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Effect of thermal annealing on ohmic contacts properties of undoped and Si-doped AlxGa1−xN on Si (1 1 1) substrate grown by PA-MBE
Optik 124 (2013) 4257–4259
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Effect of thermal annealing on ohmic contacts properties of undoped and Si-doped Alx Ga1−x N on Si (1 1 1) substrate grown by PA-MBE A. S.H. Hussein a,d,∗ , Alaa J. Ghazai b,d , Z. Hassan c,d a
Department of Energy Engineering, University of Baghdad, Iraq Physics Department, Science College, Thi-Qar University, Iraq Nano-Optoelectronics Research and Technology Laboratory, Malaysia d School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia b
c
a r t i c l e
i n f o
Article history: Received 7 August 2012 Accepted 28 December 2012
Keywords: PA-MBE Ohmic contact AlGaN TLM
a b s t r a c t In this work, we study the ohmic contact properties of titanium (Ti)/aluminum (Al) bi-layer contacts on undoped and n-type doped Alx Ga1−x N grown on silicon (1 1 1) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures of 400, 500, 600 and 700 ◦ C were investigated. Specific contact resistivity was determined using transmission line method (TLM) and current–voltage (I–V) measurements. The results reveal that the bi-layer scheme was sensitive to the change of annealing temperatures and annealing time. The optimal value of specific contact resistivities was obtained at annealing temperature of 600 ◦ C for both samples. However, the values of n-type doped sample exhibited better results compared with the undoped sample. © 2013 Published by Elsevier GmbH.
1. Introduction The III–V nitrides have long been viewed as a promising material system for optical devices applications between the visible and UV wavelength spectra; and also for various high temperature, high power and high frequency electronic devices, due to their direct, wide bandgap [1]. It is difficult to form an ohmic contact to a wide energy band gap of Alx Ga1−x N thin films; this is because a metal does not exist with a low enough work function to yield a low barrier to current transport. This excludes the possibility of thermionic emission to be a mechanism for carrier transport through contact [2,3]. In addition, the barrier formed between the contact and the semiconductor strongly depends on the type of the contacts metal. On the other hand, multi-layer metallization has been interesting to researchers because of its capability of producing phase formation, which promotes good ohmic contacts. Thus, the contact resistance for the Ti/Al bi-layer metallization was lowered by the factor of 1.6 using a DC magnetron sputtered Ti/Al (35/115 nm) bi-layer metallization [4]. A wide variety metallization for ohmic contact on AlGaN also investigated. From literature, the specific contact resistances between range 1.0 × 10−5 cm2
∗ Corresponding author at: School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia. Tel.: +60 1 639 56435. E-mail addresses: [email protected] (A.S.H. Hussein), [email protected] (A.J. Ghazai). 0030-4026/$ – see front matter © 2013 Published by Elsevier GmbH. http://dx.doi.org/10.1016/j.ijleo.2012.12.070
to 1.0 × 10−8 cm2 have been reported by [5,6], which are good enough for the optical and electronic devices. In this work, we report our initial investigation of the Ti/Al bilayer contacts on the undoped and n-type doped Alx Ga1−x N grown on silicon (1 1 1) substrates by radio frequency nitrogen plasmaassisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures (400–700 ◦ C) investigated.
2. Experiment The undoped and n-type doped Alx Ga1−x N/GaN/AlN heterostructures on Si (1 1 1) substrates were grown using Veeco model Gen II MBE system. The growth procedure details are given elsewhere [7]. The over layer on the Alx Ga1−x N consists of transparent organic and inorganic contamination as well as the native oxide layer. The native oxide parts were removed with NH4 OH:H2 O = 1:20 solution, followed by dipping the wafer in a HF:H2 O = 1:50 solution. Boiling aqua regia (HCl:HNO3 = 3:1) was selected to chemically etch the wafer because it is more effective in removing and cleaning the samples as this ex situ cleaning treatment is suggested to reduce the amount of oxygen and/or carbon contamination on the surface of the semiconductor [8]. The wafer is then rinsed with distilled water and blown dry with compressed Nitrogen and is ready for the next step. Firstly, titanium (Ti) with 50 nm thickness sputtered onto the Alx Ga1−x N samples through a metal mask, followed by the evaporation of 150 nm capping layer
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A.S.H. Hussein et al. / Optik 124 (2013) 4257–4259 Table 1 The specific contact resistivities of the undoped Alx Ga1−x N with different annealing temperatures and times. Annealing temperature (◦ C)
Specific contact resistivities (-cm2 ) Time/(cumulated time)
400 500 600 700 Fig. 1. (a) Metal mask used to fabricate the linear TLM pads and (b) samples after metallization with fabricated linear TLM pads.
