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Diamond growth in the presence of boron and sulfur
Diamond and Related Materials 12 (2003) 1627–1632
Diamond growth in the presence of boron and sulfur Sally C. Eatona, Alfred B. Andersonb, John C. Angusa,*, Yulia E. Evstefeevac, Yuri V. Pleskovc a
Chemical Engineering Department, Case Western Reserve University, Cleveland, OH 44106, USA b Chemistry Department, Case Western Reserve University, Cleveland, OH 44106, USA c Frumkin Institute of Electrochemistry, Moscow 117071, Russia
Abstract Diamond was co-doped with sulfur and boron during deposition using hydrogen sulfide and trimethylboron in the source gases. Secondary ion mass spectroscopy (SIMS) confirmed the presence of sulfur and showed that the sulfur incorporation is concentrated in the near-surface region. Also, examination by scanning tunneling spectroscopy (STS) of a cleaved cross-section showed greater conductivity in the near-surface region. Hall effect, STS, the thermoelectric effect and Mott–Schottky analysis confirmed the ntype conductivity for films grown with sulfur and limited amounts of boron. The source of the donors is not known and could arise from defects or impurity bands. Additionally, the properties of a pn-junction using co-doped diamond for the n-type layers are discussed. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulfur incorporation; Co-doping; n-Type conductivity
1. Introduction Barber and Yarbrough w1x were the first to use sulfur during the chemical vapor deposition of diamond. They showed that the growth rate was halved when CS2 replaced CH4 in the source gas; however, they did not report on sulfur incorporation or the electrical properties of the diamond films. Sakaguchi, Nishitani-Gamo and co-workers were the first to report that sulfur gave n-type conductivity in diamond w2–4x. They reported an activation energy of 0.38 eV and a Hall mobility of electrons at room temperature of 597 cm2 Vy1 sy1 for diamond grown on (100) substrates. However, others subsequently reported that the sulfur-doped samples were contaminated with boron and were p-type w5,6x. Gheeraert et al. w7x reported a color change of the diamond when the H2S concentration in the feed gas was varied. The color changed from yellow (HPHT substrate color) for SyCs500 ppm in the feed gas, to light green–gray for 1000 ppm and then light gray for 5000 ppm. Hall effect measurements showed the samples were p-type, which was ascribed to unintentional boron-incorporation. *Corresponding author. Tel.: q1-216-368-4133; fax: q1-216-3683016. E-mail address: [email protected] (J.C. Angus).
Petherbridge et al. w8x reported n-type conductivity for polycrystalline, sulfur-doped samples grown on silicon. Depth profiling by XPS indicated that the sulfur was uniformly distributed throughout the diamond; however, discrimination between sulfur in grain boundaries and in the bulk was not reported. Eaton et al. w9–11x found that sulfur was incorporated in diamond in concentrations greater than background levels only when boron was present and n-type conductivity was achieved only when small amounts of boron were present. Two series of experiments using (111) and (110) single crystal substrates were performed in which the SyC atomic ratio in the feed gas was held constant and the ByS atomic ratio increased. At low By S ratios, the thermoelectric coefficient had a small negative value that became more negative as the boron concentration increased. The thermoelectric coefficient changed from negative values to positive values at a By S atomic ratio in the feed gas of approximately 0.23 for the (111) films and 0.14 for the (110) films. As the By S ratio was further increased, the thermoelectric coefficient dropped to small positive values. Katayama-Yoshida et al. w12–15x examined theoretical aspects of co-doping for wide bandgap semiconductors. Co-doping has proved advantageous for incorporation of impurities that are difficult to achieve otherwise w16–21x. Kiyota et al. w22x found that an
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increase in boron concentration in diamond films led to an increase in the concentration of donors, which they surmised were nitrogen. 2. Growth experiments Diamond films were grown in an ASTeX microwave reactor using H2S as the sulfur source and trimethylboron (TMB) as the boron source. Homoepitaxial diamond films were grown on (111), (110) and (100) diamond substrates. The methane concentration ranged from 0.1 to 1%; the SyC atomic ratio in the feed gas varied from 15 to 40 000 ppm. For the (111) and (110) substrates, the gas flow was 200 sccm, the pressure was 25 torr, and the microwave power was 1000 W. Substrate temperatures ranged from 700 to 800 8C. For the (100) substrates, the gas flow was 200 sccm, the pressure was 35 torr, and the microwave power was 800 W; substrate temperatures ranged from 820 to 840 8C and were controlled by an RF heater. To minimize the effect of residual boron in the reactor, experiments were normally performed in order of increasing boron concentration. When a boron concentration lower than a previous concentration was required, hydrogen plasma was activated within the reactor chamber for about a day to reduce the background boron levels. Despite these precautions, the amount of residual boron varied between experiments and probably contributed to some of the scatter in the data. Additionally, most experiments were performed on natural (111) diamond macles, which do not have identical surfaces prior to film growth. 3. Analytical results Raman analyses of 50 films grown on (111) and (110) substrates showed no evidence of sp2 carbon and the 1332 cmy1 diamond peak had FWHM that ranged from 2.9 to 4.6 cmy1. Diamond grown without sulfur had a FWHM of 2.9 cmy1. Analysis by X-ray photoelectron spectroscopy (XPS), particle-induced X-ray emission (PIXE), and SIMS showed that sulfur was incorporated into the diamond. Both XPS and SIMS showed the sulfur concentration was high at the surface and decreased several orders of magnitude to a relatively constant value in the bulk diamond. Figs. 1a,b show SIMS depth profiles for diamond films grown on (100) and (111) substrates. The analyses probe the first 50 nm of depth and show a concentration gradient for sulfur. Each of these samples had an n-type thermoelectric effect. The values for rms surface roughness, as measured by atomic force microscopy (AFM), were 1 nm for the (100) sample and 6 nm for the (111) sample. The SIMS data reported in Fig. 1a,b are the ratios of the counts of sulfur to carbon and boron to carbon.
Fig. 1. (a) SIMS depth profile for a (100) diamond sample showing the ratios of the sulfur intensity to the carbon intensity and the boron intensity to the carbon intensity. The source gas for this n-type sample had a SyC atomic ratio of 10 000 ppm, a ByC ratio of 2 ppm, and a 0.5% methane concentration. The film thickness was 4.3 mm. (b) SIMS depth profile for a (111) diamond sample showing the ratios of the sulfur intensity to the carbon intensity and the boron intensity to the carbon intensity. The source gas for this n-type sample had a SyC atomic ratio of 1000 ppm, a ByC ratio of 20 ppm, and a 0.125% methane concentration. The film thickness was 1.0 mm.
Quantitative analyses were performed by comparing the diamond samples against a standard. The sulfur concentration dropped rapidly with depth from 1020 cmy3 to 1017 cmy3 at a depth of approximately 50 nm, below which the sulfur concentration varied from 5 to 200 times greater than the background level of 8=1015 cmy3. The decrease in concentration with depth may also be seen on a much smaller scale by non-linear Mott– Schottky plots, which were analyzed to yield a depth profile of uncompensated donors. Analysis of a representative plot (not shown) indicated a decrease in uncompensated donors from 5=1021 cmy3 to 1=1021 cmy3 over a penetration depth of 10 nm. SIMS results indicate that boron enhances the incor-
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4. Electrical characterization 4.1. Hall measurements
Fig. 2. Ratio of S and C peaks from SIMS analysis vs. ByC atomic ratio in source gas. Feed gas concentrations of methane and the SyC atomic ratio were held constant at 0.125% and 1000 ppm. The substrate was the (111) face of a natural macle. The data points are averages over the first 50 nm of depth.
poration of sulfur. Fig. 2 shows that the sulfur concentration increases with increased boron in the source gas. Fig. 3 shows that the sulfur and boron concentration both increase with increased sulfur in the source gas. The data points shown in Figs. 2 and 3 are averages over the first 50 nm of sputter depth. The color of the samples in which films were grown on HPHT (100) diamond substrates changed from an initial yellow color to gray at feed conditions of SyC ratio of 500 ppm and ByC ratio of 10 ppm. For samples in which films were grown on (111) diamond macles, the color change was less pronounced and changed from clear to light gray (feed conditions at SyC ratio of 1000 ppm and ByC ratio of 20 ppm).
Gold-capped titanium contacts were annealed at 450 8C under vacuum for 2 h to create ohmic junctions, which were confirmed by I–V plots. Initial data for films grown on (111) and (100) substrates showed both n-type and p-type Hall coefficients using the van der Pauw method. An n-type (100) sample had a room temperature mobility of 12 cm2 yV-s and a sheet carrier concentration of 5=1012 cmy2. A p-type (100) sample had a room temperature mobility of 5 cm2 yV-s with sheet carrier concentration of 1=1012 cmy2. Because of the concentration gradient of sulfur, the samples may exhibit the features of two separate layers: a heavilydoped, n-type layer in the near-surface region and, depending on the ByS ratio in the bulk of the film, either an n-type or p-type layer underneath. The Hall coefficients from these samples will be positive or negative depending on the mobilities, carrier concentrations and depth of the n-type layer. 4.2. Scanning tunneling spectroscopy (STS) The STS measurements were made in air using a Digital Instruments Multimode III STM with Pt-Ir tips. The sample was grounded and the current recorded while the tip voltage varied from negative to positive values at a fixed tip-to-sample distance. Measurements on highly oriented pyrolytic graphite (HOPG) gave an ohmic I–V curve. Boron-doped diamond that had a ptype thermoelectric effect gave a p-type rectifying STS curve. An n-type film grown on a (111) diamond substrate from a source feed gas with 0.75% methane,
Fig. 3. Ratios of S and C peaks and B and C peaks from SIMS analysis vs. SyC atomic ratio in the feed gas at constant methane and boron concentrations. Feed gas concentrations of methane and the ByC atomic ratio were held constant at 0.125% and 4 ppm. The substrate was a (111) face of a natural macle. The data points are averages over the first 50 nm of depth.
