p-type doping by B ion implantation into diamond at elevated temperatures

p-type doping by B ion implantation into diamond at elevated temperatures

Diamond & Related Materials 15 (2006) 157 – 159 www.elsevier.com/locate/diamond p-type doping by B ion implantation into diamond at elevated temperat...

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Diamond & Related Materials 15 (2006) 157 – 159 www.elsevier.com/locate/diamond

p-type doping by B ion implantation into diamond at elevated temperatures Nobuteru Tsubouchi a,*, M. Ogura b, H. Kato b, S.G. Ri b, H. Watanabe b, Y. Horino a, H. Okushi b a

Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan b Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 16 May 2005; received in revised form 26 July 2005; accepted 2 September 2005 Available online 19 October 2005

Abstract We have studied B ion implantation at 400 -C into undoped homoepitaxial chemical vapor deposition diamond films and high-pressure and high-temperature (HPHT) synthetic IIa substrates. The highest Hall mobility at room temperature is 268 cm2/Vs among B implanted homoepitaxial films, while it is 38 cm2/Vs for the B implanted HPHT synthetic IIa substrate. The present result reveals that the quality of a doped layer is strongly dependent upon that of a diamond substrate employed for ion implantation. D 2005 Elsevier B.V. All rights reserved. Keywords: B ion implantation; Homoepitaxial diamond film; HPHT synthetic IIa; Hall mobility

1. Introduction For successful p-type doping to diamond using B implantation, it has been understood that cold implantation at a liquid nitrogen temperature or the so-called Cold Implantation followed by Rapid Annealing (CIRA) method is effective [1,2]. The highest quality B doped layer formed by ion implantation is obtained when using a high energy MeV B ion beam at a low substrate temperature of 77 K [3]. Very recently, the formation of a high mobility B doped layer by room temperature (RT) B implantation at a MeV energy has also been reported [4]. On the other hand, it has ever been reported that B implantation at elevated temperatures in the keV energy region is not effective for p-type doping [5]. In our group, high quality undoped homoepitaxial diamond films have ever been grown using a chemical vapor deposition (CVD) technique [6]. Using these high quality films, n-type layer fabrication by N ion implantation was also attempted [7]. In this paper, the first results of B implantation to undoped homoepitaxial diamond films as well as to highpressure and high-temperature (HPHT) synthetic IIa substrates at elevated temperatures, will be described. It is found that a B doped layer with relatively high Hall mobility of holes can * Corresponding author. E-mail address: [email protected] (N. Tsubouchi). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.008

be formed using medium energy (¨keV) range B ion implantation even above RT. 2. Experimental procedure The undoped homoepitaxial diamond film employed in this study is grown on a HPHT synthetic Ib(100) substrate using plasma CVD [6]. The reaction gas during the film growth consists of 0.5% CH4 and 99.5% H2. The total gas pressure and input microwave power are maintained constant at 25 Torr and 750 W, respectively. The substrate temperature, which is controlled independently of the input microwave power, is 800 -C. The film thickness is approximately 1.8 Am. For comparison, HPHT synthetic IIa(100) substrates are also used for B implantation. Prior to ion implantation, cathodoluminescence (CL) together with current– voltage (I –V) measurements of both homoepitaxial diamond films and HPHT synthesized IIa substrates are performed. The CL measurement is performed at 77 K using a 13 keV electron beam. It is confirmed that both diamond films and IIa substrates have high resistance enough, above 109 X at 700 -C. A Boron-box profile of 30430 nm in depth is formed by multiple B implantation at different ion energies (30 –360 keV) at a substrate temperature of 400 -C. The implanted B concentration is 5  1019 cm 3. The defect concentration estimated using TRIM simulation [8] is below 1 1022 cm 3, above which ion implanted diamond

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cannot be annealed back to diamond but rather collapses to graphite. The pressure in the sample chamber during ion implantation is ¨3  10 4 Pa. The contacts are formed at the four corners of the substrate surface by B implantation at RT through a four-dot mask with a dose exceeding the above critical dose, i.e., 1 1016 cm 2 at 30 keV. Subsequent annealing at 1450 -C for 0.5 h in vacuum followed by acidetching to remove a graphite layer, results in the formation of highly conducting B dots. A tri-layer of Au, Pt and Ti is evaporated through the four-dot mask in an ultrahigh vacuum chamber (base pressure: ¨10 9 Pa). The formed electrodes display ohmic I –V characteristics in the range of  50 to 50 V. The high S / N ratio AC Hall measurement is performed in the temperature range of RT to 700 -C in the Van der Pauw configuration.