of Al. The transmission line model (TLM), refined by Berger [9] and after by Li et al. [10], is currently used to characterize electrically the ohmic contact metallization systems. The TLM design used shown in Fig. 1. The TLM pads were designed to be 2 mm (W, width) × 1 mm (d, length) in size and the spacings, l between the pads were 0.3, 0.4, 0.6, 0.9 and 1.3 mm. The specific contact resistivities, c were determined from the plot of the measured resistances against the spacing between the TLM pads. The linear-square method used to fit a straight line in the experimental data. Thermal annealing carried out in a conventional tube furnace by heating the wafers. The gas purged at a mass flow rate of approximately 4 L min−1 . The samples annealed under flowing nitrogen environment at 400, 500, 600 and 700 ◦ C for 5 min in a conventional tube furnace. Similar heat treatments carried out for additional annealing times of 10 and 20 min to investigate the thermal stability of the interface contacts. The current–voltage (I–V) measurements were performed by a Kiethley high voltage source measure unit model 237. 3. Results and discussion The measurements of the specific contact resistivity c were made using the TLM method that has been widely used in the characterization of ohmic contacts to semiconductors. Resistance, Ri , between two contacts with spacing li , is given by Ri =
Rsh li 2Rsk Lt + Wc Wc
(1)
Ri =
Rsh li + 2Rc Wc
(2)
where li is the spacing between two pads, Wc is the width of the contact pad, Rc is the resistance due to the contact, Rsh is sheet resistance of the semiconductor layer outside the contact region, Rsk is the sheet resistance of the layer directly under the contact, and Lt is the transfer length. The plot shows Ri as a function of li which produces a straight line with the slope Rsh /Wc , and 2Rc is yielded from the intercept at y-axis. The intercept at x-axis gives Lx : Lx =
2Rsk Lt Rsh
5 min
10 min/(15 min)
20 min/(35 min)
Nearly ohmic 2.460 0.901 1.178
0.534 0.397 0.230 0.351
1.290 – – –
Table 2 The specific contact resistivities of the n-type doped Alx Ga1−x N with different annealing temperatures and times. Annealing temperature (◦ C)
Specific contact resistivities ( cm2 ) Time/(cumulated time)
400 500 600 700
5 min
10 min/(15 min)
20 min/(35 min)
Schottky 2.201 0.723 1.024
0.521 0.313 0.190 0.277
1.021 – – –
and the native oxides on AlGaN. The surface oxide is known to increase effective Schottky barrier height and to deteriorate the metal/AlGaN interface, leading to the degradation of ohmic contacts [12]. Table 1 shows the specific contact resistivities for the undoped sample with different times and annealing temperatures. Sample annealed at 400 ◦ C for 5 min, still exhibits nearly ohmic behavior. While the other samples exhibited ohmic behavior at higher annealing temperatures (500–600 ◦ C). This indicates that higher annealing temperatures improve the contact resistances. The improvement and reduction in the contact resistance correspond to formation of the intermetallic alloy (TiN) with low work function on AlGaN surface [13]. In addition, when annealing Al with work function of 4.28 eV, it will be diffused through Ti and will reach the AlGaN surface. The Al then reacts with the surface of AlGaN to form a thin layer of AlN at the interface. This again results in N vacancies yielding a heavily doped interface, resulting in tunneling current which is responsible for the ohmic contact formation [14]. 10-min (15 min cumulated time) annealed sample showed a significant improvement in the characteristics of all the samples as shown in Fig. 2. The specific contact resistivity increases with the increase of annealing temperature at 700 ◦ C. This indicates the formation of the thin oxide layer on the contact surface. Table 2 shows the contact resistivities of the n-type doped sample with different times and annealing temperatures. The results for the Si doped sample were better than the undoped sample which indicates that the Si doping improves the electrical properties of the sample [15]. Furthermore, the diffusion of the N atoms
(3)
where Lx ≈ 2Lt with the assumption that Rsh = Rsk . On the other hand, the assumption of an electrically long contact d > > Lt enabled the relationship c = Rsh Lt2 to be invoked which leads to c = Rc WLt [11]. The specific contact resistivities measured for undoped and ntype doped samples summarized in Tables 1 and 2. After depositing Ti/Al bi-layer metal on the surface of the samples, the initial study revealed that all as-deposited samples have shown non-linear behavior. This non-linear behavior could be due to several factors, which include the growth quality of the AlGaN layer, the surface preparation prior to contact deposition
Fig. 2. Typical I–V characteristics for Ti/Al contacts on undoped AlGaN.
