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Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD
Diamond & Related Materials 32 (2013) 48–53
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD☆ J.L. Liu a, C.M. Li a,⁎, L.X. Chen a, J.J. Wei a, L.F. Hei a, J.J. Wang b, Z.H. Feng b, H. Guo c, F.X. Lv a a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Science and Technology on ASIC Laboratory, Hebei Semiconductor Research Institute, Shi Jia Zhuang 050051, PR China Institute of Laser, Academy of Science of Hebei Province, Shi Jia Zhuang 050000, PR China
a r t i c l e
i n f o
Available online 6 December 2012 Keyword: DC arc jet CVD Nucleation surface Growth surface H-terminated diamond Surface conductivity MESFET
a b s t r a c t High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (fT) and the maximum frequency of oscillation (fmax) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (Rs). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 1013/cm2 order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97×1011/cm2, respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (gm) for MESFET with gate length LG of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the surface conductivity of H-terminated diamond films is found, many research areas have been developed especially in high frequency and high power devices [1–3]. Although the room temperature drift mobilities of 4500 cm 2/V s for electrons and 3800 cm 2/V s for holes in intrinsic single crystal diamond have been reported [4], one main problem to limit the further application and even commercialization is the smaller size of single crystal diamond (SCD) compared with other wide gap semiconductors such as GaN and SiC, which leads to the high cost of the devices. Several approaches by controlling of alpha-parameter [5], three-dimensional growths in plasma chemical vapor deposition (CVD) [6] and cloning seed crystal substrate to form tiled clones of SCD [7,8], have been adopted to enlarge crystals size. Until now, 1 square inch is the largest area ☆ Originally presented at the International Conference of Diamond and Carbon Materials. ⁎ Corresponding author. E-mail addresses: [email protected] (J.L. Liu), [email protected] (C.M. Li). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2012.11.013
ever reported for single crystal diamond. However, it is still not enough to satisfy the practical requirement. High quality polycrystalline diamond films have been explored in advantage of its large area. In fact excellent performance of high frequency device (FET with fT of 45 GHz and fmax of 120 GHz) has been obtained based on 4 inch high quality poly-crystalline diamond films prepared by microwave CVD, which is even higher than that on single crystal diamond [9]. Diamond films prepared by DC arc jet CVD with high deposition rate may be an alternative for high frequency and high power devices industrialization. Due to the high energy activation process by DC voltage which leads to higher plasma temperature for C―H or C―H―O radicals, the deposition rate of DC arc jet CVD is much higher than that by microwave CVD or other methods. Since preparation of diamond films using DC arc jet CVD was proposed in 1980s, the diamond films' quality has been improved from black to transparent by optimizing plasma torch structure and process [10,11]. For 6 inch transparent windows, deposition rate of 50 μm/h was achieved [12]. Four inch high quality diamond films with thermal conductivity above 20 W/cm K could also
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
be obtained with deposition rate about 20 μm/h. If a recycle gas system is used, fabrication of high quality diamond films becomes more economical [13,14]. In spite of plenty of work on mechanical and optical application, little study on electronic properties of diamond films prepared by DC arc jet CVD has been reported. In this study, the grain size, surface profile, crystalline quality and impurity content of diamond films prepared by DC arc jet CVD were conducted to investigate the difference of electrical properties between the nucleation and growth diamond film surfaces, both of which were treated by common hydrogenation process using hydrogen plasma. Finally, MESFET on growth surface with 2 μm gate length was fabricated and preliminary results on DC output characteristic were reported. 2. Experimental DC arc jet CVD system with magnetic field and fluid dynamics controlled plasma torch has been reported previously [14]. In order to lower the production cost, a semi-closed gas recycling system was used, which allows more than 90% of gases be recycled while keeping a small amount (less than 10%) of gases being exhausted and renewed by the proper use of a two stage roots pumps and an exhausting mechanical pump [13,14]. In order to improve the quality of diamond films, feed gases were purified and deposition process was optimized. The deposition parameters of 4 inch diamond films are shown in Table 1. Free-standing diamond film was obtained and the calculated deposition rate was about 14 μm/h. Both the nucleation and growth surfaces of as-deposited diamond films were roughly polished by using commercial diamond grits with varying grit sizes, and then fine polishing was done with fast rotating diamond grinding wheel, until surface roughness reached below 1 nm. After polishing, the thickness is about 300 μm. In order to handle, 