High Breakdown Voltage InAlN/AlN/GaN HEMTs Achieved by Schottky-Source Technology

Qi Zhou1, Wanjun Chen1,3, Shenghou Liu2, Bo Zhang1, Zhihong Feng4, Shujun Cai4, and Kevin J. Chen2 1State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China,

Chengdu, Sichuan, P.R. China, 610054 2Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong

3Science and Technology on Reliability Physics and Application Technology of Electronic Component Lab., Guangzhou, P.R. China 4 The Science and Technology on ASIC Lab., Hebei Semiconductor Research Institute, Shijiazhuang, P.R. China

E-mail: zhouqi@uestc.edu.cn, Phone: +86-28-8320 1693

Abstract—In this paper, we demonstrate 253% improvement in the off-state breakdown voltage (BV) of the lattice-matched In0.17Al0.83N/GaN high-electron-mobility transistors (HEMTs) by using a new Schottky-Source technology. Based on this concept, the Schottky-Source (SS) InAlN/GaN HEMTs are proposed. The SS HEMTs with a LGD of 15 μm showed a three-terminal BV of 650 V, while conventional InAlN/GaN HEMTs of the same geometry showed a maximum BV of 184 V. Without using any field-plate the result measured in the proposed device is the highest BV ever achieved on InAlN/GaN HEMTs. The corresponding specific on-resistance (Rsp,on) is as low as 3.4 mΩ·cm2. A BV of 118 V was also obtained in an SS InAlN/GaN HEMTs with LGD=1 μm, which is the highest BV in GaN-based HEMTs featuring such a short LGD with GaN buffer.

Keywords-InAlN/GaN HEMTs, high breakdown voltage, Schottky-Source technology

I. INTRODUCTION The emerged InAlN/GaN heterojunction has been

predicted to be capable of delivering record device performance owing to the superior material properties of wide bandgap, ultra-high spontaneous polarization induced charge density [1]. Moreover, the InxAl1-xN ternary barrier layer is lattice-matched to GaN templates, for an Indium mole-fraction x≈0.17, leading to important benefits with regard to potentially better device reliability than in AlGaN heterostructures due to the stress-free configurations [2]. The ultra-high spontaneous polarization allows InxAl1-xN/GaN heterojunction feature 2DEG density well above 2×1013cm-2 [3], even higher than that in its AlGaN/GaN counterparts. Recently, extensive efforts have been made focusing on demonstration of high on-current [4], and ultra-high temperature operation [5] of InAlN/GaN HEMTs. The intrinsic material properties and experimentally demonstrated device characteristics imply that the InAlN/GaN HEMT is a promising high-power switching device with low specific on-resistance, good high voltage reliability, high power density, and high temperature operation capability. However, research

on high BV InAlN/GaN HEMTs is scarcely reported and the results are far behind its counterpart AlGaN/GaN HEMTs [6]-[8]. The poor BV of InAlN/GaN HEMTs originates from the relatively large gate leakage current as a result of the conductive leakage path formed by Indium segregation related screw- and mixed-type threading dislocation [9], which indicates an immature stage of the InAlN/GaN heterojunction growth and device processing techniques.

In this work, we demonstrate a significant improvement of BV on InAlN/GaN HEMTs by using Schottky-Source (SS) technology. Due to the elimination of high temperature annealing and associated metallic spikes normally observed in Ohmic contact [10], [11] the SS can effectively suppress the source carrier injection and subsequently alleviate the impact ionization in the GaN buffer beneath the drain-side gate edge [12]. As a result, the BV of the SS InAlN/GaN HEMTs can be considerably improved. In the proposed device with LGD=15 μm the three-terminal BV of 650 V was achieved which is the highest BV reported on an InAlN/GaN HEMTs up to date. The corresponding specific on-resistance (Rsp.on) is as low as 3.4 mΩ·cm2.

II. DEVICE DESIGN AND FABRICATION The detail of InAlN/GaN epilayer structure can be found

in our previous work [13]. The device fabrication commenced with mesa isolation by BCl3/Cl2 based plasma etching. The Ti/Al/Ni/Au Ohmic metal stack were deposited in the area of drain electrode for the SS HEMTs followed by a 850 oC high temperature annealing. The Ohmic contact resistance (Rc) is 0.6 Ω·mm. After that the Ti/Au Schottky metal was directly deposited on the InAlN/GaN heterostructure in the area of source contacts for the SS HEMTs followed by Ni/Au gate deposition. Finally, the devices were passivated by Al2O3 deposited by ALD at 300 oC. The schematic cross section of the SS InAlN/GaN HEMTs and process flow are depicted in Fig. 1. The control devices featuring Ohmic source & drain were also fabricated on the same wafer for reference. Both of the SS HEMTs and control HEMTs fabricated for BV

This work was supported by the National Natural Science Foundation of China (No. 61234006, 61274090) and by the Opening Project of Science and Technology on Reliability Physics and Application Technology of Electronic Component Laboratory (No. ZHD201201).

