Ga Diffusion Research Articles

Ga interaction with ZnO surfaces: Diffusion and melt-back etching

Despite being technologically very attractive, highly-doped zinc oxide whiskers with precisely defined morphology and doping level are difficult to prepare. Here, as an advancing step towards this goal, we show that pre-annealing of ZnO in oxygen-poor conditions (e.g. high vacuum) at low temperature encourages a deeper diffusion of Ga into the ZnO crystal lattice in contrast to ZnO pre-annealed in oxygen-rich conditions. We also demonstrate that gallium acts as a reactant causing ZnO etching at diffusion temperatures, contrary to Al-based doping of ZnO systems. This behaviour, being similar to gallium melt-back etching during GaN epitaxy on silicon, has not been observed for ZnO so far and can represent a significant hurdle for the post-growth diffusion doping of ZnO nanostructures. The paper suggests possible ways how to diminish this effect.

Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg

We have demonstrated a fabrication process for the Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg. An Ohmic contact with a contact resistance of 0.158 Ω cm2 is realized on p−-GaN ([Mg] = 1.3 × 1017 cm−3). The contact resistance of p-type GaN with higher Mg concentration ([Mg]=1.0 × 1019 cm−3) can also be reduced to 2.8 × 10−5 Ω cm2. A localized contact layer is realized without any etching or regrowth damage. The mechanism underlying this reduced contact resistance is studied by scanning transmission electron microscopy with energy dispersive x-ray spectroscopy and secondary ion mass spectrometry, representing a mutual diffusion of Ga and Mg atoms on the interface. Reductions in the barrier height and surface depletion width with the nitrogen-annealed Mg layer are confirmed by XPS and Hall effect measurements qualitatively.

The non-isothermal crystallization kinetics and mechanism of FeGaGeBCu alloy

In this paper, Fe76Ga5Ge5B13Cu1 ribbon was prepared by melt spinning method and its thermal stability and non-isothermal crystallization kinetics were analyzed. It is found that the primary crystallization phase is α-Fe(Ga,Ge), following by the precipitation of Fe-B compounds. Besides, Kissinger and Ozawa methods were utilized to calculate the initial activation energy Ex and apparent activation energy Ea. The results show that Ex1 > Ea1, which is due to that the diffusion of Ga and Ge is difficult during the nucleation process but left a channel for the subsequent diffusion of atoms. In addition, Avrami index n was calculated by Johnson-Mehl-Avrami-Kolmogorov (JMAK) model and the crystallization mechanism of Fe76Ga5Ge5B13Cu1 alloy was analyzed. For the alloy with grains or clusters embedded in the amorphous matrix, it is concluded that the annealing condition with a higher heating rate is more conducive to obtain homogeneous nanocrystalline and improve soft magnetic properties.

(Invited) Study of HfO2-Based High-k Gate Insulators for GaN Power Device

We discussed about usefulness of HfO2-based high dielectric constant (High-k) materials such as HfO2, HfSiOx and HfAlOx as gate insulator for GaN power device. Here, we systematically studied characteristics of Hf0.55Al0.45Ox gate insulator which fabricated by plasma-enhanced atomic layer deposition and post-deposition annealing (PDA) at 800°C in N2 ambient. The Hf0.55Al0.45Ox film had an amorphous structure but Ga diffusion into Hf0.55Al0.45Ox film was observed after PDA. The k-value of the Hf0.55Al0.45Ox film was 17.2, which was larger than that of HfSiOx (13.5). The Hf0.55Al0.45Ox also showed superior electrical properties such as a minimal flatband voltage (V fb) hysteresis ( +10 mV) and a relatively small V fb shift ( £ -0.95 V), as well as a low interface state density (~ 1 ´ 1011 cm-2eV-1 at Ec-E=0.25 eV), and a high breakdown electric field (8.6 MVcm-1). Based on these experimental data and previous our research of HfO2 and HfSiOx films, we concluded the HfSiOx and HfAlOx had candidate materials as gate insulator for GaN power device.

