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Al ohmic contacts to n-type GaN
Solid-State Electronics 46 (2002) 1975–1981 www.elsevier.com/locate/sse
High temperature characteristics of Ti/Al and Cr/Al ohmic contacts to n-type GaN N.A. Papanicolaou b
a,*
, K. Zekentes
b
a Naval Research Laboratory, 4555 Overlook Ave, Washington, DC 20375, USA Foundation for Research and Technology Hellas, P.O. Box 1527, GR-71110 Heraklion/Crete, Greece
Received 2 January 2002; received in revised form 5 February 2002; accepted 27 February 2002
Abstract The thermal stability of the Ti/Al, Ti/Al/Ni/Au, Cr/Al and Cr/Al/Ni/Au ohmic contact metallizations on n-GaN were investigated. Bilayers of Ti/Al and Cr/Al, with and without Ni/Au overlayers, were deposited on n-GaN using an e-beam deposition method and ohmic contacts formed by a vacuum annealing technique. For both the Ti/Al and Ti/Al/ Ni/Au minimum specific contact resistivities of 1 105 X cm2 were achieved after vacuum annealing at 1100 °C for 2 min. Minimum specific resistivities of similarly formed Cr/Al and Cr/Al/Ni/Au ohmic contacts were 3:8 105 and 2:3 105 X cm2 respectively, obtained at 700 and 950 °C respectively. The above mentioned contacts were exposed to long term aging at temperatures of 300, 400 and 500 °C, for periods up to 100 h and their electrical and morphological characteristics were monitored at the various stages of the aging process. The Ti/Al and Cr/Al/Ni/Au proved to be the most stable of the four metallizations in terms of both electrical performance and surface morphology. The Cr/Al had shown stable morphological stability, but displayed the poorest electrical performance characteristics as a result of the aging process. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Semiconductor; GaN; Ohmic contacts; n-type
1. Introduction Owing to their refractory characteristics, the III–V nitrides are very attractive materials for high temperature device applications. In addition, because of their wide band gap and large dielectric constants these materials are suited for blue and UV optoelectronic applications. GaN blue and green LEDs are now commercially available [1,2] and metal-semiconductor-fieldeffect transistors (MESFETs) have been successfully fabricted [3]. However, forming low resistance thermally stable and uniform ohmic contacts to wide band gap semiconductors such as GaN and related materials is
* Corresponding author. Tel.: +1-202-767-4898; fax: +1-202767-0455. E-mail address: [email protected] (N.A. Papanicolaou).
still a challenge. Single metal layer schemes such as Au or Al on n-GaN have been initially reported [4] with specific contact resistivity values in the 103 –104 X cm2 range. Bilayer metallization schemes such as Ti/Al, Ti/ Au have displayed the best contact resistivity results on n-GaN [5,6]. More recently, Cr/Al and Cr/Al/Ni/Au metallization systems on GaN have also been investigated [7] with specific contact resistivities as low as 1:2 105 X cm2 . For these metallization systems, formation of interfacial layers such as TiN or CrN have been suggested as playing a role in the formation of the ohmic contact [5–8]. These interfacial reactions tend to form N vacancies near the GaN surface which act as donors, thus resulting in a highly doped layer at the GaN/metal interface. The resulting high concentration of donors at the GaN/metal interface gives rise to a tunneling current across the barrier thus creating the ohmic contact. Another possible scheme which has been suggested for the formation of ohmic contacts to GaN is
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 2 ) 0 0 1 3 7 - 5
1976
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
the use of an InN intervening superlatice layer epitaxially grown on the GaN before the ohmic metal deposition [9]. Even though specific contact resistivities as low as 5 107 X cm2 have been reported with this approach, the thermal instability of the InN precludes this method from being a practical approach in forming useful ohmic contacts for high temperatures applications. Reproducibility and reliability under extreme (hightemperature) operating conditions are important considerations in the selection of the metal system for ohmic contacts on GaN. In this study, we report on the electrical and morphological investigation of the Ti/Al, Ti/ Al/Ni/Au, Cr/Al, and Cr/Al/Ni/Au ohmic contacts to n-GaN as a result of thermal aging at test temperatures of 300, 400 and 500 °C.
