Energy Band Profile Research Articles

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.

An accurate interband tunneling model for InAs/GaSb heterostructure devices

AbstractA numerical model that accurately describes interband tunneling in backward diodes and broken‐gap tunnel diode structures based on the InAs/GaSb material system is described. The model applies the transfer matrix method to discretized bias‐dependent energy band profiles to calculate the transmission probability for tunneling. The model has been validated against experimental results, with good agreement in the current‐voltage and curvature coefficient having been obtained for a range of hetero‐structure backward diode and interband tunnel diode structures. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

XPS Study of Energy Band Alignment between Hf-La Oxides and Si(100)

We have studied the chemical bonding features and electronic states of ultrathin Hf-La oxide using high-resolution x-ray photoelectron spectroscopy (XPS). 6 nm-thick Hf-La oxides with different La/(Hf+La) content were prepared on wet-chemical cleaned p-type Si(100) and Pt layer by thermal decomposition of Hf(DPM: dipivaloymcthanato)4 and La(DPM)3 in O2 ambient at 450℃. From XPS core-line analysis, diffusion and incorporation of Si atoms from the Si substrate into the Hf-La oxide film by 800℃ annealing in O2 ambience were pronounced with increased La content over 35%. The energy band profile of the Hf-La oxide/Si system as a function of La content has been determined by a combination of oxide bandgap (Eg) values and valence band (VB) line-ups. The conduction band (CB) offset has been also estimated using the Eg of the Si substrate, measured Eg, and VB offset. The impact of Si incorporation into Hf-La oxide on CB and VB offsets between the Hf-La(:Si) oxide and Si substrate also discussed in comparison to the energy band diagram of the Hf-La oxide/Pt structure.

Electrical properties of ferroelectric-gate FETs with SrBi2Ta2O9 formed using MOCVD technique

Ferroelectric-gate field-effect transistors (FeFETs) with a Pt/SrBi2Ta2O9/Hf-Al-O/Si gate stack were fabricated using the metal-organic chemical vapor deposition (MOCVD) technique to prepare the SrBi2Ta2O9 (SBT) ferroelectric layer. A good threshold voltage (V th) distribution was found for more than 90 n-channel FeFETs in one chip with a 170 nm SBT layer owing to the good film uniformity of the SBT layer deposited by MOCVD. The average memory window \((V_{\mathrm{w}}^{\mathrm{av}})\) and the standard deviations (σ thl,σ thr) of the left- and right-side branches of the drain-gate voltage curves of the FeFETs yielded a \(V_{\mathrm{w}}^{\mathrm{av}}/(\sigma_{\mathrm{thl}} + \sigma_{\mathrm{thr}})\) value of 5.45, indicating that the FeFETs can be adapted for large-scale-integration. The electric field, the energy band profile in the gate stack, and the gate leakage current were also investigated at high gate voltages. We found that the effect of Fowler–Nordheim tunneling appeared under these conditions. Because of the tunneling injection and trapping of electrons into the gate insulators, the operation voltage ranges of the FeFETs were limited by this tunneling.

Lateral energy band profile modulation in tunnel field effect transistors based on gate structure engineering

Choosing novel materials and structures is important for enhancing the on-state current in tunnel field-effect transistors (TFETs). In this paper, we reveal that the on-state performance of TFETs is mainly determined by the energy band profile of the channel. According to this interpretation, we present a new concept of energy band profile modulation (BPM) achieved with gate structure engineering. It is believed that this approach can be used to suppress the ambipolar effect. Based on this method, a Si TFET device with a symmetrical tri-material-gate (TMG) structure is proposed. Two-dimensional numerical simulations demonstrated that the special band profile in this device can boost on-state performance, and it also suppresses the off-state current induced by the ambipolar effect. These unique advantages are maintained over a wide range of gate lengths and supply voltages. The BPM concept can serve as a guideline for improving the performance of nanoscale TFET devices.