Proceedings of The 25th International Symposium on Power Semiconductor Devices & ICs, Kanazawa WB-P8

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Fig. 2. Top view SEM images of (a) Ti/Au Schottky contact and (b) Ti/Al/Ni/Au Ohmic contact after high temperature annealing.

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measurements feature identical LG of 2.5 μm but varied LGD from 1 to 15 μm, the width of the fabricated devices is 10 μm. However, the LGS of the SS HEMTs and control HEMTs are 0.25 and 0.75 μm, respectively. The ultra-short LGS of the proposed device is achievable due to the elimination of lateral metal overflow that normally occurs in thermally annealed Ohmic contacts, which is beneficial to reduce the on-resistance of the SS HEMTs. Fig. 2 shows the surface morphology of the Ti/Au Schottky contacts and the Ohmic contacts after high temperature annealing, which confirm that the Schottky contact features much better surface metal morphology than the Ohmic contacts.

III. MEASUREMENTS RESULTS In order to verify the current drive capability of the Ti/Au

Schottky contacts on InAlN/GaN heterostructure the Schottky-Schottky TLM structure was measured. As shown in Fig. 3, the Ti/Au Schottky contact is capable of delivering substantial current. The good electrical conduction between the Schottky metal and the 2DEG channel is a collective effects of several factors. First, the threading dislocations present in InAlN barrier provide additional conducting path between the Schottky contact and the 2DEG channel [9]. Second, low work function metal Ti used in the Schottky contact can pull down the conduction band reducing the Schottky barrier

height as the Fermi level pinning at the metal/semiconductor interface. Third, the 2DEG concentration of the InAlN/GaN sample used in this work is much higher than that in a normal AlGaN/GaN heterostructure (1.85×1013cm-2 versus 1.2×1013cm-2). The high 2DEG concentration together with the small InAlN barrier thickness results in a lower effective barrier to confine the 2DEG.

The I-V curves of the SS InAlN/GaN HEMT with dimensions of LG/LGS/LGD/WG=2.5/0.25/15/10 μm are plotted in Fig. 4. The control HEMT with dimensions of LG/LGS/LGD/WG=2.5/0.75/15/10 μm is also given for comparison. It can be seen that the SS HEMT features respectable drain current. The maximum drain current density is 334 mA/mm at VGS=2 V and the peak transconductance is 158 mS/mm (see Fig. 4). Furthermore, the static on-resistance (Ron) of the SS HEMT shows only a slight increase compared with the control device.

The SS HEMTs and control HEMTs subjected to the three-terminal off-state BV measurement feature LGD vary from 1 to 15 μm. In a SS HEMT with LGD of 1 μm, the BV of 118 V is measured while the BV of the control HEMTs with identical LGD is only 35 V as depicted in Fig. 5 (a). The BV of the SS HEMTs is about 4× higher than the result reported in [14]. Without using field-plate [15] and buffer engineering technique [16] this value is already among the best reported results in GaN-based devices even compared with state-of-the-art AlGaN/GaN HEMTs by linear scaling of LGD to 1 μm [6]-[8]. In a SS HEMT with LGD of 15 µm, the BV of 650 V is

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terminal off-state BV ever reported on InAlN/GaN HEMTs so far. Compared with the control HEMTs featuring conventional Ohmic source & drain, the BV was improved by 253 %. The corresponding specific on-resistance (Rsp,on) of the SS HEMT and control HEMT are 3.4 and 2.77 mΩ·cm2, respectively. Compared with the significant BV improvement the penalty paid in Rsp,on is moderate. It is noteworthy that the breakdown of the SS HEMTs is dominated and induced by the gate leakage instead of the source leakage as reported in [14]. The leakage current of the SS HEMTs is consistently ~1 order lower than that in the control HEMTs prior to the device breakdown revealing better off-state leakage suppression capability of the Schottky contact over the conventional Ohmic contact. Compared with the device structures of the SS HEMTs and control HEMTs, it can be confirmed that the remarkably improved BV in the SS HEMTs stem from reduced source carrier injection achieved by the Schottky-Source.

