Enhanced Forward Bias Operation of 4H-SiC PiN Diodes Using High Temperature Oxidation

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society DOI: /opl Enhanced Forward Bias Operation of 4H-SiC PiN Diodes Using High Temperature Oxidation Craig A. Fisher 1, Michael R. Jennings 1, Yogesh K. Sharma 1, Dean P. Hamilton 1, Stephen M. Thomas 1, Fan Li 1, Peter M. Gammon 1, Amador Pérez-Tomás 2, Susan E. Burrows 3 and Philip A. Mawby 1 1 School of Engineering, University of Warwick, Coventry, CV4 7AL, UK. 2 Institut Català De Nanociència i Nanotecnologia, 08193, Bellaterra, Barcelona, Spain. 3 Department of Physics, University of Warwick, Coventry, CV4 7AL, UK. ABSTRACT In this paper, high temperature (>1400 C) thermal oxidation has been applied, for the first time, to 4H-SiC PiN diodes with thick (110 µm) drift regions, for the purpose of increasing the carrier lifetime in the semiconductor. PiN diodes were fabricated using 4H-SiC material that had undergone thermal oxidation performed at 1400 C, 1500 C and 1600 C, then were electrically characterized. Forward current-voltage (I-V) measurements showed that thermally oxidized PiN diodes exhibited considerably improved electrical characteristics, with devices oxidized at 1500 C having a forward voltage drop (V F ) of 4.15 V and a differential on-resistance (R on,diff ) of 8.9 mω-cm 2 at 100 A/cm 2 and 25 C. Compared to typical control sample PiN diode characteristics, this equated to an improvement of 8% and 23% for V F and R on,diff, respectively. From analysis of the reverse recovery characteristics, the carrier lifetime of the PiN diodes oxidized at 1500 C was found to be 1.05 µs, which was an improvement of around 30% compared to the control sample PiN diodes. INTRODUCTION Due to its excellent electrical and thermal properties, 4H-silicon carbide (SiC) is widely tipped to be the successor to silicon (Si) for high voltage (>3kV) power electronics [1]. These superior material properties of 4H-SiC include a high critical electric field of ~3 MV/cm (around 10 times higher than Si), a high thermal conductivity of ~4.5 W/cm-K (around 3 times that of Si) and an energy band gap of 3.26 ev (also around 3 times that of Si). From a practical perspective, these material advantages mean that, compared to Si, 4H-SiC offers improved voltage blocking performance for a given drift region thickness and doping; this in turn means that shorter carrier lifetimes are sufficient to fully modulate the drift region in bipolar devices, thus also yielding much improved transient performance. Furthermore, the higher thermal conductivity and wider energy band gap are both beneficial for device operation under high power density and high operating temperatures, two increasingly important factors for future power electronics systems. Despite the obvious advantages of 4H-SiC for power electronics devices, the material has not yet fulfilled its true potential, and, to date, only unipolar devices (Schottky diodes and MOSFETs) are commercially available, up to a maximum voltage rating of 1.7 kv [2]. At higher voltages, MOSFETs suffer from increased conduction losses; as such, the use of bipolar devices, such as IGBTs, thyristors and PiN diodes, is preferable for high voltage applications, as the conductivity modulation effect reduces the on-state voltage drop across the thick drift region, thus reducing the power losses of the device. However, the effectiveness of this conductivity modulation is heavily dependent on the carrier lifetime in the semiconductor; for example, a 4H-

