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The spinel zinc gallate ZnGa2O4 stands out among the emerging ultra-wide bandgap (∼5 eV) semiconductors as the ternary complex oxide with the widest gap where bipolar conductivity has been demonstrated. For power and energy electronics, a fundamental property of the material is its critical electric field ( E CR ) although, for ZnGa2O4, is yet unknown. In this work, highly resistive p-type ZnGa2O4 thin films on sapphire and Si substrates were grown by metal organic chemical vapor deposition to determine both, the remote acceptor concentration and vertical breakdown voltage. Hall Effect measurements confirmed a low carrier concentration at room temperature of ∼1011 cm−3. From vertical metal-semiconductor-metal structures the average E CR has been estimated to be of at least 5.3 MV cm−1, which already is significantly larger than the one of SiC and GaN.

The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics., This study was financially supported by Ministry of Science and Technology, Taiwan (MOST 109-2622-E-009-033, 108-2618-E-009-031, 109-2224-E-009-002, 109-2634-F-009-028, 109-2221-E-009-143-MY3, 108-2628-E-002-010-MY3) and the Higher Education SPROUT Project-Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University.

Ultra-wide bandgap gallium oxide (∼5 eV) has emerged as a novel semiconductor platform for extending the current limits of power electronics and deep ultraviolet optoelectronics at a predicted fraction of cost. Finding effective acceptor dopant for gallium oxide is a hot issue. One element that quite often is considered as a potential candidate is zinc. A number of experimental works have reported the signature of Zn-acceptor, but the direct evidence of hole conductivity was missing. In this work, p-type Zn-doped Ga2O3 thin films were grown by the metal-organic chemical vapour deposition technique on sapphire substrates. By high-temperature Hall effect measurements, Zn related acceptor level ionization energy as 0.77 eV above the valence band maximum was determined. Additionally, we have carried out the simulation study regarding the application of the Zn:Ga2O3 semi-insulating material, to be used as a guard ring for improving the high voltage performance of the Schottky diode structure., The ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706). GEMaC colleagues acknowledge financial support of the French National Agency of Research (ANR), project “GOPOWER” (No. CE-50 N0015-01).

Highly resistive undoped p-type gallium oxide samples were subjected to cumulative proton irradiation with energies ranging from 25 to 70 keV and doses in the 1.6 × 1014–3.6 × 1014 cm−2 range. Proton irradiation resulted in up to a factor of 2 reduction of minority electron diffusion length in the samples for temperatures between ∼ 300 and 400 K. Electron injection into the samples under test using a scanning electron microscope beam leads to pronounced elongation of diffusion length beyond the pre-irradiation values, thus demonstrating stable (days after injection) recovery of adverse radiation impact on minority carrier transport. The activation energy of 91 meV estimated from the temperature dependent diffusion length vs electron injection duration experiments is likely related to the local potential barrier height for native defects associated with the phenomenon of interest.

International audience; Antimony-doped ZnO layers have been grown by metalorganic vapour-phase epitaxy on sapphire and ZnO substrates at high-temperature (950 °C) and low-pressure conditions (50 torr). Nitrous oxide and diethyl-zinc have been used as oxygen and zinc precursors, respectively. The incorporation of antimony has been obtained from the decomposition of triethylantimony doping molecules added in the gas phase. High Sb concentrations were measured from 1019 to 1021 at/cm−3 using secondary ion mass spectroscopy and depend on the nature and the orientation on the substrate. Low-temperature photoluminescence spectra of Sb-doped layers exhibit donor–acceptor pair transitions at 3.253 eV. Unlike Raman spectra of nitrogen-doped ZnO layers which show several local vibrational modes related to nitrogen incorporation, these modes were found to be absent in the antimony-doped ZnO layers. Transmission electron microscopy suggests that the incorporation of Sb is partly related to dislocations and other structural defects. All together, the characterizations suggest the formation of acceptor dopant–defect complexes, such as SbZn-2VZn. Finally, ZnO:Sb/n-ZnO homojunction diodes have been successfully elaborated on ZnO substrate. The current–voltage characteristics of the device exhibit a rectifying behaviour with a turn-on voltage of 3 V.

