ZnSe Layers Research Articles

Deep-level-transient spectroscopy of heavily Al-doped ZnSe layers grown by molecular-beam epitaxy

Using deep-level-transient spectroscopy, we have investigated deep levels in heavily Al-doped ZnSe layers grown by molecular-beam epitaxy. The Al concentration of the ZnSe layers lies in the range of 5×1018–9×1018cm−3. The ZnSe:Al layers exhibit two electron-trap centers with the thermal activation energies of 0.16eV (ND1) and 0.80eV (ND2). ND2 is a dominant trap center with a trap density of 3×1016cm−3, while the trap density of ND1 is estimated to be 2×1015cm−3. However, ND2 shows anomalous behaviors, different from isolated point defects, in the following points: (1) the emission peak of ND2 moves to the low temperature side with increasing filling pulse duration; (2) the emission peak of ND2 is broader than theoretically calculated one for an isolated point defect; and (3) the capacitance-transient curve is nonexponential. It is observed by high-resolution x-ray diffraction that heavy Al doping results in the relaxation and plastic deformation of the ZnSe lattice. These behaviors can be ascribed to extended defects with a broad energy spectrum. By assuming a Gaussian distribution of deep levels due to extended defect, the broad emission peak is successfully simulated.

Magnitude of magnetic field dependence of a possible selective spin filter inZnSe∕Zn1−xMnxSemultilayer heterostructures

Spin-polarized transport through a band-gap-matched $\mathrm{Zn}\mathrm{Se}∕{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}∕{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ multilayer structure is investigated. The resonant transport is shown to occur at different energies for different spins owing to the split of spin subbands in the paramagnetic layers. It is found that the polarization of current density can be reversed in a certain range of magnetic field, with the peak of polarization moving towards a stronger magnetic field for increasing the width of central ZnSe layer while shifting towards an opposite direction for increasing the width of paramagnetic layer. The reversal is limited in a small-size system. A strong suppression of the spin up component of the current density is present at high magnetic field. It is expected that such a reversal of the polarization could act as a possible mechanism for a selective spin filter device.

Role of Cation Vacancy-Related Defects in Self-Assembling of CdSe Quantum Dots

In this chapter we present the results of the photoluminescent and optical investigations of the influence of cation vacancy-related defects on CdSe/ZnSe quantum dot organization. Selfassembling growth was achieved under molecular beam epitaxy with subsequent annealing step. Number of cation vacancies was controlled by the intensity of the emission band connected with complex that includes cation vacancy and shallow donor. For the first time it is shown that increase of number of cation vacancy related defects results in the reduction of potential fluctuations in the QD layer. In this case a relatively uniform dense array of QDs with shallow localization potential is organized. It is proposed that generation of cation vacancies during the growth suppresses both Cd segregation and Cd surface diffusion as well as facilitates Cd/Zn interdiffusion. Interdiffusion process is proved by the changes in the photoluminescence and optical reflection spectra of ZnSe layers. It is showned that Cd/Zn interdiffusion can play an important role in CdSe/ZnSe intermixing during the QD formation at least under such growth conditions which can stimulate generation of cation vacancies.

Structure characterization of MBE-grown (Zn,Cr)Se layers

Cr-based diluted magnetic semiconductors (DMSs) have attracted a lot of attention within the last few years because of their possible applications in the area of spintronics. Modern growth techniques of such materials require efficient methods of characterization of their structure. In this work, the structure of Cr-rich ZnSe layers grown by MBE on (001) GaAs substrate was investigated by means of the high-resolution X-ray diffraction, atomic force microscopy and Raman scattering. Direct evidence of the mixed Zn1−xCrxSe crystal in the zinc-blende phase is demonstrated by the high resolution X-ray diffraction (a part of Cr is shown to occupy the cationic sites in the crystal lattice). The presence of amorphous or crystalline Se- and Cr-related precipitates in selected samples is demonstrated, their influence on the properties of (Zn,Cr)Se layers is discussed.

