Cd Mole Fraction Research Articles

Study on the Characteristics of ZnCdO Thin Films with Varying Cd Mole Fraction Synthesized by the Sol-Gel Method

Study on the Characteristics of ZnCdO Thin Films with Varying Cd Mole Fraction Synthesized by the Sol-Gel Method

Polychromatic ZnO/CdxZn_1-xO composite nanorods prepared by simple chemical methods: nanoscale optical characteristics

We report the fabrication and nanoscale optical characteristics of polychromatic ZnO/CdxZn1-xO composite nanorods prepared by simple hydrothermal and sol-gel chemical methods. Hydrothermally grown ~300 nm diameter and ~3.5 µm long ZnO nanorods were coated, using the sol-gel method, with a thin CdxZn1-xO layer having a spatially varying Cd mole fraction, where x ranged from x = 0 to 1. Full control of the emission color, including white emission, was achieved by simply varying the local Cd mole fraction along the single ZnO/CdxZn1-xO nanorod. The continuous variation of the optical band gap energy along the single nanorod was visualized using nanoscale confocal absorption spectral imaging.

Tuning and Enhancing White Light Emission of II–VI Based Inorganic–Organic Hybrid Semiconductors as Single-Phased Phosphors

Single-phased white light emitters made of semiconductor bulk materials are most desirable for use in white light-emitting diodes (WLEDs) based on both photoluminescence and electroluminescence. Here we demonstrate Cd and/or Se substituted double-layer [Zn2S2(ha)] (ha = n-hexylamine) hybrid semiconductors emit bright white light in the bulk form and their emission properties are systematically tunable. The ternary Zn2–2xCd2xS2(ha) hybrid compounds exhibit two photoluminescence (PL) emission peaks, one of which being attributed to band gap emission, and the other resulting from Cd doping and surface sites. The Cd concentration modulates the optical absorption edge (band gap) and the positions of the two emission bands along with their relative intensities. The ZnS-based hybrid structures (with a nominal Cd mole fraction x = 0.25) emit bright white light with significantly enhanced photoluminescence quantum yield (PLQY) compared to its CdS-based hybrid analogues. For the quaternary Zn2–2xCd2xS2–2ySe2y(ha) c...

Dimensional transition and carrier dynamics in CdxZn1−xTe/ZnTe nanostructures on Si substrates

We investigate the dimensional transition and carrier dynamics in CdxZn1−xTe/ZnTe nanostructures with various Cd mole fractions, grown on Si substrates. Atomic force microscopy images show that the dimensional transition from quantum dots (QDs) to quantum wires occurs with increasing Cd mole fraction. The activation energy of the electrons confined in CdxZn1−xTe QDs with a Cd mole fraction of 0.6 is higher than that of electrons confined in CdxZn1−xTe nanostructures. In addition, the radiative recombination rate shows a linear dependence on the length of the CdxZn1−xTe nanostructures, which is well explained in terms of the “coherence volume” of the bound excitons.

ZnO Nanorods Grown on CdxZn1-xO Seed Layers with Various Cd Mole Fractions

ZnO nanorods were grown on the seed layers with various Cd mole fractions by hydrothermal method. The effects of the Cd mole fraction for seed layers on the structural and optical properties of the ZnO nanorods were investigated by scanning electron microscopy, X-ray diffraction, and photoluminescence. The narrowest full-width at half-maximum and largest grain size of the seed layers, indicating improvement in crystal quality, were observed at the Cd mole fraction of 0.5. At the Cd mole fraction of 0.5, the largest enhancement in the density, the crystal quality, and the growth rate of the ZnO nanorods was observed while their appearance was not affected significantly by the incorporation of the Cd in the seed layers. Consequently, the luminescent properties of the ZnO nanorods were enhanced. The largest improvement in the structural and optical properties of the ZnO nanorods was observed at the Cd mole fraction of 0.5.

HgCdTe Research at FFI: Molecular Beam Epitaxy Growth and Characterization

This paper presents results from recent work on molecular beam epitaxy growth of HgCdTe at the Norwegian Defence Research Establishment (FFI), including studies of material properties and fabrication of photodiodes and nanostructures. Systematic studies of defect morphology in HgTe and Hg1� xCdxTe have revealed that there is a minimum in the area covered by defects just below the onset of Te precipitation. The shape and density of microvoids in HgTe can be used to determine the deviation from the optimal growth temperature. This can be further related to the optimal growth temperature of Hg1� xCdxTe with any Cd mole fraction by thermodynamic calculations. A mechanism for the formation of microvoids and needles has been presented. Photoluminescence (PL) has been used to study layers without doping and with Hg vacancy, Ag, and In doping. Planar photodiodes with high dynamic resistance and good quantum efficiency were fabricated by ion-milling vacancy-doped mid-wave and long-wave infrared layers. Quantum wells (QWs) with good crystallinity and high PL light output have been grown. Surface patterning has been found to enhance light emission from HgCdTe thin-film and QW samples by � 30%. Single-crystal HgTe and segmented HgTe/Te nanowires have been grown, and the resistivity of the nanowires has been measured by conductive atomic force microscopy (AFM), where the AFM tip has been used as a mobile electrode.

