ELECTRICAL PROPERTIES OF NANOROD-BASED ZnO/SiC HYBRID HETEROJUNCTIONS

ZnO nanorods have attracted increasing interest in recent years due to their potential in optoelectronic applications. The lack of p-type ZnO emphasizes the importance of rectifying junctions realized on other ptype materials. SiC is a good candidate to create hybrid heterojunctions with ZnO due to its wurtzite crystal structure and a small lattice and thermal mismatch. The ZnO/SiC heterojunctions have a potential to show intense UV electroluminescence. We investigate morphology and electrical properties of a single verticallyoriented ZnO nanorod on a SiC substrate. The current-voltage measurements are performed directly in the vacuum chamber of a scanning electron microscope. The contact to a single nanorod is obtained by a nanoprobe, which allows for the measurement of the current-voltage characteristic of a single nanorod heterojunction of choice. The influence of ZnO growth parameters and post-growth treatment of ZnO/SiC structures are studied with the aim to minimize the density of structural/interfacial defects and to create lowdimensional hybrid heterojunctions with the potential to show intense UV electroluminescence.


INTRODUCTION
Zinc oxide (ZnO) with its wide band gap of 3.37 eV and high exciton binding energy of 60 meV is a promising material for electronic and photonic applications. ZnO offers several advantages over other wide-bandgap materials: a low cost and wide availability, environmental safeness, high thermal stability, and possibility to form different kinds of nanostructures by a variety of growth techniques. As-grown ZnO is intrinsically n-type semiconductor and reproducible p-type doping is difficult to be achieved. Therefore, the advantageous properties of ZnO for light-emitting devices are frequently investigated in the form of heterostructures with other materials [1,2].
Silicon carbide (SiC) is another promising wide bandgap semiconductor material with a high thermal conductivity, high breakdown voltage, low response time, excellent mechanical properties, and good chemical stability. SiC is a good candidate to create hybrid heterojunctions with ZnO due to its wurtzite crystal structure and small lattice (~5 %) and thermal (<1 %) mismatch [3,4].
Chen et al. reported ZnO/SiC heterojunctions that were deposited by metal-organic chemical vapor deposition on n-6H-SiC [5]. The isotype heterojunctions showed rectifying characteristics with the reverse current in the picoampere range, the on/off ratio ~10 7 , and the ideality factor ~1.23. The native SiO2 layer on SiC surface was reported to have impact on the morphology of the deposited ZnO and consequently on the electrical properties of ZnO/SiC heterojunctions. Felix et al. reported that the heterojunctions formed by thermally deposited ZnO thin films on n-4H-SiC had excellent electrical properties with the leakage current lower than 1 nA and the rectification ratio ≈10 5 . The larger than unity ideality factor was ascribed to the thickness and composition variation of interfacial layer [6]. Taube  isotype heterojunction diodes showed better electrical properties than the anisotype diodes. In particular, lower values of ideality factor, series resistance, capacitance and built-in voltage were reported. The lower resistance and capacitance are especially important for the increase of the response time of UV photodetectors. Due to the absence of donor-acceptor pair transitions in n-type 4H-SiC, the isotype ZnO/4H-SiC heterojunctions showed lower response to visible light while keeping UV light response at the same level as observed for anisotype heterojunctions [7]. Rebaoui et al. reported on the effects of polytypes and doping nature on electrical properties of ZnO/SiC heterojunctions. The ZnO/SiC diodes based on n-type 4H-SiC polytype showed the best electrical performance. It was found, that the ZnO/n-4H-SiC heterojunctions had 40 times lower leakage current than the ZnO/p-4H-SiC heterojunctions and two times higher rectifying ratio than the ZnO/n-6H-SiC heterojunctions. The excellent performance of ZnO/n-4H-SiC heterojunctions was ascribed to high vertical a-axis electron mobility, higher heterojunction barrier, and suppressed trap assisted tunneling [8].
The excellent rectifying properties of isotype ZnO/SiC heterojunctions considered above were explained in detail by Zhang et al. using x-ray photoelectron and Auger electron spectroscopies of ZnO layers deposited on 6H-SiC substrates by molecular beam epitaxy from which the band alignment for the isotype ZnO/SiC interface was calculated. Significant band offsets were suggested to be the reason for high rectification ratios of isotype ZnO/SiC structures. Together with the significant band offsets, the absence of chemical interactions and additional interfacial layers on the abrupt MBE-ZnO/SiC interface was considered as the reason for highly rectifying isotype ZnO/SiC structures [9]. On the other hand, Mu et al. reported the presence of the zinc silicate species at the ZnO/SiC interface prepared by the radiofrequency magnetron sputtering. He suggested, that the interface species significantly affect the electronic structure of the ZnO/SiC interface [10].
To summarize, the 4H polytype of SiC is more attractive for the device application due to its wider bandgap and higher mobility than other polytypes. Moreover, it is widely believed that the isotype n-ZnO/n-SiC heterojunctions are more suitable for optoelectronic devices than the anisotype n-ZnO/p-SiC heterojunctions.
As was mentioned above, ZnO easily forms several types of nanostructures that are suitable for the manufacturing of vertically stacked devices (e.g. ZnO nanorods/SiC LEDs). However, further downscaling to nanoelectronic devices requires a deep understanding of electrical properties of nanoscale heterojunctions below a single nanorod. In this work, electrical properties of a single ZnO nanorod on n-4H-SiC substrates are investigated by measuring the current-voltage (I-V) characteristics. The contact to a single nanorod is obtained by a nanoprobe in the chamber of a scanning electron microscope (SEM), which allows for the measurement of the current-voltage characteristic of a single nanorod heterojunction of choice.

