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ozone treatment
Current Applied Physics 20 (2020) 293–297
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Improving Ni/GaN Schottky diode performance through interfacial passivation layer formed via ultraviolet/ozone treatment
T
Kwangeun Kima,∗, Jaewon Jangb,∗∗ a b
Department of Electronics and Electrical Convergence Engineering, Hongik University, Sejong, 30016, Republic of Korea School of Electronics Engineering, Kyungpook National University, Daegu, 41566, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: GaN Schottky diode Ultraviolet/ozone treatment Passivation Dislocation Band alignment
Electrical passivation has a significant effect on metal-semiconductor (MS) device operations including performance and reliability. In this study, the improvement in performance of Ni/GaN Schottky diodes (SDs) through an ultraviolet/ozone (UV/O3) interface treatment is investigated and the mechanism of carrier conduction at the MS junction interfaces is analyzed. The formation of surface oxide layer at the MS interface through the UV/O3 treatment is confirmed by the measurements using X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy. The atomic intensity and surface energy increased and surface roughness improved through the implementation of oxide layer. Electrical measurements reveal reduced leakage and improved breakdown voltage and are used to determine the Schottky barrier height and Richardson constant of the Ni/GaN MS SDs. The enhancement in the entire performance of the MS SDs is attributed to the passivation of defect centers at the dislocation-related pits through the formation of oxide layer with the UV/O3 treatment, which thereby improves the carrier transfer properties of Ni/GaN SDs.
1. Introduction High power switching electronics using wide bandgap semiconductors have attracted considerable attention for smart grid, renewable energy production, electric vehicle, and military applications [1–10]. These switching electronics manage electric power conversion and delivery via transmission lines as well as control power distribution and consumption. Gallium nitride (GaN)-based Schottky diodes (SDs) are a prevailing switching component in power system operation due to unique GaN properties, including a direct wide bandgap, high electron mobility, high saturation velocity, robust radiation hardness, and high breakdown field [11–18]. Owing to these properties, GaN SDs are used to switch power semiconductor devices, such as Schottky-gate highelectron-mobility transistors (Schottky-HEMTs) and metal-semiconductor field-effect transistors (MESFETs). Leakage and recombination currents at the metal-semiconductor (MS) interface, owing to threading dislocation (TD)-induced surface states, represent the main drawbacks of GaN SDs [19–21]. The unavoidable defect centers formed at the TD-related pits have a strong influence on device parameters (e.g., breakdown voltage (Vbr), current collapse, and leakage current), leading to operational reliability issues that hinder device optimization [19–24]. Therefore, the effects of
∗
interface states on the device performance must be counteracted. Several methods (e.g., thin layer deposition, wet processes, and plasma treatment) aimed at eliminating these effects have been explored [25–30]. Studies have reported that one of these techniques, i.e., an ultraviolet/ozone (UV/O3) treatment, screens polarization charge effects through the formation of a gallium-oxide (GaeO) thin layer on the GaN surface [11,31]. Moreover, surface oxidation resulted in low interface trap states in the GaeO layer [27,32]. The use of a UV/O3 treatment for improving the interface states and performance of GaN SDs remains, however, unexplored. In this work, an ultrathin oxide passivation layer was formed at a Ni/GaN interface subjected to a UV/O3 treatment and the mechanism for electrical-property enhancement of GaN SDs was explored. The leakage current, current on/off ratio, and Vbr of the Ni/GaN SD improved, owing to the formation of an interface passivation layer resulting from the treatment. Due to this layer, the defect centers located around the periphery of GaN surface dislocations were suppressed and the interface states at the Ni/GaN interface improved. 2. Methods Si-doped GaN layers were grown via metalorganic chemical vapor
Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Kim), [email protected] (J. Jang).
∗∗
https://doi.org/10.1016/j.cap.2019.11.017 Received 3 October 2019; Received in revised form 14 November 2019; Accepted 20 November 2019 Available online 23 November 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 20 (2020) 293–297
K. Kim and J. Jang
associated with the formation of GaeO and GaeN bonds, respectively. Therefore, Ga atoms may have combined with the GaeO bond after being released from the GaeN bond. The ultrathin oxide layer resulting from the UV/O3 treatment induced changes in the wettability and the surface energy of GaN, and these changes were examined via contact angle (ϴc) measurements (Fig. 2c). For each measurement, 0.6 μL DI H2O was dropped onto the GaN surface. The ϴc values obtained for the as-cleaned and as-treated GaN (50.7° and 19.7°, respectively), indicated that the wettability transformed to hydrophilicity, owing to the formation of the oxide layer. The surface energy (surface energy of solid-vapor) was calculated using the modified Berthelot's rule [36],
cos θc = −1 + 2
γsv −β(γ − γ )2 lv sv e γlv
(1)
where, γsv, γlv, and β are the surface energy, the surface energy of liquidvapor for DI H2O (72.70 mJ m−2), and an experimental coefficient (0.00012 (m2 mJ−1)2), respectively. The surface energy of GaN increased from 53.14 mJ m−2 to 68.76 mJ m−2, owing to the ultrathin oxide layer. The surface band bending (i.e., surface potential) of GaN was determined by computing the UV/O3-treatment-induced binding energy (BE) shift in the Ga 3d core levels (Fig. 2d). The BE of atomic core levels represents the energy distance from the Fermi level (EF) in the bandgap and, hence, the difference between Ga 3d BEs can be considered indicative of relative band bending. The BEs of Ga 3d core levels from the as-cleaned and as-treated GaN were 19.48 eV and 19.32 eV, respectively, corresponding to an upward energy band bending with a surface potential of 0.16 eV. Moreover, the bending direction of GaN corresponds to the direction of band bending associated with Ga-faced GaN, where spontaneous polarization (Psp) and surface ionic states contribute mutually to this bending [11]. To ascertain the formation of surface oxide layer through the UV/O3 treatment, surface topographic images of GaN without and with the treatment are obtained using atomic force microscopy scans (Fig. 3). Several dislocation-related pits are detected by scanning the GaN surface (Fig. 3a) and the intensity (height) of pits reduced after the UV/O3 treatment (Fig. 3b), implying the formation of oxide layer that passivates the periphery of dislocation-related pits. The decreased root-mean square roughness from 1.435 nm to 0.827 nm demonstrates the improved interface quality by the UV/O3 treatment. The results of the current density-voltage (J-V) measurements performed on Ni/GaN SDs are shown in Fig. 4, where the effects of a UV/ O3-induced ultrathin oxide layer on the carrier conduction of MS junctions are demonstrated. Owing to the formation of this layer, the reverse leakage current density at ‒ 0.5 V decreased by more than two orders of magnitude (from 9.33× 10−6 A/cm2 to 7.23 × 10−8 A/cm2). Furthermore, the current density on/off ratio at ± 0.5 V increased by more than one order of magnitude, from 1.06× 105 to 2.00 × 106 (Fig. 3a). These improved J-V properties confirm that the ultrathin oxide layer prepared via UV/O3 treatment effectively functions as a surface/interface passivation layer, which enhances the conduction features of GaN SDs. A linear scale J-V plot (Fig. 4b) reveals a cut-in voltage shift in the SDs, resulting from the ultrathin oxide layer located at the junction interface. The Vbr of SD is closely correlated with the occurrence of defects around the periphery of surface dislocations [6,19]. This value was further improved by the GaN surface passivation resulting from the UV/O3 treatment (see the inset of Fig. 4b). The ultrathin oxide layer accompanying this passivation could remove defect centers at the MS interface, thereby enhancing the interface quality for charge carrier transfer. Temperature-dependent current density-voltage (J-V-T) measurements were performed on the UV/O3-treated Ni/GaN SD to determine the band alignment and verify the conduction mechanism occurring at the MS junction interface (Fig. 5). The current density levels of a GaN
Fig. 1. Fabrication of Ni/GaN Schottky diode (SD) subjected to a UV/O3 treatment. (a) Processing steps: I. Material preparation, II. isolation, III. mesa formation, IV. Ohmic metal (Ti/Al/Ni/Au) deposition, V. UV/O3 treatment, and VI. Schottky metal (Ni/Au) deposition. (b) Optical image of a Ni/GaN SD (scale bar = 50 μm).
