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p-type Si devices by nanowire surface passivation
Current Applied Physics 15 (2015) 213e218
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Interface modification and trap-type transformation in Al-doped CdO/ Si-nanowire arrays/p-type Si devices by nanowire surface passivation Yow-Jon Lin a, *, Wei-Min Cho a, Hsing-Cheng Chang b, Ya-Hui Chen c a
Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan Department of Automatic Control Engineering, Feng Chia University, Taichung 407, Taiwan c Precision Instrument Support Center, Feng Chia University, Taichung 407, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2014 Received in revised form 19 October 2014 Accepted 15 December 2014 Available online 17 December 2014
The present work reports the fabrication and detailed electrical properties of Al-doped CdO/Si-nanowire (SiNW) arrays/p-type Si Schottky diodes with and without SiNW surface passivation. It is shown that the interfacial trap states influence the electronic conduction through the device. The experimental results demonstrate that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiOx/SiNW interface which can be changed by the SieO bonding on the energy barrier lowering and the charge transport property. The induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the passivation treatment time is found in this study. © 2014 Elsevier B.V. All rights reserved.
Keywords: Defects Semiconductors Passivation Electrical properties Nanostructures
1. Introduction In this paper, we reported the electrical property of Al-doped CdO (AlCdO) Schottky contact on Si nanowires (SiNWs) with and without H2O2 treatment. Due to the technological importance of Schottky diodes, a full understanding of the nature of their electrical characteristics is of great interest. Correlation effects were evaluated using the well-known expression for the thermionic emission (TE) [1e4]. TE theory is normally used to extract the Schottky diode parameters. For AlCdO/SiNWs/p-type Si (p-Si) Schottky diodes, the ideality factor (h) and Schottky barrier height (qfB) were determined from the forward bias currentevoltage (IeV) characteristics on the basis of the TE mechanism. It is shown that trap states in the SiNWs near the AlCdO/SiNW interfaces were controlled by H2O2 treatment, improving device performance. In recent years, solar cells based on SiNWs have attracted great interest [5e9]. This remarkably low broadband reflectance is a major advantage of cost-effective SiNW-based solar cells. Li et al. [10] found that SiNWs have a mean diameter of 220 nm. Sun et al. [11] found that the diameters of SiNWs are in a range from 40 nm to 420 nm. If the diameters are much smaller than the lengths of the
* Corresponding author. E-mail address: [email protected] (Y.-J. Lin). http://dx.doi.org/10.1016/j.cap.2014.12.015 1567-1739/© 2014 Elsevier B.V. All rights reserved.
SiNWs, the SiNWs will be likely to lean into each other, forming a SiNW bundle [11]. However, due to the large surface-to-volume ratio, the SiNWs have a high surface recombination rate [12,13]. SiNWs surface passivation, however, is an even more challenging task due to their small size and the fact that multiple facets with different crystalline orientation are exposed. The various surface passivation layers, such as SiOx, -Sx and -CH3, have been used in previous works [1,4,14e18]. On the other hand, CdO has shown promising results in solar cell application [19e21]. CdO thin films have high transparency in the visible region of the electromagnetic spectrum and show degenerate n-type conductivity mainly due to oxygen vacancies and high carrier concentration contributed by shallow donors resulting from self non-stoichiometry [22]. Unfortunately not much work has been done on CdO due to its toxic nature. Although, there are reservations to the application of CdO material due to environmental concerns, CdO films have shown a transmittance of over 80% in the visible and near infrared region of the spectrum [23,24].
2. Experimental details Four-inch p-Si (100) wafers purchased from Woodruff Tech Company were used in the experiment. The p-Si film thickness was about 525 mm. The resistivity of p-Si is about 5 U cm. The p-Si
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samples were cleaned in chemical cleaning solutions of acetone and methanol, rinsed with de-ionized water, and blow-dried with N2. Next, the p-Si sample was chemically etched with a diluted HF solution for 1 min, rinsed with de-ionized water and blow-dried with N2. The SiNW arrays were then formed adopting a developed silver-induced wet-chemical-etching process in an aqueous buffered HF and AgNO3 etching solution at 25 C [25]. The etching time was 15 min. The surface color of these wafers appeared black after removing the Ag remnants by immersing them in the concentrated HNO3 solution for 1 h. The length of SiNWs, as estimated from field emission scanning electron microscopy (SEM), was about 690 nm. Fig. 1(a) shows a plane SEM image of the SiNWs. The SEM operating voltage is 5 kV. It is found that SiNWs lean into each other, forming a SiNW bundle. This is because of the smaller diameters than the lengths of the SiNWs [11]. Fig. 1(b) shows a cross-sectional SEM image of the SiNWs. The SEM operating voltage is 3 kV. Then, some of the SiNWs/p-Si samples were dipped in the H2O2 solution at 60 C for 10 and 20 min (referred to as 10 min- and 20 min-H2O2-treated SiNWs/p-Si samples), respectively. Fig. 1(c) shows a cross-sectional SEM image of the H2O2-treated SiNWs. No noticeable changes in cross-sectional SEM images of the SiNWs [Fig. 1(b) and (c)] were observed. AlCdO films were deposited on the SiNW arrays by an rf magnetron sputtering system using two magnetrons, a high-purity CdO target (rf power was fixed at 30 W), and an Al target (rf power was fixed at 30 W). The target size is 2 inch and the targetesubstrate distance is 65 mm. Targets were used in Ar as an ambient gas for sputtering. The flow of Ar was 90 SCCM (SCCM denotes standard cubic centimeter per minute). The sputtering pressure was fixed at 5 103 Torr. The substrate temperature was fixed at 400 C. The sputtering time was 60 min. Next, Au contacts were deposited onto the back surface of p-Si by a sputter coater and annealed at 400 C for 2 min on a hotplate in a nitrogen-filled glovebox to form a large area ohmic contact. The IeV and current-time (I-t) curves were measured using a Keithley Model-4200-SCS semiconductor characterization system. The device photoresponse was measured under AM 1.5G condition with an illumination intensity of 100 mW/cm2 using a solar simulator. The photoresponse was measured by recording the current versus time while sunlight illumination was turned on and off by a shutter. To rectify the measurement divergence, the light intensity was calibrated using a reference silicon solar cell certificated by the National Renewable Energy Laboratory. The work function of the AlCdO films was examined with the SKP5050 Scanning Kelvin probe (KP Technology). The scan area is 20 30 mm2. The Kelvin probe is a non-contact, non-destructive vibrating capacitor device used to measure the work function of conducting materials KP Technology Systems offer very high work-function resolution of 1e3 meV. The theory and operation of the vibrating KP method for measuring work function have been well documented [26,27]. Xray photoelectron spectroscopy (XPS) was employed to examine the chemical bonding states at the SiNW surfaces with and without
Fig. 2. J-V curves of (a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2-treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes in the dark.
