Band Gap Collapse Research Articles

Influence of crystallinity and stoichiometry on the high temperature semiconductor-metal transition of lanthanum cobalt oxide deposited by reactive dc sputtering

Lanthanum cobalt oxide (LCO) has garnered growing interest in electronic switch applications based on its unique insulator to metal transition. A single-step high temperature deposition process was developed for thin film LCO via reactive dc magnetron sputtering, starting with exploration of target poisoning within the context of Berg's theory. By tracking target voltage on metal targets during reactive deposition, we reason that increasing inert gas flux to the target improves target hysteresis, thus increasing target lifetime. Furthermore, it is shown that the onset of the critical region of oxide deposition has a stronger dependence on reactive gas flow than on partial pressure and is found at approximately 1.5 sccm and 5 sccm oxygen flow rate for the La and Co targets, respectively. We subsequently probed substrate and stoichiometry influence on LCO film quality and electronic properties. Epitaxial (100) LaCoOx (Rq = 0.41 nm) films grown on lanthanum aluminate (LaAlO3) demonstrate similar reductions in resistivity over 300 K–623 K as polycrystalline LCO films on 4H–SiC and sapphire (Rq = 1.59, 2.36 nm respectively). However, La-heavy films, grown as La2.6CoO4.8, can demonstrate significantly steeper resistivity drops. Finally, we estimate the band gap collapse and insulator-metal transition of thin-film LCO over temperature by near-IR absorption.

Hydrostatic deformation potentials of narrow-gap HgCdTe

Close to the semimetal-to-semiconductor topological phase transition, the band structure of HgCdTe is represented by massive Kane fermions that, in a semirelativistic approach, are characterized by two parameters: rest mass and velocity. Using terahertz magneto-optical spectroscopy, we explore the band structure evolution of HgCdTe films with hydrostatic pressure in the vicinity of the band-gap collapse. By analyzing the energies of interband optical transitions as a function of magnetic field, we have determined the rest mass of Kane fermions at different hydrostatic pressures. The pressure dependence of the rest mass allows us to obtain the hydrostatic deformation potential ${a}_{c}\ensuremath{-}{a}_{v}$ at low temperature, where ${a}_{c}$ and ${a}_{v}$ are the deformation potentials of the conduction and valence bands, respectively.

Unique Conduction Band Minimum of Semiconductors Possessing a Zincblende-Type Framework.

Tetrahedral semiconductors such as Si adopt a diamond-type crystal structure with low packing density arising from open cavities in the crystallographic space. By taking LiAlGe as an example, we propose a zincblende-type framework as a platform for semiconductors possessing electroactive cavities. LiAlGe adopts a half-Heusler-type crystal structure including an ordered diamond-type sublattice (zincblende-type) (AlGe) and is an indirect semiconductor with a band gap of ∼0.1 eV. The conduction band minimum (CBM) is uniquely located at the cavity space surrounded by four cations (Al4) in real space. The bond ionicity and cation (Al) p orbitals located around the Fermi energy are requisite for the CBM to float in the cavity space. DFT calculations indicate the conversion of the semiconductor to a semimetallic electride under a pressure of ∼8 GPa, which is accompanied by band gap collapse due to electron transfer from valence band maximum to the cavity space. The high-pressure electride of LiAlGe formed under a very small critical pressure is derived from the presence of inherent crystallographic cavities having deep orbital levels energetically. This finding suggests the possible utilization of electroactive cavity spaces in tetrahedral semiconductors, which are widely used in modern electronic devices.

Electronic spectrum of bilayer graphene with broken P-symmetry of both intra- and inter-layers

Abstract Electronic spectra of AA- and AB- graphene bilayers have been investigated for the case when P-symmetry is broken both inside and between the layers. We have focused special attention on the new regime when the band gaps in different layers have opposite signs. For example, the spectrum of AB bilayer is gapless in the case when band gaps of monolayers are equal in magnitude and opposite in sign. At the same time, such a band gap collapse does not occur in the case of the AA bilayer. We have shown that the gate voltage strongly affects the spectrum of gapped AB bilayers. We use elementary and clear arguments to explain our main results. Finally, we proposed a new and unusual way to determine the type of laying of a graphene bilayer.

Improved polaronic transport under a strong Mott–Hubbard interaction in Cu-substituted NiO

We report the origin of improved hole conduction without band-gap collapse in Cu-doped NiO.

