M-plane ZnO Substrates Research Articles

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.

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.

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.

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.

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.

Homoepitaxial nonpolar (10-10) ZnO/ZnMgO monolithic microcavities: Towards reduced photonic disorder

Nonpolar ZnO/ZnMgO-based optical microcavities have been grown on (10-10) m-plane ZnO substrates by plasma-assisted molecular beam epitaxy. Reflectivity measurements indicate an exponential increase of the cavity quality factor with the number of layers in the distributed Bragg reflectors. Most importantly, microreflectivity spectra recorded with a spot size in the order of 2 μm show a negligible photonic disorder (well below 1 meV), leading to local quality factors equivalent to those obtained by macroreflectivity. The anisotropic character of the nonpolar heterostructures manifests itself both in the surface features, elongated parallel to the in-plane c direction, and in the optical spectra, with two cavity modes being observed at different energies for orthogonal polarizations.

Inversion of absorption anisotropy and bowing of crystal field splitting in wurtzite MgZnO

The anisotropic optical properties of wurtzite MgxZn1−xO thin films (0≤x≤0.45) grown on m-plane ZnO substrates by plasma assisted molecular beam epitaxy are studied using spectroscopic ellipsometry at room temperature. The data analysis provides the dielectric functions for electric field polarizations perpendicular and parallel to the optical axis. The splitting between the absorption edges of the two polarization directions decreases between x = 0 and x = 0.24, while an inverted absorption anisotropy is found at higher Mg content, indicating a sign change of the crystal field splitting Δcr as for the spin orbit parameter. The characteristic energies such as exciton binding energies and band gaps are determined from the analysis of the imaginary parts of the dielectric functions. In particular, these data reveal a bowing parameter of b=−283 meV for describing the compositional dependence of the crystal field splitting and indicate Δcr=−327 meV for wurtzite MgO. The inverted valence band ordering of ZnO (Γ7-Γ9-Γ7) is found to be preserved with increasing Mg content, while the optical selection rules interchange.

Photoluminescence studies of (Mg, Zn)O epilayers via metalorganic vapor phase epitaxy on m-plane ZnO substrates

Hexagonal wurtzite MgxZn1−xO intrinsic and gallium-doped epilayers were deposited on m-plane ZnO substrates by metalorganic vapor phase epitaxy. We demonstrate a linear dependence in the energy gap with increasing Mg concentration for n-type alloys 0.10 ≤ x ≤ 0.30 and for intrinsic alloys 0.20 ≤ x ≤ 0.50. We show a comparison of m-plane and c-plane MgxZn1−xO indicating a dependence of the electronic activity of Mg with crystallographic orientation. Further, we demonstrate an increase in exciton localization associated with the strong enhancement of photoluminescence intensity and reduction in the near band-edge full-width at half–maximum with increasing Mg concentration. Additionally, the thermal activation energy corresponding to the localization is considered.

InGaN growth with various InN mole fractions on m-plane ZnO substrate by metalorganic vapor phase epitaxy

We have demonstrated In x Ga 1− x N epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate.

Epitaxial growth of nonpolar AlN films on ZnO substrates using room temperature grown GaN buffer layers

The authors have grown nonpolar AlN layers on m-plane ZnO substrates using pulsed laser deposition and investigated their structural properties. The direct growth of AlN on ZnO substrates at 750°C results in the formation of polycrystalline materials due to significant interfacial reactions between AlN and ZnO. On the other hand, m-plane AlN was grown epitaxially on the ZnO substrates by using a GaN buffer layer prepared at room temperature (RT). The full width at half maximum value for AlN 11¯00 x-ray rocking curve was determined to be 468arcsec. Grazing incidence angle x-ray reflectivity measurements revealed that the heterointerface between AlN and RT GaN is quite abrupt. X-ray diffraction measurements revealed that the in-plane epitaxial relationship is ⟨0001⟩AlN‖⟨0001⟩GaN‖⟨0001⟩ZnO. These results indicate that the use of the RT GaN buffer layer makes it possible to take full advantage of small lattice mismatches and the wurtzite structure of the ZnO substrates.

Room temperature epitaxial growth of m-plane GaN on lattice-matched ZnO substrates

The authors have grown high-quality m-plane GaN (11¯00) films on ZnO (11¯00) substrates using pulsed laser deposition. They have found that annealing in a box made of ceramic ZnO improves the surface morphology of m-plane ZnO substrates and have succeeded in the layer-by-layer growth of m-plane GaN on the annealed ZnO substrates even at room temperature (RT). X-ray diffraction reveals that high crystalline quality m-plane GaN grows on the RT buffer layer at 700°C. The 300-nm-thick m-plane GaN film grown on the ZnO substrate contains the residual strains because the lattice mismatches between them are quite small. The relationship of in-plane and out-of-plane strains is consistent with the calculation using the elastic constants.