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Journal of Nanomaterials
Volume 2013 (2013), Article ID 560542, 9 pages
http://dx.doi.org/10.1155/2013/560542
Research Article

Surface Modification on the Sputtering-Deposited ZnO Layer for ZnO-Based Schottky Diode

1Institute of Electro-Optical and Material Science, National Formosa University, Yunlin 63201, Taiwan
2ITRI South, Industrial Technology Research Institute, Tainan 73445, Taiwan
3Metal Industries Research & Development Centre, Kaohsiung 81160, Taiwan

Received 14 September 2013; Accepted 5 November 2013

Academic Editor: Liang-Wen Ji

Copyright © 2013 Ren-Hao Chang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We prepare a zinc oxide- (ZnO-) based Schottky diode constructed from the transparent cosputtered indium tin oxide- (ITO-) ZnO ohmic contact electrode and Ni/Au Schottky metal. After optimizing the ohmic contact property and removing the ion-bombardment damages using dilute HCl etching solution, the dilute hydrogen peroxide (H2O2) and ammonium sulfide (NH4)2Sx solutions, respectively, are employed to modify the undoped ZnO layer surface. Both of the Schottky barrier heights with the ZnO layer surface treated by these two solutions, evaluated from the current-voltage (I-V) and capacitance-voltage (C-V) measurements, are remarkably enhanced as compared to the untreated ZnO-based Schottky diode. Through the X-ray photoelectron spectroscopy (XPS) and room-temperature photoluminescence (RTPL) investigations, the compensation effect as evidence of the increases in the O–H and OZn acceptor defects appearing on the ZnO layer surface after treating by the dilute H2O2 solution is responsible for the improvement of the ZnO-based Schottky diode. By contrast, the enhancement on the Schottky barrier height for the ZnO layer surface treated by using dilute (NH4)2Sx solution is attributed to both the passivation and compensation effects originating from the formation of the Zn–S chemical bond and acceptors.

1. Introduction

To accomplish a high-performance zinc oxide- (ZnO-) based optoelectronic device, the formation of a quality contact between ZnO and electrode is essential. A superior rectifying junction with metals and low-resistance ohmic contacts onto the ZnO surface is crucial to strengthen the diode application, UV detectors, gas sensors, piezoelectric transducers, and optical coatings [13]. Up to date, sputtering technology is a commonly used system for large-area and cost-efficiency ZnO-layered fabrication in application on the optoelectronic devices. However, limited reports on the ZnO-based Schottky diodes purely prepared using sputtering technology since significant defects formed in the films are inevitable due to the ion-bombardment damages and subsurface defects [46]. Accordingly, various ZnO surface passivation processes, such as chemical preparation with specific acid or organic solution, plasma bombardment, and activated light irradiation, were processed to achieve a quality ZnO-based Schottky diode [710]. Among the surface treatments, liquid-phase process, using the dilute hydrogen peroxide (H2O2) and ammonium sulfide ((NH4)2Sx) solutions, respectively, that has the simple and nonvacuum advantages over others, seems to be the best process method. Although reports had demonstrated that the improvement of the resulting metal/ZnO contact was attributed to the surface state passivation [1113], the dominated mechanism responsible for these surface treatments still was indefinite. Except for engineering a quality ZnO/metal Schottky contact, the low-resistance contacts between ZnO and the electrode also are crucial. Although most ZnO-based optoelectronics use metal structures contact to n-type ZnO films with a low specific contact resistance, , in the range of 10−5~10−7 Ω-cm [1416], the conversion efficiency between photons and electrons of the resulting optoelectronic devices is limited due to their opaque nature at the emission/absorption wavelengths. Accordingly, the transparent conductive oxide (TCO) film becomes a good candidate to be employed as an ohmic electrode with low light reabsorption [17, 18].

