Narrow-gap Compounds Research Articles

Enhancing the Resistive Memory Window through Band Gap Tuning in Solid Solution (Cr1-xVx)2O3.

Memories based on the insulator-to-metal transition in correlated insulators are promising to overcome the limitations of alternative nonvolatile memory technologies. However, associated performances have been demonstrated so far only on narrow-gap compounds, such as (V0.95Cr0.05)2O3, exhibiting a tight memory window. In the present study, V-substituted Cr2O3 compounds (Cr1-xVx)2O3 have been synthesized and widely investigated in thin films, single crystals, and polycrystalline powders, for the whole range of chemical composition (0 < x < 1). Physicochemical, structural, and optical properties of the annealed magnetron-sputtered thin films are in very good agreement with those of polycrystalline powders. Indeed, all compounds exhibit the same crystalline structure with a cell parameter evolution consistent with a solid solution over the whole range of x values, as demonstrated by X-ray diffraction and Raman scattering. Moreover, the optical band gap of V-substituted Cr2O3 compounds decreases from 3 eV for Cr2O3 to 0 eV for V2O3. In the same way, resistivity is decreased by almost 5 orders of magnitude as the V content x is varying from 0 to 1, similarly in thin films and single crystals. Finally, a reversible resistive switching has been observed for thin films of three selected V contents (x = 0.30, 0.70, and 0.95). Resistive switching performed on MIM devices based on a 50 nm thick (Cr0.30V0.70)2O3 thin film shows a high endurance of 1000 resistive switching cycles and a memory window ROFF/RON higher by 3 orders of magnitude, as compared to (Cr0.05V0.95)2O3. This comprehensive study demonstrates that a large range of memory windows can be reached by tuning the band gap while varying the V content in the (Cr1-xVx)2O3 solid solution. It thus confirms the potential of correlated insulators for memory applications.

Comparative Analysis of the Electroluminescence Efficiency in Type-I and Type-II Heterostructures Based on III–V Narrow-Gap Compounds

Comparative Analysis of the Electroluminescence Efficiency in Type-I and Type-II Heterostructures Based on III–V Narrow-Gap Compounds

Width Dependent Two-Photon Absorption in Monolayer Black Phosphorus Nanoribbons

Black phosphorus nanoribbons (BPNs) might offer alternatives to narrow-gap compound semiconductors for tunable optoelectronics in infrared region. In this work we present a quantum perturbation theory on two-photon absorption (TPA) in monolayer armchair-edged black phosphorus nanoribbons (acBPNs) employing the reduced two-band model within the long-wavelength BP Hamiltonian. The matrix elements for one-photon transition have been derived and the TPA spectrum associate with intra conduction band transition and inter band transition have been drawn. The calculations predict that the TPA coefficient in acBPNs is in the magnitude of 10−6 m/W in visible region, which is 4 orders higher than the conventional semiconductor quantum dots. And in infrared region, there is a giant TPA coefficient, which is mainly contributed from intra band transitions and can reach up to10−1 m/W. The TPA peaks can be tuned both by the width of BPNs and the electron relaxation energy.

Improvement of the Thermoelectric Performance of Pseudogap and Narrow-Gap Compounds via Theoretical Calculations

Improvement of the Thermoelectric Performance of Pseudogap and Narrow-Gap Compounds via Theoretical Calculations

Improvement of thermoelectric performance of pseudogap and narrow-gap compounds using theoretical calculation

Improvement of thermoelectric performance of pseudogap and narrow-gap compounds using theoretical calculation

Transforming Common III–V and II–VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices

Currently, known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. It is shown via first‐principle calculations that a heterovalent superlattice made of common semiconductor building blocks can transform its non‐TI components into a topological nanostructure, illustrated by III–V/II–VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. The relationship between the interfacial stability and the topological transition is revealed, finding a “window of opportunity” where both conditions can be optimized. Once a critical InSb layer thickness above ≈1.5 nm is reached, both [111] and [100] superlattices have a relative energy of 1.7–9.5 meV Å–2, higher than that of the atomically exchanged interface and an excitation gap up to ≈150 meV, affording room‐temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next‐generation superstructured functional materials.

Tunable optical properties of multilayer black phosphorus thin films

Black phosphorus thin films might offer attractive alternatives to narrow gap compound semiconductors for optoelectronics across mid- to near-infrared frequencies. In this work, we calculate the optical conductivity tensor of multilayer black phosphorus thin films using the Kubo formula within an effective low-energy Hamiltonian. The optical absorption spectra of multilayer black phosphorus are shown to vary sensitively with thickness, doping, and light polarization. In conjunction with experimental spectra obtained from infrared absorption spectroscopy, we also discuss the role of interband coupling and disorder on the observed anisotropic absorption spectra.