High-quality ZnSSeTe epitaxial layers grown by MBE

High-quality quaternary ZnSSeTe epitaxial layers were successively grown on GaAs substrates by molecular beam epitaxy. It was found that a ZnSSe layer was automatically formed between a ZnSe buffer layer and ZnSSeTe epitaxial layer when we increased the ZnS cell temperature. It was also found that the photoluminescence (PL) activation energy of the sample was 263 meV, which was much larger than those observed from ZnSe and/or ZnSSe epitaxial layers. The much larger PL activation energy was attributed to the fact that excitons in ZnSSeTe were still bounded at the localized Te or Te n cluster states.

Novel ZnTe‐based green light emitters grown on ZnTe substrates

Lattice matched ZnCdSeTe/(ZnMgSeTe quasi-quaternary) pn-junction green light emitting diodes (LED) grown on (001) ZnTe substrates were studied. The ZnMgSeTe quasi-quaternary layers, which consisted of stacked layers of very thin ZnSe and ZnMgTe layers, were introduced in order to obtain high carrier concentrations in the n-type layers. Two layer designs near the ZnCdSeTe active layers in the LEDs were compared to optimize the emission properties. Electrical properties from two LED structures showed no difference. On the other hand, total emission intensity from the modified LED structure was drastically improved by a factor over 30 under same operation conditions. Momentums of the electron and heavy hole wavefunctions inside the two LED structures were simulated to understand an experimentally observed difference of the emission properties. Results of the simulations showed that the modified LED structure has more efficient confinements of electrons and holes inside the ZnCdSeTe active layers by introducing thicker ZnSe and ZnMgTe layers neighboring the ZnCdSeTe active layer. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct investigation of very thin CdSe layers on ZnSe by out‐of‐plane grazing incidence X‐ray diffraction

Very thin CdSe layers grown on ZnSe and capped by ZnSe layers of various thicknesses were investigated by out-of-plane grazing incidence X-ray diffraction. An algorithm that will allow extraction of vertical and lateral lattice constants and therefore composition and relaxation state for these layers is presented. Qualitative considerations based on simulations using the dynamical diffraction theory confirm the presence of intermixing. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and interface phonons of different kind in BeTe/ZnSe superlattices

AbstractWe have studied Raman scattering for a number of type II novel superlattices BeTe/ZnSe, which have no common cation or anion atoms at the interfaces. Under off‐resonant excitation LO‐phonons in BeTe and ZnSe layers were observed as well as a high‐frequency mode due probably to the “wrong bond” BeSe at the interface. Under excitation resonant with the direct exciton in ZnSe layers we observed ZnSe LO‐phonons of different orders and electrostatic interface modes of the Fuchs–Kliever type. Calculated frequencies of the interface modes are compared with the experimental ones. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of core/multishell CdSe/Zn(S,Se) nanocrystals

CdSe nanocrystals are now known as highly efficient fluorescent light sources. Their efficiency is however strongly dependent on the quality of the passivation layers. We show here that using a ZnSe/ZnS bi-layer shell leads to a larger photoluminescence yield than both ZnSe and ZnS simple shells. The intermediate ZnSe layer acts as a lattice parameter adaptation layer to improve on the core/shell interface quality, while the ZnS outer shell maximizes the exciton confinement.

Time domain investigation on excitonic spectral diffusion in CdSe quantum dots grown on vicinal surface GaAs substrates

Two monolayers of CdSe sandwiched by ZnSe layers were grown by molecular-beam epitaxy on a vicinal GaAs surface substrate. Small ensembles of these self-assembled quantum dots (QD) have been studied at low temperatures using micro-photoluminescence spectroscopy. We observe reversible spectral diffusion in individual QD luminescence lines. The energy shift is negligibly small at low optical excitation power and amount to about 0.7meV under higher excitation (0.6kW/cm2). Because the energy shift is found to be significantly smaller than the exciton–biexciton splitting of CdSe QDs even under high excitation, the conventional spectral filtering could be used to effectively select the exciton-related emission from these QDs.