Synthesis and spectroscopic study of high quality alloy Cd x Zn1−x S nanocrystals

In the present study, we report the synthesis of high quality Cd x Zn1−x S nanocrystals alloy at 150°C with changing the composition. The shifting of absorption and emission peak in shorter wavelength is obtained with increasing the mole fraction of zinc. The quantum yield (QY) value decreases with increasing the Cd mole fraction and the values are 0·08, 0·13 and 0·40 for Cd0·62Zn0·38S, Cd0·52Zn0·48S and Cd0·31Zn0·69S nanocrystals, respectively after 120 min of reaction. However, the full width at halfmaximum (fwhm) values are 45, 34 and 28 nm and the corresponding quantum yield (QY) values are 0·52, 0·17 and 0·13 for 0·5 mM, 1·5 mM and 3 mM of initial S concentrations, respectively. It is interesting to note that the radiative decay time is dominating with increasing the Cd content. Analysis suggests that the decay dynamics depends on the composition of Cd x Zn1−x S nanocrystals.

Metal-Organic Chemical Vapor Deposition of Hg1−x Cd x Te Fully Doped Heterostructures Without Postgrowth Anneal for Uncooled MWIR and LWIR Detectors

Uncertainty in precursor pulse delays and shapes has been found to be an important factor in Hg1−x Cd x Te metal-organic chemical vapor deposition (MOCVD) growth using the interdiffused multilayer process (IMP). Herein, metal-organic concentration changes in the growth zone are examined using an in␣situ infrared (IR) absorption gas monitoring system, and modifications to the interdiffused multilayer process are applied for in␣situ control of stoichiometry, improved morphology, minimized process length, and consumption of precursors. Dimethylcadmium (DMCd) introduction during IMP flush stages in HgTe was used for stoichiometry control. The final stage of heterostructure formation was optimized to prevent Hg outdiffusion. As a result, vacancy concentration was reduced far below the equilibrium level at the growth conditions so the background of n-type doping was revealed. Acceptor doping with arsine (AsH3) and trisdimethylaminoarsenic (TDMAAs) was examined over a wide range of compositions, and doping levels of 5 × 1015 cm−3 to 5 × 1017 cm−3 were obtained. The presence of both arsenic dopants significantly increased the CdTe growth rate. This caused an increase of Cd mole fraction in the grown material. Doped heterostructures can be grown without any postgrowth anneal and used for mid- and long-wavelength infrared (MWIR and LWIR) devices operating at near-ambient temperatures.

Dimensional transition of CdxZn1−xTe nanostructures grown on ZnTe layers

The atomic force microscopy images showed that the dimensional structural transformation from CdxZn1−xTe quantum dots (QDs) with Cd mole fractions of 0.5 and 0.6 to CdxZn1−xTe quantum wires occurred at a Cd mole fraction of 0.8. Photoluminescence spectra showed that the excitonic peak corresponding to the transition from the ground electronic subband to the ground heavy-hole band in CdxZn1−xTe∕ZnTe nanostructures shifted to a lower energy with increasing Cd mole fraction. The activation energy of the electrons confined in the CdxZn1−xTe∕ZnTe QDs with a Cd mole fraction of 0.6 was higher than those of electrons in CdxZn1−xTe∕ZnTe nanostructures.

Properties of CdZnO/ZnO Heterostructures for UV Light Emitters

In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces. The paper describes some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon, which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys has been confirmed. We achieved a strong optical emission at RT in the 380 nm to 574 nm spectral range from CdxZn1-xO with various compositions. Dependence of the fundamental optical band gap on the composition of CdxZn1- xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49{plus minus}3 psec. lifetimes, suggesting that composition micro- fluctuations may be present in Cd0.16Zn0.84O film. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells.

Dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures

CdTe nanostructures were grown on CdxZn1−xTe buffer layers by using molecular-beam epitaxy and atomic-layer epitaxy. The atomic force microscopy image showed that uniform CdTe quantum dots were formed on ZnTe buffer layer. Photoluminescence measurements showed that the excitonic peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band in the CdTe∕CdxZn1−xTe nanostructure shifted to a higher energy with increasing Cd mole fraction. The activation energy of the electrons confined in the CdTe∕ZnTe quantum dots was higher than those of electrons in CdTe∕CdxZn1−xTe nanostructures. These results can help improve understanding of the dimensional structural transition in CdTe∕CdxZn1−xTe nanostructures.

Electrical and optical properties of CdxZn1−xTe single crystals for applications as terahertz electro-optic sensors

Cd x Zn 1 − x Te single crystals were grown by using the Bridgman method. Hall-effect measurements showed that the carrier type and carrier concentration of unintentionally CdxZn1−xTe crystals were p type and between approximately 1014 and 1015, respectively. Phototransmission spectra showed that the position of the band edge emission shifted to higher wavelength with increasing Cd mole fraction, and photoluminescence spectra showed that the peak corresponding to the excitons bound to neutral acceptors shifted to lower energy with increasing Cd mole fraction. The electro-optic sensors fabricated utilizing the CdxZn1−xTe single crystals were operated in the terahertz spectrum range. These results can help improve the understanding of CdxZn1−xTe single crystals for applications in terahertz electro-optic sensors.

Correlation between the ordered structure and the valence-band splitting in highly strained CdxZn1−xTe epilayers

Selected-area electron diffraction pattern (SADP) results showed two sets of {1/2 1/2 1/2} superstructure reflections with symmetrical intensities along the [110] axis, and the corresponding high-resolution transmission electron microscopy images indicated a doublet periodicity in the contrast of the {111} lattice planes. Photoluminescence spectra from highly strained CdxZn1−xTe/GaAs heterostructures showed that the valence-band splitting into the heavy hole and the light hole bands occurred as the Cd mole fraction was increased. The valence-band splitting is strongly correlated to the CuPtB-type ordered structure in highly strained heterostructures.

Passivation of HgCdTe p-n diode junction by compositionally graded HgCdTe formed by annealing in a Cd/Hg atmosphere

Cadmium telluride (CdTe) is being widely used for passivating the HgCdTe p-n diode junction. Instead of CdTe, we tried a compositionally graded HgCdTe as a passivation layer that was formed by annealing an HgCdTe p-n junction in a Cd/Hg atmosphere. During annealing, Cd diffuses into HgCdTe from the Cd vapor, while Hg diffuses out from HgCdTe, forming compositionally graded HgCdTe at the surface. The Cd mole fraction at the surface was constant regardless of the annealing temperature in the range of 250-350°C. Capacitance versus voltage (C-V) curves for p-type HgCdTe that were passivated with compositionally graded HgCdTe formed by Cd/Hg annealing at 260°C showed a smaller flat-band voltage than the one passivated by thermally deposited CdTe, indicative of the better quality of the passivation. A long-wave infrared (LWIR) HgCdTe p-n junction diode passivated by compositionally graded HgCdTe showed about a one order of magnitude smaller RdA value than the one passivated by thermally deposited CdTe, confirming the effectiveness of the compositionally graded HgCdTe as a passivant.

Analysis of carrier concentration, lifetime, and electron mobility on p-type HgCdTe

Minority carrier transport characteristics of vacancy-doped p-type HgCdTe such as carrier concentration, lifetime, and mobility are investigated. In the calculation of the carrier concentration two acceptor levels—a donor level and a trap level—were taken into account. The acceptor levels have been described by two models—two independent singly ionized levels and a divalent level with two ionization energies. When each model was examined by calculating electron mobility as a function of temperature, the latter was found to be more accurate. Electron mobility as a function of majority carrier concentration was also presented for both n-type and p-type HgCdTe with 0.225 Cd mole fraction. Steady state electron lifetime was computed assuming the acceptor levels and the trap level would act as Schokley–Read–Hall type recombination centers. The calculated results using the divalent acceptor model were in good agreement with the experimental data.

Applications of thermodynamical modeling in molecular beam epitaxy of CdxHg1-xTe

It is well known that the crystalline quality of CdxHg1-xTe grown by molecular beam epitaxy is critically dependent on the substrate temperature. The optimal growth temperature has been identified immediately below the crossing of the Te-rich phase boundary, that is just below the temperature range where Te precipitation occurs in the layer. It is potentially very useful to be able to predict the optimal temperature and its variation with other growth parameters, but no general guidelines for this can be found in the literature. We have studied experimentally the variation of the optimal growth temperature with Hg flux, Cd mole fraction and growth rate. These results are compared with a thermodynamical model published previously by Gailliard. We find that the modeled position of the phase boundary coincides well with the observed variations in optimal growth temperature for growth on Te-terminated surfaces, within the uncertainties of available thermodynamical constants. We show that the optimal substrate temperature depends mainly on the Hg flux and Cd mole fraction, while the dependence on growth rate can be neglected in practical molecular beam epitaxy conditions. The experimental observation of optimal layer quality at the phase boundary could suggest the existence of an adsorbed layer of Te, acting as a reservoir for Te atoms and reducing the supersaturation of the growth reaction. Simultaneous growth on the (211)B and (100) orientations reveals a clear, although not very large, difference in optimal growth temperature and Cd incorporation, indicating a difference in growth kinetics. This can be accounted for in the thermodynamical model by condensation and evaporation coefficients.

Hg/sub 1-x/Cd/sub x/Te metal-semiconductor-metal (MSM) photodetectors

Metal-semiconductor-metal (MSM) detectors with active layers of Hg/sub 1-x/Cd/sub x/Te (x=0.62-0.74) and electrode spacings of 2, 4, and 6 mu m have been fabricated and characterized. Direct-current measurements have shown a low dark current and high responsivity from 0.15 to 1.5 A/W at 10-V bias. The lowest values of dark current (0.16 mA cm/sup 2/) were obtained for detectors which incorporated an overlayer of CdTe. For detectors without the overlayer, increasing the Cd mole fraction resulted in a decrease in the dark current and a reduction in the 300-nm responsivity. Measurements of frequency response for these detectors show a maximum loss of 8 dB to 20 GHz. These results compare favorably with high-performance MSM detectors based on In/sub 0.53/Ga/sub 0.47/As with a lattice-matched barrier layer of In/sub 0.52/Al/sub 0.48/As.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compositional analysis of HgCdTe epitaxial layers using secondary ion mass spectrometry

The work reported here elaborates on the emission of the secondary Te− ions from secondary ion mass spectrometry (SIMS) analysis of HgCdTe using a Cs+ primary ion beam. Investigations of numerous samples covering a wide compositional range showed excellent linear correlation between negative Te ion yield and Cd mole fraction. These led to the newly developed SIMS analytical method for measuring the changes in the composition of Hg1−xCdxTe with high sensitivity and depth resolution. A physical model describing the emission of secondary Te− ions is proposed. The results can be interpreted by considering the short range interaction between reactive Cs primary beam and the CdTe sublattice via a ‘‘harpoon’’ mechanism. The relevance of this method to the development of Hg1−xCdxTe was demonstrated on metalorganic chemical vapor deposition grown heterostructures.

Novel very sensitive analytical technique for compositional analysis of Hg1−xCdxTe epilayers

This letter reports a novel analytical method using negative secondary ion mass spectrometry (SIMS) of the Te matrix element with a Cs primary ion beam to analyze and determine the composition in HgCdTe epilayers with a high sensitivity (better than Δx of 0.0015) and a depth resolution determined by SIMS. The method is based on matrix-induced ion yield variations in the Te signal observed in SIMS. The correlation between the Te− secondary ion yield and Cd mole fraction is a linear function. The material composition can be calculated by this technique when the Te yield profile is known provided that the composition of any two layers of the structure are known, or the structure contains a CdTe layer, for which case composition of no Hg1−xCdxTe layer is needed.

Specific contact resistance of indium ohmic contacts to n-type Hg1−xCdxTe

The specific contact resistant ρC of In contacts ton-Hg1−xCdxTe has been measured as a function of the Cd mole fraction and annealing treatment of the Hg1−xCdxTe. Transmission line model measurements were performed on the In/Hg1−xCdxTe junctions with the Hg1−xCdxTe doped n type in the range 3.5×1016 to 1.6×1018 cm−3 (77 K). A linear dependence of In ρC on x was obtained, with values of ρC ranging from 2.0×10−5 Ω cm2 at x=0.30 through to 2.6×10−2 Ω cm2 at x=0.68. Hg annealing of the epitaxial Hg0.38Cd0.62Te layers resulted in a decrease in ρC from 5.9×10−3 Ω cm2 unannealed to 1.2×10−3 Ω cm2 after a 300 °C anneal, corresponding to equivalent changes in the resistivity of the Hg1−xCdxTe. Isothermal annealing of the In/n-Hg1−xCdxTe contacts for x=0.30, 0.40, and 0.62 produced an enhanced indiffusion of In but with only a minor reduction in ρC.