我们阐明了材料在高应变梯度区间的挠曲电效应，其潜在机制目前尚不清晰。通过使用广义 Bloch 定理，我们在弯曲的 Si 和 Ge 薄膜中发现了一种强烈的类挠曲电效应，这是由高应变梯度引发的绝缘体-金属转变所致。我们表明，在弯曲薄膜的压缩侧和伸长侧之间形成了一种不寻常的 type-II 能带对齐，导致了电子和空穴的空间分离。因此，在发生绝缘体-金属转变时，电子从压缩侧转移到伸长侧以达到热力学平衡，从而沿薄膜厚度方向产生了显著的极化。所得的横向挠曲电系数出乎意料地高，且对薄膜厚度表现出二次依赖关系。这种新机制可扩展到其他具有适中能隙的半导体材料。我们的发现对未来挠曲电效应在半导体材料中的应用具有重要意义。
英文摘要：
We elucidate the flexoelectricity of materials in the high strain gradient regime, of which the underlying mechanism is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high strain gradient-induced insulator-to-metal transition. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film, resulting in a spatial separation of electron and hole. Therefore, upon the insulator-to-metal transition, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to pronounced polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high, with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.
Abstract: In this paper we show that experimentally realizable apparently piezoelectric thin-film super-lattices can be created from non-piezoelectric materials provided an odd-order (e.g. trilayer) stacking sequence is used. The size-dependent mechanism of flexoelectricity, which couples gradients of strain to polarization, allows such a possibility. We present closed-form analytical expressions for the response of various thin-film and super-lattice configurations. We also clarify some of the subtleties that arise in considering interface boundary conditions in the theory of flexoelectricity…
A centrosymmetric stress cannot induce a polar response in centric materials, piezoelectricity is, for example, possible only in non-centrosymmetric structures. An exception is meta-materials with shape asymmetry, which may be polarized by stress even when the material is centric. In this case the mechanism is flexoelectricity, which relates polarization to a strain gradient. The flexoelectric response scales inversely with size, thus a large effect is expected in nanoscale materials. Recent experiments in polycrystalline, centrosymmetric perovskites [e.g., (Ba,Sr)TiO3] have indicated values o…
The muffin-tin model of an effective-mass electron interacting with magnetic ions in semiconductors is extended to incorporate electrostatic potentials that are present in the case of Mn-based III-V compounds (${Ga}_{1-x} {Mn}_x {N}$, ${Ga}_{1-x} {Mn}_x {As}$). Since the conduction band electron is repelled from negatively charged magnetic ions and attracted by compensating donors, the \emph{apparent} value of the s-d exchange coupling $N_0 α$ is reduced. It is shown that the magnitude of this effect increases when x diminishes. Our model may explain an unusual behavior of electron spin splitt…
Spin splitting of photoelectrons in p-type and electrons in n-type III-V Mn-based diluted magnetic semiconductors is studied theoretically. It is demonstrated that the unusual sign and magnitude of the apparent s-d exchange integral reported for GaAs:Mn arises from exchange interactions between electrons and holes bound to Mn acceptors. This interaction dominates over the coupling between electrons and Mn spins, so far regarded as the main source of spin-dependent phenomena. A reduced magnitude of the apparent s-d exchange integral found in n-type materials is explained by the presence of repu…
Using large-scale Monte Carlo calculations, we consider strongly disordered Heisenberg models on a cubic lattice with missing sites (as in diluted magnetic semiconductors such as Ga_{1-x}Mn_{x}As). For disorder ranging from weak to strong levels of dilution, we identify Curie temperatures and calculate the critical exponents nu, gamma, eta, and beta finding, per the Harris criterion, good agreement with critical indices for the pure Heisenberg model where there is no disorder component. Moreover, we find that thermodynamic quantities (e.g. the second moment of the magnetization per spin) self …
The LDA-1/2 method expands Slater's half occupation technique to infinite solid state materials by introducing a self-energy potential centered at the anions to cancel the energy associated with electron-hole self-interaction. To avoid an infinite summation of long-ranged self-energy potentials they must be trimmed at a variationally-defined cutoff radius. The method has been successful in predicting accurate band gaps for a large number of elementary and binary semiconductors. Nevertheless, there has been some confusion regarding carbon and silicon, both in the cubic diamond structure, which …
The attainability of modification of the apparent magnetic anisotropy in (III,Mn)V ferromagnetic semiconductors is probed by means of the finite-elements-based modelling. The most representative case of (Ga,Mn)As and its in-plane uniaxial anisotropy is investigated. The hysteresis loops of the continuous films of a ferromagnetic semiconductor as well as films structured with the elliptic antidots are modelled for various eccentricity, orientation, and separation of the anti dots. The effect of anti-dots on the magnetic anisotropy is confirmed but overall is found to be very weak. The subsequen…
We present a multiscale modeling of the infrared optical properties of the rubrene crystal. The results are in very good agreement with the experimental data that point to nonmonotonic features in the optical conductivity spectrum and small optical effective masses. We find that, in the static-disorder approximation, the nonlocal electron-phonon interactions stemming from low-frequency lattice vibrations can decrease the optical effective masses and lead to lighter quasiparticles. On the other hand, the charge-transport and infrared optical properties of the rubrene crystal at room temperature…