Pressure-driven switching of magnetism in layered CrCl3.

Layered transition-metal compounds with controllable magnetic behaviors provide many fascinating opportunities for the fabrication of high-performance magneto-electric and spintronic devices. The tuning of their electronic and magnetic properties is usually limited to the change of layer thickness, electrostatic doping, and the control of electric and magnetic fields. However, pressure has been rarely exploited as a control parameter for tailoring their magneto-electric properties. Here, we report a unique pressure-driven isostructural phase transition in layered CrCl3 accompanied by a simultaneous switching of magnetism from a ferromagnetic to an antiferromagnetic ordering. Our experiments, in combination with ab initio calculations, demonstrate that such a magnetic transition hinders the bandgap collapse under pressure, leading to an anomalous semiconductor-to-semiconductor transition. Our findings not only reveal the potential applications of this material in electronic and spintronic devices but also establish the basis for exploring unusual phase transitions in layered transition-metal compounds.

Carbon nanotube array as a van der Waals two-dimensional hyperbolic material

We use an ab-initio approach to design and study a novel two-dimensional material - a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasi-metallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model low-energy Hamiltonian for its semi-analytical treatment. Periodic-potential-induced lifting of the valley degeneracy for an array of zigzag narrow-gap nanotubes leads to the band gap collapse. In contrast, the band gap is opened in an array of gapless armchair tubes. These unusual spectra, marked by pronounced van Hove singularities in the low-energy density of states, open the opportunity for interesting physical effects and prospective optoelectronic applications.

High-pressure polymorphs of gadolinium orthovanadate: X-ray diffraction, Raman spectroscopy, and ab initio calculations

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_{4}$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.

Putting the Squeeze on Lead Chromate Nanorods.

We have studied by means of X-ray diffraction and Raman spectroscopy the high-pressure behavior of PbCrO4 nanorods. We have found that these nanorods follow a distinctive structural sequence that differs from that of bulk PbCrO4. In particular, a phase transition from a monoclinic monazite-type PbCrO4 to a novel monoclinic AgMnO4-type polymorph has been discovered at 8.5 GPa. The crystal structure, Raman-active phonons, and compressibility of this novel high-pressure phase are reported for the first time. The experimental findings are supported by ab initio calculations that provide information not only on structural and vibrational properties of AgMnO4-type PbCrO4 but also on the electronic properties. The discovered phase transition triggers a band gap collapse and a subsequent metallization at 44.2 GPa, which has not been observed in bulk PbCrO4. This suggests that nanoengineering can be a useful strategy to drive metallization under compression.

High-pressure characterization of the optical and electronic properties of InVO4, InNbO4, and InTaO4

We have studied the electronic properties at ambient pressure and under high pressure of InVO4, InNbO4, and InTaO4 powders, three candidate materials for hydrogen production by means of photocatalytic water splitting using solar energy. A combination of optical absorption and resistivity measurements and band structure calculations have allowed us to determine that these materials are wide band-gap semiconductors with a band-gap energy of 3.62(5), 3.63(5), and 3.79(5) eV for InVO4, InNbO4, and InTaO4, respectively. The last two compounds are indirect band-gap materials, and InVO4 is a direct band-gap material. The pressure dependence of the band-gap energy and the electrical resistivity have been determined too. In the three compounds, the band gap opens under compression until reaching a critical pressure, where a phase transition occurs. The structural transition triggers a band-gap collapse larger than 1.2 eV in the three materials, being the abrupt decrease in the band-gap energy related to an increase in the pentavalent cation coordination number. The phase transitions also cause changes in the electrical resistivity, which can be correlated with changes induced by pressure in the band structure. An explanation to the reported results is provided based upon ab initio calculations. The conclusions attained are of significance for technological applications of the studied oxides.

Origin of Pressure‐Induced Metallization in Cu3N: An X‐ray Absorption Spectroscopy Study

High‐pressure (0–26.7 GPa) Cu K‐edge X‐ray absorption spectroscopy is used to study possible structural modifications of anti‐perovskite‐type copper nitride (Cu3N) crystal lattice. The analysis of X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS), based on theoretical full‐multiple‐scattering and single‐scattering approaches, respectively, suggests that at all pressures the local atomic structure of Cu3N remains close to that in cubic phase. Therefore, the transition to metal state above 5 GPa, observed previously using pressure‐dependent electrical resistance and optical absorption measurements, is explained by the band gap collapse due to a decrease of the unit cell volume. We found that the lattice parameter of Cu3N is reduced by ≈2% upon increasing pressure up to 26.7 GPa, and the structure is restored upon pressure release.