Improvement of the quantum confined Stark effect characteristics by means of energy band profile modulation: The case of Gaussian quantum wells

We study the quantum confined stark effect (QCSE) characteristics in Gaussian quantum wells (GQW). This special energy band profile is built varying the aluminum concentration of the AlGaAs ternary alloy in Gaussian fashion. The semi-empirical sp3s* tight-binding model including spin is used to obtain the energy Stark shifts (ESS) and the wave-function Gaussian spatial overlap (GSO) between electrons and holes for different electric field strengths, quantum well widths and aluminum concentrations. We find that both the ESS and the GSO depend parabolically with respect to the electric field strength and the quantum well width. These QCSE characteristics show an asymmetry for the electric field in the forward and reverse directions, related directly to the different band-offset of electrons and holes, being the negative electric fields (reverse direction) more suitable to reach greater ESS. Two important features are presented by this special energy band profile: (1) reductions of the ESS and (2) enhancements of the GSO of tents to hundreds with respect to parabolic and rectangular quantum wells. Even more, tailoring the quantum well width it is possible to reach GSO of thousands with respect to rectangular quantum wells. Finally, it is important to mention that similar results could be obtained in other quantum well heterostructures of materials such as nitrides, oxides (ZnO), and SiGe whenever the confinement band profiles are modulated in Gaussian form.

Microstructure and polarization fields in nitride semiconductors

The nitride semiconductors are widely used in high efficiency light emitting devices and are currently being considered for photovoltaic applications. The reduced symmetry in the wurtzite structure compared to cubic semiconductors results in the growth of large densities of crystalline defects and in the presence of strong spontaneous and piezoelectric polarization effects. A correlation between the microstructure and the polarization fields can be achieved with electron holography in the transmission electron microscope. Electron holograms thus obtained can provide energy band profiles with sub-nanometer spatial resolution. The phase of the electron beam is sensitive to the electrostatic potential, and a direct measurement of the latter can be achieved by making the electron beam signal that traverses the specimen interfere with a reference electron beam that travels through vacuum. This technique has been quite useful in probing the fields and charges at dislocations and at interfaces in semiconductors, and it is particularly useful to determine the piezoelectric effects in group III nitride semiconductor heterostructures. A review of applications to InGaN and AlGaN based heterostructures is presented in this paper.

Computational modeling of novel InN/Al 0.30In 0.70N multilayer nano-heterostructure

Most of the low dimension heterostructures that have been modeled and simulated to determine various important quantum mechanical parameters are based on GaN/AlGaN and GaAs/AlGaAs. The heterostructures of newly invented material (InN/AlInN), however, have not been well studied. In this paper, novel multilayer nano-heterostructure InN/Al 0.30In 0.70N of length 288 nm have been modeled and studied to compute the energy band profile within the frame work of eight band k.p method, which graphs the energy of conduction and valence band edges versus position, and potential distribution throughout the modeled and one dimensionally simulated nano-heterostructure. In addition, electron–hole densities along with space charge densities have also been calculated for 30% Al concentration. The novelty of the nano-heterostructure due to unusual properties of InN studied by FP-LAPW and LCAO methods is also discussed. The results obtained in this paper will be applicable to the newly invented nano-opto-electronic devices.

Red emitting LEDs formed by indium rich quantum dots incorporated in MQWs

Abstractindium rich InGaN nanostructures grown by MOCVD were incorporated in InGaN/GaN quantum wells for long wavelength emission by optimizing the growth condition for the quantum dots (QDs) and InGaN quantum wells. The indium rich InGaN QDs were about 20 nm in diameter and 1.5 nm in height with a density of ∼1 × 1010 cm−2. The InGaN nanostructures in quantum wells capped by AlN were insensitive to In out‐diffusion due to formation of stable Al‐N bonds and lower mobility of Al adatoms on the film surface at 780 °C. The piezeoelectric field in the active layers of the light emitting diodes (LEDs) was reduced with the AlN capped layer and minimal blueshift of the electroluminescence was observed as compared to conventional InGaN/GaN LEDs was noted. The energy band profile of strained AlN/InGaN/QDs/InGaN/GaN quantum well system was analyzed.

