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p-GaN ohmic contact
Materials Science and Engineering B 128 (2006) 37–43
Temperature dependent diffusion and epitaxial behavior of oxidized Au/Ni/p-GaN ohmic contact C.Y. Hu a,∗ , Z.X. Qin a , Z.X. Feng b , Z.Z. Chen a , Z.B. Ding b , Z.J. Yang a , T.J. Yu a , X.D. Hu a , S.D. Yao b , G.Y. Zhang a a
State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, Research Center for Wide Gap Semiconductor, School of Physics, Peking University, Beijing 100871, PR China b Department of Technical Physics, School of Physics, Peking University, Beijing 100871, PR China Received 3 June 2005; received in revised form 23 October 2005; accepted 5 November 2005
Abstract The temperature dependent diffusion and epitaxial behavior of oxidized Au/Ni/p-GaN ohmic contact were studied with Rutherford backscattering spectroscopy/channeling (RBS/C) and synchrotron X-ray diffraction (XRD). It is found that the Au diffuses to the surface of p-GaN to form an epitaxial structure on p-GaN after annealing at 450 ◦ C. At the same time, the O diffuses to the metal–semiconductor interface and forms NiO. Both of them are suggested to be responsible for the sharp decrease in the specific contact resistance (ρc ) at 450 ◦ C. At 500 ◦ C, the epitaxial structure of Au develops further and the O also diffuses deeper into the interface. As a result, the ρc reaches the lowest value at this temperature. However, when annealing temperature reaches 600 ◦ C, part or all of the interfacial NiO is detached from the p-GaN and diffuses out, which cause the ρc to increase greatly. © 2005 Elsevier B.V. All rights reserved. Keywords: GaN; Ohmic contact; Transmission line method (TLM); Rutherford backscattering spectroscopy (RBS); Synchrotron X-ray diffraction (XRD)
1. Introduction The III–V nitride semiconductors have wide band-gap, good thermal conductivity, high breakdown voltage, and chemical stability. All of these make the material an ideal candidate for use in high-temperature, high-frequency, and high-power electronic devices and short-wavelength optoelectronic devices [1]. In the rapid progress of III–V nitride semiconductor devices, one major concern is the development of low-resistance metal contacts to p-GaN semiconductors to substantially improve the efficiency and reliability of GaN-based electrical and optical devices [2]. Since Ho et al. [3] found that very low resistance ohmic contact to p-GaN could be achieved by oxidization of Ni/Au contact, this contact scheme has been widely adopted [4–7]. To understand the ohmic contact mechanism, groups of people investigated the microstructure of the contact scheme [2,8–11]. However, almost all the investigations of the microstructure focus on the
∗
Corresponding author. Tel.: +86 10 62754247; fax: +86 10 62751615. E-mail address: Chengyu [email protected] (C.Y. Hu).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.11.004
500 ◦ C-annealed samples and the microstructure evolution of the oxidized Ni/Au layers in oxidation process remains unclear yet. The images of the structural evolution are indispensable for understanding the ohmic contact mechanism. Synchrotron X-ray diffraction (XRD) is a very effective tool to study the structure of thin film due to its good monochromaticity and high sensitivity. Rutherford back scattering (RBS) was also widely used to study diffusion behavior of metal films because it can be used to perceive depth distributions of atomic species without modifying the samples under investigation [5,12–14]. On the other hand, RBS is also a very rapid and convenient method to study the structural changes of films under different annealing conditions [12]. Furthermore, compared with transmission electron microscope (TEM), RBS can present a wider area of image of the diffusion behavior of the contact metals. In this work, we focus on the effect of annealing temperature on the microstructure evolution of oxidized Ni/Au/p-GaN ohmic contact. Synchrotron X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS) were used to investigate the epitaxial and diffusion behaviors of the Au/Ni/p-GaN contact structure.
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2. Experimental procedure
2.4. High resolution X-ray diffraction
2.1. Sample preparation
The high resolution X-ray diffraction was carried out in Beijing synchrotron radiation facility (BSRF). A monochromatic ˚ was used as the incident X-ray beam with a wavelength of 1.54 A light. The sample was mounted on a huber five-circle diffractometer and the diffracted beam was detected by a scintillation counter.
The Mg-doped GaN films were grown by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. The thickness of Mg-doped GaN layer was 2.5 m. To activate the Mg atom, these samples were annealed for 20 min at 750 ◦ C in N2 ambient. Hall measurement showed that the hole concentration of p-GaN was 1 × 1017 cm−3 typically. The mesa structures for transmission line method (TLM) [15] were patterned by reactive ion etching (RIE) using BCl3 /Ar. The pads were 200 m × 200 m in size, and the spacings were 7, 12, 17, 22, 33, and 37 m, respectively. Prior to deposition of the metal films, the TLM-patterned samples were dipped into HCl:H2 O (1:1) for 1 min to remove the native surface oxides. The Ni(20 nm)/Au(20 nm) bilayer was deposited by an electron beam evaporation system with the base pressure of 4.3 × 10−8 mbarr. Then the Au/Ni/p-GaN specimens were annealed in a rapid thermal annealing system at temperatures ranged from 350 to 700 ◦ C for 10 min in air ambient. 2.2. Electrical measurement The current–voltage (I–V) characteristics of the contacts were examined between two pads with the spacing of 37 m. The specific contact resistance (ρc ) was measured by TLM method [15]. For the TLM measurement, the total resistance, Rtot , between two adjacent contacts is given by Eq. (1): Rtot (di ) = 2Rc +
Rs di W
(1)
where Rs denotes the sheet resistance of the semiconductor layer outside the contacts, W the width of the contact, Rc the contact resistance and di is the spacing between two adjacent contacts. The total resistance was measured for various spacing and plotted as function of di . The least squares curve fitting method was used to obtain a straight line plot of Rtot versus di data. Rs is determined from the slope of the straight line and Rc from the intercept of the fitted straight line with the resistance (y) axis. Thus, the specific contact resistance, ρc , can be obtained by Eq. (2): ρc =
R2c W 2 Rs
3. Results and discussion 3.1. Electrical properties The samples were annealed at various temperatures from 350 to 700 ◦ C in air. The I–V curve (displayed in Fig. 1) show nonlinear property for the as-deposited samples and 350 ◦ C-annealed samples, indicating that the contacts were rectifying. When the annealing temperature was higher than 350 ◦ C, the I–V curves became linear, implying that an ohmic contact to p-GaN was formed. Then the slopes rose with the increasing annealing temperature and reached the highest value at 500 ◦ C, however, above 500 ◦ C, the slopes of the I–V curves began to decrease. For samples annealed at 700 ◦ C, the I–V curves became nonlinear again. Therefore, it was shown that the contact properties strongly rely on annealing temperature. Fig. 2 shows the dependence of the measured total resistance (Rtot ) on the spacings (d). According to the TLM model [15], ρc can be determined from the intercept and the slope of the plots shown in Fig. 2. The nonlinearity of the data from the asdeposited and 700 ◦ C-annealed sample is due to the rectifying I–V property of the sample (showed in Fig. 1). However, for the purpose of comparison, the ρc still can be calculated for such samples just as lots of researchers have done [16–20]. The other plots in this figure are linear, indicating that ρc determined from this figure is reliable.
(2)
2.3. Rutherford backscattering spectroscopy In our work, a collimated 2.023 MeV He+ beam was used for RBS/channeling mseasurements. The samples were mounted on a high-precision (0.01◦ ) three-axis goniometer in vacuum chamber so that the orientation of the sample relative to the He+ beam could be precisely controlled. The backscattered particles from the target were accepted by an Au–Si surface barrier detector at a 165◦ laboratory angle. The energy resolution was around 15 keV.
Fig. 1. The I–V curves of the Ni(20 nm)/Au(20 nm) contacts on p-GaN alloyed at various temperatures for 10 min under air.
C.Y. Hu et al. / Materials Science and Engineering B 128 (2006) 37–43
Fig. 2. The dependence of Rtot on d for the oxidized Ni(20 nm)/Au(20 nm) contacts to p-GaN.
Table 1 displays the variation of ρc at different annealing temperatures. The ρc is 1.02 × 10−1 and 6.87 × 10−2 cm2 for as-deposited and 350 ◦ C-annealed samples. At 450 ◦ C, the ρc sharply decreases to 1.08 × 10−2 cm2 , which is 1/6 of the value at 350 ◦ C. Then an increase of only 50 ◦ C in annealing temperature makes the ρc decrease further to the lowest value (2.73 × 10−3 cm2 ), which is 1/4 of the value at 450 ◦ C. Above 500 ◦ C, the ρc begins to increase. At 550 ◦ C, the increase is less than 1/2 of the value at 500 ◦ C. Then with an increase of 50 ◦ C, at 600 ◦ C, the ρc increases remarkably to 2.85 × 10−2 cm2 , which is more than 10 times the value at 500 ◦ C. At 700 ◦ C, the ρc reached to the highest value 8.89 × 10−2 cm2 . The third column of the table is the values normalized at 500 ◦ C, which can show it more clearly that the sharp variation in the ρc values occurs at 450, 500 and 600 ◦ C. Accordingly, the corresponding microstructures at these temperatures may be different. 3.2. Diffusion of electrode metals For each of the following RBS spectra, at least 10 measurements have been made. Fig. 3 shows the standard RBS random spectra of Au/Ni/p-GaN samples. The arrows indicate the energy of He+ ion backscattered from corresponding surface atoms. Since the detected elements are inside the samples, their signals Table 1 The variation of ρc with different annealing temperatures for the oxidized Ni/Au contacts to p-GaN Temperature (◦ C)
ρc ( cm2 )
ρc (2.73 × 10−3 cm2 ) (normalized at 500 ◦ C)
As-deposited 350 450 500 550 600 700
1.02 × 10−1 6.87 × 10−2 1.08 × 10−2 2.73 × 10−3 3.8 × 10−3 2.85 × 10−2 8.89 × 10−2
37.36 25.16 3.96 1 1.39 10.44 32.56
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Fig. 3. Rutherford backscattering spectra for the samples with a contact of Au(20 nm)/Ni(20 nm)/p-GaN annealed at 700 ◦ C for 10 min in air.
are at the lower energy side of the corresponding arrows. The heavier the atom is, the higher the energy of the backscattered He+ is. Therefore, the signal on the right side is Au signal. The wide step on the left is the Ga signal. Due to the smaller atomic number than Ga, the Ni signal piles up on the Ga signal. The N and O signals also pile up on the Ga signal on the lower energy side. However, their backscattering signals are weak due to their small differential scattering cross section. To investigate the interdiffusion of the metals in detail, the partial RBS spectra of all samples are displayed in Fig. 4. Fig. 4a shows the shift of Au signals with different annealing temperatures. The arrow labeled Au denotes the energy of He+ backscattered from atoms on the surface. Therefore, Au is on the surface for the as-deposited sample. After 350 ◦ C-annealing Au signals begin to move to the lower energy side, denoting that the interdiffusion of Au begins. Ziegler et al. [21] investigated Au(100 nm)/Ni(100 nm)/SiO2/Si structure with RBS [21]. They found that the Au completely penetrated the individual Ni film in 10 min at 350 ◦ C, but at lower temperatures there was no detectable penetration. It is worth noting that the Au signals cease to move towards lower energy side and stabilize at the same energy for samples annealed above 450 ◦ C. Such phenomenon indicates that Au has diffused to the surface of GaN during 450 ◦ C annealing. As a result, Au almost does not diffuse into the sample further at higher annealing temperature. Thus the interdiffusion of Au to the p-GaN surface implies the layer reversal of Au/Ni layers [5] for samples annealed above 450 ◦ C. The widened Au signals can also indicate the diffusion of Au [5]. However, the Au signals are almost at the same height for the samples annealed at 450, 500 and 550 ◦ C, and has an obvious decrease for samples annealed at 600 and 700 ◦ C. According to the basic theory of RBS, Ottaviani et al. [23] have shown that island shape Ag will cause wider and lower signals than uniform Ag film, which contain the same number of atoms with the island shape Ag [23]. Therefore, lower Au signals mean the formation of more Au islands. It is suggested that the most significant conversion occurs at 600 ◦ C. Although the formation of Au islands has been found for Au/Ni/p-GaN sam-
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C.Y. Hu et al. / Materials Science and Engineering B 128 (2006) 37–43
Fig. 4. Partially magnified RBS spectrum for Au(20 nm)/Ni(20 nm)/p-GaN annealed at different temperature in air: (a) the signals of Au in random spectra; (b) the signals of Ni in random spectra; (c) the signals of O in aligned spectra.
