Influence of Oxide Layer Thickness and Silicon Carbide (SiC) Polytype on SiC MOS Capacitor Hydrogen Sensor Performance BOGDAN OFRIM, FLORIN UDREA, GHEORGHE BREZEANU, ALICE PEI-SHAN HSIEH Devices, circuits and electronic apparatus Politehnica University of Bucharest 313 Splaiul Independentei Street, sector 6, Bucharest ROMANIA bofrim@yahoo.com Abstract: A proposed SiC MOS capacitor structure used as a hydrogen sensor is defined and simulated. The effect of hydrogen, temperature and interface states concentration on C-V characteristic is analyzed and evaluated. A comparison between structures with different oxides (SiO 2, TiO 2 ) and SiC polytypes (3C, 6H, 4H) is performed. Also, several oxide layer thicknesses (50..10nm) are employed for the TiO 2 /6H-SiC case. A comparison with the 50..10nm SiO 2 /6H-SiC structure, which was analyzed in a previous work, is conducted. Results show that the 6H polytype represents the best option for the SiC layer. Also, the oxide layer thickness has a greater influence on the sensor s performance when SiO 2 is used. Key-Words: silicon carbide, SiC, MOS capacitor, hydrogen sensor, gas sensing 1 Introduction The negative impact on environment of burning fossil fuels has drawn attention to clean energy technologies. Hydrogen represents a clean energy source of high interest. It is also used in processes of many industries such as chemical, petroleum, food and semiconductor [1]. Hydrocarbons are widely used as fuels and represent a component in products like pesticides, solvents and plastics. They are found in exhaust gases and they also contribute to the formation of greenhouse gases. Hydrocarbons affect health during short exposure [1]. The processes involving hydrogen and hydrocarbons need sensors for gas distribution monitoring and leak detection. These processes require sensors to operate in high temperature and harsh environments. Besides, the sensors must have high sensitivity and reliability, fast response time and they must as well be able to detect a wide range of gas concentrations. Hence, intensive work is carried out to develop materials and structures that could withstand such severe operating conditions and high performance needs [1,2]. Silicon carbide (SiC) MOS capacitor sensors are widely used for hydrogen and hydrocarbons detection. The large band gap of silicon carbide enables the sensor to operate at high temperatures. In addition, SiC is chemically stable making it suitable for harsh environment applications [1,3]. Previous work was conducted in regards to the influence of oxide type and oxide layer thickness on the performance of a silicon carbide MOS capacitor used as a hydrogen sensor [4]. Different 50nm oxides were employed: SiO 2, ZnO and TiO 2. In the SiO 2 case, several thicknesses ranging between 50nm and 10nm were considered [4]. Results showed that the best performance is obtained by the 50nm TiO 2 structure [4]. Also, the performance of the SiO 2 structure increases when the oxide thickness decreases [4]. The current paper presents a follow-up on the previous work described above. Several oxide layer thicknesses were considered for TiO 2, too. Besides, different SiC polytypes were used for MOS capacitor structures based on both SiO 2 and TiO 2. The change in the sensor's C-V characteristic due to a hydrogen concentration, temperature or interface states was investigated by extensive simulations. 2 Structure The general SiC MOS capacitor structure used to simulate the hydrogen sensors is illustrated in Fig.1. This structure consists of a 250µm n-type SiC substrate with a doping of 5 x 10 18 cm -3. On top of the bulk, there is considered a 3µm epitaxial layer with a concentration of 2 x 10 16 cm -3. Different SiC polytypes were used: 3C, 6H and 4H. Over the epitaxial layer a 50nm oxide is deposited. Two oxides were utilized: SiO 2 and TiO 2. For the TiO 2 /6H-SiC structure, several oxide thicknesses between 50nm and 10nm were employed. Palladium ISBN: 978-1-61804-119-7 67
