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Study of activation of beryllium implantation in gallium nitride
ARTICLE IN PRESS
Journal of Crystal Growth 268 (2004) 489–493
Study of activation of beryllium implantation in gallium nitride H.T. Wang*, L.S. Tan, E.F. Chor Centre for Optoelectronics, Department of Electrical and Computer Engineering, 4 Engineering Drive 3, National University of Singapore, Singapore 117576, Singapore
Abstract In this paper, post-implantation thermal activation of beryllium in GaN by rapid thermal annealing (RTA) or pulsed laser annealing (PLA) has been investigated. The result of a two-step RTA, in which the Be-implanted GaN sample was annealed first in forming gas (12% H2, 88% N2) and followed by annealing in pure nitrogen, showed slight p-type for Hall measurement, which was also confirmed by hot probe measurement. However, low activation efficiency and reproducibility was a problem in RTA. In PLA, by optimizing the laser fluence and annealing ambient, efficient activation of Be can be obtained using a 248 nm KrF excimer laser. The photoluminescence (PL) spectra revealed the presence of a Be-related transition level. The Hall measurement results showed that a hole sheet concentration of 2.56 1013 cm3 can be achieved with a single-pulse laser irradiation of 0.2 J/cm2 in flowing nitrogen ambient. r 2004 Elsevier B.V. All rights reserved. PACS: 61.72.Vv; 78.55.m Keywords: A1. Hall measurement; A1. Photoluminescence; A1. Pulsed laser annealing; A1. Rapid thermal annealing; B1. Beryllium implantation
1. Introduction Gallium nitride has gained great technological importance for applications in blue-light emitters and high-temperature electronic devices [1,2]. However, achievement and control of substantial activation of p-type dopants remains a critical issue in this material. The large ionization energy of acceptors (Mg, Ca, Zn, Cd [3,4]), leads to a room-temperature dopant activation of 0.1–1% *Corresponding author. Tel.: +65-68745251; fax: +6568745251. E-mail address: [email protected] (H.T. Wang).
for doping level between 1018 and 1021 cm3 [5]. Theoretical ab initio studies showed that beryllium has a high solubility and the lowest ionization energy among acceptors for GaN [6]. Several research groups have reported that its optical activation energy is in the range of 90–150 meV [7– 9]. This would make it a favorite acceptor to improve the p-type conductivity in GaN. However, the problem with Be is that it is easily incorporated on interstitial sites, where it acts as a donor instead of an acceptor. Therefore, there is little achievement of electrical activation of Bedoped GaN, except for Brandt et al. [10] who obtained high-mobility p-type cubic GaN by
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.078
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H.T. Wang et al. / Journal of Crystal Growth 268 (2004) 489–493
means of molecular beam epitaxy (MBE), and Sun et al. [11] who achieved activation of Be by a twostep annealing process. Ion implantation of GaN has recently attracted interest for the integration of such devices into circuits [12–14]. It offers precise region definition, accurate depth control and flexible doping level without constraint by equilibrium solubility, thermodynamic consideration, etc. However, this process introduces severe lattice damage, which needs to be removed by suitable annealing treatment to achieve good crystalline quality recovery and high activation efficiency. In this paper, two methods to activate Be dopants and removal of implantation induced damage will be investigated. Pearton et al. reported that annealing below 1100 C left a coarse network of extended defects, which indicated that higher annealing temperatures will be needed to recover the optimum electrical and optical properties [13,14]. This is a problem for commercial RTA equipment only has modest temperature capacity. Pulsed laser annealing has only recently been introduced to GaN technology [15]. It can provide controllable high annealing temperature up to GaN melting temperature (2518 C) and ‘‘nearzero’’ thermal budget, where with such ultra short annealing duration, the decomposition of sample surface can be avoided efficiently. The purpose of this work is to examine and compare the two-step RTA and excimer laser annealing of Be-implanted GaN. The effectiveness of electrical and optical activation of the two processes is evaluated by Hall measurement and photoluminescence (PL), respectively.
