- Email: [email protected]
Effects of oxygen plasma treatments on the formation of ohmic contacts to GaN
Applied Surface Science 234 (2004) 328–332
Effects of oxygen plasma treatments on the formation of ohmic contacts to GaN J. Yan*, M.J. Kappers, Z.H. Barber, C.J. Humphreys Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Available online 7 July 2004
Abstract Oxygen plasma treatments have been performed prior to contact deposition on both n- and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n-type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 107 O cm2 was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 8C in argon. The I–V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n-type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects. # 2004 Elsevier B.V. All rights reserved.
1. Introduction High quality ohmic contacts are required for applications in GaN-based optoelectronic and electronic devices. According to the literature, a specific contact resistivity (rc) as low as 106 O cm2 can be readily achieved by using Ti-based metallisation to n-type GaN [1]. The value of rc is further reduced after a chlorine-based reactive ion etching (RIE) treatment prior to contact deposition. The contact improvement has been explained as the result of preferential sputtering of nitrogen from the GaN surface, and because nitrogen vacancies (VN) act as donors in GaN, the sputtering process results in the surface becoming
* Corresponding author. Tel.: þ44 1223 334404. E-mail address: [email protected] (J. Yan).
highly n-type [2–5]. The same RIE treatment deteriorated the contact performance for p-type GaN, which would support the argument of VN formation [3,4,6]. However, Chen et al. performed a similar chlorinebased RIE treatment on n-type GaN and observed a deterioration in the contact characteristics, which was attributed to lattice disordering at the surface [7]. In this work, we demonstrate that excellent ohmic Ti/Al contacts to n-type GaN can be achieved by treating the surface with oxygen plasma prior to contact deposition and a rapid thermal annealing (RTA) treatment post-deposition. We investigated the effects of treatment time, rf power and plasma pressure on the contacts to n- and p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in an attempt to understand the mechanism underlying the change in contact resistance which occurs following the plasma treatment.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.066
J. Yan et al. / Applied Surface Science 234 (2004) 328–332
Ga 3p, N 1s, C 1s, and O 1s) in a single sweep at a pass energy of 100 eV.
The samples used in this work were grown by metal-organic chemical vapour deposition (MOCVD) on c-plane sapphire substrates. The GaN layer was doped with Mg for p-type conductivity (p 1017 cm3) and Si for n-type conductivity (n 5– 1l 1018 cm3). The GaN wafers were cut into small (10 mm 5 mm) samples and cleaned ultrasonically for 5 min each in trichloroethylene, acetone, methanol, isopropanol, and 20 min in NH4OH (aq), followed by rinsing in de-ionised (DI) water and then blowdried with nitrogen. The samples were then patterned with photoresist to form circular transmission line method (c-TLM) structures [8] for the lift-off process, followed by oxygen plasma treatments using different combinations of time, power and pressure. Prior to contact deposition, the samples were dipped in NH4OH (aq) for 20 s, rinsed with DI water, then blow-dried with nitrogen. Contacts of Ti/Al (10/ 100 nm) or Ti/Al (20/200 nm) were deposited on ntype GaN by dc magnetron sputtering with a base pressure < 107 mbar and deposition pressures of 1 Pa for Ti and 0.5 Pa for Al. Contacts of Ni/Au (10/10 nm) were deposited on p-type GaN using electron beam evaporation. A lift-off process was then used to form the c-TLM structure with inner radius (L) of 150 mm and gap spacing (d) of 6, 8, 10, 20, 40, 80, 160 and 320 mm. Values for rc were calculated using Eq. (1), where sheet resistance (rs) and transfer length (LT) were determined by fitting Eq. (2) to the measured variation of total resistance (RT), versus ln[(L þ d)/L], where d is the gap spacing [8]. LT represents the distance over which most of the current transfers from the semiconductor into the metal or vice versa, and therefore, the lower the contact resistance, the lower the value of LT. (1) (2)
Atomic force microscopy (AFM) and Dektek profilometry were also used to determine the surface roughness and etch rate, respectively, on non-patterned samples. XPS measurements were performed on an ESCALAB using 200 W Mg Ka radiation (1253.6 eV) scanning all the relevant peaks (Ga 3d,
3. Results The effects of the oxygen plasma treatment time on the current–voltage (I–V) characteristics of as-deposited Ti/Al contacts on n-type GaN are shown in Fig. 1. The native contacts, i.e. without oxygen plasma treatment, are rectifying but become ohmic with rc ¼ 2 104 O cm2 when the GaN is treated with an oxygen plasma for 20 s, at 0.2 mbar and 30 W. For longer plasma treatment times the contact resistance is found to increase and the contacts become rectifying again when the GaN surface is exposed for 300 s to the plasma. Fig. 2 shows the I–V characteristics of as-deposited Ti/Al contacts to the n-type GaN which has been subjected to oxygen plasma at different rf power levels (30, 45, and 60 W). The contacts formed exhibit an ohmic behaviour irrespective of the power level used, but the lowest value for rc is obtained with the lowest rf power, as is shown in the inset of Fig. 2. The effects of oxygen plasma pressure on the I–V characteristics of as-deposited Ti/Al contacts on n-type GaN are shown in Fig. 3. The plasma pressure was varied from 0.2 to 0.5 mbar, for a constant treatment time and rf power. The sample treated at the
0.15 0.10
O2 plasma 0.2 mbar 30 W
0.05 Current [A]
2. Experimental
rc ¼ rs L2T r 1 1 Lþd þ RT ¼ s L T þ ln L Lþd L 2p
329
0.00 -0.05
Native 20 s 60 s 300 s
-0.10 -0.15 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Voltage [V]
Fig. 1. Effects of the oxygen plasma treatment time on the I–V characteristics of as-deposited Ti/Al (20/200 nm) contacts to GaN (n ¼ 1.1 1019 cm3), at a gap spacing of 10 mm.
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annealing temperature which leads to the lowest rc) depends upon the oxygen plasma pressure used. The lowest value obtained for rc is 6 109 O cm2 (after oxygen plasma treatment at 0.2 mbar and 500 8C annealing). It should be noted, however, that the c-TLM method is only accurate when rc > 0.2rst2, where t is the thickness of the active semiconductor layer [13,14]. In this particular case, rs 60 O/& and t ¼ 1 mm and therefore, the rc value determined using this method is accurate only when rc > 107 O cm2. Our minimum rc value of 109 O cm2 must be there-
Fig. 2. Effects of the rf power used during oxygen plasma treatment of GaN on the I–V characteristics of as-deposited Ti/Al (20/200 nm) contacts to GaN (n ¼ 1.1 1019 cm3), at a gap spacing of 10 mm. Insert: specific contact resistivity as a function of rf power.
higher pressure exhibits a rectifying behaviour, which is only slightly better than the native sample. The native contact, and the two set of samples exposed at different plasma pressures (shown in Fig. 3) were then annealed in argon for 60 s at various temperatures. In Fig. 4a, the values of rc versus the RTA temperature are shown for the three samples. The initially rectifying contacts become ohmic after thermal annealing between 500 and 700 8C. However, it seems that the optimal annealing temperature (the 0.15 0.10
O2 plasma 30 W 20 s
Current [A]
0.05 0.00 -0.05 Native 0.2 mbar 0.5 mbar
-0.10 -0.15 -1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
Voltage [V]
Fig. 3. Effects of oxygen plasma pressure on I–V characteristics of as-deposited Ti/Al (10/100 nm) contacts on GaN (n ¼ 4.6 1018 cm3), at a gap spacing of 10 mm.
Fig. 4. (a) Specific contact resistivity on n-type GaN as a function of annealing temperature following different oxygen plasma treatment pressures. The contacts used were Ti/Al (10/100 nm) and n ¼ 4.6 1018 cm3. (b) Total contact resistance as a function of ln[(L þ d)/L] after oxygen plasma treatment at 0.2 mbar for different annealing temperatures.
J. Yan et al. / Applied Surface Science 234 (2004) 328–332 Table 1 Results from XPS measurement on various oxygen plasma-treated samples Treatment conditions: power/pressure/time Native sample 30 W/0.2 mbar/20 s 30 W/0.2 mbar/300 s 30 W/0.5 mbar/20 s 60 W/0.2 mbar/20 s
Normalised ratios Ga/N
Ga/O
N/O
Ga/C
1.00 1.02 1.00 1.06 1.04
1.00 0.81 0.51 0.69 0.68
1.00 0.79 0.51 0.65 0.66
1.00 1.16 1.08 1.13 1.13
The relative variations of the surface gallium 3d, nitrogen 1s, oxygen 1s and carbon 1s were obtained by taking the native sample as the basis.
