Isaac Hernández-Calderón

Raman spectroscopy and photoluminescence of ZnTe thin films grown on GaAs

We report resonant Raman scattering and photoluminescence (PL) measurements on two ZnTe thin films grown by molecular-beam epitaxy on GaAs substrates with thicknesses around 0.5 and 1.0 μm. The data have been obtained by using the different excitation energies of an Ar+ laser to distinguish Raman from PL and analyze resonant effects. The characteristic features of the low-temperature PL spectra are the light and heavy free exciton emissions, split due to the thermal strain effect, followed by several phonon replicas of these lines. Moreover, longitudinal and transversal polariton splittings of heavy excitons are clearly observed. Their reduced masses have been obtained from the exciton binding energies. Room and low-temperature Raman spectra show, besides the typical longitudinal optical (LO) multiphonon emissions, forbidden zone-center transverse optical (TO)+(n−1)LO phonon combinations, which yield an accurate value for the LO and TO phonon energies. The breakdown of the selection rules is attributed to...

Interface and growth studies of α‐Sn/CdTe(110) superlattices

Angular‐resolved synchrotron radiation photoemission spectroscopy and reflection high‐energy electron diffraction (RHEED) are used to study the molecular‐beam epitaxy of CdTe/ α‐Sn(110) interfaces and superlattices. Core level photoemission spectra indicate that both sides of the interface are stable, nonreactive, and abrupt for growth temperatures up to 100 °C. At the α‐Sn/CdTe interface, the valence band maximum at Γ is at EV=1.1 eV below the Fermi level. This gives a valence band offset of ΔEV=1.1 eV, assuming zero band gap for the Sn. Stable superlattices of α‐Sn/CdTe(110) have been grown at 100 °C. The surface quality of the superstructure degrades after the growth of several α‐Sn/CdTe periods. After the growth of ten periods each 50‐A thick, the RHEED pattern shows mainly three‐dimensional bulk diffraction, indicating increased surface, and interface roughness.

Temperature dependence of Raman scattering and luminescence of the disordered Zn0.5Cd0.5Se alloy

Abstract We report on luminescence and Raman scattering measurements of zincblende Zn0.5Cd0.5Se thin film grown by molecular beam epitaxy. From the luminescence data of the exciton peak, the dependence of the energy gap with temperature [ d E g / d T=(4.35±0.01)×10 −4 meV / K ] and zero-temperature phonon renormalization energy ( Δ E(0)=30±1 meV ) have been obtained. The broadening of the excitonic emission as the temperature increases is mainly due to scattering processes with longitudinal optical phonons and residual ionized impurities. Raman scattering shows a multiphonon structure, which depends on the temperature. At low temperatures, up to the fifth-order phonon peaks appear due to resonant effects. The increase in the Raman intensity as the temperature decreases is discussed in terms of a model which gives a very good quantitative agreement of the relative intensity between successive phonon peaks.

Strained‐layer epitaxy of SnTe on CdTe(110)

Bulk SnTe is a IV–VI compound which crystallizes in the NaCl structure. However, deposition of SnTe on CdTe(110) can force very thin films of SnTe to grow in the zinc‐blende structure, leaving an ‘‘extra’’ electron per atom not involved in tetrahedral bonds. We present an angular‐resolved photoemission study of this exotic heterostructure, and show that indeed the first ∼4 monolayers of SnTe do grow in the zinc‐blende structure. For films of more than ∼10 layers in thickness, the film relaxes into the more favorable NaCl structure. The valence‐and conduction‐band offset between NaCl structured SnTe and CdTe(110) are ΔEν=1.01±0.05 eV, and ΔEc=0.37±0.05 eV, respectively. The Fermi level lies at the midpoint of the SnTe band gap.

Thermal quenching of the self-activated band of ZnSe:Cl thin films grown by molecular beam epitaxy

Abstract We studied the thermal quenching of the self-activated (SA) band of molecular beam epitaxy (MBE) grown ZnSe:Cl thin films by means of temperature dependent photoluminescence (PL) experiments. We analyzed the spectra of the self-activated (SA) band as a function of temperature and Cl concentration. All studied samples presented the emission of this band, however, the excitonic emission was observed only for those samples with lower Cl concentration. A different activation energy ( E a ) associated to quenching of the SA band was obtained for each sample. These values suggest that different electron and hole levels, which depend on Cl concentration, are associated to the mechanism of quenching of the SA band.

Symmetry properties and electronic band structure of ordered Zn0.5Cd0.5Se alloys

Abstract Along the [001] direction of a zincblende semiconductor we find a sequence of cation–anion planes. In the case of a random zincblende ternary alloy of the type A 1− x B x C the A and B cations are randomly distributed in each cation plane. In the particular case, when x =0.5, another possible arrangement is the sequence A–C–B–C–A–C…; we can describe this crystal as an (AC) 1 (BC) 1 superlattice, however, it is not a true superlattice, it is one of the possible ordered structures that we can obtain with A 0.5 B 0.5 C composition. This ternary alloy has the simple tetragonal structure, in contrast with the fcc crystalline structure of the random alloy with the same composition. Here, we present the results for the electronic band structure of the ordered Zn 0.5 Cd 0.5 Se alloy qualitatively determined by means of a comparative analysis of the symmetry properties of both crystals. The volume reduction of the first Brillouin zone (1BZ) of the tetragonal alloy, in relation to the cubic alloy, gives place to band folding effects. They are advantageously employed to describe the band structure of the ordered alloys in terms of the random one. Band gap reduction, removal of degeneracy at some symmetry points of the 1BZ and the doubling of the number of bands is concluded for the ordered alloy compared to the ordered one. A quite good agreement is obtained when comparing the ordered band structure obtained by symmetry arguments with an empirical tight binding calculation.

Nearly lattice-matched Zn1-zCdzSe/Zn1-xCdxSe/Zn1-yMgySe (z > x) quantum wells for yellow emission

The authors present the results of the modeling and epitaxial growth of a nearly lattice matched Zn1-zCdzSe/Zn1-xCdxSe/Zn1-yMgySe quantum well (QW) heterostructure with yellow emission. The ZnCdSe QW is composed of regions with two different Cd content: in the center, seven monolayers of Zn1-zCdzSe with z Cd content are surrounded on each side by eight Zn1-xCdxSe monolayers with x Cd content (z > x). These last regions are lattice matched to the Zn1-yMgySe barrier. The quantum well design and modeling was based on calculations employing the transfer matrix method. The ZnCdSe quantum well layers were grown in a layer-by-layer mode by submonolayer pulsed beam epitaxy within ZnMgSe barriers grown by molecular beam epitaxy. The low temperature photoluminescence spectrum presented yellow excitonic emission at 2.176 eV, which is in very good agreement with the model calculations. At room temperature, the emission shifted to 2.112 eV, a deep yellow color.

Temperature dependence of exciton localization in Zn1−xCdxSe quantum wells

We report the results of the investigation of the temperature dependence of the spontaneous emission of localized excitons in Zn1−xCdxSe quantum wells (QWs). Two main peaks, which show a strong change in relative intensities with temperature, dominate the spectra. The presence in the spectra of biexcitons and bound excitons was ruled out after the corresponding analyses. Calculation of the fundamental transitions of the QWs suggested that the peaks are due to thickness fluctuations of one and two monomolecular layers. The successful analysis of the spectra in terms of a simple two-level model indicated that localization of excitons due to QW thickness fluctuations and exciton migration are basic processes which have noticeable influence in the spontaneous emission of Zn1−xCdxSe QWs.

Chapter 14 – Epitaxial growth of thin films and quantum structures of II–VI visible-bandgap semiconductors

II–VI semiconductors have demonstrated convenient properties for application in photodetectors, solar cells, light-emitting diodes, laser diodes, solar blind detectors, radiation detectors, magneto-optical devices, biophotonics, etc. Molecular beam epitaxy (MBE) has proven to be a powerful technique for the elaboration of high-quality films and low-dimensional structures. This chapter presents a general overview and recent advances of the MBE growth of visible bandgap II–VI semiconductors for application in light-emitting devices and other novel quantum structures. A short description of the fundamentals of molecular beam epitaxy and derived techniques related to II–VI semiconductors is presented. A brief presentation of the layer-by-layer growth techniques and their applications to the growth of thin and ultra-thin quantum wells is included. This chapter contains abundant information about homo- and heteroepitaxial growth of II–VI semiconductors; particular emphasis is placed on the advantages of quaternary alloys for bandgap tuning and growth of lattice-matched systems. This chapter also contains an outline of recent results of II–VI quantum structures produced by MBE.

Local Atomic Structure Of Cdse Ultra‐Thin Quantum Wells Examined By X‐Ray Absorption Fine Structure Experiments

We have employed the X‐ray absorption fine structure (XAFS) technique for the investigation of the local atomic structure of CdSe/ZnSe ultra‐thin quantum wells (UTQWs) with 1 and 3 ML thickness. The CdSe/ZnSe system presents a large lattice mismatch of ∼7.3 % and the pseudomorphic UTQWs are under compressive biaxial strain. The analysis of the XAFS data indicate differences in the Cd–Se bond length of the 1 and 3 ML UTQWs, as a consequence, differences in the magnitude of the strain, in significant disagreement with the usual hypothesis of thickness independent strain for pseudomorphic thin films. Furthermore, the analysis of the XAFS experiments suggests the possibility of inhomogeneous strain in the 3ML UTQW. We conclude that even for ultra‐thin pseudomorphic films the strain can be inhomogeneous and depend on film thickness if this is close to the critical thickness hc.