Author name used in this publication: X. M. Tao2006-2007 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Beling, CD

Cheung, CH

Cheung, CK

Djurišić, AB

Fung, S

Tao, X

Wang, RX

Xu, SJ

Yang, H

Zhao, DG

Applied Physics Letters

PolyU Institutional Repository

APPLIED PHYSICS LETTERS 89, 033503 2006Influence of indium-tin-oxide thin-film quality on reverse leakage currentof indium-tin-oxide/n-GaN Schottky contactsR. X. Wang,a S. J. Xu,b A. B. Djurišić, C. D. Beling, C. K. Cheung,C. H. Cheung, 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 of Sciences, Beijing 100083, ChinaX. M. TaoInstitute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon, Hong Kong, ChinaReceived 21 January 2006; accepted 20 June 2006; published online 19 July 2006Indium-tin-oxide ITO/n-GaN Schottky contacts were prepared by e-beam evaporation at 200 °Cunder various partial pressures of oxygen. X-ray photoemission spectroscopy and positron beammeasurements were employed to obtain chemical and structural information of the deposited ITOfilms. The results indicated that the observed variation in the reverse leakage current of the Schottkycontact and the optical transmittance of the ITO films were strongly dependent on the quality of theITO film. The high concentration of point defects at the ITO-GaN interface is suggested to beresponsible for the large observed leakage current of the ITO/n-GaN Schottky contacts. © 2006American Institute of Physics. DOI: 10.1063/1.2227627As a result of the rapid development of optoelectronicdevices, indium tin oxide ITO has been used in a greatvariety of practical devices For example, under suitable fab-rication and postdeposition conditions, ITO can be used suc-cessfully as either a Schottky or Ohmic contact to p- andn-type III-nitride based devices such as ultravioletphotodetectors1 and LEDs.2,3 Even with these successes,some open issues still remain regarding the influence of ITOpreparation conditions on the results. The technique selectedfor ITO deposition must be optimized since growth condi-tions strongly affect the microstructures and chemical com-positions of the deposited ITO films,4 which in turn modifythe electrical and optical properties of the ITO films5–7 andtheir specific performance on GaN devices.8It is well known that the gaseous environment duringany deposition can have a large effect on the quality andoptoelectrical properties of the ITO film.4,9 The oxygen en-vironment is of particular importance with regard to ITOcontacts on III-nitrides through the diffusion and reaction ofoxygen. For example, the diffusion of oxygen into GaN froma sapphire substrate10 and a SiO2 top layer11 is known tohave a significant effect on the electrical and optical proper-ties of the GaN film. While this effect has been discussed10,11further investigation is required. In this work we focus on theeffect that varying the oxygen partial pressure has on theelectrical characteristics of ITO/n-GaN Schottky contacts.The results show that the higher oxygen partial pressures canimprove the quality of the ITO film crystalline structure andthat this has the benefit of reducing the reverse leakage cur-rent of the ITO/n-GaN Schottky contact and improving theoptical transmittance of the ITO.Lightly Si-doped n-type GaN epilayer wafer grown on0001 sapphire substrate by metal-organic chemical vaporaPresent address: Institute of Textiles and Clothing, The Hong Kong Poly-technic University, Kowloon, Hong Kong, People’s Republic of China.bAuthor to whom correspondence should be addressed; electronic mail:sjxu@hkucc.hku.hk0003-6951/2006/893/033503/3/$23.00 89, 03350Downloaded 26 Oct 2008 to 158.132.12.80. Redistribution subject to deposition were used for this study. Using Hall measure-ments, the carrier concentration and mobility of the GaN wasfound to be 3.31016 cm−3 and 819 cm2 V−1 s−1, respec-tively. Prior to making the ITO Schottky contact and the InOhmic contact, the samples were sequentially cleaned by anultrasonic cleaner with acetone and ethanol, and de-ionizedwater. ITO films were prepared by e-beam evaporation withthe substrate temperature maintained at 200 °C under differ-ent oxygen flow rates from 0 to 6 SCCM SCCM denotescubic centimeter per minute at STP. The corresponding par-tial pressures of oxygen can be found in Table I. Amorphousglass was used as a reference substrate to study the opticaltransmittance and electrical properties of the ITO films. Thetransmittance of the ITO films was measured in an HP 8453UV-visible spectrophotometer system. Current-voltage I-Vmeasurements were made using an HP 4155A semiconductorparameter analyzer. The chemical compound information foroxygen, indium, and tin was studied using a PHI 5600 x-rayphotoelectron spectroscopy XPS system. Variable energyDoppler broadening spectroscopy VEDBS measurementswere made using the low energy positron beam facility at theUniversity of Hong Kong.12Figure 1 shows typical room-temperature I-V character-istics of the ITO/n-GaN Schottky contacts fabricated underdifferent oxygen partial pressures. It can be seen that allI-V curves clearly exhibit rectification characteristics. How-ever, it is noted that the reverse-biased leakage current of thecontacts deposited with higher than 3 SCCM oxygen flowrates is much smaller as compared with contacts depositedwith lower or zero oxygen flow rate. Based on thermionic-emission theory the Schottky parameters are extracted byfitting the forward exponential section of the formula13I = AA*T2e−qb/kTeqV/nkT − 1 , 1where A, A*, b, and n are the contact area, the Richardsonconstant, the Schottky barrier height, and the ideality factor,respectively. The values of b and n are determined as givenin Table I. In sharp contrast to the substantial variation of© 2006 American Institute of Physics3-1AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp033503-2 Wang et al. Appl. Phys. Lett. 89, 033503 2006the reverse leakage current, the values of b and n are foundto be not very sensitive to the oxygen partial pressure. Ourresults certainly seem consistent with Hsu et al.’s.14 As willbe shown later, the reverse bias leakage current in our ITO/n-GaN Schottky contacts is strongly correlated with the de-fects in the ITO films, in particular, defects at the ITO-GaNinterface.Figure 2 shows the transmittance curves of the ITO filmsgrown under different oxygen partial pressures. It is ob-served that the transmittance of the ITO films deposited un-der high oxygen partial pressure is much higher than that ofthe films deposited under zero or low oxygen partial pres-sure. Moreover, the transmittance edge of the films fabri-cated under high oxygen partial pressure shifts to shorterwavelengths as compared with that of films deposited with-out any oxygen flow.To explore and investigate the reasons for the oxygenenvironment influencing the reverse-biased characteristic andoptical transmittance, an XPS study was performed. Figure 3shows the XPS Sn 3d, In 3d, and O 1s data for the filmsgrown under different oxygen partial pressures. It can beseen that the relative intensity of Sn decreases for higheroxygen partial pressures. The integrated intensity ratiosSn/ In and Sn/ In+Sn+O, as obtained from the XPS data, aregiven in Table I. The ratios of films deposited under low orzero oxygen partial pressure are almost twice that of the onesdeposited under high oxygen partial pressure. This change inSn ratio follows the change seen in the optical transmittanceresult as mentioned above in which the absorption edgeshifts to the lower energy side under excessive Sn. This re-sult agrees with that reported by Kim et al.5TABLE I. Properties of devices and ITO films depoSample labels aOxygen flow rate SCCM 0Oxygen partial pressure 10−3 torr 0Barrier height eV 0.63±0.003Ideality factor n 2.10±0.91Reverse leakage current at −10 VA 2300Sn/In ratio % 27Sn/O+In+Sn ratio % 6Concentration 1021 cm−3 4.01Mobility cm2/V s 2.62Sheet resistance  / 29.9FIG. 1. Typical I-V characteristics of ITO/n-GaN Schottky contacts fabri-cated under different partial pressures of