International audience; Surface diffusion is known to be of prime importance in the growth of semiconductor nanowires. In this work, we used ZnMgO layers as markers to analyze the growth mechanisms and kinetics during the deposition of ZnMgO/ZnO multilayered shells by molecular beam epitaxy on previously grown ZnO nanowire cores (so called core-shell heterostructures). Specifically, the influence of the O2 flow sent into the plasma cell on the adatom surface mobility was investigated. By carefully measuring the growth rate on the lateral facets as well as on the top of the nanowires, it is concluded that the surface diffusion length of adatoms, within the used MBE growth conditions, is very low. Such poor surface mobility explains why so few works can be found related to the spontaneous growth (without catalyst) of ZnO nanowires by MBE, contrary to other deposition techniques.

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The family of spinel compounds is a large and important class of multifunctional materials of general formulation AB2X4 with many advanced applications in energy and optoelectronic areas such as fuel cells, batteries, catalysis, photonics, spintronics, and thermoelectricity. In this work, it is demonstrated that the ternary ultrawide-band-gap (∼5 eV) spinel zinc gallate (ZnGa2O4) arguably is the native p-type ternary oxide semiconductor with the largest Eg value (in comparison with the recently discovered binary p-type monoclinic β-Ga2O3 oxide). For nominally undoped ZnGa2O4 the high-temperature Hall effect hole concentration was determined to be as large as p = 2 × 1015 cm–3, while hole mobilities were found to be μh = 7–10 cm2/(V s) (in the 680–850 K temperature range). An acceptor-like small Fermi level was further corroborated by X-ray spectroscopy and by density functional theory calculations. Our findings, as an important step toward p-type doping, opens up further perspectives for ultrawide-band-gap bipolar spinel electronics and further promotes ultrawide-band-gap ternary oxides such as ZnGa2O4 to the forefront of the quest of the next generation of semiconductor materials for more efficient energy optoelectronics and power electronics., H.M. acknowledges the Cultural Affairs and Massion Sector, Egyptian Ministry for Higher Education, for her fellowship giving the possibility of work in France. E.d.C. acknowledges the Spanish MINECO Juan de la Cierva Fellowship JC-2015-25201. A.P.-T. acknowledges Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under contract ENE2015-74275-JIN. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2017-0706).

The metal–organic chemical vapor deposition growth of various Ga-doped ZnO nanostructures for plasmonics is investigated, with a particular focus on the nanowire facet transformations induced by the addition of trimethylgallium in the gas phase. For nonintentionally doped spontaneous ZnO nanowires, the aspect ratio is strongly decreased due to residual Ga in the reactor, and the shape evolves rapidly toward Christmas-tree-like and hierarchical structures upon intentional Ga doping. Regarding ZnO/ZnO:Ga core–shell structures, a change of the smooth initial M-oriented facets occurs, with the development of {2021} surfaces, and further {1011} and {0001} surfaces. Interestingly, a similar evolution of the lateral roughness is observed in Au-catalyzed doped nanowires. High concentrations of Ga in the grown nanostructures are revealed by photoluminescence and confirmed by Rutherford backscattering spectrometry. First photoacoustic measurements show an optical absorption at 6 μm, evidencing that the degenerate...

Which the actual critical electrical field of the ultra-wide bandgap semiconductor β-Ga2O3 is? Even that it is usual to find in the literature a given value for the critical field of wide and ultra-wide semiconductors such as SiC (3 MV/cm), GaN (3.3 MV/cm), β-Ga2O3 (~8 MV/cm) and diamond (10 MV/cm), this value actually depends on intrinsic and extrinsic factors such as the bandgap energy, material residual impurities or introduced dopants. Indeed, it is well known from 1950's that reducing the residual doping (NB) of the semiconductor layer increases the breakdown voltage capability of a semiconductor media (e.g. as 𝑁𝑁𝐵𝐵 −3⁄4 by using the Fulop's approximation for an abrupt junction). A key limitation is, therefore, the residual donor/acceptor concentration generally found in these materials. Here, we report that doping with amphoteric Zinc a p-type β-Ga2O3 thin films shortens free carrier mean free path (0.37 nm), resulting in the ultra-high critical electrical field of 13.2 MV/cm. Therefore, the critical breakdown field can be, at least, four times larger for the emerging Ga2O3 power semiconductor as compared to SiC and GaN. We further explain these wide-reaching experimental facts by using theoretical approaches based on the impact ionization microscopic theory and thermodynamic calculations., Hagar Mohammed would like to acknowledge Cultural Affairs and Massion Sector, Egyptian Ministry for Higher Education for her fellowship giving possibility work in France. APT acknowledges Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under contract ENE2015-74275-JIN. The ICN2 is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2017-0706).