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relative to boron in the feed gas (SyCs0.04, ByCs4 ppm), the sheet resistivity increased to 109 Vysquare. Resistivity measurements were performed from 200 to 300 K. An exponential relationship was found between resistivity and inverse temperature; the calculated activation energies ranged from 0.05 to 0.12 eV. These low activation energies may arise from conduction through impurity bands. These values are lower than the activation energies reported by Sakaguchi et al. w3x and Nishitani-Gamo et al. w4x of 0.38 eV for sulfur-doped diamond. Additionally, Prins w23x reported an activation energy of 0.32 eV for sulfur-implanted diamond and suggested that the conduction mechanism was nearestneighbor hopping. Chen et al. w24x determined activation energy of 0.33 eV for sulfur-implanted diamond. Fig. 4. Plot of current vs. applied voltage for the surface of (a) HOPG showing ohmic behavior, (b) boron-doped, p-type sample showing ptype rectification and (c) a co-doped, n-type sample showning n-type rectification.
SyC ratio of 525 ppm and ByC ratio of 0.67 ppm showed n-type rectification by STS. A survey of the film surface after deposition showed only n-type conductivity with no isolated regions of p-type conductivity. Representative I–V plots are shown in Fig. 4. Current scans were also performed on the cleaved cross-section of an n-type film grown on a (110) surface. See Fig. 5. The feed gas had 0.12% methane, SyC ratio of 1000 ppm, ByC ratio of 2 ppm; the film thickness was 3.7 mm. This sample exhibited n-type rectification across the entire film thickness. The current profiles in Fig. 4 indicate the conductivity decreased with film depth in the near-surface region. The major part of this decrease occurred over 400 nm, which is greater than the depth of excess sulfur concentration shown for the (111) and (100) samples in Fig. 1a,b. The conductivity decreased by about a factor of four in this near-surface region, while the sulfur concentration for the cleaved (110) sample decreased by two orders of magnitude. The differences in spatial distribution and in the magnitude of the sulfur concentration and conductivity can arise from electrically inactive sulfur and changes in mobility with depth. STS measurements can be used to give an indication of the density of states by plotting the dimensionless conductivity, (dIydV)y(IyV), vs. the applied voltage, V. However, the data were too noisy to make reliable conclusions using this method.
4.4. PN-junction Two types of pn-junctions were fabricated using boron-sulfur co-doped diamond as the n-type side. In one junction, boron-doped p-type diamond was used and in the other, the p-type surface conductivity induced by hydrogen termination was used. In the former junction, the p- and n-type layers were grown on a virgin (111) macle surface using appropriate masking; the second junction was grown on a virgin (100) diamond. Gold-capped titanium pads annealed at 450 8C were used as contacts in both cases. The p-type and n-type conductivity of the two sides were confirmed by thermoelectric measurements after the masks were removed. The I–V curve for the junction using the hydrogen-induced surface conductivity is shown in Fig. 6. For this junction, the p-type section
4.3. Resistivity measurements Typical values for sheet resistivity were 106 –107 Vy square for co-doped (111) and (100) films approximately 5 mm thick grown from source gases with SyC atomic ratios from 10 to 1000 ppm, and ByC atomic ratio of 4 ppm. At very high concentrations of sulfur
Fig. 5. Plot of current vs. applied voltage and depth on a cross-section of an n-type film grown on a (110) substrate. Film thickness was 3.7 mm. The tip voltage (V) relative to the sample was varied from q2 V to y2 V. The film shows n-type rectification over the depth examined (1000 nm).
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Fig. 6. Current–voltage curve for a pn-junction with hydrogen-terminated (p-type) and boronysulfur doped (n-type) diamond.
has a sheet resistance of 2.4=107 Vysquare and the ntype has a sheet resistance of 5.7=107 Vysquare. The thermoelectric coefficients were q1.3 mVy8C and y 1.9 mVy8C. The rectification ratio was approximately 100 at a voltage of 10 V and the turn-on voltage was approximately 0.5 V. Breakdown was not seen for a reverse voltage up to 100 V. The I–V characteristics of the junction with the boron-doped p-type side were very similar. Although rectification was observed with both junctions, the rectification ratio is relatively low and the reverse current is high compared to other results with diamond w25x. 5. Discussion of results 5.1. Co-doping with boron and sulfur Several processes may be responsible for the facilitation of sulfur incorporation in diamond by boron: formation of relatively stable BxSyHz species in the deposition gases, enhanced adsorption of these species on the diamond surface compared to species without boron, and strain energy compensation between the smaller boron atom and the larger sulfur atom within the diamond lattice w11x. Some relatively simple calculations support these suppositions. Gas phase equilibrium calculations performed for 1300 K and 25 torr indicate that HBS is present in concentrations greater than the principal growth species, CH3, and much more than S and HS. Both BS and BS2 are present in concentrations approximately equal to that of CH3. Density functional calculations indicate that BS is more strongly adsorbed on diamond than BH2 or SH w9x. Finally, density functional calculations by Albu w26x show that BS in a divacancy site is more stable than isolated B and S atoms.