The development of semiconductor electronics is shortly reviewed, beginning with the development of germanium devices (band gap $E_g=0.66$ eV) after world war II. Quickly a tendency to alternative materials with wider band gap became apparent, starting with silicon ($E_g=1.12$ eV). This improved the signal/noise ratio for classical electronic applications. Both semiconductors have tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or several anions and cations other semiconductors with wider $E_g$ are obtained, that are transparent for visible light …
2D materials have intriguing quantum phenomena that are distinctively different from their bulk counterparts. Recently, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are attracting attention not only for their potential applications but also for exotic quantum effects such as superconductivity. By mapping momentum-resolved electronic states using time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal that monolayer Ag confined between bilayer graphene and SiC is a large gap (> 1 eV) 2D semiconductor, consistent with GW-corrected density fun…
The (In,Fe)Sb layers with the Fe content up to 13 at. % have been grown on (001) GaAs substrates using the pulsed laser deposition. The TEM investigations show that the (In,Fe)Sb layers are epitaxial and free of the inclusions of a second phase. The observation of the hysteretic magnetoresistance curves at temperatures up to 300 K reveals that the Curie point is above room temperature. The resonant character of magnetic circular dichroism confirms the intrinsic ferromagnetism in the (In,Fe)Sb layers. We suggest that the ferromagnetism of the (In,Fe)Sb matrix is not carrier-mediated and apparen…
Upon application of a uniform strain, internal sub-lattice shifts within the unit cell of a non-centrosymmetric dielectric crystal result in the appearance of a net dipole moment: a phenomenon well known as piezoelectricity. A macroscopic strain gradient on the other hand can induce polarization in dielectrics of any crystal structure, even those which possess a centrosymmetric lattice. This phenomenon, called flexoelectricity, has both bulk and surface contributions: the strength of the bulk contribution can be characterized by means of a material property tensor called the bulk flexoelectric…
Transport properties of progressively reduced graphene oxide (GO) are described. Evolution of the electronic properties reveals that as-synthesized GO undergoes insulator-semiconductor-semi-metal transitions with reduction. The apparent transport gap ranges from 10 ~ 50 meV and approaches zero with extensive reduction. Measurements at varying degrees of reduction reveal that transport in reduced GO occurs via variable-range hopping and further reduction leads to increased number of available hopping sites.
Finding stimuli capable of driving an imbalance of spin-polarised electrons within a solid is the central challenge in the development of spintronic devices. However, without the aid of magnetism, routes towards this goal are highly constrained with only a few suitable pairings of compounds and driving mechanisms found to date. Here, through spin- and angle-resolved photoemission along with density functional theory, we establish how the $p$-derived bulk valence bands of semiconducting 1T-HfSe$_2$ possess a local, ground-state spin texture spatially confined within each Se-sublayer due to stro…
Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced ordering of the crystal structure. The nature of the bandgap becomes increa…
The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and…
Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of…
Glass transition temperatures (Tg) and liquid fragilities are measured along a line of constant Ge content in the system Ge-As-Te, and contrasted with the lack of glass-forming ability in the twin system Ge-Sb-Te at the same Ge content. The one composition established as free of crystal contamination in the latter system shows a behavior opposite to that of more covalent system. Comparison of Tg vs bond density in the three systems Ge-As-chalcogen differing in chalcogen i.e. S, Se, or Te, shows that as the chalcogen becomes more metallic, i.e. in the order S<Se<Te the bond density effect on Tg…
Lead and tin-based chalcogenide semiconductors like PbTe or SnSe have long been known to exhibit an unusually low thermal conductivity that makes them very attractive thermoelectric materials. An apparently unrelated fact is that the excitonic bandgap in these materials increases with temperature, whereas for most semiconductors one observes the opposite trend. These two anomalous features are also seen in a very different class of photovoltaic materials, namely the halide-perovskites such as CsPbBr3. It has been previously proposed that emphanisis, a local symmetry-breaking phenomenon, is the…
The layers of a high-temperature novel GaAs:Fe diluted magnetic semiconductor (DMS) with an average Fe content up to 20 at. % were grown on (001) i-GaAs substrates using a pulsed laser deposition in a vacuum. The transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy investigations revealed that the conductive layers obtained at 180 and 200 C are epitaxial, do not contain any second-phase inclusions, but contain the Fe-enriched columnar regions of overlapped microtwins. The TEM investigations of the non-conductive layer obtained at 250 C revealed the embedded coherent …
X-ray diffraction measurements on the sphalerite-derivatives ZnGa2Se4 and CdGa2S4 have been performed upon compression up to 23 GPa in a diamond-anvil cell. ZnGa2Se4 exhibits a defect tetragonal stannite-type structure (I-42m) up to 15.5 GPa and in the range from 15.5 GPa to 18.5 GPa the low-pressure phase coexists with a high-pressure phase, which remains stable up to 23 GPa. In CdGa2S4, we find the defect tetragonal chalcopyrite-type structure (I-4) is stable up to 17 GPa. Beyond this pressure a pressure-induced phase transition takes place. In both materials, the high-pressure phase has bee…
This paper reports that ~10-nm-thick FeSe thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various [Fe]/[Se] were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter …
