Changes in surface state density due to chlorine treatment in GaN Schottky ultraviolet photodetectors

(1)

Changes in surface state density due to chlorine treatment in GaN

Schottky ultraviolet photodetectors

Ching-Ting Lee,1,a兲 Chih-Chien Lin,1Hsin-Ying Lee,2and Po-Sung Chen1 1

Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan

2

Department of Electro-Optical Engineering, National Cheng Kung University, 701 Tainan, Taiwan

共Received 15 September 2007; accepted 2 March 2008; published online 5 May 2008兲

A chlorination surface treatment was used to reduce the surface density of states of a n-type GaN surface, which improves the Schottky performances of the resultant metal-semiconductor contact. Using capacitance-frequency measurement, the surface state density of the chlorine-treated GaN surface was about one order less than that without chlorination treatment. The dark current of the chlorine-treated GaN ultraviolet photodetectors共UV-PDs兲 is 1.5 orders of magnitude lower than that of those without chlorination treatment. The products of quantum efficiency and internal gain of the GaN Schottky UV-PDs without and with chlorination treatment under conditions of −10 V reverse bias voltage at a wavelength of 330 nm were 650% and 100%, respectively. The internal gain in chlorine-treated GaN UV-PDs can therefore be reduced due to a decrease in the surface state density. © 2008 American Institute of Physics.关DOI:10.1063/1.2913344兴

I. INTRODUCTION

Impressive progress has recently been made in III–V nitride-based compound semiconductors, which have been widely used in electronic devices and optoelectronic devices because of their inherent advantageous properties.1–4 For GaN-based electronic and optoelectronic devices, high-quality and reliable metal-semiconductor contacts are critical for gaining satisfactory performance. However, GaN-based compound semiconductors do contain high surface state den-sities. GaN Schottky ultraviolet photodetectors 共UV-PDs兲 show a high internal gain as a result of hole trapping and electron injection by surface states.5,6The high inherent sur-face state density ultimately renders GaN semiconductors with a large dark current and a low Schottky barrier height.7 A low dark current can effectively reduce the current noise in order to deliver a lower minimum detectable power in PDs. A Schottky barrier height in a range from 0.56 共Ref. 8兲 to

1.09 eV 共Ref. 9兲 has been reported for Ni contacted with

n-type GaN. The difference in the Schottky barrier height is attributed to the relative surface state variation. Several methods have been reported to increase the Schottky barrier height of III–V nitride semiconductors.9–11 In the present work, a chlorination treatment method was used to reduce the surface states of the GaN surface. The associated dark current and the photoresponsivity of the resultant UV-PDs were also studied.

II. EXPERIMENTAL PROCEDURE

The epitaxial structure of the GaN-based UV-PDs was grown using a metallorganic chemical-vapor deposition

sys-tem. The sources of Ga and N were trimethylgallium and ammonia, respectively. The n-type dopant source was Si. The UV-PDs consisted of a 30 nm thick undoped GaN buffer layer grown at 500 ° C, and a 2 ␮m thick n-type GaN layer grown at 1100 ° C. The electron concentration and mobility as determined using Hall measurement at room temperature were 2⫻1017 cm−3 and 310 cm2/V s, respectively. The chlorination surface treatment and the associated Schottky mechanism for Ni/Au contact with chlorine-treated n-type GaN layer have been previously studied and reported.11

Using a conventional photolithography and a lift-off technique, GaN Schottky UV-PDs were fabricated and are shown in Fig.1. The Ti/Al/Pt/Au共25/100/50/150 nm兲 long-term thermally stable ohmic metals were deposited on the n-type GaN surface of the UV-PDs using an electron-beam evaporator.12 Concentric ohmic contact rings 共outer radius = 260 ␮m, inner radius= 210 ␮m兲 were patterned and then the samples were annealed in a N2 ambient at 700 ° C for 1 min by a rapid thermal annealing system. The samples were divided into two groups and labeled as either samples A or B.

a兲_{Author to whom correspondence should be addressed. Present address:} Institute of Microelectronics, Department of Electrical Engineering, Na-tional Cheng Kung University, Tainan, Taiwan, Republic of China. Tel.: 886-6-2379582. FAX: 886-6-2362303. Electronic mail:

[email protected]. FIG. 1. 共Color online兲 Schematic configuration of GaN Schottky UV-PDs.

