Red and green emissions are observed from P ion implanted ZnO. Red emission at ~680 nm (1.82 eV) is associated with the donor-acceptor pair (DAP) transition, where the corresponding donor and acceptor are interstitial zinc (Zni) and interstitial oxygen (Oi), respectively. Green emission at ~ 516 nm (2.40 eV) is associated with the transition between the conduction band and antisite oxygen (OZn). Green emission at ~516nm (2.403 eV) was observed for ZnO annealed at 800 oC under ambient oxygen, whereas, it was not visible when it was annealed in ambient nitrogen. Hence, the green emission is most likely not related to oxygen vacancies on ZnO sample, which might be related to the cleanliness of ZnO surface, a detailed study is in progress. The observed micro-strain is larger for N ion implanted ZnO than that for P ion implanted ZnO. It is attributed to the larger straggle of N ion implanted ZnO than that of P ion implanted ZnO. Similar phenomenon is also observed in Be and Mg ion implanted GaN.
We fabricated two dimensional photonic crystal structures in zinc oxide films with focused ion beam etching. Lasing is realized in the near ultraviolet frequency at room temperature under optical pumping. From the measurement of lasing frequency and spatial profile of the lasing modes, as well as the photonic band structure calculation, we conclude that lasing occurs in either localized or extended defect modes near the dielectric edge of photonic band gap. These defect modes originate from the structure disorder unintentionally introduced during the fabrication process. Fine tuning of lasing wavelength across 20nm range has been realized by varying the lattice constant of PhCS structure. A qualitative explanation for these PhCS lasers with self optimization of laser cavity quality factor has been proposed.
We report on fabrications and characteristics of high performance ZnO nanorod nanodevices including Schottky diodes, metal-oxide-semiconductor field-effect transistors (MOSFETs), metal-semiconductor field-effect transistors (MESFETs) and logic gate devices. Electrical characteristics of several ZnO nanorod MOSFETs are compared in this proceeding. In particular, after coating polymer thin films on ZnO nanorod surfaces, the nanorod MOSFETs exhibited much improved field effect transistor characteristics including field effect electron mobility as high as 3000 cm2/Vs. In addition, ZnO nanorod Schottky diodes and MESFETs were fabricated using Au/ZnO Schottky contacts without any specific oxide etching process. These devices have been used for realization of ZnO nanorod logic gates.
We fabricated ZnO nanostructures via a thermal oxidation reaction in air from Zn pellets placed in an open quartz tube heated in a furnace. The reaction started at a temperature of about 900°C and consumed the Zn pellets very rapidly. The fabrication process employed did not rely on metal catalysts. The nanostructures obtained were in the form of powder and thick layers deposited on quartz substrates. X-ray diffraction analysis showed that the crystal structure of the obtained material matched that of the hexagonal wurtzite structure of ZnO. X-ray fluorescence analysis showed no trace of metallic contamination, which confirmed that our material was synthesized without metal catalysts. Scanning electron microscope and transmission electron microscope observations revealed needle-like nanostructures, with a high yield for nanostructures having 4 legs assembled in a tetrapod-like shape. The diameter of the needles/legs varied in the 0.1 - 1 μm range while their lengths varied in the 1 - 10 μm range. Connections between long legs of different tetratpods were observed. Most of the legs were in the form of monocrystals. Material made of these tetrapods exhibited a sharp near-band-gap photoluminescence peak having its maximum at 390 nm together with a weak green below-band-gap emission. We found that a three-dimensional network made up of interconnected tetrapods could be deposited on quartz substrates. The ZnO tetrapod network formed a porous layer with a very high surface to volume ratio. The tetrapod network layer was tested as a gas sensing element by measuring changes in its electrical resistance upon exposure to ethanol vapor. A maximum in ethanol sensitivity was found at an operating temperature of 400°C. Ethanol concentration as low as 0.5 ppm was detected with a sensitivity of 2.5, suggesting a high sensitivity to ethanol vapor and possible applications in trace gas detection for the three-dimensional tetrapod network.
