Ga-rich Layer Research Articles

Air stable plasma passivation of GaAs at room temperature

GaAs surfaces require electrical and chemical passivation for semiconductor devices, but in order to have air stable passivation, high temperatures have been previously required in the passivation step. Here, we demonstrate air-stable, ex situ plasma passivation of GaAs using consecutive hydrogen and nitrogen plasmas at room temperature. No pre-clean using deoxidizing wet chemistry or other means is required. The hydrogen plasma step removes surface oxides and As, which leaves a Ga-rich layer that the nitrogen plasma then turns to GaN. The formed GaN layer efficiently passivates the surface. The plasma-passivated GaAs shows upto 5× room-temperature photoluminescence after 1 year, and room-temperature time-resolved photoluminescence demonstrates robust passivation even after 3 years, both comparisons to similarly aged unpassivated GaAs. Atomic force microscopy was used to confirm that the passivated surfaces can be made smooth enough for microelectronic applications. Grazing incidence x-ray diffraction indicated that the nitride films are amorphous, and energy-dispersive x-ray spectroscopy was used to estimate the nitrogen content. We used a common inductively coupled plasma reactive ion etching system for plasma passivation, thus enabling the rapid adoption of this technique.

Reversible Mg-Metal Batteries Enabled by a Ga-Rich Protective Layer through One-Step Interface Engineering.

Practical applications of Mg-metal batteries (MMBs) have been plagued by a critical bottleneck─the formation of a native oxide layer on the Mg-metal interface─which inevitably limits the use of conventional nontoxic electrolytes. The major aim of this work was to propose a simple and effective way to reversibly operate MMBs in combination with Mg(TFSI)2-diglyme electrolyte by forming a Ga-rich protective layer on the Mg metal (GPL@Mg). Mg metal was carefully reacted with a GaCl3 solution to trigger a galvanic replacement reaction between Ga3+ and Mg, resulting in the layering of a stable and ion-conducting Ga-rich protective film while preventing the formation of a native insulating layer. Various characterization tools were applied to analyze GPL@Mg, and it was demonstrated to contain inorganic-rich compounds (MgCO3, Mg(OH)2, MgCl2, Ga2O3, GaCl3, and MgO) roughly in a double-layered structure. The artificial GPL on Mg was effective in greatly reducing the high polarization for Mg plating and stripping in diglyme-based electrolyte, and the stable cycling was maintained for over 200 h. The one-step process suggested in this work offers insights into exploring a cost-effective approach to cover the Mg-metal surface with an ion-conducting artificial layer, which will help to practically advance MMBs.

Enhanced oxidation resistance of Ce by addition of Ga and nanocrystallization

Ce is prone to catastrophic oxidation at room temperature and its oxidation resistance is difficult to be improved by alloying. Herein, we found that the oxidation resistance of active metal Ce can be significantly improved by the addition of 20 at.% Ga. Focused ion beam lift-out technique and scanning transmission electron microscopy analysis disclosed that a discontinuous Ga-rich layer was generated beneath the oxide layer in the coarse-grained Ce-Ga alloy. The Ga-rich layer formed by selective oxidation of Ce acts as a diffusion barrier for Ce outward diffusion and ceases the O/M interfacial reaction when a critical concentration of Ga (75 at.%) is reached. After nanocrystallization, uniform distribution of Ga was achieved. After oxidation, a relatively continuous Ga-rich layer was formed which further enhanced the oxidation resistance. The introduction of noble elements combining with nanocrystallization may provide a novel strategy for the protection of metals with high activity and poor oxidation resistance.

