Effects of preoxidation NH3 treatment on p-type 6H–SiC/SiO2 interface properties were investigated as compared to conventional thermally oxidized devices. It was found that NH3 treatment before oxidation can reduce the SiC/SiO2 interface states and fixed oxide charge. Furthermore, less shift of flatband voltage, and smaller increases of effective oxide charge and interface states during high-field stress were observed for the NH3 pretreated devices. © 2000 American Institute of Physics.published_or_final_versio

Chan, CL

Cheng, YC

Lai, PT

Xu, JP

Applied Physics Letters

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Title Improved interface properties of p-type 6H–SiC/SiO2 system byNH3 pretreatmentAuthor(s) Xu, JP; Lai, PT; Chan, CL; Cheng, YCCitation Applied Physics Letters, 2000, v. 76 n. 3, p. 372-374Issued Date 2000URL http://hdl.handle.net/10722/42109Rights Applied Physics Letters. Copyright © American Institute ofPhysics.APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 3 17 JANUARY 2000Improved interface properties of p-type 6H–SiC/SiO2 systemby NH3 pretreatmentJ. P. XuDepartment of Electronic Science & Technology, Huazhong University of Science and Technology,Wuhan, 430074, People’s Republic of ChinaP. T. Lai,a) C. L. Chan, and Y. C. ChengDepartment of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road,Hong Kong~Received 11 October 1999; accepted for publication 16 November 1999!Effects of preoxidation NH3 treatment on p-type 6H–SiC/SiO2 interface properties wereinvestigated as compared to conventional thermally oxidized devices. It was found that NH3treatment before oxidation can reduce the SiC/SiO2 interface states and fixed oxide charge.Furthermore, less shift of flatband voltage, and smaller increases of effective oxide charge andinterface states during high-field stress were observed for the NH3 pretreated devices. © 2000American Institute of Physics. @S0003-6951~00!01803-9#SiC offers many advantages over Si for high-temperature, high-power, and high-freuqency devices due toits large band gap, high thermal conductivity, and high elec-tron saturation drift velocity. Moreover, one of the most im-portant advantages of SiC over other wide band gap semi-conductors is that it can be thermally oxidized to form SiO2 .However, it has been frequently reported that oxides grownon SiC have high interface-state density and a large amountof fixed oxide charges, especially for the SiO2 /p-SiCstructure.1,2 Like silicon processing, an important factor af-fecting the interface and oxide is the passivation of SiC sur-face before oxidation. Relevant research has been reportedusing a H2 surface cleaning technique3–5 and high-mobilitymetal–oxide–semiconductor field effect transistors ~MOS-FETs! were obtained by this method.5 Although this tech-nique can form a hydrogen-terminated surface for 6H–SiC,Si–H bonds have a small binding energy (ESi–H53.17 eV!,6thus degrading device resistance against hot-carrier bom-bardment. In this work, an alternative surface-passivationtechnique with NH3 substituting H2 is employed to alleviatethe above problem, because a nitrogen/hydrogen-terminatedsurface is probably formed during NH3 annealing, givingstronger Si–N bonds (ESi–N’4.6 eV!.6 As demonstrated bythe following experimental results on electrical propertiesand stress-induced degradation, this passivation step is in-deed effective in improving the reliability of p-SiC MOSdevices.p-type ~0001! Si-face 6H–SiC wafers, manufactured byCREE Research, were used in this study. The SiC wafers hada 5 mm epitaxial layer grown on heavily doped substrates.The doping level of the epitaxial layer was 431015 cm23.The wafers were cleaned using the conventional Radio Cor-poration of America ~RCA! method followed by a 60 s dip-ping in 1% HF, and then loaded into a quartz furnace at850 °C. After raising to an oxidation temperature of 1100 °Cin a N2 ambient, a 1 h NH3 treatment on the surface of thea!Electronic mail: laip@eee.hku.hk3720003-6951/2000/76(3)/372/3/$17.00Downloaded 