A.S.H. Hussein et al. / Optik 124 (2013) 4257–4259
4259
Acknowledgments Financial support from Research University (RU) grant, Postgraduate Research Grant Scheme (PRGS) and Universiti Sains Malaysia are gratefully acknowledged. References
Fig. 3. Typical I–V characteristics for Ti/Al contacts on n-type doped AlGaN.
leaves the N-vacancies in n-AlGaN layer near the metal/n-AlGaN interface. Because N-vacancy is determined to serve as a shallow donor in AlGaN, the highly n-doped region in n-AlGaN is formed near the metal/n-AlGaN interface, and thus the specific contact resistivity decreases. Fig. 3 shows the I–V characteristics of n-type doped AlGaN sample annealed at 600 ◦ C for 10 min. Therefore, with increasing the annealing temperature to 600 ◦ C, the N-vacancy concentration in n-AlGaN increases, and thus the specific contact resistivity further decreases [16]. However, the degradation and a light brown layer were visually observed after annealing the samples at 700 ◦ C. This indicates the formation of the thin oxide layer on the contact surface [11]. 4. Conclusions The characteristics of Ti/Al bi-layer ohmic contacts for the undoped and Si-doped Alx Ga1−x N samples grown on silicon (1 1 1) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) have been studied. The specific contact resistivities of this bi-layer scheme were sensitive to the change of annealing temperatures and annealing time. The best results of the specific contact resistivities have been obtained with annealing temperature of 600 ◦ C at 10 min for both samples. Moreover, the results for the Si doping sample were better than the undoped sample which indicates that the Si doped improves the electrical properties of the sample.
[1] S. Jain, M. Willander, J. Narayan, R. Overstraeten, III–nitrides: growth, characterization, and properties, J. Appl. Phys. 87 (2000) 965–1006. [2] S. Ruvimov, Z. Liliental-Weber, J. Washburn, K. Duxstad, E. Haller, Z. Fan, S. Mohammad, W. Kim, A. Botchkarev, H. Morkoc, Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN, Appl. Phys. Lett. 69 (1996) 1556–1558. [3] A. Soltani, A. BenMoussa, S. Touati, V. Hoël, J. Jaeger, J. Laureyns, Y. Cordier, C. Marhic, M. Djouadi, C. Dua, Development and analysis of low resistance ohmic contact to n-AlGaN/GaN HEMT, Diam. Rel. Mater. 16 (2007) 262–266. [4] B. Luther, S. Monhney, T. Jackson, M. Khan, Q. Chen, J. Yang, Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN, Appl. Phys. Lett. 70 (1997) 57–59. [5] L. Chuah, Z. Hassan, N. Shamsuddin, N. Amirruddin, C. Chin, H. Hassan, Ohmic metal contact to InGaN, Optoelectron. Adv. Mater. Rapid Commun. (OAM-RC) 2 (2008) 650–654. [6] W. Lau, J. Tan, B. Singh, Formation of Ohmic contacts in AlGaN/GaN HEMT structures at 500 ◦ C by Ohmic contact recess etching, Microelectron. Reliab. 