Electronic defect states at annealed metal 4 H – SiC interfacesa)S. Tumakha, S. H. Goss, L. J. Brillson, and R. S. Okojie

Citation: Journal of Vacuum Science & Technology B 23, 594 (2005); doi: 10.1116/1.1884124 View online: http://dx.doi.org/10.1116/1.1884124 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/23/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effect of ambient pressure and nickel contamination on the dimer-dangling-bond surface state of Si ( 001 ) 2 × 1 J. Vac. Sci. Technol. A 25, 721 (2007); 10.1116/1.2740295 Inhomogeneities in Ni 4 H - Si C Schottky barriers: Localized Fermi-level pinning by defect states J. Appl. Phys. 101, 114514 (2007); 10.1063/1.2745436 Electrical and deep-level characterization of GaP 1 x N x grown by gas-source molecular beam epitaxy J. Appl. Phys. 101, 103707 (2007); 10.1063/1.2732451 The geometric, electronic, and magnetic properties of Ag 5 X + ( X = Sc , Ti, V, Cr, Mn, Fe, Co, and Ni) clusters J. Chem. Phys. 124, 184319 (2006); 10.1063/1.2191495 Enhancement of Schottky barrier height on p -type GaN by ( N H 4 ) 2 S x treatment J. Appl. Phys. 99, 053706 (2006); 10.1063/1.2175446

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Electronic defect states at annealed metal/4H–SiC interfaces *S. Tumakha,a! S. H. Goss, and L. J. BrillsonDepartment of Electrical Engineering and Computer Science, The Ohio State University, 205 Dreese Lab,Columbus, Ohio 43210-1272

R. S. OkojieNASA-Glenn Research Center, 21000 Brookpark Road, M/S 77-1, Cleveland, Ohio 44135

sReceived 12 August 2004; accepted 7 February 2005; published 6 April 2005d

We have used low energy electron-excited nanoscale luminescence spectroscopysLEENd to studythe formation of electronic surface states at metal/4H–SiC contacts. These junctions were formedusing both low and high reactivity metals to study how the nature of interface chemical bondingaffects the interface state formation. We observe evidence for the formation and removal oflocalized states at energies that have been associated with morphological SiC defects. Metals suchas Au and Ag with no strong chemical reactivity exhibited the most pronounced changes.Conversely, chemically-reactive metals such as Ti and Ni exhibited only minor changes and onlywith high temperature annealing. These observations suggest that native defects rather thenmetal-specific chemical bonding dominate the interface electronic features.© 2005 American

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I. INTRODUCTION

Localized electronic states play a central role in themation of Schottky barriers, Ohmic contacts, and heterojtions. Such states can form as a result of localized chtransfer, chemical bonding, impurity diffusion, or pointfect formation. In SiC, local polytype changes, dislocaand stacking faults can produce electrically active sitewell. To measure interface states, one requires a prochemical, structural and electronic properties on nanomscale. This is a particular challenge for the conventional tniques such as photoluminescence, where optical penetdepth is typically microns, or capacitance spectroscowhere probe depth are limited by depletion width to hdreds of microns. A variant of cathodoluminescence, lowergy electron-excited nanoluminescence spectrossLEENd provides a depth-resolved technique for measuelectronic properties of semiconductor surfaces, interfand thin films. Depending on the electron beam energysEBd,the electron cascade generated by an incident beam pedifferent depths under the surface. AsEB decreases belowkeV, excitation of electron-hole pairs occurs less than senanometers below the free surface. By varying the enand excitation depth, one can use LEEN spectroscopdepth-resolved probe of buried interfaces. In this workhave used LEEN spectroscopy to provide evidence for loized states at metal–SiC junctions. Such states have a scant influence on the charge transfer properties ofcontacts.1,2 Until recently, these states at SiC–metal junctihave only been inferred, based on the variations in Flevel position with metal work function, rather than fromore direct measurements. Schottky barrier heightsSBHdstudies of 4H–SiC with a wide range of metals showstrong Fermi level “pinning” in a narrow range of energie3

