Optical Excitation Of Carriers Research Articles

Determination of Optical Constants and Thickness of Nanostructured ZnO Film by Spin Coating Technique

In this report, we have investigated optical constants and thickness of nanostructured ZnO films grown on a glass substrate by sol-gel spin coating technique using zinc acetate as precursor. Optical constants such as complex refractive index ñ and dielectric constant ϵ determined from the transmittance spectrum in the ultraviolet, visible, near infrared (UV-VIS, NIR) region by envelope method. The value of refractive index decreases from 2.34 to 1.86 and extinction coefficient increases from 0.28 to 0.64 with increasing wavelength. The decreasing behavior of refractive index is attributed due to the increase in transmission and decrease in absorption coefficient with increasing wavelength. The film exhibits reasonably high transmittance (>80%) in the visible region. Absorbance coefficient α and film thickness (d) were calculated from the interference of fringes of transmittance spectrum. The band gap and thickness of the film were found 3.02 eV and 275nm, respectively. The thickness of the film measured by envelope method is validated with cross-section micrograph of SEM images which is about 285 nm. The real part of the dielectric function of nanostructured ZnO decreases with increasing wavelength where as the imaginary part of dielectric constant increases with increasing wavelength. The observed high value of refractive index n and real part of dielectric constant ϵ at lower wavelength is due to band edge absorption of carriers. The dispersion relation shows the increase of complex refractive index and dielectric constant at the high frequency regime is due to the discharging of defect levels using optical excitation of carriers in the visible region.

Optical injection of spin current into a zigzag nanoribbon of monolayer MoS2 with antiferromagnetic Kekule distortion

The Kekule pattern of the (anti)ferromagnetic exchange field on monolayer MoS2 can be induced by proximity to the (111) surface of BiFeO3 on both sides. The three-band tight binding model of the MoS2 layer with Kekule patterned exchange field is applied to describe the heterostructures. The tight binding model is justified by the first principle calculations. The magnetization orientations of the substrates control the pattern of the exchange field, which then switches the band structures of the lowest zigzag edge states between being metallic and insulating. The lowest four zigzag edge bands provide conducting channels with a spin-polarized current. Optical excitation of carriers in these bands generates sizable spin and charge currents, which are theoretically modeled by the perturbation solution of the semiconductor Bloch equation. The injected spin currents have multiple resonant peaks at a few frequencies, which can be switched off by rotating the magnetization orientations of the substrates.

Spectral diffusion time scales in InGaN/GaN quantum dots

A detailed temporal analysis of the spectral diffusion phenomenon in single photon emitting InGaN/GaN quantum dots (QDs) is performed via measurements of both time-varying emission spectra and single photon emission intensity autocorrelation times. Excitation dependent phenomena are investigated via the optical excitation of carriers into the GaN barrier material and also directly into InGaN. Excitation into InGaN reveals that the fastest environmental fluctuations occur on timescales as long as a few hundreds of nanoseconds: an order of magnitude longer than previously measured in GaN QDs. Such long time scales may in future allow for the generation of indistinguishable photons in spite of the fact that the experimentally measured linewidths are broad.

The opportunity of using InGaAsN/AlGaAs quantum wells for extended short-wavelength infrared photodetection

We propose and demonstrate a novel concept to address high-performance, extended short wavelength (e-SWIR) photodetectors. Our approach is based on shifting the well-developed GaAs quantum-well infrared photodetector (QWIP) technology to e-SWIR wavelengths. In order to increase the available conduction band offsets (CBOs), we suggest incorporating nitrogen (N) atoms into the quantum well material. The incorporation of N atoms into III-Vs results in dilute-nitride highly mismatched alloys with lower bandgaps and higher CBOs. In our work, we demonstrate CBO values reaching up to ∼1 eV in InGaAsN/AlGaAs QWIPs. This large CBO makes these structures suitable for e-SWIR detection. The large CBO reduces the dark current dramatically and allows efficient detection at room temperature.In our study, we devised two similar InGaAsN/AlGaAs QWIP devices with twofold, 1% and 2% N composition. Based on the measured dark current data, we extracted activation energy barriers of 780 meV and 580 meV for the 1% and 2% N devices, respectively. The dark current and photocurrent spectral response behave in correlation to the change in the barriers’ height. The photocurrent response of the 1% N device peaks at ∼2.25 μm and spans at a spectral range between 1.3 and 2.95 μm. The photocurrent response of the 2% N device is blue shifted to ∼1.42 μm and spans at a spectral range between 1.1 and 2.2 μm. The 2% N device exhibits lower dark currents and a strong photoresponse at room temperature, whereas the 1% N device exhibits a clear response only at temperatures below 150 K.The detection mechanism in the InGaAsN/AlGaAs QWIP devices is based on optical excitation of carriers from the quantum wells into highly-localized nitrogen-related E+ defect-like states located energetically above the conduction band edge. For this reason, the photoresponse is insensitive to the radiation polarization and exhibits low absorption efficiency on the order of 0.15% per quantum well. However, at the same time, the responsivity and photocurrent gain are very high due to the long lifetime of the highly localized excited states.The peak responsivity of the 2% N device is ∼70 A/W at room temperature. Significant responsivity is also available at the response tail at longer wavelengths, i.e., ∼15 and 1 A/W for 1.65 μm—the peak wavelength of the night glow emission and 2 μm—at the e-SWIR wavelengths, respectively. Corresponding detectivity values are ∼1010 and ∼5 × 108 cm·Hz1/2/W at 1.65 μm and 2.0 μm, respectively. These values are similar to those of other developing technologies. In-reach potential detectivity is estimated based on the measured data and can arrive easily to values as high as ∼2 × 1012 and ∼1 × 1011 cm·Hz1/2/W for ∼1.65 μm and 2.0 μm, respectively.The presented characteristics and potential performance indicate that InGaAsN/AlGaAs quantum wells are most suitable for efficient e-SWIR photodetection.

