DIAMAT-06380; No of Pages 4 Diamond & Related Materials xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes K. Ueda ⁎, K. Kawamoto, H. Asano Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Available online xxxx Keywords: Diamond Semiconductors Schottky diodes High-temperature Breakdown voltage Power devices
a b s t r a c t High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes were investigated. Cu Schottky diodes showed clear rectification up to ~700 °C. The current–voltage characteristics of the diodes at 400 °C were almost unchanged after keeping them for 30 h, implying they have high stability at ~400 °C. The diodes showed specific on-resistance and breakdown voltage of 83.4 mΩ cm2 and 713 V at 400 °C, respectively, which are comparable to reported highest values for diamond Schottky diodes and close to the theoretical limit for 6H-SiC at several hundred °C. These results indicate that Cu/diamond Schottky diodes are promising for high-temperature power applications. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Diamond is a promising semiconductor for high-power devices using in harsh environment such as high-temperature and high radiation area because it has excellent physical properties such as wide band-gap (5.5 eV), high thermal conductivity (22 W cm−1 K−1), high electric breakdown field (N10 MV/cm), and high chemical stability [1–3]. Based on its band gap, the intrinsic carrier concentration for diamond is estimated to be ~1010 cm−3 at 1000 °C. This is comparable to that for Si at room temperature, suggesting that diamond devices can operate even at ~1000 °C. One important application for high-temperature and high-voltage devices is power devices for inverters used in automobiles working at 100 °C to several hundred °C. There are many reports on power characteristics of diamond devices at room temperature [4–8], however there are only a few reports on power characteristics of the devices such as Schottky diodes [9] and FETs [10,11] at several hundred °C, where high-temperature power devices for automobiles operate and diamond offers advantages over other semiconductors because of its high thermal conductivity, deep acceptor (boron) activation, etc. By theoretical calculation, Raynaud et al. indicated that diamond is interesting for high power devices working at higher temperature, and diamond power devices have advantage to other wide band gap semiconductors such as SiC for voltage higher than 30–40 kV at room temperature because of higher activation energy for the dopants in diamond [12]. They also suggested that diamond become superior to other semiconductors for voltage as low as ~ 1 kV at 600 K due to the activation of the deep dopants. Therefore, it is important to show that diamond
⁎ Corresponding author.
Schottky diodes have good high-voltage characteristics and high thermal stability at higher temperature (~several hundred °C). In this study, we tried fabricating diamond Schottky diodes working at highvoltage and high-temperature, and show that Cu/diamond Schottky diodes are promising as high-power devices operating at several hundred °C for the first time. 2. Experimental methods Boron (B)-doped homoepitaxial diamond films were grown on commercial diamond (100) Ib substrates by microwave plasma chemical vapor deposition (CVD) using trimethylboron (TMB) as dopant gas (TMB/CH4 ratio: 0.5–1 ppm) [13]. The microwave power and frequency was 1.3 kW and 2.45 GHz, respectively. The CH4/H2 ratio, working pressure and growth temperature were 0.1–1%, 50 Torr, and 700–900 °C, respectively. The films produced were typically ~ 1 μm thick. Typical acceptor concentration and mobility at room temperature was ~ 10 17 cm − 3 and ~ 1000 cm 2 /V·s, respectively. Cu Schottky diodes were fabricated on the oxygen (O)-terminated B-doped diamond films by usual photolithographic process [13,14]. UV/ozone treatment was applied to convert the hydrogen (H)-terminated surfaces of the as-grown B-doped diamond films to O-terminated surfaces before depositing Schottky and ohmic electrodes [15]. Lateral Schottky diodes were formed on mesa structures of B-doped diamond layers [Schottky electrodes; area: 3 or 5 × 100 μm2 (2 devices on the two different mesa structures), ohmic electrodes: 250 × 100 μm2, and distance between ohmic and Schottky electrodes: ~ 5–20 μm (Fig. 1(a) inset)]. Ohmic contacts were formed on the diamond films by evaporating Cu/Ti electrodes and annealing them in vacuum at 600 °C. The current–voltage (I–V) characteristics of Cu Schottky diodes were measured in vacuum up to 800 °C using an infrared (IR) annealing system.
