Bi Fluxes Research Articles

Exploring the Implementation of GaAsBi Alloys as Strain-Reducing Layers in InAs/GaAs Quantum Dots.

This paper investigates the effect of GaAsBi strain reduction layers (SRLs) on InAs QDs with different Bi fluxes to achieve nanostructures with improved temperature stability. The SRLs are grown at a lower temperature (370 °C) than the usual capping temperature for InAs QDs (510 °C). The study finds that GaAs capping at low temperatures reduces QD decomposition and leads to larger pyramidal dots but also increases the threading dislocation (TD) density. When adding Bi to the capping layer, a significant reduction in TD density is observed, but unexpected structural changes also occur. Increasing the Bi flux does not increase the Bi content but rather the layer thickness. The maximum Bi content for all layers is 2.4%. A higher Bi flux causes earlier Bi incorporation, along with the formation of an additional InGaAs layer above the GaAsBi layer due to In segregation from QD erosion. Additionally, the implementation of GaAsBi SRLs results in smaller dots due to enhanced QD decomposition, which is contrary to the expected function of an SRL. No droplets were detected on the surface of any sample, but we did observe regions of horizontal nanowires within the epilayers for the Bi-rich samples, indicating nanoparticle formation.

Bismuth surfactant-enhanced III-As epitaxy on GaAs(111)A

Quantum dot (QD) growth on high () symmetry GaAs{111} surfaces holds promise for efficient entangled photon sources. Unfortunately, homoepitaxy on GaAs{111} surfaces suffers from surface roughness/defects and InAs deposition does not natively support Stranski–Krastanov QD growth. Surfactants have been identified as effective tools to alter the epitaxial growth process of III-V materials, however, their use remains unexplored on GaAs{111}. Here, we investigate Bi as a surfactant in III-As molecular beam epitaxy on GaAs(111)A substrates, demonstrating that Bi can eliminate surface defects/hillocks in GaAs and (Al,Ga)As layers, yielding atomically-smooth hillock-free surfaces with RMS roughness values as low as 0.13 nm. Increasing Bi fluxes are found to result in smoother surfaces and Bi is observed to increase adatom diffusion. The Bi surfactant is also shown to trigger a morphological transition in InAs/GaAs(111)A films, directing the 2D InAs layer to rearrange into 3D nanostructures, which are promising candidates for high-symmetry QDs. The desorption activation energy () of Bi on GaAs(111)A was measured by reflection high energy electron diffraction, yielding = 1.7 ± 0.4 eV. These results illustrate the potential of Bi surfactants on GaAs(111)A and will help pave the way for GaAs(111)A as a platform for technological applications including quantum photonics.

Bi incorporation and segregation in the MBE-grown GaAs-(Ga,Al)As-Ga(As,Bi) core\u2013shell nanowires

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments.

Modelling of bismuth segregation in InAsBi/InAs superlattices: Determination of the exchange energies

InAsBi dilute alloys are potential new candidates for the improvement of infrared optoelectronic devices such as photodetectors or lasers. In this work, InAsBi/InAs superlattices (SLs) with Bi contents ranging between 1 and 3% were grown by molecular beam epitaxy with different Bi fluxes and growth temperatures to analyze Bi segregation by cross sectional transmission electron microscopy techniques. Bi segregation profiles have been described layer-by-layer using a three-layer fluid exchange mechanism, extracting the values of the As/Bi exchange energies (E1, 1.26 ± 0.01 eV and E2, 1.36 ± 0.02 eV). A relationship to calculate the activation energies for exchange from the binding energies in III–V alloys is proposed, which would allow predicting them for other hitherto unknown compounds.

Analysis of GaAsBi growth regimes in high resolution with respect to As/Ga ratio using stationary MBE growth

The control of Bi incorporation and material properties in III-V-Bi alloys has proved challenging due to their high sensitivity to the epitaxial growth parameters. Here, we present a methodology for determining the variation in the Ga, As, and Bi fluxes and the temperature across a stationary substrate in molecular beam epitaxy. By correlating the flux distributions with material properties, we identify distinct regimes for epitaxy of GaAsBi. In particular, we devise a detailed image of the interplay between Bi incorporation and structural properties of a bulk GaAs0.96Bi0.04 layer grown on GaAs(1 0 0) with respect to the As/Ga ratio. The influence of As/Ga is analyzed with high resolution over the important stoichiometric range (i.e. As/Ga = 0.6–1.6). Growth outside the near-stoichiometric As/Ga regime leads to decreased Bi incorporation, decreased structural quality and the formation of Ga, Ga/Bi or Bi droplets. On the other hand, growth at As/Ga = 1.00–1.17 leads to maximized material quality. For this regime, the surface roughness is further optimized by fine-tuning the As/Ga ratio to suppress surface mounding to a value of 0.5 nm. The results reveal the extreme sensitivity of GaAsBi growth to small variations in the As/Ga ratio, and demonstrate the applicability of stationary growth in studying these effects.

