II-VI Compound Semiconductors Research Articles

Synthesis and Tuning of Electronic, Optical, and Photoelectrochemical Properties of Copper Metavanadate Alloys via Alkaline Earth Metal Substitution

Unlike the well-studied and technologically advanced Group III-V and Group II-VI compound semiconductor alloys, alloys of ternary metal oxide semiconductors have only recently begun to receive widespread attention. Here, we describe the effect of alkaline earth metal substitution on the optical, electronic, and photoelectrochemical (PEC) properties of copper metavanadate (CuV2O6). As a host, the Cu-V-O compound family presents a versatile framework to develop such composition-property correlations. Alloy compositions of A0.1Cu0.9V2O6 (A = Mg, Ca) photoanodes were synthesized via a time and energy-efficient solution combustion synthesis (SCS) method. The effect of introducing alkaline earth metals (Mg, Ca) on the crystal structure, microstructure, electronic, and optical properties of copper metavanadates was investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), and Raman spectroscopy. The PXRD, TEM, and Raman spectroscopy data demonstrated the polycrystalline powder samples to be mutually soluble, solid solutions of copper and alkaline earth metal metavanadates and not simple mixtures of these compounds. The DRS data showed a systematic decrease in the optical bandgap with Cu incorporation. These trends were corroborated by electronic band structure calculations. Finally, the PEC properties exhibited a strong dependence on the alloy composition, pointing to possible applicability in solar water splitting, heterogeneous photocatalysis, phosphor lighting/displays, and photovoltaic devices.

Atomic structure, stability, and dissociation of dislocations in cadmium telluride

Dislocation plays a crucial role in many material properties of semiconductors, ranging from plastic deformation to electronic transport. But the dislocation structures and reactions remain controversial in many semiconductors. Here, we systematically examine the dislocation properties of cadmium telluride (CdTe), a prototype of II-VI compound semiconductors, using molecular statics and molecular dynamics simulations with a machine-learning force field. We find that dislocation cores in CdTe are not reconstructed along the dislocation line due to the significant ionic bonding characteristics. Moreover, the undissociated screw dislocation in the shuffle set is more stable than that in the glide set, and the glide dislocation tends to move to the shuffle set, followed by random movement rather than dissociation, arising from its low Peierls stress. For 60° perfect dislocation, it also prefers to locate in the shuffle set, but the 60° glide dislocation is dissociated into a pair of 30° and 90° partial dislocations connected by a stacking fault with a width of ∼9.03 nm at low temperatures through reconfiguring atomic bonds in the core. This work provides an atomic-level understanding of dislocation core properties in CdTe, laying a basis for interpreting the mechanical and electronic performance of CdTe and other II-VI semiconductors.

Transparency and room temperature ferromagnetism in diluted magnetic polycrystalline Zn1−xCrxTe non-oxide II-VI semiconductor compounds

Polycrystalline Zn1−xCrxTe samples for x = 0.00, 0.05, 0.10, 0.15, 0.18, and 0.20 were synthesized by solid-state reaction, and their physical properties were studied for the purpose of identifying a novel non-oxide transparent diluted magnetic semiconductor (DMS). Their transport properties exhibit semiconducting behavior for low Cr concentrations and become conductors for higher Cr concentration concentrations. Ferromagnetic behavior was observed in Cr-doped ZnTe with Curie temperature Tc increasing as Cr concentration increases. At x = 0.20 Cr concentration, the system has Tc above room temperature, indicating an interesting II-VI ferromagnetic semiconductor. Furthermore, optical transparency in the visible light range was found to be 30–85 % for different Cr concentration concentrations. Also, according to the electrical transport measurement data in this study, the resistivity doesn’t exhibit any significant change as the temperature increases for x = 0.20. This consistent resistivity exhibits a possible candidacy for further studies of the sample Zn0.80Cr0.20Te for half-metallic ferromagnetic properties.

