Optik 125 (2014) 785–788
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Fabrication and electrical characterization of ZnO rod arrays/CuSCN heterojunctions C. Xiong a,b,∗ , R.H. Yao b , W.J. Wan b , J.X. Xu b a b
School of Photoelectric Engineering, Changzhou Institute of Technology, Changzhou 213002, People’s Republic of China School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 12 March 2013 Accepted 10 July 2013
PACS: 81.05.Dz 81.05.Hd 73.40.Lq 81.10.Dn Keywords: ZnO rod arrays CuSCN thin ﬁlms Heterojunctions Solution processing
a b s t r a c t ZnO rod arrays/CuSCN heterojunctions are fabricated by depositing ZnO rod arrays ﬁlms using two-step chemical bath deposition (CBD) and CuSCN thin ﬁlms using successive ionic layer adsorption and reaction (SILAR) on ITO substrate successively. The structures and morphologies of ZnO ﬁlms and CuSCN ﬁlms, analyzed by X-ray diffraction (XRD) spectroscopy and metallurgical microscope, show that ZnO ﬁlms are hexagonal wurtzite structure and consisted of vertical polycrystalline rods with diameter of 1 m, CuSCN ﬁlms are ␤-phase structure and consisted of elongated grains with length of 3 m. Current–voltage (I–V) measurements of ZnO/CuSCN heterojunctions show good diode characteristics with rectiﬁcation ratio about 48.3 at 3 V. The forward conduction is, respectively, determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of ZnO/CuSCN heterojunctions has been proposed to explain the transport mechanism. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction Zinc oxide (ZnO), a II–IV semiconductor, has a wide direct gap of 3.37 eV at room temperature and large exciton binding energy of 60 meV [1,2], which has attracted much attention for its wide prospects in short-wavelength optoelectronic applications, such as light-emitting diodes (LEDs) , ultraviolet lasers , ultraviolet photodetectors , etc. In order to fabricate ZnO-based optoelectronic devices, both n-type and p-type ZnO are needed. It is easy to obtain good conductive n-type ZnO, while it is difﬁcult to fabricate p-type ZnO due to its self-compensating effect since existing a large number of native defects, such as oxygen vacancies and zinc interstitials [5,6]. Thus using other p-type semiconductors such as Si , GaN , SiC , CuAlO2  with n-type ZnO to form p–n heterojunctions becomes to another way to applications of the ZnO-based devices. ␤-CuSCN, a p-type semiconductor with a wide gap of 3.6 eV at room temperature, has been used in extremely thin absorber solar cells (ETAs) as a p-type hole conducting material [11,12].
∗ Corresponding author at: School of Photoelectric Engineering, Changzhou Institute of Technology, Changzhou 213002, People’s Republic of China. E-mail address: [email protected]
(C. Xiong). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.07.080
However, there are few reports discussing about ZnO/CuSCN heterojunctions recently. Literatures [13,14] focus on ﬁlling of CuSCN on ZnO rod arrays by the electrodeposition method to form p-CuSCN/n-ZnO rod array interpenetrating heterojunctions. Literature  reports a hybrid ﬂexible vertical nanoscale diodes, which formed by n-type ZnO and p-type CuSCN embedded in polymer foil, by using electrochemical deposition technique. Literature  reports a light-emitting diodes consisting of n-ZnO nanorods and p-CuSCN prepared by electrochemical method. Compared to ITO/ZnO nanorods/Au diodes, CuSCN layer enhances the efﬁciency of radiative recombination of ZnO nanorods in the heterojunction LEDs. These reports mostly focus on the preparation of heterojunctions, yet rarely discuss the current transport properties. In this paper, ZnO rod arrays/CuSCN heterojunctions are fabricated by using simple solution method at low temperature. The process is that ﬁrstly ZnO seed layers are grown on ITO substrates by successive ionic layer adsorption and reaction (SILAR) method, then ZnO microrod arrays are grown on seed layers by chemical bath deposition (CBD), at last CuSCN thin ﬁlms are deposited on ZnO rod arrays by SILAR. This method fabricating ZnO/CuSCN heterojunctions has not been seen in previous reports. The current transport properties of the heterojunctions are also investigated by means of current–voltage measurements.
C. Xiong et al. / Optik 125 (2014) 785–788
2. Experimental 2.1. Preparation of ZnO seed layers using SILAR To grow ZnO microrod arrays, ﬁrst ZnO seed layer ﬁlms are deposited on ITO substrates using SILAR method. The ITO substrates are cleaned, before deposition, by deionized water (resistivity ∼18 M cm) rinse and ultrasonic cleaning with acetone and alcohol. The used aqueous zinc complex solution comprises zinc acetate and ammonia, which form zinc ammonia complex ([Zn(NH3 )4 ]2+ ). For doping, aluminum nitrate is added into the zincate solution with Al/Zn molar ratios of 2%. The concentration of the complex solution is 0.05 mol/L zinc concentration. The pH value of the solution is about 11.0. The SILAR growth is a three-step process involving subsequent immersion of substrate in zinc complex solution for 10 s and deionized water for 20 s along with immersion in the deionized water bath maintained at 95 ◦ C for 30 s. In the ﬁrst immersion process, zinc ammonia is adsorbed onto the ITO substrates. In the second step, the adsorbed zinc ammonia complex is converted into zinc hydroxide (Zn(OH)2 ). In the third step, Zn(OH)2 is converted into ZnO in hot water bath. This is one deposition cycle, and thirty deposition cycles are carried out in this research.
2.2. Growth of ZnO microrod arrays using chemical bath deposition (CBD) method The chemical bath is prepared by mixing equimolar aqueous solution of zinc acetate and hexamethylene tetraammine (HMT) with the concentration of 0.1 mol/L. The pH of the aqueous solution is adjusted to 6.0 by adding acetic acid solution. To grow ZnO rod arrays, seed layered substrates are placed vertically inside the chemical bath and heated at a constant temperature of 90 ◦ C for 3 h. After deposition, the as-grown CBD ﬁlms are carefully cleaned with deionized water to remove residual materials.
2.3. Deposition of CuSCN ﬁlms using SILAR The cationic precursor is aqueous thiosulphatocuprate (I) complex, formed by dissolving 0.02 mol/L CuSO4 ·5H2 O and 0.08 mol/L Na2 S2 O3 in deionized water. The anionic precursor is NaSCN solution with the concentration of 0.08 mol/L. To deposit CuSCN ﬁlms, the substrates with ZnO rod arrays are immersed in cationic precursor for 10 s to absorb copper ions, subsequently immersed in anionic precursor for 20 s to react with SCN− ions, then rinsed in deionized water for 10 s to remove the powdery material or loosely bounded ions. After 100 such deposition cycles, the substrates are dried in an air atmosphere, and then thick Ag contact dots with area of 1 mm2 , acting as a second electrode, are prepared by printing silver paste on the top of the CuSCN ﬁlms. Fig. 1 shows the entire structures of ZnO rod arrays/CuSCN heterojunctions.
Fig. 1. Schematics of ZnO rod arrays/CuSCN heterojunctions.
Fig. 2. XRD spectrum of ZnO rod arrays/CuSCN heterojunctions.
2.4. Characterization and measurements The crystal structures of the heterojunctions are determined by X-ray diffraction using a Rigaku RINT-2100V diffractometer with Cu K˛ radiation. The surface morphologies are determined with an Olympus BX51M metallurgical microscope. The current–voltage characteristics of the heterojunctions are measured with an Agilent 4156C precision semiconductor parameter analyzer. 3. Results and discussion 3.1. Crystal structures and morphologies Fig. 2 shows XRD analysis of the crystal structures of ZnO rod arrays/CuSCN heterojunctions. Multi-peak patterns conﬁrm the polycrystalline nature of the ﬁlms, and all the peaks match well with those in JCPDC cards (No. 36-1451, No. 29-0581). As can be seen in Fig. 2, ZnO rod arrays have good crystalline quality of the wurtzite structure, strongly preferred (0 0 2) c-axis orientation. Rhombohedral crystal structure with ␤-phase for CuSCN  is observed with a preferred orientation along (0 0 3) direction. No evident diffraction peaks of ITO are found in X-ray diffraction (XRD) pattern. This may be because ITO is present in small extent compared to ZnO and CuSCN. To study the surface morphologies, metallurgical microscope images are taken for ZnO rod arrays and CuSCN ﬁlms. Fig. 3(a) shows that ZnO ﬁlms, with dense microstructure, are consisted of vertically aligned ZnO micro-rods with diameter of about 1 m. Top-view of ﬁlms reveals that the micro-rods have perfect hexagonal shape indicated by the white arrows. Fig. 3(b) is the surface morphology of CuSCN ﬁlms deposited on ZnO rod arrays ﬁlms. Elongated grains are observed in the image, and the average grain size is about 3 m. As showed in Fig. 3(b), a well surface coverage is observed for CuSCN ﬁlms on ZnO rod arrays.
Fig. 3. Metallurgical microscope images of (a) ZnO rod arrays and (b) CuSCN ﬁlms.
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Fig. 6. Plot of the ideality factor n versus V.
Fig. 4. Current–voltage (I–V) characteristic of ZnO rod arrays/CuSCN heterojunctions. Inset shows the I–V characteristic between two silver contacts on CuSCN.
3.2. Current–voltage (I–V) characteristics Fig. 4 shows I–V characteristic of ZnO rod arrays/CuSCN heterojunctions at room temperature (300 K) and inset shows the I–V characteristic between two silver contacts on CuSCN ﬁlms deposited on glass. The ohmic contact of Ag/CuSCN is conﬁrmed by the linear I–V curve in the inset of Fig. 4. The heterojunctions present a typical rectifying characteristic with a rectiﬁcation ratio of 48.3 at ±3 V. Moreover, the reverse current is as low as 5.843 A at −3 V, equivalent to 5.84 × 10−4 A/cm2 taking into account the area of the contact. Assumed turn-on voltage is corresponding to the voltage when the forward current is twice the reverse one, then the turn-on voltage of the heterojunctions is about 0.3 V. In order to identify the carrier transport mechanism, the log–log plot of I versus V at forward bias is depicted in Fig. 5. The curve can be divided into three regions according to the slope of which. At low voltage (V < 0.06 V, region I), the curve with a slope of 1 in log–log plot, exhibiting ohmic behavior, indicates that an ohmic shunt resistor in parallel to the junction is responsible for the current. This shunt is possibly due to local damage of the heterojunctions and/or leakage current at the edges of the device. For 0.06 V < V < 1 V (region II), the current exponentially increases following the equation I ∝ exp(ˇV), which is usually observed in the wide band gap p–n diodes due to recombination and tunneling mechanism [18,19], The exponential factor ˇ depends on the dominant current transport mechanism. When V > 1 V (region III), the curve becomes linear again with a constant slope m in log–log plot Fig. 5, indicating I–V characteristic follows a power law I ∝ Vm , where the current conduction is attributed to the space charge limited current (SCLC) . In general, the exponent m based on the distribution of traps,
equals 2 for the normal SCLC case without any traps or for a single trapping level, and is large than 2 for an exponential distribution of trapping levels. In our experiment, the value m equals 4, provides evidence that this SCLC is controlled by an exponential distribution of trapping levels. To investigate the behavior in the diode region (region II), an ideal heterojunction diode equation  is used as follows:
I = Is exp
where Is is the saturation current, q, k and T are elementary charge, the Boltzmann constant, and temperature, respectively. n is the ideality factor, n = 1 for a diffusion or thermionic-emission limited model, n = 2 for a recombination-limited model and n > 2 for a tunneling limited model . After taking logarithm and derivation to Eq. (1), n can be deﬁned as the following expression : n=
q dV kT d(ln I)
Based on Eq. (2), the relationship between ideal factor n and bias V can be drawn up, shown in Fig. 6. In ohmic region (0.01–0.06 V) and space charge limited current region (>1 V), the relationship between current and voltage is nonexponential, the ideality factor n in these regions is meaningless, so we focused on the range near diode region (0.06–1 V). At 0.06 V, the corresponding n equals 2, indicating that the main current transport mechanism is recombination in space charge region. At 0.1 V, n increases to 3, indicating that defect-assisted tunneling  plays an important role in current transport. As can be seen in Fig. 6, with the increase of voltage from 0.1 V to 0.9 V, the ideality factor n increases linearly from 3 to 12, indicating that the defect-assisted tunneling becomes more and more dominant. At higher voltage, the I–V characteristics enter into the region of space charge limited current. 3.3. Heterojunction band diagram analysis
Fig. 5. Forward I–V characteristic of ZnO rod arrays/CuSCN heterojunctions in log–log scale.
To explain the transport mechanism, it is necessary to understand the energy band diagrams of the heterojunctions based on Anderson’s model, given in Fig. 7. The band gap and electron afﬁnity values for ZnO and CuSCN are Egn = 3.37 eV, n = 4.35 eV , and p Eg = 3.6 eV, p = 1.5 eV , respectively. The conduction band offset is Ec = n − p with a value of 2.85 eV, and the valence band p offset is Ev = Egn − Eg + Ec with a value of 2.62 eV. Because of large values of conduction band offset and valence band offset, which yield energetic barriers for electrons and holes respectively, few carriers are able to get across the barriers, so the thermionic emission can be ruled out as the dominant current transport mechanism. As seen in Fig. 7, the conduction band bottom of ZnO is close
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and Technique Foundation of Chang Zhou (CJ20120001), Natural Science Research Project of University in Jiangsu Province (12KJD510001) and National Natural Science Foundation of China (11247323). References
Fig. 7. The energy band diagrams of the heterojunctions at (a) zero bias and (b) forward bias.
to valence band maximum of CuSCN. Besides, in our experiment, the grown ﬁlms have polycrystalline nature, with a few defects and interface states depicted in Fig. 7(b). Thus, the electrons from the conduction band of ZnO can easily tunnel the junction potential barrier into the valence band of CuSCN via defects and interface states, without climbing the barrier, to produce a tunneling current under forward bias condition, this defect-assisted tunneling [19,25] via defects and interface states plays an important role in current transport mechanism. Fig. 7(b) shows three possible tunneling paths of electrons, from left to right they are, falling into defects levels or interface states and successive tunneling, band-toband multi-tunneling process via defects and interface states, and tunneling into interface states or defects and successive recombination. In addition, it is easy that carriers recombine in space charge region via defects and interface states, so recombination mechanism is also important. Since the ﬁlms are undoped in our heterojunctions, the depletion region is wide, defect-assisted tunneling may not be more prominent than recombination at low voltage. With the increase of forward voltage, the depletion layer gets narrower, and the defect-assisted tunneling mechanism gets more remarkable, to become a dominating current transport mechanism. It is notable that defect-assisted tunneling cannot supply injection of minority carriers, and radiative recombination cannot be produced. Thus, for light-emitting diodes, the interface states and defects should be minimized to control the defect-assisted tunneling current. 4. Conclusions In conclusion, ZnO rod arrays/CuSCN heterojunctions have been prepared using a simple solution method at low temperature. The structures and surface morphologies are examined by XRD and metallurgical microscope. An attempt is made to determine the dominant current transport mechanism and the heterojunction band diagram of ZnO rod arrays/CuSCN is proposed. At low forward voltage, recombination in the space charge region is the mechanism that dominates the forward current. With the increase of forward voltage, the current transport is found to govern by defectassisted tunneling which becomes more and more remarkable. At higher forward voltage, the forward conduction is determined by exponential distribution trap-assisted space charge limited current mechanism. Acknowledgments The work is ﬁnancially supported by the Natural Science Foundation of Chang Zhou Institute of Technology (YN1105), the Science
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