Thin Semiconductor Layers Research Articles

Mid-infrared semiconductor metamaterials utilizing intersubband transitions in quantum cascade laser structure

We have analyzed metamaterials made of very thin semiconductor layers, whose composition corresponds to GaAs/AlGaAs quantum cascade laser, placed in a very strong external magnetic field (in the range 15–40 T) to maximize the optical output. The role of the magnetic field is to assist in the attainment of sufficient population inversion, which is necessary to effectively manipulate the permittivity. By discretizing the in-plane electron motion, the external magnetic field influences all the relevant relaxation processes in the structure. This enables one to control and tune the value of the complex dielectric permittivity so as to satisfy negative refraction conditions. In addition to illustrating the influence of magnetic field magnitudes on permittivity, we also present the effects of varying electron surface densities and the operating temperature.

Optimising the efficiency of carbazole co-polymer solar-cells by control over the metal cathode electrode

We have explored the effect of a range of different cathode materials on the power conversion efficiency of organic (polymer) solar cells based on a blend of the conjugated polymer poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) with the fullerene acceptor PC70BM. We use a transfer matrix reflectivity model to quantify the optical properties of the cathode and the device structure on its operational efficiency and compare this with the results of experimental measurements. We show that both optical and electrical effects play a role in determining overall device efficiency through their impact on short-circuit current, open circuit voltage and fill-factor. We use our model to demonstrate that devices composed of a thin (60–70nm) active semiconductor layer and a composite cathode composed of a 5nm thick layer of calcium capped by aluminium combine low optical loss and improved charge extraction and optimised power conversion efficiency.

Determination of parameters of thin semiconductor layers on the basis of one-dimensional microwave photon crystals

Determination of parameters of thin semiconductor layers on the basis of one-dimensional microwave photon crystals

Flexible low-voltage organic thin-film transistors and circuits based on C10-DNTT

Using the recently developed organic semiconductor, 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT), we have fabricated organic thin-film transistors and ring oscillators on flexible polymeric substrates. By utilizing a gate dielectric with a small thickness (5.3 nm) and a large capacitance (800 nF cm−2), the transistors can be operated with relatively low voltages of about 2 to 3 V. To improve the charge exchange between the organic semiconductor and the gold source and drain contacts, a thin layer of a non-alkylated organic semiconductor (DNTT) sandwiched between two thin layers of a strong organic dopant (NDP-9) was inserted between the C10-DNTT and the gold contacts. The optimized transistors have a field-effect mobility of 4.3 cm2 V−1 s−1, an on/off current ratio of 108, and a subthreshold swing of 68 mV per decade. The ring oscillators have a signal propagation delay of 5 μs per stage at a supply voltage of 3 V, making these the fastest organic circuits at supply voltages below 7 V reported to date.

Ultrathin functional films of titanium(IV) oxide

AbstractChemistry and physics of thin semiconducting layers of various types are subjects of intense research. Especially when nanotechnology methods such as self-assembly are involved, amazing structural and/or functional properties may appear. Also modern physical methods using variously organized plasma arrangements are able to produce uniform structures with distinctive functionality. In this review, based virtually on our own work, discussions on the preparation, structure, morphology, and function of titanium(IV) oxide nanoscopic thin films are presented. It was shown that structurally and functionally similar titanium(IV) oxide films can be prepared via completely different preparation techniques. Function tests were arranged as “primary”, covering the assessment of the light induced charge separation efficiency, and “secondary”, based on photocatalytic surface oxidations.

Ｘ線トポグラフィの進展 微小角入射Ｘ線トポグラフィによる半導体薄膜の結晶評価

X-ray topographic studies on characterization of semiconductor thin layer were reviewed. Reflection topographs of thin layer were taken by synchrotron radiation X-ray from a storage ring. In these papers, relaxation mechanisms of mismatch in lattice constant between thin layer and substrate were clarified and critical thickness for formation of misfit dislocations was determined for various hetero-epitaxial grown semiconductor thin film. Investigations using grazing incident X-ray topography on semiconductor thin film were also surveyed.

The overall device resistance in organic thin film transistor: Application to octithiophene (8T)

Organic thin film transistors were grown with vapor-deposited polycrystalline octithiophene on silicon oxide insulating layers. The performance of an organic thin film transistor may also be affected by morphological details of the interface between the metal and organic-semiconductor near the source and the drain contact. The most important variable contribution to the contact resistance is due to the charge carrier injection at the metal/OSC interface. We have investigated how traps, at the grain boundaries of an organic semiconductor thin film layer placed between the metal electrode and the active layer can contribute to the contact resistance. In this paper we have compared two models, Meyer–Neldel rule-grain boundary trapping model (MNR–GBT) and variable range hopping model (VRH) to extract the overall device resistance of the organic thin film transistor based on octithiophene. We have reproduced very well the output characteristic I DS( V DS) using the good model of overall device resistance.

Exciton diffusion length in some thermocleavable polythiophenes by the surface photovoltage method

Poly-3-(2-methylhexyloxycarbonyl) dithiophene (P3MHOCT), poly-3-carboxydithiophene (P3CT) and polythiophene (PT) polymers were studied by optical and optoelectronic methods to find diffusion length as one of the important parameters characterizing them as candidates for solar cells. Their important property is that P3MHOCT can serve as a precursor which, after thermal annealing, converts into more rigid and insoluble P3CT and further thermal treatment produces native unsubstituted PT. Ellipsometric measurement yielded data on the thickness of the spin coated layers; absorption coefficients were obtained from transmittance and reflectance spectra. Our method of surface photovoltage extended to thin semiconducting layers was utilized for extraction of diffusion length of photogenerated excitons. Higher diffusion lengths in P3CT and PT materials as compared with that in P3MHOCT can probably be ascribed to the increase of the crystalline fraction. The highest diffusion length was found in P3CT polymer but its large resistivity represents a disadvantage in application in solar cells. Taking into account just these parameters, relatively low resistivity together with quite high diffusion length (13±2nm) predetermine the native polythiophene among the polymers studied in the present work as the best candidate for construction of solar cells.

Photo-induced Mass Transfer in Chalcogenides

This work is devoted to the topical issue – photo-induced formation of surface relief gratings (SRG) in thin layers of chalcogenide vitreous semiconductors (ChVS) without following complicated chemical processing. Holographic recording technique is one of the advanced methods for direct surface relief patterning in light sensitive materials. Because of the high light and electric field gradient it is possible to obtain surface structures directly in time with illumination. This incompletely explored process which is based on the photo-induced plasticity has been discussed in this paper from the side of a light intensity and its electric field on As2S3 and other ChVS.

Transport Properties of CdTe X/$\gamma$-Ray Detectors With $p$-$n$ Junction

Charge transport mechanism in X- and γ-ray detectors based on CdTe diodes with a p-n junction is studied. Shallow p-n junctions were formed in semi-insulating p -like CdTe crystals by laser-induced doping of a thin semiconductor layer with In atoms and, finally, In/CdTe/Au diode structures were fabricated. The energy diagram was developed to explain the reverse I-V characteristics of the diodes particularly increased leakage current. It was shown that the I-V characteristics at low bias voltages were described by the Sah-Noyce-Shockley theory. At higher voltages, an additional increase in leakage current was observed and it was attributed to injection of minority carriers (electrons) from the forward-biased Au/CdTe Schottky contact to the reverse-biased p -n junction (near the In/CdTe contact) through the CdTe crystal. Spectral properties of In/CdTe/Au diode detectors have also been analyzed.

Toward Printable Sensitized Mesoscopic Solar Cells: Light-Harvesting Management with Thin TiO2Films

Sensitized mesoscopic solar cells can exhibit high performance and processability by the use of a thin semiconductor layer in combination with a sensitizer of superior light-harvesting ability. This combination is an important key toward the realization of high-efficiency flexible solar cells using solution-based, rapid roll-to-roll printing processes. To this goal, preparation of a binder-free TiO2 paste is presented as a key process to low-temperature plastic electrode fabrication. For the future challenges of printable sensitized solar cells, this Perspective is focused on the importance of sensitizer choice for thin TiO2 films with regards to optimum light harvesting, showing photovoltaic behaviors of organic dye sensitizers and lead halide perovskite nanocrystals as inorganic sensitizers. Further, the author offers a comparison of the light-harvesting ability between organic dye sensitizers and inorganic quantum dot size sensitizers based on optical extinction coefficients and the surface density of ...

Long-range surface plasmon polaritons at THz frequencies in thin semiconductor layers (Invited Paper)

We present a theoretical investigation of THz long-range surface plasmon polaritons propagating on thin layers of InSb. The metallic behavior of doped semiconductors at THz frequencies allows the excitation of surface plasmon polaritons with propagation and confinement lengths that can be actively controlled. This control is achieved by acting on the free carrier density, which can be realized by changing the temperature of InSb.

