Changing the ratio of carbon to silicon during the epitaxial 4H–SiC growth is expected to alter the dominant deep level trap, which has been attributed to a native defect. The C∕Si ratio was changed from one to six during epitaxialgrowth of SiC. Diodes fabricated on the epitaxial layer were then characterized using current-voltage and deep level transient spectroscopy. The single peak at 340K (Z1/Z2 peak), was deconvolved into two traps, closely spaced in energy. The concentration of one of the Z1/Z2 traps decreased with increasing C∕Si ratio. This result opposes theoretical predictions of carboninterstitial components, and supports assignment to a silicon antisite or carbonvacancy relationship. The concentration of the second component of the peak at 340K did not depend on the C∕Si ratio, which would indicate an impurity in an interstitial site

C. W. Litton

D. Johnstone

Dalibor T.

E. F. Schubert

I. Bhat

J. K. Kim

K. S. Ramaiah

S. Akarca-Biyikli

T. P. Chow

Litton, C. W.

Johnstone, D.

Akarca-Biyikli, S.

Ramaiah, K. S.

Bhat, I.

Chow, T. P.

Kim, J. K.

Schubert, E. F.

VCU Scholars Compass

Virginia Commonwealth UniversityVCU Scholars CompassElectrical and Computer Engineering Publications Dept. of Electrical and Computer Engineering2006Effect of C∕Si ratio on deep levels in epitaxial4H–SiCC. W. LittonAir Force Research Laboratory, cole.litton@sbcglobal.netD. JohnstoneSEMETROLS. Akarca-BiyikliVirginia Commonwealth UniversitySee next page for additional authorsFollow this and additional works at: http://scholarscompass.vcu.edu/egre_pubsPart of the Electrical and Computer Engineering CommonsLitton, C.W., Johnstone, D., Akarca-Biyikli, S., et al. Effect of C∕Si ratio on deep levels in epitaxial 4H–SiC. AppliedPhysics Letters, 88, 121914 (2006). Copyright © 2006 AIP Publishing LLC.This Article is brought to you for free and open access by the Dept. of Electrical and Computer Engineering at VCU Scholars Compass. It has beenaccepted for inclusion in Electrical and Computer Engineering Publications by an authorized administrator of VCU Scholars Compass. For moreinformation, please contact libcompass@vcu.edu.Downloaded fromhttp://scholarscompass.vcu.edu/egre_pubs/117AuthorsC. W. Litton, D. Johnstone, S. Akarca-Biyikli, K. S. Ramaiah, I. Bhat, T. P. Chow, J. K. Kim, and E. F. SchubertThis article is available at VCU Scholars Compass: http://scholarscompass.vcu.edu/egre_pubs/117Effect of C/Si ratio on deep levels in epitaxial 4H–SiCC. W. LittonaAir Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/MLPS),Wright–Patterson Air Force Base, Ohio 45433D. JohnstoneSEMETROL, 13312 Shore Lake Turn, Chesterfield, Virginia 23838S. Akarca-BiyikliDepartment of Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia 23284K. S. Ramaiah, I. Bhat, T. P. Chow, J. K. Kim, and E. F. SchubertDepartment of Electrical, Computer and System Engineering, Rensselaer Polytechnic Institute,110 8th Street, Troy, New York 12180Received 27 June 2005; accepted 3 November 2005; published online 23 March 2006Changing the ratio of carbon to silicon during the epitaxial 4H–SiC growth is expected to alter thedominant deep level trap, which has been attributed to a native defect. The C/Si ratio was changedfrom one to six during epitaxial growth of SiC. Diodes fabricated on the epitaxial layer were thencharacterized using current-voltage and deep level transient spectroscopy. The single peak at 340 KZ1/Z2 peak, was deconvolved into two traps, closely spaced in energy. The concentration of oneof the Z1/Z2 traps decreased with increasing C/Si ratio. This result opposes theoretical predictionsof carbon interstitial components, and supports assignment to a silicon antisite or carbon vacancyrelationship. The concentration of the second component of the peak at 340 K did not depend on theC/Si ratio, which would indicate an impurity in an interstitial site. © 2006 American Institute ofPhysics. DOI: 