deposition (Fig. 1). The doping concentration for the top layer was 1 × 1017 cm−3 (400 nm) and for the Ohmic contact layer was 5 × 1018 cm−3 (400 nm). The as-grown GaN wafer was treated in RCA cleaning steps where: ammonium hydroxide solution (NH4OH: hydrogen peroxide (H2O2): DI H2O = 1 : 1: 5), hydrochloric acid solution (HCl: H2O2: DI H2O = 1 : 1: 5), and hydrofluoric acid solution (HF: DI H2O = 1:100) were used to eliminate the organic debris, ionic debris, and surface oxide, respectively. A circular Schottky (r = 50 μm) area and Ohmic contact area were obtained via photolithography and inductively coupled plasma reactive-ion etching (ICP-RIE). Deposition of a Ti/Al/Ni/Au (20/180/20/80 nm) metal stack for Ohmic contact was followed by annealing at 650 °C for 30 s in ambient N2. Subsequently, the Schottky contact region was treated for 7 min with UV/O3 (O3 dose: 1 mg/L, substrate temperature: 100 °C). A Ni/Au (20/180 nm) stack was then deposited as a Schottky contact metal. Fig. 1b shows an optical image of the constructed Ni/GaN SD subjected to the UV/O3 treatment. 3. Results and discussion X-ray photoelectron spectroscopy (XPS) using a monochromatic AlKα X-ray (1486.60 eV) was used to assess the modifications in surface composition and energy band bending induced by UV/O3 treatment of GaN (Fig. 2). The XPS spectra corresponding to Ga 3d core levels of GaN without (as-cleaned) and with (as-treated) the UV/O3 treatment are shown in Fig. 2a and b. Considering spin-orbital splitting, the Ga 3d spectra were de-convoluted into gallium-oxide (GaeO) and galliumnitrogen (GaeN) bonding components. The GaeO and GaeN peaks occurred at 20.66 eV and 19.47 eV, and then shifted to 20.33 eV and 19.30 eV, respectively. Furthermore, owing to the UV/O3 treatment, the ratio of the GaeO/GaeN peak intensity increased from 8.33% to 14.06% and, consequently, an ultrathin oxide layer was formed on the GaN surface (Table 1).[33−35] Values of ‒ 285 kJ/mol and ‒ 157 kJ/ mol were obtained for the change in the Gibbs free energy (ΔGf °) 294
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Fig. 2. Surface-chemistry modification through a UV/O3 treatment. X-ray photoelectron spectroscopy (XPS) scanning of Ga 3d core levels in the (a) as-cleaned and (b) UV/O3-treated GaN. (c) Contact angle (ϴc) and surface energy changes induced by UV/O3 treatment of GaN. (d) Surface band bending of UV/O3-treated GaN.
respectively. The values of the ideality factor (n) were extracted from the J-V-T curves and plotted as a function of temperature. As the inset in Fig. 5a shows, the ideality factors increased with increasing measurement temperature. These results indicated that the TE mechanism overwhelms the carrier transport at the Ni/GaN Schottky interface. The Jo can be expressed in terms of the Richardson constant (A*) and Schottky barrier height (ΦB) as follows [37,38].
Table 1 Binding energies of intensity peaks comprising the Ga 3d spectra. Type
Ga 3d [eV]
GaeO [eV]
GaeN [eV]
GaeO/GaeN [%]
GaN without UV/O3 GaN with UV/O3
19.48 19.32
20.66 20.33
19.47 19.30
8.33 14.06
SD with an interface oxide layer were investigated at temperatures ranging from 300 K to 510 K (step: 70 K; see Fig. 5a). The current density (J) of SD was determined from the thermionic emission (TE) model, where [37,38].
ΦB
JO = A∗ T 2⋅e− kT
Fig. 5b shows an Arrhenius plot of the GaN SD subjected to the UV/ O3 treatment. The linear trend observed in the plot validates the TE mechanism for carrier conduction at the MS interface. Through linearfitting of Jo/T2-1/kT, values of 0.83 eV and 15.03 A/cm2/K2 were determined for ΦB and A*, respectively. The ΦB and A* obtained from J-VT measurement were similar or slightly smaller than those reported in
qV
J = JO⋅e nkT
(3)
(2)
where, Jo, q, n, k, and T are the saturation current density, electron charge, ideality factor, Boltzmann constant, and absolute temperature,
Fig. 3. Surface topographic measurements. Atomic force microscopy (AFM) surface scans of (a) GaN with several dislocation-related pits and (b) UV/O3-treated GaN with a reduced surface roughness (scale bar = 200 nm). Rrms denotes root-mean square roughness. 295
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K. Kim and J. Jang
Fig. 4. Electrical characteristics of GaN SDs. Current density-voltage (J–V) curves of Ni/GaN SDs with and without UV/O3 treatment in (a) log and (b) linear scales. Inset in Fig. 3(b) shows breakdown voltage (Vbr) in Ni/GaN SDs without and with UV/O3 interface.
Fig. 5. Temperature dependent electrical characteristics (J-V-T) of GaN SDs. (a) J-V-T curves of GaN SDs with UV/O3 treatment. Inset shows ideality factors as a function of temperature. (b) Arrhenius plot of GaN SD with UV/O3 treatment. Fig. 6. Mechanism yielding improved performance of GaN SDs subjected to a UV/O3 interface treatment. Schematic illustrations showing the reduction in the number of defect centers at the dislocationrelated pits (a) without and (b) with the interface oxide layer formed by UV/O3 treatment. Band alignment and charge carrier transfer of Ni/GaN SDs without and with a UV/O3 treatment under (c) forward bias, (d) moderate reverse bias, and (e) high reverse bias (~Vbr).
