II-VI Semiconductor Materials Research Articles

A review on quantum well structures in photonic devices for enhanced speed and span of the transmission network

Quantum well devices feature heterostructures of very thin epitaxial layers of group III-V and II-VI semiconductor materials. Quantum well devices are integrated monolithically with various optoelectronics devices to provide photonic integrated circuits. The representative structure could be realized with GaAs wells with GaAlAs barriers for wavelengths around 0.9 μm and InGaAsP are used for longer wavelengths. Together with quantum well, superlattice structure is another popular design for InGaAs Avalanche Photo Diode (APD). Quantum well structures find their applications in improved lasers, superlattice for photodiodes, modulators and switches. Consequences of quantum well theory are available today in terms of quantum wires and quantum dots. Upon the application of the normal electric field to quantum well structures, exciton pairs becomes more and more confined and the sharp exciton absorption peaks are observed. The effect is termed as “Quantum Confined Stark Effect”. The electro-absorption effect is approximately 50 times larger in multiple quantum well structures than it is in bulk semiconductors. Another electro-absorption effect known as “Franz Keldysh Effect” has been employed in monolithic waveguide detector. These effects lead to electro-absorption lasers (EAL) as well as electro-absorption laser modulators (EML).

Theoretical and experimental studies of stressed nanoparticles of II-VI semiconductors

A theoretical and experimental study of isolated nanoparticles of II-VI semiconductor materials has been done. Using the framework of the effective mass model, the optical absorption spectrum of distributions of spherical quantum dots, freestanding, and under compressive or tensile stress, has been examined theoretically. The theoretical results allow one to foresee the absorption spectra of quantum dots made of a series of materials and having any size. The syntheses of colloidal quantum dots of CdS and CdSe has also been performed through wet chemical routes and characterized by means of optical techniques. The values of the strains in the synthesized quantum dots were inferred from a correlation established between the theoretical and the experimental results.

Recent Progress in Transition-Metal-Doped II–VI Mid-IR Lasers

Recent progress in transition metal (TM)-doped II-VI semiconductor materials (mainly Cr2+:ZnSe) makes them the sources of choice for laser when one needs a compact continuous wave (CW) system with tunability over 1.9-3.1 mum, output powers up to 2.7 W, and high (up to 70%) conversion efficiency. The unique combination of technological (low-cost ceramic material) and spectroscopic characteristics make these materials ideal candidates for new regimes of operation such as microchip and multiline lasing. This paper reviews these nontraditional Cr-doped middle-infrared (mid-IR) lasers as well as describes emerging Fe2+:ZnSe lasers having potential to operate at room temperature (RT) over the spectral range extended to 3.7-5.1 mum. In addition to being wideband semiconductors, effective RT mid-IR lasing TM-doped II-VI media also hold potential for direct electrical excitation. This paper shows the initial steps toward achieving this goal by studying Cr2+-Co2+- and-doped quantum dots. We have demonstrated a novel method of TM-doped II-VI quantum dots fabrication based on laser ablation in liquid environment. TM-doped II-VI quantum dots demonstrated strong mid-IR luminescence. It opens a new pathway for future optically and electrically pumped mid-IR lasers based on TM-doped quantum confined structures.

CdSe Thin Films Grown by MOCVD Method Using New Single-source Precursors

E-mail: iwshim@cau.ac.krRecevied September 5 20, 05Key Words : Cadmium selenide C, dSe Dis, elenocarbamate, MOCVD Sing, le-source precursorCdSe is one of the well known II-VI semiconductormaterials having a band gap of 1.74 eV, suitable for photo-voltaic cell, light emitting diodes, and thin film transistors(TFTs).

Synthesis and characterisation of zinc oxide tetrapod nanocrystals

Zinc oxide is an important group II-VI semiconductor material with optical properties that permit stable emission at room temperature. We report on the synthesis of highly uniform nanocrystalline ZnO tetrapod (ZnO-T) nanostructures through a modified chemical vapour transport process. These self assembled nanocrystals are characterised by four cylindrical arms with a hexagonal facet all of which are joined at a tetrahedral core. Studies are carried out on ZnO tetrapods using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), photoluminescence spectroscopy (PLS) and Raman measurements. We find a simple technique to quench visible emission found in ZnO tetrapods as grown. We also observe Raman active modes suggesting that nitrogen is incorporated within our samples.

On the Structural and Optical Properties of ZnO Nanoparticles Formed in Silica by Ion Implantation

ABSTRACTZinc Oxide (ZnO) is a II-VI semiconductor material with a wide direct band-gap of 3.37 eV at room temperature (RT). In the past decades, the material has been used for a variety of applications such as gas sensors, surface acoustic wave devices, or transparent contacts. Recently, ZnO has gained a new substantial interest primarily because to its potentialities for optoelectronic and spintronic applications. The renewed interest has been fueled by the availability of high-quality bulk substrates, reports of p-type conduction and theoretical predictions of its ferromagnetic behavior at room temperature when doped with transition metals. In the domain of optoelectronics, its main applications include devices emitting in the blue and UV regions by exploiting its wide band-gap such as light-emitting and laser diodes. With respect to several wide band-gap semiconductor materials, ZnO has the advantage of a larger exciton binding energy (about 60 meV) which paves the way for an intense near-band-edge excitonic emission at room and higher temperatures. On the other hand, a band gap engineering can be also achieved by the incorporation of Cadmium and Magnesium atoms into the ZnO lattice.Many techniques have been used to prepare ZnO in various forms, such as single crystals, powders and films. In the past few years, the great attention toward materials with nanometric size have motivated a number of studies on the synthesis of ZnO nanocrystals. Ion implantation is one of the most effective and versatile technique to obtain nanoparticles. ZnO particles embedded in silica matrix have been successfully prepared by ion implantation followed by thermal oxidation.In this work, we report on a detailed structural and optical characterization of the ZnO-silica nanocomposites by using several complementary techniques; in particular, Glancing Incidence X-ray Diffraction (GIXRD), Rutherford Backscattering Spectrometry (RBS), linear Optical Absorption (OA) in the UV-near IR spectrum and Photo-Luminescence (PL). The ZnO nanoparticles embedded in SiO2matrix were prepared by implanting the substrates with 130 keV Zn+ions at doses of 1, 1.5 and 2´1017 ions/cm2. Subsequently, the implanted samples were annealed for 1h in a furnace at a temperature between 500 and 800°C under flowing O2 gas. X-ray diffraction results indicate the formation of Zn and ZnO nanoparticles in the as-implanted and annealed samples, respectively. Moreover, the ZnO nanocrystals embedded in the SiO2matrix have a (002) preferred orientation. After the oxidation, the optical absorption spectra show an absorption edge at about 374 nm by confirming the presence of the ZnO particles. A relatively strong exciton photoluminescence peak was observed at room temperature under pulsed N2 laser excitation at l=337nm. The results obtained, peculiarly related to the implantation doses and annealing temperature, are discussed.

CdTe nuclear detector electroless contact Studies-new results on contact structures, interfaces, and stress

Although electroless contacts are the most used contacts on II-VI semiconductor materials (CdTe family) to solve many problems, they are difficult to understand and to control. For this paper, we have chosen two II-VI materials, CdTe and ZnTe, and three metals: Au, Pt, and Pd. For the characterization methods, we have chosen Rutherford backscattering (RBS), secondary ions mass spectroscopy (SIMS), and partially Auger for the contacts and interface structures, stoichiometry, and composition profiles. Electrical measurements, PICTS and TEES, are done to find out defect level concentrations and nature, as well as the possible correlation with the composition concentrations. As first parameter change, we have chosen the solution dilution in the electroless deposition process, while the other parameters were frozen. This allows us to drive and to see a drastic variation in the profile of the metal, the two constituent materials and the oxygen in the metal layer. The same has been seen at the interface where the V/sub Cd/ concentration profile shows sensible variation too. We have found a clear connection with specific electrical defect concentrations. The validity of these concentration measurements in the bulk are then subject to questions. The comparison with evaporated contacts is done for bulk and surface property separations. Contact induced stress is simulated by static pressure in longitudinal and transversal direction. We have driven out the induced defect natures and concentrations. The limit of operation under stress is also given.