Enhanced optical performance of amber emitting quantum dots incorporated InGaN/GaN light-emitting diodes with growth on UV-enhanced electrochemically etched nanoporous GaN

Phosphor-free amber light emitting quantum dots with bimodal size distribution have been effectively incorporated in InGaN/GaN light emitting diodes (LEDs). With overgrowth of the LEDs structure on electrochemically etched nanoporous GaN templates, a reduction in density of threading dislocations and lower biaxial stress are achieved. A higher density of smaller quantum dots ∼4.5×109 cm−2 is incorporated in the multiple quantum well of LEDs on the nanoporous GaN template, generating higher energy emission, and enhanced light output by 1.45 times over LEDs grown on conventional GaN. The carrier capture kinetic by the bimodal distributed quantum dots is discussed with its energy band profile.

Heteromaterial gate tunnel field effect transistor with lateral energy band profile modulation

A tunnel field effect transistor (TFET) device with a heteromaterial gate structure was introduced, and the influence of the lateral energy band profile modulation effect on the performance of TFET devices has been investigated. The two-dimensional numerical simulations demonstrated that a local minimum of the conduction band in the channel was formed by work function mismatch, which resulted in the abrupt transition between the on- and off-states and significant drive current enhancement. In particular, these unique features could be manipulated by engineering the heteromaterial and the gate length. This work reveals an effective method to improve the performance of nanoscale TFETs.

Electrostatic energy profiles at nanometer‐scale in group III nitride semiconductors using electron holography

AbstractElectron holography in the transmission electron microscope can provide energy band profiles with sub‐nanometer spatial resolution. The phase of the electron beam is sensitive to the electrostatic potential, and a direct measurement of the latter can be achieved by making the electron beam signal that traverses the specimen interfere with a reference electron beam that travels through vacuum. This technique has been quite useful in probing the fields and charges at dislocations and at interfaces in semiconductors. This article presents a review of work done with the participation of the author in the past decade in the use of this technique to determine the piezoelectric effects in group III nitride semiconductor heterostructures.

Generation of amber III-nitride based light emitting diodes by indium rich InGaN quantum dots with InGaN wetting layer and AlN encapsulation layer

Indium rich InGaN nanostructures grown by metalorganic chemical vapor deposition were incorporated in InGaN/GaN quantum wells for long wavelength generation. These results were achieved by optimizing the growth temperature of the nanostructures, InGaN quantum well, the AlN capping layer and the GaN barrier layers. Before the growth of nanostructures, a thin InGaN wetting layer was included to reduce the lattice mismatch as well as to enhance the deposition of indium-rich InGaN nanostructures These individual quantum wells were each subsequently capped with an AlN layer which better preserved the In-rich phase in the nanostructures and prevented the indium interdiffusion between the InGaN/GaN heterojunctions. The AlN capping layer also reduces the effect of piezeoelectric field in the active layers of the light emitting diodes as seen from the reduction in the blueshift in the electroluminescence peaks with higher injection currents. The energy band profile of such a structure is discussed.

Theoretical study of the SiO2/Si interface and its effect on energy band profile and MOSFET gate tunneling current

Two SiO2/Si interface structures, which are described by the double bonded model (DBM) and the bridge oxygen model (BOM), have been theoretically studied via first-principle calculations. First-principle simulations demonstrate that the width of the transition region for the interface structure described by DBM is larger than that for the interface structure described by BOM. Such a difference will result in a difference in the gate leakage current. Tunneling current calculation demonstrates that the SiO2/Si interface structure described by DBM leads to a larger gate leakage current.

Quantum well infrared photodetectors hardiness to the nonideality of the energy band profile

We report results on the effect of a nonsharp and disordered potential in quantum well infrared photodetectors (QWIP). Scanning electronic transmission microscopy is used to measure the alloy profile of the structure which is shown to present a gradient of composition along the growth axis. Those measurements are used as inputs to quantify the effect on the detector performance (peak wavelength, spectral broadening, and dark current). The influence of the random positioning of the doping is also studied. Finally we demonstrate that QWIP properties are quite robust with regard to the nonideality of the energy band profile.

Modeling and simulation of GaN/Al 0.3Ga 0.7N new multilayer nano-heterostructure

In this paper we have proposed a simple model and performed one-dimensional simulation for GaN/Al 0.3Ga 0.7N multi-layer new nano-heterostructure in order to obtain the energy band profile, which graphs the energy of the conduction and valence band edges versus position. Also, potential distribution has been studied throughout the modeled nano-heterostructure along with electron–hole densities and space charge densities. The total size of the sample was 288 nm grown on pseudomorph GaN including ohmic contact with the metal on edges. The quantum region is over the whole heterostructure but the interesting region in the heterostructure has been found in between 100 and 196 nm, which can be applicable for a particular device application.

Analysis of current-voltage characteristics of Fe/MgO/GaAs junctions using self-consistent field modeling

Current-voltage data in metal-insulator-semiconductor devices are usually interpreted by a model of the tunneling current or the Schottky thermionic emission current. In these models, the barrier through which the electrical current flows is normally assumed to be rectangular or at best trapezoidal. For metal-insulator-metal junctions, this is a reasonable assumption. However, when one electrode is a doped semiconductor, the parameters of the current-voltage models require a self-consistent field description and the bias-dependent band bending within the semiconductor must be taken into account. These bias-dependent energy-band profiles are modeled with a Schr\"odinger-Poisson solver and incorporated into the fitting procedure for the current-voltage data. We find that the ratio of the Schottky thermionic emission to the tunneling current, as well as the tunnel barrier heights can be determined using this approach. With this approach, one can quantitatively distinguish between tunneling and thermionic transport regimes and this is particularly applicable to the interpretation of spin-transport experiments in metal-insulator-semiconductor devices.

Atomic-layer-resolved bandgap structure of an ultrathin oxynitride-silicon film epitaxially grown on6H-SiC(0001)

Electronic structures of a silicon-oxynitride (SiON) layer ($\ensuremath{\sim}0.6\text{ }\text{nm}$ in thickness) epitaxially grown on $6H\text{-SiC}(0001)$ were investigated on atomic-layer scale using soft x-ray absorption spectroscopy and x-ray emission spectroscopy (XAS and XES) and first-principles calculations. The SiON layer has a hetero-double-layered structure: an interfacial silicon nitride layer and a silicon oxide overlayer. The element-specific XAS and XES measurements revealed layer-resolved energy-band profiles. Measured gap sizes are $6.3\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{eV}$ at the nitride layer and $8.3\ifmmode\pm\else\textpm\fi{}0.8\text{ }\text{eV}$ at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being $\ensuremath{\sim}3\text{ }\text{eV}$ higher than CBM of the SiC substrate. The energy-band profiles of the SiON layer are qualitatively reproduced by the calculations. The calculations show that broadening of bandgap of the substrate occurs only at an interfacial SiC bilayer.

Ultralow dark current Ge/Si(100) photodiodes with low thermal budget

Vertical incidence photodiodes were fabricated from Ge grown epitaxially on Si(100) by low-energy plasma-enhanced chemical vapor deposition. Consideration of the energy band profiles of n-i-p and p-i-n heterostructures, and optimization of growth processes and thermal budget, allowed the performance of Ge photodectors integrated on Si(100) substrates to be optimized. Record low dark current density of Js=4.1×10−5 A/cm2 and external quantum efficiency at 1550 nm of η=32% were measured.

Photoinduced current transient spectroscopy technique applied to the study of point defects in polycrystalline CdS thin films

CdS thin films of variable thickness (between 160 and 1200 nm) were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the ⟨002⟩ axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height ϕ decreases with the increase in film thickness. Values of ϕ between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08–1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities (mainly Cu, Au, and Ag). Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary.