Photothermal Deflection Spectroscopy Spectra Research Articles

Spatial Localization of Defects in Halide Perovskites Using Photothermal Deflection Spectroscopy.

Photothermal deflection spectroscopy (PDS) emerges as a highly sensitive noncontact technique for measuring absorption spectra and serves for studying defect states within semiconductor thin films. In our study, we applied PDS to methylammonium lead bromide single crystals. By analyzing the frequency dependence of the PDS spectra and the phase difference of the signal, we can differentiate between surface and bulk deep defect absorption states. This methodology allowed us to investigate the effects of bismuth doping and light-induced degradation. The identified absorption states are attributed to MA+ vibrational states and structural defects, and their influence on the nonradiative recombination probability is discussed. This distinction significantly enhances our capability to characterize and analyze perovskite materials at a deeper level.

Structural, optical and annealing studies of nitrogen implanted GaAs

Nitrogen implanted III-V compounds semiconductors have attracted a great deal of attention of the research community due to their interesting fundamental physical properties and potential applications in optoelectronic devices. A small amount of nitrogen incorporation in gallium arsenide (GaAs) causes significant reduction in the band gap energy of fabricated alloys. In this study the GaAsN layer was prepared by implantation of nitrogen ion on thick GaAs wafer and the samples were thermally annealed for studying annealing effects. The structural, compositional and optical properties have been investigated by X-Ray Diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), Scanning Electron Microscopy (SEM), Photoluminescence (PL) and Photo-thermal Deflection Spectroscopy (PDS). The results reveal that the samples prepared with higher implantation dose and annealed at higher temperature exhibit strong optical signal of GaN, while the samples annealed at lower temperature show weak optical signal. Furthermore in PDS spectra, the band gap reduction was observed indicating the diluted nitrogen content in GaAs. We have demonstrated that annealing conditions and implanted nitrogen doses could be helpful to achieve the desired variation in band gap and low physical damages on surface.

Study of the surface properties of ZnO nanocolumns used for thin-film solar cells.

Densely packed ZnO nanocolumns (NCs), perpendicularly oriented to the fused-silica substrates were directly grown under hydrothermal conditions at 90 °C, with a growth rate of around 0.2 μm/h. The morphology of the nanostructures was visualized and analyzed by scanning electron microscopy (SEM). The surface properties of ZnO NCs and the binding state of present elements were investigated before and after different plasma treatments, typically used in plasma-enhanced CVD solar cell deposition processes, by X-ray photoelectron spectroscopy (XPS). Photothermal deflection spectroscopy (PDS) was used to investigate the optical and photoelectrical characteristics of the ZnO NCs, and the changes induced to the absorptance by the plasma treatments. A strong impact of hydrogen plasma treatment on the free-carrier and defect absorption of ZnO NCs has been directly detected in the PDS spectra. Although oxygen plasma treatment was proven to be more efficient in the surface activation of the ZnO NC, the PDS analysis showed that the plasma treatment left the optical and photoelectrical features of the ZnO NCs intact. Thus, it was proven that the selected oxygen plasma treatment can be of great benefit for the development of thin film solar cells based on ZnO NCs.

Bandedge optical properties of MBE grown GaAsBi films measured by photoluminescence and photothermal deflection spectroscopy

The bandedge optical properties of GaAsBi films, as thick as 470nm, with Bi content varying from 0.7% Bi to 2.8% Bi grown by molecular beam epitaxy on GaAs substrates are measured by photoluminescence (PL) and photothermal deflection spectroscopy (PDS). The PDS spectra were fit with a modified Fernelius model which takes into account multiple reflections within the GaAsBi layer and GaAs substrate. Three undoped samples and two samples that are degenerately doped with silicon are studied. The undoped samples show a clear Urbach absorption edge with a composition dependent bandgap that decreases by 56meV/% Bi and a composition independent Urbach slope parameter of 25meV due to absorption by Bi cluster states near the valence band. The doped samples show a long absorption tail possibly due to absorption by gap states and free carriers in addition to a Burstein–Moss bandgap shift. PL of the undoped samples shows a lower energy emission peak due to defects not observed in the usually available thin samples (50nm or less) grown under similar conditions.

