Low-temperature ZnO Buffer Layer Research Articles

Type-II ZnO/ZnS core-shell nanowires: Earth-abundant photoanode for solar-driven photoelectrochemical water splitting.

A core-shell structure, formed in a nanostructured photoanode, is an effective strategy to achieve high solar-to-hydrogen conversion efficiency. In this study, we present a facile and simple synthesis of a unique vertically aligned ZnO/ZnS core-shell heterostructure nanowires (NWs) on a Si substrate. Well-aligned ZnO NWs were grown on Si (100) substrates on a low-temperature ZnO buffer layer by metal-organic chemical vapor deposition. The ZnO NWs were then coated with various thicknesses of ZnS shell layers using atomic layer deposition. The structural characterizations exhibit the well-developed ZnO/ZnS core-shell NWs heterostructure. The as-prepared ZnO/ZnS core-shell NWs was applied as photoanode for photoelectrochemical (PEC) water splitting. This unique ZnO/ZnS core-shell NWs photoanode shows photocurrent density of 1.21 mA cm-2, which is 8.5 times higher than bare ZnO NWs. The PEC performance and the applied-bias-photon-to-current conversion efficiency of ZnO/ZnS core-shell NWs photoanode are further improved with the optimized ZnS shell. The type-II band alignment of the heterostructure photoanode is the key factor for their excellent PEC performance. Importantly, this type of core-shell NWs heterostructure provides useful insights into novel electrode design and fabrication based on earth abundant materials for low-cost solar fuel generation.

Facile Synthesis of Well-Aligned ZnO Nanowires on Various Substrates by MOCVD for Enhanced Photoelectrochemical Water-Splitting Performance

We present results related to the growth of high-aspect-ratio ZnO nanowires (NWs) using metal–organic chemical vapor deposition based on the study of the NW growth mechanism on various substrates. The ZnO NWs on Si, sapphire, and GaN substrates were optically and structurally characterized, and the effect of a low-temperature ZnO buffer layer was investigated. It was demonstrated that NWs deposited under the same conditions could be randomly oriented or vertically aligned. Well-aligned ZnO NWs were grown on Si (100) substrates with a ZnO buffer layer which was grown via atomic layer deposition. The NWs are single-crystalline wurtzite structures with a preferential growth in the (002) direction. Room-temperature photoluminescence measurements exhibited a strong ultraviolet emission and suppressed visible emission. This result affirms the absence of defects related to ZnO or oxygen vacancies. On changing the substrate, different morphologies of the ZnO NWs can be attained, which present different photoelect...

Fast-response photoconductive ultraviolet light detectors fabricated using high-quality ZnO films obtained by plasma-assisted molecular beam epitaxy

Abstract ZnO films form the core of numerous ultraviolet (UV) optoelectronic devices. Herein, a ZnO buffer layer produced by thermal plasma oxidation of metallic Zn was used to grow an active ZnO layer, with Zn film and active layer growth achieved by plasma-assisted molecular beam epitaxy. Although low-temperature ZnO buffer layer and Zn film growth was mainly three-dimensional (3D) due to a large lattice mismatch between ZnO/Zn and Si, the use of thermal plasma oxidation changed the growth mode from 3D to 2D, producing a smooth ZnO active layer. The thus fabricated ZnO active layer (sample 2) exhibited a lower tensile stress than that fabricated on a ZnO buffer layer using a conventional method (sample 1). Both samples were used to fabricate UV sensors, with that based on sample 2 exhibiting a faster photoresponse.

