Electrical transport properties of an isolated CdS microrope composed of twisted nanowires
- Gui-Feng Yu†1, 2, 3,
- Miao Yu†1, 4,
- Wei Pan1, 5,
- Wen-Peng Han1, 2,
- Xu Yan1, 2,
- Jun-Cheng Zhang1, 2,
- Hong-Di Zhang1, 2 and
- Yun-Ze Long1, 2, 6Email author
© Yu et al.; license Springer. 2015
Received: 2 December 2014
Accepted: 5 January 2015
Published: 28 January 2015
CdS is one of the important II-VI group semiconductors. In this paper, the electrical transport behavior of an individual CdS microrope composed of twisted nanowires is studied. It is found that the current–voltage (I-V) characteristics show two distinct power law regions from 360 down to 60 K. Space-charge-limited current (SCLC) theory is used to explain these temperature- and electric-field-dependent I-V curves. The I-V data can be well fitted by this theory above 100 K, and the corresponding carrier mobility, trap energy, and trap concentration are also obtained. However, the I-V data exhibit some features of the Coulomb blockade effect below 80 K.
One-dimensional (1D) semiconductor nanostructures (nanotubes, nanorods, nanowires, nanobelts, etc.) have gained tremendous attention within the last two decades due to their unique electronic, optical, and mechanical properties. Among the huge variety of 1D nanostructures, CdE (E = S, Se, and Te) 1D nanostructures have attracted much attention for their potential applications in solar cells , biosensors , electrochemical detection , and photocathodes . In order to fulfill these potential applications, it is very essential to properly identify some physical characteristics which play important roles on electrical transport characteristics such as conductivity, I-V characteristic, and carrier mobility. Moreover, the extractions of material parameters (i.e., carrier mobility and trap energy) rely on analysis with specific models.
In the past decades, various theoretical models such as Fowler-Nordheim field-emission tunneling , Luttinger liquid theory , Wigner crystal model , variable range hopping (VRH) theory , fluctuation-induced tunneling (FIT) theory , scaling theory , space-charge-limited current (SCLC) theory , phonon-assisted tunneling theory , and Coulomb blockade effect  have been used to explain the conduction mechanism of such quasi-one-dimensional (quasi-1D) inhomogeneous structures. However, it is still an open issue how to explain the nonlinear I-V characteristics for the complexities of the structure and conduction mechanism in 1D nanofibers . For instance, the non-ohmic I-V characteristic curve of single-wall carbon nanotube network was measured at 7 K, and the non-ohmic regime could be fitted well by the FIT model, indicating the importance of inter-tubular contacts or inherent energy barriers inside the tubes . However, Kaiser et al. fitted the same curve to the calculated behavior with fluctuation-assisted tunneling and thermal activation model, giving a good account of the feature of the I-V curve . In addition, the electrical transport mechanism of nano-CdS has been discussed by various theories [17-19]. For example, the single CdS nanowire synthesized by aqueous chemical growth showed a high electrical conductivity of 0.82 S cm−1 at room temperature and a small band gap of 0.055 eV, and the resistance of the nanowire increased exponentially with decreasing temperature, namely, the temperature dependence of resistance followed the typical thermal activation model . The rectifying characteristics of Cu/CdS/SnO2/In-Ga structure were investigated in the temperature range of 130 to 325 K, indicating that the mechanism of charge transport was performed by tunneling among interface states/traps or dislocations intersecting the space charge region . Analysis of the voltage and temperature dependencies of the SCLC theory in n-type CdS nanowire showed that the nanowire surface traps were exponentially distributed in energy with a characteristic depth about 0.28 ± 0.04 eV, showing that the surface traps were an essential ingredient for proper understanding of SCLC in nanowires .
