Photothermal Deflection Spectroscopy Measurements Research Articles

Defect reduction in silicon nanoparticles by low-temperature vacuum annealing

Using electron paramagnetic resonance, we find that vacuum annealing at 200 °C leads to a significant reduction in the silicon dangling bond (Si-db) defect density in silicon nanoparticles (Si-NPs). The best improvement of the Si-db density by a factor of 10 is obtained when the vacuum annealing is combined with an etching step in hydrofluoric acid (HF), whereas HF etching alone only removes the Si-dbs at the Si/SiO2 interface. The reduction in the Si-db defect density is confirmed by photothermal deflection spectroscopy and photoconductivity measurements on thin Si-NPs films.

Dielectric function of hydrogenated amorphous silicon near the optical absorption edge

We report the results of the optical spectra modeling for hydrogenated amorphous silicon (a-Si:H) thin films produced for photovoltaic cell applications, which allowed us to accurately determine the material’s optical gap (Eg). While for thick films of a-Si:H, as well as for other amorphous semiconductors, Eg is normally estimated from a so called Tauc plot, this is hardly possible for thin films because of the interference effects. We developed a physically founded semianalytical model for the complex dielectric function of a-Si:H, valid below and above the optical gap and containing a small number of adjustable meaningful parameters, including Eg and the characteristic energy scales of the optical transition matrix element distribution and the joint density of states in the absorption tail region. With this model and using the transfer matrix formalism for multilayer optics, we have achieved a good agreement between the calculated and experimental transmittance spectra, which allowed us to self-consistently determine the values of the above parameters. We found that both Eg and the characteristic scale of the subgap absorption tail increase with the hydrogen addition. We have also determined the absorption rate spectra of the films, relevant to any optical spectroscopy of subgap states, such as the photothermal deflection spectroscopy and constant photocurrent measurements.

Optical stability of small-molecule thin-films determined by Photothermal Deflection Spectroscopy

AbstractIn this paper the optical absorption properties of n-type C60and PTCDA, and p-type CuPc small molecule semiconductors are investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). The results show the usual absorption bands related to HOMO-LUMO transitions in the high absorption region of the transmission spectra. PDS measurements also evidences exponential absorption shoulders with different characteristic energies. In addition, broad bands in the low absorption level are observed for C60and PTCDA thin-films. These bands have been attributed to contamination due to air exposure. In order to get deeper understanding of the degradation mechanisms single and co-evaporated thin-films have been characterized by PDS. The dependence of the optical coefficient on exposure to light and air have been studied and correlated to the structural properties of the films (as measured by X-Ray Diffraction Spectroscopy). The results show that CuPc and PTCDA are quite stable against light and air exposure, while C60shows important changes in its absorption coefficient. The bulk heterojunctions show stability in agreement with what observed for single layers, since the absorption coefficient of CuPc:PTCDA is almost not altered after the degradation treatments, while CuPc:C60shows changes for low energy values.

Properties of n-type μc-Si:H films by Cat-CVD for c-Si heterojunction solar cells

The electrical and structural properties of n-type μc-Si films prepared by Cat-CVD with different doping ratios R = PH 3/SiH 4, deposition pressures P g, hydrogen dilution ratios S H, and filament temperatures T f have been characterized by dark conductivity, Raman scattering and Photothermal Deflection Spectroscopy (PDS) measurements. The crystalline volume fraction ( X c) of n-type μc-Si:H films is sensitive to T f rather than S H and R. The density of states in n-type μc-Si:H films were investigated by PDS. The heterojunction solar cells of n-type μc-Si:H on p-type c-Si (100) with an undoped a-Si:H buffer layer were performed at a deposition rate of 0.35 nm/s. The best J– V parameters of solar cell are J sc = 29.5 mAcm − 2 , V oc = 483 mV, FF = 70%, and η = 10.2%.

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.

Optical study of disorder and defects in hydrogenated amorphous silicon carbon alloys

We have studied the defect density and disorder in hydrogenated amorphous silicon carbon alloys produced by rf glow discharge of silane-methane-hydrogen mixtures, by combining spectroscopic ellipsometry and photothermal deflection spectroscopy measurements. Increasing the methane flow rate leads to a widening of the optical gap and to an increase of the apparent disorder, deduced from the standard analysis of the exponential absorption edge; the so-called Urbach energy. Interestingly, the subgap absorption decreases with increasing methane flow rate. This points towards a lower density of defects with increasing carbon content and is in contrast with the increased disorder. This apparent contradiction results from the presence of three absorption bands within the gap of this material, as reported by [Ivashchenko et al., J. Phys.: Condens. Matter 14, 1799 (2002)], and which make unreliable the standard analysis of the disorder in silicon carbon alloys.

Growth and characterization of GaN:Mn epitaxial films

The oxidation states of Mn in epitaxial GaN films grown by plasma induced molecular beam epitaxy were investigated by electron spin resonance (ESR), elastic recoil detection, superconducting quantum interference device magnetization, and photothermal deflection spectroscopy measurements. Comparison of the measured Mn2+ spin densities with the total Mn concentrations in GaN of about 1020 cm−3 indicates that the majority of Mn is present in the neutral Mn3+ acceptor state. In samples codoped with Si, electrons are transferred to the Mn acceptors, which is observed by a strong increase of the Mn2+ spin densities in ESR. Simultaneously, this charge transfer reduces the characteristic optical absorption features at 1.5 and above 1.8 eV, suggesting their assignment to a Mn3+-related gap state.

Measurement of absolute minority species concentration and temperature in a flame by the photothermal deflection spectroscopy technique.

It is experimentally demonstrated that absolute concentrations of minority species in flames can be measured by the photothermal deflection spectroscopy (PTDS) technique. In addition, the PTDS signal simultaneously yields the flame temperature the measurement point. Absolute concentration profiles of OH have been measured in a flat-flame burner with methane as fuel. The PTDS measurements agree well with those obtained independently by the absorption technique. The flame temperature measurements by PTDS are also in good agreement with those obtained by the Boltzmann distribution among the rotational levels of OH.

Effect of the Nature of the Hydrogen Bonding on the Light-Induced Metastable Defects in Hydrogenated Amorphous Silicon Prepared by Radiofrequency Magnetron Sputtering

The effects of the nature of the hydrogen bonding and distribution on light-induced metastable defects are investigated in detail in a-Si:H films deposited by radiofrequency magnetron sputtering at high rates (∼15 Å/s) with different hydrogen dilution in the gas phase mixture (Ar + x% H2) (x = 5–20%). A combination of optical transmission and photothermal deflection spectroscopy measurements, correlated with infrared absorption ones, is used to characterise the samples in their as-deposited, annealed and light-soaked states. The results indicate that the increase in the light-induced density of defects is strongly dependent on the amount of the relative proportion of the polyhydride (Si–H2 and (Si–H2)n) groups present in the as-deposited films, which favour the formation of structural inhomogeneities and increase the disorder. The results also show that optimised films can be elaborated and exhibit better stability than optimised samples elaborated at much lower rates (∼1 Å/s) by other techniques. The results are discussed as a whole in the context of the potential fluctuation model.

Highest optical gap tetrahedral amorphous carbon

The deposition and characterisation of tetrahedral amorphous carbon (ta-C) with an E04 optical gap of 3.5 eV and Tauc gap of 3 eV is presented. This is the highest optical gap reported in literature for ta-C and was directly measured using photothermal deflection spectroscopy (PDS) and UV-Vis spectrophotometry. Independent PDS, UV-Vis and electron energy loss spectroscopy (EELS) measurements confirmed the high gap. A large Urbach slope of 600 meV was measured, which indicated that there are many tail states. Electron spin resonance (ESR) measurements confirmed that this material has a high spin density of 7.5×1020 spins/cm3. Post-deposition vacuum annealing of the samples to 500 °C resulted in a small increase of the optical gap, E04∼3.6 eV, and a decrease of the defects to 2.7×1020 spins/cm3. Post-deposition annealing at this temperature did not significantly change the sp3 fraction, but caused a 2–3 order of magnitude increase of the conductivity and a significant reduction of the stress. Thus, this is the first demonstration that we can engineer the defects in ta-C by increasing the electron delocalisation whilst maintaining the high sp3 and optical gap of the material.

Excitation of Er 3+ ions in mixed amorphous-nanocrystalline GaN:Er films

We relate the strong enhancement of the Er 3+ emission in amorphous+nanocrystalline GaN thin films alter annealing at 750°C with changes in the microstructure and respective changes in the electronic system. We use high resolution transmission electron microscopy (HRTEM), photothermal deflection spectroscopy and photoluminescence excitation measurements before and after annealing. The main results concern distinct resonant absorption bands of the Er 3+ ions superimposed on a broad background absorption from the amorphous matrix and very marked Stark splitting of the 4F 9/2 absorption peak after optimal annealing.

Optical properties of amorphous hydrogenated carbon thin films

The optical absorption of various amorphous hydrogenated carbon thin films (a-C:H) grown by radiofrequency (RF) magnetron sputtering were investigated through photothermal deflection spectroscopy (PDS) and transmission measurements in the visible and near IR region. We compare our data to two models: the standard Tauc model and the Gaussian bands model of Dasgupta. A good agreement with both models was found and we report a semi-empirical correlation between the so-called Dasgupta's band-width and the spacing of the π and π * bands of amorphous carbon. The problem of a connection between the bandgap and the disorder is raised. We propose a generalization of the Dasgupta's assumptions on the electronic density of states which accounts well for our results and show that the density of states at the Fermi level can be assessed from PDS data.

Structural and optical properties of Si-doped GaN

Structural and optical properties of Si-doped GaN thin films grown by metal-organic chemical vapor deposition have been studied by means of high resolution x-ray diffraction (XRD), atomic force microscopy, photoluminescence, photothermal deflection spectroscopy, and optical transmission measurements. The incorporation of silicon in the GaN films leads to pronounced tensile stress. The energy position of the neutral donor bound excitonic emission correlates with the measured stress. The stress induced near band gap luminescence shift is estimated to $19\ifmmode\pm\else\textpm\fi{}2 \mathrm{m}\mathrm{e}\mathrm{V}/\mathrm{G}\mathrm{P}\mathrm{a}.$ An increasing concentration of dopant impurities in the films leads to asymmetries of the XRD and photoluminescence spectra, which are probably related to a stress induced inhomogeneous distribution of dopants. Atomic force microscopy observations of surface modulation with increasing silicon doping support this latter statement. Transmission and photothermal deflection spectroscopy measurements are used to determine the band gap energy and Urbach energy of highly doped samples.