4 inch diamond films were cut into 15 mm × 15 mm × 0.3 mm pieces. A series of fundamental characterizations was used to evaluate characteristic differences between the nucleation surface and growth surfaces. AFM was used to observe the surface morphology of polished surfaces. Crystal orientation and crystal quality was revealed by XRD and Raman, respectively. Impurity information was obtained by PL spectrum at room temperature. Before hydrogen plasma treatment, the samples were boiled in the solution of H2SO4/HNO3 (3:1) for 1 h to remove amorphous carbon and other contaminants and form O-termination on the surfaces of diamond films. Then they were rinsed by deionized water, acetone, ethanol and deionized water in order. Hydrogen plasma treatment started when limiting vacuum of microwave CVD system reached 10 −3 Pa. Both the nucleation surface and growth surface were hydrogenated under the same conditions, listed in Table 2. Hydrogen plasma was turned off until samples temperatures decreased below 300 °C and H2 flow was persistent until samples temperatures reached room temperature. The surface morphology of the treated surfaces was compared with the untreated surface by AFM and the bonding characteristic of the nucleation and growth surfaces was analyzed by XPS with valence band spectrum and core level spectrum. After hydrogen plasma treatment, the dangling bonds on the surface of diamond were terminated by hydrogen atoms. Van der Pauw– Hall test was conducted on both the nucleation and growth surfaces of diamond films with H-termination using Au electrodes with
49
Table 2 Hydrogen plasma treatment parameters by microwave CVD. Temperature/°C
Power/W
Chamber pressure/kPa
Time/min
780–800
1200–1300
5
40
contact resistance of 10−5 Ω cm2. Meanwhile, MESFETs with standard double-finger structures were fabricated on the H-terminated growth surface, shown in Fig. 1. The source and drain Au ohmic contacts were formed using same procedure with Van der Pauw–Hall pattern. Electron beam (EB) lithography and self-alignment techniques were used to form Al Schottky gate contacts. The devices isolation was achieved by forming O-termination. The nominal drain–source gap of the MESFETs was 4 μm and the gate length was 2 μm. The DC characteristics were given by Keithley 4200. 3. Results and discussion 3.1. Polished samples pre-treated by hydrogen plasma After polishing, the morphology of the nucleation and growth surfaces is shown in Fig. 2. It can be observed that both surfaces are very smooth even with several scratches in different directions due to abrasion of diamond grits. Besides the scratches, the residual diamond grits are also observed, which will disappear after ultrasonic cleaning. The surface roughness (Ra) of nucleation and growth surfaces are 1.165 nm and 2.066 nm within the region of 10 μm×10 μm area, respectively. When tested area is confined in 2 μm×2 μm, Ra of both surfaces becomes 0.31 nm and 0.869 nm, respectively. According to height bar, maximum height difference of both surfaces is below 10 nm and drops to below 5 nm regardless the effect of residual diamond grits. It is enough to obtain 2 dimensional (2D) hole channel on H-terminated diamond surface, which generally exists at or up to 30 nm below the surface [15]. Fig. 3 shows the X-ray diffraction (XRD) pattern of both the nucleation and growth surfaces. It can be seen that the peaks mainly consist of (111), (220) and (311). Ratio of peak intensity for (220) and (111), I(220)/I(111), is used to estimate preferred orientation of diamond film surfaces. Based on origin software, I(220)/I(111) ranging from 1.25 for nucleation surface to 2.53 for growth surface is obtained. The (220) orientation is preferred as it grows in later stage. Chen et al. [16] reported relationship between the relative intensities of I(220)/I(111) and deposition parameters such as CH/C2, temperature and deposition time for as-deposited diamond films by DC arc jet CVD. They concluded that as deposition time increases, (111) orientation will be dominant. In our study, the rough surface was polished and the exposed (111) plane on growth surface was weakened dramatically, which leads to (220) preferred orientation in this order of thickness. Fig. 4 shows the Raman spectrum of both the nucleation and growth surfaces. It is shown that intensity of characteristic peak for
Table 1 Deposition parameters of 4 inch diamond films by DC arc jet CVD. Temperature/°C
1050–1070
Power/kW
15-–7
Pc/kPa
2.8–3
Pr/kPa
13–14
Feed gas flow H2/L min−1
Ar/L min−1
CH4/ml min−1
7
1.5
50
Note: Pc is chamber pressure and Pr is recycling gas pressure.
Fig. 1. Photograph of MESFETs with typical double-finger structure on H-terminated growth.
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Fig. 2. Morphology of nucleation surface and growth surface after polishing. (a) Nucleation surface in 10 μm× 10 μm; (b) growth surface in10 μm × 10 μm; (c) nucleation surface in 1 μm × 1 μm; (d) growth surface in1 μm × 1 μm.
diamond at 1332 cm −1 is stronger and the background is steeper on growth surface compared to that on nucleation surface. No obvious peaks of non-diamond phases such as amorphous carbon and graphite are observed for both surfaces. Through Lorentz fitting, it is calculated that FWHM is 6.7 cm −1 for growth surface and 8.4 cm −1 for nucleation surface, which illuminates better crystal quality on growth surface than on nucleation surface.