oxygen.Downloaded 26 Oct 2008 to 158.132.12.80. Redistribution subject to Sn acts as a cationic dopant when it substitutes for Inand is surrounded by In2O3. As such, it provides an electronto the conduction band making the ITO conducting. How-ever, too high a Sn/ In ratio negatively influences the electri-cal properties of ITO film.4,6 A number of suggestions havebeen made to account for this behavior.4,6,15 1 When one ofthe nearest-neighbor In2O3 sites around the Sn ion is occu-pied with a second Sn ion, the donor action is compensated.2 If excess Sn doping occupies an interstitial site then itforms a deep level electron trap rather than an electron donorand 3 some other hitherto unknown reason exists for Snbeing electrically inactive in amorphous microstructure. Inour case, the Sn/ In weight ratio in the source was 10% andthe substrate temperature during ITO film deposition was200 °C which was around the ITO crystallizationtemperature.16 It is thus that the microstructure of the ITOfilm would be polycrystalline16 which places the Sn dopingin a regime where behavior is difficult to interpret. Interest-ingly, it is noted that for zero and low oxygen partial pres-sures, the sheet resistance of ITO film decreases with in-creasing oxygen partial pressure. This can best be understoodas the primary Sn dopant effect.17 However, for higher oxy-gen partial pressures, the sheet resistance of ITO film in-creases with increasing oxygen flow rate. One possible ex-planation for this effect is the decrease in the concentrationof oxygen vacancies.7To test the idea that there exist defects that form deeplevel traps and to investigate the crystal structure of the ITOfilm and the interface between ITO and GaN, positron beammeasurements were performed. Figure 4 shows the VEDBSscans taken on the different contacts grown under differentpartial pressures of oxygen. The low electron momentum Sparameter is usually higher in material that contains a highat 200 °C.b c d e3 5 6.3 3.6 6.2 7.9.67±0.03 0.77±0.04 0.75±0.03 0.77±0.02.75±0.38 1.87±0.65 1.70±0.42 1.75±0.691.5 0.031 0.073 0.0785 12 ¯ 124 ¯ 3.58 1 4.85 5.721.2 4.13 4.72 4.370.8 75.5 136.5 124FIG. 2. Transmittance of ITO film deposited under different partial pres-sited1101914211sures of oxygen.AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp033503-3 Wang et al. Appl. Phys. Lett. 89, 033503 2006concentration of open volume point defects. This is a resultof the positron trapping into open volume defect sites whereit predominantly samples low momentum valence electrons.The VEDBS data in the 0–10 keV implantation energyrange are indicative of annihilations occurring in the ITOoverlayer. At 10 keV, the positron beam is maximally sam-pling the ITO-GaN interface while at higher energies, the Sparameter drops to the value characterizing the GaN film.The remarkable feature of this positron annihilation data isthat the ITO S parameter is significantly higher in the zerooxygen partial pressure case and drops with increasing oxy-gen partial pressure. This clearly supports the oxygen va-cancy model, oxygen vacancies being more predominant inthe films grown without oxygen. Also noted, particularly inthe zero oxygen case a peaking in the S parameter is a peak-ing at the ITO/GaN interface. This indicates the presence ofvacancy-type defects at the interface. This peaking phenom-enon seems to reduce under higher oxygen partial pressures,presumably as a result of the point defects being removedunder oxygenation.The above evidence for the presence of oxygen vacancydefects both in the ITO film and at the ITO-GaN interfacecan lend some understanding to the observed pattern of re-verse leakage current. The reverse leakage current expectedfor an ideal ITO/GaN Schottky contact can be estimated us-ing the standard equation AA*T2e−qb/kT and the Richardsonconstant A* which may be taken as 2.64105 A m−2 K−2 forGaN.18 The values obtained in this way are much smallerthan the measured reverse leakage current. This indicatesthat for contacts made under zero or low oxygen concentra-tion, the oxygen vacancy defects can act as carrier-generation centers at the interface causing larger than ex-pected leakage current. The real picture is likely to be morecomplicated with other types of point defects making contri-butions to the generation and tunneling currents. Miller etal.19 have shown that, at higher temperatures, the reverseleakage current of Ni/n-GaN Schottky contact is mainly dueto a two-step, trap-assisted tunneling. For our case, it is alsopossible that the increased crystallinity and oxygen contentof the ITO film under high oxygen partial pressure results in8FIG. 3. Sn 3d top, In 3d middle, and O 1s bottom normalized XPScurves of the ITO films deposited on GaN wafer under different partialpressures of oxygen.forming a GaOx insulator layer that can reduce tunneling.Downloaded 26 Oct 2008 to 158.132.12.80. Redistribution subject to In conclusion, ITO/GaN Schottky contacts fabricated un-der different oxygen partial pressures have been studied byI-V, XPS, and positron beam techniques. Contacts grownunder zero or low oxygen partial pressure show poor reversecharacteristics. This finding has been related to the higherSn/ In ratio found in low oxygenated films and the poor ITOcrystallinity. The higher concentration of point defects mostlikely oxygen vacancies as seen through positron annihila-tion is a source of increased generation current in theSchottky contacts exhibiting high reverse leakage current.This work was supported by the HKSAR RGC CERGgrants under Contract Nos. HKU7036/03P and HKU7021/04P. Financial support from the University DevelopmentFund grant administrated by The University of Hong Kongis also acknowledged.1C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. Y. Chi, C. A. Chang, J. K.Sheu, and J. F. Chen, IEEE Photonics Technol. Lett. 13, 848 2001.2D. W. Kim, H. Y. Lee, M. C. Yoo, and G. Y. Yeom, Appl. Phys. Lett. 86,052108 2005.3C. S. Chang, S. J. Chang, Y. K. Su, Y. C. Lin, Y. P. Hsu, S. C. Shei, S. C.Chen, C. H. Liu, and U. H. Liaw, Semicond. Sci. Technol. 18, L212003.4M. Rottmann, H. Hennig, B. Ziemer, R. Kalahna, and K. H. Heckner,J. Mater. Sci. 31, 6465 1996.5H. Kim, C. M. Gilmore, A. Pique, J. S. Horwitz, H. Mattoussi, H. Murata,Z. H. Kafafi, and D. B. Chrisey, J. Appl. Phys. 86, 6451 1999.6F. O. Adurodija, H. Izumi, T. Ishihara, H. Yoshioka, and M. Motoyama,J. Appl. Phys. 88, 4175 2000.7W. G. Haines and R. H. Bube, J. Appl. Phys. 49, 304 1978.8J. K. Sheu, Y. K. Su, G. C. Chi, M. J. Jou, and C. M. Chang, Appl. Phys.Lett. 72, 3317 1998.9W. F. Wu and B. S. Chiou, Semicond. Sci. Technol. 11, 196 1996.10J. E. Van Nostrand, J. Solomon, A. Saxler, Q. H. Xie, D. C. Reynolds, andD. C. Look, J. Appl. Phys. 87, 8766 2000.11X. C. Wang, S. J. Xu, S. J. Chua, K. Li, X. H. Zhang, Z. H. Zhang, K. B.Chong, and X. Zhang, Appl. Phys. Lett. 74, 818 1999.12P. Y. Kuan, S. Fung, and C. D. Beling, Appl. Surf. Sci. 194, 32 2002.13S. M. Sze, Physics of Semiconductor Devices, 2nd ed. Wiley, New York,1981.14J. 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 2001.15H. Kim, C. M. Gilmore, A. Piqué, J. S. Horwitz, H. Mattoussi, H. Murata,Z. H. Kafafi, and D. B. Chrisey, J. Appl. Phys. 86, 6451 1999.16R. X. Wang, C. D. Beling, S. Fung, A. B. Djurišić, C. C. Ling, C. Kwong,and S. Li, J. Phys. D 38, 2000 2005.17S. Noguchi and H. Sakata, J. Phys. D 14, 1523 1981.18A. C. Schmitz, A. T. Ping, M. Asif Khan, Q. Chen, J. W. Yang, and I.Adesida, Semicond. Sci. Technol. 11, 1464 1996.19E. J. Miller, E. T. Yu, P. Waltereit, and J. S. Speck, Appl. Phys. Lett. 84,FIG. 4. Positron beam VEDBS scans of the ITO/GaN Schottky contactsdeposited under different oxygen partial pressures.535 2004.AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Influence of indium-tin-oxide thin-film quality on reverse leakage current of indium-tin-oxide/n-GaN Schottky contacts

Influence of indium-tin-oxide thin-film quality on reverse leakage current of indium-tin-oxide/n-GaN Schottky contacts

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