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Device processing, fabrication and analysis of diamond pseudo-vertical Schottky barrier diodes with low leak current and high blocking voltage
Diamond & Related Materials 18 (2009) 299–302
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Device processing, fabrication and analysis of diamond pseudo-vertical Schottky barrier diodes with low leak current and high blocking voltage R. Kumaresan a,⁎, H. Umezawa a, N. Tatsumi b, K. Ikeda a, S. Shikata a a b
Diamond Research Centre, AIST, Umezono, Tsukuba, Japan Sumitomo Electric Industries, Japan
a r t i c l e
i n f o
Available online 6 November 2008 Keywords: Diamond Schottky barrier diode Psuedo-vertical structure Device processing ICP etching Fabrication Analysis High blocking voltage Breakdown Model
a b s t r a c t Diamond pseudo-vertical structure Schottky barrier diodes (PVSBD) have been fabricated by developing a simple and efficient fabrication technology, in which 14 µm thick p− layer was selectively etched out and ohmic contact was made onto the low resistive p+ layer from topside. The electrical characteristics were evaluated by fabricating Mo/Diamond Schottky barrier diodes in pseudo-vertical structure. With the fabricated structure, a high blocking voltage of 1.6 kV with a low leakage current density in order of 10− 7 ~ 10− 6 A/cm2 could be obtained, without any edge termination. The reverse characteristics of the SBDs exhibited a hard breakdown at 1.6 kV, and causing breakdown of the diamond surface itself in a particular case. The later case indicated the presence of sub-surface leak path causing current leakage in one direction in near surface region, and a model has been proposed to explain this. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among the numerous semiconductors available, diamond has unique electronic properties which make the popular Baliga figure of merit and Huang figure of merit to a high value of 25106 and 23.8 respectively [1,2] in comparison to other semiconductors, which reduce the power loss of diamond power devices. Thus diamond is an ideal, and important candidate for electronic device applications, which can be exploited for fabricating high power devices that can be operated in high voltage, high temperature, high radiation, and corrosive environments. Despite the advancement since last decade in different areas of diamond research such as epi-growth [3,4], surface analysis [5,6], bulk analysis [7,8], device fabrication technologies [9,10] etc., still the state-of-art technology for fabrication of high power devices such as Schottky barrier diodes, finds place on the challenging roadmap due to the difficulty in making low resistive base substrates, and a well suitable technology is much anticipated. Generally, in power device structures such as SBDs, vertical configuration is adopted using highly doped low resistive substrate allowing the electric current flow in a vertical direction. In case of diamond, although reports are available on the fabrication of devices using bulk single crystal p+ substrates with a back contact layer, these low resistive substrates by HPHT are very limited [11,12] for the size and resistivity, and are commercially unavailable. Thus, diamond SBDs have generally been ⁎ Corresponding author. Fax: +81 29 861 2771. E-mail address: [email protected] (R. Kumaresan). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.055
fabricated in lateral structure in which the ohmic contact is made either on the low doped p− layer at the top or to be at p+/p− layer interface through the p− layer. Some reports are available on the high voltage diamond SBDs fabricated in lateral geometry, but they exhibited large RonS (300 Ω.cm2) in comparison to theoretical value, which is due to the poor ohmic contacts and non-optimum device geometry [13]. The high Ron than the one estimated from the film thickness, carrier concentration and mobility is due to the high resistance of the ohmic contact fabricated. For a boron doping density lower than 1 × 1018/cm3, the contact resistance is determined by thermionic emission with the corresponding barrier height [9]. Hence good ohmic contact is very important for getting low RonS. Besides, in the operation of high-power and high-voltage electronic devices, electrical breakdown is one of the main limitations [14–16]. The breakdown voltage of the device depends directly on the thickness of the active layer used, since the current transport can take place either in punch through or a non-punch through mode. Basically, thick drift layers are needed to fabricate both bipolar and unipolar high-voltage diodes [17]. In spite of the available reports on thin film diamond SBDs and their current transport mechanism [18,19], detailed study about the electrical properties and reverse breakdown of thick, lightly doped diamond epilayers for fabricating high voltage SBDs has been more intriguing, and more light is to be thrown in this direction. With this background, in our present work we have developed a new and efficient fabrication technology for fabrication of diamond Schottky barrier diodes in pseudo-vertical structure, by utilizing ICP etching process. In this process, the low doped layer was selectively etched out optimally at
300
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Fig. 2. Forward I-V plot of Diamond/Mo VSBD (barrier height is 2.1 eV).