In this paper, we study the growth of thin films of the Mg1–xCdxO alloy in the Mg-rich range of compositions by using the metal organic chemical vapour deposition (MOCVD) method at low pressure. X-ray diffraction (XRD) has been used to analyse the compound formation and the progressive incorporation of Cd2+ ions into the cubic MgO lattice. Both, layers with a single-cubic phase of Mg1–xCdxO and layers with a phase separation, where Cd1–xMgxO and Mg1–xCdxO coexist, have been studied. Finally, a morphological study of the layers has been carried out by using scanning electron microscopy (SEM) and the layers' composition has been measured by energy dispersive X-ray analysis (EDX). (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO nanowires have been grown by metalorganic chemical vapor deposition on sapphire substrates. The use of N2O and DEZn as oxygen and zinc precursors combined with high temperature leads to the spontaneous growth of vertical ZnO nanowires (NWs) on underlying three dimensional islands present at the bottom of each nanowire. These nanowires are grown along the c axis of the wurtzite structure, exhibit well defined m-plane facets and are free of extended defects. This configuration is favorable to band-gap engineering studies. Based on this, we have grown a series of radial ZnO/ZnMgO shells and quantum wells with different thicknesses and Mg concentrations. ZnxMg1-xO alloys have been grown using O2, (MCp)2Mg and DEZn as oxygen, magnesium and zinc precursors. Low temperature photoluminescence spectroscopy and transmission electron microscopy have assessed Mg concentrations in the shell up to 20 %. Electron diffraction reveals that the ZnMgO alloyed structure is, at least partially, ordered. Micro-photoluminescence performed on a single ZnO/ZnMgO coreshell nanowire including a quantum well exhibits luminescence properties, with a lateral quantum confined emission. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coaxially periodic ZnO/ZnMgO core-multishell nanowire (NW) heterostructures were grown via a metal organic chemical vapor deposition method. We investigated their electrical properties via the application of two locally resolved electrical scanning probe microscopy techniques, i.e., scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), following a planarization process. As a result, ZnO and ZnMgO layers can be unambiguously distinguished by both techniques on NWs with diameters

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International audience; Quantitative characterization of electrically active dopants and surface charges in nano-objects is challenging, since most characterization techniques using electrons [1–3], ions [4] or field ionization effects [5–7] study the chemical presence of dopants, which are not necessarily electrically active. We perform cathodoluminescence and voltage contrast experiments on a contacted and biased ZnO nanowire with a Schottky contact and measure the depletion length as a function of reverse bias. We compare these results with state-of-the-art off-axis electron holography in combination with electrical in situ biasing on the same nanowire. The extension of the depletion length under bias observed in scanning electron microscopy based techniques is unusual as it follows a linear rather than square root dependence, and is therefore difficult to model by bulk equations or finite element simulations. In contrast, the analysis of the axial depletion length observed by holography may be compared with three-dimensional simulations, which allows estimating an n-doping level of 1 × $10^{18}$ $cm^{−3}$ and negative sidewall surface charge of 2.5 × $10^{12}$ $cm^{−2}$ of the nanowire, resulting in a radial surface depletion to a depth of 36 nm. We found excellent agreement between the simulated diameter of the undepleted core and the active thickness observed in the experimental data. By combining TEM holography experiments and finite element simulation of the NW electrostatics, the bulk-like character of the nanowire core is revealed.

In this study, the epitaxial growth of ( 11 2 ¯ 0 ) (≡ a -plane) Zn (1− x ) Cd x O thin films on ( 01 1 ¯ 2 ) (≡ r -plane) sapphire substrates was performed by metal organic chemical deposition (MOCVD) with the same cadmium flow. The influence of growth temperature (850 °C≤ T g ≤950 °C) and position of sample in the MOCVD reactor on the microstructural and optical properties of Zn (1− x ) Cd x O thin films was investigated using high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM), optical transmission measurements and photoluminescence spectroscopy (PL). A maximum Cadmium incorporation about 5.5% is obtained at T g =850 °C for the sample placed as a nearest reactor source. X-ray diffraction study revealed that all films had wurtzite phase and a -plane≡ ( 11 2 ¯ 0 ) film are epitaxially grown on r -plane sapphire. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 3.10 eV for the highest Cd content (5.5%) at low temperature (2 K). The uniform striated morphology along the [0001] ZnO direction which is a typical feature observed in non-polar ZnO is lost with an increase in the Cd incorporation content. This morphology change is due to the crystal lattice deformation which is revealed by HRXRD.

International audience; The diffusion of excitons injected in ZnO/Zn 0.92 Mg 0.08 O quantum well heterostructures grown by metal-organic-vapor-phase-epitaxy on non-polar ZnO substrates is investigated at room temperature. Cathodoluminescence linescans in a field-emission-gun scanning-electronmicroscope are performed across cleaved cross-sections. A 55 nm diffusion length is assessed for excitons in bulk ZnMgO. When prepared as small angle bevels using focused ion beam (FIB), the effective diffusion length of excitons is shown to decrease down to 8 nm in the thinner part of the slab. This effect is attributed to non-radiative surface recombinations, with a 7 × 10 4 cm s−1 recombination velocity estimated at the FIB-machined ZnMgO surface. The strong reduction of the diffusion extent in such thin lamellae usually used for transmission electron microscopy could be use improve the spatial resolution of cathodoluminescence images, often limited by diffusion processes.

The characterization of nanowires (NWs) often requires the use of scanning electron beam techniques because of their high spatial resolution. However, the impact of the high energetic electron beam on the physical parameters under investigation is rarely taken into account. In this work, a combination of optical and electrical techniques is involved for the measurement of the electron beam dose (EBD) dependence of cathodoluminescence intensity, exciton diffusion length and electrical resistance in ZnO NWs. Large EBD dependences of these key parameters are observed and their reversibility is investigated. The results are discussed in terms of bulk and surface reversible modifications. In particular, the behaviors of surface recombination velocity and surface space charge under electron beam exposure are determined and simulated. This study points out that caution must be taken and experimental protocols must be well defined when measuring physical parameters of NWs using electron beam techniques.

Zn (1− x ) Cd x O solid solutions have been grown by us on Zn- and O-polar surfaces of ZnO substrate by metal organic chemical vapor deposition (MOCVD), with the same cadmium flow. We carried out photoluminescence spectroscopy (PL) and optical transmission measurements to investigate the incorporation of cadmium in the layers, as well as its influence on the optical properties. The lattice parameters and the morphology of these films were examined using high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). In our case, the Cd incorporation is obtained at 2.5% for Zn (1− x ) Cd x O grown on the Zn-polar surface of ZnO substrate, which is confirmed by the greatest energy shift (237 meV) toward lower energies of the PL emission, and by the appearance of a new peak located at 270 cm −1 in the Raman spectra. HRXRD studies show that all layers exhibit a wurtzite phase with c -axis orientation. In particular, regarding the Zn 0.975 Cd 0.025 O layer grown on Zn-face, the in-plane lattice parameter remains unchanged while the c -axis parameter is elongated to 5.399 A.

ZnO nanowires with different nitrogen concentrations were grown by metalorganic chemical vapour deposition (MOCVD) using DEZn, NO2 and NH3 as zinc, oxygen and nitrogen doping sources respectively. The NH3 concentration in the vapour phase varied from 0 to 13.8 μmol min−1 to favour the nitrogen incorporation. The ZnO nanowires were characterised by scanning electron microscopy, photoluminescence and micro-Raman spectroscopy. Two additional intense Raman peaks at 272 and 580 cm−1 are found to be related to the nitrogen incorporation. Micro-Raman scattering of a single nanowire probes the uniformity of the nitrogen concentration along the nanowire for highly doped samples. The photoluminescence (PL) spectra exhibit donor–acceptor pair band which confirms the incorporation of the nitrogen in the ZnO nanowires.

Whereas nanowire (NW)-based devices offer numerous advantages compared to bulk ones, their performances are frequently limited by an incomplete understanding of their properties where surface effect should be carefully considered. Here, we demonstrate the ability to spatially map the electric field and determine the exciton diffusion length in NW by using an electron beam as the single excitation source. This approach is performed on numerous single ZnO NW Schottky diodes whose NW radius vary from 42.5 to 175 nm. The dominant impact of the surface on the NW properties is revealed through the comparison of three different physical quantities recorded on the same NW: electron-beam induced current, cathodoluminescence, and secondary electron signal. Indeed, the space charge region near the Schottky contact exhibits an unusual linear variation with reverse bias whatever the NW radius. On the contrary, the exciton diffusion length is shown to be controlled by the NW radius through surface recombination. This systematic comparison performed on a single ZnO NW demonstrates the power of these complementary techniques in understanding NW properties.

Scanning spreading resistance microscopy (SSRM) was performed on non-intentionally doped (nid) ZnO nanowires (NWs) grown by metal-organic chemical vapor deposition in order to measure their residual carrier concentration. For this purpose, an SSRM calibration profile has been developed on homoepitaxial ZnO:Ga multilayer staircase structures grown by molecular beam epitaxy. The Ga density measured by SIMS varies in the 1.7 × 1017 cm−3 to 3 × 1020 cm−3 range. From measurements on such Ga doped multi-layers, a monotonic decrease in SSRM resistance with increasing Ga density was established, indicating SSRM being a well-adapted technique for two dimensional dopant/carrier profiling on ZnO at nanoscale. Finally, relevant SSRM signal contrasts were detected on nid ZnO NWs, and the residual carrier concentration is estimated in the 1–3 × 1018 cm−3 range, in agreement with the result from four-probe measurements.

We present Raman scattering study of wurtzite ZnO nanowires deposited by metal–organic chemical vapor deposition (MOCVD) on (0001) sapphire. It is shown that the misorientation of the nanowires, i.e. the inclination with respect to the vertical direction, mixes A1 and E1 Raman modes, giving rise to new modes for which the propagation directions make various angles with the C-axis of ZnO nanowires. Two particular bands depending on the tilt of the nanowires are observed at 400 and 585 cm−1. They are attributed to TO and LO quasi-phonons, and explained using Loudon's model. Morphological information of the nanowires was extracted and an average orientation of nanowires is calculated.

ZnO thin films were grown by metalorganic chemical vapor deposition on (0001) Zn- and O-polar surfaces of ZnO substrate. The surface morphology of these films was investigated by micro-Raman scattering and scanning electron microscopy. In the case of Zinc-polar face and using back scattering configuration, two new Raman bands were observed at 398 and 584.5 cm−1. They have been interpreted as quasi phonons TO (qTO) and LO (qLO) respectively. The origin of these modes was attributed to the particular morphology of ZnO films grown on Zn-polar face, which consists in c -axis nano-platelets, randomly tilted from the substrate. qTO and qLO frequencies have been calculated using the model of Loudon. SEM observation is in agreement with our Raman interpretation.

Nanowires with different nitrogen concentrations were grown by Metal-Organic Chemical Vapor Deposition (MOCVD) using DEZn, N2O and NH3as zinc, oxygen and nitrogen doping sources respectively. Low temperature photoluminescence, Raman spectroscopy and Transmission Electron Microscopy are combined to study the incorporation of nitrogen in the wires. The observation of donor-acceptor pair band confirms that the incorporation nitrogen in ZnO nanowires is responsible for the creation of acceptor centers. The additional peaks observed in Raman are correlated to nano-sized inter-atomic distance fluctuations observed in TEM. These domains combined with a resonance effect are probably the explanation of the huge Raman cross section observed for the impurity related peaks.

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As 2 or As 4 molecules. In a wide range of growth temperatures, As 4 leads to growth rates twice faster than As 2 . The shape of the NWs also depends on the arsenic species: with As 4 , regular rods can be obtained, while pencil-like shape results from growth with As 2 . From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As 2 flux as compared to that under As 4 flux. It follows that As 2 flux is favourable to the formation of radial heterostructures, whereas As 4 flux is preferable to maintain pure axial growth.

Nitrogen- and antimony-doped thin layers have been grown by metal organic vapour phase epitaxy (MOVPE) at 950 °C. The nitrogen- and antimony-doped layers have been characterized by Raman scattering and photoluminescence (PL) spectroscopy. In nitrogen-doped layers, we have observed several Raman lines from local vibrational modes related to nitrogen. Low temperature PL spectra exhibit donor-acceptor pair (DAP) transitions on both nitrogen- and antimony-doped layers. The DAP transitions have been analysed versus temperature and excitation power experiments. The acceptor binding energy of antimony in ZnO is evaluated to 200 meV.

Various one-dimensional (1D) ZnO-based nanostructures, including ZnO nano-wires (NWs) grown using vapour-liquid-solid (VLS) process, ZnO/ZnSe core/shell, nitrogen-doped ZnO and ZnMgO NWs were grown by metalorganic chemical vapour deposition (MOCVD). Transmission electron microscopy (TEM) analysis is presented. For all the samples, a high crystalline quality is observed. Some features are emphasized such as the gold contamination of ZnO wires grown under the metal droplets in the VLS process. It is concluded that MOCVD is a suitable technique for the realization of original ZnO nanodevices.

ZnO and ZnO:Sb films were deposited using low pressure metal organic chemical vapor deposition on C- and R-oriented sapphire and O-polar ZnO substrates. Surface morphologies were studied using scanning electron microscopy. Whereas ZnO films grown on C-sapphire show a rough surface and hexagonal rods, the doped ZnO:Sb layers exhibit a relatively smoother surface, indicating a possible surfactant effect of antimony. The secondary ion mass spectrometry measurements permit to determine the antimony profile in the doped layers. Sb concentrations from 10 18 to 10 20 at/cm 3 were measured, depending on the growth conditions and substrate nature. Photoluminescence spectra exhibit donor-acceptor pair emission at 3.22 eV.

Possibilities of obtaining laterally ordered arrays of GaAs nanowhiskers on GaAs (110) and GaAs(111)As surfaces during the molecular beam epitaxy are considered. As in the case of the GaAs(111)As substrate, nanowhiskers are formed in the hexagonal phase on the GaAs(110) surface, which is also confirmed by the patterns of the reflection high-energy electron diffraction (obtained during the growth of nanowhiskers) and by the photoluminescence spectra.

We bury vertical free-standing core-shell GaAs/AlGaAs nanowires by a planar GaAs overgrowth. As the nanowires get buried, their crystalline structure progressively transforms: whereas the upper emerging part retains its initial wurtzite structure, the buried part adopts the zinc blende structure of the burying layer. The burying process also suppresses all the stacking faults that existed in the wurtzite nanowires. We consider two possible mechanisms for the structural transition upon burying, examine how they can be discriminated from each other, and explain why the transition is favorable.

UV scanning photoluminescence spectroscopy is applied to 4H-SiC epitaxy characterization. In a first part, the technique is used for triangular defects characterization. The results show that two kinds of defects are present. Some defects are inclusion of cubic SiC which is confirmed by Raman spectroscopy. The other ones consist of staking faults of different thicknesses acting as quantum wells. In a second part, the effective lifetime profile is mapped for a whole 50 mm epitaxy using the variation of room temperature PL intensity versus excitation intensity.

The conduction through unintentionally doped ZnO nanowires grown by metalorganic chemical vapor deposition versus temperature (4 to 300 K) and nanowire radius (50 to 175 nm) was investigated. To obtain a reliable picture of the electrical conduction, controlled experimental conditions were used to master the influence of the surrounding environment. Conduction in the nanowires was found to be metallic through the core of the degenerate semiconductor. The origin of the high residual doping level [(1–5) × 1018 cm−3] is discussed in terms of incorporation of impurities during the growth process.