*No proof corrections received from author prior to publication.ad

Electronic mail: tumakha.1@osu.edu

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suggesting only a moderate influence of localized statesviously, we reported 1.9 eV process-dependent interfaceemission at Pt/Ti/4H–SiC interfaces.4,5 Furthermore, higtemperature annealing can produce stacking faults andtive gap states in highly-dopedn-type 4H–SiC.3 At issue iswhether these and other interface states due to chebonding and/or diffusion are present at metal–SiC interfand play a role in barrier formation. Here we presentenergy, electron-excited nanoluminescencesLEENd spectroscopy measurements of both free and metallized 4H–SiCface surfaces at room and elevated temperatures.

II. EXPERIMENT

Specimens consisted of 8° off-axisffrom the s0001dplaneg, Si-face substrates with 2µm thick n-type fNg, 2.231019 cm−3 doped epilayer grown homoepitaxially by CrInc. sResearch Triangle Park, NCd. LEEN spectra were aquired with energies ranging from 0.5 keV to 5 keV, cosponding to peak excitation depthsU0 of 3 nm to 60 nmrespectively.6 The experiments were conducted in four sts1d ,2–4 nm of metal were thermally evaporatedsolvent-cleanedsacetone, methanol, DI waterd SiC surfaceMetal thickness was limited in order to permit detectionlight emission through the overlayer;s2d This was followedby LEEN analysissin a different chamberd both in the regioexposed to metal deposition and the adjacent bare SiCs3dThe sample was then annealed in ultrahigh vacuumsUHVd ata given temperature for one hour. Temperatures weresured with an optical pyrometer focused directly onsample surface;s4d After a sufficient time for the samplecool down to room temperature in ultrahigh vacuumsUHVd,LEEN spectra were retaken. This LEEN measurement/acycle was repeated with increased temperatures. LEENtra are acquired using a glancing incidence electronquartz window and lens, an Oriel monochromator, and A

CCD detector. Secondary ion mass spectrometrysSIMSd

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measurements were performed using a time-of-flightsTOFdSIMS instrumentsPHI Trift III d. Depth profiles were peformed with Ga ions at a rate of 1 nm per s.

III. RESULTS

Figure 1 presents LEEN spectra for Au, an unreacmetal on the SiCs0001d surface. Deposition of 2–4 nm Aintroduced a new, near surface luminescence at 1.7–2without any other processing steps. The changes areevident in the spectra shown forEB=0.5 keV, correspondinto an electron cascade peak atU0=2–4 nmbelow the freesurface. For direct comparison, spectra before and aftedeposition were normalized to the near-band edgesNBEdpeak to emphasize the changes taking place within thesample volume. The increase in luminescence was appon an absolute scale as well. The difference of the two Lcurves from the same area before and after Au deposappears as a broad peak centered at 1.9 eV with a ranenergies indicated.

FIG. 1. EB=0.5 keV LEEN spectra of 4 nm Au on SiC vs bare SiC, normized to NBE emission. Curve subtraction yields a broad peak at,1.9 eV.

FIG. 2. EB=1 keV LEEN spectra of 4 nm Au on SiC vs bare SiC, normized to NBE emission. The 1.9 eV difference curve has less intens

EB=1 keV vs 0.5 keV and lacks the 2.3 eV shoulder.

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nof IncreasingEB=1 keV, corresponding toU0=4–6 nm, oneprobes deeper into the epilayer and further away frominterface. Figure 2 shows a set of spectra taken at 1analogous to those in Fig. 1. The changes in the 1.9 eVinduced by Au deposition are similar to those atEB

=0.5 keV but appear less intense. The analogous set oftra for EB=2 keV sU0=15 nmd showed no significanchanges with Au deposition. Therefore the changes didextend beyond the first several nanometers below theface. It should be noted that LEEN spectra taken at 2and higher accelerating voltages displayed a close corredence, indicating that the Au film introduces no significspectral changes due to any wavelength-depenabsorption.

Annealing of the Au on the 4H–SiC sample in UHV800 °C for 1 h produced well-pronounced changes asFigures 3sad and 3sbd show the comparison of the specobtained before and after anneal. The spectra were noized to the same NBE emission level. Similar to the chawith Au deposition, the most pronounced effect was seelowest penetration depth spectra taken withEB=0.5 keVt

FIG. 3. sad Annealing removes the 1.9 eV,EB=0.5 keV LEEN feature at thAu–SiC interface, reversing changes induced by Au deposition.sbd Thesechanges are less pronounced with deeperEB=1 keV excitation.

beam penetratingU0=2–4 nm.Again, these changes were

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located in the 1.7–2.1 eV range. The subtraction of thecurves taken before and after annealing revealed a remof the broad peak at 1.7–2.1 eV nearly identicalssee Fig. 4dto that introduced by deposition. As with Figs. 1 and 2,EB=1 keV data showed similar but less pronounced chacentered around 1.9 eV. Likewise, a deeper probing 2beam showed no significant changes in LEEN spectra. Bground changes due to sample positioning are noticeonly above 3.2 eV. In general, the induced changesnearly equal and opposite to those produced after thedeposition alone. This is illustrated in Fig. 4, comparing schanges forEB=0.5 keV due to Au deposition vs Au depsition plus annealing. This figure demonstrates the ncomplete reversibility of 1.9 eV emission by 800 °C anning. Comparison of these difference spectra also showsannealing removes additional emission in the 2.3 eV raIn contrast, the annealing does not cause any pronouchanges in the adjacent SiC bare surface with no Au deited. snot shownd. The 2.3 eV emission also exhibits a strodepth dependence, showing the most pronounced decwith annealing right at the interface and less changeincreasing distance from the interface.

A negative SIMS, Cs depth profile of the annealed AuSiC surface indicates a small but significant diffusion ofinto the SiC lattice. Figure 5 illustrates a SIMS depth proobtained from a SiC sample after Au deposition, anneafollowed by an aqua regia etch to remove the metalthereby eliminate any knock-on effects from the overlaAu. As shown, the Au diffuses,15 nm into the SiC. Notalso the,40% decrease in Si within the first few nm ofinterface.

The LEEN features of other metals on 4H–SiC furtcharacterize the nature of the near-interface states. Icase of Ag, no strong chemical bonding is expected. Thanalogous to the chemical nature of the Au/SiC interfEB=0.5 keV excitation probes the first few nm below

FIG. 4. Comparison ofEB=0.5 keV LEEN difference spectra from Figsand 3 showing near complete reversibility of 1.9 eV emission chaAnnealing also reduces pre-existing 2.3 eV emission.

,3 nm thick Ag layer. For this SiC specimen, 1.9 eV emis-

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sion was evident even before deposition, exten,200 nm into the SiC. Unlike the Au/SiC interface, thwere no changes induced by Ag deposition alone. Anneat temperatures up to 500 °C also had little effect on esion, as shown in Fig. 6. However, consistent withAu/SiC interface, Ag/4H–SC LEEN spectra show an apent decrease in 1.9 eV emission with 700 °C annealing.similar to the Au/SiC case, the changes due to annewith EB=1 keV excitation are less apparent than at 0.5These changes diminish further with deeper excitation, aconsistent with the Au–SiC results.

In the case of Ti, strong chemical bonding can occurshould not be reversible with annealing. We usedEB

=1 keV excitation to probe the first 4–5 nm, i.e., the mand the first few nm below the metal. Indeed, comparisothe Ti/SiC plus 800 °C anneal vs Ti/SiC before anneaFig. 7sbd showed apparent changes only in the,2.9–3 eV

.FIG. 5. Negative SIMS profile of SiC after Au deposition, annealingremoval by aqua regia etch. Sputter rate was,1 nm/s. Au has diffused inthe first 10–15 nm.

FIG. 6. LEEN spectra of 3 nm Ag on SiC. Analogous to the Au/4H–

interface, 1.9 eV emission is reduced by 700 °C annealing.

