Electric characterization of GaAs deposited on porous silicon by electrodeposition technique

GaAs thin films were synthesized on porous Si substrate by the electrodeposition technique. The X-ray diffraction studies showed that the as-grown films were crystallised in mixed phase nature orthorhombic and cubic of GaAs. The GaAs film was then electrically characterized using current-voltage (I-V) and capacitance-voltage (C-V) techniques by the way of Al/GaAs Shottky junctions. The electric analysis allowed us to determine the n factor and the barrier height Ф_b0 parameters of Al/GaAs Schottky junctions. The (C-V) characteristics were recorded at frequency signal 1 MHz in order to identify the effect of the surface states on the behaviour of the capacitance of the device.     

Incorporation of deep centres in VPE GaAs

The incorporation of deep levels in high purity vapour phase epitaxy (VPE) GaAs is studied as a function of the crystal growth conditions. Two deep levels, at 0.4 eV and 0.75 eV above the valence band, are investigated using photocapacitance. It is shown that their concentrations are always equal and vary together as a function of the AsCl₃ mole fractionX. Two regimes are observed, respectively, characterized by different variations of the total deep level concentrationN _T:N _TαX for lowX, andN _TαX⁻² for highX. In this last range,N _T andN _D are found to vary similarly. This work has been supported by the DRME.     

Thermal and mechanical damage of GaAs in picosecond regime

An in-depth study of the single pulse and multiple pulse laser (35 ps, 10 Hz and 1064 nm) damage for threshold fluence and greater fluence of GaAs 1 0 0 single crystal is presented. Damage which starts at a power 2×10¹¹ W/cm² in the form of pits occurs due to accumulation of laser induced microscopic defects. Effect of multiple pulse at first makes the pits more prominent in the form of Ga emission. Then the topmost layer is removed. If the number of pulses is further increased new pits are formed in the new surface (beneath the removed surface) and the above process is repeated. The thermal model is sufficient to explain this morphology. However, for larger fluences, a large cracking and fracture and the possibility of both Ga and As emission in different ratios suggest that mechanical damage is a dominant feature for higher fluences which arises due to generation of shock waves and rapid vaporization of material. Damage threshold has been calculated with the help of the thermal model given by Meyer et al. which is in good agreement with our experimental results.     

Electrical and XPS studies of 100 MeV Si ion irradiated Pd/n-GaAs devices

The effect of Swift Heavy Ion (100 MeV Si⁷⁺) irradiation on electronic-transport of Pd/n-GaAs devices has been studied by I–V and C–V techniques. The chemical compositions of the interface have been studied by XPS/EDAX techniques. It is observed that the irradiated devices show a reduction in current and capacitance by few orders of magnitude. The C–V characteristics show a change in conductivity type from n- to p-type after the irradiation. On hydrogenation, the irradiated devices show a capacitance peak in C–V characteristics, which has been ascribed to As vacancies. The XPS studies of these devices, for various etching durations, show that the ratio of As:Ga has reduced after the irradiation, which indicates the formation of irradiation-induced As vacancies. This reduction in As:Ga ratio is also confirmed by EDAX measurement. The observed conductivity type change from n- to p-type (on the irradiation) seems to be due to the change of substitutional sites of dopant silicon atoms from Ga to As sites due to the irradiation-induced As vacancies.     

The effect of hydrogen as an additive in reactive ion etching of GaAs for obtaining polished surface

The effect of hydrogen on the reactive ion etching (RIE) of GaAs in the CF₂Cl₂ plasma is discussed. The addition of hydrogen into the reaction mixture improves the sharpness of etch borders; the etched surface is smooth for etching depth > 1 μm, etching rate is time-constant.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

Elimination of interface states in the GaAs band-gap by cyanide treatment: XPS measurements under bias

Energy distribution of interface states for GaAs-based metal-oxide-semiconductor structure with an ultra-thin silicon oxide layer is obtained from “XPS measurements under bias.” The interface state spectra have peaked-structure at 0.7 and 0.9 eV above the valence band maximum and they are attributed to (++/+) and (+/0) transitions of As_Ga antisite defects at the interface. When cyanide treatment (i.e., the immersion in a KCN aqueous solution followed by the rinse in boiling water) is performed after the deposition of the silicon oxide layer, the interface state density is decreased to ∼50%, resulting in the partial unpinning of the Fermi level.     

Energetics of the growth mode transition in InAs/GaAs(0 0 1) small quantum dot formation: A first-principles study

Based on first-principles density-functional pseudopotential calculations, the growth of InAs on the GaAs(0 0 1) surface has been studied. By analyzing the free energies of the surfaces with different thicknesses of the InAs coverages, the critical thickness of the layer-by-layer (2D) to island (3D) growth mode transition is predicted to be around 1.5 ML. Comparing the total energy differences between layer-by-layer growth models and 3D island models, the mechanism of the 2D-3D growth mode transition near the critical thickness (θ_crit) is studied which indicates that at the initial stage of InAs quantum dots formation, small 3D islands are formed randomly.     

Early stages of interface formation of C60 on GaAs(1 0 0)

We present a detailed investigation of the electronic properties of C₆₀ grown on GaAs(1 0 0) substrates, as a function of the fullerene coverage, from the very early stages of interface formation up to the development of a bulk-like fullerene film. We monitor the chemical interactions and the energy levels alignment by means of X-rays, ultraviolet and inverse photoemission spectroscopies. The two latter techniques allow to investigate the electronic structure close to the Fermi level. Energy levels alignment at the interfaces of C₆₀ with p-doped and GaAs(1 0 0) are obtained and discussed.     

Surface reactions of 2-iodopropane on GaAs(1 0 0)

The surface reactions of 2-iodopropane ((CH₃)₂CHI) on gallium-rich GaAs(1 0 0)-(4 × 1), was studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). CH₃CHICH₃ adsorbs molecularly at 120 K but dissociates below room temperature to form chemisorbed 2-propyl ((CH₃)₂CH) and iodide (I) species. Thermal activation causes desorption of the molecular species at 240 K, and this occurs in competition with the further reactions of the (CH₃)₂CH and I chemisorbed species. Self-coupling of the (CH₃)₂CH results in the formation of 2,3-dimethylbutane ((CH₃)₂CH-CH(CH₃)₂) at 290 K. β-Hydride elimination in (CH₃)₂CH yields gaseous propene (CH₃CHCH₂) at 550 K while reductive elimination reactions of (CH₃)₂CH with surface hydrogen yields propane (CH₃CH₂CH₃) at 560 K. Recombinative desorption of the adsorbed hydrogen as H₂ also occurs at 560 K. We observe that the activation barrier to carbon-carbon bond formation with 2-propyls on GaAs(1 0 0) is much lower than that in our previous investigations involving ethyl and 1,1,1-trifluoroethyl species where the β-elimination process was more facile. The difference in the surface chemistry in the case of 2-propyl species is attributable to its rigid structure resulting from the bonding to the surface via the second carbon atom, which causes the methyl groups to be further away from the surface than in the case of linear ethyl and 1,1,1-trifluoroethyl species. The β-hydride and reductive elimination processes in the adsorbed 2-propyl species thus occurs at higher temperatures, and a consequence of this is that GaI desorption, which is expected to occur in the temperature range 550-560 K becomes suppressed, and the chemisorbed iodine leaves the surface as atomic iodine.     

Evidence of formation of tetravacancies in uniformly oxygen irradiated n-type silicon

High purity n-type silicon single crystal with resistivity in the order of 4000 Ω cm has been irradiated with high-energy oxygen ions at room temperature up to a fluence of 5E15 ions/cm². The energy of the beam was varied from 3 to 140 MeV using a rotating degrader to achieve a depthwise near-uniform implantation profile. Radiation induced defects and their dynamics have been studied using positron annihilation spectroscopy along with isochronal annealing up to 700 °C in steps of 50 °C for 30 min. After annealing the sample at 200 °C for 30 min, formation of silicon tetravacancies has been noticed. The formation of the tetravacancies was found to be due to agglomeration of divacancies present in the irradiated sample. An experimentally obtained positron lifetime value of 338±10 ps has been reported for silicon tetravacancies, which has a very close agreement with the value obtained from recent theoretical calculations. The tetravacancies were found to dissociate into trivacancy clusters upon further annealing. The trivacancies thus obtained were observed to agglomerate beyond 400 °C to form larger defect clusters. Finally, all the defects were found to anneal out after annealing the sample at 650 °C.     

Boron and indium substitution in GaAs(0 0 1) surfaces: Density-functional supercell calculations of the surface stability

Using the density-functional method and surface supercells the surface formation energies are calculated for the most stable GaAs(0 0 1) surface reconstructions without and with up to four indium or/and boron substitutions. Optimal conditions for the growth of the alloys are derived from calculated surface stability diagrams. The incorporation of indium into GaAs without phase separation is possible under strong As-rich conditions and medium to In-rich conditions. Less As-rich conditions can lead to the formation of an InAs phase. Ga-rich conditions give an InGa phase. Isovalent boron incorporation into GaAs without phase separation is possible under strong arsenic and reduced boron exposure. A BAs phase can be formed under more B-rich conditions. More Ga-rich conditions lead to the boron substitution in arsenic positions. The formed boron dimers can be a starting point for the formation of a boron phase. A true antisite boron substitution is less probable. Using the suitable growth conditions obtained for the ternary alloys it is energetically more favourable to incorporate both indium and boron (formation of BInGaAs) than to incorporate only one of the two elements (In or B). The antisite boron incorporation is not favoured in combination with isovalent boron or indium.     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Trench formation and lateral damage induced by gallium milling of silicon

Molecular dynamics simulations are performed to model the nanomachining of materials via focused ion beams (FIBs). The goal of this research is to investigate the fundamental dynamics which govern the interaction of FIB with materials which are vital to the semiconductor industry, namely silicon. Specifically, we focus on the formation of trenches/holes within the sample and the dynamics responsible for their characteristic v-shape, as well as the extent of lateral damage due to a gallium beam. These phenomena have been successfully modelled, with evidence that the lateral and subsurface damage created is much larger than the beam itself. The results presented here begin to elucidate the dynamics governing the spatial resolution of these experiments, and provide an idea of some of the technical issues associated with these beams.     

Surface reconstructions and phase transitions on the GaAs(111)B surface

The (111)B surface of GaAs has been investigated using scanning tunneling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)_LT, ( ) and (1 × 1)_HT with increasing annealing temperature, corresponding to decreasing surface As concentration. The (1 × 1)_LT is of particular interest, since it only occurs in a narrow temperature window between the two more established reconstructions, the (2 × 2) and the ( ). We find the (1 × 1)_LT to take the form of a mixture of the local structures of both the (2 × 2) and ( ) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)_LT transition, atomic resolution images of the ( ) surface reveal only a three-fold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding due to the 23.4° rotation of the surface unit cell relative to the substrate. This is responsible for the disorder found in the ( ) reconstruction, since the structure may form in one of two domains. At lower surface As concentration, the (1 × 1)_HT surface adopts a structure combining small domains of a 19.1° structure and random disorder. There is no apparent similarity between the (1 × 1)_LT and (1 × 1)_HT structures, which may be due to our measurements being conducted at room temperature and without an As flux to control the surface As concentration.     

GaAs光生载流子动力学机制的超快光谱学分析

利用飞秒激光泵浦探测技术,通过改变光学参数,如中心波长、功率,分别对未故意掺杂高纯n型砷化镓的差分反射谱进行研究,进而分析室温下砷化镓光生载流子动力学过程.首先,当泵浦光功率恒为100mW,探测光功率恒为10mW时,随着中心波长的增大,差分反射率峰值随之增大,信噪比也随之增加.其次,通过拟合不同延迟时间下泵浦光功率和差分反射率的实验曲线,并和理论模型比较后发现,在一定范围内的泵浦功率和差分反射率呈线性相关,未故意掺杂高纯n型砷化镓的饱和载流子浓度为(3.590 1±0.310 3)×1017 cm~(-3).在此基础上,把光生载流子动力学过程分为3个过程:804±67fs的光激发过程、134~268fs的初始散射过程、1ps和3~6ps的复合过程.研究表明,差分反射率与探测功率不存在显著的依赖性,但差分反射谱的信噪比与探测功率存在相关性.     

Accurate Small-Signal Model Extraction for pHEMT on GaAs

An accurate small-signal modeling approach applied to GaAs-based pHEMT devices is presented. The procedure for extracting equivalent-circuit model parameters is illustrated in detail. A genetic algorithm (GA) program is developed to optimize the model parameters in order to improve the modeling accuracy. The validity of modeling approach is verified by comparing the simulated and measured result of two pHEMTs and a fabricated ka-band power amplifier. The conclusion can be drawn that the proposed modeling method is rather accurate and efficient.     

Chemisorption of Cl on GaAs: a density functional-LCAO approach

A linear combination of atomic orbitals method supplemented by a local density approximation is applied to the calculation of Cl adsorbed on GaAs(110). We present results for coverages of and 1 monolayer. Our results show that Cl in the early stages of deposition tends to adsorb on Ga, although the deposition of Cl-pairs on Ga and As nearest neighbours is very likely. We also find that the majority deposition of Cl on Ga tends to pin the interface Fermi level in the As-like dangling bond, near the valence band top. These results are in good agreement with the available experimental data.     

Reconfigurations and diffusion of trivacancy in silicon

Disappearance of the divacancy (V₂) and trivacancy (V₃) complexes upon isochronal and isothermal annealing of electron irradiated Si:O crystals has been studied by means of deep level transient spectroscopy. The annealing studies have shown that the V₂ and V₃ defects are mobile in Si at T>200 °C and in oxygen-rich material are trapped by interstitial oxygen atoms so resulting in the appearance of V₂O and V₃O defects. The activation energies for diffusion of the V₂ and V₃ centers have been determined. Density functional modeling calculations have been carried out to investigate the migration and reorientation mechanisms of V₃ in large silicon supercells. It is proposed that these comprise a sequence of transformations between V₃(D₃) and V₃(C_2v) configurations.