5.2. Enhanced near-surface sulfur concentration The enhanced sulfur composition found in the nearsurface region is a striking observation. It was noted in all samples for which depth profile analysis was performed. One can speculate that it arises from a kinetic capture of non-equilibrium amounts of sulfur, followed by out-diffusion of the sulfur as the growth front proceeds. If this is true, one can expect to find that the enhanced sulfur concentration in the near-surface region is reduced by annealing. We have performed preliminary experiments that indicate that samples held under hydrogen plasma after growth show a significant decrease in sulfur content. Baral et al. w27x found that the sulfur concentration in polycrystalline diamond was reduced significantly by annealing at 400 8C under vacuum and disappeared entirely at 800 8C. Other workers have seen enhanced concentration of phosphorous w28x, lithium w29x and other defects w30,31x in the near-surface region of diamond grown by CVD. Of special interest is the report by Allers and Mainwood w31x indicating enhanced number of vacancies in the near-surface region. One can speculate that these may play a role in the sulfur incorporation. 5.3. Electronic properties It is not known what center is responsible for the observed n-type behavior. However, the thermoelectric results show that the donor centers are compensated by boron in a systematic fashion w11x. The observed conductivity may be due to the formation of impurity bands or defect sites. It has not been established if the electronic properties are stable at higher temperatures. 6. Summary Sulfur incorporation in diamond is promoted by the presence of boron. Depth profile analysis shows the
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sulfur incorporation is greatest in the near-surface region. Hall effect, thermoelectric power, Mott–Schottky analysis and scanning tunneling spectroscopy confirm that the conductivity is n-type if the ByS ratio in the source gas is below a critical value. The nature of the donors is not known; the conductivity could arise from defect or impurity bands. Acknowledgments The National Science Foundation (Grant CHE9816345), the Civilian Research and Development Foundation (Grant RC1-2053) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work. Hall measurements were performed by Lakeshore Cryogenics. Quantitative SIMS analyses were performed by Charles Evans Associates, Inc.
w10x
w11x w12x w13x w14x w15x w16x w17x w18x w19x
References w1x G.D. Barber, W.A. Yarbrough, J. Am. Ceram. Soc. 80 (1997) 1560. w2x M. Nishitani-Gamo, C. Xiao, K. Ushizawa, E. Yasu, I. Sakaguchi, H. Haneda, U. Kikuchi, Proceedings of the Applied Diamond Conference Tokyo, Japan, 1999, 54. w3x I. Sakaguchi, M. Nishitani-Gamo, Y. Kukuchi, E. Yasu, H. Haneda, T. Suzuki, et al., Phys. Rev. B 60 (1999) 2139. w4x M. Nishitani-Gamo, C. Xiao, Y. Zhang, E. Yasu, Y. Kukuchi, I. Sakaguchi, et al., Thin Solid Films 382 (2001) 113. w5x R. Kalish, A. Resnick, C. Uzan-Saguy, C. Cytermann, App. Phys. Lett. 76 (2000) 757. w6x H. Sternschulte, M. Schreck, B. Stritzker, Diam. Relat. Mater. 11 (2002) 296. w7x E. Gheeraert, N. Casanova, A. Tajani, A. Deneuville, E. Bustarret, J.A. Garrido, et al., Diam. Relat. Mater. 11 (2002) 289. w8x J.R. Petherbridge, P.W. May, G.M. Fuge, G.F. Robertson, K.N. Rosser, M.N.R. Ashfold, J. Appl. Phys. 91 (2002) 3605. w9x S.C. Eaton, A.B. Anderson, J.C. Angus, Y.E. Evstefeeva, Y.V. Pleskov, Proceedings of the Sixth Applied Diamond Confer-