Two-color luminescence from a single type-II InAsSbP/InAs heterostructure

Gradient scanning Kelvin-probe microscopy has been for the first time used to study p-n hetero-junctions based on narrow-gap compounds in the In-As-Sb-P solid-solution system. Redistribution of the flow of injected carriers in the sample along the direction of the external electric field is demonstrated for the example of a single type-II p-InAsSbP/n-InAs heterostructure. When a forward bias is applied to the hetero-structure, two-band luminescence is observed, i.e., that of the interface type at the p-InAsSbP/p-InAs heterointerface and of the bulk type in indium arsenide. It is shown that indirect (interface) transitions exhibit a higher radiative-recombination efficiency than direct interband transitions. The observation of multiband electroluminescence spectra opens up a means of developing single-heterostructure multicolored light-emitting diodes for the mid-IR spectral range (3–5 μm).

Temperature dependence of the energy gap and spin-orbit splitting in a narrow-gap InGaAsSb solid solution

Temperature dependence of the energy gap and the spin-orbit split off transition in a thick layer of narrow-gap InGaAsSb material with high In content has been determined by a combination of photoluminescence and photoreflectance. The respective temperature coefficients have been found to be equal for both the transitions and determined to be α = −0.41 meV/K. For the investigated In0.86Ga0.14As0.83Sb0.17 alloy, the separation energy of the split-off band has been obtained to be Δso = 0.460 eV and experimentally evidenced to be independent on temperature, which opens broad application prospects for these multinary (multicomponent) narrow gap compounds and their heterostructures.

Thermomagnetic Effects in High-Mobility PbTe Films

The narrow-gap compound semiconductor PbTe has high Hall mobility. The Fermi surface at the L-point in the Brillouin zone has large anisotropy. In this work, we measured thermomagnetic effects in PbTe thin films to confirm anisotropy of the Nernst coefficient A Ne and show Nernst mobility from the ratio of A Ne and the Seebeck coefficient S: μ Ne = A Ne/S. Angular dependences of the Nernst voltage show that A Ne is independent of the angle between the temperature gradient and the magnetic field, because of the high L-point symmetry. The calculated Nernst mobility was compared with the Hall mobility. Because the former is smaller, the Mott equation cannot explain the Seebeck coefficient at room temperature.

Composition dependence of thermoelectric properties of binary narrow-gap Ga 67− xRu 33+ x compound

The composition and carrier concentration variations of the thermoelectric properties of the binary narrow-gap compound Ga 67− x Ru 33+ x have been investigated. The measured samples were synthesized by a combination of arc-melting and spark plasma sintering (SPS), succeeded in crack-free samples. We report that the temperature dependences of the electrical resistivity and Seebeck coefficient of the Ga 67− x Ru 33+ x are strongly affected by nominal Ru concentration. The Seebeck coefficients showed large positive values from 170 to 350 μV K −1 at 373 K. Also, large power factors from 2.2 to 3.0 mW m −1 K −2 were obtained at 773 K. The dimensionless figures of merit ZT beneficially increased with increasing temperature and reached a maximum value of 0.50 at about 773 K.

Manganese concentration effect on the lattice dynamics in multinary spin doped semiconductors

The vibration states of diluted magnetic narrow-gap Hg1−x−yCdxMnyTe (x=0–0.34, y=0–0.04) semiconductors are studied by means of far infrared reflection. The comparison of spectral features common for Hg1−x−yCdxMnyTe and Hg1−xCdxTe single crystals demonstrates the substantial effect of magnetic doping on the optical spectra, providing indirect evidence of improved crystal structure in quaternary compounds. The inherent fundamental phonon modes of Hg1−xCdxTe become significantly narrower in Hg1−x−yCdxMnyTe whereas the defect and some cluster modes are of much lower intensity. These modifications of the lattice dynamics are assumed to be caused by a manganese concentration effect on the relaxation of crystalline defects. For Hg1−x−yCdxMnyTe single crystals this concentration is found to be within 0.01<y<0.025. The particular mechanisms of the manganese-assisted relaxation of crystal structure in quaternary narrow-gap compounds are not yet completely clear.

Optical Study of Electronic Structure in Perovskite-TypeRMO3(R=La, Y;M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu)

Optical study of trivalent 3 d transition metal oxide compounds R M O 3 with perovskite-type structure has revealed the variation of their electronic structures with the 3 d element M and rare earth element R . The low energy feature is governed by electronic states in the M - O network. The charge gap, either of the Mott type or charge-transfer type, is observed for a series of R M O 3 compounds, except for metallic LaNiO 3 and LaCuO 3 compounds. The charge gap systematics has been accounted for in terms of a simple ionic model. However, the deviation from the ionic model is magnified in narrow gap compounds, where the M 3 d - O 2 p mixing appears to reshuffle the electronic states. The variation of the covalent nature of the M - O network in the RMO 3 compounds is also discussed in terms of the shell-model analysis of the phonon spectra.

Interband dipole matrix elements of narrow band gap semiconductor films

The absorption spectra of size-quantized films of IV–VI narrow-gap compounds were investigated theoretically using the two-band effective mass theory. The optical conductivity tensor for (111) and (001) PbTe film was calculated including the k dependence of the interband dipole matrix elements. We found that mixing of the conduction and valence bands had two effects: (i) it leads to dependence of the squared optical matrix elements on k □: however, changes in polarization of the incident light or configuration of the valley lead to significant changes in this dependence: (ii) it gives rise to direct optical transitions between all pairs of valence and conduction sub-bands during which the Δn = 0 selection rule is violated.