The PL spectrum at room temperature for both surfaces using laser with wavelength 532 nm is presented in Fig. 5. It is shown that the peaks at 575 nm and 638 nm appear for both surfaces, which correspond to vacancy–nitrogen complexes, [N–V]0 and [N–V] −, respectively. Compared to nucleation surface, these two peaks are stronger on growth surface, which indicates it contains more nitrogen impurities on growth surface. It also explains why steep background is observed
Fig. 3. X-ray diffraction (XRD) pattern of both the nucleation surface and growth surface.
Fig. 4. Raman spectrum of both the nucleation surface and growth surface.
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
Fig. 5. PL spectrum of both the nucleation surface and growth surface.
on growth surface in Fig. 3. That is due to fluorescence caused by nitrogen impurities [17]. 3.2. Hydrogen plasma treated samples After hydrogen plasma treatment, the morphology of diamond films on both nucleation and growth surfaces in 1 μm × 1 μm is shown in Fig. 6. The grains can be seen clearly, and the nucleation and growth surfaces can be distinguished according to the grain size. The average grain size is about 100 nm for nucleation surface and 400 nm for growth
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surface. The grain size difference mainly stems from the columnar growth and competitive growth in DC arc jet CVD process. Both nucleation and growth surfaces become rough and Ra increase to 3.739 nm and 8.406 nm respectively after hydrogen treatment. From 3D morphology, it is found that height difference increases to 35 nm for nucleation surface and 60 nm for growth surface compared with polished samples. That means the larger surface roughness results from hydrogen plasma etching, especially preferential etching on the grain boundary. The terminations on diamond surface after acid cleaning and hydrogen plasma treatment are characterized by XPS and compared in Fig. 7. After acid cleaning the peaks of C 1s, O 1s and O KLL are observed in full score in Fig. 7(a). The peak of O KLL disappears and O 1s becomes much weaker after hydrogen plasma treatment. Further atomic concentration ratio of O 1s to C 1s is obtained by calculating corresponding peaks intensity ratio. It is 21.2% for O-termination, and 3.4% for H-termination. Although they are a little higher than those obtained by Yamada [18], which may be due to exposure to atmosphere for a long time, it is inferred that O-termination is replaced by H-termination after treatment. The C 1s core level spectra for both the H-termination and O-termination are shown in Fig. 7(b). Through Gaussian fitting, C 1s consisting of C―H at 282.6 eV and C―C bonding at 281.9 eV is obtained for H-terminated surface. Compared to O-terminated diamond surface, the lower bonding energy of C―C indicates the band bends upwards induced by holes on H-terminated diamond surface. Further termination information such as molecular orbital on diamond surface is given by valence band spectrum in Fig. 7(c). For O-terminated surface, the peaks at 8 eV, 12.6 eV, 16.7 eV and 22.9 eV correspond to C 2p–O 2p orbital interaction, C 1s–C 2p hybrid orbital, C 2s orbital and O 2s, respectively [19]. The characteristic peaks of C―C bonding moved in the direction of
Fig. 6. Morphology of both the nucleation surface and growth surface after hydrogen plasma treatment. Nucleation surface: (a) 2D morphology; (c) 3D morphology; Growth surface: (b) 2D morphology; (d) 3D morphology.
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J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
Fig. 8. (a) DC drain–current (IDS)–voltage (VDS) characteristics for various gate biases (VGS = −6 − 6V, ΔVGS = 1V) for FET on growth surface of H-terminated diamond (LG = 2 μm, WG = 20 μm). (b) Gate bias dependence of transconductance gm and dc drain–current IDS of the FET measured at VDS of −10 V.
Fig. 7. XPS of H-terminated and O-terminated diamond film surface. (a) XPS in full score; (b) C 1s core level spectra; (c) valence band spectrum.
Table 3 Comparison of conductive property of both nucleation and growth surfaces. Surface type
Carrier mobility/cm2/V s
Carrier density/cm−2
Rs/Ω
Nucleation surface Growth surface
0.898 20.2
2.16 × 1013 9.97 × 1011
3.2 × 105 3.1 × 105
Note: Rs is the square resistance.