We report on the growth, structural and optical properties of GaN/AlN free standing NWs. The NW lateral growth rate was investigated using thin AlN marker layers. The lateral growth is found to be homogeneous along the NW axis. Its rate can be tuned from ∼1% to ∼10% of the axial growth rate by changing III/V flux ratio. The resulting axial and lateral heterostructures were studied by micro-photoluminescence and micro-cathodoluminescence, which revealed sharp (several meV broad) emission lines. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience; Current–voltage and Kelvin probe force microscopy (KPFM) measurements were performedon single ZnO nanowires. Measurements are shown to be strongly correlated with the contactbehavior, either Ohmic or diode-like. The ZnO nanowires were obtained by metallo-organicchemical vapor deposition (MOCVD) and contacted using electronic-beam lithography.Depending on the contact geometry, good quality Ohmic contacts (linear I–V behavior) ornon-linear (diode-like) contacts were obtained. Current–voltage and KPFM measurements onboth types of contacted ZnO nanowires were performed in order to investigate their behavior.A clear correlation could be established between the I–V curve, the electrical potential profilealong the device and the nanowire geometry. Some arguments supporting this behavior aregiven based on technological issues and on depletion region extension. This work will help tobetter understand the electrical behavior of Ohmic contacts on single ZnO nanowires, forfuture applications in nanoscale field-effect transistors and nano-photodetectors.

International audience; We present work on the fabrication and the characterization of ohmic contacts on single zinc oxide nanowires grown by metalorganic chemical vapour deposition. We demonstrate pre‐positionning of single nanowires before electronic‐beam lithography for 2 point metallic contacts. We characterize single nanowires by microphotoluminescence and by current‐voltage measurements. I–V results present a linear curve down to 1 mV with resistivities from 0.23 to 2.4 Ω ċ cm are measured, consistent with previous observation on ZnO nanowires.

ZnO/ZnO:Sb core-shell structured nanowires (NWs) were grown by the metal organic chemical vapor deposition method where the shell was doped with antimony (Sb) in an attempt to achieve ZnO p-type conduction. To directly investigate the Sb doping effect in ZnO, scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) were performed on the NWs' cross-sections mapping their two dimensional (2D) local electrical properties. Although no direct p-type inversion in ZnO was revealed, a lower net electron concentration was pointed out for the Sb-doped ZnO shell layer with respect to the non-intentionally doped ZnO core, indicating an evident compensating effect as a result of the Sb incorporation, which can be ascribed to the formation of Sb-related acceptors. The results demonstrate SCM/SSRM investigation being a direct and effective approach for characterizing radial semiconductor one-dimensional (1D) structures and, particularly, for the doping study on the ZnO nanomaterial towar...

We describe and analyze the growth of III-V semiconductor nanowires by molecular beam epitaxy activated with gold particles. We focus on (Al)GaAs(P) and InP(As) compounds. Optimal conditions of substrate surface preparation and adequate growth parameters are reported. The catalyst particles are shown to be rich in group-III atoms and liquid when nanowires form. The favorable growth temperatures thus depend on which group-III element is used. We explain why the nanowires often adopt the unusual wurtzite structure, pointing out that the crystalline phase is determined at the nucleation stage and that high liquid supersaturation is necessary. The kinetics of nanowire elongation is investigated with an original method based on modulations of the incident fluxes. The group-III flux intercepted by the nanowire sidewall facets is the main contribution to axial growth. Conditions leading to a short diffusion length of the group-III adatoms on these facets, produce lateral growth that we use to form abrupt core-shell heterostructures. We also present a method to bury vertical freestanding nanowires by a planar epitaxial growth. This process induces the layer-by-layer transformation of the nanowire phase from wurtzite to zinc-blende. Finally, the statistics of nucleation at the liquid-solid interface is revealed by using the flux modulation method. We show that the diluted concentration of group-V atoms in the nano-sized catalyst drop is at the origin of the self-regulation of the nucleation events.

International audience; Heteroepitaxial growths of Zn1−xCdxO films were performed on R-oriented sapphire substrates by metal-organic chemical-vapor deposition. The authors carried out secondary ion mass spectrometry analysis, scanning electron microscopy, photoluminescence, and ellipsometric measurements to investigate the incorporation of cadmium in the layers, as well as its influence on the optical properties. Compositions up to 5.5% Cd were obtained, tuning the photoluminescence emission from 3.36 (ZnO) to 3.1 eV and increasing the refractive index at 600 nm from 1.94 to 2.

