Au for the Fabrication of
Copyright © 2013 American Scientific Publishers All rights reserved Printed in the United States of America
Journal of Nanoscience and Nanotechnology Vol. 13, 5715–5718, 2013
Seung Cheol Han1 † , Jae-Kwan Kim1 , Jun Young Kim1 , Dong Min Lee1 , Jae-Sik Yoon2 ∗ , Jong-Kyu Kim3 , E. F. Schubert4 , and Ji-Myon Lee1 ∗ 1
Department of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Korea 2 Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Korea 3 Department of Materials Science and Technology, POSTECH, Pohang 790-784, Korea 4 Department for Electrical, Computer, and Systems Engineering, RPI, Troy, NY 12180, USA
The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01 cm2 to 905 × 10−3 cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature. However, the specific contact resistance could be decreased down to 103× 10−4 cm2 by means of surface treatment using an alcohol-based (NH4 2 S solution. The depth profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed that the alcohol-based (NH4 2 S treatment was effective in removing the surface oxide layer of GaN.
Keywords: N-Face p-GaN, LED, Surface Treatment, XPS. Delivered by Publishing Technology to: Rensselaer Polytechnic Institute IP: 128.113.123.68 On: Mon, 06 May 2013 06:26:45 Copyright American Scientific Publishers on the p-type GaN surface, resulting in a low-resistance 1. INTRODUCTION ohmic contact to p-type GaN.9 In the area of GaAs The group III nitrides including GaN are attractive matedevices, several studies have reported that the alcoholrials for optoelectronic devices because of the success in based (NH4 2 Sx solution removes the native oxide on the commercialization of blue light emitting diodes (LEDs).1–5 GaAs surface more efficiently than the normal (NH4 2 Sx Generally, nitrides exhibit high spontaneous or piezosolution due to the low dielectric constant of the alcoholelectric polarization fields that are oriented along the based (NH4 2 Sx solution, resulting in a larger improvement c direction. This large polarization filed cause spatial in device characteristics.10 11 separation of injected electrons and holes that results in Zhilyaev et al.12 have reported that the photoluminesreduced radiative recombination efficiency. To overcome cence intensity of n-type GaN is considerably enhanced these polarization effects in LEDs, devices grown on nonas a result of the surface treatment with an alcoholic sulpolar or semipolar templates have been developled. fide solution. Our previous results13 also showed that, for Furthermore, polarization inverted p-side down LEDs the case of n-type GaN, the surface treatment with an were also realized and showed only 10% efficiency droop alcohol-based (NH4 2 Sx solution removes the insulating up to 500 A/cm2 by using hydride vapor epitaxy (HVPE).6 layer on the n-type GaN surface more effectively than the However, fabrication of p-down LEDs were known to be normal, (NH4 2 Sx solution, leading to more enhanced elecdifficult due to vulnerability for dry-etch damage of p-type trical properties. In this paper, we investigated the effect of GaN. an alcohol-based (NH4 2 Sx solution [t-C4 H9 OH1 (NH4 2 S] On the other hand, to obtain a low-resistance ohmic on the characteristics of ohmic contacts to N -face p-type contact to p-type GaN, a variety of surface treatments of GaN. p-GaN using solutions such as aqua-regia solution,7 and (NH4 2 Sx ,8 9 have been proposed. It has been reported that the surface treatment with a (NH4 2 Sx solution is 2. EXPERIMENTAL DETAILS especially efficient in removing the native oxide layer The p-GaN samples were grown on a c-plane sapphire ∗ substrate by using a MOCVD system. After the growth of Authors to whom correspondence should be addressed. † Current address: Selcos Co. Suwon, 441-813, Korea. the GaN buffer layer and subsequent undoped GaN layer, J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 8
1533-4880/2013/13/5715/004
doi:10.1166/jnn.2013.7072
5715
RESEARCH ARTICLE
Ohmic Contacts to N-Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
Han et al.
