THE EFFECTS OF DOPING CONCENTRATION ON THE ELECTRICAL PERFORMANCE OF DC-SPUTTERED p-zno/n-si HETEROJUNCTION

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1 THE EFFECTS OF DOPING CONCENTRATION ON THE ELECTRICAL PERFORMANCE OF DC-SPUTTERED p-zno/n-si HETEROJUNCTION Dao Anh Tuan, Bui Khac Hoang, Nguyen Van Hieu, Le Vu Tuan Hung Department of Applied Physics, Faculty of Physics and Engineering Physics, The University of Science of HCM city, Vietnam ABSTRACT In this paper, In-N co-doped p - type ZnO thin film (ZnO:(In, N)) is deposited by the sputtering magnetron dc technique from ceramic target ZnO:In on n-si (1 0 0) wafer to fabricate p- ZnO:(In, N)/n-Si hetero-junctions. The doping concentration of In is changed from 0 to 3% wt In to study the electrical performance of the p-zno/n-si heterojunction. The optical properties and thickness of films are investigated by UV-VIS, Swanepoel interference method and stylus profile. The electrical properties are determined by four-point probe and Hall measurements. The structure and concentration of films is determined by X-ray diffraction and RBS method. ZnO:(In, N) p type films under optimum conditions show p- type conductivity, with a low resistivity of 0.09 Ω.cm, hole concentration of cm -3 and transmission over 85%. The electrical junction properties are investigated by I V measurement, which reveals that the hetero-junction shows a rectifying I V curves as good diode. The ideality factor and the saturation current of this diode is n=10.14, I= (A). KEYWORDS: doped, carrier concentration, hetero-junction, current voltage (I V) characteristics, rectifying. 1 INTRODUCTION Zinc oxide (ZnO) has been regarded as promising materials for optical devices, due to its wide direct band gap of 3.37 ev and large exciton binding energy of 60 mev [1-3]. Therefore, ZnO has potential applications in light-emitting diodes (LEDs), laser diodes (LDs) and ultraviolet (UV) detection devices [3-6]. Some especially interesting properties of ZnO are low cost, availability, non toxicity and high chemical stability non reducing environments. For optoelectronic applications, the growth of high quality n- and p- type ZnO is very important. However, the fabrication of p- type ZnO thin films for high performance have not been demonstrated. The reproducibility and stability of p-type conductivity is still controversial. The codoped method was first proposed by Yamamoto and Yoshida, this method can increase the solubility of nitrogen in ZnO with acceptable stability and reproducibility by various techniques, such as ultrasonic spray pyrolysis [1], [2], [3], RF co-sputtering [6]. In this article, In-N codoped p-type ZnO thin films are grown on glass to study microstructure, optical and electric properties. In order to verify p- type conduction of ZnO:(In, N) thin films, they are deposited on n-si (1 0 0) substrate. Then p-zno/n-si heterojunction and its I- V characteristics are reported. 2 EXPERIMENT ZnO:(In, N) thin films are prepared by dc magnetron sputtering equipped with ZnO:In ceramic target. The concentration of dopant In in ceramic targets is varied from 0 to 3wt% In. The working pressure, direct current, electrical potential and the substrate temperatures T s are: 9 x 10-3 torr, 0.35 A, 520 V, C, respectively. The target substrate distance is kept at 3.5 cm, the substrate edge track of target is kept about 2.5 cm. 293

2 The thickness of thin films is measured by Stylus method (DEKTAK 6M, UAS). Electrical measurements are carried out by a Hall measurement system (HMS-300, ECOPIA) and four-point probe method. The structure of the films is analyzed by an X-ray diffraction (XRD) system (D/max-II with CuKa radiation). To study the optical properties of thin films, transmission spectra of the samples is considered by a UV-Vis spectrophotometer (Model PB-10, power 200W, Taiwan), and photoluminescence measurements are carried out by using a 325 nm He-Cd laser as the excitation source with PL spectrometer (HORIBA JOBIN YVON, USA). Nitrogen concentration in the films is detected by back scattering spectroscopy (RBS) using 2 MeV He+ ion beam with a backscattering angle of and detector solid angle of 3.4 msr. For the purpose of fabricating heterojunction structures with good behaviors, the n-type Si(100) wafers are prepared carefully by standard procedure. Firstly, they are cleaned and dipped into (H2SO4:H2O2 =1:1) solution for 10 min, then they are dipped into buffered HF (HF:H2O2 = 1:6) to remove native oxides and finally, they are dried in a flow Argon. After the p-zno/n-si heterojunction structures are established, the Ag metal ohmic contact layer is deposited onto the p-type ZnO film as the anode electrode, and is formed onto the n-type Si substrate as the cathode electrode. The diode-likerectifying-characteristic of thin films is determined by current-voltage (I-V) measurements.. 3 RESULT AND DISCUSSION 3.1 Microstructure, optical and electrical properties of ZnO:(In, N) film For the purpose of finding the optimal p- type conduction condition with respect to the concentration In in thin films, the doping concentrations of In are varied from 0-3%wt (see table 1). The conditions of sputtering are direct current of 0.35 A, electrical potential of 520 V, working pressure of torr, substrate temperature of C, the ratio gas mixture of 40%N2+60%Ar, and period sputtering of 15 minutes. The average value of the film thickness is found to be in the range 700 nm nm. Figure 1. XRD patterns of In-N co-doped ZnO thin films with various In doping concentration (0 wt%, 1.0 wt%, 2.0 wt%, 3.0 wt%). Table 1. The electrical and optical parameters of films with various doping concentration (1-3wt% In) Sample In content (wt %) Mobility (cm 2 /V.s) Bulk resitivit y (Ω.cm) Carrier density (cm -3 ) S % S % S % S % Figure 1 shows the typical XRD patterns of the as-grown ZnO films using a series of targets with various In contents (0 wt.%, 1wt.%, 2wt.% and 3wt%) on glass substrates. Only one diffraction peak corresponding to the (0 0 2) plane is observed indicating preferred c-axis orientation, no other phases corresponding to Zn 3 N 2 or In-N peaks are detected obviously. With the increase of In content in the targets (from 0wt. % to 3wt. %), the intensity of the (0 0 2) peak decreases evidently, indicating the degrading crystallinity due to incorporation of In atoms. As In atoms are incorporated into ZnO structure, they substitute for Zn atoms, causing the d-spacing value of co-doped 294

3 ZnO to decrease because the radius of In is smaller than that of Zn. Figure 2. Transmission spectra of ZnO:(In, N) films on glass substrate with various In content Table 1 shows the electronic properties of co-doped ZnO thin films deposited on glass substrates with various dopant concentrations (1 3 wt% In). All the films co-doped by Indium and nitrogen show p-type conduction with the hole concentration is positive change. It may indicate that the impurity conduction is the dominant factor for p-type conduction. With the increase of In dopant concentration (from 0-2 wt %), p-type conduction of as- grown ZnO thin films is improved. Sample S2.0 shows the best p- type conductivity with good quality (hole concentration cm -3 and bulk resistivity 0.09 Ω.cm was obtained). It is believed that, with the reasonable increase of In atoms, N atoms are effectively doped into ZnO thin films, because the incorporation of In enhances the doped efficiency, acceptor concentration is improved largely, and resistivity of thin films also drop significantly. On the contrary, as concentration In in films are excess, the solubility N in ZnO is low, so acceptor may be compensated by natural donor defects such as oxygen vacancies or interstitial zinc. Thus thin films indicate p-type conductivity with hole density is low. From figure. 2 and table 1, we find that the transmission of films is quite high, over 85%. Comparing ZnO:(In,N) thin films with ZnO:N (0 wt% In), we find that their edge absorption shift drastically to visible region. It indicates that nitrogen is involved in thin films ZnO:(In, N). Figure 3. Room temperature PL spectra for undoped ZnO thin film and N In co-doped ZnO:(In,N) thin film which were excited by the 325 nm line of a He Cd laser. The inset on right top corner is the selected magnification area Photoluminescence spectra for undoped and co-doped ZnO films are shown in Fig. 3, the strong near-band-edge (NBE) ultraviolet emission peaks, at 375 nm for undoped sample and at 380 nm for codoped ZnO:(In,N) thin film, was observed. As author Jiming Bian [5], the small redshift of peak position maybe the result of introduction of defect energy level in band gap in co-doped ZnO sample. Determining by RBS measurement in Fig. 4, we recognize that concentration of nitrogen impurities in ZnO(In, N) thin film is 20 wt% and concentration of In is 1wt%. We suggest that nitrogen implant completely in lattice of ZnO. Figure 4. RBS spectra of In-N co-dope ZnO film was deposited from ceramic target containing 2 wt% of In 375 nm nm ZnO ZnO:(In, N) 295

4 Energy Journal of Engineering Technology and Education The International Conference on Green 1600 Technology 1800 and Sustainable Development (GTSD2012) Counts Channel 3.2 The survey of p-n heterojunctions S2 S1 that the p-type conduction is realized in the co-doped thin films. Sample S3 has the threshold voltage of about 0.4V and a forward current between 0.25 and 1.2 ma; Sample S1 has the threshold voltage of about 0.5 V and a S2 S1 S3 S3 Figure 5. Current Voltage characteristics of the p ZnO:(In, N)/n-Si hetero-junction: the ZnO:(In, N) thin film was deposited from ceramic target containing 1.0 wt% In for S1; 2.0 wt% In for S2; 3.0 wt% In for S3 respectively. For the fabrication of p n hetero-junctions, the p-type ZnO:(In,N) thin films were prepared on n-type Si substrate (100) with the optimal conditions such as: direct current of 0.35A; electrical potential of 520 V; working pressure of Torr; substrate temperature of C; the impurities concentrations of In were varied from 1-3%wt, the gas ratio is 40%N 2 :60%Ar. After the thin films are synthesized, the Ag metal to cover on thin film and substrate as electrode. In order to verify further p-type conduction of the ZnO:(In, N) thin films, room temperature I V characteristics of p- ZnO:(In N)/n-Si heterojunction diode is investigated and is shown in Fig.5. The heterojunction clearly demonstrates rectifying diode like behavior, indicating forward current between 0.1 and 1.4 ma, samples S2 shows a higher forward current between 0.55 and 6.6 ma. So that, the characteristic current rectifiers of the sample S2 is better than that of sample S1 and S3. Figure 6. Logarithmic scale in current with forward bias condition of sample S1, S2 and S3 A semilog plot of I-V characteristics is shown in Fig. 6, which indicates that the current at low voltage (V<1.0V) varies exponentially with voltage. The characteristics can be described by the stand diode equation [6]: I=I 0 (e qv/nkt -1) (1) n = q dv KT d(lni) Where q is the electronic charge, V the voltage at the junction, k the Boltzmann (2) 296

5 constant, n is the junction ideality factor, I 0 the reverse saturation current, and T is the absolute temperature. The value of the diode ideality factor of p-zno:(in, N)/ n- Si heterojunction is determined from the slope of the straight line region of the forward bias log I V characteristics and using Eq. (2) [7]. At low forward bias (V < 1 V), the typical values of ideality factors and the reverse saturation current are n=14.4, I s = (A) for S1.; n=10.14, I= (A) for S2 and n=13.6, I= (A) for S3, respectively. The value of the ideality factor is higher than 2 (value of the ideality factor of diode), which may be due to the effect of a series resistance [1]. At higher voltages the curves deviate from the exponential behavior, which is seen from the flattening of the characteristic curve (see Fig. 6). It is worth mentioning that the forward bias ideality factor value of p- ZnO:(In N)/n-Si heterojunction is relatively good compared to the previous reports [6], [7], [8] (see table 2). Table 2. The forward bias ideality factor value of the previous reports Technique structure Ideality RF cosputtering DC magnetron sputtering Vapor liquid solid p-zno(al,n) /n-si n-zno:al /p-si n-zno /p-si factor 3.87 [6] 20.1 [7] 5.47 [8] References 4 CONCLUSION P-type ZnO:(In,N) thin film has been fabricated successfully with high quality (high hole concentration and the low bulk resistivity). By analyzing the I-V measurements in detail of p- ZnO:(In, N)/n-Si heterojunction structures, we see that rectifying characteristic of films are fairly good, and their photoelectronic behaviours are good as well. They can be used not only for low cost solar cell, but also for high quantum efficiency UV and visible light enhanced photodiode in various applications. 5 ACKNOWLEDGEMENTS The RBS spectra of In-N co-dope ZnO film described in this article was supported by A.G. Balogh and Nhu-T. H. Kim-Ngan, Institute of Nuclear Physics of the University Frankfurt/Main Germany. 6 REFERENCES [1] Bian JM, Li XM, Gao XD, Yu WD, Chen LD, Deposition and electrical properties of N In codoped p-type ZnO films by ultrasonic spray pyrolysis, Appl Phys Lett. 84, 2004, pp [2] Chen LL, Ye ZZ, Lu JG, Chu PK, Control and improvement of p-type conductivity in indium and nitrogen codoped ZnO thin films, Appl Phys Lett. 89, 2006, pp [3] Lung-Chien Chen and Chun-Nan Pan, P-ZnO/n-Si Photodiodes Prepared by Ultrasonic Spraying Pyrolysis Metho, The Open Crystallography Journal. 1, 2008, pp [4] Young SJ, Ji LW, Fang TH, Chang SJ, Su YK, Du XL, ZnO ultraviolet photodiodes with Pd contact electrodes, Acta Mater. 55, 2007, pp [5] Jiming Bian, Xiaomin Li, Lidong Chen, Qin Yao, Properties of undoped n-type ZnO film and N In codoped p- type ZnO film deposited by ultrasonic spray pyrolysis, Chemical Physics Letters. 393, 2004, pp [6] Manoj Kumar, Sang-Kyun Kim, Se- Young Choi, Formation of Al N codoped p-zno/n-si (1 0 0) heterojunction structure by RF cosputtering technique, Applied Surface Science. 256, 2009, pp [7] He Bo, Ma Zhong Quan, Xu Jing Yin Yan Ting, Characterization of AZO/p- Si heterojunction prepared by DC magnetron sputtering, Materials Science in Semiconductor Processing. 12, 2009, pp

6 [8] P. Klason, M.M. Rahman, Q.-H. Hu,,R.Turan, M. Willander, Fabrication and characterization of p- Si/n-ZnO heterostructured junctions, Microelectronics Journal. 40, 2009, pp Contact: Dao Anh Tuan - Tel: datuan@phys.hcmuns.edu.vn 298