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N-type implantation doping of GaN
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 515–517
N-type implantation doping of GaN Yoshitaka Nakanoa,b,*, Tetsu Kachia, Takashi Jimbob a
Toyota Central Research and Development Laboratories, Inc., Nagakute Aichi 480-1192, Japan b Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
Abstract Doping characteristics of N/Si and N/Ge co-implanted GaN have been systematically investigated. N-type regions were produced in undoped GaN films by the co-implantation and subsequent annealing with an SiO2 encapsulation layer at high temperatures. The annealing procedures above 1100 and 1200 C were required to achieve an n-type activation for N/Si and N/Ge co-implanted GaN, respectively. The both samples show effective activation efficiencies of B50% after annealing at 1300 C. However, actual Si activation seems to be much higher than the Ge activation due to the different behaviors of implantation-induced damage. r 2003 Elsevier Ltd. All rights reserved. PACS: 72.80.Ey; 61.72.Vv; 73.61.Ey Keywords: GaN; Implantation; Doping; Si; Ge
1. Introduction Ion implantation of semiconductors is one of the most convenient techniques to realize selective-area doping and device isolation. In general, group-IV and -II elements such as Si and Mg are promising donor and acceptor impurities for GaN, respectively. These elements occupying a Ga-lattice site in GaN are theoretically suggested to have a low formation energy and to form donor and acceptor levels. As for implantation doping of GaN, several n- and p-type implantation techniques have already been reported with the use of Si for n type and Mg and Ca for p-type [1–5]. However, in the case of conventional implantation, where only one kind of dopant is used, the generation of many N vacancies and self-compensation induced by site switching may occur in the implanted region after activation annealing at high temperatures. In order to suppress the generation of N vacancies, an N-rich condition needs to be created prior to implantation of the dopant atoms, *Corresponding author. Toyota Central Research and Development Laboratories, Inc., Nagakute Aichi 480-1192, Japan. Tel.: +81-561-63-4721; fax: +81-561-63-6042. E-mail address: [email protected] (Y. Nakano).
and so the implantation of additional N atoms into GaN might be expected to increase the probability of the particular dopant atoms occupying a Ga-lattice site [6]. Thus, we propose a co-implantation technique with N atoms and dopant atoms substituted on favored Galattice sites in GaN for high electrical activation of the dopant, based on a site-competition effect. In this study, we have investigated n-type doping characteristics of N/ Si and N/Ge co-implanted GaN, and the results are compared to each other.
2. Experimental The epitaxial GaN films used in these experiments were 1 mm thick grown on a-plane sapphire substrates by metalorganic chemical-vapor deposition (MOCVD) at 1130 C with B20 nm AlN buffer layer grown at 420 C. The GaN films were not intentionally doped with background n-type carrier concentrations of B5 1015 cm 3. After growth, N/Si and N/Ge coimplantation was performed at room temperature, with an incidence angle 7 off the normal surface. First the 14 + N ions were implanted at 35 keV to position the ion peak B50 nm from the surface. The 28Si+ or 72Ge+ ions
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.06.001
ARTICLE IN PRESS 516
Y. Nakano et al. / Materials Science in Semiconductor Processing 6 (2003) 515–517
were then implanted at 65 and 150 keV to place the ion peak range at the same position as that for the N+ implanted at 35 keV. The N/Si and N/Ge ratios were kept 1 for an optimum doping. The implant dosages were varied between 1 1013 and 1 1016 cm 2. Conventional Si- and Ge-implanted GaN samples were also prepared for reference. After implantation, a 500 nm thick SiO2 capping layer was deposited by radio-frequency sputtering at room temperature. All the samples were annealed at temperatures between 1100 C and 1300 C for 5 min in flowing H2 gas. Following the anneal step, HF was used to remove the SiO2 cap, and then Al-contacts were formed at the corners of each sample by electron-beam evaporation. Carrier activation was characterized by room-temperature Hall-effect measurements. The surface morphology of the implanted region of the GaN samples was analyzed by atomic force microscopy (AFM).