Fig. 6 and 7 plot the BV of the SS and control HEMTs versus LGD. It can be seen that the SS HEMTs obtained considerably improved BV than the control HEMTs. Moreover, the BV of the SS HEMTs increases linearly with LGD as shown in Fig. 7. On the contrary, the BV of the control HEMTs saturates at ~180 V while LGD beyond 10 μm. The linear dependence on LGD of BV for the SS HEMTs indicates that higher BVs can be achieved in the SS HEMTs with larger LGD. We believe that the reduced off-state leakage current and improved BV of the SS HEMTs attribute to the elimination of Ohmic source and associate metallic spikes formed during the high temperature annealing for Ohmic contact formation [10], [11]. As the spikes are small in dimensions (tens of

nanometers), at sufficient high drain bias, these deep spikes may cause the electric-field lines concentrate at the spikes and form a high electric-field region in GaN buffer where the electrons could be injected into GaN buffer. Since the existence of the leakage path formed by the background impurity doping (e.g. O or Si) in GaN buffer the injected electrons would then drift to the peak electric-field region beneath the drain side gate edge where they can be accelerated to high energies to trigger intraband or interband impact ionization and subsequently induce gate breakdown. On the contrary, a smooth surface morphology can be obtained in the Schottky region in the SS HEMTs. Taking the advantage of smooth Schottky source a uniform electric-field distribution can be obtained in the proposed devices leading to effective alleviation of electron injection at source, which is responsible for the BV enhancement in the SS HEMTs.

As discussed above the Schottky-Source in the SS HEMTs responsible for the dramatically improved BV while the Ohmic drain was designed to obtain a low Ron. Fig. 8 gives the comparison of Rsp,on versus BV of the SS and control HEMTs. The Rsp,on exhibits a linear dependence on BV in the SS HEMTs, which is different from that obtained in the control devices. By delivering high BV and low Rsp,on simultaneously, the proposed SS InAlN/GaN HEMTs achieve the optimized balance between BV and Rsp,on for devices with BV beyond 200 V. The Rsp,on-BV relations of the fabricated InAlN/GaN HEMTs were plotted in Fig. 9, also given are the InAlN/GaN HEMTs [14], [15] and AlGaN/GaN HEMTs [6]-[8], [11] reported in literatures for comparison. It is obvious that the

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BVs of the InAlN/GaN HEMTs were significantly improved by using the SS technology while paying a moderate cost in Rsp,on compared with the control HEMTs. The performance of the proposed devices is much better than the reported InAlN/GaN HEMTs. By linear extrapolation the performance of the SS InAlN/GaN HEMTs is comparable with AlGaN/GaN HEMTs. The results validate the feasibility of Schottky-Source technology for BV improvement on InAlN/GaN HEMTs, suggesting a new degree of freedom for designing high voltage InAlN/GaN HEMTs with simplified device fabrication process and buffer layer structure.

IV. CONCLUSION In this work, we proposed the Schottky-Source technology

for breakdown voltage improvement of InAlN/GaN HEMTs. By simply replacing the conventional Ohmic source with Ti/Au Schottky contact, the record three-terminal off-state BV of 650 V with Rsp,on as low as 3.4 mΩ·cm2 is obtained in the Schottky-Source InAlN/GaN HEMTs with an LGD of 15 µm realizing 253 % improvement in BV paying only 23 % penalty in Rsp,on compared with the conventional HEMTs, which realizes the optimized balance of BV enhancement and Rsp,on. The significantly improved BV in the proposed devices is a result of the effective suppression of source carrier injection due to the smooth metal morphology of the Schottky-Source. It is expected that the BV of the proposed devices could be further improved by LGD extension.

ACKNOWLEDGMENT The devices in this work were fabricated in the

Nanoelectronics Fabrication Facility of Hong Kong University of Science and Technology

REFERENCES [1] J. Kuzmik, “Power Electronic on InAlN/(In)GaN: Prospect for a

Record Performance,” IEEE Electron Device Lett., vol. 22, no. 11, pp. 510–512, Nov. 2001.

[2] J. Joh, and J. A del Alamo, “Critical Voltage for Electrical Degradation of GaN High-Electron Mobility Transistors,” IEEE Electron Device Lett., vol. 29, no. 4, pp. 287-289, Apr. 2008.