2. Experimental procedure The epitaxial GaN layers used in this study were grown by metal-organic-chemical-vapor-deposition (MOCVD) on sapphire substrates with R-plane orientation. A 200 nm thick undoped semi-insulating GaN buffer layer was initially grown, followed by a 2.0 lm thick, Si-doped n-type GaN active layer. The GaN layers had a net n-type carrier concentration 1:1 1018 cm3 . The TLM test patterns were formed on the samples using a standard photolithographic and lift-off technique. Metal layers consisting of Ti(25 nm)/Al(150 nm), Ti(25 nm)/ Al(80 nm)/Ni(25 nm)Au(100 nm) and Cr(25 nm)/Al(150 nm) and Cr(25 nm)/Al(80 nm)/Ni(25 nm)/Au(100 nm) were deposited on two sets of four discrete GaN samples, using an e-beam deposition method in a cryopumped vacuum system with a base pressure of 1:5 107 Torr. The first set of samples was used for determining the formation temperature for each metallization system, whereas the second set was used for the aging experiments. Prior to the aging experiments ohmic contacts were formed on the first set of samples by a vacuum annealing technique described in Ref. [10]. The annealing of the samples in this part of the experiment was cumulative, in other words each metallization was subjected to the annealing temperature, the contact resistivity measured and then the same sample annealed at the next higher temperature until the desired temperature range was covered. The annealing set-up consisted of a 1 in. wide, 5 in. long, and 0.015 in. thick tungsten strip heater inside a diffusion-pumped bell-jar vacuum system with a liquid N2 trap. The aging experiments which were performed on the second set of samples, were carried out in a conventional horizontal quartz tube furnace in an untreated air ambient. The outlet of the tube was connected to a ventilation system, which created a small negative pressure at the output of the tube thus causing
a flow of air at 35 sscm through the furnace tube. All four metalliizations were aged simultaneously in a series of three isothermal experiments at temperatures of 300, 400 and 500 °C with aging times ranging from 0.5 to 100 h. After each aging step, the samples were electrically characterized by means of specific contact resistivity (qc ) measurements. These measurements were made using test patterns conforming to the linear transmission line model (TLM) as described by Reeves and Harrison [11]. The TLM pattern consists of 200 lm 200 lm contact pads with separations of 5, 10, 25, 40, and 60 lm. Each sample contained six identical TLM test patterns which were measured separately and then averaged to produce a single (qc ) value. A Nomarsky optical microscope was used to monitor apparent changes in the surface morphology and roughness of the samples after each aging step. If significant changes were observed with the optical microscope, we proceeded to perform atomic force microscopy (AFM) measurements for more detailed morphological evaluation.
3. Results and discussion The first part of this study was aimed at determining the optimum annealing conditions for forming the ohmic contacts for the four different metallization systems under investigation, and for this purpose the first set of four samples were used. Fig. 1 shows the specific contact resistivity of the vacuum annealed (2-min isochronal) Ti/Al and Ti/Al/Ni/Au ohmic contacts as a function of anneal temperature. Both metallizations are ohmic as deposited, then become rectifying (non-linear) after annealing at the intermediate temperatures in the range of 500–800 °C. The contacts become ohmic again after the 900 and 950 °C anneals for the Ti/Al and Ti/Al/ Ni/Au, respectively. For the Ti/Al and the Ti/Al/Ni/Au,
Fig. 1. Specific contact resistivity vs. anneal temperature for the Ti/Al and Ti/Al/Ni/Au ohmic contacts on n-GaN.
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
the specific contact resistivity values drop to their minimum levels of 1:2 105 and 1:3 105 X cm2 , respectively, both achieved after the 1100 °C anneal. The effect of the Ni/Au overlayer on the Ti/Al system was to shift the ohmic contact formation temperatures upward by approximately 200 °C. These plots show that the Ti/Al reaches its optimum value after the 950 °C, 2 min anneal, whereas the Ti/Al/Ni/Au requires a 1100 °C anneal in order to reach the same minimum qc level. Similar contact resistivity vs. anneal temperature plots are shown in Fig. 2 for the Cr/Al and Cr/Al/Ni/Au metallizations. The as-deposited contacts are ohmic with initial qc values of 2:3 104 X cm2 for both metal systems. The behavior of both the Cr/Al and Cr/Al/Ni/ Au metallizations is very similar up to 800 °C. Initially, the qc values drop after anneals of 500, 600 and 700 °C, and then increase as a result of the 800 °C anneals. However, beyond 800 °C the values of the Cr/Al/Ni/Au drop again to a minimum value of 2:3 105 X cm2 , and remain relatively stable up to 1200 °C. The Cr/Al reaches a minimum of 3:8 105 X cm2 at 700 °C but then exhibits a drastic increase in specific contact resistivity after the 800 and 900 °C anneals. Another small drop to a second minimum value of 1:1 103 X cm2 , occurs at 1000 °C, and then degrades again at anneal temperatures beyond 1000 °C. It can be clearly seen that in the temperature range of 950–1200 °C, the contact resistivity values are nearly two orders of magnitude lower for the Cr/Al/Ni/Au compared to those of the Cr/ Al, thus demonstrating the enhancing effect of the Ni/Au overlayer in the case of the Cr/Al ohmic contact. The qc values of the Cr/Al degrade for temperatures above 700 °C, whereas the Cr/Al/Ni/Au contacts remain stable in this temperature range, except for a small increase after the 800 °C anneal. These plots exhibit peaks, at 900 and 800 °C for the Cr/Al and Cr/Al/Ni/Au respectively, where they reach maximum levels before they recover at
Fig. 2. Specific contact resistivity vs. anneal temperature for the Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-GaN.
