Intrinsic defects in ZnO and GaN crystals

The kinetics of defect formation in the system of a crystal with a nonmetal component in the activated gas phase has been investigated. The data obtained has made it possible to develop physicochemical methods of regulating the defect-formation processes depending on the adsorption—desorption—crystallization equilibrium on the surface of a crystal. A kinetic model of defect formation in the A^IIB^VI and A^IIIB^V compounds is proposed. Results of the kinetic analysis of the intrinsic defects in the ZnO and GaN compounds are presented. The photoluminescence spectra of GaN films annealed in a flow of nonmetal-component radicals (atoms) have been considered. This work was reported at the Vth International Scientific-Technical Conference on Quantum Electronics, November 22–25, 2004, Minsk, Belarus. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 760–765, November–December, 2005.     

Violet luminescence from ZnO nanorods grown by room temperature pulsed laser deposition

ZnO thin films were deposited at room temperature by pulsed laser deposition (PLD) varying the oxygen pressure. Morphological analysis using scanning electron microscope (SEM) and atomic force microscopy (AFM) demonstrated the formation of ZnO nanorods at a particular oxygen pressure. Room temperature violet luminescence was observed from these ZnO nanorods and temperature dependence of luminescence was studied. Influence of oxygen pressure on the growth of ZnO thin films by PLD was studied using the X-ray photoelectron spectroscopy of both post ablated targets and deposited films. The ZnO films were crystalline and the formation of crystalline phase is found to follow a pressure–temperature (P–T) scaling with increase of temperature.     

Effect of high temperature treatments on defect centers and impurities in hydrothermally grown ZnO

Deep level transient spectroscopy (DLTS) has been employed to study electron traps in hydrothermally grown n-type ZnO samples after thermal treatments up to 1500 °C. Schottky barrier contacts were formed by e-beam evaporation of Pd, followed by DLTS and secondary ion mass spectrometry (SIMS) measurements in order to investigate possible correlations between electron traps in the upper part of the band gap and the concentration of the most prominent impurities. The DLTS results show three different levels having energy positions of , , and (E_c denotes the conduction band edge). The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Fe, Mn, and Ni with concentrations up to . A decrease in the level is observed after temperature treatments above 1300 °C, and in the same temperature range the Li concentration drops from ∼10¹⁷ to . However, based on absolute concentration values an association between Li and the level can be ruled out. In contrast, the level, which is not stable above 1300 °C, may be associated with Li but further experimental data are needed to substantiate this assignment. The level occurred in selected samples and is presumably impurity-related but no correlation was found with the main impurities detected by SIMS. Except for Li, the concentration of all the impurities remained essentially constant as a function of heat treatment temperature.     

Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods

A variety of different metal catalyst coatings (Pt, Pd, Au, Ag, Ti and Ni) deposited on multiple ZnO nanorods have been compared for their effectiveness in enhancing sensitivity for detecting hydrogen at room temperature. Pt-coated nanorods show a relative response of up to 8% in room-temperature resistance upon exposure to a hydrogen concentration in N₂ of 500 ppm. This is a factor of two larger than that obtained with Pd and more than an order of magnitude larger than that achieved with the remaining metals. The power levels for these sensors were low, ∼0.4 mW for the responses noted above. Pt-coated ZnO nanorods easily detected hydrogen down to 100 ppm, with a relative response of 4% at this concentration after 10-min exposure. The nanorods show a return to their initial conductance upon switching back to a pure-air ambient with time constants of the order of a few minutes at room temperature. This slow response at room temperature is a drawback in some applications, but the sensors do offer low-power operation and ppm detection sensitivity. PACS 78.66.Hf; 73.61.Ga; 73.40.Qv     

Characterisation of ZnO nanorod arrays grown by a low temperature hydrothermal method

In this paper, growth steps of well defined ZnO nanorod arrays deposited on seeded substrates were investigated. To obtain ZnO seed layer on glass substrates, a successive ionic layer adsorption and reaction (SILAR) method was used and then ZnO nanorods were grown on seed layer using a chemical bath deposition (CBD) method. The effects of seed layer and deposition time on morphology, crystallographic structure (e.g. grain size, microstrain and dislocation density) and electrical characteristics of ZnO nanorods were studied. From the SEM micrographs, it could be seen that the ZnO nanorods densely covered the substrate and were nearly perpendicular to the substrate surface. The XRD patterns showed that the ZnO nanorod arrays had a hexagonal wurtzite structure with a preferred orientation along the (002) plane. An increase in deposition time resulted in an increase in the intensity of the preferred orientation and grain size, but a decrease in microstrain and dislocation density. Electrical activation energies of the structures were calculated as 0.15–0.85?eV from current–temperature characteristics. It was concluded that the morphologies of the structures obtained in this study via a simple and fast solution method can provide high surface areas which are important in area-dependent applications, such as solar cells, hydrogen conversion devices, sensors, etc.     

Intrinsic point defects in niobium: Trapping at100Pd and111In impurities observed by PAC

Radiation-induced lattice defects in high-purity niobium have been investigated in the temperature range of 30K to 540 K by means of - perturbed angular correlation (PAC) measurements using the radioactive probes¹⁰⁰Pd/¹⁰⁰Rh and¹¹¹In/¹¹¹Cd. Both probes were produced within the niobium samples by means of heavy-ion nuclear reactions. At the Pd impurities trapping of defects occurred during heavy-ion irradiation at about 30 K in two defined configurations: defect 1(Pd) withv _Q1=e ² qQ/h=42(±2) MHz, ₁=0 and defect 2 (Pd) withv _Q2=(±2) MHz, ₂=1. Two defects were observed at the In impurities in annealing stage III (around 250 K) after heavy-ion as well as electron irradiations: defect 1(In) withv _Q1=87(±1) MHz, ₁=0 and defect 2(In) withv _Q2=105(±2) MHz, ₂=0.65(±0.02). A third defect (defect 3(In):v _Q3=177(±2) MHz, ₃0.2) appeared above 260 K after heavy-ion irradiation only. The data are interpreted in terms of interstitial trapping at the Pd impurities and vacancy trapping at the In impurities. Information on the microscopic structure of defect 1(In) and 2(In) is obtained from a PAC-single-crystal experiment. For defect 1(In) axial 111-symmetry is found, which leads us to identify this defect with a monovacancy as nearest neighbor with respect to the In probe. Defect 2(In) is the trapped divacancy for which an orientation is found that is consistent with both vacancies being nearest neighbor to the probe but second nearest neighbors to each other.     

X-ray analysis of ZnO nanorods grown by microwave irradiation heating on ZnO films

Utilizing microwave irradiation heating, 100-nm-diameter ZnO nanorods were grown from aqueous solution on sputtered ZnO films on glass substrates. Its out-of-plane X-ray diffraction (XRD) measurement indicated that the ZnO nanorods were grown with c-axis orientation, similar with the underlying ZnO films. In the in-plane XRD measurement, intensity of the () diffraction was comparable with that of the () one, suggesting their intensity ratio would contain useful information on nanorods density.     

Intrinsic low temperature paramagnetism in B-DNA

We present an experimental study of magnetization in lambda-DNA in conjunction with structural measurements. The results show the surprising interplay between the molecular structures and their magnetic property. In the B-DNA state, lambda-DNA exhibits paramagnetic behavior below 20 K that is nonlinear in an applied magnetic field whereas, in the A-DNA state, it remains diamagnetic down to 2 K. We propose orbital paramagnetism as the origin of the observed phenomena and discuss its relation to the existence of long range coherent transport in B-DNA at low temperature.     

Comparative PL study of individual ZnO nanorods,grown by APMOCVD and CBD techniques

The photoluminescence properties of individual ZnO nanorods, grown by atmospheric pressure metalorganic chemical vapor deposition (APMOCV) and chemical bath deposition (CBD) are investigated by means of temperature dependent micro-PL. It was found that the low temperature PL spectra are driven by neutral donor bound exciton emission D⁰X, peaked at 3.359 and 3.363 eV for APMOCVD and CBD ZnO nanorods, respectively. The temperature increase causes a red energy shift of the peaks and enhancement of the free excitonic emission (FX). The FX was found to dominate after 150 K for both samples. It was observed that while APMOCVD ZnO nanorods possess a constant low signal of visible deep level emission with temperature, the ZnO nanorods grown by CBD revealed the thermal activation of deep level emission (DLE) after 130 K. The resulting room temperature DLE was a wide band located at 420–550 nm. The PL properties of individual ZnO nanorods can be of importance for their forthcoming application in future optoelectronics and photonics.     

Enhanced optical and field emission properties of CTAB-assisted hydrothermal grown ZnO nanorods

We have developed a simple N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB)-assisted hydrothermal route for the production of ZnO one-dimensional (1D) nanostructures on zinc foil at reaction temperature of 160 °C. With the increase of CTAB concentration, the one-dimensional structures change from microrod to a mixture of nano- and microrod and finally to nanorods. X-ray diffraction studies confirmed the proper phase formation of the grown nanostructures. The room temperature photoluminescence spectra showed that ZnO nanostructures prepared with increased CTAB concentration exhibited enhanced band edge UV emission and also blue shift of the emission peak. All the samples show no defect related green emission. Field emission property of the 1D structures has been investigated in detail. By tuning the CTAB concentration, the field emission property was optimized. The nanorods synthesized with high CTAB showed turn-on and threshold fields of 3.2 and 5 V/μm, respectively, which are comparable to the values for vapour phase synthesized high field emitting ZnO nanostructures.     

High-quality ZnO nanorods grown on graphite substrates by chemical solution method

ZnO nanorods (NRs) were grown perpendicularly on graphite substrates using low-temperature wet chemical bath deposition (CBD) with sputtered ZnO film as seed layer. The individual ZnO NRs exhibit single-crystalline feature with well defined hexagonal prism shape and smooth side facets. The high optical qualities of ZnO NRs on graphite substrates were demonstrated by the dominant near-band edge emission and nearly undetectable deep level emissions in room-temperature photoluminescence spectra. The extremely low average reflectance of 0.45 % was obtained for the ZnO NRs/graphite structure in the spectra range from 200 to 1100 nm, indicating that the reported ZnO NRs/graphite structures have significant opportunity for potential application in high-performance photovoltaic devices. Considering the excellent material characteristics of ZnO NRs and the versatile and fascinating features of graphite substrates, the achievements make it possible for the development of high-performance ZnO-based nano-devices even in transferable, flexible, or stretchable forms.     

Photoresistor based on ZnO nanorods grown on a p-type silicon substrate

In this work we discuss a method of preparation of a highly sensitive light detector based on ZnO nanorods. A photoresistor constructed by us is based on a heterojunction between high quality ZnO nanorods and high resistivity p-type Si used as a substrate for nanorods’ deposition. ZnO nanorods are grown by a modified version of a microwave assisted hydrothermal method which allows for growth of high quality ZnO nanorods in a few minutes. The obtained photoresistor responds to a wide spectral range of light starting from near infrared (IR) to ultraviolet (UV). Properties of the detector are evaluated. We propose the use of the detector as an optical switch.     

Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition

The photoluminescence study of self-assembled ZnO nanorods grown on a pre-treated Si substrate by a simple chemical bath deposition method at a temperature of 80 °C is hereby reported. By annealing in O₂ environment the UV emission is enhanced with diminishing deep level emission suggesting that most of the deep level emission is due to oxygen vacancies. The photoluminescence was investigated from 10 K to room temperature. The low temperature photoluminescence spectrum is dominated by donor-bound exciton. The activation energy and binding energy of shallow donors giving rise to bound exciton emission were calculated to be around 13.2 meV, 46 meV, respectively. Depending on these energy values and nature of growth environment, hydrogen is suggested to be the possible contaminating element acting as a donor.     

Nucleation control for ZnO nanorods grown by catalyst-driven molecular beam epitaxy

We report a study of the annealing temperature and time on Ag catalyst size and density for subsequent growth of ZnO nanorods by catalyst-driven molecular beam epitaxy (MBE). Two different substrates (SiO₂ and SiN_X) for the Ag deposition were used and the thickness of the Ag held constant at 25 Å. Annealing between 600 and 800 °C produced Ag cluster sizes in the range 8-30 nm diameter on SiO₂ and 10-65 nm on SiN_X with a cluster density from 100 to 2500 mm⁻² for SiO₂ and 30 to 1900 mm⁻² for SiN_X. ZnO nanorods grown on these clusters show single-crystal, wurtzite-phase nature and strong band-edge photoluminescence at 380 nm. The nanorods can also be grown selectively on lithographically-patterned dielectric stripes with Ag clusters formed on top by e-beam evaporation and annealing.     

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.     

Strong blue emission from ZnSe nanowires grown at low temperature

We report optimized photoluminescence of ZnSe nanowires grown by molecular beam epitaxy, obtained by lowering the growth temperature down to 300 °C. The low‐temperature growth method has been developed using Si(111) and GaAs(111)B substrates. On the latter, vertical oriented blue‐emitting nanowires have been obtained. The growth mecha‐ nism is discussed with the help of in‐situ and ex‐situ electronic and structural measurements. We also report strong blue luminescence from ZnSe nanowires grown on ITO‐coated glasses, demonstrating that ZnSe nanowires are optimal candidates for transparent optoelectronics. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A new emission line in highly excited ZnO at low temperature

A new emission line from ZnO has been observed under strong u.v.-excitation at 4.2 K, which shifts to longer wavelengths with increasing excitation. The experimental results are described by a model involving the recombination of a bound exciton which transfers part of its energy to a free electron.     

Growth of Fe-doped ZnO nanorods using aerosol-assisted chemical vapour deposition via in situ doping

In situ Fe doping of ZnO nanorods (NRs) was performed using aerosol assisted chemical vapour deposition (AA-CVD) technique. As the aerosol generator is located outside the reactor, AA-CVD provides the flexibility to control doping parameters, such as doping timing, doping duration and a wider choice of dopant precursors. The Fe dopant aerosol was flowed into the reactor during the growth of ZnO NRs to achieve in situ doping. The X-ray diffraction analysis indicates that the Fe dopants were introduced into the ZnO lattice and present mainly in the form of Fe²⁺. This result is supported by the X-ray photoelectron spectroscopy analysis as the doublet separation is 13.6 eV, although there is a shift of Fe_1/2 and Fe_3/2 peaks to a lower binding energy levels. A strong green emission of PL of Fe-doped ZnO NRs shows that the NRs have poor crystal quality attributed to the Fe-induced defects (recombination centres). The poor photocatalytic performance in degrading Rhodamine B solution of Fe-doped ZnO NRs further proves that the Fe-induced defects were recombination centres rather than traps. Lastly, the growth mechanism of in situ Fe doping of ZnO NRs was discussed.     

ZnO nanorods grown on ZnSe particles by the chemical vapor deposition method

A novel structure of ZnO nanorods on microsized ZnSe particles has been prepared through a chemical vapor deposition technique using Zn and Se powders as the sources. The dimension of the nanorods can be controlled by adjusting the growth temperature, time and the Zn : Se ratio. Through the investigation of the effects of synthesis time and Zn : Se ratio on the formation of ZnO nanorods on ZnSe microparticles, it is proposed that the synthesis of the ZnO–ZnSe structures involves a two-stage formation. The growth of ZnO nanorods can be described by the vapor–solid mechanism. The photoluminescence of the ZnO–ZnSe structures has also been studied. PACS 73.21.-b; 78.55.Et; 61.10.Nz; 61.46.+w; 68.65.-k     

ZnO nanorods/plates on Si substrate grown by low-temperature hydrothermal reaction

The zinc oxide (ZnO) nanorods/plates are obtained via hydrothermal method assisted by etched porous Al film on Si substrate. The products consist of nanorods with average diameter of 100 nm and nanoplates with thickness of 200-300 nm, which are uniformly distributed widely and grown perpendicularly to the substrate. The ZnO nanoplates with thickness of 150-300 nm were grown on Si substrate coated with a thin continuous Al film (without etching) in the same aqueous solution. The growth mechanism and room temperature photoluminescence (PL) properties of ZnO nanorods/plates and nanoplates were investigated. It is found that the introduction of the etched Al film plays a key role in the formation of ZnO nanorods/plates. The annealing process is favorable to enhance the UV PL emissions of the ZnO nanorods/plates.