Transparent indium zinc oxide ohmic contact to phosphor-doped n -type zinc oxideGuangxia Hu, Bhupendra Kumar, Hao Gong, E. F. Chor, and Ping Wu Citation: Applied Physics Letters 88, 101901 (2006); doi: 10.1063/1.2178404 View online: http://dx.doi.org/10.1063/1.2178404 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highly transparent and low resistance gallium-doped indium oxide contact to p -type GaN Appl. Phys. Lett. 87, 042109 (2005); 10.1063/1.1999012 Low-resistance and highly transparent Ni/indium-tin oxide ohmic contacts to phosphorous-doped p -type ZnO Appl. Phys. Lett. 86, 211902 (2005); 10.1063/1.1935030 Low-resistivity and transparent indium-oxide-doped ZnO ohmic contact to p -type GaN Appl. Phys. Lett. 85, 6191 (2004); 10.1063/1.1826231 Highly low resistance and transparent Ni/ZnO ohmic contacts to p-type GaN Appl. Phys. Lett. 83, 479 (2003); 10.1063/1.1591236 Investigation of transparent and conductive undoped Zn 2 In 2 O 5−x films deposited on n-type GaN layers J. Appl. Phys. 92, 274 (2002); 10.1063/1.1481207

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Transparent indium zinc oxide ohmic contact to phosphor-doped n-typezinc oxide

Guangxia Hu, Bhupendra Kumar, and Hao Gonga�

Department of Materials Science and Engineering, National University of Singapore,10 Kent Ridge Crescent, Singapore 119260, Singapore

E. F. ChorDepartment of Electrical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore119260, Singapore

Ping WuInstitute of High Performance Computing, 1 Science Park Road, No. 01-01 The Capricorn,Singapore 117528

�Received 23 August 2005; accepted 5 January 2006; published online 6 March 2006�

Transparent indium zinc oxide �IZO� ohmic contacts to phosphor-doped n-type ZnO have beenformed. The resistance, transmittance, and phase reliability of the contacts were investigated. Asdeposited, an ohmic contact was formed with a specific contact resistance of about 1.1�10−4 � cm2 and the transmittance of the ZnO/IZO �520/350 nm� film was more than 75% in the450–1100 nm wavelength range. After annealing at 400 °C for 5 min in a vacuum �2�10−5 mbar�, the specific contact resistance was reduced by about two orders of magnitude to3.8�10−6 � cm2, while maintaining the contact stability and high optical transparency. © 2006American Institute of Physics. �DOI: 10.1063/1.2178404�

Recently, zinc oxide �ZnO� has attracted much attentionas a direct wide band-gap semiconductor for applications inthe field of short-wavelength optoelectronics, light-emittingdiodes �LEDs�, and laser diodes due to its promising proper-ties, such as larger exciton binding energy �60 meV� at roomtemperature, lower cost, and higher chemical etching rate incomparison with GaN.1–3 To fabricate high-performance op-toelectronic devices, it is necessary to form reliable and lowresistance ohmic contacts with low light absorption. Manyschemes with low resistance ohmic contacts to ZnO havebeen reported, including Ti/Al/Pt/Au, Ti/Au, Al/Pt, nonal-loyed Al, Pt–Ga, Pt/Au, Ti/Al, and Ni/Au.4–11 However,such contacts involve metals and are opaque. Recently, Kanget al.12 reported the Ni/indium tin oxide �5/50 nm� contactsto p-type ZnO, with a specific resistance about 6.2�10−5 � cm2. The indium zinc oxide �IZO� system has beenreported to be an ideal transparent electrode for optoelec-tronic devices, solar cells, and organic LEDs, due to a largerwork function, a wide transmittance window from400 to 2500 nm, and a higher chemical etching rate in com-parison with indium tin oxide thin films.13–20 If transparentIZO can be used as an ohmic contact to ZnO optoelectronicdevices, the efficiency of photon emission and detection ofLEDs and detecting devices will be significantly increased.In this letter, we report, for the first time, a transparent IZOohmic contact to phosphorus doped n-type ZnO.

The 520 nm phosphor-doped ZnO films were depositedon a glass substrate by using a radio-frequency sputteringapparatus �Coaxial power system: MN600�. A ZnO targetcontaining 1.5 wt % P2O5 was employed. The substrate tem-perature was 480 °C. The base pressure was 4�10−7 Torr.The working Ar pressure was 1�10−2 Torr. The films wereannealed at 600 °C in a vacuum �2�10−5 mbar� for 20 min

after the sputtering deposition. The annealed films were sub-sequently cleaned ultrasonically with organic reagents�acetone and ethanol� and then blown dry by using pure N2gas. Transmission line method �TLM� patterns with IZO ascontacts were formed by the standard optical lithography,mesa wet etch �5% CH3COOH solution�, contact windowsopening, deposition of IZO by sputtering, and lift-off tech-nique. An IZO layer of 350 nm was deposited by co-sputtering of zinc oxide and indium oxide targets, followingthe deposition conditions reported by our group previously.18

The mobility and carrier concentration, measured with aBIO-RAD HL55WL Hall effect system in a van der Pauwconfiguration, were 3.8�1019 cm−3, 1.4 cm2/V s, and 5�1019 cm−3, 82.4 cm2/V s for the ZnO film �after mesa wetetch� and the as-deposited IZO film, respectively. The area ofeach of the IZO contact pads is 100�100 �m2 and the spac-ing between IZO contact pads varies from 5 to 30 �m. Afterthe formation of the IZO pads, the samples were ultrasoni-cally cleaned with acetone and ethanol, and then blown dryusing pure N2 gas. Some samples were annealed at 300 °C,400 °C, or 500 °C in a vacuum �2�10−5 mbar� for 5 min.Optical properties of the films were measured with a UV-1600 PC spectrophotometer �Shimadzu�. The current-voltage�I-V� data were measured by a parameter analyzer�HP4155A� using the four-point probe measurement. X-raydiffraction �XRD� spectra were obtained with an X’Pert dif-fractometer �Model PW3040, �Cu K�=0.154 18 nm�. Thecomposition of the IZO/ZnO structures was measured usingsecondary ion mass spectroscopy �SIMS�.

Figure 1 shows the optical transmittance of the as-deposited 300 °C, 400 °C, and 500 °C annealed IZO con-tacts to the phosphor-doped ZnO films. The ZnO film on aglass substrate was used as the reference to calibrate the lighttransmittance measurements. The as-deposited sample showsa transmittance window ranging from a wavelength of370 nm to well above 1100 nm. From a wavelength of

a�Author to whom correspondence should be addressed; electronic mail:msegongh@nus.edu.sg

APPLIED PHYSICS LETTERS 88, 101901 �2006�

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450 nm onward, the transmittance varies between 75% and95%. Transmittance of the samples remains high even afterannealing up to 500 °C. The I-V curves of the IZO contactsto the n-type ZnO as a function of annealing temperature,measured between the contact pads with a spacing of 5 �m,are shown in the inset of Fig. 1. The I-V curve of the as-deposited sample exhibits a linear characteristic which indi-cates the formation of an ohmic contact in the as-depositedstate. With an increased annealing temperature, the slope ofthe I-V curve increases, which suggests a reduction in thetotal resistance. The specific contact resistances were ob-tained from the plots of the measured resistance between twocontact pads versus the pad spacing. The details of the cal-culation of specific contact resistance using the TLM patternsare referred to in literature.21,22 The specific contact resis-tance is 1.1�10−4 � cm2 for the as-deposited sample. Afterannealing, the specific contact resistance is 1.7�10−5, 3.8�10−6, and 3.6�10−5 � cm2 for the 300 °C, 400 °C, and500 °C annealed samples, respectively. It is observed thatannealing at 400 °C can lower the specific contact resistanceby nearly two orders of magnitude compared to that of theas-deposited contact.

Figure 2 shows the SIMS depth profile of the as-deposited and 500 °C annealed samples of the same thick-ness. In the as-deposited sample, a small amount of indium isfound to diffuse into the ZnO layer, as shown in Fig. 2�a�.There is little difference between the depth profiles of theas-deposited sample and the 400 °C annealed sample �TheSIMS depth profile of the 400 °C annealed sample is notshown here�, which shows that the interdiffusion between theZnO/IZO layers does not change significantly for annealingtemperatures below 400 °C. After being annealed at 500 °Cin a vacuum for 5 min, a significant amount of indium hasdiffused into the ZnO layer and, at the same time, there isalso a substantial outdiffusion of zinc into the IZO layer, asshown in Fig. 2�b�. In addition, it is noted that the sputteringtime taken to complete the depth profiling was substantiallylonger for the 500 °C annealed sample, even though bothsamples are equally thick. However, the times required toprofile the IZO and the ZnO layers for both samples are

about the same, so this could indicate that the formation ofcompounds in the interfacial layer are harder to sputter.

In order to investigate the interfacial reaction betweenIZO and ZnO layers, XRD examinations were carried out.Figure 3�a� shows the XRD spectra of the as-deposited IZOfilm grown on glass substrate and ZnO/IZO samples beforeand after annealing. The diffraction peak of the as-depositedIZO sample at 2 theta �2��=31° corresponds to the �008�diffraction planes of IZO.13,15,20,23 Three peaks are observed

FIG. 1. Optical transmittance spectra of the IZO films at the as-depositedstate and after being annealed at 300 °C, 400 °C, and 500 °C in vacuum�2�10−5 mbar� for 5 min. The inset shows the I-V curve of the IZO con-tacts to phosphorus doped n-type ZnO at the as-deposited state and the sameannealing temperatures in a vacuum for 5 min.