49 (2009) 558–561. [7] A. Hussein, Z. Hassan, S. Thahab, H. Hassan, M. Abid, C. Chin, Optical and electrical properties of undoped and Si-doped Alx Ga1−x N thin films on Si (1 1 1), Phys. B Condens. Matter 406 (2011) 1267–1271. [8] D. Kim, J. Bae, W. Kim, H. Baik, J. Myoung, S. Lee, The improvement of electrical properties of Pd-based contact to p-GaN by surface treatment, J. Electron. Mater. 30 (2001) 183–187. [9] H. Berger, Models for contacts to planar devices, Solid State Electron. 15 (1972) 145–158. [10] Y. Li, H. Harrison, G. Reeves, Correcting separation errors related to contact resistance measurement, Microelectron. J. 29 (1998) 21–30. [11] S. Pearton, J. Zolper, R. Shul, F. Ren, GaN: processing, defects, and devicesm, J. Appl. Phys. 86 (1999) 1–78. [12] H. Morkoc, Springer-Verlag, Berlin, 1999, p. 99. [13] L. Chuah, Z. Hassan, H.A. Hassan, Ohmic contacts properties of Ni/Ag metallization scheme on p-type GaN, J. Non-Cryst. Solids 365 (2010) 181. [14] Q. Feng, L. Li, Y. Hao, J. Ni, J. Zhang, The improvement of ohmic contact of Ti/Al/Ni/Au to AlGaN/GaN HEMT by multi-step annealing method, Solid State Electron. 53 (2009) 955–958. [15] L. Tan, S. Prakash, K. Ng, A. Ramam, S. Chua, A. Wee, S. Lim, Formation of Ti/Al ohmic contacts on Si-doped GaN epilayers by low temperature annealing, Semicond. Sci. Technol. 15 (2000) 585–588. [16] B. Shen, H. Zhou, J. Liu, Y. Zhou, R. Zhang, Y. Shi, Y. Zheng, T. Someya, Y. Arakawa, Ohmic contact and interfacial reaction of Ti/Al/Pt/Au metallic multi-layers on n-Alx Ga1−x N/GaN heterostructures, Opt. Mater. 23 (2003) 197–201.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Effect of thermal annealing on ohmic contacts properties of undoped and Si-doped Alx Ga1−x N on Si (1 1 1) substrate grown by PA-MBE A. S.H. Hussein a,d,∗ , Alaa J. Ghazai b,d , Z. Hassan c,d a
Department of Energy Engineering, University of Baghdad, Iraq Physics Department, Science College, Thi-Qar University, Iraq Nano-Optoelectronics Research and Technology Laboratory, Malaysia d School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia b
c
a r t i c l e
i n f o
Article history: Received 7 August 2012 Accepted 28 December 2012
Keywords: PA-MBE Ohmic contact AlGaN TLM
a b s t r a c t In this work, we study the ohmic contact properties of titanium (Ti)/aluminum (Al) bi-layer contacts on undoped and n-type doped Alx Ga1−x N grown on silicon (1 1 1) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures of 400, 500, 600 and 700 ◦ C were investigated. Specific contact resistivity was determined using transmission line method (TLM) and current–voltage (I–V) measurements. The results reveal that the bi-layer scheme was sensitive to the change of annealing temperatures and annealing time. The optimal value of specific contact resistivities was obtained at annealing temperature of 600 ◦ C for both samples. However, the values of n-type doped sample exhibited better results compared with the undoped sample. © 2013 Published by Elsevier GmbH.
1. Introduction The III–V nitrides have long been viewed as a promising material system for optical devices applications between the visible and UV wavelength spectra; and also for various high temperature, high power and high frequency electronic devices, due to their direct, wide bandgap [1]. It is difficult to form an ohmic contact to a wide energy band gap of Alx Ga1−x N thin films; this is because a metal does not exist with a low enough work function to yield a low barrier to current transport. This excludes the possibility of thermionic emission to be a mechanism for carrier transport through contact [2,3]. In addition, the barrier formed between the contact and the semiconductor strongly depends on the type of the contacts metal. On the other hand, multi-layer metallization has been interesting to researchers because of its capability of producing phase formation, which promotes good ohmic contacts. Thus, the contact resistance for the Ti/Al bi-layer metallization was lowered by the factor of 1.6 using a DC magnetron sputtered Ti/Al (35/115 nm) bi-layer metallization [4]. A wide variety metallization for ohmic contact on AlGaN also investigated. From literature, the specific contact resistances between range 1.0 × 10−5 cm2
∗ Corresponding author at: School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia. Tel.: +60 1 639 56435. E-mail addresses: [email protected] (A.S.H. Hussein), [email protected] (A.J. Ghazai). 0030-4026/$ – see front matter © 2013 Published by Elsevier GmbH. http://dx.doi.org/10.1016/j.ijleo.2012.12.070
to 1.0 × 10−8 cm2 have been reported by [5,6], which are good enough for the optical and electronic devices. In this work, we report our initial investigation of the Ti/Al bilayer contacts on the undoped and n-type doped Alx Ga1−x N grown on silicon (1 1 1) substrates by radio frequency nitrogen plasmaassisted molecular beam epitaxy (PA-MBE). The electrical stability of the contacts at various annealing temperatures (400–700 ◦ C) investigated.
2. Experiment The undoped and n-type doped Alx Ga1−x N/GaN/AlN heterostructures on Si (1 1 1) substrates were grown using Veeco model Gen II MBE system. The growth procedure details are given elsewhere [7]. The over layer on the Alx Ga1−x N consists of transparent organic and inorganic contamination as well as the native oxide layer. The native oxide parts were removed with NH4 OH:H2 O = 1:20 solution, followed by dipping the wafer in a HF:H2 O = 1:50 solution. Boiling aqua regia (HCl:HNO3 = 3:1) was selected to chemically etch the wafer because it is more effective in removing and cleaning the samples as this ex situ cleaning treatment is suggested to reduce the amount of oxygen and/or carbon contamination on the surface of the semiconductor [8]. The wafer is then rinsed with distilled water and blown dry with compressed Nitrogen and is ready for the next step. Firstly, titanium (Ti) with 50 nm thickness sputtered onto the Alx Ga1−x N samples through a metal mask, followed by the evaporation of 150 nm capping layer
4258
A.S.H. Hussein et al. / Optik 124 (2013) 4257–4259 Table 1 The specific contact resistivities of the undoped Alx Ga1−x N with different annealing temperatures and times. Annealing temperature (◦ C)
Specific contact resistivities (-cm2 ) Time/(cumulated time)
400 500 600 700 Fig. 1. (a) Metal mask used to fabricate the linear TLM pads and (b) samples after metallization with fabricated linear TLM pads.
of Al. The transmission line model (TLM), refined by Berger [9] and after by Li et al. [10], is currently used to characterize electrically the ohmic contact metallization systems. The TLM design used shown in Fig. 1. The TLM pads were designed to be 2 mm (W, width) × 1 mm (d, length) in size and the spacings, l between the pads were 0.3, 0.4, 0.6, 0.9 and 1.3 mm. The specific contact resistivities, c were determined from the plot of the measured resistances against the spacing between the TLM pads. The linear-square method used to fit a straight line in the experimental data. Thermal annealing carried out in a conventional tube furnace by heating the wafers. The gas purged at a mass flow rate of approximately 4 L min−1 . The samples annealed under flowing nitrogen environment at 400, 500, 600 and 700 ◦ C for 5 min in a conventional tube furnace. Similar heat treatments carried out for additional annealing times of 10 and 20 min to investigate the thermal stability of the interface contacts. The current–voltage (I–V) measurements were performed by a Kiethley high voltage source measure unit model 237. 3. Results and discussion The measurements of the specific contact resistivity c were made using the TLM method that has been widely used in the characterization of ohmic contacts to semiconductors. Resistance, Ri , between two contacts with spacing li , is given by Ri =
Rsh li 2Rsk Lt + Wc Wc
(1)
Ri =
Rsh li + 2Rc Wc
(2)