lower bonding energy for H-terminated surface. The stronger O 2s peak can be explained that it is easier to absorb oxygen on H-termination surface when exposed to air than that on O-termination surface. The weak peak at 5 eV may be related to C―H σ-bonding. The surface conductive properties of both nucleation and growth surfaces of H-terminated diamonds in 100 μm × 100 μm are given by Van der Pauw–Hall test, shown in Table 3. After hydrogen treatment, the p-type conduction on both surfaces is obtained. Meanwhile, it is found that the square resistance (Rs) is almost equal on both surfaces. However, the carrier mobility of growth surface is two orders of magnitude higher and the carrier density is correspondingly two orders of magnitude lower than those of nucleation surface. Generally, the carrier density is 10 13/cm 2 order of magnitude on diamond film surfaces treated by hydrogen plasma with appropriate process and the carrier mobility ranges from 10 to 1000 cm 2/V s for both single and polycrystalline diamonds. In this paper, DC arc jet CVD diamond films with the grain size of 100-400 nm and (110) preferred orientation are used. It is reported (220) surface owns the highest carrier density based on calculating H-C dipole charge density [20]. The low carrier density may be mainly due to impurities, especially content of vacancy– nitrogen complexes such as higher [N–V]– concentration on growth
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
surface resulting from impurity accumulation effect in circulating gas. Holes generated by hydrogen plasma treatment will be compensated by [N–V] –. The low carrier mobility on nucleation surface mainly roots in the relatively small grains. That is because the possibility of boundaries impeding the migration of carriers will increase while the tested area is large enough to cross several grains. Especially, while the grain boundaries are etched preferentially, it will be easier to block the carriers to cross the boundary for 2D hole channel existing at or up to 30 nm below the surface. Therefore larger crystal grain is essential to obtain excellent performance for H-terminated polycrystalline diamond films [9]. 3.3. DC characteristics of MESFET on H-terminated growth surface Fig. 8 shows the DC drain-current (IDS)–voltage (VDS) characteristics for various gate biases of the FET on the growth surface of H-terminated diamond film with gate length LG of 2 μm and gate width WG of 20 μm. The maximum drain current density (IDS) is 22.5 mA/mm at VGS = −6 V. The low IDS results from high resistance of p-type channel and long drain–source gap. In particular, it contains high concentration of [N-V] − on the growth surface and the low IDS is also attributed to the low carrier density caused by [N-V] − compensation of the hole carriers. The maximum transconductance (gm) is 4 mS/mm at VGS = 1.3 V for the 2-μm-gate FET, as shown in Fig. 8(b). gm decreased sharply showing that more work on optimization of gate structure should be done. Considering the grain size of 100–400 nm, the drain–source gap of 4 μm and the gate length of 2 μm is enough to cross several grains. The irregular atomic arrangement and the preferential etching in hydrogen plasma at the boundary increase the work instability of devices, so larger grains are needed to obtain better performance. In fact, the DC properties had been improved and the RF characteristic was obtained by using larger grain and optimizing device fabrication techniques in our recent work. 4. Conclusions Four inch high quality DC arc jet CVD diamond film with deposition rate of 14 μm/h was used to study surface conductivity of both the nucleation and growth surfaces after hydrogen plasma treatment.
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The XPS results show the formation of O-termination and H-termination after acid cleaning and after hydrogen plasma treatment, respectively. The energy band bent upwards for H-termination diamond surface. By Hall test, the similar square resistance was obtained while the carrier mobility and the carrier density differed by two orders of magnitude for the nucleation surface and growth surface. The small grain size was the reason of low mobility on nucleation surface. The vacancy–nitrogen complex of [N-V]− may reduce the carrier density on growth surface. The maximum drain current density of 22.5 mA/mm and maximum gm of 4 mS/mm were obtained for MESFET with gate length LG of 2 μm and gate width WG of 20 μm on H-terminated growth surface. It is concluded that the device performance could be further improved by increasing the grain size and optimizing device fabrication techniques. Acknowledgments This work was sponsored by the Chinese National Natural Science Foundation Program (No. 51272024) and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20110006110011). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Y. Gurbuz, O. Esame, I. Tekin, et al., Solid State Electron. 49 (2005) 1055. M. Kasu, K. Ueda, H. Ye, et al., Electron. Lett. 41 (2005) 1249. E. Kohn, M. Adamschik, P. Schmid, et al., J. Phys. D: Appl. Phys. 34 (2001) 77. J. Isberg, J. Hammersberg, E. Johansson, et al., Science 297 (2002) 1670. F. Silva, J. Achard, O. Brinza, et al., Diamond Relat. Mater. 18 (2009) 683. Y. Mokuno, A. Chayahara, H. Yamada, Diamond Relat. Mater. 18 (2009) 1258. H. Yamada, A. Chayahara, Y. Mokuno, et al., Appl. Phys. Express 3 (2010) 051301. H. Yamada, A. Chayahara, H. Umezawa, et al., Diamond Relat. Mater. 