ZnSe Substrates Research Articles

Characterization of self-assembled quantum dots using the phonon-induced features of PL spectra

We consider the photoluminescence spectra for self-assembled CdSe quantum dots on the ZnSe substrate. The spectra of optical phonons are derived for these structures. The PL features due to the LO-phonon-assisted transitions from the remnant-layer exciton state to the quantum-dot exciton state are analyzed. They compare well with the recently observed fine structure of PL in self-assembled CdSe quantum dots formed by thermally activated surface reorganization.

Surface Effects on the Dynamics of Liquid Crystalline Thin Films Confined in Nanoscale Cavities

The dynamics of 4-n-pentyl-4‘-cyanobiphenyl (5CB) nematic liquid crystalline thin films have been studied in real time using step-scan Fourier transform infrared spectroscopy (FTIR). In these studies, the liquid crystal was confined in a nanocavity defined and bounded by an interdigitated gold electrode array. The gold microstructures were microfabricated on a zinc selenide (IR-transparent) window. The 5CB interactions with the ZnSe substrate result in surface-induced ordering of the ultrathin layers (on the order of 40 nm). As the films increase in thickness, the nanoscale organization induced by the surface layer becomes a less significant contributor to the overall bulk structure of the sample. Time-resolved FTIR studies have enabled the measurement of rate constants for the orientation and relaxation of the thin films under an applied electric field as a direct function of confinement dimensions. Cell thicknesses ranging from 40 to 300 nm were studied. The measured rate behaviors demonstrate the stron...

Fabrication of an interdigitated array electrode on ZnSe and its application to electrooptical measurements using FT-IR spectroscopy

An interdigitated array electrode (IDA) is fabricated on an IR-transparent substrate for use in electrooptical measurements using Fourier transform infrared (FT-IR) spectroscopy. The fabrication of the IDA and its unique sampling geometry for transmission spectroscopy is detailed. The gold IDA was patterned on the ZnSe substrate using a photolithographically defined liftoff process. An IR flow cell was modified to enable the substrate containing the IDA to be used for electrooptical measurements in transmission. The utility of the electrooptical cell is demonstrated by application to two model systems. In the first, changes in the conductivity and spectral features of a receptor membrane (Nafion) upon dehydration are measured. In the second, the measurement of the electric-field-induced orientation of a liquid-crystalline film of 4-n-pentyl-4'-cyanobiphenyl was conducted in a new experimental geometry, with IR transmission normal to the direction of the applied electric field. Difference spectroscopy facilitated the observation of the change in orientation of the liquid crystal with applied potential. These systems demonstrate the general utility of the fabrication scheme described here. It also illustrates its facile adaptation to infrared difference spectroscopy as a means of studying complex phenomena in electrochemical and electrooptical systems.

Studies of heteroepitaxial growth of thin II–VI semiconductor layers by sequential ultrahigh vacuum dosing

An in situ molecular-level study of material growth using a binary reaction sequence of hydride and metalorganic precursors is presented. The study used a model material system of CdS/ZnSe(100) and focused on the material chemistry of heteroepitaxy growth. In the growth process, dimethylcadmium and H2S precursors were sequentially dosed onto a c(2×2) ZnSe(100) substrate under high-vacuum conditions. At temperatures of ∼300 K, saturated chemisorption of a Cd and a S monolayer occurred during each cycle of the binary reaction sequence. Characterization of the growth surface was accomplished in the growth chamber using Auger electron spectroscopy, x-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy for probing surface chemical composition and low-energy electron diffraction for determining surface order. These measurements showed layer-by-layer growth at a substrate temperature of ∼300 K, yielding an ordered stoichiometric CdS film. Strong variations in the composition of the grown surface layer were observed at different substrate temperatures; these variations were found to be related to the temperature dependence of the precursor reactions with the growth surfaces.

Resonant Raman scattering in self-assembled quantum dots

A theoretical treatment for first-order resonant Raman scattering in self-assembled quantum dots (SAQD's) of different materials is presented. The dots are modeled as cylindrical disks with elliptical cross section, to simulate shape and confinement anisotropies obtained from the SAQD growth conditions. Coulomb interaction between electron and hole is considered in an envelope function Hamiltonian approach and the eigenvalues and eigenfunctions are obtained by a matrix diagonalization technique. By including excitonic intermediate states in the Raman process, the scattering efficiency and cross section are calculated for long-range Fr\ohlich exciton-phonon interaction. The Fr\ohlich interaction in the SAQD is considered in an approach in which both the mechanical and electrostatic matching boundary conditions are fulfilled at the SAQD interfaces. Exciton and confined phonon selection rules are derived for Raman processes. Characteristic results for SAQD's are presented, including InAs dots in GaAs, as well as CdSe dots in ZnSe substrates. We analyze how Raman spectroscopy would give information on carrier masses, confinement anisotropy effects, and SAQD geometry.

Autocorrelation measurement of femtosecond laser pulses by use of a ZnSe two-photon detector array

We demonstrate autocorrelation measurements of 85-fs Ti:sapphire laser pulses, using a 32-pixel ZnSe detector array in a single-shot geometry. The two-photon photoconductor is fabricated by deposition of an array of interdigitated gold fingers on a single-crystal ZnSe substrate.

Phase content and photoluminescence of ZnO layers obtained on ZnSe substrates by radical-beam gettering epitaxy

The influence of parameters of Radical-Beam Gettering Epitaxy (substrate temperature, annealing time, atomic oxygen concentration) on the structure of obtained ZnO layers was studied. The conditions of obtaining of single crystal and textured ZnO layers were determined. With the help of Auger-spectroscopy, X-rays photoelectronic spectroscopy the process of oxidation of ZnSe surface in the vast interval of temperatures was studied. It was found that SeO 2, ZnSeO 3, and ZnO phases were produced on the surface of ZnSe in the interval of temperatures 250÷350°C. At growth temperatures equal or higher 400°C there exist only ZnO phase with approximately equal content of Zn and O on the surface of ZnSe. The state of the boundary ZnOZnSe was studied. The conditions of obtaining of ZnOZnSe structures with abrupt interfaces were determined.

New Heterostructures n-PbS/n-ZnSe: Long-Term Stability of Electrical Characteristics

The long-term stability of electrical parameters had been investigated for heterostructures based on lead sulphide films and zinc selenide during 3.5 years samples storage at normal ambient conditions. In order to clarify the time stability of electrical properties characterized the examined heterostructures the investigations of current-voltage and capacitance-voltage dependencies were performed. It was found that the heterostructures characterized by the carrier concentration of ZnSe substrate in range of (1.5-3.5)x1017 cm-3 manifested more stable current-voltage behavior than the heterostructures with the lower carrier concentration of the ZnSe substrate.

Homoepitaxial laser diodes grown on conducting and insulating ZnSe substrates

The different steps of ZnSe substrate preparation are described. In situ hydrogen plasma cleaning to get rid of contaminations like thermally stable oxides on ZnSe substrate surfaces before growth is the crucial point to fabricate reproducibly pseudomorphic (Mg,Zn) (S,Se) layers. It is found that the crystalline quality of the layers depends strongly on that of the ZnSe substrates itself as well as on the initial growth start conditions. Finally, ZnSe-based laser diodes were grown on insulating and different kinds of conducting ZnSe substrates. They operated at room temperature under pulsed conditions.

Annealing Effect of ZnSe:N/ZnSe Grown by Metalorganic Chemical Vapor Deposition

High-quality nitrogen-doped ZnSe (ZnSe:N) epitaxial layers were grown on ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system under optimum growth conditions using hydrogen as a carrier gas and ammonia as a dopant source. In order to enhance the activation efficiency of nitrogen in ZnSe epitaxial layers, ZnSe:N/ZnSe was annealed in melted zinc using the rapid thermal annealing technique. The dependence of net acceptor concentration on annealing duration, VI/II flux ratio, and ammonia flux was examined. The highest net acceptor concentration value calculated from capacitance–voltage (C–V) measurements reached 2×1017 cm-3. This is the highest value obtained so far for ZnSe homosystem epitaxial layers grown by the MOCVD method.

Homoepitaxy of ZnSe on the citric acid etched (1 0 0)ZnSe surface

ZnSe substrate surfaces were treated by the citric acid-based etchant. Homoepitaxial layers of ZnSe grown by MBE using this etching treatment showed excellent film qualities based on X-ray rocking curve and transmission electron microscopy observations. The electrical properties of the n-ZnSe/n-ZnSe interface prepared using this etchant was studied, and the C– V measurement showed that a good electrical interface was formed.

Growth and annealing effect of high-quality ZnSe:N/ZnSe by MOCVD

The ZnSe : N epitaxial layers were grown on (1 1 0) ZnSe substrates in a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas, and using ammonia as a dopant source. In order to obtain highly doped ZnSe : N epitaxial layers, the optimum growth and doping conditions were determined by studying the photoluminescence (PL) spectra from the ZnSe epitaxial layers grown at different ammonia flux and VI/II flux ratio. Furthermore, in order to enhance the concentration of active nitrogen in ZnSe epitaxial layer, a rapid thermal anneal technique was used for post-heat-treating. The results show that the annealing temperature of over 1023 K is necessary. Beside, a novel treatment method to obtain a smooth substrate surface for growing high quality ZnSe epitaxial layers is also described.

Device Properties of Homo- and Heteroepitaxial ZnSe-Based Laser Diodes

The characteristics of ZnSe-based laser diodes grown on GaAs and ZnSe substrates are discussed. There is no significant difference observed in the dynamic behavior and in the operating voltages between the two cases. The degradation mechanism is similar with the developing of dark line defects and a 1/t-like decrease in light intensity at constant current for t →∞. The width of the dark line defects is in homoepitaxy almost constant in time, although their number is higher. This difference is also reflected in the lifetimes of our devices during lasing, which is in heteroepitaxy three minutes and about one second in homoepitaxy, for both in cw operation at room temperature.

An investigation of the surface reaction mechanisms of alternating-grown, ordered atomic layers: CdS on ZnSe(100)

Temperature-programmed desorption (TPD) is used for detailed investigation of the surface chemistry of a binary reaction sequence by sequential gas-phase dosing of a ZnSe(100) substrate with (CH3)2Cd and H2S. Analysis of the TPD spectra shows that adsorbed DMCd irreversibly dissociates on a ZnSe(100)-c(2×2) surface at room temperature to form a fully methyl-terminated surface; this termination is responsible for the previously reported, self-limiting reaction. At ∼370K, DMZn desorbs from this surface due to a methyl-exchange reaction. This desorption temperature is independent of coverage, indicating a first-order reaction. In addition, at high DMCd exposures, the adsorption/desorption process leads to replacement of surface Zn by Cd. The experiments also examine the reaction of H2S with the methyl-terminated surface. This reaction, which is also self-limiting, forms a sulfur-hydride-terminated surface after the release of surface CH3 groups in the form of CH4.Studies of surfaces formed by more than one binary reaction sequence are also reported, which show that the alternating growth surfaces are terminated with either methyl groups or hydrogen. The methyl-passivated growth surface preferentially desorbs methyl radicals at ∼390K instead of the metal-alkyl species. For the sulfur-hydride-terminated surface the recombinative reaction of the HS species causes desorption of H2S at 480K. In this case, the symmetric peak shape of the desorbed H2S signal and its shift to lower temperature with increasing the coverage suggest a second-order reaction mechanism. In more general terms, these results indicate that the relative strengths of bond for methyl–metal (II) and metal–VI element play an important role in the surface reactions.

The effect of surface roughness on infrared external reflection spectroscopy

The effect of surface roughness of the back-side of a film-supporting material on Fourier transform infrared (FTIR) external reflection (ER) spectra was studied by using 9-monolayer cadmium stearate Langmuir–Blodgett (LB) films. The LB films are prepared on two infrared-transparent ZnSe substrates whose top surfaces are optically polished and bottom surfaces have controlled surface roughness with 1.2 and 0.1 μm of protrusions. Although the roughness of 0.1 μm is smaller than the wave-length of the infrared ray, both LB films show typical ER spectra qualitatively. On closer inspection, however, the LB film on the substrate with 0.1 μm protrusions is irregular and the LB film on 1.2 μm protrusions is better. These results indicate that the surface roughness of the backside of substrate is necessary for ER analyses.

Substrate/layer relationships in II–VIs

This review explores the scope of substrate/layer relationships in II–VI epitaxy. In addition to the lattice parameter mismatch, other important factors such as valence mismatch, polarity and thermal expansion match are considered. In some cases the lattice parameter mismatch is not the dominant factor in determining material quality such as ZnSe on GaAs. The ideal for high quality epilayers is to use lattice matched II–VI substrates but these have not always been available and alternative substrates have in some cases looked more attractive. Recent progress with CdTe and CdZnTe substrates is changing the prospect for II–VI substrates. However, issues such as surface preparation and cleaning are highlighted with newly available ZnSe substrates. Finally, the potential for II–VI growth onto microcrystalline and amorphous substrates is considered and possible approaches for promoting grain growth are discussed.

Investigations of ZnSe based laser structures on ZnSe substrates by highresolution x-ray diffraction

The structural quality of homoepitaxially grown ZnSe based layers depends sensitively on the ZnSe substrate preparation prior to the growth. In this paper the consequences of an in situ hydrogen plasma cleaning of ZnSe substrates to the crystalline quality of II-VI based laser structures are discussed. The crystalline properties of the resulting laser structures are compared with those of homoepitaxially grown structures on non-hydrogen cleaned substrates as well as with the characteristics of heteroepitaxially grown lasers on GaAs. The samples were grown by molecular beam epitaxy and characterized by high-resolution x-ray diffraction. Remarkable differences in the strain state of the homoepitaxially grown samples are observed. While the structures grown on hydrogen cleaned ZnSe substrates are pseudomorphic, layers of comparable thickness and lattice mismatch on non-hydrogen cleaned substrates show clear strain relaxation. Triple axis (004) rocking curves reveal distinct differences between the three sample types investigated. According to these measurements, the in situ hydrogen plasma cleaning of ZnSe substrates results in a drastic improvement of crystalline perfection of the homoepitaxial laser structures. However, even the defect density of the optimized homoepitaxial samples exceeds that of heteroepitaxial reference lasers by two orders of magnitude as estimated from the diffuse scattered intensity.

The band lineups at highly strained ZnS/CdS and ZnSe/ZnTe interfaces: effects of the quadratic deformation potentials and the relaxation of the semicore d-electrons

The strain dependence of the band lineups at ZnS/CdS and ZnSe/ZnTe (001) interfaces has been investigated using a first principles pseudopotential planewave technique and the local density approximation for the exchange-correlation potential. Similarly, the linear and quadratic deformation potentials (DPs) of the cubic ZnS, ZnSe, ZnTe and CdS have been calculated. It is found that the ZnSe/ZnTe superlattices are of type II, with a valence band offset varying almost linearly between 0.54 and 1.14 eV by going from ZnTe to ZnSe substrates. The ZnS/CdS superlattices have a very small and show a transition from type I to type II by changing the strain state. Moreover, the conduction band offset at the ZnS/CdS interface is found to have a quite strong strain dependence. Our results for linear deformation potentials of the semiconductors considered are in the range of the available experimental data and theoretical results, and strong non-linear effects have been predicted. The quadratic DPs and the relaxation of the semicore d-electrons are found to have small, but not negligible, effects on the calculated band lineups.

A near-edge X-ray absorption fine structure study of atomic layer epitaxy: the chemistry of the growth of CdS layers on ZnSe(100)

Atomic layer epitaxy (ALE) using a binary reaction sequence of the precursors dimethylcadmium (DMCd) and hydrogen sulfide (H 2S) is shown to produce epitaxial layers of CdS on a ZnSe(100) substrate. Near-edge X-ray adsorption fine structure (NEXAFS) spectra taken at the carbon K-edge and sulfur L-edge are consistent with a growth model based on self-limiting surface reactions and support a mechanism based on sequential surface ligand displacement. This evidence, combined with a TPD study, shows the presence of alternating methyl- and hydride-terminated surfaces, which are passivated to further uptake of Cd or S, respectively. Layer-by-layer growth of good quality CdS was confirmed by comparison of NEXAFS spectra of bulk CdS with our grown films.

Interfacial stability of the As–Se–S/ZnS, As–Se–S/ZnSe, As–Se–Tl/ZnS, As–Se–Tl/ZnSe, and related systems

As–Se–S and As–Se–Tl chalcogenide glasses are brazes used in the bonding of ZnSe and ZnS substrates. Their interfacial stability was examined using the reaction couple technique. Twelve kinds of reaction couples were prepared: As–Se/Zn, As–Se/ZnSe, As–Se/ZnS, As–S/Zn, As–S/ZnSe, As–S/ZnS, As–Se–Tl/Zn, As–Se–Tl/ZnSe, As–Se–Tl/ZnS, As–Se–S/Zn, As–Se–S/ZnSe, and As–Se–S/ZnS. The reaction couple experimental results indicate that As–Se/ZnSe, As–S/ZnS, As–Se–S/ZnS, As–Se–Tl/ZnSe are stable contacts; however, the ZnSe substrate reacts with As–S and As–Se–S, and the ZnS reacts with As–Se–Tl. Based on the compositional analysis and the results of a microstructural examination, it is possible that there is a new ternary compound with the As 21S 23Zn 56 stoichiometry in the As–S–Zn system. Layered structure with a planar interface morphology was observed in most of the reaction couples in this study, except those made of As–Se–Tl which had faceted interfaces. The planar morphology of the interface together with the parabolic growth rate indicate that the interfacial reactions are diffusion-controlled and the convection effect is much less in the viscous molten glass.