Band gaps and spin-orbit splitting of ordered and disordered AlxGa1-xAs and GaAsxSb1-x alloys

Band gap engineering and photoluminescence tuning in halide double perovskites

Halide double perovskites (HDPs) present a convenient alternative to the unstable and toxic lead halide perovskites for various optoelectronic applications. Because of their compositional and structural tunability, many HDP phases have been synthesized in the past decades. While efficient photovoltaic applications remain largely out of reach for the HDP phases due to their wide band gaps and structures with pseudoisolated metal centers, their electronic structures favor light conversion applications. Since the field of HDP witnesses rapid growth and development, this article is aimed at providing a brief snapshot of the recent advances on all-inorganic HDPs, primarily focusing on the relationship between their compositions and optical properties, and some aspects of their applications for visible light conversion.

Synthesis of two-dimensional phenylethylamine tin-lead halide perovskites with bandgap bending behavior

Recently, two-dimensional (2D) metal halide perovskite materials with wide application in perovskite-based solar cells have attracted significant attention. Among them, 2D mixed lead-tin perovskites have not been systematically explored. Herein, we synthesize a 2D phenethylammonium (PEA) tin-lead bromide perovskite, PEA₂Sn _x Pb_1-x Br₄, via a simple solution-phase approach without toxic reagents and high temperatures. By tuning the ratio of Sn and Pb, the UV-vis absorption spectra showed unique bandgap bending behaviors. DFT calculations indicate the key effects of spin-orbital coupling (SOC) without the interference of lattice distortion. Moreover, we provided the standard equation with a correction term to introduce the influence of SOC. These results not only provide a step forward towards the bandgap engineering of perovskites, but also help to expand the application of 2D perovskite materials.

Unique Behavior of Halide Double Perovskites with Mixed Halogens

Engineering halide double perovskite (A₂M⁺M³⁺X^VII₆) by mixing elements is a viable way to tune its electronic and optical properties. In spite of many emerging experiments on halide double perovskite alloys, the basic electronic properties of the alloys have not been fully understood. In this work, we chose Cs₂AgBiCl₆ as an example and systematically studied electronic properties of its different site alloys Cs₂Na_xAg_1-xBiCl₆, Cs₂AgSb_xBi_1-xCl₆, and Cs₂AgBi(Br_xCl_1-x)₆ (x = 0.25, 0.5, 0.75) by first-principles calculations. Interestingly, the halogen site alloy shows opposite behavior to M⁺ and M³⁺ cation site alloys; that is, Cs₂AgBi(Br_xCl_1-x)₆ displays virtual crystal behavior without substantial broadening, while Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆ show split-band behaviors with substantial broadening, which indicates that lifetimes of electrons and holes in Cs₂AgBi(Br_xCl_1-x)₆ would be longer than those in Cs₂Na_xAg_1-xBiCl₆ and Cs₂AgSb_xBi_1-xCl₆. We further found that long lifetimes of electrons and holes are common for mixed halide perovskites. Moreover, the band alignment is provided to determine the band gap change of alloys and to understand the transport of electrons and holes when these pure compounds form heterostructures. Our systematical studies should be helpful for future optoelectronic applications of halide perovskites.

Iuliacumite: A Novel Chemical Short-Range Order in a Two-Dimensional Wurtzite Single Monolayer InAs1-xSbx Shell on InAs Nanowires

A chemical short-range order is found in single monolayer InAs_1-xSb_x shells, which inherit a wurtzite structure from the underlying InAs nanowire, instead of crystallizing in the energetically preferred zincblende structure. The chemical order is characterized by an anticorrelation ordering vector in the ⟨112̅0⟩ direction and arises from strong Sb-Sb repulsive interactions along the atomic chains in the ⟨112̅0⟩ direction.

Transition of radiative recombination channels from delocalized states to localized states in a GaInP alloy with partial atomic ordering: a direct optical signature of Mott transition?

Anderson localization is a predominant phenomenon in condensed matter and materials physics. In fact, localized and delocalized states often co-exist in one material. They are separated by a boundary called the mobility edge. Mott transition may take place between these two regimes. However, it is widely recognized that an apparent demonstration of Anderson localization or Mott transition is a challenging task. In this article, we present a direct optical observation of a transition of radiative recombination dominant channels from delocalized (i.e., local extended) states to Anderson localized states in the GaInP base layer of a GaInP/GaAs single junction solar cell by the means of the variable-temperature electroluminescence (EL) technique. It is found that by increasing temperature, we can boost a remarkable transition of radiative recombination dominant channels from the delocalized states to the localized states. The delocalized states are induced by the local atomic ordering domains (InP/GaP monolayer superlattices) while the localized states are caused by random distribution of indium (gallium) content. The efficient transfer and thermal redistribution of carriers between the two kinds of electronic states was revealed to result in both a distinct EL mechanism transition and an electrical resistance evolution with temperature. Our study gives rise to a self-consistent precise picture for carrier localization and transfer in a GaInP alloy, which is an extremely technologically important energy material for fabricating high-efficiency photovoltaic devices.

Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1-x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4-300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2.

Electronically nonalloyed state of a statistical single atomic layer semiconductor alloy

Using atomically and momentum resolved scanning tunneling microscopy and spectroscopy, we demonstrate that a two-dimensional (2D) √3 × √3 semiconducting Ga-Si single atomic alloy layer exhibits an electronic structure with atomic localization and which is different at the Si and Ga atom sites. No indication of an interaction or an electronic intermixing and formation of a new alloy band structure is present, as if no alloying happened. The electronic localization is traced back to the lack of intra alloy bonds due to the 2D atomically confined structure of the alloy overlayer.

Reducing thermal conductivity of binary alloys below the alloy limit via chemical ordering

Substitutional solid solutions that exist in both ordered and disordered states will exhibit markedly different physical properties depending on their exact crystallographic configuration. Many random substitutional solid solutions (alloys) will display a tendency to order given the appropriate kinetic and thermodynamic conditions. Such order-disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, the dominant scattering mechanism in ordered alloys will be different than that in disordered alloys. In this study, we present a hypothesis that ordered alloys can exhibit lower thermal conductivities than their disordered counterparts at elevated temperatures. To validate this hypothesis, we investigate the phononic transport properties of disordered and ordered AB Lennard-Jones alloys via non-equilibrium molecular dynamics and harmonic lattice dynamics calculations. It is shown that the thermal conductivity of an ordered alloy is the same as the thermal conductivity of the disordered alloy at ≈0.6T(melt) and lower than that of the disordered alloy above 0.8T(melt).

Compositional Ordering in Semiconductor Alloys

Compositional ordering has been observed in a wide variety of III/V semiconductor alloys as well as in SiGe alloys. The thermodynamic driving force is now understood in terms of minimization of the microscopic strain energy of the bonds in the solid. However, the mechanism leading to the specific ordered structures formed is only now beginning to be understood. It appears to be intimately related to the physical processes occurring on the surface during epitaxial growth, specifically surface reconstruction and the attachment of atoms at steps and kinks. Thus, an improved understanding the ordering process may lead to a better understanding of the surface processes occurring during epitaxial growth from the vapor.

This paper will review the current understanding of the ordering process, including discussions of the arrangement of atoms on the surface and the nature of surface steps. The emphasis will be on the use of patterned surfaces to investigate and control the ordered structures formed during organometallic vapor phase epitaxial growth of GaInP. Using photolithography and chemical etching, [110]-oriented steps are formed on the (001) GaAs substrate. The direction of motion of these steps determines the specific variant of the Cu-Pt ordered structure (with ordering on (111) planes) formed. The step density at the edge of the groove apparently determines the degree of order. Highly stepped surfaces suppress ordering or lead to small domains of a single variant. When the steps are very shallow, the large domain of the predominant variant is filled with “inclusions” of the second variant. Step edges that are oriented at nearly 160 from (001) form a {511} variant during growth. This facet is observed to grow at the expense of adjacent (001) surfaces and to produce material that is completely disordered.

Growing on intentionally misoriented substrates leads to interesting structures consisting of both large, highly-ordered domains and disordered material. This allows, using cathodoluminescence(CL) imaging, a direct determination of the effect of ordering on the energy band gap. In the GaInP samples studied, the CL images show that the disordered material has a distinct emission pattern consisting of a single, sharp peak at an energy more than 100 meV higher than that observed in the adjacent ordered region.

Mutual passivation of donors and isovalent nitrogen in GaAs

We study the mutual passivation of shallow donor and isovalent N in GaAs. We find that all the donor impurities, SiGa, GeGa, SAs, and SeAs, bind to N in GaAs:N, which has a large N-induced band-gap reduction relative to GaAs. For a group-IV impurity such as Si, the formation of the nearest-neighbor SiGa-NAs defect complex creates a deep donor level below the conduction band minimum (CBM). The coupling between this defect level with the CBM pushes the CBM upwards, thus restoring the GaAs band gap; the lowering of the defect level relative to the isolated SiGa shallow donor level is responsible for the increased electrical resistivity. Therefore, Si and N mutually passivate each other's electrical and optical activities in GaAs. For a group-VI shallow donor such as S, the binding between SAs and NAsdoes not form a direct bond; therefore, no mutual passivation exists in the GaAs:(S+N) system.