Hexagonal ZnO Films Research Articles

Luminescent Properties of (004) Highly Oriented Cubic Zinc Blende ZnO Thin Films.

Photoluminescence properties of cubic zinc blende ZnO thin films grown on glass substrates prepared by the spray pyrolysis method are discussed. X-ray diffraction spectra show the crystalline wurtzite with preferential growth in the (002) orientation and a metastable cubic zinc blende phase highly oriented in the (004) direction. Raman measurements support the ZnO cubic modification growth of the films. Photoluminescence (PL) spectra of zinc blende films are characterized by a new PL band centerd at 2.70 eV, the blue emission, in addition there are two principal bands that are also found in hexagonal ZnO films with the peak positions at 2.83 eV and 2.35 eV. The origin of the 2.70 eV band can be attributed to transitions from Zn-interstitial to Zn-vacancies. It is also important to mention that the PL intensity of the 2.35 eV band of the zinc blende thin films is relatively higher than in the band present in hexagonal ZnO films, which means that zinc blende films have more oxygen vacancies, as was corroborated by means of the energy dispersion spectroscopy (EDS) measurements. PL spectra at 77 °K were measured and the 2.70 eV band was confirmed for the zinc blende films. Some PL bands of cubic films also appeared for the hexagonal phase, which is due, to a certain extent, to the similar ions stacking of both wurtzite and zinc blende symmetries.

Structural and optical characteristics of the hexagonal ZnO films grown on cubic MgO (001) substrates.

In this Letter, we report on the structural and optical characteristics of ZnO films with a wurtzite structure grown on MgO (001) substrates with cubic structures. The ZnO films were prepared through the molecular beam epitaxy method, and growth orientation transformation from [0001] to [10-10] direction was observed with the change of growth temperature and thickness. The x-ray diffraction pole figures and in situ RHEED patterns demonstrated that the rotational relationship among grains within the ZnO films appeared in a typical two-fold rotation of about 30° for the [0001] growth orientation and four-fold rotation of about 30° or 60° for the [10-10] growth orientation, respectively. Last, we investigated their optical properties through measuring the transmission and photoluminescence spectra of the ZnO films, which showed the bulk-like bandgap feature of the ZnO films in spite of the existing growth orientation transformation.

Zinc Oxide Wire-Like Thin Films as Nitrogen Monoxide Gas Sensor

We present an excellent detection for nitrogen monoxide (NO) gas using polycrystalline ZnO wire-like films synthesized via a simple method combined with sputtering of Zn metallic films and subsequent thermal oxidation of the sputtered Zn nanowire films in dry air. Structural and morphological characterization revealed that it would be possible to synthesize polycrystalline hexagonal wurtzite ZnO films of a wire-like nanostructure with widths of 100-150 nm and lengths of several microns by controlling the sputtering conditions. It was found from the gas sensing measurements that the ZnO wire-like thin film gas sensor showed a significantly high response, with a maximum value of 29.2 for 2 ppm NO at , as well as a reversible fast response to NO with a very low detection limit of 50 ppb. In addition, the ZnO wire-like thin film gas sensor also displayed an NO-selective sensing response for NO, , , , and CO gases. Our results illustrate that polycrystalline ZnO wire-like thin films are potential sensing materials for the fabrication of NO-sensitive high-performance gas sensors.

Low temperature preparation p-CuI/n-ZnO wide gap heterojunction diode

Wide gap diode structures are attractive for various applications including tandem solar cells, light emission devices, UV detectors, and display technology. p-CuI/n-ZnO wide gap heterojunction diode have been prepared at a low cost by chemical method. The prepared hexagonal ZnO and γ-CuI films are polycrystalline nature and observed preferential orientation along the (002) and (111) axis aligning with the growth direction, respectively. The heterojunction shows a good rectifying behavior with a IF/IR ∼600 at 3V. The turn on voltage (2.05eV) from I–V characteristic is agree with the built-in potential (2.1eV) from C–V characteristic. The current transport mechanism is dominated by the recombination tunneling at low forward bias voltages and by the space-charge limited current (SCLC) conduction at higher forward bias voltages. This heterojunction diode can be good used for light emission devices and UV detectors.

Evolution of wurtzite ZnO films on Cubic MgO (001) substrates: a structural, optical, and electronic investigation of the misfit structures.