Although metal/semiconductor and oxide/semiconductor junctions have long been studied in the areas of microelectronics, new phenomena and interests arise from time to time. In particular, in the realm of nanotechnology where materials are shrunk at a length scale of nanometers, the role of heterojunctions in controlling the overall characteristics of the system will become more and more important. In this paper, we will show our recent results on the light emission and charge transport properties of metal/ZnO and oxide/ZnO system at different dimensionalities. On one hand, it is found that by capping metal on ZnO, it is possible to excite the surface plasmon polaritorn at the metal/ZnO interface and resonantly couple it with the spontaneous recombination of ZnO. This results in a significant enhancement of emission efficiency of ZnO. On the other hand, providing an oxidic overlayer (AlOx) is present on ZnO, a focused electron beam can be used to locally modify optical and electrical properties of ZnO. Under electron bombardment, we find the emission profile of ZnO gradually changes from green-yellow emitting into ultra-violet emitting while the conductivity decreases by more than two orders of magnitude at the same time. Well-defined sub-micron patterns with tunable optical and electrical properties can be fabricated on 2-D ZnO films and 1-D nanoribbons by carefully controlling the dose and energy density of the electron beam. Since ZnO is a versatile material, we believe our studies will shed light on the further use of ZnO in frontier technologies such as gas sensing, display technology, catalysis, spintronics, etc.
Optical properties of ZnO doped with Mn and V were studied. Zn(Mn)O layers were grown by peroxide MBE, and Zn(V)O was prepared by high-dose ion implantation of bulk ZnO prepared by hydrothermal technique. The Zn(Mn)O layers containing up to 50% of Mn were characterized by high-resolution x-ray diffraction, photoluminescence, and optical absorption. A blue shift of the band edge revealed from optical absorption measurements points to the incorporation of at least a part of Mn atoms on the lattice sites. An increase in the Zn(Mn)O band gap and an enhancement of the broad below band gap absorption associated with Mn ions were observed with increasing Mn composition. Correlating structural and optical transmission data, we suggest that the band edge of Zn(Mn)O rises linearly with the amount of Mn ions substituting Zn on the lattice sites. Photoluminescence of ZnO moderately doped with Mn shows several emission lines (the strongest ones are located at 3.34 and 3.36 eV). Surprisingly, no shift in the near-band-edge emission (3.36 eV) was detected in the photoluminescence data. Photoluminescence excitation studies revealed that the near-band-edge peak and the peak centered around 3.34 eV have different origin. Most probably, the second line is due to Mn intracenter transitions. Photoluminescence studies of ZnO bulk samples implanted with V+ have revealed that thermal annealing at 800 °C restores to a large extent the optical quality of the material. A new emission line centered at 3.307 eV has been found in the photoluminescence spectrum of the highly conductive samples implanted with a V dose of 1 × 1016 cm-2.
The light emitting property of zinc oxide (ZnO) nanoparticles, which were spontaneously precipitated and dispersed in fluorinated polyimide films has been studied because this hybrid material exhibits interesting properties and could be widely applied in optoelectronics and photonics. Polyimides (PI) provide high thermal and chemical stability and outstanding electrical properties. In addition, ZnO presents excellent optical properties due to its wide band gap (3.37 eV) at room temperature and large exciton bonding energy (60 meV), which can be used in light-emitting diodes, transparent electrodes, and piezoelectric devices. By adding small amount of Zinc compounds (5 mol %), either Zinc hexafluoroacetylacetonate dihydrate or Zinc nitrate hexahydrate, to precursor solutions of polyimides followed by thermal curing at 350~390°C, the green light emission at ca. 520 nm of polyimides has been significantly enhanced by 10 to 15 times respectively.
When zinc concentration excesses the saturation level, light emission decreases and emission peak was shifted to higher wavelengths due to the aggregation of ZnO. This can be explained by the quantum confinement mechanism and the interaction between the oxygen of ZnO and PI functional groups. Zinc compound precursors and curing temperature and atmosphere affect the light emitting behavior which will be discussed in detail.
Sb-doped p-type ZnO films were grown on n-Si (100) by electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). Room temperature Hall effect measurements reveal that a heavily Sb-doped ZnO sample exhibits a low resistivity of 0.2 Ω cm, high hole concentration of 1.7×1018 cm-3, and high mobility of 20.0 cm2/V s. Low-temperature photoluminescence (PL) measurements show an Sb-associated acceptor-bound exciton (AoX) emission exists at 3.358 eV at 8.5 K. The acceptor energy level of the Sb dopant is estimated to be 0.14 eV above the valence band. Based on these electrical and optical properties, p-n hetero- and homojunction photodetectors employing Sb-doped p-type ZnO films were designed and fabricated. The heterojunction photodiode consists of Sb-doped p-type ZnO grown on n-Si (100) substrate. An Sb-doped p-type ZnO layer with an n-type Ga-doped ZnO layer was grown on a p-Si (111) substrate to form the homojunction. Current-Voltage (I-V) characterizations reveal rectifying characteristics. Good photoresponse to UV light has been demonstrated for both hetero and homojunction photodetectors.