2 um-thick p-GaN:Mg was grown. The GaN layers were annealing temperatures were increased to 300 C, the spe cific contact resistance was also decreased to the value rapid-thermal annealed at 650 C for 1 min in N2 ambient of 91 × 10−3 cm2 . However, compared to the value of to activate the Mg dopant. Hall measurements showed that 17 3 reported N -face n-type GaN sample and for the purpose the carrier concentration was about 1 × 10 cm . Using a of fabricating high efficiency LEDs, the specific contact KrF excimer laser (248 nm), a laser lift-off process was resistance should be decreased further. We have tried the then performed to separate the sapphire substrate from the surface treatment method in order to quench the specific GaN layer, followed by HCl:DI (de-ionized) water (= 1:2) contact resistance and results are shown in Figure 1(b). cleaning for 5 min to remove Ga droplets. The undoped By treating an alcohol-based (NH4 2 S solution to N -face GaN surface was then dry etched to a thickness of 1.7 um p-type GaN, the specific contact resistance was decreased for inductively coupled plasma (ICP). The samples were by four and by two orders of magnitude for untreated and first etched in HCl:H2 O (1:1) solution for 1 min and then 300 C annealed sample, achieving 103 × 10−4 cm2 for dipped into alcoholic solution for 5 min at room temthe alcohol-based (NH4 2 S treated samples. perature, which was composed of 90 vol% tert-butanol To study the mechanisms for the improvement of the (t-C4 H9 OH) and 10 vol% (NH4 2 S before metal deposiohmic characteristics, XPS spectra were obtained from tion. After the sulfur treatment, conventional photo resist the Ni/sulfur-treated N -face p-type GaN sample. Figure 2 mask was used for the fabrication of the circular transfer shows the depth-profile data of Ga, Ni, O, C, and N length method (CTLM) patterns. The size of the pads was peak from the Ni/GaN for untreated and alcohol-based 200 ×200 um2 and the spacings between the pads were 10, (NH4 2 S-treated samples, respectively. 20, 40, 60, 80, and 100 um. The Ni (20 nm)/Au (80 nm) As shown in Figure 2, the atomic percent of oxide for films were then deposited by electron-beam evaporation. untreated sample is about 7% at the interface of Ni and Contact resistivity data were measured at room temperaGaN. However, atomic percent of O peak at the interface ture using a parameter analyzer (HP4145B). To characterof the samples treated with alcohol-based (NH4 2 S soluize the chemical bonding at the interface between the Ni tion were significantly decreased compared to that of the layer and sulfur-treated p-type GaN, XPS was performed untreated sample, indicating that the treatment effectively using the Mg Ka line (1253.6 eV) as an excitation source remove the oxide layer at the interface. in a ultra-high-vacuum system. It was known that the suitable Delivered by Publishing Technology to: Rensselaer Polytechnic Institutesurface treatment can decrease the Schottky barrier height (SBH). IP: 128.113.123.68 On: Mon,effectively 06 May 2013 06:26:45 3. RESULTS AND DISCUSSION According to the metal semiconductor band theory14 15 Copyright American Scientific Publishers the effective SBHs can be influenced by the presence Figure 1(a) shows the specific contact resistance of the of native oxide at the Ni/p-GaN interface, which can be samples which had been deposited on N -face p-type GaN described as,15 as a function of annealing temperatures. Specific contact resistances for the samples were determined from a plot of qb = qb0 + 4kT /h2m 1/2
the measured resistance versus the spacings between the CTLM pads. The specific contact resistance was measured where b0 is the SBH without native oxide. The second to be 101 × 100 cm2 for the untreated samples due to term is associated with the presence of an oxide layer, where m is the mean tunneling effective mass of the the high Schottky barrier height of surface state. As the 101
101
(b)
Contact resistivity (ohm cm2)
(a) Contact resistivity (ohm cm2)
RESEARCH ARTICLE
Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
100
10–1
10–2
N-face p-GaN
10–3
10–4 0
50
100
150
200
Annealing Temperature (ºC)
250
300
100 10–1 10–2 10–3 10–4 10–5
As-deposited
300 ºC annealed
sulfur treated
Treatment
Fig. 1. Variation of contact resistivity for Ni (20 nm)/Au (80 nm) deposited on N -face p-type GaN as (a) a function of annealing temperatures and (b) surface treatment.
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Han et al.
Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
(a)
Photoemission intensity (arb.Units) 1112
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Ga 2p
(b)
1116
1120
1124
Binding energy (eV) Fig. 3. Ga2p core-level spectra of the Ni/N -face p-type GaN interface for (a) an untreated sample and (b) an alcohol-based (NH4 2 S-treated sample.
where EF is surface Fermi level position relative to theRensselaer valence band maximum Institute and EGa2p core-level binding Delivered by Publishing Technology to: Polytechnic Fig. 2. XPS depth profiles of (a) untreated and (b) an alcohol-based energy for Ga N bonds. IP: 128.113.123.68 On: Mon, 06 May 2013 06:26:45 (NH4 2 S-treated N -face p-type GaN. This implies that the alcohol-based (NH4 2 S-treated Copyright American Scientific Publishers samples causes the surface Fermi level to shift toward the carriers, the mean tunneling barrier for carrier injecvalence-band edge, resulting in a reduction in the band tion form a metal to a semiconductor, and the oxide bending in the p-GaN. A previous study17 reported that layer thickness. This indicates that the effective removal sulfur does not bond with nitrogen but bonds with gallium of native oxide due to the surface treatment (Fig. 2) could or occupies nitrogen-related vacancies. make a contribution to the observed reduction of the SBHs The S ions in the solution would then bond to the Ga due to the decreased . This result shows that the oxide atoms on the N -face GaN sub-surface, producing Ga sullayer was efficiently removed from the N -face p-GaN surfides and this sulfides are soluble in the sulfur solution.18 face by the surface treatment with alcohol-based (NH4 2 S Hence, the simultaneous process of reaction and dissolusolution. In other words, increased barrier height for hole tion can leave a very thin sulfide layer on the sulfur-treated injection from metal to p-type GaN by formation of oxide GaN surfaces, leading to the effective removal of surface layer could be reduced by treatment with the alcohol-based oxide, formation of Ga vacancies, and protection from the (NH4 2 S solution. Hence, the improved ohmic characterformation of the native oxide on exposure to air prior to istics for samples can be attributed to the removal of the metal deposition. interfacial oxide layer. Figure 4 shows the schematic energy band diagram Figure 3 shows the Ga2p peak in the Ni/GaN interface of Ni/p-GaN ohmic contact to N -face p-type GaN. As regions of the samples before and after the treatment. It is the binding energy of Ga2p peak of surface treated samshown that the Ga2p core level of the treated sample shifts ple shifted to valence-band, the valence-band maxima of toward the lower binding energy side by 0.3 eV, compared the GaN also shifted to the Fermi level, resulting in the to that of the untreated sample. decrease of band bending between metal and GaN interThe position of surface Fermi level can be determined face. According to this, the Schottky barrier height should from the energy position of the Ga N peaks in the corebe decreased as follows; level spectra. The difference between the Ga2p core-level binding energy and valence band maximum in bulk GaN T NT qp = EGa2p − EGa2p − EV is 1116.7 eV. Therefore, the following equation is used to 16 determine the position of surface Fermi level: T , where qp is SBH between Ni and p-GaN, and EGa2p NT EGa2P , and Ev is a binding energy of core-level spectrum EF = EGa2p − 11167 eV J. Nanosci. Nanotechnol. 13, 5715–5718, 2013
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The Ga2p core-level peak for the alcohol-based (NH4 2 Streated sample showed a red-shift by 0.3 eV toward the valence-band edge compared to that for the untreated sample. The drastic improvement in the ohmic characteristics of the alcohol-based (NH4 2 S-treated sample can be attributed to the effective removal of the surface oxide and the shift of the surface Fermi level toward the valenceband edge, resulting in the lowering of SBH. Acknowledgment: This work was supported by the Ministry of Knowledge Economy (MKE) through the project of Regional Innovation Center (RIC) and by the Korea Basic Science Institute (KBSI) grant (T32604).