Enhanced Light Scattering by Preferred Orientation Control of Ga Doped ZnO Films Prepared through MOCVD

We have explored the effective approach to fabricate GZO/ZnO films that can make the pyramidal surface structures of GZO films for effective light scattering by employing a low temperature ZnO buffer layer prior to high temperature GZO film growth. The GZO thin films exhibit the typical preferred growth orientations along the (002) crystallographic direction at deposition temperature of 400°C and SEM showed that column-like granule structure with planar surface was formed. In contrast, GZO films with a pyramidal texture surface were successfully developed by the control of (110) preferred orientation. We found that the light diffuse transmittance of the film with a GZO (800 nm)/ZnO (766 nm) exhibited 13% increase at 420 nm wavelength due to the formed large grain size of the pyramidal texture surface. Thus, the obtained GZO films deposited over ZnO buffer layer have high potential for use as front TCO layers in Si-based thin film solar cells. These results could develop the potential way to fabricate TCO based ZnO thin film using MOCVD or sputtering techniques by depositing a low temperature ZnO layer to serve as a template for high temperature GZO film growth. The GZO films exhibited satisfactory optoelectric properties.

Stable Inverted Low-Bandgap Polymer Solar Cells with Aqueous Solution Processed Low-Temperature ZnO Buffer Layers

Efficient inverted low-bandgap polymer solar cells with an aqueous solution processed low-temperature ZnO buffer layer have been investigated. The low-bandgap material PTB-7 is employed so that more solar light can be efficiently harvested, and the aqueous solution processed ZnO electron transport buffer layer is prepared at 150°C so that it can be compatible with the roll-to-roll process. Power conversion efficiency (PCE) of the inverted device reaches 7.12%, which is near the control conventional device. More importantly, the inverted device shows a better stability, keeping more than 90% of its original PCE after being stored for 625 hours, while PCE of the conventional device is only 75% of what it was. In addition, it is found that the ZnO thin film annealed in N2can obviously increase PCE of the inverted device further to 7.26%.

Properties of ZnO/ZnMgO nanostructures grown on r-plane Al2O3 substrates by molecular beam epitaxy

Growths of ZnMgO/ZnO/ZnMgO multiple quantum well (MQW) structures were performed on semi-polar r-plane sapphire substrates by molecular beam epitaxy. It has been shown that it is possible to grow self-organized ZnMgO nanocolumns with MQW structures at a temperature of 550 °C without employing a catalyst. The nanocolumns are symmetrically tilted at an angle of 62° to the substrate surface. Deposition of a low-temperature (450 °C) ZnO buffer layer prior to the growth of ZnO/ZnMgO MQWs results in good quality planar structures. The crystalline quality of the nanostructures was characterized by X-ray diffraction and scanning electron microscopy. Luminescence properties of ZnO/ZnMgO heterostructures and MQWs were studied. The luminescence was dominated by localized exciton emissions below 100K, while free exciton recombination was the most intense emission at temperatures above 100 K.

MgO(111)上ZnO薄膜的外延生长及其结构和光学特性

由于ZnO在光电器件的应用前景，其高质量薄膜的制备是研究热点之一。本文通过分子束外延生长法在MgO(111)单晶衬底上生长ZnO薄膜，表征了其结构和特性，探讨了不同生长条件对薄膜质量的影响。结果表明，先低温生长ZnO缓冲层，再高温生长ZnO薄膜，有望提高ZnO薄膜的质量。原位反射高能电子束衍射(RHEED)和异位的X射线衍射(XRD)分别测量出薄膜的面内结构和沿[0001]的单晶域高取向结构，并确定薄膜和衬底的外延关系为ZnO[01-10]//MgO[1-10]和ZnO[2-1-10]//MgO[11-2]。透射谱显示了ZnO的特征光学带隙。 The growth of high quality ZnO films is highly desirable due to the promising applications of ZnO in optoelectronics. In this paper, ZnO films were grown on the MgO(111) substrates via the growth technique of molecular- beam epitaxy and their structural and optoelectronic properties were characterized. In particular, the influence of growth condition on the film qualify was investigated. The results show that, inducing a low temperature ZnO buffer layer before the high temperature growth of ZnO films will help to improve the film quality. In situ reflection high-energy electron diffraction (RHEED) and ex situ X-ray Diffraction (XRD) measurements indicate that the ZnO film and the MgO substrate follow the epitaxial relationship: ZnO[01-10]//MgO[1-10] and ZnO[2-1-10]//MgO[11-2]. Transmission Spectra show the characteristic optical bandgap of ZnO.