We investigated the interface between hexagonal ZnO films and cubic MgO (001) substrates, fabricated via molecular beam epitaxy. X-ray diffraction and (scanning) transmission electron microscopy revealed that growth follows the single [0001] direction when the temperature of the substrate is above 200 °C, while when the substrate temperature is below 150 °C, growth initially is along [0001] and then mainly changes to [0-332] variants beyond a thickness of ∼10 nm. Interestingly, a double-domain feature with a rotational angle of 30° appears during growth along [0001] regardless of the temperature, experimentally demonstrating the theoretical predictions for the occurrence of double rotational domains in such a heteroepitaxy [Grundmann et al., Phys. Rev. Lett. 105, 146102 (2010)]. We also found that the optical properties of the ZnO film are influenced greatly by the mutation of growth directions, stimulated by the bond-length modulations, as we determined from X-ray absorption spectra at Zn K edge. These results also showed the evolution of the 4p(xy) and 4p(z) states in the conduction band with the rise in the temperature for growth. We consider that our findings may well promote the applications of ZnO in advanced optoelectronics for which its integration with other materials of different phases is desirable.

Investigation of ZnO thin films deposited on ferromagnetic metallic buffer layer by molecular beam epitaxy toward realization of ZnO-based magnetic tunneling junctions

Deposition of ZnO thin films on a ferromagnetic metallic buffer layer (Co3Pt) by molecular beam epitaxy technique was investigated for realization of ZnO-based magnetic tunneling junctions with good quality hexagonal ZnO films as tunnel barriers. For substrate temperature of 600 °C, ZnO films exhibited low oxygen defects and high electrical resistivity of 130 Ω cm. This value exceeded that of hexagonal ZnO films grown by sputtering technique, which are used as tunnel barriers in ZnO-MTJs. Also, the effect of oxygen flow during deposition on epitaxial growth conditions and Co3Pt surface oxidation was discussed.

Effect of sputtering pressure on growth behavior of heteroepitaxial ZnO/SrTiO3 (001) films grown by radio frequency magnetron sputtering

The effect of sputtering pressure on the growth behavior of heteroepitaxial ZnO films grown by radio-frequency magnetron sputtering on SrTiO3 (STO) (001) substrates was investigated. Columnar growth of ZnO films at a growth pressure of 3×10−2Torr was observed, while typical planar growth was dominant at lower growth pressures. This can be explained by the complicated interplay between the surface diffusivity of ZnO films and their growth rates. We also found that wurtzite hexagonal ZnO films with a preferred (0002) orientation could be grown epitaxially on the perovskite cubic STO (001) substrates in the substrate normal direction. Azimuthal angle scans of off-specular ZnO (101¯1) Bragg peaks revealed that the domains of ZnO films tilted by ±15° with respect to the STO [100] direction in the in-plane direction. The epitaxial relationship was confirmed as being ZnO[101¯0]//STO [110] and ZnO [101¯0]//STO [1¯10].

Strain Mismatch Induced Tilted Heteroepitaxial (000l) Hexagonal ZnO Films on (001) Cubic Substrates

AbstractA novel strain mismatch induced tilted epitaxy method has been demonstrated for producing high quality (000l) hexagonal films on (001) cubic substrates. Highly oriented hexagonal (000l) ZnO films are grown on cubic (001) MgO substrates using Sm0.28Zr0.72O2−δ (SZO) as a template. The large lattice mismatch of >13% between the obvious crystallographic matching directions of the template and substrate means that cube‐on‐cube epitaxy is energetically unfavorable, leading to growth instead of two high index, low energy compact planes, close to the {111} orientation. These planes give three different in‐plane orientations resulting from coincidence site lattice matching (12 in‐plane orientations in total) and provide a pseudo‐hexagonal symmetry surface for the ZnO to grow on. The texture of the ensuing (000l) ZnO layer is markedly improved over the template. The work opens up both a new avenue for growing technologically important hexagonal structures on a range of readily available, (001) cubic substrates, as well as showing that there are wide possibilities for heteroepitaxial growth of a range of dissimilar materials.