References and Notes 1. S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, Jpn. J. Appl. Phys. 34, L797 (1995). 2. S. Nakamura, M. Senoh, S. Nagahama, Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, Jpn. J. Appl. Phys. 35, L217 (1996). 3. M. A. Khan, A. R. Bhattarai, J. N. Kuznia, and D. T. Olson, Appl. Phys. Lett. 63, 1214 (1993). 4. M. A. Khan, J. N. Kuznia, A. R. Bhattarai, and D. T. Olson, Appl. Phys. Lett. 62, 1786 (1993). Fig. 4. Schematic energy band diagram of Ni/p-GaN ohmic contact. 5. M. A. Khan, J. N. Kuznia, D. T. Olson, J. M. Van Hove, M. Blasingame, and L. F. Reitz, Appl. Phys. Lett. 60, 2917 (1993). 6. M. L. Reed, E. D. Readinger, H. Shen, M. Wraback, and A. Syrkin, for surface treated and non-treated sample, and valence Appl. Phys. Lett. 93, 133505 (2008). Delivered Publishing Technology Polytechnic Institute band maxima, respectively. Fromby these results, the reduced to: Rensselaer K. Kim, J. L.06:26:45 Lee, J. W. Lee, H. E. Shin, Y. J. Park, and T. Kim, IP: 128.113.123.68 On: Mon, 7.06J.May 2013 band-bending renders the decrease of effective SBH result- Scientific Appl. Publishers Phys. Lett. 73, 2953 (1998). Copyright American 8. J.-S. Jang, S.-J. Park, and T.-Y. Seong, J. Vac. Sci. Technol. B ing in the enhanced transfer of holes between metal and 17, 2667 (1999). valence band of GaN, indicating that the alcohol based 9. J.-L. Lee, J. K. Kim, J. W. Lee, Y. J. Park, and T. Kim, Electrochem. (NH4 2 S treatment effectively enhance the ohmic contact Solid-State Lett. 3, 53 (2000). properties of Ni/N -face p-type GaN. 10. V. N. Bessolov, M. V. Lebedev, and D. R. T. Zahn, J. Appl. Phys. 82, 2640 (1997). 11. V. N. Bessolov, A. F. Ivankov, E. V. Konenkova, and M. V. Lebedev, Tech. Phys. Lett. 21, 20 (1995). 4. CONCLUSION 12. Y. V. Zhilyaev, M. E. Kompan, E. V. Konenkova, and S. D. Raevskii, MRS Internet J. Nitride Semicond. Res. 4S1, G6.14 (1999). We report on results about electrical properties of Ni13. C. Huh, S.-W. Kim, H.-S. Kim, I.-H. Lee, and S.-J. Park, J. Appl. based ohmic contact on N -face p-type GaN. Contact resisPhys. 87, 4591 (2000). tance are decreased as the annealing temperature was 14. Y. E. H. Rhoderick and R. H. Williams, Metal-Semiconductor Conincreased. In order to lower a contact resistance further, tacts, Clarendon, Oxford, U.K. (1998). alcohol-based (NH4 2 S surface treatment on N -face p-type 15. K. Hattori and Y. Izumi, J. Appl. Phys. 53, 6906 (1982). 16. G. Landgren, R. Ludeke, Y. Jugnet, J. F. Morar, and F. J. Himpsel, GaN were conducted. Compared to the untreated sample, J. Vac. Sci. Technol. B 2, 351 (1984). the specific contact resistance was drastically decreased 17. Y.-J. Lin, C.-D. Tsai, Y.-T. Lyu, and C.-T. Lee, Appl. Phys. Lett. by four orders of magnitude. The oxygen depth-profiling 77, 687 (2000). data showed that the alcohol-based (NH4 2 S treatment is 18. R. C. Weast, M. J. Astle, and W. H. Beyer (eds.), Handbook of Chemistry and Physics, 64th edn., CRC, New York (1985). very effective in the removal of the surface oxide layer.
Received: 23 March 2012. Accepted: 4 May 2012.
