We demonstrate how to tailor the size-dependent enhanced transmission and absorption of 1D subwavelength semiconductor-based metamaterial high contrast gratings. We focus on 3 spectral regimes generic for most semiconductors, where the refractive index (n) and extinction coefficient (k) of the semiconducting material satisfy the following conditions: n >> k, n ~ k and n < k. We show that the transmission into such structures can be enhanced by reducing the bar width, increasing the grating period, or tapering the grating sidewalls. Moreover, thanks to the slow-light phenomenon, the absorption of the grating can be enhanced as compared to bulk semiconductors.
Understanding the interactions of light with periodic arrays of Si fins is of the utmost importance for nanoelectronics, where laser light is used for the fabrication and metrology of fin field effect transistors (finFETs). However, due to their nanoscale dimensions and periodic arrangement, these structures exhibit complex photonic properties. In this work, we explain theoretically how the reflectance of semi-infinite periodic arrays of Si fins embedded in SiO2 varies with the fin pitch (i.e. spatial periodicity) and fin width. The results are corroborated with band structure calculations, showing that the spectra of both polarizations, parallel (TE) and perpendicular (TM) to the fin sidewalls, can be understood based on the excitation of one waveguide mode. First, we demonstrate that increasing the pitch decreases the reflectance from the arrays, for both polarizations. Moreover, TE spectra resemble that of bulk Si and are much higher as compared to TM, which are similar to the bulk SiO2 spectrum. The difference is attributed to the fact that TE mode is mostly confined inside the fin, whereas TM is spread in the SiO2. Subsequently, we show that the reflectance from the arrays increases as a function of the fin width. TE reflectances are again mostly sensitive towards Si dispersion and higher than TM counterparts. Interestingly, for TM illumination a transition from the SiO2- to Si-like spectra is observed for the fins of increasing width. The transition is caused by the change in the fraction of the electric field propagating inside the fin. The developed insight will facilitate design, fabrication and metrology of optoelectronic, photovoltaic and nanoelectronic devices.
Thin (50-90 m) non-doped and doped (by Al atoms) Mg2Sn0.6Si0.4 and Mg2Sn0.4Si0.6films with roughness of 1.9-3.7 nm have been grown by multiple deposition and single annealing at 150 °C of multilayers formed by repetition deposition of three-layers (Si-Sn-Mg) on Si(111) p-type wafers with 45 Ω-cm resistivity. Transmission electron microscopy has shown that the first forming layer is an epitaxial layer of hex-Mg2Sn(300) on Si(111) substrate with thickness not more than 5-7 nm. Epitaxial relationships:hex-Mg2Sn(300)|| Si(111), hex-Mg2Sn[001]|| Si[-112] and hex-Mg2Sn[030]||Si[110] have been found for the epitaxial layer. But inclusions of cub-Mg2Si were also observed inside hex-Mg2Sn layer. It was found that the remaining part of the film thickness is in amorphous state and has a layered distribution of major elements: Mg, Sn and Mg without exact chemical composition. It was established by optical spectroscopy data that both type films are semiconductor with undispersed region lower 0.18 eV with no= 3.59 ± 0.01, but only two direct interband transitions with energies 0.75-0.76 eV and 1.2 eV have been determined. The last interband transition has been confirmed by photoreflectance data at room temperature. Fourier transmittance spectroscopy and Raman spectroscopy data have established the formation of stannide, silicide and ternary compositions.
We present result of optical studies on InAs/GaIn(As)Sb/InAs type II quantum wells predicted for the active region in interband cascade lasers, and further for laser-based gas sensors operating at room temperature in a broad wavelength range of mid infrared. Using photoreflectance spectroscopy supported by electronic structure calculations we determine the oscillator strength of the fundamental optical transition in structures with GaIn(As)Sb material of various compositions hole confinement layer. We show that incorporation of arsenic into this layer can affect several crucial properties significantly like transition wavelength and its probability, but also the structural material quality affecting the radiative efficiency. Also, by using photoluminescence we investigate one of the crucial parameters for the performance of interband cascade lasers, the spectral emission width of type II quantum wells constituting the laser active region.
There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb
substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade
lasers. There are presented their crucial optical properties and the related current challenges with respect to the device
performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap
discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used
(photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level
calculations based on effective mass approximation and kp theory. Eventually, the potential for further material
optimization and prospects for the improved device performances are also discussed.
The nowadays quite indispensable enhancement of PV conversion efficiency cannot be obtained without new
mechanisms. The most useful of these mechanisms have to appear in the front face of the device, i.e. in the emitter, so
as to allow exploitation of the energetic photons of the solar spectrum. Such an improvement can be realized through a
multistage PV conversion starting by primary generation (photon absorption) followed by secondary generations (hot
carrier collisions with low-energy generation centers). This cascade-like process is possible, for example, in
multiinterface devices containing several emitter strata. Some of these strata assume the primary free-carrier generation
while others do the secondary free-carrier generation.
In this work we report investigations of new mechanisms based on I(V) curves measured on test samples with
different multiinterface architectures, electronic passivations, front grids, collecting electrodes and so on. The
measurements have been performed under a variable intensity incident light beam conserving always its spectral (solar)
composition, except for analogous measurement cycle without a UV component. The same beam intensities with a
filtered UV component complemented these investigations. The measurements have been compared with those of a
weak excitation from a typical halogen lamp (relatively stable flux without a UV component). The test structures show
a clear improvement of the PV conversion in the UV range induced by impact ionization within the superficial
nanostratum.
We report on the modulation spectroscopy investigation (in a form of photoreflectance (PR)) of self-assembled InAs/GaInAsP quantum dash structures grown by gas source molecular beam epitaxy on InP (100) substrates and designed for laser applications at 1.55 μm wavelength range in two different architectures: dash-in-a-barrier and dash-in-a-well. The dashes parameters have been determined by cross-sectional and plane-view transmission electron microscopy to be of typical height and width of about 2 per 20 nm and length from 50 to more than 200 nm, depending on the growth conditions. The part of the PR spectra related to the quantum dash layer has revealed several well distinguished transitions with the energy separation between the ground state and first excited state ones of about 150 meV independently of the length of the dashes for the dash-in-a-barrier and twice less for dash-in-a-well-designs, respectively. Our theoretical analysis based on effective mass approximation calculations has shown that all the higher order state transitions are related to the InAs wetting layer quantum well, where its parameters, as WL thickness and conduction band offset ratio, have been estimated on the base of agreement with the experimental data. The PR-based optical properties have found confirmation in the PL thermal quenching and device characteristics.
We have investigated structures grown on InP substrate by gas source molecular beam epitaxy and composed of InAs/In0.53Ga0.23Al0.24As quantum dash (QDash) layer and In0.53Ga0.47As/In0.53Ga0.23Al0.24As quantum well (QW) separated by a thin barrier allowing the carriers tunneling between these parts. The growth conditions for QDashes have been optimized to achieve their ground state emission at 1.55 ?m suitable for telecommunication laser applications, whereas the well parameters have been chosen to get the QW ground state levels above the dash ones, i.e. a construction allowing the injection of carriers from the QW layer (injector) into the dashes (emitter). Basing on photoreflectance spectra, supported by effective mass calculations, we have studied the electronic structure of the system, determining the energy levels and band offsets. The QW-QDash energy transfer has been probed by temperature dependent photoluminescence with changed excitation wavelength, where an efficient tunneling has been evidenced directly in the photoluminescence excitation spectra up to 130 K.
The purpose of this work was to investigate the optical properties of GaN nanocrystals (GaN-nc) doped by Eu3+
ions. The total photoluminescence excitation spectroscopy (TPLE) (where the full emission spectra were recorded for the
different excitation wavelengths) has been performed to investigate the absorption properties and the energy transfer
between GaN-nc and Eu3+ ions.
Nanosized GaN:1%Eu3+-nc with the average grain sizes of ~ 8 nm have been synthesized as a powder by the
combustion method with some modifications. In PL spectra the strong emission lines related to Eu3+ ions have been
observed with the most intense line at ~614 nm. Additionally, the broad yellow/red emission band related to GaN
surface/defect states has been also observed.
In recorded TPLE spectra an efficient excitation energy transfer from GaN-nc to Eu3+ has been observed. It has
been shown that there are three channels for the excitation of Eu3+ ions: (i) through quantized states in GaN nanocrystals,
(ii) through defect-related states in the GaN, (iii) and directly through the excited states of Eu+3 ions. It has been found that for investigated GaN powder the most efficient is the excitation of Eu+3 ions through quantized states in GaN
nanocrystals.
Two dimensional hole gas (2DHG) created at modulated doped Al1- xGaxAs/GaAs heterostructures was studied by optical methods. Samples of different 2DHG concentration p=9.8*1011 cm$min2 were examined by means of photo luminescence (PL) and photo luminescence excitation (PLE) measurements under magnetic field up to 8T in Faraday configuration. In PL spectra two groups of lines are observed: a broad line at lower energy so called H-band and at higher energy exciton related lines. In magnetic field H-band splits and shifts linearly following Landau level quantization whereas higher energy lines exhibit diamagnetic shift. A strong absorption of H-band in PLE spectra is observed. We propose consistent theoretical model which explains both the nature of H-band and the formation of excitons on 2DHG states.
Photoreflectance (PR) spectroscopy has been applied to the investigation of Si (delta) -doped AlxGa1-xAs layers grown by metal-organic vapor phase epitaxy on GaAs substrates. Measurements have been carried out on samples with aluminum content of 0,0.35 and 1. The observation of Franz-Keldysh oscillations (FKO) in a number of more than 10 (in the best case) and application of the fast Fourier transformation has allowed us to determine the internal electric field with high accuracy. Thus, the potential barrier between surface and (delta) -doped region has been estimated. Finally, the contribution of heavy and light hole related transitions to the FKO has been resolved.
MBE-grown In (formula available in paper)As/GaAs double quantum dot structure has been investigated by photo reflectance (PR) spectroscopy. Features related to all the relevant portions of the sample including the quantum dots and wetting layers have been observed at 10 K. The PR results have been supported by the standard high-excitation photo luminescence (PL) measurements revealing excited state transition due to the effect of higher level filling. The experimental transition energies have been compared to the results of the effective mass approximation calculations for double quantum wells and lens-shaped double quantum dots. The evidence of the dot-dot and wetting layer well-well interaction has been found.
The interband transitions in thin epitaxial films grown by metalorganic chemical vapour deposition (MOCVD) have been studied as a function of temperature (1 0-300K) by photoluminescence (PL) and reflectance measurements. In photoluminescence at low temperature bound and free excitons are observed. Temperature dependence of free exciton energies have been studied in reflectance measurement. In the reflectance spectra excitonic interband transitions f9V.4'7C (exciton FX(A)), f7V (upper band)JT7C (exciton FX(B)) and F7"(lower band)I7C (exciton FX(C)) were observed. From temperature dependence of the excitonic spectra the energy gap dependence is determined using both the Varshni E(T)=E(O)-aT2/(T0+T) and Bosse-Einstein E(T)=E(O)-A/[exp(3/T)-1] expressions.
The room temperature photoreflectance spectroscopy (PR) was used to investigate AlxGa1-xAs/GaAs structures. All structures were grown by molecular beam epitaxy technique. The aluminum content changes from x equals 0.11 to x equals 0.53. The alloy composition was determined from X-ray diffraction measurements. The dependence of the direct band gap energy on the Al content was under investigations. To obtain the direct band gap energy, the measured PR spectra were analyzed using Aspnes lineshape procedure. The determined Eg(AlGaAs) dependence on x for doped Al1-xGaxAs layers was compared to previous results.
Titanium dioxide films doped with erbium were prepared by sol-gel technique from Ti(OC2H5)4 or Ti(OC4H9)4 precursors. Room-temperature luminescence at 1.53 micrometers associated with 4I13/2 - 4I15/2 transitions of Er3+ ions in TiO2 films derived from the first precursor has been detected. The films have a refractive index about 2.4 and may be used in the planar erbium doped waveguides.
The amorphous Zn3P2 thin films were elaborated by direct evaporation in vacuum. The temperature dependence of the energy gap in the range T equals 10 divided by 300 K was determined from optical measurements. The polycrystalline samples were obtained by annealing in vacuum. The ordering effects were studied in transmission and reflectivity measurements in wide energy region E equals 1.38 divided by 5.0 eV.
Mariusz Ciorga, Krzysztof Jezierski, Leszek Bryja, Jan Misiewicz, Regina Paszkiewicz, Ryszard Korbutowicz, Marek Panek, Bogdan Paszkiewicz, Marek Tlaczala, Ib Trabjerg
Photoluminescence and reflectance studies of MOVPE grown GaN samples were performed. From reflectance measurements optical constants were calculated by means of Kramers-Kronig analysis in the energy region 0 divided by 6 eV.
A double quantum well molecular beam epitaxy grown GaAs/AlxGa1-xAs structure was studied. To investigate the coupling effects in such a system 1 monolayer thick AlAs barrier was inserted at the center of the GaAs/AlxGa1-xAs single well. Due to the strong coupling between wells each confined state splits into two: symmetric and antisymmetric ones. In the room temperature photoreflectance spectrum features related to transitions between all these states were observed. Theoretical considerations based on the envelope function approximation were performed to obtain the energies of expected optical transitions. An excellent agreement between experiment and theory was obtained.
Partially strained Si1-xGex epilayers with germanium composition up to 25% have been studied by room temperature photoreflectance spectroscopy. Observed transitions were identified as E1 and E0' direct transitions at Si1-xGex structure. The E1 transition energy dependence on alloy composition shows that the studied Si1-xGex epilayers are partially relaxed and the degree of relaxation differ from one structure to another. For the germanium content close to 10% the crossing between E1 and E0' optical transitions was observed.
Cd0.9Mn0.1Se0.3Te0.7 photoluminescence and reflectivity were studied in magnetic field up to 5T in T equals 4.2 K. In luminescence spectra two lines were observed. Comparing results to previous studies in Cd1-xMnxTe and Cd1-xMnxSe they were identified as the free exciton X and exciton bound to neutral acceptor A0X. Influence of high alloy disorder on bound magnetic polaron effect was considered.
Photoreflectance spectroscopy as the nondestructive, contactless, room temperature method to investigate semiconductor layers, interfaces, structures and devices is presented. Principles of the method are described. Application to the investigations of the III-V and II-VI compounds structures, including quantum wells, heterojunction bipolar transistors, high electron mobility transistors, vertical cavity surface emitting lasers and quantum dots arrays are shown.
We have measured the photoreflectance (PR) spectra at room and liquid nitrogen temperatures of two MBE grown GaAs/AlGaAs structures. The first one is HEMT type system with buried 10 periods of 2.5 micrometers GaAs/2.5nm AlGaAs superlattice. Oscillations-like signal associated with this SL have been observed and detailed analyzed. The second investigated structure is the sequence of 10 different quantum wells. Transitions in almost all wells and those associated with above barrier states have been observed. The experimental transitions are well described in terms of envelope function model.
Thick intentionally undoped GaAs epitaxial layers grown by LPE from Ga-Bi solution with different contents of Bi in liquid solvent were studied by photoluminescence at temperature T equals 2K. The dependence of photoluminescence spectra on contents of Bi in solution was analyzed.
InGaAs/GaAs quantum wells have been grown in MOCVD system equipped with horizontal Aixtron reactor. Photoreflectance spectra have shown, even at room temperature, sharp heavy and light holes excitonic transitions in quantum wells. The obtained splitting energies have been compared with values derived from theoretical considerations using envelope function model including lattice mismatch-related stress. Heavy and light holes transitions have been identified as excitonic transitions type I and type II, respectively. Photoluminescence measurements have been also done. For quantum wells, transitions between first heavy hole and first electron subbands have been observed. Additionally the temperature dependence of observed transitions have been performed.
The photoluminescence (PL) spectra of MOCVD-grown GaAs samples have been measured at 10K, and for the most representative sample, PL spectra were recorded from 10 to 150K. Besides of the well-established transitions, a hitherto unreported peak was seen at 1.454 eV. The identification of a line at 1.408 eV is discussed.
Photoreflectance technique is used to investigate two types of GaAs/(AlGa)As heterostructures with 2D hole systems: triangular and rectangular quantum wells. The attention is focused on the few extra features visible above FKO oscillations. This extra features are found as a result of the 2D holes presence in our structures.
Photoreflectance spectroscopy as the nondestructive, contactless, room temperature method to investigate semiconductor layers, interfaces, structures and devices is presented. Principles of the method are described. Application to the investigations of the III-V compounds structures, including heterojunction bipolar transistors, high electron mobility transistors vertical cavity surface emitting lasers and quantum dots arrays are shown.
Experimental set-up has been established by using high resolution monochromator as well as He-Ne and Ar multiline lasers. Epitaxial, undoped and doped (Si and Zn) GaAs and GaAlAs layers as well as heterostructures of GaAs/GaAlAs have been grown in atmospheric pressure vertical MOCVD system. Room temperature photoreflectance (PR) have been applied to characterize layers, heterostructures as well as the multiple quantum wells. The surface and interface related PR have been studied by means of Kramers-Kronig analysis. Decomposition of PR spectrum into the spectra connected with surface region and with the interface has been proposed. Modulus of the complex photoreflectance gives us the critical point energy, whereas the phase of this function can be used for a carrier concentration topography.
Interband optical transitions and optical lattice modes are presented and discussed in terms of real tetragonal symmetry of Zn3P2 crystal. Data of defect-related optical transitions in this compound are also shown.
Mg-Zn3P2 structures are examined as solar energy converter and broad-range photodetector. A distinct photodichroism observed for junctions prepared on oriented single crystal is applied in light polarization step indicator.
A method of investigation of the optical properties of anisotropic materials using modulation of the light polarization is presented. Measuring set-up was constructed and used to examine the optical properties of interesting tetragonal semiconducting Zn3P2 compound.
Fundamental optical and electronic parameters of Zn3P2 are presented. Mg-Zn3P2 structures are examined as a solar energy converter and broad-range photodetector. A distinct photodichroism, observed for junctions prepared on oriented single crystal, is applied in light polarization step indicator.
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