Sub-bandgap Region Research Articles

Photoacoustic spectroscopy of sprayed CuGa xIn 1 −xSe 2 thin films

CuGa x In 1 −x Se 2 thin films have been produced using spray-pyrolysis. The layers were deposited onto Mo-coated soda-lime glass at a substrate temperature of 623 K, and the gallium content of the layers varied. The layers were investigated using photoacoustic spectroscopy. The normalised photoacoustic spectra of CuGa x In 1− x Se 2 films with x=0, 0.2, 0.4 and 0.5 showed a sudden fall in the photoacoustic signal at photon energies of 0.99, 1.15, 1.27 and 1.36 eV , respectively, which correspond to the respective direct energy band gaps of these films. The photoacoustic spectra also showed peaks at 0.75, 0.78, 0.82, 0.86 and 0.88 eV, respectively, in the sub-band gap region, which have been attributed to the defect states within these films.

Surface photovoltage spectroscopy of n-n+ and p-n+ AlGaAs/GaAs heterojunctions

A comparative study of n-n+ and p-n+ semiconductor heterojunctions has been done using the surface photovoltage spectroscopy (SPS) in the chopped light geometry. Heterojunctions n-Al0.4Ga0.6As/n+ GaAs and p-Al0.37Ga0.67As/n+ GaAs and substrate n+ doped GaAs have been studied in the wavelength range 600–1000 nm. A sharp decrease in SPS at the band-gap energies of AlGaAs and GaAs with a broad peak in the subband-gap region of GaAs has been observed. The magnitude of surface photovoltage (SPV), for a constant photon flux for n-n+ samples, is less by more than two orders of magnitude than that for p-n+ samples in the wavelength range 645–870 nm. Changes in the dipole moments due to the redistribution of excess carriers in the space-charge regions are in opposite directions of the n-n+ heterojunctions giving less SPV, while for p-n+ heterojunction, the changes in the dipole moments are in the same direction giving more SPV in comparison to the n-n+ samples.

Polarization anisotropy of sub-band gap oscillatory features in contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates

We report the observation of strongly polarization sensitive sub-band gap oscillatory features in the contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates. At a given energy in the sub-band gap region, the peak strength of these oscillatory features decreases from a positive maximum to a negative minimum passing through zero as the polarization of the incident probe beam is rotated by 90° from [11̄0] direction to [110] direction in the (001) plane. The origin of this phenomenon is explained on the basis of optical interference coupled with linear electro-optic effect induced changes in the sub-band gap refractive index of the InxGa1−xP layers. Numerical simulations based on the above mechanism are shown to reproduce the polarization dependent observations quite well.

New Measurement Technique for Sub-Bandgap Impact Ionization Current by Transient Characteristics of Partially Depleted SOI MOSFETs

We have developed a novel, sensitive measurement technique for the sub-bandgap impact ionization current in scaled metal-oxide-semiconductor field effect transistors (MOSFETs). In this technique, partially depleted silicon on insulator MOSFETs is utilized where the floating body potential is gradually charged by the impact ionization current. The transient increase of the body potential causes a decrease in the threshold voltage due to the body effect, resulting in a transient increase in the drain current. The impact ionization current is derived from the transient increase in the body potential. The derived impact ionization current is in good agreement with the direct current measurement. Furthermore, the new measurement technique is very sensitive even in the sub-bandgap region and measurements of less than 50 fA are demonstrated.

A photoelectrochemical and ac impedance study of anodic titanium oxide films

Abstract The electrochemical behavior of sulphuric acid anodised titanium prepared galvanostatically under high field conditions over a wide range of maximum potentials (Vmax) was examined using photoelectrochemistry (PEC) and ac impedance spectroscopy (EIS). Film structure and morphology were examined by transmission electron microscopy (TEM). The variation in reciprocal capacitance against Vmax obtained from EIS was evaluated using a constant phase element model. For Vmax=0–10 V a straight line relationship was observed, enabling evaluation in terms of a parallel plate capacitor. This was followed by a region of decreasing reciprocal capacitance, indicating a substantial change in the nature of the material. Finally a slow rise occurred beyond 30 V. Large changes in the band structure of the anodic oxide were observed by PEC near the optical band edge for 0–10 V and in the sub-band gap region for 10–20 V. A defective TiO2 structure with impurity bands located deeply inside the band gap was still observed for 60 V and 84 V, where microcrystals of anatase are present. Strong correlation between the reciprocal capacitance, optical band gap, sub-band gap photoresponse and changes in crystallinity and degree of order within the anodic films were established.

Systematic study of the influence of contacts on CPM results

The constant photocurrent method (CPM) 1 is now widely used for the basic characterization of amorphous hydrogenated silicon (a-Si:H). For detailed deconvolution procedures 2 reliable data are of great importance. We therefore studied the influence of different contacts on the measured spectra of identical samples. Undoped a-Si:H samples were covered with coplanar contacts made from NiCr, Ag, or Ti. Different contact spacings were used to investigate the influence of scattering in the samples 3 . An additional n + layer between a-Si:H and metal was used on some samples to ensure good ohmic contacts as a reference. The influence of SF 6 plasma etching (necessary for removal of the n + layer) was also tested. We found that for all the above arrangements, the subbandgap shoulder increases with increasing distance of coplanar contacts 3 . However, significant differences were found in the magnitude and slope of the shoulder for different contact metals. These differences are not related to contact ohmicity (addition of n + layer has negligible influence). The preliminary results indicate that this effect can be at least partially explained by the reflection of light at the contact edges, whose height and shape can be critical in the subbandgap region.