JOURNAL OF APPLIED PHYSICS 103, 094504共2008兲

(2)

Prior to the deposition of the Ni/Au 共2.5/2.5 nm兲 Schottky contact metals on the n-type GaN layer, the B samples were treated using chlorination surface treatment. The Schottky contact area of the B samples was connected with the Pt anodic electrode; the remaining area was protected by AZ-4620 photoresist film. The Ni/Au共2.5/2.5 nm兲 metals used as a thin transparent conductive layer were first deposited. The Ni/Au共100/100 nm兲 pad with a diameter of 80 ␮m was then deposited on the center of the Ni/Au 共2.5/2.5 nm兲 Schottky region using photolithography and the lift-off techniques.

III. EXPERIMENTAL RESULTS AND DISCUSSION

The surface states of the GaN Schottky UV-PDs do not contribute to the capacitance at high frequencies since the associated charges cannot follow the high frequency signal. At high frequencies, the measured capacitance only consist of space-charge capacitance 共Csc兲, which can be expressed as13–17

C = Csc共at high frequency兲. 共1兲

At low frequencies, the total capacitance is the sum of the space-charge capacitance共C_sc兲 and the interface capacitance 共Css兲, which can be expressed as

13–17

C = C_sc+ C_ss共at low frequency兲. 共2兲

The surface state density共Nss兲 can be expressed as13–17

Nss= Css/q2A, 共3兲

where A and q are the area of Schottky region and the elec-tron charge, respectively. The surface state energy Essbelow the conduction band共Ec兲 can be expressed as14,18

Ec− Ess= q⌽b− qV, 共4兲

where q⌽b and V are the Schottky barrier height and the applied bias voltage, respectively. Figures2共a兲and2共b兲show the capacitance-frequency 共C-f兲 curves measured at various forward voltages for the UV-PDs with and without chlorina-tion surface treatment, respectively. At high frequencies, the similar capacitance values for chlorine-treated and untreated GaN Schottky UV-PDs were obtained. At a frequency of 100 Hz, the capacitance of chlorine-treated GaN Schottky UV-PDs is about one ninth of that for those without chlorination treatment at zero bias. According to our previous experimen-tal results,11 the ideality factor n of 1.04 and 1.16 and Schottky barrier height q⌽b of 0.95 and 0.75 eV were ob-tained for the Ni/Au contact deposited on n-type GaN with and without chlorination treatment, respectively. Using Eqs.

共1兲–共4兲, the surface state density Nssas a function of surface state energy Ess can be determined and is hereby shown in Fig. 3. The surface state density Nss of GaN Schottky UV-PDs without subjecting to chlorination treatment was varied from 1.9⫻1012 eV−1cm−2 共Ec− 0.75 eV兲 to 4.4 ⫻1012 _eV−1_cm−2 _共E

c− 0.45 eV兲. On the other hand, the surface state density of chlorine-treated UV-PDs was var-ied from 2.1⫻1011 eV−1cm−2 共Ec− 0.95 eV兲 to 3.5 ⫻1011 _eV−1_cm−2 _共E

c− 0.65 eV兲. The surface state density of the chlorine-treated GaN surface is about one order less than that of GaN without chlorination treatment. The

chlorine-treated GaN Schottky UV-PDs can induce more Ga vacancies while causing the numbers of N vacancies to de-crease due to the formation of GaClxand GaOx, respectively. Therefore, the lower surface state density of chlorine-treated UV-PDs can be attributed to the reduction in Ga dangling bonds and the passivation of nitrogen vacancies as a result of GaOxformation.11

FIG. 2. 共Color online兲 Capacitance-frequency curves measured at various forward voltages for GaN Schottky UV-PDs共a兲 with and 共b兲 without chlo-rination treatment.

FIG. 3. 共Color online兲 The energy distribution of surface states for GaN Schottky UV-PDs with and without chlorination treatment.

