K.G. Saw

New Insights on the Burstein-Moss Shift and Band Gap Narrowing in Indium-Doped Zinc Oxide Thin Films

The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.

STRUCTURAL AND OPTICAL PROPERTIES OF LARGE-SCALE ZnO NANOWIRES AND NANOSHEETS PREPARED BY DRY THERMAL OXIDATION

In this work, we report the morphology and optical properties of zinc oxide (ZnO) layers prepared by dry thermal oxidation at different annealing conditions. Morphology studies using scanning electron microscope (SEM) show that the amount of nanowires and nanosheets increases with the introduction of a flow of O2 gas. High-resolution X-ray diffraction (HR-XRD) data show that typical polycrystalline ZnO nanostructure layers have been deposited. Near-perfect stoichiometry of Zn and O atom vacancies has been observed from energy dispersion spectroscopy (EDS) spectrum. Photoluminescence (PL) spectra show strong peaks at UV and green regions. An increase in the stoichiometry of ZnO has been achieved with the oxygen gas flow during annealing indicating that deep-level defects represented by interstitial oxygen and antisite oxygen are gas pressure dependent. A single exciton peak with binding energy 60 meV has been observed at room temperature.

Ohmic-rectifying conversion of Ni contacts on ZnO and the possible determination of ZnO thin film surface polarity.

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.

Insights on semiconductor-metal transition in indium-doped zinc oxide from x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and x-ray diffraction

ZnO thin films doped with various amounts of In impurities were prepared by magnetron sputtering at a substrate temperature of 150°C. The shift in optical bandgap of the In-doped ZnO films is studied as a function of carrier concentration. Nominally doped ZnO films exhibit an increase in the measured optical band gap known as the Burstein-Moss effect. Dominant band gap narrowing is observed with increased doping. XPS and TOFSIMS analyses confirm that In is incorporated in the ZnO material. The In 3d peaks show that no metallic In is present as a result of heavy doping. The XRD phase analysis shows a preferential c-axis growth but a shift of the ZnO (002) peak to lower 2-theta values with increasing FWHM as the carrier concentration increases indicates the decline in the quality of crystallinity. An elongation of the c lattice constant is also observed and is likely to be caused by intersitital In as the amount of In dopants increases. The incorporation of In induces a semiconductor-metal transition betwee...

The Effects of Thermal Treatments on Microstructure Phosphorus-Doped ZnO Layers Grown by Thermal Evaporation

The ZnO films doped with 3 wt% phosphorus (P) were produced by activating phosphorus doped ZnO (ZnO:P) thin films in oxygen (O2) ambient at 600°C for 30, 60, 90 and 120 min, respectively. As-deposited films doped with phosphorus are highly conductive and n type. All the films showed p-type conduction after annealing, in an O2 ambient atmosphere. The activation energies of the phosphorus dopant in the p-type ZnO under O2 ambient gases indicate that phosphorus substitution on the O site yielded a deep level in the gap. With a further increase of the annealed durations, the crystalline quality of the ZnO:P sample is degraded. The best p-type ZnO:P film deposited at 600°C for 30 min shows a resistivity of 1.85 Ω cm and a relatively high hole concentration of 5.1 × 1017cm–3 at room temperature. The films exhibit a polycrystalline hexagonal wurtzite structure without preferred orientation. The mean grain sizes are calculated to be about 60, 72, 78, 85 and 90 nm for the p-type ZnO films prepared at 600°C for 30, 60, 90 and 120 min, respectively. Room temperature photoluminescence (PL) spectra of the ZnO film exhibit two emission bands — paramount excitonic ultraviolet (UV) emission and weak deep level visible emission. The excellent emission from the film annealed at 600°C for 30 min is attributed to the good crystalline quality of the p-type ZnO film and the low rate of formation of intrinsic defects at such short duration. The visible emission consists of two components in the green range.

Effect of zinc on the growth mechanism of zinc oxide nanostructures in the nitrogen environment

The effect of zinc on the growth mechanism of zinc oxide (ZnO) nanostructures is investigated by annealing zinc-rich sputtered ZnO thin films and ZnO single crystals that are partially coated with metallic zinc (Zn). Evidence from scanning electron microscopy, x-ray photoelectron spectroscopy as well as Raman and photoluminescence measurements suggests that excess Zn may induce the growth of ZnO nanostructures. This mechanism provides the possibility of large area selective growth.

STRUCTURAL AND OPTICAL PROPERTIES OF ZnO SYNTHESIZED BY TEMPERATURE GRADIENT THERMAL OXIDATION

In this work, a series of polycrystalline ZnO samples have been synthesized from Zn thin films deposited on Si(100) substrates by using thermal oxidation technique. The ZnO thin film samples grown by this technique were then characterized by a variety of structural and optical characterization tools. The results revealed that the use of novel annealing process i.e. the application of temperature gradient in the thermal treatment could enhance the structural and optical quality of the ZnO thin films significantly as compared to the normal annealing process, i.e. a fixed temperature under different durations. Apart from the improvement of structural and optical properties of ZnO thin films, another striking feature of this novel annealing process was the promotion of the growth of ZnO nanostructures.

Ultraviolet Photoresponse Properties of Zinc Oxide Nanorods on Heavily Boron-Doped Diamond Heterostructure

Heterostructures consisting of ZnO and diamond appear to have an elusive nature. A rectifying behaviour was previously observed only for heterojunctions with very lightly doped p-type diamond using residual boron gas during the chemical vapour deposition process or type IIb diamond. Other studies, however, claimed to obtain a rectifying behaviour for heterojunctions with p-type diamond with higher carrier densities between 1018 1019 cm-3. In this work we investigate the behaviour of n-type ZnO on heavily boron-doped p-type diamond. This heterostructure that is sensitive to UV light has been fabricated using ZnO nanorods grown on heavily boron-doped chemical vapour deposition diamond substrates. The I -V measurements show a rectifying characteristic. The threshold voltages under dark and UV conditions are 3.66 and 2.52 V, respectively. The UV illumination also results in an increased current flow. The electrical behaviour due to the UV illumination will be discussed.

Thermal Degradation of Single Crystal Zinc Oxide and the Growth of Nanostructures

Heat treatment of (0001) single crystal zinc oxide (ZnO) seems to degrade the surface morphology at high temperature. The degradation, however, does not suppress the growth of ZnO nanostructures on selective regions of the single crystal ZnO that have been sputtered with metallic zinc (Zn) and annealed at 800 °C. On the uncoated regions, no growth occurs but the presence of pits suggests material loss from the surface. The formation of ZnO nanostructures on the selective regions could be aided by the preferential loss of oxygen as well as zinc suboxides from the uncoated regions. Indirect evidence of the role of oxygen and zinc suboxides can be inferred from the formation of nickel zinc oxide Ni0.9Zn0.1O and nickel oxide NiO2 when Zn is replaced by Ni and annealed under similar conditions.