Superlattice Layer Thickness Research Articles

InGaAs-Based MSM Photodetector: Researching Absorption Layer, Barrier Layer, and Digital Graded Superlattice Layer with 3D Simulation

The effects of absorption, barrier, and digital graded layer thickness on dark current and photocurrent in Metal Semiconductor Metal (MSM) photodetectors by using 3D Silvaco TCAD are reported. The photo-dark current ratio (Iphoto/Idark) is calculated using the photocurrent and dark current values obtained by simulation. In study A (absorption layer thickness variation) the photocurrent, dark current and photo-dark current ratio are increased with increased absorption layer thickness, and the dark current is 1.138x10-7 A levels. In Study B (barrier layer thickness variation), when the barrier layer is added to the absorption layer, the dark current is decreased to 1.56x10-11 A levels. It is reported that the photo-dark current ratio with increasing barrier layer thickness increases. In study C (digital graded superlattice layer thickness variation), the dark current increases, and photocurrent decreases with the increase of the digital graded superlattice layer thickness. However, the photo-dark current ratio with increasing digital graded superlattice layer thickness decreases. Furthermore, a similar trend of development is observed on photo-dark current with adding of the barrier layer and digital graded superlattice layer on the absorption layer. These findings demonstrate the importance of optimizing layer thickness in MSM photodetectors for improved device performance.

First-Principles Study of n*AlN/n*ScN Superlattices with High Dielectric Capacity for Energy Storage.

As a paradigm of exploiting electronic-structure engineering on semiconductor superlattices to develop advanced dielectric film materials with high electrical energy storage, the n*AlN/n*ScN superlattices are systematically investigated by first-principles calculations of structural stability, band structure and dielectric polarizability. Electrical energy storage density is evaluated by dielectric permittivity under a high electric field approaching the uppermost critical value determined by a superlattice band gap, which hinges on the constituent layer thickness and crystallographic orientation of superlattices. It is demonstrated that the constituent layer thickness as indicated by larger n and superlattice orientations as in (111) crystallographic plane can be effectively exploited to modify dielectric permittivity and band gap, respectively, and thus promote energy density of electric capacitors. Simultaneously increasing the thicknesses of individual constituent layers maintains adequate band gaps while slightly reducing dielectric polarizability from electronic localization of valence band-edge in ScN constituent layers. The AlN/ScN superlattices oriented in the wurtzite (111) plane acquire higher dielectric energy density due to the significant improvement in electronic band gaps. The present study renders a framework for modifying the band gap and dielectric properties to acquire high energy storage in semiconductor superlattices.

Carrier Transport in the Valence Band of nBn III–V Superlattice Infrared Detectors

Mid-wavelength infrared detectors have been fabricated in the nBn configuration using the InAs/InAsSb superlattice as the absorber. Possible impediments in the valence band can interfere with the transport of holes that represent the signal. We demonstrate that the thermal activation energy of the photocurrent density, as a function of the applied bias voltage, can be a very sensitive probe of the valence band features. We identify and measure two types of impediments, the hole-block due to a band misalignment and the localization sites formed by fluctuations in the superlattice layer thicknesses. The latter are found to dominate the temperature dependence of the hole mobility. Our inferred localization characteristics are consistent with published results obtained by other techniques.

GaAsSb layer thickness dependence of arsenic incorporation on InAs/GaAsSb superlattice on InAs substrate grown by metalorganic vapor phase epitaxy for mid-infrared device

We investigated the gas flow sequence to realize a short-period and lattice-matched InAs/GaAsSb superlattice on an InAs substrate by metalorganic vapor phase epitaxy (MOVPE) growth for applications in mid-infrared photonic devices. The arsenic composition of GaAsSb for lattice matching was adjusted by adding AsH3 flow to residual arsenic incorporation. The growth conditions for lattice-matched superlattices at a period of 5 nm were found. The superlattices with period thicknesses of 1 to 9 nm were successfully confirmed by the photoluminescence (PL) peak in the range from 2.4 to 5 µm depending on layer thickness at 20 K. The emission wavelength can be controlled by changing the layer thickness of superlattices. The results led to the improvement of InAs/GaAsSb type II superlattice growth.

