Si heterojunctions

Si heterojunctions

Applied Surface Science 252 (2006) 3449–3453 www.elsevier.com/locate/apsusc Fabrication and electrical characterization of nanocrystalline ZnO/Si het...

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Applied Surface Science 252 (2006) 3449–3453 www.elsevier.com/locate/apsusc

Fabrication and electrical characterization of nanocrystalline ZnO/Si heterojunctions Yang Zhang, Jin Xu, Bixia Lin, Zhuxi Fu *, Sheng Zhong, Cihui Liu, Ziyu Zhang Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China Received 23 March 2005; received in revised form 20 April 2005; accepted 20 April 2005 Available online 1 July 2005

Abstract Nanocrystalline zinc oxide (nc-ZnO) films were prepared by a sol–gel process on p-type single-crystalline Si substrates to fabricate nc-ZnO/p-Si heterojunctions. The structure and morphology of ZnO films on Si substrates, which were analyzed by X-ray diffraction (XRD) spectroscopy and atomic force microscopy (AFM), showed that ZnO films consisted of 50–100 nm polycrystalline nanograins with hexagonal wurtzite structure. The electrical transport properties of the nc-ZnO/p-Si heterojunctions were investigated by temperature-dependent current–voltage (I–V) measurements and room temperature capacitance– voltage measurements. The temperature-dependent I–V characteristics revealed that the forward conduction was determined by multi-step tunneling current, and the activation energy of saturation current was about 0.26 eV. The 1/C2–V plots indicated the junction was abrupt and the junction built-in potential was 1.49 V at room temperature. # 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Sol–gel; Nanostructure; Heterojunction; Tunneling

1. Introduction Nanocrystalline semiconductors present an increasing interest for electronic and optoelectronic applications due to their superior characteristic properties. Their electrical properties are influenced by sizes and orientations of the crystallites and the grain boundary characteristics [1]. For instance, nanocrystalline silicon films show high conductivity * Corresponding author. Fax: +86 551 360 6004. E-mail address: [email protected] (Z. Fu).

and low activation energy. ZnO-based materials have received considerable attention for many potential applications, such as piezoelectric transducers [2], phosphors, optoelectronic devices, and space applications [3,4]. Nowadays, many scientists focus on its applications in optoelectronic areas [5,6], especially in blue and ultraviolet lasers and light emitting diodes. These applications benefit from a hetero-epitaxial system. Therefore, it is important to formation ZnObased heterostructures. Although monocrystalline ZnO films are actively pursued as semiconductor materials, nanocrystalline ZnO (nc-ZnO) thin films

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.053

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have the peculiar physical properties different from those of single crystals [7]. The nc-ZnO films were also even more proper for fabrication of junctions [8], ultraviolet diode lasers [9], and photodiodes [10]. Therefore, it is important to clarify the transport mechanism of nc-ZnO heterojunctions. The purpose of the present work is to report the electrical properties of the nc-ZnO/p-Si heterojunctions. It was found that the forward conduction was determined by multi-step tunneling current.

2. Experimental Un-doped nc-ZnO films were prepared on singlecrystalline p-Si substrates by a sol–gel method. The sol solution was prepared by zinc acetate dihydrate (Zn(CH3COO)2H2O) and polyvinyl alcohol (PVA). To prepare a sol, 1.50 g zinc acetate dihydrate and 1.50 g PVA were added to 30 ml de-ionized water, heated to 80 8C with stirring, then kept for 30 min. The substrates were single-crystalline p-type silicon, one side polished, and (1 0 0)-oriented with resistance of 8–12 V cm. The Si substrates were treated in a standard wet cleaning procedure, and chemically etched in a solution of H2SO4, H2O2 and H2O with the ratio of 6:1:1, then dipped in diluted HF solution for 3 min to remove SiO2, then thoroughly rinsed in deionized water. Then the cleaned Si wafers were immediately placed on the sample holder of a spinning coater. The sol was spin-coated on the polished side of Si substrate at 3000 rpm for 30 s, and then dried at 120 8C in a tubular furnace in air. This process was repeated six times for each sample. At last, the obtained films were heat-treated at 600 8C for 30 min in air. The I–V characteristics were measured by a laboratory assembled system consisting of a function generator, an X–Y function recorder, a low-temperature cryostat, and a specially designed sampled holder. Thin films samples are mounted in a specially designed sampled holder. The samples are then rapidly cooled down to 77 K. The temperature of the films is controlled by mounting a heater inside the sample holder, and measured by a calibrated copper– constantan thermocouple mounted very near to the films. Voltage on the system was sawtooth waveform with linear variation generated by Model 459 function

