Thin Superlattices Research Articles

Structural state of III nitride layers implanted with erbium ions

The defect structure of AlGaN/GaN superlattices and GaN layers grown through vapor-phase epitaxy from organometallic compounds is investigated using x-ray diffraction analysis before and after implantation with erbium ions at an energy of 1 MeV and a dose of 3 × 1015 cm−2, as well as after annealing. For a superlattice with a total thickness larger than the implantation depth, the satellites of the superlattice region strained under the action of ions disappear in the x-ray diffraction pattern after annealing at temperatures higher than 900°C. This suggests that the radiation-induced defects responsible for the positive deformation in the layer are annealed at these temperatures. However, annealing even at a temperature of 1050°C does not lead to complete recovery of the initial state and the positive deformation in the remaining regions is caused by residual defects. An analysis of the x-ray diffraction patterns demonstrates that, in samples with thin superlattices located at the depth corresponding to maximum radiation damage, the periodic structure that disappears after implantation at a dose of 3 × 1015 cm−2 is not recovered even after annealing at a temperature of 1050°C. This inference is confirmed by the results of examinations with an electron microscope.

Interfaces as design tools for short-period InAs/GaSb type-II superlattices for mid-infrared detectors

AbstractThe effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. Although the oscillator strengths are predicted to be larger for thinner period superlattices, the absorption coefficients are comparable because of the compensating effect of larger band widths. However, larger intervalence band separations for thinner-period samples should lead to longer minority electron Auger lifetimes and higher operating temperatures in p-type SLs. In addition, the hole masses for thinner-period samples are on the order the free-electron mass rather than being effectively infinite for the wider period samples. Therefore, holes should also contribute to photoresponse. A number of superlattices with periods ranging from 50.6 to 21.2 Å for the 4 μm detection threshold were grown by molecular beam epitaxy based on the model design. Low temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5. Overall, the present study points to the importance of interfaces as a tool in the design and growth of thin superlattices for mid-IR detectors for room temperature operation.

Strength of coherently strained layered superlattices

Electronic-grade single-crystal semiconductor structures provide a means to study the mechanical effects of coherency strain in isolation. In this work, thin coherently strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500°C. The force-deflection curves of the specimens were measured and the beams were found to be significantly strengthened by the presence of the superlattices. Analysis provides estimates of the stress supported by the thin superlattices in the plastic regime. The superlattices are found to display mechanical strength up to a hundred times greater than the strength of the bulk substrate material. This effect can be attributed only to the coherency strain in the superlattices.

FINITE SIZE EFFECTS OF DIELECTRIC CONSTANT IN BaTiO3/SrTiO3 SUPERLATTICE

ABSTRACT BaTiO3/SrTiO3 superlattices have been deposited onto (La,Sr)CoO3/MgO (100) substrate using pulsed laser deposition. As the thickness of BTO/STO superlattice was varied from 100 nm to 10 nm, the dielectric constant decreased, which is know to be the finite size effects. The finite size effect was investigated with structural characteristics such as lattice distortion and volume change. These structural analyses were accomplished by synchrotron X-ray scattering. The anisotropic lattice distortion of the superlattice was significantly influenced by the variation in the thickness. Moreover, the superlattice exhibited the volume change of BTO and STO layers while maintaining the volume of a supercell unchanged, suggesting that the hydrostatic strain (i.e., volume change) as well as the anisotropic lattice distortion exist in the consisting BTO and STO layers. Oxygen vacancy defect, possibly causing the hydrostatic strain, did not affect the dielectric constant of the superlattice significantly, excluding the oxygen nonstoichiometry in thin superlattices from the cause of the finite size effect. The finite size effect observed in BTO/STO superlattices is mainly attributed to the lattice strain (i.e., hydrostatic and anisotropic strains) developed in the superlattices.

Theoretical investigations of epitaxial strain effects in ferroelectric oxide thin films and superlattices

Epitaxial strain in perovskite oxides has been shown to play an important role in determining the structure and properties of thin films and superlattices, especially properties related to polarization. In this short review, we describe theoretical investigations of the epitaxial strain effects in a variety of perovskite oxides, including ferroelectrics, with particular emphasis on first-principles methods, as well as Landau–Devonshire functionals and the phase-field approach.