ples annealed at 500 ◦ C [3,8], continuous Au layers also have been found on the GaN surface for such samples by transmission electron microscopy (TEM) [4,11]. Fig. 4a also shows that the height of the Au signals only decreases a little for samples annealed at 500 ◦ C. Thus it can be implied that the Au in the 500 ◦ C-annealed samples is mainly in the form of platelets on the surface of p-GaN. However, higher annealing temperature, 600 ◦ C, forms vast Au islands and causes a sharp decrease of the Au signals shown in Fig. 4a. Liu et al. [22] also verified the formation of vast Au islands for Au/Ni/p-GaN samples oxidized at 600 ◦ C. There are two possible reasons for such conversion. One is the interdiffsion of Ni and Au during the oxidization process [8,22]. The grain boundaries of the as-deposited Au films
may serve as diffusion channels for outdiffusion of Ni atoms and indiffusion of O atoms. The condition tends to break the thin Au film during NiO formation. The other is the tendency of lowing down the surface energy of the whole contact scheme [22]. Fig. 4b shows the shift of Ni signals at different annealing temperatures. An obvious outdiffusion of Ni can be found in this figure. Especially for the as-deposited sample, a small shoulder formed by the Ga signal can be found at the high energy side of Ni signal. However, 350 ◦ C-annealing makes the shoulder vanish due to the outdiffusion of Ni. Therefore, it can be concluded that Ni tend to outdiffuse easily, even at such a low annealing temperature as 350 ◦ C. It results from the stronger affinity of Ni to O than that of Au to O. However, when the 350 ◦ C-annealed
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3.3. Evolution of epitaxial structures of electrode metals Fig. 6(a) and (b)shows the synchrotron XRD profiles of Au/Ni/p-GaN for samples annealed at different temperatures. The samples were detected along the surface normal direction of GaN in ω scan mode and ω–2θ scan mode. Glancing angle XRD (GAXRD) scan was also performed. This is the first investigation of the epitaxial evolution of Au on p-GaN, since other investigation of the epitaxial structures only focused on the samples annealed at 500 ◦ C [2,8]. For as-deposited and 350 ◦ C-annealed samples, as shown in Fig. 6a, Au peaks are absent from the ω scan profiles, indicating that Au has not formed an epitaxial structure on the GaN and is still in a polycrystalline structure [8]. However, when annealing temperature get to 450 ◦ C, Au peaks begin to appear in the ω scan profiles. Then the Au peaks become stronger and stronger with increasing annealing temperature. At the same Fig. 5. Random (star) and <0 0 0 1> aligned (circle) spectrum of Ni(20 nm)/Au(20 nm) sample annealed at 450 ◦ C in air for 10 min.
sample is observed under a microscope, the color of the sample is still mainly gold, indicating that most of the Ni has not diffused to the surface and that O must indiffuse into the electrodes to react with Ni. It is well known that the backscattering yield of O signals is very low due to its small atomic number [12], so it is difficult to study the diffusion behavior of O element in the oxidized Ni/Au electrodes with random spectra of RBS. However the O signals may be obvious in the channeling measurement due to the low backscattering yield of Ga (showed in Fig. 5). For the first time, we investigated the influence of annealing temperatures on the O signals with channeling measurement. Fig. 4c shows the O signals in samples annealed at different temperatures obtained by channeling measurement. It is found that O signals move towards lower energy side with increasing annealing temperature from 350 to 500 ◦ C, indicating that O element keeps diffusing into the samples to react with Ni. Therefore, it can be concluded that during outdiffusion of Ni there is still Ni left in the interface when annealing temperature is below 500 ◦ C. As a result, O keeps diffusing into the contact metal. However, when annealing temperature is higher than 500 ◦ C, O signals begin to move towards higher energy side by 15 keV (obtained from Fig. 4c), which is within the energy resolution of the RBS system. A possible explanation is that all of the Ni has been oxidized above 500 ◦ C. Therefore O element stops to diffuse into the metal film. As a result, it becomes evident that the outdiffusion of O element with interfacial NiO. Chen et al. [9] found that the Ni/Au contacts heat treated at 600 ◦ C had larger and more voids compared with such samples annealed at 500 ◦ C. The voids may be formed by the disassociated nitrogen from interfacial reactions. Higher annealing temperature may release more N2 to enlarge the voids or form new voids. These voids separate the interfacial NiO from p-type GaN. As a result, the detached NiO tend to diffuse to the surface, which can be verified by the degraded epitaxial structure of NiO for samples annealed above 550 ◦ C presented in the following.
Fig. 6. Microstructure evolution of Ni/Au contact to p-GaN annealed at different temperatures for 10 min in air ambient: (a) ω scan of Au(1 1 1) along the surface normal direction, (b) ω–2θ scan and GAXRD scan of the sample annealed at 600 ◦ C. The inset of (b) are the ω scan of NiO(1 1 1) along the surface normal direction.