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higher temperatures. This behavior may be attributed to intermetallic phase formations, such as CrN, with subsequent reactions and penetration of the overlying metals (Al, Ni, or Au). These contact formation temperatures, which in our case are obtained by vacuum annealing, are by approximately 200 °C higher than those reported for oven annealed ohmic contacts to GaN. This difference is due to the fact that in the furnace anneal case, the entire sample, including the surface where the metal/semiconductor reactions take place, is exposed to the same temperature as that of the furnace ambient, whereas in the case of the vacuum annealing, the source of the heat is located on the rear of the sample and thus the sample surface temperature is thought to be considerably lower than that of the strip heater temperature. In the range of 700–1000 °C, the differential in temperature between the strip heater and the sample surface has been estimated to be approximately 160–180 °C, as confirmed by experiments using melting points of various known metals placed on the surface of GaN test samples. The contact formation temperatures for the four metallization systems were selected on the basis of the plots of Figs. 1 and 2. For the Ti/Al and Ti/Al/Ni/Au, 1100 °C was chosen as the contact formation temperature because the minima in the contact resistivity plots occur at this temperature for both metallization systems. For the Cr/Al/Ni/Au the contact formation temperature chosen was 950 °C. However, for the Cr/Al metallization, the minimum in contact resistivity occurs at 700 °C which corresponds to an oven temperature of approximately 520 °C. Since this formation temperature is very close to one of the temperatures chosen for this aging study (500 °C), we expected a rapid degradation of the Cr/Al contacts at the 500 °C aging temperature. For this reason it was decided that, for a more thermally stable Cr/Al contact, and to be more consistent with the annealing temperatures of the other three metallizations, selection of the second minimum at 1000 °C would be a more meaningful choice. It may be noted here that, the two different sets of samples used, were annealed at different times, and since the first set was annealed cumulatively, the values of the contact resistivity for the as formed contacts, may differ slightly between the two sets of samples. The results of the thermal stability of the Ti/Al, Ti/Al/ Ni/Au, Cr/Al, and Cr/Al/Ni/Au as a result of the 300 °C aging is shown in Fig. 3. A common trend can be observed for all four metallizations which seems to cause the contact resistivity to increase somewhat after the initial 0.5 h aging exposure. The contacts exhibit a gradual decrease in qc after the 1, 10, and 50 h aging. Subsequently, they exhibit a steeper degradation as a result of the 100 h aging. At 300 °C, both the Ti/Al and the Cr/Al/Ni/Au maintain relatively good stability with qc values between 2–4 105 X cm2 across the entire
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Fig. 3. Specific contact resistivity as a function of aging time at 300 °C.
aging time range. The Cr/Al which shows a steady drop to a value of 2:6 104 X cm2 when aged at 1, 10 and 50 h, and then abruptly increases after the 100 h aging. The stability of the contacts at the 400 °C aging temperature is shown by the plots of Fig. 4. Again the Ti/Al and Cr/Al/Ni/Au show the best characteristics in terms of electrical performance with the lowest qc values as well as superior thermal stability from 0.5 to 100 h of aging. The Cr/Al showed variations in qc with a noticable drop after the 0.5 h aging step. The Ti/Al/Ni/Au also exhibited a small increase after the initial 0.5 h aging, with a small drop at the 1 h aging time, followed by an incremental change to 12:5 105 X cm2 recorded after the 5, 10 and 100 h aging steps. At this point it may be noted that after the 400 °C, 10 h aging step both the Ti/Al and Cr/Al contacts start to show signs of surface oxidation. Probing of the contact pads became increasingly difficult and erratic with aging above 400 °C. Making appropriate TLM measurements required repeated scrubbing of the metal surface with the tungsten needle probes in order to penetrate the surface oxide. In addition, it was necessary to increase the voltage compliance from 1 to 5 V in order to electrically break
Fig. 4. Specific contact resistivity as a function of aging time at 400 °C.
Fig. 5. Specific contact resistivity as a function of aging time at 500 °C.
through the oxide layer. The metallizations with the Ni/ Au overlay did not encounter this problem of surface oxidation thus confirming the protective nature of the Ni/Au capping layer. The plots shown in Fig. 5 represent the behavior of the specific contact resistivity of the four metallizations as a result of the 500 °C aging. Again the Ti/Al shows the highest stability with the lowest qc values maintained in the vicinity of 2 105 X cm2 . The Cr/Al/Ni/Au, which starts at about the same level as that of the Ti/Al, experiences a gradual increase after the 0.5, 1 and 10 h steps, but fully recovers to its original level after further aging of 50 and 100 h. On the other hand, the Cr/Al and Ti/Al/Ni/Au display a gradual degradation after 0.5, 1, 50 h aging steps, followed by partial recovery after the longer 100 h aging. Fig. 6 shows AFM micrographs of the Ti/Al metallization system after various stages of the aging process. The surface morphology of the as-formed Ti/Al contact is depicted by the AFM scan of Fig. 6a with a recorded mean roughness of Ra ¼ 18:9 nm. There were no observable changes in the morphology up to the 400 °C, 10 h aging. However, after the 400 °C, 100 h aging (Fig. 6b), we observe a slight reduction in the overall surface roughness with a few scattered peaks, of the order of 500 nm, beginning to emerge. This changed morphology, which coincides with the aging step where the probing of the Ti/Al contacts became difficult to perform (400 °C, 100 h) is believed to be attributed to the onset of oxidation of the Al which has the effect of reducing the overall roughness of the surface. Further aging at 500 °C, 10 h (Fig. 6c) causes an increase in the density of the larger peaks. As the density of these peaks increases, as a result of aging, the rest of the surface seems to become smoother, with a net effect of maintaining the overall mean roughness constant in the range of 17.9–22.0 nm. Similar AFM scans of the Ti/Al/Ni/Au system are shown in Fig. 7. The as-formed contact (Fig. 7a) is very rough with a mean roughness of 78.3 nm and with
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
Fig. 6. AFM micrographs of the Ti/Al ohmic contacts after various stages of aging. (a) Ti/Al as formed (before aging), Ra ¼ 18:9 nm. (b) Ti/Al aged at 400 °C, 100 h, Ra ¼ 7:9 nm, (c) Ti/Al aged at 500 °C, 10 h, Ra ¼ 22:0 nm.
