Under identical preparation conditions, AuGaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers. © 2006 American Institute of Physics.published_or_final_versio

Beling, CD

Fung, S

Shi, SL

Tao, XM

Wang, RX

Xu, SJ

Yang, H

Zhao, DG

Applied Physics Letters

R. X. Wang

S. J. Xu

S. L. Shi

C. D. Beling

S. Fung

D. G. Zhao

H. Yang

X. M. Tao

Crossref

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au∕GaN Schottky contacts

HKU Scholars Hub

TitleProbing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of AuGaNSchottky contactsAuthor(s) Wang, RX; Xu, SJ; Shi, SL; Beling, CD; Fung, S; Zhao, DG; Yang,H; Tao, XMCitation Applied Physics Letters, 2006, v. 89 n. 14Issued Date 2006URL http://hdl.handle.net/10722/80737Rights Creative Commons: Attribution 3.0 Hong Kong LicenseAPPLIED PHYSICS LETTERS 89, 143505 2006Probing deep level centers in GaN epilayers with variable-frequencycapacitance-voltage characteristics of Au/GaN Schottky contactsR. X. Wang,a S. J. Xu,b S. L. Shi, C. D. Beling, and S. FungDepartment of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, ChinaD. G. Zhao and H. YangState Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy ofSciences, Beijing 100083, ChinaX. M. TaoInstitute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong, ChinaReceived 3 July 2006; accepted 10 August 2006; published online 3 October 2006Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds ofGaN epilayers with significantly different background electron concentrations and mobility as wellas yellow emission intensities. Current-voltage I-V and variable-frequency capacitance-voltageC-V characteristics show that the Schottky contacts on the GaN epilayer with a higher backgroundcarrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-Vcharacteristics. This is attributed to the presence of deep level centers. Theoretical simulation of thelow-frequency C-V curves leads to a determination of the density and energy level position of thedeep centers. © 2006 American Institute of Physics. DOI: 10.1063/1.2358207In the past decade, GaN and related III-nitride com-pounds have attracted a great deal of interest because of theirwide optoelectronic and electronic applications, such as blue/ultraviolet light-emitting diodes, ultraviolet detectors, laserdiodes, and high-temperature/high-power transistors. Metal-GaN contacts including Schottky contacts are of great impor-tance for device applications of GaN-based materials. De-spite the commercialization of GaN-based bright light-emitting diodes and even laser diodes,1 a number of issuesrelated to metal-GaN contacts especially Schottky contactsstill remain. These are not well understood and are thus be-ing actively investigated.2–8 Having a work function of5.1 eV, gold Au is one of the most used metals in thefabrication of metal-semiconductor Schottky and Ohmiccontacts. For Au/GaN Schottky contacts, several groupshave reported their results.2,3,5 However, few studies havebeen reported to investigate the influence of deep level cen-ters which are closely associated with the yellow emissionband on the capacitance-voltage characteristics of metal-GaN Schottky contacts. In this letter, we show experimen-tally that the deep level centers located at about 1.3 eV canlead to typical anomalous reverse-bias C-V characteristics ofAu/GaN Schottky diodes, particularly the low-frequencyC-V characteristics.Two n-type GaN epilayers, namely, S1 and S2, grown on0001 sapphire substrate by metal organic chemical vapordeposition, were used as the starting materials for the study.Using standard Hall measurements, the electron concentra-tion and mobility of samples S1 and S2 at room temperaturewere determined to be 1.91017 cm−3 and 331 cm2 V−1 s−1,as well as 3.31016 cm−3 and 836 cm2 V−1 s−1, respectively.Prior to making metal contacts, the samples were cleaned byimmersing the samples sequentially in boiling acetone andaPresent address: Institute of Textiles and Clothing, Hong Kong PolytechnicUniversity, Hung Hom, Kowloon, Hong Kong, China.bAuthor to whom correspondence should be addressed; electronic mail:sjxu@hkucc.hku.hk0003-6951/2006/8914/143505/3/$23.00 89, 