Schottky mechanism for Ni/ Au contact with chlorine-treated
n-type GaN layer
Po-Sung ChenInstitute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China
Tsung-Hsin Lee
Institute of Optical Sciences, National Central University, Chung-Li, 32054 Taiwan, Republic of China Li-Wen Lai and Ching-Ting Leea兲
Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, 701 Taiwan, Republic of China
共Received 30 August 2006; accepted 7 November 2006; published online 22 January 2007兲 To investigate the function of chlorination treatment, Schottky diodes with Ni/ Au contact and chlorine-treated n-type GaN were fabricated. The resultant Schottky barrier height and ideality factor of the chlorine-treated Schottky diodes were improved. The corresponding increase in photoluminescence intensity and carrier lifetime of the chlorine-treated n-type GaN was achieved using photoluminescence and time-resolved photoluminescence measurements. The improved performance of chlorine-treated Schottky diodes was attributed to the reduction of surface states as a result of reduced Ga dangling bonds and the N vacancies being passivated by the formation of GaOxon the surface of n-type GaN. © 2007 American Institute of Physics.
关DOI:10.1063/1.2427100兴
I. INTRODUCTION
In recent years, we have witnessed an impressive progress being achieved on III–V nitride-based compound semiconductor related technologies. These materials have found numerous applications in electronic and optoelectronic devices because of their inherent material properties.1–4 In particular, the issue involving metal contacts on III–V nitride semiconductors plays a key role in influencing the perfor-mance and subsequent reliability of devices. The Ohmic and Schottky characteristics of metal/ III–V nitride semiconduc-tor contacts could potentially be influenced by several pa-rameters. Among them, surface state has received a substan-tial amount of attention because of its tendency to render surface Fermi-level pinning and a reduction in Schottky bar-rier height.5,6 Schottky barrier height is dependent on the surface state density. In previous reports, various values of Schottky barrier height of Ni/ Au contact on n-type GaN were reported due to their differences in surface state density.7,8In this work, we propose a method to reduce sur-face state of n-type GaN by using chlorinated sursur-face treat-ment via the electrolysis of a dilute HCl共aq兲 chemical solu-tion. To investigate the reduction of surface state density as a function of chlorinated surface treatment, current-voltage characteristics, photoluminescence 共PL兲, and time-resolved photoluminescence共TRPL兲 measurements were performed. II. EXPERIMENTAL PROCEDURE
The n-type GaN layers used in this work were grown on c-plane sapphire substrates using metal organic chemical
va-por deposition共MOCVD兲 system. Trimethylgallium and am-monia were used as the Ga and N sources, respectively. Si was the dopant source for the n-type GaN. Following the epitaxial growth of a 1-m-thick undoped GaN buffer layer conducted at 500 ° C, a 2-m-thick Si-doped GaN layer was grown at 1100 ° C. Using Hall measurement carried out at room temperature, the electron concentration and mobility of as-grown n-type GaN layer were 3⫻1017cm−3 and 310 cm2/ V s, respectively.
For the chlorinated surface treatment of n-type GaN, di-lute HCl共1HCl+10H2O兲 chemical solution was used as the electrolytic solution. Figure 1 illustrates the chlorination treatment system. The grown sample was placed in the elec-trolytic solution underneath a Pt anodic electrode. The dis-tance between anode and cathode electrodes was 5 cm. By applying a voltage of 20 V for 60 min, the HCl共aq兲was elec-trolyzed into H+ ions and Cl− ions. The chlorine was then produced underneath the Pt anodic electrode and its chemical reaction could be expressed as9
2Cl− Cl2+ 2e−. 共1兲
There was a tendency for the chlorine thus produced to
a兲Author to whom correspondence should be addressed; FAX:
886-6-2362303; electronic mail: [email protected] FIG. 1. Chlorination treatment system.
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adhere to the n-type GaN surface which reacted with Ga dangling bonds of Ga-terminated n-type GaN layer. The GaClxwas formed via the following process:
2Ga3++ xCl2+ 6e− 2GaClx. 共2兲
The produced GaClxrelative halides could be dissolved in the chemical solution quite easily.9 Ga vacancies were then introduced to the n-type GaN surface as a result of the formation of GaClx. Since holes could be induced by Ga vacancies,10the associated net electron concentration on the n-type GaN surface was thereby decreased. On the other hand, the HClO could be produced in the chemical solution by reacting chlorine with water via the following process:
Cl2+ H2O HCl + HClO. 共3兲
The HClO in turn oxidized the n-type GaN to form GaOxvia the process9
xHClO + Ga xHCl + GaOx. 共4兲 The resultant GaOx generated on the n-type GaN then in-duced more Ga vacancies and made a function of passivation on the GaN surface.11,12
III. EXPERIMENTAL RESULTS AND DISCUSSION Using a He–Cd laser with a wavelength of 325 nm as an excitation source, the PL spectra at room temperature of the n-type GaN with and without chlorination treatment are shown in Fig. 2. It can be seen that the PL intensity of the chlorine-treated n-type GaN sample is larger than that of the sample without chlorination treatment. The nonradiative re-combination rate can be enhanced by the surface states which act as nonradiative recombination centers.13The larger PL intensity of the chlorine-treated n-type GaN can be attrib-uted to the effective passivation of surface states by the for-mation of GaOx.