Bandedge absorption of GaAsN films measured by the photothermal deflection spectroscopy

The optical absorption of GaAsN films grown by molecular beam epitaxy on GaAs substrates is measured using the mirage effect photothermal deflection spectroscopy (PDS). The PDS spectra were fitted with a modified Fernelius model, which takes into account multiple reflections within the GaAsN layer and GaAs substrate. This allowed the extraction of bandedge parameters for a series of GaAsN films with N content varying from 0.24% to 1.4% N. All films show a clear Urbach absorption edge with a composition-dependent bandgap consistent with literature and Urbach slope parameters roughly 3 times larger than GaAs values.

Determination of Absorption Coefficients and Thermal Diffusivity of Modulated Doped GaAlAs/GaAs Heterostructure by Photothermal Deflection Spectroscopy

We report in this study theoretical and experimental studies of photothermal deflection spectroscopy (PDS) for planar doped heterostructures AlGaAs/GaAs. The PDS spectra at T=300K are recorded by varying the wavelength of the excited radiation for different modulation frequencies. They show two different regions related to the fundamental electronic transitions from GaAs and Si doped AlGaAs. The DX levels have been observed approximately at 0.150 eV below the conduction band edge of Si doped Al0.33Ga0.67As (Eg=1.835 eV at 300 K). We give in this study a method of measuring the absorption coefficient of AlGaAs/GaAs heterostructures from the amplitude measurement of PDS signal. The optical absorption coefficient found was between 10 and 105 cm-1 in the energy range 1-2.5eV for samples. The PDS data are compared with those given by conventional transmission technique. The evolution of PDS amplitude spectra by varying the modulation frequency from 5 to 100 Hz (usually called depth profile analysis) and using He-Ne laser probe beam shows that the PDS signal amplitude increases with decreasing the modulated frequency. A numerical procedure is applied to fit the measured amplitude of the PDS signal. The value of thermal diffusivity (Ds=0. 27cm2s-1) shows a very good agreement with results given by spectroscopic ellipsometry.

Sub-bandgap optical absorption spectroscopy of hydrogenated microcrystalline silicon thin films prepared using hot-wire CVD (Cat-CVD) process

Hydrogenated microcrystalline silicon (μc-Si:H) thin films with different silane concentration (SC) have been prepared using the HW-CVD technique. Dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS), and transmission measurements have been used to investigate the optical properties of the μc-Si:H films. Two different sub-bandgap absorption, α( hν), methods have been applied and analyzed to obtain a better insight into the electronic states involved. A good agreement has been obtained in the absorption spectrum obtained from the PDS and DBP measurements at energies above the bandgap. Differences between PDS and DBP spectra exist below the bandgap energy where DBP spectra always give lower α( hν) values and show a dependence on the SC. For some films, differences exist in the α( hν) spectra when the DBP measurements are carried out through the film and substrate side. In addition, for some films, there remains fringe pattern left on the spectrum after the calculation of the fringe-free absorption spectrum, which indicates structural inhomogeneities present throughout the film.

Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method

The applicability of photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chemical vapor deposited and physical vapor deposited CuGaSe2 (CGSe) thin films is investigated. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a-Si theory of PDS and CPM analysis, a two-layer system consisting of a defect-rich near-surface layer and a less defective bulk layer is found in device-quality films. Optical improvements due to the Ga+Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near-surface defect density remains unchanged.

Comparison of CPM, PDS and Optical Transmittance of Amorphous Carbon Nitride Films Made by a Nitrogen Radical Sputter Method

ABSTRACTAmorphous carbon nitride films a-CNX, deposited in our laboratory by a radical sputter method, show high photosensitivity PS, where PS is the ratio of photoconductivity sP and dark-electrical conductivity sd. A-CNX made a layer-by-layer method, LLa-CNX, has the highest photosensitivity in our various preparation conditions. The photoconductivity in a-CNX and LLa-CNX shows dependence on photon energy in the range 2 eV to 6.2 eV. The constant photocurrent method (CPM), photothermal deflection spectroscopy (PDS) and optical transmittance spectra are used to obtain the information in the optical energy band gap and defect states. A-CNX and LLa-CNX are good photoconductors especially at energy higher than 3 eV. Therefore it is not difficult to obtain CPM spectra in the high photon energy region. CPM spectra are obtained by dc- and ac- measurements. The value of the absorption coefficient a spectra obtained by dc-CPM is larger than that of ac-CPM, which increases with increasing frequency of the measurement. In this paper, CPM data is used to discuss a model of density of states (DOS) of a-CNX by comparison with PDS and optical transmittance spectra.

