Influence of an ultra-thin buffer layer on the growth and properties of pseudomorphic GaAsBi layers (original) (raw)
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Electronic properties of GaAsBi(001) alloys at low Bi content
Physical Review Materials, 2019
Despite its potential in the fields of optoelectronics and topological insulators, experimental electronic band structure studies of Bi-doped GaAs are scarce. The reason is the complexity of growth which tends to leave bulk and in particular surface properties in an undefined state. Here we present an in depth investigation of structural and electronic properties of GaAsBi epilayers grown by molecular beam epitaxy with high (001) crystalline order and well-defined surface structures evident from low-energy electron diffraction. X-ray and ultraviolet photoemission spectrocopy as well as angleresolved photoemission data at variable photon energies allows to disentangle a Bi-rich surface layer with (1 × 3) symmetry from the effects of Bi atoms incorporated in the GaAs bulk matrix. The influence of Bi concentrations up to ≈ 1% integrated in the GaAs bulk are visible in angle-resolved photoemission spectra after mild ion bombardment and subsequent annealing steps. Interpretation of our results is obtained via density functional theory simulations of bulk and β2(2 × 4) reconstructed slab geometries with and without Bi. Bi-induced energy shifts in the dispersion of GaAs heavy and light hole bulk bands are evident both in experiment and theory, which are relevant for modulations in the optical band gap and thus optoelectronic applications.
The Growth and Assessment of GaAs Epitaxial Layers Obtained From Ga-As-Bi Solutions
Active and Passive Electronic Components, 1987
X-Ray investigations of GaAs epitaxial layers obtained from Ga-As-Bi solutions with different amounts of bismuth are presented. An equilibrium cooling and two phase technique for the deposition of the GaAs epitaxial layers on semi-insulating GaAs:Cr(100) substrates has been used.It has been observed that independently of the growing solution composition, the epitaxial layers were single crystal of (100) crystallographic orientation. Bismuth and chromium were not identified as impurities in the investigated layers although this may be due to the low sensitivity (0.1 at . %) of the x-ray microprobe used.
Impact of Rotation Rate on Bismuth Saturation in GaAsBi Grown by Molecular Beam Epitaxy
Journal of Electronic Materials
GaAs 1Àx Bi x has been grown by solid-source molecular beam epitaxy using varying substrate rotation rates. Changes in local bismuth saturation were studied by varying the Bi/Ga pressure ratio across the wafer. Films were grown on both GaAs and InGaAs buffer layers with varying indium content to change the strain conditions of the bismide layer and the out-of-plane growth rate. All samples demonstrated vertical composition modulations with a period of $ 4 nm that tracked with the rate of growth per substrate rotation cycle. The thermal stability of these composition modulations was shown to behave similarly to bulk GaAsBi. Bismide composition modulations are attributed to the low growth temperature and the varying Bi/Ga pressure ratio across the sample rather than the varying V/III ratio.
Bulk GaAs 1 − x Bi x /GaAs alloys with various bismuth compositions are studied using power-and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni's law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm −1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm −1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.
Migration-enhanced epitaxy of thin GaAsBi layers
Lithuanian Journal of Physics, 2014
Thin GaAsBi layers and bismide-based multiple quantum structures were grown by migration-enhanced epitaxy onto GaAs(100) substrates at 140-240 °C temperatures. The Bi content in GaAs, evaluated from high-resolution X-ray diffraction scans, varied from 2.5 to 10.3% depending on growth temperature, Bi and As atomic ratio as well as on the sequence of Ga, Bi, and As molecular supplies. The atomic force microscopy revealed a tendency of smoothening of the GaAsBi surface with substrate temperature decreasing. Transmission electron microscopy investigations evidenced high crystalline material quality of GaAsBi quantum wells and GaAs barriers.
Ab initio study of the strain dependent thermodynamics of Bi doping in GaAs
Physical Review B, 2012
The thermodynamics of Bi incorporation into bulk and epitaxial GaAs was studied using density functional theory (DFT) and anharmonic elasticity calculations. The equilibrium concentration of Bi was determined as a function of epitaxial strain state, temperature, and growth conditions. For a bulk, unstrained system, Bi in GaAs under typical growth conditions (Ga-rich and Bi-metal-rich at 400° C) has a dilute heat of solution of 572 meV/Bi and a solubility of 5 5.2 10 x − = × in GaAs 1-x Bi x. However, epitaxial strain can greatly enhance this solubility, and under the same conditions an epitaxial film of GaAs 1-x Bi x with 5% in-plane tensile strain is predicted to have a Bi solubility of 3 7.3 10 x − = × , representing approximately a hundred times increase in solubility over the unstrained bulk case. Despite these potentially large increases in solubility, the equilibrium solubility is still very low compared to values that have been achieved experimentally through non-equilibrium growth. These values of solubility are also sensitive to choice of the Bi reference state. If the primary route for phase separation is the formation of GaBi within the same structure, rather than Bi metal, GaBi would serve as the source/sink for Bi. If GaBi is used as the Bi reference state, the epitaxial formation energy on a bulk unstrained GaAs substrate is reduced dramatically to 144 meV/Bi, yielding a Bi solubility of 0.083 x = in GaAs 1-x Bi x. These calculations suggest that Bi solubility could be greatly enhanced if Bi metal formation is inhibited and the system is forced to remain constrained to the GaAs 1-x Bi x structure. Although GaBi is not a naturally stable compound, it could potentially be stabilized through a combination of kinetic limitations and alloying.
Semiconductor Science and Technology, 2018
In this work, the electronic bandstructure of GaAs1-xBix/GaAs single quantum well (QW) samples grown by molecular beam epitaxy (MBE) is investigated by photomodulated reflectance (PR) measurements as a function of Bi content (0.0065≤ x ≤0.0215) and substrate orientation. The Bi composition is determined via simulation of high-resolution X-ray diffraction (HR-XRD) measurement and found to be maximized at 2.15%Bi and 2.1%Bi samples grown on (100) and (311)B GaAs substrate. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi QW layer but extends out of the GaAsBi QW towards to GaAs barrier and forms GaAsBi epilayer. PR spectra are fitted with the third derivative function form (TDFF) to identify the optical transition energies. We analyze the TDFF results by considering strain-induced modification on conduction band (CB) and splitting of the valence band (VB) due to its interaction with the localized Bi level and VB interaction. The PR measurements confirm the existence of a GaAsBi epilayer via observed optical transitions belong to GaAsBi layers with various Bi compositions. It is found that both Bi composition and substrate orientation have strong effects on the PR signal. Comparison between TDFF and calculated optical transition energies provides a bandgap reduction of 92meV/%Bi and 36meV/%Bi and an interaction strength of the isolated Bi atoms with host GaAs valence band (CBiM) as 1.7eV and 0.9eV for (100) and (311)B GaAs substrate, respectively.
Bi incorporation in GaAs(100)-2×1 and 4×3 reconstructions investigated by RHEED and STM
Journal of Crystal Growth
Bi acts as a surfactant in molecular beam epitaxy (MBE) growth on GaAs(100). Incorporation is achieved by disequilibrium at growth temperatures below $ 450 1C. Bi can however affect the static reconstruction up to 600 1C. Two reconstructions are considered in this work: dynamic (2 Â 1) and static c(8 Â 3)/ (4 Â 3), which are shown to be the dominant reconstructions for GaAsBi MBE. Bi storage in these two reconstructions provides an explanation of RHEED transitions that cause unintentional Bi incorporation in the GaAs capping layer. Finally dynamic observations of the (2 Â 1) reconstruction are used to explain growth dynamics, atomic ordering and clustering observed in GaAsBi epilayers which have a direct influence on photoluminescence linewidth broadening in mixed anion III-V alloys.
Strain and the two-dimensional electronic structure of monolayers of Bi/InAs(110) and Bi/GaAs(110)
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1992
The two-dimensional band structure of monolayers of the BilInAs(110) (1 X I) and Bil GaAs (110) (1 X I) systems have been measured using the technique of angle-resolved photoemission spectroscopy with a synchrotron light source. Measurements of the dispersion and bandwidth of the uppermost occupied electronic state nearest the Fermi level revealed that the bandwidth was significantly smaller for the BilInAs(110) system. A comparison to available theoretical calculations of Sb overlayers on III-V semiconductor surfaces implies that the differences between the two Bi systems is greater than would be predicted by a standard theoretical analysis. The authors interpret the results as indicating the importance of strain in these systems.