Homogeneous Semiconductor Research Articles

Cascade photoconverter of concentrated radiations based on homogenous tunnel structures

A new photoelectric system with a cascade photoconverter (PhC) based on tunnel structures of a homogenous semiconductor of the n +-p-p +(t)n +-p-p +(t)n +-p-p + type with a quantum-mechanical tunneling effect for charge carriers at the p +(t)n + junction and with a radiation concentrator is presented and investigated. The problem on determining the optimal parameters of the structure and limit efficiency of an idealized cascade photoconverter is solved by neglecting the recombination of charge carriers in the thin photoactive layers. The optimal width of cascade photoconverting elements under different radiation spectra can be written via the distribution of the integral flux of the radiation quanta over a distance from an illuminated surface of the semiconductor structure. It is revealed that under light concentration the cascade photoconverters are preferable with respect to ordinary planar photoconverters, since their internal power losses are lower. The limit theoretical and real efficiencies of the examined systems as a function of radiation intensity and of the number of photoconverters in the cascade are determined.

Thermal and Electric Cloaking Effect in Concentric Composite Made of Homogeneous Bulk and Porous Semiconductors

A new material platform is presented to manipulate heat and charge transportation in steady-state conditions. More precisely, we investigate the conceptual realization of a concentric composite made of the same isotropic and homogeneous semiconductor material with layers of different porosity, and show the possibility of a simultaneous cloaking performance in such a device for both heat flux and electric current without disturbing external fields. The background medium in the composite is a porous material with a periodical 3D cubic lattice of spherical hollow pores while the cylindrical shell is made from the same bulk material with zero porosity. A sound analytical expression is found for the volume fraction of the pores at which bi-functional cloaking effect can be realized. To validate our theoretical results, we also demonstrate the temperature and heat flux profiles as well as the voltage and current profiles in numerical simulations for a composite consisting of bulk (cylindrical shell) and porous (background) n-type silicon layers.

New Photoelectric System on the Basis of Cascade Homogeneous Photoconverters and Solar Radiation Concentrators

Ultimate characteristics of the cascade solar cell based on a homogeneous semiconductor with tunneling transitions between the individual elements are researched. The advantage of the use of such structures is the reduction of internal losses, which is especially important when converting concentrated radiation. Ultimate theoretical and physical characteristics of the photocurrent, open circuit voltage and efficiency are found depending on the concentration of radiation and the number of individual elements in the cascade.

Extension of the radiative lifetime of Wannier-Mott excitons in semiconductor nanoclusters

The purpose of the study is to calculate the radiative lifetime of Wannier-Mott excitons in three-dimensional potential wells formed of direct-gap narrow-gap semiconductor nanoclusters in wide-gap semiconductors and assumed to be large compared to the exciton radius. Calculations are carried out for the InAs/GaAs heterosystem. It is shown that, as the nanocluster dimensions are reduced to values on the order of the exciton radius, the exciton radiative lifetime becomes several times longer compared to that in a homogeneous semiconductor. The increase in the radiative lifetime is more pronounced at low temperatures. Thus, it is established that the placement of Wannier-Mott excitons into direct-gap semiconductor nanoclusters, whose dimensions are of the order of the exciton radius, can be used for considerable extension of the exciton radiative lifetime.

Fractional dispersive transport in inhomogeneous organic semiconductors

The study of transport dynamics of charge carriers in homogeneous and inhomogeneous organic semiconductor using the variable order time-fractional drift-diffusion equation (VO-TFDDE) is presented in this paper. The fractional-time derivative operator and spatial derivative operator of the time-fractional drift-diffusion equation are discretized respectively using the implicit difference scheme and the centered difference scheme. Self-consistent Poisson solver was incorporated in the model to solve for the electric potential and the localized electric field that sweeps the charge carriers across the device. The homogeneity of the material is represented by the different values or functions of the fractional derivative order. Diffusion transport dynamics is observed when charge carriers are moving in homogeneous crystalline-like structure. In contrast, dispersive transport dynamics is observed when charge carriers are moving in homogeneous amorphous-like structure. For inhomogeneous amorphous-crystalline-mixed structure, pulse broadening effect is impeded as charge carriers are moving towards the crystalline-like region at the other end of the device. Conversely, pulse broadening effect is getting severe if charge carriers are moving across the device with inhomogeneous crystalline-amorphous-mixed structure. Therefore, in order to achieve diffusive-like transport dynamics that could reduce the pulse broadening effect, homogeneous crystalline-like structure or inhomogeneous amorphous-crystalline-mixed structure is recommended for device fabrication.