Surface and bulk light scattering in microcrystalline silicon for solar cells

We present here an overview of the light scattering influence on the absorption coefficient spectra α( E) measured by constant photocurrent method (CPM) and/or by photothermal deflection spectroscopy (PDS). We compare results of numerical simulation with the measured CPM and PDS spectra and verify our theory by polishing the as-grown nanotextured layers. We show that the experimentally observed apparent absorption coefficient can overestimate the true α( E) by a factor of 10.

In-plane photoconductivity in amorphous silicon doping multilayers

Abstract We prepared amorphous Si p-i-p-i and n-4-n-i doping multilayers to study their in-plane carrier transport and interfacial defect densities. The structures were grown using glow discharge or electron-cyclotron-resonance-enhanced chemical vapour deposition. We measured the following: composition by secondary-ion mass spectrometry; transport by dark conductivity SsGd and its thermal activation energy, by continuous-wave photoconductivity SsGph as a function of intensity and wavelength, and also by photoconductive decay; defect density by photothermal deflection spectroscopy (PDS) and the constant-photocurrent method (CPM). SsGd is dominated by the doped layers. The intensity dependence of SsGph suggests that it also is controlled by the doped layers. The fast component of the photoconductivity response time is comparable with that of the doped bulk. The PDS spectra of the multilayers are nearly identical with those of the bulk doped hydrogenated amorphous silicon. The pronounced wavelength dependenc...

Large range subgap absorption measurements in thin semiconducting films

A detailed description of an experimental configuration based on photothermal deflection spectroscopy (PDS) and transmittance techniques that enables high sensitivity room temperature absorption measurements over a spectral region extending between 300 nm and 3.4 μm is reported. It is shown that the simultaneously determined PDS and transmittance spectra can be processed to eliminate completely the interference induced oscillations in the spectra which are observed when the film refractive index differs substantially from the one of the substrate. A configuration based on the photopyroelectric and transmittance techniques, though less sensitive than the previous one, is shown to be suitable for the same kind of measurements at cryogenic temperatures. Absorption bands in implanted Si between 2.8 and 3.4 μm have thus been detected and tentatively associated with multiphonon defect assisted absorption bands.

Subbandgap absorption spectra of slightly doped a-Si:H measured with constant photocurrent method (CPM) and photothermal deflection spectroscopy (PDS)

The authors report on systematic measurements of the absorption spectra in the visible near-infrared range performed on a series of slightly phosphorus- and boron-doped amorphous hydrogenated silicon (a-Si:H) samples. Optical transmission spectra, Constant Photocurrent Method (CPM) and Photothermal Deflection Spectroscopy (PDS) have been used to measure the absorption coefficient (α) in differently doped samples. Variation of the gas phase doping levels in the ppm range is shown to lead to relevant differences between CPM and PDS spectra. The latter demonstrate that additional care has to be taken when evaluating deep defect density from CPM when the Fermi level is not constant, and especially so, if the Fermi level is below midgap.

Near-ir absorption in chemically vapor deposited a-SiNx:H films.

We have measured the optical-absorption properties of plasma-enhanced chemically vapor deposited films of hydrogenated amorphous silicon nitride (a-${\mathrm{SiN}}_{\mathit{x}}$:H) over the energy range from 0.54 to 2 eV using photothermal deflection spectroscopy (PDS). Typical PDS spectra show a broad absorption tail with a large number of narrow absorption peaks. Using the phase sensitive feature of this spectroscopy we are able to separate nitride-related absorption from that arising from the fused quartz substrates. In addition to the broad, featureless band tail, whose magnitude is quite sensitive to optical excitation with light above \ensuremath{\sim}3.9 eV, we observe a number of narrow absorption peaks which are most likely overtones of localized vibrational modes. Specifically, we see peaks at 0.82 and 1.20 eV which we identify as the first and second harmonics of N-H stretching vibrations, and several other peaks whose origins remain undetermined. Prolonged UV excitation produces no detectable change in these vibrational modes, although large changes are seen in the underlying absorption tail. This reinforces previous suggestions that photoinduced changes in these films do not arise from rearrangement of the bonded hydrogen.

Photoluminescence in B-doped μc-Si:H

ABSTRACTPhotoluminescence (PL) has been investigated in hydrogenated microcrystalline silicon (μc-Si:H) samples as a function of boron doping for films prepared by remote plasma enhanced chemical vapor deposition. When the dark conductivity a is below about 10-5 S/cm, the PL spectra exhibit a shape which is close to that of the so-called band tail PL in undoped hydrogenated amorphous silicon (a-Si:H) at 77 K. When a increases, the PL intensity decreases at 77 K. For samples with a on the order of 10-3 S/cm, the PL spectra show only a narrow, low energy PL band which peaks around 0.8–0.9 eV. In these samples, the PL at higher energy is essentially not observable. This trend is similar to that which occurs in doped a-Si:H. However, for higher doping levels (σ ∼ 1 S/cm) the PL in μc-Si:H, although very weak, exhibits a broad band which contains intensity at higher energies. The absorption spectra in these samples, as measured by photothermal deflection spectroscopy (PDS), show the same relationships with the corresponding PL spectra as do the PDS spectra in doped a-Si:H.

Charge Trapping Centers in Ferroelectric Ceramics

ABSTRACTElectron paramagnetic resonance (EPR), photo-thermal deflection spectroscopy (PDS), and electrical measurements have been used to characterize as-received and UV-lluminated lead lanthanum zirconate titanate (PLZT) and PZT ceramics. Following optical illumination we observe the activation of positively charged Pb+3 and negatively charged Ti+3 ions, as well as an increased broad absorption peak around 2.6 eV. The spatial distribution of the induced absorption correlates well with the location of the absorbed UV, suggesting that photo-produced carrier pairs are trapped at Ti+4 and Pb+2 producing the observed paramagnetism. The Ti+3 EPR spectra were successfully fit using the crystal field parameters derived from the PDS spectra. Preliminary results suggest that these charged paramagnetic ions can influence the electrical properties of PZT thin films. We investigated the effects of UV light and dc bias sequences by polarization-electric field (PE) measurements and find that the coercive fields are dependent on the sign of the applied bias during UV exposure. It is reasonable to expect that Pb+3 and/or Ti+3 ions act as the charge trapping sites which cause the threshold voltage shifts in the PE loop.

Optical Absorption Spectra Under DC Light Bias in Undoped a-Si:H

ABSTRACTUnder a dc light bias, the ac constant photocurrent method (CPM) yields an anomalous subgap optical absorption over that without the bias. When the intensity of the bias is high, an absorption band appears. However, absorption measured by ac photothermal deflection spectroscopy (PDS) is much less affected by the bias. A simple model qualitatively explains the effect on the PDS spectra and suggests that the anomalous CPM absorption is an apparent one which represents a variation of the recombination lifetime.

The Effect of UV Light on IR Absorption in Chemically Vapor Deposited a-SiNx:H Films

ABSTRACTWe have measured the optical absorption properties of Plasma Enhanced Chemically Vapor Deposited (PECVD) films of hydrogenated amorphous silicon nitride (a-SiNx:H) over the energy range from 0.54 to 2 eV using Photothermal Deflection Spectroscopy (PDS). The near-infrared absorption properties of these films provide a quantitative estimate of the amount of H bonded to Si or N as well as a measure of the the density of electronic gap states due to species like Si dangling bonds. Typical PDS spectra obtained on as-deposited films show a broad absorption tail with a large number of narrow absorption peaks. Using the phase sensitive feature of this spectroscopy we are able to separate nitride-related absorption from that arising from the fused quartz substrates. In addition to the broad, featureless band tail, whose magnitude is quite sensitive to optical excitation with light above ̃3.9 eV, we observe a number of narrow absorption peaks which we identify as overtones of localized vibrational modes. Specifically, we see peaks at 0.82 and 1.20 eV which we identify as the 1st and second harmonics of N-H stretching vibrations, and several other peaks whose origins remain undetermined. Prolonged UV excitation produces no detectable change in these vibrational modes, although large changes are seen in the underlying absorption tail. This reinforces previous suggestions that photoinduced changes in these films do not arise from rearrangement of the bonded hydrogen.

Study of surface/interface and bulk defect density in a-Si:H by means of photothermal deflection spectroscopy and photoconductivity

From comparison of PDS (Photothermal Deflection Spectroscopy) spectra with photoconductivity (mobility-lifetime product) data over four decades of sample thickness we find experimental evidence for the presence of material inhomogeneities along the film growth axis.