Radiation dosimetry in medicine using II-VI semiconductors

For several decades, radiation detection has been implemented using mainly silicon (Si) and germanium (Ge). However, group II-VI compound semiconductors such as cadmium telluride (CdTe), cadmium sulfide (CdS), cadmium selenide (CdSe), zinc selenide (ZnSe), zinc oxide (ZnO), zinc sulfide (ZnS), and zinc telluride (ZnTe) have been recently investigated extensively as alternative radiation detectors. Different studies have explored these semiconductors for radiation detection in diagnostic radiology, radiation oncology, nuclear medicine, and non-medical fields such as astronomy. This article aims at demonstrating that II-VI semiconductors are potential and alternative medical radiation detectors, based on the current studies. II-VI semiconductor dosimetry mechanisms and compliance to desirable dosimetry characteristics (e.g., linearity of detector signals to the radiation doses, precision, radiation hardness, and sensitivity not depending on beam energy, dose, dose rate, and irradiation angle) are presented and discussed. Additionally, the performances of II-VI-based detectors applied for dosimetry in mainly diagnostic radiology, radiation oncology, and nuclear medicine are evaluated. Eventually, the dose, dose rate, and energy ranges applicable to the II-VI-based detectors herein are recapitulated. II-VI semiconductors are promising radiation detectors because they comply to the preferred dosimetric metrics as per the current literature. Nevertheless, there is still need for more research to further (a) characterize II-VI semiconductors for medical radiation dosimetry, and (b) compare II-VI semiconductor detectors to conventional detectors. Additionally, more II-VI semiconductor detectors should be commercialized to increase the availability of these detectors on the medical dosimetry market.

Transparency and Room Temperature Ferromagnetism in Diluted Magnetic Polycrystalline Zn1-Xcrxte Non- Oxide Ii-Vi Semiconductor Compounds

Transparency and Room Temperature Ferromagnetism in Diluted Magnetic Polycrystalline Zn1-Xcrxte Non- Oxide Ii-Vi Semiconductor Compounds

Reflectivity simulation by using transfer matrix method

Efficiency of optoelectronic devices can be enhanced by using Distributed Bragg Reflector (DBR). Distributed Bragg Reflector (DBR) is a multipair layered structure of high and low refractive indices. High reflectivity of DBR is due to constructive interference of reflected light from interfacial layers of DBR. In the present paper, reflectivity of DBR composed of II-VI compound semiconductor is simulated using Transfer Matrix Method (TMM). In this paper, reflectivity of 4, 8, 12, 16 and 20 periods for the incidence angle of 45+ is simulated. Also, the effect of interchange of constituent layers in DBR stack, on reflectivity is simulated.

CdSe0.4Te0.6 Quantum Well-Based Photodetector Toward Imaging Vision Sensors

CCD and CMOS sensors used in imaging vision have a sensitivity range for wavelengths from approximately 0.4 to 0.9μm when silicon (Si) technology is used. Imaging in adverse weather conditions such as fog, rain, and snow is related to imaging density variations in media where the sensed image is better detected in infra-red wavelengths or at even longer wavelengths.Near-wave infra-red (NIR) and shortwave infra-red (SWIR) technologies based on arrays of indium gallium arsenide (InGaAs) photodiodes are typically defined as light sensing in the 0.75μm - 0.9μm and 0.9μm - 1.7μm wavelength ranges, respectively and are used as alternatives for this purpose. Although infra-red and higher spectra can be sensed using NIR and SWIR technologies, there is strong market demand for more efficient, cheaper, faster, and smaller camera sensors. Because of their wide bandgap and easy and relatively inexpensive deposition techniques, II-VI compound semiconductors are at the forefront of materials that are being studied for developing imaging sensors. In this paper, a CdSeTe based photodiode is proposed as an alternative for imaging sensor under adverse conditions. A thermal evaporation technique was used to fabricate a Glass/ITO/CdS/CdSe/CdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> /CdSe/Ag photodetector. A comprehensive analysis of the structural and electrical properties of the device confirmed a quantum well (QW) heterostructure with an 18 nm CdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub> active layer. Practical measurements demonstrated responsivity and specific detectivity for wavelengths between 0.5 and 1.1 μm with power conversion efficiency (PCE) of 10.4%.

INFLUENCE OF NICKEL ON STRUCTURAL AND OPTICAL BEHAVIOUR OF CADMIUM SULPHIDE NANOPARTICLES

Cadmium Sulphide being an important II-VI compound semiconductor has wide applications in various optoelectronic and biolabeling devices. In the present work, undoped and Ni (2%, 4%, 6% and 8%) doped CdS nanoparticles were synthesized in aqueous medium by microwave assisted solvo thermal method using cadmium acetate, nickel chloride and sodium sulphide as precursors. The effect of Ni doping on the Structural, Morphological and Optical Properties of CdS nanoparticles have been studied by X- ray diffraction, SEM, FTIR, PL and UV-Visible spectroscopy. The crystal structure and morphology of the as synthesized nanoparticles have been investigated by X- ray diffraction and scanning electron microscopy (SEM). The XRD results revealed the crystalline nature of the nanoparticles. The average grain size of the nanoparticles was estimated using Scherrer equation which was found to be in the range of 4.2 - 6 nm. The FTIR spectra were also recorded to identify the presence of various functional groups in the synthesized CdS nanoparticles. For the estimation of the optical energy band gap of as synthesized nanoparticles, absorption spectra were recorded using UV- Visible spectrophotometer. The result of UV-Visible spectra revealed the increment of energy band gap in CdS nanoparticles on increasing the concentration of Ni.

Investigation of the n-side structures of II-VI compound semiconductor optical devices on InP substrates

MgSe/ZnCdSe superlattices (SLs) and n-MgZnCdSe quaternaries were investigated as an n-cladding layer material of II-VI compound semiconductor optical devices on InP substrates. n-type samples consisting of n-doped ZnCdSe and n-MgSe/ZnCdSe SL or n-MgZnCdSe layers were fabricated on n-InP substrates by molecular beam epitaxy. From the injection current density versus applied voltage (J-V) characteristics of the samples, it was found that the applied voltage and resistivity of the MgZnCdSe sample were lower than those of the SL sample. This shows that MgZnCdSe is more suitable for the n-cladding layer. On the other hand, Schottky characteristics were observed for both samples, which were ascribed to conduction band hetero-barriers between the n-ZnCdSe and the SL or MgZnCdSe layers. The Schottky characteristics cause high applied voltages of the II-VI optical devices. To solve this problem (i.e., the Schottky characteristics), the incorporation of ZnCdSe/MgZnCdSe graded SLs at the interface between the n-ZnCdSe and MgZnCdSe layers was proposed. From the J-V characteristics, it was confirmed that the Schottky characteristics and applied voltages were reduced by introducing the graded SLs. This shows that the graded SLs are effective for lowering the applied voltages of II-VI optical devices on InP substrates.

Structural and Optical Properties of ZnS Thin Films by SILAR Technique obtained by acetate Precursor

In the group of II-VI compound semiconductor, zinc sulphide (ZnS) has numerous potential applications in optoelectronic devices. In this effort, we have prepared ZnS thin films on glass substrates by Successive Ion Layer Adsorption and Reaction (SILAR) method. Zinc acetate and sodium sulphide were used as cationic and anionic precursors for the films. The crystal structure and surface morphology of the films were studied by X-ray diffractometer (XRD) and Scanning electron microscope (SEM). The deposited ZnS thin films showed polycrystalline with cubic phase. The increment in grain size is an effect of the increase in thickness of the films in accordance with the immersion cycles. The SEM images of the films confirmed that films were uniformly distributed in substrates and pin hole free. The band-gap of the films was estimated.

Photoluminescence and electrical properties from CdO/Cd-nanocrystallites on Cd foil

CdO, which is an n-type II-VI semiconductor compound, plays an increasingly important role in the optoelectronic field. In this paper, CdO micro-rods have been synthesized through using Cd foil as the Cd2+ source and the substrate by a solvothermal method, which are approximately perpendicular to the substrate. Meanwhile, Cd nanocrystallites (nc-Cd) have been fabricated, which are observed to locate at the bottom of CdO micro-rods. Photoluminescence at 10 K has been measured and shown multiple peak emissions. Through analyzing the temperature-dependent photoluminescence and the Varshni formula, the peak located at ~ 487 nm has been confirmed as the emission of band gap. The peaks of ~ 501 nm and ~ 588 nm are disappeared with the increasing of temperature, which are ascribed to the emissions from Cd interstitials to valence band and excitonic transitions, respectively. The peak located at ~ 715 nm is attributed to the emission of surface defects and shows blue shift with increasing temperature. It may be due to the influence of the existence of nc-Cd located at the bottom of CdO micro-rods and/or the interface between CdO and nc-Cd. The current density vs voltage (I-V characterization) from CdO/nc-Cd on Cd foil shows the obvious rectifying effect. Through analyzing the I-V characterization, it is indicated that there are a lot of defects in the CdO to impede the performance. It is believed that through optimizing the preparation process CdO/nc-Cd on Cd foil will be potentially applied in the future optoelectronic field.