There are also many reasons that the SCLC theory could be used in CdS microrope; the following are two of them. One is that for semiconductor nanowires that are intrinsic or depleted of charge carriers, one would expect to observe SCLC when the nanowire resistance greatly exceeds the contact resistance . In the present article, the room-temperature conductivity of the measured nanowire is about 2.9 × 10−4 S cm−1, and the resistance of the CdS microrope is about 320 MΩ at room temperature, while the resistance of the Pt microlead is 2 kΩ using the widely recognized conductivity of 2 × 103 S cm−1 for the focused ion beam (FIB) deposited Pt film. Accordingly, for the two-probe method, the contact resistance and the microleads' resistance can be ignored by contrasting to the nanowire's resistance [20,21]. The other is that the total number of surface traps can dominate over the total number of traps when the dimension reduces to the nanoscale. Moreover, the adsorbates on the nanowire surface can capture the free carriers and modify the electrostatic profile inside the nanowire; accordingly, the charges trapped at the nanowire surface can greatly influence the SCLC .
In this paper, the I-V behavior of an isolated CdS microrope composed of twisted nanowires has been measured from 360 down to 60 K. The electronic conduction mechanism is attempted to be discussed based on the SCLC theory. It is proposed that the conduction mechanism could be attributed to the SCLC theory from 360 to 100 K. Nevertheless, the current is near zero below 80 K and around zero bias, which could be attributed to Coulomb blockade transport , because electron–electron interaction should also be taken into account especially at low temperatures in quasi-1D systems where electron states are more localized due to confinement effect or disorder .
The CdS microropes composed of twisted nanowires were prepared by a simple aqueous chemical growth route . At first, 0.032 g Cd(CH3COO)2 · 2H2O was dissolved into 120 ml 35 mol% aqueous solution of ethylenediamine at room temperature. Then, a stoichiometric amount of Na2S · 9H2O was added to the solution under vigorous stirring. After kept out of light and heated to 50°C with moderate stirring until the milk-white mixture gradually turned to a little yellow about 2 days later, it was continuously heated at 60°C with stirring for a long term up to about 6 days. The final product was obtained by centrifugation and washed with distilled water and ethanol for several times. At last, the twisted CdS microropes composed of nanowires with diameters of 6 to 10 nm were prepared.
The obtained samples were characterized by scanning electron microscopy (SEM; JEOL JSM-6700 F, JEOL Ltd., Akishima, Tokyo, Japan) and transmission electron microscopy (TEM; JEOL JEM-2100 F). A pair of platinum micro-leads on isolated CdS microrope was fabricated by FIB (FEI Company, Hillsboro, OR, USA) deposition. The I-V characteristics of a section of the microrope between two micro-leads were measured by a Physical Property Measurement System (PPMS, Quantum Design, San Diego, CA, USA) by applying bias voltage from −6.0 to 6.0 V with a step of 0.05 V.
Results and discussion
I-V characteristic curves
The SCLC theory was first discussed by Mott and Gurney  in 1940 for a trap-free insulator. It is based on the barrier at the metal electrode-nanowire interfaces, using in the condition that the number of injected charge is higher than the number of neutralized thermal free carriers recently [26-28]. Now, it has been used in many systems. For example, a transition from linear I-V behavior at a low bias to a SCLC behavior at a large bias has been found by Xu et al. in unintentionally doped GaSb nanowires, showing that the trap energy distribution in the nanowires has been reduced after thermal annealing . Kirchartz et al. discussed the influence of charged defects on the information derived from fitting space-charge-limited current models to the experimental data . Simpkins et al. exploited this theory to extract size-dependent carrier densities and demonstrated surface-dominated behavior for individual heterostructure AlGaN/GaN nanowires . Cheon et al. studied diketopyrrolopyrrole-based polymers (PDPPDTSE) using the SCLC theory and time-off-light (TOL) methods; the mobility of the hole-only device based on PDPPDTSE was found to be dependent upon the electric field over the range of 10−3 to 10−2 cm2 V−1 s−1 .