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enceySecond Frontier Carbon Technology Joint Conference, NASAyCP-2001-210948 2001, 164. S.C. Eaton, A.B. Anderson, J.C. Angus, Y.E. Evstefeeva, Y.V. Pleskov, in Diamond Materials VII, PV 2001-25, The Electrochemical Society Proceedings Series, Pennington, NJ, 2001, 139. S.C. Eaton, A.B. Anderson, J.C. Angus, Y.E. Evstefeeva, Y.V. Pleskov, Electrochem. Solid-State Lett. 5 (2002) G65. H. Katayama-Yoshida, T. Yamamoto, Phys. Stat. Sol. (b) 202 (1997) 763. H. Katayama-Yoshida, T. Nishimatsu, T. Yamamoto, N. Orita, Phys. Stat. Sol. (b) 210 (1998) 429. T. Yamamoto, H. Katayama-Yoshida, Phys. B 273–274 (1999) 113. H. Katayama-Yoshida, T. Nishimatsu, T. Yamamoto, N. Orita, J. Phys.: Cond. Matter 13 (2001) 8901. G.Z. Cao, L.J. Giling, P.F.A. Alkemade, Diam. Relat. Mater. 4 (1995) 775. T. Yamamoto, H. Katayama-Yoshida, Jpn. J. Appl. Phys. Pt. 2 36 (1997) L180. K.H. Ploog, O. Brandt, J. Vac. Sci. Technol. A 16 (1998) 1609. T. Yamamoto, H. Katayama-Yoshida, J. Cryst. Growth 189– 190 (1998) 532. A. Kozanecki, K. Homewood, B.J. Sealy, Appl. Phys. Lett. 75 (1999) 793. V.G. Shengurov, S.P. Svetlov, V.Y. Chalkov, G.A. Maksimov, Z.F. Krasil’nik, B. An.Andreev, et al., Semiconductors 35 (2001) 918. H. Kiyota, E. Matsushima, K. Sato, H. Okushi, T. Ando, J. Tanaka, Diam. Relat. Mater. 6 (1997) 1753. J.F. Prins, Diam. Relat. Mater. 10 (2001) 1756. Y.G. Chen, M. Hasegawa, H. Okusi, S. Koizumi, H. Yoshida, T. Sakai, Diam. Relat. Mater. 11 (2002) 451. S. Koizumi, K. Watanabe, M. Hasegawa, H. Kanda, Diam. Relat. Mater. 11 (2002) 307. T. Albu, unpublished results, 2000. B. Baral, L.Y.S. Pang, R.B. Jackman, J.S. Foord, Diam. Relat. Mater. 9 (2000) 1167. T. Nishimori, K. Nakano, H. Sakamoto, Y. Takakuwa, S. Kono, Appl. Phys. Lett. 71 (1997) 945. M. Nesladek, K. Meykens, L.M. Stals, C. Quaeyhaegens, M. D’Olieslaeger, T.D. Wu, et al., Diam. Relat. Mater. 5 (1996) 1006. L.H. Robins, E.N. Garabaugh, A. Feldman, J. Mater. Res. 7 (1992) 394. L. Allers, A. Mainwood, Diam. Relat. Mater. 7 (1998) 261.
Diamond growth in the presence of boron and sulfur Sally C. Eatona, Alfred B. Andersonb, John C. Angusa,*, Yulia E. Evstefeevac, Yuri V. Pleskovc a
Chemical Engineering Department, Case Western Reserve University, Cleveland, OH 44106, USA b Chemistry Department, Case Western Reserve University, Cleveland, OH 44106, USA c Frumkin Institute of Electrochemistry, Moscow 117071, Russia
Abstract Diamond was co-doped with sulfur and boron during deposition using hydrogen sulfide and trimethylboron in the source gases. Secondary ion mass spectroscopy (SIMS) confirmed the presence of sulfur and showed that the sulfur incorporation is concentrated in the near-surface region. Also, examination by scanning tunneling spectroscopy (STS) of a cleaved cross-section showed greater conductivity in the near-surface region. Hall effect, STS, the thermoelectric effect and Mott–Schottky analysis confirmed the ntype conductivity for films grown with sulfur and limited amounts of boron. The source of the donors is not known and could arise from defects or impurity bands. Additionally, the properties of a pn-junction using co-doped diamond for the n-type layers are discussed. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulfur incorporation; Co-doping; n-Type conductivity
1. Introduction Barber and Yarbrough w1x were the first to use sulfur during the chemical vapor deposition of diamond. They showed that the growth rate was halved when CS2 replaced CH4 in the source gas; however, they did not report on sulfur incorporation or the electrical properties of the diamond films. Sakaguchi, Nishitani-Gamo and co-workers were the first to report that sulfur gave n-type conductivity in diamond w2–4x. They reported an activation energy of 0.38 eV and a Hall mobility of electrons at room temperature of 597 cm2 Vy1 sy1 for diamond grown on (100) substrates. However, others subsequently reported that the sulfur-doped samples were contaminated with boron and were p-type w5,6x. Gheeraert et al. w7x reported a color change of the diamond when the H2S concentration in the feed gas was varied. The color changed from yellow (HPHT substrate color) for SyCs500 ppm in the feed gas, to light green–gray for 1000 ppm and then light gray for 5000 ppm. Hall effect measurements showed the samples were p-type, which was ascribed to unintentional boron-incorporation. *Corresponding author. Tel.: q1-216-368-4133; fax: q1-216-3683016. E-mail address: [email protected] (J.C. Angus).