3. Results and discussion Figs. 1(a) and (b) show typical AFM images of the Siand N/Si-implanted GaN after annealing at 1300 C, respectively. Here, the Si dosage was fixed at 1 1015 cm 2. In the co-implanted GaN sample, a number of growth steps are clearly observed, which surface morphology is identical with that of the asgrown GaN before implantation. Thus, the surface morphology is found to be unchanged even after the implantation and subsequent annealing processes for the co-implanted GaN. Here, the dark points correspond to threading dislocations. In sharp contrast, many whitecolored islands with B25 nm in height and B140 nm in diameter can be seen for the conventional Si-implanted sample. These islands are found to be mainly composed of Ga from Auger electron spectroscopy (AES) measurements. These Ga islands in surface region are considered to be formed in accordance with GaN dissociation caused by the high-temperature annealing. In particular, in the case of the conventional Si
Fig. 1. AFM images of Si- and N/Si-implanted GaN samples with a Si dosage of 1 1015 cm 2 after annealing at 1300 C. Both images are 5 5 mm2.
implantation, the Ga islands are expected to be more easily formed owing to there being insufficient N atoms in the implanted region to achieve stoichiometric GaN compared to the N/Si co-implantation process. Therefore, the N/Si co-implantation turns out to suppress the Ga-island formation significantly in view of GaN stoichiometry. However, implantation-induced microdefects can be seen even after the high-temperature annealing process for the both Si- and N/Si-implanted GaN samples. Fig. 2 shows room-temperature sheet carrier concentration (ns) and electron mobility (me) as a function of annealing temperature for the N/Si and N/Ge coimplanted GaN samples. Here, the Si and Ge dosages were fixed at 1 1015 cm 2, respectively. With the rising of annealing temperature from 1100 C to 1200 C, the N/Si-implanted samples show a significant increase in ns from 2 1012 to 3 1014 cm 2 with high me of B75 cm2/ Vs. This indicates that the Si electrical activation starts to occur at around 1200 C. On the other hand, the Ge activation is found to occur after annealing at 1300 C. This difference of the activation temperature between the N/Si- and N/Ge-implanted samples is probably associated with the implantation-induced damage. That is, the activation temperature for the N/Si implantation is much lower than that for the N/Ge implantation since Si is much lighter than Ge and consequently might introduce much less damage in GaN. Here, the effective activation efficiency (Zeff) is defined as a ratio of the ns to the dosage, assuming that all of the carriers are generated from the Si or Ge donors. The Zeff are estimated to be B50% for the both N/Si- and N/Geimplanted GaN after annealing at 1300 C. However, the me for the N/Si implantation is much higher than that for the N/Ge implantation. This difference of the me also seems to be related to an improvement of crystallinity in connection with the recovery of the damage in the implanted region, as mentioned above.
Fig. 2. Sheet carrier concentration (,J) and mobility (’,&) as a function of annealing temperature for N/Si and N/Ge coimplanted GaN samples with Si and Ge dosages of 1 1015 cm 2.
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crystallinity of the electrically n-type activated region compared to the N/Ge co-implantation. However, the behavior of the me for the both samples does not seem to be simply consistent with the variation seen for the ns in view of ionized impurity scattering. The me may be primarily dependent on the combined effects of ionized impurity scattering and space charge scattering induced by the presence of the implantation-introduced defects.
4. Conclusions Fig. 3. Sheet carrier concentration (,J) and mobility (’,&) as a function of Si or Ge implant dosage for N/Si and N/Ge coimplanted GaN samples after annealing at 1300 C.
Fig. 3 shows the room-temperature ns and me as a function of Si or Ge dosage for the N/Si- and N/Geimplanted GaN samples after annealing at 1300 C, together with an ideal Zeff line of 100%. In the case of the N/Si co-implantation, the ns increases monotonically with increasing Si implant dosage up to 1 1015 cm 2 and the ns and me seem to be saturated at around 1 1015 cm 2 and 71 cm2/Vs, respectively, with increasing the Si dosage above 3 1015 cm 2. When the Si dosage becomes above 1 1016 cm 2, the implanted GaN is found to get partially separated during the annealing procedure. These results suggest that the Sidoping characteristics by the implantation techniques may attain to the solid-solubility limit of the implanted Si atoms into GaN. On the other hand, the ns increases monotonically with increasing Ge implant dosage up to 3 1015 cm 2 under the N/Ge co-implantation process. However, the implanted GaN with a Ge dosage of 1 1016 cm 2 peels off during the annealing. Thus, the N/Ge co-implanted GaN samples are easily subject to the implantation-induced damage. In addition, much improved me are obtained for the N/Si-implanted samples compared to the N/Ge-implanted samples regardless of the implant dosage. This indicates that the N/Si co-implantation might significantly improve the
We have demonstrated that the N/Si co-implantation into GaN and subsequent annealing at high temperatures can enhance the actual n-type activation compared to the N/Ge co-implantation. However, numerous implantation-induced micro-defects can be seen to remain even after the high-temperature annealing process for the both N/Si- and N/Ge-implanted GaN.