Photoluminesce properties of doped and unintentionally doped hot wall deposited polycrystalline Zn3P2 films

Zn 3 P 2 films were subjected to photoluminescence measurement. Polycrystalline Zn 3 P 2 thin films were grown under optimized conditions of substrate, source and wall temperatures using the hot wall deposition technique. Unintentionally doped, silver doped and air annealed thin films were individually excited using an HeNe laser and the emission spectra were recorded at various temperatures from 14 to 350 K. Sub-band gap luminescence peaks were found to be weak compared to the broad and intense emission peak lying between the direct band gap at nearly 1.5 eV (at room temperature) and the fundamental indirect transition at 1.39 eV. The broad emission band is believed to originate from the transition from the conduction bands at the Γ point to the nearly uniformly distributed grain boundary energy states in the forbidden gap. The emissions due to acceptor impurity levels were detected in the sub-band gap region.

Femtosecond Dynamics and Nonlinear Optical Properties of Polyacetylene

ABSTRACTWe discuss two important aspects of the nonlinear optical response of polyacetylene, the prototype conjugated polymer. In the subband gap region, χ(3) of polyacetylene has a rich spectral response and its magnitude is found to be the largest among semiconductors. In the superband gap region, absorption of photons leads to direct photogeneration of charged solitons on a femtosecond-time-scale. These results are discussed in the context of a one-dimensional, coupled electronlattice system, and are generalized to other conjugated polymers.

Photovoltaic spectra of undoped GaAsAl 0.25Ga 0.75As multiple quantum well structures: Correlation with photoluminescence

The photovoltaic spectra of molecular beam epitaxy GaAsAl 0.25Ga 0.75As multiple quantum wells in the vicinity of the n=1 sub-bandgap region and their correlation with the corresponding photoluminescence spectra have been studied in the temperature range 2–297K. Photovoltaic and photoluminescence spectra are found to agree for the transition of the n=1 heavy- and light-hole excitons. The photovoltaic spectra in the Schottky-barrier configuration show as the relative maxima and minima at the corresponding exciton photoluminescence lines. A possible model of the observed coincidence of photoluminescence and photovoltage is discussed.

Deep electron traps in As2Se3 single crystals studied by photoconductivity and TSC measurements

Abstract The dependence of the steady-state photoconductivity on temperature, illumination and photon energy has been investigated over wide ranges. At low temperatures, excitonic effects reduce the photosensitivity near the absorption edge. Thermal dissociation of the excitons points to a binding energy of 50–70 meV. Measurements of thermally stimulated currents reveal three efficient electron traps, at 0·41 eV, 0·47 eV and 0–6 eV, of densities in the range of 1016−1017 cm−3. The high density of traps leads to a temperature-independent photoconductivity for T < 200 K, whereas at higher temperatures the shift of the Fermi level through these traps results in a sublinear dependence of the current on illumination, and in a photosensitivity that increases with temperature. Non-stationary photocurrents are studied in the sub-band gap region. Filling of the traps at low temperature extends the photosensitivity down to 0·6 eV. Comparison of optical and thermal excitation of trapped electrons points to considera...

CHARACTERIZATION OF SURFACE STATES AT A SEMICONDUCTOR ELECTROLYTE INTERFACE BY ELECTROLYTE ELECTROREFLECTANCE SPECTROSCOPY

Supra bandgap and subband gap Electrolyte Electroreflectance is being used to characterize surface states at semiconductor liquid interfaces. The surface states can manifest themselves either through direct optical transitions as in the case of n Ti02 aqueous electrolyte interface or through their effect on the response of the Fermi level to small changes in the electrode potential as in the case of single crystal CdT^Se/ in polysulfide solutions. We will present here, two modes in which Electrolyte Electroreflectance (EER) is being used to detect and characterize surface states at the semiconductor electrolyte interface. In the first mode, which is being demonstrated on single crystal Ti02, direct optical transitions between the surface states and the conduction band, are being observed. In the second mode, demonstrated here on single crystal CdIn2Se4 in polysulfide solution, the surface states are responsible for Fermi level pinning which quenches the EER signal. Sub-Bandgap EER -. Ti02: The potential distribution at the Ti02 aqueous electrolyte interface with particular emphasis on the surface states and their dependence on various electrolytes was investigated in detail in our laboratory, using a variety of techniques. These techniques include impedance spectroscopy (1-4). supra and sub-bandgap photorepsonse spectroscopies (2) ; photoelectrochemistry with single and double beam excitation) and EERTM) . Three main groups of surface states were identified: One state that tails from the conduction band edge and is primarily responsible for the recombination of light generated minority carriers(3), second state which resides 0.8 eV below the conduction band and is being controlled by hydrogen adsorption (2) and a third state 1.3 eV below the conduction band which can be observed only when an adsorbing anion penetrates into the inner Helmholtz layer) and is responsible for catalysis of water oxidation. This last state is the subject of the work that will be presented in this section. Whenever comparisons can be made, satisfactory agreement exist between UPS measurements under UHV(5) an(j the in situ measurements. Details about the electrode, cell and the experimental techniques were previously publishedC-^) . The doping level of the TiC>2 Is 5 x 10l9/cm3, Fig. 1 shows the EER spectra of TiC^ in various electrolytes in the sub-bandgap region. All spectra were taken at a potential which shows the maximum response. The broad peak is centered around 1.3 eV and the peak position does not change with the electrolyte but the intensity of the peak does. Within the halogen series it follows the same trend as the expected strength of adsorption, if the latter is dominated by substitution of the hydration shell of the ionsW. In all cases, the potential of the maximum response is 0.1 0.3 V more negative than the corresponding flatband potential. We interpret these results by concluding that the sub-bandgap EER originates from optical transitions between filled Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831032