Morphological control of MgxZn1−xO layers grown on Ga:ZnO/glass substrates for photovoltaics

MgxZn1−xO has been used in various photovoltaic cells because its energy bandgap can be tailored by controlling the Mg composition in this ternary compound. The MgxZn1−xO layers with different surface morphologies including two-dimensional (2-D) films and one-dimensional (1-D) nanostructures are preferred for conventional p–n junction solar cells and polymer–inorganic hybrid solar cells, respectively. The MgxZn1−xO layers are sequentially grown on Ga-doped ZnO (GZO) transparent conductive electrode using metalorganic chemical vapor deposition (MOCVD). The effect of the buffer layers on MgxZn1−xO surface morphology is investigated. It is observed that MgxZn1−xO deposited at ∼500°C on a low-temperature (∼250°C) ZnO buffer layer is in the form of 2-D dense and smooth films, whereas, on a high-temperature (∼520°C) ZnO buffer layer is in the form of 1-D nanostructures. Based on the structure characterization results, a growth mechanism in terms of nucleation and texturing is proposed to explain the buffer layer effect.

Influence of thermally diffused aluminum atoms from sapphire substrate on the properties of ZnO epilayers grown by metal-organic chemical vapor deposition

In this study, the authors investigate the evolution of the structural and electrical properties of ZnO epilayers grown by the metal-organic chemical vapor deposition method on c-sapphire substrates. The inserting of a low-temperature ZnO buffer layer not only significantly improves the structural quality of the high-temperature (HT)-grown ZnO epilayer on a sapphire substrate but also results in high background electron concentration in it from the Hall-effect measurement. After subtracting the conductive contribution from a thin degenerated layer mostly formed between the buffer layer and the substrate based on the two-layer model, the deduced electron-carrier concentration is still in the order of 1018 cm−3, which is much larger than the 1016 cm−3 obtained from capacitance-voltage measurement near the top surface. This indicates that a much thicker layer with high carrier concentration should be formed in the HT-grown ZnO epilayer, which is significantly different from that observed in GaN epitaxy, where only a thin degenerated interfacial layer is suggested to form in the GaN buffer layer. Al atoms’ distribution acquired from secondary-ion mass spectrometry shows a strong dependence on the temperature of the ZnO growth process, indicating that a thermally enhanced diffusion mechanism should be responsible for the observation of the enhanced Al atom concentration in the HT-grown ZnO epilayer. As substituted Al atoms on the Zn site act as donors in ZnO, the one-to-one correspondence between Al content and the carrier concentration, as well as the analysis of temperature-dependent Hall-effect measurement, indicates that diffusion-induced gradient-distributed AlZn shallow donors should be the main origin of the high background-carrier concentration in the HT-grown ZnO epilayers.

Growth and characteristics of ZnO films on growth side of freestanding diamond substrate dependent on buffer layer thickness

We report the growth and properties of highly c-axis oriented ZnO films, by radio-frequency magnetron sputtering, on the growth side of freestanding chemical vapor deposited diamond film-substrate. Low-temperature ZnO buffer layer is required for the formation of continuous ZnO films. The morphology, structure, and optical properties of the ZnO films deposited are strongly dependent on the thickness of the buffer layer. The optimized thickness of ZnO buffer layer is about 10 nm to realize high-quality ZnO films having small compressive stress and high intensity ultraviolet emission. The ZnO/diamond (growth side) system is available for the applications in numerous fields, especially for high performance surface acoustic wave devices.

Improvement in crystallinity and optical properties of ZnO epitaxial layers by thermal annealed ZnO buffer layers with oxygen plasma

ZnO epitaxial layers with treated low-temperature (LT) ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si (1 0 0) substrates. The LT-ZnO buffer layers were treated by thermal annealing in O 2 plasma with various radio frequency (RF) power ranging from 100 to 300 W before the ZnO epilayers growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD), and room-temperature (RT) photoluminescence (PL) were carried out to investigate their structural and optical properties. The surface roughness measured by AFM was improved from 2.71 to 0.59 nm. The full-width at half-maximum (FWHM) of the rocking curve observed for ZnO (0 0 2) XRD and photoluminescence of the ZnO epilayers was decreased from 0.24° to 0.18° and from 232 to 133 meV, respectively. The intensity of the XRD rocking curve and the PL emission peak were increased. The XRD intensity ratio of the ZnO (0 0 2) to Si substrates and PL intensity ratio of the near-band edge emissions (NBEE) to the deep-level emissions (DLE) as a function of the RF power was increased from 0.166 to 0.467 and from 2.54 to 4.01, respectively. These results imply that the structural and optical properties of ZnO epilayers were improved by the treatment process.

Improvement of ZnO thin film properties by application of ZnO buffer layers

The effect of ZnO buffer layers prepared at different temperatures on the structural, optical and morphological properties of the ZnO main layer is reported. ZnO thin films (comprising a buffer and a main layer) were deposited on (0 0 0 1) c-sapphire substrates by PEMOCVD. Two-step growth regimes were applied to realize a homoepitaxial growth on ZnO buffers: low-temperature ZnO buffer layer deposited at T s=300 °C and the main layer at T s=500 °C; high-temperature ZnO buffer layer deposited at T s=500 °C and the main layer at T s=300 °C. For comparison, a sample grown at high-temperature T s=500 °C by one-step procedure was used. The low-temperature buffer layer has shown the most beneficial effect on the structural and morphological properties, as expressed by the narrowing of the (0 0 2) diffraction peak (FWHM=0.07°) and crystallite size enlargement. However, the surface roughness of this sample is higher then that of the sample grown by one-step procedure and this needs further considerations. The photoluminescence results seem to support a conclusion that the application of a low-temperature buffer layer among the studied temperature regimes is the most advantageous.

Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient

ZnO films with improved crystallinity were grown on a Si (111) substrate by a two-step growth process using low-temperature ZnO buffer layers. The effect of the ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural properties of the films was investigated by X-ray diffraction (XRD), photoluminescence, and transmission electron microscopy. The use of argon as the ambient gas during the thermal treatment of the buffer layer leads to the enhancement of the (0002) diffraction peak intensity at 2 θ ∼ 34.4° and the reduction of the full width at half maximum value in the XRD rocking curve, which means that well-defined and c-axis oriented ZnO film was obtained. The relationship between the thickness of the SiO 2 layer between the ZnO buffer layers and Si substrates and the structural and optical properties of the ZnO films is discussed.

Post-annealing Induced Formation of ZnO Nanowires on the ZnO Films in the N2O Ambient

AbstractZnO films were grown on (0001) sapphire substrates by atomic layer deposition (ALD) using diethylzinc (DeZn) and nitrous oxide (N2O) in an inductively heated reactor operated at atmospheric pressure. Low-temperature (LT) ZnO buffer layers having various thicknesses were deposited at 400¢J followed by subsequent growth of ZnO films at 600¢J. Some of the ZnO films were then post-annealed at 1000¢J in the N2O flow. Under certain growth conditions, ZnO nanowires were formed on the post-annealed ZnO samples. Room temperature (RT) photoluminescence (PL) spectra of the ZnO nanowires show strong ultraviolet (UV) near band edge emissions at 3.27 eV with a typical full width at half-maximum ( FWHM ) of ~130 meV and quenched defect luminescence at 2.8 eV. 10 K PL spectra of the post-annealed ZnO all exhibit sharp excitonic emissions with the dominant emission being located at 3.36 eV having a FWHM of 4.6 meV.

Effects of low-temperature-grown ZnO buffer layer and Zn/O ratio on the properties of high-temperature-overgrown ZnO main layer on Si substrate by MBE

The undoped ZnO thin films grown on Si (1 1 1) substrates by plasma-assisted molecular beam epitaxy (P-MBE) were reported. The effects of growth temperature, the thickness of low temperature ZnO buffer layer and Zn/O ratio on the quality of high-temperature-overgrown ZnO main layer were studied by atomic force microscopy (AFM), X-ray diffraction (XRD) and room-temperature photoluminescence (PL) spectra. These results showed that it was difficult to directly obtain high quality ZnO films on Si substrate. By introducing a thin ZnO buffer layer at 350 °C, c-axis preferred orientation ZnO films with improved optical properties were obtained at 750 °C. However, the thickness of ZnO buffer layer and Zn/O ratio in the ZnO main layer greatly influenced the quality of high-temperature-overgrown ZnO main layer.