10.1063/1.2161388Both 4H and 6H polytypes of SiC have a defect at0.6–0.7 eV below the conduction band, and this is a domi-nant defect in 4H–SiC. Various models have been proposedfor the composition of this Z1/Z2 defect, and various experi-mental studies have also been performed,1 primarily basedon generating intrinsic defects by irradiation.2 In thiswork, we have characterized the Z1/Z2 defect in detail byfabricating Schottky diodes on a C-face 4H–SiC grownunder different carbon to silicon ratios.Epitaxial growth of SiC on C-face 4H–SiC substrateswas carried out in a horizontal low-pressure coldwall reactor.The 000-1 4H–SiC substrates, misoriented 8° toward112¯0 n1018 cm−3, obtained from Cree Research, Inc.,were cleaned sequentially in acetone, xylene, and methanolusing an ultrasonic bath, then cleaned with Caro’s etchH2SO4:H2O2:H2O, rinsed in deionized water, and finallydried in N2. Prior to the epitaxial deposition, the substrateswere etched in H2/propane mixture at 1400 °C for 10 min.Silane 2% SiH4 in H2, and propane 2% C3H8 in H2 wereused as the precursors for Si and C, respectively. Palladiumpurified H2 was used as the carrier gas. Nitrogen 2% N2 inH2 was used as the n-type dopant. The substrate temperatureand the reactor pressure were kept at 1550 °C and 80 Torr,respectively. The C/Si ratio was varied from 1.0 to 6.0 bychanging the propane flow, keeping the silane flow constant.Schottky junctions were fabricated on 5 m thick epi-taxial layers employing shadow mask with hole diameters of0.5, 1.0, and 1.5 mm. To make ohmic contact to the backside, Ti200 Å /Ni800 Å /Al100 Å metals were depos-ited by electron-beam evaporation and then annealed at1000 °C for 5 min in Ar by rapid thermal processing. Ni300 Å /Au500 Å metals were deposited through theshadow mask on the front surface for Schottky contacts. Therectifying properties of the Schottky contacts were improvedfurther by annealing the samples at 500 °C for 15–20 minin Ar.The carrier concentration in the samples was measuredby C–V as 6.61015/cm3 for the sample grown with C/Siof one, and 1.51015 cm3 for the C/Si of three and six.Figure 1 shows the current-voltage I-V characteristics ofSchottky diodes fabricated on layers grown with differentC/Si ratio. The reverse bias current is the lowest for the C/Siratio of three. A plot of the current versus voltage0.5aElectronic mail: cole.litton@sbcglobal.netFIG. 1. The I-V characteristics of Schottky diodes fabricated on layersgrown with different C/Si ratios.APPLIED PHYSICS LETTERS 88, 121914 20060003-6951/2006/8812/121914/3/$23.00 © 2006 American Institute of Physics88, 121914-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 18:07:55not shown, proportional to the volume of generation cen-ters, was superlinear for each of the samples, pointing outthat the dominant mechanism is not via a generation center,in which case the reverse bias current would be expected toscale linearly with the volume of the depletion region.From the deep-level transient spectroscopy DLTS mea-surements, a ratewindow plot of the spectra from each of thesamples, grown with carbon to silicon ratios of one, three,and six, is shown in Fig. 2. The spectra all show the samefeatures, although in different proportions, dominated by theZ1/Z2 trap at 330 K. Increasing the C/Si ratio from one tothree did not result in an appreciable change in the trap con-centration, where the trap concentration is given by the peakheight. However, increasing the ratio to six resulted in adecrease in the Z1/Z2 trap concentration by a factor of 3.Note that the spectrum for the C/Si ratio of one has higherseries resistance than the other two, and required a correctionfactor, increasing the spectrum by 30%, according to3Cm =C1 + rsC21 − 2rsC21 + rsC2 .The other two samples had series resistances in the rangeof 10 , which has a negligible effect on the measured ca-pacitance. The Arrhenius plot shown in Fig. 3 is obtained byplotting the emission rate factor, adjusted by T2 for the tem-perature dependence of the thermal velocity and the conduc-tion band density of states, versus 1/kT.4,5 The emission rateis extracted from a fit of the capacitance transient. The ener-gies and capture cross sections, and trap concentrations aregiven in Table I. The designations used are the same as inRef. 6. The energy and capture cross section are taken fromthe slope and intercept of the Arrhenius plot. The concentra-tion is taken from the amplitude of the transients, correctedfor the volume scanned. The volume was determined fromthe depletion width at the biases used for filling and mea-surement, from the capacitance-voltage profile, and the di-ameter of the diode. In each case, 0.0 V was used for thefilling pulse, and −2.0 V was used for the measurement bias.The filling pulse width was 10 ms for each.The trap at 0.17 eV has been reported as due to Ti in acubic site.6,7 The capacitance transients used to generate thesingle Z1/Z2 peak at 320 K in the ratewindow plot, werebest fit for two exponential components, given in Table I asZ1/Z2 a and Z1/Z2 b. The square of the capacitance is plot-ted and fit versus time according toCt,T2 =qns − NT2bi + V − kT/q= A0 + iAi exp− t/i ,where ns is the shallow carrier concentration, NT is the trapconcentration, bi is the built in potential, k is Boltzmann’sconstant,  is the permittivity, and q is the electron charge.The second part of the equation is the form that is used forfitting, where i is the number of exponential components, andA0 is the offset. The Levenberg–Marquardt nonlinear least-squares error method was used to fit the transients.8 An ex-ample of the quality of fit, comparing fits for one and twoexponential components to the data for the sample with C/Siof three, is given in Fig. 4. The mean square error for a fitassuming one exponential component is 34 times larger thanthe mean square error for a fit assuming two exponentialcomponents at 318 K.The peak at 320 K is expected to be a double acceptor,and other studies have also identified the peak as an overlap-ping acceptor trap, but extracting the trap characteristics hasbeen difficult.9,10 The concentration of the lower-energy trap,which appears at a higher temperature due to the differencesin capture cross section, does not change appreciably withC/Si. The higher energy trap, Z1/Z2 b, decreases as the C/Siincreases from one to three, and is below the sensitivity limitof 51011/cm3 for the C/Si of six. In Ref. 9, the peak waspartially annealed, reduced to a minimum of 80% at 750 °C.From theoretical work, the peak at 320 K is possibly due toa dicarbon interstitial complex next to a nitrogen interstitial.1FIG. 2. DLTS spectra from n-type 4H–SiC grown with different carbon tosilicon ratios of one, three, and six. The ratewindow time constant is 228 s.FIG. 3. Arrhenius plot for the traps in 4H–SiC. The trap energies, capturecross sections, and concentrations if present are listed in Table I for the threeC/Si ratios.121914-2 Litton et al. Appl. Phys. Lett. 88, 121914 2006 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 18:07:55However, other experimental work where the authors used aSi-face SiC instead of a C-face SiC as used here, in additionto the results given here, shows that increasing the carbondecreases the height of this peak, suggesting that the defectmay be due to SiC or VC.11 Experimental results that divergefrom theoretical predictions may indicate that the epitaxytakes place under strongly nonequilibrium conditions. Fur-ther clues on the origin are obtained from the field depen-dence of the trap to confirm the expected acceptor nature. Ifthe trap is charged when empty, the Coulomb potential pro-file is shifted by the electric field.12 There was very little fielddependence, less than 10% of what it should be for a singlycharged defect. From this result, we can conclude that thedefect