To investigate the carrier recombination dynamics and related carrier lifetime of the n-type GaN with and without chlorination treatment, a focused picosecond Ti: sapphire la-ser with a wavelength of 266 nm was used as an excitation source for a TRPL system. The associated TRPL curves for the energy band edge of 3.4 eV are shown in Fig.3. It can be found that the carrier lifetimes of the n-type GaN with and without chlorination treatment are 0.65 and 0.44 ns, respec-tively. The total recombination rate is split into a sum of
radiative and nonradiative recombination rates. The carrier lifetime, radiative lifetimer, and nonradiative lifetimenr are related as follows:
1 = 1 r + 1 nr . 共5兲
Since the PL intensity of the chlorine-treated n-type GaN is larger than that of the sample without chlorination treatment as shown in Fig. 2, the radiative recombination rate of the former one is expected to be larger than that of the latter one. Furthermore, the recombination rate of chlorine-treated n-type GaN is smaller because of its longer carrier lifetime. Therefore, we can conclude that the nonradiative recombina-tion rate of the n-type GaN without chlorinarecombina-tion treatment is larger. A larger nonradiative recombination rate is attributed to the enhanced recombination via surface states. Based on the PL and TRPL measurements, the surface state density of the n-type GaN can be reduced by using chlorinated surface treatment.
Using a conventional photolithography and lift-off tech-nique, Schottky diodes were fabricated and shown in Fig.4. To obtain the Ohmic performance with long-term thermal stability,14Ti/ Al/ Pt/ Au共25/100/50/200 nm兲 Ohmic metals were deposited on the n-type GaN samples using electron-beam evaporator. After using a lift-off process, concentric Ohmic contact rings 共outer radius=400m, inner radius = 150m兲 were patterned and then annealed using a rapid thermal annealing system in a N2 ambient at 700 ° C for 1 min. After the samples were cleaned, the samples were divided into group A and group B. Chlorinated surface treat-FIG. 2. Photoluminescence spectra of n-type GaN with and without
chlori-nation treatment.
FIG. 3. Time-resolved PL spectra of n-type GaN with and without chlori-nation treatment.
FIG. 4. Schematic configuration of Schottky diodes.
024507-2 Chen et al. J. Appl. Phys. 101, 024507共2007兲
ment was performed on group A samples as mentioned be-fore, while group B samples received no chlorination treat-ment. Ni/ Au共100/100 nm兲 Schottky circular pattern with a radius of 100m was defined using photoresist and then placed at the center of previously fabricated concentric Ohmic contact ring for both the groups A and B samples.
Using an HP4145B semiconductor parameter analyzer, the forward current-voltage characteristics of the Schottky diodes were measured and shown in Fig.5. According to the thermionic emission theory, the current共I兲 transport over the Schottky barrier height FB as a function of applied voltage 共V兲 can be expressed as
I = A*ST2exp共− q⌽B/KT兲exp共qV/nKT兲
⫻关1 − exp共− qV/KT兲兴, 共6兲
where A*共=26.4 A cm−2K−2兲 is the effective Richardson constant of GaN,15 S is the Schottky contact area, T is the absolute temperature, q is the electronic charge, and n is the ideality factor. From the logarithmic plot of 关I exp共qV/KT兲/关exp共qV/KT兲−1兴兴 as a function of applied voltage V, the associated ideality factor n and Schottky bar-rier height FB can be determined. The ideality factors n of 1.04 and 1.16 and Schottky barrier heights FB of 0.95 and 0.75 eV were obtained for the n-type GaN with and without chlorination treatment, respectively. Since the ideality factor of the chlorine-treated Schottky diode is close to 1, the result indicates the surface states can be effectively reduced using chlorination treatment.16 In general, the presence of surface states tends to lower the Schottky barrier height.17 Because the Schottky barrier height of Ni/ Au contact on chlorine-treated n-type GaN is larger compared to that of n-type GaN sample without chlorination treatment, chlorinated surface treatment effectively brings about a reduction on the surface state density.
Figure 6 shows the reverse current-voltage characteris-tics of the Schottky diodes with and without chlorinated sur-face treatment. The breakdown voltages of −72 and −50 V were obtained for the Schottky diodes with and without chlo-rination treatment, respectively. The breakdown voltage is defined as the voltage with which the reverse leakage current 共1⫻10−7 A, in Fig.6兲 rapidly increases. In general, nitrogen vacancy related surface states present on the surface of n-type GaN would lead to a thin n+region on the surface.18 The n+region served as a tunneling path when reverse
bias-ing the Schottky diodes, and the reverse leakage current in-creased accordingly. As the chlorinated surface treatment of n-type GaN had demonstrated, the reduction of Ga dangling bonds and the occupation of nitrogen vacancies as a result of GaOxformation jointly contributed to a decrease in the sur-face states. In return, the breakdown voltage of the chlorine-treated Schottky diodes became larger compared to that of the one without chlorination treatment, as evidently shown in Fig.6.