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

Electron transport in high quality undoped ZnO film grown by plasma-assisted molecular beam epitaxy

High quality ZnO films were grown on c-plane sapphire substrate using low temperature ZnO buffer layer by plasma-assisted molecular beam epitaxy. The film deposited at 720 °C showed the lowest value of full-width at half maximum for the symmetric (0002) diffraction peak of about 86 arcsec. The highest electron mobility in the films was about 103–105 cm 2/V s. From temperature-dependent Hall effect measurements, the mobility strongly depends on the dislocation density at low temperature region and the polar optical phonon scattering at high temperature, respectively. Moreover, by obtaining the activation energy of the shallow donors, it was supposed that hydrogen was source of n-type conductivity in as-grown ZnO films.

Theoretical investigation on the structural properties of ZnO grown on sapphire

We have investigated the growth mechanisms of ZnO on the c-plane sapphire(0001) substrates by the first-principles calculation. The first-principles total energy calculations of a sapphire slab with an isolated zinc adatom and an isolated oxygen adatom explain the experimental results such that the in-plane alignment between the ZnO and the sapphire is [10-10]ZnO//[11-20] sapphire. Moreover, from the experimental and theoretical results of GaN/sapphire growth mechanisms, the change of the polarity is very important for the migration barrier energy. The relative total energies for the isolated zinc adatom on the sapphire(0001) surface is very small while that for the isolated oxygen adatom on the sapphire(0001) surface is very large. When analogizing these relative energies with the experimental and theoretical results of GaN/sapphire growth mechanisms, we expect that the polarity of grown ZnO film on the sapphire substrate is the O-polarity. And the polarity of the direct growth of ZnO on the sapphire (0001) substrate does not change from the O-polarity under any growth conditions without using the low-temperature amorphous ZnO buffer layer. [DOI: 10.1380/ejssnt.2006.544]

The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-beam Epitaxy

ABSTRACTEpitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is -C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer.

ZnO growth on Si with low-temperature ZnO buffer layers by ECR-assisted MBE

High-quality ZnO films were grown on Si(1 0 0) substrates with low-temperature (LT) ZnO buffer layers by an electron cyclotron resonance (ECR)-assisted molecular-beam epitaxy (MBE). In order to investigate the optimized buffer layer temperature, ZnO buffer layers of about 1.1 μm were grown at different growth temperatures of 350, 450 and 550 °C, followed by identical high-temperature (HT) ZnO films with the thickness of 0.7 μm at 550 °C. A ZnO buffer layer with a growth temperature of 450 °C (450 °C-buffer sample) was found to greatly enhance the crystalline quality of the top ZnO film compared to others. The root mean square (RMS) roughness (3.3 nm) of its surface is the smallest, compared to the 350 °C-buffer sample (6.7 nm), the 550 °C-buffer sample (7.4 nm), and the sample without a buffer layer (6.8 nm). X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurements were carried out on these samples at room temperature (RT) in order to characterize the crystalline quality of ZnO films. The preferential c-axis orientations of (0 0 2) ZnO were observed in the XRD spectra. The full-width at half-maximum (FWHM) value of the 450 °C-buffer sample was the narrowest as 0.209°, which indicated that the ZnO film with a buffer layer grown at this temperature was better for the subsequent ZnO growth at elevated temperature of 550 °C. Consistent with these results, the 450 °C-buffer sample exhibits the highest intensity and the smallest FWHM (130 meV) of the ultraviolet (UV) emission at 375 nm in the PL spectrum. The ZnO characteristic peak at 438.6 cm −1 was found in Raman scattering spectra for all films with buffers, which is corresponding to the E 2 mode.