Resonant stokes and anti-stokes Raman scattering of light in CdSe/ZnSe nanostructures

Raman scattering spectra of CdSe/ZnSe multilayer nanostructures with a CdSe insert of a nominal thickness of 2.1 monolayers were studied. A heavy dependence of the intensity and of the frequency position of the multiphonon Stokes and anti-Stokes LO bands on the exciting photon energy was detected. The results obtained are interpreted as a resonance with various exciton transitions in the CdSe insert and barrier ZnSe layers. A difference between the Stokes and anti-Stokes frequencies of LO bands observed as the resonance conditions are varied confirms the inhomogeneous nature of the photoluminescence band of CdSe quantum dots.

Influence of the cap layer growth temperature on the Cd distribution in CdSe/ZnSe heterostructures

We investigated the influence of the temperature of the ZnSe cap layer growth on the Cd distribution in CdSe/ZnSe heterostructures by transmission electron microscopy, high-resolution X-ray diffraction and photoluminescence spectroscopy. ZnSe buffer layers were grown by molecular-beam epitaxy on GaAs (001). For the CdSe layers and ZnSe cap layers, migration-enhanced epitaxy was used. Increasing the temperature for the cap layer growth reduces the Cd concentration and the total CdSe amount in the (Cd,Zn)Se layers leading to a blue shift of the photoluminescence spectra. This is owing to a stronger Cd desorption during the cap layer growth for a higher substrate temperature.

Suppression of impurity interdiffusion in heteroepitaxy by inserting a low-temperature buffer layer in between the epilayer and the substrate

This article demonstrates that interdiffusion, which is inherent in heteroepitaxy, can be well suppressed by inserting a thin buffer layer grown at sufficiently lower temperature with taking a ZnSe/GaAs heteroepitaxy as an example. Two types of samples are prepared by molecular beam epitaxy (MBE): a 1114 Å thick ZnSe epilayer grown on a 150 Å thick low-temperature ZnSe buffer layer on a GaAs (0 0 1) substrate, and a 1356 Å thick ZnSe layer grown directly on GaAs. The composition profile at the heterointerface measured by secondary ion mass spectrometry (SIMS) clearly shows that the Ga diffusion is greatly suppressed in the ZnSe/GaAs system prepared with the low-temperature buffer layers, while considerable Ga interdiffusion from the GaAs substrate into the ZnSe epilayer is observed in the sample without the low-temperature buffer layer. The suppression of interdiffusion is also evidenced by low-temperature photoluminescence (PL) spectroscopy which shows no trace of donor–acceptor pair (DAP) emission at around 2.7 eV normally observed in ZnSe epilayers. In addition to the suppression of interdiffusion, Se reaction with GaAs surfaces is suppressed, which leads to the reduction of built-in defect generation due to heterovalency. Consequently, the ZnSe layer grown on a thin low-temperature ZnSe buffer layer shows high crystallinity as indicated by strong and sharp exciton emission in low-temperature photoluminescence spectrum and narrow X-ray line width.

Low-temperature growth of nitrogen-doped BeTe and ZnSe/BeTe:N superlattices for delta doping

We propose ZnSe/BeTe:N superlattice (SL) as a new p-type cladding layer with improved p-type dopability, in which nitrogen-doped BeTe (N-doped) layers are inserted into undoped ZnSe layers. We have optimized the growth temperature for ZnSe/BeTe:N SLs. Both equivalent beam pressure (BEP) ratio and growth rate are further optimized in low-temperature growth of BeTe:N layers. The structural properties of BeTe:N layers were analyzed by reflection high-energy electron diffraction and high-resolution X-ray diffraction. Electrical properties were evaluated by the van der Pauw method and capacitance–voltage ( C– V) measurements. We achieve the net acceptor concentrations of 1.2×10 19 cm −3 at the growth temperature of 350°C, BEP ratio of around 6, and growth rate of 35 nm/h.