The thermo-electric nature of the Debye temperature

The Debye temperature is typically associated with the heat capacity of a solid and the cut-off of the possible lattice vibrations, but not necessarily to the electric conductivity of the material. By investigating III-V and II-VI compound semiconductors, we reveal that the Debye temperature represents a thermo-electric material parameter, connecting the thermal and electronic properties of a solid via a distinct power law.

Heterojunctions Based on II-VI Compound Semiconductor One-Dimensional Nanostructures and Their Optoelectronic Applications

Wide band gap II-VI semiconductor nanostructures have been extensively studied according to their great potentials for optoelectronic applications, while heterojunctions are fundamental elements for modern electronic and optoelectronic devices. Subsequently, a great deal of achievements in construction and optoelectronic applications of heterojunctions based on II-VI compound semiconductor one-dimensional nanostructures have been obtained in the past decade. Herein, we present a review of a series of progress in this field. First, construction strategies towards different types of heterojunctions are reviewed, including core-shell heterojunctions, one-dimensional axial heterojunctions, crossed nanowires heterojunctions, and one-dimensional nanostructure/thin film or Si substrate heterojunctions. Secondly, optoelectronic applications of these constructed heterojunctions, such as photodetectors, solar cells, light emitting diodes, junction field effect transistors, etc., are discussed briefly. This review shows that heterojunctions based on II-VI compound semiconductor 1-D nanostructures have great potential for future optoelectronic applications.

Transient photo-response properties of CdTe thin films synthesized by screen printing technique

Group II-VI compound semiconductors are an important class of opto-electronic materials. They find important applications in detector and photovoltaic cell using both in bulk as well as thin film form. In this reference Cadmium Telluride thin films were prepared by screening printing technique and sintered at different temperatures in open environment. In this paper the comparative detailed analysis of transient photo response of CdTe (Cadmium Telluride) thin films sintered at three different temperatures are presented. For this pulsed photo-response was measured as function of three different intensity of illumination within range of 40–120mW/cm2, along with three different applied bias voltages i.e. 10V, 13V and 15V. From these photo-response analysis, different photo-response parameters e.g. photo-current at termination of illumination (Io), probability of escape of an electron from trap per second (p) and trap depth (Et) have been calculated and compared for all three films along with all three applied bias voltage and intensity of illumination. Estimation of rise and decay time is made from pulsed photo response curves and a comparative study is presented. It shows that photo-current is a strong function of intensity of illumination, sintering temperature and bias voltage. However, different photo-response parameters e.g. probability of escape of an electron from trap per second, trap depth, rise time and decay time are less dependent on sintering temperature. Observed value of long decay time is due to the process induced trap inclusion in the prepared samples which make these films suitable candidate for low processing cost solar cell material.

Study of extending carrier lifetime in ZnTe quantum dots coupled with ZnCdSe quantum well

We demonstrated the growth of a self-assembled type-II ZnTe/ZnSe quantum dot (QD) structure coupled with a type-I Zn0.88Cd0.12Se/ZnSe quantum well (QW) on the (001) GaAs substrate by molecular beam epitaxy (MBE). As the spacer thickness is less than 2nm, the carrier lifetime increasing from 20ns to nearly 200ns was successfully achieved. By utilizing the time-resolved photoluminescence (TRPL) and PL with different excitation power, we identify the PL emission from the coupled QDs consisting of two recombination mechanisms. One is the recombination between electrons in ZnSe barrier and holes confined within ZnTe QDs, and the other is between electrons confined in Zn0.88Cd0.12Se QW and holes confined within ZnTe QDs. According to the band diagram and power-dependent PL, both of the two recombinations reveal the type-II transition. In addition, the second recombination mechanism dominates the whole carrier recombination as the spacer thickness is less than 2nm. A significant extension of carrier lifetime by increasing the electron and hole separation is illustrated in a type-II ZnTe/ZnSe QD structure coupling with a type-I ZnCdSe/ZnSe QW. Current sample structure could be used to increase the quantum efficient of solar cell based on the II-VI compound semiconductors.

