Band Diagram Calculations Research Articles

Computational study of ridge states in GaAs nanopillars

Semiconductor nanopillars have unique geometries that make them very promising materials for a variety of devices. In order to improve their performance, we need to understand how they are affected by ridge states that lie on the six corners of the nanopillar hexagon. Although the GaAs nanopillars are primarily zinc blende (ABC), stacking faults of wurtzite (AB) stacking occur. We use density-functional theory to study stacking faults using one-dimensional periodic geometries that have a combination of zinc blende and wurtzite stacking. In contrast to perfect zinc blende nanopillars, energetically favorable midgap ridge states created by stacking faults are found in these geometries using density-functional theory. The calculated band diagrams and densities of state help us to understand how these midgap states lead to a reduced mobility and carrier localization. We also study how sulfur passivation affects and potentially improves the performance by modifying the ridges.

Relationship between transport properties and band diagrams in InAsxSb1−x/Al0.1In0.9Sb quantum wells

The resistivity of InAs0.1Sb0.9/Al0.1In0.9Sb quantum wells (QWs) is much lower than that of InSb/Al0.1In0.9Sb QWs, staying low resistivity even at low temperature. Fundamental difference in low temperature transport properties between InSb/Al0.1In0.9Sb and InAs0.1Sb0.9/Al0.1In0.9Sb QWs was revealed, based on the band diagram calculations of these QWs. Band diagrams of InAsxSb1−x/Al0.1In0.9Sb QWs showed that the energy band of the InAsxSb1−x layer moves downward with increasing As content x. The QW is type I at x equal to 0, becomes type II at x equal to 0.1. The Fermi level (EF) of the InSb QWs lies in the band gap and below apart from the bottom of the conduction band, while EF of the InAs0.1Sb0.9 QWs is above the bottom of the conduction band of the well. The calculated sheet carrier densities are in good agreement with the experimental results. It well explains that the sheet carrier density difference between InSb and InAs0.1Sb0.9 QWs mainly originates from this band diagram difference and the position of EF.

Low temperature transport property of the InSb and InAsSb quantum wells with Al0.1In0.9Sb barrier layers grown by MBE

We investigated difference in the transport properties between Al0.1In0.9Sb/InSb and Al0.1In0.9Sb/InAs0.1Sb0.9 quantum wells (QWs). The resistivity of InSb QWs increases exponentially with decreasing temperature, while that of InAs0.1Sb0.9 QWs is much lower than that of InSb QWs, showing low resistivity even at low temperature. Energy band diagram calculations of InAsxSb1−x/Al0.1In0.9Sb quantum wells (QWs) revealed that the bottom of the conduction band of the InAsxSb1−x well moves downward with increasing As content x. The bottom of the InSb QWs lies above the Fermi level (EF), resulting in depletion of the QW, while that in InAs0.1Sb0.9 QWs lies under EF at low temperature. These calculation results support the experimental difference of the resistivity and sheet carrier density between two types of the QWs well.

BROADBAND CALCULATIONS OF BAND DIAGRAMS IN PERIODIC STRUCTURES USING THE BROADBAND GREEN'S FUNCTION WITH LOW WAVENUMBER EXTRACTION (BBGFL)

We apply the method of the Broadband Green's Functions with Low wavenumber extraction (BBGFL) to calculate band diagrams in periodic structures. We consider 2D impenetrable objects placed in a 2D periodic lattice. The low wavenumber extraction is applied to the 2D periodic Green's function for the lattice which is used to formulate the surface integral equation. The low wavenumber extraction accelerates the convergence of the Floquet modes expansion. Using the BBGFL to the surface integral equation and the Method of Moments gives a linear eigenvalue equation that gives the broadband (multi-band) solutions simultaneously for a given point in the first Brillouin zone. The method only requires the calculation of the periodic Green's function at a single low wavenumber. Numerical results are illustrated for the 2D hexagonal lattice to show the computational efficiency and accuracy of the method. Because of the acceleration of convergence, an eigenvalue problem with dimensions 49 plane wave Floquet modes are sufficient to give the multi-band solutions that are in excellent agreement with results of the Korringa Kohn Rostoker (KKR) method. The multiband solutions for the band problem and the complementary band problem are also discussed.