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time, GAXRD scan and ω–2θ XRD scan were performed for the samples annealed at 600 ◦ C, as shown in Fig. 6b. Since only Au(1 1 1) and Au(2 2 2) peaks appear in the ω–2θ XRD spectrum and no Au(1 1 1) or Au(2 2 2) signals are shown in the GAXRD spectrum, we conclude that Au forms an epitaxial structure on p-GaN with a direction relationship of Au(1 1 1)//GaN(0 0 0 2). Recently, Omiya et al. [11] directly confirmed the epitaxial structure of Au on p-GaN by analyzing TEM images of annealed Au/Ni/p-GaN contacts [11]. On the contrary, Fig. 6b does not show such an obvious evidence for the formation of epitaxial NiO on GaN since the NiO(1 1 1) and NiO(2 2 2) peaks are very weak in the the ω–2θ XRD spectrum. However, as shown in the inset of Fig. 6b, obvious NiO(1 1 1) peaks were detected in the ω scan mode, indicating that part of NiO still formed crystalline on p-GaN with a direction relationship of NiO(1 1 1)//GaN(0 0 0 2). With nanobeam diffraction patterns, Chen et al. [8] also have identified the orientation relationship of the crystalline NiO and p-GaN film as NiO(1 1 1)//GaN(0 0 0 2) [8]. Therefore, it is suggested that NiO forms an orientation preferred structure on pGaN for annealed samples. However, the orientation preferred structure of NiO degraded with increasing annealing temperature from 550 to 700 ◦ C, which is shown by the decreased intensity of NiO(1 1 1) signals in the inset of Fig. 6b. Such a temperature dependent epitaxial structure of Au and orientation preferred NiO on p-GaN are presented for the first time. 4. Discussions As shown above, in the diffusion behavior of the electrode metals, Au diffuses to the interface between metals and GaN when annealing temperature increases to 450 ◦ C. Contacting pGaN is an essential premise for Au to form an epitaxial structure on p-GaN. Accordingly, Au peaks appear in the XRD ω scan profiles of samples annealed at 450 ◦ C, indicating that Au forms an epitaxial structure on GaN with a direction relationship of Au(1 1 1)//GaN(0 0 0 2) at 450 ◦ C, which is a beneficial structure for formation of ohmic contact [5,24,25]. Therefore, it is suggested that the sharp decrease of ρc at 450 ◦ C is related to the formation of epitaxial structure of Au on p-GaN. At 500 ◦ C, Au peaks in the ω scan profile become stronger and O diffuses into the interface further at the same time. Consequently, the decrease of ρc at 500 ◦ C may be due to a dual effect of improved epitaxial structure of Au [11] and strengthened oxidation of Ni. When annealing temperature increases to 600 ◦ C, Au peaks in the ω scan profile still become stronger. On the contrary, a sharp increase of ρc occurs at this temperature. One possible reason is the outdiffusion of interfacial NiO as shown in Fig. 4c. Chen co-workers [2] once suggested the beneficial effect of the low contact barrier of NiO. However, Yu et al. [26] provided a corrected energy band graph to oppose such a viewpoint. The other possible reason is that voids are formed between electrode metals and p-GaN, which decreases the contact area of electrode metals and p-GaN. Chen et al. [9] once used TEM to find such voids in the metal–semiconductor interface of Au(5 nm)/Ni(5 nm)/p-GaN samples annealed at 600 ◦ C. Both the outdiffusion of interfacial NiO and the decreased contact area will cause the orientation preferred structure of NiO to
degrade, as shown in the inset of Fig. 6b. At 700 ◦ C, the evolution of the microstructure is similar to the samples annealed at 600 ◦ C. Consequently, ρc keeps increasing at 700 ◦ C. 5. Conclusions The microstructure evolution of Au(20 nm)/Ni(20 nm)/pGaN ohmic contact is investigated with RBS/C and synchrotron XRD for samples oxidized at temperature range from 350 to 700 ◦ C. Referring to the variation of the specific contact resistance at 450, 500 and 600 ◦ C, It is suggested that at 450 and 500 ◦ C the formation of epitaxial Au on p-GaN and the formation of NiO have a critical influence on forming ohmic contact. However, when annealing temperature is higher (>550 ◦ C), part of NiO is detached from the surface of p-GaN and diffuses out due to the formation of more voids in the interface. As a result, the electrical property of the contact degrades significantly. Acknowledgments This work was supported by the National Nature Science Foundation of China (60376005, 60476028, 60477011, 60406007, 60276010) and the National High Technology Program of China (863-2002AA31120Z, 2001AA313060 and 2001AA313140). A part of the experiments was carried out at Beijing Synchrotron Radiation Facility (BSRF) of Institute of High Energy, Chinese Academy of Science. References [1] E.T. Yu, M.O. Manasreh, III–V Nitride Semiconductors: Applications and Devices, Taylor & Francis, New York, 2003, p. 2. [2] J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, K.-K. Shih, L.-C. Chen, F.-R. Chen, J.-J. Kai, J. Appl. Phys. 86 (1999) 4491–4497. [3] J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, C.-Y. Chen, K.-K. Shih, Appl. Phys. Lett. 76 (1999) 1275–1277. [4] Y. Koide, T. Maeda, T. Kawakami, S. Fujita, T. Uemura, N. Shibata, M. Murakami, J. Electron. Mater. 28 (1999) 341–346. [5] D. Qiao, L.S. Yu, S.S. Lau, J.Y. Lin, H.X. Jiang, T.E. Haynes, J. Appl. Phys. 88 (2000) 4196–4200. [6] S.H. Wang, S.E. Mohney, R. Birkhahn, J. Appl. Phys. 96 (2003) 3711–3716. [7] Z.X. Qin, Z.Z. Chen, H.X. Zhang, X.M. Ding, X.D. Hu, D.J. Yu, G.Y. Zhang, Appl. Phys. A 78 (2004) 729–731. [8] L.-C. Chen, F.-R. Chen, J.-J. Kai, L. Chang, J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, C.-Y. Chen, K.-K. Shih, J. Appl. Phys. 86 (1999) 3826–3832. [9] L.-C. Chen, J.-K. Ho, F.-R. Chen, J.-J. Kai, L. Chang, C.-S. Jong, C.C. Chiu, C.-N. Huang, K.-K. Shih, Phys. Stat. Sol (a) 176 (1999) 773–777. [10] A.V. Davydov, L.A. Bendersky, W.J. Boettinger, Appl. Surf. Sci. 223 (2004) 24–29. [11] H. Omiya, F.A. Ponce, H. Marui, S. Tanaka, T. Mukai, Appl. Phys. Lett. 85 (2004) 6143–6145. [12] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978, p. 12. [13] G. Stareev, Appl. Phys. Lett. 62 (1993) 2801–2803. [14] J.E.E. Baglin, M.H. Tabacniks, A.J. Kellock, Nucl. Instrum. Meth. B 137 (1998) 241–246. [15] H.H. Berger, Solid-State Electron. 15 (1972) 145. [16] M.E. Lin, Z. Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, H. Morkoc¸, Appl. Phys. Lett. 64 (1994) 1003–1005.
