Arsenic Precipitates Research Articles

A study of the vacancy-defect distribution in a GaAs/Al xGa 1− xAs multi-layer structure grown at low temperature

We have grown a multilayer structure of GaAs and Al x Ga 1− x As ( x=0.25) by molecular beam epitaxy at low substrate temperature (250°C) in order to investigate the effects of annealing on arsenic precipitation and vacancy-defect formation. The as-grown heterostructure contained ∼5×10 17 cm −3 gallium vacancies, but information about the individual layers was lost because the layer width (∼45 nm) was smaller than the average positron diffusion length (∼70 nm). Annealing at 500 and 600°C showed increases in the S-parameter (above the bulk value) of ∼2.5% and ∼1.5%, respectively, smaller than for a single LT-GaAs layer (∼3–4%). We explain this phenomenon by suggesting that the excess arsenic which migrates from the material with the higher precipitate/matrix energy (LT-Al x Ga 1− x As) reduces the gallium vacancy concentration in the LT-GaAs and hence the S-parameter. This hypothesis is supported by SIMS data which shows a build-up of As in the LT-GaAs layers, and TEM images which indicate that arsenic precipitation occurs to a greater extent in the LT-GaAs than in the LT-Al x Ga 1− x As.

Reciprocal Space X-Ray Mapping and Transmission Electron Microscopic Studies of Coincided δ-Inas and As-Cluster Superlattices in Gaas Films Grown by Molecular-Beam Epitaxy at Low Temperature

AbstractInAs/GaAs superlattices with thin (0.5–1 monolayer) δ-InAs insertions were grown by molecular beam epitaxy at low (150–200°C) temperature. The as-grown samples contained up to 1020 cm−3 arsenic antisite defects. Transmission electron microscopic study revealed no extended defect and showed that the real thickness of δ-InAs insertions is 3–4 monolayers. This thickness seems to be due to short-range roughness of the growth surface. Low diffuse scattering and extended interference picture were observed for such superlattices by x-ray diffraction study. Superlattices of two-dimensional cluster sheets were produced by annealing of the δ-InAs/GaAs superlattices at 500–600°C. Precipitation of excess arsenic at InAs δ-layers was found to be accompanied by enhanced In-Ga intermixing, roughening the InAs δ-layers, and smoothing the x-ray interference picture. No evidence for any self-ordering in the system of nanoscale As clusters was revealed using x-ray mapping in reciprocal space.

Strain Directed Assembly of Nanoparticle Arrays Within a Semiconductor

The use of strain to direct the assembly of nanoparticle arrays in a semiconductor is investigated experimentally and theoretically. The process uses crystal strain produced by a surface structure and variations in layer composition to guide the formation of arsenic precipitates in a GaAs-based structure grown at low temperature by molecular beam epitaxy. Remarkable patterning effects, including the formation of single and double one-dimensional arrays with completely clear fields are achieved for particles in the 10-nm size regime at a depth of about 50-nm from the semiconductor surface. Experimental results on the time dependence of the strain patterning indicates that strain controls the late stage of the coarsening process, rather than the precipitate nucleation. Comparison of the observed particle distributions with theoretical calculations of the stress and strain distributions reveals that the precipitates form in regions of maximum strain energy, rather than near extremum points of hydrostatic stress or dilatation strain. It is therefore concluded that the patterning results from modulus differences between the particle and matrix materials rather than from other strain related effects. The results presented here should be useful for extending strain directed assembly to other materials systems and to other configurations of particles.

Transient enhanced diffusion and defect microstructure in high dose, low energy As+ implanted Si

(001) CZ silicon wafers were implanted with As+ at 100 keV to a dose of 1×1015/cm2 in order to produce a continuous amorphous layer to a depth of about 120 nm. Furthermore, the implant condition was such that the peak arsenic concentration was below the arsenic clustering threshold. Subsequently, a second As+ or Ge+ implant was performed at 30 keV to doses of 2×1015/cm2, 5×1015/cm2 and 1×1016/cm2, respectively, into the as-implanted samples. All of the samples were annealed at 800 °C for 1 h. The second implant was designed to be contained within the amorphous region created by the initial implant. The second As+ implant was also designed to provide the additional arsenic needed to exceed the critical concentration for clustering at the projected range. Of the three samples with the dual As+ implant the clustering threshold was exceeded for the two lower doses while the SiAs precipitation threshold was exceeded at the highest dose. In the case of the dual As+/Ge+ implants the clustering and precipitation thresholds were not reached. Since arsenic and germanium are similar in mass the extent of damage created by these implants would be comparable. The implanted and annealed specimens were analyzed using secondary ion mass spectroscopy and transmission electron microscopy. The difference in the defect evolution and the transient-enhanced diffusion of arsenic beyond the end-of-range region between the As+ and Ge+ implanted and annealed samples was used to isolate the effects of arsenic clustering and precipitation. The results showed that point defects induced during clustering and/or precipitation did not contribute to the enhanced diffusion of arsenic although these defects did coalesce to form extended defects at the projected range. However, damage beyond the end-of-range region did cause enhanced diffusion of arsenic.