ZnO and N-doped ZnO thin films were grown by MOCVD on sapphire and ZnO substrates. Diethyl zinc and O2 were used as sources for Zn and O, respectively. A specially designed plasma system was employed to produce atomic N dopant for in-situ doping. Proper disk rotation speeds were found for ZnO growth on different size wafers. High crystal quality N-doped ZnO films were grown based on optimized growth conditions. Wet chemical etch of ZnO was investigated by using NH4Cl, and etch activation energy was calculated to be 463meV. Ohmic contact on N-doped ZnO film was achieved by using Ni/Au/Al multiple layers. ZnO based p-n junction has demonstrated rectification. Electroluminescence at about 384nm was obtained from ZnO based LED.
We have used molecular beam epitaxy (MBE) to deposit gallium (Ga) doped ZnO (ZnO:Ga) films. The as-deposited ZnO:Ga films have worked as ohmic contacts for the p-type GaN layers without any kinds of post annealing process. The as-deposited ZnO:Ga films on a-plane sapphire substrates have resistivities of 2-4×10-4 Ωcm, and over 80 % transparency in the near-UV and visible wavelength regions. The brightness of InGaN light-emitting diodes (LEDs) with ZnO:Ga p-contacts has doubled compared to LEDs with conventional Ni/Au semi-transparent p-contacts when measuring the brightness from right above the device surfaces. In addition, using MBE, we have grown homoepitaxial polar ZnO films on (000+1)-plane (+c-plane) ZnO substrates, and also grown non-polar ZnO films on (1-100)-plane (m-plane) and (11-20)-plane (a-plane) ZnO substrates. Growth temperatures have not affected nitrogen-doping levels for +c-axis oriented (Zn-polar) nitrogen doped ZnO (ZnO:N) films. The phenomena were quite different from that for (000-1)-axis (-c-axis) oriented (oxygen-polar) growth, where nitrogen concentrations in ZnO decrease with increasing growth temperatures. We have observed c-axis direction growth for both of m-axis and a-axis oriented films. Oxygen-rich growth conditions flatten surfaces for both m-axis and a-axis oriented films, and the surfaces of m-axis oriented ZnO films flatten with increasing growth temperatures. Nitrogen concentrations in m-axis oriented ZnO:N films have been independent on growth temperatures.
Physical vapor transport (PVT) growth of mm-size, polycrystalline ZnO has been demonstrated at temperatures exceeding 1600°C under air at atmospheric pressure. Scanning electron microscopy (SEM) analysis revealed the growth of grains and microcrystals with strong faceted morphologies suggesting near-equilibrium growth conditions. In addition, a temperature-dependent formula for the O2 sticking coefficient has been developed to predict the maximum growth rate of PVT ZnO. Combining this formula with an existing one-dimensional analytical model for PVT growth of bulk AlN, the value of the growth rate of PVT ZnO as a function of temperature and oxygen vapor partial pressure has been studied. This analysis predicts that growth rates in the order of 1mm/h could be theoretically achieved using the PVT method under non-stoichiometric oxygen-rich vapor pressures and temperatures exceeding 1600°C.
Zn0.9Mg0.1O/ZnO heterostructures were grown on both sapphire and bulk ZnO substrates via pulsed laser deposition (PLD). Electron-beam deposited 100nm Au and Ti/Au (20nm/80nm) were used as the p-Ohmic contact and n-Ohmic contact, respectively. Post-annealing at above 450°C of the contacts showed improved ohmic characteristics. I-V dependences showed good rectifying diode-like behaviors with threshold voltage of 1.36V and 2.16V for the devices fabricated on sapphire and ZnO substrates, respectively. 0.01at%Al-doped n-ZnO (n ~1019 cm-3) was deposited on MgO buffer layer via PLD. The electrical and optical properties strongly depend on the growth temperature, working pressure and laser energy. Room temperature photoluminescence showed band edge emission at ~377nm with very low deep level emission. The intensity of the band edge emission increased with growth temperature and deposition laser energy. Atomic force microscopy (AFM) results also showed that the root-mean-square (RMS) roughness increases with growth temperature and oxygen partial pressure. The full-width-at-half maximum (FWHM) for the ZnO (0002) peak is of 0.26-0.64°.
We present numerical solutions for low order hexagonal whispering gallery modes to simulate the resonant behaviour of single zinc oxide (ZnO) nanopillars. Experimental resonance spectra of such nanocavities, determined by polarization-resolved micro-photoluminescence spectroscopy, are well described by the results of our numerics. The spectral analysis yields the particular birefringence of every investigated nanopillar, consistent with current literature values for ZnO bulk material. Hence, the whispering gallery effect has been utilized to detect optical constants of individual nanostructures.