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J. Nanosci. Nanotechnol. 13, 5715–5718, 2013
Journal of Nanoscience and Nanotechnology Vol. 13, 5715–5718, 2013
Seung Cheol Han1 † , Jae-Kwan Kim1 , Jun Young Kim1 , Dong Min Lee1 , Jae-Sik Yoon2 ∗ , Jong-Kyu Kim3 , E. F. Schubert4 , and Ji-Myon Lee1 ∗ 1
Department of Materials Science and Metallurgical Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Korea 2 Division of Materials Science, Korea Basic Science Institute, Daejeon 305-333, Korea 3 Department of Materials Science and Technology, POSTECH, Pohang 790-784, Korea 4 Department for Electrical, Computer, and Systems Engineering, RPI, Troy, NY 12180, USA
The electrical properties of Ni-based ohmic contacts to N-face p-type GaN were investigated. The specific contact resistance of N-face p-GaN exhibits a linear decrease from 1.01 cm2 to 905 × 10−3 cm2 for the as-deposited and the annealed Ni/Au contacts, respectively, with increasing annealing temperature. However, the specific contact resistance could be decreased down to 103× 10−4 cm2 by means of surface treatment using an alcohol-based (NH4 2 S solution. The depth profile data measured from the intensity of O1s peak in the X-ray photoemission spectra showed that the alcohol-based (NH4 2 S treatment was effective in removing the surface oxide layer of GaN.
Keywords: N-Face p-GaN, LED, Surface Treatment, XPS. Delivered by Publishing Technology to: Rensselaer Polytechnic Institute IP: 128.113.123.68 On: Mon, 06 May 2013 06:26:45 Copyright American Scientific Publishers on the p-type GaN surface, resulting in a low-resistance 1. INTRODUCTION ohmic contact to p-type GaN.9 In the area of GaAs The group III nitrides including GaN are attractive matedevices, several studies have reported that the alcoholrials for optoelectronic devices because of the success in based (NH4 2 Sx solution removes the native oxide on the commercialization of blue light emitting diodes (LEDs).1–5 GaAs surface more efficiently than the normal (NH4 2 Sx Generally, nitrides exhibit high spontaneous or piezosolution due to the low dielectric constant of the alcoholelectric polarization fields that are oriented along the based (NH4 2 Sx solution, resulting in a larger improvement c direction. This large polarization filed cause spatial in device characteristics.10 11 separation of injected electrons and holes that results in Zhilyaev et al.12 have reported that the photoluminesreduced radiative recombination efficiency. To overcome cence intensity of n-type GaN is considerably enhanced these polarization effects in LEDs, devices grown on nonas a result of the surface treatment with an alcoholic sulpolar or semipolar templates have been developled. fide solution. Our previous results13 also showed that, for Furthermore, polarization inverted p-side down LEDs the case of n-type GaN, the surface treatment with an were also realized and showed only 10% efficiency droop alcohol-based (NH4 2 Sx solution removes the insulating up to 500 A/cm2 by using hydride vapor epitaxy (HVPE).6 layer on the n-type GaN surface more effectively than the However, fabrication of p-down LEDs were known to be normal, (NH4 2 Sx solution, leading to more enhanced elecdifficult due to vulnerability for dry-etch damage of p-type trical properties. In this paper, we investigated the effect of GaN. an alcohol-based (NH4 2 Sx solution [t-C4 H9 OH1 (NH4 2 S] On the other hand, to obtain a low-resistance ohmic on the characteristics of ohmic contacts to N -face p-type contact to p-type GaN, a variety of surface treatments of GaN. p-GaN using solutions such as aqua-regia solution,7 and (NH4 2 Sx ,8 9 have been proposed. It has been reported that the surface treatment with a (NH4 2 Sx solution is 2. EXPERIMENTAL DETAILS especially efficient in removing the native oxide layer The p-GaN samples were grown on a c-plane sapphire ∗ substrate by using a MOCVD system. After the growth of Authors to whom correspondence should be addressed. † Current address: Selcos Co. Suwon, 441-813, Korea. the GaN buffer layer and subsequent undoped GaN layer, J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 8
1533-4880/2013/13/5715/004
doi:10.1166/jnn.2013.7072
5715
RESEARCH ARTICLE
Ohmic Contacts to N-Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
Han et al.