In this work, with the aim to achieve a quality ZnO-based Schottky diode, a homogeneity indium tin oxide- (ITO-) ZnO cosputtered film was deposited onto the undoped ZnO layer as an ohmic contact electrode, followed by the surface modification processes combined with the preetching process, using the diluted HCl solution, and the surface treatment, using the dilute H2O2 and (NH4)2Sx solutions, respectively, on the undoped ZnO layer prior to the Ni/Au Schottky metal deposition. The mechanisms responsible for the improvement of the ZnO-based Schottky diode were comprehensively investigated by using the X-ray photoelectron spectroscopy (XPS) and room-temperature photoluminescence (RTPL) measurements.

2. Experiment

A 1 μm thick undoped ZnO (i-ZnO) layer was deposited onto the silicon substrate using a radio frequency (rf) magnetron cosputtering system. The undoped ZnO layer was subsequently annealed at 700°C for 30 min under oxygen ambient to improve the crystalline structure with c-axis growth orientation [10]. A homogeneity 300 nm thick ITO-ZnO cosputtered film at an atomic ratio of 33% [Zn/(Zn + In) at.%] was deposited onto the ZnO layer, followed by window of the ITO-ZnO film with a diameter of 300 μm which was completely eliminated using the dilute hydrochloric acid (HCl) solution for 40 sec. The residual ITO-ZnO/i-ZnO contact system was then optimized using an RTA treatment at 400°C for 1 min under vacuum ambient. After optimizing the ohmic contact electrode, the exposed ZnO layer surface was etched by the dilute HCl solution (referred as the preetching process hereafter) for 20 and 40 sec, respectively, to effectively remove the defects induced from the ion-bombardment damages during the cosputtered film deposition. Afterwards, the preetched ZnO layer surface was then, respectively, dipped in the dilute H2O2 solution at 100°C or dilute (NH4)2Sx solution at 60°C for 3 min. Eventually, Ni/Au (20/100 nm) Schottky metal was deposited onto the surface-modified area with a diameter of 200 μm using e-beam evaporation, followed by liftoff using a standard photolithography technique. Figure 1 illustrates a schematic structure of the ZnO-based Schottky diodes.

560542.fig.001
Figure 1: Schematic structure of the ZnO-based Schottky diodes.

Film thickness of the ZnO-based Schottky diode structure was measured using a surface profile system (Dektak 6 M). Carrier concentration and Hall mobility of these cosputtered films were measured using the Van der Pauw method (Ecopia HMS-5000) at room temperature. The radiation spectra and the chemical bond configurations near the undoped ZnO layer surface with and without the surface treatments were conducted from the photoluminescence (PL) spectra measured at room temperature using a He-Cd laser ( nm) pumping source and characterized using an X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) with a monochromatic Al K source. Current-voltage (I-V) and capacitance-voltage (C-V) properties of the resulting Schottky diodes were characterized using a semiconductor parameter analyzer (HP4156C) and a LCR meter (HP4284A) at 1 MHz.

3. Results and Discussions

The electrical properties of the annealed undoped ZnO (i-ZnO) and cosputtered ITO-ZnO and ITO films are summarized in Table 1. Both the electron concentration and Hall mobility of the asdeposited cosputtered ITO-ZnO film were higher than those of the asdeposited ITO film, resulting in a low resistivity of  2.9 × 10−4 Ω cm [19]. The I-V characteristics of the asdeposited cosputtered ITO-ZnO and ITO films contact to the i-ZnO layer as well as the ITO-ZnO/i-ZnO contact system annealed at 400°C for 1 min under vacuum ambient using the transmission line method (TLM) are shown in Figure 2. All these contacts exhibited ohmic contact behavior. The transparent cosputtered ITO-ZnO film contact to the undoped ZnO layer resulted in a specific contact resistance (2.9 × 10−3 Ω cm2) lower than that of the ITO film contact to the undoped ZnO layer (6.0 × 10−2 Ω cm2). In addition, the specific contact resistance of the ITO-ZnO/i-ZnO contact system was further decreased to 4.6 × 10−5 Ω cm2 after the RTA treatment. The mechanism responsible for the reduction in the contact resistance was ascribed to the interdiffusion between the ITO-ZnO/i-ZnO interfaces [20].

tab1
Table 1: Electrical properties of the annealed i-ZnO, and cosputtered ITO-ZnO and ITO films.
560542.fig.002
Figure 2: I-V characteristics of the asdeposited cosputtered ITO-ZnO and ITO films contact to the i-ZnO layer as well as the annealed ITO-ZnO/i-ZnO contact system.