previous studies [6,37,39]. These results lend support to the notion that the carrier transfer at the Schottky junction interface may involve an additional conduction mechanism during the high-temperature
operation. The effects of the UV/O3 treatment on the interface states of Ni/GaN Schottky junctions are schematically illustrated in Fig. 6. Several 296
Current Applied Physics 20 (2020) 293–297
K. Kim and J. Jang
dislocation-related pits, which yielded (primarily) leakage current and premature Vbr, occurred at the MS interface (Fig. 6a) [19]. An oxide passivation layer was formed due to the UV/O3 treatment and, hence, the number of defect centers surrounding the dislocation-related pits decreased and the electronic states at the Ni/GaN Schottky interface improved (Fig. 6b). Owing to the energy band alignment of Ni/GaN SD during the treatment, the interfacial oxide layer located at the MS interface compensated for the defect centers, thereby improving the interface states (Fig. 6c). Under reserve bias, the number of tunneling and recombination currents at or around defect centers decreased due to dislocation passivation induced by the treatment. This resulted in improved leakage current and prevented early breakdown of Ni/GaN SDs (Fig. 6d and e).
[2] S. Jiang, H. Hu, V. Laude, Phys. Status Solidi RRL 12 (2017) 2. [3] M. Johnson, P.R. Wilson, L. Empringham, L. De Lillo, IEEE Power Energy Mag. 5 (2018) 2. [4] S. Wang, V. Smirnov, T. Chen, X. Zhang, S. Xiong, Y. Zhao, F. Finger, Status Solidi RRL 9 (2015) 8. [5] T.P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, V. Pala, IEEE Trans. Electron Devices 64 (2017) 3. [6] C. Fares, F. Ren, S.J. Pearton, ECS J. Solid State Sci. Technol. 8 (2019) 7. [7] P. Darvishzadeh, M. Babanezhad, R. Ahmadi, N.E. Gorji, Mater. Des. 114 (2017) 339–344. [8] P. Saxena, N.E. Gorji, IEEE J. Photovolt. 9 (2019) 1693–1698. [9] H. Nazem, H.P. Dizaj, N.E. Gorji, Superlattice Microstruct. 128 (2019) 421–427. [10] I. Moeini, M. Ahmadpour, A. Mosavi, N. Alharbi, N.E. Gorji, Superlattice Microstruct. 122 (2018) 557–562. [11] K. Kim, J.H. Ryu, J. Kim, S.J. Cho, D. Liu, J. Park, I.-K. Lee, B. Moody, W. Zhou, J. Albrecht, Z. Ma, ACS Appl. Mater. Interfaces 9 (2017) 20. [12] T.L. Duan, J.S. Pan, D.S. Ang, Appl. Phys. Lett. 102 (2013) 20. [13] T.L. Duan, J.S. Pan, D.S. Ang, ECS J. Solid State Sci. Technol. 4 (2015) 9. [14] M. Kocan, A. Rizzi, H. Luth, S. Keller, U.K. Mishra, Phys. Status Solidi B 234 (2002) 3. [15] T. Palacios, Phys. Status Solidi A 206 (2009) 6. [16] X. Tang, B. Li, Y. Lu, K.J. Chen, Phys. Status Solidi C 13 (2016) 5–6. [17] H.-S. Kim, S.-W. Han, W.-H. Jang, C.-H. Cho, K.-S. Seo, J. Oh, H.-Y. Cha, IEEE Electron. Device Lett. 38 (2017) 8. [18] S.-W. Han, J.-G. Lee, C.-H. Cho, H.-Y. Cha, Appl. Phys. Express 7 (2014) 11. [19] J.W.P. Hsu, M.J. Manfra, R.J. Molnar, B. Heying, J.S. Speck, Appl. Phys. Lett. 81 (2002) 1. [20] H.-Y. Shih, M. Shiojiri, C.-H. Chen, S.-F. Yu, C.-T. Ko, J.-R. Yang, R.-M. Lin, M.J. Chen, Sci. Rep. 5 (2015) 13671. [21] S. Takeuchi, Y. Mizuta, M. Imanishi, M. Imade, Y. Mori, K. Sumitani, Y. Imai, S. Kimura, A. Sakai, J. Appl. Phys. 122 (2017) 105303. [22] D. Yan, J. Jiao, J. Ren, G. Yang, X. Gu, J. Appl. Phys. 114 (2013) 14. [23] E.H. Rhoderick, R.H. Williams, Metal-semiconductor Contacts, Clerendon, Oxford, UK, 1988. [24] M.E. Aydin, F. Yakuphanoglu, J.-H. Eom, D.-H. Hwang, Physica B 387 (2007) 1. [25] H. Cetin, B. Sahin, E. Ayyildiz, A. Turut, Physica B 364 (2005) 1–4. [26] W. Wei, Z. Qin, S. Fan, Z. Li, K. Shi, Q. Zhu, G. Zhang, Nanoscale Res. Lett. 7 (2012) 1. [27] B.S. Eller, J. Yang, R.J. Nemanich, J. Vac. Sci. Technol. A 31 (2013) 5. [28] X. Liu, R. Yeluri, J. Lu, U.K. Mishra, J. Electron. Mater. 