generator (Kikusui Electronics Corp.). I–V characteristics have been recorded using an X–Y function recorder in the temperature range of 207–339 K in the dark. C–V measurement was performed by CTG-1 model high frequency C–V characteristics measurement apparatus (Shanghai third electronics apparatus factory) at room temperature, where the input voltage frequency was fixed at 1 MHz with a sweeping rate of 0.2 V/s at room temperature. The Ohmic contacts to ZnO layers forms through the fusion of indium alloy. From careful check, we were guaranteed that the metal contacts were of adequate non-rectification. Crystal structure and morphology were analyzed by X-ray diffractometer (XRD; Philips X’Pert PRO X-ray diffractometer) and atomic force microscope (AFM) in tapping mode (IIIa model, Shanghai Aijian Nanotechnology Corp.).

3. Results and discussion Fig. 1 displays the XRD spectrum for nc-ZnO films grown on p-Si(1 0 0) substrates, which shows that the films are polycrystalline with a hexagonal wurtzite structure. It can be seen that there is not a clearly preferential orientation in the nanocrystalline ZnO thin films. The peaks at 69.58 and 33.08 correspond to the diffraction of Si(4 0 0) and (2 0 0) planes, respectively. The lattice constants of ZnO calculated ˚ and c = 5.202 A ˚, from the present data are a = 3.249 A

Fig. 1. XRD spectrum of a typical ZnO film grown on a p-Si(1 0 0) substrate.

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˚ which are reasonably in agreement with a = 3.253 A ˚ of ZnO (JCPDS Card No. 80–00075). and c = 5.209 A Fig. 2 shows the AFM micrograph of a typical ZnO film grown on a p-Si substrate. It can be easily seen that the film is crack-free and quite smooth. Grains are tightly packed, and the size varies from 50 to 100 nm. Fig. 3 shows the I–V characteristics of nc-ZnO/p-Si heterojunctions measured with temperature ranging from 270 to 339 K in a cryostat. The sample was kept in the dark condition during the measurements. It is obvious from Fig. 3(a) that the heterostructures are rectifying in nature with a turn-on voltage of 3.0 V, which is much larger than that of ZnO:Al/n-Si prepared by a magnetron sputtering [11]. Under reverse voltages, the breakdown voltage for the sample is as large as 20 V (not shown in figure). Generally, the native ZnO is n-type semiconductor due to the selfcompensation. In the case of ZnO/Si, the work function of p-type Si is larger than that of n-type Si. The potential barrier of p-type Si is higher compared with n-type Si. Therefore, the turn-on voltage of ncZnO/p-Si heterojunction is larger in comparison of ZnO:Al/n-Si. Fig. 3(b) shows the temperature dependence of the forward current in logarithmic scale. The forward currents can be classified into two regions according to the applied voltages. In region B, above 3.8 V, the forward current deviates from linearity due to the effect of a series resistance on the system. In region A,

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Fig. 3. Current–voltage characteristics of the nc-ZnO/p-Si heterojunctions at various temperatures in dark. The current is plotted as a function of the applied voltage using a linear (a) and (b) semilogarithmic scale.

below 3.8 V, the temperature dependence of the forward currents can be expressed by [12]: I ¼ I0 expðBVÞ

Fig. 2. AFM image of a typical ZnO film grown on a p-Si substrate.

(1)

where I is the current subject to an applied voltage V, I0 the saturation current, and B is a coefficient. The temperature dependence of the parameter B depends on the dominant current transport mechanism. If the current is controlled by tunneling, B is a constant independent of temperature. If the current is controlled by some other mechanisms, B is generally dependent on temperature. In the plots of log I versus V of ncZnO/p-Si heterojunctions, B is independent of the measurement temperature T, which indicates that the current in this region is dominated by a tunneling mechanism. The parameter B deduced from the plots

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of log I versus V is about 1.93 V1. The forward current can be explained by a multi-step tunneling model, which is attributed to the recombination of electrons, tunneling from ZnO into the gap states in Si, and holes tunneling across the heterojunction barrier from p-Si to nc-ZnO, where they hop between localized states through a multi-step tunneling process. The model for a multi-step tunneling captureemission process across a semiconductor junction shows the dependence of the currents on both the temperatures and the voltages. This model has successfully explained the voltage and temperature dependencies observed for an amorphous n-Si/crystalline p-Si heterojunction [13]. It is believed that it is also applicable to the case of nc-ZnO/p-Si heterojunctions. The temperature dependence of the pre-exponential factor I0 can be obtained by extrapolating the forward current curves, and the relation can be expressed as   DEa I0 / exp  (2) kT where Ea is the activation energy of carrier conduction, k Boltzmann’s constant, and T is absolute temperature. Fig. 4 shows the Arrhenius plot of log I0 versus 1000/T. From Fig. 4, I0 varies exponentially with 1/T, indicating a multi-step tunneling model [14]. Ea determined from this plot is given about 0.26 V, which

is reasonably in agreement with 0.27 V of ZnO/nSi(1 0 0) by magnetron sputtering [11]. In the charge transportation process of the nc-ZnO/ p-Si heterostructures, there are many interface states, due to the contacts of nanograins with Si substrates, which plays an important role in the depletion layer. If a positive voltage is gradually applied to the p side with respect to the n side, the potential barrier decreases, and the depletion layer gets narrower. The electrons from the conduction band of ZnO may tunnel through the junction potential barrier to the empty interface states and then transfer into the valence band of Si, without climbing the barrier, to produce a tunneling current. Under negative voltages, holes in the valence band of Si hardly transfer from the p to the n side due to the interface states. The breakdown voltage for this heterostructure is very large, in agreement with experimental results. Fig. 5 shows a typical C–V relation obtained from the formed nc-ZnO/p-Si heterojunctions measured at a high frequency of 1 MHz in dark and at room temperature. As observed from the figure, 1/C2–V variation is linear in the voltage range studied, indicating that there is little number of deep levels [15]. The typical features of an n-type metal oxide semiconductor capacitor with an accumulation region are observed, indicating that the junction is abrupt. The variation of capacitance as the bias decreases is caused by changes of the thickness of the carrier depletion region. The straight lines extrapolated to

Fig. 4. Arrhenius plot of the saturation current at 3.1 V for nc-ZnO/ c-Si heterojunctions.

Fig. 5. 1/C2 versus voltage plot of an nc-ZnO/c-Si heterojunction measured at 1 MHz in dark.

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1/C2 = 0 give the value of built-in potential Vbi = 1.49V, which is quite large compared with the reported value for sputtered ZnO:Al/Si 0.56 V [11]. The discrepancy is due to the different interface state densities at the heterojunction interfaces [16]. The difference of built-in potential indicates on a different interface state density at the heterojunction interface or composition and strain. In nanocrystalline ZnO films, the ratio of volume to surface is larger than that of columnar films, and the strain in nanocrystalline films is smaller than that of columnar films. Moreover, the conduction type of substrates and Al dopant are different. These induced the different interface state density at the heterojunction interface. Therefore, the built-in potentials of nc-ZnO/p-Si is larger than that of ZnO:Al/n-Si heterojunctions. Moreover, it should be noted that qVbi (q the electronic charge) obtained from C–V data is larger than the activation energy Ea got from I–V data. Though the value of qVbi is close to the activation energy, the forward current in heterojunctions is determined by the thermionic emission of carriers over a potential barrier of qVbi [17]. In the nc-ZnO/p-Si heterostructures, the qVbi obtained from the C–V plot is larger than the activation energy got from the temperature dependence of the saturation current. Thus tunneling seems to be an important factor in determining the current flow mechanism [18]. This result further supports the tunneling mechanism of the nc-ZnO/p-Si heterostructures.

4. Conclusions In summary, the n-type nc-ZnO/p-Si heterostructures have been successfully fabricated on Si(1 0 0) substrates using a simple sol–gel process. XRD and AFM measurements showed that ZnO films consisted of 50–100 nm polycrystalline nanograins with hexagonal wurtzite. The I–V characteristics of nc-ZnO/pSi heterostructures showed rectification clearly. Temperature-dependent I–V measurements revealed that at forward bias, the multi-tunneling model was applicable to the heterostructures, and the activation energy of saturation current was about 0.26 eV. C–V measurements confirmed the existence of an abrupt junction. The built-in potential 1.49 V much larger

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than the activation energy further confirmed the tunneling model. It is promising that nc-ZnO/p-Si heterostructures would lead to many exciting new devices.

Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 90201038, 50472009, 10474091, and by Knowledge Innovation Program of Chinese Academy of Sciences No. KJCX2-SW-04-02.

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