High thermoelectric power factors in polycrystalline germanium thin films

The high potential of polycrystalline Ge as a thin-film thermoelectric material was demonstrated. We synthesize a polycrystalline Ge layer on an insulating substrate at 450 °C via advanced solid-phase crystallization and control its carrier concentration through the solid-phase diffusion of various p- and n-type dopants. The heating deposition (150 °C) of the amorphous precursor considerably improves the crystal quality of the polycrystalline Ge layer as well as the doping properties. The solid-phase diffusion of Ga and P dopants onto the Ge layers allows for control of the carrier concentration in the ranges of 1017–1020 cm−3 for p-type and 1018–1019 cm−3 for n-type, respectively, by modulating the diffusion annealing temperature and time. Because of the high electrical conductivities reflecting the carrier mobilities and carrier concentrations, the maximum power factors reach a value of 1080 μW m−1 K−2 for p-type and 2300 μW m−1 K−2 for n-type at room temperature. These power factors are higher than those of most polycrystalline semiconductor thin films formed at temperatures below 1000 °C. Therefore, this study serves as a milestone toward high-performance and reliable thin-film thermoelectric generators based on an environmentally friendly semiconductor.

A highly controllable doping technique via interdiffusion between epitaxial germanium layers and GaAs

Interdiffusion between epitaxial Ge layers and GaAs substrates was used in this study to precisely control the doping concentration and doping type of the Ge layers. An extremely high n-type doping concentration (> 1020 cm−3) was achieved in a Ge layer grown at a low temperature of 300 °C; the layer exhibited a smooth surface morphology and excellent crystalline quality. Subsequent post-growth annealing processes led to a decrease of the n-type doping concentration followed by an n- to p-type transition because of the diffusivity difference between As and Ga atoms. Our experimental results indicated the preferential diffusion of As atoms and their consequent escape through the Ge top surface, resulting in the p-type transition via time-lagged Ga diffusion. We fabricated a Ge-on-insulator junctionless field-effect transistor using a heavily n-doped 10 nm-thick Ge channel, and the device demonstrated good performance.

Magnetic properties evolution with grain boundary phase transformation and their growth in Nd-Fe-Cu-Ga-B sintered magnet during post-sinter annealing process

The formation of Nd6Fe13Ga phase promoted the continuity and non-ferromagnetism of grain boundary (GB) phases of Nd-Fe-Cu-Ga-B magnets during proper post-sintering annealing process. The coercivity increased dramatically with a decrease of remanence. After further annealing at the sintering temperature, the remanence recovered with a drop of coercivity. It indicated that Nd2Fe14B phase decomposed with the formation of Nd6Fe13Ga phase. Overextending the annealing time led to the gradual decrease of coercivity as the growth of Nd6Fe13Ga phase weakened the continuity of GB phases. Increasing the annealing temperature to the optimum temperature for Nd6Fe13Ga phase can aggravate the decrease of coercivity and remanence. Owing to the intensifying segregation of Ga and Fe in GB phases with the formation of Nd6Fe13Ga phase by dehydrogenation (HD) at 560 °C, high coercivity can be obtained by annealing at a lower temperature or for shorter time compared with HD at 480 °C. The enrichment of Cu in Nd-rich phase ensured its wettability and non-ferromagnetism when the diffusion of Fe and Ga became more local with the formation of Nd6Fe13Ga phase. Therefore, the combined effect of Cu doping and reasonable high temperature HD was conducive to obtain high coercivity in a larger range of annealing time and temperature for Nd-Fe-Ga-B magnet, which was a potential method for its mass production.

Depth-resolved analysis of the effect of RbF post deposition treatment on CIGSe with two different Cu concentrations

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGSe) absorbers are among the most attractive non-Si alternatives. The key to their steadily increasing efficiency is a post-deposition treatment (PDT) with alkali salts. For co-evaporated CIGSe, a RbF-PDT was demonstrated as the most efficient, however, the mechanism of the RbF influence on the CIGSe absorber is not completely understood. Here, we focus on the impact of RbF on the surface of co-evaporated CIGSe absorbers in dependence on their bulk composition. Surface-as well as bulk-sensitive methods with overlapping information depths are used to examine an overall depth profile of RbF-free and RbF-treated CIGSe samples with different Cu contents. We show a gradual depletion of copper towards the surface for the as-deposited CIGSe absorber layers. The following RbF-PDT sharpens this effect especially for the sample grown with the overall lower Cu content. Under the Cu-depleted surface layer, the composition of the CIGSe gradually changes until it reaches the respective bulk composition. As a result of the RbF-PDT, Ga diffuses towards the surface and Rb gets incorporated at the surface where GaF3 and RbInSe2 secondary phases are formed, respectively. A higher Cu content leads to less surface oriented Ga diffusion, less Rb incorporation and to a thinner RbInSe2 layer. A thinner RbInSe2 barrier layer, in turn, maintains the gain in open-circuit voltage and prevents the fill factor loss of the CIGSe device.

