Chemical Bath Deposition of ZnO/ZnGa2O4 Core-Shell Nanowire Heterostructures Using Partial Chemical Conversion.

The development of innovative heterostructures made of ZnO nanowires is of great interest for enhancing the performances of many devices in the fields of optoelectronics, photovoltaics, and energy harvesting. We report an original fabrication process to form ZnO/ZnGa2O4 core-shell nanowire heterostructures in the framework of the wet chemistry techniques. The process involves the partial chemical conversion of ZnO nanowires grown via chemical bath deposition into ZnO/ZnGa2O4 core-shell nanowire heterostructures with a high interface quality following their immersion in an aqueous solution containing gallium nitrate heated at a low temperature. The double-step process describing the partial chemical conversion relies on successive dissolution and reaction mechanisms. The present finding offers the possibility to fabricate ZnO/ZnGa2O4 core-shell nanowire heterostructures at low temperatures and over a wide variety of substrates with a large surface area, which is attractive for nanostructured solar cells, deep-UV photodetectors, and piezoelectric devices.

Exploring the degradation of silver nanowire networks under thermal stress by coupling in situ X-ray diffraction and electrical resistance measurements.

The thermal instability of silver nanowires (AgNWs) leads to a significant increase of the electrical resistance of AgNW networks. A better understanding of the relationship between the structural and electrical properties of AgNW networks is primordial for their efficient integration as transparent electrodes (TEs) for next-generation flexible optoelectronics. Herein, we investigate the in situ evolution of the main crystallographic parameters (i.e. integrated intensity, interplanar spacing and peak broadening) of two Ag-specific Bragg peaks, (111) and (200), during a thermal ramp up to 400 °C through in situ X-ray diffraction (XRD) measurements, coupled with in situ electrical resistance measurements on the same AgNW network. First, we assign the (111) and (200) peaks of χ-scans to each five crystallites within AgNWs using a rotation matrix model. Then, we show that the thermal transition of bare AgNW networks occurs within a temperature range of about 25 °C for the electrical properties, while the structural transition spans over 200 °C. The effect of a protective tin oxide coating (SnO2) on AgNW networks is also investigated through this original in situ coupling approach. For SnO2-coated AgNW networks, the key XRD signatures from AgNWs remain constant, since the SnO2 coating prevents Ag atomic surface diffusion, and thus morphological instability (i.e. spheroidization). Moreover, the SnO2 coating does not affect the strain of both (111) and (200) planes. The thermal expansion for bare and SnO2-coated AgNW networks appears very similar to the thermal expansion of bulk Ag. Our findings provide insights into the underlying failure mechanisms of AgNW networks subjected to thermal stress, helping researchers to develop more robust and durable TEs based on metallic nanowire networks.

Chemical Bath Deposition of α-GaOOH with Tunable Morphology on Silicon Using the pH Adjustment.

The growth of GaOOH by chemical bath deposition has received great attention over the past years as a first step to form Ga2O3 with the α- or ß-phases by combining a wet chemical route with thermal annealing in air. By using gallium nitrate and sodium hydroxide in aqueous solution, we show that the structural morphology of GaOOH deposits is thoroughly tunable in terms of both dimensions, density, and nature by varying the initial pH value from acidic to basic conditions. In the low-pH region associated with a low supersaturation level and where Ga3+ ions represent the dominant Ga(III) species, GaOOH microrods with a low aspect ratio and low density prevail. In the intermediate-pH region associated with a high supersaturation level and where GaOH2+ ions represent the dominant Ga(III) species, GaOOH prismatic nanorods with a high aspect ratio and high density are preferentially formed. In the high-pH region where Ga(OH)4- complexes are predominantly formed, the growth of partially crystallized GaOOH thin films with a typical thickness of about 1 µm proceeds. These findings show the correlation between the characteristics of the chemical bath and the resulting structural morphology of GaOOH deposits. They further open great perspectives to grow GaOOH and hence Ga2O3-based materials on silicon with a dedicated structural morphology using chemical bath deposition for engineering devices in the fields of gas sensing, solar-blind UV-C photodetection, and power electronics.

Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition.

The simultaneous co-doping of ZnO nanowires grown by chemical bath deposition is of high interest for a large number of engineering devices, but the process conditions required and the resulting physicochemical processes are still largely unknown. Herein, we show that the simultaneous co-doping of ZnO nanowires with Al and Ga following the addition of Al(NO3)3 and Ga(NO3)3 in the chemical bath operates in a narrow range of conditions in the high-pH region, where the adsorption processes of respective Al(OH)4- and Ga(OH4)- complexes on the positively charged m-plane sidewalls are driven by attractive electrostatic forces. The structural morphology and properties of ZnO nanowires are significantly affected by the co-doping and mainly governed by the effect of Al(III) species. The incorporation processes of Al and Ga dopants are characterized by significant interplay effects, and the amount of incorporated Ga dopants into ZnO nanowires is found to be larger than the amount of incorporated Al dopants owing to energetic considerations. The Al and Ga dopants are located in the bulk of ZnO nanowires, but a part of Al and Ga lies on their surfaces, their incorporation processes in the bulk being enhanced by thermal annealing under oxygen atmosphere. Eventually, the Al and Ga dopants directly affect the incorporation of hydrogen-related defects, notably by annihilating the formation of VZn-nH defect complexes. These findings present an efficient strategy to proceed with the co-doping of ZnO nanowires grown by chemical bath deposition, opening perspectives to control their electronic structure properties with a higher precision.

Optimization of the Sb2S3 Shell Thickness in ZnO Nanowire-Based Extremely Thin Absorber Solar Cells.

Extremely thin absorber (ETA) solar cells made of ZnO/TiO2/Sb2S3 core-shell nanowire heterostructures, using P3HT as the hole-transporting material (HTM), are of high interest to surpass solar cell efficiencies of their planar counterpart at lower material cost. However, no dimensional optimization has been addressed in detail, as it raises material and technological critical issues. In this study, the thickness of the Sb2S3 shell grown by chemical spray pyrolysis is tuned from a couple of nanometers to several tens of nanometers, while switching from a partially to a fully crystallized shell. The Sb2S3 shell is highly pure, and the unwanted Sb2O3 phase was not formed. The low end of the thickness is limited by challenges in the crystallization of the Sb2S3 shell, as it is amorphous at nanoscale dimensions, resulting in the low optical absorption of visible photons. In contrast, the high end of the thickness is limited by the increased density of defects in the bulk of the Sb2S3 shell, degrading charge carrier dynamics, and by the incomplete immersion of the P3HT in the structure, resulting in the poor hole collection. The best ETA solar cell with a short-circuit current density of 12.1 mA/cm2, an open-circuit voltage of 502 mV, and a photovoltaic conversion efficiency of 2.83% is obtained for an intermediate thickness of the Sb2S3 shell. These findings highlight that the incorporation of both the absorber shell and HTM in the core-shell heterostructures relies on the spacing between individual nanowires. They further elaborate the intricate nature of the dimensional optimization of an ETA cell, as it requires a fine-balanced holistic approach to correlate all the dimensions of all the components in the heterostructures.

Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping.

The controlled incorporation of dopants like copper into ZnO nanowires (NWs) grown by chemical bath deposition (CBD) is still challenging despite its critical importance for the development of piezoelectric devices. In this context, the effects of the addition of copper nitrate during the CBD of ZnO NWs grown on Au seed layers are investigated in detail, where zinc nitrate and hexamethylenetetramine are used as standard chemical precursors and ammonia as an additive to tune the pH. By combining thermodynamic simulations with chemical and structural analyses, we show that copper oxide nanocrystals simultaneously form with ZnO NWs during the CBD process in the low-pH region associated with large supersaturation of Cu species. The Cu(II) and Zn(II) speciation diagrams reveal that both species show very similar behaviors, as they predominantly form either X2+ ions (with X = Cu or Zn) or X(NH3)42+ ion complexes, depending on the pH value. Owing to their similar ionic structures, Cu2+ and Cu(NH3)42+ ions preferentially formed in the low- and high-pH regions, respectively, are able to compete with the corresponding Zn2+ and Zn(NH3)42+ ions to adsorb on the c-plane top facets of ZnO NWs despite repulsive electrostatic interactions, yielding the significant incorporation of Cu. At the highest pH value, additional attractive electrostatic interactions between the Cu(NH3)42+ ion complexes and negatively charged c-plane top facets further enhance the incorporation of Cu into ZnO NWs. The present findings provide a deep insight into the physicochemical processes at work during the CBD of ZnO NWs following the addition of copper nitrate, as well as a detailed analysis of the incorporation mechanisms of Cu into ZnO NWs, which are considered beyond the only electrostatic forces usually driving the incorporation of dopants such as Al and Ga.

Chemical Synthesis of ß-Ga2O3 Microrods on Silicon and Its Dependence on the Gallium Nitrate Concentration.

ß-Ga2O3 microrods have attracted increasing attention for their integration into solar blind/UV photodetectors and gas sensors. However, their synthesis using a low-temperature chemical route in aqueous solution is still under development, and the physicochemical processes at work have not yet been elucidated. Here, we develop a double-step process involving the growth of α-GaOOH microrods on silicon using chemical bath deposition and their further structural conversion to ß-Ga2O3 microrods by postdeposition thermal treatment. It is revealed that the concentration of gallium nitrate has a drastic effect on tuning the morphology, dimensions (i.e., diameter and length), and density of α-GaOOH microrods over a broad range, in turn governing the morphological properties of ß-Ga2O3 microrods. The physicochemical processes in aqueous solution are investigated by thermodynamic computations yielding speciation diagrams of Ga(III) species and theoretical solubility plots of GaOOH(s). In particular, the qualitative evolution of the morphological properties of α-GaOOH microrods with the concentration of gallium nitrate is found to be correlated with the supersaturation in the bath and discussed in light of the standard nucleation and growth theory. Interestingly, the structural conversion following the thermal treatment at 900 °C in air results in the formation of pure ß-Ga2O3 microrods without any residual minor phases and with tunable morphology and improved structural ordering. These findings reporting a double-step process for forming high-quality pure ß-Ga2O3 microrods on silicon open many perspectives for their integration onto a large number of substrates for solar blind/UV photodetection and gas sensing.

Morphology Transition of ZnO from Thin Film to Nanowires on Silicon and its Correlated Enhanced Zinc Polarity Uniformity and Piezoelectric Responses.

ZnO thin films and nanostructures have received increasing interest in the field of piezoelectricity over the last decade, but their formation mechanisms on silicon when using pulsed-liquid injection metal-organic chemical vapor deposition (PLI-MOCVD) are still open to a large extent. Also, the effects of their morphology, dimensions, polarity, and electrical properties on their piezoelectric properties have not been completely decoupled yet. By only tuning the growth temperature from 400 to 750 °C while fixing the other growth conditions, the morphology transition of ZnO deposits on silicon from stacked thin films to nanowires through columnar thin films is shown. A detailed analysis of their formation mechanisms is further provided. The present transition is associated with strong enhancement of their crystallinity and growth texture along the c-axis together with a massive relaxation of the strain in nanowires. It is also related to a prevailed zinc polarity, for which its uniformity is strongly improved in nanowires. The nucleation of basal-plane stacking faults of I1-type in nanowires is also revealed and related to an emission line at about 3.326 eV in cathodoluminescence spectra, further exhibiting fairly low phonon coupling. Interestingly, the transition is additionally associated with a significant improvement of the piezoelectric amplitude, as determined by piezoresponse force microscopy measurements. The Zn-polar domains exhibit a larger piezoelectric amplitude than the O-polar domains, showing the importance of controlling the polarity in these deposits as a prerequisite to enhance the performances of piezoelectric devices. The present findings demonstrate the high potential in using the PLI-MOCVD system to form ZnO with different morphologies and polarity uniformity on silicon. They further reveal unambiguously the superiority of nanowires over thin films for piezoelectric devices.