Fig.1 General structure for the simulated SiC MOS capacitor (Pd) was considered for the metal electrode because of hydrogen s high solubility in this material. When the structure is introduced into a hydrogen environment, gas molecules dissociate in contact with the Pd electrode at temperatures as low as 150 C. Some of the hydrogen atoms remain at the surface of the metal and others diffuse into the metal until they reach the metal-oxide interface. Here, they create a dipole layer which decreases the Pd work function. This phenomenon reduces the flat band voltage of the MOS capacitor and causes a parallel shift of the structure's C-V characteristic towards negative voltages [5]. At high temperatures, over 700K, hydrogen atoms diffuse further into the structure until they reach the oxide-semiconductor interface. Here, they passivate the interface traps causing a reduction of charged states' concentration. This shortens the transition from accumulation to inversion of the sensor's C-V characteristic [5]. The sensor's response to hydrogen ambient is the voltage shift needed to keep a constant capacitance in the depletion region of the structure [6]. The passivation of interface traps is a reversible but not very stable process. Thus, in order to perform hydrogen concentration measurements of high reliability, the influence of charged states' density on the sensor's response must be negligible [6]. 3 Simulation The SiC MOS capacitor structures were simulated using MEDICI in both inert and hydrogen environments, at different temperatures and interface states densities [7]. The voltage shift in C-V characteristics with hydrogen concentration was investigated. MEDICI does not offer the possibility to specify different gas concentrations for the external environment. Thus, in order to simulate the presence of hydrogen, the metal work function was changed in accordance to the experimental C-V characteristics shifts published in literature [1,8,9]. A reduction of the work function from 5.12eV (the value for Pd in an inert environment) to 4.12eV was employed in order to simulate the hydrogen concentration. The same hydrogen absorption was considered for all structures. Previous work has shown that the oxide type and oxide layer thickness do not influence the voltage shift of the sensor's C-V curve when considering the same hydrogen absorption [4]. Therefore, in this paper the impact of hydrogen on C-V characteristics is investigated only for the structures based on SiO 2 with 3C, 4H, and 6H SiC polytypes. The simulations were performed at a temperature of 1000K with an interface states' concentration of 1 x 10 12 cm -2 /ev [10]. The effect of the temperature variation on C-V characteristic is studied as well. Because the SiC MOS capacitor structures operate at high temperatures, simulations were performed at 700K and 1000K in an inert environment. The influence of interface states' density on the structures' C-V characteristics is also analyzed. Simulations were executed with traps concentrations of 1 x 10 12 cm -2 /ev and 6 x 10 10 cm -2 /ev at 1000K in an inert environment. The difference between the dielectric constants of the oxides tested (3.9 for SiO 2 and 31 for TiO 2 ) and the variation of TiO 2 layer thickness both cause a large change in the accumulation region's capacitance. For a better comparison between the structures, the C/C OX ratio is plotted versus bias voltage. 4 Results 4.1 Oxide thickness variation 4.1.1 Temperature variation The C-V characteristics of the TiO 2 /6H-SiC structure with different oxide layer thicknesses at temperatures of 700K and 1000K are illustrated in Fig.2. The results were compared to the ones obtained for the SiO 2 structure (Fig.3) [4]. It can be observed that as the oxide layer thickness decreases, the variation with temperature of the C-V curves also decreases for both structures. ISBN: 978-1-61804-119-7 68
