M-plane ZnO Research Articles

NaCl flux growth of non-polar m-plane ZnO epitaxial thin film on c-plane sapphire substrate

We have developed a technique to fabricate non-polar m-plane ZnO thin films: the post NaCl flux treatment of Zn-containing precursors, such as amorphous zinc carbonate hydroxide, Zn5(CO3)2(OH)6. Conventionally, single-crystal m-plane sapphire or MgO (001) substrates have been utilized to fabricate m-plane ZnO thin films. In contrast, this method facilitates the growth of m-plane ZnO thin films on various substrates, such as sapphire and quartz glass, despite its c-plane growth habit. Furthermore, the m-plane ZnO thin film was demonstrated to epitaxially grow even on a c-plane sapphire substrate. Owing to the good lattice matching with the substrate, the m-plane ZnO epitaxial film had high crystallinity comparable to a c-plane ZnO epitaxial film that was created using a conventional pulsed laser deposition method, and exhibited similar photocatalytic activity to the c-plane ZnO film. This research offers valuable insights into the fundamental mechanisms of flux growth orientation control, which can be beneficial in designing ZnO-based electronic devices and photocatalysts.

Preparation of single crystalline AlN thin films on ZnO nanostructures by atomic layer deposition at low temperature

The growth of hetero-epitaxial ZnO-AlN core–shell nanowires (NWs) and single crystalline AlN films on non-polar ZnO substrate at temperature of 380 °C by atomic layer deposition (ALD) was investigated. Structural characterization shows that the AlN shells have excellent single-crystal properties. The epitaxial relationship of [0002]ZnO//[0002]AlN, and [10−10]ZnO//[10−10]AlN between ZnO core and AlN shell has been obtained. The ZnO NW templates were subsequently removed by annealing treatment in forming gas, resulting in ordered arrays of AlN single-crystal nanotubes. The impact factors on the epitaxial growth of AlN films are thoroughly investigated. It turned out that the growth parameters including lattice mismatch between substrate and AlN, growth temperature, and the polarity of ZnO substrate play important roles on the growth of single-crystal AlN films by ALD. Finally, non-polar AlN films with single-crystalline structure have been successfully grown on m-plane ZnO (10−10) single-crystal substrates. The as-grown hollow AlN nanotubes arrays and non-polar AlN films with single-crystalline structures are suggested to be highly promising for applications in nanoscale devices. Our research has developed a potential method to obtain other inorganic nanostructures and films with single-crystalline structure at fairly low temperature.

Identification by deuterium diffusion of a nitrogen-related deep donor preventing the p-type doping of ZnO

Deuterium diffusion is investigated in nitrogen-doped homoepitaxial ZnO layers. The samples were grown under slightly Zn-rich growth conditions by plasma-assisted molecular beam epitaxy on m-plane ZnO substrates and have a nitrogen content [N] varied up to 5 × 1018 at cm−3 as measured by secondary ion mass spectrometry (SIMS). All were exposed to a radio frequency deuterium plasma during 1 h at room temperature. Deuterium diffusion is observed in all epilayers, while its penetration depth decreases as the nitrogen concentration increases. This is strong evidence of a diffusion mechanism limited by the trapping of deuterium on a nitrogen-related trap. The SIMS profiles are analyzed using a two-trap model including a shallow trap, associated with a fast diffusion, and a deep trap, related to nitrogen. The capture radius of the nitrogen-related trap is determined to be 20 times smaller than the value expected for nitrogen–deuterium pairs formed by coulombic attraction between D+ and nitrogen-related acceptors. The (N2)O deep donor is proposed as the deep trapping site for deuterium and accounts well for the small capture radius and the observed photoluminescence quenching and recovery after deuteration of the ZnO:N epilayers. It is also found that this defect is by far the N-related defect with the highest concentration in the studied samples.

Surface modification by high-energy heavy-ion irradiation in various crystalline ZnO facets.

Self-assembled surface nanoscale structures on various ZnO facets are excellent templates for the deposition of semiconductor quantum dots and manipulation with surface optical transparency. In this work, we have modified the surface of c-, m- and a-plane ZnO single-crystals by high-energy W-ion irradiation with an energy of 27 MeV to observe the aspects of surface morphology on the optical properties. We kept ion fluences in the range from 5 × 109 cm-2 to 5 × 1011 cm-2 using the mode of single-ion implantation and the overlapping impact mode to see the effect of various regimes on surface modification. Rutherford backscattering spectroscopy in the channeling mode (RBS-C) and Raman spectroscopy have identified a slightly growing Zn-sublattice disorder in the irradiated samples with a more significant enhancement for the highest irradiation fluence. Simultaneously, the strong suppression of the main Raman modes and the propagation of the modes corresponding to polar Zn-O vibrations indicate disorder mainly in the O-sublattice in non-polar facets. The surface morphology, analysed by atomic force microscopy (AFM), shows significant changes after ion irradiation. The c- and a-plane ZnO exhibit the formation of small grains on the surface. The m-plane ZnO forms a sponge-like surface for lower fluences and grains for the highest fluence. The surface roughness itself increases with the irradiation fluence as shown by AFM measurement as well as spectroscopic ellipsometry (SE) analysis. The damage caused by high-energy irradiation leads to non-radiative processes and suppression of the near-band-edge peak as well as the deep-level emission peak in the photoluminescence spectra. Furthermore, the refraction index n and the extinction coefficient k of irradiated samples, determined by SE, have features corresponding to the particular exciton states blurred and are slightly lower in the optical bandgap region especially for the polar c-plane ZnO facet.

Terahertz Intersubband Electroluminescence from Nonpolar m-Plane ZnO Quantum Cascade Structures

The ZnO-based heterostructures are predicted to be promising candidates for optoelectronic devices in the infrared and terahertz (THz) spectral domains owing to their intrinsic material properties. Specifically, the large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, which is predicted to greatly improve the temperature performance of THz quantum cascade lasers. However, to date, no experimental observation of intersubband emission from ZnO optoelectronic devices has been reported. Here, we report the observation of THz intersubband electroluminescence from ZnO/MgxZn1–xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence peak shows a line width of ∼20 meV at a center frequency of ∼8.5 THz at 110 K, which is not accessible for GaAs-based quantum cascade structures because of the reststrahlen band absorption from 8 to 9 THz. This result is an important step toward the realization of ZnO-based THz quantum cascade lasers.

Creation of Gold Nanoparticles in ZnO by Ion Implantation-DFT and Experimental Studies.

Three different crystallographic orientations of the wurtzite ZnO structure (labeled as c-plane, a-plane and m-plane) were implanted with Au+ ions using various energies and fluences to form gold nanoparticles (GNPs). The ion implantation process was followed by annealing at 600 °C in an oxygen atmosphere to decrease the number of unwanted defects and improve luminescence properties. With regard to our previous publications, the paper provides a summary of theoretical and experimental results, i.e., both DFT and FLUX simulations, as well as experimental results from TEM, HRTEM, RBS, RBS/C, Raman spectroscopy and photoluminescence. From the results, it follows that in the ZnO structure, implanted gold atoms are located in random interstitial positions —experimentally, the amount of interstitial gold atoms increased with increasing ion implantation fluence. During ion implantation and subsequent annealing, the metal clusters and nanoparticles with sizes from 2 to 20 nm were formed. The crystal structure of the resulting gold was not cubic (confirmed by diffraction patterns), but it had a hexagonal close-packed (hcp) arrangement. The ion implantation of gold leads to the creation of Zn and O interstitial defects and extended defects with distinct character in various crystallographic cuts of ZnO, where significant O-sublattice disordering occurred in m-plane ZnO.

