Materials Science Forum Vols. 740-742 (2013) pp 733-736 Online available since 2013/Jan/25 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.740-742.733
Influence of nitrogen implantation on electrical properties of Al/SiO 2/4HSiC MOS structure K. Krol1,2,a, M. Sochacki1,b, M. Turek3, J. Zuk3, H. M. Przewlocki4, T. Gutt4, P. Borowicz4,5, M. Guziewicz4, J.Szuber6, M. Kwoka6, P. Koscielniak6, J. Szmidt1,c 1
Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland
2
Tele- and Radio Research Institute, Ratuszowa 11, 03-450 Warsaw, Poland
3
Maria Curie-Sklodowska University, Maria Sklodowska-Curie Sq. 1, 20-031 Lublin, Poland 4
Institute of Electron Technology, Lotnikow 32/47, 02-668 Warsaw, Poland
5
Institute of Physical Chemistry, Kasprzaka 44/52, 01-224 Warsaw, Poland
6
Silesian University of Technology, Krzywoustego 2, 44-100 Gliwice, Poland
a
E-mail:
[email protected],
[email protected],
[email protected] Keywords: silicon carbide, thermal oxidation, ion implantation, ion implantation damage.
Abstract: An influence of nitrogen implantation dose on the properties of MOS structure is analyzed. The properties are investigated using C-V, I-V measurements, Raman spectroscopy, and XPS profiling. It has been shown that the trap density is directly related to implantation damage and conditions. Introduction Silicon carbide is only one wide-bandgap material creating SiO2 layer by thermal oxidation. Unfortunately, the oxidation is a complex and multistage process. An efficient way to improve the channel mobility of MOSFETs is the annealing in NO or N2O [1]. The mobility is related to nitrogen concentration in the vicinity of MOS interface [2]. Some investigated method for introducing nitrogen to interface region is ion implantation [3,4]. The published results are inconsistent considering nitrogen dose in relation to traps profile. The objective of this work is to determine the effect of nitrogen implantation dose on traps properties. Experiment
Fig.1. Depth profile of implanted nitrogen concentration. Vertical lines marks the location of oxide/SiC interface
The experiment was performed on n-type 4H-SiC (0001) with an epitaxial layer of 10μm and doping of 1x1016 cm-3. First, a masking layer of PECVD SiO2 was deposited. Next, samples #1 - #4 were implanted with N2 at energy of 100keV at room temperature. Samples were processed in pairs, one used as a monitor for material characterization (#Xm) and sample used in MOS device fabrication (#X). Sample #5 left unimplanted. No postimplatation annealing was carried out. The simulated nitrogen concentration profiles are shown in
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Fig. 1. After implantation, the masking layers were removed. The monitor surface was measured by micro-Raman spectrometer with UV laser. Then, samples were oxidized in dry O2 at temperature of 1200oC. The implantation conditions were calculated using SRIM. Table 1 shows the measured and calculated properties of the samples. The electrical properties of MOS capacitors were measured using circular gate devices with gate area of 1.8 x 10-4 cm2. An C-V and I-V measurements were performed using Keithley 4200 Semiconductor Characterization System. Samples #1m-#4m were characterized using XPS profiling measurement to determine chemical and structural composition of dielectric layers as a function of depth. Results and discussion Table 1 shows the calculated parameters of MOS structures. Sample #5 exhibits relatively high positive flatband voltage. As the implantation dose is increased, the flatband voltage decreases that it is caused by accumulation of positive charge in the oxide due to nitrogen incorporation. Similar behavior was observed in case of annealing in nitrous oxide. Figure 2 shows a cumulative failure rate using a Weibull distribution. We have obtained higher mean value of critical electric field for implanted samples in comparison to the reference. No
#1 #2 #3 #4 #5
Table 1 - Calculated average values of electrical parameters EOT UFB Oxide Implantation Interface N thickness dose concentration [nm] [V] [nm] [cm-2] [cm-3] 49.5 33.7 50.4 35.8 77.1
56 38 59 42 86
~1.5∙10 ~1.5∙1014 ~1.5∙1015 ~1.5∙1016 13
3.3∙10 1.0∙1017 3.4∙1018 2.5∙1019 16
0.90 -0.10 -3.72 -8.74 2.89
Qeff/q [cm-2] -2.28∙1011 2.24∙1011 1.46∙1012 4.64∙1012 -6.04∙1011
The implantation also reduces the standard deviation of critical electric field. The critical field increases slightly in the investigated dose range for samples #1 - #3. An abrupt increase is observed for sample #4. Fig. 3 shows the calculated distribution of interface traps near the conduction band. No improvement of deep trap density can be observed for samples #1 and #2. On the other hand, trap density near the conduction band for samples #1 and #2 is lower than in the reference. Sample #3 behaves in a different way. No improvement of trap density near the conduction band edge is observed. However, a slight decrease of trap density occurs deeper in the bandgap. This change is expected, since nitrogen reduces mainly the traps located deeper in the bandgap [5] and the implanted nitrogen concentration for sample #3 is higher than its concentration in epitaxial layer. The trap density near the conduction band rises for sample #4 and tends to the specific level of the reference below 2.8 eV. The effect of implantation doe and damage can be observed at Raman spectra. Typical one-phonon Raman spectra for silicon carbide is shown at Fig. 4. All implanted samples exhibit a slight shift of peak towards higher frequency comparing to unimplanted one. Moreover, the Raman spectra for samples #1 and #2 are more intensive than the reference. Similarly to trap density near the conduction band, the peak value for sample #3 is at the same level as in the reference. A rapid decrease of all peaks intensity can be observed for sample #4. The substrate damage was simulated using SRIM to correlate the results to the amount of vacancies. This procedure included the depth profile of damage events (knock-outs, both silicon and carbon) normalized to SiC atomic density. Figure 5 shows the results of this calculation. The damage level for samples #1-#3 in the vicinity of interface region is lower than 5% whereas the level is approx. 25% for sample #4. The significant change in the damage level strongly interferes the electrical properties. In order to explain an observed phenomena, the following hypothesis is proposed. The states in the midgap are presumed to be traps associated with carbon impurities by Deak et. al [3]. In contract to the results presented in [3], we conclude that the rising edge of the U-shaped trap profile near the conduction band is not related to near interface traps (NIT), since these traps cannot be detected using HF method, although they may exist in the examined sample. Only sample #4
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exhibits a large hysteresis probably caused by NITs. The structural defects related to bond length mismatch between SiO2 and SiC and/or non-stoichiometric SiOX bonds and transitional phase at the interface are assumed as the cause of those traps.
Fig.2. Cumulative breakdown failure of measured samples.
Fig. 3 Distribution of Dit near Ec for implanted samples and the reference
Fig.4. One phonon spectra of silicon carbide substrates implanted with nitrogen. The existence of these phases was predicted theoretically by Gavrikov et. al. [6]. In this model, low dose nitrogen implantation reduces stress near the interface region and decreases the amount of stress-related defects. It can be observed as a lowering of trap density near the conduction band. A small dose of nitrogen is enough to achieve the improvement. Unfortunately, low implantation dose is not enough to efficiently reduce the carbon related traps deeper in the bandgap. Thus, no improvement was observed at energy lower than ~3eV. Simultaneously, the increasing dose generates more structural defects, making a contribution to trap density near the conduction band. As a result, the traps density calculated for sample #3 and sample #5 is almost the same. It is confirmed by Raman spectroscopy results. Higher nitrogen concentration starts to interact with carbon related impurities deeper in the bandgap. It is Fig.5. The calculated damage level observed at trap energy profile and critical field plots. Sample #4 is heavily damaged after the implantation. The damage promotes the formation of the interphase at the interface resulting in increased trap density near the conduction band. The effect on the carbon related defects is negligible and the trap density for sample #4 tends to the level of the reference deeper in the bandgap. The damage also promotes nitrogen incorporation into dielectric layer and formation of SiOxNy phase improving the critical field value.
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The presented hypothesis is supported by XPS depth profiling measurements (Fig.6). An increase of intensity of interphase induced SiO O1S signal related to creation of SiOx or SiOxNy is measured for highly implanted samples (Fig. 6) whereas it is negligible for the other samples ( #1 and #2) with flat profile of this signal over the entire depth profiling. a) b)
Fig.6. XPS profiles for samples with high implantation dose - sample #3 (a) and #4 (b) The hypothesis is useful to explain the properties of the traps for implanted samples reported by other groups [3, 4]. Summary The hypothesis on the origin of traps near the conduction band is proposed. The hypothesis is useful to explain the results achieved for nitrogen implanted samples within wide range of implantation dose. It was confirmed that traps near the conduction band are mostly due to structural defects and are not related to carbon impurities. These traps can be controlled by ion implantation causing either decrease or increase of its density that depends on implantation conditions. Acknowledgement This work was supported by Polish National Science Center under project no. N N515 498340 "Influence of shallow ion implantation of 4H-SiC on electro-physical properties of MOS structure" References [1]. G. V. Soares, I. J. R. Baumvol, L. Hold, F. Kong, J. Han, S. Dimitrijev, C. Radtke, F. C. Stedile, Sequential thermal treatments of SiC in NO and O2: Atomic transport and electrical characteristics, Appl. Phys. Lett., 91, (2007) id. 041906 [2]. Rozen, John; Dhar, Sarit; Zvanut, M. E.; Williams, J. R.; Feldman, L. C., "Density of interface states, electron traps, and hole traps as a function of the nitrogen density in SiO2 on SiC", Journal of Applied Physics, Volume 105, Issue 12, pp. 124506-124506-11 (2009) [3]. Moscatelli, F.; Poggi, A.; Solmi, S.; Nipoti, R., Nitrogen Implantation to Improve Electron Channel Mobility in 4H-SiC MOSFET, IEEE Trans. Electron Devices, 55, (2008) 961-967, [4]. D. Okamoto, H. Yano, T. Hatayama, T. Fuyuki, Systematic Investigation of Interface Properties in 4H-SiC MOS Structures Prepared by Over-Oxidation of Ion-Implanted Substrates, Mat. Sci. Forum, Vol. 645 - 648, (2009) pp. 495-498, [5]. P. Deák, J. Knaup, C. Thill, T. Frauenheim, T. Hornos, The mechanism of defect creation and passivation at the SiC/SiO2 interface, J. Phys. D: Appl. Phys., 41, (2008) pp. 049801 [6]. Gavrikov, A.; Knizhnik, A.; Safonov, A.; Scherbinin, A.; Bagatur'yants, A.; Potapkin, Boris; Chatterjee, A.; Matocha, K.,”First-principles-based investigation of kinetic mechanism of SiC(0001) dry oxidation including defect generation and passivation”, J. Appl. Phys., 104, (2008) 093508-093508-9
Silicon Carbide and Related Materials 2012 10.4028/www.scientific.net/MSF.740-742
Influence of Nitrogen Implantation on Electrical Properties of Al/SiO2/4H-SiC MOS Structure 10.4028/www.scientific.net/MSF.740-742.733