(Invited) (Photo)Electrochemical Cells for Hydrogen Production and Carbon Dioxide Utilization

Photoelectrochemical (PEC) and electrochemical cells can produce hydrogen from water and/or can produce useful chemicals from carbon dioxide, and, thus, are the key technologies for construction of carbon neutral society.Direct water splitting using photoelectrochemical cell is one of the promising means to produce hydrogen utilizing solar energy. The most important issue for photoelectrode the central of photoelectrochemical cell is narrow bandgap combined with large reaction driving force. Cu(In,Ga)Se2 (CIGS) which is employed as an absorber material in photovoltaic devices is one of the promising photocathode materials because of its long absorption edge of >1000 nm. [1] However, its driving force for water splitting is limited because of relatively shallow valence band maximum (VBM). The solid solution between ZnSe and CIGS (ZnSe-CIGS) is one of the promising candidates of photocathode material for water splitting because of its long absorption edge, ~900 nm, and large driving force, ~1.0 V. [2] In the present study, we investigated introduction of tellurium during ZnSe-CIGS thin films, and found the tellurium introduction increase grain size of ZnSe-CIGS film. The sequential deposition of Ga-rich layer and In-rich layer resulted in formation of composition gradient which facilitate charge separation thorough conduction band minimum (CBM) gradient. The ZnSe-CIGS base photocathode prepared with employing tellurium introduction and composition gradient showed significantly increased incident photon-to-current conversion efficiencies (IPCEs), close to unity. [3]The electrochemical cell with gas diffusion electrode (GDE) for carbon dioxide reduction reaction (CO2RR) can produce useful chemicals efficiently. Copper species are the catalysts with capable of producing C2 products such as C2H5OH and C2H4. In the present study, Cu2O was examined as an electrocatalyst for CO2RR. A GDE composed of carbon paper coated with Cu2O by electroplating method showed C2H4 production with faradaic efficiency (FE) of >50% and C2H5OH production with FE of >20% under the optimized conditions for >10 hours.References H. Kumagai, T. Minegishi, N. Sato, T. Yamada, J. Kubota, K. Domen, J. Mater. Chem. A, 3, 8300 (2015). H. Kaneko, T. Minegishi, M. Nakabayashi, N. Shibata, K. Domen, Angew.Chem. Int. Ed. 55,15329 (2016). T. Minegishi, S. Yamaguchi, M. Sugiyama, Appl. Phys. Lett. 119, 123905 (2021).

Efficient hydrogen evolution from water over thin film photocathode composed of solid solutions between ZnSe and Cu(In, Ga)Se2 with composition gradient

Solid solutions between ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) have promising properties as photocathodes for solar hydrogen production from water, such as a long absorption edge of about 900 nm and a large driving force for reaction, >0.9 V, originated by a deep valence band maximum of 1.0–1.1 V vs normal hydrogen electrode (NHE). However, their performance is limited with an incident photon-to-current conversion efficiency (IPCE) of 65% at 400 nm and a half-cell solar-to-hydrogen energy conversion efficiency (HC-STH) of 3.6% so far. Te addition during ZnSe:CIGS thin film deposition by vacuum co-evaporation clearly decreased the optimal deposition temperature from 450 to 380 °C and resulted in columnar shaped grains of submicrometer size in diameter and with almost no grain boundary between the film surface and the backside electrode of Mo, which is significantly larger than the case of without Te addition. Interestingly, a Ga/In compositional ratio gradient was further introduced to the depth profile, which can facilitate charge separation. Structural characterizations using XRD and cross-sectional transmission electron microscopy revealed that the composition gradient was mainly formed by the diffusion of In through grain boundaries in the Ga-rich layer, and thus, the mixing between the Ga-rich and In-rich layers was more significant at a lower deposition temperature owing to the smaller grain size. The photocathode fabricated from the ZnSe:CIGS thin film with the composition gradient showed a very high IPCE of 89% at 540 nm and 0 V vs the reversible hydrogen electrode, and HC-STH of 3.7%, which is higher than values reported thus far. In conclusion, it is clarified that the potential gradient, which can be introduced by composition gradient, is beneficial for photoelectrodes and photocatalysts to achieve higher performance.