EXPERIMENTAL METHODS
Conventional chemical bath deposition (CBD) method was used to synthesize ZnO nanorods on n-4H-SiC substrates [11]. Commercially available nitrogen doped 4H-SiC substrates with a thickness of 350 mm and an electron concentration of n=10 17 cm -3 were used for the fabrication of the ZnO/SiC junctions. The substrate was sequentially cleaned in H2SO4:H2O2, NH4OH:H2O2:H2O (1:1:5) at 70°C, HF:H2O (1:20) and H2O:HCl:H2O2 (6:1:1) at 70 °C. After each step, the substrate was rinsed in deionized water and dried with nitrogen. A backside ohmic contact was created by thermal evaporation of 150 nm of Ni and subsequently annealed at 950 °C for 10 min in Ar atmosphere [12].
The 5mM aqueous solution of zinc nitrate hexahydrate, which is a source of Zn 2+ ions, and hexamethylenetetramine, which is a source of OHions, was used for the CBD of ZnO nanorods. To activate the reaction, the temperature of the solution was elevated to 95 °C for 2 hours.
The morphology of ZnO nanorods was observed using Tescan Lyra 3 GM FIB/SEM system. The I-V characteristics of the nanoscale heterojunction formed by a single ZnO nanorod on SiC substrate were measured in the chamber of the same FIB/SEM system (Figure 1).     The I-V measurements between two as-deposited Ni contacts show the presence of two back-to-back Schottky diodes. However, after the thermal annealing, a clear ohmic behavior of Ni/SiC contact was observed. The top W/ZnO contact also demonstrates almost linear behavior. Therefore, we can suggest that the diodelike I-V characteristics of n-ZnO nanorod/n-4H-SiC heterostructures are determined by the charge transport through the ZnO/SiC interface rather than by the metal contacts. The parameters extracted from the current-voltage measurements are summarized in Table 1.

Figure 4 Schematic energy band diagram for the n-ZnO/n-4H-SiC heterojunction. The unit is eV in all cases
To explain the rectifying behavior of n-ZnO/n-4H-SiC heterojunctions, in the first approach, the Anderson model was used to construct the n-ZnO/n-4H-SiC heterojunction band diagram (Figure 4). The electron affinities of ZnO and of 4H-SiC were taken as XZnO = 4.35 eV and XSiC=3.24 eV, and the bandgap energies of ZnO and SiC as Eg, ZnO = 3.37 eV and Eg, SiC = 3.29 eV, respectively.
The conduction band offset is therefore ΔEC = XZnO-XSiC = 1.1 eV, while the valence band offset is ΔEV = (XZnO+Eg, ZnO)-(XSiC+Eg, SiC) = 1.19 eV. Adjustment can be made to account for the position of the Fermi level relative to the conduction band in each material. The positions of Fermi level below the conduction band edge Ec-EF is taken from the Boltzmann's statistics assuming that all dopants are fully ionized: where, k is the Boltzmann constant, T is the absolute temperature, ND is the donor concentration and NC is the effective density of states in the conduction band [13].
where I0 is the reverse bias saturation current of the junction, n is the ideality factor of the junction, q is the electron charge, k is the Boltzmann's constant, T is the temperature and Rser is the serial resistance [13].
At higher biases, when the impact of potential barrier is negligible, the current is limited by the conductance of the space charge region. In general case the space charge limited current (SCLC) is given by: where A is a coefficient that is related to the length and conductivity of the current flow path and β is a scaling exponent of the power-law current-voltage dependence, which should be equal to 2 in the case of trap-free space charge limited current, or >2 in the case of traps distributed over the bandgap [14].
The leakage current that bypassed the heterojunction follows the Ohm's law: where Rsh is the shunt resistance of the leakage path.
According to the Kirchhoff's rule, the total current injected to a single nanorod n-ZnO/n-4H-SiC heterojunction is: The I-V curve modelled using Equations (2) -(5) fits well experimental I-V characteristics of the single nanorod n-ZnO/n-4H-SiC heterojunction. The fitting variables are summarized in Table 2. The ideality factor larger than unity, relatively high saturation current I0, and the power law exponent larger than two allow us to consider the trap assisted tunneling process through the interface states distributed over the band gap as the main transport mechanism for the single nanorod n-ZnO/n-4H-SiC heterojunction. The leakage current determined by the shunt resistance is on the order of 1 pA, which is the noise level of our measurement setup. Taking into account the experimental I-V characteristics measured on the reference bulk ZnO substrate (Figure 3c), a large serial resistance is ascribed to the resistance of the top W/ZnO contact and of the ZnO nanorod.

CONCLUSIONS
In summary, nanoscale heterojunctions between a single free-standing n-type ZnO nanorod grown on n-4H-SiC substrate were obtained by chemical bath deposition. Using the nanomanipulator mounted in the scanning electron microscope, the current-voltage characteristics of individual n-ZnO nanorod/n-4H-SiC heterojunctions were measured. Heterojunctions showed diode-like characteristics with a high rectification ratio and low leakage current. The equivalent circuit of the isotype n-ZnO/n-4H-SiC heterojunction was proposed. The parameters of the heterojunctions were extracted from the experimental current-voltage characteristics using a proposed fitting model. We believe that the method of characterization of a single nanorod-based heterojunction proposed in this paper opens door for a detailed analysis of charge transport in a variety of nanoscaled optoelectronic devices.