Non-linear properties of nitride-based nanostructures for optically controlling the speed of light at 1.5 μm

Future bandwidth demand in optical communications requires all-optical devices based on optical non-linear behavior of materials. InN, with a room temperature direct bandgap well below 0.82 eV (1.5 μm) is very attractive for these applications. In this work, we characterize the non-linear optical response and recombination lifetime of the interband transition of InN layers grown on GaN template and Si(1 1 1) by molecular beam epitaxy. Non-linear characterization shows a decrease of the third-order susceptibility, χ⁽³⁾, and an increase of recombination lifetime when decreasing the energy difference between the excitation and the apparent optical band-gap energy of the analysed samples. Taking into account the non-linear characterization, an optically controlled reduction of the speed of light by a factor S=4.2 is obtained for bulk InN at 1.5 μm. The S factor of InN (5 nm)/In_0.7Ga_0.3N (8 nm) multiple quantum well heterostructures at the same operation wavelength is analysed, predicting an increase of this factor of three orders of magnitude. This result would open the possibility of using InN-based heterostructures for all-optical devices applications.     

Parameter-dependent third-order nonlinear susceptibility of parabolic InGaN/GaN quantum dots

The electron states confined in wurtzite In_xGa_1−xN/GaN-strained quantum dots (QDs) have been investigated in the effective-mass approximation by solving the Schrödinger equation, in which parabolic confined potential and strong built-in electric field effect due to the piezoelectricity and spontaneous polarization have been taken into account. The third-order nonlinear susceptibility of the QDs in various directions (both parallel to z direction and vertical to z direction) have been calculated, and the magnitude reaches 10⁻¹⁴ m²/V². It has been shown from the results that the order of the built-in electric field in the strained QD is of MV/cm. Furthermore, the results of how the third-order nonlinear susceptibility depend on the radius R of QDs, the height L of QDs, the In content x of QDs and the relaxation rate Γ₁₀ have been given.     

Polaron effect-dependent third-order optical susceptibility in a ZnS/CdSe quantum dot quantum well

The effect of the electron-phonon interaction on the third-harmonic is investigated theoretically for electrons confined in a core-shell quantum dot. The interactions of electrons with different phonon modes in the core-shell system, including the confined longitudinal optical (LO) and the interface optical (IO) phonon modes, are investigated. We carried a detailed calculation of third-harmonic generation (THG) process on a ZnS/CdSe core-shell quantum dot as a function of pump photon energy with different incident photon energy and under different sizes. The results reveal that the polaron effects are quite important especially around the peak value of the third-order susceptibility. By increasing the size of the quantum dots, the peaks of χ_THG⁽³⁾ will shift to lower energy, and the intensities of the peaks will increase.     

Measurement of second-order nonlinear optical susceptibility of cBN crystal synthesized at high pressure and high temperature

We demonstrate a transverse electro-optical modulator based on a tiny and irregular octahedral wafer of cubic boron nitride (cBN) crystal that is prepared by hexagonal boron nitride at high pressure and high temperature using nitride as the catalyst. A continuous wave semiconductor laser at the wavelength of 650 nm is used as a light source. A novel electrode fabrication is designed, a developed method different from the conventional transverse electro-optical modulator is introduced and the expression of the intensity of output beam is thought over. We obtain the half-wave voltage based on experiments of transverse electro-optical modulation. The second-order nonlinear optical susceptibility χ_ijk⁽²⁾(ω,0)=1.919×10⁻¹² m/V of cBN crystal is calculated by means of the half-wave voltage.     

Band offsets at the (1 0 0)GaSb/Al2O3 interface from internal electron photoemission study

From electron internal photoemission and photoconductivity measurements at the (1 0 0)GaSb/Al₂O₃ interface, the top of the GaSb valence band is found to be 3.05 ± 0.10 eV below the bottom of the Al₂O₃ conduction band. This interface band alignment corresponds to conduction and valence band offsets of 2.3 ± 0.10 eV and 3.05 ± 0.15 eV, respectively, indicating that the valence band in GaSb lies energetically well above the valence band of In_xGa_1−xAs (0 ? x ? 0.53) or InP.     

Defect-engineering in SiC by ion implantation and electron irradiation

Three examples are given, which show that ion implantation and electron irradiation can drastically modify the electrical properties of SiC and SiC-based MOS capacitors. (1) It is demonstrated that sulphur ions (S⁺) implanted into 6H-SiC act as double donors with ground states ranging from 310 to 635 meV below the conduction bandedge. (2) Co-implantation of nitrogen (N⁺) - and silicon (Si⁺) - ions into 4H-SiC leads to a strong deactivation of N donors. Additional experiments with electron (e⁻)-irradiated 4H-SiC samples (E(e⁻) = 200 keV) support the idea that this deactivation is due to the formation of an electrically neutral (N_x-V_C, y)-complex. (3) Implantation of a surface-near Gaussian profile into n-type 4H-SiC followed by a standard oxidation process leads to a strong reduction of the density of interface traps D_it close to the conduction bandedge in n-type 4H-SiC/SiO₂ MOS capacitors.     

Quadratic electro-optic effects and electro-absorption process in InGaN/GaN cylinder quantum dots

We carried a detailed calculation of quadratic electro-optic effects (QEOE) and electro-absorption (EA) process as a function of pump photon energy ?ω in InGaN/GaN cylinder quantum dots. The third-order susceptibility dispersion behaviors of direct current are obtained. It is found that with the increase of the quantum dot (QD) height and radius, the magnitudes of the real part of the quadratic electro-optic susceptibility and the imaginary part of the EA susceptibility increase at the resonant frequency, and its resonant position shifts to the lower energy region. In addition, lower In content induces larger χ⁽³⁾.     

AP-MOVPE of InGaAs on GaAs (0 0 1): Analysis of in situ reflectivity response

We have investigated in situ monitoring of growth rate and refractive index by laser reflectometry during InGaAs on GaAs (0 0 1) substrate growth in atmospheric pressure metalorganic vapour-phase epitaxy (AP-MOVPE). The indium solid composition (x_In^s) was varied by changing the substrate temperature or the indium vapour composition (x_In^v). The refractive index of InGaAs alloys as a function of temperature and composition was quantified and compared which that of GaAs for 632.8 nm wavelength by simulation of experimental reflectivity responses. Composition analyses were carried out by high-resolution X-ray diffraction (HRXRD) and optical absorption (OA). The layers thicknesses were estimated by scanning electron microscopy (SEM) observations. The temperature dependence of InGaAs growth rate has been investigated in the temperature range 420-680 °C using trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH₃) sources. It shows Arrhenius-type behaviour with an apparent activation energy E_a of 0.62 eV (14.26 kcal/mol). This value is close to that determinate in the AP-MOVPE of GaAs.     

Effects of tail states on the conduction mechanisms in silicon carbide thin films with high carbon content

Hydrogenated amorphous silicon carbide (a-SiC_x:H) films of different carbon content (x) were deposited by radio frequency plasma enhanced chemical vapor deposition (PECVD) system. Apart from the X-ray photoelectron spectroscopy (XPS) and UV-Visible transmission analyses, the resistivity measurements between 293 K and 450 K were emphasized to assess the eventual transport mechanisms. The film resistivities are unexpectedly found relatively low, especially for high carbon content. In the frame of exclusive band conduction, the apparent thermal activation energies, evaluated from Arrhenius type plot remain too low compared to half values of the optical gaps.Numerical analyses were undertaken by extending conduction from the band conduction about the mobility edge inside the band gap by including the nearest neighbor hopping (NNH) conduction across the localized tail states. By successfully fitting the formulated conductivity expression to the experimental results, parameters such as tail states distributions, true activation energies to the mobility edge have been retrieved.     

Ferroelectric and leakage current characteristics of (Pb0.72La0.28)Ti0.93O3 thin films prepared by a sol-gel process

(Pb_1 − xLa_x)Ti_1 − x/4O₃(x = 28 mol%, denoted as PLT) thin films were grown on Pt/Ti/SiO₂/Si substrates by using a sol-gel process. The Pt/PLT/Pt film capacitor showed well-saturated hysteresis loops at an applied electric field of 500 kV/cm with spontaneous polarization (P_s), remanent polarization (P_r) and coercive electric field (E_c) values of 9.23 μC/cm², 0.53 μC/cm² and 19.7 kV/cm, respectively. At 100 kHz, the dielectric constant and dissipation factor of the film were 748 and 0.026, respectively. The leakage current density is lower than 1.0 × 10⁻⁷ A/cm²over the electric field range of 0 to 200 kV/cm. And the Pt/PLT interface exist a Schottky emission characteristics.     

A novel proposal for ultra-high optical nonlinearity in GaN/AlGaN spherical centered defect quantum dot (SCDQD)

In this paper a novel idea for enhancement and tunable optical nonlinearity based on spherical centered defect quantum dot (SCDQD) is proposed. The proposed structure is a special quantum dot including a spherical defect inside it. Complete analysis of the proposed structure based on the effective mass equation is done and optical properties (third-order susceptibilities of quadratic electro-optic effect (QEOE) and third harmonic generation (THG) associated with intersubband transition) of the introduced structure using density matrix method are investigated also. Effects of system parameters including defect and dot on energy levels and optical nonlinearity are examined. We observed that the proposed structure has high nonlinear and tunable susceptibilities suitable for implementation of optical active and passive devices. It is shown that the magnitudes of dipole transition matrix element, third-order susceptibilities of THG and QEOE can be increased significantly compared traditional cases (4.5-10 nm, 10⁻¹¹-10⁻⁹ m²/V² and 10⁻¹²-10⁻⁹ m²/V², respectively). Also, the resonance wavelengths are displaced using these parameters that can be used for realization of tunable devices.     

Electronic structure and optical transitions in Sn and SnGe quantum dots in a Si matrix

Self-assembled quantum dots in the Si-Ge-Sn system have attracted research attention as possible direct band gap materials, compatible with Si-based technology, with potential applications in optoelectronics. In this work, the electronic structure near the Γ-point and the interband optical matrix elements of strained Sn and SnGe quantum dots in a Si matrix are calculated using the eight-band k.p method, and the competing L-valley conduction band states were found by the effective mass method. The strain distribution in the dots was found within the continuum mechanical model. The bulk band-structure parameters, required for the k.p or effective mass calculation for Sn were extracted by fitting to the energy band structure calculated by the non-local empirical pseudopotential method (EPM). The calculations show that the self-assembled Sn/Si dots, with sizes between 4 and 12 nm, have indirect interband transition energies (from the size-quantized valence band states at Γ to the conduction band states at L) between 0.8 and 0.4 eV, and direct interband transitions between 2.5 and 2.0 eV, which agrees very well with experimental results. Similar good agreement with experiment was also found for the recently grown SnGe dots on Si substrate, covered by SiO₂. However, neither of these is predicted to be direct band gap materials, in contrast to some earlier expectations.     

Synthesis and nonlinear optical behavior of Ag nanoparticles in PMMA

In this work we have synthesized silver nanoparticles in Poly (methyl methacrylate) (PMMA). This was achieved by polymerizing the mixture of monomer and corresponding metal compound, followed by post-heating treatment. The linear absorption coefficient of the samples was measured using a spectrophotometer, where an absorption peak at 420 nm was observed. This peak grows up and shifts as a function of the concentration of the radical initiator. The linear refractive index was measured using the Fresnel equations and agrees with previous reported results. The nonlinear properties were obtained using the single lens Z-scan method, where the nonlinear absorption coefficient (Δα) was found between 5.5975514 and 17.9483493 cm⁻¹. The nonlinear refractive index coefficient (Δη) was found to be negative and its value oscillates between 12.9099 E-06 and 22.4276 E-06. Finally, the third-order coefficient (χ⁽³⁾) was calculated in the range of 233-787 E-9 esu.     

Study of electrical and optical properties of CNx thin films deposited by reactive magnetron sputtering

The study concerns the CN_x thin films deposited by Low Pressure Hot Target Reactive Magnetron Sputtering (LP-HTRMS). The thin film resistance changes with relative humidity (RH) and optical properties have been studied in the range of 300-653 K. The temperature coefficients of resistivity changes were −2.5%/K at 300 K and −0.5%/K at 500 K. The activation energy of conductivity E_ρ was found to be 0.21 eV in the case of unannealed sample and 0.44 eV when the sample was annealed at 653 K. The CN_x thin films fastness to light was tested in the range of 200-2500 nm by measuring their transmittance. The calculations of absorption carrying out with Tauc formula proved the dominance of indirect optical transitions with Eg energy of 1.04 eV and direct transitions of Eg 2.05 eV. The UV radiation was fully absorbed and light transmission was ca. 90% in the range from visible radiation to far infrared of 1000-2500 nm. The CN_x thin films showed the high resistance sensitivity to RH changes. At T = 300 K resistance changed from 882 M Ω for 36% RH to 386 k Ω for 85% RH. The CN_x thin films susceptibility to humidity was observed in case of both DC and AC current (100 Hz to 10 kHz) measurements. The Si₃N₄ or SiC buffer adhesive layer was incorporated between CN_x film and substrate and its influence on CN_x electrical properties was observed.     

Characterization of UV photodetectors with MgxZn1−xO thin films

In this study the metal-semiconductor-metal (MSM) structure ultraviolet (UV) photodetectors (PDs) based on Mg_xZn_1−xO thin films were fabricated. The Mg_xZn_1−xO thin films were grown on glass substrates by sol-gel method. The results show that the optical absorption has a blue shift and higher transmittance with increasing Mg dopant. The optical band gap were modified by 3.28-3.52 eV, which corresponded to x = 0 and x = 0.16. For a 10 V applied bias, the dark currents of the Mg_xZn_1−xO MSM-PDs were 637 nA (x = 0) to 0.185 nA (x = 0.16) and showed good Schottky contacts. This UV-visible rejection ratio of the Mg_xZn_1−xO UV PDs at x = 0, 0.16, 0.21 and 0.33 were 18.82, 35.36, 40.91 and 42.92, respectively.     

Effect of spin-orbit split-off bands on GaInP/AlGaInP strainedquantum well lasers

We theoretically analyzed the effect of the spin-orbit split-off band on GaInP/AlGaInP strained quantum well lasers using 6×6 Luttinger-Kohn Hamiltonian. Because of the small spin splitting energy of GaInP, we show that the spin-orbit split-off subbands strongly couple with heavy and light hole subbands, and linear gain properties are greatly different from those without the effect of spin-orbit split-off bands. The unstrained quantum well structure is most influenced by the spin-orbit split-off bands, and laser characteristics such as the differential gain and threshold current are degraded. The compressive-strained quantum well has the lowest threshold current, and the tensile-strained quantum well has largest differential gain, which is improved by the effect of the spin-orbit split-off bands     

Influence of Two-Photon Absorption Anisotropy on Terahertz Emission Through Optical Rectification in Zinc-Blende Crystals

We report for the first time on the observation of an angular anisotropy of the THz signal generated by optical rectification in a < 111 > ZnTe crystal. This cubic (zinc-blende) crystal in the <?111 > orientation exhibits both transverse isotropy for optical effects involving the linear χ⁽¹⁾ and nonlinear χ⁽²⁾ susceptibilities. Thus, the observed anisotropy can only be related to χ⁽³⁾ effect, namely two-photon absorption, which leads to the photo-generation of free carriers that absorb the generated THz signal. Two-photon absorption in zinc-blende crystals is known to be due to a spin-orbit interaction between the valence and higher-conduction bands. We perform a couple of measurements that confirm our hypothesis, as well as we fit the recorded data with a simple model. This two-photon absorption effect makes difficult an efficient generation, through optical rectification in <?111 > zinc-blende crystals, of THz beams of any given polarization state by only monitoring the laser pump polarization.     

HfAlOx high-k gate dielectric on SiGe: Interfacial reaction, energy-band alignment, and charge trapping properties

Ultra thin HfAlO_x high-k gate dielectric has been deposited directly on Si_1−xGe_x by RF sputter deposition. The interfacial chemical structure and energy-band discontinuities were studied by using X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectroscopy (TOF-SIMS) and electrical measurements. It is found that the sputtered deposited HfAlO_x gate dielectric on SiGe exhibits excellent electrical properties with low interface state density, hysteresis voltage, and frequency dispersion. The effective valence and conduction band offsets between HfAlO_x (E_g = 6.2 eV) and Si_1−xGe_x (E_g = 1.04 eV) were found to be 3.11 eV and 2.05 eV, respectively. In addition, the charge trapping properties of HfAlO_x/SiGe gate stacks were characterized by constant voltage stressing (CVS).     

Studies on dielectric relaxation and defect generation for reliability assessments in ultrathin high-k gate dielectrics on Ge

In this work, we present the results of dielectric relaxation and defect generation kinetics towards reliability assessments for Zr-based high-k gate dielectrics on p-Ge (1 0 0). Zirconium tetratert butoxide (ZTB) was used as an organometallic source for the deposition of ultra thin (∼14 nm) ZrO₂ films on p-Ge (1 0 0) substrates. It is observed that the presence of an ultra thin lossy GeO_x interfacial layer between the deposited high-k film and the substrate, results in frequency dependent capacitance-voltage (C-V) characteristics and a high interface state density (∼10¹² cm⁻² eV⁻¹). Use of nitrogen engineering to convert the lossy GeO_x interfacial layer to its oxynitride is found to improve the electrical properties. Magnetic resonance studies have been performed to study the chemical nature of electrically active defects responsible for trapping and reliability concerns in high-k/Ge systems. The effect of transient response and dielectric relaxation in nitridation processes has been investigated under high voltage pulse stressing. The stress-induced trap charge density and its spatial distribution are reported. Charge trapping/detrapping of stacked layers under dynamic current stresses was studied under different fluences (−10 mA cm⁻² to −50 mA cm⁻²). Charge trapping characteristics of MIS structures (Al/ZrO₂/GeO_x/Ge and Al/ZrO₂/GeO_xN_y/Ge) have been investigated by applying pulsed unipolar (peak value - 10 V) stress having 50% duty-cycle square voltage wave (1 Hz-10 kHz) to the gate electrode.     

Effect of a silicon nitride buffer layer on the electrical properties of tantalum pentoxide films

Ta₂O₅ films with a buffer layer of silicon nitride of various thicknesses were deposited on Si substrate by reactive sputtering and submitted to annealing at 700 °C in nitrogen atmosphere. The microstructure and the electrical properties of thin films were studied. It was found that with a buffer layer of silicon nitride the electrical properties of Si_xN_y/Ta₂O₅ film can be improved than Ta₂O₅ film. When the thickness of the buffer layer was 3 nm, the Si_xN_y/Ta₂O₅ film has the highest dielectric constant of 27.4 and the lowest leakage current density of 4.61 × 10⁻⁵ A/cm² (at −1 V). For the Si_xN_y (3 nm)/Ta₂O₅ film, the conduction mechanism of leakage current was also analyzed and showed four types of conduction mechanisms at different applied voltages.