Structure and properties of NiFe∕NiFeO magnetic superlattice

Using a sequence of the sputtering deposition and the natural oxidation at low oxygen partial pressure, a thin (total thickness of 533–900Å) superlattice was produced consisting of NiFe18 and (NiFe18)O layers on Si, Si∕SiO2, and GaAs substrates. The thickness of the as-deposited NiFe layers was in the range 9–533Å. It was found that the thickness of the NiFe film oxidizing as a result of the natural oxidation is limited to 10Å and produces about 16Å of the resulting NiFeO. It was determined that resistivity, saturation magnetization, magnetic anisotropy, and permeability could be controlled in the wide range of values by changing the thickness of the NiFe layers of the NiFe∕NiFeO superlattice. Resistivity could be increased by up to two orders of magnitude and the Hk value could be increased by up to four times with respect to those of the NiFe18 film of a comparable thickness. Stress measurements were performed in a wide range of temperature to characterize the coefficient of thermal expansion and the biaxial modulus of the superlattice. Both values were found to be essentially identical to those of NiFe18 alloy. Thermal stability of the transport and magnetic properties of the NiFe∕NiFeO superlattice with respect to 225°C magnetic annealing was also determined.

Modeling of ferroelectric domains in thin films and superlattices

We have developed the set of numerical (finite-element) and analytical tools that are based on the self-consistent solution of the electrostatic equations coupled with material Ginzburg–Landau equations with an objective to model the profile of domain textures in the entire temperature region. We calculated the evolution of principal parameters of domain texture: modulation vector, distribution of polarization, renormalization of critical temperature etc. In contrast to the Kittel approximation we conclude that the profile of polarization across domains in nanometric ferroelectric thin films is highly nonuniform.

Pulsed laser deposition of thin films and superlattices based on ZnO

The pulsed laser deposition (PLD) technique has been applied for the epitaxial growth of ZnO for more than two decades. The emergence of high-temperature stability of the excitonic lasing was first demonstrated in a microcrystalline ZnO film grown by PLD leading to recent remarkable growth in this field. A number of attempts have been made to improve the crystallinity for realizing p-type materials in the quest for ZnO-based short wavelength light emitting devices (LEDs). In this paper, we describe practical advantages of PLD and currently accomplished intrinsic properties of ZnO films according to the abundant literature. We find that correlation between Hall mobility and lateral grain size captures the effect of grain boundaries for the films grown on sapphire substrates. On the other hand, advantages of the use of lattice-matched ScAlMgO4 substrate are evidenced by the lower residual electron density, higher mobility and sharper exciton peaks in the photoluminescence and absorption spectra. We also focus on the wide-band-gap ternary alloy, MgxZn1−xO, especially in terms of the composition dependence of its lattice parameters and band-gap in two different crystallographic phases, to discuss the stability of this metastable compound. The studies on the PLD growth of multilayer and superlattices are briefly reviewed. We finally present the current capability of electron and hole doping by incorporating Ga and N into films grown on (0001) ScAlMgO4 substrates. We conclude that the PLD technique and related technologies have now mature to meet the requirements for fabricating UV-LEDs.

Spin-polarized ballistic transport in a thin superlattice of zinc blende half-metallic compounds

We examine theoretically ballistic conduction in thin layers of zinc blende half metals, considering as an example a superlattice consisting of monolayers of GaAs and MnAs, a bilayer of CrAs, and a bilayer of GaAs. By artificially separating bilayers, we show that surface states thwart half metallicity. However, capping the metal-As bilayers restores half metallicity, and ballistic conduction of electrons within $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ of the Fermi level will give nearly 100% spin-polarized transmission in the direction of the superlattice. Recent developments suggest atomic layer epitaxy can be used to produce such thin layers for spintronic applications.

Bi2Te3: Structural Modulations in Epitaxially Grown Superlattices and Bulk Materials

ABSTRACTMultiquantum well structures of Bi2Te3 are predicted to show an enhancement of the thermoelectric figure of merit ZT. Electron-conducting Bi2Te3 thin films and superlattices (SL) with a period of 12 nm were epitaxially grown on BaF2 substrates by molecular beam epitaxy. The microstructure was investigated by transmission electron microscopy. The SL could be imaged with strong contrast yielding a period of 12.0±0.5 nm. The SL is slightly bent with an amplitude of 30 nm and a wave length of 400 nm. Threading dislocations were found with a density of 2·109 cm−2. The SL interfaces are strongly bent close to threading dislocations, undisturbed regions have a maximum lateral size of 500 nm. A structural modulation (nns) with a wave length of 10 nm was found in Bi2Te3 thin films, superlattices and bulk materials. The structural modulation is found to be of general character for Bi2Te3 materials and is superimposed to the average structure. It was analysed in detail by stereomicroscopy in bulk material yielding a pure structural modulation with a displacement vector parallel to [5,-5,1] and a wave vector parallel to (1,0,10). The investigations showed the presence of none, one or two (nns). The number of (nns) and thereby the thermoelectric properties might be controlled by the growth parameters. Phonons should be scattered on the sinusoidal strain field of the (nns) yielding (i) a significantly decreased thermal conductivity, (ii) a reduced dimensionality and (iii) anisotropic transport coefficients in the basal plane.