[3] M. Gonschorek, J. F. Carlin, E. Feltin, M. A. Py, and N. Grandjean, “High electron mobility lattice-matched AlInN/GaN field-effect transistor heterostructures,” Appl. Phys. Lett., vol. 89, 062106, 2006.

[4] H. Wang, J. W. Chung, X. Gao, S. Guo and T. Palacios, “Al2O3 passivated InAlN/GaN HEMTs on SiC substrates with record current density and transconductance,” Phys. Status Solid-C. vol. 7, no. 10, pp. 2440-2444, 2010.

[5] D. Maier, M. Alomari, N. Grandjean, J.-F. Carlin, M.-A. Diforte-Poisson, C. Dua, S. Delage, and E. Kohn, “InAlN/GaN HEMTs for Operation in the 1000 oC Regime: A First Experiment,” IEEE Electron Device Lett., vol. 33, no. 7, pp. 985–987, Jul. 2012.

[6] N. Tipirneni, A. Koudymov, V. Adivarahan, J. Yang, G. Simin, and M. Asif Khan, “The 1.6-kV AlGaN/GaN HFETs,” IEEE Electron Device Lett., vol. 27, no. 9, pp. 716-718, 2006.

[7] B. Lu, and T. Palacios, “High Breakdown (>1500 V) AlGaN/GaN HEMTs by Substrate-Transfer Technology,” IEEE Electron Device Lett., vol. 31, no. 9, pp.951-953, Sep. 2010.

[8] P. Srivastava, J. Das, D. Visalli, M. V. Hove, P. E. Malinowski, D. Marcon, S. Lenci, K. Geens, K. Cheng, M. Leys, S. Decoutere, R. P. Mertens, and G. Borghs, “Record Breakdown Voltage (2200 V) of GaN DHFETs on Si With 2-µm Buffer Thickness by Local Substrate Removal,” IEEE Electron Device Lett.,vol. 32, no. 1, pp. 30-32, Jan. 2011.

[9] J. Song, F. J. Xu, X. D. Yan, F. Lin, C. C. Huang, L. P. You, T. J. Yu, X. Q. Wang, B. Shen, K. Wei, and X. Y. Liu, “High conductive gate leakage current channels induced by In segregation around screw- and mixed-type threading dislocations in lattice-matched InxAl1-xN/GaN heterostructures,” Appl. Phys. Lett. vol. 97, 232106, 2010.

[10] Y. Dora, A. Chakraborty, S. Heikman, L. McCarthy, S. Keller, S. P. DenBaars, and U. K. Mishra, “Effect of Ohmic Contacts on Buffer Leakage of GaN Transistors,” IEEE Electron Device Lett., vol. 27, no. 7, pp. 529–531, July. 2006.

[11] B. Lu, E. L. Piner, and T. Palacios, “Schottky-drain technology for AlGaN/GaN high-electron mobility transistors,” IEEE Electron Device Lett., vol. 31, no. 4, pp. 302–304, Apr. 2010.

[12] Maojun Wang, and Kevin J. Chen, “Off-State Breakdown Characterization in AlGaN/GaN HEMT Using Drain Injection Technique,” IEEE Trans. Electron Devices, vol. 57, no. 7, pp. 1492–1496, Jul. 2010.

[13] Qi Zhou, Hongwei Chen, Chunhua Zhou, Z. H. Feng, S. J. Cai, and Kevin J. Chen, “Schottky Source/Drain InAlN/AlN/GaN MISHEMT With Enhanced Breakdown Voltage,” IEEE Electron Device Lett., vol. 33, no. 1, pp. 38–40, Jan. 2012.

[14] J. Kuzmik, G. Pozzovivo, J. F. Carlin, M. Gonschorek, E. Feltin, N. Grandjean, G. Strasser, D. Pogany, and E. Gornik, “Off-state breakdown in InAlN/AlN/GaN high electron mobility transistors,” Phys. Status Solid-C, no. s2, pp. s925-s928, 2009.

[15] Q. Fareed, A. Tarakji, J. Dion, M. Islam, V. Adivarahan, and A. Khan, “High voltage operation of field-plated AlInN HEMTs,” Phys. Stat. Sol. (C), vol. 8, no. 7-8, pp. 2454-2456, Jul. 2011.

[16] H.S. Lee, D. Piedra, M. Sun, X. Gao, S. Guo, and T. Palacios, “3000-V 4.3-mΩ·cm2 InAlN/GaN MOSHEMTs With AlGaN Back Barrier,” IEEE Electron Device Lett., vol. 33, no. 7, Jul. 2012.

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