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Temperature dependent diffusion and epitaxial behavior of oxidized Au/Ni/p-GaN ohmic contact C.Y. Hu a,∗ , Z.X. Qin a , Z.X. Feng b , Z.Z. Chen a , Z.B. Ding b , Z.J. Yang a , T.J. Yu a , X.D. Hu a , S.D. Yao b , G.Y. Zhang a a
State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, Research Center for Wide Gap Semiconductor, School of Physics, Peking University, Beijing 100871, PR China b Department of Technical Physics, School of Physics, Peking University, Beijing 100871, PR China Received 3 June 2005; received in revised form 23 October 2005; accepted 5 November 2005
Abstract The temperature dependent diffusion and epitaxial behavior of oxidized Au/Ni/p-GaN ohmic contact were studied with Rutherford backscattering spectroscopy/channeling (RBS/C) and synchrotron X-ray diffraction (XRD). It is found that the Au diffuses to the surface of p-GaN to form an epitaxial structure on p-GaN after annealing at 450 ◦ C. At the same time, the O diffuses to the metal–semiconductor interface and forms NiO. Both of them are suggested to be responsible for the sharp decrease in the specific contact resistance (ρc ) at 450 ◦ C. At 500 ◦ C, the epitaxial structure of Au develops further and the O also diffuses deeper into the interface. As a result, the ρc reaches the lowest value at this temperature. However, when annealing temperature reaches 600 ◦ C, part or all of the interfacial NiO is detached from the p-GaN and diffuses out, which cause the ρc to increase greatly. © 2005 Elsevier B.V. All rights reserved. Keywords: GaN; Ohmic contact; Transmission line method (TLM); Rutherford backscattering spectroscopy (RBS); Synchrotron X-ray diffraction (XRD)
1. Introduction The III–V nitride semiconductors have wide band-gap, good thermal conductivity, high breakdown voltage, and chemical stability. All of these make the material an ideal candidate for use in high-temperature, high-frequency, and high-power electronic devices and short-wavelength optoelectronic devices [1]. In the rapid progress of III–V nitride semiconductor devices, one major concern is the development of low-resistance metal contacts to p-GaN semiconductors to substantially improve the efficiency and reliability of GaN-based electrical and optical devices [2]. Since Ho et al. [3] found that very low resistance ohmic contact to p-GaN could be achieved by oxidization of Ni/Au contact, this contact scheme has been widely adopted [4–7]. To understand the ohmic contact mechanism, groups of people investigated the microstructure of the contact scheme [2,8–11]. However, almost all the investigations of the microstructure focus on the
∗
Corresponding author. Tel.: +86 10 62754247; fax: +86 10 62751615. E-mail address: Chengyu [email protected] (C.Y. Hu).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.11.004
500 ◦ C-annealed samples and the microstructure evolution of the oxidized Ni/Au layers in oxidation process remains unclear yet. The images of the structural evolution are indispensable for understanding the ohmic contact mechanism. Synchrotron X-ray diffraction (XRD) is a very effective tool to study the structure of thin film due to its good monochromaticity and high sensitivity. Rutherford back scattering (RBS) was also widely used to study diffusion behavior of metal films because it can be used to perceive depth distributions of atomic species without modifying the samples under investigation [5,12–14]. On the other hand, RBS is also a very rapid and convenient method to study the structural changes of films under different annealing conditions [12]. Furthermore, compared with transmission electron microscope (TEM), RBS can present a wider area of image of the diffusion behavior of the contact metals. In this work, we focus on the effect of annealing temperature on the microstructure evolution of oxidized Ni/Au/p-GaN ohmic contact. Synchrotron X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS) were used to investigate the epitaxial and diffusion behaviors of the Au/Ni/p-GaN contact structure.
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2. Experimental procedure
2.4. High resolution X-ray diffraction
2.1. Sample preparation
The high resolution X-ray diffraction was carried out in Beijing synchrotron radiation facility (BSRF). A monochromatic ˚ was used as the incident X-ray beam with a wavelength of 1.54 A light. The sample was mounted on a huber five-circle diffractometer and the diffracted beam was detected by a scintillation counter.
The Mg-doped GaN films were grown by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. The thickness of Mg-doped GaN layer was 2.5 m. To activate the Mg atom, these samples were annealed for 20 min at 750 ◦ C in N2 ambient. Hall measurement showed that the hole concentration of p-GaN was 1 × 1017 cm−3 typically. The mesa structures for transmission line method (TLM) [15] were patterned by reactive ion etching (RIE) using BCl3 /Ar. The pads were 200 m × 200 m in size, and the spacings were 7, 12, 17, 22, 33, and 37 m, respectively. Prior to deposition of the metal films, the TLM-patterned samples were dipped into HCl:H2 O (1:1) for 1 min to remove the native surface oxides. The Ni(20 nm)/Au(20 nm) bilayer was deposited by an electron beam evaporation system with the base pressure of 4.3 × 10−8 mbarr. Then the Au/Ni/p-GaN specimens were annealed in a rapid thermal annealing system at temperatures ranged from 350 to 700 ◦ C for 10 min in air ambient. 2.2. Electrical measurement The current–voltage (I–V) characteristics of the contacts were examined between two pads with the spacing of 37 m. The specific contact resistance (ρc ) was measured by TLM method [15]. For the TLM measurement, the total resistance, Rtot , between two adjacent contacts is given by Eq. (1): Rtot (di ) = 2Rc +
Rs di W
(1)
where Rs denotes the sheet resistance of the semiconductor layer outside the contacts, W the width of the contact, Rc the contact resistance and di is the spacing between two adjacent contacts. The total resistance was measured for various spacing and plotted as function of di . The least squares curve fitting method was used to obtain a straight line plot of Rtot versus di data. Rs is determined from the slope of the straight line and Rc from the intercept of the fitted straight line with the resistance (y) axis. Thus, the specific contact resistance, ρc , can be obtained by Eq. (2): ρc =
R2c W 2 Rs
3. Results and discussion 3.1. Electrical properties The samples were annealed at various temperatures from 350 to 700 ◦ C in air. The I–V curve (displayed in Fig. 1) show nonlinear property for the as-deposited samples and 350 ◦ C-annealed samples, indicating that the contacts were rectifying. When the annealing temperature was higher than 350 ◦ C, the I–V curves became linear, implying that an ohmic contact to p-GaN was formed. Then the slopes rose with the increasing annealing temperature and reached the highest value at 500 ◦ C, however, above 500 ◦ C, the slopes of the I–V curves began to decrease. For samples annealed at 700 ◦ C, the I–V curves became nonlinear again. Therefore, it was shown that the contact properties strongly rely on annealing temperature. Fig. 2 shows the dependence of the measured total resistance (Rtot ) on the spacings (d). According to the TLM model [15], ρc can be determined from the intercept and the slope of the plots shown in Fig. 2. The nonlinearity of the data from the asdeposited and 700 ◦ C-annealed sample is due to the rectifying I–V property of the sample (showed in Fig. 1). However, for the purpose of comparison, the ρc still can be calculated for such samples just as lots of researchers have done [16–20]. The other plots in this figure are linear, indicating that ρc determined from this figure is reliable.