References [1] Zolper JC, Shul RJ, Baca AG, Wilson RG, Pearton SJ, Stall RA. Ion-implanted GaN junction field effect transistor. Appl Phys Lett 1996;68(16):2273. [2] Zhang AP, Johnson JW, Ren F, Han J, Polyakov AY, Smirnov NB, Govorkov AV, Redwing JM, Lee KP, Pearton SJ. Lateral AlxGa1–xN power rectifiers with 9.7 kV reverse breakdown voltage. Appl Phys Lett 2001; 78(6):823. [3] Zhang AP, Dang G, Ren F, Han J, Polyakov AY, Smirnov NB, Govorkov AV, Redwing JM, Cho H, Pearton SJ. Temperature dependence and current transport mechanisms in AlxGa1–xN Schottky rectifiers. Appl Phys Lett 2000;76(25):3816. [4] Pearton SJ, Vartuli CB, Zolper JC, Yuan C, Stall RA. Ion implantation doping and isolation of GaN. Appl Phys Lett. 1995;67(10):1435. [5] Zolper JC, Wilson RG, Pearton SJ, Stall RA. Ca and O ion implantation doping of GaN. Appl Phys Lett 1996;68(14): 1945. [6] Kobayashi H, Gibson WH. Lattice location of Ca in GaN using ion channeling. Appl Phys Lett 1999;74(16):2355.
Materials Science in Semiconductor Processing 6 (2003) 515–517
N-type implantation doping of GaN Yoshitaka Nakanoa,b,*, Tetsu Kachia, Takashi Jimbob a
Toyota Central Research and Development Laboratories, Inc., Nagakute Aichi 480-1192, Japan b Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
Abstract Doping characteristics of N/Si and N/Ge co-implanted GaN have been systematically investigated. N-type regions were produced in undoped GaN films by the co-implantation and subsequent annealing with an SiO2 encapsulation layer at high temperatures. The annealing procedures above 1100 and 1200 C were required to achieve an n-type activation for N/Si and N/Ge co-implanted GaN, respectively. The both samples show effective activation efficiencies of B50% after annealing at 1300 C. However, actual Si activation seems to be much higher than the Ge activation due to the different behaviors of implantation-induced damage. r 2003 Elsevier Ltd. All rights reserved. PACS: 72.80.Ey; 61.72.Vv; 73.61.Ey Keywords: GaN; Implantation; Doping; Si; Ge
1. Introduction Ion implantation of semiconductors is one of the most convenient techniques to realize selective-area doping and device isolation. In general, group-IV and -II elements such as Si and Mg are promising donor and acceptor impurities for GaN, respectively. These elements occupying a Ga-lattice site in GaN are theoretically suggested to have a low formation energy and to form donor and acceptor levels. As for implantation doping of GaN, several n- and p-type implantation techniques have already been reported with the use of Si for n type and Mg and Ca for p-type [1–5]. However, in the case of conventional implantation, where only one kind of dopant is used, the generation of many N vacancies and self-compensation induced by site switching may occur in the implanted region after activation annealing at high temperatures. In order to suppress the generation of N vacancies, an N-rich condition needs to be created prior to implantation of the dopant atoms, *Corresponding author. Toyota Central Research and Development Laboratories, Inc., Nagakute Aichi 480-1192, Japan. Tel.: +81-561-63-4721; fax: +81-561-63-6042. E-mail address: [email protected] (Y. Nakano).
and so the implantation of additional N atoms into GaN might be expected to increase the probability of the particular dopant atoms occupying a Ga-lattice site [6]. Thus, we propose a co-implantation technique with N atoms and dopant atoms substituted on favored Galattice sites in GaN for high electrical activation of the dopant, based on a site-competition effect. In this study, we have investigated n-type doping characteristics of N/ Si and N/Ge co-implanted GaN, and the results are compared to each other.
2. Experimental The epitaxial GaN films used in these experiments were 1 mm thick grown on a-plane sapphire substrates by metalorganic chemical-vapor deposition (MOCVD) at 1130 C with B20 nm AlN buffer layer grown at 420 C. The GaN films were not intentionally doped with background n-type carrier concentrations of B5 1015 cm 3. After growth, N/Si and N/Ge coimplantation was performed at room temperature, with an incidence angle 7 off the normal surface. First the 14 + N ions were implanted at 35 keV to position the ion peak B50 nm from the surface. The 28Si+ or 72Ge+ ions
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.06.001
ARTICLE IN PRESS 516
Y. Nakano et al. / Materials Science in Semiconductor Processing 6 (2003) 515–517
were then implanted at 65 and 150 keV to place the ion peak range at the same position as that for the N+ implanted at 35 keV. The N/Si and N/Ge ratios were kept 1 for an optimum doping. The implant dosages were varied between 1 1013 and 1 1016 cm 2. Conventional Si- and Ge-implanted GaN samples were also prepared for reference. After implantation, a 500 nm thick SiO2 capping layer was deposited by radio-frequency sputtering at room temperature. All the samples were annealed at temperatures between 1100 C and 1300 C for 5 min in flowing H2 gas. Following the anneal step, HF was used to remove the SiO2 cap, and then Al-contacts were formed at the corners of each sample by electron-beam evaporation. Carrier activation was characterized by room-temperature Hall-effect measurements. The surface morphology of the implanted region of the GaN samples was analyzed by atomic force microscopy (AFM).
3. Results and discussion Figs. 1(a) and (b) show typical AFM images of the Siand N/Si-implanted GaN after annealing at 1300 C, respectively. Here, the Si dosage was fixed at 1 1015 cm 2. In the co-implanted GaN sample, a number of growth steps are clearly observed, which surface morphology is identical with that of the asgrown GaN before implantation. Thus, the surface morphology is found to be unchanged even after the implantation and subsequent annealing processes for the co-implanted GaN. Here, the dark points correspond to threading dislocations. In sharp contrast, many whitecolored islands with B25 nm in height and B140 nm in diameter can be seen for the conventional Si-implanted sample. These islands are found to be mainly composed of Ga from Auger electron spectroscopy (AES) measurements. These Ga islands in surface region are considered to be formed in accordance with GaN dissociation caused by the high-temperature annealing. In particular, in the case of the conventional Si
Fig. 1. AFM images of Si- and N/Si-implanted GaN samples with a Si dosage of 1 1015 cm 2 after annealing at 1300 C. Both images are 5 5 mm2.
implantation, the Ga islands are expected to be more easily formed owing to there being insufficient N atoms in the implanted region to achieve stoichiometric GaN compared to the N/Si co-implantation process. Therefore, the N/Si co-implantation turns out to suppress the Ga-island formation significantly in view of GaN stoichiometry. However, implantation-induced microdefects can be seen even after the high-temperature annealing process for the both Si- and N/Si-implanted GaN samples. Fig. 2 shows room-temperature sheet carrier concentration (ns) and electron mobility (me) as a function of annealing temperature for the N/Si and N/Ge coimplanted GaN samples. Here, the Si and Ge dosages were fixed at 1 1015 cm 2, respectively. With the rising of annealing temperature from 1100 C to 1200 C, the N/Si-implanted samples show a significant increase in ns from 2 1012 to 3 1014 cm 2 with high me of B75 cm2/ Vs. This indicates that the Si electrical activation starts to occur at around 1200 C. On the other hand, the Ge activation is found to occur after annealing at 1300 C. This difference of the activation temperature between the N/Si- and N/Ge-implanted samples is probably associated with the implantation-induced damage. That is, the activation temperature for the N/Si implantation is much lower than that for the N/Ge implantation since Si is much lighter than Ge and consequently might introduce much less damage in GaN. Here, the effective activation efficiency (Zeff) is defined as a ratio of the ns to the dosage, assuming that all of the carriers are generated from the Si or Ge donors. The Zeff are estimated to be B50% for the both N/Si- and N/Geimplanted GaN after annealing at 1300 C. However, the me for the N/Si implantation is much higher than that for the N/Ge implantation. This difference of the me also seems to be related to an improvement of crystallinity in connection with the recovery of the damage in the implanted region, as mentioned above.