Rigorous analysis of image force barrier lowering in bounded geometries: application to semiconducting nanowires

Bounded geometries introduce a fundamental problem in calculating the image force barrier lowering of metal-wrapped semiconductor systems. In bounded geometries, the derivation of the barrier lowering requires calculating the reference energy of the system, when the charge is at the geometry center. In the following, we formulate and rigorously solve this problem; this allows combining the image force electrostatic potential with the band diagram of the bounded geometry. The suggested approach is applied to spheres as well as cylinders. Furthermore, although the expressions governing cylindrical systems are complex and can only be evaluated numerically, we present analytical approximations for the solution, which allow easy implementation in calculated band diagrams. The results are further used to calculate the image force barrier lowering of metal-wrapped cylindrical nanowires; calculations show that although the image force potential is stronger than that of planar systems, taking the complete band-structure into account results in a weaker effect of barrier lowering. Moreover, when considering small diameter nanowires, we find that the electrostatic effects of the image force exceed the barrier region, and influence the electronic properties of the nanowire core. This study is of interest to the nanowire community, and in particular for the analysis of nanowire I−V measurements where wrapped or omega-shaped metallic contacts are used.

A wire-form emitter metal–insulator–semiconductor tunnel junction

Physical effects arising due to change of configuration of a MIS system from planar to cylindrical, are theoretically analyzed. Attention is paid to the voltage partitioning and all the components of tunneling current. A simple simulation model is developed enabling prediction of the band diagram details and calculation of the currents. The trends expected with decreasing system radius are elucidated. Cylindrical geometry can be faced with when quantum wire is used as an electron emitter. Similar form may also be roughly attributed to an edge region of conventional MIS capacitors.

Band parameters of InGaAs/GaAs quantum dots: electronic properties study

We have made a systematic investigation of the band diagram calculation of strained and unstrained InxGa1 − xAs alloys in order to extract accurate and adapted parameters which are useful to the electronic properties of InxGa1 − xAs/GaAs quantum dots. As an application, the 40-band k.p model is used to describe the band offsets as well as the band parameters in the strained InxGa1 − xAs/GaAs system. The κ valence band parameter as well as g* Landé factor depending of the indium concentration were estimated. These results are analyzed and compared with experiment.

Photoluminescence Studies of Polarization Effects in InGaN/(In)GaN Multiple Quantum Well Structures

The impact of polarization fields in multiple quantum well (MQW) structures is revealed by photoluminescence measurements and band diagram calculations. We observe a blue shift of luminous energy of 33 meV and an increased light emission of 19% for InxGa1-xN/InyGa1-yN MQWs with respect to InxGa1-xN/GaN MQWs. Band diagram calculations show a lowering of the polarization fields and an increase in wave function overlap of 22% by adding indium into the barriers. We therefore attribute the observed blue shift and increased emission to an improved electron and hole wave function overlap due to lower electric fields in InxGa1-xN/InyGa1-yN structures.

Design of thin–film photonic metamaterial Lüneburg lens using analytical approach

We design an all-dielectric Lüneburg lens as an adiabatic space-variant lattice explicitly accounting for finite film thickness. We describe an all-analytical approach to compensate for the finite height of subwavelength dielectric structures in the pass-band regime. This method calculates the effective refractive index of the infinite-height lattice from effective medium theory, then embeds a medium of the same effective index into a slab waveguide of finite height and uses the waveguide dispersion diagram to calculate a new effective index. The results are compared with the conventional numerical treatment - a direct band diagram calculation, using a modified three-dimensional lattice with the superstrate and substrate included in the cell geometry. We show that the analytical results are in good agreement with the numerical ones, and the performance of the thin-film Lüneburg lens is quite different than the estimates obtained assuming infinite height.

Finite-Difference Frequency-Domain Algorithm for Band-Diagram Calculation of 2-D Photonic Crystals Composed of Debye-Type Dispersive Materials

Wave propagation characteristics in metamaterials including photonic crystals have been widely investigated due to their inherent capabilities for developing a novel devices in the fields of optics, microwave, and antenna engineering. This letter propose a novel finite-difference frequency-domain (FDFD) algorithm for calculating band diagrams of photonic crystals composed of Debye-type dispersive materials. It will be shown that this method is highly accurate and stable in band-diagram calculation by comparison to the finite-difference time-domain (FDTD) method.