The growth of inclined GaAs nanowires (NWs) during molecular beam epitaxy (MBE) on the rotating substrates is studied. The growth model provides explicitly the NW length as a function of radius, supersaturations, diffusion lengths and the tilt angle. Growth experiments are carried out on the GaAs(211)A and GaAs(111)B substrates. It is found that 20° inclined NWs are two times longer in average, which is explained by a larger impingement rate on their sidewalls. We find that the effective diffusion length at 550°C amounts to 12 nm for the surface adatoms and is more than 5,000 nm for the sidewall adatoms. Supersaturations of surface and sidewall adatoms are also estimated. The obtained results show the importance of sidewall adatoms in the MBE growth of NWs, neglected in a number of earlier studies.

We study theoretically and experimentally nonlinear effects during the ``vapor-liquid-solid'' growth of semiconductor nanowires. Nonlinear growth equation considered contains kinetic coefficients from the surface and sidewall diffusion which can be of either signs. We predict four possible growth scenarios: (I) infinite growth; (II) decomposition; (III) averaging growth to a finite length; and (IV) continuing growth such that nanowires of small initial length decay and longer nanowires grow infinitely. We present the experimental evidence of nontrivial scenarios (II) and (IV) during the Au-assisted molecular-beam epitaxy of GaAs nanowires. Scenario (II) corresponds to the evaporation of GaAs nanowires during the annealing. Scenario (IV) is observed during two-step growth procedure where a low-temperature growth is followed by deposition at $630\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. We show that the growth via scenario (IV) enables to control the crystal phase and to obtain the stacking-fault-free sections of zinc-blende GaAs NWs having only 15--20 nm in radius.

The use of ZnO template layers grown Pulsed Laser Deposition (PLD) has been seen to produce dramatic improvements in the surface morphology, crystallographic quality and optical properties of ZnO layers grown on c-sapphire substrates by Metal Organic Chemical Vapor Deposition. This paper provides complementary details on the PLD-grown ZnO template properties.

In this work, we report on the Au-assisted MBE of various InP and related material NWs on InP(111)B substrates. The influence of growth conditions on the InAsP insertion geometry and composition in InP nanowire is evidenced. Ex-situ thermal annealing produces significant changes to the PL spectra of these heterostructures. It is shown that InP/InAsP/InP heterostructures are well suited for fabrication of emitters in the telecommunication wavelength range 1.3-1.55 mum.

ZnO one-dimensional nanostructures were synthesized using the vapour-liquid-solid (VLS) process. Highly C-axis oriented ZnO nanowires, exhibiting a gold droplet on their top, were observed by scanning electron microscopy. Growth mechanisms are discussed, and the issue of supersaturation and precipitation of zinc in the gold droplet is debated. Transmission electron microscopy was carried out to investigate the crystalline properties and the polarity of the synthesized structures. In particular, an unexpected result shows that VLS ZnO nanowires grow with O-polarity, whereas spontaneous (catalyst-free) ZnO nanowires grow with Zn-polarity. The process at the origin of this polarity inversion is discussed.

Homoepitaxial growths of ZnO were performed by low pressure metal organic chemical vapor deposition (MOCVD) on both bulk hydrothermal and thin film buffer layers of ZnO grown by pulsed laser deposition on c-sapphire substrates. The surface roughness, photoluminescence, and x-ray diffraction (0002) peak omega rocking-curve line widths for the bulk and thin film substrates were similar prior to MOCVD growth. After MOCVD growth, it was found that the surface morphology, crystallographic quality, and optical properties of the ZnO layers were far superior to those typically observed for heteroepitaxial growth of ZnO layers on c-sapphire by MOCVD. The MOCVD layers were comparable for growths on both the bulk substrate and the thin film buffer layer. Furthermore, no significant impact of carrier gas was observed. The quality of the ZnO layer appeared to be correlated with the surface morphology and the crystalline quality of the substrate rather than being dependent on whether the substrate was in bulk or thin f...