(2)
2.3. Rutherford backscattering spectroscopy In our work, a collimated 2.023 MeV He+ beam was used for RBS/channeling mseasurements. The samples were mounted on a high-precision (0.01◦ ) three-axis goniometer in vacuum chamber so that the orientation of the sample relative to the He+ beam could be precisely controlled. The backscattered particles from the target were accepted by an Au–Si surface barrier detector at a 165◦ laboratory angle. The energy resolution was around 15 keV.
Fig. 1. The I–V curves of the Ni(20 nm)/Au(20 nm) contacts on p-GaN alloyed at various temperatures for 10 min under air.
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Fig. 2. The dependence of Rtot on d for the oxidized Ni(20 nm)/Au(20 nm) contacts to p-GaN.
Table 1 displays the variation of ρc at different annealing temperatures. The ρc is 1.02 × 10−1 and 6.87 × 10−2 cm2 for as-deposited and 350 ◦ C-annealed samples. At 450 ◦ C, the ρc sharply decreases to 1.08 × 10−2 cm2 , which is 1/6 of the value at 350 ◦ C. Then an increase of only 50 ◦ C in annealing temperature makes the ρc decrease further to the lowest value (2.73 × 10−3 cm2 ), which is 1/4 of the value at 450 ◦ C. Above 500 ◦ C, the ρc begins to increase. At 550 ◦ C, the increase is less than 1/2 of the value at 500 ◦ C. Then with an increase of 50 ◦ C, at 600 ◦ C, the ρc increases remarkably to 2.85 × 10−2 cm2 , which is more than 10 times the value at 500 ◦ C. At 700 ◦ C, the ρc reached to the highest value 8.89 × 10−2 cm2 . The third column of the table is the values normalized at 500 ◦ C, which can show it more clearly that the sharp variation in the ρc values occurs at 450, 500 and 600 ◦ C. Accordingly, the corresponding microstructures at these temperatures may be different. 3.2. Diffusion of electrode metals For each of the following RBS spectra, at least 10 measurements have been made. Fig. 3 shows the standard RBS random spectra of Au/Ni/p-GaN samples. The arrows indicate the energy of He+ ion backscattered from corresponding surface atoms. Since the detected elements are inside the samples, their signals Table 1 The variation of ρc with different annealing temperatures for the oxidized Ni/Au contacts to p-GaN Temperature (◦ C)
ρc ( cm2 )
ρc (2.73 × 10−3 cm2 ) (normalized at 500 ◦ C)
As-deposited 350 450 500 550 600 700
1.02 × 10−1 6.87 × 10−2 1.08 × 10−2 2.73 × 10−3 3.8 × 10−3 2.85 × 10−2 8.89 × 10−2
37.36 25.16 3.96 1 1.39 10.44 32.56
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Fig. 3. Rutherford backscattering spectra for the samples with a contact of Au(20 nm)/Ni(20 nm)/p-GaN annealed at 700 ◦ C for 10 min in air.
are at the lower energy side of the corresponding arrows. The heavier the atom is, the higher the energy of the backscattered He+ is. Therefore, the signal on the right side is Au signal. The wide step on the left is the Ga signal. Due to the smaller atomic number than Ga, the Ni signal piles up on the Ga signal. The N and O signals also pile up on the Ga signal on the lower energy side. However, their backscattering signals are weak due to their small differential scattering cross section. To investigate the interdiffusion of the metals in detail, the partial RBS spectra of all samples are displayed in Fig. 4. Fig. 4a shows the shift of Au signals with different annealing temperatures. The arrow labeled Au denotes the energy of He+ backscattered from atoms on the surface. Therefore, Au is on the surface for the as-deposited sample. After 350 ◦ C-annealing Au signals begin to move to the lower energy side, denoting that the interdiffusion of Au begins. Ziegler et al. [21] investigated Au(100 nm)/Ni(100 nm)/SiO2/Si structure with RBS [21]. They found that the Au completely penetrated the individual Ni film in 10 min at 350 ◦ C, but at lower temperatures there was no detectable penetration. It is worth noting that the Au signals cease to move towards lower energy side and stabilize at the same energy for samples annealed above 450 ◦ C. Such phenomenon indicates that Au has diffused to the surface of GaN during 450 ◦ C annealing. As a result, Au almost does not diffuse into the sample further at higher annealing temperature. Thus the interdiffusion of Au to the p-GaN surface implies the layer reversal of Au/Ni layers [5] for samples annealed above 450 ◦ C. The widened Au signals can also indicate the diffusion of Au [5]. However, the Au signals are almost at the same height for the samples annealed at 450, 500 and 550 ◦ C, and has an obvious decrease for samples annealed at 600 and 700 ◦ C. According to the basic theory of RBS, Ottaviani et al. [23] have shown that island shape Ag will cause wider and lower signals than uniform Ag film, which contain the same number of atoms with the island shape Ag [23]. Therefore, lower Au signals mean the formation of more Au islands. It is suggested that the most significant conversion occurs at 600 ◦ C. Although the formation of Au islands has been found for Au/Ni/p-GaN sam-
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Fig. 4. Partially magnified RBS spectrum for Au(20 nm)/Ni(20 nm)/p-GaN annealed at different temperature in air: (a) the signals of Au in random spectra; (b) the signals of Ni in random spectra; (c) the signals of O in aligned spectra.