Fig. 1. Process flow for fabrication of pseudo-vertical SBD device structure. a) CVD grown homo-epilayer on 1b substrate. b) Partial masking of P- layer, for etching process. c) ICP etching of P- layer in unmasked area. d) Selectively etched epi-wafer. e) Ohmic and Schottky metallization. f) Pseudo-vertical SBD device: cross sectional view.
the edges of the substrate by masking the centre area and the ohmic contact was made appropriately on the highly doped low resistive layer (p+) in the etched out area. Electrical characteristics of low doped, thick diamond homo-epilayers grown by MPCVD technique were evaluated, by fabricating Mo/Diamond pseudo-vertical Schottky barrier diodes. With this device configuration, we could obtain a blocking voltage of more than 1 kV, without using any edge termination.
etch surface with very minimum nano-whiskers formed during the etching process. While etching out p− drift layer, detection of reaching p+ layer was carried out by direct “on wafer” I-V testing of low resistive p+ contact layer. After the etching process, wafer was wet chemically cleaned again, which consisted of Sulfuric-Peroxide mixture (H2SO4: H2O2 = 3:1, 350 °C) cleaning followed by hot acid treatment (a mixture of nitric and sulfuric acids), and all the surfaces were oxygen terminated. Ti/Pt/Au ohmic contact was made on p+ layer by e-beam evaporation followed by annealing at 420 °C for 30 min. The ohmic contact was made at the edge, keeping a gap of about few microns from the etched side wall. The ohmic metal sheet resistance and the contact layer sheet resistance were 1 × 10− 8 Ω/square, and 1 × 10− 4 Ω/square respectively. Mo Schottky metal layer (300 Å thick, 30 µm and 50 µm sized) was evaporated on the top p− layer. The Schottky patterns were obtained by e-beam lithography, followed by O2 plasma ashing process to remove any residual resist, then Schottky metallization was carried out by e-beam evaporation, and the final structure was realized by lift off process. The devices were not fabricated on individual mesa structure and were similar to the planar type structure concerning the SBDs. Fig. 1 depicts the over-all process flow that we have developed for fabrication of present pseudo-vertical SBD structure. It also exhibits the schematic of pseudo-vertical structure SBDs (PVSBDs) fabricated in this research, exhibiting the ohmic contact at the edges on p+ layer and the Schottky contacts on top of p− layer. In this study SBDs were fabricated without utilizing any edge termination structures. Forward conduction and reverse blocking characteristics of the fabricated PVSBDs were analyzed using Keithley-237 source measurement unit. Reverse I-V characteristics beyond the measurement limit of this electrometer (N1.1 kV), was studied by Tektronix 371A model curve tracer at room temperature. During reverse I-V study, the devices were immersed in fluorine based insulating liquid to avoid any electrical arcing. The C-V analysis was carried out using a 4284-A
2. Experimental Boron doped diamond homo-epilayers of p− drift layer and p+ contact layer were grown with a thickness of 14 µm and 10 µm respectively, by MPCVD technique on 3 × 3 mm sized Ib (001) single crystal HPHT substrate with an off angle of 1.1°. Boron doping concentration was controlled by varying B/C ratio, with the manipulation of TMB and methane gas concentrations. As-grown epi-wafer was wet chemically cleaned and towards realizing ohmic contact, epi-grown p− layer was etched out by masking the centre 2 × 2 mm area of the substrate. In our present research, diamond etching was carried out by inductively coupled plasma (ICP) etching process using O2 and CF4 reactant gases [20,21], to a total depth of 19 µm starting from the top of p− layer into p+ layer (14 µm p− layer plus a 5 µm p+ layer). ICP process conditions were optimized to get after
Fig. 3. Reverse I-V characteristics of Diamond/Mo PVSBD (up to 1.1 kV, the limit of Keithley 237; Diode-B exhibits leak current path within this range).