Calculated charge carrier density n 0 of the CdS microrope at different temperatures
n 0 (1018 cm−3)
The values of the exponent M can be obtained from fitting, as shown in Figure 2b. Here, M 1 represents the value of M at lower voltages, indicating Ohm's characteristic of the I-V curves, and M 2 represents the value of M at higher voltages, indicating the SCLC characteristic of the I-V curves. The transition from M 1 to M 2 means the transition from Ohm's regime to SCLC regime, which depends markedly on the distribution of the trapping levels in energy, because the appearance of SCLC is inhibited until a sufficiently large electric field is applied . M 1 suggests that the concentration of the thermally generated free carriers is superior to the concentration of injected carriers. M 1 is about 1.05 at 360 K, and it increases gradually with decreasing temperature. To our surprise, M 1 decreases sharply from 1.7 to 0.5 ~ 0.6 when the temperature is below 100 K. This obvious deviation from the SCLC theory may be attributed to another conduction mechanism, and it will be discussed in the following context. M 2 implies that the concentration of injected carriers is overwhelmingly large to overcome the influence of thermal free carriers. It is evident that M 2 increases with decreasing temperature possibly due to the enhanced thermal emission of trapped charges into the conduction band , revealing the reduction of deeper level traps inside the CdS microrope. For example, M 2 is 2 when the temperature is 200 K, implying the material is in trap-free state according to the SCLC theory. For the temperature lowering from 180 to 100 K, the depth of the trap state increases gradually.
The voltage and temperature dependencies of SCLC are extremely sensitive to the presence of defects, which can be used to characterize the density and energy distribution of the defect states . As we know, trap plays an important role in understanding the I-V characteristics in solid-state physics; in addition, the corresponding characteristic energy could be extracted from a linear fit to the temperature dependence of M 2. From the explanation by Rose , I ∞ V Tc/T + 1, where T c is the characteristic temperature relating to the trap energy distribution; furthermore, the relation T c/T + 1 = M 2 can be obtained from the SCLC theory. The trap energy E which is measured from the bottom of the conduction band can be E = k B T c, where k B is the Boltzmann constant. The E is 11.7 meV at 100 K, which increases from 11.7 to 17.5 meV (200 K). For comparison, the trap energy dropped from 0.26 eV before annealing to 0.12 eV after annealing. It was consistent with the explanation that the annealing process reduced the deep level traps inside the individual GaSb nanowire . In addition, Simpkins et al. obtained the trap energy of coaxial AlGaN/GaN nanowires which was 75 meV .
The current density is given by the Mott-Gurney Law without any trapping effects . In this theory, the current is assumed to be due to carriers of one sign only, the effect of diffusion is neglected, and the mobility is assumed to be independent of the field . The current density J is defined as J = ε 0 ε r μV 2/8d 3, where μ is the free charge carrier mobility, d the distance between the two Pt microleads, ε 0 the vacuum permittivity, and ε r the dielectric constant which is about 5 . The carrier mobility is the most important parameter in understanding the transport in some electric devices. For the reason of trap free, only the I-V characteristic of 200 K is chosen here (the parameter M 2 = 2); thus, the carrier mobility is 87.73 cm2 V−1 s−1, which is in the range of the typical mobility of inorganic semiconductors 10−5 ~ 103 cm2 V−1 s−1. For comparison, the mobility of CdTe thin films was 2.539 × 10−8 cm2 V−1 s−1 at 303 K . The data of CdSe/ZnS quantum dot composites reported by Hikmet et al. was 1.0 × 10−6 cm2 V−1 s−1 at about 423 K , and the mobility of Ge2Sb2Te5 layers made by Lebedev et al. was about 10−3 cm2 V−1 s−1 at room temperature .
Further discussion: I-V curves below 80 K
In summary, the electrical transport properties of an individual CdS microrope composed of twisted nanowires are studied in the temperature range from 360 down to 60 K. The results show that the I-V curves can be well fitted by the SCLC theory at higher temperatures. For example, the conduction mechanism is dominated by trap-free space-charge-limited current at 200 K. Trap concentration and carrier mobility are calculated to be 4.54 × 1015 cm−3 and 87.73 cm2 V−1 s−1 separately. However, the conduction mechanism may be attributed to the Coulomb blockade effect when temperature is below 80 K. It shows that the electron–electron interaction should be taken into account especially at low temperatures in inhomogeneous quasi-1D systems.
This work was supported by the National Natural Science Foundation of China (51373082 and 11404181), the Natural Science Foundation of Shandong Province for Distinguished Young Scholars (JQ201103), the Taishan Scholars Program of Shandong Province, China (ts20120528) and the National Key Basic Research Development Program of China (973 special preliminary study plan, 2012CB722705), the Natural Science Foundation of Shandong Province (ZR2013EMQ003), and the Program of Science and Technology in Qingdao City (13-1-4-195-jch).
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