Effects of annealing temperature, thickness and substrates on optical properties of m-plane ZnO films studied by photoluminescence and temperature dependent ellipsometry

A series of m-plane ZnO films grown on sapphire and Si substrates with different thicknesses (186–371 nm) and different annealing temperatures (150–200 °C) were studied in detail by comparing X-ray diffraction, photoluminescence and spectroscopic ellipsometry measurements. The impact of thickness, annealing temperature and substrates on optical properties was investigated by the systematical analysis of the highly correlated stain, grain size, exciton effect, absorption tailing and crystallinity. Our finding indicates that the improvement of crystal quality along with the increase of grain size and refractive index for m-plane ZnO can be observed with increasing annealing temperature, increasing thickness or by introducing sapphire instead of Si substrate. The refractive index and bandgap have stronger dependence on annealing temperature than on thickness, causing the blue-shift of bandgap with rising annealing temperature despite thicker film. This bandgap shift is more pronounced for ZnO in ZnO/Si system. Photoluminescence combined with ellipsometry analysis demonstrate that higher annealing temperature can strengthen the excitonic feature. Bose-Einstein equation, employed to fit the dependence of bandgap on experimental temperature (300–583 K) illustrates that, the reduction of bandgap at elevated temperatures is more significant for ZnO in ZnO/sapphire system due to the corresponding stronger electron/exciton-phonon interactions involved with larger longitudinal optical phonon energies.

Coupling behaviors of large lattice mismatch interfaces between hexagonal ZnO and cubic (001)MgO

The combination of materials with large lattice mismatch due to different phase structures plays a significant role on designing devices. Precise control of heteroepitaxial thin films is still a big challenge because of the ambiguous understanding of the interfacial coupling behaviors. Here, by combing of molecular beam epitaxy and X-ray reflection diffraction techniques, as well as the density function theory method, we investigate the mechanisms of large mismatch interface coupling behaviors by taking ZnO film heteroepitaxial on cubic MgO(001) substrate. The X-ray reflection diffraction pole figures show a two- and four-fold rotations for the c- and m-plane ZnO films respectively. The density function theory calculations show the interface energy of c-plane ZnO is larger than that of the m-plane, and the interface azimuthal registry is determined by the interaction energy between epitaxy layers and substrates. Moreover, the evolution of the density of states crossover the interfaces shows a large band bending at the polar interface due to the internal spontaneous field.

Cathodoluminescence study of electric field induced migration of defects in single crystal m-plane ZnO

Internal electric fields can have a significant effect on the behavior of charged defects, dopants, and impurities in operating electronic devices that can adversely impact on their long-term performance and reliability. In this paper, we investigate the redistribution of charged centers in single crystal m-plane ZnO under the action of a DC electric field at 873 K using in-plane and in-depth spatially resolved cathodoluminescence (CL) spectroscopy. The CL intensities of the ultra-violet near band edge (NBE) emission at 3.28 eV and green luminescence (GL) at 2.39 eV were observed to both uniformly increase on the anode side of the electrode gap. Conversely, toward the cathode, the NBE and GL steadily decrease and increase, respectively. The GL quenched after hydrogen donor doping, confirming that the emission is related to acceptor-like centers. Based on the electro-migration and hydrogen doping results, the GL is attributed to radiative recombination involving ZniandVZn pairs. The intensity of an orange luminescence centered at 2.01 eV was unaffected by the electric field and is assigned to substitutional Li acceptors.

Non-polar ZnO facet implanted with Au ions and subsequently modified using energetic O ion irradiation

The possible precipitation or dispersion of Au was followed in non-polar ZnO (10-10) implanted with Au+ 1 MeV ions at the ion fluence 1.5 × 1016 cm−2 and subsequent annealing at 600 °C for 1 h in ambient atmosphere. Afterward irradiation using O3+ 10 MeV ions with fluence 5 × 1014 cm−2 was used to modify Au distribution and internal morphology as well as the structure modification of ZnO host matrix. Rutherford Backscattering Spectrometry (RBS) in channelling mode (RBS-C) and Raman spectroscopy show that the subsequent O irradiation with high electronic energy transfer slightly decreased Zn sub-lattice disorder in m-plane ZnO and increased significantly O sub-lattice disorder. Optical absorbance in the wavelengths ranging from 350 to 700 nm shows the increasing absorbance mainly between 450 and 500 nm and progressive shift of absorbance edge to the higher wavelengths after Au implantation and oxygen irradiation. The distinct absorbance curves appeared in the case where polarized light were used at angles 0° and 90° in oxygen irradiated ZnO:Au samples indicating Au dispersion modification. Transmission Electron Microscopy (TEM) with Energy Dispersive Spectroscopy (EDS) observed Au cluster morphology and an elemental composition showing Au rich clusters of the sizes 10–20 nm in the as-implanted and as-annealed samples containing probably ZnO nanocrystallites.

Distinct defect appearance in Gd implanted polar and nonpolar ZnO surfaces in connection to ion channeling effect

(0001) c-plane, (11-20) a-plane, and m-plane (10-10) ZnO bulk crystals were implanted with 400-keV Gd+ ions using fluences of 5 × 1014, 1 × 1015, 2.5 × 1015, and 5 × 1015 cm−2. Structural changes during the implantation and subsequent annealing were characterized by Rutherford backscattering spectrometry in channeling mode (RBS-C); the angular dependence of the backscattered ions (angular scans) in c-, a-, and m-plane ZnO was realized to get insight into structural modification and dopant position in various crystallographic orientations. X-ray diffraction (XRD) with mapping in reciprocal space was also used for introduced defect identification. Defect-accumulation depth profiles exhibited differences for c-, a-, and m-plane ZnO, with the a-plane showing significantly lower accumulated disorder in the deeper layer in Zn-sublattice, accompanied by the preservation of ion channeling phenomena in a-plane ZnO. Enlargement of the main lattice parameter was evidenced, after the implantation, in all orientations. The highest was evidenced in a-plane ZnO. The local compressive deformation was seen with XRD analysis in polar (c-plane) ZnO, and the tensile deformation was observed in nonpolar ZnO (a-plane and m-plane orientations) being in agreement with RBS-C results. Raman spectroscopy showed distinct structural modification in various ZnO orientations simultaneously with identification of the disordered structure in O-sublattice. Nonpolar ZnO showed a significant increase in disorder in O-sublattice exhibited by E2(high) disappearance and enhancement of A1(LO) and E1(LO) phonons connected partially to oxygen vibrational modes. The lowering of the E2(low) phonon mode and shift to the lower wavenumbers was observed in c-plane ZnO connected to Zn-sublattice disordering. Such observations are in agreement with He ion channeling, showing channeling effect preservation with only slight Gd dopant position modification in a-plane ZnO and the more progressive diminishing of channels with subsequent Gd movement to random position with the growing ion fluence and after the annealing in c-plane and m-plane ZnO.

The Dependence of Crystal Plane on Hydrogen Sensing Properties of ZnO Bulk Substrates

We investigated the dependence of ZnO crystal plane on hydrogen sensing properties using Pt Schottky diodes on nonpolar m-plane ZnO (m-ZnO) and Zn-polar c-plane (0001) ZnO (c-ZnO) bulk substrates. Both of the Pt/ZnO Schottky diode sensors showed the increase in forward and reverse currents when exposed to hydrogen gas at room temperature. The maximum relative current change of m-ZnO diode sensor was measured to be 2748%, which was 3 times higher than that of the c-ZnO diode (844%). The current responses of both the Pt/ZnO diode sensors increased almost linearly with log of hydrogen concentrations. The Pt/m-ZnO Schottky diode showed promising results for the use in hydrogen gas detection at room temperature, possibly due to more active sites available on m-ZnO surface for hydrogen adsorption.