Ultrafast all-optical switching in AlGaAs photonic crystal waveguide interferometers

We have demonstrated ultrafast all-optical switching with photonic crystals integrated into AlGaAs Mach–Zehnder interferometers. The nonlinearity is induced by optical excitation of carriers into one arm of the interferometer, and switching times as short as 3 ps are achieved by surface recombination at the air holes in the photonic crystal. The fast recombination times and high nonlinearities of the AlGaAs material make this design suitable for high speed all-optical switching applications.

Fullband Particle-Based Simulation of High-Field Transient Transport in III–V Semiconductors

Motivated by recent experimental measurements (A. Leitenstorfer et al., 2000. Physical Review B 61(24): 16642–16652), this work presents the transient analysis of photogenerated electron-hole pairs in GaAs and InP pin diodes (S. M. Sze, 1981. Physics of Semiconductor Devices, 2nd edn., John Wiley) using a fullband particle-based simulator (M. Saraniti and S. Goodnick, 2000. IEEE Transactions on Electron Devices 47(10): 1909–1915). The fullband simulation tool is based on a particle-based technique that has been developed to reduce the computational time required for modeling charge transport phenomena in semiconductors. Excellent agreement is found between experiment and simulation of transient acceleration and velocity overshoot in GaAs and InP pin diodes due the femto-second optical excitation of carriers.

Resonatorless Optical Bistability Based on Increasing Nonlinear Absorption

AbstractA new type of optical bistability is presented. Instead of a resonator the intrinsic feedback between the optical excitation of carriers and their recombination is used. Optical bistability appears, if the absorption coefficient increases with growing carrier density. Both propagation effects and carrier diffusion are considered. Analytical model calculations are carried out for an abrupt behaviour of the absorption coefficient. The results explain the hysteresis in the transmittivity of CdS, recently observed.

Surface photovoltage spectroscopy and thermally stimulated contact potential difference studies of real ZnS surfaces

Surface photovoltage spectroscopy and thermally stimulated contact potential difference measurements are reported for real ZnS surfaces. The measurements allow the determination of the surface states existing via two different processes, i.e. optical excitation of carriers into and from the existing surface states. Changing the surface condition by annealing at 350°C produces different electronic features which are attributed to different bondings at surface vacancy sites. Furthermore, annealing changes the polarity of the surface photovoltage.

Measurement of lateral variation of hole diffusion lengths in GaAs

A technique for making rapidly scanned, high spatial resolution minority-carrier diffusion length measurements in direct band-gap semiconductor thin films is described. Using optical excitation of carriers through a semitransparent Schottky barrier, the technique is nondestructive, and at the highest resolution of 1 μm, permits measurements over a 100-μm square area to be made in a matter of minutes. The technique is applied to the investigation of grain boundaries in large- grained polycrystalline n-type GaAs films and is illustrated by measurements over a region containing an electrically active grain boundary. Application of the method for assessment of uniformity of minority-carrier properties of epitaxial layers is feasible.

Photovoltage inversion effect and its application to semiconductor surface studies: CdS

Photovoltage inversion, a new surface photoelectric effect, was discovered in CdS, and was manifested as an increase in surface barrier height under illumination of sub-bandgap energy. A small, but measurable, decrease in surface conductance was found to be associated with such barrier height increase; the time constants of the barrier and conductance changes were found to be identical and inversely proportional to the intensity of illumination. These findings could not be explained on the basis of the classical photovoltage theories, but were found consistent with a model involving photo-stimulated population of surface states. Photovoltage inversion, in conjunction with surface photovoltage spectroscopy 1), allows the study of surface states via two different processes, i.e., optical excitation of carriers into and from existing surface states. This approach is particularly effective in the case of wide bandgap materials, where standard methods are not readily applicable. Experimental results on the surface states of the basal plane of CdS are presented and discussed in the light of a photovoltage inversion model.

Persistent Internal Polarization of Insulators induced by Electron Bombardment

EARLIER investigations of the effect known as persistent internal polarization of insulators have been concerned principally with optical excitation of carriers which travel through the insulator under the influence of an applied electric bias field and afterwards become trapped1,2. Polarization phenomena have been reported in investigations of electron bombardment induced conductivity in insulators, although no detailed investigation has been published3. Investigations of persistent internal polarization induced by pulses of electron bombardment are being carried out in these Laboratories. Measurements have been made on targets consisting of a layer of insulator 0.3–1.0µ thick, produced by vacuum evaporation, sandwiched between aluminium contacts 0.1µ thick.