Band gap calculation of 2-D photonic crystal and its application towards thin film solar cell

Two-dimensional (2-D) photonic crystals (PhCs) of square lattice have been investigated to find the complete photonic band gap. Band diagram calculations have been performed using the simulator "PWE band solver" of the software package optiFDTD, based on plane wave expansion method. Air holes in dielectric background with four different shapes have been considered. Parameters like filling fraction, rod/air hole orientation angle and material dielectric constant have been varied. It has been found that PhCs with square air holes have the largest gap; the gap-midgap ratio is 13.66% for GaAs (when filling fraction, f=0.680 and orientation angle ϑ = 30°). Finally, a proposal is made to improve the efficiency of thin film solar cell using this photonic crystal as back reflector. Full Text: PDF References J.D. Joannopoulos, S.G. Johnson, J. Winn, R. Meade, Photonic Crystals: Molding the Flow of Light (Princeton University Press, New Jersey 2008) L. Zeng, Y. Yi, C. Hong, J. Liu, N. Feng, X. Duan, L.C. Kimerlin, B.A. Alamariu, "Efficiency enhancement in Si solar cells by textured photonic crystal back reflector", Appl. Phys. Lett. 89, 111111 (2006) [CrossRef] P. Bermel, C. Luo, L. Zeng, L.C. Kimerling, J.D. Joannopoulos, "Improving thin-film crystalline silicon solar cell efficiencies with photonic crystals", Opt. Exp. 15 16986 (2007) [CrossRef] M. Qiu, S. He, "Large complete band gap in two-dimensional photonic crystals with elliptic air holes", Phys. Rev. B 60, 10610 (1999) [CrossRef] G. Qiua, F. Lina, Y.P. Licompelte, Complete two-dimensional bandgap of photonic crystals of a rectangular Bravais lattice (Elsevier Science, 2003) R. Wang, X.H. Wang, B.Y. Gu, G.Z. Yang, "Effects of shapes and orientations of scatterers and lattice symmetries on the photonic band gap in two-dimensional photonic crystals", J. Appl. Phys. 90, 4307 (2001) [CrossRef] J. Gee, "Optically enhanced absorption in thin silicon layers using photonic crystals", 29th IEEE Photovolt. Spec. Conf., pp. 150?153 (2002) J.I. Pankove, Optical Process in Semiconductors (Dover, New York 1971).

Solid and hollow inorganic replicas of biological photonic crystals

Photonic crystals in nature give rise to structural colors in a wide variety of organisms. We combine intriguing biological photonic structures, taken from weevils, with sol–gel templating to create a pallet of robust silica and titania replicas. Structural and optical characterization results are used to calculate band diagrams and density of optical states distributions, revealing that these replicas possess multiple overlapping photonic stop gaps with strongly modified density of optical states.

Temperature dependence of photonic crystals based on thermoresponsive magnetic fluids

The influence mechanisms of temperature on the band gap properties of the magnetic fluids based photonic crystals are elaborated. A method has been developed to obtain the temperature-dependent structure information ( A sol / A) from the existing experimental data and then two critical parameters, i.e. the structure ratio ( d/ a) and the refractive index contrast (Δ n) of the magnetic fluids photonic crystals are deduced for band diagram calculations. The temperature-dependent band gaps are gained for z-even and z-odd modes. Band diagram calculations display that the mid frequencies and positions of the existing forbidden bands are not very sensitive to the temperature, while the number of the forbidden bands at certain strengths of magnetic field may change with the temperature variation. The results presented in this work give a guideline for designing the potential photonic devices based on the temperature characteristics of the magnetic fluids based photonic crystals and are helpful for improving their quality.

Proposal of High-Electron Mobility Transistors With Strained InN Channel

By using a Schrödinger-Poisson equation solver, we calculate band diagrams of potentially record fast III-N high electron mobility transistors (HEMTs), which are based on strained InN channels. Assuming cation polarity, pseudomorphic HEMT devices are proposed with a relaxed InAIN buffer layer having Al mole fraction in the range of 0.10-0.15 and with an InAIN barrier layer. Calculations indicate highly confined electrons in the channel with a density of 1.4-2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> if the 5-10-nm-thick InN channel is separated from the barrier by ≤ 0.8-nm-thick GaN spacer. Alternatively, for a nonpolar structure with an Al mole fraction of 0.3 in the InAIN buffer and for a doping 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in the InAIN barrier, we calculate the InN channel carrier density of approximately 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . We propose to use high-k dielectrics to insulate the gate from the barrier for both of the transistor structures.