Electrical Properties of Low-Temperature Processed Sn-Doped In2O3 Thin Films: The Role of Microstructure and Oxygen Content and the Potential of Defect Modulation Doping.

Low-temperature-processed ITO thin films offer the potential of overcoming the doping limit by suppressing the equilibrium of compensating oxygen interstitial defects. To elucidate this potential, electrical properties of Sn-doped In 2 O 3 (ITO) thin films are studied in dependence on film thickness. In-operando conductivity and Hall effect measurements during annealing of room-temperature-deposited films, together with different film thickness in different environments, allow to discriminate between the effects of crystallization, grain growth, donor activation and oxygen diffusion on carrier concentrations and mobilities. At 200 ∘ C , a control of carrier concentration by oxygen incorporation or extraction is only dominant for very thin films. The electrical properties of thicker films deposited at room temperature are mostly affected by the grain size. The remaining diffusivity of compensating oxygen defects at 200 ∘ C is sufficient to screen the high Fermi level induced by deposition of Al 2 O 3 using atomic layer deposition (ALD), which disables the use of defect modulation doping at this temperature. The results indicate that achieving higher carrier concentrations in ITO thin films requires a control of the oxygen pressure during deposition in combination with seed layers to enhance crystallinity or the use of near room temperature ALD.

Formation mechanisms of ZnO nanowires on polycrystalline Au seed layers for piezoelectric applications.

ZnO nanowires are considered as attractive building blocks for piezoelectric devices, including nano-generators and stress/strain sensors. However, their integration requires the use of metallic seed layers, on top of which the formation mechanisms of ZnO nanowires by chemical bath deposition are still largely open. In order to tackle that issue, the nucleation and growth mechanisms of ZnO nanowires on top of Au seed layers with a thickness in the range of 5-100 nm are thoroughly investigated. We show that the ZnO nanowires present two different populations of nano-objects with a given morphology. The majority primary population is made of vertically aligned ZnO nanowires, which are heteroepitaxially formed on top of the Au (111) grains. The resulting epitaxial strain is found to be completely relieved at the Au/ZnO interface. In contrast, the minority secondary population is composed of ZnO nanowires with a significant mean tilt angle around 20° with respect to the normal to the substrate surface, which are presumably formed on the (211) facets of the Au (111) grains. The elongation of ZnO nanowires is further found to be limited by the surface reaction at the c-plane top facet in the investigated conditions. By implementing the selective area growth using electron beam lithography, the position of ZnO nanowires is controlled, but the two populations still co-exist in the ensemble. These findings provide an in-depth understanding of the formation mechanisms of ZnO nanowires on metallic seed layers, which should be taken into account for their more efficient integration into piezoelectric devices.

Integration of LaMnO3+δ films on platinized silicon substrates for resistive switching applications by PI-MOCVD.

The next generation of electronic devices requires faster operation velocity, higher storage capacity and reduction of the power consumption. In this context, resistive switching memory chips emerge as promising candidates for developing new non-volatile memory modules. Manganites have received increasing interest as memristive material as they exhibit a remarkable switching response. Nevertheless, their integration in CMOS-compatible substrates, such as silicon wafers, requires further effort. Here the integration of LaMnO3+δ as memristive material in a metal-insulator-metal structure is presented using a silicon-based substrate and the pulsed injection metal organic chemical vapour deposition technique. We have developed three different growth strategies with which we are able to tune the oxygen content and Mn oxidation state moving from an orthorhombic to a rhombohedral structure for the active LaMnO3+δ material. Furthermore, a good resistive switching response has been obtained for LaMnO3+δ-based devices fabricated using optimized growth strategies.

Well-ordered ZnO nanowires with controllable inclination on semipolar ZnO surfaces by chemical bath deposition.