Current [A]
fore treated with caution. The c-TLM data for this treatment condition (oxygen plasma treatment at 0.2 mbar) and for a range of annealing temperatures are shown in Fig. 4b. The GaN surface following oxygen plasma treatment under a range of conditions has been analysed using XPS. The normalised Ga/N, Ga/O, N/O and Ga/C ratios are listed in Table 1. The relative variations of the signals from gallium 3d, nitrogen 1s, oxygen 1s and carbon 1s were obtained and normalised against the ratios for the native sample. No significant change in the nitrogen signal as a result of the plasma treatment is observed. The carbon contamination on the surface and/or within the GaN layers decreases to an apparently constant value of the Ga-to-C ratio after the various plasma treatments. The 1.5x10
-5
1.0x10
-5
5.0x10
-6
O2 plasma 0.2 mbar 30 W 20 s
0.0
-5.0x10
-6
-1.0x10
-5
-1.5x10
-5
-6
Native Oxygen plasma treated
-4
-2
0
2
4
6
Voltage [V]
Fig. 5. Effect of pre-contact deposition oxygen plasma treatment on as-deposited Ni/Au (10/10 nm) contacts to GaN (p 1017 cm3), at a gap spacing of 10 mm.
331
largest variation in chemical composition is observed in the oxygen signal that seems to increase with increasing the severity of the plasma treatment. Similar treatment was also performed on p-type GaN. Fig. 5 illustrates the effect of a mild (low power, short treatment time) low pressure oxygen plasma treatment on p-type GaN prior to Ni/Au contact deposition. Although the Ti/Al contact to n-type GaN showed ohmic behaviour after a similar treatment, the Ni/Au contact on plasma-treated p-type GaN exhibits a degraded contact with respect to the contacts on untreated p-type GaN.
4. Discussion The oxygen plasma treatment of n-type GaN prior to metallisation improves the electrical characteristics and renders the samples ohmic provided the plasma treatment time is short and the plasma power and pressure are relatively low. State-of-the-art Ti/Al contacts are formed when the samples are subjected to an additional RTA anneal in an Ar gas ambient. However, when plasma conditions become more severe, i.e. through longer exposure times, or higher rf power, we observe degradation of the Ti/Al contacts to GaN. It is proposed that the plasma-induced ion damage may also lead to surface disorder in the crystal lattice and/or form defects on the sample surface [7]. It is clear that careful control of the plasma parameters is required to optimise the GaN surface conditions prior to metallisation. Overlooking this phenomenon may explain the controversy observed between Chen et al. [7] and other workers [2–5]. The effect of the plasma treatment on GaN is consistent with the formation of a highly n-type surface layer, as the results presented seem to suggest. The local increase in carrier concentration would lead to the formation of a tunnelling contact, as the Schottky barrier width is inversely proportional to the carrier concentration [9]. When this Schottky barrier at the metal–GaN interface is thinner, electrons can tunnel more easily through the barrier. During plasma treatment, the sample surface may be damaged by the bombardment of energetic ions which may lead to the creation of point defects such as donor-like nitrogen vacancies (VN) [10,11]. This would then create a thin, highly n-type layer just under
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the Ti/Al contact. An alternative explanation is that the oxygen plasma results in nitrogen atoms being replaced by oxygen atoms (ON) in the surface of the GaN, which act as donors in GaN, thus improving the contact properties. The latter explanation would be favoured by the lower formation energy of ON than VN [12]. However, increasing the amount of oxygen may not increase the concentration of oxygen atoms incorporated in the GaN, as the solubility of oxygen is limited by the formation of Ga2O3 [12] and this may explain the observation of deteriorated contact behaviour when the high pressure plasma treatment was used. Our experimental XPS data suggest a high concentration of oxygen on or in the GaN surface layers after oxygen plasma treatment. However, the constant Ga-to-N ratio as measured by XPS seems to oppose the oxygen atom replacement argument. Nonetheless, the deteriorated I–V characteristic of the Ni/Au contacts to p-type GaN after treatment in an oxygen plasma is in line with the creation of a highly n-doped layer on the GaN surface, and the occurrence of a dopant compensation effect. Further studies are currently underway in order to understand the mechanism behind the reduction in rc on the annealed Ti/Al contacts to oxygen-plasma-treated n-type GaN. The advantage of using an oxygen plasma treatment over chlorine-based RIE is that no actual etching is done on the surface, since there was no detectable change in thickness even after 30 min of treatment. Also, AFM analysis (results not shown) revealed no change in the surface morphology after performing these treatments. 5. Conclusion We have shown in this study that an oxygen plasma treatment to n-type GaN prior to Ti/Al contact deposition is advantageous provided the optimum conditions are used. Prolonged treatment times and/or high plasma pressure deteriorated the contact and resulted in rectifying behaviour. The deteriorated behaviour upon plasma treatment of p-type GaN compared to the native samples of Ni/Au contacts top-type GaN strongly suggests that the improvements of Ti/Al contacts to n-type GaN were due to the creation of
a highly n-doped layer on the surface. We have also demonstrated that when the plasma-treated samples are consequently subjected to an optimised RTA treatment, specific contact resistivities on the order of 107 O cm2 can be achieved.