Novel resonant cavity-enhanced absorber structures for high-efficiency midinfrared photodetector application

A new dielectric Fabry-Perot cavity is considered for enhanced optical absorption in a thin semiconductor layer embedded within the resonant cavity. In this design, the front (furthest from the illuminated side) mirror is a grating structure with nearly perfect retroreflection. Proof of concept, including semianalytical calculation, and computer-aided design and simulation is performed for application in a midinfrared wavelength band based on a HgCdTe absorbing layer. The results indicate that this new type of cavity meets the combined challenges of significantly increasing the absorption efficiency and reducing the overall complexity and size of the entire device, in comparison to a conventional resonant cavity, in which both mirrors are formed from quarter-wavelength multilayer stacks.

The spray-ILGAR ® (ion layer gas reaction) method for the deposition of thin semiconductor layers: Process and applications for thin film solar cells

The spray Ion Layer Gas Reaction (ILGAR) process starts with ultrasonic nebulisation of the precursor solution, e.g. InCl 3/ethanol for our successful buffer material In 2S 3. In an aerosol assisted chemical vapour deposition (AACVD) type reaction an In(O,OH,Cl) film is deposited on a heated substrate and is subsequently converted to In 2S 3 by H 2S gas. The cycle of these steps is repeated until the required layer thickness is obtained. The robust and reproducible process allows a wide control of composition and morphology. Results of this “spray-ILGAR” method with respect to process, material properties and its application depositing the buffer layer in chalcopyrite solar cells are reviewed. New aspects such as the investigation of the complex chemical mechanism by mass spectrometry, the process acceleration by the addition of H 2S gas to the aerosol, the controlled deposition of ZnS nano-dot films and finally the latest achievements in process up-scaling are also included. Solar cells based on industrial Cu(In,Ga)(S,Se) 2 absorbers (Avancis GmbH) with a Spray-ILGAR In 2S 3 buffer reached 14.7% efficiency (certified) and 15.3% with a ZnS/In 2S 3 bi-layer buffer comparable to reference cells using standard CdS buffer layers deposited by chemical bath deposition (CBD). The quasi-dry, vacuum-free ILGAR method for In 2S 3 buffer layers is well suited for industrial in-line production and is capable of not only replacing the standard buffer material (the toxic CdS) but also the often slow CBD process. A tape coater for 10 cm wide steel tape was constructed. It was shown that In 2S 3 layers could be produced with an indium yield better than 30% and a linear production speed of 1m/min. A roll-to-roll pilot production line for electrochemically deposited Cu(In,Ga)Se 2 with ILGAR buffer is running in industry (CIS-Solartechnik, Hamburg). A 30x30 cm 2 prototype of an ILGAR in-line coater developed by Singulus and Helmholtz Zentrum Berlin is currently being optimised. First 30×30 cm 2 encapsulated modules achieved efficiencies up to 13.0% (CdS buffered reference 13.3%).

Strong interference enhancement of terahertz emission from a photoexcited semiconductor surface

To enhance terahertz emission from an optically excited semiconductor surface, we propose to sandwich a thin (as compared to the terahertz wavelength) semiconductor layer between a dielectric hyperhemispherical lens and metal substrate. The layer is excited through the lens. The substrate provides constructive interference of terahertz waves emitted to the lens directly from the layer and reflected by the substrate. The lens outcouples terahertz radiation into free space. For InAs layer sandwiched between MgO (or sapphire) lens and metal substrate, our theory predicts order of magnitude increase in the terahertz yield as compared to the previous schemes of terahertz emission from semiconductor surfaces.

Iteration methods and algorithms for dielectric-resonator integral equations of the field

Quasi-eigenmodes of open cylindrical and rectangular dielectric resonators (DRs) are determined by the method of iterative solution of the volume integral and integro-differential equations with corresponding functionals. New forms of equations and iteration algorithms for the nonlinear input of the desired complex parameter are proposed. Frequencies and Q-factors of the H01δ and H011 modes of a cylindrical DR and the H mode of a rectangular DR for the uniform and nonuniform cases are obtained numerically. The influence of a thin semiconductor layer located at the ends of the DR and irradiated by high-power laser pulses on the frequencies and Q-factors of the DR modes is examined. It is shown that an up to ten or more percent tuning of resonant frequencies can be reached by transformation of a low conducting state to a high conducting state.

Effect of picosecond strain pulses on thin layers of the ferromagnetic semiconductor (Ga,Mn)(As,P)

The effect of picosecond acoustic strain pulses (ps-ASP) on a thin layer of (Ga,Mn)As co-doped with phosphorus was probed using magneto-optical Kerr effect (MOKE). A transient MOKE signal followed by low amplitude oscillations was evidenced, with a strong dependence on applied magnetic field, temperature and ps-ASP amplitude. Careful interferometric measurement of the layer's thickness variation induced by the ps-ASP allowed us to model very accurately the resulting signal, and interpret it as the strain modulated reflectivity (differing for $\sigma_{\pm}$ probe polarizations), independently from dynamic magnetization effects.

Anisotropy, gyrotropy and dispersion properties of the periodical thin-layer structure of magnetic–semiconductor

In this paper the expression for components of the permittivity and permeability tensors of the periodical magnetic–semiconductor structure for basic directions of external magnetic field and periodicity, with respect to eigenwave propagation direction, is obtained using the thin-layer approach. The periodical structure is formed by the interleaving thin layers of semiconductor and ferromagnetic. For each direction regarded, the expressions for effective permittivity and permeability indices are derived. The structure's symmetry and gyrotropy influence on character of the eigenwave dispersion spectrum is studied. The numerical analysis of dispersion relation and search for the frequency region of refraction index negativity is performed.

Spin injection/detection using an organic-based magnetic semiconductor

The new paradigm of electronics, 'spintronics', promises to extend the functionality of information storage and processing in conventional electronics. The principal spintronics device, the 'spin valve', consists of two magnetic layers decoupled by a spin-transporting spacer, which allows parallel (on) and antiparallel (off) alignment of the magnetizations (spins) of the two magnetic layers. The device resistance then depends on the spin alignment controlled by the external magnetic field. In pursuit of semiconductor spintronics, there have been intensive efforts devoted to develop room-temperature magnetic semiconductors and also to incorporate both inorganic semiconductors and carbon-based materials as the spin-transporting channels. Molecule/organic-based magnets, which allow chemical tuning of electronic and magnetic properties, are a promising new class of magnetic materials for future spintronic applications. Here, we report the realization of an organic-based magnet as an electron spin polarizer in the standard spintronics device geometry. A thin non-magnetic organic semiconductor layer and an epitaxial ferromagnetic oxide film were employed to form a hybrid magnetic tunnel junction. The results demonstrate the spin-polarizing nature of the organic-based magnetic semiconductor, vanadium(TCNE: tetracyanoethylene)(x) (x approximately 2; T(c) approximately 400 K), and its function as a spin injector/detector in hybrid magnetic multilayer devices.

WE‐E‐204B‐04: TiO2 Nanotube Field Emitter as a New Cold Cathode for X‐Ray Generation

Introduction/Purpose: Titanium dioxide (TiO2) nanotubes (NTs) grown by anodization are proposed as a new candidate for a cold cathode X‐ray source. This kind of oxide nanotube can potentially be an excellent choice to overcome the major limitations that exist in relation to carbon nanotubes (CNT). Being an oxide, TiO2 is not affected by residual oxygen present in the tube in contrast to CNTs, which can severely degrade from exposure to oxygen. Second, since TiO2 nanotubes are grown on a titanium (Ti) sheet as the latter oxidizes, a good electrical contact between TiO2 nanotube film and conductive Ti substrate is guaranteed. On the other hand, poor electrical contact between CNT and deposited substrate causes heating effects. The goal of this work was to simulate the effect of the NT parameters on field TiO2 NT emission properties. Methods: The field emission from two sources was considered: 1) from the top of NTs, and 2) the bottom of the NTs which serve as a thin semiconductor layer. Numerical calculations were performed to study the dependence of field emission on NT parameters for each emission mechanism. The calculations were performed in Matlab. Results: It is shown that the current decreased significantly with an increase of NT diameter, while it increased with the height of NTs. The rate of current increase with diameter is stronger for smaller diameter NTs. Regarding field emission from the bottom of the TiO2 layer, this current is strongly dependent on the TiO2 layer thickness and decreases as the latter increases. This current linearly increases with an increase of TiO2 electron charge carrier concentration. Conclusions: Effect of TiO2 NT geometrical and physical parameters on field emission can be simulated and the results can be used as a guide for design purposes.