pronounced large peaks up to 500 nm in height. After the 400 °C, 100 h aging, where the first morphological changes were observed (see Fig. 7b), with some of the peaks growing in size with the net effect of increasing the mean roughness to Ra ¼ 133:00 nm. Additional aging at 500 °C for 10 h caused a reduction of the surface roughness to Ra ¼ 88:6 nm (see Fig. 7c) with no further
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Fig. 7. AFM micrographs of the Ti/Al/Ni/Au ohmic contacts after various stages of aging. (a) Ti/Al/Ni/Au as formed (before aging), Ra ¼ 78:3 nm, (b) Ti/Al/Ni/Au aged at 400 °C, 100 h, Ra ¼ 133:0 nm, (c) Ti/Al/N/Au aged at 500 °C, 10 h, Ra ¼ 88:6 nm.
surface changes observed as a result of the longer 100 h, 500 °C aging. Morphologically the Cr/Al contact metallization, whose AFM scans are shown in Fig. 8, was the smoothest and most stable of the four metallization systems we investigated. The as-formed ohmic contact had a mean
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N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
Fig. 8. AFM micrographs of the Cr/Al ohmic contacts after various stages of aging. (a) Cr/Al as formed (before aging), Ra ¼ 4:6 nm, (b) Cr/Al aged at 400 °C, 100 h, Ra ¼ 3:3 nm, (c) Cr/Al aged at 500 °C, 10 h, Ra ¼ 9:3 nm.
roughness of Ra ¼ 4:6 nm (Fig. 8a), with a slight improvement in the morphology observed after the 400 °C, 100 h aging (Fig. 8b). A small increase in roughness to Ra ¼ 9:3 nm resulted from the 500 °C, 10 h aging (see Fig. 8c) with no additional changes occurring after the longer 100 h, 500 °C exposure. The evolution of the surface morphology of the Cr/Al/ Ni/Au as a result of long term aging is represented by the AFM scans of Fig. 9. The as-formed contact shown in
Fig. 9. AFM micrographs of the Cr/Al/Ni/Au ohmic contacts after various stages of aging. (a) Cr/Al/Ni/Au as formed (before aging), Ra ¼ 26:4 nm, (b) Cr/Al/Ni/Au aged at 400 °C, 100 h, Ra ¼ 74:6 nm, (c) Cr/Al/Ni/Au aged at 500 °C, 10 h, Ra ¼ 18:5 nm.
Fig 9a has a surface roughness of Ra ¼ 26:4 with a uniform distribution of peaks in the range of 100–200 nm in height. After the 400 °C, 100 h aging step (Fig. 9b) the size of these peaks increased to more than 500 nm
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
thus increasing the overall roughness to 74.6 nm. However, further aging at 500 °C for 10 h, had the effect of reducing the roughness to 18.5 nm, possibly due to oxidation or some solid state diffusion process. It is possible that the observed peaks are conglomerations with a high concentration of one the constituent metal components of the Cr/Al/Ni/Au system, most likely Al. As the contact system is exposed to the higher (500 °C) temperatures for longer periods of time, the accumulated metals, which are present in these peaks, diffuse out into the contact layer thus effectively dissipating the peaks.
4. Conclusions We have investigated the thermal stability of four Albase ohmic contacts to n-GaN as a result of long term aging at 300, 400, and 500 °C. The Ti/Al and Ti/Al/Ni/ Au contact systems achieved contact resistivity values of 1:2 105 and 1:3 105 X cm2 respectively, whereas for the Cr/Al and Cr/Al/Ni/Au contact resistivities of 4:0 105 and 2:8 105 X cm2 were obtained, respectively. When one considers both the electrical performance as well as the surface morphology of the contacts, the Ti/Al system fared the best with stable qc levels in the range of 2–3 105 X cm2 and surfaces with mean roughness (Ra ) in the range of 17.9–25.0 nm. However, the Ti/Al system was the first one to show signs of surface oxidation starting at the 400 °C, 100 h aging step. The Cr/Al/Ni/Au followed closely behind the Ti/Al system with qc values in the range of 2–6 105 X cm2 and a mean roughness in the range of Ra ¼ 18:5–74:6 nm. Even though the Cr/Al displayed the best morphological characteristics, with surface mean roughness in the range between 3.3 and 8.8 nm, it showed the poorest and most unstable electrical performance with qc values in the range of 5:2–16 104 X cm2 .
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Acknowledgements The authors would like to thank Mr. J. Mittereder for his valuable assistance.