H2O2 treatment. XPS measurements (ULVAC-PHI, PHI 5000) were performed using a monochromatic Al Ka x-ray source. The binding energy scale was calibrated using the position of the Au 4f peak, measured on a clean gold foil in electrical contact with the substrate.
3. Results and discussions Fig. 2 shows the current densityevoltage (JeV) characteristics of the AlCdO/SiNWs/p-Si and AlCdO/H2O2-treated SiNWs/p-Si Schottky diodes in the dark, respectively. The rectifying JeV characteristics suggest that Schottky junctions are formed at the AlCdO/ SiNW interfaces. The ratio of the forward to reverse current at a bias voltage of ±2 V for the AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diode was calculated to be 714 (4184 and 21235), implying a good rectification behavior for AlCdO/20 min-H2O2treated SiNWs/p-Si Schottky diodes. With consideration that our measurements were made at room temperature and the SiNWs had a low doping, the dominant transport property at the barrier is thermionic emission and diffusion, and the contribution made by tunneling is negligible [28]. However, the nonlinear ln (I)eV0.25 curve is found, making the classic thermionic emission-diffusion theory inapplicable in our case. In a Schottky contact, the forward bias IeV relation obtained by using the TE theory is given by [2e4]
Fig. 1. (a) Plane and (b) cross-sectional SEM images of the SiNWs and (c) a cross-sectional SEM image of the H2O2-treated SiNWs.
Y.-J. Lin et al. / Current Applied Physics 15 (2015) 213e218
q4 V IRs J ¼ I=S ¼ A*T 2 exp B exp 1 1 kT q hkT
(1)
where q is the electron charge, T is the measurement temperature, k is the Boltzmann constant, S is the Schottky contact area, Rs is the series resistance and A* is the effective Richardson constant (32 A cm2 K2 for p-Si [29]). The diode properties of the nanoscale devices have been analyzed using the TE equation [30,31]. h was determined from the slope of the linear regions of the forward JeV plots. From the curve fitting of the AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diode, h of 2.5 (2.2 and 1.7) was derived. Deviation of h from unity may be attributed to a combined effect of the recombination of electrons and holes in the depletion region and the presence of a large number of the interfacial defects [32]. h > 2 is found for AlCdO/SiNWs/p-Si devices, suggesting that interface states may play important roles in the conduction process. However, H2O2 treatment may lead to the reduction of h and the JeV characteristic of the AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diode is limited by the forward-bias current behavior of the conventional Schottky junction. qfB was also obtained from the forward JeV characteristics, according to Equation (1). For AlCdO/ SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/ 20 min-H2O2-treated SiNWs/p-Si) Schottky diode, qfB of 0.76 (0.82 and 0.93) eV was derived. For estimating the accurate Schottky contact area, the filling ratio, the cross section and the length of SiNWs should be considered. Due to the absence of reliable values for cross sectional area of the AlCdO/SiNWs junctions, the extracted barrier height can only serve as an approximation. However, Ruan and Lin [4] found that qfB is slightly sensitive to the contribution of the contact-area variation. It is shown that the barrier heights produced on SiNW, by exploiting this effect of 20 min-H2O2 treatment, can achieve a value of 0.93 eV which is 0.17 eV higher than the derived value for devices without SiNW surface passivation. In addition, the reverse-bias leakage current is affected by H2O2 treatment and decreases with increasing the H2O2 treatment time. A probable reason for the large leakage current is the existence of the high interfacial state density. We suggest that H2O2 treatment may lead to the dramatically reduced number of charge traps in the SiNWs, thus, increasing qfB and reducing the reverse-bias leakage current. Trap states in the SiNWs near the AlCdO/SiNW interfaces were considered to affect interfacial barriers with contacts and consequently electrical leakage [33]. This is because of the energy barrier lowering caused by trapped charge carriers jumping between the continuous potential well. The current will flow preferentially through the lower barriers in the potential distribution (that is, electrons hopping from one trap state to another trap state). The details of this mechanism are discussed later. Fig. 3 shows the images of the work-function difference (DW) between the probe and AlCdO (Au). The Au and AlCdO samples were placed together. The AlCdO sample is on the right-hand side and Au is on the left-hand side. Relative methods make use of the contact potential difference between the probe and a reference material (that is, Au) or between the probe and AlCdO. Au is used as a reference material for obtaining the work function of AlCdO (Wa), owing to its high chemical stability. It is found that the average work function of Au is higher than that of the probe (DW ¼ 295 ± 10 meV) and the average work function of AlCdO is lower than that of the probe (DW ¼ 585 ± 10 meV). It is known that the value of the work function of Au is 5.10 eV [34]. Thus, Wa is calculated to be 4.22 ± 0.02 eV. For AlCdO/SiNWs/p-Si Schottky diodes, the ideal Schottky barrier height (qfI) is given by [35] qfI ¼ (c þ Eg) Wa ¼ 5.17 Wa
215
(2)
Fig. 3. Two-dimensional image of the work-function difference between the probe and AlCdO (Au).
