Date Published: January 23, 2014
Publisher: Public Library of Science
Author(s): Kim Guan Saw, Sau Siong Tneh, Gaik Leng Tan, Fong Kwong Yam, Sha Shiong Ng, Zainuriah Hassan, Sefer Bora Lisesivdin.
The current-voltage characteristics of Ni contacts with the surfaces of ZnO thin films as well as single crystal (0001) ZnO substrate are investigated. The ZnO thin film shows a conversion from Ohmic to rectifying behavior when annealed at 800°C. Similar findings are also found on the Zn-polar surface of (0001) ZnO. The O-polar surface, however, only shows Ohmic behavior before and after annealing. The rectifying behavior observed on the Zn-polar and ZnO thin film surfaces is associated with the formation of nickel zinc oxide (Ni1-xZnxO, where x = 0.1, 0.2). The current-voltage characteristics suggest that a p-n junction is formed by Ni1-xZnxO (which is believed to be p-type) and ZnO (which is intrinsically n-type). The rectifying behavior for the ZnO thin film as a result of annealing suggests that its surface is Zn-terminated. Current-voltage measurements could possibly be used to determine the surface polarity of ZnO thin films.
In recent years, development in optoelectronic devices using ZnO has attracted great interest as this semiconducting material has a wide bandgap (∼3.4 eV), a high exciton binding energy (∼60 meV) and a relatively non-toxic nature . ZnO normally crystallizes into the wurtzite structure but the ratio of its lattice parameters c:a is 1.602, which is lower than that for perfectly hexagonally close-packed atoms (1.633). The smaller ratio of the lattice parameters and the ionic nature of the Zn-O bond as well as the lack of inversion symmetry result in a net dipole moment along the c-axis of the unit cell. While the dipole moments cancel each other in the bulk they cause equal and opposite bound polarization charges on the Zn-polar and O-polar surfaces. The surfaces of single crystal (0001) ZnO can exist as Zn- or O-terminated. A previous study on the surface of ZnO thin film identified it as O-terminated . Sasaki et al., however, reported that the polarity of ZnO thin films depends on the growth temperatures. The electrical behavior of metal contacts on the different surfaces of ZnO may exhibit Ohmic, near-linear or rectifying behavior. For instance, Pt contact on ZnO thin film exhibits a rectifying behavior while Al turns out to be Ohmic in nature . Intrinsic ZnO typically exhibits n-type conductivity which is attributed to donor defects such as interstitial Zn, oxygen vacancy or hydrogen –. Thus undoped ZnO in the form of single crystals or thin films is typically n-type. In many cases the successful application of ZnO-based devices would depend on the understanding and control of the current-voltage (I–V) characteristics of the metal films that act as electrical contacts and the related surface physics and chemistry involved during the contact formation. The Schottky model suggests that when a metal is in contact with a semiconductor, the Ohmic or rectifying character of the contact is dependent on the work function of the metal and the electron affinity of the semiconductor. A metal-semiconductor junction is Ohmic if the barrier formed by the contact is zero. In n-type semiconductors, an Ohmic junction happens when the work function of the metal is close to or smaller than the electron affinity of the semiconductor. It is important that in the fabrication of semiconductor devices, care is taken to ensure that Ohmic contacts are not inadvertently created where a rectifying connection is desired and vice versa. A recent study showed a conversion from Ohmic to rectifying behavior can occur as a result of remote room temperature O2/He plasma treatment . Recent studies have largely focused on multilayer or bilayer metallization schemes involving pre- and post- deposition treatments which resulted in different I–V behaviors –. There were also some studies on single metals such as Cu, Ag, Au, Pd and Pt on either the thin film surface or on the polar surfaces of single crystal ZnO –. An important transition metal that is widely used as contact material is Ni. However, investigation into the use of this transition metal remains inconclusive. Ni/Au bilayer contacts on ZnO have yielded different results upon annealing. Motayed et al.  reported that as-deposited Ni/Au contacts and those annealed below 800°C demonstrated a near-linear behavior. A conversion to linear behavior was observed for 800°C while a higher annealing temperature started to degrade electrical properties as well as the surface morphology of the contacts. Ryu et al. , however, noted that the Ni/Au contacts remained linear before and after annealing at high temperatures although a lower contact resistance is associated with annealing at 600°C. It should be noted that the possibility of Ni contacts being oxidized to NiO (stoichoimetric or non-stoichoimetric oxides) could be partly the reason for the different I–V behaviors. Initially regarded as a Mott insulator, NiO has also been reported to be semiconducting in certain conditions –. For instance, a p-NiO/n-ZnO diode was fabricated recently by Ohta et al. . Charge transport in NiO that shows p-type conductivity is attributed to thermally-activated hopping. NiO has a direct band gap of 3.7–4.0 eV with a cubic lattice.
I–V characteristics were investigated by evaporating metallic Ni on sputtered ZnO thin films supported by α-(0001) sapphire. A mask was used to create areas or regions of metallic Ni on ZnO. The thickness of the ZnO film is ∼200 nm. A co-planar configuration was used in this work. I–V measurements were taken before and after the samples were annealed in a controlled furnace at 800°C for 2 h in a nitrogen atmosphere. The flow rate of the pure nitrogen gas was 4 L/min. All I–V measurements were taken at room temperature. The x-ray diffraction (XRD) 2θ measurements were taken with the X’Pert PRO system. The XRD data were collected using the Cu Kα radiation with a 0.02o 2θ step size, and a 3 s count time. The annealing temperature was chosen after considering the XRD data which indicated that as-deposited ZnO thin films annealed at 800°C experienced a decrease of ∼1.4% in the c lattice constant to values comparable to the strain-free lattice constant (∼0.5205 nm). The higher c lattice constants of the as-deposited films indicated that the ZnO unit cell was elongated along the direction of growth and implied that the films were compressively strained. The annealing temperature for ZnO thin films was thus maintained at 800°C. The annealing temperature of 800°C is also chosen for the following reasons:
Fig. 1(a) shows the Ohmic behavior of evaporated Ni contacts on the ZnO thin film before annealing while Fig. 1(b) shows the forward and reverse I–V characteristics after annealing. The annealing process results in the conversion of the Ohmic behavior to a rectifying feature. The threshold voltage, VTH, is 3.50 V while the corresponding current is 1.71×10−7A. The sputtered ZnO thin film shows a preferential c-axis orientation. In principle, an Ohmic contact is formed when the work function of the evaporated Ni is less than the electron affinity of ZnO. Ni is known to have work functions in the range of 4.1–5.4 eV . Evaporated Ni thin films are believed to have work functions less than 4.5 eV due to the atomically rough surface while the electron affinity of ZnO is around 4.5 eV .
Ni contacts on ZnO demonstrate both Ohmic and rectifying behavior depending on the surface polarity of ZnO as well as thermal annealing. The as-sputtered ZnO thin film as well as the as-received Zn- and O-polar surfaces of (0001) ZnO generally show an Ohmic behavior. Ohmic-rectifying conversion is observed for the ZnO thin film and Zn-polar surface of (0001) ZnO that have been annealed and is associated with the formation of nickel zinc oxide (Ni1-xZnxO, where x = 0.1, 0.2). No conversion is observed for the O-polar surface of (0001) ZnO. The Ni contacts are oxidised to NiO but no nickel zinc oxide has been detected. The surface of the sputtered ZnO thin film that has been annealed at 800°C is likely to be Zn-terminated.