We discove the decrement,comparing the Electroreflectance spectroscopy of theforward biased voltage is 0.5V after the photon energy is 1.8eV.We discuss the case from Asymptotic form and the sample.

The electroreflectance (ER) of surface-intrinsic-n+ type doped GaAs has exhibited many Franz-Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy and light-hole transitions. In this work, we can get the dependence of surface electric field on external biased voltage from analyzing the Franz- Keldysh oscillations and the way of fast Fourier transform on condition that weakly modulated field, further more we can get the dependence of effective reduced mass on changed photon energy.

Electroreflectance (ER) spectra of Schottky-barrier Au/n-GaN have been measured at various dc biased voltages (Vbias). The ER spectra have exhibited excitonic signals beneath band-gap energy (Eg ). In addition, Franz-Keldysh oscillations (FKOs) were also observed above Eg. The FKOs come from the former region, and the excitonic signals come from the latter region. When Vbias = 0, they are mixed. As reverse Vbias is increased, they become more separated. Furthermore, strength of surface electric field (Fs) can be deduced from the period of the FKOs. From the plot of versus Vbias, barrier height of 1.2 V and carrier concentration were obtained.

Electroreflectance spectra of AlGaN/AlN/GaN heterostructures were measured at various biased voltages (Vdc). Strengths of the internal electric field in AlGaN (FAlGaN) were evaluated from periods of Franz-Keldysh oscillations (FKOs), which were observed above band-gap energy of AlGaN. The relation between FAlGaN and Vdc exhibits an anomalous behavior, which is different from the previous results of the AlGaN/GaN heterostructure. It agrees with the theoretical result of a Poisson-Schrödinger calculation, which shows that two dimensional electron gas (2DEG) exists not only in quantum well (QW) at AlN/GaN interface, but also in QW at AlGaN/AlN interface. This is also consistent with electron-density distribution obtained by capacitance-voltage measurements. When Vdc becomes more negative, the previous mechanism of depleting 2DEG is through flatting one side of QW. However, it was found that the depletion of 2DEG can also occur when the top of valence band at surface becoming higher than bottom at QW.

Photo reflectance¡]PR¡^ and Contactless electroreflectance¡]CER¡^spectra of Zn and O-faces of a c-plane ZnO bulk have been measured at room temperature, respectively. It was found that the phase of PR is the same as that of CER for the Zn-face and they are inverted for the O-face. This indicates a polarization induced field existing in the c- plane ZnO bulk due to nonzero spontaneous polarization. In addition, a mercury lamp was focused on the ZnO sample in the CER measurements to provide a photovoltaic voltage to reduce electric field in the sample. The CER spectrum with Hg lamp is more blue-shifted and its amplitude is smaller than that without Hg lamp. Hence, the type of transitions was classified as excitonic transition. The A, B, and C excitonic transition energies were obtained by fitting experimental spectra.

Potential modulated attenuated total reflectance (PM-ATR) spectroscopy is a novel technique that makes it possible to sensitively monitor spectroscopic changes in an adsorbed molecular film as a function of applied potential. Here, PM-ATR was used to study charge transfer processes in Prussian blue (PB) and cytochrome c (cyt c) films deposited on indium tin oxide (ITO) electrodes.The electron transfer rate of PB films determined by PM-ATR was found to be in good agreement with the rate determined by conventional cyclic voltammetry, which validates the optical technique.The relationship between molecular orientation and electron transfer in adsorbed cyt c monolayers was investigated using PM-ATR. The electron transfer rate measured using TM polarized light was four-fold greater than that measured using TE polarized light. These data are the first to correlate a distribution of molecular orientations with a distribution of electron transfer rates in a redox-active molecular film.

Understanding interfacial charge injection processes is one of the key factors needed for development of efficient organic electronic devices, such as biosensors and energy conversion systems, since these processes control the electrical characteristics of these devices. Spectroelectrochemical characterization of electron transfer processes occurring at the electrode - electroactive thin film interface has been evaluated to improve our understanding of charge transfer kinetics using a novel form of electroreflectance spectroscopy, potential-modulated attenuated total reflectance (PM-ATR), which makes it possible to sensitively monitor spectroscopic changes in thin films as a function of applied potential.PM-ATR was used to evaluate three different redox-active films deposited on indium tin oxide (ITO) electrodes to investigate: i) the orientation dependence of charge transfer rates of thin films of biomolecules, ii) surface treatment and modification effects on charge transfer kinetics of conducting polymers and, iii) estimation of rates of electron injection and conduction band edge of semiconductor nanocrystalline materials.First, Prussian blue film as a model system was used successfully to examine the PM-ATR technique for determination of the charge transfer rate constant between ITO and a molecular film.Second, an anisotropic and redox active protein film, cytochrome c, was used to probe charge transfer rates with respect to molecular orientation. The electron transfer rate measured using TM polarized light was four-fold greater than that measured using TE polarized light. These data are the first to correlate a distribution of molecular orientations with a distribution of electron transfer rates in a redox-active molecular film.Third, the effects of ITO surface treatment and modification on charge transfer kinetics on a conducting polymer, poly(3,4-ethylenedioxythiophene/)/poly(styrenesulfonate) (PEDOT/PSS), were studied. The apparent interfacial charge transfer rate constant for PEDOT/PSS on ITO has been reported for the first time which cannot be measured otherwise with conventional electrochemistry due to high non-Faradaic background of PEDOT/PSS films.Fourth, PM-ATR enabled characterization of reversible redox processes between submonolayer coverages of surface-tethered, CdSe nanocrystals and ITO for the first time. Optically determined onset potentials for electron injection were used for estimation for the conduction band and valance band energies (ECB and EVB, respectively).

The photoreflectance(PR) and Electroreflectance(ER) of surface-intrinsic-n+ type doped GaAs exhibit many Franz-Keldysh oscillations (FKOs), which enable the electric field (F) to be determined from the technique of the fast Fourier transform (FFT). It is known that F's determined from PR are subjected to photovoltaic effect, but it is difficult to estimate the strength of modulating field (dF) of the pump beam in the PR measurements . Alperovich et. al. have used imaginary part of FFT to determine the strengths of dF's in the ER measurements [V. L. Alperovich, et. al. Appl. Phys . Lett. 71, 2788 (1997)]. Here, we will apply this method to the PR measurements. The dF's thus obtained will be compared with those deduced from photo-voltage measurements. The result shows that the method of Alperovich's can be used to determine the strength of dF in the PR measurements.

It is known that electroreflectance (ER) of surface-intrinsic-n+ type doped GaAs has exhibited many Franz-Keldysh oscillations to enable the application of fast Fourier transform to separate the heavy and light-hole transitions. However each peak still contains two components, which belong to F+ F/2 and F- F/2 respectively, where F is the built-in field and F is the modulating field of applied voltage (Vac). In this work, we have used a larger Vac to modulate the field, and hence the peaks can be further separated. The peak belonging to heavy hole-transition and F- F/2 can be singled out to compare with Airy function-theory.

A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (V_OC) and decrease in short circuit current (J_sc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in V_OC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in J_sc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in J_sc, due primarily to poor overall charge transport in the blend. V_OC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C₇₀ solar cells. Due to graphene’s high transparency, graphene devices have enhanced J_sc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. V_OC is raised by using MoO₃ as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C₇₀ layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C₇₀.