IV. CONCLUSIONS
In summary, the Schottky mechanism for chlorine-treated n-type GaN by electrolyzing a chemical solution of dilute was investigated. According to the experimental re-sults of current-voltage characteristics, photoluminescence, and time-resolved photoluminescence measurements, the im-provement in the Schottky barrier height and ideality factor of the chlorine-treated Schottky diodes was attributed to the reduction of surface states. Using chlorinated surface treat-ment of n-type GaN, the reduction of surface states could be achieved through the removal of Ga dangling bonds and the passivation of nitrogen vacancies as a result of forming GaOx on the chlorine-treated n-type GaN surface. The nonradiative recombination rate could also be minimized by reducing sur-face states; this in turn helped enhance the photolumines-cence intensity and carrier lifetime. The lowering of reverse leakage current and an increase in the breakdown voltage of the chlorine-treated Schottky diodes were all benefited from the reduction of surface states.
ACKNOWLEDGMENTS
This work was supported by the program of Top 100 Universities Advancement, Ministry of Education, and the National Science Council of Taiwan, Republic of China un-der Contact No. NSC-94-2215-E006-013.
1S. Haffour et al., J. Vac. Sci. Technol. B 23, 1199共2005兲.
2R. J. Xie, N. Hirosaki, M. Mitomo, K. Takahashi, and K. Sakuma, Appl.
Phys. Lett. 88, 101104共2006兲.
3T. K. Kim, S. S. Yang, J. K. Son, Y. G. Hong, and G. M. Yang, Appl. Phys.
Lett. 89, 041129共2006兲.
4C. T. Lee, U. Z. Yang, C. S. Lee, and P. S. Chen, IEEE Photonics Technol.
Lett. 18, 2029共2006兲.
5W. Mönch, Semiconductor Surfaces and Interfaces, 2nd ed. 共Springer,
Berlin, 1993兲.
6F. Bechstedt and R. Enderlein, Semiconductor Surfaces and Interfaces:
FIG. 5. Forward current-voltage characteristics of Schottky diodes with and without chlorination treatment.
FIG. 6. Reverse current-voltage characteristics for the Schottky diodes with and without chlorination treatment.
024507-3 Chen et al. J. Appl. Phys. 101, 024507共2007兲
Their Atomic and Electronic Structures共Acadamie, Berlin, 1988兲.
7J. D. Guo, F. M. Pan, M. S. Feng, R. J. Guo, P. F. Chou, and C. Y. Chang,
J. Appl. Phys. 80, 1623共1996兲.
8A. C. Schmitz, A. T. Ping, M. A. Khan, Q. Chen, J. W. Yang, and I.
Adesida, Semicond. Sci. Technol. 11, 1464共1996兲.
9CRC Handbook of Chemistry and Physics, 75th ed., edited by D. R. Lide
and A. P. R. Friderikse共CRC, Boca Raton, FL, 1995兲.
10C. S. Lee, Y. J. Lin, and C. T. Lee, Appl. Phys. Lett. 79, 3815共2001兲. 11L. H. Peng, C. H. Liao, Y. C. Hsu, C. S. Jong, C. N. Huang, J. K. Ho, C.
C. Chiu, and C. Y. Chen, Appl. Phys. Lett. 76, 511共2000兲.
12O. Ambacher, J. Phys. D 31, 2653共1998兲.
13S. J. Cho, S. Dogan, S. Sabuktagin, M. A. Reshchikov, D. K. Johnstone,
and H. Morkoc, Appl. Phys. Lett. 84, 3070共2004兲.
14C. T. Lee and H. W. Kao, Appl. Phys. Lett. 76, 2364共2000兲.
15M. Sawada, T. Sawada, Y. Yamagata, K. Imai, H. Kimura, M. Yoshino, K.
Iizuka, and H. Tomozawa, J. Cryst. Growth 189/190, 706共1998兲.
16H. Jiang, N. Nakata, G. Y. Zhao, H. Ishikawa, C. L. Shao, T. Egawa, T.
Jimbo, and M. Umeno, Jpn. J. Appl. Phys., Part 2 40, L505共2001兲.
17K. A. Ricket, A. B. Ellis, F. J. Himpsel, J. Sun, and T. F. Kuech, Appl.
Phys. Lett. 80, 204共2002兲.
18S. J. Pearton, J. C. Zolper, R. J. Shul, and F. Ren, J. Appl. Phys. 86, 1
共1999兲.
024507-4 Chen et al. J. Appl. Phys. 101, 024507共2007兲