2 um-thick p-GaN:Mg was grown. The GaN layers were annealing temperatures were increased to 300 C, the spe cific contact resistance was also decreased to the value rapid-thermal annealed at 650 C for 1 min in N2 ambient of 91 × 10−3 cm2 . However, compared to the value of to activate the Mg dopant. Hall measurements showed that 17 3 reported N -face n-type GaN sample and for the purpose the carrier concentration was about 1 × 10 cm . Using a of fabricating high efficiency LEDs, the specific contact KrF excimer laser (248 nm), a laser lift-off process was resistance should be decreased further. We have tried the then performed to separate the sapphire substrate from the surface treatment method in order to quench the specific GaN layer, followed by HCl:DI (de-ionized) water (= 1:2) contact resistance and results are shown in Figure 1(b). cleaning for 5 min to remove Ga droplets. The undoped By treating an alcohol-based (NH4 2 S solution to N -face GaN surface was then dry etched to a thickness of 1.7 um p-type GaN, the specific contact resistance was decreased for inductively coupled plasma (ICP). The samples were by four and by two orders of magnitude for untreated and first etched in HCl:H2 O (1:1) solution for 1 min and then 300 C annealed sample, achieving 103 × 10−4 cm2 for dipped into alcoholic solution for 5 min at room temthe alcohol-based (NH4 2 S treated samples. perature, which was composed of 90 vol% tert-butanol To study the mechanisms for the improvement of the (t-C4 H9 OH) and 10 vol% (NH4 2 S before metal deposiohmic characteristics, XPS spectra were obtained from tion. After the sulfur treatment, conventional photo resist the Ni/sulfur-treated N -face p-type GaN sample. Figure 2 mask was used for the fabrication of the circular transfer shows the depth-profile data of Ga, Ni, O, C, and N length method (CTLM) patterns. The size of the pads was peak from the Ni/GaN for untreated and alcohol-based 200 ×200 um2 and the spacings between the pads were 10, (NH4 2 S-treated samples, respectively. 20, 40, 60, 80, and 100 um. The Ni (20 nm)/Au (80 nm) As shown in Figure 2, the atomic percent of oxide for films were then deposited by electron-beam evaporation. untreated sample is about 7% at the interface of Ni and Contact resistivity data were measured at room temperaGaN. However, atomic percent of O peak at the interface ture using a parameter analyzer (HP4145B). To characterof the samples treated with alcohol-based (NH4 2 S soluize the chemical bonding at the interface between the Ni tion were significantly decreased compared to that of the layer and sulfur-treated p-type GaN, XPS was performed untreated sample, indicating that the treatment effectively using the Mg Ka line (1253.6 eV) as an excitation source remove the oxide layer at the interface. in a ultra-high-vacuum system. It was known that the suitable Delivered by Publishing Technology to: Rensselaer Polytechnic Institutesurface treatment can decrease the Schottky barrier height (SBH). IP: 128.113.123.68 On: Mon,effectively 06 May 2013 06:26:45 3. RESULTS AND DISCUSSION According to the metal semiconductor band theory14 15 Copyright American Scientific Publishers the effective SBHs can be influenced by the presence Figure 1(a) shows the specific contact resistance of the of native oxide at the Ni/p-GaN interface, which can be samples which had been deposited on N -face p-type GaN described as,15 as a function of annealing temperatures. Specific contact resistances for the samples were determined from a plot of qb = qb0 + 4kT /h2m 1/2
the measured resistance versus the spacings between the CTLM pads. The specific contact resistance was measured where b0 is the SBH without native oxide. The second to be 101 × 100 cm2 for the untreated samples due to term is associated with the presence of an oxide layer, where m is the mean tunneling effective mass of the the high Schottky barrier height of surface state. As the 101
101
(b)
Contact resistivity (ohm cm2)
(a) Contact resistivity (ohm cm2)
RESEARCH ARTICLE
Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
100
10–1
10–2
N-face p-GaN
10–3
10–4 0
50
100
150
200
Annealing Temperature (ºC)
250
300
100 10–1 10–2 10–3 10–4 10–5
As-deposited
300 ºC annealed
sulfur treated
Treatment
Fig. 1. Variation of contact resistivity for Ni (20 nm)/Au (80 nm) deposited on N -face p-type GaN as (a) a function of annealing temperatures and (b) surface treatment.