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3. Results and discussion Fig. 1 shows a CL spectrum of the diamond film prior to ion implantation. It should be noted that a broad peak at ¨440 nm (the so-called ‘‘band-A’’ originating from stacking faults [9], which is often found in low-quality epitaxial diamond films) is very low and the free exciton peak at 235 nm is relatively very high. This indicates that the quality of the homoepitaxial film employed for B implantation in this study is very high. Both the diamond film and the IIa substrate after B implantation and annealing demonstrate p-type carrier conduction in the Hall measurements. The resistivities of the film and IIa samples are 5.2  103 and 8.2  103 V cm at RT, respectively. Fig. 2 shows temperature dependence of hole concentrations in the implanted diamond film (solid squares) and IIa sample (solid circles), respectively, calculated from data of Hall measurements. Since hole concentrations of both samples increase without saturation even at a high temperature,

Fig. 2. Temperature dependence of hole concentrations in the implanted diamond film and IIa samples. Data are fitted with linear lines using the Eq. (1) in the text.

the data is fitted by the following approximated formula, which is valid for p ? N d (freeze-out regime): 3=2 expð  Ea =kT Þ; ð1Þ p ¼ ð1=K  1Þð2=gÞ 2pmh 4kT =h2 where K = N d / N a is the compensation ratio; N a and N d are acceptor and donor concentrations, respectively; E a is the ionization energy of the acceptor; k is the Boltzmann constant; T is the absolute temperature; h is the Planck’s constant. In this work, we assume that a hole effective mass is m*h = 0.75m e and degeneracy factor is g = 2 [10], where m e is electron mass. In this fitting procedure, K and E a are fitting parameters. As shown in Fig. 2, both data of the B implanted homoepitaxial film and IIa substrate are fitted well with linear lines in the Arrhenius plot. In the homoepitaxial film, the slope of the fitted T (K) 400

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wavelength (nm) Fig. 1. CL spectrum measured at 77 K of an as-grown homoepitaxial diamond film prior to B implantation. Note that the ‘‘band-A’’ peak at ¨440 nm is low, whereas the free exciton peak at 235 nm is relatively very high.

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log T (K) Fig. 3. Temperature dependence of the Hall mobility in the implanted diamond film and IIa samples. The data using the formula l = l 0T  1.5 are shown as dotted lines.

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line shows that the activation energy of the B acceptor is 0.37 eV, in agreement with the well-known activation energy of the B doped CVD film. This indicates that implanted B atoms are placed in substitutional sites of the diamond lattice. The compensation ratio K of the B implanted film is K = 0.77. If g = 6 and m*h = 0.8m e which were used in a previous report [4] are assumed for comparison, K in this study is K = 0.25. In the IIa substrate, the activation energy is E a = 0.32 eV, which is slightly smaller than in the homoepitaxial film, but is most likely that valence band conduction of activated holes is realized. The compensation ratio of the IIa sample is K = 0.79 (K = 0.32, if g = 6 and m h* = 0.8m e are assumed for comparison), which is slightly larger than that of the diamond film. The reason for an increase of the B concentration above 700 K is not understood clearly. A possible explanation is that total signals originating not only from the film but from the interface and the Ib substrate are measured at such high temperatures. Fig. 3 displays temperature dependence of the Hall mobility. As shown in Fig. 3, the mobility at RT reaches 268 cm2/Vs in the implanted homoepitaxial film, which is the highest value among published data of diamond B-implanted above RT and is ¨70% of that of the B doped layer formed using the CIRA technique previously published [1]. Unlike the B-implanted film, the Hall mobility of the implanted IIa sample is 36 cm2/ Vs at RT. When fitting the data with l = l 0T  s (l is Hall mobility), the fit shown as dotted lines in Fig. 3 gives a suitable value s = 1.5 below 500 K in the homoepitaxial film and from 500 to 1000 K in the IIa sample, respectively. This indicates that mechanism of the carrier scattering in this temperature range is due to acoustic phonon. A rapid drop of the Hall mobility above 500 K in the diamond film suggests that low l electrical conduction of the Ib substrate at high temperatures contributes partially to the measured values. This rapid drop of the mobility is likely to be correlated with an increase in the hole concentration above 600 K as indicated in Fig. 2. In the IIa sample, hopping conduction is dominant at low temperatures unlike the diamond film sample. This can be seen in the decrease of the mobility with decreasing a temperature below ¨450 K for the HPHT sample. In fact, the s value decreases down to less than s = 1.5 below 600 K. The B concentration 5  1019 cm 3 implanted into diamond in this experiment is theoretically high enough to form an impurity band. In fact, even in B doped CVD diamond films with a lower B concentration than in this study, electrical conductivity is higher and the activation energy of acceptor is lower than those in the band conduction [11]. Also in other studies on B doped diamond formed by B implantation previously reported, the band conduction of holes is observed in spite of a high B concentration such as 5  1019 cm 3. This suggests that there are still a lot of unannealed donor-like defects compensating acceptors in the B implanted samples unlike B doped CVD films. What induces the difference between the electrical properties of the B implanted homoepitaxial film and IIa substrate? In this study, the high quality synthetic HPHT IIa substrates and homoepitaxial films were employed. ESR measurements by Mizuochi et al. [12] demonstrate that while

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H related defects similar to the H1 center distribute uniformly (¨5  1018 cm 3) along the depth direction in the diamond films, there are no such defects in HPHT synthetic IIa substrates. The major difference between the electrical properties of the homoepitaxial film and the IIa substrate is likely to be associated with hydrogen and related defects. Such H related defects introduced during the film growth prior to ion implantation might affect recovery of radiation damage and movement of B atoms to substitutional sites, and give the difference between the effective number of scattering centers of holes in B-implanted diamond films and IIa substrates. For understanding detailed mechanism, further studies will be required in the future. 4. Conclusions Boron implantation into high quality homoepitaxial diamond films and HPHT synthetic IIa substrates at 400 -C has been performed. The values of the Hall mobility of carriers in the implanted homoepitaxial film and the HPHT synthesized IIa substrate are 268 and 38 cm2/Vs at RT, respectively. This suggests that electrical properties of the B implanted layer are dependent not only on the ion implantation condition (e.g. CIRA or ion implantation at elevated temperatures), but also strongly on the quality of the diamond substrates employed—impurity, microscopic structures, crystalinity. Acknowledgements The authors are grateful to K. Sekine for a technical assistance of ion implantation and to N. Fujimori for valuable discussions. A part of this study was financially supported by the Budget for Nuclear Research of the Ministry of Education, Culture, Sports, Science and Technology, based on the screening and counseling by the Atomic Energy Commission. References [1] F. Fontaine, C. Uzan-Saguy, B. Philosoph, R. Kalish, Appl. Phys. Lett. 68 (1996) 2264. [2] J.F. Prins, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 80/81 (1993) 1433. [3] C. Uzan-Saguy, R. Kalish, R. Walker, D.N. Jamieson, S. Prawer, Diamond Relat. Mater. 7 (1998) 1429. [4] T. Vogel, J. Meijer, A. Zaitsev, Diamond Relat. Mater. 13 (2004) 1822. [5] J.R. Zeidler, C.H. Hewett, R.G. Wilson, Phys. Rev., B 47 (1993) 2065. [6] H. Watanabe, K. Hayashi, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, Appl. Phys. Lett. 73 (1998) 981. [7] M. Hasegawa, Y. Yamamoto, H. Watanabe, H. Okushi, M. Watanabe, T. Sekiguchi, Diamond Relat. Mater. 13 (2004) 600. [8] J.F. Ziegler, J.P. Biersack, U. Littmark, Stopping and Range of Ions in Matter, Pergamon, New York, 1985. [9] D. Takeuchi, H. Watanabe, S. Yamanaka, H. Okushi, H. Sawada, H. Ichinose, T. Sekiguchi, K. Kajimura, Phys. Rev., B 63 (2001) 245328. [10] A.T. Collins, E.C. Lightowlers, in: F.E. Field (Ed.), The Properties of Diamond, Academic Press, London, 1979, (Chap.3). [11] T. Inushima, A. Ogasawara, T. Shiraishi, S. Ohya, S. Karasawa, H. Shiomi, Diamond Relat. Mater. 7 (1998) 874. [12] N. Mizuochi, H. Watanabe, J. Isoya, H. Okushi, S. Yamasaki, Diamond Relat. Mater. 13 (1998) 765.