2 SiC bipolar device designed to block 10 kv requires a carrier lifetime of around 5 µs to ensure sufficient conductivity modulation for optimum on-state losses [3]. Unfortunately, the carrier lifetime of as-grown 4H-SiC is typically less than 1 µs [4], hence some form of post-growth treatment to increase the carrier lifetime is required if low on-state losses are to be realized from high voltage 4H-SiC bipolar devices. It is widely accepted that the predominant cause of short carrier lifetimes in 4H-SiC is the presence of the carbon vacancy-related Z 1/2 defect center in the semiconductor bulk [5]. However, by employing either a thermal oxidation [4] or carbon implantation (and subsequent anneal) process [6], the carrier lifetime of 4H-SiC has been found to be dramatically increased. This increase in carrier lifetime is due to the presence of excess carbon interstitials near the 4H-SiC surface that arise from the thermal oxidation or annealing process, which then diffuse into the semiconductor bulk and effectively repair the carbon vacancy defects [7]. On evaluation of these two methods, the use of carbon implantation has the disadvantage of requiring two separate fabrication processes (the implantation and a high-temperature anneal), as well as generating new defect levels in the surface region due to the damage caused by the ion implantation process [4]. However, the use of thermal oxidation typically requires very long oxidation times to eliminate the Z 1/2 defect center in the thick epitaxial layers needed for high voltage 4H-SiC devices; for instance, when performing the oxidation at 1300 C, the process takes over 50 hours when applied to a 100 µm thick epitaxial layer [8]. Encouragingly, it was also shown in [8] that by increasing the oxidation temperature to 1400 C, the time taken to eliminate the Z 1/2 center to a depth of 100 µm was reduced to 16.5 hours, as the oxidation rate was dramatically increased. By employing temperatures higher than 1400 C, it has been shown that the oxidation rate can be increased much further still [9]. However, to the best of the authors knowledge, performing this high temperature oxidation has not yet been applied for increasing the carrier lifetime in 4H-SiC bipolar devices. From a device manufacturing perspective, the use of high temperature oxidation for lifetime-enhancement can offer significantly reduced processing time, and thus lower overall processing cost. As such, this paper investigates the effect of performing thermal oxidation at temperatures up to 1600 C on the electrical characteristics of 4H-SiC PiN diodes. EXPERIMENT Device Fabrication The substrates employed in this work were n-type Si-face 4 off-axis 4H-SiC, with a micropipe density less than 1 cm -2. Epitaxial layers (details shown in Figure 1) were grown in a continuous growth run to minimize the effects of interface recombination on the overall carrier lifetime of the PiN diodes [10]. After epitaxial growth, the epiwafer was laser-cut into mm dies for subsequent device fabrication, and the dies then underwent a solvent- and acidbased cleaning process. The dies then underwent thermal oxidation at 1400 C, 1500 C or 1600 C, with one die being processed at each temperature. One further die underwent no thermal oxidation, to provide a control sample. Each thermal oxidation was performed for a duration of 5 minutes, in a dry pure oxygen (O 2 ) environment. Following the oxidation process, the thermally grown oxide layer was removed in dilute HF solution, and the dies rinsed in DI water. Next, individual device anodes were mesa-isolated, with active areas ranging from cm 2 to cm 2. Deposited tetraethyl orthosilicate (TEOS) SiO 2 was used for surface passivation, and an optimized titanium (Ti) / aluminum (Al) ohmic contact was used for the

3 anode [11]. A Ti / nickel (Ni) ohmic contact was used for the cathode, and thick layers of Al and silver (Ag) were deposited onto the front and back sides of the dies respectively, to enable wire bonding and soldering to direct copper bond (DCB) substrates. A cross-sectional schematic of the fabricated PiN diodes is shown in Figure 1. Figure 1. Cross-sectional schematic of fabricated PiN diodes. Test Setup In order to measure the current-voltage (I-V) characteristics of the fabricated PiN diodes, a Tektronix 371B high power curve tracer has been used in conjunction with a heated chuck and probe station. The use of a heated chuck has enabled device characteristics to be obtained at temperatures ranging from 25 C to 300 C. In order to minimize self-heating effects of the device under test (DUT), pulsed measurement mode has been used. Reverse recovery characteristics have been evaluated using a custom-built clamped inductive switching test rig operated in double pulse mode. In order to measure the transient characteristics at elevated temperatures (up to 125 C), the DUT has been submerged in an inert fluid that was heated using a ceramic hotplate. DISCUSSION Figure 2 shows the forward J-V characteristics of a typical control sample PiN diode across a range of measurement temperatures. It can be seen that the diodes exhibit a negative temperature coefficient, having a lower V F at elevated temperatures; this is clearly beneficial for high temperature operation. This improvement in V F is expected, as, in addition to the decrease in turn-on voltage that arises due to a higher intrinsic carrier concentration at elevated temperatures, dopant ionisation in the p-type anode is increased, the carrier lifetime in the drift region is also increased and the ohmic contact resistance is reduced. These effects combine to overcome the degraded carrier mobility at higher temperatures. However, above 150 C the improvement in V F is less significant, as the previously outlined improvements are outweighed by the higher substrate resistance caused by reduced carrier mobility [12]. The V F of the diode is 4.53 V at 100 A/cm 2 and 25 C, dropping to 3.83 V at 300 C. The corresponding R on,diff as a function of forward current density (J F ) plot for a typical control sample PiN diode is shown in Figure 3; it can be seen that R on,diff drops from 11.6 mω-cm 2 at 100 A/cm 2 and 25 C to 9.0 mωcm 2 at 300 C. It is also evident from this Figure that, even at 300 C, R on,diff continues to decrease

4 at higher current density, indicating that a longer carrier lifetime is required to minimize power losses at low forward bias [13] Current density (A/cm 2 ) degc degC 300degC Voltage (V) Figure 2. Forward J-V characteristics of a control sample PiN diode o C 300 o C R on,diff (mω cm 2 ) Current density (A/cm 2 ) Figure 3. R on,diff as a function of J F of a control sample PiN diode. Figure 4 shows the typical forward J-V characteristics at 25 C for the thermally oxidized PiN diodes, as well as a typical characteristic for a control sample PiN diode, for comparison. It is evident that the thermally oxidized PiN diodes exhibit a lower V F when compared to the control sample PiN diodes, with the devices that had undergone thermal oxidation at 1500 C showing the lowest V F for a given value of J F. The typical V F of the PiN diodes oxidized at 1500 C was measured to be 4.15 V at 100 A/cm 2 and 25 C, and R on,diff was 8.9 mω-cm 2 ; compared to the control sample PiN diodes, this equated to an improvement of around 8% and 23% for the forward voltage and R on,diff, respectively. Though this improvement is significant, it is noted that these results are inferior to those recently published by Salemi et al [14]. However, the oxidation times used in this work were only 5 minutes, the purpose being to evaluate the effectiveness of different oxidation temperatures; it is expected that the device characteristics would be improved further if longer oxidation durations were applied. Prior to the experiment, it was expected that the devices fabricated on 4H-SiC material thermally oxidized at 1600 C would have yielded the best improvement in forward I-V characteristics, due to an increased oxidation

5 rate [9] in addition to the fact that processing at 1600 C has been shown not to generate additional lifetime-killing intrinsic defects in 4H-SiC [15]. As such, further work is planned to better understand this result Current Density (A/cm 2 ) Control sample degC ox. 1500degC ox. 1600degC ox Voltage (V) Figure 4. Forward J-V characteristics of PiN diodes having undergone thermal oxidation at a range of temperatures. Control sample results are also shown. Measurements were taken at 25 C. Figure 5 shows the typical reverse recovery characteristics of both the control sample and the 1500 C thermally oxidized PiN diodes across the temperature range 25 C to 125 C. It is evident that the reverse recovery current, and thus charge, of the devices increases with increasing temperature, as a result of the increased carrier lifetime. Using the equation presented in [11], the carrier lifetime of the PiN diode has been calculated from the reverse recovery characteristics; for the control sample PiN diode a lifetime of 743 ns was calculated at 25 C, compared to 1.05 µs for the PiN diode oxidized at 1500 C. This equates to an increase in lifetime of around 30%. CONCLUSIONS In this paper, high temperature thermal oxidation has been applied to PiN diodes with 110 µm thick drift regions, in order to improve the characteristics of the devices. Performing the oxidation at 1500 C was found to yield the largest improvement in forward characteristics of the diodes, with a typical V F of 4.15 V and a R on,diff of 8.9 mω-cm 2 being measured at 100 A/cm 2 and 25 C, an improvement of 8% and 23%, respectively, when compared to control sample PiN diodes. The carrier lifetime of the PiN diodes oxidized at 1500 C was calculated to be 1.05 µs at 25 C, an improvement of around 30% when compared to control sample PiN diodes. It is noted that very short (5 minute) oxidation times were used; it is expected that the application of longer oxidation times would further improve the electrical characteristics of the PiN diodes. As such, it has been shown that the application of high temperature thermal oxidation is an effective and efficient way of improving the electrical characteristics of 4H-SiC PiN diodes. ACKNOWLEDGMENTS The authors gratefully acknowledge the Power Electronics Centre and the High Voltage Microelectronics and Sensors group at the University of Cambridge, for use of their electrical

6 characterization facilities. This research has been funded by the EPSRC grants EP/I013636/1 and EP/K035304/1. -I A (A) I RP = o C 25 o C 75 o C 125 o C t rr = o C µ 23.7µ 23.8µ 23.9µ I RP = o C -I A (A) t rr = o C µ 23.7µ 23.8µ 23.9µ Time (s) Figure 5. Reverse recovery current waveforms of a control sample PiN diode (bottom) and a 1500 C thermally oxidized PiN diode (top). REFERENCES 1. T. P. Chow, Mater. Sci. Forum , (2014). 2. Y. K. Sharma, A. C. Ahyi, T. Issacs-Smith, A. Modic, M. Park, Y. Xu, E. L. Garfunkel, S. Dhar, L. C. Feldman and J. R. Williams, IEEE Electron Device Lett. 34 (2), (2013). 3. T. Kimoto, K. Danno and J. Suda, Phys. Stat. Sol. (B). 245 (7), (2008). 4. T. Hiyoshi and T. Kimoto, Appl. Phys. Expr. 2, (2009). 5. P. B. Klein, Mater. Sci. Forum , (2012). 6. L. Storasta and H. Tsuchida, Appl. Phys. Lett. 90, (2007). 7. K. Kawahara, J. Suda and T. Kimoto, J. Appl. Phys. 111, (2012). 8. K. Kawahara, J. Suda and T. Kimoto, Mater. Sci. Forum , (2012). 9. S. M. Thomas, M. R. Jennings, Y. K. Sharma, C. A. Fisher and P. A. Mawby, Mater. Sci. Forum , (2014). 10. K. Nakayama, A. Tanaka, M. Nishimura, K. Asano, T. Miyazawa, M. Ito and H. Tsuchida, IEEE Trans. Electron Devices. 59 (4), (2012). 11. M. R. Jennings, C. A. Fisher, D. Walker. A. Sanchez, A. Pérez-Tomás, D. P. Hamilton, P. M. Gammon, S. E. Burrows, S. M. Thomas, Y. Sharma, F. Li and P. A. Mawby, Mater. Sci. Forum , (2014). 12. C. A. Fisher, M. R. Jennings, A. T. Bryant, A. Pérez-Tomás, P. M. Gammon, P. Brosselard, P. Godignon and P. A. Mawby, Mater. Sci. Forum , (2012). 13. L. Cheng, A. K. Agarwal, M. O Loughin, C. Capell, K. Lam, C. Jonas, J. Richmond, A. Burk, J. W. Palmour, A. A. Ogunniyi, H. K. O Brien and C. J. Scozzie, Mater. Sci. Forum , (2013). 14. A. Salemi, B. Buono, A. Hallén, J. U. Hassan, P. Bergman, C. M. Zetterling and M. Östling, Mater. Sci. Forum , (2014). 15. B. Zippelius, J. Suda and T. Kimoto, Mater. Sci. Forum , (2012).