Multistep transition of diamond to warm dense matter state revealed by femtosecond X-ray diffraction

Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon energy, 5 fs duration, at the ~19–25 eV/atom absorbed doses, is studied theoretically on its way to warm dense matter state. Simulations with our hybrid code XTANT show disordering on sub-100 fs timescale, with the diffraction peak (220) vanishing faster than the peak (111). The warm dense matter formation proceeds as a nonthermal damage of diamond with the band gap collapse triggering atomic disordering. Short-living graphite-like state is identified during a few femtoseconds between the disappearance of (220) peak and the disappearance of (111) peak. The results obtained are compared with the data from the recent experiment at SACLA, showing qualitative agreement. Challenges remaining for the accurate modeling of the transition of solids to warm dense matter state and proposals for supplementary measurements are discussed in detail.

Pressure-induced phase transition and band-gap collapse in the wide-band-gap semiconductorInTaO4

A pressure-induced phase transition, associated with an increase of the coordination number of In and Ta, is detected beyond 13 GPa in InTaO4 by combining synchrotron x-ray diffraction and Raman measurements in a diamond anvil cell with ab-initio calculations. High-pressure optical-absorption measurements were also carried out. The high-pressure phase has a monoclinic structure which shares the same space group with the low-pressure phase (P2/c). The structure of the high-pressure phase can be considered as a slight distortion of an orthorhombic structure described by space group Pcna. The phase transition occurs together with a unit-cell volume collapse and an electronic bandgap collapse observed by experiments and calculations. Additionally, a band crossing is found to occur in the low-pressure phase near 7 GPa. The pressure dependence of all the Raman-active modes is reported for both phases as well as the pressure dependence of unit-cell parameters and the equations of state. Calculations also provide information on IR-active phonons and bond distances. These findings provide insights into the effects of pressure on the physical properties of InTaO4.

Tuning the band gap of PbCrO4 through high-pressure: Evidence of wide-to-narrow semiconductor transitions

The electronic transport properties and optical properties of lead(II) chromate (PbCrO4) have been studied at high pressure by means of resistivity, Hall-effect, and optical-absorption measurements. Band-structure first-principle calculations have been also performed. We found that the low-pressure phase is a direct band-gap semiconductor (Eg = 2.3 eV) that shows a high resistivity. At 3.5 GPa, associated to a structural phase transition, a band-gap collapse takes place, becoming Eg = 1.8 eV. At the same pressure the resistivity suddenly decreases due to an increase of the carrier concentration. In the HP phase, PbCrO4 behaves as an n-type semiconductor, with a donor level probably associated to the formation of oxygen vacancies. At 15 GPa a second phase transition occurs to a phase with Eg = 1.2 eV. In this phase, the resistivity increases as pressure does probably due to the self-compensation of donor levels and the augmentation of the scattering of electrons with ionized impurities. In the three phases the band gap red shifts under compression. At 20 GPa, Eg reaches a value of 0.8 eV, behaving PbCrO4 as a narrow-gap semiconductor.

Photon energy dependence of graphitization threshold for diamond irradiated with an intense XUV FEL pulse

We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24–275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.Received 8 March 2013DOI:https://doi.org/10.1103/PhysRevB.88.060101©2013 American Physical Society

Role of a nontrivial quantum phase in the dynamical band gap collapse

In the quasi-one-dimensional lattice system with a band gap, the effective magnitude of the gap may be reduced, and even collapsed, by applying a monochromatic oscillating external field. The internal relationship between this dynamical band gap collapse (BGC) and the well-known coherent destruction of tunneling (CDT) in the periodically driven two-state model is clarified from a unified point of view. The destructive interference between the resonant scattering paths plays an essential role in both the dynamical BGC and the CDT. This leads to the suppression of the Bragg reflection in the momentum space for the dynamical BGC, and to the suppression of tunneling in the coordinate space for the CDT.