Ultralow Lattice Thermal Conductivity of the Random Multilayer Structure with Lattice Imperfections

Randomizing the layer thickness of superlattices (SL) can lead to localization of coherent phonons and thereby reduces the lattice thermal conductivity κl. In this work, we propose strategies that can suppress incoherent phonon transport in the above random multilayer (RML) structure to further reduce κl. Molecular dynamics simulations are conducted to investigate phonon heat conduction in SLs and RMLs with lattice imperfections. We found that interfacial species mixing enhances thermal transport across single interfaces and few-period SLs through the phonon “bridge” mechanism, while it substantially reduces the κl of many-period SLs by breaking the phonon coherence. This is a clear manifestation of the transition from incoherent-phonon-dominated to coherent-phonon-dominated heat conduction in SLs when the number of interface increases. In contrast, interfacial species mixing always increases the κl of RMLs owing to the dominance of incoherent phonons. Moreover, we found that doping a binary RML with impurities can reduce κl significantly, especially when the impurity atom has an atomic mass lower or higher than both of the two base elements. This work reveals the critical effect of lattice imperfections on thermal transport in SLs and RMLs, and provides a unique strategy to hierachically suppress coherent and incoherent phonon transport concurrently.

Form factor dispersion at La M5,4 edges and average density of resonant atoms

Resonant soft x-ray scattering on complex oxide superlattices shows very large variations in the superlattice reflection position and intensity near La M5,4 edges. Resonant dispersion of the La x-ray form factor describes the observations well. We determine the average density of resonant La atoms and the thickness of superlattice layers.

Terahertz electroluminescence of fibonacci superlattices

The electroluminescence of AlGaAs Fibonacci superlattices in the terahertz range is studied theoretically. Linear behavior of the optical amplification coefficient G and multicolored emission with peaks of G higher than 20 cm−1 without accompanying absorption peaks are observed in electric fields of F = 11–13 kVcm−1 at the frequencies f = 2–4 THz. The parameters of the peaks can be conveniently controlled by varying the electric field and the superlattice layer thicknesses. Such emitters based on quasi-periodic super-lattices offer promise for terahertz technology.

CdSe/CdTe type-II superlattices grown on GaSb (001) substrates by molecular beam epitaxy

CdSe/CdTe superlattices are grown on GaSb substrates using molecular beam epitaxy. X-ray diffraction measurements and cross-sectional transmission electron microscopy images indicate high crystalline quality. Photoluminescence (PL) measurements show the effective bandgap varies with the superlattice layer thicknesses and confirm the CdSe/CdTe heterostructure has a type-II band edge alignment. The valence band offset between unstrained CdTe and CdSe is determined as 0.63 ± 0.06 eV by fitting the measured PL peak positions using the envelope function approximation and the Kronig-Penney model. These results suggest that CdSe/CdTe superlattices are promising candidates for multi-junction solar cells and other optoelectronic devices based on GaSb substrates.

Growth and properties of digitally-alloyed AlGaInP by solid source molecular beam epitaxy

Digital alloying using molecular beam epitaxy (MBE) was investigated to produce AlGaInP quaternary alloys for bandgap engineering useful in 600-nm band optoelectronic device applications. Alternating Ga0.51In0.49P/Al0.51In0.49P periodic layers ranging from 4.4 monolayers (ML) to 22.4 ML were used to generate 4,000-A-thick (Al0.5Ga0.5)0.51In0.49P quaternary materials to understand material properties as a function of constituent superlattice layer thickness. High-resolution x-ray diffraction (XRD) analysis exhibited fine satellite peaks for all the samples confirming that digitally-alloyed (Al0.5Ga0.5)0.51In0.49P preserved high structural quality consistent with cross-sectional transmission electron microscopy (X-TEM) images. Low-temperature photoluminescence (PL) measurements showing a wide span of luminescence energies ∼ 170 meV can be obtained from a set of identical composition digitally-alloyed (Al0.5Ga0.5)0.51In0.49P with different superlattice periods, indicating the bandgap tunability of this approach and its viability for III-P optoelectronic devices grown by MBE.

Lattice-dynamical calculation of phonon scattering at ideal Si–Ge interfaces

Detailed phonon scattering at an ideal Si–Ge interface is studied with a linear lattice dynamics model. Frequency dependent transmission coefficients indicate the significance of acoustic-optical phonon mode conversion at the interface. Applied to multiple interfaces, the method shows how the overall thermal resistance approaches a finite (Bloch mode) limit with the increasing number of interfaces in absence of other scattering mechanisms. The dependence of thermal resistance on the superlattice layer thickness is not significant even in the interface-scattering-only limit we study. We also assess errors incurred by the finite domain size and classical statistics in molecular dynamics simulations of interface thermal resistance. Results suggest that using 6×6 unit cells in the transverse directions, a tractable size for such simulations, will incur only a 5% error in the predicted thermal resistance. Similarly, the error due to the classical (Boltzmann) phonon distribution in molecular dynamics simulations is predicted to be less than 10% for temperatures above 300 K.

IR reflection of optical phonons in GaN/AlGaN superlattices

We present the IR study of vibrational properties of GaN/Al0.28Ga0.72N superlattices with different thickness of superlattice layers. IR reflection spectra of the superlattices were taken at normal and off-normal incidence using p-polarised light in order to analyze optical phonons propagating both along and perpendicular to the layer surface. Optical phonon frequencies as well as thickness of the structure layers were determined from a comparison of the experimental IR reflection spectra to those calculated in the framework of the dielectric continuum model. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnMnSe/ZnSSe Type-II semimagnetic superlattices: Growth and magnetoluminescence properties

A ZnSSe/ZnMnSe type-II semimagnetic superlattice was pseudomorphically grown via molecular beam epitaxy on a GaAs substrate. The superlattice-layer thicknesses and compositions were chosen so that compressive strains in the ZnMnSe layer compensated tensile strains in the ZnSSe layer. The photoluminescence spectra in an external magnetic field demonstrate the effect of “giant Zeeman splitting of an exciton”. Simulation of the luminescence-line shift in a magnetic field allowed us to determine more accurately the band offsets at the ZnSSe/ZnMnSe interface.

Critical thickness for transformation of epitaxially stabilized cubic AlN in superlattices

The epitaxial stabilization and transformation of cubic AlN layers in AlN/VN and AlN/TiN superlattices, grown by reactive sputtering on MgO (001), is described. In AlN/VN, the critical AlN thickness lC for transformation from cubic to hexagonal increased from ≈3.0 to >4 nm when the VN superlattice layer thickness was increased from 2.0 to 6.0 nm. The effect of lattice mismatch was observed by comparing AlN/VN (mismatch=1.46%) and AlN/TiN (mismatch=3.84%). The lC values were smaller, 2–2.5 nm, for the larger mismatch AlN/TiN system. The dependence of lC on the lattice mismatch and stabilizing layer thickness is discussed based on models of epitaxial stabilization.

Growth and operation tolerances for Sb-based mid-infrared lasers

The potential performance of InAs/InGaSb-based mid-wavelength infrared lasers is examined theoretically. Band structure engineering is employed to determine the superlattice growth parameters which minimize intersubband absorption and Auger recombination losses, resulting in optimized designs which minimize threshold current densities. Calculations are performed to assess the impact of uncertainties in superlattice layer thicknesses and the temperature of laser operation on these optimized laser designs. We find that growth accuracies of ±3.5 Å for InGaSb and ±0.25 Å for InAs layers are required to retain optimization. Moreover, it is demonstrated that threshold current densities of the optimized superlattices do not obey a simple T o characteristic temperature parameterization due to sharp structure in the intersubband absorption spectrum, whereas nonoptimized structures have characteristic temperatures in the 30–65 K range. The microscopic calculations employ accurate K·p superlattice band structures and matrix elements, and evaluations of the gain, intersubband absorption, threshold carrier densities, and radiative and Auger recombination rates to obtain threshold current densities.

Experimental Study of Phonon-Folding in Si/Ge and Si/Sige Structures Designed for Thermoelectric Applications

AbstractWe report data of Raman study of Si/Ge and Si/SiGe superlattices designed for thermoelectric applications. The obtained Raman spectra clearly indicate the presence of folded doublets from longitudinal acoustic phonons. Due to the significant difference in the sound velocities for Si and Ge the position of the doublets strongly depends on the superlattice layer thickness and ordering. Comparison of the Raman data for different samples with measured thermal and thermoelectric properties allows us to determine a correlation between the strength of the phonon confinement in these structures and their thermoelectric properties. Our experimental results are consistent with recent theoretical prediction of increased thermoelectric figure of merit in semiconductor quantum wells.

Resonance Raman Study of Folded Acoustic Modes in StrainedInSb/In1-xAlxSb Superlattices

The folded acoustic modes in a series of InSb/In1-xAlxSb (0.2<x<0.5) strained-layer superlattices were investigated using resonance Raman spectroscopy. Folded modes up to fourth order in the folding index were observed. Analysis of the wavenumbers and intensities of these modes allowed estimates of the superlattice periods and layer thicknesses, and also the ratioP2/P1 of the photoelastic coefficients for the InSb (P1) and alloy (P2) layers at 647.1 nm. TheP2/P1 ratio remarkably changes sign forx≈0.1 under near-resonance excitation owing to the variation in the alloyE1 optical gap withx.

Resonance Raman Study of Folded Acoustic Modes in StrainedInSb/In1-xAlxSb Superlattices

The folded acoustic modes in a series of InSb/In1-xAlxSb (0.2<x<0.5) strained-layer superlattices were investigated using resonance Raman spectroscopy. Folded modes up to fourth order in the folding index were observed. Analysis of the wavenumbers and intensities of these modes allowed estimates of the superlattice periods and layer thicknesses, and also the ratioP2/P1 of the photoelastic coefficients for the InSb (P1) and alloy (P2) layers at 647.1 nm. TheP2/P1 ratio remarkably changes sign forx≈0.1 under near-resonance excitation owing to the variation in the alloyE1 optical gap withx.

Growth and x-ray characterization of Co/Cu (111) superlattices.

We have prepared Co/Cu(111) superlattices by molecular-beam-epitaxy methods on sapphire (112\ifmmode\bar\else\textasciimacron\fi{}0) substrates with a Nb buffer layer. The structural properties with various superlattice periodicities and layer thicknesses have been thoroughly analyzed by high-resolution x-ray Bragg-scattering and surface-scattering methods. The characterization includes the structural coherence lengths and mosaic distributions parallel and perpendicular to the film plane, the epitaxial relation to the Nb buffer, the stacking sequence, and in particular the interfacial roughness. These parameters are crucial for the interpretation of magnetic data concerning the exchange coupling in the Co/Cu superlattice. For all the superlattices studied we find that they are coherently strained with complete in-plane matching and corresponding out-of-plane Poisson response. The stacking is always completely fcc-like, with no hcp contribution even for a Co/Cu thickness ratio of 2/1. The interfacial roughnesses as obtained from fits to low- and high-angle x-ray reflectivity data are 14 and 6 \AA{}, respectively. These parameters are discussed within the framework of random and correlated roughness effects.

Deep levels in type-II InAs/GaSb superlattices.

The theory of ${\mathit{sp}}^{3}$-bonded substitutional deep impurity levels is extended to type-II gap-misaligned InAs/GaSb superlattices. The theory predicts that some shallow impurities (donors or acceptors) in either bulk InAs or bulk GaSb can become deep traps in a thin-layer superlattice. This happens because the deep levels associated with point defects in either InAs or GaSb layers (when measured relative to the valence-band maximum of InAs) are much less sensitive to changes of the layer thicknesses of the superlattice than the superlattice band edges. Some common column-IV dopants may lose their amphoteric character [i.e., being a donor when substituting for the column-III host atom (${\mathit{C}}_{\mathrm{In}}$ in InAs) and an acceptor when substituting for the column-V host atom (${\mathit{C}}_{\mathrm{As}}$ in InAs)] and become deep traps for some superlattice layer thicknesses and can even become ``false valence'' dopants (e.g., ${\mathit{C}}_{\mathrm{In}}$ is predicted to be an acceptor in a 10\ifmmode\times\else\texttimes\fi{}10 InAs/GaSb [001] superlattice) for other layer thicknesses. The deep-level splitting and shifting in the type-II superlattice is found to follow the same physics as in the type-I superlattice. A semiconductor-semimetal-semiconductor transition is predicted to occur as the InAs layer thickness increases.

Photoconductive properties of PbSrS/PbSnS superlattices

PbS/Pb 1− x Sn x S ( x = 0.03) and Pb 1− y Sr y S/Pb 1− x Sn x S ( x = 0.03, y = 0.02) superlattices were prepared by hot wall epitaxy and electrical or photoconductive properties parallel to the superlattice layer were measured. In the PbSnS, Sn makes deep donor level just above the valence band edge. So the films doped with acceptor impurity show high resistivity and large photoconductivity. For the PbS/PbSnS superlattices, it was observed that the activation energy of ionized Sn levels becomes larger than that in PbSnS, and also that the photoconduction edge shifts to high energy side with decreasing PbS or PbSnS layer thickness. These results show that the superlattice has a type-I' structure where conduction band edge of the PbSnS is higher than that of the PbS. For PbSrS/PbSnS type-I superlattices, oscillations depending on superlattice period or PbSnS layer thickness were observed in photoconductive properties. The photoconduction edge and the oscillation peak positions in the PbSrS/PbSnS superlattices are discussed in terms of the intersubband transition between valence and conduction bands.