Martin Muñoz

Band offset determination of Zn0.53Cd0.47Se/Zn0.29Cd0.24Mg0.47Se

The interband transitions of a single quantum well structure of Zn0.53Cd0.47Se/Zn0.27Cd0.23Mg0.50Se (lattice matched to InP) were evaluated using contactless electroreflectance at room temperature. From a comparison of the measured optical transitions with those calculated using the envelope function approximation we determined that the conduction band offset for this system is given by the parameter Qc=ΔEc/ΔE0=0.82±0.02, which yields ΔEc of 590 meV. Such a large conduction band offset may be useful for the design of quantum cascade lasers and other devices based on intersubband transitions.

Optical characterization and evaluation of the conduction band offset for ZnCdSe∕ZnMgSe quantum wells grown on InP(001) by molecular-beam epitaxy

Lattice matched ZnMgSe grown on InP is of considerable interest for its potential applications as a cladding layer due to the high band-gap energy (∼3.6eV) and for use in intersubband devices such as quantum cascade lasers. Several lattice matched Zn0.5Cd0.5Se∕Zn0.13Mg0.87Se quantum wells (QWs) were grown on InP (001) substrates. Emission ranging from the near UV to the visible spectral range was achieved by varying the thickness of the wells. The QW fundamental transition as function of the QW thickness was experimentally studied and modeled using an envelope calculation. The contactless electroreflectance measurements of a Zn0.5Cd0.5Se∕Zn0.13Mg0.87Se single QW yielded multiple transitions from the QW, allowing us to estimate the conduction band offset of this heterostructure to be as high as 1.12eV.

Optical constants of Ga1−xInxAsySb1−y lattice matched to GaSb (001): Experiment and modeling

The optical constants e(E)[=e1(E)+ie2(E)] of two epitaxial layers of GaInAsSb/GaSb have been measured at 300 K using spectral ellipsometry in the range of 0.35–5.3 eV. The e(E) spectra displayed distinct structures associated with critical points (CPs) at E0 (direct gap), spin-orbit split E0+Δ0 component, spin-orbit split (E1,E1+Δ1) and (E0′,E0′+Δ0′) doublets, as well as E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Holden et al., Phys. Rev. B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb-enhancement effects at E0, E0+Δ0, and the E1, E1+Δ1 doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R1) of the two-dimensional exciton related to the E1, E1+Δ1 CPs was obtained. The value of R1 was in good agreement with effective mass/k⋅p theory. The ability to evaluate R1 has imp...

Midinfrared intersubband absorption in ZnxCd1−xSe∕Znx′Cdy′Mg1−x′−y′Se multiple quantum well structures

The authors report the observation of intersubband absorption in ZnxCd(1−x)Se∕Znx′Cdy′Mg(1−x′−y′)Se multiple quantum wells. Lattice-matched samples were grown by molecular beam epitaxy on InP (001) substrates. Photoluminescence measurements indicate that the samples have excellent material quality. The peak absorption wavelengths measured by Fourier transform infrared spectroscopy are 3.99 and 5.35μm for two samples with ZnxCd(1−x)Se well widths of 28 and 42A, respectively. These values fall within the 3–5μm wavelength range, which is of interest for midinfrared intersubband devices, such as quantum cascade lasers and quantum well infrared photodetectors. Their experimental results fit well with theoretical predictions based on the envelope function approximation. The results indicate that these wide band gap II-VI materials are very promising for midinfrared intersubband device applications.

CdSe self-assembled quantum dots with ZnCdMgSe barriers emitting throughout the visible spectrum

Self-assembled quantum dots of CdSe with ZnCdMgSe barriers have been grown by molecular beam epitaxy on InP substrates. The optical and microstructural properties were investigated using photoluminescence (PL) and atomic force microscopy (AFM) measurements. Control and reproducibility of the quantum dot (QD) size leading to light emission throughout the entire visible spectrum range has been obtained by varying the CdSe deposition time. Longer CdSe deposition times result in a redshift of the PL peaks as a consequence of an increase of QD size. AFM studies demonstrate the presence of QDs in uncapped structures. A comparison of this QD system with CdSe∕ZnSe shows that not only the strain but also the chemical properties of the system play an important role in QD formation.