14350Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP liceethanol and de-ionized water. To remove any native oxideand contamination, they were dipped into different acid so-lutions after which the samples were rinsed in de-ionizedwater and dried with N2. Au and indium In were used tomake Schottky and Ohmic contacts, respectively. I-V mea-surements were carried out using an HP 4155A parameteranalyzer, and frequency-dependent C-V characteristics of theSchottky contacts were measured using an HP 4294A param-eter analyzer. For the photoluminescence PL measure-ments, the samples were excited by the 325 nm line of aKimmon He–Cd continuous-wave laser with an outputpower of 34 mW at room temperature. The PL setup hasbeen described in detail elsewhere.9Figure 1 shows typical room-temperature I-V character-istics of the Au/GaN Schottky contacts fabricated on the S1and S2 GaN wafers. The I-V curves clearly exhibit rectifica-tion behavior characteristic of common metal-semiconductorjunctions. However, the reverse-bias characteristics of theFIG. 1. Typical I-V characteristics of the Au/GaN Schottky contacts fabri-cated on the S1 and S2 GaN epilayers.© 2006 American Institute of Physics5-1nse or copyright; see http://apl.aip.org/about/rights_and_permissions143505-2 Wang et al. Appl. Phys. Lett. 89, 143505 2006two kinds of Schottky contacts are very different. For thecontacts fabricated on S2, their reverse-bias characteristicsare not only uniform but also reproducible and stable. Thereverse-bias leakage current is around 10−11 A at −20 V. Incontrast, the contacts fabricated on S1 show a much largerreverse leakage current and lower breakdown voltage. Thereproducibility and stability of the reverse-bias I-V charac-teristics of the devices are poor. Based on the thermionic-emission theory, the Schottky parameters can be extracted byfitting the forward exponential section of the I-V character-istic using the equation10I = AA*T2e−qb/kTeqV/nkT − 1 , 1where A, A*, b, and n are the contact area, Richardsonconstant, Schottky barrier height, and the ideality factor, re-spectively. The extracted average barrier heights b ofAu/GaN contacts fabricated on S1 and S2 are 0.77 and0.72 eV, respectively, whereas their average ideality factorsn are 1.45 and 1.73, respectively. The values of b and n ofthe contacts fabricated on S1 look better than those on S2.Now we turn to the C-V characteristics of the devices. Itis well known that defects strongly affect the electrical char-acteristics of metal-semiconductor junctions, in particular,the capacitance-voltage characteristics of the junctions.10 Infact, the effect of defects especially deep traps on the ca-pacitance of a metal-semiconductor Schottky contact affordsa very convenient method for detecting and characterizing anextremely low concentration of defects.11,12 Figure 2 showsthe frequency-dependent C-V curves of Au–GaN Schottkycontacts fabricated on S1 and S2. It can be observed that thefrequency-dependent C-V curves of both the samples arequite different from each other. The most noteworthy differ-ence is that a peak structure around 1.3 V appears whichbecomes more prominent with decreasing applied frequencyfor the capacitance CS1 of the device fabricated on S1. Thisis a typical C-V characteristic of metal-semiconductorSchottky contact due to the existence of defect centers onsemiconductor surface.12 In sharp contrast to the case of S1,the capacitance CS2 of the devices fabricated on S2 simplyand monotonically decreases when the applied voltage islower than 1.3 V, which is the typical behavior of an idealmetal-semiconductor Schottky contact without surface states.The simple monotonic decrease of the device capacitance isDownloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP licedue to the variation of the space-charge capacitance withapplied voltage. For the device with the surface states, anadditional surface-state capacitance should be considered.Thus, the total capacitance of the device is written asCS = CSS + CSC, 2where CSC and CSS are, respectively, the differential space-charge and surface-state capacitances. According to the workof Many et al., CSC and CSS are given by12CSC  dQSC/dVS = q/kTdQSC/dS , 3CSS  dQSS/dVS = q/kTdQSS/dS , 4where QSC is the space-charge density, QSS is the surface-state-charge density which is equal to −qNSS for acceptorlikesurface states and +qNSS for donorlike surface states, q is theelectron charge, k is the Boltzmann constant, T is tempera-ture, and SqVS /kT is dimensionless barrier height. For thecase of fully ionized impurities and single-charge surfacestates characterized with both the density NSS and effectiveenergy level Et, one yields12CSC = 0/LdFS/dvS, CSS = q2NSS/kTdfEt/dS ,5where  and 0 are, respectively, the relative and vacuumdielectric constants of GaN, L is the effective Debye lengthcharacterizing the width of the space-charge region, FS is thevalue of a function F at the surface, and fEt is the Fermi-Dirac distribution function. Usually, the exact expression ofthe function F for a metal-semiconductor Schottky contact isnot easily found. A study of the characteristics of the func-tions FS and fEt reveals that CSC achieves a minimumvalue in the depletion layers and increases rapidly in bothaccumulation and inversion layers. On the other hand, CSSreaches its maximum value when the Fermi level at the sur-face crosses the effective energy level Et which is induced bythe surface states. Based on the work of Many et al.,12 CSSFIG. 2. Color online Frequency-dependent C-Vcurves of both Au–GaN Schottky contacts. The peakstructure in the C-V curves of the gold Schottky contacton the S1 GaN epilayer is due to the surface statesinduced by the deep level centers. The solid line is thefitting curve using Eqs. 2–6 while the dashed line isthe differential space-charge capacitance.can be derived asnse or copyright; see http://apl.aip.org/about/rights_and_permissions143505-3 Wang et al. Appl. Phys. Lett. 89, 143505 2006CSS =q2NSSkTexpEt − Ei/kT − us1 + expEt − Ei/kT − us	2, 6where Ei is the Fermi level of intrinsic semiconductor at 0 Kand uS is the dimensionless surface potential which is relatedto S in a relationship of SuS−ub. Here ub is a constant.Using Eqs. 2–6, we did a fitting to the C-V curve mea-sured at 1 kHz for the device fabricated on S1, as shown inFig. 2. NSS of the device fabricated on S1 is determined to bearound 1.21014 cm−2 and the effective energy level Et is1.3 eV.As the energy gaps of GaN epilayers range between 3.4and 3.5 eV, the energy difference  between the defect levelEt and the conduction band, i.e., =Egap−Et, is around2.1–2.2 eV. This energy difference corresponds to the yel-low band emission which has been widely assigned to theoptical transition from the shallow donors e.g., residual Sito the deep level centers. The room-temperature PL spectraof the samples were measured as shown in Fig. 3. As ex-pected, the intensity of the yellow luminescence band IYLof sample S1 is much stronger than that of S2 whose theband-edge transition at 3.44 eV IBE is much stronger. Theratio IYL/ IBE of S1 is ten times higher than that of S2, whichindicates that the density of deep level centers of S1 is muchhigher than that of S2. As mentioned earlier, sample S1 hashigher background electron concentration and much lowermobility compared to S2. A recent investigation on the mi-crostructural origin of the yellow emission of GaN epilayersreveals that the emission is closely related to edgedislocations.13 Such edge dislocations more likely defectcomplexes may well be the deep level centers as detected inthe variable-frequency C-V measurements. Incidentally, dif-ferent acid solutions were used to clean the surface of S1.However, it is found that the surface treatment has no sig-nificant influence on the electrical properties of the Schottkydevices. For example, the peak structure in the reverse-biasFIG. 3. Color online Room-temperature PL spectra of samples S1 and S2.The band-edge transition and yellow luminescence band are denoted by BEand YL, respectively. The inset depicts the band diagram of the Au/n-GaN Schottky contact under reverse bias. qVbi and qVR are the barrier heightat thermal equilibrium of the contact and the potential drop due to thereverse bias, respectively. Efm represents the Fermi level of gold metal.C-V characteristic is not affected by the surface treatment.Downloaded 19 Apr 2012 to 147.8.21.150. Redistribution subject to AIP liceThis means that the deep level centers involved are prettywell distributed throughout the body of the GaN epilayer.The inset in Fig. 3 shows a schematic diagram of Schottkybarrier with deep level centers under reverse bias. The band-edge BE and yellow luminescence YL transitions are alsoindicated schematically by vertical arrows. When a reversebias is applied, the uniform Fermi level across the entirestructure will be split into quasi-Fermi levels for electronsand holes. The energy level of the deep centers varies withrespect to the Fermi level in the depletion region of theSchottky barrier where the bands are bent. Hence, the occu-pation of the level will also change. As a result, the capaci-tance of the contact structure will be affected. On the otherhand, as the reverse bias is increased, more electrons cangain enough energy to pass through the barrier, resulting inan enhancement of the tunneling and recombination currents,and the reverse current thus becomes larger.14In conclusion, Au Schottky contacts on different n-GaNepilayers were prepared and studied. The results show thatthe deep level centers play an important role in determiningthe electrical characteristics of the Schottky contacts. Thedensity and energy level of the deep level centers can beextracted from the measured C-V curves of the devices.Support from RGC-CERG Grants, HKSAR, under Con-tract Nos HKU7036/03P and HKU7021/04P is acknowl-edged. Two of the authors R.X.W. and S.J.X. wish to thankS.-J. Xiong for his helpful discussion on the theoretical fit-ting and Y. Wang and J. Y. Dai for sharing their variable-frequency C-V measurement system.1S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode Springer,Berlin, 2000.2T. G. G. Maffeis, M. C. Simmonds, S. A. Clark, F. Peiro, P. Haines, and P.J. Parbrook, J. Appl. Phys. 92, 3179 2002.3K. Wang, R. X. Wang, S. Fung, C. D. Beling, X. D. Chen, Y. Huang, S. Li,S. J. Xu, and M. Gong, Mater. Sci. Eng., B 117, 21 2005.4P. Kozodoy, J. P. Ibbetson, H. Marchand, P. T. Fini, S. Keller, J. S. Speck,S. P. DenBaars, and U. K. Mishra, Appl. Phys. Lett. 73, 975 1998.5J. W. P. Hsu, M. J. Manfra, D. V. Lang, S. Richter, S. N. G. Chu, A. M.Sergent, R. N. Kleiman, L. N. Pfeiffer, and R. J. Molnar, Appl. Phys. Lett.78, 1685 2001.6Y. C. Chou, D. Leung, I. Smorchkova, M. Wojtowicz, R. Grundbacher, L.Callejo, Q. Kan, R. Lai, P. H. Liu, D. Eng, and A. Oki, Microelectron.Reliab. 44, 1033 2004.7Z. Y. Liu, D. A. Saulys, and T. F. Kuech, Appl. Phys. Lett. 85, 43912004.8M. R. H. Khan, T. Detchprohm, P. Hacke, K. Hiramatsu, and N. Sawaki, J.Phys. D 28, 1169 1995.9R. X. Wang, S. J. Xu, S. Fung, C. D. Beling, K. Wang, S. Li, Z. F. Wei, T.J. Zhou, J. D. Zhang, Y. Huang, and M. Gong, Appl. Phys. Lett. 87,031906 2005.10S. M. Sze, Physics of Semiconductor Devices, 2nd ed. Wiley, New York,1981, p. 264.11E. H. Rhoderick, Metal-Semiconductor Contacts Oxford UniversityPress, Oxford, 1978, p. 142.12A. Many, Y. Goldstein, and N. B. Grover, Semiconductor Surface North-Holland, Amsterdam, 1965, p. 220.13D. G. Zhao, D. S. Jiang, H. Yang, J. J. Zhu, Z. S. Liu, S. M. Zhang, J. W.Liang, X. Li, X. Y. Li, and H. M. Gong, Appl. Phys. Lett. 88, 2419172006.14R. X. Wang, S. J. Xu, A. B. Djurišić, C. D. Beling, C. K. Cheung, C. H.Cheung, S. Fung, D. G. Zhao, H. Yang, and X. M. Tao, Appl. Phys. Lett.89, 033503 2006.nse or copyright; see http://apl.aip.org/about/rights_and_permissions

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of AuGaN Schottky contacts

Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers.Institute of Textiles and ClothingAuthor name used in this publication: X. M. Ta

Tao, X

The Hong Kong Polytechnic University Pao Yue-kong Library

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au/GaN Schottky contacts

http://hub.hku.hk/bitstream/10722/80737/1/content.pdf

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of AuGaN Schottky contacts

Abstract

Similar works

Full text

Available Versions

Crossref

HKU Scholars Hub

The Hong Kong Polytechnic University Pao Yue-kong Library