Wide Band Gap II-VI Research Articles

Thin Films of Wide Bandgap II-VI Compounds Grown by Atomic Layer Epitaxy - Properties and Application

Thin Films of Wide Bandgap II-VI Compounds Grown by Atomic Layer Epitaxy - Properties and Application

Self-Organized Formation of Wide-Bandgap II-VI Quantum Dots. Thermally Activated Surface Re-Organization versus 2D Stabilization

The thermally activated transition from a 2D CdSe layer grown on ZnSe by MBE to a quantum dot assemblage is studied by in situ RHEED and UHV-AFM. The observed morphologies are a result of the interference of competitive processes. Both surface states, 2D and 3D, can be obtained and stabilized under the same conditions.

Excitons with large binding energies in MgS/ZnSe/MgS andZnMgS/ZnS/ZnMgS quantum wells

The wide bandgap II-VI semiconductors have unique properties which allow thepossibility of suppressing the exciton-phonon scattering up to roomtemperature in quantum well structures designed so that the exciton excitationE1s→2s>hνLO. In particular, magnetic field and temperaturedependent measurements are used to study the exciton binding energies and toinvestigate the exciton-LO phonon scattering processes of high quality ZnSequantum wells in MgS grown by MBE. The small inhomogeneous broadening of theexciton transitions in these samples allows the observation of higher excitedexciton states. Due to the large difference in band gap between ZnSe and MgSthe exciton binding energy in a 5 nm well is found to be 43.9 meV, which isthe largest reported for this material system. The FWHM of the heavy holeabsorption transitions measured as a function of temperature shows that thescattering of the excitons by the LO phonons is partially suppressed. Theseresults are compared with ZnS quantum wells where the exciton g-values havebeen measured and the exciton binding energies have been deduced from theexciton diamagnetic shifts. The results show the possibility of suppressingexciton-LO phonon scattering in these structures.

Spin Polarization and Injection in ZnMnSe/ZnCdSe Heterostructures

ABSTRACTMagneto-optical spectroscopy in combination with tunable laser excitation spectroscopy is employed to carry out a detailed study of spin alignment and spin injection in II-VI wide-bandgap semiconductor heterostructures, aiming at optimization of structural design for nano-scale spintronic applications. The use of tunable excitation is shown to provide a valuable opportunity to monitor separately spin relaxation and spin injection processes in the structures. Efficient spin alignment is achieved by using a diluted magnetic semiconductor (DMS) (a layer of ZnMnSe or a ZnMnSe/CdSe superlattice) as thin as 10 nm. The spin alignment efficiency is shown to depend critically on the ratio between the rates of spin relaxation and spin transport within the DMS layer. This allows the realization of spin alignment and spin switching functions by varying the structural design.

The problem of conductivity-type inversion in wide band gap II-VI compounds

To solve the problem of conductivity-type inversion in wide band gap II-VI compounds the thermodynamical analysis of intrinsic point defects has been performed. The existence of certain critical temperature of heat treatment in equilibrium conditions is shown. Above this temperature it is impossible to obtain the samples with stoichiometry deviation toward non-metals. At the temperatures lower than Tc, the diffusion processes in crystals are retarded and the equilibrium between II-VI crystals and B component vapour cannot be established (B is a component of a binary compound AB). Thus, it is shown that under thermodynamical equilibrium conditions it is impossible to obtain wide band gap II-VI compounds of p-type conductivity.

Acceptor Neutralisation by Hydrogen in GaN and Wide Band Gap II-VI Materials

One of the major problems encountered for the elaboration of blue light emitting devices processed with either nitrides or II–VI compounds is the difficulty for p-type doping of the materials. It is shown that the unintentional introduction of hydrogen, issued for instance from the organometallic precursors during the process is responsible for at least part of the problem. Various experimental techniques for investigating hydrogen in semiconductors are reviewed. They show unambiguously that hydrogen forms complexes with the acceptors and neutralises them. The microscopic structure of the complexes is discussed. The processes which allow to reactivate the neutralised acceptors are discussed; their limitations are outlined.

