap, typically 1 mm, in a transmission line structure. The gap isilluminated by a <10 ps pulse of UV laser light with a photon energy 6.11 eV which isabove the bandgap of diamond. This radiation penetrates about 2pm into the material.The resulting current pulse is recorded on a high-speed oscilloscope. The decay of thepulse is related to the carrier lifetime, r, and the integral of the current pulse is thecollected charge, q. TMs is a surface measurement of the material characteristics,typically the applied electric field is perpendicular to the growth direction (see fig. 1).The collection distance is also measured using high-energy electrons fim a mSrradiation which emits 2.3 MeV particles or electrons from a storage ring with energiesabove 0.5 GeV. These are called minimum ionizing particles (hIIP). Thesemeasurements are referred to as charged-particle induced conductivity (CPIC) tests.This method employs the measurement of the collected charge. The energy depositionrate in the charge twack left by a MIP is well determined and can be considered to be-!,.D.R.IiMilALx’LophotonsUuuL _-charge& particle(a) cross jsection (b) d~ume (C) dPc ,.:columnar structure average over bulk surfaceFig. 1.- A cross-section of a PC and CPIC radiationdetectors on CVD diamond.uniform through the thickness of the material. The particle leaves 36 electron-holepairs per micrometer, creating a track density of 1013paim /crn3. The collectiondistance can be CXMilycalculated from this measurement. These are vertical devices.The electxic field and the particle path are parallel to the growth direction.In an isotropic sample, such as a geologic IIa diamond, the PC and CPICmeasurements should agree on the measurement of d. Tests were under perfornkl fich:~which verified this result. Therefore, it has been established that in the absence of anymaterial anisotropy the collection distance through the bulk of a diamond sample is thesame as that of the surface [1].3. - Detector fabricationBefore any metalization is applied the diamond surface must be cleaned. Chemicalcleaning is usually performed, although excellent results are obtained with in situsputter cleaning. A typical procedure is to degrease the sample with an acetone bathin a ultrasonic cleaner foIlowed by boiling in a chromic-acid and sulfuric-acid bath.The sample is rinsed in a boiling mixture of NH4 OH and Hz Oz. A final rinse is:DIAMOND RADIATIONDSTECTY)RSII 3performed in deionized water. This procedure terminates the surface of the diamondin oxygen [2].All of the detectors discussed in this lecture use multilayer metal contacts on thediamond surface. Typically, titanium is first deposited to form a carbide sticking layer.Sputtering is the preferred approach, the large incident kinetic energy providessufficient energy for carbide fopnation to take place [3]. If the contacts are evaporateda postdeposition thermal anneal is required. Platinum may be deposited as a difi%sionbarrier and then thick gold for contact to the external electric circuit. This approachwill result in ohmic contacts to the diamond, resulting in linear IV ch~cteristics andminimum polarization of the detector. The contacts can be removed by boiling thesample in hot H2S04 or other strong oxidizing agenlsIn summary, as shown in fig. 1, two different detector geometries are used in ourtests. The PC measurements interrogate the surface of the sample. The CPICmeasurements integrate over the path of the particle horn the growth surface to thesubstrate. As illustrated, as the CVD material grows the crystallite size increases withincreasing thickness.4. - The evolution of CVD diamond for detectorsOver the past several years we have studied a large number of CVD samples. In thissection we will touch on the highlights of this development As the CVD diamondgrowth communi~ matured in the 1980’s and cdy 90’s, so did the quality of thematerial. Table I summarizes the evolution of polycrystalline CVD diamond during thisperiod.The earliest measurements on submicrometer thicknes% DC plasma CVD sampleswere of very poor qutilty. The Raman spectrum indicated the presence of significantnondiamond bonded carbon along with a low resistivity of Id Q cm and large opticalabsorption. This material was unsuitable for detectors and exhibited microsecondlifetime, persistent photoconductive signal with picosecond pulse excitation. Thesepersistent sigmals were associated with the nondlamond bonded carbon content of theTABLEI. - Evolution of th pqwrties of polycystaUin.e CVD diamond.Date d (pm)@ Id V/cm109BC (geologic IIa) 2oto50Dc plasma (1989) ~()-3microwave(1990) 1o-1microwave (1991) 2.5microwave (1992) 20D.fL KANIA~ Singleaystal Ha1000100 r“-10 ~1 1 I 1 I .1014 1015 1016 1(317 1018 1019n (cm-a)Fig. 2.- Mobility measuredwith PC A(1990), B(1991) and C(1992).The mobility falls off at highdensity as a result of electron-hole scattering. The solid lines are calculated nobilities usingelectron-hole scattering theory.fihns. As this material was eliminated in the growth, the collection distance increased,the resistivity increased and the persistent conductivity disappeared.In 1990, higherquality films approximately 5pm thick produced by microwaveCVD were tested. The nondiamond component was reduced dramatically, theresistivity was > K? S2cm resulting in a collection distance of 0.1 ~m. A criticaldevelopment used to increase the resistivity was the use of a post-growth anneal step todrive off a conducting hydrogen fji@n the diamond [4]. (JJIt was discovered tlxat the Raman spectra were an %xcellent guide in developingdetector material as it was for other material properties being developed at the sametime. The goal was to eliminate any evidence of a non-diamond component and reducethe width of the 1332 em-1 Raman peak characteristic of diamond bonding. Additionalcorrelations were noted with the optical clarity, W absorption coefficient and lllmresistivity.The continued optimization of the microwave grown films lead to a collectiondistance increase to 2.5pm in 1991 and 20pm in 1992. The latter result was verysignificant because the collection distance of polycrystalliie diamond entered therange spanned by geologic Ha diamonds. In particular, the mobility of these tilmsequaled that of the best geologic IIa dmmonds. F@me 2 shows these dataj the mobilityof the ftis measured with PC. The mobility falls at high density as a result ofelectron-hole scattering. This work was reported by Pkano et UL[51 in 1993. Threesamples were grown with 2.45 GHz microwave excitation. The growth conditions aresummarized in table II and the material characteristics in table III.The trends are clear in the improvements in the diamond with growth conditions.Higher power, pressure and substrate temperature contribute to the quality. The. ..— — — -DIAMOND1MDIATIONDETECTORS:II 5TABLEII. - M&mwavs CVD deposition parametersSample T.* rc) P (ToIT) Power (W) W (%) Grainsize (urn)1990 650 50 1000 0.1 41991 700 q 1900 3.0 111992 950 100 1900 1.0 15TABLEIII. - Summmy of the md4mid properties.Sample Mobility (electron Lifetime d RamanwH~and hole, cmzV/s) (ps) (urn) (cm-l)1990 50 50 c 0.1 7.11991 1000 100 2 4.81992 4000 150 15 3.3correlation with the Raman FWHM and grain size is quite obvious. The collectiondistance as a function of electric field is shown in fig. 3.The lifetime of the best material was the limiting factor in the collection distance.As noted in the previous lecture, nitrogen is a significant factor in determining the100n5.G1‘z xx0.1 I 1x 1 f-10,000 -6,000 -2,000 2,000 6,000 10,000E(v cm-qFig. 3.- Collection distance as a function electric field 1991 (x), 1992 (+) and geologic IIa(o).G D.R.KANIA60 i I I I ,● CVD series 93-1 -50– (E=lOkVkm)“40 -30-20–10-ro I 1 I I 10 100 200 Xx3 400 Soox (W@Fig. 4. - Collection distance as a fknction of film thickness showing a gradient in the materialproperties.lifetime in geologic diamond. A SIMS analysis was performed on highquality samplesto determine the density of nitrogen and other elements. The results indicated thatthere was significantly less nitrogen in the CV’D material than was in a geologic IIa byabout a factor of 3 and no other impurities of signiikance were observed. Nitrogen wasnot limiting the lifetime. In addition, temperature4ependent conductivity measure-ments to 1000 “C were performed on these samples. The electrical conductivity of theCVD samples is an order of magnitude smaller than a geologic 11%perhaps indicatinghigher purity.With the advent. of high-quality diamond films CPIC measurements became morecommon using CVD diamond detectors. A comparison of the collection distancemeasured with CPIC and PC revealed a factor of two difference between the twomeasurements on every sample. The PC measurements were consistently larger. Thisindicated that, like the thermal conductivity of CVD tihns [6], there exists a gradient inmaterial properties in the iilrns. This was first identified in the very same microwaveCVD films used by Graebner et UL in ref. [6]. It has also been identified in DC arcjetgrown ftis (see fig. 4). It was ako noted that d never exceeded the grain size of themateriaL Again in analogy to the thermal/ conductivity measurements, grain 1-7boundaries or the region near the grain boundaries are the probable locdion of thestructural, not impurity, defects in the CVD intragrain materiaL.. .