094504-2 Lee et al. J. Appl. Phys. 103, 094504共2008兲

(3)

The current-voltage 共I-V兲 characteristics of the fabri-cated GaN UV-PDs were measured using an HP4145B semi-conductor parameter analyzer. Figure 4shows the dark cur-rent as a function of the reverse voltage of the GaN UV-PDs with and without the chlorination surface treatment. Nitrogen vacancy related surface states existed on the surface of n-type GaN generally result in a thin n+ _{region on the} surface.19 The n+ _{region can allow electrons to tunnel} through the Schottky barrier under reverse bias, thereby in-creasing the reverse leakage current. An increase in dark cur-rent with applied reverse voltage can also be attributed to the band bending, reduction in Schottky barrier height, and ther-mionic field emission.5 In chlorine-treated n-type GaN, the formation of GaOxdecreases the surface states by reducing Ga dangling bonds and filling nitrogen vacancies. The dark current of chlorine-treated Schottky UV-PDs was 1.5 orders of magnitude smaller than that of those without chlorination treatment.

Figures 5共a兲 and 5共b兲 show the photoresponsivity as a function of wavelength for the GaN Schottky UV-PDs with and without the chlorination surface treatment, respectively. A xenon共Xe兲 lamp through a calibrated monochromator was used as the pumping source. The photoresponsivity of UV-PDs without chlorination treatment is larger than that of those being treated with such treatment. The photoresponsiv-ity measured under conditions of −10 V reverse bias voltage and the wavelength of 330 nm of UV-PDs with and without chlorination treatment were 0.27 and 1.73 A/W, respectively. When a reverse voltage of −5 V was applied, the UV-visible rejection ratio of 104 _{and 10}3 _{was obtained for the GaN} UV-PDs with and without chlorination treatment, respec-tively. The UV-visible rejection ratio of chlorine-treated GaN UV-PDs is about one order higher than that of those without chlorination treatment, which is ascribed to the reduction in surface states.

Figure 6 shows the product of quantum efficiency and internal gain as a function of reverse voltage at a wavelength of 330 nm of the resultant UV-PDs with and without chlori-nation surface treatment. The product of quantum efficiency and internal gain was measured using an HP4145B semicon-ductor parameter analyzer and a xenon共Xe兲 lamp through a

calibrated monochromator as the pumping source in a dark box. The product of quantum efficiency and internal gain in the GaN Schottky UV-PDs can be calculated by the equation

R =␩G␭/1240, 共5兲

where R is the photoresponsivity, ␩ is the quantum effi-ciency, G is the internal gain, and ␭ is the wavelength of FIG. 4.共Color online兲 Dark current as a function of reverse voltages of GaN

Schottky UV-PDs with and without chlorination treatment.

FIG. 5. 共Color online兲 Photoresponsivity as a function of wavelength at various voltages for GaN Schottky UV-PDs共a兲 with and 共b兲 without chlo-rination treatment.

FIG. 6. 共Color online兲 The product of quantum efficiency and internal gain as a function of reverse voltages of GaN Schottky UV-PDs with and without chlorination treatment.

(4)

pumping source. The product of quantum efficiency and in-ternal gain in untreated UV-PDs is larger than 100%, indi-cating the existence of an internal gain. The internal gain can be attributed to the reduction in the Schottky barrier height and the injection of additional electrons into the surface state traps. The surface states of the chlorine-treated GaN Schottky UV-PDs can be effectively passivated, resulting in a smaller reduction in the Schottky barrier height and a smaller internal gain.6,20As shown in Fig.6, the product of quantum efficiency and internal gain in chlorine-treated GaN Schottky UV-PDs slowly increased with increasing applied reverse voltage. An increase in the product of quantum effi-ciency and internal gain under optical illumination indicates an additional injection of electrons at a higher reverse voltage.21Because surface states at the metal-semiconductor interface could trap photogenerated holes, lowering the Schottky barrier height and producing additional gain in the UV photoresponse,6the smaller internal gain in the chlorine-treated GaN samples could be attributed to the effective re-duction and passivation of surface states. The hole trapping effect, which is obvious at higher reverse voltages due to the narrow Schottky barrier width, causes more electrons to tun-nel through the Schottky barrier. The product of quantum efficiency and internal gain of the GaN Schottky UV-PDs with and without chlorination treatment under a reverse volt-age of −10 V at a wavelength of 330 nm are 100% and 650%, respectively.

IV. CONCLUSIONS

Chlorination surface treatment for n-type GaN reduces the surface states as a result of the decrease in Ga dangling bonds and the occupation of nitrogen vacancies caused by the passivation function of the GaOx formation on the chlorine-treated n-type GaN surface. Compared to the GaN Schottky UV-PDs without chlorination surface treatment, the chlorine-treated GaN Schottky UV-PDs show a lower dark current, a lower surface state density, a higher Schottky bar-rier, an ideal ideality factor, and a smaller internal gain. In-ternal gain in Schottky photodetectors is evidently induced

by surface states; therefore, high performances of GaN Schottky UV-PDs could be expected if the surface states of the metal-semiconductor interface are significantly and effec-tively reduced.

ACKNOWLEDGMENTS

This work was supported by the National Science Coun-cil of Taiwan, Republic of China.

1_{T. Murata, M. Hikita, Y. Hirose, Y. Uemoto, K. Inoue, T. Tanaka, and D.} Ueda,IEEE Trans. Electron Devices52, 1042共2005兲.

2_{C. T. Lee, U. Z. Yang, C. S. Lee, and P. S. Chen,}_{IEEE Photon. Technol.}

Lett.18, 2029共2006兲.

3_{R. J. Xie, N. Hirosaki, M. Mitomo, K. Takahashi, and K. Sakuma,}_Appl.

Phys. Lett.88, 101104共2006兲.

4_{D. J. Rogers, F. H. Teherani, A. Yasan, K. Minder, P. Kung, and M.} Razeghi,Appl. Phys. Lett.88, 141918共2006兲.

5_{J. C. Carrano, T. Li, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C.} Campbell,J. Appl. Phys.83, 6148共1998兲.

6_{O. Katz, V. Garber, B. Meyler, G. Bahir, and J. Salzman,}_{Appl. Phys. Lett.}

79, 1417共2001兲.

7_{S. M. Sze, Physics of Semiconductor Devices, 2nd ed.}_{共Wiley, New York,} 1981兲.

8_{J. 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.

9_{C. T. Lee, Y. J. Lin, and D. S. Liu,}_{Appl. Phys. Lett.}_{79, 2573}_共2001兲. 10_{K. A. Rickert, A. B. Ellis, F. J. Himpsel, J. Sun, and T. F. Kuech,}_Appl.

Phys. Lett.80, 204共2002兲.

11_{P. S. Chen, T. H. Lee, L. W. Lai, and C. T. Lee,}_{J. Appl. Phys.}_{101, 024507} 共2007兲.

12_{C. T. Lee and H. W. Kao,}_{Appl. Phys. Lett.}_{76, 2364}_共2000兲.

13_{E. H. Nicollian and A. Goetzberger, Bell Syst. Tech. J. 46, 1055}_共1967兲. 14_{A. Singh,}_{Solid-State Electron.}_{28, 223}_共1985兲.

15_{A. F. Özdemir, A. Türüt, and A. Kőkce,}_{Thin Solid Films}_{425, 210}_共2003兲. 16_{R. X. Wang, S. J. Xu, S. L. Shi, C. D. Beling, S. Fung, D. G. Zhao, H.}

Yang, and X. M. Tao,Appl. Phys. Lett.89, 143505共2006兲.

17_{P. Chattopadhyay and B. Ray Chaudhuri,} _{Solid-State Electron.}_{36, 605} 共1993兲.

18_{B. Bati, C. Nuhoglu, M. Saglam, E. Ayyildiz, and A. Türüt, Phys. Scr. 61,} 209共2000兲.

19_{S. J. Pearton, J. C. Zolper, R. J. Shul, and F. Ren,}_{J. Appl. Phys.}_{86, 1} 共1999兲.

20_{S. J. Chang, M. L. Lee, J. K. Sheu, W. C. Lai, Y. K. Su, C. S. Chang, C.} J. Kao, G. C. Chi, and J. M. Tsai,IEEE Electron Device Lett. 24, 212

共2003兲.

21_{H. Jiang, N. Nakata, G. Y. Zhao, H. Ishikawa, C. L. Shao, T. Egawa, T.} Jimbo, and M. Umeno,Jpn. J. Appl. Phys., Part 240, L505共2001兲.