References [1] Amano H, Kito M, Hiramatsu K, Akasaki I. P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI). Jpn J Appl Phys 1989;28(12):L2112–4. [2] Nakamura S, Mukai T, Senoh M. Candela-class high brightness InGaN/AlGaN double heterostructure bluelight emitting diodes. Appl Phys Lett 1994;64(13):1687–9. [3] Khan MA, Battarai A, Kuznia JN, Olson D. High electron mobility transistor based on a GaN–Alx Ga1x N heterojunction. Appl Phys Lett 1993;63(9):1214–5. [4] Foressi JS, Moustakas TD. Metal contacts to GaN. Appl Phys Lett 1993;62(22):2859–61. [5] Lin ME, Ma Z, Huang FY, Fan ZF, Allen LH, Morkocß H. Low resistance ohmic contacts on wide band-gap GaN. Appl Phys Lett 1994;64(8):1003–5. [6] Fan Z, Mohammed SN, Kim W, Aktas O, Botchkarev AE, Morkoc H. Very low resistance ohmic contacts to n-GaN. Appl Pys Lett 1996;68(12):1672–4. [7] Papanicolaou NA, Edwards A, Rao MV, Mittereder J, Anderson WT. Cr/Al and Cr/Al/Ni/Au ohmic contacts to n-type GaN. J Appl Phys 2000;87(1):380–6. [8] Cordes H, Chang YA. Interfacial reactions and electrical properties of Ti/n-GaN contacts. MRS Internet. J Nitride Semicond Res 1997;2(2):1–6. [9] Lin ME, Huang FY, Morkoc H. Nonalloyed ohmic contacts on GaN using InN/GaN short-period superlattices. Appl Phys Lett 1994;64(19):2557–9. [10] Papanicolaou NA, Edwards A, Rao MV, Wickenden AE, Koleske DD, Henry RL, et al. A high temperature annealing method for forming ohmic contacts to GaN and SiC, Proceedings of the High Temperature Electronics Conference (HiTEC), Albuquerque, NM, 1998. p. 122–7. [11] Reeves GK, Harrison HB. Obtaining the specific contact resistance from transmission line model measurements. IEEE Electron Dev Lett 1982;EDL-3(5):111–3.
High temperature characteristics of Ti/Al and Cr/Al ohmic contacts to n-type GaN N.A. Papanicolaou b
a,*
, K. Zekentes
b
a Naval Research Laboratory, 4555 Overlook Ave, Washington, DC 20375, USA Foundation for Research and Technology Hellas, P.O. Box 1527, GR-71110 Heraklion/Crete, Greece
Received 2 January 2002; received in revised form 5 February 2002; accepted 27 February 2002
Abstract The thermal stability of the Ti/Al, Ti/Al/Ni/Au, Cr/Al and Cr/Al/Ni/Au ohmic contact metallizations on n-GaN were investigated. Bilayers of Ti/Al and Cr/Al, with and without Ni/Au overlayers, were deposited on n-GaN using an e-beam deposition method and ohmic contacts formed by a vacuum annealing technique. For both the Ti/Al and Ti/Al/ Ni/Au minimum specific contact resistivities of 1 105 X cm2 were achieved after vacuum annealing at 1100 °C for 2 min. Minimum specific resistivities of similarly formed Cr/Al and Cr/Al/Ni/Au ohmic contacts were 3:8 105 and 2:3 105 X cm2 respectively, obtained at 700 and 950 °C respectively. The above mentioned contacts were exposed to long term aging at temperatures of 300, 400 and 500 °C, for periods up to 100 h and their electrical and morphological characteristics were monitored at the various stages of the aging process. The Ti/Al and Cr/Al/Ni/Au proved to be the most stable of the four metallizations in terms of both electrical performance and surface morphology. The Cr/Al had shown stable morphological stability, but displayed the poorest electrical performance characteristics as a result of the aging process. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Semiconductor; GaN; Ohmic contacts; n-type
1. Introduction Owing to their refractory characteristics, the III–V nitrides are very attractive materials for high temperature device applications. In addition, because of their wide band gap and large dielectric constants these materials are suited for blue and UV optoelectronic applications. GaN blue and green LEDs are now commercially available [1,2] and metal-semiconductor-fieldeffect transistors (MESFETs) have been successfully fabricted [3]. However, forming low resistance thermally stable and uniform ohmic contacts to wide band gap semiconductors such as GaN and related materials is
* Corresponding author. Tel.: +1-202-767-4898; fax: +1-202767-0455. E-mail address: [email protected] (N.A. Papanicolaou).
still a challenge. Single metal layer schemes such as Au or Al on n-GaN have been initially reported [4] with specific contact resistivity values in the 103 –104 X cm2 range. Bilayer metallization schemes such as Ti/Al, Ti/ Au have displayed the best contact resistivity results on n-GaN [5,6]. More recently, Cr/Al and Cr/Al/Ni/Au metallization systems on GaN have also been investigated [7] with specific contact resistivities as low as 1:2 105 X cm2 . For these metallization systems, formation of interfacial layers such as TiN or CrN have been suggested as playing a role in the formation of the ohmic contact [5–8]. These interfacial reactions tend to form N vacancies near the GaN surface which act as donors, thus resulting in a highly doped layer at the GaN/metal interface. The resulting high concentration of donors at the GaN/metal interface gives rise to a tunneling current across the barrier thus creating the ohmic contact. Another possible scheme which has been suggested for the formation of ohmic contacts to GaN is
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 2 ) 0 0 1 3 7 - 5
1976
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
the use of an InN intervening superlatice layer epitaxially grown on the GaN before the ohmic metal deposition [9]. Even though specific contact resistivities as low as 5 107 X cm2 have been reported with this approach, the thermal instability of the InN precludes this method from being a practical approach in forming useful ohmic contacts for high temperatures applications. Reproducibility and reliability under extreme (hightemperature) operating conditions are important considerations in the selection of the metal system for ohmic contacts on GaN. In this study, we report on the electrical and morphological investigation of the Ti/Al, Ti/ Al/Ni/Au, Cr/Al, and Cr/Al/Ni/Au ohmic contacts to n-GaN as a result of thermal aging at test temperatures of 300, 400 and 500 °C.