In situ surface analysis of an ion-energy-dependent chlorination layer on GaN during cyclic etching using Ar+ ions and Cl radicals

Gallium nitride (GaN) semiconductor devices must be fabricated using plasma etching with precise control of the etching depths and minimal plasma-induced damage on the atomic scale. A cyclic process comprising etchant adsorption and product removal may be suitable for this purpose but an understanding of the associated etching surface reactions is required. The present work examined the formation of a chlorinated layer based on Cl radical adsorption on a GaN surface in conjunction with Ar ion irradiation. This research employed beam experiments and in situ x-ray photoelectron spectroscopy. The results show that N atoms are preferentially desorbed during exposure to Ar ions to produce Ga-rich layers at depths of 0.8 and 1.1 nm at an Ar ion dosage on the order of 1016 cm−2 and ion energies of 116.0 and 212.6 eV, respectively. Subsequent exposure of the irradiated Ga-rich layer to Cl radicals removes some Ga atoms and produces a chlorinated layer over the GaN surface. This chlorinated layer has a thickness on the order of 1 nm following Cl radical dosages on the order of 1019 cm−2. This study of plasma-treated surfaces is expected to assist in developing means of controlling the etching depth during the atomic layer etching of GaN via Ar ion bombardment.

Unveiling the Different Physical Origins of Magnetic Anisotropy and Magnetoelasticity in Ga-Rich FeGa Thin Films

The aim of this work is to clarify how in-plane magnetic anisotropy and magnetoelasticity depend on the thickness of Ga-rich FeGa layers. Samples with an Fe72Ga28 composition were grown by sputtering in the ballistic regime in oblique incidence. Although for these growth conditions uniaxial magnetic anisotropy could be expected, in-plane anisotropy is only present when the sample thickness is above 100 nm. By means of differential X-ray absorption spectroscopy, we have determined the influence of both Ga pairs and tetragonal cell distortion on the evolution of the magnetic anisotropy with the increase of FeGa thickness. On the other hand, we have used the cantilever beam technique with capacitive detection to also determine the evolution of the magnetoelastic parameters with the thickness increase. In this case, experimental results can be understood considering the grain distribution. Therefore, the different physical origins for anisotropy and magnetoelasticity open up the possibility to independently tune these two characteristics in Ga-rich FeGa films.

Device performances and instabilities of the engineered active layer with different film thickness and composition ratios in amorphous InGaZnO thin film transistors

The device performances and instabilities of the engineered active layer with different film thickness and composition ratios in amorphous InGaZnO thin film transistor prepared by RF-sputtering have been investigated. The engineered active layer devices were composed of an In and Zn rich front layer with thickness of 10 nm and a Ga rich back layer with thickness from 20 to 30 nm. The device instabilities were investigated under positive bias stress (PBS), negative bias illumination stress (NBIS) and high VGS and VDS stress. The device performances and the stability have been enhanced in channel engineered active layer a-IGZO TFTs due to the combination of the high conductive channel with In and Zn rich front layer and the passivation effects with the Ga rich back layer. The concurrent device degradation mechanism is suggested to explain the more severe device degradation under high VGS and VDS stress than the device degradation under PBS.

Tuning the magnetic anisotropy of Ga-rich FeGa thin films deposited on rigid substrates

We have investigated the possibility of growing on outward bent rigid substrates to create an in-plane magnetic anisotropy in Ga-rich FeGa layers grown in the diffusive flow that in principle are expected to be magnetically isotropic in the sample plane when using non-bent rigid substrates. In this work, samples were grown by sputtering in the diffusive regime in rigid outward bent substrates, i.e. substrates under tensile stress. After the growth of the layers, the release of the substrate promotes a compression in the sample plane confirmed by x-ray diffractometry. This compression strain induces a uniaxial magnetic anisotropy axis in the sample plane perpendicular to the mechanical stress as expected for a material with a positive magnetostriction. Moreover, a value of 73 ppm for the magnetostriction constant has been inferred from the magnetic anisotropy measurements in the as-grown layer. A thermal treatment at a moderate temperature of 300 °C partially release the strain, but it does not modify the magnetic anisotropy either in magnitude or direction.