Controlling the formation of ZnO nanowire (NW) arrays on a wide variety of substrates is crucial for their efficient integration into nanoscale devices. While their nucleation and growth by chemical bath deposition (CBD) have intensively been investigated on non-polar and polar c-plane ZnO surfaces, their formation on alternatively oriented ZnO surfaces has not been addressed yet. In this work, the standard CBD technique of ZnO is investigated on [Formula: see text] and [Formula: see text] semipolar ZnO single crystal surfaces. A uniform nanostructured layer consisting of tilted ZnO NWs is formed on the [Formula: see text] surface while elongated nanostructures are coalesced into a two-dimensional compact layer on the [Formula: see text] surface. By further combining the CBD with selective area growth (SAG) using electron beam-assisted lithography, highly tilted well-ordered ZnO NWs with high structural uniformity are grown on the [Formula: see text] patterned surface. The structural analysis reveals that ZnO NWs are homoepitaxially grown along the polar c-axis. The occurrence of quasi-transverse and -longitudinal optical phonon modes in Raman spectra is detected and their origin and position are explained in the framework of the Loudon's model. These results highlight the possibility to form ZnO NWs on original semipolar ZnO surfaces. It also opens the way for comprehensively understanding the nucleation and growth of ZnO NW arrays on poorly and highly textured polycrystalline ZnO seed layers composed of nanoparticles with a wide range of non-polar, semipolar, and polar plane orientations. Eventually, the possibility to tune both the inclination and dimensions of well-ordered ZnO NW arrays by using SAG on semipolar surfaces is noteworthy for photonic and optoelectronic nanoscale devices.

The initial stages of ZnO atomic layer deposition on atomically flat In0.53Ga0.47As substrates.

InGaAs is one of the III-V active semiconductors used in modern high-electron-mobility transistors or high-speed electronics. ZnO is a good candidate material to be inserted as a tunneling insulator layer at the metal-semiconductor junction. A key consideration in many modern devices is the atomic structure of the hetero-interface, which often ultimately governs the electronic or chemical process of interest. Here, a complementary suite of in situ synchrotron X-ray techniques (fluorescence, reflectivity and absorption) as well as modeling is used to investigate both structural and chemical evolution during the initial growth of ZnO by atomic layer deposition (ALD) on In0.53Ga0.47As substrates. Prior to steady-state growth behavior, we discover a transient regime characterized by two stages. First, substrate-inhibited ZnO growth takes place on InGaAs terraces. This leads eventually to the formation of a 1 nm-thick, two-dimensional (2D) amorphous layer. Second, the growth behavior and its modeling suggest the occurrence of dense island formation, with an aspect ratio and surface roughness that depends sensitively on the growth condition. Finally, ZnO ALD on In0.53Ga0.47As is characterized by 2D steady-state growth with a linear growth rate of 0.21 nm cy-1, as expected for layer-by-layer ZnO ALD.

Exploring the optical properties of Vernier phase yttrium oxyfluoride thin films grown by pulsed liquid injection MOCVD.

In this work, we report on the first successful deposition of Vernier phase yttrium oxyfluoride (V-YOF) thin films on Si (100) wafers using pulsed liquid injection metal organic chemical vapor deposition (PLI-MOCVD). The formation of V-YOF has been confirmed by X-ray diffraction measurements and electron probe microanalysis. The infrared phonon modes of V-YOF thin films and their corresponding optical constants as inferred from spectroscopic ellipsometry are reported here for the first time. The V-YOF thin films are featured by a broad absorption band centering around 370-373 cm-1; their refractive index is located between those of YF3 and Y2O3, and which shows an intimate relationship with the oxygen content in the film. The luminescence properties of Er3+ doped V-YOF thin films are finally examined and compared with that of YF3. Our results highlight that the as-deposited V-YOF thin films by PLI-MOCVD serve as promising candidates as efficient host material for spectral conversion for photovoltaic applications.

Identifying and mapping the polytypes and orientation relationships in ZnO/CdSe core-shell nanowire arrays.