Observation of Coulomb Staircases in Arsenic Precipitates in Low-Temperature Grown GaAs

Observation of Coulomb Staircases in Arsenic Precipitates in Low-Temperature Grown GaAs

Annealing cycle dependence of preferential arsenic precipitation in AlGaAs/GaAs layers

The spatial distribution of arsenic precipitates formed in a nonstoichiometric AlGaAs/GaAs quantum well is examined for different annealing temperatures and times. Preferential precipitation in the GaAs layer of samples annealed at 600 °C is found to be much weaker than in samples annealed at 850 °C because of the reduced diffusion of arsenic at lower temperatures. Nevertheless, it is demonstrated that strong preferential precipitation is possible at low annealing temperatures, provided that the annealing time is sufficiently long. Limitations to the preferential precipitation process imposed by interface mixing and the decrease in gallium vacancy concentration during annealing are also examined.

Formation of two-dimensional arsenic precipitation in superlattice structures of alternately undoped and heavily Be doped GaAs with varying periods grown by low-temperature molecular beam epitaxy

As precipitates in superlattice structures of alternately undoped and [Be]=2.4×10 20 cm −3 doped GaAs with varying periods grown by molecular beam epitaxy at low substrate temperatures were studied by transmission electron microscopy. Novel arsenic precipitate microstructures were observed in annealed samples, including preferential accumulation of precipitates inside the Be-doped GaAs but near each interface of Be-doped GaAs and the following grown undoped GaAs. The confinement reaches the extreme for samples annealed at 800°C, where the precipitates appear as dot arrays along such interfaces and leave other areas almost free of precipitates. The incorporation of substitutional Be acceptors is believed to cause a smaller lattice constant in the heavily Be-doped regions than in the undoped regions. A strain-induced mechanism was proposed to account for the preferential segregation of As clusters, though the underlying mechanism is not fully clear.

Two-dimensional arsenic precipitation in superlattice structures of alternately undoped and heavily Be-doped GaAs grown by low-temperature molecular beam epitaxy

The precipitation of arsenic in superlattice structures of alternately undoped and [Be]=2.4×1019 cm−3 doped GaAs grown at low temperatures has been studied using transmission electron microscopy. Novel precipitate microstructures were observed in annealed samples, including preferential accumulation of precipitates toward each interface of Be-doped GaAs and the following grown undoped GaAs. Specifically, after 800 °C annealing, the precipitates are totally confined in Be-doped regions, forming two-dimensional dot arrays near the aforementioned interfaces. Data are also presented to show that the heavily Be-doped GaAs has a smaller lattice constant than the undoped GaAs. A strain-induced mechanism was proposed to account for the segregation of As clusters.

Time Dependence of Arsenic Precipitates' Size Distribution in Low Temperature GaAs

ABSTRACTThe time dependence of the size distribution of arsenic precipitates during annealing for both large (~10nm) and small size (~4nm) regimes is investigated. A narrowing of the size distribution is observed in the small size regime. This improvement in size uniformity is in marked contrast to what is observed for larger precipitates, which coarsen with a widening distribution similar to that of classical Ostwald ripening. Inverse coarsening caused by an elastic interaction between small precipitates due to coherency strain is a possible mechanism for this interesting and potentially useful behavior.

A preliminary investigation of the ferric leaching of a pyrite/arsenopyrite flotation concentrate

Although considerable work on the leaching of pyrite using Fe(III) is reported in the literature, to date very little work on the chemical leaching of arsenopyrite, using Fe(III) has been reported. Batch, ferric leaching experiments, each 48 hours in length, were carried out in sealed, baffled, agitated vessels. Varying quantities of Fe(III), ranging from 0 to 0.54 M were added to a slurry, containing 10 g.ℓ −1 concentrate. The redox potential was monitored continuously. Samples were taken at regular intervals and the total iron and arsenic concentration of the supernatant was determined The results showed that no leaching occurred in the absence of Fe(III). However, considerable leaching of arsenic occurred in the reactors to which Fe(III) was added Furthermore the results suggest that the chemical leaching rate may be a function of the redox potential, and not of the absolute Fe(III) concentration. The stoichiometry determined for the ferric leaching of arsenopyrite agreed with the stoichiometry postulated by previous researchers [1], viz.: FeAsS + 5Fe 3+ → S 0 + As 3+ + 6Fe 2+ Furthermore, the results obtained are consistent with the hypothesis that the leaching of arsenopyrite, the oxidation of As(III) to As(V), the leaching of pyrite, and the precipitation of ferric arsenate compete for the available Fe(III).

Characterization of Natural Enargite/Aqueous Solution Systems by Electrochemical Techniques

The electrochemical behavior of natural enargite electrodes in stirred and quiescent aqueous solutions covering a wide pH range was investigated using ac and dc electrochemical techniques. It is proposed that in the first stage of enargite oxidation the release of copper ions takes place and a copper‐depleted passivating film is formed on the electrode surface. At more positive potential the film becomes oxidized but depending on solution pH, a secondary passivation can be attained due to the precipitation of cupric arsenate. The electroreduction of enargite or a possible oxidized film formed during the preliminary treatment of the mineral surface initially releases S(II‐) species and generates a surface film containing and . At more negative potentials the electroreduction of to metallic As predominates. A dynamic system analysis carried out at pH 9.2 using electrochemical impedance spectroscopy gave additional information about the processes occurring on the enargite surface under anodic polarization.

Structures and defects in arsenic-ion-implanted GaAs films annealed at high temperatures

The structures and defects are studied in arsenic-ion-implanted GaAs(As+–GaAs) films annealed at temperatures higher than 600 °C by using transmission electron microscopy, deep level transient spectroscopy, temperature-dependent conductance, and photoluminescence. The estimated concentration of arsenic precipitates in films decreases from ∼4×1016 cm−3 to ∼6×1015 cm−3 and the corresponding size increases from ∼3 to ∼10 nm as the annealing temperature increases from 600 to 800 °C. A defect with an energy level at about 0.3 eV from the band edge is found and its concentration increases with the increasing annealing temperatures. The electrical transport of free carriers is replaced by hopping conduction, through the defect band at about 0.26 eV below conduction band, when the film is annealed at temperature 800 °C. It indicates that during high-temperature annealing the defect of the arsenic and gallium vacancies due to the diffusion of As and Ga atoms is the dominant factor to change its electrical and structural properties.

Diffusion‐Induced Precipitation Model of Arsenic in Arsenosilicate Glass

A model for the precipitation of arsenic in arsenosilicate glass (AsSG) is developed to simulate the increased arsenic concentration at the glass/silicon interface after high temperature heat‐treatment. In this model, the precipitation occurs at the interface when arsenic concentration exceeds the solubility limit and grows laterally until it covers the entire interface. This assumption is partly supported by secondary ion mass spectrometry and transmission electron microscopy results showing the presence of silicon arsenide precipitates near the interface. Reasonable agreement with published experimental data is achieved. This model can be used to predict the precipitation, and thereby develop novel techniques to avoid precipitate formation, and to optimize the material developments.

Photoreflectance of low-temperature-grown GaAs on Si-δ-doped GaAs

Photoreflectance has been used to study the Fermi-level of annealed low temperature (200°C) grown GaAs which is passivated on Si-δ-doped GaAs. The Fermi-level of the samples are measured by the photovoltaic effect of the built-in electric field between the interface of the low temperature grown cap layer and the Si-δ-doped layer. The annealing temperature of the low temperature cap is 600–900°C. The Fermi-levels of annealed low temperature GaAs are found to decrease from 0.55 eV to 0.40 eV below the conduction band when the annealing temperatures are increased from 600°C to 900°C. These results are connected to the arsenic precipitation at the different annealed temperatures.

Properties of GaAs single crystals grown by molecular beam epitaxy at low temperatures

Properties of GaAs single crystals grown at low temperatures by molecular beam epitaxy (LTMBE GaAs) have been studied. The results show that excessive arsenic atoms of about 1020 cm−3 exist in LTMBE GaAs in the form of arsenic interstitial couples, and cause the dilation in lattice parameter of LTMBE GaAs. The arsenic interstitial couples will be decomposed, and the excessive arsenic atoms will precipitate during the annealing above 300°C. Arsenic precipitates accumulate in the junctions of epilayers with the increase in the temperature of annealing. The depletion regions caused by arsenic precipitates overlap each other in LTMBE GaAs, taking on the character of high resistivity, and the effects of backgating or sidegating are effectively restrained.