Short infrared wavelength quantum cascade detectors based on m-plane ZnO/ZnMgO quantum wells

This paper reports on the demonstration of quantum cascade detectors (QCDs) based on ZnO/ZnMgO quantum wells (QWs) grown by molecular beam epitaxy on an m-plane ZnO substrate. The TM-polarized intersubband absorption is peaked at a 3 μm wavelength. The sample has been processed in the form of square mesas with sizes ranging from 10 × 10 μm2 up to 100 × 100 μm2. The I-V characteristics reveal that 86% of the 260 devices are operational and that the surface leakage current is negligible at room temperature, which is not the case at 77 K. The photocurrent spectroscopy of 100 × 100 μm2 QCDs reveals a photocurrent resonance at a 2.8 μm wavelength, i.e., slightly blue-shifted with respect to the intersubband absorption peak. The photocurrent persists up to room temperature. The calibrated peak responsivity amounts to 0.15 mA/W under irradiation at Brewster's angle of incidence. This value allows us to estimate the transfer efficiency (1.15%) of the photoexcited electrons into the active QW of the next period.

Epitaxially grown semi-vertical and aligned GaTe two dimensional sheets on ZnO substrate for energy harvesting applications

Two-dimensional (2D) materials are particularly interesting for increasing the efficiency of optoelectronic devices. Here, we report a solar cell structure based on 2D GaTe nanosheets on m-plane ZnO substrate. Specifically, the 2D GaTe nanosheets form a semi-vertical/tilted array to combine sufficient solar light absorption and efficient carrier conduction. The formed GaTe layer of about 70 nm and the basal plane have a tilted angle of 53 degrees with respect to the substrate. Such an alignment is found to be induced by the interface lattice mismatch between GaTe and ZnO. The heterojunction photovoltaic device achieves a power conversion efficiency (PCE) of 6.64% demonstrating that the controlled integration of 2D materials in 3D configurations is promising for increasing the efficiency in various kinds of energy devices.

Damage formation and Er structural incorporation in m-plane and a-plane ZnO

The various crystallographic orientations in semiconductors as ZnO exhibit different resistivity under the ion beam irradiation/implantation. Study of the various crystallographic orientations is mandatory for nano-structured semiconductor system development. This paper reports on the implantation damage build-up, structural modification and Er dopant position in a-plane and m-plane ZnO implanted with Er+ 400 keV ions at the ion fluences 5 × 1014, 2.5 × 1015, 5 × 1015 cm−2 and subsequently annealed at 600 °C in O2 atmosphere using Rutherford Back-Scattering spectrometry (RBS) in channelling mode as well as using Raman spectroscopy. Strongly suppressed surface damage formation was observed in both crystallographic orientations compared to the deep damage growth with the increased ion implantation fluence. More progressive damage accumulation appeared in m-plane ZnO compared to a-plane ZnO. Simultaneously, the strong Er out-diffusion depth profile in m-plane ZnO accompanied by the damage accumulation at the surface was observed after the annealing. Contrary, the surface recovery accompanied by Er concentration depth profiles keeping a normal distribution with a small maximum shift to the surface was observed in a-plane ZnO. Different structure recovery and Er behaviour was evidenced in a-plane and m-plane ZnO by RBS-C, moreover Raman spectroscopy proved a lower damage at higher ion fluences introduced in a-plane ZnO compared to m-plane. The structure modifications were discussed in connection with a damage accumulation and Er concentration depth profile shape in various ZnO crystallographic orientations in as-implanted and as-annealed samples.

Homoepitaxy of non-polar ZnO/(Zn,Mg)O multi-quantum wells: From a precise growth control to the observation of intersubband transitions

We have developed a method to grow and characterize the state of the art non-polar ZnO/(Zn,Mg)O multi-quantum wells on m-plane ZnO substrates as a prerequisite for applications based on intersubband transitions. The epilayer interfaces exhibit a low roughness, and the layer thickness remains constant within one monolayer in these heterostructures. The optical properties have been studied in the UV and IR domains by means of photoluminescence and absorption experiments, respectively. In the UV, the photoluminescence is very well described by an excitonic transition, with the clear effect of quantum confinement as a function of the well thickness in the absence of the internal field. In the IR, the intersubband transitions can be precisely modeled if a large depolarization shift is taken into account. Overall, we demonstrate a very good control in the design and fabrication of ZnO quantum wells (QWs) for intersubband transitions. Our result gives a clear understanding of the ISBTs in ZnO QWs.