Carbon-sulfur bond cleavage in Bis(N-alkyldithiocarbamato)cadmium(II) complexes: heterolytic desulfurization coupled to topochemical proton transfer.

Three bis(N-alkyldithiocarbamato)cadmium(II) complexes [Cd(S(2)CNHR)(2)] (1, R = n-C(3)H(7); 2, R = n-C(5)H(11); 3, n-C(12)H(25)) were prepared by metathesis of the corresponding lithium salt, Li[S(2)CNHR], with cadmium chloride. The crystal structures of 2 and 3 consist of planar molecular units of [Cd(S(2)CNHR)(2)] connected by intermolecular Cd.S interactions to give a one-dimensional chain. The chains are connected by a network of intermolecular N-H.S hydrogen bonds between the dithiocarbamato nitrogen atom and bridging sulfur atoms in neighboring chains. In solution, the (113)Cd NMR spectrum of 2 is dependent on concentration and temperature, indicative of a dimerization equilibrium mediated by similar Cd.S intermolecular bridging interactions. In the solid state, thermal gravimetric analyses show that all three complexes decompose smoothly via a heterolytic C-S bond cleavage reaction to give the corresponding alkyl isothiocyanate and cadmium sulfide as the primary products, with the formation of primary amine and CS(2) as coproducts. These products can result only from the net transfer of protons between N-alkyldithiocarbamato ligands in the solid state. Thus, the C-S bond cleavage reaction is interpreted in terms of the topochemical arrangement of molecular units in the crystalline state, which provides a pathway for proton transfer between ligands via N-H.S hydrogen bonds. Decomposition was also initiated by addition of a tertiary amine to a solution of [Cd(S(2)CNHR)(2)]. This confirms that C-S bond cleavage must be coupled to deprotonation of the -NH group, and explains why dialkylated derivatives [Cd(S(2)CNR(2))(2)] are inert to this particular mode of C-S bond cleavage. This system thus constitutes an unusual example of heterolytic, nonoxidative C-S bond cleavage that appears to proceed by a topochemical transfer of protons, which has implications for C-S bond cleavage processes in single-source precursors for II-VI semiconductor materials.

Borrowing Ideas from Nature: Peptide Specific Binding to Gallium Arsenide

AbstractBiological systems have a unique ability to control crystal structure, phase, orientation and nanostructural regularity of inorganic materials. An example is seen with the control of crystallographic phase and orientation of calcium carbonate with the polyanionic proteins isolated from shells of the marine gastropod abalone. We are currently investigating the principles of natural biological molecular recognition in materials and developing new methods to pattern useful non-biological electronic and magnetic materials on new length scales. A peptide combinatorial approach has been employed to identify proteins that select for and specifically bind to inorganic structures such as semiconductor wafers, nanoparticles and quantum confined structures. This approach utilizes the inherent self-organizing, highly selective properties of biologically derived molecules. We are currently investigating peptide recognition and interaction with III-V and II-VI semiconductor materials, magnetic materials, calcium carbonates and phosphates. We have selected peptides that can specifically bind and discriminate between zinc-blende III-V semiconductor surfaces. These peptides show crystal face specificity and are being used to organize nanoparticle heterostructures. Long term potential application of these materials would include opto-electronic devices such as light emitting displays, optical detectors, lasers, and nanometer scale computer components.

Variation of the effective extinction coefficient during pyrolytic and photo-assisted II-VI MOVPE growth, measured by in situ laser interferometry

In situ laser reflectometry has been used to monitor the growth of II-VI semiconductor materials under pyrolytic and photo-assisted conditions. The fit of effective extinction coefficient, , at each peak and trough in the growth interferograms showed that different growth modes are dominant under pyrolytic and photo-assisted conditions. The fit of for several single layers of ZnTe grown under pyrolytic conditions indicated an overall good quality epitaxial growth, with layer quality increasing with layer thickness, whereas for layers grown with photo-assistance, for the same growth temperatures and precursor mixtures, the fit of was anomalously high. In situ monitoring of these layers revealed that they begin to roughen after a maximum of of film deposition. Thus the anomalously high values for would seem to indicate that this fit of is revealing insights into the degradation of film growth for these layers. Through post-growth spectral reflectance analysis of a thin epilayer of ZnSe, where the fringes due to variations in film thickness are clearly visible, an argument for the increase in based on non-linear growth rate across the surface under photo-assisted conditions is presented.

Role of Ionic Processes in Degradation of Wide-Gap II-VI Semiconductor Materials

A role of mobile defects in processes responsible for II—VI compound semiconductor characteristic instability is under consideration. These defects have been shown to be responsible for electron-enhanced reactions in these materials, in particular, shallow donor creation in CdS crystals. Accumulation of mobile defects near dislocations results in some specific effects: anisotropy of conductivity induced by electric field and distortion of edge emission spectrum shape. These effects side by side with electron-enhanced defect reactions have been found to influence considerably semiconductor device characteristics.

New developments in CdTe and CdZnTe detectors for X and γ-ray applications

There has been considerable recent progress in II-VI semiconductor material and in methods for improving performance of the associated radiation detectors. New high resistivity CdZnTe material, new contact technologies, new detector structures, new electronic correction methods have opened the field of nuclear and x-ray imaging for industrial and medical applications. The purpose of this paper is to review new developments in several of these fields. In addition, we will present some recent results at LETI concerning first the CdTe 2-D imaging system (20 × 30 mm2 with 400 × 600 pixels) for dental radiology and second the CdZnTe fast pulse correction method applied to a 5 × 5 × 5 mm3 CdZnTe detector (energy resolution = 5% for detection efficiency of 85% at 122 keV) for medical imaging.

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

Blue/green lasers focus on the market

The authors present a review of past, present, and future blue/green laser technologies. They discuss: II-VI semiconductor materials and alternate approaches (including the use of SiC and III-V nitrides, e.g. GaN, AlN); laser diode design; multiple quantum well approaches; contacting to p-ZnSe; and remaining challenges. >

Semiconductor Nanocrystals formed in SiO2 by Ion Implantation

ABSTRACTNanocrystals of group IV (Si, Ge and SiGe), III-V (GaAs), and II-VI (CdSe) semiconductor materials have been fabricated inside SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystalline semiconductor materials has been studied by transmission electron microscopy (TEM). The nanocrystals form in near-spherical shape with random crystal orientations in amorphous SiO2 Extensive studies on the nanocrystal size distributions have been carried out for the Ge nanocrystals by changing the implantation doses and the annealing temperatures. Remarkable roughening of the nanocrystals occurs when the annealing temperature is raised over the melting temperature of the implanted semiconductor material. Strong red photoluminescence peaked around 1.67 eV has been achieved in samples with Si nanocrystals in SiO2.

Application of phosphine tellurides to the preparation of Group II-VI (2-16) semiconductor materials

In order to find low-temperature routes to metal tellurides, the authors have investigated some of the reactions of phosphine tellurides. Via the phosphine telluride, tellurium is chemically transported by PMe/sub 3/ under very mild conditions (T approx. = 150/sup 0/C). Triethylphosphine telluride reacts with Hg at room temperature and with HgR/sub 2/ (R = Et, Ph) at 115/sup 0/C, in each case giving HgTe as the sole solid-sate product. They studied the reaction with HgPh/sub 2/ in greater detail, showing that Ph/sub 2/Te and Ph/sub 2/Te/sub 2/ are the major organic products.