The I-V curve of the Ni/Au metal contact to the H2O2-treated ZnO layer without an additive preetching process using the dilute HCl solution is shown in Figure 3. The ZnO-based Schottky diode with the ZnO layer surface treated by dilute H2O2 solution showed rectifying behavior. The Schottky diode performance is evaluated by the forward current on the logarithmic scale, as shown in the inset figure, according to the thermionic theory: where the saturation current, , and ideality factor, , of the Schottky diode indicated in (1) are determined as where, the parameter is the magnitude of electronic charge, is the Boltzmann constant, is the operating temperature, is the applied forward bias, is the Schottky barrier height (SBH), and is the effective Richardson constant. In accordance with previous reports with an effective mass of 0.27 m0, the theoretical effective Richardson constant used to derive these Schottky diodes was 32 A cm2 K−2 [21]. The derived parameters of the Schottky diode and the forward turn-on current to reverse leakage current ratio measured at 2 and −2 V, respectively, are listed in Table 2. Although the Schottky diode constructed from the Ni/Au metal contact to the H2O2-treated i-ZnO layer surface led to a barrier height of 0.68 eV and an ideality factor of 1.45, a low current ratio was obtained as a consequence of the high leakage current (~5 × 10−7 A), which is unfavorable for the Schottky diode application. Such apparent leakage current was attributed to the ion-bombardment damages on the ZnO layer during sputtering deposition. Accordingly, the preetching process using the dilute HCl solution was firstly carried out to remove the defects on the ZnO layer surface induced from the ion-bombardment damages during the cosputtered film deposition before the surface treatment on the ZnO layer using the dilute H2O2 solution. The resulting I-V characteristics of the ZnO layer surface contact to the Ni/Au metal with an additive preetching process for 20 and 40 sec prior to the surface treatment using the dilute H2O2 solution are shown in Figure 3 (the inset figure highlights the reverse current of these diodes in logarithmic scale). Evidently, the reverse current was significantly decreased to 3.64 × 10−11 A as the preetching process reached 40 sec, which also corresponded to a higher Schottky barrier height (0.90 eV) and ideality factor (1.20) as compared to the nonetched sample. This revealed that the additive preetching process on the i-ZnO layer prior to the surface treatment using the dilute H2O2 solution also was essential for further idealizing the sputtering-deposited ZnO-based Schottky diode.

tab2
Table 2: Parameters of the Schottky diodes with and without an additive preetching process prior to the surface treatment using the dilute H2O2 solution.
560542.fig.003
Figure 3: I-V characteristics of the ZnO layer contact to the Ni/Au metal with and without an additive preetching process prior to the surface treatment using the dilute HCl solution (the inset figure highlights the current in logarithmic scale).