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range, i.e., an emission increase, as illustrated by the dence spectrum. Emission at that energy has been attribusingle stacking faults as well as native defects.7 There wereno changes in the 1.9 eV region after annealing, in conto the Au–SiC or Ag–SiC junctions.

In the case of Ni also, strong chemical bonding ispected that should not be reversible with annealing. AsTi, there are no observed reversible changes due to annof the Ni/4H–SiC interface. In fact, comparison ofNi/4H-SiC vs the bare SiC surface both after a 500 °Cneal showed no differences. Similarly, only minor chanoccur above 900 °C, and these are confined in the,2.8 eVrange. See difference curve. Therefore, these spectrathat the Ni on the 4H–SiC interface exhibits emission beior analogous to that of Ti.8

IV. DISCUSSION

The LEEN spectra above demonstrate the presenelectronic states localized within nanometersmetal/4H–SiC interfaces. Furthermore, the energies antensities of the emissions associated with these states aas their changes with annealing and chemical reactivityvide an indication of their physical origin. Previouslyreported the emergence of a 2.47 eV midgap emission d

FIG. 7. LEEN spectra of 3 nmsad Ni and sbd Ti on SiC. No significanchanges observed for 900 °C annealing of Ni film. For Ti film, chaappear at 2.85 eV at 800 °C annealing.

stacking faults for bare 4H–SiC surfaces annealed in UHV a

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1150 °C for 2 h. Additional 2 h, 1150 °C anneals increathis emission along with a 2.2 eV shoulder and a broad1.9 eV shoulder. Electroluminescence measurementGaleckaset al. of electrically-stressed 4H–SiC diodes shthat broad 1.8 eV emission can be associated with pdislocations that decorate stacking faults and 2.8 eV esion with the stacking faults themselves.9 Liu et al. showedthat the structure of such stacking faults were isolated silayer Shockley faults bound by partial dislocations.10 Previ-ously, Okojieet al. showed that the structure of the stackfaults formed by high temperature annealing of hign-doped 4H–SiC were double stacking faults with optemission at 2.5 eV.4 Thus, optical emissions at 1.8, 2.5, a2.8 eV are all characteristic of either stacking faults ordislocations that bound them.

For Au deposited on bare SiC in UHV, similar 1.9 eV a2.3 eV emissions appear at room temperature undemetal. Depth-resolved spectra with increasing incident benergyEB and excitation depth show that the 1.9 eV emsion is localized within the first 10 nm. A 1 h, 800 °C annremoves this 1.9 eV feature, along with some 2.3 eV esion. These changes may be related to the Au–Si intersion, as indicated by Fig. 5. For Ag deposited on b4H–SiC in UHV, the interface state emissions again inva 1.9 eV transition. As with Au, annealing decreased itstensity. Furthermore, these changes are all localized to wnanometers of the metal/SiC junction.

The more reactive metals Ti and Ni exhibit qualitativdifferent behavior, showing changes only at energies asated previously with stacking fault formation.4 The samplewere annealed up to 900 °C, in the range of the temperaknown to produce Ohmic contacts due to metal–silicompound formation.11 The changes induced by Ni af900 °C annealing appear in Fig. 7sad and are negligible compared to minor spatial variations in LEEN features acrossample. The changes induced by Ti after 900 °C arepronounced. They indicate increased emission in the 2eV range, consistent with single stacking fault formation9,10

The absence of new defect emission features underconditions is surprising, given the movement and bondinSi and C atoms expected at reacted interfaces. Howeven though such silicide and carbide reactions are fable, they may consume Si and C at the surface eqresulting in no net defect formation. Alternatively, reacand diffusion may be inhibited by residual surface O ctamination, since the thermodynamic driving force for moxide formation is much larger than those of the silicidecarbide counterparts12 and the high bond strength of suoxides could result in barriers to further chemical interac

The LEEN results show the appearance of emissionsat energies of 1.9, 2.5, and 2.8 eV as well as the absenany emissions due to new gap states related to partmetals. They indicate that extrinsic metal-induced statenot dominate interface state behavior. Instead, the similabetween the interface features and those associated withphological defects suggest a physical mechanism comm

tboth. Native point defects associated with both SiC inter-

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faces and dislocations can provide a mechanism conswith the observations presented here. Such defectsknown to diffuse at temperatures comparable to thosehere13 and to form with metal-semiconductor interdiffusio1

Furthermore, point defects can coalesce around dislocadue to strain fields or charging. If such defects are ressible for the 1.9 eV emission at 4H–SiC dislocations, ttheir spatial redistribution with metal–SiC interdiffusion aannealing could account for the localized changes in 1.emission in the Au or Ag LEEN spectra. For example,intermixes with Si in the outermost layer of SiC. This pcess is possible without intentional annealing since Au foa eutectic with Si at relatively lows363 °Cd temperatures.14

Likewise, Ag forms a eutectic with Si, albeit at higher teperatures84 °Cd.14 Both result in the formation of interfacidefects such asVSi, and metal interstitials. Such intermixiis relatively localized, even after multiple, high temperaanneals. The depth profile in Fig. 5 showed that interdsion occurs at Au–SiC interfaces. Si vacancies are a scandidate for the deep levels controlling the barrier heig15

and they have been shown16 to produce photoluminescenfeatures at,1.35–1.44 eV. The complementary optical trsition within the 3.26 eV 4H–SiC band gap17 would fall intothe 1.8–1.9 eV range, in agreement with both our spectrawith the Galeckaset al. assignment.9 While the peak observed in this work is significantly broader then the featobserved by Wagneret al.16 in low-doped samples, thmight be expected due to defect complexing with othertice imperfections as well as the higher doping. Carboncancy complexes may also play a role since annealinknown to convertVSi to theVC–CSi pair with optical emission in the 1.1–1.15 eV range.18 The complement of thoptical transition is,2.2 eV, near the peak energy of emsion reduced by annealing. The smaller reduction ofemission is consistent with the higher thermal stabilitycarbon complexes vsVSi.

19 The decrease in 1.9 eV emisswith annealing may be the result of vacancy removal ametal interface or free surface. Alternatively, the diffuspromotes atomic Au diffusion, complex formation, and psivation with other chemical impurities or morphologicalfects such as stacking and dislocations.

Finally, the energies of these metal- and anneainduced interface features can account for the rang4H–SiC s0001d SBH’s measured previously.3,20 In generafor n-type 4H–SiC SBH 1.0øFSB

n ø1.8 eV, i.e., 1.45–2.2eV above the valence band. This range of SBH correspto energy positions in the band gap ofEC −1.8 eV andEV

+2.25 eV. Significantly, the near interface changes inemission with metallization and annealing reported herepear at similar energies of,1.9 eV and 2.3 eV, indicatintransitions to or from deep levels and the SiC band edSince the filling and unfilling of such states during mesemiconductor charge transfer acts to constrain the ranFermi level movement,1 our results suggest that deferather than metal-specific localized states play a primary

in SiC Schottky barrier formation.

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V. CONCLUSIONS

LEEN spectroscopy of different metals on 4H–SiCveals interface states localized to within only a few naneters of the intimate junction. These metal- and therminduced interface state observations suggest that ndefects rather than any metal-specific chemical bondinimpurity dominates the interface electronic states. Theregood correspondence between the Schottky barrier hlimits and changes in the interface features. These statepear to be of two distinct types, depending on the mreactivity with the SiC. Those metals that have no strchemical reactivity exhibit states whose intensities varyannealing. Those metals that chemically react with SiC squalitatively different behavior and exhibit these chanonly at high temperatures.

ACKNOWLEDGMENTS

The authors gratefully acknowledge support by the Oof Naval ResearchsColin Woodd, the Department of EnergsJane Zhud sdepth-dependent spectrad, the National SciencFoundationsVerne Hessd, and the National Aeronautics aSpace Administration under the Glennan MicrosystInitiative.

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