is an acceptor, as determined also by others.13 Increas-ing the C/Si ratio also caused other traps at a higher energyto emerge. RD1/2, RD3, and RD4 were only seen in appre-ciable concentrations in the sample grown with C/Si of six.There was some evidence of a higher-energy trap in thesamples grown with a C/Si of one and three at temperaturesabove 700 K for the ratewindow of 228/s. However, the trapparameters could not be determined.This series of samples, including carbon to silicon ratiosof one, three, and six, demonstrated that one of the primarydefects in 4H–SiC is related to a carbon vacancy, as deter-mined by a reduction in the Z1/Z2 DLTS peak height withhigher C/Si ratio. This result opposes predictions of carboninterstitial components, and supports assignment to a siliconantisite or carbon vacancy relationship. The Z1/Z2 peak wasalso deconvolved into two component traps, one of whichwas independent of the C/Si ratio, perhaps related to animpurity in an interstitial site. The I-V data showed that theintermediate C/Si ratio of three had lower reverse biasleakage current. The slight increase in leakage current for theC/Si of six reflects the presence of deeper traps seen byDLTS in this sample, which can reduce the thermal barrierthrough the Schottky barrier by tunneling into the deeplevels.The work at RPI was partially supported by DARPAContract No. DAAD19-02-1-0246. Work at VCU was per-formed under USAF SBIR Contract No. F33615-02-M-2250.One of the authors K.S.R. would like to thank S. Rao forhis constant help during the growth of samples; another au-thor C.W.L. would like to acknowledge the AFOSR supportunder AFOSR/NE In-House Laboratory Task No. 2305EL1P.1T. A. G. Eberlein, R. Jones, and P. R. Briddon, Phys. Rev. Lett. 90,225502 2003.2A. Kawasuso, F. Redmann, R. Krause-Rehberg, M. Weidner, T. Frank,G. Pensl, P. Sperr, W. Triftshauser, and H. Itoh, Appl. Phys. Lett. 79, 39502001.3A. Broniatowski, A. Blosse, P. C. Srivastava, and J. C. Bourgoin, J. Appl.Phys. 54, 2907 1983.4D. Lang, J. Appl. Phys. 45, 3023 1974.5L. Stolt and K. Bohlin, Solid-State Electron. 28, 1215 1985.6T. Dalibor, G. Pensl, H. Matsunami, T. Kimoto, W. J. Choyke, A. Shoner,and N. Nordell, Phys. Status Solidi A 162, 199 1997.7M. Weidner, T. Frank, G. Pensl, A. Kawasuso, H. Itoh, andR. Krause-Rehberg, Physica B 308, 633 2001.8Numerical Recipes in C, The Art of Scientific Computing Cambridge Uni-versity Press, New York, 1988.9C. Hemmingsson, N. T. Son, O. Kordina, J. P. Bergman, E. Janzen, J. L.Lindstrom, S. Savage, and N. Nordell, J. Appl. Phys. 81, 6157 1997.10C. G. Hemmingsson, N. T. Son, A. Ellison, J. Zhang, and E. Janzen,Phys. Rev. B 58, R10119 1998.11K. Fujihira, T. Kimoto, and H. Matsunami, J. Cryst. Growth 255, 1362003.12W. R. Buchwald and N. M. Johnson, J. Appl. Phys. 64, 958 1988.13T. Dalibor, C. Peppermuller, G. Pensl, S. Sridhara, R. P. Devaty, W. J.Choyke, A. Itoh, T. Kimoto, and H. Matsunami, Inst. Phys. Conf. Ser.142, 517 1996.TABLE I. Trap energies, capture cross sections, and concentrations measured by DLTS for 4H–SiC grown withC/Si of one, three, and six.Ti Z1/Z2a Z1/Z2b RD1/2 RD3 RD4ET eV 0.171 0.582 0.654 0.92 0.89 1.24	 cm2 710−14 310−15 110−13 410−14 810−17 710−15C/Si NT cm−31 51012 51012 1.510133 21012 51012 1.010136 51012 51012 11012 91011 11012FIG. 4. Representative fit for transient for sample with C/Si of three show-ing the best fit for two exponential components for the peak appearing at320 K in the ratewindow plot of Fig. 2. The fit for two components overlapswith the experimental data.121914-3 Litton et al. Appl. Phys. Lett. 88, 121914 2006 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:128.172.48.59 On: Tue, 14 Apr 2015 18:07:55

Effect of C∕Si ratio on deep levels in epitaxial 4H–SiC

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Effect of C∕Si ratio on deep levels in epitaxial 4H–SiC

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