Adsorption and Diffusion of Aluminum on β-Ga2O3(010) Surfaces.

Epitaxial growth of aluminum gallium oxide is important for forming heterojunctions on Ga2O3 for high power electronics applications. We use density functional theory to explore the co-adsorption of Al, Ga, and O adatoms on the Ga2O3(010) surface and the surface reconstructions during the growth of the alloy. We find that Al can adsorb in tetrahedral sites in many of the reconstructions. The migration barrier escaping from a tetrahedral site to an octahedral site is 1.72 eV for an Al adatom and 0.56 eV for a Ga adatom, indicating that Al diffusion is much more restricted than Ga diffusion on the surface. Our findings indicate that kinetic limitations are responsible for Al occupying both octahedral and tetrahedral sites in (AlxGa1-x)2O3, in spite of the fact that thermodynamically the octahedral site is preferred.

Liquid metal intercalation of epitaxial graphene: Large-area gallenene layer fabrication through gallium self-propagation at ambient conditions

We demonstrate the fabrication of an ultra thin gallium film, also known as gallenene, beneath epitaxial graphene on 6H-SiC under ambient conditions triggered by liquid gallium intercalation. Gallenene has been fabricated using the liquid metal intercalation, achieving lateral intercalation and diffusion of Ga atoms at room temperature on square centimeter areas limited only by the graphene samples' size. The stepwise self-propagation of the gallenene film below the epitaxial graphene surface on the macroscopic scale was observed by optical microscopy shortly after the initial processing without further physical or chemical treatment. Directional Ga diffusion of gallenene occurs on SiC terraces since the terrace steps form an energetic barrier (Ehrlich-Schwoebel barrier),retarding the gallenene propagation. The subsequent conversion of the epitaxial graphene into quasi free-standing bilayer graphene (QFBLG) and the graphene-gallenene heterostack interactions have been analyzed by XPS and Raman measurements. The results reveal a novel approach for controlled fabrication of wafer-scale gallenene as well as for two-dimensional heterostructures and stacks based on the interaction between liquid metal and epitaxial graphene. Please note, this work was also titled as Graphene meets gallenene -- A straightforward approach to developing large-area heterostacks by gallium self-propagation https://www.researchgate.net/publication/333451130_Graphene_meets_gallenene_-_A_straightforward_approach_to_developing_large-area_heterostacks_by_gallium_self-propagation

Ge-GaN deposition: An assistant kMC model

The present work provides a parametric simulation tool to assist the experimental deposition of GaN and Ge-GaN material. For this purpose, a kinetic Monte Carlo (kMC) model was developed and implemented to simulate the deposition, diffusion, and desorption of Ge, Ga, and N and subsequent material growth on GaN (0001). The kMC is a Monte Carlo algorithm that simulates the dynamics of a given on-the-lattice system by computing on-the-fly every event rate. In the present model, the deposition rates were computed by means of the collision theory and the diffusion and desorption rates were calculated with the usual Arrhenius form based on knowledge of the activation energies and the local energy configuration. Ge diffusion energies as well as experimental deposition conditions were simulated to investigate their impact on the resulting Ge-GaN layers. Proposed kMC model outcomes, which are consistent with the observed experimental results, are discussed in detail and conclusions are provided.

A kinetic study of liquid gallium diffusion in a tin-based solder alloy and its role in solder embrittlement

A first quantitative analysis of the liquid gallium (Ga) embrittlement of a tin (Sn)-based solder alloy is reported here. Ga penetration into Sn96.5-Ag3-Cu0.5 (SAC305) solder was found to follow a diffusion-controlled process using simultaneous intergranular and transgranular paths. A model of the diffusion kinetics of liquid Ga into SAC305 solder thin films was established and appears to obey an Arrhenius relationship. Diffusivity in a 1000 nm thin film was found to be of the same order of magnitude (10–10 m2 s−1) as those determined for other systems of liquid metal penetration into solid metal at similar relative temperatures, i.e., close to the liquid metal melting point and significantly below the solid metal melting point. Determined activation energies in the range of 30–37 kJ mol−1 support a mixed grain boundary and lattice diffusion process that is consistent with the observed intergranular/transgranular penetration. This exponential temperature dependence of liquid Ga diffusion and resultant SAC solder degradation suggests that a liquid metal embrittlement-based solder rework process can be established under favorable time–temperature conditions.