2. Experimental procedure The nominally undoped GaN epilayers with thickness of about 2 mm were grown on (0 0 0 1) sapphire substrate by MBE by SVT Associate, Inc, USA. The electron background concentration of the as-grown GaN wafer was about of 1015 cm3.The wafer was implanted at room temperature with Be at a dose of 5 1014 cm2 and an energy of 40 keV at a 7 tilt angle. After implantation, a 100 nm reactively sputtered AlN
layer was deposited on the top surface of the implanted GaN wafer for RTA annealing to provide the encapsulant to avoid GaN decomposition [16]. Then the wafer was cut into 7 7 mm2 samples. In two-step RTA, the implanted samples were first annealed in forming gas (12% H2 and 88% N) for 120 s at 550 C, and followed by annealing in flowing N2 ambient for 60 s at 1100 C using an AST Electronik SHS10 Rapid Thermal Process system. After annealing, the AlN encapsulant was removed with a heated (75 C) 0.4 mol KOH solution. In the PLA process, the implanted samples were irradiated by a 248 nm KrF excimer laser (Lambda Physik LPX 100) in flowing N2 ambient. The laser pulses were produced at a repetition rate of 1 Hz and the pulse duration was 30 ns. The annealed samples were characterized by several methods. Room temperature photoluminescence (RT PL) was performed using a Renishaw 2000 System. Low-temperature photoluminescence (LT PL) spectra were obtained at 12 K with the samples mounted in a liquid-helium cryostat with optical access. The signal was dispersed to a monochromator and detected by a cooled PMT detector. The He-Cd laser source emitting at 325 nm was used for both LT PL and RT PL. Hall effect measurements were carried out using a Bio-Rad HL 5500PC Hall system.
3. Results and discussion 3.1. Electrical activation of Be Owing to the n-type autodoping background present in most as-grown GaN samples, it is difficult to detect p-type doping by Hall measurement even if the hole concentration is comparable to the electron concentration. According to the Hall voltage expression: VH ¼ IBðm2p p 2 mn nÞ=tðmn n þ mp pÞ; where mp is the hole mobility, mn is the electron mobility, p is the hole concentration, n is the electron concentration, and t is the sample thickness. Hence the hole concentration has to be one or two orders of magnitude higher than the electron concentration in order for ppolarization to be indicated in Hall measurement
ARTICLE IN PRESS
because of the much smaller hole mobility in GaN. Therefore, the hot probe measurement was also performed to confirm the realization of p-type conversion. The unintentionally doped MBE grown GaN wafer presented here had a very low n-type background sheet concentration of 2.68 1011 cm2. After implantation, the sheet resistance was four orders of magnitude higher than that of the unimplanted one, hence the Hall measurement was not possible. For the two-step RTA activation, the result showed slight p-type conversion with hole sheet concentration of 1.13 1011 cm2. This was also confirmed by the hot probe method, in which a positive electrostatic voltage with respect to hot probe end was detected. As proposed by Sun et al. [11], the activation mechanism was possibly that the Beint–H–N interstitial complex firstly formed during annealing in forming gas ambient. It moved into Ga vacancies (VGa ) and converted to be BeGa–H–N substitutional complex, then the H-bond in this complex was broken by subsequent N2 annealing [11]. Lower energy might be required in this process than that needed to move Be from interstitial sites directly to substitutional sites. For PLA activation, a hole sheet concentration of 2.56 1013 cm3 was obtained after irradiation by a single laser pulse of 0.2 J/cm2 in pure nitrogen ambient. It is much higher than that of the RTA annealed sample (1.13 1011 cm2). The hole mobility is 10.36 cm2/V s, compared to 5.51 cm2/ V s of the RTA annealed case. Hence, the Hall measurement indicated that excimer laser annealing can provide sufficiently high activation energy for the Be dopants to move to substitutional Ga sites and to set the holes free by repairing implantation damage-related deep traps. Moreover, the mobility improvement also indicated that the electrically active defects, which were most likely candidate for the dominant ionized impurity scattering in low-mobility GaN samples [17], can be removed by laser annealing efficiently. 3.2. Optical properties of Be-implanted GaN Fig. 1 shows the room-temperature PL of the thermally annealed Be-implanted GaN films. The
PL Intensity (a.u.) (logarithmic scale)
H.T. Wang et al. / Journal of Crystal Growth 268 (2004) 489–493
491
(a) as-grown GaN (b) Be-implanted GaN After two-step RTA (c) Be-implanted GaN After PLA
(c) (b)
(a)
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
Energy (eV)
Fig. 1. Room temperature PL of GaN samples. The PL intensity ratios of near band edge and yellow band are (a) 400:1 for as-grown sample; (b) 1.3:1 for Be-implanted sample after RTA; and (c) 19:1 for Be-implanted sample after PLA.
