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Study of interface state density and effective oxide charge in post-metallization annealed SiO2SiC structures

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Study of interface state density and effective oxide charge in post-metallization annealed SiO2SiC structures
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  CAMPI  et al .: STUDY OF INTERFACE STATE DENSITY AND EFFECTIVE OXIDE CHARGE 513 (a) (b)Fig. 2. Normalized HFCV characteristics of (a) Mo/SiO   /4HSiC and (b) Mo/SiO   /6HSiC structures demonstrating the effect of PMA in forming gas.All devices were measured in the dark at 25    C. The theoretical curves were calculated assuming an electron affinity, doping concentration, and oxidethickness of 3.6 eV,      2      cm 0    , and 315 ˚A for 4HSiC and 3.85 eV,   2      cm 0    , and 295 ˚A for 6HSiC. The work function for Mo, Al,and Au were assumed to be 4.69 eV, 4.1 eV, and 5.0 eV, respectively. be taken when determining interface state density deep inthe energy band gap of SiC because the information on holecapture cross sections is too limited.Standard approaches have been taken to ensure accuratemeasurement of quasi-static capacitance such as monitoringand subtracting the effect of the leakage current and parasiticseries resistance [7].IV. S AMPLE  P REPARATION Si-faced p-type 4H and 6HSiC wafers, purchased fromCREE Research, Inc. were used in this study. For all thesamples, there was a 4.5- m thick CVD grown epilayer onthe p substrate. Three p-type 4HSiC wafers of differentdoping concentrations ( cm , cm ,and cm ) were used. The doping concentrationfor p-type 6HSiC samples was either cm orcm . Prior to oxidation, the wafers were cleaned bythe RCA cleaning process in a class-100 cleanroom followedby a dip in a buffered hydrogen fluoride solution. Afterthe cleaning, the samples were immediately loaded into theoxidation tube at room temperature in a nitrogen atmosphereand the furnace temperature ramped up over a 2 h periodto the oxidation temperature. Wet oxidation was performedat 1100 C in a horizontal quartz tube for 2 h to form a300-˚A thick oxide layer. The wet oxygen was formed bybubbling oxygen through deionized water at 90 C. Followingthe oxidation, a 60 min  in situ  annealing was done in argon,also at 1100 C. The samples were unloaded after the oxidationfurnace was cooled down to room temperature in the argonatmosphere. Next, a layer of 200 nm of metal was depositedonto the oxide to form the front side contacts. Three differentmetals, molybdenum, aluminum, and gold, were used in thisstudy. Molybdenum and gold were deposited by sputtering,while both sputtering and thermal evaporation were usedfor aluminum deposition. Gate contacts were then patternedusing standard photolithography techniques and defined by wetetching. All MOS capacitors were circular with a diameterof 500 m and surrounded by an isolation ring 25 m inwidth, while the remaining open area comprised a large areacapacitor. The area of this large capacitor was more than 100times larger than the area of the measured devices and servedas the other front contact for – measurements. The PMAwas then performed either in forming gas or in nitrogen. Theforming gas used throughout our experiments was a mixtureof 5% hydrogen in nitrogen. The optimum PMA duration of 45 min., identified earlier for Mo [3], [4], was used for allthe experiments reported in this paper. By using the front sidecontact scheme, we have repeated each PMA experiment byreusing the same samples to confirm the results reported inthis paper. The samples were reused by simply removing theSiO along with the gate metal on top of the oxide without theconcern of possible heavy metal contamination to the oxidationsystem.V. E XPERIMENTAL  R ESULTS AND  D ISCUSSIONS  A. Total Effective Oxide Charge Density For measurements, high-frequency room-temperature– characterization was performed at 100 kHz using aKeithley 590 – meter. All measurements, performed inthe dark, were made by applying the negative signal terminalto the gate and making small signal contact to the substratethrough the large area capacitor on the top surface. Fig. 2(a)and (b) shows the typical high-frequency – characteristicsfor the control and annealed samples for p-type 4HSiC andp-type 6HSiC, respectively, along with the ideal theoretical– curve. The total effective charge, , was determinedfrom the difference between the experimental and theoreticalflat-band voltages. It is seen from Fig. 2 that PMA can resultin a – curve with a very small voltage deviation from thetheoretical one, indicating a very low . The work functionsneeded for the calculation of the theoretical – curves wereassumed to be 4.69, 4.1, and 5.0 V for Mo, Al, and Au [10],respectively.
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