Wide Band Gap II-VI Compounds Research Articles

Luminescence of Doped Nanoparticles of Wide Band Gap II-VI Compounds

Nanoparticles of wide band gap II-VI compounds doped with transition metal (TM) or rare earth (RE) ions are perspective phosphor materials and fluorescence labels for optoelectronic, biology and medical applications. The efficiency of 3d-3d and 4f-4f intra-shell transitions is shown to be enhanced in TM, RE doped nanoparticles. Two mechanisms of emission enhancement related to spin dependent interactions of free carriers with impurities are discussed. These interactions enhance the TM, RE intra-shell transitions by increasing the rate of host to impurity energy transfer. It is shown also that Al doping increases the intensity of light emission from ZnO nanoparticles.

UV-A sensors of MgCdS /ZnCdS

Ultra violet radiation sensors are a new, but interesting market, which has been studied by Professor Masakazu Kobayashi of Waseda University and his co-authors. In a paper given at ICMBE in August, he examines the application of visible blind UV-A sensors which was extensively studied for wide bandgap II-VI compound epitaxial layers, where the growth of ZnMgCdS quaternary alloys and MgCdS/ZnCdS short period superlattices were performed, and UV-A sensors fabricated.

Blue Anti-Stokes Emission in Chromium and Iron Doped Wide Band Gap II-VI Compounds

We demonstrate that blue anti-Stokes luminescence (ASL) in ZnSe : Cr is induced by the 2+ → 1+ ionization transition of chromium ions. The role of metastability of the photo-excited Cr 1+ charge state is discussed. We also discuss properties of the ASL in ZnSe: Fe and in Cr-doped solid alloys of ZnCdS and ZnSSe.

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

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.

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.

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.

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.

The first compact blue/green diode lasers-wide-bandgap II-VI semiconductors come of age

The first blue/green laser diode demonstrations reported in the summer of 1991 presented a hope that viable light-emitting devices based on the wide-bandgap II-VI compound were truly possible. In this paper we place those developments into the context of the 30-year research effort which has opened the possibility for the new laser diode, LED, and display devices operating in the short visible wavelengths. Active research interests worldwide have led to rapid progress in the field, culminating in the recent demonstration of room-temperature continuous wave diode lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Light-induced effects on the growth and doping of wide-bandgap II-VI compounds

The effects of bandgap light irradiation on the growth and the doping from the vapour phase of wide-gap II-VI compounds are reviewed. These effects rely on the interaction of the photogenerated carriers with the species present at the surface: molecular reagents (for MOVCD), adatoms, vacancies. The main observations are: enhanced decomposition of adsorbed molecules, enhanced desorption of chemisorbed atoms, higher surface mobility leading to a better crystalline quality, reduction of impurity compensation. The mechanisms of interaction are illustrated by means of simple microscopic models.

The study of compound semiconductor heterostructures by High-resolution transmission electron microscopy

Heteroepitaxial compound semiconductor systems have increasing importance for the fabrication of advanced electronic devices. Microstructural characterisation of the finest features of these materials can be carried out using transmission electron microscopy (TEM) and this paper demonstrates the use of high resolution work for heterointerface and related investigations in two different areas.The epitaxial growth of wide band gap II-VI compounds offers potential for a number of optoelectronic device applications. Indeed, in some cases, such as for CdS, it is possible to prepare very high quality Wurtzite-structure layers on Sphalerite substrates, despite the presence of substantial misfit. When hexagonal close-packed CdS is grown by metal-organic chemical vapour deposition on (111)A GaAs, the layer is oriented with (0001)cds //(111)GaAs and [110]cds//[10]GaAs. A cross-sectional, high resolution micrograph of the interface is shown in Fig. 1a where the different atomic stacking in the CdS (ABAB....) and GaAs (ABCABC....) regions is evident.

Materials for Heterojunction Devices

Heterojunction devices are playing an increasingly important role in opto­ electronics. They are produced by combining semiconductors that have differing bandgap energies but closely matching lattice parameters. These devices are epitaxially formed on single-crystal substrates. Laser diodes ( 1) and photodetectors are the most important optoelectronic hetero­ structures, although other types of devices, such as transistors, can also be fabricated. The use of heterostructures has made possible dramatic improvements in 111-V compound laser diode performance. However, the addition of heterojunctions to optical detectors has had a lesser impact on their performance. Therefore, this review concentrates mostly on material considerations bearing on laser diode fabrication. The 111-V compound heterojunction lasers have been extensively developed in the past decade starting with the single-heterojunction AIGaAs/GaAs close confinement laser diode (2,3). Since then, structures have been developed with up to four heterojunctions and other III-V and IV-VI materials have been used for heterojunction laser fabrication. The addition of heterojunctions to lasers of the GaAs alloy family has led to a large reduction in the threshold current density at room tempera­ ture, thus making continuous wave operation possible. This development led to practical light sources for high data rate, optical fiber communication. Semiconductor lasers can emit in a spectral range extending from the far infrared (A � 33 11m) to the visible (A � 0.5 11m). However, because some interesting direct bandgap semiconductors cannot be doped both nand p-type, only part of the range is attainable with p-n junction injection devices. For example, the wide-bandgap II-VI compounds CdS and ZnO cannot be doped p-type, and therefore the production of the

The electric and photoresponse characteristics of Ge/ZnSe heterojunctions

Wide band gap II-VI compounds deposited epitaxially upon semiconducting substrates have possible applications as solid-state infra-red detectors and imaging devices. The Ge/ZnSe heterojunction has been prepared by vacuum evaporation of epitaxial layers of zinc selenide on to orientated, single crystal, p-type, germanium substrates. Measurements have been made of the electrical characteristics, capacitance properties and photoresponse of these junctions. From these measurements a realistic band model has emerged involving intrinsic and extrinsic defects present in the bulk and interfacial region of the zinc selenide. The data presented suggest that a Mott-type barrier rather than a Schottky barrier is present at the germanium-zinc selenide interface. Techniques are described for reducing the magnitude of this Mott barrier and the resulting change in the physical properties and band structure are discussed.