24 (2012) 29. K. Ueda, M. Kasu, Y. Yamauchi, et al., IEEE Electron Device Lett. 27 (2006) 570. F.X. Lu, Adv. Mater. Res. 211–212 (2011) 766. V.G. Ralchenko, E. Pleiler, D.N. Sovyk, et al., Inorg. Mater. 2 (2011) 439. G. Lu, Appl. Phys. Lett. 59 (1991) 1556. F.X. Lu, W.Z. Tang, G.F. Zhong, et al., Diamond Relat. Mater. 9 (2000) 1655. F.X. Lu, W.Z. Tang, T.B. Huang, et al., Diamond Relat. Mater. 10 (2001) 1551. F. Maier, M. Riedel, B. Mantel, et al., Phys. Rev. Lett. 85 (2000) 3472. G.C. Chen, B. Li, H. Lan, et al., Diamond Relat. Mater. 16 (2007) 477. S. Harris, A.M. Weiner, K.W. Nugent, et al., J. Appl. Phys. 80 (1996) 2187. T. Yamada, T. Yokoyama, A. Sawabe, Diamond Relat. Mater. 11 (2002) 780. K.P. Loh, X.N. Xie, Y.H. Lim, et al., Surf. Sci. 505 (2002) 93. K. Hirama, H. Takayanagi, S. Yamauchi, et al., Appl. Phys. Lett. 92 (2008) 112107.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD☆ J.L. Liu a, C.M. Li a,⁎, L.X. Chen a, J.J. Wei a, L.F. Hei a, J.J. Wang b, Z.H. Feng b, H. Guo c, F.X. Lv a a b c
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Science and Technology on ASIC Laboratory, Hebei Semiconductor Research Institute, Shi Jia Zhuang 050051, PR China Institute of Laser, Academy of Science of Hebei Province, Shi Jia Zhuang 050000, PR China
a r t i c l e
i n f o
Available online 6 December 2012 Keyword: DC arc jet CVD Nucleation surface Growth surface H-terminated diamond Surface conductivity MESFET
a b s t r a c t High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (fT) and the maximum frequency of oscillation (fmax) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (Rs). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 1013/cm2 order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97×1011/cm2, respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (gm) for MESFET with gate length LG of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the surface conductivity of H-terminated diamond films is found, many research areas have been developed especially in high frequency and high power devices [1–3]. Although the room temperature drift mobilities of 4500 cm 2/V s for electrons and 3800 cm 2/V s for holes in intrinsic single crystal diamond have been reported [4], one main problem to limit the further application and even commercialization is the smaller size of single crystal diamond (SCD) compared with other wide gap semiconductors such as GaN and SiC, which leads to the high cost of the devices. Several approaches by controlling of alpha-parameter [5], three-dimensional growths in plasma chemical vapor deposition (CVD) [6] and cloning seed crystal substrate to form tiled clones of SCD [7,8], have been adopted to enlarge crystals size. Until now, 1 square inch is the largest area ☆ Originally presented at the International Conference of Diamond and Carbon Materials. ⁎ Corresponding author. E-mail addresses: [email protected] (J.L. Liu), [email protected] (C.M. Li). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2012.11.013
ever reported for single crystal diamond. However, it is still not enough to satisfy the practical requirement. High quality polycrystalline diamond films have been explored in advantage of its large area. In fact excellent performance of high frequency device (FET with fT of 45 GHz and fmax of 120 GHz) has been obtained based on 4 inch high quality poly-crystalline diamond films prepared by microwave CVD, which is even higher than that on single crystal diamond [9]. Diamond films prepared by DC arc jet CVD with high deposition rate may be an alternative for high frequency and high power devices industrialization. Due to the high energy activation process by DC voltage which leads to higher plasma temperature for C―H or C―H―O radicals, the deposition rate of DC arc jet CVD is much higher than that by microwave CVD or other methods. Since preparation of diamond films using DC arc jet CVD was proposed in 1980s, the diamond films' quality has been improved from black to transparent by optimizing plasma torch structure and process [10,11]. For 6 inch transparent windows, deposition rate of 50 μm/h was achieved [12]. Four inch high quality diamond films with thermal conductivity above 20 W/cm K could also
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
be obtained with deposition rate about 20 μm/h. If a recycle gas system is used, fabrication of high quality diamond films becomes more economical [13,14]. In spite of plenty of work on mechanical and optical application, little study on electronic properties of diamond films prepared by DC arc jet CVD has been reported. In this study, the grain size, surface profile, crystalline quality and impurity content of diamond films prepared by DC arc jet CVD were conducted to investigate the difference of electrical properties between the nucleation and growth diamond film surfaces, both of which were treated by common hydrogenation process using hydrogen plasma. Finally, MESFET on growth surface with 2 μm gate length was fabricated and preliminary results on DC output characteristic were reported. 