ples annealed at 500 ◦ C [3,8], continuous Au layers also have been found on the GaN surface for such samples by transmission electron microscopy (TEM) [4,11]. Fig. 4a also shows that the height of the Au signals only decreases a little for samples annealed at 500 ◦ C. Thus it can be implied that the Au in the 500 ◦ C-annealed samples is mainly in the form of platelets on the surface of p-GaN. However, higher annealing temperature, 600 ◦ C, forms vast Au islands and causes a sharp decrease of the Au signals shown in Fig. 4a. Liu et al. [22] also verified the formation of vast Au islands for Au/Ni/p-GaN samples oxidized at 600 ◦ C. There are two possible reasons for such conversion. One is the interdiffsion of Ni and Au during the oxidization process [8,22]. The grain boundaries of the as-deposited Au films
may serve as diffusion channels for outdiffusion of Ni atoms and indiffusion of O atoms. The condition tends to break the thin Au film during NiO formation. The other is the tendency of lowing down the surface energy of the whole contact scheme [22]. Fig. 4b shows the shift of Ni signals at different annealing temperatures. An obvious outdiffusion of Ni can be found in this figure. Especially for the as-deposited sample, a small shoulder formed by the Ga signal can be found at the high energy side of Ni signal. However, 350 ◦ C-annealing makes the shoulder vanish due to the outdiffusion of Ni. Therefore, it can be concluded that Ni tend to outdiffuse easily, even at such a low annealing temperature as 350 ◦ C. It results from the stronger affinity of Ni to O than that of Au to O. However, when the 350 ◦ C-annealed
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3.3. Evolution of epitaxial structures of electrode metals Fig. 6(a) and (b)shows the synchrotron XRD profiles of Au/Ni/p-GaN for samples annealed at different temperatures. The samples were detected along the surface normal direction of GaN in ω scan mode and ω–2θ scan mode. Glancing angle XRD (GAXRD) scan was also performed. This is the first investigation of the epitaxial evolution of Au on p-GaN, since other investigation of the epitaxial structures only focused on the samples annealed at 500 ◦ C [2,8]. For as-deposited and 350 ◦ C-annealed samples, as shown in Fig. 6a, Au peaks are absent from the ω scan profiles, indicating that Au has not formed an epitaxial structure on the GaN and is still in a polycrystalline structure [8]. However, when annealing temperature get to 450 ◦ C, Au peaks begin to appear in the ω scan profiles. Then the Au peaks become stronger and stronger with increasing annealing temperature. At the same Fig. 5. Random (star) and <0 0 0 1> aligned (circle) spectrum of Ni(20 nm)/Au(20 nm) sample annealed at 450 ◦ C in air for 10 min.
sample is observed under a microscope, the color of the sample is still mainly gold, indicating that most of the Ni has not diffused to the surface and that O must indiffuse into the electrodes to react with Ni. It is well known that the backscattering yield of O signals is very low due to its small atomic number [12], so it is difficult to study the diffusion behavior of O element in the oxidized Ni/Au electrodes with random spectra of RBS. However the O signals may be obvious in the channeling measurement due to the low backscattering yield of Ga (showed in Fig. 5). For the first time, we investigated the influence of annealing temperatures on the O signals with channeling measurement. Fig. 4c shows the O signals in samples annealed at different temperatures obtained by channeling measurement. It is found that O signals move towards lower energy side with increasing annealing temperature from 350 to 500 ◦ C, indicating that O element keeps diffusing into the samples to react with Ni. Therefore, it can be concluded that during outdiffusion of Ni there is still Ni left in the interface when annealing temperature is below 500 ◦ C. As a result, O keeps diffusing into the contact metal. However, when annealing temperature is higher than 500 ◦ C, O signals begin to move towards higher energy side by 15 keV (obtained from Fig. 4c), which is within the energy resolution of the RBS system. A possible explanation is that all of the Ni has been oxidized above 500 ◦ C. Therefore O element stops to diffuse into the metal film. As a result, it becomes evident that the outdiffusion of O element with interfacial NiO. Chen et al. [9] found that the Ni/Au contacts heat treated at 600 ◦ C had larger and more voids compared with such samples annealed at 500 ◦ C. The voids may be formed by the disassociated nitrogen from interfacial reactions. Higher annealing temperature may release more N2 to enlarge the voids or form new voids. These voids separate the interfacial NiO from p-type GaN. As a result, the detached NiO tend to diffuse to the surface, which can be verified by the degraded epitaxial structure of NiO for samples annealed above 550 ◦ C presented in the following.
Fig. 6. Microstructure evolution of Ni/Au contact to p-GaN annealed at different temperatures for 10 min in air ambient: (a) ω scan of Au(1 1 1) along the surface normal direction, (b) ω–2θ scan and GAXRD scan of the sample annealed at 600 ◦ C. The inset of (b) are the ω scan of NiO(1 1 1) along the surface normal direction.