High performance GZO/p-Si heterojunction diodes fabricated by reactive co-sputtering of Zn and GaAs through the control of GZO layer thickness.

The effect of thickness of Ga doped ZnO (GZO) layer on the performance of GZO/p-Si heterojunctions fabricated by reactive co-sputtering of Zn–GaAs target is investigated. GZO films were deposited at 375 °C with 0.5% GaAs area coverage of Zn target and 5% O2 in sputtering atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy show that c-axis orientation of crystallites, Ga/Zn ratio and oxygen related defects depend substantially on the thickness of films. The 200–350 nm thick GZO films display low carrier concentration ∼1017 cm−3, which increases to >1020 cm−3 for thicker films. The diodes fabricated with >500 nm thick GZO layers display non-rectifying behaviour, while those fabricated with 200–350 nm thick GZO layers display nearly ideal rectification with diode factors of 1.5–2.5, along with, turn-on voltage ∼1 V, reverse saturation current ∼10−5 A, barrier height ∼0.4 eV and series resistance ∼200 Ω. The drastically improved diode performance is attributed to small Ga/Zn ratio (∼0.01) and extremely low dopant activation (∼0.3%), owing to diffusion and non-substitutional incorporation of Ga in thin GZO layers, which cause self-adjustment of doping concentration. These factors, together with c-axis orientation and chemisorbed oxygen at grain boundaries, facilitate ideal diode characteristics, not reported earlier for GZO/p-Si heterojunctions.

Направление диффузионного потока галлия при осаждении на поверхность с регулярными массивами отверстий

Self-catalyzed growth of GaAs and GaP nanowires by molecular beam epitaxy is often performed on processed SiOx/Si(111) substrates with regular arrays of lithographically defined holes. Ga droplets form in the holes during Ga pre-deposition step in the absence of As supply. It was considered evident that the Ga diffusion flux in such a process is directed from the oxide surface into the holes. Here, we show that it is not always true and that the diffusion flux can change its direction depending on the growth conditions. Our model can be useful for modeling the incubation and growth times of group III droplets within the holes and explains long nucleation delays of the droplets.

Tuning Ga Grading in Selenized Cu(In,Ga)Se2 Solar Cells by Formation of Ordered Vacancy Compound

Severe Ga accumulation at the back contact is regarded as one of the major limiting factors for high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells fabricated by the sequential process. The Ga deficiency near the front surface of absorber leads to the reduction of open‐circuit voltage; however, controlling the Ga grading is quite challenging during the selenization process. Herein, the tuning of Ga profile is demonstrated by forming the ordered vacancy compound (OVC) phase at the beginning of selenization process under high Se partial pressure (PSe) with the precursor composed of a relatively high In content. The OVC phase formed with a columnar structure promotes Ga diffusion through Cu vacancies, resulting in Ga increase at the front side of CIGSe. However, high PSe also increases the thickness of MoSe2, significantly raising the series resistance. Thereby, a 5 nm TiN layer is deposited on top of Mo to suppress the formation of MoSe2. In addition, the extra Na‐contained precursor is added to increase the carrier concentration because the TiN layer also blocks the Na outdiffusion from the substrate. By controlling PSe, interlayer TiN thickness, and carrier concentration, a 16.04% record high efficiency of CIGSe solar cells (sulfur‐free) via elemental Se source is achieved.

Growth Uniformity in Selective Area Epitaxy of AlGaN/GaN Heterostructures for the Application in Semiconductor Devices