spectrum of as-grown GaN film is also presented for reference. It should be noted that the PL intensity of the implanted film after RTA was only partially recovered. This is usually due to the residual damage remaining after post-implant annealing, which in turn contributes to the nonradiative recombination centers and act as deep level centers of PL emission. This shows that RTA at or below 1100 C is insufficient to completely remove the implantation damage, which was also confirmed by some other research group [18]. On the other hand, after laser annealing, the PL signal was improved a lot. The PL intensity is around six times that of the as-grown sample. This indicates that the implantation induced damage could be removed efficiently and crystalline quality greatly improved by laser annealing. The PL spectra show that Be implantation results in large enhancement of a broad yellow band (YB) transition centered around 2.4 eV, which was also reported by some groups [19,20]. This transition could be assigned to some complex involving Be at deep level as reported by Dewsnip et al. [19]. Another possibility which cannot be excluded is that gallium vacancy (VGa ) contributes to the YB, as reported in Refs. [21,22]. The implantation process knocks Ga atoms away from their original sites, however, Be dopants incorporate as Beit rather than substitute into Ga sites and
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492
Low temperature PL (12K)
3.395eV 1LO (DBE)
3.363eV (DAP)
3.303eV 2LO (DBE)
PL Intensity (a.u.)
(a) As-grown (b) laser annealed
3.492eV (FAX)
3.485eV (DBE)
leave them unoccupied in the from of gallium vacancies. Therefore, both of above possibilities do not result in effective activation of shallow level Be dopants, which in turn means that the YB is not a good indicator of Be activation. On the contrary, this implies that, to maximize the activation of Be, the YB transition should be suppressed. As shown in the PL spectra, after twostep RTA, the YB intensity is still comparable to that of near band edge emission, whereas for the PLA case, YB decreases significantly relative to the near band edge emission (ratio B1:19). This shows that PLA is a more efficient activation technique compared to two-step RTA, an observation which is in good agreement with the results obtained from Hall measurement. To further study the optical activation of the Be dopants, low-temperature PL spectra (12 K) of the implanted films and the as-grown film are shown in Fig. 2. From the spectra, we assign the peak at 3.485 eV to donor bound excitons (DBE) by considering its energy position and narrow linewidth. The peaks at 3.395 and 3.303 eV can be reasonably assigned to the 1st and 2nd optical phonon replica of the DBE at 3.485 eV, for their energy separations are around 92 meV.The peak at 3.492 eV can be assigned to the free A exciton (FAX) line. The DBE line is 7 meV lower in energy than the FAX transition, this localization energy
(b)
(a)
3.275 3.300 3.325 3.350 3.375 3.400 3.425 3.450 3.475 3.500 3.525 3.550
Energy (eV)
Fig. 2. PL spectra (12 K) for laser annealed Be-implanted sample and as-grown sample. New DAP peak appeared after laser annealing.