Fig. 2. Sheet carrier concentration (,J) and mobility (’,&) as a function of annealing temperature for N/Si and N/Ge coimplanted GaN samples with Si and Ge dosages of 1 1015 cm 2.
ARTICLE IN PRESS Y. Nakano et al. / Materials Science in Semiconductor Processing 6 (2003) 515–517
517
crystallinity of the electrically n-type activated region compared to the N/Ge co-implantation. However, the behavior of the me for the both samples does not seem to be simply consistent with the variation seen for the ns in view of ionized impurity scattering. The me may be primarily dependent on the combined effects of ionized impurity scattering and space charge scattering induced by the presence of the implantation-introduced defects.
4. Conclusions Fig. 3. Sheet carrier concentration (,J) and mobility (’,&) as a function of Si or Ge implant dosage for N/Si and N/Ge coimplanted GaN samples after annealing at 1300 C.
Fig. 3 shows the room-temperature ns and me as a function of Si or Ge dosage for the N/Si- and N/Geimplanted GaN samples after annealing at 1300 C, together with an ideal Zeff line of 100%. In the case of the N/Si co-implantation, the ns increases monotonically with increasing Si implant dosage up to 1 1015 cm 2 and the ns and me seem to be saturated at around 1 1015 cm 2 and 71 cm2/Vs, respectively, with increasing the Si dosage above 3 1015 cm 2. When the Si dosage becomes above 1 1016 cm 2, the implanted GaN is found to get partially separated during the annealing procedure. These results suggest that the Sidoping characteristics by the implantation techniques may attain to the solid-solubility limit of the implanted Si atoms into GaN. On the other hand, the ns increases monotonically with increasing Ge implant dosage up to 3 1015 cm 2 under the N/Ge co-implantation process. However, the implanted GaN with a Ge dosage of 1 1016 cm 2 peels off during the annealing. Thus, the N/Ge co-implanted GaN samples are easily subject to the implantation-induced damage. In addition, much improved me are obtained for the N/Si-implanted samples compared to the N/Ge-implanted samples regardless of the implant dosage. This indicates that the N/Si co-implantation might significantly improve the
We have demonstrated that the N/Si co-implantation into GaN and subsequent annealing at high temperatures can enhance the actual n-type activation compared to the N/Ge co-implantation. However, numerous implantation-induced micro-defects can be seen to remain even after the high-temperature annealing process for the both N/Si- and N/Ge-implanted GaN.
References [1] Zolper JC, Shul RJ, Baca AG, Wilson RG, Pearton SJ, Stall RA. Ion-implanted GaN junction field effect transistor. Appl Phys Lett 1996;68(16):2273. [2] Zhang AP, Johnson JW, Ren F, Han J, Polyakov AY, Smirnov NB, Govorkov AV, Redwing JM, Lee KP, Pearton SJ. Lateral AlxGa1–xN power rectifiers with 9.7 kV reverse breakdown voltage. Appl Phys Lett 2001; 78(6):823. [3] Zhang AP, Dang G, Ren F, Han J, Polyakov AY, Smirnov NB, Govorkov AV, Redwing JM, Cho H, Pearton SJ. Temperature dependence and current transport mechanisms in AlxGa1–xN Schottky rectifiers. Appl Phys Lett 2000;76(25):3816. [4] Pearton SJ, Vartuli CB, Zolper JC, Yuan C, Stall RA. Ion implantation doping and isolation of GaN. Appl Phys Lett. 1995;67(10):1435. [5] Zolper JC, Wilson RG, Pearton SJ, Stall RA. Ca and O ion implantation doping of GaN. Appl Phys Lett 1996;68(14): 1945. [6] Kobayashi H, Gibson WH. Lattice location of Ca in GaN using ion channeling. Appl Phys Lett 1999;74(16):2355.