Valence Band Lineup Research Articles

Room Temperature Electroluminescence from Mechanically Formed van der Waals III–VI Homojunctions and Heterojunctions

Room temperature electroluminescence from semiconductor junctions is demonstrated. The junctions are fabricated by the exfoliation and direct mechanical adhesion of InSe and GaSe van der Waals layered crystals. Homojunction diodes formed from layers of p‐ and n‐type InSe exhibit electroluminescence at energies close to the bandgap energy of InSe (Eg = 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p‐type GaSe and n‐type InSe emit photons at lower energies, which is attributed to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. These results demonstrate the technological potential of mechanically formed heterojunctions and homojunctions of direct‐bandgap layered GaSe and InSe compounds with an optical response over an extended wavelength range, from the near‐infrared to the visible spectrum.

Determination of Energy Band Alignment in Ultrathin Hf-based Oxide/Pt System

Effect of incorporating a third element into HfO2 on the electronic structures has been studied by high resolution x-ray photoelectron spectroscopy (XPS). Hf-IIIa (La, Y, Gd, and Dy) oxide and Hf-Ti oxide films were deposited on a Pt layer by metal organic chemical vapor deposition (MOCVD) and co-sputtering and followed by post-deposition annealing in O2 ambience at 500°C. The energy bandgap (Eg) of these Hf-based oxide films was determined by analyzing the energy loss spectra of O 1s photoelectrons in consideration of the overlap with Hf 4s core-line signals. From analyses of the valence band signals and the cut-off energy for photoelectrons, the valence band offset between the Hf based-oxide, and the Pt electrode and the work function value of the Pt layer were evaluated. By combining the oxide bandgap values, the valence band line-ups, and the Pt work function value, the energy band profile of the Hf-based oxide/Pt has been determined.

界面エレクトロニクス 金属／高誘電率絶縁膜ゲートスタックの光電子分光分析

The method to determine the energy band alignments of high-k dielectrics to Si(100) is described, in which the energy band gap values and the valence band lineups are measured from the analyses of energy loss signals of core line and valence band spectra measured by X-ray photoelectron spectroscopy (XPS), respectively. Also, it is demonstrated how useful the measurement of the cut-of energy for photoemission is to evaluate the effective work function in metal/high-k dielectric. Total photoelectron yield spectroscopy (PYS) is highlighted as a powerful way to quantify energy distribution of electronic defect states in a dielectric or at the dielectric/Si interface in the energy region corresponding to the Si band gap.

Chemical and electronic structure of ultrathin zirconium oxide films on silicon as determined by photoelectron spectroscopy

We have measured the energy bandgaps of thermally evaporated ZrO 2 on Si(1 0 0) and a very-thin interfacial silicate (SiO x :Zr) layer formed by 500 °C annealing in dry O 2 from the onset of the energy loss spectra of O 1s photoelectrons. The valence band lineups at the interfaces among ZrO 2, SiO x :Zr and Si(1 0 0) have also been evaluated by analyzing the valence band spectra of thin heterostructures. By combination of the measured energy bandgaps and valence band lineups, we have determined the energy band alignments of as-evaporated ZrO 2/Si(1 0 0) and annealed ZrO 2/SiO x :Zr/Si(1 0 0) systems. The influence of 300–500 °C O 2-annealing on the energy distribution of electronic defect states in the films and at the interfaces has been demonstrated by total photoelectron yield measurements.

Characterization of high-k gate dielectric/silicon interfaces

We have shown that, for thermally evaporated Ta 2O 5 or ZrO 2 thin films on Si(1 0 0), O 2 annealing at 300–500 °C causes the formation of an interfacial silicon oxide layer as thin as 1–2 nm which can be interpreted in terms of their high permeability to oxygen. And we have demonstrated how useful the energy loss spectra of photoexcited electrons from core levels such as O 1s are to measure the energy bandgaps of very thin insulators. With the combination of measured bandgaps and valence band lineups determined for X-ray photoelectron spectroscopy valence band spectra, we have determined the energy band alignments of Ta 2O 5 and ZrO 2 with Si(1 0 0) before and after the O 2 annealing at 500 °C. In addition, we have demonstrated that total photoelectron yield spectroscopy provides us direct information to quantify the energy distributions of both the defect states in the high-k dielectrics and the dielectric/Si(1 0 0) interface states over nearly entire Si bandgap.

Photoemission study of energy-band alignments and gap-state density distributions for high-k gate dielectrics

The determination of the energy band gaps of thin-gate insulators has been demonstrated from the onsets of the energy-loss spectra of O 1s (or N 1s) photoelectrons. The valence-band lineups of thin high-dielectric-constant (high-k) dielectrics such as Ta2O5, Al2O3, and ZrO2 formed on metals and Si(100) have also been determined by measuring the energy difference between the valence-band density-of-states curves. The energy band diagrams for metal/high-k dielectrics/Si(100) systems have been derived explicitly from considering the measured band gaps, valence-band lineups, electron affinities, and metal work functions in the systems. It is also demonstrated that total photoelectron yield spectroscopy can be used to quantify the energy distributions of both the defect states in high-k gate dielectrics and at the dielectric/Si(100) interfaces over the entire Si band gap without gate formation.

Low-energy yieldspectroscopy measurements applied to determine valence band line-upat interfaces with non-homogeneous overlayers

The technique of low-energy yield spectroscopy is applied to determine the valence band line-up at heterojunctions in which the overlayer does not cover the substrate. It is shown that by tuning the analysis energy, the contribution made by electrons that traverse the surface in the uncovered regions can be suppressed from the interface spectrum obtained from low-energy yield spectroscopy operating in the constant-final-state mode, thus allowing the determination of the band line-up without ambiguity. The method was applied to the c-Si/c-SiC heterostructure. A value of EV = 0.78±0.06 eV was found for the valence band discontinuity.

Valence-band lineups at highly strained SiInP, GeInAs, and SiGe interfaces

This paper reports a theoretical study on the valence-band lineups at highly strained SiInP, GeInAs and SiGe interfaces by combining average bond energy theory and deformation potential method. The effect of the shallow d-orbitals in In-atom on the valence-band lineup is investigated. Three typical strain conditions are calculated for each interface system. The dependence of the lineups on the strain condition are demonstrated quantitatively. It is found that the offset between the average energies of valence-band maximum is affected little by the elastic strain for the three interface systems, however, those between the topmost valence-band states (ΔE v ) is closely related to the strain condition. The change regions of ΔE v are −0.39∼−1.23eV, −0.49∼−1.32eV and 0.22∼0.74eV for SiInP, GeInAs and SiGe systems, respectively. It is demonstrated that this strong dependence is due to the uniaxial component of the strain and its interaction with the spin-orbit splitting. Our results for SiGe system are in good agreement with the data given by x-ray photoemission measurement and supercell selfconsistent calculations by ab initio pseudopotential methods. Our results for Ge InP system are consistent with the experimental data of synchrotron radiation photoemission.

Electronic properties and valence-band offset of strained ZnTe/CdTe (001) superlattices.

We present a systematic study of the electronic and structural properties of strained (ZnTe${)}_{\mathit{n}}$/(CdTe${)}_{\mathit{n}}$ (001) superlattices using the full-potential linearized-augmented-plane-wave method. We simulate three different growth conditions: a free-standing mode (no substrate is assumed) and the growth on ZnTe and CdTe substrates. The heat of formation of ultrathin superlattices and the valence-band lineup are determined in the different strain conditions. The agreement between the most recent experiments and the theoretical results is very good. The effects of the strain on the valence-band discontinuity are discussed.

Photo-induced screening of the excitonic interaction in ZnSe-ZnTe type II strained-layer superlattices

We report the observation of nonlinear optical properties of ZnSe-ZnTe superlattices under photo-injected carriers, at pumped liquid helium temperature. Identification of several transitions measured by transmission on samples grown by metalorganic vapour phase epitaxy and etched away from the GaAs substrate is made in the context of the envelope function approach. This reveals that the conduction to valence band line-ups are type II in these samples. This situation is invoked in order to interpret the efficiency of the exciton screening.

Structural and electronic properties of CdTe-based heterostructures

This paper reviews the structural and optical properties of CdTe/Cd 1− x Zn x Te strained heterostructures grown by molecular beam epitaxy. Special attention is given to the control of strain and interface roughness and sharpness. It is shown that high quality superlattices are obtained with well width fluctuations on the monolayer scale. The electronic properties are also strain dependent, especially the valence band line-up. Two-dimensional exciton states are the dominant features of the optical spectra of CdTe/Cd 1− x Zn x Te quantum wells. By appropriate quantum well engineering, the exciton-phonon interaction is reduced and large room-temperature absorption observed.