-......— -------- / ———cn 2oi- ‘T l’”flio 1.0 2.0 3.0 4.0 :,,/Normalized Growth Rate /’“Fig. 5.- Collection distance as a function of normalizedgrowth rate.The gradient of the collection d=tance along the growth direction was mapped bymeasuring the collection distance prot51e. Two approaches were used. One approachused thick diamond samples cut from the same wafer which were tinned tlom thesubstrate side. Another set of samples was generated by varying the growth time atconstant growth conditions to vary the sample thickness. Analysis of CPICmeasurements and PC measurements on these samples uncovered a linear change in das a function of thickness. This was true in the microwave and DC arcjet samples (seefig. 4).The existence of a linear gradient in the CVD diamond reveals a strategy forproducing improved particle detectors, remove material from the substrate side. Thiswill increase the average properties of a CVD diamond sample.Another observation was made at this time, highquality fllrns grow at reducedgrowth rate. Many applications required thick films, this stimulated an interest in ahigher growth rate process, DC arcjet. Afl.er significant process optimization whichwas analogous to the development of microwave CVD films, the collection distance wasimproved to 50pm in films several hundred micrometers thick. A clear correlationpersisted between the collection distance and the growth rate (see fig. 5). The work ofZhao [’71 showed several other distinct correlations of collection distance with thecxystal lattice parameter, Raman FWHM, X-ray diffraction peak width and filmtransparency (see fig. 6). The clear trend in all of the correlation analysis is to push thediamond growth to lower stress and large grained films.—-—— —_—-— -:.“-1 d●cvoohmond +g-30(B=Iokvan’)6 tawlhmmlmvl(lloj= &lokvcm‘)%+:20U*10Fig.L..l-m ..,]..]s-=- -3:623.5643.5663.5663.570 3.572130 “20+ “go%%?)*Oww-mlkummd(llcj(Edokvcmj+/I+-+10I01 ~-- - 10.000 0.100 0.200 0.300a (A) FWHM (degrees)6.- Collection distance as a fiction of lattice parameter and X-ray dfiction FWHM.Fig. 7. - A summary of the developmentof detector quali~ WI) diamond.Development has continuedfdms with a collection distancedetector grade CVD diamond ison the growth chemistry. This work has resulted inin excess 100pm.shown in fig. 7.A summary of the development of...“DIIkMONDRADIATIONDETECTORS II 95. - ConclusionDiamond CVD growth has made great strides in the past several years.High-purity, high-mobility diamond samples have b@en grown. In analogy to thermalproperties, a grdent exists in the detector properties which improves from thesubstrate to the growth surface. Although it has not been established with certainty,grain boundaries or the defective region around graind boundaries limit the carriertransport in diamond. Collection distances of > 100pm are achievable. This worksuggests a growth stiategy which emphasizes the growth of material with large ~ainsin as thin a material as possible.REFERENCES* [1][2]‘ [3][4][5]. [6]v[7]PANL. S. et QL,J. AppL Phy&, 74 (1993) 1086.MORIY., KAWARADAH. and HmMUA., AppL Phys M, 5S (1991) 940.NISIUMURAK. et aL, MRS Symposiu~162 (1990)341. /$LANDSTRASSM. and RAW K Vv Appl Phye. Lett., 55 (19S9)1391.PLANOM. A. et al, Science, 260 (1993) 1310.GRAEBNERJ. E. et aL, ~atuw, 369(1992)401.Zmo s., Chum.demat. ion of the Electrical Pmpwties of Polycrystdim Diumond Fikns, Ph.D. Thesis, Ohio State University, 1994.This work was performed under the auspices of the U.S.Department of Energy by LawrenceLivermore National Laboratory under contract no. W-7405-Eng-48.— ——.- 4s=Technical Information Department  • Lawrence Livermore National LaboratoryUniversity of California • Livermore, California  94551

Diamond radiation detectors II. CVD diamond development for radiation detectors

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