Deformation of small volumes of material using nanostructured strained layered superlattices

A key aspect of nanostructured materials is that large coherency strains can readily exist between nano-sized phases. This can result in strengthening or in improved ductility. However, in conventional materials, it can be very difficult to separate the effects of coherency strain from other phenomena. Electronic grade single crystal semiconductor structures provide a means to study the effects of coherency strain in isolation. In this work, thin coherently strained InGaAs superlattices grown on thick InP substrates were tested in three-point bending at 500°C. The stress–strain curves of the specimens were measured, and from them, analysis yields the actual stress supported by the thin superlattice. The superlattices display extraordinary strength compared to the corresponding bulk material. This effect can be attributed only to the coherency strain in the superlattices.

Surfaces and interfaces in short-period GaAs/AlAs superlattices

Photoelectron spectroscopy, in particular the angular resolved photoemission excited by ultraviolet radiation (ARUPS), provides the most direct experimental information about the electron structure of crystals, both of the bulk and of the low-index surfaces. The sensitivity of the method, as well as its difficulties, when applied to GaAs/AlAs superlattices are described. The new periodicity of these man-made crystals in the direction of their growth (e.g., in the layer-by-layer growth in molecular beam epitaxy), is responsible for opening of the new energy gaps (so-called minigaps) in the electron energy bands of crystals forming the superlattice. In addition to the well-known confinement of electrons at the valence and conduction band edges in long-period superlattices, the electron confinement to the interfaces has also been found in the vicinity of minigaps in short-period superlattices. The role of this confinement in the intensities of electrons photoemitted from superlattice surfaces is discussed. Superlattices with different thicknesses in the topmost layers represent systems with a simple change of the surface atomic structure. The predictions of one-dimensional models about a change of the surface-state energy within the band gap with a change of crystal potential termination are tested for the ideally terminated (1 0 0) surface of a very thin superlattice (GaAs) 2 (AlAs) 2 . The results of the energy distributions of photoemitted electrons, calculated in the one-step model of photoemission, show that the ARUPS experimental observation of surface-state shifts should be possible, at least in larger minigaps. The results indicate the possibility of a straightforward tuning of the electronic structure of the superlattice surface by geometrical means.

Thermal conductivity in strain symmetrized Si/Ge superlattices on Si(111)

We have presented systematic cross-plane thermal conductivity (λ) data for the undoped strain-symmetrized Si/Ge superlattices grown on Si(111) with superlattice (SL) period thickness varying from 3.6 to 16 nm. In thin SL period (L⩽7 nm) samples, the data have shown considerable reductions of λ, by more than 50% and 30% compared to the SiGe alloy and to the earlier reported values in (100)-oriented Si/Ge superlattice structures (SLS), respectively. For the thick SL period samples (L>10 nm), λ has shown a tendency to saturate at the SiGe alloy value. This is understood as, with increasing L, the SLS breaks and the SiGe alloying starts to grow. This structural behavior is clearly observed in the cross-plane transmission electron microscope images as well. In addition to these, for the thin SL period (L⩽7 nm) samples, the data have shown a shallow minimum which is attributed to the competing behavior of the wave nature and the classical particle nature of the localized phonons. Nevertheless, the present study of thermal conductivity on undoped strain-symmetrized Si/Ge SLs in (111) orientation suggests that an enhancement of thermoelectric figure-of-merit Z is possible.

Electronic structure of thin heterocrystalline superlattices in SiC and AlN

The spontaneous polarization, the valence band offset, and the quantum confinement effect for thin SiC and AlN cubic/hexagonal heterocrystalline superlattices are studied by use of a full potential linear muffin-tin orbital method (FP-LMTO). We find that the polarization is screened and suppressed while the length of the cubic region grows. The band offsets do not change with layer thickness. For thin superlattices, the quantum confinement effects dominate and result in a band gap which stays larger than the gap of the bulk cubic structure. Furthermore, the energy levels of the bound states in the quantum well resemble the pattern of the energy levels of conventional III-V based quantum wells and superlattices but at a much smaller length scale, which is due to the higher quantum well depth and the larger effective masses in SiC and AlN systems.