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time, GAXRD scan and ω–2θ XRD scan were performed for the samples annealed at 600 ◦ C, as shown in Fig. 6b. Since only Au(1 1 1) and Au(2 2 2) peaks appear in the ω–2θ XRD spectrum and no Au(1 1 1) or Au(2 2 2) signals are shown in the GAXRD spectrum, we conclude that Au forms an epitaxial structure on p-GaN with a direction relationship of Au(1 1 1)//GaN(0 0 0 2). Recently, Omiya et al. [11] directly confirmed the epitaxial structure of Au on p-GaN by analyzing TEM images of annealed Au/Ni/p-GaN contacts [11]. On the contrary, Fig. 6b does not show such an obvious evidence for the formation of epitaxial NiO on GaN since the NiO(1 1 1) and NiO(2 2 2) peaks are very weak in the the ω–2θ XRD spectrum. However, as shown in the inset of Fig. 6b, obvious NiO(1 1 1) peaks were detected in the ω scan mode, indicating that part of NiO still formed crystalline on p-GaN with a direction relationship of NiO(1 1 1)//GaN(0 0 0 2). With nanobeam diffraction patterns, Chen et al. [8] also have identified the orientation relationship of the crystalline NiO and p-GaN film as NiO(1 1 1)//GaN(0 0 0 2) [8]. Therefore, it is suggested that NiO forms an orientation preferred structure on pGaN for annealed samples. However, the orientation preferred structure of NiO degraded with increasing annealing temperature from 550 to 700 ◦ C, which is shown by the decreased intensity of NiO(1 1 1) signals in the inset of Fig. 6b. Such a temperature dependent epitaxial structure of Au and orientation preferred NiO on p-GaN are presented for the first time. 4. Discussions As shown above, in the diffusion behavior of the electrode metals, Au diffuses to the interface between metals and GaN when annealing temperature increases to 450 ◦ C. Contacting pGaN is an essential premise for Au to form an epitaxial structure on p-GaN. Accordingly, Au peaks appear in the XRD ω scan profiles of samples annealed at 450 ◦ C, indicating that Au forms an epitaxial structure on GaN with a direction relationship of Au(1 1 1)//GaN(0 0 0 2) at 450 ◦ C, which is a beneficial structure for formation of ohmic contact [5,24,25]. Therefore, it is suggested that the sharp decrease of ρc at 450 ◦ C is related to the formation of epitaxial structure of Au on p-GaN. At 500 ◦ C, Au peaks in the ω scan profile become stronger and O diffuses into the interface further at the same time. Consequently, the decrease of ρc at 500 ◦ C may be due to a dual effect of improved epitaxial structure of Au [11] and strengthened oxidation of Ni. When annealing temperature increases to 600 ◦ C, Au peaks in the ω scan profile still become stronger. On the contrary, a sharp increase of ρc occurs at this temperature. One possible reason is the outdiffusion of interfacial NiO as shown in Fig. 4c. Chen co-workers [2] once suggested the beneficial effect of the low contact barrier of NiO. However, Yu et al. [26] provided a corrected energy band graph to oppose such a viewpoint. The other possible reason is that voids are formed between electrode metals and p-GaN, which decreases the contact area of electrode metals and p-GaN. Chen et al. [9] once used TEM to find such voids in the metal–semiconductor interface of Au(5 nm)/Ni(5 nm)/p-GaN samples annealed at 600 ◦ C. Both the outdiffusion of interfacial NiO and the decreased contact area will cause the orientation preferred structure of NiO to
degrade, as shown in the inset of Fig. 6b. At 700 ◦ C, the evolution of the microstructure is similar to the samples annealed at 600 ◦ C. Consequently, ρc keeps increasing at 700 ◦ C. 5. Conclusions The microstructure evolution of Au(20 nm)/Ni(20 nm)/pGaN ohmic contact is investigated with RBS/C and synchrotron XRD for samples oxidized at temperature range from 350 to 700 ◦ C. Referring to the variation of the specific contact resistance at 450, 500 and 600 ◦ C, It is suggested that at 450 and 500 ◦ C the formation of epitaxial Au on p-GaN and the formation of NiO have a critical influence on forming ohmic contact. However, when annealing temperature is higher (>550 ◦ C), part of NiO is detached from the surface of p-GaN and diffuses out due to the formation of more voids in the interface. As a result, the electrical property of the contact degrades significantly. Acknowledgments This work was supported by the National Nature Science Foundation of China (60376005, 60476028, 60477011, 60406007, 60276010) and the National High Technology Program of China (863-2002AA31120Z, 2001AA313060 and 2001AA313140). A part of the experiments was carried out at Beijing Synchrotron Radiation Facility (BSRF) of Institute of High Energy, Chinese Academy of Science. References [1] E.T. Yu, M.O. Manasreh, III–V Nitride Semiconductors: Applications and Devices, Taylor & Francis, New York, 2003, p. 2. [2] J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, K.-K. Shih, L.-C. Chen, F.-R. Chen, J.-J. Kai, J. Appl. Phys. 86 (1999) 4491–4497. [3] J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, C.-Y. Chen, K.-K. Shih, Appl. Phys. Lett. 76 (1999) 1275–1277. [4] Y. Koide, T. Maeda, T. Kawakami, S. Fujita, T. Uemura, N. Shibata, M. Murakami, J. Electron. Mater. 28 (1999) 341–346. [5] D. Qiao, L.S. Yu, S.S. Lau, J.Y. Lin, H.X. Jiang, T.E. Haynes, J. Appl. Phys. 88 (2000) 4196–4200. [6] S.H. Wang, S.E. Mohney, R. Birkhahn, J. Appl. Phys. 96 (2003) 3711–3716. [7] Z.X. Qin, Z.Z. Chen, H.X. Zhang, X.M. Ding, X.D. Hu, D.J. Yu, G.Y. Zhang, Appl. Phys. A 78 (2004) 729–731. [8] L.-C. Chen, F.-R. Chen, J.-J. Kai, L. Chang, J.-K. Ho, C.-S. Jong, C.C. Chiu, C.-N. Huang, C.-Y. Chen, K.-K. Shih, J. Appl. Phys. 86 (1999) 3826–3832. [9] L.-C. Chen, J.-K. Ho, F.-R. Chen, J.-J. Kai, L. Chang, C.-S. Jong, C.C. Chiu, C.-N. Huang, K.-K. Shih, Phys. Stat. Sol (a) 176 (1999) 773–777. [10] A.V. Davydov, L.A. Bendersky, W.J. Boettinger, Appl. Surf. Sci. 223 (2004) 24–29. [11] H. Omiya, F.A. Ponce, H. Marui, S. Tanaka, T. Mukai, Appl. Phys. Lett. 85 (2004) 6143–6145. [12] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978, p. 12. [13] G. Stareev, Appl. Phys. Lett. 62 (1993) 2801–2803. [14] J.E.E. Baglin, M.H. Tabacniks, A.J. Kellock, Nucl. Instrum. Meth. B 137 (1998) 241–246. [15] H.H. Berger, Solid-State Electron. 15 (1972) 145. [16] M.E. Lin, Z. Ma, F.Y. Huang, Z.F. Fan, L.H. Allen, H. Morkoc¸, Appl. Phys. Lett. 64 (1994) 1003–1005.
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