Epitaxial ZnO thin films grown by pulsed electron beam deposition

In this work, the pulsed electron beam deposition method (PED) is evaluated by studying the properties of ZnO thin films grown on c-cut sapphire substrates. The film composition, structure and surface morphology were investigated by means of Rutherford backscattering spectrometry, X-ray diffraction and atomic force microscopy. Optical absorption, resistivity and Hall effect measurements were performed in order to obtain the optical and electronic properties of the ZnO films. By a fine tuning of the deposition conditions, smooth, dense, stoichiometric and textured hexagonal ZnO films were epitaxially grown on (0001) sapphire at 700 °C with a 30° rotation of the ZnO basal plane with respect to the sapphire substrate. The average transmittance of the films reaches 90% in the visible range with an optical band gap of 3.28 eV. Electrical characterization reveals a high density of charge carrier of 3.4 × 1019 cm−3 along with a mobility of 11.53 cm²/Vs. The electrical and optical properties are discussed and compared to ZnO thin films prepared by the similar and most well-known pulsed laser deposition method.

Organozinc Aminoalcoholates: Synthesis, Structure, and Materials Chemistry

A series of novel organozinc aminoalcoholates have been synthesized by the reaction of the amino alcohols HOCH(x)(CH(2)NMe(2))(3-x) (x = 2, Hdmae; 1, Hbdmap; 0, Htdmap) with R(2)Zn (R = Me, Et). The 1:1 reaction with Hdmae leads to the tetramers [RZn(dmae)](4) [R = Me (1), Et (2)], while with Htdmap, dimeric [RZn(tdmap)](2) [R = Me (7), Et (8)] are produced. Reaction with Hbdmap only yields [MeZn(bdmap)](n) (3), an oil which mass spectral data suggests contains a mixture of Zn(2)-Zn(7) species, when a 2-fold excess of ligand is used. Crystals of dimeric [Zn(bdmap)(2).Hbdmap](2) (4) deposit from this oil on prolonged standing. Reaction of Et(2)Zn with Hbdmap (1:1) affords [EtZn(bdmap)](n) (5), also an oil made up of Zn(3) and Zn(4) clusters on the basis of mass spectral evidence. Crystals of EtZn(3)(bdmap)(5) (6) are formed within the oil that is 5 on standing. [MeZn(tdmap)](2) (7) has been used as a single-source precursor for hexagonal ZnO films under low pressure chemical vapor deposition (LPCVD) conditions and a substrate temperature of 500 degrees C.

Sputtering ZnO films on langasite and its saw properties

C-axis-oriented ZnO films were sputtered on Langasite substrate (LGS, La(3)Ga(5)SiO(14)). The crystalline structure of the films was determined by grazing incident angle X-ray diffraction, the surface microstructure of films was investigated by scanning electron microscopy and atomic force microscopy, the atom composition ratio O/Zn of films was determined by energy dispersive X-ray spectroscopy, and the resistivity of films was determined by the four-point probe instrument. The measurement results showed those films prepared were all polycrystalline hexagonal ZnO films. By analyzing the microstructure of the ZnO films, those prepared at the oxygen flow rate (O(2)/O(2)+Ar) of 20%, the RF power of 200 W, and the substrate temperature of 200 degrees C had the best performance: highly c-axis-oriented microstructures, dense surface morphology, and the atom composition ratio 1.02. The measured scattering parameters of the SAW device fabricated on the composite substrate (ZnO/LGS) with film thickness 1.76 microm showed an average shifted velocity around 2741 m/s at 57.1 MHz and a electromagnetic coupling coefficient greater than 1%.

Dependence of photoconductivity on the crystallite orientations and porosity of polycrystalline ZnO films

Hexagonal ZnO films deposited on quartz glass, sapphire and glass substrates by sol–gel coating are found to be randomly oriented; maximum randomness is found in the film on quartz glass substrate. All the films are ultra-violet (UV) sensitive at around 360 nm sensitivity being maximum for the film with maximum randomness in the crystallite orientations. The film on quartz showed the lowest dark current and maximum photoresponse, which is related to the lowering of the barrier heights, introduced by the adsorbed oxygen at the grain boundaries. Faster decay in photocurrent is observed for the film deposited on glass, which is attributed due to the smaller crystallite sizes with porous microstructure of the film.

Comparison between ZnO films grown by femtosecond and nanosecond laser ablation

We have studied the structural properties of ZnO thin films grown on Al2O3 (00.1) single-crystal substrates by pulsed-laser deposition using either a femtosecond or a nanosecond laser. Although hexagonal ZnO films deposited on sapphire substrate were epitaxially grown in both cases, the crystalline quality was found to be very different: ZnO films grown with the femtosecond laser are characterized by a higher mosaicity, a smaller crystallite size, a larger content of defects but also smaller residual stresses than ZnO films obtained by nanosecond laser ablation. These differences can be explained according to the kinetic energy of the species evolved during laser ablation as deduced from plasma characterization with a charged-coupled device camera: close to 1 KeV in the femtosecond regime for the population species emitted from the target with the highest velocity, versus a few hundreds of eV in the case of nanosecond pulses. The high energy species irradiation associated with a femtosecond laser is likely to induce a large structural disorder together with stress relaxation during ZnO films growth.

Growth of heteroepitaxial ZnO thin films by femtosecond pulsed-laser deposition

ZnO thin films have been grown on various substrates by femtosecond pulsed-laser deposition. According to optical microscopy and atomic force microscopy analyses, the production of droplets is not significant using femtosecond pulses. Smooth, dense, stoichiometric, crystalline, and textured hexagonal ZnO films are epitaxially grown on (0001) sapphire at 700 °C with an in-plane epitaxial relationship corresponding to a 30° rotation of the ZnO basal plane with respect to the sapphire. Nevertheless, channeling experiments and rocking curve measurements show that the crystalline quality is not as good as that obtained with nanosecond pulses.

Growth of a high quality ZnO film on sapphire by atmospheric pressure halide vapor phase epitaxy using ZnO buffer layers

Hexagonal ZnO films have been grown on a sapphire(0001) substrate by atmospheric pressure halide vapor phase epitaxy using ZnO buffer layers. The full width at half maximum of the X-ray (0002) diffraction peak for the ZnO films with the buffer layer was found to be smaller than that of the ZnO films without the buffer layer. Reflection high-energy electron diffraction measurements of the former revealed a diffraction pattern similar to that of a single crystal. The photoluminescence spectra showed a strong peak at 370 nm up to 180 K.

Oxide Quantum Structures for Future Electro-Photonics

ABSTRACTWe show our recent development on the epitaxial growth of oxide thin films by laser molecular beam epitaxy directed towards a possible new field of electro-photonics based on metal-oxide quantum structures. Examples of quantum structures include superlattices and tunnel junctions (2-dimensional), superconducting quantum wire arrays (1-dimensional), and quantum dots for photonic application (O-dimensional). For fabricating such structures, it is vitally important to prepare the substrate surface and to understand the growth dynamics. Such electro-phptonic functions as Josephson effect in high Tc tunnel junctions and excitonic UV-laser operation at room temperature in self-organized hexagonal nanocrystal ZnO films are presented.

Fast photoresponse and the related change of crystallite barriers for ZnO films deposited by RF sputtering

Hexagonal ZnO films were deposited on a glass substrate at room temperature by RF sputtering a ZnO target using a mixture of oxygen and argon. Polycrystalline ZnO films prepared in the presence of excess oxygen or at a high oxygen pressure tend to have crystallites with the orientation of the c-axis parallel to the substrate and a large and relatively fast photoresponse. The time constants of the photoresponse can be much reduced by doping the surface layer with nitrogen. This fast photoresponse of the resulting films is related to the changes of barrier heights of grain boundaries due to photodesorption of the chemisorbed oxygen on the grain boundaries when the films are irradiated with uv light.

Crystallite orientation and the related photoresponse of hexagonal ZnO films deposited by r.f. sputtering

Hexagonal ZnO films with different observed orientations were deposited on glass substrates at temperatures below 200°C by r.f. sputtering of a ZnO target using an oxygen and argon gas mixture. Orientations with the (002) planes of the crystallites parallel to the substrate, or with the (100) planes parallel to the substrate, or films in which the (101) peak is dominant in their X-ray diffraction spectra have been obtained by selecting the deposition parameters. The partial pressure of the oxygen in the sputtering gas, the distance between target and substrate, the substrate temperature during film nucleation and the input power each affect (to varying degrees) the observed crystallite orientation obtained. Polycrystalline ZnO films made in the presence of excess oxygen or at a high sputtering gas pressure tend to have the c axis parallel to the substrate and they exhibit a large photoresponse, while films sputter-deposited with a low partial pressure of oxygen tend to have the c axis perpendicular to the substrate and may have no detectable d.c. photoresponse. The availability of oxygen during sample preparation has a significant affect on the slow photoresponse of the resulting ZnO films.