Surfaces and interfaces in short-period GaAs/AlAs superlattices

Photoelectron spectroscopy, in particular the angular resolved photoemission excited by ultraviolet radiation (ARUPS), provides the most direct experimental information about the electron structure of crystals, both of the bulk and of the low-index surfaces. The sensitivity of the method, as well as its difficulties, when applied to GaAs/AlAs superlattices are described. The new periodicity of these man-made crystals in the direction of their growth (e.g., in the layer-by-layer growth in molecular beam epitaxy), is responsible for opening of the new energy gaps (so-called minigaps) in the electron energy bands of crystals forming the superlattice. In addition to the well-known confinement of electrons at the valence and conduction band edges in long-period superlattices, the electron confinement to the interfaces has also been found in the vicinity of minigaps in short-period superlattices. The role of this confinement in the intensities of electrons photoemitted from superlattice surfaces is discussed. Superlattices with different thicknesses in the topmost layers represent systems with a simple change of the surface atomic structure. The predictions of one-dimensional models about a change of the surface-state energy within the band gap with a change of crystal potential termination are tested for the ideally terminated (1 0 0) surface of a very thin superlattice (GaAs) 2(AlAs) 2. The results of the energy distributions of photoemitted electrons, calculated in the one-step model of photoemission, show that the ARUPS experimental observation of surface-state shifts should be possible, at least in larger minigaps. The results indicate the possibility of a straightforward tuning of the electronic structure of the superlattice surface by geometrical means.

Relation Between Surface Crystallography and Surface Electron Structure of the Superlattice

Profound gradual changes of surface state energies were predicted for varying surface terminations of the periodic crystal potential in one-dimensional models.1 This situation can be realized in superlattices with different thicknesses of topmost layers. For the ideally terminated (100) surface of a very thin superlattice (GaAs)2(AlAs)2, the shift of the energy of the surface state over the whole minigap in the lower part of the valence band has been found for different terminations of the topmost layer. In the center of the surface Brillouin zone the surface state shift follows model trends. The changes of the energy distribution of photoemitted electrons as determined from the one-step photoemission calculation2 indicate that experimental observation by the surface-sensitive technique of angle-resolved photoemission should be feasible, and preliminary data indicate this. The results show a straigthforward tuning of surface electron structure by geometrical means.

K � p Calculations of Electronic and Optical Properties of p-doped (001) AlGaN/GaN Thin Superlattices

In this work we present theoretical photoluminescence (PL) and absorption spectra as calculated for modulation p-doped (001) AlGaN/GaN thin superlattices (SLs). We apply a self-consistent k · p method, in the framework of the effective-mass theory, which solves the full 8 × 8 Kane Hamiltonian, in conjunction with the Poisson equation in order to calculate the electronic and optical properties of these systems. The trends in the calculated Stokes shift, due to many-body effects within the quasi-two-dimensional hole gas (2DHG), are analysed as a function of the acceptor doping concentration. The in-plane hole effective mass for the first occupied energy level in the systems is predicted.

Functional behaviour of thin film dielectric superlattices

Pulsed laser deposition has been used to fabricate thin film capacitor structures in which the dielectric layer is a superlattice. The properties of two superlattice systems were investigated as a function of superlattice wavelength Λ, one based on barium strontium titanate and the other on lead based relaxor electroceramics. In both systems the dielectric constant was significantly enhanced at stacking wavelengths of a few unit cells. However, the dielectric enhancement seen in the barium strontium titanate superlattices was found to be due to Maxwell-Wagner effects, whereas in the relax or superlattices Maxwell-Wagner behaviour was not evident; rather, the dielectric enhancement was associated with the onset of polar coupling at around Λ=20 nm.

Growth and characterization of Al 1− XMn Xas ( X⩽4%) magnetic semiconductor: thin film and superlattices

A new diluted magnetic semiconductor, AlMnAs, was grown using stoichiometric low-temperature molecular beam epitaxy, with Mn composition up to 4%. In situ (RHEED) and ex situ (XRD, SIMS) characterization of uniform thin films and a GaAs/AlMnAs superlattice revealed good crystal quality and a very high Mn incorporation. Magnetic and electrical characterization showed the samples to be semi-insulating and hence paramagnetic, due to the lack of free carriers. The magnetic moment can be explained assuming Mn 2+ in the AlMnAs.

On the structure of epitaxial YH x films

Hydrogen in thin epitaxial films and superlattices has gained renewed interest because of the hydrogen-induced novel optical and magnetic switching properties that have recently been revealed. In this contribution we report on structural details on the effect of hydrogen in epitaxial Y-films. X-ray diffraction reveals the response of the host metal lattice upon hydrogen loading. The hydrogen ordering within the hydride phases as well as the hydrogen concentration can be measured by neutron diffraction. The macroscopic aspects of the hydride formation like domain nucleation and growth are accessible by in-situ real time X-ray diffraction topography.