Characteristics of deep levels in Al-doped ZnSe grown by molecular beam epitaxy

We investigated the characteristics of deep levels in heavily Al-doped ZnSe layers grown by molecular beam epitaxy, whose electron concentration is saturated. Low-temperature photoluminescence showed deep level emission around 2.25 eV, and its intensity increases with Al concentration. This deep-level is located at 0.55 eV above valence band maximum, implying a point defect such as a self-activated center, Al ZnV Zn. Deep-level transient spectroscopy was used to investigate non-radiative trap centers in Al-doped ZnSe layers, and showed the presence of two electron trap centers at depths of 0.16 and 0.80 eV below conduction band minimum, with the electron capture cross-sections of 810 −12 and 1×10 −7 cm 2, respectively. It is suggested the carrier compensation in heavily Al-doped ZnSe layers be ascribed to the deep levels.

Deep hole trap levels of Sb-doped ZnSe grown by MOVPE

Sb-doped p-type ZnSe layers grown epitaxially on GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated using deep-level transient spectroscopy (DLTS). All samples prepared at three different flow rate ratios of DMSe/DMZn (VI/II) have three hole traps. At VI/II=2.45, which is a stoichiometric condition, the total concentration of the three deep traps is the lowest compared with the others. The trap at 0.46 eV is tentatively ascribed to be related to an Sb atom. The origin of a major trap at VI/II=1.5 which has an activation energy of 0.62 eV is likely to be defects related to a Se vacancy. Although a major trap at VI/II=3.27 has the same activation energy of 0.62 eV, its origin seems to be a Zn vacancy.

Acoustic, optical, and interface phonons in BeTe/ZnSe superlattices

Raman scattering in a number of BeTe/ZnSe type-II superlattices which share no common cations or anions in the interfaces was studied. Folded acoustic phonons; LO phonons of the first, second, and third order in the ZnSe layers; and Kliewer-Fuchs-type electrostatic interface phonons were observed when excited in resonance with the direct exciton transition in the ZnSe layers. Nonresonant excitation produced LO phonons in the ZnSe and BeTe layers and a high-frequency mechanical interface mode, assigned tentatively to a local vibration of the interface Be-Se bond.

Interface structure and chemistry in ZnSe/Ga1−xMnxAs/ZnSe heterostructures

The structure and chemical composition of ZnSe/Ga1−xMnxAs/ZnSe multilayers grown on (100) GaAs substrates are investigated by high-resolution transmission electron microscopy imaging and spectroscopy techniques. While all layers grow epitaxially and the Ga1−xMnxAs layer is free of planar defects, a high density of stacking faults is observed in the ZnSe layer over Ga1−xMnxAs. The composition of the ferromagnetic layer is measured to be Ga0.93Mn0.07As, and the Mn valence was determined to be 2+. Compositional profiles across the interfaces quantified by electron energy-loss spectroscopy show that the ZnSe/Ga1−xMnxAs interfaces are wider than the ZnSe/GaAs–substrate interface, which is mainly attributed to interfacial roughness.

Single quantum dot spectroscopy of CdSe/ZnSe grown on vicinal GaAs substrates

We investigated the optical properties of two monolayers of CdSe sandwiched by ZnSe layers grown by molecular-beam epitaxy on GaAs substrates with a vicinal tilt of 2° in the [111] direction. By varying the spatial resolution from 10 μm down to 500 nm, sharp photoluminescence lines due to the recombination of excitons confined into quantum dots could be observed at low temperature. The dot density could be as low as ≈109 dots/cm2, which is smaller than previously reported values by at least one order of magnitude.

PL characteristics of MBE-grown, Pb-doped ZnSe crystal layers

Abstract Investigations have been performed for MBE-grown, Pb-doped ZnSe crystal layers on crystallographic as well as PL characteristics. An intense green emission has been observed in ZnSe layers at low temperatures under focused 325 nm wavelength excitation. The green emission was shown to have a nonlinear dependence upon excitation intensity. A correlation between the green emission intensity and Pb flux during the growth has shown that the green emission is related to the presence of Pb atoms, particularly isolated Pb atoms. Excitation intensity and temperature dependence of the green emission has been studied and the mechanism leading to the green emission is discussed.