Current oscillations as a manifestation of spatio-temporal inhomogeneity of temperature distribution in vanadium dioxide films at semiconductor-metal phase transition

A spatial temperature distribution in VO2 film was first investigated at current oscillations using infrared microscope. The oscillations are revealed to arise from the periodic formation and disappearance of a narrow high-temperature channel in VO2 film. The nature of the oscillations in VO2 films is considered from the standpoint of a well-known phenomenon: spatio-temporal instability of current flow in homogeneous semiconductors. The temperature of the channel significantly exceeds the semiconductor-metal transition temperature being the cause of film destruction and oscillations cessation.

Giant Photogalvanic Effect in Noncentrosymmetric Plasmonic Nanoparticles

Photoelectric properties of metamaterials containing non-centrosymmetric, similarly oriented metallic nanoparticles embedded in a homogeneous semiconductor matrix are theoretically studied. Due to the asymmetric shape of the nanoparticle boundary, photoelectron emission acquires a preferred direction, resulting in a photocurrent flow in that direction when nanoparticles are uniformly illuminated by a homogeneous plane wave. This effect is a direct analogy of the photogalvanic (or bulk photovoltaic) effect known to exist in media with non-centrosymmetric crystal structure, such as doped lithium niobate or bismuth ferrite, but is several orders of magnitude stronger. Termed the giant plasmonic photogalvanic effect, the reported phenomenon is valuable for characterizing photoemission and photoconductive properties of plasmonic nanostructures, and can find many uses for photodetection and photovoltaic applications.

Empiric k·p Hamiltonian calculation of the band-to-band photon absorption in semiconductors

The Empiric k·p Hamiltonian method is usually applied to nanostructured semiconductors. In this paper, it is applied to a homogeneous semiconductor in order to check the adequacy of the method. In this case, the solutions of the diagonalized Hamiltonian, as well as the envelope functions, are plane waves. The procedure is applied to the GaAs and the interband absorption coefficients are calculated. They result in reasonable agreement with the measured values, further supporting the adequacy of the Empiric k·p Hamiltonian method.

Growth of high-homogeneity GaSb:Te crystals for thermophotovoltaic energy converters

Problems concerning the technology of growing highly homogeneous semiconductor crystals are discussed. The dependence between the inhomogeneity of GaSb:Te substrates and the efficiency of thermophotovoltaic converters (TPVCs) fabricated on their base via the diffusion technique is examined. The macro- and microhomogeneities of grown crystals can be substantially increased on account of the theoretically substantiated and experimentally implemented approximation to diffusive mass transfer conditions in the melt. The characteristics of TPVCs on substrates made of crystals with the most homogeneous properties exceed the analogous characteristics of other samples.

Time-dependent transport in amorphous semiconductors: Instability in the field-controlled regime

A time-dependent trap-limited conduction scheme is used to analyze the transient behavior of bistable homogeneous amorphous semiconductors when either the electric field or the current density is prescribed. Numerical outcomes confirm that, for a current-controlled system, the working point is unique and stable in any region of the current-voltage characteristic, while in a field-controlled system the negative differential-resistance region is unstable even in absence of circuit parasitics. The proposed theoretical approach represents a valid tool to grasp the relevant time-dependent features of the Ovonic switching in chalcogenide materials.

Analysis of optical re-modulation by multistage modeling of RSOA

The time-resolved multistage reservoir model well-known for semiconductor optical amplifier (SOA) is extended to analyze the behavior of a bulk homogeneous InP-InGaAsP buried heterostructure reflective semiconductor optical amplifier (RSOA). Parameters for simulation have been extracted from the experimental RSOA characteristics. We have employed the model to explain the steady-state and re-modulation dynamics in the RSOA. Electrical modulation bandwidth and intermodulation distortion in the RSOA have been derived from the model and close agreement is obtained with the reported data. It is found out that the ripples in the upstream output from the RSOA for incomplete modulation erase of downstream modulated data follow Gaussian distribution, which simplifies the calculation of upstream SNR and bit error rate. It is explained in detail that amplitude ripples in the upstream data can be reduced by judicious choice of optical and electrical parameters of the RSOA. In particular, for an average low downstream power level (<−20dBm) a good downstream modulation erase factor about 89% and 23dB extinction ratio in the upstream modulated signal can be achieved.

Preparation of nanostructured PbS thin films as sensing element for NO2 gas

Abstract In this work, we demonstrate that semiconducting films of AIVBVI compounds, in particular, of nanostructured lead sulfide (PbS) which prepared by chemical bath deposition (CBD), can be used as a sensing element for nitrogen dioxide (NO2) gas. The CBD method is versatile, simple in implementation and gives homogeneous semiconductor structures. We have prepared PbS nanocrystalline thin film at different reaction baths and temperatures. In the course of deposition, variable amounts of additives, such as organic substances among them, were introduced into the baths. The energy dispersive analysis (EDX) confirms the chemical composition of PbS films. A current–voltage (I–V) characterization of Pd/nc-PbS/a-SiC:H pSi(100)/Al Schottky diode structures were studied in the presence of NO2 gas. The gas sensing behavior showed that the synthesized PbS nanocrystalline thin films were influenced by NO2 gas at room temperature. The results can be used for developing an experimental sensing element based on chemically deposited nanostructured PbS films which can be applicable in gas sensors.

The stochastic model for ternary and quaternary alloys: Application of the Bernoulli relation to the phonon spectra of mixed crystals

To understand and interpret the experimental data on the phonon spectra of the solid solutions, it is necessary to describe mathematically the non-regular distribution of atoms in their lattices. It appears that such description is possible in case of the strongly stochastically homogenous distribution which requires a great number of atoms and very carefully mixed alloys. These conditions are generally fulfilled in case of high quality homogenous semiconductor solid solutions of the III–V and II–VI semiconductor compounds. In this case, we can use the Bernoulli relation describing probability of the occurrence of one n equivalent event which can be applied, to the probability of finding one from n configurations in the solid solution lattice. The results described in this paper for ternary HgCdTe and GaAsP as well as quaternary ZnCdHgTe can provide an affirmative answer to the question: whether stochastic geometry, e.g., the Bernoulli relation, is enough to describe the observed phonon spectra.

Subcycle control of terahertz waveform polarization using all-optically induced transient metamaterials

Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible using femtosecond laser technology. These terahertz (THz) waves have already been applied in spectroscopy and imaging and can be manipulated using static optical elements such as lenses, polarizers and filters. However, ultrafast modulation of THz radiation is required as well, for instance, in short-range wireless communication or for preparing shaped THz transients for the coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that allows us to manipulate the polarization of THz waveforms with subcycle switch-on times. The polarization-modulated pulses are potentially interesting for controlling elementary motions such as the vibration of crystal lattices, the rotation of molecules and the precession of spins. N. Kamaraju and Tobias Kampfrath from the Fritz Haber Institute and international collaborators have developed a fast metamaterial switch for terahertz radiation. Terahertz waves are important for imaging and sensing applications, but are difficult to manipulate in a dynamic manner. Metamaterials made from on-chip metallic resonators are able to control terahertz waves through a variation in the effective refractive index of the device, but also lack the necessary intrinsic flexibility. To achieve flexible and ultrafast switching, the researchers used an optical excitation pulse that writes spatially varying patterns of excited charge carriers into a homogeneous semiconductor slab. Because the refractive index of the material is different between areas that are illuminated and those that are not, this approach creates a metamaterial capable of controlling terahertz waves and whose properties can be dynamically varied by changing the spatial structure of the optical excitation pulse.

Graded-gap semiconductors and their application

In graded-gap semiconductors spatial dependence of energy gap leads to quasi-electrical embedded layers of different size for holes and electrons, respectively alter their mobility. This results in diffusion-drift mechanism of transfer of alignment-grown carriers, change of coordinate distribution of concentrations, changing of conditions in the surface recombination compared to homogeneous semiconductors. The view of graded-gap semiconductor band structure is determined by the spatial distribution of built-in micro, and the presence of internal and external microfields. The heterogeneity of such fields in space leads to a corresponding different efficiency of band structure that is the result of two mechanisms.Solar cells based graded-gap semiconductors have a much better power characteristics compared to homogeneous causes considerable interest in their use.

Analysis of Effective Plasma Frequency in a Magnetized Extrinsic Photonic Crystal

The effective plasma frequency (EPF) in an extrinsic photonic crystal is theoretically analyzed. The extrinsic PC is a single homogeneous doped semiconductor ( n-GaAs) that is influenced by an externally and periodically applied magnetic field. Based on the calculated photonic band structure, we investigate the magnetic-field dependence of EPF. The results reveal that the EPF will be smaller in the absence of the magnetic field and lowered down when the magnetic field increases. The EPF is shown to be a decreasing function of the filling factor of the magnetized region. Additionally, investigation of the first passband and band gap is also given. The study illustrates that such an extrinsic PC possesses tunable optical properties that are of technical use in semiconductor photonic applications.

On a modified projection scheme of the least-squares method for the modeling of the distribution of minority charge carriers generated by an electron beam in a homogeneous semiconductor material

A modified projection scheme of the least-squares method is considered for modeling of the distribution of minority charge carriers generated by an electron beam in a homogeneous semiconductor material. The order estimate is given, and the condition for the computational stability of the proposed modified projection scheme is obtained in the form of the limiting relation.

Bipolar-driven large linear magnetoresistance in silicon at low magnetic fields

Large linear magnetoresistance (MR) in electron-injected $p$-type silicon at very low magnetic field is observed experimentally at room temperature. The large linear MR is induced in electron-dominated space-charge transport regime, where the magnetic field modulation of electron-to-hole density ratio controls the MR, as indicated by the magnetic field dependence of Hall coefficient in the silicon device. Contrary to the space-charge-induced MR effect in unipolar silicon device, where the large linear MR is inhomogeneity-induced, our results provide a different insight into the mechanism of large linear MR in nonmagnetic semiconductors that is not based on the inhomogeneity model. This approach enables homogeneous semiconductors to exhibit large linear MR at low magnetic fields that until now has only been appearing in semiconductors with strong inhomogeneities.

Plasma-Enhanced Synthesis of Poly(allylamine)-Encapsulated Ruthenium Dye-Sensitized Titania Photocatalysts

Photocatalysis for sustainable hydrogen production from water is becoming one of the core challenges for an efficient conversion of solar energy into a clean and renewable fuel, enabling the growth of a carbon-free hydrogen economy. In dye-sensitized TiO2, performances were obtained to be unstable over time due to weak covalent bonding of the molecule at the surface leading to detachment of the dye from the TiO2 surface and dye self-aggregation, with recombination of charge carriers at the dye/TiO2 interface being the second major factor for efficiency losses. Our strategy for improvement of the stability of the dye/TiO2 catalyst is the encapsulation of the assembly by a plasma-polymerized allylamine layer, providing a chemically and mechanically stable nanoconfinement of the dye in close proximity of the titania surface. The TiO2-layer was synthesized by a plasma-enhanced DC magnetron sputtering process on FTO substrates, resulting in homogeneous porous semiconductor layers. The stability of the PPAAm la...

Transformation of laser light by a semiconductor microcylinder

Theoretical simulation of the transformation of the electromagnetic field by microcavities in the shape of homogeneous semiconductor microcylinders has shown that the morphology of microcavities affects the transformation of the optical radiation in modes of the microlaser in a rather complicated way. We show that, in homogeneous semiconductor microcylinders, there can be realized the regime of single- or two-mode lasing on morphological resonances differing both by the quality factor and by the spatial structure of the electromagnetic field. The region of single- or two-mode lasing is determined by the ratio of the gain and loss factors and by the field magnitudes in the spatial structure of morphological resonances.