R. Kumaresan et al. / Diamond & Related Materials 18 (2009) 299–302
Fig. 4. Reverse I-V plot as measured by curve tracer; exhibiting 1.6 kV reverse breakdown.
impedance spectroscope. The surface of the diamond substrate was analyzed by Nomarski optical microscope and Scanning electron microscope (SEM), both before and after the I-V characteristics analysis. 3. Results and discussion The forward I-V characteristics of 30 µm diameter sized circular Mo/Diamond PVSBD is shown in Fig. 2. Forward characteristics of SBDs are governed by the thermionic emission model and the current is proportional to the surface area. A forward current density of 60 A/cm2 was attained at −8 V with a low RonS in the range of 0.1 Ω.cm2. Comparing with the literature we could get higher current density and low RonS for the similar thick drift layer [12,13], and this is due to good ohmic contact obtained (on p+ layer) in the present geometry of the device structure. This is justified since it is clearly reported for the case of heavily boron doped diamond, that the activation energy decreases for B concentrations higher than some 1018 cm− 3, and falls to zero for the sample doped to 3 × 1020 cm− 3 and the hopping conduction appears
301
at room temperature [22]. The ideality factor of SBDs is an important factor, and the present fabricated diodes exhibit ‘n’ value very close to 1 (in range of 1.00 to 1.04 with a mean value of 1.02) which infers that the diamond surface processing results in spatial uniformity. This also implies that the internal resistance of the device structure is considerably low. In present study, the Ron value was estimated to be as low as 1.4 × 104 Ω, in comparison to the high Ron value of 9.5 × 105 Ω as reported in literature [13] in which the authors claim it to be due to the poor ohmic contacts, and non-optimum device geometry. Also, the ICP etching process followed in this work is suitable for the fabrication of high power device structures, since the n values of the PVSBDs indicate the absence of any plasma induced damage to the p− layer (similar to our previous SBD without any ICP etching step), otherwise which will degrade the n values. From the forward I-V characteristics, Schottky barrier height of the diodes was estimated to be a consistent value of 2.1 eV and the fabricated Mo SBDs show a rectifying factor of 8 orders of magnitude. Fig. 3 shows the reverse I-V characteristics for two different diodes which were analyzed using Keithley 237 electrometer. In this, diode-A depicted a blocking voltage without any breakdown up to 1.1 kV which is the limit of the measurement unit. Also, this diode showed a low reverse current density of the order of 10− 7 ~ 10− 6 A/cm2, even without any edge termination structure, while diode-B exhibited a leak current path in high bias regions above 900 V. Beyond the measurement limit of Keithley electrometer, reverse I-V characteristics was analyzed by a curve tracer. As shown in Fig. 4 the reverse breakdown occurred at 1.6 kV in case of diode-A, without any edge termination. Here, the Schottky diodes were not fabricated on an individual mesa structure, but similar to be on a planar type structure, and were fabricated without utilizing any edge termination structures. These SBDs could withstand an electric field of 1.4 MV/cm, which is similar to other literature reports. Our SBDs fabricated on a different substrate grown under different conditions, exhibited a still higher breakdown field of 3.1MV/cm with a breakdown voltage of 300 V [23], and we are elucidating out further optimal growth conditions towards achieving an epilayer for much higher breakdown field operation. Meanwhile, in the case of diode-B which exhibited a leak current path could sustain only a lower breakdown voltage of 1.4 kV. Despite this substrate contained a large density of pyramidal hillocks distributed throughout the substrate, they showed negligible influence on the reverse leakage characteristics of the diodes, and they are considered to be harmless morphological
Fig. 5. A model to explain the hard breakdown of diamond surface in reverse bias mode. a & b) SEM and Microscopic top view of diamond breakdown occurred due to leak current path in particular direction causing very high current passage in one direction. c) Figure illustrating the breakdown occurred. d) A model explaining reverse leak current paths due to the defects and charge states in subsurface region, causing in diamond breakdown. e) Another diode showing distributed leak current path causing less damage to diamond surface compared to figure-a.