of the donor was generally reported in the range of 6–9 meV [23–26]. Besides these peaks, there is a new strong peak at 3.363 eV for the laser-annealed sample, compared with the as-grown sample and Si-implanted sample (not shown here). Several groups have reported a Be-related deep emission detected by PL in Be-doped [7,8] and Be-implanted GaN [9]. We attribute this transition to a Be-related donor– acceptor-pair (DAP) as reported by Dewsnip and co-workers [7]. This DAP involving Be is approximately 90 meV shallower than the usually observed Mg-related DAP at 3.27 eV. This result is in agreement with theoretical calculations, predicating that Be is a shallow acceptor in GaN [27]. The energy of a DAP transition is given by: EDAP ¼ Eg ðEA þ ED Þ þ Ecoul : Considering the general validity of the Haynes rule for semiconductors [28], which states that the localization energy is aED ; where ED is the donor binding energy, we obtained a value of 35 meV for the donor binding energy by taking the constant a as 0.2. This value agrees well with some independent measurements [29]. The bandgap energy Eg is around 3.518 eV taking free exciton binding energy as B26 meV [30]. The coulombic interaction energy can be estimated as Ecoul B15 meV [31]. Hence, an ionization energy EA B135 meV is derived for the Be acceptor. This is much shallower than the acceptor level of EA B250 meV B250 meVinducedbyMgdoping½32 :
4. Conclusion Pulsed laser annealing showed advantages over RTA for activation of Be-implanted GaN. The Hall measurement results proved that reasonable electrical activation can be achieved, and PL measurements showed the presence of Be-related DAP transition at 3.363 eV which provided strong evidence for the presence of optically active Be acceptors. However, the achievement and control of substantial activation of p-type dopants in GaN remains a critical issue. Especially for sheet resistance of implanted material after annealing, the result is not comparable to the value of good conductive n-doped GaN. Therefore, further work
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Table 1 Summary of electrical parameters obtained from Hall measurements at room temperature Sample
Sheet concentration (cm2)
Bulk concentration (cm3)
Sheet resistance (O/&)
Mobility (cm2/V s)
As-grown As-implanted Two-step RTA annealed Excimer laser annealed
2.68 1011 (ns ) N/Aa +1.13 1011 (ps ) +2.56 1013 (ps )
1.34 1015 N/Aa +5.65 1014 +1.28 1017
1.2 106 N/Aa 9.98 106 6.79 105
36 N/Aa 5.51 10.36
ns and ps are electron sheet concentration and hole sheet concentration, respectively. a As-implanted sample is highly resistive.
on Be-doped GaN needs to be carried out to obtain good p-type conductivity (Table 1).
Acknowledgements We would like to thank the kind assistance of Dr M. H. Hong and Mr P. K. Tan of Data Storage Institute, Singapore. The current work is supported by research grant R-263-000-176-112, and H.T.Wang is supported by a research scholarship from the National University of Singapore.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [2] S. Yoshida, J. Suzuki, Jpn. J. Appl. Phys. Part 2 37 (1998) L482. [3] U. Kaufmann, M. Kunzer, M. Maier, H. Obloh, A. Ramakrishnan, B. Santic, P. Schlotter, Appl. Phys. Lett. 72 (1998) 1326. [4] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (1992) 1237–1266. [5] J.W. Orton, Semicond. Sci. Technol. 10 101 (1995). [6] F. Bernardini, V. Fiorentini, A. Bosin, Appl. Phys. Lett. 70 (1997) 2990. [7] D.J. Dewsnip, A.V. Andrianov, I. Harrison, J.W. Orton, D.E. Lacklison, G.B. Ren, S.E. Hooper, T.S. Cheng, Semicond. Sci. Technol. 13 (1998) 500. [8] F.J. S"anchez, F. Calle, M.A. Sanchez-Garica, E. Calleja, $ E. Munoz, C.H. Molloy, MRS Internet J. Nitride Semicond. Res. 3 (1998) 19. [9] C. Ronning, E.P. Carlson, D.B. Thomson, R.F. Davis, Appl. Phys. Lett. 73 (1998) 1622.
