xBix Films Research Articles

Strain stabilization of far from equilibrium GaAsBi films

GaAs1−xBix was grown on GaAs and InGaAs underlayers to determine the effect of global strain on bismuth (Bi) incorporation. Reducing compressive strain aids in Bi incorporation and results in higher Bi content films by preventing Ga-Bi surface droplet formation. A maximum Bi fraction of x = 0.088 was achieved in a 100 nm film on an In0.105GaAs buffer layer, a 24% relative improvement compared to layers grown on GaAs. Increasing Bi flux further did not result in incorporations past x = 0.088 on In0.105GaAs and strain could not be manipulated to incorporate more bismuth. Strain stabilization is thought to lessen Bi rejection and segregation, which increases the overall Bi content and yields homogenous films. The use of strain compensating layers can enable x = 0.06–0.09 content pseudomorphic GaAs1−xBix films with uniform composition at thicknesses relevant for optoelectronic devices.

Droplet-mediated formation of embedded GaAs nanowires in MBE GaAs1−xBix films

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs1−xBix using high angle annular dark field (‘Z-contrast’) imaging in an aberration-corrected scanning transmission electron microscope. Samples were grown in Ga-rich growth conditions on a stationary GaAs substrate. Ga-rich droplets are observed on the surface with lateral trails extending from the droplet in the [110] direction. Cross-sectional scanning transmission electron microscopy of the film reveals epitaxial nanowire structures of composition ∼GaAs embedded in the GaAs1−xBix epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (∼4°). They typically are 4 μm long and have a lens-shaped cross section with major and minor axes dimensions of 800 and 120 nm. The top surface of the nanowires exhibits a linear trace in longitudinal cross-section, across which the composition change from ∼GaAs to GaAs1−xBix appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over ∼25 nm. The droplets have phase separated into Ga- and Bi-rich components. A qualitative model is proposed in which Bi is gettered into Ga droplets, leaving Bi depleted nanowires in the wakes of the droplets as they migrate in one direction across the surface during GaAs1−xBix film growth.

Increased bismuth concentration in MBE GaAs1−xBix films by oscillating III/V flux ratio during growth

The authors have examined bismuth concentration profiles in GaAs1−xBix films grown by molecular beam epitaxy using high angle annular dark field imaging (Z-contrast imaging) in an aberration-corrected scanning transmission electron microscope in conjunction with x-ray diffraction. Samples were grown with a gradient in each of the component fluxes, and therefore, the III/V ratio across the substrate. Rotating the sample during growth exposed the growth surface to an oscillating III/V flux ratio. Sinusoidal [Bi] profiles resulted in the growth direction, the wavelength and number of which were consistent with the growth rate and the rate of substrate rotation. However, the magnitude of [Bi] in the observed fluctuations was greater than the maximum [Bi] achieved using the same Bi flux and Ga/As flux ratios in steady-state conditions on a stationary substrate, suggesting that varying the III/V flux ratio during growth promotes the incorporation of Bi in GaAs1−xBix films. A proposed qualitative model for how this enhancement might occur hypothesizes a critical role for alternating growth and shrinkage of Ga-Bi predroplet clusters on the surface as the growing material is rotated through Ga-rich and As-rich flux compositions.

Bandgap and optical absorption edge of GaAs1−xBix alloys with 0 < x < 17.8%

The compositional dependence of the fundamental bandgap of pseudomorphic GaAs1−xBix layers on GaAs substrates is studied at room temperature by optical transmission and photoluminescence spectroscopies. All GaAs1−xBix films (0 ≤ x ≤ 17.8%) show direct optical bandgaps, which decrease with increasing Bi content, closely following density functional theory predictions. The smallest measured bandgap is 0.52 eV (∼2.4 μm) at 17.8% Bi. Extrapolating a fit to the data, the GaAs1−xBix bandgap is predicted to reach 0 eV at 35% Bi. Below the GaAs1−xBix bandgap, exponential absorption band tails are observed with Urbach energies 3–6 times larger than that of bulk GaAs. The Urbach parameter increases with Bi content up to 5.5% Bi, and remains constant at higher concentrations. The lattice constant and Bi content of GaAs1−xBix layers (0 < x ≤ 19.4%) are studied using high resolution x-ray diffraction and Rutherford backscattering spectroscopy. The relaxed lattice constant of hypothetical zincblende GaBi is estimated to be 6.33 ± 0.05 Å, from extrapolation of the Rutherford backscattering spectrometry and x-ray diffraction data.

