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Ohmic contact of p-doped ZnO

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https://www.eduzhai.net International Journal of M aterials Engineering 2013, 3(1): 1-3 DOI: 10.5923/j.ijme.20130301.01 Ohmic Contacts to P-Type Doped ZnO L. S. Chuah1,*, S. S. Tneh2, Z. Hassan2, K. K. Saw1, F. F. Yam2 1Physics Section, School of Distance Education, Universiti Sains M alaysia, 11800 Penang, M alaysia 2School of Physics, Universiti Sains M alaysia, 11800 Penang, M alaysia Abstract For the purpose of enhance the ohmic contacts, we have utilizing Pt to act as ohmic contacts on p-type phosphorus doped ZnO (Zn O:P) by a metal with h igh work function. The Pt oh mic contacts in terms of electrical property and therma l stability were reported in this paper. The ZnO films doped with 3 wt . % phosphorus (P) were produced by activating phosphorus doped ZnO (Zn O:P) thin films in air amb ient at 300℃, 400℃, 500℃, and 600℃ for 60 min without any catalyst. The activation energies of the phosphorus dopant in the p-type ZnO under air environment show that phosphorus replacement on the O-site co mpliance a deep level in the gap. Keywords ZnO, Doping, Semiconducting II– VI Materials, Oh mic 1. Introduction Zinc o xide (ZnO), with a high exciton binding energy (60 meV) and a large band gap (3.37 eV at roo m temperature). ZnO is a material strong agreed to imp lementations in ultra-vio let (UV) optoelectronic devices, for examp le light-emitting diodes (LEDs), laser diodes (LDs), field-effect transistors (FETs), and solar b lind ultravio let (UV) photo d etecto rs [1]. In consequence, two categorization of p-type oh mic contact with high reflectivity and lo w specific contact resistance have been investigated to improve low resistance ohmic contacts to p-type GaN with light reflectance to perform h igh light extraction efficiency in flip chip light-emitting diodes (FCLEDs). The primary one is situated on the multilayer metal contact, such as Ir/Ag, Pd/Ni/Al/Ti/Au, Ni/Ag/Au and Ni/Au/Rh. The additional is founded on transparent conductive oxide contact (antireflection coating), such as MIO/Ag and ICO/Ag[2]. Simu ltaneous discoveries in obtaining high quality p-type ZnO and comp rehension of electrolu minescence (EL) fro m ZnO bas ed pn junctions confirm that practical utilizations of Zn O LEDs can be ant icipat e soon [3– 7]. Noneth eless, devices with high contact resistance between the electrode metals and the semiconducto rs wou ld g reat ly limit the perfo rmance. Consequent ly , fab ricat ion o f h igh qu ality ohmic contacts with low contact resistivity and high thermal stability is advantages for ZnO-based optoelectronic devices. Becaus e o f d ifficu lty in g ro wing h igh qualit y p -ty pe conductivity ZnO, reports on ohmic contact to p-ZnO are * Corresponding author: chuahleesiang@yahoo.com (L. S. Chuah) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved only few. In those limited literatures, metal such as Ti/Au and Ni/Au were often chosen as ohmic contacts to p-type ZnO or p-ZnMgO due to their h igh wo rk function[8,9]. But the earlier was found to be not so thermal stable, the mo rphology of which became rough even after annealing at temperatures up to 150℃[10]. While Ni/Au bilayer electrodes was used to form oh mic contacts to other p-type semiconductors, such as GaN and CuCrO2[11,12]. Lu et. al. provided an approach, in their study, ohmic contacts to N-doped p-type ZnO was studied[13, 14]. As a means to enhance the ohmic contacts, we have apply Pt to perform as ohmic contacts on p-type ZnO by choice of metal with needed high work function. In this paper, the Pt ohmic contacts in terms of electrical property and thermal stability were investigated. 2. Experiment The phosphorus (3 wt %) p-doped ZnO films of about 500 nm thickness were grown on g lass substrates by the thermal evaporation technique. Subsequently, activating thin films by thermal o xidation in air without any catalysts. The oxidation was conducted at 300℃, 400℃, 500℃, 600℃ for 1 hours without any catalyst. The essential factor is that an oxygen-rich condition was raised to minimize the formation of zinc interstitials and oxygen vacancies. After the growth of ZnO:P films, a Pt (100 n m) contact metal layers were deposited sequentially by dc sputtering. Atomic force microscopy (AFM) was used to analyze surface morphology. The metal mask wh ich was applied for oh mic contacts fabrication includes of an array o f dots with diameter of 100 µm. Contact was formed on the top surface of the sample. The contact is characterized by Keithley high-voltage-sourc e-measure-unit (model 237) for its I-V characteristics. 2 L. S. Chuah et al.: Ohmic Contacts to P-Type Doped ZnO 3. Results and Discussion Fig. 1 shows the I– V characteristic of the fabricated Pt contacts. The sample annealed at 600℃ exhib ited the best I–V characteristics of all samples. Fro m the plots, we found that samples annealed at 400℃, 500℃, 600℃ were oh mic. The current conducted through the 300 ℃ was in mini-A mpere (mA). For 400℃, 500℃, 600℃ samp les, the current increased visible reached nano-Ampere (nA) grade. The contacts at 500℃ annealing revealed the largest current conduction. Consequently, the contact resistance depends on the annealed temperature[15]. AFM 3D analysis was performed to see the surface morphology of the ZnO:P thin films (see Fig. 2). The AFM image scale was 5 × 5 μm2. We have proved the small-grained, co lu mnar structure of the Zn O:P thin film. The ZnO thin film diffused at 600 ℃ appeared to be composed of converging grains with increased surface roughness. Specifically, the surface roughnesses of the ZnO:P thin films were 11.23, 12.53, 16.70 and 19.21 n m, for 300℃, 400℃, 500℃, 600℃, respectively. The increased surface roughness after d iffusion is due to recrystallization of the crystalline structure. (c) 500℃ (d) 600℃ Figure 1. The I–V characteristics of Pt/ZnO:P deposited on glass (a) 300℃ (a) 300℃ (b) 400℃ (b) 400℃ International Journal of M aterials Engineering 2013, 3(1): 1-3 3 REFERENCES [1] S.J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, T. Steiner, Recent progress in processing and properties of ZnO, Prog M ater Sci, 50(3), pp. 293–340, 2005. [2] D. S. Leem, T. W. Kim, T. Lee, J. S. Jang, Y. W. Ok and T. Y. Seong, Appl. Phys. Lett. 89 262115, 2006. (c) 500℃ [3] W. Z. Xu, Z. Z. Ye, T. Zhou, B. H. Zhao, L. P. Zhu, J. Y. Huang. Low-pressure M OCVD growth of p-type ZnO thin films by using NO as the dopant source, J Cryst Growth 265, pp. 133–136, 2004. [4] J. F. Rommeluere, L. Svob, F. Jomard, J. M imila-Arroyo, A. Lusson, V. Sallet, Electrical activity of nitrogen acceptors in ZnO films grown by metalorganic vapor phase epitaxy, Appl Phys Lett 83, pp. 287–289, 2003. [5] W. Z. Xu, Z. Z. Ye, Y. J. Zeng, L. P. Zhu, B. H. Zhao, L. Jiang, ZnO light-emitting diode grown by plasma-assisted metal organic chemical vapor deposition, Appl Phys Lett 88, 173506, 2006. [6] J. G. Lu, Z. Z. Ye, F. Zhuge, Y. J. Zeng, B. H. Zhao, L. P.Zhu, p-Type conduction in N–Al codoped ZnO thin films, Appl Phys Lett 85, 3134–5, 2004. (d) 600℃ Fi gure 2. AFM 3D images of the Pt /ZnO:P deposited on glass 4. Conclusions ZnO:P films were prepared by annealing film in air at temperatures fro m 300 to 600℃. The sample annealed at 600 ℃ exhib ited the best I–V characteristics of all samples. Fro m the plots, we found that samples annealed at 400℃, 500℃, 600℃ were oh mic. The current conducted through the 300℃ was in min i-A mpere (mA ). For 400℃, 500℃, 600 ℃ samp les, the current increased visible reached nano-Ampere (nA) grade. The AFM image scale was 5 × 5 μm2. We confirmed the small-grained, colu mnar structure of the ZnO:P thin film. The ZnO:P thin film at 600℃ appeared to be composed of converging grains with increased surface roughness. Specifically, the surface roughnesses of the ZnO:P thin films were 11.23, 12.53, 16.70 and 19.21 n m, for 300℃, 400℃, 500℃, 600℃, respectively. ACKNOWLEDGEMENTS The support fro m Universit i Sains Malaysia and FRGS (203/ PJJAUH/6711159) are gratefully acknowledged. [7] J. Zeng, Z. Z. Ye, Y. F. Lu, W. Z. Xu, L. P. Zhu, J. Y. Huang, Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO, J Phys D – Appl Phys 41(16), 165104, 2008. [8] K. Ip, G. T. Thaler, H. Yang, S. Y. Han, Y. Li, D. P. Norton, Contacts to ZnO, J Cryst Growth 287, 149–56, 2006. [9] L.J. M andalapu, Z. Yang, J. L. Liu. Low-resistivity Au/Ni Ohmic contacts to Sb-doped p-type ZnO, Appl Phys Lett 90, 252103, 2007. [10] J. J. Chen, S. Jang, T.J. Anderson, F. Ren, Y. Li, H.S. Kim, Low specific contact resistance Ti/Au contacts on ZnO. Appl Phys Lett, 88, 122107-3, 2006. [11] W. T. Lim, L. Stafford, P. W. Sadik, D. P. Norton, S. J. Pearton, Y. L. Wang, Ni/Au Ohmic contacts to p-type M g-doped CuCrO2 epitaxial layers. Appl Phys Lett, 90, 142101-3, 2007. [12] M . S. Oh, D. K. Hwang, J. H. Lim, C. G. Kang, S. J. Park, Low resistance nonalloyed Ni/Au ohmic contacts to p-GaN irradiated by KrF excimer laser, Appl Phys Lett, 89, 2006. [13] Y.K. Lu, Z.Z. Ye, Y.J. Zeng, J.Y. Huang, B.H. Zhao, Low-resistivity Ni/Pt ohmic contacts to p-type N-doped ZnO, Solid-State Electronics, 54, 732-735, 2010. [14] Y. F. Lu, Z. Z. Ye, Y. J. Zeng, L. P. Zhu, B. H. Zhao, Ni/Au Ohmic contacts to p-type N-doped ZnO, Electrochem Solid-State Lett, 11, H185–8, 2008. [15] L.S. Chuah, Z. Hassan, S.G. Teo, “Effect of thermal annealing on the Ir/A g contact to p-GaN”, Journal of Non-Crystalline Solids, 356, 1863-1866, 2010.

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