High-quality ZnO Epilayers Research Articles

ZnO film with ultra-low background electron concentration grown by plasma-assisted MBE using Mg film as the buffer layer

High quality ZnO epilayer with background electron concentration as low as 2.6×1014cm−3 was obtained by plasma-assisted MBE on c-sapphire using a thin Mg film as the buffer layer. High-resolution XRD measurement shows a sharp (002) peak with full width at half maximum (FWHM) of only 0.029°. Photoluminescence spectroscopy presents a weak defect-related near-edge emission. A metal–semiconductor–metal (MSM) typed photodetector based on the material demonstrates a response of ∼43A/W under the bias of 1V and an ON/OFF ratio of 104. This un-doped ZnO with ultra-low background electron concentration could be a promising starting material for p-type doping.

Optimization study of metal-organic chemical vapor deposition of ZnO on sapphire substrate

Metal-organic chemical vapor deposition (MOCVD) of ZnO with dimethylzinc (DMZn) and tert-butanol (t-BuOH) as Zn and oxygen precursors has been investigated and optimized in this work. Growth experiments show hydrogen addition, usually used to suppress carbon incorporation, should be kept at a low concentration due to its strong etching effect on ZnO. We found that reduction of hydrogen concentration also results in smoother films. Growth at a reactor pressure of 4–8kPa resulted in smoother films as compared to growth at 2kPa, and growth temperature of 470°C is also observed to result in smoother films, while fully suppressing gas phase pre-reactions. A low growth rate also helps to obtain smooth surface morphology, which is helpful to achieve improved crystal quality of the subsequent high temperature epilayer. The optimized growth parameters provided by this work are helpful for achieving high quality ZnO epilayer growth on sapphire substrates.

Structure and stimulated emission of a high-quality zinc oxide epilayer grown by atomic layer deposition on the sapphire substrate

A high-quality ZnO epilayer was grown on the (0001) sapphire substrate by atomic layer deposition (ALD) and followed by high-temperature rapid thermal annealing (RTA). The layer-by-layer growth and low deposition temperature of ALD, as well as the RTA treatment, prevent the formation of columnar structures in the ZnO epilayer. A distorted ZnO layer at the ZnO/sapphire interface, which relaxes the misfit in ZnO leads to a low threading dislocation density in the ZnO epilayer. Optically-pumped stimulated emission was achieved with a low-threshold intensity of 153 kW/cm 2 at room temperature. The good crystalline quality and low-threshold stimulated emission indicate that the ALD approach is appropriate for preparing high-quality ZnO epilayers.

UV Electroluminescence and Structure of n-ZnO/p-GaN Heterojunction LEDs Grown by Atomic Layer Deposition

Atomic layer deposition technique and subsequent rapid thermal annealing (RTA) were implemented to grow high-quality ZnO epilayers for the fabrication of n-ZnO/p-GaN heterojunction LEDs. The X-ray diffraction measurement reveals that the ZnO epilayer has high crystallinity with c axis orientation. Transmission electron microscopy images present that the ZnO layer is a single crystal, including only a few survivals of threading dislocations, which were generated in the GaN layer deposited by metal-organic chemical vapor deposition on the c-Al2O3 substrate and most of which were eliminated at the n-ZnO/p-GaN interface. An interfacial layer 4-5 nm thick caused by the RTA treatment was observed between the n-ZnO and p-GaN layers. Room temperature UV electroluminescence (EL) at 391 nm from ZnO was achieved at a low injection current about 10 mA. It is concluded that the competition between the ELs from the n-ZnO and p-GaN (around 425 nm) may be ascribed to the ZnO/GaN interface states coupled with the differences between the n-ZnO and p-GaN in carrier concentration and light emission efficiency.

Epitaxial growth and luminescence properties of ZnO-based heterojunction light-emitting diode on Si(1 1 1) substrate by pulsed-laser deposition

An epitaxial ZnO heterojunction light-emitting diode with an n-ZnO/MgO/TiN/n+-Si structure is produced by pulsed-laser deposition. By introducing a thin MgO/TiN buffer and a low temperature (LT) ZnO buffer, layer-by-layer growth of high quality ZnO epi-layer on Si(1 1 1) has been realized, which was confirmed by in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy, high-resolution x-ray diffraction, resonant Raman spectra and photoluminescence spectroscopy. Combining in situ RHEED with Phi-scan XRD analysis, the in-plane epitaxial growth of ZnO[1 1 0]||MgO[1 0 ]||Si[1 0 ] has been demonstrated. The strong room temperature electroluminescence (EL) with a broad emission band ranging from 1.46 to 3.5 eV and centred at 2.31 eV could be observed from the diode under relative low injection current. Furthermore, the EL output light intensity is enhanced obviously by improving the ZnO crystal quality via inserting a ZnO LT buffer layer.

Influence of oxygen pressure on elastic strain and excitonic transition energy of ZnO epilayers prepared by pulsed laser deposition

High quality ZnO epilayers ( χ min ∼ 10%) were prepared on Al 2O 3 (0 0 0 1) substrates at a temperature of 750 °C by pulsed laser deposition (PLD) with oxygen pressure of 0.015, 0.15, 1.5, and 15 Pa. The best crystalline quality and strongest intensity of UV photoluminescence were observed on ZnO layer with oxygen pressure of 15 Pa. It is probable due to the higher oxygen pressure lessens oxygen deficiency in the film. The tetragonal distortion e T, which is caused by elastic strain in the epilayer, was determined by Rutherford backscattering/channeling. It reduces as a whole (from 0.93 to 0.65%) with the increase of oxygen pressure from 0.015 to 15 Pa and the excitonic transition energy simultaneously shows a weak blue shift.

Growth kinetics and properties of ZnO/ZnMgO hetero‐ structures grown by radical‐source molecular beam epitaxy

A phenomenological approach to quantitative description of Zn(Mg)O growth by radical-source molecular beam epitaxy, based on the experimental studies of RHEED intensity oscillations, has been developed. It allows a precise control of growth rate, composition and stoichiometry at any growth temperature, Along with optimization of a growth initiation procedure on a c-sapphire, it is necessary condition for fabrication of high quality ZnO epilayers and ZnO/ZnMgO heterostructures in a wide Mg composition range. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling analysis of the MOCVD growth of ZnO film

The fluid dynamics of a vertical metal-organic chemical vapor deposition (MOCVD) reactor for the growth of ZnO film have been studied. The relevant transport equations were solved by numerical integration using a finite volume difference method. Computation results show that the deposition rate strongly depends on the operating parameters of the MOCVD reactor. Simulation results show that the reactor pressure has effect on the deposition rate, which becomes larger with the increment of the reactor pressure in the range of low pressure in our study. The simulated deposition rate is observed to increase firstly and then decrease with the main carrier gas flow rate. The computational study demonstrates that the distance between the O 2 inlet and substrate in the vertical MOCVD reactor plays a key role to obtain a balance between the deposition rate and the uniformity of films across the substrate. The optimization of the MOCVD operational parameters has been applied in the MOCVD growth of high-quality ZnO epilayers.

Production of high-quality ZnO films by the two-step annealing method

In this study, a two-step annealing method is advanced to produce high-quality ZnO films with excellent structural, electrical, and optical properties. The effects of oxygen and nitrogen annealing on the properties of undoped ZnO films are reversible to each other and are attributed to the creation and annihilation of extrinsic trap states of antisite oxygen OZn and oxygen vacancies VO, which result from the chemisorption and desorption of oxygen, respectively. Moreover, annealing in nitrogen causes slight nitrogen incorporation, subsequently increasing the resistivity and inducing compressive stress in the film. The key to this two-step method is to keep the chemisorption and desorption of oxygen in equilibrium. Due to the similarity of annealing ambient with the growth condition, this process can be transplanted and employed in the in situ preparation of high-quality ZnO epilayers.

High-quality ZnO epilayers grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy

High-quality ZnO epilayers have been grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O-rich, the growth mode changed from three- to two-dimensional growth and the line widths of (0 0 0 2) and (1 0 1 ̄ 0) ω-rocking curves showed dramatic narrowing from 471 to 42 arcsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device-grade MOCVD-grown GaN. Moreover, A-, B-excitons (FE A, FE B), and the n=2 state of FE A at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low-temperature (4.2 K) photoluminescence spectrum of ZnO grown under O-rich flux conditions. Our results show that growth under high O-rich flux conditions is required to produce high-quality Zn-polar ZnO films.

MBE‐grown ZnO film on sapphire substrate with double buffer layers

ZnO film was grown on c-Al2O3 substrate by rf-plasma-assisted molecular beam epitaxy. A low temperature (LT) GaN thin layer and a LT-ZnO layer were used as double buffer layers. A high quality ZnO epilayer was obtained by using the double buffer layers. The FWHM values of (002) symmetric and (102) asymmetric ω scans were 90 and 430 arcsec, respectively. Very smooth surface was obtained in step flow growth with rms roughness of 0.9 nm in 10 μm ×10 μm scanned area. The electron mobility of 1 μm thick ZnO epilayer was as high as 152 cm2V−1s−1 with residual carrier concentration of 2.8 × 1016 cm−3. Furthermore, the ZnO film was proved to be Zn polar by coaxial ion collision impact scattering spectroscopy (CAICISS) measurement. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoreflectance spectra of a ZnO heteroepitaxial film on the nearly lattice-matched ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy

Photoreflectance spectra of a high-quality ZnO epilayer on the ScAlMgO4 (0001) substrate grown by laser molecular-beam epitaxy exhibited clear excitonic resonances due to three excitons associated with uppermost valence bands (A, B, and C excitons). The oscillator strengths of the B and C excitons are larger than that of the A exciton. Their broadening was interpreted to be due to the contribution by exciton–polaritons in terms of large longitudinal–transverse splitting of respective excitons. Dependence of the exciton energy on temperature was well fitted assuming the Bose–Einstein statistics giving the Einstein characteristic temperature ΘE of 380 K (33 meV).

Stimulated emission and optical gain in ZnO epilayers grown by plasma-assisted molecular-beam epitaxy with buffers

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.

Plasma-assisted molecular-beam epitaxy of ZnO epilayers on atomically flat MgAl2O4(111) substrates

Plasma-assisted molecular-beam epitaxy of ZnO epilayers on MgAl2O4(111) substrates is described. Acid mixed by H2SO4 and H3PO4 was used for the substrate etching, which provides atomically flat surfaces with a regular terrace array. The influence of the substrate on lateral growth and coalescence of ZnO islands during epitaxy is discussed with corresponding morphological and structural properties. The epitaxial relationship between the ZnO epilayer and MgAl2O4 substrate is determined as: ZnO[011̄0]∥MgAl2O4[1̄1̄2] and ZnO[21̄1̄0]∥MgAl2O4[1̄10]. X-ray diffraction, photoluminescence, and Hall-effect measurements are reported, which indicate that high-quality ZnO epilayers are obtained by using atomically flat MgAl2O4(111) substrates to suppress columnar growth.

Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization

ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a very weak deep level emission at room temperature. Recently, these high quality ZnO epilayers have allowed the observation of optically pumped lasing at room temperatures as well as stimulated emission up to 550 K, both of which are due to an exciton related mechanism.

High temperature excitonic stimulated emission from ZnO epitaxial layers

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

Growth of ZnO single crystal thin films on c-plane (0 0 0 1) sapphire by plasma enhanced molecular beam epitaxy

ZnO single crystal thin films were grown by plasma enhanced molecular beam epitaxy on (0 0 0 1) sapphire. The growth modes of ZnO epilayers were investigated by reflection high-energy electron diffraction. A transition from two-dimensional nucleation to three-dimensional nucleation is found at the initial growth stage. Optical properties of the films, studied by photoluminescence spectroscopy, exhibit a dominant bound exciton emission at 3.361 eV at 4 K, and a deep level emission centered at 2.42 eV which is associated with either impurities or native defects. The deep level emission which is successfully suppressed to 1 500 of intensity of the excitonic emission. Fabrication of these high-quality ZnO epilayers had lead to observation of stimulated emission at room temperature.