Synthesis and characterization of Bi doped Se–Te nanostructured thin films

The nanostructure formation has been revealed in different types of materials and therefore an increasing interest has been witnessed in recent years to study the amorphous materials. Nano-structured materials show dramatic changes in their structural, optical and electrical characteristics due to the decrease in size below excitonic limit and provide new physical and chemical properties. In the present research work amorphous Se80Te20−xBix (x=3 and 6) was prepared by melt quenching techniques. Nano-structured thin films of amorphous Se80Te20−xBix have been obtained using physical vapor condensation process on glass/Si wafer substrate. The morphology of amorphous Se80Te20−xBix films was scanned by employing field emission scanning electron microscope (FESEM). The optical absorption and reflection spectra of nano-structured films were inspected by UV/VIS/NIR spectrophotometer. The optical absorption analysis indicates the non-direct transitions predominate in nano-structured Se80Te20−xBix films. The extinction coefficient and absorption coefficient are found to decrease while optical energy band gap and the refractive index increase with Bi contents in Se–Te system.

Growth of GaAs1−xBix by molecular beam epitaxy: Trade-offs in optical and structural characteristics

Recent work has shown that Bi incorporation increases during molecular beam epitaxy (MBE) when surface processes are kinetically limited through increased growth rate. Herein we explore how the structural and optical properties of GaAs1−xBix films are modified when grown under conditions with varying degrees of kinetic limitations realized through growth temperature and growth rate changes. Within the typical window of MBE growth conditions for GaAs1−xBix, we compare films with similar (∼3%) compositions grown under conditions of reduced kinetic limitations, i.e., relatively low gallium supersaturation achieved at higher temperatures (∼350 °C) and lower growth rates (∼0.5 μm/h), to those grown farther from equilibrium, specifically, higher supersaturation achieved at lower growth temperatures (∼290 °C) and higher growth rates (∼1.4 μm/h). Both the x-ray diffraction full width at half maximum of the omega-2theta scan and the 300 K photoluminescence intensity increase when samples are grown under less kinetically limited conditions. We interpret these findings in relation to the incorporation of Bi-related microstructural defects that are more readily formed during less kinetically limited growth. These defects lead to enhanced luminescence efficiency due to the spatial localization of carriers.

Optical properties of amorphous (GeS)1 − x Bi x (0 ≤ x ≤ 0.15) films and a tentative cluster model for their structure

This paper examines the effect of Bi doping on the optical properties of amorphous (GeS)1 − xBix (0 ≤ x ≤ 0.15) films. Experimental data in conjunction with first-principles electronic structure calculations for GenSm and BinSm clusters are used to derive a cluster model for the structure of the amorphous (GeS)1 − xBix films.

Effect of Bi doping on the structure and conduction mechanism of amorphous GeSe films

This paper examines the effect of Bi doping on the short-range order in amorphous (GeSe)1 − xBix (0 ≤ x ≤ 0.15) films and their electrical properties. Electron diffraction results demonstrate that the structure of the films can be described by a model typical of GeSe-Bi2Se3 solid solutions. Bi doping changes the charge transport mechanism and leads to type inversion in the amorphous (GeSe)1 − xBix films.

Growth and phase stability of epitaxial metastable InSb1−xBix films on GaAs. I. Crystal growth