Fig.2 C-V characteristics of SiC MOS capacitors based on TiO 2 with different oxide thicknesses at temperatures of 700K and 1000K Fig.4 C-V characteristics of TiO 2 based SiC MOS capacitors of different oxide thickness with interface states of 1x10 12 cm -2 /ev and 6x10 10 cm -2 /ev Fig.3 C-V characteristics of SiC MOS capacitors based on SiO 2 with different oxide thicknesses at temperatures of 700K and 1000K,[4]. For TiO 2, the influence of the oxide layer's thickness on the temperature variation is smaller than in the SiO 2 case due to the higher dielectric constant of TiO 2 oxide. 4.1.2 Interface states variation The change in the C-V characteristics with oxidesemiconductor interface traps' concentration for the TiO 2 /6H-SiC structure is presented in Fig.4. The results were also confronted to the ones obtained for SiO 2 (Fig.5) [4]. It can be observed that when the oxide layer thickness is reduced, the variation of C-V plots is diminished. Also, the TiO 2 structure is less sensitive to the change of interface states' concentration and oxide thickness. The SiO 2 structure does not reach the performance of the 50nm TiO 2 structure even when its oxide layer thickness is reduced to 10nm. In the case of 10nm TiO 2, the variation of interface states' concentration produces a negligible change Fig.5 C-V characteristics of SiO 2 based SiC MOS capacitors of different oxide thickness with interface states of 1x10 12 cm -2 /ev and 6x10 10 cm -2 /ev [4]. in the C-V characteristics. Thus, the reversible passivation/creation of charged traps at the oxidesemiconductor interface has an insignificant influence on the sensor's output signal. This offers high reliability for the hydrogen concentration measurements. 4.2 SiC polytype variation 4.2.1 Work function variation The C-V curves of the MOS capacitors based on SiO 2 with different SiC polytypes (3C, 6H and 4H) in inert and hydrogen environments are illustrated in Fig.6. The continuous lines signify the inert environment for which a work function value of 5.12eV was used in simulations. The hydrogen environment is represented by the interrupted lines and was simulated by using a value of 4.12eV for the Pd work function. ISBN: 978-1-61804-119-7 69
Fig.6 C-V characteristics of SiC MOS capacitors based on SiO 2 with different SiC polytypes in both inert and hydrogen environments By comparing the plots from Fig.6, one can observe that each MOS capacitor structure based on a different SiC polytype has a similar voltage shift induced by hydrogen in the C-V characteristic. This signifies that the SiC polytype does not influence the change in the sensor's C-V curve when the same hydrogen absorption is considered. 4.2.2 Temperature variation The C-V characteristics of the structures based on SiO 2 and TiO 2 with different SiC polytypes at 700K and 1000K are shown in Figs.7-8. For the MOS capacitors with SiO 2, the 3C-SiC structure has the largest variation with temperature. The lower energy band gap of 3C-SiC (2.36eV) enables more electrons to jump from the valence band to the conduction one when the temperature increases. The 6H and 4H structures have similar dependence on temperature, in spite of the difference between their band gaps (2.86eV for 6H and 3.23eV for 4H). Fig.7 C-V characteristics of SiC MOS capacitors based on SiO 2 with different SiC polytypes at temperatures of 700K and 1000K Fig.8 C-V characteristics of SiC MOS capacitors based on TiO 2 with different SiC polytypes at temperatures of 700K and 1000K For the TiO 2 structures, all SiC polytypes have the same temperature dependence in the depletion region of the C-V characteristic. The change of the capacitance in the inversion region decreases when the SiC polytype band gap increases. As mentioned before, the sensor's response to hydrogen concentration is measured by keeping the structure at a constant capacitance in the depletion region and by measuring the voltage necessary to maintain that capacitance. Thus, it can be concluded that the SiC polytype does not influence the temperature variation of the TiO 2 sensor's output signal. 4.2.3 Interface states variation The change in C-V characteristics with interface traps' concentration is illustrated in Fig.9 (SiO 2 ) and Fig.10 (TiO 2 ). It can be observed that the 3C-SiC structures have the lowest variation. Also, the 4H-SiC structures have the highest change in the C-V curve. The reason for this behavior is the following: for a fixed interface states' concentration, the number of charged traps at the oxide-semiconductor interface increases when the band gap increases. Thus, when the concentration of interface states is reduced, the change in the number of charged traps increases when the band gap increases. A larger decrease of interface states causes a greater variation of C-V characteristic. Thus, it can be concluded that when the SiC band gap increases, the impact of interface traps' concentration increases as well. For the SiO 2 structure, the SiC polytype has a significant influence on the C-V characteristic's variation with interface states' concentration. On the other hand, in the TiO 2 case, the impact of the SiC polytype is much reduced. ISBN: 978-1-61804-119-7 70
Fig.9 C-V characteristics of SiC MOS capacitors based on SiO 2 with different SiC polytypes and interface states concentrations of 1x10 12 cm -2 /ev and 6x10 10 cm -2 /ev The voltage shift of the sensor's C-V characteristic in a hydrogen environment is similar for all SiC polytypes employed, when considering the same hydrogen absorption. For the TiO 2 structure, the impact of the SiC polytype on the C-V curve's change with temperature and interface states' concentration is very small. For SiO 2, there is a significant distinction in performance between the SiC polytypes. The 6H polytype represents a compromise between the variations with temperature and interface states' densities. The best performance was obtained by the 10nm TiO 2 structure. Therefore, a 6H-SiC MOS capacitor with a 10nm TiO 2 oxide layer is recommended for hydrogen detection at elevated temperatures up to 1000K. Fig.10 C-V characteristics of SiC MOS capacitors based on TiO 2 with different SiC polytypes and interface states concentrations of 1x10 12 cm -2 /ev and 6x10 10 cm -2 /ev 4 Conclusion A silicon carbide MOS capacitor structure used as a hydrogen sensor was simulated extensively. Certain SiC polytypes and oxides were employed. For the TiO 2 /6H-SiC structure, different oxide layer thicknesses were used. The variation of the C-V characteristic in a hydrogen environment, at different temperatures or interface states' concentrations was investigated by extended simulations. Results have shown that the change in the TiO 2 /6H-SiC structure's C-V characteristics with temperature and interface states' concentration diminishes when the oxide layer thickness decreases. However, the change is very small in comparison to the one obtained in the previous work for the SiO 2 /6H-SiC MOS capacitor [4]. Thus, it can be concluded that the oxide layer thickness is important only for SiO 2, not for TiO 2. References: [1] A. Trinchi, S. Kandasamy, W. Wlodarski, High temperature field effect hydrogen and hydrocarbon gas sensors based on SiC MOS devices, Sensors and Actuators B, No.133, Elsevier, 2008, pp. 705 716. [2] Xi Dong Qu, MOS Capacitor Sensor Array for Hydrogen Gas Measurement, Simon Fraser University, summer 2005. [3] P. Tobias, B. Golding, R. N. Ghosh, Sensing Mechanisms of High Temperature Silicon Carbide Field-Effect Devices, Proc. Of 12 th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, June 8-12, 2003, pp. 416-419. [4] B. Ofrim, F. Udrea, G. Brezeanu, A. Hsieh, Hydrogen sensor based on silicon carbide (SiC) MOS capacitor, sent to International Semiconductor Conference (CAS), Sinaia, Romania, October 2012. [5] M. T. Soo, K. Y. Cheong, A. F. M. Noor, Advances of SiC-based MOS capacitor hydrogen sensors for harsh environment applications, Sensors and Actuators B, No.151, Elsevier, 2010, pp.39-55. [6] R. N. Ghosh, P. Tobias, S. G. Ejakov, B. Golding, Interface States in High Temperature SiC Gas Sensing, Proceedings of IEEE Sensors, Vol.2, 2002, pp. 1120-1125. [7] Synopsys, Inc., Taurus Medici User Guide, Version Y-2006.06, June 2006. [8] Md H. Rahman, J. S. Thakur, L. Rimai, S. Perooly, R. Naik, L. Zhang, G. W. Auner, G. Newaz, Dual-mode operation of a Pd/AlN/SiC device for hydrogen sensing, Sensors and ISBN: 978-1-61804-119-7 71
Actuators B, No. 129, Elsevier, 2008, pp. 35-39. [9] S. Kandasamy, Investigation of SiC Based Field Effect Sensors with Gas Sensitive Metal Oxide Layers for Hydrogen and Hydrocarbon Gas Sensing at High Temperatures, Doctor of Philosophy Thesis, School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia, December 2007. [10] W. J. Choyke, H. Matsunami, G. Pensl, Silicon Carbide: Recent Major Advances, Springer- Verlag Berlin Heidelberg, 2004. ISBN: 978-1-61804-119-7 72