Effects of high-pressure on the structural, vibrational, and electronic properties of monazite-type PbCrO4

We have performed an experimental study of the crystal structure, lattice-dynamics, and optical properties of PbCrO4 (the mineral crocoite) at ambient and high pressures. In particular, the crystal structure, Raman-active phonons, and electronic band-gap have been accurately determined. X-ray-diffraction, Raman, and optical-absorption experiments have allowed us also to completely characterize two pressure-induced structural phase transitions. The first transition is isostructural, maintaining the monoclinic symmetry of the crystal, and having important consequences in the physical properties; among other a band-gap collapse is induced. The second one involves an increase of the symmetry of the crystal, a volume collapse, and probably the metallization of PbCrO4. The results are discussed in comparison with related compounds and the effects of pressure in the electronic structure explained. Finally, the room-temperature equation of state of the low-pressure phases is also obtained.

Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator–metal transition in VO2, by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.

A combined high-pressure experimental and theoretical study of the electronic band-structure of scheelite-type AWO4 (A = Ca, Sr, Ba, Pb) compounds

The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and PbWO4 has been measured under high pressure up to ∼20 GPa at room temperature. From these measurements, we have obtained the evolution of the band-gap energy with pressure. We found a low-pressure range (up to 7–10 GPa) where alkaline-earth tungstates present a very small Eg pressure dependence (− 2.1 < dEg/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a pressure coefficient of − 62 meV/GPa. The high-pressure range is characterized in the four compounds by an abrupt decrease of Eg followed by changes in dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also calculated the electronic-band structures and their pressure evolution. The calculations allow us to interpret experiments considering the different electronic configurations of divalent metals. Changes in the pressure evolution of Eg are correlated with the occurrence of pressure-induced phase transitions. The band structures for the low- and high-pressure phases are also reported. No metallization of any of the compounds is detected in experiments nor is it predicted by the calculations.

I−Vcurve signatures of nonequilibrium-driven band gap collapse in magnetically ordered zigzag graphene nanoribbon two-terminal devices

Motivated by the very recent fabrication of sub-10-nm-wide semiconducting graphene nanoribbons [X. Li et al., Science 319, 1229 (2008)], where some of their band gaps extracted from transport measurements were closely fitted to density-functional theory predictions for magnetic ordering along zigzag edges that is responsible for the insulating ground state, we compute current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of finite-length zigzag graphene nanoribbons (ZGNRs) attached to metallic contacts. The transport properties of such devices, at source-drain bias voltages beyond the linear-response regime, are obtained using the nonequilibrium Green's function formalism combined with the mean-field version of the Hubbard model fitted to reproduce the local spin-density approximation description of magnetic ordering. Our results indicate that magnetic ordering and the corresponding band gap in ZGNR can be completely eliminated by passing large enough direct current through it. The threshold voltage for the onset of band gap collapse depends on the ZGNR length and the contact transparency. If the contact resistance is adjusted to experimentally measured value of $\ensuremath{\simeq}60\text{ }\text{k}\ensuremath{\Omega}$, the threshold voltage for sub-10-nm-wide ZGNR with intercontact distance of $\ensuremath{\simeq}7\text{ }\text{nm}$ is $\ensuremath{\approx}0.4\text{ }\text{V}$. For some device setups, including $60\text{ }\text{k}\ensuremath{\Omega}$ contacts, the room-temperature $I\text{\ensuremath{-}}V$ curves demonstrate steplike current increase by one order of magnitude at the threshold voltage and can exhibit a hysteresis as well. On the other hand, poorly transmitting contacts can almost completely eliminate abrupt jump in the $I\text{\ensuremath{-}}V$ characteristics. The threshold voltage increases with the ZGNR length (e.g., reaching $\ensuremath{\approx}0.8\text{ }\text{V}$ for $\ensuremath{\simeq}13\text{-nm}$-long ZGNR) which provides possible explanation of why the recent experiments [Wang et al., Phys. Rev. Lett. 100, 206803 (2008)] on $\ensuremath{\sim}100\text{-nm}$-long GNR field-effect transistors with bias voltage $<1\text{ }\text{V}$ did not detect the $I\text{\ensuremath{-}}V$ curve signatures of the band gap collapse. Thus, observation of predicted abrupt jump in the $I\text{\ensuremath{-}}V$ curve of two-terminal devices with short ZGNR channel and transparent metallic contacts will confirm its zigzag edge magnetic ordering via all-electrical measurements, as well as a current-flow-driven magnetic-insulator--nonmagnetic-metal nonequilibrium phase transition.