where Eg is the energy band gap of Si (Eg ¼ 1.12 eV) and c is the electron affinity of Si (c ¼ 4.05 eV [36]). According to Equation (2), the Schottky limit could be calculated to be 0.95 eV. For AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes, qfB evaluated from J-V measurements is close to the Schottky limit, indicating a suitable treatment time may lead to a better Schottky contact with the SiNWs. To confirm the dominance of charge trapping, time domain measurement was performed on the AlCdO/SiNWs/p-Si (AlCdO/ H2O2-treated SiNWs/p-Si) Schottky diode. The photocurrent (IP) values were measured for V varying from 0 to 1 mV (DV ¼ 1 mV) for 300 s (Illumination was turned on at t ¼ 0 s). Fig. 4 shows the normalized photocurrent decay for AlCdO/SiNWs/p-Si (AlCdO/ H2O2-treated SiNWs/p-Si) Schottky diodes and the fitting curve. jIP0 j is the normalized jIPj. jIP0 j should rapidly decay for AlCdO/ SiNWs/p-Si Schottky diodes without H2O2 treatment, since the electron traps in the SiNWs influence the electronic conduction through the device. jIP0 j should slowly decay for AlCdO/10 minH2O2-treated SiNWs/p-Si Schottky diodes, since H2O2 treatment leads to the reduced number of electron traps in the SiNWs. However, jIP0 j should rapidly grow for AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes, implying that the remarkably reduced
Fig. 4. jIP0 j-t characteristics of (a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2-treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes and the fitting curves.
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number of electron traps may lead to the dominance of hole traps. This is a strong evidence of the induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the H2O2 treatment time. In this study, we consider two possible physical pictures to explain the observed jIP0 j decay [Fig. 4(a) and (b)]: It is mainly dominated by the long-lifetime or short-lifetime electron traps for AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si) Schottky diodes. To estimate the time constants for jIP0 j decay, we fit the data to exponential decay functions. The transient was fitted to second-order exponential decay jIP0 j ¼ A1e(t/t1) þ A2e(t/t2) using the nonlinear least-squares method [37e41], with four fitting parameters. This equation reflects two different electron trapping mechanisms with time constants t1 and t2 (t1 > t2). The values of A1 and A2, where A1 þ A2 ¼ 1, represent weighing factors that quantify the contribution of each mechanism to the decay process. The first (second) term, which can be attributed to long-lifetime (short-lifetime) electron trapping, dominates the decay process. The fitting parameters are listed in Table 1. For AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment, the data show that jIP0 j rapidly decayed, owing to the dominance of short-lifetime electron trapping (A1 < A2). However, for AlCdO/10 min-H2O2-treated SiNWs/p-Si Schottky diodes, the data show that jIP0 j slowly decayed, owing to the dominance of long-lifetime electron trapping (A1 ¼ 1). For long-lifetime electron trapping, t1 stayed the same at 90 s, which implies that the underlying relaxation mechanism is the same with or without H2O2 treatment. Incident photon illumination can create electronehole pairs in the space charge region at the AlCdO/SiNW interfaces that will be swept out producing the photocurrent. Holes originating from the photogenerated electronehole pairs were swept out of the space charge region by the built-in electric field into the SiNWs and electrons originating from the photogenerated electronehole pairs were swept out of the space charge region by the built-in electric field into the AlCdO layer. The detrapped electrons and holes may recombine in pairs in the SiNWs, resulting in a decaying hole population. The photoresponse result confirms that the photocurrent decay is due to the dominance of electron trapping with short-second (long-second) lifetime in the SiNWs for AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2treated SiNWs/p-Si) Schottky diodes [40,42]. As compared to AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment, the number of electron traps in the SiNWs is significantly decreased for AlCdO/10 min-H2O2-treated SiNWs/p-Si Schottky diodes. Consequently, the 10 min-H2O2 treatment may lead to the removal of the short-lifetime electron traps, causing the slow jIP0 j decay. On the other hand, we consider one possible physical picture to explain the observed jIP0 j growth [Fig. 4(c)]: It is mainly dominated by hole traps in the SiNWs for AlCdO/20 min-H2O2-treated SiNWs/ p-Si Schottky diodes. The growing jIP0 j is described by jIP0 j ¼ {1 exp [(t/t3)d]} [41,43,44]. The parameter d, which reflects the relaxation mechanism, is an exponent between 0 and 1. The time constant is represented by t3. The fitting parameters are listed in Table 1. The small t3 indicates the AlCdO/20 min-H2O2treated SiNWs/p-Si Schottky diode having the low hole-trap density and good stability. The 20 min-H2O2 treatment may lead to the
dramatically reduced number of electron traps in the SiNWs, exhibiting the dominance of hole trapping. The number of holes reaching the electrode in the SiNW array and the number of electrons reaching the electrode in the AlCdO layer should determine the measured photocurrent under a near-constant bias voltage (DV ¼ 1 mV). Thus, the growing jIP0 j may be explained in terms of the effect of hole trapping in achieving the steady state current after illumination. We consider that conducting holes can be captured at trap centers in the SiNWs. As a result, the photocurrent onset is slowed because steady state current will be reached only after all the traps are filled. Under this condition, an equilibrium between multiple hole trapping and detrapping events is observed. We suggest that the electron-trapping effect competes with the hole-trapping effect for AlCdO/H2O2-treated SiNWs/p-Si Schottky diodes and the remarkably reduced number of electron traps may lead to the dominance of hole traps. Fig. 5 shows the Si 2p core-level spectra at the SiNW surfaces with and without H2O2 treatment, respectively. The peak positioned at ~99.4 eV is attributed to SieSi bonds and the peak positioned at ~103 eV is attributed to SieO bonds [2,45]. It is found the SiNW/p-Si sample with no SieO peak and the H2O2-treated SiNW/ p-Si sample with strong SieO peak. An explanation to the origin of AlCdOeSiNW interface state is provided by the dangling bonds