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features, in contradiction to the device killing defect such as nonepitaxial crystallite (not included in the present work). However, some of the diodes exhibited high reverse leak currents, even though those diodes did not include any visible defects. This can be speculated to be due to the presence of high density of trap concentration and/or dislocations in the specific diode area, or to the formation of random graphitic phase inclusion during the CVD growth, and further detailed study is to be carried out in this aspect. While both the diode-A and diode-B were reverse biased up to breakdowns, diode-B caused a damage to diamond itself, as was evidenced from the hard breakdown of the diamond surface beneath the diode, which occurred during reverse bias measurement. The diode-B was located at about 150 µm far from the etched edge. Fig. 5a and b show the SEM and microscopic view of diamond breakdown that occurred in our study. This breakdown started at the periphery of the SBD and proceeded in a lateral direction (not in a vertical direction through the bulk) forming a valley like dimple and this is correlated to the high reverse current leak path exhibited by the diode. Fig. 5c and d show the schematic of the leak path in lateral direction and the resulting diamond breakdown in one direction, which started from the p− layer and continued to end up in p+ layer. Fig. 5e shows another diode after breakdown, in which the leak paths spread around the diode and hence the high leak current flow in one direction was prevented, and thus by the diamond surface escaped from any hard damage. In general, the defects such as dislocations cause pre-mature breakdown associated with high leakage current. But, the X-ray topographic analysis of the substrate used in the present study (not shown in this article), that was carried out before the device fabrication did not exhibit any dislocations at the site of the diode which caused diamond breakdown. Hence, the diamond breakdown that occurred in a lateral direction is explained due to the presence of near surface defective leak path. Although the nature and origin of the associated defect is not clearly known, this might include charged states at the surface and sub surface regions [24], causing carrier multiplication in one direction leading to crystal damage. Detailed study is further to be carried out to understand this mechanism clearly. In general, the breakdown at the periphery of the SBD is due to the electric field crowding at the edges, and this can be significantly improved by an edge termination technique. In our continuing part of research we studied about the significance of field plate structure SBD, and it exhibited an improved breakdown field in comparison to SBD without any edge termination [25]. 4. Conclusions We have developed an alternate and efficient technique for fabrication of diamond pseudo-vertical SBD structure. The SBDs fabricated in this structure, by our recently developed process flow exhibited a high blocking voltage (1.6 kV) with low leakage current density (1 µA/cm2) and a satisfactorily high forward current density
(60 A/cm2), without any edge termination. To realize still higher forward current density, SBDs have to be fabricated in a vertical geometry, and accordingly we are modifying the present fabrication process further. To understand the diamond breakdown mechanism that occurred in reverse blocking mode, a model has been proposed exhibiting the leak current path in single lateral direction. Acknowledgements This study was partially supported by Industrial Technology Research Grant Program in 2007 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