where li is the spacing between two pads, Wc is the width of the contact pad, Rc is the resistance due to the contact, Rsh is sheet resistance of the semiconductor layer outside the contact region, Rsk is the sheet resistance of the layer directly under the contact, and Lt is the transfer length. The plot shows Ri as a function of li which produces a straight line with the slope Rsh /Wc , and 2Rc is yielded from the intercept at y-axis. The intercept at x-axis gives Lx : Lx =
2Rsk Lt Rsh
5 min
10 min/(15 min)
20 min/(35 min)
Nearly ohmic 2.460 0.901 1.178
0.534 0.397 0.230 0.351
1.290 – – –
Table 2 The specific contact resistivities of the n-type doped Alx Ga1−x N with different annealing temperatures and times. Annealing temperature (◦ C)
Specific contact resistivities ( cm2 ) Time/(cumulated time)
400 500 600 700
5 min
10 min/(15 min)
20 min/(35 min)
Schottky 2.201 0.723 1.024
0.521 0.313 0.190 0.277
1.021 – – –
and the native oxides on AlGaN. The surface oxide is known to increase effective Schottky barrier height and to deteriorate the metal/AlGaN interface, leading to the degradation of ohmic contacts [12]. Table 1 shows the specific contact resistivities for the undoped sample with different times and annealing temperatures. Sample annealed at 400 ◦ C for 5 min, still exhibits nearly ohmic behavior. While the other samples exhibited ohmic behavior at higher annealing temperatures (500–600 ◦ C). This indicates that higher annealing temperatures improve the contact resistances. The improvement and reduction in the contact resistance correspond to formation of the intermetallic alloy (TiN) with low work function on AlGaN surface [13]. In addition, when annealing Al with work function of 4.28 eV, it will be diffused through Ti and will reach the AlGaN surface. The Al then reacts with the surface of AlGaN to form a thin layer of AlN at the interface. This again results in N vacancies yielding a heavily doped interface, resulting in tunneling current which is responsible for the ohmic contact formation [14]. 10-min (15 min cumulated time) annealed sample showed a significant improvement in the characteristics of all the samples as shown in Fig. 2. The specific contact resistivity increases with the increase of annealing temperature at 700 ◦ C. This indicates the formation of the thin oxide layer on the contact surface. Table 2 shows the contact resistivities of the n-type doped sample with different times and annealing temperatures. The results for the Si doped sample were better than the undoped sample which indicates that the Si doping improves the electrical properties of the sample [15]. Furthermore, the diffusion of the N atoms
(3)
where Lx ≈ 2Lt with the assumption that Rsh = Rsk . On the other hand, the assumption of an electrically long contact d > > Lt enabled the relationship c = Rsh Lt2 to be invoked which leads to c = Rc WLt [11]. The specific contact resistivities measured for undoped and ntype doped samples summarized in Tables 1 and 2. After depositing Ti/Al bi-layer metal on the surface of the samples, the initial study revealed that all as-deposited samples have shown non-linear behavior. This non-linear behavior could be due to several factors, which include the growth quality of the AlGaN layer, the surface preparation prior to contact deposition
Fig. 2. Typical I–V characteristics for Ti/Al contacts on undoped AlGaN.
A.S.H. Hussein et al. / Optik 124 (2013) 4257–4259
4259
Acknowledgments Financial support from Research University (RU) grant, Postgraduate Research Grant Scheme (PRGS) and Universiti Sains Malaysia are gratefully acknowledged. References
Fig. 3. Typical I–V characteristics for Ti/Al contacts on n-type doped AlGaN.
leaves the N-vacancies in n-AlGaN layer near the metal/n-AlGaN interface. Because N-vacancy is determined to serve as a shallow donor in AlGaN, the highly n-doped region in n-AlGaN is formed near the metal/n-AlGaN interface, and thus the specific contact resistivity decreases. Fig. 3 shows the I–V characteristics of n-type doped AlGaN sample annealed at 600 ◦ C for 10 min. Therefore, with increasing the annealing temperature to 600 ◦ C, the N-vacancy concentration in n-AlGaN increases, and thus the specific contact resistivity further decreases [16]. However, the degradation and a light brown layer were visually observed after annealing the samples at 700 ◦ C. This indicates the formation of the thin oxide layer on the contact surface [11]. 4. Conclusions The characteristics of Ti/Al bi-layer ohmic contacts for the undoped and Si-doped Alx Ga1−x N samples grown on silicon (1 1 1) substrates by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PA-MBE) have been studied. The specific contact resistivities of this bi-layer scheme were sensitive to the change of annealing temperatures and annealing time. The best results of the specific contact resistivities have been obtained with annealing temperature of 600 ◦ C at 10 min for both samples. Moreover, the results for the Si doping sample were better than the undoped sample which indicates that the Si doped improves the electrical properties of the sample.