Influence of growth conditions on the structural and opto-electronic quality of GaAsBi

A systematic series of GaAsBi pin diodes was grown by MBE using different growth temperatures and Bi fluxes, to study the effect on the structural and opto-electronic properties of GaAsBi. The Bi contents of the diodes show both growth temperature and Bi flux dependences. The diodes grown at higher temperatures show evidence of long range inhomogeneity from X-ray diffraction (XRD) measurements, whereas samples of comparable Bi content grown at lower temperatures appear to have well defined, uniform GaAsBi regions. However, the high temperature grown diodes exhibit more intense photoluminescence (PL) and lower dark currents. The results suggest that growth temperature related defects have a greater influence on the dark current than bismuth related defects, and therefore GaAsBi devices should be grown at the highest temperature possible for the desired Bi content.

Bi flux-dependent MBE growth of GaSbBi alloys

The incorporation of Bi in GaSb1−xBix alloys grown by molecular beam epitaxy is investigated as a function of Bi flux at fixed growth temperature (275°C) and growth rate (1μmh−1). The Bi content is found to vary proportionally with Bi flux with Bi contents, as measured by Rutherford backscattering, in the range 0<x≤4.5%. The GaSbBi samples grown at the lowest Bi fluxes have smooth surfaces free of metallic droplets. The higher Bi flux samples have surface Bi droplets. The room temperature band gap of the GaSbBi epitaxial layers determined from optical absorption decreases linearly with increasing Bi content with a reduction of ~32meV/%Bi.

Effect of bismuth on structural and electrical properties of InAs films grown on GaAs substrates by MBE

InAs films were grown on GaAs substrates by molecular beam epitaxy (MBE) without or with different Bi fluxes. The effect of Bi on the structural and electrical properties of the InAs films was studied. Atomic Force Microscopy (AFM) measurement showed clear surface steps in all samples, indicating an over-flow growth mode. A more uniform distribution and narrower spacing of the surface steps were observed with increased Bi fluxes. Small area AFM scans showed reduced surface roughness with increased Bi fluxes. Whereas from large area scans, reduced surface roughness was only observed in samples grown under low Bi fluxes, and a deteriorated surface was obtained in the sample grown under the highest Bi flux. Bi was not compositionally incorporated to the InAs films confirmed by X-ray diffraction (XRD) and Second Ion Mass Spectroscopy (SIMS) measurements. The electron mobility of the InAs films, measured at room temperature, decreased monotonically with increased Bi fluxes. This is correlated to the Transmission Electron Microscopy (TEM) results in which increased threading dislocation (TD) densities were shown with increased Bi fluxes. Therefore, Bi-mediated growth deteriorates the electron mobility of the MBE-grown InAs films, whereas smoother surface can be obtained by applying low Bi fluxes. Possible mechanisms were proposed to explain these phenomena.

Kinetically limited growth of GaAsBi by molecular-beam epitaxy

The growth of GaAsBi alloys is plagued by the appearance of Bi droplets due to excess Bi that accumulates during growth. Here we present an alternate growth regime that kinetically limits the amount of Bi on the surface, eliminating Bi droplets for a wide range of Bi compositions, while yielding atomically smooth surfaces. Growth rate plays a major role in the amount of Bi that accumulates on the surface, with high growth rates and low Bi fluxes leading to less surface Bi. A balance can be achieved between low Bi coverage, the resultant rough surfaces, and the excessive Bi coverage that leads to Bi droplets. Bi incorporation in this growth regime is linear with Bi flux and scales inversely with growth rate. Unlike previous studies, there is no sign of saturating Bi incorporation with increasing Bi flux, allowing for intuitive prediction and control of Bi content in this regime.

Metastable Cd4Sb3: A Complex Structured Intermetallic Compound with Semiconductor Properties