N -type doping of lattice-matched ZnCdSe and ZnxCdyMg1−x−ySe epilayers on InP using ZnCl2

Chlorine on Se-site forms a shallow donor for ZnSe. In this article, we use Cl, obtained from ZnCl2, as the n-type dopant for ZnxCdyMg1−x−ySe lattice matched to InP, a new wide band gap II-VI material grown by molecular beam epitaxy. An 800 Å p-type doped InGaAs buffer layer was grown to improve the doping behavior, consistent with improved crystalline quality. The highest free-carrier concentrations measured by Hall effect are 7×1018 cm−3 with mobility of 240 cm2/V s and 3×1018 cm−3 with mobility of 230 cm2/V s for Zn0.5Cd0.5Se (77 K Eg=2.17 eV) and ZnCdMgSe (77 K Eg=2.74 eV), respectively. A small systematic reduction of maximum carrier concentration was observed as the quaternary layer band gap is increased. No deep level emission is introduced by the chlorine dopant. High n-type doping levels, consistent with semiconductor laser applications were achieved for quaternaries of band gaps as high as 2.9 eV.

Self-organized growth, ripening, and optical properties of wide-bandgap II-VI quantum dots

Self-organized growth, ripening, and optical properties of wide-bandgap II-VI quantum dots

Dynamic vapour pressure measurements of di-tertiarybutyl sulphide using an ultrasonic monitor

Di-tertiarybutyl sulphide is an attractive precursor for the growth, using metal-organic vapour-phase epitaxy (MOVPE), of sulphur-based wide band-gap II-VI compounds due to its lower volatility compared to other more commonly used sulphur sources and likely suppression of unwanted gas-phase pre-reactions. The vapour pressure of MOVPE precursors is a key parameter and needs to be precisely known in order to control the concentration of source materials entering the reactor. Only scattered and incomplete vapour pressure data exist for di-tertiarybutyl sulphide. The concentration under dynamic conditions was measured with an Epison concentration monitor and used to predict the saturated vapour pressures. The vapour pressure equation (log 10 p=8.559(±0.082)−(2338(±23)/ T) was obtained and the measured vapour pressures were found to be consistently smaller than the reported literature values. The saturated vapour pressure for tertiarybutyl thiol under dynamic conditions has also been measured and new data are reported.

Low damage thermally assisted electron cyclotron resonance etch technology for wide bandgap II-VI materials

A flexible and low damage dry etch technology for the fabrication of ZnSe-based nanostructures is presented. Thermally assisted electron cyclotron resonance etching using gas mixtures of chlorine compounds and Ar and N2, respectively, combines plasma etching at low ion energies with process temperatures between 60 and 250 °C. With increasing process temperatures, rising etch rates and reduced surface roughness indicate a thermal activation of the etching process. The etch profile can be controlled by varying the plasma power causing a transition from partially physical to prevailing chemical etch properties. High quantum efficiencies in CdZnSe/ZnSe quantum wires with lateral sizes down to 20 nm were obtained, indicating a significantly reduced etch damage compared to conventionally dry etched II-VI nanostructures. The potential of the etch technology is demonstrated by realizing quantum wires with a blue shift of the photoluminescence signal (e.g., 8 meV for 20-nm-wide wires) caused by lateral carrier confinement effects.

Structural quality of pseudomorphic Zn0.5Cd0.5Se layers grown on an InGaAs or InP buffer layer on (0 0 1) InP substrates

Abstract The structural quality of pseudomorphic Zn1 − xCdxSe(x ≈ 0.5) − a II-VI wide bandgap semiconductor that can be lattice-matched to (0 0 1)InP - is studied using high resolution X-ray diffraction and transmission electron microscopy. When ZnCdSe is grown by molecular beam epitaxy directly on de-oxidized InP substrates, a density of 109/cm2 of Shockley-type stacking faults nucleate at the InP/ZnCdSe interface and grow into the micron-thick ZnCdSe film. The density of stacking faults is reduced by two orders of magnitude (down to 107/cm2) by first growing an InP buffer layer. Films grown on an InP buffer layer are compared to ZnCdSe films grown on InGaAs buffer layers: both types of buffer layer lead to identical defect densities in the ZnCdSe. The ZnCdSe X-ray diffraction peak becomes as narrow as 73 arcsec. To obtain the 107/cm2 stacking fault density, the ZnCdSe growth needs to be initiated at a temperature well below the growth temperature of 270°C. All the results are explained in terms of achieving two-dimensional nucleation of the II-VI growth.

(ZnCd)S, (ZnCd)Se and Zn(SeTe) downconverting phosphors

The efficiency of (ZnCd)S, (ZnCd)Se and Zn(SeTe) solid solutions, doped with copper and chlorine, as x-ray downconverting phosphors for photodetectors is investigated. It is found, that the phosphors based on the solid solutions of the group II-VI wide-bandgap compounds and prepared in oxygen-free medium, can increase the photodetectors sensitivity to x-ray radiation more than one order of magnitude and enhance the detectors photocurrent versus radiant flux characteristics linearity.

Nondestructive analysis of structural defects in wide bandgap II-VI heterostructures

X-ray scattering measurements of wide bandgap II-VI heterostructures show that stacking faults (which nucleate defects that are responsible for optical degradation of light emitting diodes and lasers) introduce significant levels of diffuse scattering near Bragg reflections of both the epitaxial layers and the GaAs substrate. We employed triple axis x-ray diffraction techniques to investigate stackingfault diffuse scattering and used cathodoluminescence and transmission electron microscopy to independently measure the stacking fault density. For comparison, double axis scans from the same samples were largely incapable of detecting the presence of these defects. Measurements taken at different azimuthal positions exhibit different levels of diffuse scattering and the diffuse scattering intensity is related to the stacking fault intensity in each direction, which suggests that this technique can provide a non-destructive assessment of defects present in these systems. For some samples, the ZnSe buffer layer exhibited a tilt with respect to the substrate along a direction; this tilt was greater than the tilt which would be attributed to growing a strained layer on the slightly miscut substrates which were used here.

MBE Growth of lattice-matched ZnCdMgSe quaternaries and ZnCdMgSe/ZnCdSe quantum wells on InP substrates

We report the growth and characterization of a new wide bandgap II-VI alloy, ZnxCdyMg1-x-ySe, grown lattice-matched to InP. High quality quaternary layers with bandgaps ranging from 2.4 to 3.1 eV were grown by molecular beam epitaxy. The bandgaps and lattice constants were measured using photoluminescence and single crystal Θ-2Θ scans. Quantum well structures with quaternary barriers and ZnCdSe wells were also grown, entirely lattice matched to InP. Their photoluminescence properties suggest that these materials are suitable for the design of visible semiconductor lasers spanning the blue, green, and yellow regions of the visible range. The absence of strain in these heterostructures is expected to improve the reliability of the materials in device applications.

Nonradiative recombination processes in wide band GAP II-VI phosphor materials

Abstract Transition metal ions limit the light emission from wide band gap II-VI phosphor materials. Nickel, iron and chromium deactivate the visible luminescence of ZnSe due to the bypassing and three center Auger processes. For ZnMnS the donor-acceptor pair (DAP) emission is deactivated by spin dependent energy transfer transitions from DAPs to Mn ions.

Wide-Bandgap II-VI Heterostructures for Blue/Green Optical Sources: Key Materials Issues

Substantial advances have been realized in the quest to achieve blu e/green light-emitting devices and injection lasers based on wide-bandgap II-VI semiconductor materials. The lasers operate continuously at room tem­ peratur e and emit a signifi cant amount of power with wavelengths ranging from 463 to 51 4 nm (depending on the actual structure). The various laser structures are composed of (Zn,Mg)(Se,S) and Zn(Se,S) cladding layers with (Zn,Cd)Se quantum wells and possess a graded ohmic contact con­ sisting of Au metal on a pseudo-alloy of Zn(Se, Te). In addition to describ­ ing the advantages and disadvantages offered by various epitaxial growth

II-VI Blue-Green Laser Diodes: A Frontier of Materials Research

The current interest in the wide bandgap II-VI semiconductor compounds can be traced back to the initial developments in semiconductor optoelectronic device physics that occurred in the early 1960s. The II-VI semiconductors were the object of intense research in both industrial and university laboratories for many years. The motivation for their exploration was the expectation that, possessing direct bandgaps from infrared to ultraviolet, the wide bandgap II-VI compound semiconductors could be the basis for a variety of efficient light-emitting devices spanning the entire range of the visible spectrum.During the past thirty years or so, development of the narrower gap III-V compound semiconductors, such as gallium arsenide and related III-V alloys, has progressed quite rapidly. A striking example of the current maturity reached by the III-V semiconductor materials is the infrared semiconductor laser that provides the optical source for fiber communication links and compact-disk players. Despite the fact that the direct bandgap II-VI semiconductors offered the most promise for realizing diode lasers and efficient light-emitting-diode (LED) displays over the green and blue portions of the visible spectrum, major obstacles soon emerged with these materials, broadly defined in terms of the structural and electronic quality of the material. As a result of these persistent problems, by the late 1970s the II-VI semiconductors were largely relegated to academic research among a small community of workers, primarily in university research laboratories.

Growth of Highly Doped P-Type Znte Films by Pulsed Laser ablation in Molecular Nitrogen

AbstractHighly p-doped ZnTe films have been grown on semi-insulating GaAs (001) substrates by pulsed-laser ablation (PLA) of a stoichiometric ZnTe target in a high-purity N2 ambient without the use of any assisting (DC or aC) plasma source. Free hole concentrations in the mid-1019 cm-3 to 1020 cm-3 range were obtained for a range of nitrogen pressures the maximum hole concentration equals the highest hole doping reported to date for any wide band gap II-VI compound. the highest hole mobilities were attained for nitrogen pressures of 50–100 mTorr (~6.5–13 Pa). Unlike recent experiments in which atomic nitrogen beams, extracted from RF and DC plasma sources, were used to produce p-type doping during molecular beam epitaxy deposition, spectroscopic measurements carried out during PLA of ZnTe in N2 do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. This appears to be the first time that any wide band gap (Eg 2 eV) II-VI compound (or other) semiconductor has been impurity-doped from the gas phase by laser ablation. In combination with the recent discovery that epitaxial ZnSe1-xSx films and heterostructures with continuously variable composition can be grown by ablation from a single target of fixed composition, these results appear to open the way to explore PLA growth and doping of compound semiconductors as a possible alternative to molecular beam epitaxy.

DX-Like Centers in IV-VI.

AbstractUp to the recent times it was believed that observation of the DX-centers is restricted to the rather wide-bandgap III-V and II-VI semiconductors. However it becomes obvious now that some of the narrow-gap IV-VI semiconductors doped with the group III elements reveal the features of DX-centers. A range of features, such as negative-U behavior, large lattice relaxation, metastability of the local excited states, narrow bandgap - make the effects resulting from the DX-like behavior much more complicated than in the “classical” materials with the DX-centers. We review the most unusual recent results obtained in the field that originate from the specifics of the DX-like centers in IV-VI.

Photo-ESR Studies of Ni doped ZnS and ZnSe

Transition metal impurities, such as iron [1, 2], chromium [2, 3], or nickel, belong to the most active centers of nonradiative recombination in wide band gap II-VI compounds [4, 5]. In this communication we present the results of electron spin resonance experiments performed under the photo-excitation (photoESR) for Ni doped ZnS and ZnSe. Nonradiative recombination processes in Ni doped samples are discussed. Efficiency of electron and hole trapping by Fe and Ni centers is compared. We verify also previous estimations of energy level position of Nil+ center in ZnS and ZnSe.