[1] S. Jain, M. Willander, J. Narayan, R. Overstraeten, III–nitrides: growth, characterization, and properties, J. Appl. Phys. 87 (2000) 965–1006. [2] S. Ruvimov, Z. Liliental-Weber, J. Washburn, K. Duxstad, E. Haller, Z. Fan, S. Mohammad, W. Kim, A. Botchkarev, H. Morkoc, Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN, Appl. Phys. Lett. 69 (1996) 1556–1558. [3] A. Soltani, A. BenMoussa, S. Touati, V. Hoël, J. Jaeger, J. Laureyns, Y. Cordier, C. Marhic, M. Djouadi, C. Dua, Development and analysis of low resistance ohmic contact to n-AlGaN/GaN HEMT, Diam. Rel. Mater. 16 (2007) 262–266. [4] B. Luther, S. Monhney, T. Jackson, M. Khan, Q. Chen, J. Yang, Investigation of the mechanism for Ohmic contact formation in Al and Ti/Al contacts to n-type GaN, Appl. Phys. Lett. 70 (1997) 57–59. [5] L. Chuah, Z. Hassan, N. Shamsuddin, N. Amirruddin, C. Chin, H. Hassan, Ohmic metal contact to InGaN, Optoelectron. Adv. Mater. Rapid Commun. (OAM-RC) 2 (2008) 650–654. [6] W. Lau, J. Tan, B. Singh, Formation of Ohmic contacts in AlGaN/GaN HEMT structures at 500 ◦ C by Ohmic contact recess etching, Microelectron. Reliab. 49 (2009) 558–561. [7] A. Hussein, Z. Hassan, S. Thahab, H. Hassan, M. Abid, C. Chin, Optical and electrical properties of undoped and Si-doped Alx Ga1−x N thin films on Si (1 1 1), Phys. B Condens. Matter 406 (2011) 1267–1271. [8] D. Kim, J. Bae, W. Kim, H. Baik, J. Myoung, S. Lee, The improvement of electrical properties of Pd-based contact to p-GaN by surface treatment, J. Electron. Mater. 30 (2001) 183–187. [9] H. Berger, Models for contacts to planar devices, Solid State Electron. 15 (1972) 145–158. [10] Y. Li, H. Harrison, G. Reeves, Correcting separation errors related to contact resistance measurement, Microelectron. J. 29 (1998) 21–30. [11] S. Pearton, J. Zolper, R. Shul, F. Ren, GaN: processing, defects, and devicesm, J. Appl. Phys. 86 (1999) 1–78. [12] H. Morkoc, Springer-Verlag, Berlin, 1999, p. 99. [13] L. Chuah, Z. Hassan, H.A. Hassan, Ohmic contacts properties of Ni/Ag metallization scheme on p-type GaN, J. Non-Cryst. Solids 365 (2010) 181. [14] Q. Feng, L. Li, Y. Hao, J. Ni, J. Zhang, The improvement of ohmic contact of Ti/Al/Ni/Au to AlGaN/GaN HEMT by multi-step annealing method, Solid State Electron. 53 (2009) 955–958. [15] L. Tan, S. Prakash, K. Ng, A. Ramam, S. Chua, A. Wee, S. Lim, Formation of Ti/Al ohmic contacts on Si-doped GaN epilayers by low temperature annealing, Semicond. Sci. Technol. 15 (2000) 585–588. [16] B. Shen, H. Zhou, J. Liu, Y. Zhou, R. Zhang, Y. Shi, Y. Zheng, T. Someya, Y. Arakawa, Ohmic contact and interfacial reaction of Ti/Al/Pt/Au metallic multi-layers on n-Alx Ga1−x N/GaN heterostructures, Opt. Mater. 23 (2003) 197–201.