The metastable binary intermetallic compound Cd4Sb3 was obtained as polycrystalline ingot by quenching stoichiometric Cd-Sb melts and as mm-sized crystals by employing Bi or Sn fluxes. The compound crystallizes in the monoclinic space group Pn with a = 11.4975(5) A, b = 26.126(1) A, c = 26.122(1) A, beta = 100.77(1) degrees, and V = 7708.2(5) A(3). The actual formula unit of Cd4Sb3 is Cd13Sb10 and the unit cell contains 156 Cd and 120 Sb atoms (Z = 12). Cd4Sb3 displays a reversible order-disorder transition at 373 K and decomposes exothermically into a mixture of elemental Cd and CdSb at around 520 K. Disordered beta-Cd4Sb3 is rhombohedral (space group R3c, a approximately = 13.04 A, c approximately = 13.03 A) with a framework isostructural to beta-Zn4Sb3. The structure of monoclinic alpha-Cd4Sb3 bears resemblance to the low-temperature modifications of Zn4Sb3, alpha- and alpha'-Zn4Sb3, in that randomly distributed vacancies and interstitial atoms of the high-temperature modification aggregate and order into distinct arrays. However, the nature of aggregation and distribution of aggregates is different in the two systems. Cd4Sb3 displays the properties of a narrow gap semiconductor. Between 10 and 350 K the resistivity of melt-quenched samples first increases with increasing temperature until a maximum value at 250 K and then decreases again. The resistivity maximum is accompanied with a discontinuity in the thermopower, which is positive and increasing from 10 to 350 K. The room temperature values of the resistivity and thermopower are about 25 mohms cm and 160 microV/K, respectively. Flux synthesized samples show altered properties due to the incorporation of small amounts of Bi or Sn (less than 1 at. %). Thermopower and resistivity appear drastically increased for Sn doped samples. Characteristic for Cd4Sb3 samples is their low thermal conductivity, which drops below 1 W/mK above 130 K and attains values around 0.75 W/mK at room temperature, which is comparable to vitreous materials.

Effects of bismuth on wide-depletion-width GaInNAs solar cells

GaInNAs solar cells could be useful in next-generation multijunction solar cells if issues surrounding low photocurrents and photovoltages are surmounted. Wide-depletion-width devices generate significant photocurrent using a p-i-n structure grown by molecular beam epitaxy, but these depletion widths are only realized in a region of parameter space that leads to rough surface morphologies. Here, bismuth is explored as a surfactant for the growth of GaInNAs solar cells. Very low fluxes of Bi are effective at maintaining smooth surfaces, even at high growth temperatures and In contents. However, Bi also increases the net donor concentration in these materials, manifested in our n-on-p device structures as a pn-junction that moves deeper into the base layer with increasing Bi fluxes. Quantum efficiency modeling and scanning kelvin probe microscopy measurements confirm the type conversion of the base layer from p type to n type. Bi incorporation in GaAsBi samples shows signs of surface segregation, leading to a finite buildup time, and this effect may lead to slow changes in the electrical properties of the GaInNAs(Bi) devices. Bi also appears to create a defect level, although this defect level is not deleterious enough to increase the dark current in the devices.

Optical characterisation of Bi‐doped GaN films grown by molecular beam epitaxy

AbstractWe use cathodoluminescence and Raman spectroscopy to investigate GaN layers grown by plasma‐assisted molecular beam epitaxy with different Bi fluxes. We find that the growth of GaN with an additional Bi flux favours the formation of submicron cubic domains, which results in layers with reduced crystalline quality and lower luminescence emission. We detect a Bi‐related Raman feature at low frequencies that we tentatively assign to local Bi modes in GaN, which indicates that a fraction of Bi atoms incorporates into the GaN lattice. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects, formation of GaN 1− xBi x alloys and co-doping with arsenic

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN 1− x Bi x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.

Source strength assessment of volcanic trace elements emitted from the Indonesian arc

We have for the first time estimated volcanic emission of volatile metals for a large regional volcanic source showing permanent activity, namely the Indonesian Arc. We have done this by normalizing concentrations of metals ratios in volcanic plumes to SO 2 emission rates. Metal/SO 2 ratios measured in magmatic gases are used to estimate emissions from eruptive activity, whereas those measured in fumarolic gases are used to estimate emissions from non-eruptive activity. The SO 2 emission rate obtained is 3.5 × 10 6 tons per year, which represents about 20% of the annual worldwide volcanic flux of SO 2. The obtained trace metal ( 210Po, Pb, Bi, Cd, Zn and Cu) fluxes account for only about 5 to 30% of their global volcanic flux. These relatively low values for trace metal emissions (relative to the volcanic activity and the number of active volcanoes of the region) appear to reflect the andesitic magma composition of the volcanoes of the arc.

Crystal Growth of RuX 2 (X = S, Se, Te) by chemical vapour transport and high temperature solution growth

Single crystals of RuS 2 haave been prepared by chemical vapour transport (CVT) with chlorine in a temperature gradient from 1100 to 1050° of RuX 2 ( rmX = S, Se, Te) were grown from high temperataure Se, Te, and Bi fluxes. Best results were achieved in bismuth melts. Four different flux growth techniques have been applied: recrystallization of a fine RuS 2 powder, crystal growth by Ostwald ripening, crystallization of the solute in a temperature gradient and crystallization in a time-varying temperature profile.