(Invited) Zinc Oxide As a Potential Material for Future Electronic Device Applications

Zinc oxide is a II-VI compound semiconductor that is in possession of a unique constellation of interesting material properties. While at the present moment zinc oxide is primarily being used as an electronic material for low-field thin-film transistors, transparent conducting oxide contacts, sensing, and field emitter device applications, recent electron transport work has pointed to the potential of this material for high-field and high-frequency electronic device applications. In this paper, we present some recent results on the steady-state and transient electron transport within zinc oxide, suggesting that this material, having higher peak and saturation electron drift velocities than silicon carbide and gallium nitride, may also be considered as an alternative material to silicon carbide and gallium nitride for high-power and high-frequency field effect transistors. Limits on expectations for zinc oxide based device performance will be projected and contrasted with those expected for silicon carbide and gallium nitride based devices. Upon the basis provided by these results, possible future applications for zinc oxide based devices will be suggested.

The role of Mg dopant on the morphological, structural and optical properties of Mg doped zinc oxide grown through hydrothermal method

ZnO nanorods is a low cost II-VI semiconductor compound with huge potential to be applied in optoelectronic devices i.e. light emitting diodes, solar cells, gas sensor, spintronic devices and lasers. In order to improve the electrical and optical properties, group II, III and IV elements were widely investigated as dopand elements on ZnO. In this work, magnesium (Mg) was doped into ZnO nanorods. Samples were prepared firstly by deposition of undoped ZnO seed layer on indium thin oxide coated glass substrates by ultrasonic spray pyrolysis method and then followed by the growth of ZnO nanorods doped by three different Mg concentrations by hydrothermal method. Based on the morphological, microstructural and optical characterizations results, it is concluded that the increase of magnesium concentration tends to reduce the diameter of ZnO nanorods, increases the bandgap energy and decreases the UV absorption the luminescence in UV and visible range.

Band Structure, Density of States, Structural Phase Transition and Metallization in CdO Under High Pressure

The structural phase transition and metallization of the group II-VI compound semiconductor cadmium oxide (CdO) is investigated through its band structure obtained using the full potential linear muffin-tin orbital (FP-LMTO) method. The ground state properties and band gap values are compared with the experimental results. Under high pressure CdO undergoes a first order structural phase transition from the NaCl structure (B1) to the CsCl structure (B2). The band structure, density of states (DOS) and total energy are computed as a function of reduced volume for both NaCl and CsCl phases. The values of pressure under reduced volume are calculated using Birch-Murnaghan’s equation of state. The pressure corresponding to structural phase transition from NaCl structure (B1) to the CsCl structure (B2) is 0.9 Mbar in CdO. The metallization pressure PM is 1.2 Mbar. DOI: http://dx.doi.org/10.17807/orbital.v0i0.884

Green Stimulated Emission Boosted by Nonradiative Resonant Energy Transfer from Blue Quantum Dots.

Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II-VI semiconductor compound offer the potential to bridge the "green gap" in conventional semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Förster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and cascaded exciton transfer. This would potentially pave the way for an efficient multicolor laser for lighting and display applications.

Density functional theory based tight binding study on theoretical prediction of low-density nanoporous phases ZnO semiconductor materials

Polymorphs or phases - different inorganic solids structures of the same composition usually have widely differing properties and applications, thereby synthesizing or predicting new classes of polymorphs for a certain compound is of great significance and has been gaining considerable interest. Herein, we perform a density functional theory based tight binding (DFTB) study on theoretical prediction of several new phases series of II-VI semiconductor material ZnO nanoporous phases from their bottom-up building blocks. Among these, three phases are reported for the first time, which could greatly expand the family of II- VI compound nanoporous phases. We also show that all these generally can be categorized similarly to the aluminosilicate zeolites inorganic open-framework materials. The hollow cage structure of the corresponding building block ZnkOk (k= 9, 12, 16) is well preserved in all of them, which leads to their low-density nanoporous and high flexibility. Additionally the electronic wide-energy gap of the individual ZnkOk is also retained. Our study reveals that they are all semiconductor materials with a large band gap. Further, this study is likely to be the common for II-VI semiconductor compounds and will be helpful for extending their range of properties and applications.