Contactless electroreflectance of CdSe/ZnSe quantum dots grown by molecular-beam epitaxy

The interband transitions of a capped CdSe quantum-dot structure have been investigated using contactless electroreflectance. The electroreflectance spectrum shows transitions originating from all the portions of the sample including the quantum dots and the wetting layer. The transitions of the two-dimensional layers have been modeled using an envelope approximation calculation which takes into account the biaxial strain in the wetting layer. A good agreement was found between the experimental values for the transition energies and the calculated ones. From atomic force microscopy measurements, a lens shape was observed for the uncapped quantum dots. Taking into account the lens shape geometry and assuming that the effective height-to-radius ratio is preserved, the size of the capped quantum dots was determined using the observed electroreflectance transitions, in the framework of the effective mass approximation.

Molecular beam epitaxial growth and characterization of zinc-blende ZnMgSe on InP (001)

High crystalline quality zinc-blende structure Zn(1−x)MgxSe epitaxial layers were grown on InP (001) substrates by molecular beam epitaxy. Their band gap energies were determined as a function of Mg concentration and a linear dependence was observed. The band gap of the Zn(1−x)MgxSe closely lattice matched to InP was found to be 3.59 eV at 77 K and the extrapolated value for zinc-blende MgSe was determined to be 3.74 eV. Quantum wells of Zn(1−x)CdxSe with Zn(1−x)MgxSe as the barrier layer were grown which exhibit near ultraviolet emission.

Gallium desorption kinetics on "0001… GaN surface during the growth of GaN by molecular-beam epitaxy

Gallium (Ga) surface desorption behavior was investigated using reflection high-energy electron diffraction during the GaN growth. It was found that the desorption of Ga atoms from the (0001) GaN surfaces under different III-V ratio dependents on the coverage of adsorbed atoms. Doing so led to desorption energies of 2.76 eV for Ga droplets, 1.24–1.89 eV for Ga under Ga-rich growth conditions, and 0.82 eV – 0.94 eV for Ga under stoichiometric growth conditions. Moreover, the variation of the GaN surface morphology under different III-V ratios on porous templates supports the conclusion that Ga desorption energy depends on the coverage, and the III/V ratio dominates the growth mode.

Temperature Dependence of the Band-Edge Transitions of ZnCdBeSe

We have characterized the temperature dependence of band-edge transitions of three (Zn0.38Cd0.62)1-xBexSe II–VI films with different Be concentrations x by using contactless electroreflectance (CER) and piezoreflectance (PzR) in the temperature range of 15 to 450 K. By a careful comparison of the relative intensity of PzR and CER spectra, the identification of light-hole (lh) and heavy-hole (hh) character of the excitonic transitions of the samples has been accomplished. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the energy (including thermal expansion effects) and broadening parameter of the band edge transitions of ZnCdBeSe. The study shows that Be incorporation can effectively reduce the rate of temperature variation of the energy gap.

Optical constants of In0.53Ga0.47As/InP: Experiment and modeling

The optical constants e(E)=e1(E)+ie2(E) of unintentionally doped In0.53Ga0.47As lattice matched to InP have been measured at 300 K using spectral ellipsometry in the range of 0.4 to 5.1 eV. The e(E) spectra displayed distinct structures associated with critical points at E0 (direct gap), spin-orbit split E0+Δ0 component, spin-orbit split E1, E1+Δ1, E0′ feature, as well as E2. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Holden et al., Phys. Rev. B 56, 4037 (1997)], plus a Kramers–Kronig consistent correction, described in this work, that improves the fitting at low energies. This extended model is based on the electronic energy-band structure near these critical points plus excitonic and band-to-band Coulomb-enhancement effects at E0, E0+Δ0, and the E1, E1+Δ1, doublet. In addition to evaluating the energies of these various band-to-band critical points, information about the binding energy (R1) of the two-dimen...