1986 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 7, JULY 2011 Analysis of Reverse Leakage Current and Breakdown Voltage in GaN and InGaN/GaN Schottky Barriers Wei Lu, Student Member, IEEE, Lingquan Wang, Member, IEEE, Siyuan Gu, Student Member, IEEE, David P. R. Aplin, Daniel M. Estrada, Paul K. L. Yu, Fellow, IEEE, and Peter M. Asbeck, Fellow, IEEE Abstract—A study of the reverse-leakage-current mechanisms in metal–organic-chemical-vapor-deposition (MOCVD)-grown GaN Schottky-barrier diodes is presented. An analysis is carried out of the characteristics of GaN Schottky diodes as well as of diodes with an InGaN surface layer to suppress the reverse leakage current and increase the breakdown voltage. The experimental results of the diodes with InGaN surface layers showed a ∼40-V breakdown voltage increase and a significant leakage-current reduction under high reverse bias, in comparison with the design with GaN only. Such improvements are attributed to the reduced surface electric field and the increased electron tunneling distance induced by the polarization charges at the InGaN/GaN interface. We also report the effect of a high-pressure (near atmospheric pressure) MOCVD growth technique of the GaN buffer layer to further improve the leakage current and breakdown voltage. Index Terms—Diode breakdown voltage, high-pressure (HP) metal–organic-chemical-vapor-deposited (MOCVD) buffer, InGaN/GaN heterojunction, polarization charge, Schottky-diode leakage current. I. I NTRODUCTION III–nitride-based Schottky diodes have achieved many appli- cations, such as ultraviolet photodetectors [1], gas sensors [2], high-voltage rectifiers [3], and varactors [4]. One key factor to improve the performance of these devices is to minimize re- verse leakage current, particularly under high reverse voltages. Many efforts have been reported to reduce the leakage cur- rent in GaN-based Schottky barriers, such as electrochemical surface treatment [5], SiO 2 dielectric surface passivation [6], capping with low-temperature GaN layers [7], and oppositely doped surface layers to increase the Schottky-barrier height Manuscript received December 8, 2010; revised March 13, 2011; accepted April 12, 2011. Date of publication May 19, 2011; date of current version June 22, 2011. This work was supported in part by the University of California San Diego Center for Wireless Communications, by the FutureWei Technologies Inc., and by the UC Discovery Grant Program. The work of D. M. Estrada was supported by the U.S. Department of Homeland Security Fellowship administered by the Oakridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the Department of Homeland Security. The review of this paper was arranged by Editor S. Bandyopadhyay. W. Lu, S. Gu, D. P. R. Aplin, P. K. L. Yu, and P. M. Asbeck are with the Department of Electrical and Computer Engineering, University of California San Diego, San Diego, CA 92093 USA (e-mail: w8lu@ucsd.edu). L. Wang is with the SuVolta Inc., Los Gatos, CA 95032 USA, and also with the Department of Electrical and Computer Engineering, University of California San Diego, San Diego, CA 92093 USA. D. M. Estrada is with the Materials Science and Engineering Program, University of California San Diego, San Diego, CA 92093 USA. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2011.2146254 (SBH) [8]. However, these studies do not discuss in detail the leakage-current-suppression mechanisms particularly under high reverse voltages, which is important in high-power appli- cations. We have recently reported that, for the development of high-Q high-voltage varactors, using a thin in situ grown InGaN surface layer, the reverse leakage current of GaN-based Schottky barriers under high reverse voltages is significantly suppressed and the breakdown voltage is increased by ∼40 V [4]. In this paper, we present a detailed analysis of the char- acteristics of the resultant devices and discuss the mechanisms for leakage-current reduction. We also report that the leakage current can be further significantly suppressed by using a high-pressure (HP, near atmospheric pressure) metal–organic- chemical-vapor-deposition (MOCVD) growth technique [9] for the GaN buffer layers (i.e., an HP GaN buffer). In this paper, the details of our experimental work are present first, followed by the discussions on the reverse-leakage-current mechanisms in GaN Schottky barriers and the InGaN-surface- layer design. Capacitance–voltage (C–V ) and current–voltage (I –V ) characteristics of these diodes are studied. Lastly, the improvements stemming from the HP GaN buffer are discussed. II. EXPERIMENTAL PROCEDURE The samples used in this paper were grown using a Thomas Swan close-coupled showerhead 3 × 2 ′′ MOCVD system with an adjustable gap between the showerhead and the susceptor. The substrates used were 2-in c-sapphire. For comparison pur- poses, five samples have been grown using a conventional LP MOCVD growth technique [10], and their key growth details are shown in Table I. Thick InGaN samples (> 300 nm to ensure full strain relaxation [11]) were grown for estimating the indium concentration and optimizing the InGaN growth by X-ray-diffraction (XRD) measurements. To fabricate the Schot- tky diode, a 120-nm-thick Ni Schottky contact was deposited immediately after the sample growth and surface cleaning. The Ni film was then patterned, and circular mesa diodes with diameters in the range of 80–300 μm were formed using BCl 3 /Cl 2 in a Trion reactive-ion-etching/inductively-coupled- plasma dry-etch system, which was followed by an ∼90 ◦ C KOH (0.1 mol/L) treatment to reduce the dry-etch residuals. A Ti/Al/Pd/Au (20 nm/80 nm/50 nm/100 nm) metal stack was used to form ohmic contacts to n + GaN [4]. Fig. 1 shows the schematic diagram of the cross section of the GaN/InGaN/GaN sample structure and the corresponding scanning-electron- microscopy (SEM) top view. 0018-9383/$26.00 © 2011 IEEE