Petherbridge et al. w8x reported n-type conductivity for polycrystalline, sulfur-doped samples grown on silicon. Depth profiling by XPS indicated that the sulfur was uniformly distributed throughout the diamond; however, discrimination between sulfur in grain boundaries and in the bulk was not reported. Eaton et al. w9–11x found that sulfur was incorporated in diamond in concentrations greater than background levels only when boron was present and n-type conductivity was achieved only when small amounts of boron were present. Two series of experiments using (111) and (110) single crystal substrates were performed in which the SyC atomic ratio in the feed gas was held constant and the ByS atomic ratio increased. At low By S ratios, the thermoelectric coefficient had a small negative value that became more negative as the boron concentration increased. The thermoelectric coefficient changed from negative values to positive values at a By S atomic ratio in the feed gas of approximately 0.23 for the (111) films and 0.14 for the (110) films. As the By S ratio was further increased, the thermoelectric coefficient dropped to small positive values. Katayama-Yoshida et al. w12–15x examined theoretical aspects of co-doping for wide bandgap semiconductors. Co-doping has proved advantageous for incorporation of impurities that are difficult to achieve otherwise w16–21x. Kiyota et al. w22x found that an
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increase in boron concentration in diamond films led to an increase in the concentration of donors, which they surmised were nitrogen. 2. Growth experiments Diamond films were grown in an ASTeX microwave reactor using H2S as the sulfur source and trimethylboron (TMB) as the boron source. Homoepitaxial diamond films were grown on (111), (110) and (100) diamond substrates. The methane concentration ranged from 0.1 to 1%; the SyC atomic ratio in the feed gas varied from 15 to 40 000 ppm. For the (111) and (110) substrates, the gas flow was 200 sccm, the pressure was 25 torr, and the microwave power was 1000 W. Substrate temperatures ranged from 700 to 800 8C. For the (100) substrates, the gas flow was 200 sccm, the pressure was 35 torr, and the microwave power was 800 W; substrate temperatures ranged from 820 to 840 8C and were controlled by an RF heater. To minimize the effect of residual boron in the reactor, experiments were normally performed in order of increasing boron concentration. When a boron concentration lower than a previous concentration was required, hydrogen plasma was activated within the reactor chamber for about a day to reduce the background boron levels. Despite these precautions, the amount of residual boron varied between experiments and probably contributed to some of the scatter in the data. Additionally, most experiments were performed on natural (111) diamond macles, which do not have identical surfaces prior to film growth. 3. Analytical results Raman analyses of 50 films grown on (111) and (110) substrates showed no evidence of sp2 carbon and the 1332 cmy1 diamond peak had FWHM that ranged from 2.9 to 4.6 cmy1. Diamond grown without sulfur had a FWHM of 2.9 cmy1. Analysis by X-ray photoelectron spectroscopy (XPS), particle-induced X-ray emission (PIXE), and SIMS showed that sulfur was incorporated into the diamond. Both XPS and SIMS showed the sulfur concentration was high at the surface and decreased several orders of magnitude to a relatively constant value in the bulk diamond. Figs. 1a,b show SIMS depth profiles for diamond films grown on (100) and (111) substrates. The analyses probe the first 50 nm of depth and show a concentration gradient for sulfur. Each of these samples had an n-type thermoelectric effect. The values for rms surface roughness, as measured by atomic force microscopy (AFM), were 1 nm for the (100) sample and 6 nm for the (111) sample. The SIMS data reported in Fig. 1a,b are the ratios of the counts of sulfur to carbon and boron to carbon.
Fig. 1. (a) SIMS depth profile for a (100) diamond sample showing the ratios of the sulfur intensity to the carbon intensity and the boron intensity to the carbon intensity. The source gas for this n-type sample had a SyC atomic ratio of 10 000 ppm, a ByC ratio of 2 ppm, and a 0.5% methane concentration. The film thickness was 4.3 mm. (b) SIMS depth profile for a (111) diamond sample showing the ratios of the sulfur intensity to the carbon intensity and the boron intensity to the carbon intensity. The source gas for this n-type sample had a SyC atomic ratio of 1000 ppm, a ByC ratio of 20 ppm, and a 0.125% methane concentration. The film thickness was 1.0 mm.
Quantitative analyses were performed by comparing the diamond samples against a standard. The sulfur concentration dropped rapidly with depth from 1020 cmy3 to 1017 cmy3 at a depth of approximately 50 nm, below which the sulfur concentration varied from 5 to 200 times greater than the background level of 8=1015 cmy3. The decrease in concentration with depth may also be seen on a much smaller scale by non-linear Mott– Schottky plots, which were analyzed to yield a depth profile of uncompensated donors. Analysis of a representative plot (not shown) indicated a decrease in uncompensated donors from 5=1021 cmy3 to 1=1021 cmy3 over a penetration depth of 10 nm. SIMS results indicate that boron enhances the incor-
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4. Electrical characterization 4.1. Hall measurements
Fig. 2. Ratio of S and C peaks from SIMS analysis vs. ByC atomic ratio in source gas. Feed gas concentrations of methane and the SyC atomic ratio were held constant at 0.125% and 1000 ppm. The substrate was the (111) face of a natural macle. The data points are averages over the first 50 nm of depth.
poration of sulfur. Fig. 2 shows that the sulfur concentration increases with increased boron in the source gas. Fig. 3 shows that the sulfur and boron concentration both increase with increased sulfur in the source gas. The data points shown in Figs. 2 and 3 are averages over the first 50 nm of sputter depth. The color of the samples in which films were grown on HPHT (100) diamond substrates changed from an initial yellow color to gray at feed conditions of SyC ratio of 500 ppm and ByC ratio of 10 ppm. For samples in which films were grown on (111) diamond macles, the color change was less pronounced and changed from clear to light gray (feed conditions at SyC ratio of 1000 ppm and ByC ratio of 20 ppm).
Gold-capped titanium contacts were annealed at 450 8C under vacuum for 2 h to create ohmic junctions, which were confirmed by I–V plots. Initial data for films grown on (111) and (100) substrates showed both n-type and p-type Hall coefficients using the van der Pauw method. An n-type (100) sample had a room temperature mobility of 12 cm2 yV-s and a sheet carrier concentration of 5=1012 cmy2. A p-type (100) sample had a room temperature mobility of 5 cm2 yV-s with sheet carrier concentration of 1=1012 cmy2. Because of the concentration gradient of sulfur, the samples may exhibit the features of two separate layers: a heavilydoped, n-type layer in the near-surface region and, depending on the ByS ratio in the bulk of the film, either an n-type or p-type layer underneath. The Hall coefficients from these samples will be positive or negative depending on the mobilities, carrier concentrations and depth of the n-type layer. 4.2. Scanning tunneling spectroscopy (STS) The STS measurements were made in air using a Digital Instruments Multimode III STM with Pt-Ir tips. The sample was grounded and the current recorded while the tip voltage varied from negative to positive values at a fixed tip-to-sample distance. Measurements on highly oriented pyrolytic graphite (HOPG) gave an ohmic I–V curve. Boron-doped diamond that had a ptype thermoelectric effect gave a p-type rectifying STS curve. An n-type film grown on a (111) diamond substrate from a source feed gas with 0.75% methane,
Fig. 3. Ratios of S and C peaks and B and C peaks from SIMS analysis vs. SyC atomic ratio in the feed gas at constant methane and boron concentrations. Feed gas concentrations of methane and the ByC atomic ratio were held constant at 0.125% and 4 ppm. The substrate was a (111) face of a natural macle. The data points are averages over the first 50 nm of depth.
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relative to boron in the feed gas (SyCs0.04, ByCs4 ppm), the sheet resistivity increased to 109 Vysquare. Resistivity measurements were performed from 200 to 300 K. An exponential relationship was found between resistivity and inverse temperature; the calculated activation energies ranged from 0.05 to 0.12 eV. These low activation energies may arise from conduction through impurity bands. These values are lower than the activation energies reported by Sakaguchi et al. w3x and Nishitani-Gamo et al. w4x of 0.38 eV for sulfur-doped diamond. Additionally, Prins w23x reported an activation energy of 0.32 eV for sulfur-implanted diamond and suggested that the conduction mechanism was nearestneighbor hopping. Chen et al. w24x determined activation energy of 0.33 eV for sulfur-implanted diamond. Fig. 4. Plot of current vs. applied voltage for the surface of (a) HOPG showing ohmic behavior, (b) boron-doped, p-type sample showing ptype rectification and (c) a co-doped, n-type sample showning n-type rectification.
SyC ratio of 525 ppm and ByC ratio of 0.67 ppm showed n-type rectification by STS. A survey of the film surface after deposition showed only n-type conductivity with no isolated regions of p-type conductivity. Representative I–V plots are shown in Fig. 4. Current scans were also performed on the cleaved cross-section of an n-type film grown on a (110) surface. See Fig. 5. The feed gas had 0.12% methane, SyC ratio of 1000 ppm, ByC ratio of 2 ppm; the film thickness was 3.7 mm. This sample exhibited n-type rectification across the entire film thickness. The current profiles in Fig. 4 indicate the conductivity decreased with film depth in the near-surface region. The major part of this decrease occurred over 400 nm, which is greater than the depth of excess sulfur concentration shown for the (111) and (100) samples in Fig. 1a,b. The conductivity decreased by about a factor of four in this near-surface region, while the sulfur concentration for the cleaved (110) sample decreased by two orders of magnitude. The differences in spatial distribution and in the magnitude of the sulfur concentration and conductivity can arise from electrically inactive sulfur and changes in mobility with depth. STS measurements can be used to give an indication of the density of states by plotting the dimensionless conductivity, (dIydV)y(IyV), vs. the applied voltage, V. However, the data were too noisy to make reliable conclusions using this method.
4.4. PN-junction Two types of pn-junctions were fabricated using boron-sulfur co-doped diamond as the n-type side. In one junction, boron-doped p-type diamond was used and in the other, the p-type surface conductivity induced by hydrogen termination was used. In the former junction, the p- and n-type layers were grown on a virgin (111) macle surface using appropriate masking; the second junction was grown on a virgin (100) diamond. Gold-capped titanium pads annealed at 450 8C were used as contacts in both cases. The p-type and n-type conductivity of the two sides were confirmed by thermoelectric measurements after the masks were removed. The I–V curve for the junction using the hydrogen-induced surface conductivity is shown in Fig. 6. For this junction, the p-type section
4.3. Resistivity measurements Typical values for sheet resistivity were 106 –107 Vy square for co-doped (111) and (100) films approximately 5 mm thick grown from source gases with SyC atomic ratios from 10 to 1000 ppm, and ByC atomic ratio of 4 ppm. At very high concentrations of sulfur
Fig. 5. Plot of current vs. applied voltage and depth on a cross-section of an n-type film grown on a (110) substrate. Film thickness was 3.7 mm. The tip voltage (V) relative to the sample was varied from q2 V to y2 V. The film shows n-type rectification over the depth examined (1000 nm).
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Fig. 6. Current–voltage curve for a pn-junction with hydrogen-terminated (p-type) and boronysulfur doped (n-type) diamond.
has a sheet resistance of 2.4=107 Vysquare and the ntype has a sheet resistance of 5.7=107 Vysquare. The thermoelectric coefficients were q1.3 mVy8C and y 1.9 mVy8C. The rectification ratio was approximately 100 at a voltage of 10 V and the turn-on voltage was approximately 0.5 V. Breakdown was not seen for a reverse voltage up to 100 V. The I–V characteristics of the junction with the boron-doped p-type side were very similar. Although rectification was observed with both junctions, the rectification ratio is relatively low and the reverse current is high compared to other results with diamond w25x. 5. Discussion of results 5.1. Co-doping with boron and sulfur Several processes may be responsible for the facilitation of sulfur incorporation in diamond by boron: formation of relatively stable BxSyHz species in the deposition gases, enhanced adsorption of these species on the diamond surface compared to species without boron, and strain energy compensation between the smaller boron atom and the larger sulfur atom within the diamond lattice w11x. Some relatively simple calculations support these suppositions. Gas phase equilibrium calculations performed for 1300 K and 25 torr indicate that HBS is present in concentrations greater than the principal growth species, CH3, and much more than S and HS. Both BS and BS2 are present in concentrations approximately equal to that of CH3. Density functional calculations indicate that BS is more strongly adsorbed on diamond than BH2 or SH w9x. Finally, density functional calculations by Albu w26x show that BS in a divacancy site is more stable than isolated B and S atoms.
5.2. Enhanced near-surface sulfur concentration The enhanced sulfur composition found in the nearsurface region is a striking observation. It was noted in all samples for which depth profile analysis was performed. One can speculate that it arises from a kinetic capture of non-equilibrium amounts of sulfur, followed by out-diffusion of the sulfur as the growth front proceeds. If this is true, one can expect to find that the enhanced sulfur concentration in the near-surface region is reduced by annealing. We have performed preliminary experiments that indicate that samples held under hydrogen plasma after growth show a significant decrease in sulfur content. Baral et al. w27x found that the sulfur concentration in polycrystalline diamond was reduced significantly by annealing at 400 8C under vacuum and disappeared entirely at 800 8C. Other workers have seen enhanced concentration of phosphorous w28x, lithium w29x and other defects w30,31x in the near-surface region of diamond grown by CVD. Of special interest is the report by Allers and Mainwood w31x indicating enhanced number of vacancies in the near-surface region. One can speculate that these may play a role in the sulfur incorporation. 5.3. Electronic properties It is not known what center is responsible for the observed n-type behavior. However, the thermoelectric results show that the donor centers are compensated by boron in a systematic fashion w11x. The observed conductivity may be due to the formation of impurity bands or defect sites. It has not been established if the electronic properties are stable at higher temperatures. 6. Summary Sulfur incorporation in diamond is promoted by the presence of boron. Depth profile analysis shows the
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sulfur incorporation is greatest in the near-surface region. Hall effect, thermoelectric power, Mott–Schottky analysis and scanning tunneling spectroscopy confirm that the conductivity is n-type if the ByS ratio in the source gas is below a critical value. The nature of the donors is not known; the conductivity could arise from defect or impurity bands. Acknowledgments The National Science Foundation (Grant CHE9816345), the Civilian Research and Development Foundation (Grant RC1-2053) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan supported this work. Hall measurements were performed by Lakeshore Cryogenics. Quantitative SIMS analyses were performed by Charles Evans Associates, Inc.
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