ZnTe Buffer Layer Research Articles

IR spectroscopy of lattice vibrations and comparative analysis of the ZnTe/CdTe quantum-dot superlattices on the GaAs substrate and with the ZnTe and CdTe buffer layers

A comparative analysis of multiperiod ZnTe/CdTe superlattices with the CdTe quantum dots grown by molecular beam epitaxy on the GaAs substrate with the ZnTe and CdTe buffer layers is carried out. The elastic-stress-induced shifts of eigenfrequencies of the modes of the CdTe- and ZnTe-like vibrations of materials forming similar superlattices but grown on different buffer ZnTe and CdTe layers are compared. The conditions of formation of quantum dots in the ZnTe/CdTe superlattices on the ZnTe and CdTe buffer layers differ radically.

Structural Characterization of Integrated II–VI and III–V Heterostructures for Solar Cell Applications

We report on the microstructural characterization of monolithically integrated II-VI and III-V heterostructures that are being developed for possible solar cell applications. Observations by transmission electron microscopy have shown that the growth of lattice-matched ZnTe layers on GaSb(100) substrates resulted in very high quality interfaces and very few structural defects. We then investigated short-period superlattices (SSLs) of CdSe and CdTe digital alloys grown using ZnTe buffer layers on GaSb substrates. The effect of rapid thermal annealing was also studied. High-resolution electron micrographs showed that the CdSe-CdTe SSL had very high quality for approximately 25-50 periods closest to the substrate but that considerable stacking faults and microtwins were visible in layers near the top surface of the sample. For comparison purposes, CdSe layers grown on InAs(100) substrates, again using ZnTe as a buffer layer, were also characterized. The quality of the CdSe layer was not as good as that observed in the SSL sample, possibly because of the larger lattice mismatch between the materials.

Infrared spectroscopy of lattice vibrations in ZnTe/CdTe superlattices with quantum dots on the GaAs substrate with the ZnTe buffer layer

The results of the analysis of the infrared lattice reflectance spectra of multiperiod ZnTe/CdTe superlattices with CdTe quantum dots are reported. The samples are grown by molecular beam epitaxy on the GaAs substrate with the ZnTe buffer layer. Due to the large number of periods of the superlattices, it is possible to observe CdTe-like vibration modes in the quantum dots, i.e., the dislocation-free stressed islands formed during the growth due to relaxation of elastic stresses between the ZnTe and CdTe layers are markedly different in their lattice parameters. From the frequency shifts of the CdTe- and ZnTe-like vibration modes with respect to the corresponding modes in the unstressed materials, it is possible to estimate the level of elastic stresses.

Strain Distributions of Self-Assembled CdxZn1-xTe Quantum Wires Grown on ZnTe Buffer Layers

Strain Distributions of Self-Assembled CdxZn1-xTe Quantum Wires Grown on ZnTe Buffer Layers

Strain distributions and electronic subband energies of self-assembled CdTe quantum wires grown on ZnTe buffer layers

The structural properties and the shape of self-assembled CdTe/ZnTe quantum wires (QWRs) grown by using molecular beam epitaxy and atomic layer epitaxy were determined by using atomic force microscopy (AFM) measurements, and the interband transitions in the CdTe/ZnTe QWRs were investigated by using temperature-dependent photoluminescence (PL) measurements. The shape of the CdTe/ZnTe QWRs on the basis of the AFM image was modeled to be a half-ellipsoidal cylinder approximately. The temperature-dependent PL spectra showed that the PL peaks corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E1-HH1) shifted to lower energy with increasing temperature. Strain distributions and electronic subband energies at several temperatures were numerically calculated by using a finite-difference method (FDM) with and without taking into account shape-based strain and nonparabolicity effects. The excitonic peak corresponding to (E1-HH1) interband transitions, as determined from the PL spectra, was in reasonable agreement with that corresponding to the (E1-HH1) transitions obtained, as determined from the FDM calculations taking into account shape-based strain and nonparabolicity effects. The present results help improve understanding of the electronic structures of CdTe/ZnTe QWRs.

WITHDRAWN: Effect of strain and nonparabolicity on electronic properties of self-assembled CdTe quantum wires grown on ZnTe buffer layer

This article has been withdrawn consistent with Elsevier Policy on Article Withdrawal ( http://www.elsevier.com/locate/withdrawalpolicy ). The Publisher apologizes for any inconvenience this may cause.

Molecular beam epitaxy of GaSb layers on GaAs (0 0 1) substrates by using three-step ZnTe buffer layers

We report on the MBE growth of GaSb layer by using three-step ZnTe buffer layer: a thin ZnTe buffer layer was grown at 250 °C (LT-buffer), then annealed at 330 °C (HT-annealing) and a thick buffer layer was grown at 310 °C (HT-buffer). The effect of a three-step buffer on the surface and structural quality of the GaSb layer has been investigated. Two-dimensional reciprocal space mapping (RSM) results of the GaSb layers clearly indicate that the structural deformation of the GaSb layer is greatly reduced by introducing the ZnTe buffer, which remarkably improves the crystallinity of the GaSb layer.

Electronic Properties of Self-Assembled CdTe Quantum Wires Grown on ZnTe Buffer Layers

Electronic Properties of Self-Assembled CdTe Quantum Wires Grown on ZnTe Buffer Layers

Effect of ZnTe buffer layer on the structural and optical properties of CdTe/ZnTe quantum dots

We have studied growth of self-assembled CdTe quantum dots (QDs) grown on various thicknesses of ZnTe buffer layer by using molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE). The excitonic peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E 1−HH 1) in the CdTe/ZnTe QDs shifted to a lower energy with increasing thickness of the ZnTe buffer layer. The results of the atomic force microscopy (AFM) images showed that the size of the CdTe/ZnTe QDs increases with increasing thickness of the ZnTe buffer layer. The activation energy of the electrons confined in the CdTe/ZnTe QDs, as obtained from the temperature-dependent PL spectra, increases with increasing thickness of the ZnTe buffer layer. These results can help improve our understanding of the effect of the ZnTe buffer layer on the structural and the optical properties of CdTe/ZnTe QDs.

Structural and optical properties of Cd xZn 1−xTe/ZnTe quantum dots grown on Si(1 0 0) substrates by using molecular beam epitaxy

We have studied structural and optical properties of self-assembled Cd 0.6Zn 0.4Te/ZnTe quantum dots (QDs) grown on Si(1 0 0) substrates by using molecular beam epitaxy. X-ray diffraction patterns indicated that the ZnTe buffer layers grown on the Si substrates were hetero-epitaxial films with the (1 0 0) orientation. The atomic force microscopy images showed that Cd 0.6Zn 0.4Te/ZnTe QDs were formed on Si(1 0 0) substrates. The photoluminescence spectra at 32 K showed the dominant excitonic peaks corresponding to the interband from the ground-state electronic sub-band to the ground-state heavy-hole band in the Cd 0.6Zn 0.4Te/ZnTe QDs. The present results can help to improve the understanding of the structural and interband transition properties in Cd 0.6Zn 0.4Te/ZnTe QDs grown on Si(1 0 0) substrates.

Formation mode of self-assembled CdTe quantum dots directly grown on GaAs substrates

Atomic force microscopy (AFM) measurements were carried out to investigate the formation of CdTe quantum dots (QDs) grown on GaAs substrates by using molecular beam epitaxy. The AFM images showed that uniform CdTe QDs were formed on GaAs substrates due to the large lattice mismatch between the CdTe QDs and the GaAs substrate. The size and the density of the CdTe QDs were varied by changing the thickness and the substrate temperature. The formation behavior for the CdTe QDs on the GaAs substrate is in reasonable agreement with a Volmer–Weber (V–W) growth mode. The size and the density of the CdTe QDs grown on the GaAs substrate by using the V–W growth mode are compared with those of the CdTe QDs grown on the ZnTe buffer layer by using the Stranski–Krastanow mode. The present results can help improve understanding of the formation and the structural properties of CdTe QDs directly grown on GaAs substrates.

Optical spectroscopy of CdTe/ZnTe quantum dot multilayers

AbstractElectronic and vibrational states in CdTe/ZnTe quantum dot multilayers grown on a thick ZnTe buffer layer are studied using optical spectroscopy methods (photoluminescence in a wide temperature range, Raman scattering, and IR reflection). It has been established that a new band appears in the Raman spectra of sample with a small ZnTe spacer layer thickness, which can be ascribed to a vibrational mode of two closely‐spaced quantum dots. A comparison of results obtained under studies of structures grown on ZnTe and CdTe buffer layers shows that change of strain distribution pattern substantially modifies electron and optical spectra of QD structures. In particular, it can result in change of the band diagram type. A reason of anomalous peak position shift of quantum dot emission band is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and activation energy of CdxZn1−xTe nanostructures with different dimensions grown on ZnTe buffer layers

Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to investigate the formation process and the activation energy of different-dimensional CdxZn1−xTe∕ZnTe nanostructures. The results of the AFM images show that CdxZn1−xTe quantum dots (QDs) are formed and that the dimensional transformation from CdxZn1−xTe QDs to CdxZn1−xTe quantum wires is caused by coalescence. The excitonic peak corresponding to the transition from the ground electronic subband to the ground heavy-hole transitions in CdxZn1−xTe∕ZnTe nanostructures shifts to lower energy with increasing thickness of the CdxZn1−xTe layer due to variations in the thickness and the dimension of the layer. The activation energy of the electrons confined in the CdxZn1−xTe∕ZnTe nanostructures, as obtained from the temperature-dependent PL spectra, was significantly affected by the thickness and the dimension of the CdxZn1−xTe layer.

Growth of CrTe thin films by molecular-beam epitaxy

We report the growth of Cr 1− δ Te films on (100) GaAs substrates using ZnTe buffer layers by solid-source molecular-beam epitaxial technique. RHEED patterns indicate a clear structural change during the initial stages of deposition. Temperature-dependent magnetization results reveal that different NiAs-related phases of Cr 1− δ Te can be obtained at different substrate temperatures. By varying the film thickness, a metastable zinc blende structure of CrTe could be obtained at lower substrate temperature.

Homoepitaxy of ZnTe on (100) oriented substrates: Technology issues and MOVPE growth aspects

AbstractThe metalorganic vapour phase epitaxy of ZnTe on single crystal (100)ZnTe:P wafers is reported. The technological steps to prepare a substrate surface suitable for the high quality homoepitaxy of ZnTe are identified and optimised in terms of structural and morphological properties of overgrown epilayers. Removal of ∼7 µm of material from the ZnTe:P wafers by chemical etching in 1% Br2‐methanol solution proved necessary to achieve a sufficiently smooth and homogeneous surface; in‐situ H2 heat treatment of the wafers at 350°C immediately before growth ensures optimal desorption of residual oxides, allowing epilayer crystalline quality comparable to the substrate. However, the structure of epilayers degrades for growth temperatures (TG) above 350 °C due to the occurrence of stacking faults (SFs) within ∼200‐300 nm from the epilayer‐substrate interface. Accordingly, the epilayer band‐edge luminescence vanishes below 350 nm, indicating a worsening of the material radiative efficiency in very thin epilayers. The epilayer surface morphology is the result of a complex interplay between SF nucleation and Te:Zn ad‐atom stoichiometry during growth. Almost featureless morphologies are obtained for growth at 350 °C, i.e. under nearly stoichiometric surface conditions. Pyramid‐like hillocks develop instead for TG ≥ 360 °C, corresponding to Te‐rich surface conditions, their density rapidly increasing up to around 9 × 106 cm–2 at TG = 400 °C. Hillocks occur in close pairs on the epilayer surface, their nucleation being strongly reduced if a thin ZnTe buffer layer is grown at low (325 °C) temperature, i.e. if SFs do not occur at the epilayer‐substrate interface. This demonstrates that hillocks form as a result of three‐dimensional growth around partial dislocations pairs bounding SFs, the phenomenon being driven by Te ad‐atoms experiencing a Schwoebel potential barrier at the surface step edges around the dislocations. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures

CdTe nanostructures were grown on CdxZn1−xTe buffer layers by using molecular-beam epitaxy and atomic-layer epitaxy. The atomic force microscopy image showed that uniform CdTe quantum dots were formed on ZnTe buffer layer. Photoluminescence measurements showed that the excitonic peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band in the CdTe∕CdxZn1−xTe nanostructure shifted to a higher energy with increasing Cd mole fraction. The activation energy of the electrons confined in the CdTe∕ZnTe quantum dots was higher than those of electrons in CdTe∕CdxZn1−xTe nanostructures. These results can help improve understanding of the dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures.

Microstructural and interband transition properties in CdTe quantum dots grown on ZnTe buffer layers by using atomic layer epitaxy

The microstructural and the optical properties of CdTe/ZnTe quantum dots (QDs) grown by atomic layer epitaxy (ALE) were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The AFM image showed that uniform CdTe QDs are formed, and the TEM image showed that the CdTe QDs were embedded into the ZnTe buffer layers. The temperature-dependent PL spectra showed that the peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band of the CdTe/ZnTe QDs shifted to lower energy with increasing temperature. The full widths at half maxima of the PL peaks for CdTe/ZnTe QDs grown by ALE remained almost constant regardless of the temperature variation. These present observations can help to improve understanding for the microstructural and the optical the properties of the CdTe/ZnTe QDs grown by using the ALE method.

A microstructural study of the CdTe/ZnTe film morphology as related to the Si substrate orientation

A series of CdTe/ZnTe films was grown by molecular beam epitaxy (MBE) onto Si substrates. We examine the relationships between the film morphology, the ZnTe growth method, and the Si growth orientation. The substrates' degree of miscut from the [1 1 1] direction ranged from 0° to 29.5°. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies show that the substrate orientation is maintained by the ZnTe/CdTe epilayers only for low miscut angles. For higher miscut angles, the film orientation does not match that of the substrate, and in some cases is dependent on the growth methods used to deposit the ZnTe buffer layer.

Asymmetric AlAsSb/InAs/CdMgSe quantum wells grown by molecular-beam epitaxy

Asymmetric (6.7–300)-nm-thick InAs quantum wells (QWs) confined between AlAsSb and CdMgSe barriers have been fabricated by molecular-beam epitaxy. A special procedure of the CdMgSe-on-InAs growth initiation, exploiting an ex situ S passivation of InAs and in situ deposition of an ultrathin ZnTe buffer layer, results in the fabrication of high quality structures with a density of extended defects below 106 cm2. QW photoluminescence studies demonstrate a confinement effect and confirm the type I band alignment at the heterovalent InAs/CdMgSe interface mediated by the ZnTe interlayer. Observation of Shubnikov de Haas oscillations of magnetoresistance for an asymmetric 19-nm-thick InAs QW indicates an existence of the two-dimensional electron gas with the low-temperature sheet electron density of 1.3×1012 cm−2 and the mobility as high as ∼10 000 cm2/V s.

Existence of the CuPt-type ordering due to the surface undulation in CdxZn1−xTe epilayers grown on ZnTe buffer layers

The relationship between the CuPt-type ordering and the surface undulation in CdxZn1−xTe epitaxial layers grown on ZnTe buffer layers was investigated. The results of selected area electron diffraction pattern and transmission electron microscopy measurements showed that CuPt-type ordered structures were formed in the CdxZn1−xTe epitaxial layers. The atomic force microscopy image showed that the surface undulations were created from lattice mismatch between the CdxZn1−xTe ZnTe thin films and the GaAs substrate. The surface undulations provided the [110] steps, which enhanced the formation of CuPt-type ordering in highly strained CdxZn1−xTe epilayers. These results provide important information on the relationship between the microstructural and surface properties in lattice mismatched heteroepilayers in the CdxZn1−xTe/ZnTe system.