at the SiNW surfaces. The presence of SieO bonds might have an effect on the Schottky barrier by reducing the interface state density, indicating that a good passivation is formed at the interface as a result of the reduction of the interface state density and an appropriate interfacial SiOx layer plays an important role in the conduction process of a Schottky diode. He et al. [45] found that an ultra thin SiOx layer was grown on the p-Si surface by immersing the wafer in a H2O2 solution. Zhang et al. [16] suggested that the Si surface termination state plays a key role on the electrical output of the Si-based devices. Note, thin SiOx layer grown by H2O2 treatment could be utilized to suppress the recombination velocity [46]. The improved device performance is attributed to SiNW surface passivation. The presence of a large number of trap and defect levels at the AlCdO/SiNW interfaces is a major impediment to the realization of junction devices. These defects will strongly influence
Table 1 Fitting parameters and results [(a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes].
(a) (b) (c)
t1 (s)
A1
t2(s)
A2
t3(s)
d
90 90 x
0.15 1 x
23 x x
0.85 x x
x x 4
x x 0.5
Fig. 5. Si 2p core-level spectra at the (a) as-cleaned, (b) 10 min-H2O2-treated and (c) 20 min-H2O2-treated SiNW surfaces.
Y.-J. Lin et al. / Current Applied Physics 15 (2015) 213e218
carrier dynamics, and are deleterious for device application. In this report, we focused on the effect of the dangling bonds at the SiNW surface and suggested that it can be changed by the SieO bonding. We believe that the increased Schottky barrier height has a strong relation to the remarkably reduced number of electron traps in the SiNWs; and, such result can be attributed to the SieO bonding. The existence of trap states at the AlCdO/SiNW interfaces may lead to the energy barrier lowering, owing to trapped electrons jumping between the continuous potential well. The current will flow preferentially through the lower barriers in the potential distribution. These can explain why the AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment exhibit larger h, lower qfB and higher leakage current. In addition, Rs was obtained from the forward JeV characteristics, according to Equation (1). For AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 minH2O2-treated SiNWs/p-Si) Schottky diodes, the value of Rs was found as about 200 (270 and 590) U. It is found that H2O2 treatment may lead to an increase in Rs, owing to the contribution of a thin SiOx layer. The ratio of the SieO to SieSi peak intensities was calculated to be 0.00 (0.20 and 0.26) for AlCdO/SiNWs/p-Si (AlCdO/ 10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diodes, suggesting that the thickness of SiOx layer increases with increasing the time of H2O2 treatment. However, the resistance of SiOx (70 or 390 U) is much lower than the depletion resistance under low forward bias (that is, > 104 U at 0.2 V). We suggested that the influence of a SiOx layer on junction conductance at low forward voltages is negligible and the increased qfB cannot be attributed to the contribution of a SiOx interlayer. On the other hand, Kirichenko et al. [47] found that the SiO2/Si interface may serve as a limited sink for Si vacancies. Single vacancy and vacancy cluster defects are substantially more stable at SiO2/Si interface compared to the bulk Si layers away from interface, mainly due to termination of dangling bonds with bridging O atoms and reduction of interface strain [47]. In addition, Kim et al. [48] investigated the oxidative behavior of SiNWs. In this report, we focused on the effect of Si vacancies at the SiOx/SiNW interface and suggested that it can be changed by the SieO bonding. This may explain why the dominance transformation from electron traps to hole traps is induced by controlling the H2O2 treatment time.
4. Conclusions In summary, we have discussed the electrical and optoelectronic properties of AlCdO Schottky contact on SiNWs with and without H2O2 treatment. For AlCdO/SiNWs/p-Si Schottky diodes without SiNW surface passivation, the JeV result shows a deviation from that expected from TE for the current mechanism. Compared to the AlCdO/SiNWs/p-Si Schottky diodes without SiNW surface passivation, the AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes exhibit lower h (h ¼ 1.7), higher qfB (qfB ¼ 0.93 eV) and lower leakage current. Both h and qfB are affected by H2O2 treatment, implying that charge traps in the SiNWs have a noticeable effect on the electronic conduction through the AlCdO/SiNWs/p-Si Schottky diodes. It is shown the induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the H2O2 treatment time. The photoresponse, J-V and XPS results demonstrated that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiOx/SiNW interface which can be changed by the SieO bonding on the energy barrier lowering and the charge transport property. The current will flow preferentially through the low barriers caused by trapped charge carriers jumping between the continuous potential well. This helps explain why the AlCdO/ SiNWs/p-Si Schottky diodes without SiNW surface passivation exhibit larger h, lower qfB and higher leakage current.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Interface modification and trap-type transformation in Al-doped CdO/ Si-nanowire arrays/p-type Si devices by nanowire surface passivation Yow-Jon Lin a, *, Wei-Min Cho a, Hsing-Cheng Chang b, Ya-Hui Chen c a
Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan Department of Automatic Control Engineering, Feng Chia University, Taichung 407, Taiwan c Precision Instrument Support Center, Feng Chia University, Taichung 407, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2014 Received in revised form 19 October 2014 Accepted 15 December 2014 Available online 17 December 2014
The present work reports the fabrication and detailed electrical properties of Al-doped CdO/Si-nanowire (SiNW) arrays/p-type Si Schottky diodes with and without SiNW surface passivation. It is shown that the interfacial trap states influence the electronic conduction through the device. The experimental results demonstrate that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiOx/SiNW interface which can be changed by the SieO bonding on the energy barrier lowering and the charge transport property. The induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the passivation treatment time is found in this study. © 2014 Elsevier B.V. All rights reserved.