42 (2013) 1. [29] N. Nepal, N.Y. Garces, D.J. Meyer, J.K. Hite, M.A. Mastro, C.R. Eddy Jr., Appl. Phys. Express 4 (2011) 5. [30] L.L. Smith, S.W. King, R.J. Nemanich, R.F. Davis, J. Electron. Mater. 25 (1996) 5. [31] S.T. Bradley, S.H. Goss, J. Hwang, W.J. Schaff, L.J. Brillson, Appl. Phys. Lett. 85 (2004) 8. [32] C. Bae, G.B. Rayner, G. Lucovsky, Appl. Surf. Sci. 216 (2003) 1. [33] R. Carin, J.P. Deville, J. Werckmann, Surf. Interface Anal. 16 (1990) 1–12. [34] R. Carli, C.L. Bianchi, Appl. Surf. Sci. 74 (1994) 1. [35] Thermo Fisher Scientific Inc., XPS interpretation of Gallium, https://xpssimplified. com , Accessed date: November 2019. [36] D.Y. Kwok, A.W. Neumann, Adv. Colloid Interface Sci. 81 (1999) 3. [37] L. Wang, M.I. Nathan, T.-H. Lim, M.A. Khan, Q. Chen, Appl. Phys. Lett. 68 (1996) 9. [38] H. Yamada, H. Chonan, T. Takahashi, M. Shimizu, Appl. Phys. Express 10 (2017) 4. [39] S.M. Sze, K.K. Ng, Physics Of Semiconductor Devices, Wiley, NJ, USA, 2007.
4. Conclusions In summary, through a UV/O3 treatment, a passivation oxide layer was formed on the surface of Ni/GaN SDs and the corresponding mechanism governing electrical-property enhancement of these SDs was investigated. This treatment led to the passivation of defect centers and, in turn, improvement in the leakage current density, current on/off ratio, and Vbr of the SDs. These results and the mechanism can be utilized for enhancing the operation of GaN-based power devices, such as HEMT and MESFET, where improved Vbr and reduced leakage are required. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by 2019 Hongik University Research Fund. This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2009-0082580). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1G1A1099677). The work was supported by the Hongik University new faculty research support fund. References [1] S.K. Barman, M.N. Huda, Phys. Status Solidi RRL 13 (2019) 5.
297
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Improving Ni/GaN Schottky diode performance through interfacial passivation layer formed via ultraviolet/ozone treatment
T
Kwangeun Kima,∗, Jaewon Jangb,∗∗ a b
Department of Electronics and Electrical Convergence Engineering, Hongik University, Sejong, 30016, Republic of Korea School of Electronics Engineering, Kyungpook National University, Daegu, 41566, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: GaN Schottky diode Ultraviolet/ozone treatment Passivation Dislocation Band alignment
Electrical passivation has a significant effect on metal-semiconductor (MS) device operations including performance and reliability. In this study, the improvement in performance of Ni/GaN Schottky diodes (SDs) through an ultraviolet/ozone (UV/O3) interface treatment is investigated and the mechanism of carrier conduction at the MS junction interfaces is analyzed. The formation of surface oxide layer at the MS interface through the UV/O3 treatment is confirmed by the measurements using X-ray photoelectron spectroscopy, contact angle, and atomic force microscopy. The atomic intensity and surface energy increased and surface roughness improved through the implementation of oxide layer. Electrical measurements reveal reduced leakage and improved breakdown voltage and are used to determine the Schottky barrier height and Richardson constant of the Ni/GaN MS SDs. The enhancement in the entire performance of the MS SDs is attributed to the passivation of defect centers at the dislocation-related pits through the formation of oxide layer with the UV/O3 treatment, which thereby improves the carrier transfer properties of Ni/GaN SDs.
1. Introduction High power switching electronics using wide bandgap semiconductors have attracted considerable attention for smart grid, renewable energy production, electric vehicle, and military applications [1–10]. These switching electronics manage electric power conversion and delivery via transmission lines as well as control power distribution and consumption. Gallium nitride (GaN)-based Schottky diodes (SDs) are a prevailing switching component in power system operation due to unique GaN properties, including a direct wide bandgap, high electron mobility, high saturation velocity, robust radiation hardness, and high breakdown field [11–18]. Owing to these properties, GaN SDs are used to switch power semiconductor devices, such as Schottky-gate highelectron-mobility transistors (Schottky-HEMTs) and metal-semiconductor field-effect transistors (MESFETs). Leakage and recombination currents at the metal-semiconductor (MS) interface, owing to threading dislocation (TD)-induced surface states, represent the main drawbacks of GaN SDs [19–21]. The unavoidable defect centers formed at the TD-related pits have a strong influence on device parameters (e.g., breakdown voltage (Vbr), current collapse, and leakage current), leading to operational reliability issues that hinder device optimization [19–24]. Therefore, the effects of
∗
interface states on the device performance must be counteracted. Several methods (e.g., thin layer deposition, wet processes, and plasma treatment) aimed at eliminating these effects have been explored [25–30]. Studies have reported that one of these techniques, i.e., an ultraviolet/ozone (UV/O3) treatment, screens polarization charge effects through the formation of a gallium-oxide (GaeO) thin layer on the GaN surface [11,31]. Moreover, surface oxidation resulted in low interface trap states in the GaeO layer [27,32]. The use of a UV/O3 treatment for improving the interface states and performance of GaN SDs remains, however, unexplored. In this work, an ultrathin oxide passivation layer was formed at a Ni/GaN interface subjected to a UV/O3 treatment and the mechanism for electrical-property enhancement of GaN SDs was explored. The leakage current, current on/off ratio, and Vbr of the Ni/GaN SD improved, owing to the formation of an interface passivation layer resulting from the treatment. Due to this layer, the defect centers located around the periphery of GaN surface dislocations were suppressed and the interface states at the Ni/GaN interface improved. 2. Methods Si-doped GaN layers were grown via metalorganic chemical vapor
Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Kim), [email protected] (J. Jang).
∗∗
https://doi.org/10.1016/j.cap.2019.11.017 Received 3 October 2019; Received in revised form 14 November 2019; Accepted 20 November 2019 Available online 23 November 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
Current Applied Physics 20 (2020) 293–297
K. Kim and J. Jang
associated with the formation of GaeO and GaeN bonds, respectively. Therefore, Ga atoms may have combined with the GaeO bond after being released from the GaeN bond. The ultrathin oxide layer resulting from the UV/O3 treatment induced changes in the wettability and the surface energy of GaN, and these changes were examined via contact angle (ϴc) measurements (Fig. 2c). For each measurement, 0.6 μL DI H2O was dropped onto the GaN surface. The ϴc values obtained for the as-cleaned and as-treated GaN (50.7° and 19.7°, respectively), indicated that the wettability transformed to hydrophilicity, owing to the formation of the oxide layer. The surface energy (surface energy of solid-vapor) was calculated using the modified Berthelot's rule [36],
cos θc = −1 + 2
γsv −β(γ − γ )2 lv sv e γlv
(1)
where, γsv, γlv, and β are the surface energy, the surface energy of liquidvapor for DI H2O (72.70 mJ m−2), and an experimental coefficient (0.00012 (m2 mJ−1)2), respectively. The surface energy of GaN increased from 53.14 mJ m−2 to 68.76 mJ m−2, owing to the ultrathin oxide layer. The surface band bending (i.e., surface potential) of GaN was determined by computing the UV/O3-treatment-induced binding energy (BE) shift in the Ga 3d core levels (Fig. 2d). The BE of atomic core levels represents the energy distance from the Fermi level (EF) in the bandgap and, hence, the difference between Ga 3d BEs can be considered indicative of relative band bending. The BEs of Ga 3d core levels from the as-cleaned and as-treated GaN were 19.48 eV and 19.32 eV, respectively, corresponding to an upward energy band bending with a surface potential of 0.16 eV. Moreover, the bending direction of GaN corresponds to the direction of band bending associated with Ga-faced GaN, where spontaneous polarization (Psp) and surface ionic states contribute mutually to this bending [11]. To ascertain the formation of surface oxide layer through the UV/O3 treatment, surface topographic images of GaN without and with the treatment are obtained using atomic force microscopy scans (Fig. 3). Several dislocation-related pits are detected by scanning the GaN surface (Fig. 3a) and the intensity (height) of pits reduced after the UV/O3 treatment (Fig. 3b), implying the formation of oxide layer that passivates the periphery of dislocation-related pits. The decreased root-mean square roughness from 1.435 nm to 0.827 nm demonstrates the improved interface quality by the UV/O3 treatment. The results of the current density-voltage (J-V) measurements performed on Ni/GaN SDs are shown in Fig. 4, where the effects of a UV/ O3-induced ultrathin oxide layer on the carrier conduction of MS junctions are demonstrated. Owing to the formation of this layer, the reverse leakage current density at ‒ 0.5 V decreased by more than two orders of magnitude (from 9.33× 10−6 A/cm2 to 7.23 × 10−8 A/cm2). Furthermore, the current density on/off ratio at ± 0.5 V increased by more than one order of magnitude, from 1.06× 105 to 2.00 × 106 (Fig. 3a). These improved J-V properties confirm that the ultrathin oxide layer prepared via UV/O3 treatment effectively functions as a surface/interface passivation layer, which enhances the conduction features of GaN SDs. A linear scale J-V plot (Fig. 4b) reveals a cut-in voltage shift in the SDs, resulting from the ultrathin oxide layer located at the junction interface. The Vbr of SD is closely correlated with the occurrence of defects around the periphery of surface dislocations [6,19]. This value was further improved by the GaN surface passivation resulting from the UV/O3 treatment (see the inset of Fig. 4b). The ultrathin oxide layer accompanying this passivation could remove defect centers at the MS interface, thereby enhancing the interface quality for charge carrier transfer. Temperature-dependent current density-voltage (J-V-T) measurements were performed on the UV/O3-treated Ni/GaN SD to determine the band alignment and verify the conduction mechanism occurring at the MS junction interface (Fig. 5). The current density levels of a GaN
Fig. 1. Fabrication of Ni/GaN Schottky diode (SD) subjected to a UV/O3 treatment. (a) Processing steps: I. Material preparation, II. isolation, III. mesa formation, IV. Ohmic metal (Ti/Al/Ni/Au) deposition, V. UV/O3 treatment, and VI. Schottky metal (Ni/Au) deposition. (b) Optical image of a Ni/GaN SD (scale bar = 50 μm).
deposition (Fig. 1). The doping concentration for the top layer was 1 × 1017 cm−3 (400 nm) and for the Ohmic contact layer was 5 × 1018 cm−3 (400 nm). The as-grown GaN wafer was treated in RCA cleaning steps where: ammonium hydroxide solution (NH4OH: hydrogen peroxide (H2O2): DI H2O = 1 : 1: 5), hydrochloric acid solution (HCl: H2O2: DI H2O = 1 : 1: 5), and hydrofluoric acid solution (HF: DI H2O = 1:100) were used to eliminate the organic debris, ionic debris, and surface oxide, respectively. A circular Schottky (r = 50 μm) area and Ohmic contact area were obtained via photolithography and inductively coupled plasma reactive-ion etching (ICP-RIE). Deposition of a Ti/Al/Ni/Au (20/180/20/80 nm) metal stack for Ohmic contact was followed by annealing at 650 °C for 30 s in ambient N2. Subsequently, the Schottky contact region was treated for 7 min with UV/O3 (O3 dose: 1 mg/L, substrate temperature: 100 °C). A Ni/Au (20/180 nm) stack was then deposited as a Schottky contact metal. Fig. 1b shows an optical image of the constructed Ni/GaN SD subjected to the UV/O3 treatment. 3. Results and discussion X-ray photoelectron spectroscopy (XPS) using a monochromatic AlKα X-ray (1486.60 eV) was used to assess the modifications in surface composition and energy band bending induced by UV/O3 treatment of GaN (Fig. 2). The XPS spectra corresponding to Ga 3d core levels of GaN without (as-cleaned) and with (as-treated) the UV/O3 treatment are shown in Fig. 2a and b. Considering spin-orbital splitting, the Ga 3d spectra were de-convoluted into gallium-oxide (GaeO) and galliumnitrogen (GaeN) bonding components. The GaeO and GaeN peaks occurred at 20.66 eV and 19.47 eV, and then shifted to 20.33 eV and 19.30 eV, respectively. Furthermore, owing to the UV/O3 treatment, the ratio of the GaeO/GaeN peak intensity increased from 8.33% to 14.06% and, consequently, an ultrathin oxide layer was formed on the GaN surface (Table 1).[33−35] Values of ‒ 285 kJ/mol and ‒ 157 kJ/ mol were obtained for the change in the Gibbs free energy (ΔGf °) 294
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Fig. 2. Surface-chemistry modification through a UV/O3 treatment. X-ray photoelectron spectroscopy (XPS) scanning of Ga 3d core levels in the (a) as-cleaned and (b) UV/O3-treated GaN. (c) Contact angle (ϴc) and surface energy changes induced by UV/O3 treatment of GaN. (d) Surface band bending of UV/O3-treated GaN.
respectively. The values of the ideality factor (n) were extracted from the J-V-T curves and plotted as a function of temperature. As the inset in Fig. 5a shows, the ideality factors increased with increasing measurement temperature. These results indicated that the TE mechanism overwhelms the carrier transport at the Ni/GaN Schottky interface. The Jo can be expressed in terms of the Richardson constant (A*) and Schottky barrier height (ΦB) as follows [37,38].
Table 1 Binding energies of intensity peaks comprising the Ga 3d spectra. Type
Ga 3d [eV]
GaeO [eV]
GaeN [eV]
GaeO/GaeN [%]
GaN without UV/O3 GaN with UV/O3
19.48 19.32
20.66 20.33
19.47 19.30
8.33 14.06
SD with an interface oxide layer were investigated at temperatures ranging from 300 K to 510 K (step: 70 K; see Fig. 5a). The current density (J) of SD was determined from the thermionic emission (TE) model, where [37,38].
ΦB
JO = A∗ T 2⋅e− kT
Fig. 5b shows an Arrhenius plot of the GaN SD subjected to the UV/ O3 treatment. The linear trend observed in the plot validates the TE mechanism for carrier conduction at the MS interface. Through linearfitting of Jo/T2-1/kT, values of 0.83 eV and 15.03 A/cm2/K2 were determined for ΦB and A*, respectively. The ΦB and A* obtained from J-VT measurement were similar or slightly smaller than those reported in
qV
J = JO⋅e nkT
(3)
(2)
where, Jo, q, n, k, and T are the saturation current density, electron charge, ideality factor, Boltzmann constant, and absolute temperature,
Fig. 3. Surface topographic measurements. Atomic force microscopy (AFM) surface scans of (a) GaN with several dislocation-related pits and (b) UV/O3-treated GaN with a reduced surface roughness (scale bar = 200 nm). Rrms denotes root-mean square roughness. 295
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Fig. 4. Electrical characteristics of GaN SDs. Current density-voltage (J–V) curves of Ni/GaN SDs with and without UV/O3 treatment in (a) log and (b) linear scales. Inset in Fig. 3(b) shows breakdown voltage (Vbr) in Ni/GaN SDs without and with UV/O3 interface.
Fig. 5. Temperature dependent electrical characteristics (J-V-T) of GaN SDs. (a) J-V-T curves of GaN SDs with UV/O3 treatment. Inset shows ideality factors as a function of temperature. (b) Arrhenius plot of GaN SD with UV/O3 treatment. Fig. 6. Mechanism yielding improved performance of GaN SDs subjected to a UV/O3 interface treatment. Schematic illustrations showing the reduction in the number of defect centers at the dislocationrelated pits (a) without and (b) with the interface oxide layer formed by UV/O3 treatment. Band alignment and charge carrier transfer of Ni/GaN SDs without and with a UV/O3 treatment under (c) forward bias, (d) moderate reverse bias, and (e) high reverse bias (~Vbr).
previous studies [6,37,39]. These results lend support to the notion that the carrier transfer at the Schottky junction interface may involve an additional conduction mechanism during the high-temperature
operation. The effects of the UV/O3 treatment on the interface states of Ni/GaN Schottky junctions are schematically illustrated in Fig. 6. Several 296
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dislocation-related pits, which yielded (primarily) leakage current and premature Vbr, occurred at the MS interface (Fig. 6a) [19]. An oxide passivation layer was formed due to the UV/O3 treatment and, hence, the number of defect centers surrounding the dislocation-related pits decreased and the electronic states at the Ni/GaN Schottky interface improved (Fig. 6b). Owing to the energy band alignment of Ni/GaN SD during the treatment, the interfacial oxide layer located at the MS interface compensated for the defect centers, thereby improving the interface states (Fig. 6c). Under reserve bias, the number of tunneling and recombination currents at or around defect centers decreased due to dislocation passivation induced by the treatment. This resulted in improved leakage current and prevented early breakdown of Ni/GaN SDs (Fig. 6d and e).
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4. Conclusions In summary, through a UV/O3 treatment, a passivation oxide layer was formed on the surface of Ni/GaN SDs and the corresponding mechanism governing electrical-property enhancement of these SDs was investigated. This treatment led to the passivation of defect centers and, in turn, improvement in the leakage current density, current on/off ratio, and Vbr of the SDs. These results and the mechanism can be utilized for enhancing the operation of GaN-based power devices, such as HEMT and MESFET, where improved Vbr and reduced leakage are required. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by 2019 Hongik University Research Fund. This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2009-0082580). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1G1A1099677). The work was supported by the Hongik University new faculty research support fund. References [1] S.K. Barman, M.N. Huda, Phys. Status Solidi RRL 13 (2019) 5.
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