http://dx.doi.org/10.1016/j.diamond.2015.03.006 0925-9635/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: K. Ueda, et al., High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.03.006
2
K. Ueda et al. / Diamond & Related Materials xxx (2015) xxx–xxx
were almost unchanged after keeping them for 30 h (Fig. 2(a)), implying that they have high stability at ~400 °C. The ideality factor (n) and Schottky barrier heights (ϕB) of the diodes estimated by the fitting using the TE model considering series resistance were almost unchanged after annealing (n = ~ 1.2, ϕB = ~ 1.6 eV), also suggesting high stability of the Cu Schottky diodes at 400 °C. On the other hand, in the case of the I–V characteristics at 600 °C, the reverse current density (JR) systematically increased as annealing time increased though forward current density was almost the same independent of annealing time (Fig. 2(b)). This means that severe deterioration of the Cu Schottky diodes occurred at 600 °C. Fig. 2(b) inset shows annealing time dependence of the JR at 6 V for the diodes. The JR was almost unchanged for 400 °C after keeping them for above 10 h, however it systematically increased after keeping them at 500–700 °C, and the increase ratio of JR increased as temperature increased. Increase of JR started at 500 °C and drastic increase of JR occurred above ~600 °C, indicating that deterioration of Schottky junctions started at ~500 °C and speed of deterioration became higher at higher temperature. These behaviors correspond to the behaviors of dissociation of oxygen from O-terminated diamond surface, which became maximum at ~600 °C [17]. The mechanism responsible for the deterioration is not clear at present, but we consider it is related to interfacial reactions and/or interdiffusion at Cu/diamond interfaces induced by breakdown of the O-termination at the diamond surface above ~ 500 °C [17,18]. These behaviors are detrimental for using as power devices working above ~ 500 °C. We think a simple way to solve the problem is to change O-termination of the diamond surfaces to H-termination. There is also another possibility for
Fig. 1. (a) Current–voltage characteristics of Cu/diamond Schottky diodes measured from room temperature (R. T.) to 800 °C in vacuum. The inset shows schematic cross section and typical top view of the Schottky junctions. (b) Forward current–voltage characteristics from R. T. to 600 °C. The dotted lines show the results of fitting using the equation for the thermionic emission (TE) model considering series resistance R. The inset shows temperature dependence of inverse of series resistance R, which was estimated from fitting of forward I–V characteristics.
3. Results and discussion Fig. 1(a) shows typical current–voltage (I–V) characteristics of Cu/diamond Schottky diodes measured from room temperature (R. T.) to 800 °C. A high rectification ratio of ~105 was observed at R. T. The forward current systematically increased as temperature increased because of activation of the B acceptors and/or improvement of ohmic resistance at higher temperature. A rectification ratio of ~103 was obtained at ~600 °C though the reverse leakage current increased as temperature increased. Clear rectification with a rectification ratio of more than 10 was observed even at 700 °C, indicating that the Cu/diamond Schottky diodes could operate up to ~ 700 °C. The Schottky barrier height (ϕB) was estimated to be 1.4 ± 0.2 eV with ideality factor n = ~ 1.7 below 600 °C by the fitting of forward I–V characteristics using equation for thermionic emission (TE) model considering series resistance (R) [14] by taking the Richardson constant as 90 A cm− 2 K− 2 [16] (Fig. 1(b)). The estimated R values became less than 1 Ω above 300 °C, which were much smaller than interfacial resistance of the Schottky junctions. The effect of series resistance R was negligibly small above the temperature. The activation energy evaluated by 1/R vs. 1/T plot became ~ 0.4 eV (Fig. 1(b) inset), which agrees well with the activation energy of B in diamond semiconductors (~0.37 eV). In order to evaluate high-temperature stability, I–V characteristics of the Cu Schottky diodes were compared before and after keeping them at 400–700 °C for ~ 30 h. The I–V characteristics of the diodes at 400 °C
Fig. 2. Current–voltage characteristics of the diodes at (a) 400 °C and (b) 600 °C after keeping them at the temperature for ~30 h. The inset in Fig. 2 (a) and (b) shows annealing temperature dependence of the ideality factor n and Schottky barrier heights ϕB, and annealing time dependence of the reverse current density JR at 6 V for the diodes, respectively.
Please cite this article as: K. Ueda, et al., High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.03.006
K. Ueda et al. / Diamond & Related Materials xxx (2015) xxx–xxx
increasing the leakage current above ~500 °C such as the formation of surface graphite or other materials. We have to also consider about this aspect by detailed TEM measurements of Cu/diamond interfaces. These will be important subjects for our future study to clarify the deterioration mechanism of the diamond Schottky diodes at higher temperature. Fig. 3(a) shows bias-voltage dependence of JR of the Cu Schottky diodes measured at 400 °C. The temperature of 400 °C was adopted because it is well below the oxygen desorption temperature for Oterminated diamond (~500 °C) and is above the typical operation temperature for power devices of automobiles (~100–300 °C). The JR of the diodes gradually increased as reverse bias voltage increased and it suddenly increased drastically at 713 V. These behaviors indicate that hard breakdown of the diodes occurred at the voltage. The breakdown voltage (VBR) and specific on-resistance (Ron) estimated from forward I–V characteristics were 713 V (~1.0 MV/cm) and 83.4 mΩ cm2 at 400 °C, respectively. As a note, the distance between ohmic and Schottky electrodes is ~ 7 μm in this case. On the other hand, at R. T., the VBR and Ron values of the diodes were 952 V and 13.3 Ω cm2 (Fig. 3(b)), respectively. These values are comparable to the reported highest values for diamond Schottky diodes at 250 °C (9.4 mΩ cm2 and 840 V) [9] and close to theoretical limit for 6H-SiC at several hundred °C (~ 1 m Ω cm2@~1000 V) [12]. However, these values are far from theoretical limit of diamond semiconductors (~ 3 × 10− 5 Ω cm2@~1000 V). The
Fig. 3. Reverse leakage current characteristics (JR) of Cu/diamond Schottky diodes measured at (a) 400 °C and (b) R. T. The dotted lines represent theoretical JR estimated by using the thermionic emission (TE), thermionic emission combined with barrier lowering (TE + BL), thermionic-field emission (TFE), and TFE combined with barrier lowering (TFE + BL) model.
3
reason is not clear, however one of the possible reasons is the lower breakdown field of the Schottky diodes of ~ 1.0 MV/cm, which was much smaller than the typical breakdown field of diamond semiconductors (N10 MV/cm [19]). The diamond films also have electrically active defects such as hillock [20] and crystal quality of them should be improved by further optimization of growth conditions. The JR characteristics were analyzed by using various models such as the thermionic emission (TE) [21], thermionic emission combined with barrier lowering (TE + BL) [21,22], thermionic-field emission (TFE) [22–24], and TFE combined with barrier lowering (TFE + BL) model [16,21,25]. The Schottky barrier height (ϕB) was estimated by fitting the JR characteristics by using the following equations. −qϕB 2 J R ¼ A T exp ðTEÞ kT q qE 1=2 2 J R ¼ A T exp − ϕB − ðTE þ BLÞ kT 4πε s " #1=2 ϕB 2 pffiffiffiffiffiffiffiffiffiffiffiffiffi qπE00 =kT V þ JR ¼ A T cosh2 ðE00 =kT Þ ðT FEÞ expð−qϕB =ðE00 cothðE00 =kT ÞÞÞ exp qV= E00= ðE00 =kT− tanhðE00 =kT ÞÞ " ! # rffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A TqℏE π q qðℏEÞ2 ðT FE þ BLÞ: exp − V bn − qE=4πε s − JR ¼ k 2mkT kT 24mðkT Þ2
The notation of the physical constants was in the references [16, 21–25]. As a note, in the case of fitting by the TFE model, E00, which is the characteristic energy related to the tunneling probability of a triangular potential barrier, was also used as a fitting parameter (E00 = (qh/4π)(N/m⁎ε0εs)1/2 [23]). The fitting results of the JR at 400 °C with ϕB of 1.13 eV are shown in Fig. 3(a). For TFE, the E00 value was estimated to be 1.5 meV at 400 °C, which corresponds to a carrier concentration of ~ 3 × 1016 cm− 3. As a note, the effective mass (m⁎) and dielectric constant (εs) for diamond was taken as m⁎ = 0.908m0 [26] and εs = 5.7, respectively, for estimation of acceptor concentration for the B-doped films from E00 values. The E00 value was acceptable because the estimated carrier concentration was comparable to the typical acceptor concentration (~1017 cm−3) of the B-doped diamond films. In the case of fitting by TE or TE + BL models, deviation of the fitting curves from experimental JR was much larger. This means that the behaviors of JR cannot be explained by the TE or TE + BL model. On the other hand, in the case of fitting by the TFE or TFE + BL model, which includes a tunneling transport component thorough the Schottky barrier, theoretical JR agree well with experimental ones, and the TFE + BL model showed better results, especially for lower reverse bias region. In the case of Si Schottky diodes the JR characteristics can be well explained by using the TE + BL model, however in the case of Schottky diodes of wide-gap semiconductors such as GaN and diamond, the JR is much higher, and we should take into account a tunneling transport component based on the TFE model [16,23–25]. Therefore, we consider the fact that the experimental JR can be explained at 400 °C by the TFE + BL or TFE model as adequate. JR curves at R. T were also fitted by using equation for the TFE + BL model. The experimental JR behaviors agree with the theoretical one, however the estimated ϕB (=0.76 eV) was much lower than those for ϕB (forward) of ~1.5 eV. The behaviors of experimental JR can be explained by using the TFE + BL (or TFE) model, however the estimated ϕB of ~ 1.1 eV at 400 °C was ~0.3 eV smaller than those estimated from forward I–V characteristics. For I–V curves at R. T., difference of the ϕB estimated using reverse I–V (0.76 eV) and ϕB (forward) became much larger. The ϕB estimated from reverse I–V should be the same independent of temperature and comparable to ϕB (forward). These results indicate that we cannot totally explain temperature dependence of JR by using TFE (+BL) with fixed ϕB (=~1.1 eV) and the TFE + BL model is insufficient to explain total behaviors of experimental JR, and another models which incorporate other factors such as the edge effects should be considered to explain the mechanism for additional barrier height lowering, and this will be a subject for a future study.
Please cite this article as: K. Ueda, et al., High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.03.006
4
K. Ueda et al. / Diamond & Related Materials xxx (2015) xxx–xxx
4. Conclusions High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes were examined. Cu Schottky diodes showed clear rectification up to 700 °C, indicating they can operate at ~700 °C. However they have problems in long time stability above ~500 °C. The I–V characteristics at 400 °C indicate that the Cu Schottky diodes have excellent high-temperature stability at ~400 °C. The diodes showed Ron and VBR of 83.4 mΩ cm2 and 713 V at 400 °C, respectively, which were comparable to reported highest values for diamond Schottky diodes at 250 °C. We found that the experimental JR can be explained by the TFE + BL (or TFE) model by using smaller ϕB. These results indicate that Cu/diamond Schottky diodes are promising for high-temperature and high-voltage applications. Prime novelty We show that Cu/diamond Schottky diodes are promising as highpower devices operating at several hundred °C for the first time. Cu/diamond Schottky diodes showed specific on-resistance and breakdown voltage of 83.4 mΩ cm2 and 713 V at 400 °C, respectively, which are comparable to reported highest values for diamond Schottky diodes and close to the theoretical limit for 6H-SiC at several hundred °C. Acknowledgments This work was supported by JSPS KAKENHI (grant number: 24560372), the Nippon Sheet Glass Foundation for Materials Science and Engineering, the Kurata memorial Hitachi science and technology foundation, and the SCAT Technology Research Foundation. References [1] M.R. Werner, W.R. Wolfgang, Review on materials, microsensors, systems, and devices for high-temperature and harsh-environment applications, IEEE Trans. Ind. Electron. 48 (2001) 249–257. [2] A. Veskan, P. Gluche, W. Ebert, E. Kohn, Very high temperature operation of diamond Schottky diode, IEEE Electron Device Lett. 18 (1997) 222–224. [3] A. Veskan, I. Daumiller, P. Gluche, W. Ebert, E. Kohn, High temperature, high voltage operation of diamond Schottky diode, Diam. Relat. Mater. 7 (1998) 581–584. [4] A. Traoré, P. Muret, A. Fiori, D. Eon, E. Gheeraert, J. Pernot, Zr/oxidized diamond interface for high power Schottky diodes, Appl. Phys. Lett. 104 (2014) (052105-1-4). [5] P.N. Volpe, P. Muret, J. Pernot, F. Omne`s, T. Teraji, Y. Koide, F. Jomard, D. Planson, P. Brosselard, N. Dheilly, B. Vergne, S. Scharnholz, Extreme dielectric strength in boron doped homoepitaxial diamond, Appl. Phys. Lett. 97 (2010) (223501-1-3).
[6] D. Twitchen, A. Whitehead, S. Coe, J. Isberg, J. Hammersberg, T. Wikstrom, E. Johansson, High-voltage single-crystal diamond diodes, IEEE Trans. Electron Devices 51 (2004) 826–828. [7] E. Butler, M.W. Geis, K.E. Krohn, J. Lawless Jr., S. Deneault, T.M. Lyszczarz, D. Flechtner, R. Wright, Exceptionally high voltage Schottky diamond diodes and low boron doping, Semicond. Sci. Technol. 18 (2003) S67–S71. [8] H. Umezawa, T. Matsumoto, S. Shikata, Diamond metal—semiconductor field-effect transistor with breakdown voltage over 1.5 kV, IEEE Electron Device Lett. 35 (2014) 1112–1114. [9] H. Umezawa, Y. Kato, S. Shikata, 1 Ω on-resistance diamond vertical-Schottky barrier diode operated at 250 °C, Appl. Phys. Express 6 (2013) (011302-1-4). [10] T. Iwasaki, J. Yaita, H. Kato, T. Makino, M. Ogura, D. Takeuchi, H. Okushi, S. Yamasaki, M. Hatano, 600 V diamond junction field-effect transistors operated at 200 °C, IEEE Electron Device Lett. 35 (2014) 241–243. [11] H. Kawarada, H. Tsuboi, T. Naruo, T. Yamada, D. Xu, A. Daicho, T. Saito, A. Hiraiwa, C– H surface diamond field effect transistors for high temperature (400 °C) and high voltage (500 V) operation, Appl. Phys. Lett. 105 (2014) (013510-1-4). [12] C. Raynaud, D. Tournier, H. Morel, D. Planson, Comparison of high voltage and high temperature performance of wide bandgap semiconductors for vertical power devices, Diam. Relat. Mater. 19 (2010) 1–6. [13] K. Ueda, K. Kawamoto, T. Soumiya, H. Asano, High-temperature characteristics of Ag and Ni/diamond Schottky diodes, Diam. Relat. Mater. 38 (2013) 41–44. [14] K. Ueda, K. Kawamoto, H. Asano, High-temperature characteristics and stability of Cu/diamond Schottky diodes, Jpn. J. Appl. Phys. 53 (2014) (04EP05-1-4). [15] T. Sakai, K.-S. Song, H. Kanazawa, Y. Nakamura, H. Umezawa, M. Tachiki, H. Kawarada, Ozone-treated channel diamond field effect transistors, Diam. Relat. Mater. 12 (2003) 1971–1975. [16] H. Umezawa, T. Saito, N. Tokuda, M. Ogura, S. Ri, H. Yoshikawa, S. Shikata, Leakage current analysis of diamond Schottky barrier diode, Appl. Phys. Lett. 90 (2007) (073506-1-3). [17] T. Ando, K. Yamamoto, M. Ishii, M. Kamo, Y. Sato, Vapour-phase oxidation of diamond surfaces in O2 studied by diffuse reflectance Fourier-transform infrared and temperature-programmed desorption spectroscopy, J. Chem. Soc. Faraday Trans. 89 (1993) 3635–3640. [18] T. Teraji, Y. Koide, T. Ito, High-temperature stability of Au/p-type diamond Schottky diode, Phys. Status Solidi Rapid Res. Lett. 3 (2009) 211–213. [19] C.A. Clain, R. Desalbo, Thresholds for dielectric breakdown in laser-irradiated diamond, Appl. Phys. Lett. 63 (1993) 1895–1897. [20] D. Takeuchi, H. Watanabe, S. Yamanaka, H. Okushi, H. Sawada, H. Ichinose, T. Sekiguchi, K. Kajimura, Origin of band-A emission in diamond thin films, Phys. Rev. B 63 (2001) (245328-1-7). [21] S.M. Sze, Physics of Semiconductor Devices, 2nd ed. Wiley, New York, 1981. [22] A. Veskan, E. Ebert, T. Borst, E. Kohn, IV characteristics of epitaxial Schottky Au barrier diode on p+ diamond substrate, Diam. Relat. Mater. 4 (1995) 661–665. [23] Zs.J. Horváth, Domination of the thermionic-field emission in the reverse I–V characteristics of n-type GaAs Schottky contacts, J. Appl. Phys. 64 (1988) 6780–6784. [24] S. Oyama, T. Hashizume, H. Hasegawa, Mechanism of current leakage thorough metal/n-GaN interfaces, Appl. Surf. Sci. 190 (2002) 322–325. [25] T. Hatakeyama, T. Shinohe, Reverse characteristics of a 4H-SiC Schottky barrier diode, Mater. Sci. Forum 389–393 (2002) 1169. [26] P.-N. Volpe, J. Pernot, P. Muret, F. Omnès, High hole mobility in boron doped diamond for power device applications, Appl. Phys. Lett. 94 (2009) (092102-1-3).
Please cite this article as: K. Ueda, et al., High-temperature and high-voltage characteristics of Cu/diamond Schottky diodes, Diamond Relat. Mater. (2015), http://dx.doi.org/10.1016/j.diamond.2015.03.006