Figure 4 gives the I-V curves of the metallic Ni/Au contact to the surface-treated ZnO layer, using dilute H2O2 and (NH4)2Sx solutions, respectively, to study how these two surface treatments contributed to the Schottky diode performance (the Ni/Au contact to the untreated ZnO layer also is shown for comparison). It can be seen that the rectifying property of the Au/Ni/i-ZnO contact system was obviously improved using these two surface treatments. The device only etched by the dilute HCl solution (untreated sample) showed an almost symmetric I-V curve in the voltage range from −2 to 2 V, whereas the measured I-V characteristics of the Schottky diodes constructed for the H2O2- and (NH4)2Sx-treated ZnO layers exhibited the excellent rectifying behavior without significant breakdown at a reverse bias of −2 V. The leakage currents for both of the ZnO layer surfaces treated by using the dilute H2O2 and (NH4)2Sx solutions, as shown in the inset figure, were apparently reduced from 0.16 mA to 3.64 pA and 3.28 pA, respectively. The derived barrier height, ideality factor, and forward turn-on current to reverse leakage current ratio of these three Schottky diodes by adopting the thermionic theory given in (1)–(3) are summarized in Table 3. The Schottky barrier high and ideality factors of the Ni/Au contact onto the untreated ZnO layer were 0.59 eV and 2.01, respectively, indicating that the preetching process still was insufficient for idealizing the Schottky contact property and multiple current pathways still existed in addition to thermionic emission. By contrast, the low reverse leakage current of the Ni/Au contact to the surface-treated ZnO layer led to a high current ratio of about 107, which also corresponded to a Schottky barrier height of approximately 0.9 eV, a value close to the ideal value (~0.85 eV), revealing that thermionic emission predominated the current transition. Figure 5 further illustrates the as a function of the applied voltage for the metallic Ni/Au contact to the surface-treated ZnO layer, using dilute H2O2 and (NH4)2Sx solutions, respectively. The C-V relation for the ZnO-based Schottky diodes is described as [22] where  for ZnO [23], is the permittivity in vacuum, is the carrier concentration, is the area of the Schottky contact, and is the built-in voltage. Accordingly, the carrier concentration and the built-in voltage are derived from the slope and intercept, respectively, of the curves shown in Figure 5. The carrier concentrations of 1.5 × 1015 and 2.1 × 1015 cm−3 were in turn calculated from the H2O2- and (NH4)2Sx-treated ZnO layer surfaces. Both of these values were significant lower than those of the untreated ZnO layer surface (6.9 × 1015 cm−3), as shown in Table 3. In addition, the carrier concentration of the undoped ZnO layer surface etched by the dilute HCl solution for 40 sec also was calculated to be lower than that of the nonetched ZnO layer (7.4 × 1015 cm−3), revealing that the preetching process was also beneficial for the reduction in the carrier concentration. The intercepts on the x-axis for the C-V curves of the H2O2- and (NH4)2Sx-treated Schottky diodes shown in Figure 5 were 0.76 and 0.71 V, respectively, whereas that of the Schottky diode without the surface treatment was 0.39 V (not shown). The relationship between the Schottky barrier height and the built-in voltage is expressed as where is the potential difference between the conduction band and the Fermi level of the undoped ZnO layer. is the effective density of states in the conduction band of ZnO (~4.98 × 1018 cm−3) [11]. The Schottky barrier heights of the Ni/Au contact to the ZnO layer surface treated by the surface treatment dilute H2O2 and (NH4)2Sx solutions, respectively, were calculated to be 0.97 and 0.92 eV, as listed in Table 3. In addition, the ZnO layer surface only etched by using the dilute HCl solution resulted in a Schottky barrier height of 0.66 V while contacting to the Ni/Au metal. It also can be seen that the barrier heights derived from the I-V curves typical were lower than those calculated from the C-V characteristics. The difference in the Schottky barrier height between the I-V and C-V measurements was demonstrated to be the presence of the interface states which led to the image force barrier-lowering (IFBL) as derived from the I-V characteristics [11, 24]. Accordingly, the present surface modification processes, combined with the preetching process and the surface treatments using the dilute H2O2 and (NH4)2Sx solutions, respectively, on the undoped ZnO layer were helpful to improve the resulting Schottky diode performance. The ion-bombardment damages on the sputtering-deposited ZnO layer were removed by an additive preetching process using the dilute HCl solution. By contrast, both of the surface treatments using the dilute H2O2 and (NH4)2Sx solutions, respectively, were functional to significantly reduce the surface states on the ZnO layer surface, thereby improving the Schottky barrier height between the undoped ZnO layer and Ni/Au metal more effectively.

tab3
Table 3: Parameters of the Schottky diodes with and without the surface treatments calculated from the - and - characteristics.
560542.fig.004
Figure 4: I-V curve of the ZnO layer contact to the Ni/Au metal with and without the surface treatments (the inset figure highlights the current in logarithmic scale).
560542.fig.005
Figure 5: 1/C2 as a function of the applied voltage for the metallic Ni/Au contact to the surface-treated ZnO layer.