03 Nov 2006 to 147.8.21.97. Redistribution subject to wafers was first performed at a flow of 250 ml/min, and thena 380 min oxidation was carried out in a wet ambient bybubbling oxygen through deionized water at 95 °C ~denotedas NH3 sample! at a flow rate of 250 ml/min. The controlsample without preoxidation NH3 treatment was also ther-mally oxidized under exactly the same conditions ~denotedas OX!. After the oxidation, the two samples were annealedin N2 at a flow of 250 ml/min at 1100 °C for 1.5 h. Finally,aluminum was thermally evaporated, and then patterned asgate electrode of MOS capacitors ~120 mm3150 mm!, fol-lowed by a forming gas anneal at 410 °C for 30 min. Thefinal oxide thickness, determined from high frequency ~hf!accumulation capacitance, was 508 Å. High-field stress~27.87 MV/cm!, with the capacitor biased in accumulation,was used to examine device reliability. The SiO2/SiC inter-face properties were characterized by hf ~1 MHz!capacitance–voltage (C – V) measurements at room tempera-ture under dark condition and after light illumination at deepdepletion. The total deep interface states and fixed chargeswere estimated from a ledge feature occurred on hf C–Vcurves and flatband voltage shift.7 All measurements werecarried out under a light-tight and electrically shielded con-dition.Figure 1 shows the typical hf C–V curves of the twosamples under dark conditions, swept in both directions, andno appreciable hysteresis occurs, implying little slow traps.A smaller flatband shift of the NH3 sample than the OXsample indicates reduced positive fixed charges and deepdonor-type interface states in NH3 oxide, as further demon-strated below.Depicted in Fig. 2 are the hf C–V curves of the twosamples after light illuminations at deep depletion. A 100 Wtungsten lamp was used as the light source,7 and the capaci-tors were illuminated for 3 min with a 10 V bias to form aninversion layer. After removing the illumination, C–V curveswere recorded first from depletion to accumulation and thenfrom accumulation to depletion. It can be seen that aninterface–state ledge8 appears when sweeping from deple-tion to accumulation, implying that deep donor-type inter-© 2000 American Institute of PhysicsAIP license or copyright, see http://apl.aip.org/apl/copyright.jsp373Appl. Phys. Lett., Vol. 76, No. 3, 17 January 2000 Xu et al.face states exist at the interface. Moreover, smaller shifts ofgate voltage at the ledge and flatband voltage ~DVG andDV fb) are observed for the NH3 sample than the OX sample.The total deep interface-state density and fixed charge den-sity can be estimated as follows:7D it5COXuDVGuqAEg, ~1!Q f5COX~ uDV fbu2uDVGu!qA , ~2!FIG. 1. High-frequency C–V curves of the two kinds of capacitors underdark conditions, swept in two directions: depletion to accumulation ~dashedlines! and accumulation to depletion ~solid lines!. Area of capacitor is 1.831024 cm2.FIG. 2. High-frequency C–V curves of the two samples after light illumi-nation at deep depletion, swept in two directions. ~a! Before and ~b! after a5000 s high-field stressing ~27.87 MV/cm! at room temperature. Area ofcapacitor is 1.831024 cm2.Downloaded 03 Nov 2006 to 147.8.21.97. Redistribution subject to where COX is the oxide capacitance, q electronic charge, Adevice area, and Eg energy gap ~3.0 eV for 6H–SiC at roomtemperature!.9 DV fb is determined from the flatband capaci-tance C fb ~Ref. 10! by using 6H–SiC dielectric constant of9.66:9C fbCOX5S 11150AT/300tOXANAD 21. ~3!Here, tOX is oxide thickness, NA acceptor doping density,and T temperature ~K!. In Fig. 2~a!, it can be found thatDVG51.25 V and DV fb524.4 V for the OX sample, andDVG520.75 V and DV fb523.3 V for the NH3 sample.Hence, D it and Q f are estimated to be 1.7431011cm22 eV21 and 1.3131012 cm22 for the fresh OX sample,and 1.0431011 cm22 eV21 and 1.0631012 cm22 for thefresh NH3 sample. Smaller D it and Q f of the NH3 sampleshould be associated with NH3-treatment-induced nitrogen-hydrogen-terminated SiC surface, because the initial surfacestructure is succeeded by the SiO2 /SiC interface even afteroxidation.5 So, it is reasonable to consider the surface passi-vation as an important issue in improving the interface qual-ity of SiC MOS devices.Figure 3 shows the flatband voltage shifts of the twosamples during high-field stressing ~27.87 MV/cm! at roomtemperature. Obviously, a much smaller DV fb is obtained forthe NH3 sample. The negative DV fb means generations ofdonor-type interface states and positive oxide charges. FromFig. 2~b! and Eq. ~1!, it can be found that after stressing for5000 s. D it increases to 3.3331011 cm22 eV21 for the OXsample and 2.2231011 cm22 eV21 for the NH3 sample, i.e.,DD it51.5931011 and 1.1831011 cm22 eV21, respectively.On the other hand, from the maximum DV fb at 5000 s stresstime, the increase of effective oxide charge is estimated to be1.631012 cm22 for the OX sample and 8.331011 cm22 forthe NH3 sample. Similar to NO-annealed devices,11 the en-hanced resistance of the NH3 device against the high-fieldstress is likely linked to the formation of strong Si–N bondsduring NH3 treatment. In addition, during the initial 100 sstress time, DV fb should mainly result from pre-existingdonor-type interface states and near-interface hole traps,which are both neutral due to electron occupation beforestress. From this, it can be deduced that NH3 oxide has muchfewer donor-type interface state and oxide traps than OXFIG. 3. Flatband voltage shifts of the two samples under high-field stressing~27.87 MV/cm! at room temperature. Area of capacitor is 1.831024 cm2.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp374 Appl. Phys. Lett., Vol. 76, No. 3, 17 January 2000 Xu et al.oxide. This is consistent with the results in Fig. 2~a!. There-fore, it can be suggested that NH3 treatment before oxidationis significantly conductive to improving p-SiC/SiO2 interfaceand oxide qualities, probably due to formation of a nitrogen-and hydrogen-terminated and atomically flat surface.Improved p-SiC/SiO2 interface and oxide qualities weredemonstrated in MOS devices with NH3 pretreatment. En-hanced reliability of the NH3-treated oxide was also ob-served under high-field stress. These facts indicate that NH3treatment before oxidation is effective in forming a nitrogen/hydrogen-passivated layer on SiC surface, thus reducing theinterface states and oxide charge, and enhancing the stabilityof MOS devices. The reported results here will encourageresearch on the optimization of surface-passivation condi-tions for better SiC device properties.This work is financially supported by the RGC andCRCG Research Grants, the University of Hong Kong.Downloaded 03 Nov 2006 to 147.8.21.97. Redistribution subject to 1 S. T. Sheppard, J. A. Cooper, Jr., and M. R. Melloch, J. Appl. Phys. 75,3205 ~1994!.2 D. Alok, P. K. McLarty, and B. J. Baliga, Appl. Phys. Lett. 65, 2177~1994!.3 H. Tsuchida, I. Kamata, and K. Izumi, Appl. Phys. Lett. 70, 3072 ~1997!.4 H. Tsuchida, I. Kamata, and K. Izumi, Jpn. J. Appl. Phys. 36, L6990~1997!.5 K. Ueno, R. Asai, and T. Tsuji, IEEE Electron Device Lett. 19, 244~1998!.6 J. Ahn, M. Arendt, J. M. White, and D. L. Kwong, International Confer-ence on SSDM, 1992, p. 416.7 H. Yano, F. Katafuchi, T. Kimoto, and H. Matsunami, IEEE Trans. Elec-tron Devices 46, 504 ~1999!.8 A. Goetzberger and J. C. Irvin, IEEE Trans. Electron Devices 15, 1009~1968!.9 M. Ruff, H. Mitlehner, and R. Helbig, IEEE Trans. Electron Devices 41,1040 ~1994!.10 D. K. Schroder, Semiconductor Material and Device Characterization,2nd. ed. ~Wiley, New York, 1998!.11 H. F. Li, S. Dimitrijev, and H. B. Harrison, IEEE Electron Device Lett.19, 279 ~1998!.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Improved interface properties of p-type 6H–SiC/SiO2 system by NH3 pretreatment

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Improved interface properties of p-type 6H–SiC/SiO2 system by NH3 pretreatment

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