Acknowledgements The authors would like to thank S. Whelan for his help in the evaporation of the p-GaN contacts and the RTA, A. Crossley and R.A. Oliver from Oxford University for the XPS measurements and analysis, R.F. Broom and A. Phillips for their helpful advice and discussions, and one of the authors (J.Y.) would like to thank The Cambridge Overseas Trust, The Department for Education and Skills, Schlumberger Cambridge Research Ltd. and Queens’ College Cambridge for their financial support.
References [1] S.J. Pearton, F. Ren, A.P. Zhang, K.P. Lee, Mater. Sci. Eng. R30 (2000) 55. [2] Z.F. Fan, S.N. Mohammad, W. Kim, O. Aktas, A.E. Botchkarev, H. Morkoc¸, Appl. Phys. Lett. 68 (1996) 1672. [3] H.W. Choi, S.J. Chua, X.J. Kang, Phys. Stat. Sol. 188 (a) (2001) 399. [4] H.W. Jang, J.L. Lee, J. Electrochem. Soc. 150 (2003) G513. [5] S. Ruvimov, Z. Liliental-Weber, J. Washburn, K.J. Duxstad, E.E. Haller, Z.F. Fan, S.N. Mohammad, W. Kim, A.E. Botchkarev, H. Morkoc, Appl. Phys. Lett. 69 (1996) 1556. [6] K.M. Chang, C.C. Cheng, J.Y. Chu, J. Electrochem. Soc. 149 (2002) G367. [7] J.Y. Chen, C.J. Pan, G.C. Chi, Solid-State Electron. 43 (1999) 649. [8] M. Ahmad, B.M. Arora, Solid-State Electron. 35 (1992) 1441. [9] S.M. Sze, Semiconductor Devices Physics and Technology, John Wiley & Sons, New York, 1985. [10] D.W. Jenkins, J.D. Dow, Phys. Rev. B39 (1989) 3317. [11] S. Nakamura, S.F. Chichibu, Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes, Taylor and Francis, London, 2000. [12] C.G. Van de Walle, J. Vac. Sci. Technol. A21 (2003) S182. [13] D.K. Schroder, Semiconductor Material and Device Characterization, John Wiley & Sons, 1998. [14] C. Lu, H. Chen, X. Lv, X. Xie, S.N. Mohammad, J. Appl. Phys. 91 (2002) 9218.
Effects of oxygen plasma treatments on the formation of ohmic contacts to GaN J. Yan*, M.J. Kappers, Z.H. Barber, C.J. Humphreys Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Available online 7 July 2004
Abstract Oxygen plasma treatments have been performed prior to contact deposition on both n- and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n-type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 107 O cm2 was obtained using a plasma treatment of 20 s at 30 W and 0.2 mbar, followed by RTA at 500 8C in argon. The I–V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n-type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects. # 2004 Elsevier B.V. All rights reserved.
1. Introduction High quality ohmic contacts are required for applications in GaN-based optoelectronic and electronic devices. According to the literature, a specific contact resistivity (rc) as low as 106 O cm2 can be readily achieved by using Ti-based metallisation to n-type GaN [1]. The value of rc is further reduced after a chlorine-based reactive ion etching (RIE) treatment prior to contact deposition. The contact improvement has been explained as the result of preferential sputtering of nitrogen from the GaN surface, and because nitrogen vacancies (VN) act as donors in GaN, the sputtering process results in the surface becoming
* Corresponding author. Tel.: þ44 1223 334404. E-mail address: [email protected] (J. Yan).
highly n-type [2–5]. The same RIE treatment deteriorated the contact performance for p-type GaN, which would support the argument of VN formation [3,4,6]. However, Chen et al. performed a similar chlorinebased RIE treatment on n-type GaN and observed a deterioration in the contact characteristics, which was attributed to lattice disordering at the surface [7]. In this work, we demonstrate that excellent ohmic Ti/Al contacts to n-type GaN can be achieved by treating the surface with oxygen plasma prior to contact deposition and a rapid thermal annealing (RTA) treatment post-deposition. We investigated the effects of treatment time, rf power and plasma pressure on the contacts to n- and p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in an attempt to understand the mechanism underlying the change in contact resistance which occurs following the plasma treatment.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.066
J. Yan et al. / Applied Surface Science 234 (2004) 328–332
Ga 3p, N 1s, C 1s, and O 1s) in a single sweep at a pass energy of 100 eV.
The samples used in this work were grown by metal-organic chemical vapour deposition (MOCVD) on c-plane sapphire substrates. The GaN layer was doped with Mg for p-type conductivity (p 1017 cm3) and Si for n-type conductivity (n 5– 1l 1018 cm3). The GaN wafers were cut into small (10 mm 5 mm) samples and cleaned ultrasonically for 5 min each in trichloroethylene, acetone, methanol, isopropanol, and 20 min in NH4OH (aq), followed by rinsing in de-ionised (DI) water and then blowdried with nitrogen. The samples were then patterned with photoresist to form circular transmission line method (c-TLM) structures [8] for the lift-off process, followed by oxygen plasma treatments using different combinations of time, power and pressure. Prior to contact deposition, the samples were dipped in NH4OH (aq) for 20 s, rinsed with DI water, then blow-dried with nitrogen. Contacts of Ti/Al (10/ 100 nm) or Ti/Al (20/200 nm) were deposited on ntype GaN by dc magnetron sputtering with a base pressure < 107 mbar and deposition pressures of 1 Pa for Ti and 0.5 Pa for Al. Contacts of Ni/Au (10/10 nm) were deposited on p-type GaN using electron beam evaporation. A lift-off process was then used to form the c-TLM structure with inner radius (L) of 150 mm and gap spacing (d) of 6, 8, 10, 20, 40, 80, 160 and 320 mm. Values for rc were calculated using Eq. (1), where sheet resistance (rs) and transfer length (LT) were determined by fitting Eq. (2) to the measured variation of total resistance (RT), versus ln[(L þ d)/L], where d is the gap spacing [8]. LT represents the distance over which most of the current transfers from the semiconductor into the metal or vice versa, and therefore, the lower the contact resistance, the lower the value of LT. (1) (2)
Atomic force microscopy (AFM) and Dektek profilometry were also used to determine the surface roughness and etch rate, respectively, on non-patterned samples. XPS measurements were performed on an ESCALAB using 200 W Mg Ka radiation (1253.6 eV) scanning all the relevant peaks (Ga 3d,
3. Results The effects of the oxygen plasma treatment time on the current–voltage (I–V) characteristics of as-deposited Ti/Al contacts on n-type GaN are shown in Fig. 1. The native contacts, i.e. without oxygen plasma treatment, are rectifying but become ohmic with rc ¼ 2 104 O cm2 when the GaN is treated with an oxygen plasma for 20 s, at 0.2 mbar and 30 W. For longer plasma treatment times the contact resistance is found to increase and the contacts become rectifying again when the GaN surface is exposed for 300 s to the plasma. Fig. 2 shows the I–V characteristics of as-deposited Ti/Al contacts to the n-type GaN which has been subjected to oxygen plasma at different rf power levels (30, 45, and 60 W). The contacts formed exhibit an ohmic behaviour irrespective of the power level used, but the lowest value for rc is obtained with the lowest rf power, as is shown in the inset of Fig. 2. The effects of oxygen plasma pressure on the I–V characteristics of as-deposited Ti/Al contacts on n-type GaN are shown in Fig. 3. The plasma pressure was varied from 0.2 to 0.5 mbar, for a constant treatment time and rf power. The sample treated at the
0.15 0.10
O2 plasma 0.2 mbar 30 W
0.05 Current [A]
2. Experimental
rc ¼ rs L2T r 1 1 Lþd þ RT ¼ s L T þ ln L Lþd L 2p
329
0.00 -0.05
Native 20 s 60 s 300 s
-0.10 -0.15 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Voltage [V]
Fig. 1. Effects of the oxygen plasma treatment time on the I–V characteristics of as-deposited Ti/Al (20/200 nm) contacts to GaN (n ¼ 1.1 1019 cm3), at a gap spacing of 10 mm.
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J. Yan et al. / Applied Surface Science 234 (2004) 328–332
annealing temperature which leads to the lowest rc) depends upon the oxygen plasma pressure used. The lowest value obtained for rc is 6 109 O cm2 (after oxygen plasma treatment at 0.2 mbar and 500 8C annealing). It should be noted, however, that the c-TLM method is only accurate when rc > 0.2rst2, where t is the thickness of the active semiconductor layer [13,14]. In this particular case, rs 60 O/& and t ¼ 1 mm and therefore, the rc value determined using this method is accurate only when rc > 107 O cm2. Our minimum rc value of 109 O cm2 must be there-
Fig. 2. Effects of the rf power used during oxygen plasma treatment of GaN on the I–V characteristics of as-deposited Ti/Al (20/200 nm) contacts to GaN (n ¼ 1.1 1019 cm3), at a gap spacing of 10 mm. Insert: specific contact resistivity as a function of rf power.
higher pressure exhibits a rectifying behaviour, which is only slightly better than the native sample. The native contact, and the two set of samples exposed at different plasma pressures (shown in Fig. 3) were then annealed in argon for 60 s at various temperatures. In Fig. 4a, the values of rc versus the RTA temperature are shown for the three samples. The initially rectifying contacts become ohmic after thermal annealing between 500 and 700 8C. However, it seems that the optimal annealing temperature (the 0.15 0.10
O2 plasma 30 W 20 s
Current [A]
0.05 0.00 -0.05 Native 0.2 mbar 0.5 mbar
-0.10 -0.15 -1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
Voltage [V]
Fig. 3. Effects of oxygen plasma pressure on I–V characteristics of as-deposited Ti/Al (10/100 nm) contacts on GaN (n ¼ 4.6 1018 cm3), at a gap spacing of 10 mm.
Fig. 4. (a) Specific contact resistivity on n-type GaN as a function of annealing temperature following different oxygen plasma treatment pressures. The contacts used were Ti/Al (10/100 nm) and n ¼ 4.6 1018 cm3. (b) Total contact resistance as a function of ln[(L þ d)/L] after oxygen plasma treatment at 0.2 mbar for different annealing temperatures.
J. Yan et al. / Applied Surface Science 234 (2004) 328–332 Table 1 Results from XPS measurement on various oxygen plasma-treated samples Treatment conditions: power/pressure/time Native sample 30 W/0.2 mbar/20 s 30 W/0.2 mbar/300 s 30 W/0.5 mbar/20 s 60 W/0.2 mbar/20 s
Normalised ratios Ga/N
Ga/O
N/O
Ga/C
1.00 1.02 1.00 1.06 1.04
1.00 0.81 0.51 0.69 0.68
1.00 0.79 0.51 0.65 0.66
1.00 1.16 1.08 1.13 1.13
The relative variations of the surface gallium 3d, nitrogen 1s, oxygen 1s and carbon 1s were obtained by taking the native sample as the basis.
Current [A]
fore treated with caution. The c-TLM data for this treatment condition (oxygen plasma treatment at 0.2 mbar) and for a range of annealing temperatures are shown in Fig. 4b. The GaN surface following oxygen plasma treatment under a range of conditions has been analysed using XPS. The normalised Ga/N, Ga/O, N/O and Ga/C ratios are listed in Table 1. The relative variations of the signals from gallium 3d, nitrogen 1s, oxygen 1s and carbon 1s were obtained and normalised against the ratios for the native sample. No significant change in the nitrogen signal as a result of the plasma treatment is observed. The carbon contamination on the surface and/or within the GaN layers decreases to an apparently constant value of the Ga-to-C ratio after the various plasma treatments. The 1.5x10
-5
1.0x10
-5
5.0x10
-6
O2 plasma 0.2 mbar 30 W 20 s
0.0
-5.0x10
-6
-1.0x10
-5
-1.5x10
-5
-6
Native Oxygen plasma treated
-4
-2
0
2
4
6
Voltage [V]
Fig. 5. Effect of pre-contact deposition oxygen plasma treatment on as-deposited Ni/Au (10/10 nm) contacts to GaN (p 1017 cm3), at a gap spacing of 10 mm.
331
largest variation in chemical composition is observed in the oxygen signal that seems to increase with increasing the severity of the plasma treatment. Similar treatment was also performed on p-type GaN. Fig. 5 illustrates the effect of a mild (low power, short treatment time) low pressure oxygen plasma treatment on p-type GaN prior to Ni/Au contact deposition. Although the Ti/Al contact to n-type GaN showed ohmic behaviour after a similar treatment, the Ni/Au contact on plasma-treated p-type GaN exhibits a degraded contact with respect to the contacts on untreated p-type GaN.
4. Discussion The oxygen plasma treatment of n-type GaN prior to metallisation improves the electrical characteristics and renders the samples ohmic provided the plasma treatment time is short and the plasma power and pressure are relatively low. State-of-the-art Ti/Al contacts are formed when the samples are subjected to an additional RTA anneal in an Ar gas ambient. However, when plasma conditions become more severe, i.e. through longer exposure times, or higher rf power, we observe degradation of the Ti/Al contacts to GaN. It is proposed that the plasma-induced ion damage may also lead to surface disorder in the crystal lattice and/or form defects on the sample surface [7]. It is clear that careful control of the plasma parameters is required to optimise the GaN surface conditions prior to metallisation. Overlooking this phenomenon may explain the controversy observed between Chen et al. [7] and other workers [2–5]. The effect of the plasma treatment on GaN is consistent with the formation of a highly n-type surface layer, as the results presented seem to suggest. The local increase in carrier concentration would lead to the formation of a tunnelling contact, as the Schottky barrier width is inversely proportional to the carrier concentration [9]. When this Schottky barrier at the metal–GaN interface is thinner, electrons can tunnel more easily through the barrier. During plasma treatment, the sample surface may be damaged by the bombardment of energetic ions which may lead to the creation of point defects such as donor-like nitrogen vacancies (VN) [10,11]. This would then create a thin, highly n-type layer just under
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the Ti/Al contact. An alternative explanation is that the oxygen plasma results in nitrogen atoms being replaced by oxygen atoms (ON) in the surface of the GaN, which act as donors in GaN, thus improving the contact properties. The latter explanation would be favoured by the lower formation energy of ON than VN [12]. However, increasing the amount of oxygen may not increase the concentration of oxygen atoms incorporated in the GaN, as the solubility of oxygen is limited by the formation of Ga2O3 [12] and this may explain the observation of deteriorated contact behaviour when the high pressure plasma treatment was used. Our experimental XPS data suggest a high concentration of oxygen on or in the GaN surface layers after oxygen plasma treatment. However, the constant Ga-to-N ratio as measured by XPS seems to oppose the oxygen atom replacement argument. Nonetheless, the deteriorated I–V characteristic of the Ni/Au contacts to p-type GaN after treatment in an oxygen plasma is in line with the creation of a highly n-doped layer on the GaN surface, and the occurrence of a dopant compensation effect. Further studies are currently underway in order to understand the mechanism behind the reduction in rc on the annealed Ti/Al contacts to oxygen-plasma-treated n-type GaN. The advantage of using an oxygen plasma treatment over chlorine-based RIE is that no actual etching is done on the surface, since there was no detectable change in thickness even after 30 min of treatment. Also, AFM analysis (results not shown) revealed no change in the surface morphology after performing these treatments. 5. Conclusion We have shown in this study that an oxygen plasma treatment to n-type GaN prior to Ti/Al contact deposition is advantageous provided the optimum conditions are used. Prolonged treatment times and/or high plasma pressure deteriorated the contact and resulted in rectifying behaviour. The deteriorated behaviour upon plasma treatment of p-type GaN compared to the native samples of Ni/Au contacts top-type GaN strongly suggests that the improvements of Ti/Al contacts to n-type GaN were due to the creation of
a highly n-doped layer on the surface. We have also demonstrated that when the plasma-treated samples are consequently subjected to an optimised RTA treatment, specific contact resistivities on the order of 107 O cm2 can be achieved.
Acknowledgements The authors would like to thank S. Whelan for his help in the evaporation of the p-GaN contacts and the RTA, A. Crossley and R.A. Oliver from Oxford University for the XPS measurements and analysis, R.F. Broom and A. Phillips for their helpful advice and discussions, and one of the authors (J.Y.) would like to thank The Cambridge Overseas Trust, The Department for Education and Skills, Schlumberger Cambridge Research Ltd. and Queens’ College Cambridge for their financial support.
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