Δ1 Transitions Research Articles

Photoreflectance study in the E1 and E1+Δ1 transition regions of CdTe

Photoreflectance (PR) measurements have been carried out to determine the E1 and E1+Δ1 critical-point (CP) parameters in CdTe at temperatures between T=77 and 300 K using a He–Ne laser as modulation light source. The measured PR spectra give very weak, but distinct, structures at ∼3.5 eV(E1) and ∼4 eV(E1+Δ1). When an Ar+-ion laser is illuminated as bias light, the spectra measured for T⩽150 K become very strong. This effect is considered to be due to a bias-laser-induced band bending. The experimental PR spectra can be successfully explained by an excitonic model of the interband transitions. The temperature dependence of the CP parameters (energy, amplitude, and broadening parameter) are analyzed using the Varshni equation and an empirical expression of Bose–Einstein type.

Optical characterisation of MOVPE-grown Ga1−xMnxAs semimagnetic semiconductor layers

For the first time, a successful growth of Ga1−xMnxAs layers on (100)GaAs substrates by metal-organic vapour-phase epitaxy is reported. Optical and magneto-optical spectroscopy of the E0, E1 and E1+Δ1 transitions is possible due to the high structural quality of the samples. A strong exchange interaction is found in the Ga1−xMnxAs layers between the magnetic moments of manganese and the excitonic states, which exhibits an effective ferromagnetic coupling. The Mn concentrations in the layers can be estimated by two different and independent optical methods (i) from the Zeeman splitting of the E0 excitons and (ii) from the red-shift of the E1 transition due to the p-doping with Mn.

Quantum confinement of E1 and E2 transitions in Ge quantum dots embedded in an Al2O3 or an AlN matrix

Alternating layers of Ge quantum dots embedded in either Al2O3 or AlN matrices were deposited on sapphire substrates by pulsed-laser deposition. The characteristics of the dots are shown to be independent of the surrounding matrix. The dots size (73, 130, 160, and 260 ű5%) was controlled by the laser energy density and deposition time, and was characterized by high-resolution transmission electron microscopy. The dots were single crystalline with no apparent GeOx interfacial layers. Transmission spectroscopy at room temperature and 77 K was used to probe the above-band-edge absorption of the Ge nanodots. The spectral positions of both E1/E1+Δ1 and E2 transitions were found to shift to higher energy in the absorption spectra with decreasing nanodot sizes. This indicates that strong quantum-confinement effect permits the optical properties of Ge dots to be modified in a controlled manner.

Optical Constants, Critical Points, and Phonon Modes of GaAsN Single Layers

ABSTRACTSpectroscopic ellipsometry (SE) is employed to study the optical properties of GaAs1−yNy [0% ≤ y ≤ 3.7%] single layers for photon energies from 0.75 eV to 4.5 eV and for wavenumbers from 100 cm−1 to 600 cm−1. We provide parametric model functions for the dielectric function spectra of GaAsN in both photon energy ranges. The model functions for photon energies from 0.75 eV to 4.5 eV excellently match dielectric function data obtained from a numerical wavelength-by-wavelength inversion of the experimental data. Criticalpoint analysis of the ellipsometric data is performed in the spectral regions of the fundamental band gap and the critical points E1 and E1+δ1. The band-gap energy is red shifted whereas the E1 and E1+δ1 transition energies are blue shifted with increasing y. For y ≤ 1.65% the observed blue shift of the E1 energy is well explained by the sum of the effects of biaxial (001) strain and alloying. The GaAsN layers show two-mode behaviour in the infrared spectral range (100 cm−1 to 600 cm−1). We detect the transverse GaAs- and GaN- sublattice modes at wavenumbers of about 267 cm−1 and 470 cm−1, respectively. The polar strength of the GaN TO mode increases linearly with y. This effect can be used to monitor the nitrogen composition in GaAsN layers.

Optical Spectroscopic Studies of N-Related Bands in Ga(N,As)

We have investigated the unusual band formation at the Γ-point and in the vicinity of the L-point in the alloy system Ga(N, As) by various spectroscopic methods. A series of GaNxAs1—x epitaxial layers with x varying from 0.05 to 2.8% was grown on (100) GaAs by metal-organic vapour phase epitaxy. The samples were studied by photoluminescence (PL) as well as photoluminescence excitation (PLE) spectroscopy, photomodulated reflectance (PR), and conventional reflectance (R) spectroscopy at room temperature and liquid helium temperature. The low-temperature PL and PLE spectra in the spectral region of the E0 band gap show clear evidence for in-gap nitrogen-pair and cluster states at low concentrations (x < 0.1%), and for higher nitrogen concentrations the formation of a new band. The dependence of the E0 band gap on N-content for x < 1% at 8 K is considerably stronger than at 300 K. Furthermore, R spectra of the E1 and E1 + Δ1 transitions show an uncommonly strong disorder-induced broadening with increasing N-content.

In situ reflectance difference spectroscopy of II–VI compounds: A real time study of N plasma doping during molecular beam epitaxy

Reflectance difference spectroscopy (RDS) has been performed during the growth and nitrogen-doping process of II–VI layers fabricated by molecular beam epitaxy (MBE). To the MBE chamber, equipped with an electron cyclotron resonance cell for N plasma generation, a RDS system has been attached via a normal incidence viewport which allows the acquisition of spectra during the doping process in the spectral range from 1.5 to 5.5 eV. ZnTe (001) surfaces have been studied under varying exposure conditions, like Zn, Te, and/or N plasma flux onto the sample surface. Furthermore, RDS features in the vicinity of the E1 and E1+Δ1 transitions were used to optimize online the doping performance of the N plasma cell by varying the source parameters, like N pressure and input power. Doping induced surface processes and surface saturation with activated N species have been investigated. Ex situ measured spectra are compared with in situ acquired data to study the surface Fermi level pinning occurring at ambient pressures. Finally, in situ acquired RDS data of δ-doped ZnSe are presented.

In situ reflectance difference spectroscopy of N-plasma doped ZnTe grown by molecular beam epitaxy

In situ reflectance difference spectroscopy (RDS) has been performed during growth and nitrogen-doping of ZnTe thin films fabricated by molecular beam epitaxy. The doping level of the ZnTe samples can be determined by evaluating the RD spectra in the vicinity of the E1 and E1+Δ1 transitions. RDS features in this spectral range were used to optimize online the doping performance of the N-plasma cell. Furthermore, doping-induced surface processes have been investigated, like surface saturation with activated N species and surface Fermi level pinning occurring at ambient pressure.

Ellipsometry Studies, Optical Properties, And Band Structure of Gel.1-yCy, Ge-RICH Si1-x-yGexCy, And Boron-Doped Si1-xGexAlloys

AbstractWe measured the pseudodielectric function (PDF) of Ge1-yCy and Ge-rich Si1-x-yGexCy alloys from 1.1 to 5.2 eV using spectroscopic ellipsometry. These alloys were grown by molecular beam epitaxy at 6000C on (001) Si substrates. Analytical lineshapes fitted to numerically calculated derivatives of their PDFs determined the critical-point parameters of the E1, E1+Δ1, E0', and E2 transitions. The critical-point energies of the Ge1-yCy alloys were found to be similar to bulk Ge. This indicates that the presence of C in these alloys only has a small influence on the band structure. For some samples, the amplitude of the PDF is much lower than in bulk Ge, which can be attributed to surface roughness and explained within the framework of the Kirchhoff theory of diffraction or using effective medium theory. The degree of surface roughness indicated by optical measurements was confirmed by atomic force microscopy. We also studied bulk Czochralski-grown Si1-xGex alloys (0&lt;x&lt;0.28) doped with boron. Due to doping, the critical points shift to lower energies as reported previously for bulk Si and Ge.

Crystal field transitions in Co 2 [Al 4 Si 5 ]O 18 cordierite and CoAl 2 O 4 spinel determined by neutron spectroscopy

Inelastic magnetic neutron scattering has been used to determine the energy of the 4A2→4T2 transition in CoAl2O4 spinel and the δ1 transition in Co2[Al4Si5]O18 cordierite. The observed crystal field splitting in Co-spinel is 485 meV (3900 cm−1), which corresponds to a crystal field stabilization energy of 56.2 kJmol−1. The transition energy of the δ1 transition in Co-cordierite has been determined to be 21 meV (170 cm−1). The present data demonstrate that magnetic neutron scattering can be used to measure crystal field transitions at energies of interest in the study of 3d-containing silicates. It may be used to measure transition energies when the use of optical spectroscopy is inappropriate.

Dielectric response of strained and relaxed Si1−x−yGexCy alloys grown by molecular beam epitaxy on Si(001)

Using spectroscopic ellipsometry, we measured the pseudodielectric function of Si1−x−yGexCy alloys (0≤x≤0.48,0≤y≤0.05) grown on Si(001) using molecular beam epitaxy. For pseudomorphically strained layers, the energy shifts of the E1, E1+Δ1, E0′, and E2 transitions are determined by line shape analysis and are due to alloy composition effects, as well as hydrostatic and shear strain. We developed expressions for hydrostatic and shear shift from continuum elasticity theory, using deformation potentials for Si and Ge, for biaxial stress parallel to the (001) growth plane in a diamond or zinc blende-type crystal and applied this to the ternary Si–Ge–C alloy. The energies of E1 and its spin-orbit split partner E1+Δ1 agree fairly well with theory. The E2 transitions in Si1−xGex at around 4.3 eV depend linearly on Ge concentration. In case of relaxed layers, the E1 and E1+Δ1 transitions are inhomogeneously broadened due to the influence of misfit and threading dislocations. For a silicon cap on top of a dislocated, relaxed SiGe layer, we recovered the bulk Si dielectric function.

Ellipsometric characterization of Cd1−x Mn x Te thin films in the presence of perturbative fields

Ellipsometric studies in the 1·5 to 5 eV range have been carried out on thin films of Cd1−x Mn x Te and exhibitE 0,E 1 andE 1+Δ1 transitions of the cubic semiconductors. The effect on these transitions upon irradiation by HeNe laser light, application of small alternating magnetic fields and the influence of both the fields, applied simultaneously, has been investigated. The observed critical point shifts have been interpreted on the basis of changes in the band structure of these thin films. This has been corroborated by theoretically calculating the effective number of electrons contributing to the transition per atom —N eff.

Ellipsometric studies on Cd1−xMnxTe thin films under the influence of HeNe laser and small alternating magnetic fields

Ellipsometric studies in the energy range 1.5–5 eV have been performed on thin films of Cd1−xMnxTe deposited by sublimation from alloys prepared by melt quenching. The spectra show E0, E1, and E1+Δ1 transitions of cubic semiconductors. The changes in peak positions under influence of HeNe laser light, small alternating magnetic field, and under the influence of both applied simultaneously have been investigated. Shifts in peak positions observed for all the three transitions have been interpreted on the basis of changes in the band structure of Cd1−xMnxTe thin films at these critical points. This has been corroborated by theoretically calculating the effective number of electrons contributing to transition per atom Neff and the density-of-states data.

Study of the optical properties of In0.52(AlxGa1−x)0.48As by variable angle spectroscopic ellipsometry

The optical properties of In0.52(AlxGa1−x)0.48As epilayers with various x values were systematically studied using variable angle spectroscopic ellipsometry in the wavelength range of 310–1700 nm. The refractive indexes were determined and could be given as n(x)=0.12x2–0.51x+3.6 at the wavelength of 1.55 μm. The measured thickness of the epilayers agrees within 5.2% of the nominal thickness. The energies and broadening parameters of the E1 and E1+Δ1 transitions as a function of Al composition were also examined based on the second-derivative spectra of the dielectric function. The comparison between the results and the reported data is presented.

Photoreflectance of InSb/GaAs heterostructures at E1 and E1 + Δ1 transitions

The photoreflectance spectra of InSb/GaAs heterostructures at the E1 and E1 + Δ1 transitions have been measured as a function of layer thickness. The spectra show an increase in lineshape broadening, similar to that observed in the increase in FWHM of the X-ray diffraction peaks, as the film thickness decreases. The data indicate increased crystal quality with increasing layer thickness. The results show that photoreflectance can be used as a tool to study the structural quality of lattice-mismatched heterostructures.

Photoreflectance of InSb at the E1 and E1 + Δ1 transitions

The Photoreflectance at high interband transitions ( E 1 and E 1 + Δ 1) is obtained at 8 K for InSb bulk samples. For a p-type sample, the lineshape is well reproduced by Aspens third derivative model, while for an undoped sample, the photoreflectance spectra is better reproduced by a first derivative of an exciton lineshape, suggesting the presence of a saddle-point exciton at low temperature. The transition energies determined from the spectra indicate an excitonic resonance at about 12 meV below the interband transitions in agreement with the 2D exciton model. The transition broadenings obtained from the first derivative function agree with the rigid pseudo-ion model predictions.

Optical Constants of Zn1-xCdxTe Ternary Alloys: Experiment and Modeling

The optical spectra of Zn1-xCdxTe ternary alloys in the 1.1-5.6-eV photon-energy range at room temperature have been measured by spectroscopic ellipsometry (SE). The optical constants reported here are the complex dielectric constant (ε=ε1+iε2), complex refractive index (n*=n+ik), absorption coefficient (α) and normal-incidence reflectivity (R). The measured data are analyzed on the basis of a simplified model of the interband transitions. The experimental data reveal distinct structures in the optical spectra at energies of the E0, E0+Δ0, E1, E1+Δ1 and E2 transitions, and are in reasonable agreement with the theoretical model. The model proposed here enables us to obtain the optical response in Zn1-xCdxTe alloys of arbitrary composition (x) and photon energy (E=\\hbarω).

Strain determination in InxGa1−xAs/GaAs strained-layer superlattices by photomodulated reflectance

A series of Raman and photoreflectance measurements was performed on several InxGa1−xAs/GaAs strained-layer superlattices of the same period but of different alloy compositions and substrate orientations. In the photoreflectance spectra the photon energy region containing the E1 and E1+Δ1 transitions of the GaAs barrier material is analyzed. Both types of measurements are used in order to estimate of the in-plane strain in these layers. The values obtained by both methods are in good mutual agreement, thus showing that photoreflectance is an effective method for strain determination.

Optical transitions of a GaSb-AlSb superlattice in the 2–5 eV range

We report reflectance measurements in the range 2–5 eV of a GaSb-AlSb superlattice grown by molecular-beam epitaxy. In the region of the E1 and E1 + Δ1 transitions, the electronic properties can be related to the formation of electronic subbands of a type II superlattice at the L point.

Ellipsometric study of GaAs/GaP superlattices

Ellipsometric measurements were performed to study the E1 and E1+Δ1 transitions on a set of GaAs/GaP strained-layer superlattices. The experimental results show the existence of small quantum confinement effects.

Modulated reflectance study of strain in epitaxial GaAs on silicon

The electrolyte electroreflectance technique was used to study the nature of the strain in epitaxial layers of GaAs grown by laser-assisted metalorganic chemical vapor deposition on (100) Si substrate. The GaAs epilayer, about 140 nm thick, was chemically etched in steps of about 20 nm and spectra of the differential reflectivity (ΔR/R) versus energy were recorded in the 2.4–4.0 eV region immediately following each etch procedure. The E1 and E1+Δ1 transitions of GaAs and their broadening parameters were calculated using the low-field analysis of the electrolyte electroreflectance line shapes. The uniformity of carriers and the effects of the etching procedure were studied with three-dimensional amplitude maps. The variations of the spectral characteristics are interpreted in terms of strains and electronic changes as one approaches the GaAs-Si interface.