2. Experimental DC arc jet CVD system with magnetic field and fluid dynamics controlled plasma torch has been reported previously [14]. In order to lower the production cost, a semi-closed gas recycling system was used, which allows more than 90% of gases be recycled while keeping a small amount (less than 10%) of gases being exhausted and renewed by the proper use of a two stage roots pumps and an exhausting mechanical pump [13,14]. In order to improve the quality of diamond films, feed gases were purified and deposition process was optimized. The deposition parameters of 4 inch diamond films are shown in Table 1. Free-standing diamond film was obtained and the calculated deposition rate was about 14 μm/h. Both the nucleation and growth surfaces of as-deposited diamond films were roughly polished by using commercial diamond grits with varying grit sizes, and then fine polishing was done with fast rotating diamond grinding wheel, until surface roughness reached below 1 nm. After polishing, the thickness is about 300 μm. In order to handle, 4 inch diamond films were cut into 15 mm × 15 mm × 0.3 mm pieces. A series of fundamental characterizations was used to evaluate characteristic differences between the nucleation surface and growth surfaces. AFM was used to observe the surface morphology of polished surfaces. Crystal orientation and crystal quality was revealed by XRD and Raman, respectively. Impurity information was obtained by PL spectrum at room temperature. Before hydrogen plasma treatment, the samples were boiled in the solution of H2SO4/HNO3 (3:1) for 1 h to remove amorphous carbon and other contaminants and form O-termination on the surfaces of diamond films. Then they were rinsed by deionized water, acetone, ethanol and deionized water in order. Hydrogen plasma treatment started when limiting vacuum of microwave CVD system reached 10 −3 Pa. Both the nucleation surface and growth surface were hydrogenated under the same conditions, listed in Table 2. Hydrogen plasma was turned off until samples temperatures decreased below 300 °C and H2 flow was persistent until samples temperatures reached room temperature. The surface morphology of the treated surfaces was compared with the untreated surface by AFM and the bonding characteristic of the nucleation and growth surfaces was analyzed by XPS with valence band spectrum and core level spectrum. After hydrogen plasma treatment, the dangling bonds on the surface of diamond were terminated by hydrogen atoms. Van der Pauw– Hall test was conducted on both the nucleation and growth surfaces of diamond films with H-termination using Au electrodes with
49
Table 2 Hydrogen plasma treatment parameters by microwave CVD. Temperature/°C
Power/W
Chamber pressure/kPa
Time/min
780–800
1200–1300
5
40
contact resistance of 10−5 Ω cm2. Meanwhile, MESFETs with standard double-finger structures were fabricated on the H-terminated growth surface, shown in Fig. 1. The source and drain Au ohmic contacts were formed using same procedure with Van der Pauw–Hall pattern. Electron beam (EB) lithography and self-alignment techniques were used to form Al Schottky gate contacts. The devices isolation was achieved by forming O-termination. The nominal drain–source gap of the MESFETs was 4 μm and the gate length was 2 μm. The DC characteristics were given by Keithley 4200. 3. Results and discussion 3.1. Polished samples pre-treated by hydrogen plasma After polishing, the morphology of the nucleation and growth surfaces is shown in Fig. 2. It can be observed that both surfaces are very smooth even with several scratches in different directions due to abrasion of diamond grits. Besides the scratches, the residual diamond grits are also observed, which will disappear after ultrasonic cleaning. The surface roughness (Ra) of nucleation and growth surfaces are 1.165 nm and 2.066 nm within the region of 10 μm×10 μm area, respectively. When tested area is confined in 2 μm×2 μm, Ra of both surfaces becomes 0.31 nm and 0.869 nm, respectively. According to height bar, maximum height difference of both surfaces is below 10 nm and drops to below 5 nm regardless the effect of residual diamond grits. It is enough to obtain 2 dimensional (2D) hole channel on H-terminated diamond surface, which generally exists at or up to 30 nm below the surface [15]. Fig. 3 shows the X-ray diffraction (XRD) pattern of both the nucleation and growth surfaces. It can be seen that the peaks mainly consist of (111), (220) and (311). Ratio of peak intensity for (220) and (111), I(220)/I(111), is used to estimate preferred orientation of diamond film surfaces. Based on origin software, I(220)/I(111) ranging from 1.25 for nucleation surface to 2.53 for growth surface is obtained. The (220) orientation is preferred as it grows in later stage. Chen et al. [16] reported relationship between the relative intensities of I(220)/I(111) and deposition parameters such as CH/C2, temperature and deposition time for as-deposited diamond films by DC arc jet CVD. They concluded that as deposition time increases, (111) orientation will be dominant. In our study, the rough surface was polished and the exposed (111) plane on growth surface was weakened dramatically, which leads to (220) preferred orientation in this order of thickness. Fig. 4 shows the Raman spectrum of both the nucleation and growth surfaces. It is shown that intensity of characteristic peak for
Table 1 Deposition parameters of 4 inch diamond films by DC arc jet CVD. Temperature/°C
1050–1070
Power/kW
15-–7
Pc/kPa
2.8–3
Pr/kPa
13–14
Feed gas flow H2/L min−1
Ar/L min−1
CH4/ml min−1
7
1.5
50
Note: Pc is chamber pressure and Pr is recycling gas pressure.
Fig. 1. Photograph of MESFETs with typical double-finger structure on H-terminated growth.
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Fig. 2. Morphology of nucleation surface and growth surface after polishing. (a) Nucleation surface in 10 μm× 10 μm; (b) growth surface in10 μm × 10 μm; (c) nucleation surface in 1 μm × 1 μm; (d) growth surface in1 μm × 1 μm.
diamond at 1332 cm −1 is stronger and the background is steeper on growth surface compared to that on nucleation surface. No obvious peaks of non-diamond phases such as amorphous carbon and graphite are observed for both surfaces. Through Lorentz fitting, it is calculated that FWHM is 6.7 cm −1 for growth surface and 8.4 cm −1 for nucleation surface, which illuminates better crystal quality on growth surface than on nucleation surface.
The PL spectrum at room temperature for both surfaces using laser with wavelength 532 nm is presented in Fig. 5. It is shown that the peaks at 575 nm and 638 nm appear for both surfaces, which correspond to vacancy–nitrogen complexes, [N–V]0 and [N–V] −, respectively. Compared to nucleation surface, these two peaks are stronger on growth surface, which indicates it contains more nitrogen impurities on growth surface. It also explains why steep background is observed
Fig. 3. X-ray diffraction (XRD) pattern of both the nucleation surface and growth surface.
Fig. 4. Raman spectrum of both the nucleation surface and growth surface.
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
Fig. 5. PL spectrum of both the nucleation surface and growth surface.
on growth surface in Fig. 3. That is due to fluorescence caused by nitrogen impurities [17]. 3.2. Hydrogen plasma treated samples After hydrogen plasma treatment, the morphology of diamond films on both nucleation and growth surfaces in 1 μm × 1 μm is shown in Fig. 6. The grains can be seen clearly, and the nucleation and growth surfaces can be distinguished according to the grain size. The average grain size is about 100 nm for nucleation surface and 400 nm for growth
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surface. The grain size difference mainly stems from the columnar growth and competitive growth in DC arc jet CVD process. Both nucleation and growth surfaces become rough and Ra increase to 3.739 nm and 8.406 nm respectively after hydrogen treatment. From 3D morphology, it is found that height difference increases to 35 nm for nucleation surface and 60 nm for growth surface compared with polished samples. That means the larger surface roughness results from hydrogen plasma etching, especially preferential etching on the grain boundary. The terminations on diamond surface after acid cleaning and hydrogen plasma treatment are characterized by XPS and compared in Fig. 7. After acid cleaning the peaks of C 1s, O 1s and O KLL are observed in full score in Fig. 7(a). The peak of O KLL disappears and O 1s becomes much weaker after hydrogen plasma treatment. Further atomic concentration ratio of O 1s to C 1s is obtained by calculating corresponding peaks intensity ratio. It is 21.2% for O-termination, and 3.4% for H-termination. Although they are a little higher than those obtained by Yamada [18], which may be due to exposure to atmosphere for a long time, it is inferred that O-termination is replaced by H-termination after treatment. The C 1s core level spectra for both the H-termination and O-termination are shown in Fig. 7(b). Through Gaussian fitting, C 1s consisting of C―H at 282.6 eV and C―C bonding at 281.9 eV is obtained for H-terminated surface. Compared to O-terminated diamond surface, the lower bonding energy of C―C indicates the band bends upwards induced by holes on H-terminated diamond surface. Further termination information such as molecular orbital on diamond surface is given by valence band spectrum in Fig. 7(c). For O-terminated surface, the peaks at 8 eV, 12.6 eV, 16.7 eV and 22.9 eV correspond to C 2p–O 2p orbital interaction, C 1s–C 2p hybrid orbital, C 2s orbital and O 2s, respectively [19]. The characteristic peaks of C―C bonding moved in the direction of
Fig. 6. Morphology of both the nucleation surface and growth surface after hydrogen plasma treatment. Nucleation surface: (a) 2D morphology; (c) 3D morphology; Growth surface: (b) 2D morphology; (d) 3D morphology.
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Fig. 8. (a) DC drain–current (IDS)–voltage (VDS) characteristics for various gate biases (VGS = −6 − 6V, ΔVGS = 1V) for FET on growth surface of H-terminated diamond (LG = 2 μm, WG = 20 μm). (b) Gate bias dependence of transconductance gm and dc drain–current IDS of the FET measured at VDS of −10 V.
Fig. 7. XPS of H-terminated and O-terminated diamond film surface. (a) XPS in full score; (b) C 1s core level spectra; (c) valence band spectrum.
Table 3 Comparison of conductive property of both nucleation and growth surfaces. Surface type
Carrier mobility/cm2/V s
Carrier density/cm−2
Rs/Ω
Nucleation surface Growth surface
0.898 20.2
2.16 × 1013 9.97 × 1011
3.2 × 105 3.1 × 105
Note: Rs is the square resistance.
lower bonding energy for H-terminated surface. The stronger O 2s peak can be explained that it is easier to absorb oxygen on H-termination surface when exposed to air than that on O-termination surface. The weak peak at 5 eV may be related to C―H σ-bonding. The surface conductive properties of both nucleation and growth surfaces of H-terminated diamonds in 100 μm × 100 μm are given by Van der Pauw–Hall test, shown in Table 3. After hydrogen treatment, the p-type conduction on both surfaces is obtained. Meanwhile, it is found that the square resistance (Rs) is almost equal on both surfaces. However, the carrier mobility of growth surface is two orders of magnitude higher and the carrier density is correspondingly two orders of magnitude lower than those of nucleation surface. Generally, the carrier density is 10 13/cm 2 order of magnitude on diamond film surfaces treated by hydrogen plasma with appropriate process and the carrier mobility ranges from 10 to 1000 cm 2/V s for both single and polycrystalline diamonds. In this paper, DC arc jet CVD diamond films with the grain size of 100-400 nm and (110) preferred orientation are used. It is reported (220) surface owns the highest carrier density based on calculating H-C dipole charge density [20]. The low carrier density may be mainly due to impurities, especially content of vacancy– nitrogen complexes such as higher [N–V]– concentration on growth
J.L. Liu et al. / Diamond & Related Materials 32 (2013) 48–53
surface resulting from impurity accumulation effect in circulating gas. Holes generated by hydrogen plasma treatment will be compensated by [N–V] –. The low carrier mobility on nucleation surface mainly roots in the relatively small grains. That is because the possibility of boundaries impeding the migration of carriers will increase while the tested area is large enough to cross several grains. Especially, while the grain boundaries are etched preferentially, it will be easier to block the carriers to cross the boundary for 2D hole channel existing at or up to 30 nm below the surface. Therefore larger crystal grain is essential to obtain excellent performance for H-terminated polycrystalline diamond films [9]. 3.3. DC characteristics of MESFET on H-terminated growth surface Fig. 8 shows the DC drain-current (IDS)–voltage (VDS) characteristics for various gate biases of the FET on the growth surface of H-terminated diamond film with gate length LG of 2 μm and gate width WG of 20 μm. The maximum drain current density (IDS) is 22.5 mA/mm at VGS = −6 V. The low IDS results from high resistance of p-type channel and long drain–source gap. In particular, it contains high concentration of [N-V] − on the growth surface and the low IDS is also attributed to the low carrier density caused by [N-V] − compensation of the hole carriers. The maximum transconductance (gm) is 4 mS/mm at VGS = 1.3 V for the 2-μm-gate FET, as shown in Fig. 8(b). gm decreased sharply showing that more work on optimization of gate structure should be done. Considering the grain size of 100–400 nm, the drain–source gap of 4 μm and the gate length of 2 μm is enough to cross several grains. The irregular atomic arrangement and the preferential etching in hydrogen plasma at the boundary increase the work instability of devices, so larger grains are needed to obtain better performance. In fact, the DC properties had been improved and the RF characteristic was obtained by using larger grain and optimizing device fabrication techniques in our recent work. 4. Conclusions Four inch high quality DC arc jet CVD diamond film with deposition rate of 14 μm/h was used to study surface conductivity of both the nucleation and growth surfaces after hydrogen plasma treatment.
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The XPS results show the formation of O-termination and H-termination after acid cleaning and after hydrogen plasma treatment, respectively. The energy band bent upwards for H-termination diamond surface. By Hall test, the similar square resistance was obtained while the carrier mobility and the carrier density differed by two orders of magnitude for the nucleation surface and growth surface. The small grain size was the reason of low mobility on nucleation surface. The vacancy–nitrogen complex of [N-V]− may reduce the carrier density on growth surface. The maximum drain current density of 22.5 mA/mm and maximum gm of 4 mS/mm were obtained for MESFET with gate length LG of 2 μm and gate width WG of 20 μm on H-terminated growth surface. It is concluded that the device performance could be further improved by increasing the grain size and optimizing device fabrication techniques. Acknowledgments This work was sponsored by the Chinese National Natural Science Foundation Program (No. 51272024) and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20110006110011). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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