Electrical properties of n-GaAs epilayers, FET and HEMT structures grown on LT-GaAs by MBE

We have investigated the dependence of the conductive layer width and Hall mobility in n-GaAs/LT-GaAs structures on the conditions of low temperature (LT) buffer growth and on annealing parameters. Both the conductive layer width, as determined from electrochemical C- V profiling, as well as the carrier mobility of the doped epilayer decrease as the LT-GaAs growth temperature is reduced and decrease with duration and temperature of annealing. This degradation is attributed to the outdiffusion of excess arsenic and the formation of arsenic precipitates in the n-GaAs epilayer and to the outdiffusion of point defects, as indicated by Photoluminescence observations. The use of Al x Ga 1 − x As/GaAs superlattice and GaAs as intermediate layers between the LT buffer and the active layers in field effect transistor (FET) and high electron mobility transistor (HEMT) structures results in maintaining both carrier mobility and doping profile at conventional layer levels. The improvement is attributed to the suppression of excess As and point defect outdiffusion from the LT-GaAs to the active layer.

Fundamental Modeling of Transient Enhanced Diffusion through Extended Defect Evolution

Prediction of transient enhanced diffusion (TED) requires modeling of extended defects of many types, such as {311} defects, dislocation loops, boron-interstitial clusters, arsenic precipitates, etc. These extended defects not only form individually, but they also interact with each other through changes in point defect and solute concentrations. We have developed a fundamental model which can account for the behavior of a broad range of extended defects, as well as their interactions with each other. We have successfully applied and parameterized our model to a range of systems and conditions, some of which are presented in this paper.

Aggregation and precipitation in high dose As ion implanted Ge 0.5Si 0.5 alloy

Thermally stimulated redistribution and precipitation of excess arsenic in Ge 0.5Si 0.5 alloy has been studied by X-ray photoelectron spectroscopy (XPS), cross sectional transmission electron microscopy (XTEM) and X-ray energy disperse spectrometry (EDS). Samples were prepared by the implantation of 6 × 10 6 As + cm −2 and 100 keV with subsequent thermal processing at 800°C and 1000°C for 1 h. The XPS depth profiles from the implanted samples before and after the thermal annealing indicate that there is marked redistribution of the elements in heavily arsenic-implanted Ge 0.5Si 0.5 alloys during the annealing, including: (1) diffusion of As from the implanted region to the surface; (2) aggregation of Ge in the vicinity of the surface. A high density of precipitates was observed near the surface which were by XTEM and EDS identified as an arsenide. It is suggested that most of the implanted As in Ge 0.5Si 0.5 alloy exists in the form of GeAs.

The effect of arsenic overpressure on the structural properties GaAs grown at low temperature

The structural properties of GaAs grown by molecular-beam epitaxy at low temperatures have been investigated by scanning electron microscopy, transmission electron microscopy, and high-resolution x-ray double-crystal rocking curves as a function of arsenic overpressure during growth. It was found that surface smoothness and excess arsenic incorporation both depend strongly on growth temperature and on As/Ga flux ratio. For each growth temperature there is a ‘‘window’’ in the flux ratio which results in smooth surfaces. As-grown layers have an increased lattice constant in the growth direction. This relative lattice expansion increases with flux ratio at a constant growth temperature and eventually saturates. Transmission electron micrographs have revealed the presence of arsenic precipitates in material annealed at 600 °C. Increasing the As4 pressure during growth results in increases in precipitate diameter by almost 50% while their density and shape remain constant. Based on these observations a model has been developed to explain the lattice expansion dependence on arsenic overpressure.

Improvement in low energy ion-induced damage with a low temperature GaAs capping layer

A thin capping layer of annealed GaAs (∼210 Å) grown at low temperature (LT-GaAs) can effectively block incident ions from penetrating into the growth substrate. Ion-bombarded, multiple quantum well structures capped by an annealed LT-GaAs layer show a dramatic improvement in the photoluminescence, compared to samples capped with ‘‘normal’’ GaAs. The improvement appears to be correlated with the microstructures of the LT-GaAs, since the improvement is particularly notable for samples annealed at 600 °C. This improvement in low energy ion-induced damage is primarily the result of the reduced channeling of ions through the LT-GaAs layer. These results suggest a potential application of LT-GaAs in reducing ion damage and underscore the importance of the microstructure of arsenic precipitates in LT-GaAs layers.