The design of modern semiconductor devices often requires the fabrication of three-dimensional (3D) structures to integrate microelectronic components with photonic, micromechanical, or sensor systems within one semiconductor substrate. It is a technologically challenging task, as a strictly defined profile of the device structure is obligatory. This can be achieved either by chemical etching or selective deposition on a masked substrate. In this paper, the growth uniformity of AlGaN/GaN heterostructures during selective-area metalorganic vapour-phase epitaxy (SA-MOVPE) was studied. Such structures are typically used in order to fabricate high-electron-mobility transistors (HEMT). The semiconductor material was deposited through 200 μm long stripe-shaped open windows in a SiO2 mask on GaN/sapphire templates. The window width was varied from 5 μm to 160 μm, whereas mask width separating particular windows varied from 5 μm to 40 μm. The experiment was repeated for three samples differing in GaN layer thickness: 150 nm, 250 nm, and 500 nm. Based on theoretical models of the selective growth, a sufficiently uniform thickness of epitaxially grown AlGaN/GaN heterostructure has been achieved by selecting the window half-width that is smaller than the diffusion length of the precursor molecules. A Ga diffusion length of 15 μm was experimentally extracted by measuring the epitaxial material agglomeration in windows in the dielectric mask. Measurements were conducted while using optical profilometry.

Raman Spectroscopy of GaN Epitaxial Layers Synthesized on Si(111) by Molecular Beam Epitaxy with Nitridation

Investigation of GaN epitaxial layers on silicon substrates is driven by high potential for fabrication of high efficiency and relatively low-cost electronic devises. Growth process of GaN layers on Si by molecular-beam epitaxy is complicated by mutual diffusion of Si and Ga and differences in thermal expansion coefficients and lattice parameters causing large number of defects. To improve the quality of GaN layers high temperature nitridation of silicon substrates was applied leading to stress relaxation and decrease of diffusion.

Gallium Liquid Metal Embrittlement of Tin-based Solder Alloys

We report on the embrittlement of tin (Sn)-based solder by liquid gallium (Ga). Exposure to liquid Ga results in a degradation of the intrinsic mechanical properties of the solder due to its ductile-to-brittle transition. Important changes in the solder microstructure accompany this degradation and are predominantly associated with a spontaneous recrystallization of Sn during Ga diffusion into the solder. A detailed investigation of the solder microstructure evolution identifies the recrystallization process as a Continuous Static Recrystallization (C-SRX). Taking into account mechanisms of atomic transport as well as the creation and propagation of defects, a novel liquid metal embrittlement (LME) model is herein proposed, namely the Diffusion-Induced Recrystallization Evolution (DIRE) model.

Epitaxial GaN layers synthesized on Si (111)

Realization of epitaxial synthesis of high quality gallium nitride layers on silicon substrates is driven by the high potential of GaN-on-silicon technology for fabrication of high efficiency and relatively low-cost electronic devises. Growth process of GaN layers on Si substrates by molecular-beam epitaxy is complicated by differences in thermal expansion coefficients and lattice parameters causing large number of defects and dislocations. Another issue is mutual diffusion of Si and Ga. To improve the quality of GaN layers high temperature nitridation of silicon substrates was applied. We used Raman spectroscopy and Hall measurements to study the effect of nitridation. The shift of E2 H phonon mode allowed to calculate biaxial stress and this indicated stress relaxation in GaN epitaxial layers. P-type conductivity was observed in GaN/Si(111) structures and nitridation was proven to reduce carrier concentration. Thus, high temperature nitridation of Si substrate led to stress relaxation and decrease of Ga diffusion into substrate.

Molecular beam epitaxy of GaN on 2H–MoS2

Recent interest in the heterogeneous integration of two-dimensional (2D) materials with three-dimensional (3D) semiconductors has resulted in the implementation of direct heteroepitaxial growth via various epitaxy techniques. In this study, we demonstrated the synthesis of wafer-scale direct growth of GaN on MoS2 using plasma-assisted molecular beam epitaxy. Systematic studies were employed to understand the growth diagram of GaN on large area multi-layer MoS2, and various growth parameters such as substrate temperature and Ga to N flux ratio were investigated to study the degradation mechanism of the underlying MoS2 layer. The growth diagram of GaN on MoS2 was proposed based on the studies. The microstructure of GaN (i.e., N-rich and 550 °C) was evaluated by scanning transmission electron microscopy. Phase segregation was found in the N-rich GaN layer. A two-step growth process involving N-rich condition at a lower temperature and Ga-rich condition at a higher growth temperature was attempted to obtain the smooth and high crystalline quality of GaN. While the two-step growth method led to substantial improvement in the film quality, the removal of MoS2 was observed and the degradation of MoS2 substrate was attributed to Ga diffusion through dislocation lines based on energy-dispersive x-ray spectroscopy mapping. This work helps illuminate critical issues in the epitaxial growth of conventional semiconductors on 2D crystals for various device applications.