Identifying and mapping the crystalline phases and orientation relationships on the local scale in core-shell ZnO nanowire heterostructures are of primary importance to improve the interface quality, which governs the performances of the nanoscale devices. However, this represents a major difficulty, especially when the expected polytypes exhibit very similar properties as in the case of CdSe. In the present work, we address that issue in ZnO nanowire heterostructures involving a uniform and highly conformal CdSe shell grown by molecular beam epitaxy. It is shown by x-ray diffraction and Raman spectroscopy through the occurrence of the (101̄0) and (101̄1) diffraction peaks and of the [Formula: see text] mode at 34 cm-1, respectively, that the CdSe shell is mostly crystallized into the wurtzite phase. By using automated crystal phase and orientation mapping with precession (ASTAR) in a transmission electron microscope and thus by benefiting from highly precise electron diffraction patterns, the CdSe shell is found to crystallize also into the minority zinc blende phase. The wurtzite CdSe shell is epitaxially grown on the top of ZnO nanowires, and some specific orientation relationships are mapped and revealed when grown on their vertical sidewalls. Zinc blende CdSe domains are also formed exclusively in the center of wurtzite CdSe grains located on the vertical sidewalls; both wurtzite and zinc blende CdSe crystalline phases have a strong orientation relationship. These findings reveal that ASTAR is a powerful technique to elucidate the structural properties on the local scale and to gain a deeper insight into their crystalline phases and orientation relationships, which is highly promising for many types of semiconducting nanowire heterostructures.

The quest towards epitaxial BaMgF4 thin films: exploring MOCVD as a chemical scalable approach for the deposition of complex metal fluoride films.

Conventional and Pulsed Liquid Injection MOCVD processes (C-MOCVD and PLI-MOCVD) have been explored as synthetic routes for the growth of BaMgF4 on Si (100) and single crystalline SrTiO3 (100) substrates. For the two applied approaches, the volatile, thermally stable ß-diketonate complexes Ba(hfa)2tetraglyme and Mg(hfa)2(diglyme)2(H2O)2 have been used as single precursors (C-MOCVD) or as a solution multimetal source (PLI-MOCVD). Structural characterization through X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) analyses confirmed the formation of epitaxial BaMgF4 films on SrTiO3 substrates. Energy dispersive X-ray (EDX) analyses have been used to confirm composition and purity of deposited films. The impact of process parameters on film properties has been addressed, highlighting the strong influence of precursor ratio, deposition temperature and oxygen partial pressure on composition, microstructure and morphology of the films. Both methods appear well suited for the growth of the BaMgF4 phase, but while PLI-MOCVD yields a more straightforward control of the precursor composition that reflects on film stoichiometry, C-MOCVD provides easier control of the degree of texturing as a function of temperature.

Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires.

The growth of semiconductor (SC) nanowires (NW) by CVD using Au-catalyzed VLS process has been widely studied over the past few years. Among others SC, it is possible to grow pure Si or SiGe NW thanks to these techniques. Nevertheless, Au could deteriorate the electric properties of SC and the use of other metal catalysts will be mandatory if NW are to be designed for innovating electronic. First, this article's focus will be on SiGe NW's growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGex NW has been successfully varied by modifying the gas flow ratio: R = GeH4/(SiH4 + GeH4). Characterization (by Raman spectroscopy and XRD) revealed concentrations varying from 0.2 to 0.46 on NW grown at 375°C, with R varying from 0.05 to 0.15. Second, the results of Si NW growths by CVD using alternatives catalysts such as platinum-, palladium- and nickel-silicides are presented. This study, carried out on a LPCVD furnace, aimed at defining Si NW growth conditions when using such catalysts. Since the growth temperatures investigated are lower than the eutectic temperatures of these Si-metal alloys, VSS growth is expected and observed. Different temperatures and HCl flow rates have been tested with the aim of minimizing 2D growth which induces an important tapering of the NW. Finally, mechanical characterization of single NW has been carried out using an AFM method developed at the LTM. It consists in measuring the deflection of an AFM tip while performing approach-retract curves at various positions along the length of a cantilevered NW. This approach allows the measurement of as-grown single NW's Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement.