Interfacial reactions between palladium thin films and InP

Palladium appears to be an important component in ohmic contact metallizations to III–V semiconductors. Very little is known about its interaction with InP. Consequently, the reaction between a thin layer of Pd (100 nm) and an InP substrate has been studied at annealing temperatures ranging from 250–450 °C for up to 30 sec, i.e., typical annealing conditions encountered during contact fabrication. Palladium reacts readily with InP, initially forming an amorphous ternary phase, which transforms to crystalline Pd₂InP on annealing. Pd₂InP has an ordered cubic structure, with a lattice parameter of 0.830 nm, and grows epitaxially on InP. Microtwins, 2–3 atomic layers thick, have been identified in the ternary phase and these form along the (110) and ( 10) planes.     

Interfacial electron density profile in Nb/Si bilayer films: an X-ray reflectivity study

This article describes a systematic study of the nature of interfaces involved in a Nb layer deposited on Si (Nb-on-Si) and Si layer deposited on Nb (Si-on-Nb) bilayer films by using a UHV electron beam evaporation technique, having individual layer thickness of 35 and 100 Å each. By using Grazing angle X-ray reflectivity and adopting a proper modelling technique the electron density profile (EDP) as a function of depth has been determined in the samples. EDP determined in as-deposited 35 Å Nb and 35 Å Si bilayer films show that the width of Si-on-Nb and Nb-on-Si interfaces are 20 Å and 40 Å, respectively. The difference observed in the width of two interfaces is attributed to the different growth morphology of 35 Å Nb and 35 Å Si single-layer films as revealed by atomic force microscopy (AFM) investigations. EDP determined from measured XRR data for 100 Å Nb and 100 Å Si deposited bilayer film shows that the width of Si-on-Nb interface is 10 Å. This observed width is smaller than the similar interface in the case of samples having an individual layer thickness of 35 Å. The corresponding interface width of Nb-on-Si is found to be 45 Å and marginally more than the similar interface in the case of the 35 Å Nb/35 Å Si bilayer samples. AFM studies carried out on 100 Å Nb and Si layers deposited separately on float glass substrate indicate similar gross as well as subtle morphological features and cannot be attributed to the observed asymmetry in this case. The observed asymmetry in EDP of two interfaces in this case is due to the enhanced diffusion of Si into the formed metal layer relative to the diffusion into the already deposited metal layer.     

Interfacial reactions between eutectic Sn–Pb solder and Co substrate

This study examined the reaction couples of Co substrate with liquid and solid Sn-37 wt% Pb alloys, respectively. In the liquid/solid Sn–Pb/Co couples, one single-phase CoSn₃ layer was found at the interface at temperatures from 210 to 250 °C. The layer thickness was measured as a function of the reaction time. The layer growth rate increased with increasing temperature. The CoSn₃ layer growth was linear in the early stage and then became parabolic. In addition, the similar CoSn₃ growth behavior results were observed in the solid/solid Sn–Pb/Co couples from 150 to 170 °C. Nevertheless, one Pb-rich phase layer accumulated at the solder/CoSn₃ interface due to Sn consumption. Also, the reaction kinetics of Sn-37 wt% Pb/Co was symmetrically studied at various temperatures. The chemical reaction constants, diffusion constants, and the relevant activation energies were reported.     

Thermal Oxidation of Co/GaAs Structures

The kinetics and mechanism of GaAs oxidation in Co/GaAs structures are studied. The interfacial transition layer is shown to influence Co/GaAs oxidation and to lead to predominant growth of cobalt arsenate. This improves the dielectric properties of the structure compared to the oxidation of free GaAs surfaces. A model for the thermal oxidation of Co/GaAs structures is proposed which takes into account the formation of an interfacial transition layer at the initial stages of oxidation.     

Thickness dependent reactivity and coercivity for ultrathin Co/Si(111) films

For Co/Si(111) films thinner than 15 ML, the thickness dependent reactivity and magnetic properties have been systematically studied. As the Co coverage increases, Co adatoms on the Si(111) surface show enhanced chemical reactivity for oxidation due to the change of the chemical state. After the saturation oxygen exposure, oxygen atoms interact with a thick Co layer to form a rougher interface. Complex adsorption kinetics of oxygen in the Co layer is observed. From the depth-profiling measurements for Co layers close to the Co-Si interface, the sputtering rate is enhanced due to that the solid surfaces of Si and Co-Si compounds are resistive against oxidation. The descending of the Kerr intensity by saturation oxygen exposure shows the limited diffusion length of oxygen atoms into the films. The inertness of the Co-Si interface, the reduction of pure cobalt and imperfection introduced by oxygen influence the coercivity of O/Co/Si(111).     

氨化Si基Ga2O3/Co薄膜制备GaN纳米线

采用磁控溅射技术先在硅衬底上制备Ga2O3/Co薄膜,然后在900℃时于流动的氨气中进行氨化反应制备GaN薄膜.X射线衍射(XRD)、傅立叶红外吸收光谱(FTIR)、选区电子衍射(SAED)和高分辨透射电子显微镜(HRTEM)的分析结果表明,采用此方法得到了六方纤锌矿结构的GaN单晶纳米线.通过扫描电镜(SEM)观察发现纳米线的形貌,纳米线的尺寸在 50～200nm之间.     

Interfacial reactions for the non-stoichiometric TiB x /(100)Si system

In order to evaluate the interfacial reactions in the TiB _x /(100)Si system and the thermal stability of non-stoichiometric TiB _x films (0 B/Ti 2.5), TiB _x /Si samples prepared by a co-evaporation process were annealed in vacuum at temperatures between 300 and 1000°C. The solid phase reactions were investigated by means of sheet resistance, X-ray diffraction, transmission electron microscopy, X-ray photo-electron spectroscopy, and stress measurement. For TiB _x samples with a ratio of B/Ti 2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with a ratio of B/Ti < 2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide process. The sheet resistance and the film stress in the Ti/Si and TiB _x /Si systems are well explained by the solid phase reactions.     

GaAs/Si界面的化学态的研究

利用俄歇峰形分析技术对GaAs/Si界面的化学态和电子态进行了研究。提出了一种新的俄歇信号强度标定法——频域法。该方法利用低能俄歇电子较低的逸出深度,在复频域内标定来自最表层原子的俄歇信号强度,避免了二次电子本底和电子逸出平均自由程的影响,从而可以准确判定在界面这种突变结构中元素成分的深度分布信息。利用谱峰修正技术,研究了Ga、As、Si价带跃迁几率在界面不同深度内的变化情况,发现在界面处As-Si有明显的成键作用,Si原子的一部分p电子转移到了As原子空的p轨道中,由此产生了一个新的界面态,该界面态位于GaAs价带顶上方2eV的地方。在对Ga原子价带俄歇跃迁几率进行的研究中,并未发现其价电荷分布有明显的变化,表明Ga原子与其他原子在界面处并未发生明显的成键作用。     

Photoluminescence study of GaAs thin films and nanowires grown on Si(111)

Mg-doped GaAs nanowires have been grown by molecular beam epitaxy on a partially Au-coated Si(111) substrate by the vapor–liquid–solid mechanism. Outside the coated areas, a thin film of GaAs was grown epitaxially at the same time. The optical properties in both parts of the sample were investigated by photoluminescence spectroscopy, as a function of temperature. A structured emission in the range ～1.25–1.55 eV was observed at 10 K and the resemblances in both cases were identified. The radiative transitions are discussed with relevance to known defect centers in the GaAs thin films and to their possible relation with the zinc-blende and wurtzite phases in the nanowires. The presence of both crystalline phases in the nanowires was confirmed by μ-Raman spectroscopy.     

Interfacial properties and characterization of Sc/Si multilayers

We investigate the intermixing of layers in Sc/Si and Sc/B₄C/Si/B₄C multilayers using electron and synchrotron excited soft X-ray emission and absorption spectroscopy. The multilayers are annealed at 100, 200, 300, 400 and 500 °C after preparation by magnetron sputtering. Silicon K_β emission and reflectivity measurements verify that the non-annealed multilayer systems are composed of distinct layers with only a minor interdiffusion in Sc/Si samples whereas annealing Sc/Si multilayers at 400 °C leads to a degradation of the multilayer structure and the formation of intermittent scandium silicide, ScSi. The presence of B₄C barriers in Sc/B₄C/Si/B₄C hinders this degradation from developing for the entire temperature range considered. The barrier layers continue to be effective for the entire temperature range even after an extended shelf-life.     

Solid phase reactions between Fe thin films and Si-Ge layers on Si

Solid phase reactions in Fe thin films on epi-Si_0.8Ge_0.2, poly-Si_0.7Ge_0.3, a-Si_0.8Ge_0.2, and a-Si_0.7Ge_0.3 layers on silicon have been investigated. The as-deposited samples were in situ annealed in the ultrahigh vacuum chamber at 400-800 °C for 30 min. The island structure was found to cause the abrupt increase in the sheet resistance of the annealed Fe/SiGe samples at 700-800 °C. The formation of FeSi islands containing a small amount of Ge is attributed to the preferential reactions of Fe with Si to Ge. As the annealing temperature was raised to 800 °C, the Fe(Si_1−xGe_x) phase is the only phase found in the annealed Fe/epi-Si_0.8Ge_0.2 and Fe/poly-Si_0.7Ge_0.3 samples. On the other hand, at the annealing temperature above 700 °C, the β-Fe(Si_1−xGe_x)₂ phase was observed in the annealed Fe/a-Si_0.8Ge_0.2 and Fe/a-Si_0.7Ge_0.3 but the Fe(Si_1−xGe_x) is still the dominant phase. The results indicate that the formation of Fe disilicide was retarded by the presence of Ge atoms.     