Keywords: Defects Semiconductors Passivation Electrical properties Nanostructures
1. Introduction In this paper, we reported the electrical property of Al-doped CdO (AlCdO) Schottky contact on Si nanowires (SiNWs) with and without H2O2 treatment. Due to the technological importance of Schottky diodes, a full understanding of the nature of their electrical characteristics is of great interest. Correlation effects were evaluated using the well-known expression for the thermionic emission (TE) [1e4]. TE theory is normally used to extract the Schottky diode parameters. For AlCdO/SiNWs/p-type Si (p-Si) Schottky diodes, the ideality factor (h) and Schottky barrier height (qfB) were determined from the forward bias currentevoltage (IeV) characteristics on the basis of the TE mechanism. It is shown that trap states in the SiNWs near the AlCdO/SiNW interfaces were controlled by H2O2 treatment, improving device performance. In recent years, solar cells based on SiNWs have attracted great interest [5e9]. This remarkably low broadband reflectance is a major advantage of cost-effective SiNW-based solar cells. Li et al. [10] found that SiNWs have a mean diameter of 220 nm. Sun et al. [11] found that the diameters of SiNWs are in a range from 40 nm to 420 nm. If the diameters are much smaller than the lengths of the
* Corresponding author. E-mail address: [email protected] (Y.-J. Lin). http://dx.doi.org/10.1016/j.cap.2014.12.015 1567-1739/© 2014 Elsevier B.V. All rights reserved.
SiNWs, the SiNWs will be likely to lean into each other, forming a SiNW bundle [11]. However, due to the large surface-to-volume ratio, the SiNWs have a high surface recombination rate [12,13]. SiNWs surface passivation, however, is an even more challenging task due to their small size and the fact that multiple facets with different crystalline orientation are exposed. The various surface passivation layers, such as SiOx, -Sx and -CH3, have been used in previous works [1,4,14e18]. On the other hand, CdO has shown promising results in solar cell application [19e21]. CdO thin films have high transparency in the visible region of the electromagnetic spectrum and show degenerate n-type conductivity mainly due to oxygen vacancies and high carrier concentration contributed by shallow donors resulting from self non-stoichiometry [22]. Unfortunately not much work has been done on CdO due to its toxic nature. Although, there are reservations to the application of CdO material due to environmental concerns, CdO films have shown a transmittance of over 80% in the visible and near infrared region of the spectrum [23,24].
2. Experimental details Four-inch p-Si (100) wafers purchased from Woodruff Tech Company were used in the experiment. The p-Si film thickness was about 525 mm. The resistivity of p-Si is about 5 U cm. The p-Si
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samples were cleaned in chemical cleaning solutions of acetone and methanol, rinsed with de-ionized water, and blow-dried with N2. Next, the p-Si sample was chemically etched with a diluted HF solution for 1 min, rinsed with de-ionized water and blow-dried with N2. The SiNW arrays were then formed adopting a developed silver-induced wet-chemical-etching process in an aqueous buffered HF and AgNO3 etching solution at 25 C [25]. The etching time was 15 min. The surface color of these wafers appeared black after removing the Ag remnants by immersing them in the concentrated HNO3 solution for 1 h. The length of SiNWs, as estimated from field emission scanning electron microscopy (SEM), was about 690 nm. Fig. 1(a) shows a plane SEM image of the SiNWs. The SEM operating voltage is 5 kV. It is found that SiNWs lean into each other, forming a SiNW bundle. This is because of the smaller diameters than the lengths of the SiNWs [11]. Fig. 1(b) shows a cross-sectional SEM image of the SiNWs. The SEM operating voltage is 3 kV. Then, some of the SiNWs/p-Si samples were dipped in the H2O2 solution at 60 C for 10 and 20 min (referred to as 10 min- and 20 min-H2O2-treated SiNWs/p-Si samples), respectively. Fig. 1(c) shows a cross-sectional SEM image of the H2O2-treated SiNWs. No noticeable changes in cross-sectional SEM images of the SiNWs [Fig. 1(b) and (c)] were observed. AlCdO films were deposited on the SiNW arrays by an rf magnetron sputtering system using two magnetrons, a high-purity CdO target (rf power was fixed at 30 W), and an Al target (rf power was fixed at 30 W). The target size is 2 inch and the targetesubstrate distance is 65 mm. Targets were used in Ar as an ambient gas for sputtering. The flow of Ar was 90 SCCM (SCCM denotes standard cubic centimeter per minute). The sputtering pressure was fixed at 5 103 Torr. The substrate temperature was fixed at 400 C. The sputtering time was 60 min. Next, Au contacts were deposited onto the back surface of p-Si by a sputter coater and annealed at 400 C for 2 min on a hotplate in a nitrogen-filled glovebox to form a large area ohmic contact. The IeV and current-time (I-t) curves were measured using a Keithley Model-4200-SCS semiconductor characterization system. The device photoresponse was measured under AM 1.5G condition with an illumination intensity of 100 mW/cm2 using a solar simulator. The photoresponse was measured by recording the current versus time while sunlight illumination was turned on and off by a shutter. To rectify the measurement divergence, the light intensity was calibrated using a reference silicon solar cell certificated by the National Renewable Energy Laboratory. The work function of the AlCdO films was examined with the SKP5050 Scanning Kelvin probe (KP Technology). The scan area is 20 30 mm2. The Kelvin probe is a non-contact, non-destructive vibrating capacitor device used to measure the work function of conducting materials KP Technology Systems offer very high work-function resolution of 1e3 meV. The theory and operation of the vibrating KP method for measuring work function have been well documented [26,27]. Xray photoelectron spectroscopy (XPS) was employed to examine the chemical bonding states at the SiNW surfaces with and without
Fig. 2. J-V curves of (a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2-treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes in the dark.
H2O2 treatment. XPS measurements (ULVAC-PHI, PHI 5000) were performed using a monochromatic Al Ka x-ray source. The binding energy scale was calibrated using the position of the Au 4f peak, measured on a clean gold foil in electrical contact with the substrate.
3. Results and discussions Fig. 2 shows the current densityevoltage (JeV) characteristics of the AlCdO/SiNWs/p-Si and AlCdO/H2O2-treated SiNWs/p-Si Schottky diodes in the dark, respectively. The rectifying JeV characteristics suggest that Schottky junctions are formed at the AlCdO/ SiNW interfaces. The ratio of the forward to reverse current at a bias voltage of ±2 V for the AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diode was calculated to be 714 (4184 and 21235), implying a good rectification behavior for AlCdO/20 min-H2O2treated SiNWs/p-Si Schottky diodes. With consideration that our measurements were made at room temperature and the SiNWs had a low doping, the dominant transport property at the barrier is thermionic emission and diffusion, and the contribution made by tunneling is negligible [28]. However, the nonlinear ln (I)eV0.25 curve is found, making the classic thermionic emission-diffusion theory inapplicable in our case. In a Schottky contact, the forward bias IeV relation obtained by using the TE theory is given by [2e4]
Fig. 1. (a) Plane and (b) cross-sectional SEM images of the SiNWs and (c) a cross-sectional SEM image of the H2O2-treated SiNWs.
Y.-J. Lin et al. / Current Applied Physics 15 (2015) 213e218
q4 V IRs J ¼ I=S ¼ A*T 2 exp B exp 1 1 kT q hkT
(1)
where q is the electron charge, T is the measurement temperature, k is the Boltzmann constant, S is the Schottky contact area, Rs is the series resistance and A* is the effective Richardson constant (32 A cm2 K2 for p-Si [29]). The diode properties of the nanoscale devices have been analyzed using the TE equation [30,31]. h was determined from the slope of the linear regions of the forward JeV plots. From the curve fitting of the AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diode, h of 2.5 (2.2 and 1.7) was derived. Deviation of h from unity may be attributed to a combined effect of the recombination of electrons and holes in the depletion region and the presence of a large number of the interfacial defects [32]. h > 2 is found for AlCdO/SiNWs/p-Si devices, suggesting that interface states may play important roles in the conduction process. However, H2O2 treatment may lead to the reduction of h and the JeV characteristic of the AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diode is limited by the forward-bias current behavior of the conventional Schottky junction. qfB was also obtained from the forward JeV characteristics, according to Equation (1). For AlCdO/ SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/ 20 min-H2O2-treated SiNWs/p-Si) Schottky diode, qfB of 0.76 (0.82 and 0.93) eV was derived. For estimating the accurate Schottky contact area, the filling ratio, the cross section and the length of SiNWs should be considered. Due to the absence of reliable values for cross sectional area of the AlCdO/SiNWs junctions, the extracted barrier height can only serve as an approximation. However, Ruan and Lin [4] found that qfB is slightly sensitive to the contribution of the contact-area variation. It is shown that the barrier heights produced on SiNW, by exploiting this effect of 20 min-H2O2 treatment, can achieve a value of 0.93 eV which is 0.17 eV higher than the derived value for devices without SiNW surface passivation. In addition, the reverse-bias leakage current is affected by H2O2 treatment and decreases with increasing the H2O2 treatment time. A probable reason for the large leakage current is the existence of the high interfacial state density. We suggest that H2O2 treatment may lead to the dramatically reduced number of charge traps in the SiNWs, thus, increasing qfB and reducing the reverse-bias leakage current. Trap states in the SiNWs near the AlCdO/SiNW interfaces were considered to affect interfacial barriers with contacts and consequently electrical leakage [33]. This is because of the energy barrier lowering caused by trapped charge carriers jumping between the continuous potential well. The current will flow preferentially through the lower barriers in the potential distribution (that is, electrons hopping from one trap state to another trap state). The details of this mechanism are discussed later. Fig. 3 shows the images of the work-function difference (DW) between the probe and AlCdO (Au). The Au and AlCdO samples were placed together. The AlCdO sample is on the right-hand side and Au is on the left-hand side. Relative methods make use of the contact potential difference between the probe and a reference material (that is, Au) or between the probe and AlCdO. Au is used as a reference material for obtaining the work function of AlCdO (Wa), owing to its high chemical stability. It is found that the average work function of Au is higher than that of the probe (DW ¼ 295 ± 10 meV) and the average work function of AlCdO is lower than that of the probe (DW ¼ 585 ± 10 meV). It is known that the value of the work function of Au is 5.10 eV [34]. Thus, Wa is calculated to be 4.22 ± 0.02 eV. For AlCdO/SiNWs/p-Si Schottky diodes, the ideal Schottky barrier height (qfI) is given by [35] qfI ¼ (c þ Eg) Wa ¼ 5.17 Wa
215
(2)
Fig. 3. Two-dimensional image of the work-function difference between the probe and AlCdO (Au).
where Eg is the energy band gap of Si (Eg ¼ 1.12 eV) and c is the electron affinity of Si (c ¼ 4.05 eV [36]). According to Equation (2), the Schottky limit could be calculated to be 0.95 eV. For AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes, qfB evaluated from J-V measurements is close to the Schottky limit, indicating a suitable treatment time may lead to a better Schottky contact with the SiNWs. To confirm the dominance of charge trapping, time domain measurement was performed on the AlCdO/SiNWs/p-Si (AlCdO/ H2O2-treated SiNWs/p-Si) Schottky diode. The photocurrent (IP) values were measured for V varying from 0 to 1 mV (DV ¼ 1 mV) for 300 s (Illumination was turned on at t ¼ 0 s). Fig. 4 shows the normalized photocurrent decay for AlCdO/SiNWs/p-Si (AlCdO/ H2O2-treated SiNWs/p-Si) Schottky diodes and the fitting curve. jIP0 j is the normalized jIPj. jIP0 j should rapidly decay for AlCdO/ SiNWs/p-Si Schottky diodes without H2O2 treatment, since the electron traps in the SiNWs influence the electronic conduction through the device. jIP0 j should slowly decay for AlCdO/10 minH2O2-treated SiNWs/p-Si Schottky diodes, since H2O2 treatment leads to the reduced number of electron traps in the SiNWs. However, jIP0 j should rapidly grow for AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes, implying that the remarkably reduced
Fig. 4. jIP0 j-t characteristics of (a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2-treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes and the fitting curves.
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number of electron traps may lead to the dominance of hole traps. This is a strong evidence of the induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the H2O2 treatment time. In this study, we consider two possible physical pictures to explain the observed jIP0 j decay [Fig. 4(a) and (b)]: It is mainly dominated by the long-lifetime or short-lifetime electron traps for AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si) Schottky diodes. To estimate the time constants for jIP0 j decay, we fit the data to exponential decay functions. The transient was fitted to second-order exponential decay jIP0 j ¼ A1e(t/t1) þ A2e(t/t2) using the nonlinear least-squares method [37e41], with four fitting parameters. This equation reflects two different electron trapping mechanisms with time constants t1 and t2 (t1 > t2). The values of A1 and A2, where A1 þ A2 ¼ 1, represent weighing factors that quantify the contribution of each mechanism to the decay process. The first (second) term, which can be attributed to long-lifetime (short-lifetime) electron trapping, dominates the decay process. The fitting parameters are listed in Table 1. For AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment, the data show that jIP0 j rapidly decayed, owing to the dominance of short-lifetime electron trapping (A1 < A2). However, for AlCdO/10 min-H2O2-treated SiNWs/p-Si Schottky diodes, the data show that jIP0 j slowly decayed, owing to the dominance of long-lifetime electron trapping (A1 ¼ 1). For long-lifetime electron trapping, t1 stayed the same at 90 s, which implies that the underlying relaxation mechanism is the same with or without H2O2 treatment. Incident photon illumination can create electronehole pairs in the space charge region at the AlCdO/SiNW interfaces that will be swept out producing the photocurrent. Holes originating from the photogenerated electronehole pairs were swept out of the space charge region by the built-in electric field into the SiNWs and electrons originating from the photogenerated electronehole pairs were swept out of the space charge region by the built-in electric field into the AlCdO layer. The detrapped electrons and holes may recombine in pairs in the SiNWs, resulting in a decaying hole population. The photoresponse result confirms that the photocurrent decay is due to the dominance of electron trapping with short-second (long-second) lifetime in the SiNWs for AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2treated SiNWs/p-Si) Schottky diodes [40,42]. As compared to AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment, the number of electron traps in the SiNWs is significantly decreased for AlCdO/10 min-H2O2-treated SiNWs/p-Si Schottky diodes. Consequently, the 10 min-H2O2 treatment may lead to the removal of the short-lifetime electron traps, causing the slow jIP0 j decay. On the other hand, we consider one possible physical picture to explain the observed jIP0 j growth [Fig. 4(c)]: It is mainly dominated by hole traps in the SiNWs for AlCdO/20 min-H2O2-treated SiNWs/ p-Si Schottky diodes. The growing jIP0 j is described by jIP0 j ¼ {1 exp [(t/t3)d]} [41,43,44]. The parameter d, which reflects the relaxation mechanism, is an exponent between 0 and 1. The time constant is represented by t3. The fitting parameters are listed in Table 1. The small t3 indicates the AlCdO/20 min-H2O2treated SiNWs/p-Si Schottky diode having the low hole-trap density and good stability. The 20 min-H2O2 treatment may lead to the
dramatically reduced number of electron traps in the SiNWs, exhibiting the dominance of hole trapping. The number of holes reaching the electrode in the SiNW array and the number of electrons reaching the electrode in the AlCdO layer should determine the measured photocurrent under a near-constant bias voltage (DV ¼ 1 mV). Thus, the growing jIP0 j may be explained in terms of the effect of hole trapping in achieving the steady state current after illumination. We consider that conducting holes can be captured at trap centers in the SiNWs. As a result, the photocurrent onset is slowed because steady state current will be reached only after all the traps are filled. Under this condition, an equilibrium between multiple hole trapping and detrapping events is observed. We suggest that the electron-trapping effect competes with the hole-trapping effect for AlCdO/H2O2-treated SiNWs/p-Si Schottky diodes and the remarkably reduced number of electron traps may lead to the dominance of hole traps. Fig. 5 shows the Si 2p core-level spectra at the SiNW surfaces with and without H2O2 treatment, respectively. The peak positioned at ~99.4 eV is attributed to SieSi bonds and the peak positioned at ~103 eV is attributed to SieO bonds [2,45]. It is found the SiNW/p-Si sample with no SieO peak and the H2O2-treated SiNW/ p-Si sample with strong SieO peak. An explanation to the origin of AlCdOeSiNW interface state is provided by the dangling bonds at the SiNW surfaces. The presence of SieO bonds might have an effect on the Schottky barrier by reducing the interface state density, indicating that a good passivation is formed at the interface as a result of the reduction of the interface state density and an appropriate interfacial SiOx layer plays an important role in the conduction process of a Schottky diode. He et al. [45] found that an ultra thin SiOx layer was grown on the p-Si surface by immersing the wafer in a H2O2 solution. Zhang et al. [16] suggested that the Si surface termination state plays a key role on the electrical output of the Si-based devices. Note, thin SiOx layer grown by H2O2 treatment could be utilized to suppress the recombination velocity [46]. The improved device performance is attributed to SiNW surface passivation. The presence of a large number of trap and defect levels at the AlCdO/SiNW interfaces is a major impediment to the realization of junction devices. These defects will strongly influence
Table 1 Fitting parameters and results [(a) AlCdO/SiNWs/p-Si, (b) AlCdO/10 min-H2O2treated SiNWs/p-Si, and (c) AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes].
(a) (b) (c)
t1 (s)
A1
t2(s)
A2
t3(s)
d
90 90 x
0.15 1 x
23 x x
0.85 x x
x x 4
x x 0.5
Fig. 5. Si 2p core-level spectra at the (a) as-cleaned, (b) 10 min-H2O2-treated and (c) 20 min-H2O2-treated SiNW surfaces.
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carrier dynamics, and are deleterious for device application. In this report, we focused on the effect of the dangling bonds at the SiNW surface and suggested that it can be changed by the SieO bonding. We believe that the increased Schottky barrier height has a strong relation to the remarkably reduced number of electron traps in the SiNWs; and, such result can be attributed to the SieO bonding. The existence of trap states at the AlCdO/SiNW interfaces may lead to the energy barrier lowering, owing to trapped electrons jumping between the continuous potential well. The current will flow preferentially through the lower barriers in the potential distribution. These can explain why the AlCdO/SiNWs/p-Si Schottky diodes without H2O2 treatment exhibit larger h, lower qfB and higher leakage current. In addition, Rs was obtained from the forward JeV characteristics, according to Equation (1). For AlCdO/SiNWs/p-Si (AlCdO/10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 minH2O2-treated SiNWs/p-Si) Schottky diodes, the value of Rs was found as about 200 (270 and 590) U. It is found that H2O2 treatment may lead to an increase in Rs, owing to the contribution of a thin SiOx layer. The ratio of the SieO to SieSi peak intensities was calculated to be 0.00 (0.20 and 0.26) for AlCdO/SiNWs/p-Si (AlCdO/ 10 min-H2O2-treated SiNWs/p-Si and AlCdO/20 min-H2O2-treated SiNWs/p-Si) Schottky diodes, suggesting that the thickness of SiOx layer increases with increasing the time of H2O2 treatment. However, the resistance of SiOx (70 or 390 U) is much lower than the depletion resistance under low forward bias (that is, > 104 U at 0.2 V). We suggested that the influence of a SiOx layer on junction conductance at low forward voltages is negligible and the increased qfB cannot be attributed to the contribution of a SiOx interlayer. On the other hand, Kirichenko et al. [47] found that the SiO2/Si interface may serve as a limited sink for Si vacancies. Single vacancy and vacancy cluster defects are substantially more stable at SiO2/Si interface compared to the bulk Si layers away from interface, mainly due to termination of dangling bonds with bridging O atoms and reduction of interface strain [47]. In addition, Kim et al. [48] investigated the oxidative behavior of SiNWs. In this report, we focused on the effect of Si vacancies at the SiOx/SiNW interface and suggested that it can be changed by the SieO bonding. This may explain why the dominance transformation from electron traps to hole traps is induced by controlling the H2O2 treatment time.
4. Conclusions In summary, we have discussed the electrical and optoelectronic properties of AlCdO Schottky contact on SiNWs with and without H2O2 treatment. For AlCdO/SiNWs/p-Si Schottky diodes without SiNW surface passivation, the JeV result shows a deviation from that expected from TE for the current mechanism. Compared to the AlCdO/SiNWs/p-Si Schottky diodes without SiNW surface passivation, the AlCdO/20 min-H2O2-treated SiNWs/p-Si Schottky diodes exhibit lower h (h ¼ 1.7), higher qfB (qfB ¼ 0.93 eV) and lower leakage current. Both h and qfB are affected by H2O2 treatment, implying that charge traps in the SiNWs have a noticeable effect on the electronic conduction through the AlCdO/SiNWs/p-Si Schottky diodes. It is shown the induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the H2O2 treatment time. The photoresponse, J-V and XPS results demonstrated that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiOx/SiNW interface which can be changed by the SieO bonding on the energy barrier lowering and the charge transport property. The current will flow preferentially through the low barriers caused by trapped charge carriers jumping between the continuous potential well. This helps explain why the AlCdO/ SiNWs/p-Si Schottky diodes without SiNW surface passivation exhibit larger h, lower qfB and higher leakage current.
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