Influences of sulfurization on performances of Cu(In,Ga)(Se,S)2 cells fabricated based on the method of sputtering CIGSe quaternary target

The sequential processes of sputtering quaternary CIGSe ceramic targets and annealing in sulfur-containing atmosphere were applied to prepare Cu(In,Ga)(Se,S)2 (CIGSeS) absorbers. Solar cell devices were fabricated with CIGSeS absorbers annealed in H2S-containing atmosphere with different H2S concentrations. Composition and microstructure of absorbers and performances of cells were investigated. Gallium tends to diffuse towards CIGSeS absorber surface as H2S concentration increases, furthermore leading to a Ga-rich layer at the surface. The Ga gradient and the incorporation of S at surface result in the enlarged band gap (Eg) in the space charge region (SCR). In this work, the conversion efficiency of CIGSeS solar cells could be promoted with the highest efficiency of 13.3%, with a VOC of 566 mV. The increasing Eg gradient towards surface enhances the suppress of deep level recombination, and the best device performance was achieved with the improvement of band gap structure as the annealing H2S concentration reached 15 vol%.

Effect of sodium diffusion on the properties of CIGS solar absorbers prepared using elemental Se in a two-step process

The influence of Na diffusion from various glass substrates during a high-temperature slenization process on the microstructure and morphology of two-step formed CIGS absorber layers is investigated. In order to minimise the CIGS absorber formation time, elemental Se vapour is used to prepare the CIGS absorber. The grain sizes of the CIGS films are found to increase with increasing sodium in the glass substrates (extra clear glass, soda-lime glass, borosilicate glass). TiN and SiN thin films are used as diffusion barrier layers inserted between the glass substrate and the Mo rear conatct to tune the Na diffusion from the soda-lime glass. The interdiffusion between the In-rich CuInSe2 surface layer and the Ga-rich CuGaSe2 layer is promoted by the barrier layer, leading to larger CIGS grains. Efforts are also taken to understand the differences in Na diffusion (from the glass substrates) and their effects on the MoSe2 intermediate layer formation during the high-temperature CIGS absorber formation processes. We find that excess amounts of Na and Se are essential for the MoSe2 growth. The excessive Na in the form of Na2Sex at the CIGS/Mo interface works as a Se source and catalyses the MoSe2 formation. The Se flow in the two-step CIGS formation process must be sufficiently high to obtain high-efficiency CIGS solar cells.

Annealing Induced Interfacial Evolution of Titanium/Gold Metallization on Unintentionally Doped β-Ga2O3

Here, we study the kinetic evolution of the interface between a Ti/Au metal stack and bulk (010) β-Ga2O3 substrate under different annealing conditions using scanning / transmission electron microscopy. We observe distinct processes of interfacial reaction and interdiffusion between the metal films and at the metal-semiconductor junction. Upon rapid thermal annealing (RTA), the as-deposited Ti readily reacts at the β-Ga2O3 interface, driven by redox favorability. After a 1-min 470°C N2 RTA, the interface exhibits two segregated crystalline layers: a ∼5 nm Ti-rich (Ti-TiOx) layer lattice-matched to the β-Ga2O3 substrate and a ∼3 nm Ga-rich (TiGax) layer. A substitutional mechanism is proposed based on the similarity in ionic radii of Ti+3, Ti+4, and Ga+3. After 15-min RTA, the Ga-rich layer is diluted within the Ti-Au matrix, while the Ti-TiOx layer does not significantly change, and there is no further observable Ga out-diffusion from the substrate. Thus, we propose that the Ti-TiOx layer acts as a diffusion barrier, even when it is no longer lattice-matched with β-Ga2O3. In addition, Ti-rich nanocrystals form within the Ti-Au layer, presumably via the proceeding reactions. The observations here provide insights for contact stack evolution during operation of power electronic devices at elevated temperature.

Study of Gallium Front Grading at Low Deposition Temperature on Polyimide Substrates and Impacts on the Solar Cell Properties

Cu(In,Ga)Se2 (CIGS) solar cells have achieved the highest efficiencies among thin-film solar technologies. At low temperatures ( V oc and FF with high Ga content at the interface. We show that the insertion of a Ga-rich layer favors the formation of ordered vacancy compound phases, and the absence of Ga during the last stage of the process impacts the overall element diffusion. Optimized Ga gradient leads to 17.8% efficiency solar cells without antireflecting coating or KF postdeposition treatment.

Role of gallium diffusion in the formation of a magnetically dead layer at the Y3Fe5O12/Gd3Ga5O12 epitaxial interface

We have clarified the origin of magnetically dead interface layer formed in yttrium iron garnet (YIG) films grown at above 700{\deg}C onto gadolinium gallium garnet (GGG) substrate by means of laser molecular beam epitaxy. The diffusion-assisted formation of a Ga-rich region at the YIG / GGG interface is demonstrated by means of composition depth profiling performed by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy and X-ray and neutron reflectometry. Our finding is in sharp contrast to the earlier expressed assumption that Gd acts as a migrant element in the YIG/GGG system. We further correlate the presence of Ga-rich transition layer with considerable quenching of ferromagnetic resonance and spin wave propagation in thin YIG films. Finally, we clarify the origin of the enigmatic low-density overlayer that is often observed in neutron and X-ray reflectometry studies of the YIG / GGG epitaxial system.

He-Ion Microscopy as a High-Resolution Probe for Complex Quantum Heterostructures in Core-Shell Nanowires.

Core-shell semiconductor nanowires (NW) with internal quantum heterostructures are amongst the most complex nanostructured materials to be explored for assessing the ultimate capabilities of diverse ultrahigh-resolution imaging techniques. To probe the structure and composition of these materials in their native environment with minimal damage and sample preparation calls for high-resolution electron or ion microscopy methods, which have not yet been tested on such classes of ultrasmall quantum nanostructures. Here, we demonstrate that scanning helium ion microscopy (SHeIM) provides a powerful and straightforward method to map quantum heterostructures embedded in complex III-V semiconductor NWs with unique material contrast at ∼1 nm resolution. By probing the cross sections of GaAs-Al(Ga)As core-shell NWs with coaxial GaAs quantum wells as well as short-period GaAs/AlAs superlattice (SL) structures in the shell, the Al-rich and Ga-rich layers are accurately discriminated by their image contrast in excellent agreement with correlated, yet destructive, scanning transmission electron microscopy and atom probe tomography analysis. Most interestingly, quantitative He-ion dose-dependent SHeIM analysis of the ternary AlGaAs shell layers and of compositionally nonuniform GaAs/AlAs SLs reveals distinct alloy composition fluctuations in the form of Al-rich clusters with size distributions between ∼1-10 nm. In the GaAs/AlAs SLs the alloy clustering vanishes with increasing SL-period (>5 nm-GaAs/4 nm-AlAs), providing insights into critical size dimensions for atomic intermixing effects in short-period SLs within a NW geometry. The straightforward SHeIM technique therefore provides unique benefits in imaging the tiniest nanoscale features in topography, structure and composition of a multitude of diverse complex semiconductor nanostructures.

A highly efficient Cu(In,Ga)(S,Se)2 photocathode without a hetero-materials overlayer for solar-hydrogen production

Surface modification of a Cu(In,Ga)(S,Se)2 (CIGSSe) absorber layer is commonly required to obtain high performance CIGSSe photocathodes. However, surface modifications can cause disadvantages such as optical loss, low stability, the use of toxic substances and an increase in complexity. In this work, we demonstrate that a double-graded bandgap structure (top-high, middle-low and bottom-high bandgaps) can achieve high performance in bare CIGSSe photocathodes without any surface modifications via a hetero-materials overlayer that have been fabricated in a cost-effective solution process. We used two kinds of CIGSSe film produced by different precursor solutions consisting of different solvents and binder materials, and both revealed a double-graded bandgap structure composed of an S-rich top layer, Ga- and S-poor middle layer and S- and Ga-rich bottom layer. The bare CIGSSe photocathode without surface modification exhibited a high photoelectrochemical activity of ~6 mA·cm−2 at 0 V vs. RHE and ~22 mA·cm−2 at −0.27 V vs. RHE, depending on the solution properties used in the CIGSSe film preparation. The incorporation of a Pt catalyst was found to further increase their PEC activity to ~26 mA·cm−2 at −0.16 V vs. RHE.