B.J. Baliga, J. Appl. Phys. 53 (1982) 1759. Alex Q. Huang, IEEE Electron Device Lett. 25 (2004) 298. C. Findeling-Dufour, A. Vignes, A. Gicquel, J. Cryst. Growth 183 (1998) 338. G. Bogdan, M. Nesladek, J. D’Haen, K. Haenen, M. D’Olieslaeger, Diam. Relat. Mater. 15 (2006) 508. Y. Fan, A.G. Fitzgerald, P. John, C.E. Troupe, J.I.B. Wilson, Surf. Interface Anal. 34 (2002) 703. J.I.B. Wilson, J.S. Walton, G. Beamson, J. Electron Spectrosc. Relat. Phenom. 121 (2001) 183. R.E. Harper, C. Johnston, N.G. Blamires, P.R. Chalker, I.M. Buckley-Golder, Surf. Coat. Technol. 47 (1991) 344. R.S. Sussmann, J.R. Brandon, G.A. Scarsbrook, C.G. Sweeney, T.J. Valentine, A.J. Whitehead, C.J.H. Wort, Diam. Relat. Mater. 3 (1994) 303. H. Umezawa, N. Tokuda, M. Ogura, S. Ri, S. Shikata, Diam. Relat. Mater. 15 (2006) 1949. M. Kubovic, H. El-Hajj, J.E. Butler, E. Kohn, Diam. Relat. Mater. 16 (2007) 1033. D.J. Twitchen, A.J. Whitehead, S.E. Coe, J. Isberg, J. Hammersberg, T. Wikstrom, E. Johansson, IEEE Trans. Electron Devices 51 (2004) 826. S.J. Rashid, L. Coulbeck, A. Tajani, M. Brezeanu, A. Garraway, T. Butler, N.L. Rupesinghe, D.J. Twitchen, G.A.J. Amaratunga1, F. Udrea1, P. Taylor, M. Dixon, J. Isberg, Proc. Int. Symp. Power Semicond. Devices & ICs 17 (2005) 315. J.E. Butler, M.W. Geis, K.E. Krohn, J. Lawless Jr., S. Deneault, T.M. Lyszczarz, D. Flechtner, R. Wright, Semicond. Sci. Technol. 18 (2003) S67. D.G. Jeng, H.S. Tuan, R.F. Salat, G.J. Fricano, J. Appl. Phys. 68 (1990) 5902. Bohr-Ran Huang, Wen-Cheng Ke, Jung-Fu Hsu, Wei-Kuo Chen, Mater. Chem. Phys. 72 (2001) 214. W. Ebert, A. Vescan, P. Gluche, T. Borst, E. Kohn, Diam. Relat. Mater. 6 (1997) 329. H. Tsuchida, I. Kamata, T. Jikimoto, K. Izumi, ICSCRM 2001, Mater. Sci. Forum 389 (2002) 171. H. Umezawa, T. Saito, N. Tokuda, M. Ogura, S. Ri, H. Yoshikawa, S. Shikata, Appl. Phys. Lett. 90 (2007) 073506. A. Vescan, W. Ebert, T. Borst, E. Kohn, Diam. Relat. Mater. 4 (1995) 661. Johannes Enlund, Jan Isberg, Mikael Karlsson, Fredrik Nikolajeff, Jörgen Olsson, Daniel J. Twitchen, Carbon 43 (2005) 1839. H. Yoshikawa, S. Shikata, N. Fujimori, N. Sato, T. Ikehata, New Diam. Front. Carbon Technol. 16 (2006) 97. J.P. Lagrange, A. Deneuville, E. Gheeraert, Diam. Relat. Mater. 7 (1998) 1390. H. Umezawa, K. Ikeda, N. Tatsumi, R. Kumaresan, S. Shikata, The 34th International Symposium on compound semiconductors, Kyoto, Japan, 2007. Kazushi Hayashi, Hideyuki Watanabe, Sadanori Yamanaka, Hideyo Okushi, Koji Kajimura, Takashi Sekiguchi, Appl. Phys. Lett. 69 (8) (1996). Kazuhiro Ikeda, Hitoshi Umezawa, Natsuo Tatsumi, Kumaresan Ramanujam, Shinichi Shikata, Diam. Relat. Mater. 18 (2009) 292 (this issue).
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Device processing, fabrication and analysis of diamond pseudo-vertical Schottky barrier diodes with low leak current and high blocking voltage R. Kumaresan a,⁎, H. Umezawa a, N. Tatsumi b, K. Ikeda a, S. Shikata a a b
Diamond Research Centre, AIST, Umezono, Tsukuba, Japan Sumitomo Electric Industries, Japan
a r t i c l e
i n f o
Available online 6 November 2008 Keywords: Diamond Schottky barrier diode Psuedo-vertical structure Device processing ICP etching Fabrication Analysis High blocking voltage Breakdown Model
a b s t r a c t Diamond pseudo-vertical structure Schottky barrier diodes (PVSBD) have been fabricated by developing a simple and efficient fabrication technology, in which 14 µm thick p− layer was selectively etched out and ohmic contact was made onto the low resistive p+ layer from topside. The electrical characteristics were evaluated by fabricating Mo/Diamond Schottky barrier diodes in pseudo-vertical structure. With the fabricated structure, a high blocking voltage of 1.6 kV with a low leakage current density in order of 10− 7 ~ 10− 6 A/cm2 could be obtained, without any edge termination. The reverse characteristics of the SBDs exhibited a hard breakdown at 1.6 kV, and causing breakdown of the diamond surface itself in a particular case. The later case indicated the presence of sub-surface leak path causing current leakage in one direction in near surface region, and a model has been proposed to explain this. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Among the numerous semiconductors available, diamond has unique electronic properties which make the popular Baliga figure of merit and Huang figure of merit to a high value of 25106 and 23.8 respectively [1,2] in comparison to other semiconductors, which reduce the power loss of diamond power devices. Thus diamond is an ideal, and important candidate for electronic device applications, which can be exploited for fabricating high power devices that can be operated in high voltage, high temperature, high radiation, and corrosive environments. Despite the advancement since last decade in different areas of diamond research such as epi-growth [3,4], surface analysis [5,6], bulk analysis [7,8], device fabrication technologies [9,10] etc., still the state-of-art technology for fabrication of high power devices such as Schottky barrier diodes, finds place on the challenging roadmap due to the difficulty in making low resistive base substrates, and a well suitable technology is much anticipated. Generally, in power device structures such as SBDs, vertical configuration is adopted using highly doped low resistive substrate allowing the electric current flow in a vertical direction. In case of diamond, although reports are available on the fabrication of devices using bulk single crystal p+ substrates with a back contact layer, these low resistive substrates by HPHT are very limited [11,12] for the size and resistivity, and are commercially unavailable. Thus, diamond SBDs have generally been ⁎ Corresponding author. Fax: +81 29 861 2771. E-mail address: [email protected] (R. Kumaresan). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.055
fabricated in lateral structure in which the ohmic contact is made either on the low doped p− layer at the top or to be at p+/p− layer interface through the p− layer. Some reports are available on the high voltage diamond SBDs fabricated in lateral geometry, but they exhibited large RonS (300 Ω.cm2) in comparison to theoretical value, which is due to the poor ohmic contacts and non-optimum device geometry [13]. The high Ron than the one estimated from the film thickness, carrier concentration and mobility is due to the high resistance of the ohmic contact fabricated. For a boron doping density lower than 1 × 1018/cm3, the contact resistance is determined by thermionic emission with the corresponding barrier height [9]. Hence good ohmic contact is very important for getting low RonS. Besides, in the operation of high-power and high-voltage electronic devices, electrical breakdown is one of the main limitations [14–16]. The breakdown voltage of the device depends directly on the thickness of the active layer used, since the current transport can take place either in punch through or a non-punch through mode. Basically, thick drift layers are needed to fabricate both bipolar and unipolar high-voltage diodes [17]. In spite of the available reports on thin film diamond SBDs and their current transport mechanism [18,19], detailed study about the electrical properties and reverse breakdown of thick, lightly doped diamond epilayers for fabricating high voltage SBDs has been more intriguing, and more light is to be thrown in this direction. With this background, in our present work we have developed a new and efficient fabrication technology for fabrication of diamond Schottky barrier diodes in pseudo-vertical structure, by utilizing ICP etching process. In this process, the low doped layer was selectively etched out optimally at
300
R. Kumaresan et al. / Diamond & Related Materials 18 (2009) 299–302
Fig. 2. Forward I-V plot of Diamond/Mo VSBD (barrier height is 2.1 eV).
Fig. 1. Process flow for fabrication of pseudo-vertical SBD device structure. a) CVD grown homo-epilayer on 1b substrate. b) Partial masking of P- layer, for etching process. c) ICP etching of P- layer in unmasked area. d) Selectively etched epi-wafer. e) Ohmic and Schottky metallization. f) Pseudo-vertical SBD device: cross sectional view.
the edges of the substrate by masking the centre area and the ohmic contact was made appropriately on the highly doped low resistive layer (p+) in the etched out area. Electrical characteristics of low doped, thick diamond homo-epilayers grown by MPCVD technique were evaluated, by fabricating Mo/Diamond pseudo-vertical Schottky barrier diodes. With this device configuration, we could obtain a blocking voltage of more than 1 kV, without using any edge termination.
etch surface with very minimum nano-whiskers formed during the etching process. While etching out p− drift layer, detection of reaching p+ layer was carried out by direct “on wafer” I-V testing of low resistive p+ contact layer. After the etching process, wafer was wet chemically cleaned again, which consisted of Sulfuric-Peroxide mixture (H2SO4: H2O2 = 3:1, 350 °C) cleaning followed by hot acid treatment (a mixture of nitric and sulfuric acids), and all the surfaces were oxygen terminated. Ti/Pt/Au ohmic contact was made on p+ layer by e-beam evaporation followed by annealing at 420 °C for 30 min. The ohmic contact was made at the edge, keeping a gap of about few microns from the etched side wall. The ohmic metal sheet resistance and the contact layer sheet resistance were 1 × 10− 8 Ω/square, and 1 × 10− 4 Ω/square respectively. Mo Schottky metal layer (300 Å thick, 30 µm and 50 µm sized) was evaporated on the top p− layer. The Schottky patterns were obtained by e-beam lithography, followed by O2 plasma ashing process to remove any residual resist, then Schottky metallization was carried out by e-beam evaporation, and the final structure was realized by lift off process. The devices were not fabricated on individual mesa structure and were similar to the planar type structure concerning the SBDs. Fig. 1 depicts the over-all process flow that we have developed for fabrication of present pseudo-vertical SBD structure. It also exhibits the schematic of pseudo-vertical structure SBDs (PVSBDs) fabricated in this research, exhibiting the ohmic contact at the edges on p+ layer and the Schottky contacts on top of p− layer. In this study SBDs were fabricated without utilizing any edge termination structures. Forward conduction and reverse blocking characteristics of the fabricated PVSBDs were analyzed using Keithley-237 source measurement unit. Reverse I-V characteristics beyond the measurement limit of this electrometer (N1.1 kV), was studied by Tektronix 371A model curve tracer at room temperature. During reverse I-V study, the devices were immersed in fluorine based insulating liquid to avoid any electrical arcing. The C-V analysis was carried out using a 4284-A
2. Experimental Boron doped diamond homo-epilayers of p− drift layer and p+ contact layer were grown with a thickness of 14 µm and 10 µm respectively, by MPCVD technique on 3 × 3 mm sized Ib (001) single crystal HPHT substrate with an off angle of 1.1°. Boron doping concentration was controlled by varying B/C ratio, with the manipulation of TMB and methane gas concentrations. As-grown epi-wafer was wet chemically cleaned and towards realizing ohmic contact, epi-grown p− layer was etched out by masking the centre 2 × 2 mm area of the substrate. In our present research, diamond etching was carried out by inductively coupled plasma (ICP) etching process using O2 and CF4 reactant gases [20,21], to a total depth of 19 µm starting from the top of p− layer into p+ layer (14 µm p− layer plus a 5 µm p+ layer). ICP process conditions were optimized to get after
Fig. 3. Reverse I-V characteristics of Diamond/Mo PVSBD (up to 1.1 kV, the limit of Keithley 237; Diode-B exhibits leak current path within this range).
R. Kumaresan et al. / Diamond & Related Materials 18 (2009) 299–302
Fig. 4. Reverse I-V plot as measured by curve tracer; exhibiting 1.6 kV reverse breakdown.
impedance spectroscope. The surface of the diamond substrate was analyzed by Nomarski optical microscope and Scanning electron microscope (SEM), both before and after the I-V characteristics analysis. 3. Results and discussion The forward I-V characteristics of 30 µm diameter sized circular Mo/Diamond PVSBD is shown in Fig. 2. Forward characteristics of SBDs are governed by the thermionic emission model and the current is proportional to the surface area. A forward current density of 60 A/cm2 was attained at −8 V with a low RonS in the range of 0.1 Ω.cm2. Comparing with the literature we could get higher current density and low RonS for the similar thick drift layer [12,13], and this is due to good ohmic contact obtained (on p+ layer) in the present geometry of the device structure. This is justified since it is clearly reported for the case of heavily boron doped diamond, that the activation energy decreases for B concentrations higher than some 1018 cm− 3, and falls to zero for the sample doped to 3 × 1020 cm− 3 and the hopping conduction appears
301
at room temperature [22]. The ideality factor of SBDs is an important factor, and the present fabricated diodes exhibit ‘n’ value very close to 1 (in range of 1.00 to 1.04 with a mean value of 1.02) which infers that the diamond surface processing results in spatial uniformity. This also implies that the internal resistance of the device structure is considerably low. In present study, the Ron value was estimated to be as low as 1.4 × 104 Ω, in comparison to the high Ron value of 9.5 × 105 Ω as reported in literature [13] in which the authors claim it to be due to the poor ohmic contacts, and non-optimum device geometry. Also, the ICP etching process followed in this work is suitable for the fabrication of high power device structures, since the n values of the PVSBDs indicate the absence of any plasma induced damage to the p− layer (similar to our previous SBD without any ICP etching step), otherwise which will degrade the n values. From the forward I-V characteristics, Schottky barrier height of the diodes was estimated to be a consistent value of 2.1 eV and the fabricated Mo SBDs show a rectifying factor of 8 orders of magnitude. Fig. 3 shows the reverse I-V characteristics for two different diodes which were analyzed using Keithley 237 electrometer. In this, diode-A depicted a blocking voltage without any breakdown up to 1.1 kV which is the limit of the measurement unit. Also, this diode showed a low reverse current density of the order of 10− 7 ~ 10− 6 A/cm2, even without any edge termination structure, while diode-B exhibited a leak current path in high bias regions above 900 V. Beyond the measurement limit of Keithley electrometer, reverse I-V characteristics was analyzed by a curve tracer. As shown in Fig. 4 the reverse breakdown occurred at 1.6 kV in case of diode-A, without any edge termination. Here, the Schottky diodes were not fabricated on an individual mesa structure, but similar to be on a planar type structure, and were fabricated without utilizing any edge termination structures. These SBDs could withstand an electric field of 1.4 MV/cm, which is similar to other literature reports. Our SBDs fabricated on a different substrate grown under different conditions, exhibited a still higher breakdown field of 3.1MV/cm with a breakdown voltage of 300 V [23], and we are elucidating out further optimal growth conditions towards achieving an epilayer for much higher breakdown field operation. Meanwhile, in the case of diode-B which exhibited a leak current path could sustain only a lower breakdown voltage of 1.4 kV. Despite this substrate contained a large density of pyramidal hillocks distributed throughout the substrate, they showed negligible influence on the reverse leakage characteristics of the diodes, and they are considered to be harmless morphological
Fig. 5. A model to explain the hard breakdown of diamond surface in reverse bias mode. a & b) SEM and Microscopic top view of diamond breakdown occurred due to leak current path in particular direction causing very high current passage in one direction. c) Figure illustrating the breakdown occurred. d) A model explaining reverse leak current paths due to the defects and charge states in subsurface region, causing in diamond breakdown. e) Another diode showing distributed leak current path causing less damage to diamond surface compared to figure-a.
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features, in contradiction to the device killing defect such as nonepitaxial crystallite (not included in the present work). However, some of the diodes exhibited high reverse leak currents, even though those diodes did not include any visible defects. This can be speculated to be due to the presence of high density of trap concentration and/or dislocations in the specific diode area, or to the formation of random graphitic phase inclusion during the CVD growth, and further detailed study is to be carried out in this aspect. While both the diode-A and diode-B were reverse biased up to breakdowns, diode-B caused a damage to diamond itself, as was evidenced from the hard breakdown of the diamond surface beneath the diode, which occurred during reverse bias measurement. The diode-B was located at about 150 µm far from the etched edge. Fig. 5a and b show the SEM and microscopic view of diamond breakdown that occurred in our study. This breakdown started at the periphery of the SBD and proceeded in a lateral direction (not in a vertical direction through the bulk) forming a valley like dimple and this is correlated to the high reverse current leak path exhibited by the diode. Fig. 5c and d show the schematic of the leak path in lateral direction and the resulting diamond breakdown in one direction, which started from the p− layer and continued to end up in p+ layer. Fig. 5e shows another diode after breakdown, in which the leak paths spread around the diode and hence the high leak current flow in one direction was prevented, and thus by the diamond surface escaped from any hard damage. In general, the defects such as dislocations cause pre-mature breakdown associated with high leakage current. But, the X-ray topographic analysis of the substrate used in the present study (not shown in this article), that was carried out before the device fabrication did not exhibit any dislocations at the site of the diode which caused diamond breakdown. Hence, the diamond breakdown that occurred in a lateral direction is explained due to the presence of near surface defective leak path. Although the nature and origin of the associated defect is not clearly known, this might include charged states at the surface and sub surface regions [24], causing carrier multiplication in one direction leading to crystal damage. Detailed study is further to be carried out to understand this mechanism clearly. In general, the breakdown at the periphery of the SBD is due to the electric field crowding at the edges, and this can be significantly improved by an edge termination technique. In our continuing part of research we studied about the significance of field plate structure SBD, and it exhibited an improved breakdown field in comparison to SBD without any edge termination [25]. 4. Conclusions We have developed an alternate and efficient technique for fabrication of diamond pseudo-vertical SBD structure. The SBDs fabricated in this structure, by our recently developed process flow exhibited a high blocking voltage (1.6 kV) with low leakage current density (1 µA/cm2) and a satisfactorily high forward current density
(60 A/cm2), without any edge termination. To realize still higher forward current density, SBDs have to be fabricated in a vertical geometry, and accordingly we are modifying the present fabrication process further. To understand the diamond breakdown mechanism that occurred in reverse blocking mode, a model has been proposed exhibiting the leak current path in single lateral direction. Acknowledgements This study was partially supported by Industrial Technology Research Grant Program in 2007 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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