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Ohmic Contacts to N -Face p-GaN Using Ni/Au for the Fabrication of Polarization Inverted Light-Emitting Diodes
(a)
Photoemission intensity (arb.Units) 1112
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Ga 2p
(b)
1116
1120
1124
Binding energy (eV) Fig. 3. Ga2p core-level spectra of the Ni/N -face p-type GaN interface for (a) an untreated sample and (b) an alcohol-based (NH4 2 S-treated sample.
where EF is surface Fermi level position relative to theRensselaer valence band maximum Institute and EGa2p core-level binding Delivered by Publishing Technology to: Polytechnic Fig. 2. XPS depth profiles of (a) untreated and (b) an alcohol-based energy for Ga N bonds. IP: 128.113.123.68 On: Mon, 06 May 2013 06:26:45 (NH4 2 S-treated N -face p-type GaN. This implies that the alcohol-based (NH4 2 S-treated Copyright American Scientific Publishers samples causes the surface Fermi level to shift toward the carriers, the mean tunneling barrier for carrier injecvalence-band edge, resulting in a reduction in the band tion form a metal to a semiconductor, and the oxide bending in the p-GaN. A previous study17 reported that layer thickness. This indicates that the effective removal sulfur does not bond with nitrogen but bonds with gallium of native oxide due to the surface treatment (Fig. 2) could or occupies nitrogen-related vacancies. make a contribution to the observed reduction of the SBHs The S ions in the solution would then bond to the Ga due to the decreased . This result shows that the oxide atoms on the N -face GaN sub-surface, producing Ga sullayer was efficiently removed from the N -face p-GaN surfides and this sulfides are soluble in the sulfur solution.18 face by the surface treatment with alcohol-based (NH4 2 S Hence, the simultaneous process of reaction and dissolusolution. In other words, increased barrier height for hole tion can leave a very thin sulfide layer on the sulfur-treated injection from metal to p-type GaN by formation of oxide GaN surfaces, leading to the effective removal of surface layer could be reduced by treatment with the alcohol-based oxide, formation of Ga vacancies, and protection from the (NH4 2 S solution. Hence, the improved ohmic characterformation of the native oxide on exposure to air prior to istics for samples can be attributed to the removal of the metal deposition. interfacial oxide layer. Figure 4 shows the schematic energy band diagram Figure 3 shows the Ga2p peak in the Ni/GaN interface of Ni/p-GaN ohmic contact to N -face p-type GaN. As regions of the samples before and after the treatment. It is the binding energy of Ga2p peak of surface treated samshown that the Ga2p core level of the treated sample shifts ple shifted to valence-band, the valence-band maxima of toward the lower binding energy side by 0.3 eV, compared the GaN also shifted to the Fermi level, resulting in the to that of the untreated sample. decrease of band bending between metal and GaN interThe position of surface Fermi level can be determined face. According to this, the Schottky barrier height should from the energy position of the Ga N peaks in the corebe decreased as follows; level spectra. The difference between the Ga2p core-level binding energy and valence band maximum in bulk GaN T NT qp = EGa2p − EGa2p − EV is 1116.7 eV. Therefore, the following equation is used to 16 determine the position of surface Fermi level: T , where qp is SBH between Ni and p-GaN, and EGa2p NT EGa2P , and Ev is a binding energy of core-level spectrum EF = EGa2p − 11167 eV J. Nanosci. Nanotechnol. 13, 5715–5718, 2013
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Han et al.
RESEARCH ARTICLE
The Ga2p core-level peak for the alcohol-based (NH4 2 Streated sample showed a red-shift by 0.3 eV toward the valence-band edge compared to that for the untreated sample. The drastic improvement in the ohmic characteristics of the alcohol-based (NH4 2 S-treated sample can be attributed to the effective removal of the surface oxide and the shift of the surface Fermi level toward the valenceband edge, resulting in the lowering of SBH. Acknowledgment: This work was supported by the Ministry of Knowledge Economy (MKE) through the project of Regional Innovation Center (RIC) and by the Korea Basic Science Institute (KBSI) grant (T32604).
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Received: 23 March 2012. Accepted: 4 May 2012.
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