Polarization effects in 2-DEG and 2-DHG AlGaN/AlN/GaN multi-heterostructures measured by electron holography

The electrostatic potential profiles and charge distributions in modulation-doped n-type and p-type AlGaN/AlN/GaN heterostructures have been measured by electron holography with high spatial resolution. For n-type two-dimensional electron gas structure a negative curvature and for p-type two-dimensional hole gas structure a positive curvature in the potential profile at the AlN/GaN interface were observed, which demonstrated the accumulation of two-dimensional carriers. The measured electrostatic potential profiles were also compared with the calculated band diagram in the heterostructures.

Imaging of THz waves in 2D photonic crystal structures embedded in a slab waveguide

We present space- and time-resolved simulations and measurements of single-cycle terahertz (THz) waves propagating through two-dimensional (2D) photonic crystal structures embedded in a slab waveguide. Specifically, we use a plane wave expansion technique to calculate the band structure and a time-dependent finite-element method to simulate the temporal evolution of the THz waves. Experimentally, we measure the space–time evolution of the THz waves through a coherent time-resolved imaging method. Three different structures are laser machined in LiNbO3 crystal slabs and analyzing the transmitted as well as the reflected THz waveforms allows determination of the bandgaps. Comparing the results with the calculated band diagrams and the time-dependent simulations shows that the experiments are consistent with 3D simulations, which include the slab waveguide geometry, the birefringence of the material, and a careful analysis of the excited modes within the band diagrams.

Optical near-field study of photonic crystal tapers

A scanning near-field optical microscopy study is undertaken on photonic crystal tapers. These tapers are used to couple a 3 microm wide multimode dielectric waveguide to a single-line defect monomode photonic crystal waveguide. Two kinds of taper, with or without a localized defect, are compared. Higher transmission efficiency is obtained when a defect is utilized. The near-field study at 1550 nm shows experimentally that this defect prevents leaky resonant states to appear in the taper and thus permits a decrease in out-of-plane losses. These observations are supported by band diagram calculations.

Observation of hole accumulation at the interface of an undoped InGaN/GaN heterostructure

A pronounced hump structure at about −5 V in the high-frequency capacitance-voltage (C-V) curve of an undoped InGaN/GaN heterostructure is observed and this hump weakens gradually with decreasing measurement frequency, indicating the occurrence of an inversion behavior in the InGaN/GaN heterostructure. The inversion behavior in the C-V curve is attributed to hole accumulation at the heterointerface where a hole well is formed due to the strong piezoelectric polarization effect in the InGaN/GaN heterostructure. The acceptor traps related to Ga vacancies in the InGaN layer are thought to be the source of the minority carriers. The theoretical calculation of band diagram of the InGaN/GaN heterostructure confirms the formation of the hole well at the heterointerface and supports the behavior of hole accumulation under negative bias voltage.

Tunable photonic crystals based on MnFe 2O 4 magnetic fluids by magnetic fields

The model of photonic crystals (PCs) formed in magnetic fluids under externally magnetic fields is established and the mechanisms of tunability are expounded. The magnetic-field-dependent structure ratio and refractive index contrast of the photonic crystals are derived from the experimental data about the structures and refractive indices for a MnFe 2O 4 magnetic fluid under externally magnetic fields. Band diagram calculations display the tunability of the forbidden bands with magnetic field strength for both TE and TM modes. Furthermore, the number of the forbidden bands for the TE mode increases with the magnetic field strength.

Type II broken-gap GaSb1 − x Asx/InAs heterojunction (x &lt; 0.15): Evolution of the band diagram for the ternary solid solution

Layers of the GaSb1 − xAsx alloy with arsenic content in the range x = 0.06–0.15 have been grown for the first time on InAs (100) substrates by metal-organic vapor-phase epitaxy. A new approach to the calculation of the band diagram of the GaSbAs alloy is suggested. It is demonstrated on the basis of magnetotransport measurements in p-GaSbAs/p-InAs heterostructures and with the method suggested by the authors for the calculation of band diagrams for alloys in the GaSbAs system that, in the composition range under study, the GaSbAs/InAs heterojunction is a type II broken-gap heterojunction.