FIG. 2. SIMS depth profile spectra of the IZO/ZnO samples: �a� As-deposited and �b� after annealing at 500 °C for 5 min in a vacuum.

FIG. 3. XRD spectra of the as-deposited IZO sample and the IZO/ZnOsamples at various annealing conditions. The 2� values of ZnO �002� plane,ZnO �004� plane, Zn2In2O5 �008� planes, and Zn3In2O5 �00 15� plane are34.400 �Ref. 23�, 72.516 �Ref. 23�, 31.008 �Ref. 24�, and 31.547 �Ref. 25�,respectively. The inset shows an expanded view of the diffraction peakbetween 2�=31 and 32°.

101901-2 Hu et al. Appl. Phys. Lett. 88, 101901 �2006�

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for the ZnO/IZO samples, two of which belong to ZnO�002� �2�=34.40°, where � is the Bragg angle� and ZnO�004� �2�=72.52° � diffraction planes.24 The third or lowestdiffraction peak of the as-deposited IZO/ZnO sample has ashift of about 0.22° toward the higher Bragg angle from thatof �008� IZO at 2�=31°, as seen in the inset of Fig. 3. Thisreveals that the interfacial layer, formed by the reaction be-tween IZO and ZnO layers during the cosputtering process,has a higher atomic ratio of Zn. No difference is observed inthe XRD spectra of the as-deposited and 400 °C annealedZnO/IZO samples. This indicates that the ZnO/IZO contactis thermally stable up to 400 °C. However, there is a shift ofanother 0.18° in the diffraction peak toward the higher Braggangle after annealing at 500 °C, as shown in the inset of Fig.3. A similar trend was observed in Refs. 15 and 20 for theIZO system with increasing amounts of zinc in the films.15,20

Therefore, we suggest that the shift in the diffraction peakmight be due to the interdiffusion of the indium and zincatoms between the ZnO and IZO layers at 500 °C, leading tothe formation of Zn3In2O6 which has a �0 0 15� diffractionplane at 2�=31.547° in the interfacial layer. This observationis consistent with our SIMS results.

The IZO has a band gap about 2.9 eV and a work func-tion of about 4.9–5.2 eV.17,19 The n-type ZnO has a bandgapof about 3.3 eV �Refs. 1–3� and a work function of about4.2–4.5 eV.26,27 When the IZO layer is contacted to the ZnOlayer, a barrier is formed. In the as-deposited condition, theformation of the ohmic contact may be attributed to the in-terface reactions between the ZnO and IZO during thecosputtering process, where some indium atoms diffused intothe ZnO layer, hence increasing the interface doping concen-tration, and decreasing the barrier width of the interface.When the tunneling effect is dominant, the specific contactresistance �c is proportional to exp��2��sm

* /��B /�ND��,28

where �s is the semiconductor permittivity, m* is the effec-tive mass, B is the barrier height, and the ND is the dopingconcentration. The specific contact resistance dependsstrongly on the doping concentration. The carrier concentra-tion and electron mobility of the n-type ZnO and n-type IZOfilms �on a glass substrate� are listed in Table I. The carrierconcentrations of ZnO films vary from 3.8�1019 to 4.7�1019 cm−3, and the carrier concentrations of IZO films varyfrom 5�1019 to 1.7�1020 cm−3. It seems that with increas-ing the annealing temperature, the carrier concentration in-creases; most likely due to an increase in the number ofoxygen vacancies. With an increase in the carrier concentra-

tion, the barrier width and the work function of IZO will bereduced. A reduction in the work function of IZO causes adecrease in the barrier height. The combined decrease of thebarrier width and height leads to a reduced specific contactresistance. At the annealing temperature of about 500 °C, theinterdiffusion of indium and zinc and the associated interfa-cial layer formation would lead to the formation of manydefects in the interfacial layer which might contribute tochange of the specific contact resistance.

In summary, we have shown that sputtered IZO can yieldohmic contact with low specific contact resistance and hightransmittance on phosphor-doped n-typed ZnO. Annealing at400 °C leads to greatly improved specific contact resistanceof 3.8�10−6 � cm2 with a high transparency ratio of morethan 75% in the 450–1100 nm wavelength range, whilemaintaining a good stability.

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TABLE I. The carrier concentrations and mobilities of ZnO films �after wetetch� and as-deposited IZO films and subsequent annealing at 400 °C, and500 °C in a vacuum �2�10−5 mbar� for 5 min.

SampleAnnealing temperature

�°C�Carrier concentration

�cm−3�Mobility

�cm2/V s�

ZnO After wet etch 3.8�1019 1.4ZnO 400 3.9�1019 2.2ZnO 500 4.7�1019 2.8

IZO As deposited 5.0�1019 82.4IZO 400 1.1�1020 40.4IZO 500 1.7�1020 37.2

101901-3 Hu et al. Appl. Phys. Lett. 88, 101901 �2006�

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