Features of the magnetic properties of Co/Si/Co thin-film systems

The magnetic properties of Co/Si/Co thin-film structures grown by magnetron sputtering have been studied using magnetooptical techniques. It is established that the saturation field (H _S) of trilayers exhibits oscillations as a function of the thickness of the semiconductor (silicon) interlayer. This behavior is explained by structural features of the Co/Si/Co system and the presence of antiferromagnetic exchange coupling between magnetic layers via the silicon interlayer.     

Microstructural change and interfacial reactions of Pt/Ti thin films on SiNx/Si during annealing in various ambients

The microstructural changes and interfacial reactions of Pt/Ti/SiN_x/Si during annealing were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD), and Auger electron spectroscopy (AES). Pt/Ti thin films were deposited on SiN_x/Si substrates by a d.c. magnetron sputtering system. Pt/Ti/SiN_x/Si specimens were annealed at 600°C in an oxygen ambient or a vacuum. Annealing ambients strongly influenced the microstructural change in the Pt/Ti thin films. When the specimens were annealed in oxygen, the oxygen diffused into metal layers from the ambient and reacted with Ti to form the rutile phase. When the specimens were annealed in vacuum, Ti reacted with nitrogen which was dissociated from SiN_x to form TiN and reacted with Pt to form Pt₃Ti.     

Initial stage and reconstruction of GaAs/Si heterostructures

GaAs epitaxial layers on Si(1 0 0) substrates having a single or a double domain 2×1 have been grown by molecular beam epitaxy using the two-step growth mode and thermal regrowth techniques. The initial stage and the reconstruction of the GaAs/Si heterostructures have been investigated in situ by Auger electron spectroscopy and reflection high-energy electron diffraction. GaAs layers grown by both methods show the reconstruction of a single domain, and models for the process of GaAs growth have been presented to explain the self-annihilation of the antiphase boundary.     

Interfacial reactions between Sn-3.5 Ag solder and Ni–W alloy films

Nickel based alloys are currently being investigated in an effort to develop stable barrier films between lead free solder and copper substrate. In this study, interfacial reactions between Ni–W alloy films and Sn-3.5 Ag solder have been investigated. Ni–W alloys films with tungsten content in the range of 5.0–18.0 at.% were prepared on copper substrate by electrodeposition in ammonia citrate bath. Solder joints were prepared on the Ni–W coated substrate at a reflow temperature of 250 °C. The solder joint interface was investigated by Cross-sectional scanning electron microscopy, energy dispersive X-ray spectroscopy and electron back scatter diffraction. It has been observed that a Ni₃Sn₄ layer with faceted morphology formed on the Ni–W alloy film after reflow. The thickness of the bright layer was found to decrease with the increase of tungsten content in the Ni–W film. An additional layer with a bright appearance was also found to form below the Ni₃Sn₄ layer. The bright layer was identified to be a ternary phase containing Sn, W and Ni. The bright layer is found to be amorphous and is suggested to have formed through solid state amorphization caused by anomalously fast diffusion of Sn into Ni–W film.     

Selective Nucleation of GaAs on Si Nanofacets

The early growth stage of GaAs by metal organic vapor phase epitaxy on a novel kind of Si substrate is investigated. The substrate consists of nanotips (NTs) fabricated on a Si(001) wafer by means of lithography and reactive ion etching. 3D GaAs nanocrystals are found to nucleate with a probability of 90% on the (n0m), (–n0m), (0nm), and (0–nm) facets (n, m integers) of these NTs. Additionally, in terms of nucleation yield, an average of 2 GaAs nanocrystals in each of those facets is observed. By contrast, facets of type {±nnm} remain virtually free of any 3D nuclei. A simple model based on the kinetics of the growth is used to explain the facet selective 3D nucleation. The model is consistent with a similar selectivity observed on micrometer‐sized substrate features.     

Interfacial reaction in bimetallic Sn/Cu thin films

Interfacial reaction in electroplated bimetallic Sn/Cu (the layer grown last is given first) thin films was studied by Auger depth profiling and X-ray diffraction measurements. Direct experimental evidence was found for the formation of intermetallic compounds in the SnCu interface, i.e. η'-Cu₆Sn₅ at room temperature and both η'-Cu₆Sn₅ and ε-Cu₃Sn at 150°C. The results of a quantitative analysis of the film composition and sputtering-induced effects are also discussed.     

Si diffusion in GaAs

Theoretical studies are carried out to ascertain the dominant mechanism of Si diffusion in GaAs. Lattice dynamical model calculations have shown that the most probable diffusion mechanism is through a single vacancy even though several experiments cannot fix the mechanism as substitutional, substitutional-interstitial pair or neutral defect pair.