Pulsed sputtering epitaxial growth of m-plane InGaN lattice-matched to ZnO

m-Plane GaN and InGaN films were grown on m-plane ZnO substrates at ~350 °C by pulsed sputtering deposition. It was found that the critical thickness of the m-plane GaN films grown on ZnO lies between 25 and 62 nm, whereas 180-nm-thick m-plane In0.12Ga0.88N can be coherently grown on ZnO substrates, which is explained well by theoretical calculations based on an energy-balance model. The coherently grown m-plane InGaN on ZnO exhibited narrow X-ray rocking curves compared with the m-plane GaN grown on ZnO. These results demonstrate the benefit of lattice-matched ZnO substrates for epitaxy of high-quality nonpolar InGaN films.

Metal/nonpolar m-plane ZnO contacts with and without thin Al2O3 interlayer deposited by atomic layer deposition

Using the temperature dependent current–voltage (I–V) measurements, the electrical properties of Au/nonpolar m-plane ZnO Schottky diodes with an Al2O3 interlayer prepared by atomic layer deposition (ALD) was investigated. With an Al2O3 interlayer, it was found to have higher barrier heights and higher rectifying ratio. Modified Richardson plots produced effective Richardson constants of 30.0 and 37.6 Acm−2K−2 for the samples with and without Al2O3 interlayer, respectively, which are similar to the theoretical value of 32.0 Acm− 2K− 2 for n-ZnO. Scanning transmission electron microscope (STEM) results showed that the oxygen-contained layer on ZnO surface degraded the film quality of subsequently deposited Al2O3 layer. In addition, the inter-diffusion of Au and Al atoms into ZnO subsurface region also modulated the electrical properties of Au/ZnO contacts.

Asymmetric ZnO/ZnMgO double quantum well structures grown on m-plane ZnO substrates by MBE

Asymmetric ZnO/ZnMgO double quantum well (QW) structures grown on non-polar m-plane ZnO bulk substrates are investigated. Pairs of 2 and 5nm wide ZnO QWs were grown on ZnMgO buffer/barrier layers with Mg concentrations over 20%. The QWs are separated with 7nm or 20nm ZnMgO barriers. Temperature dependent photoluminescence (PL) and low temperature cross-sectional cathodoluminescence allowed to identify the origin of the observed spectral lines. Well resolved and narrow luminescence lines due to localized and free excitons are observed, confirming excellent quality of the structures. The results of luminescence studies suggest that at the barrier width of 7nm coupling between the wells enhances excitonic emission in 5nm well via tunneling of charge carriers from 2nm to 5nm QW. No trace of inter-well coupling in a structure with a 20nm thick barrier is observed. It is found that the binding energies of excitons in 2 and 5nm wells are 52 and 101 meV, respectively. Such large enhancement of the binding energies is due to the quantum confinement in the wells.

Water-modulated oxidation in the growth of m-ZnO epitaxial thin film by atomic layer deposition

The effects of extra H2O-modulated oxidation are reported on the structural, optical, and electrical properties of nonpolar m-plane ZnO thin films grown on m-plane Al2O3 substrates by atomic layer deposition. Films without modulation, one modulated layer, and two modulated layers are compared. Structural properties studied using x-ray reflectivity, x-ray diffraction, and transmission electron microscopy show that all the films have a largely similar thickness and epitaxial relations with their substrates, but the rocking curves grow broader as the number of modulations increases. However, the extra layer of water modulation reduces the surface roughness drastically and also improves the electrical properties as compared to the unmodulated ZnO films. Water modulation is believed to serve as a source of atomic oxygen that promotes compensation of the pre-existing oxygen vacancies. The films tend to exhibit larger mosaicity around the a-axis as compared to that around the c-axis.