Epitaxial metastable InSb1−xBix films with InBi concentrations up to 12 mole % have been grown on (110) GaAs substrates by multitarget rf sputtering. Tetragonal InBi is a semimetal whose equilibrium solid solubility in zinc blende structure InSb is 2.6 mole %. Low-energy ion bombardment of the growing film was used to modify the elemental Sb and Bi incorporation probabilities σSb and σBi through preferential resputtering in order to maintain overall stoichiometry and to allow the growth of single-crystal extended solid solution alloys at elevated temperatures. The effects of the Bi to Sb impingement flux ratio JBi/JSb, film growth temperature TS, and the Ar sputtering pressure PAr on the composition and structure of the as-deposited films as well as their metastable solid solubility, were also investigated. Holding all growth variables except one constant in a given set of experiments, the ratio σBi/σSb was found to decrease with increasing JBi/JSb, increasing TS, and decreasing PAr . The metastable solid solubility limit increased with decreasing PAr . Single-phase metastable films were n-type semiconductors with the 100 °K carrier concentration increasing from 1.8×1017 to 4.5×1017 cm−3 as the mole % of InBi was increased from 3 to 12. Electron mobilities at this temperature ranged from a few hundred to ∼1000 cm2/V sec. The mobility increased with increasing temperature yielding an activation energy of 0.02 eV. The optical band gap decreased with increasing InBi concentrations indicating a semiconductor-semimetal transition at ∼11 mole % InBi at 20 °K.

Growth and phase stability of epitaxial metastable InSb1−xBix films on GaAs. II. Phase stability

Single-crystal metastable InSb1−xBix films have been grown on GaAs substrates by multitarget sputtering. The films were metastable in two directions on the In-Sb-Bi ternary phase diagram—the solid solubility of tetragonal InBi in zinc blende structure InSb was increased by more than a factor of 4, and the width of the InSb-InBi pseudobinary phase field was increased from ∼10−3% to ?0.5%. The transition from the single-phase metastable state M1 to the equilibrium state occurred through the intermediate metastable state M2. The thermal stability of single-phase films was found by high-temperature growth and post-annealing experiments to depend on film orientation in the following order: (110), (111), and (100). The metastable phase diagram was established for (110) oriented films in which it was found that the transition temperature from the M1 state to the M2 state ranged from 150 to 300 °C, depending on film composition, above the equilibrium eutectic temperature at ∼100 °C and to be only about 125 °C less than the liquidus temperature.

Growth of single-crystal metastable InSb1−xBix and (GaSb)1−xGex semiconducting films

Epitaxial metastable InSb1-xBix alloys with InBi concentrations up to 12 mol % and (GaSb)1−xGex alloys with compositions across the pseudobinary phase diagram have been grown on GaAs substrates by multitarget rf sputtering. The InSb1−xBix film were n-type while the (GaSb)1−xGex films were p-type. Polycrystalline films from both alloy systems grown on Corning 7059 glass substrates exhibited strong (220) preferred orientation with lattice constants obeying Vegard’s law. Annealing experiments showed that the InSb1−xBix films were metastable in two directions on the In–Sb–Bi ternary phase diagram—the solid solubility of tetragonal InBi in zinc blende structure InBi was increased by more than a factor of 4 and the width of the InSb–InBi pseudobinary ’’line compound’’ phase field was increased to ?0.5%. Both InSb1−xBix and (GaSb)1−xGex films were found to be stable to decomposition during long anneals which, in the case of InSb1−xBix, were within 100°C of the equilibrium liquidus temperature. Annealing studies carried out on (GaSb)0.38Ge0.62 showed that such films transform from the as-deposited (220) oriented alloy to GaSb–rich and Ge–rich phases in which the solute concentrations continuously decrease until an equilibrium two phase alloy is obtained. The activation energy for this process is 3.1 eV.

Growth of metastable InSb1−xBix thin films by multitarget sputtering

Metastable solid-solution polycrystalline InSb1−xBix films with InBi concentrations up to ∼12 mole% have been grown by multitarget sputtering. This exceeds the solid-solubility limit of 2.6 mole% InBi reported for melt-grown bulk InSb1−xBix. The films were deposited on substrates which rotated through separate InSb and Bi discharges maintained at an Ar pressure of 15 mTorr. The relative target voltages were used to control the composition of the deposited films and were chosen such that the deposition rate was always less than a monolayer per target pass. The film growth temperatures investigated were 135 and 165 °C and a (110) preferred orientation was obtained in films grown on Corning 7059 glass substrates. All solid-solution films were stoichiometric within the accuracy of our electron microprobe, ∼0.5 at.%. The incorporation probability of Bi was found to decrease with increasing Bi impingement flux and with increasing growth temperature. The films were n type with a net room-temperature carrier concentration increasing monotonically from 1017 to 1018 cm−3 with increasing InBi concentration. Precipitation of metallic Bi was observed in films containing greater than 12 mole% InBi.