Cu(In,Ga)Se2 surface treatment with Na and NaF: A combined photoelectron spectroscopy and surface photovoltage study in ultra-high vacuum

Either metallic Na or NaF were deposited onto Cu(In,Ga)Se2 surfaces and studied by photoelectron spectroscopy and surface photovoltage spectroscopy without breaking the ultra-high vacuum. The deposition of elemental Na at room temperature led to the formation of an intermediate Cu and Ga rich layer at the CIGSe surface, whereas for NaF the composition of the CIGSe surface remained unchanged. A metal like surface induced by an inverted near surface region with a reduced number of defect states was formed after the deposition of Na. Under the chosen experimental conditions, the near surface layer was independent on the amount of Na and stable in time. In contrast, the usage of NaF weakened the inversion and led to an increased band bending compared to the untreated CIGSe sample. The SPV signals decreased with proceeding time after the deposition of NaF.

Bifacial CIGS solar cells grown by Low Temperature Pulsed Electron Deposition

In this paper we report on the single stage deposition of CuInxGa1−xSe2 (CIGS)-based bifacial solar cells on glass coated with Fluorine-doped Tin Oxide (FTO) or Indium Tin Oxide (ITO) by single-stage low-temperature (250°C) pulsed electron deposition (LTPED).We show that the mechanism of Sodium incorporation during the low-temperature deposition of CIGS on both FTO and ITO leads to the formation of a stable n+/p+ ohmic tunnel junction and photovoltaic efficiencies exceeding 14% can be obtained without any intentional bandgap grading of CIGS.The significant degradation of the cell fill factor with decreasing CIGS thickness is found to be related to the presence of craters left behind by micro-fragments of CIGS target, which are weakly incorporated in the film during the LTPED growth and removed during the subsequent process steps. Evidence is also presented that the low-temperature deposition of CIGS on ITO leads to the formation of a Ga-rich CIGS layer at the interface and to an unintentional compositional grading propagating towards the active region of the solar cells. The defects associated with this grading may be responsible for the loss in FF and Voc with respect to the cells deposited on FTO and Mo back contacts.

Focused ion beam surface treatments of single crystal zinc oxide for device fabrication

We investigated 30kVGa+ ions treatments on (0001) single crystal ZnO in order to assess the potentiality of ion beam based device fabrication on such material. A multi-technique approach combining atomic force microscopy, Raman and energy dispersive X-Ray spectroscopies, and transmission electron microscopy was used to study morphological and structural properties of ZnO upon varying the ion dose. At low doses a shallow defective layer develops showing an increasing defect density as the dose is increased. At higher dose a thinner defective layer with an amorphous layer on top is produced. The ion beam damaged layer on high resistivity ZnO shows enhanced conductivity. KOH based etching removed selectively the damaged ZnO and was found to dissolve rapidly the Ga-rich amorphous layer. The defective layer has an etch rate which depends on the ion dose, and even for prolonged etching processes it was not completely removed. However, conductivity measurements on ion beam fabricated pillars showed that the residual defects do not give a detectable electrical response. These findings indicate that electronic devices and micro-structures with pristine electrical properties of the ZnO crystal can be reliably fabricated by focused ion beam, provided the damaged surface layer is removed by proper etching procedures.

Local gate leakage current induced by inhomogeneous epitaxial growth in AlGaN/GaN high-electron-mobility transistors

We found that inhomogeneous epitaxial growth in a superlattice near a Si substrate creates an area where local leakage current occurs at the interface between an AlGaN surface and a Schottky electrode. Here, electroluminescence (EL) through a transparent gate of an AlGaN/GaN high-electron-mobility transistor enables us to identify the area in the entire gate periphery. Further, the superlattice near the Si substrate supports clear observation of inhomogeneous growth under the EL spots. The energy-dispersive X-ray spectroscopy profile indicates that a Ga-rich layer was grown in the early stage of inhomogeneous area creation.