2. Experimental procedure The epitaxial GaN layers used in this study were grown by metal-organic-chemical-vapor-deposition (MOCVD) on sapphire substrates with R-plane orientation. A 200 nm thick undoped semi-insulating GaN buffer layer was initially grown, followed by a 2.0 lm thick, Si-doped n-type GaN active layer. The GaN layers had a net n-type carrier concentration 1:1 1018 cm3 . The TLM test patterns were formed on the samples using a standard photolithographic and lift-off technique. Metal layers consisting of Ti(25 nm)/Al(150 nm), Ti(25 nm)/ Al(80 nm)/Ni(25 nm)Au(100 nm) and Cr(25 nm)/Al(150 nm) and Cr(25 nm)/Al(80 nm)/Ni(25 nm)/Au(100 nm) were deposited on two sets of four discrete GaN samples, using an e-beam deposition method in a cryopumped vacuum system with a base pressure of 1:5 107 Torr. The first set of samples was used for determining the formation temperature for each metallization system, whereas the second set was used for the aging experiments. Prior to the aging experiments ohmic contacts were formed on the first set of samples by a vacuum annealing technique described in Ref. [10]. The annealing of the samples in this part of the experiment was cumulative, in other words each metallization was subjected to the annealing temperature, the contact resistivity measured and then the same sample annealed at the next higher temperature until the desired temperature range was covered. The annealing set-up consisted of a 1 in. wide, 5 in. long, and 0.015 in. thick tungsten strip heater inside a diffusion-pumped bell-jar vacuum system with a liquid N2 trap. The aging experiments which were performed on the second set of samples, were carried out in a conventional horizontal quartz tube furnace in an untreated air ambient. The outlet of the tube was connected to a ventilation system, which created a small negative pressure at the output of the tube thus causing
a flow of air at 35 sscm through the furnace tube. All four metalliizations were aged simultaneously in a series of three isothermal experiments at temperatures of 300, 400 and 500 °C with aging times ranging from 0.5 to 100 h. After each aging step, the samples were electrically characterized by means of specific contact resistivity (qc ) measurements. These measurements were made using test patterns conforming to the linear transmission line model (TLM) as described by Reeves and Harrison [11]. The TLM pattern consists of 200 lm 200 lm contact pads with separations of 5, 10, 25, 40, and 60 lm. Each sample contained six identical TLM test patterns which were measured separately and then averaged to produce a single (qc ) value. A Nomarsky optical microscope was used to monitor apparent changes in the surface morphology and roughness of the samples after each aging step. If significant changes were observed with the optical microscope, we proceeded to perform atomic force microscopy (AFM) measurements for more detailed morphological evaluation.
3. Results and discussion The first part of this study was aimed at determining the optimum annealing conditions for forming the ohmic contacts for the four different metallization systems under investigation, and for this purpose the first set of four samples were used. Fig. 1 shows the specific contact resistivity of the vacuum annealed (2-min isochronal) Ti/Al and Ti/Al/Ni/Au ohmic contacts as a function of anneal temperature. Both metallizations are ohmic as deposited, then become rectifying (non-linear) after annealing at the intermediate temperatures in the range of 500–800 °C. The contacts become ohmic again after the 900 and 950 °C anneals for the Ti/Al and Ti/Al/ Ni/Au, respectively. For the Ti/Al and the Ti/Al/Ni/Au,
Fig. 1. Specific contact resistivity vs. anneal temperature for the Ti/Al and Ti/Al/Ni/Au ohmic contacts on n-GaN.
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
the specific contact resistivity values drop to their minimum levels of 1:2 105 and 1:3 105 X cm2 , respectively, both achieved after the 1100 °C anneal. The effect of the Ni/Au overlayer on the Ti/Al system was to shift the ohmic contact formation temperatures upward by approximately 200 °C. These plots show that the Ti/Al reaches its optimum value after the 950 °C, 2 min anneal, whereas the Ti/Al/Ni/Au requires a 1100 °C anneal in order to reach the same minimum qc level. Similar contact resistivity vs. anneal temperature plots are shown in Fig. 2 for the Cr/Al and Cr/Al/Ni/Au metallizations. The as-deposited contacts are ohmic with initial qc values of 2:3 104 X cm2 for both metal systems. The behavior of both the Cr/Al and Cr/Al/Ni/ Au metallizations is very similar up to 800 °C. Initially, the qc values drop after anneals of 500, 600 and 700 °C, and then increase as a result of the 800 °C anneals. However, beyond 800 °C the values of the Cr/Al/Ni/Au drop again to a minimum value of 2:3 105 X cm2 , and remain relatively stable up to 1200 °C. The Cr/Al reaches a minimum of 3:8 105 X cm2 at 700 °C but then exhibits a drastic increase in specific contact resistivity after the 800 and 900 °C anneals. Another small drop to a second minimum value of 1:1 103 X cm2 , occurs at 1000 °C, and then degrades again at anneal temperatures beyond 1000 °C. It can be clearly seen that in the temperature range of 950–1200 °C, the contact resistivity values are nearly two orders of magnitude lower for the Cr/Al/Ni/Au compared to those of the Cr/ Al, thus demonstrating the enhancing effect of the Ni/Au overlayer in the case of the Cr/Al ohmic contact. The qc values of the Cr/Al degrade for temperatures above 700 °C, whereas the Cr/Al/Ni/Au contacts remain stable in this temperature range, except for a small increase after the 800 °C anneal. These plots exhibit peaks, at 900 and 800 °C for the Cr/Al and Cr/Al/Ni/Au respectively, where they reach maximum levels before they recover at
Fig. 2. Specific contact resistivity vs. anneal temperature for the Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-GaN.
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higher temperatures. This behavior may be attributed to intermetallic phase formations, such as CrN, with subsequent reactions and penetration of the overlying metals (Al, Ni, or Au). These contact formation temperatures, which in our case are obtained by vacuum annealing, are by approximately 200 °C higher than those reported for oven annealed ohmic contacts to GaN. This difference is due to the fact that in the furnace anneal case, the entire sample, including the surface where the metal/semiconductor reactions take place, is exposed to the same temperature as that of the furnace ambient, whereas in the case of the vacuum annealing, the source of the heat is located on the rear of the sample and thus the sample surface temperature is thought to be considerably lower than that of the strip heater temperature. In the range of 700–1000 °C, the differential in temperature between the strip heater and the sample surface has been estimated to be approximately 160–180 °C, as confirmed by experiments using melting points of various known metals placed on the surface of GaN test samples. The contact formation temperatures for the four metallization systems were selected on the basis of the plots of Figs. 1 and 2. For the Ti/Al and Ti/Al/Ni/Au, 1100 °C was chosen as the contact formation temperature because the minima in the contact resistivity plots occur at this temperature for both metallization systems. For the Cr/Al/Ni/Au the contact formation temperature chosen was 950 °C. However, for the Cr/Al metallization, the minimum in contact resistivity occurs at 700 °C which corresponds to an oven temperature of approximately 520 °C. Since this formation temperature is very close to one of the temperatures chosen for this aging study (500 °C), we expected a rapid degradation of the Cr/Al contacts at the 500 °C aging temperature. For this reason it was decided that, for a more thermally stable Cr/Al contact, and to be more consistent with the annealing temperatures of the other three metallizations, selection of the second minimum at 1000 °C would be a more meaningful choice. It may be noted here that, the two different sets of samples used, were annealed at different times, and since the first set was annealed cumulatively, the values of the contact resistivity for the as formed contacts, may differ slightly between the two sets of samples. The results of the thermal stability of the Ti/Al, Ti/Al/ Ni/Au, Cr/Al, and Cr/Al/Ni/Au as a result of the 300 °C aging is shown in Fig. 3. A common trend can be observed for all four metallizations which seems to cause the contact resistivity to increase somewhat after the initial 0.5 h aging exposure. The contacts exhibit a gradual decrease in qc after the 1, 10, and 50 h aging. Subsequently, they exhibit a steeper degradation as a result of the 100 h aging. At 300 °C, both the Ti/Al and the Cr/Al/Ni/Au maintain relatively good stability with qc values between 2–4 105 X cm2 across the entire
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Fig. 3. Specific contact resistivity as a function of aging time at 300 °C.
aging time range. The Cr/Al which shows a steady drop to a value of 2:6 104 X cm2 when aged at 1, 10 and 50 h, and then abruptly increases after the 100 h aging. The stability of the contacts at the 400 °C aging temperature is shown by the plots of Fig. 4. Again the Ti/Al and Cr/Al/Ni/Au show the best characteristics in terms of electrical performance with the lowest qc values as well as superior thermal stability from 0.5 to 100 h of aging. The Cr/Al showed variations in qc with a noticable drop after the 0.5 h aging step. The Ti/Al/Ni/Au also exhibited a small increase after the initial 0.5 h aging, with a small drop at the 1 h aging time, followed by an incremental change to 12:5 105 X cm2 recorded after the 5, 10 and 100 h aging steps. At this point it may be noted that after the 400 °C, 10 h aging step both the Ti/Al and Cr/Al contacts start to show signs of surface oxidation. Probing of the contact pads became increasingly difficult and erratic with aging above 400 °C. Making appropriate TLM measurements required repeated scrubbing of the metal surface with the tungsten needle probes in order to penetrate the surface oxide. In addition, it was necessary to increase the voltage compliance from 1 to 5 V in order to electrically break
Fig. 4. Specific contact resistivity as a function of aging time at 400 °C.
Fig. 5. Specific contact resistivity as a function of aging time at 500 °C.
through the oxide layer. The metallizations with the Ni/ Au overlay did not encounter this problem of surface oxidation thus confirming the protective nature of the Ni/Au capping layer. The plots shown in Fig. 5 represent the behavior of the specific contact resistivity of the four metallizations as a result of the 500 °C aging. Again the Ti/Al shows the highest stability with the lowest qc values maintained in the vicinity of 2 105 X cm2 . The Cr/Al/Ni/Au, which starts at about the same level as that of the Ti/Al, experiences a gradual increase after the 0.5, 1 and 10 h steps, but fully recovers to its original level after further aging of 50 and 100 h. On the other hand, the Cr/Al and Ti/Al/Ni/Au display a gradual degradation after 0.5, 1, 50 h aging steps, followed by partial recovery after the longer 100 h aging. Fig. 6 shows AFM micrographs of the Ti/Al metallization system after various stages of the aging process. The surface morphology of the as-formed Ti/Al contact is depicted by the AFM scan of Fig. 6a with a recorded mean roughness of Ra ¼ 18:9 nm. There were no observable changes in the morphology up to the 400 °C, 10 h aging. However, after the 400 °C, 100 h aging (Fig. 6b), we observe a slight reduction in the overall surface roughness with a few scattered peaks, of the order of 500 nm, beginning to emerge. This changed morphology, which coincides with the aging step where the probing of the Ti/Al contacts became difficult to perform (400 °C, 100 h) is believed to be attributed to the onset of oxidation of the Al which has the effect of reducing the overall roughness of the surface. Further aging at 500 °C, 10 h (Fig. 6c) causes an increase in the density of the larger peaks. As the density of these peaks increases, as a result of aging, the rest of the surface seems to become smoother, with a net effect of maintaining the overall mean roughness constant in the range of 17.9–22.0 nm. Similar AFM scans of the Ti/Al/Ni/Au system are shown in Fig. 7. The as-formed contact (Fig. 7a) is very rough with a mean roughness of 78.3 nm and with
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Fig. 6. AFM micrographs of the Ti/Al ohmic contacts after various stages of aging. (a) Ti/Al as formed (before aging), Ra ¼ 18:9 nm. (b) Ti/Al aged at 400 °C, 100 h, Ra ¼ 7:9 nm, (c) Ti/Al aged at 500 °C, 10 h, Ra ¼ 22:0 nm.
pronounced large peaks up to 500 nm in height. After the 400 °C, 100 h aging, where the first morphological changes were observed (see Fig. 7b), with some of the peaks growing in size with the net effect of increasing the mean roughness to Ra ¼ 133:00 nm. Additional aging at 500 °C for 10 h caused a reduction of the surface roughness to Ra ¼ 88:6 nm (see Fig. 7c) with no further
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Fig. 7. AFM micrographs of the Ti/Al/Ni/Au ohmic contacts after various stages of aging. (a) Ti/Al/Ni/Au as formed (before aging), Ra ¼ 78:3 nm, (b) Ti/Al/Ni/Au aged at 400 °C, 100 h, Ra ¼ 133:0 nm, (c) Ti/Al/N/Au aged at 500 °C, 10 h, Ra ¼ 88:6 nm.
surface changes observed as a result of the longer 100 h, 500 °C aging. Morphologically the Cr/Al contact metallization, whose AFM scans are shown in Fig. 8, was the smoothest and most stable of the four metallization systems we investigated. The as-formed ohmic contact had a mean
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Fig. 8. AFM micrographs of the Cr/Al ohmic contacts after various stages of aging. (a) Cr/Al as formed (before aging), Ra ¼ 4:6 nm, (b) Cr/Al aged at 400 °C, 100 h, Ra ¼ 3:3 nm, (c) Cr/Al aged at 500 °C, 10 h, Ra ¼ 9:3 nm.
roughness of Ra ¼ 4:6 nm (Fig. 8a), with a slight improvement in the morphology observed after the 400 °C, 100 h aging (Fig. 8b). A small increase in roughness to Ra ¼ 9:3 nm resulted from the 500 °C, 10 h aging (see Fig. 8c) with no additional changes occurring after the longer 100 h, 500 °C exposure. The evolution of the surface morphology of the Cr/Al/ Ni/Au as a result of long term aging is represented by the AFM scans of Fig. 9. The as-formed contact shown in
Fig. 9. AFM micrographs of the Cr/Al/Ni/Au ohmic contacts after various stages of aging. (a) Cr/Al/Ni/Au as formed (before aging), Ra ¼ 26:4 nm, (b) Cr/Al/Ni/Au aged at 400 °C, 100 h, Ra ¼ 74:6 nm, (c) Cr/Al/Ni/Au aged at 500 °C, 10 h, Ra ¼ 18:5 nm.
Fig 9a has a surface roughness of Ra ¼ 26:4 with a uniform distribution of peaks in the range of 100–200 nm in height. After the 400 °C, 100 h aging step (Fig. 9b) the size of these peaks increased to more than 500 nm
N.A. Papanicolaou, K. Zekentes / Solid-State Electronics 46 (2002) 1975–1981
thus increasing the overall roughness to 74.6 nm. However, further aging at 500 °C for 10 h, had the effect of reducing the roughness to 18.5 nm, possibly due to oxidation or some solid state diffusion process. It is possible that the observed peaks are conglomerations with a high concentration of one the constituent metal components of the Cr/Al/Ni/Au system, most likely Al. As the contact system is exposed to the higher (500 °C) temperatures for longer periods of time, the accumulated metals, which are present in these peaks, diffuse out into the contact layer thus effectively dissipating the peaks.
4. Conclusions We have investigated the thermal stability of four Albase ohmic contacts to n-GaN as a result of long term aging at 300, 400, and 500 °C. The Ti/Al and Ti/Al/Ni/ Au contact systems achieved contact resistivity values of 1:2 105 and 1:3 105 X cm2 respectively, whereas for the Cr/Al and Cr/Al/Ni/Au contact resistivities of 4:0 105 and 2:8 105 X cm2 were obtained, respectively. When one considers both the electrical performance as well as the surface morphology of the contacts, the Ti/Al system fared the best with stable qc levels in the range of 2–3 105 X cm2 and surfaces with mean roughness (Ra ) in the range of 17.9–25.0 nm. However, the Ti/Al system was the first one to show signs of surface oxidation starting at the 400 °C, 100 h aging step. The Cr/Al/Ni/Au followed closely behind the Ti/Al system with qc values in the range of 2–6 105 X cm2 and a mean roughness in the range of Ra ¼ 18:5–74:6 nm. Even though the Cr/Al displayed the best morphological characteristics, with surface mean roughness in the range between 3.3 and 8.8 nm, it showed the poorest and most unstable electrical performance with qc values in the range of 5:2–16 104 X cm2 .
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Acknowledgements The authors would like to thank Mr. J. Mittereder for his valuable assistance.
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