To further determine the improved mechanism of the Schottky diodes by these two surface treatments on the ZnO layer surface, using dilute H2O2 and (NH4)2Sx solutions, respectively, the evolutions on the binding energies of the , , and core levels conducted from the XPS spectra were carried out. Figures 6(a) and 6(b), respectively, show the binding energy of the core level with and without the H2O2 surface treatment on the ZnO layer. It can be seen that the core level of the untreated ZnO surface exhibited a dominated peak of approximately 530.6 eV with a tail extending to high binding energy. This curve was deconvoluted into two overlapping peaks, located at 530.4 and 531.8 eV which were in turn associated with the O–Zn and O–H chemical bonds [25], by means of a combination of Gaussian and Lorentzian functions (70% Gaussian and 30% Lorentzian) with background subtraction. Different from the untreated sample, the intensity of the O–H chemical bond in the spectrum of the ZnO layer surface treated by dilute H2O2 solution apparently enhanced to result in a broad feature. Table 4 illustrates the composition of the O–Zn and O–H chemical bonds deconvoluted from the ZnO layer surface with and without the H2O2 surface treatment. The composition of the O–Zn chemical bond (59%) in the untreated ZnO layer surface was higher than that of the O–H chemical bond (41%), whereas the O–H chemical bond (61%) dominated over the H2O2-treated ZnO layer surface. The increase in the O–H chemical bond in the ZnO layer surface was closely related to the decomposition of the H2O2 solution. Since the O–H chemical bond is a well-known acceptor-like defect in the ZnO material [26], the Fermi level in the H2O2-treated ZnO layer surface was prone to be close to the vacuum level and thus led to the upward band bending. In addition, the red shift of the O–Zn (529.9 eV) and O–H (531.6 eV) binding energies after the H2O2-treated ZnO layer surface also gave evidence of the band bending of the ZnO layer surface [27]. The binding energies of the core level on the ZnO layer surface with and without the H2O2 treatment are shown in Figure 7. The binding energy of the core level in the untreated ZnO layer surface located at 1021.7 eV, a value lower than that of the bulk ZnO (1022.0 eV) owing to the native oxygen vacancies () formed in the sputtering-deposited ZnO layer [28]. For the ZnO layer treated by the dilute H2O2 solution, the donors were compensated by the appearance of the O–H acceptors, thereby resulting in the blue shift of binding energy (~0.5 eV).

tab4
Table 4: Compositions of the O–Zn and O–H chemical bonds for the untreated and H2O2-treated ZnO layer surfaces.
fig6
Figure 6: Binding energy of the O 1s core level for the (a) untreated and (b) H2O2-treated ZnO layer surfaces.
560542.fig.007
Figure 7: Binding energy of the Zn 2p3/2 core level for the untreated and H2O2-treated ZnO layer surfaces.

Figure 8 shows the core level of the ZnO layer surface treated by the dilute (NH4)2Sx solution. A binding energy at 162.0 eV overlapped by the and with an interval of 1.2 eV was obtained from the (NH4)2Sx-treated ZnO layer surface, which was associated with the Zn–S chemical bond [29]. In addition, the binding energy located at 1021.7 eV of the core level for the untreated ZnO layer surface, as shown in Figure 9(a), was composed of two chemical bonds which in turn were denoted as Zn–Zn (1021.3 eV) and Zn–O (1021.8 eV). By contrast, the binding energy located at 1021.8 eV of the core level for the (NH4)2Sx-treated ZnO layer surface shown in Figure 9(b) was deconvoluted into three chemical bonds of Zn–Zn (1021.4 eV), Zn–S (1021.6 eV), and Zn–O (1022.0 eV), respectively [12]. The compositions in the ZnO layer surface with and without the (NH4)2Sx treatment extracted from each area of the correspondent chemical bonds are summarized in Table 5. The composition of the Zn–Zn chemical bond (30%) which was linked to the defects on the untreated ZnO layer surface had been effectively replaced by the appearance of the Zn-S chemical bond (26%), revealing that the ZnO layer surface treated by the dilute (NH4)2Sx was favorable to passivate the formation of the donors.

tab5
Table 5: Compositions of the Zn–Zn, Zn–S, and Zn–O chemical bonds for the untreated and (NH4)2-treated ZnO layer surfaces.
560542.fig.008
Figure 8: Binding energy of the S 2p core level for the (NH4)2Sx-treated ZnO layer surface.
fig9
Figure 9: Binding energy of the Zn 2p3/2 core level for the (a) untreated and (b) (NH4)2Sx-treated ZnO layer surfaces.

Figures 10(a)10(c) show the RTPL spectra of the untreated and surface-treated ZnO layers. Two distinct emissions which in turn were denoted as the UV emission associated with near band edge emission (NBE) and the green-yellow emission associated with the deep level emission (DLE) were observed in these spectra. As given in these figures, the DLE emission was divided into several feature peaks of (~2.11 eV), antisite oxygen ( eV), interstitial oxygen ( eV), and zinc vacancies ( eV), respectively [3032]. The donor-related emission dominated over the PL spectra of the untreated ZnO layer, whereas the acceptor-related OZn emission was the most intense peak in the H2O2-treated ZnO layer. From the XPS and RTPL measurements, the undoped ZnO layer surface treated by the dilute H2O2 solution thus was demonstrated to be beneficial for the enhancement on the OZn and O–H acceptors, which was functional to compensate the native donors and even bring about the upward band bending of the ZnO layer surface, resulting in the increase in the Schottky barrier height. In contrast to the H2O2-treated sample, another deep level emission denoted as the was observed in the RTPL spectra of the (NH4)2Sx-treated ZnO layer other than the apparent decrease in the -related radiation compared to the untreated ZnO layer. Combined with the XPS and the RTPL measurements, the undoped ZnO layer surface treated by the dilute (NH4)2Sx solution not only facilitated to suppress the formation of the defects originating form to the formation of the Z–S chemical bonds but also induced the acceptors, thereby causing the reduction in the electron concentration on the ZnO layer surface and improving the contact behavior to the Ni/Au metal.

fig10
Figure 10: RTPL spectra of the (a) untreated, (b) H2O2-treated, and (c) (NH4)2Sx-treated ZnO layers.

4. Conclusions

A quality ZnO-based Schottky diode was achieved using the transparent cosputtered ITO-ZnO ohmic electrode and Ni/Au Schottky metal. The homogeneity ITO-ZnO film deposited onto the undoped ZnO layer resulted in a low specific contact resistance of 4.6 × 10−5 Ω cm2 after an RTA treatment. In terms of the Schottky contact surface, an additive preetching process using the dilute HCl solution prior to the surface treatment on the undoped ZnO layer surface was crucial to remove the ion-bombardment damages during sputtering deposition. The increase in the Schottky barrier height between the ZnO and Ni/Au contact was found to be deeply correlated to the decrease in the carrier concentration of the ZnO layer surface modified by the combination process of the etching and the surface treatments, as derived from the measurements. Accordingly, an optimal Schottky barrier height was obtainable from the undoped ZnO layer with the carrier concentration decreased from 7.4 × 1015 to 1.5 × 1015 cm−3, which was etched by the dilute HCl solution and sequentially treated by the dilute H2O2 solution, contact to the Ni/Au metal. From the XPS and RTPL measurements, the mechanisms responsible for the enhancement on the Schottky barrier height of the H2O2-treated ZnO layer contact to Ni/Au were attributed to the compensation effect originating from the increase in the O-H and OZn acceptors. By contrast, the donors passivation due to the formation of the Zn–S chemical bond and the compensation effect of the acceptor formation in the ZnO layer treated by the dilute (NH4)2Sx solution was the key factor to cause the improvement of the resulting Schottky diode performance. The transparent ZnO-based Schottky diode was fabricated from the surface-treated ZnO layer, using dilute H2O2 and (NH4)2Sx solutions with an additive preetching process, thus exhibiting the high Schottky barrier heights of 0.90 and 0.87 eV, respectively.

Acknowledgments

The authors would like to acknowledge the support of National Science Council under Grant no NSC 102-2221-E-150-067 and the fund supports by ITRI South and MIRDC.

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