[10] O. Brandt, H. Yang, H. Kostial, K.H. Ploog, Appl. Hys. Lett. 69 (1996) 2707. [11] Y.J. Sun, L.S. Tan, S.J. Chua, S. Prakash, MRS Internet J. Nitride Semicond. Res. 5S1 (2000) W3.82. [12] J.C. Zolper, Appl. Phys. Lett. 68 (1996) 2273. [13] H.H. Tan, J.S. Williams, J. Zou, D.J.H. Cockayne, S.J. Pearton, J.C. Zolper, R.A. Stall, Appl. Phys. Lett. 72 (1998) 1190. [14] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435. [15] S.J. Rhee, S. Kim, C.W. Sterner, J. Appl. Phys. 90 (2001) 2760. [16] J.C. Zopler, D.J. Rieger, A.G. Baca, S.J. Pearton, J.W. Lee, R. A. Stall Appl. Phys. Lett. 69 (1996) 538. [17] G. Li, S.J. Chua, W. Wang, Sol. Stat. Commun. 111 (1999) 659. [18] H.H. Tan, J.S. Williams, Appl. Phys. Lett. 72 (1998) 1190. [19] D.J. Dewsnip, A.V. Andrianov, I. Harrison, J.W. Orton, C.T. Foxton, Semicond. Sci. Technol. 13 (1998) 500. [20] C. Ronning, E.P. Carlson, D.B. Thomson, R.F. Davis, Appl. Phys. Lett. 73 (1998) 1622. [21] J. Neugebauer, C.G. Van de Walle, Phys. Rev. B 50 (1994) 8067. [22] J. Neugebauer, C.G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503. [23] S. Chichibu, A. Azuhata, T. Sota, S. Nakamura, J. Appl. Phys. 79 (1996) 2784. [24] D. Volm, et al., Phys. Rev. B 53 (1996) 16 543. [25] W. Shan, T.J. Schmidt, X.H. Yang, S.J. Hwang, J.J. Song, Appl. Phys. Lett. 66 (1995) 985. [26] G.D. Chen, M. Smith, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 67 (1995) 653. [27] C.H. Park, D.J. Chadi, Phys. Rev. B 55 (1997) 12 995. [28] J.R. Haynes, Phys. Rev. Lett. 4 (1960) 351. [29] C. Wetzel, D. Volm, B.K. Meyer, K. Pressel, S. Nilsson, P.G. Baranov, Appl. Phys. Lett. 65 (1994) 1033. [30] A.K. Viswanath, J.I. Lee, S. Yu, D. Kim, J. Appl. Phys. 84 (1998) 3848. [31] O. Lagerstedt, B. Monemar, J. Appl. Phys. 45 (1974) 2266. [32] A. Salvador, W. Kim, O. Aktas, A. Botchkarev, Z. Fan, H. Morkoc, Appl. Phys. Lett. 69 (1996) 2692.
Journal of Crystal Growth 268 (2004) 489–493
Study of activation of beryllium implantation in gallium nitride H.T. Wang*, L.S. Tan, E.F. Chor Centre for Optoelectronics, Department of Electrical and Computer Engineering, 4 Engineering Drive 3, National University of Singapore, Singapore 117576, Singapore
Abstract In this paper, post-implantation thermal activation of beryllium in GaN by rapid thermal annealing (RTA) or pulsed laser annealing (PLA) has been investigated. The result of a two-step RTA, in which the Be-implanted GaN sample was annealed first in forming gas (12% H2, 88% N2) and followed by annealing in pure nitrogen, showed slight p-type for Hall measurement, which was also confirmed by hot probe measurement. However, low activation efficiency and reproducibility was a problem in RTA. In PLA, by optimizing the laser fluence and annealing ambient, efficient activation of Be can be obtained using a 248 nm KrF excimer laser. The photoluminescence (PL) spectra revealed the presence of a Be-related transition level. The Hall measurement results showed that a hole sheet concentration of 2.56 1013 cm3 can be achieved with a single-pulse laser irradiation of 0.2 J/cm2 in flowing nitrogen ambient. r 2004 Elsevier B.V. All rights reserved. PACS: 61.72.Vv; 78.55.m Keywords: A1. Hall measurement; A1. Photoluminescence; A1. Pulsed laser annealing; A1. Rapid thermal annealing; B1. Beryllium implantation
1. Introduction Gallium nitride has gained great technological importance for applications in blue-light emitters and high-temperature electronic devices [1,2]. However, achievement and control of substantial activation of p-type dopants remains a critical issue in this material. The large ionization energy of acceptors (Mg, Ca, Zn, Cd [3,4]), leads to a room-temperature dopant activation of 0.1–1% *Corresponding author. Tel.: +65-68745251; fax: +6568745251. E-mail address: [email protected] (H.T. Wang).
for doping level between 1018 and 1021 cm3 [5]. Theoretical ab initio studies showed that beryllium has a high solubility and the lowest ionization energy among acceptors for GaN [6]. Several research groups have reported that its optical activation energy is in the range of 90–150 meV [7– 9]. This would make it a favorite acceptor to improve the p-type conductivity in GaN. However, the problem with Be is that it is easily incorporated on interstitial sites, where it acts as a donor instead of an acceptor. Therefore, there is little achievement of electrical activation of Bedoped GaN, except for Brandt et al. [10] who obtained high-mobility p-type cubic GaN by
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.078
ARTICLE IN PRESS 490
H.T. Wang et al. / Journal of Crystal Growth 268 (2004) 489–493
means of molecular beam epitaxy (MBE), and Sun et al. [11] who achieved activation of Be by a twostep annealing process. Ion implantation of GaN has recently attracted interest for the integration of such devices into circuits [12–14]. It offers precise region definition, accurate depth control and flexible doping level without constraint by equilibrium solubility, thermodynamic consideration, etc. However, this process introduces severe lattice damage, which needs to be removed by suitable annealing treatment to achieve good crystalline quality recovery and high activation efficiency. In this paper, two methods to activate Be dopants and removal of implantation induced damage will be investigated. Pearton et al. reported that annealing below 1100 C left a coarse network of extended defects, which indicated that higher annealing temperatures will be needed to recover the optimum electrical and optical properties [13,14]. This is a problem for commercial RTA equipment only has modest temperature capacity. Pulsed laser annealing has only recently been introduced to GaN technology [15]. It can provide controllable high annealing temperature up to GaN melting temperature (2518 C) and ‘‘nearzero’’ thermal budget, where with such ultra short annealing duration, the decomposition of sample surface can be avoided efficiently. The purpose of this work is to examine and compare the two-step RTA and excimer laser annealing of Be-implanted GaN. The effectiveness of electrical and optical activation of the two processes is evaluated by Hall measurement and photoluminescence (PL), respectively.
2. Experimental procedure The nominally undoped GaN epilayers with thickness of about 2 mm were grown on (0 0 0 1) sapphire substrate by MBE by SVT Associate, Inc, USA. The electron background concentration of the as-grown GaN wafer was about of 1015 cm3.The wafer was implanted at room temperature with Be at a dose of 5 1014 cm2 and an energy of 40 keV at a 7 tilt angle. After implantation, a 100 nm reactively sputtered AlN
layer was deposited on the top surface of the implanted GaN wafer for RTA annealing to provide the encapsulant to avoid GaN decomposition [16]. Then the wafer was cut into 7 7 mm2 samples. In two-step RTA, the implanted samples were first annealed in forming gas (12% H2 and 88% N) for 120 s at 550 C, and followed by annealing in flowing N2 ambient for 60 s at 1100 C using an AST Electronik SHS10 Rapid Thermal Process system. After annealing, the AlN encapsulant was removed with a heated (75 C) 0.4 mol KOH solution. In the PLA process, the implanted samples were irradiated by a 248 nm KrF excimer laser (Lambda Physik LPX 100) in flowing N2 ambient. The laser pulses were produced at a repetition rate of 1 Hz and the pulse duration was 30 ns. The annealed samples were characterized by several methods. Room temperature photoluminescence (RT PL) was performed using a Renishaw 2000 System. Low-temperature photoluminescence (LT PL) spectra were obtained at 12 K with the samples mounted in a liquid-helium cryostat with optical access. The signal was dispersed to a monochromator and detected by a cooled PMT detector. The He-Cd laser source emitting at 325 nm was used for both LT PL and RT PL. Hall effect measurements were carried out using a Bio-Rad HL 5500PC Hall system.
3. Results and discussion 3.1. Electrical activation of Be Owing to the n-type autodoping background present in most as-grown GaN samples, it is difficult to detect p-type doping by Hall measurement even if the hole concentration is comparable to the electron concentration. According to the Hall voltage expression: VH ¼ IBðm2p p 2 mn nÞ=tðmn n þ mp pÞ; where mp is the hole mobility, mn is the electron mobility, p is the hole concentration, n is the electron concentration, and t is the sample thickness. Hence the hole concentration has to be one or two orders of magnitude higher than the electron concentration in order for ppolarization to be indicated in Hall measurement
ARTICLE IN PRESS
because of the much smaller hole mobility in GaN. Therefore, the hot probe measurement was also performed to confirm the realization of p-type conversion. The unintentionally doped MBE grown GaN wafer presented here had a very low n-type background sheet concentration of 2.68 1011 cm2. After implantation, the sheet resistance was four orders of magnitude higher than that of the unimplanted one, hence the Hall measurement was not possible. For the two-step RTA activation, the result showed slight p-type conversion with hole sheet concentration of 1.13 1011 cm2. This was also confirmed by the hot probe method, in which a positive electrostatic voltage with respect to hot probe end was detected. As proposed by Sun et al. [11], the activation mechanism was possibly that the Beint–H–N interstitial complex firstly formed during annealing in forming gas ambient. It moved into Ga vacancies (VGa ) and converted to be BeGa–H–N substitutional complex, then the H-bond in this complex was broken by subsequent N2 annealing [11]. Lower energy might be required in this process than that needed to move Be from interstitial sites directly to substitutional sites. For PLA activation, a hole sheet concentration of 2.56 1013 cm3 was obtained after irradiation by a single laser pulse of 0.2 J/cm2 in pure nitrogen ambient. It is much higher than that of the RTA annealed sample (1.13 1011 cm2). The hole mobility is 10.36 cm2/V s, compared to 5.51 cm2/ V s of the RTA annealed case. Hence, the Hall measurement indicated that excimer laser annealing can provide sufficiently high activation energy for the Be dopants to move to substitutional Ga sites and to set the holes free by repairing implantation damage-related deep traps. Moreover, the mobility improvement also indicated that the electrically active defects, which were most likely candidate for the dominant ionized impurity scattering in low-mobility GaN samples [17], can be removed by laser annealing efficiently. 3.2. Optical properties of Be-implanted GaN Fig. 1 shows the room-temperature PL of the thermally annealed Be-implanted GaN films. The
PL Intensity (a.u.) (logarithmic scale)
H.T. Wang et al. / Journal of Crystal Growth 268 (2004) 489–493
491
(a) as-grown GaN (b) Be-implanted GaN After two-step RTA (c) Be-implanted GaN After PLA
(c) (b)
(a)
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
Energy (eV)
Fig. 1. Room temperature PL of GaN samples. The PL intensity ratios of near band edge and yellow band are (a) 400:1 for as-grown sample; (b) 1.3:1 for Be-implanted sample after RTA; and (c) 19:1 for Be-implanted sample after PLA.
spectrum of as-grown GaN film is also presented for reference. It should be noted that the PL intensity of the implanted film after RTA was only partially recovered. This is usually due to the residual damage remaining after post-implant annealing, which in turn contributes to the nonradiative recombination centers and act as deep level centers of PL emission. This shows that RTA at or below 1100 C is insufficient to completely remove the implantation damage, which was also confirmed by some other research group [18]. On the other hand, after laser annealing, the PL signal was improved a lot. The PL intensity is around six times that of the as-grown sample. This indicates that the implantation induced damage could be removed efficiently and crystalline quality greatly improved by laser annealing. The PL spectra show that Be implantation results in large enhancement of a broad yellow band (YB) transition centered around 2.4 eV, which was also reported by some groups [19,20]. This transition could be assigned to some complex involving Be at deep level as reported by Dewsnip et al. [19]. Another possibility which cannot be excluded is that gallium vacancy (VGa ) contributes to the YB, as reported in Refs. [21,22]. The implantation process knocks Ga atoms away from their original sites, however, Be dopants incorporate as Beit rather than substitute into Ga sites and
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492
Low temperature PL (12K)
3.395eV 1LO (DBE)
3.363eV (DAP)
3.303eV 2LO (DBE)
PL Intensity (a.u.)
(a) As-grown (b) laser annealed
3.492eV (FAX)
3.485eV (DBE)
leave them unoccupied in the from of gallium vacancies. Therefore, both of above possibilities do not result in effective activation of shallow level Be dopants, which in turn means that the YB is not a good indicator of Be activation. On the contrary, this implies that, to maximize the activation of Be, the YB transition should be suppressed. As shown in the PL spectra, after twostep RTA, the YB intensity is still comparable to that of near band edge emission, whereas for the PLA case, YB decreases significantly relative to the near band edge emission (ratio B1:19). This shows that PLA is a more efficient activation technique compared to two-step RTA, an observation which is in good agreement with the results obtained from Hall measurement. To further study the optical activation of the Be dopants, low-temperature PL spectra (12 K) of the implanted films and the as-grown film are shown in Fig. 2. From the spectra, we assign the peak at 3.485 eV to donor bound excitons (DBE) by considering its energy position and narrow linewidth. The peaks at 3.395 and 3.303 eV can be reasonably assigned to the 1st and 2nd optical phonon replica of the DBE at 3.485 eV, for their energy separations are around 92 meV.The peak at 3.492 eV can be assigned to the free A exciton (FAX) line. The DBE line is 7 meV lower in energy than the FAX transition, this localization energy
(b)
(a)
3.275 3.300 3.325 3.350 3.375 3.400 3.425 3.450 3.475 3.500 3.525 3.550
Energy (eV)
Fig. 2. PL spectra (12 K) for laser annealed Be-implanted sample and as-grown sample. New DAP peak appeared after laser annealing.
of the donor was generally reported in the range of 6–9 meV [23–26]. Besides these peaks, there is a new strong peak at 3.363 eV for the laser-annealed sample, compared with the as-grown sample and Si-implanted sample (not shown here). Several groups have reported a Be-related deep emission detected by PL in Be-doped [7,8] and Be-implanted GaN [9]. We attribute this transition to a Be-related donor– acceptor-pair (DAP) as reported by Dewsnip and co-workers [7]. This DAP involving Be is approximately 90 meV shallower than the usually observed Mg-related DAP at 3.27 eV. This result is in agreement with theoretical calculations, predicating that Be is a shallow acceptor in GaN [27]. The energy of a DAP transition is given by: EDAP ¼ Eg ðEA þ ED Þ þ Ecoul : Considering the general validity of the Haynes rule for semiconductors [28], which states that the localization energy is aED ; where ED is the donor binding energy, we obtained a value of 35 meV for the donor binding energy by taking the constant a as 0.2. This value agrees well with some independent measurements [29]. The bandgap energy Eg is around 3.518 eV taking free exciton binding energy as B26 meV [30]. The coulombic interaction energy can be estimated as Ecoul B15 meV [31]. Hence, an ionization energy EA B135 meV is derived for the Be acceptor. This is much shallower than the acceptor level of EA B250 meV B250 meVinducedbyMgdoping½32 :
4. Conclusion Pulsed laser annealing showed advantages over RTA for activation of Be-implanted GaN. The Hall measurement results proved that reasonable electrical activation can be achieved, and PL measurements showed the presence of Be-related DAP transition at 3.363 eV which provided strong evidence for the presence of optically active Be acceptors. However, the achievement and control of substantial activation of p-type dopants in GaN remains a critical issue. Especially for sheet resistance of implanted material after annealing, the result is not comparable to the value of good conductive n-doped GaN. Therefore, further work
ARTICLE IN PRESS H.T. Wang et al. / Journal of Crystal Growth 268 (2004) 489–493
493
Table 1 Summary of electrical parameters obtained from Hall measurements at room temperature Sample
Sheet concentration (cm2)
Bulk concentration (cm3)
Sheet resistance (O/&)
Mobility (cm2/V s)
As-grown As-implanted Two-step RTA annealed Excimer laser annealed
2.68 1011 (ns ) N/Aa +1.13 1011 (ps ) +2.56 1013 (ps )
1.34 1015 N/Aa +5.65 1014 +1.28 1017
1.2 106 N/Aa 9.98 106 6.79 105
36 N/Aa 5.51 10.36
ns and ps are electron sheet concentration and hole sheet concentration, respectively. a As-implanted sample is highly resistive.
on Be-doped GaN needs to be carried out to obtain good p-type conductivity (Table 1).
Acknowledgements We would like to thank the kind assistance of Dr M. H. Hong and Mr P. K. Tan of Data Storage Institute, Singapore. The current work is supported by research grant R-263-000-176-112, and H.T.Wang is supported by a research scholarship from the National University of Singapore.
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