Photothermal deflection spectroscopy and transmission measurements of a-C:H films

Absorption coefficient of a-C:H film prepared by saddle-field glow-discharge chemical vapor deposition was measured by means of photothermal deflection spectroscopy and transmission measurements. The absorption spectra of these films are divided into a larger absorption region and a smaller absorption region. The larger absorption region can be interpreted in terms of the Tauc model and an optical gap is obtained for the a-C:H film. The smaller absorption region can be analyzed in terms of the Urbach tail and yields information about the band tails of the a-C:H film. The effect of doping on the absorption tails and the optical gap is also reported. We found that both the optical gap and the breadth of the band tail decreased with the concentration of the impurities, phosphorous or hydrogen. Also there is a correlation between the band tail spread and the optical gap. The breadth of the band tail increases with the optical gap.

Diamond-like carbon films with extremely low stress

We in this paper report different ways to realise thick diamond-like carbon (DLC) films with stress values lower than 0.5 GPa, Thick DLC films grown by conventional r.f. self bias technique often delaminate from the substrates due to the presence of high compressive stresses of the order of 4–7 GPa. We have made an in-depth study of the delamination problem of DLC films at NPL and found that only for substrates kept away from the plasma (plume) it is possible to grow thick DLC films. This goes to show the heating of the substrates, when m contact with the plasma, appears to be one of the most important factors giving rise to the high stress values. Techniques that have produced consistently low stress values (0.2–0.5 GPa) in this laboratory are pulse plasma PECVD and the one using dc saddle field fast atom beam source. Electronic properties of the materials so produced have been estimated by evaluating Urbach energy using photothermal deflection spectroscopy (PDS) measurements. A correlation between the unbound hydrogen in these films, as measured by a nuclear technique (ERDA), and the stress levels has been found. Deposition rate, room temperature conductivity, optical bandgap and refractive index have also been measured for these films.

The hydrogen effusion induced structural changes and defects in hydrogenated amorphous SiGe films: dependence upon the microstructure

To better understand the relations between deposition conditions, microstructure, H incorporation and the optoelectronics properties of undoped a-Si1−x Gex:H alloys, we performed a comparative study of samples (with x≈0.5) prepared by plasma enhanced chemical vapour deposition at total pressures varying from 900 to 2200 mTorr. The hydrogen bonding, detected by infra-red absorption measurements and H thermal desorption experiments, as well as the optoelectronic properties, determined by a combination of standard optical and photothermal deflection spectroscopy measurements, were found to depend on the total pressure. The results obtained in the as-deposited state and after annealing at increasing temperatures are analysed as a whole in terms of specific local H bonding environment, degree of order, and defects.

An improved analysis for band edge optical absorption spectra in hydrogenated amorphous silicon from optical and photoconductivity measurements

The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap.

Incorporation of lithium in single crystal diamond: diffusion profiles and optical and electrical properties

Abstract Doping of diamond by in-diffusion of lithium in natural type Ia and IIa single crystal diamond plates has been investigated. The lithium incorporation was realized by direct in-diffusion from a Li2O source at temperatures between 400 and 650 °C. Using secondary ion mass spectroscopy (SIMS) depth profiling, high concentrations of lithium, up to a level of 8.0 × 1021 cm−3 and gradually decreasing to 1.0 × 1019 cm−3 at a depth of 0.5 μm below the surface, were measured. From these profiles the diffusion constant of lithium in diamond was determined to be DLi(T) = 2 × 10−10exp (−0.9 eV/kT) cm2s−1. The the activation energy for diffusion of 0.9 ± 0.3 eV is in agreement with the theoretical literature value of 0.85 eV. Cathodoluminescence spectroscopy showed that the blue band-A luminescence, which is characteristic for natural type IIa diamond, is largely quenched by the presence of lithium. A shift of the maximum luminescence intensity towards higher energy values has been found. From photothermal deflection spectroscopy and optical transmission measurements no new optically active defect is observed after lithium in-diffusion. However, these measurements indicated the occurrence of a large concentration of light scattering centres, the size of which is 10 nm or less. This suggests that lithium is clustered in the crystal volume just below the specimen surface and therefore is not electrically active.

Electronic Properties of Microcrystalline Silicon

ABSTRACTThe electronic and optical properties of microcrys tall ine silicon films prepared by plasma enhanced chemical vapour deposition are investigated with Hall-effect, electrical conductivity, photothermal deflection spectroscopy and photoluminescence measurements. In particular, the influence of the grain size and the crystalline volume fraction on the conductivity, the carrier density and the Hall mobility is investigated in highly doped films. A percolation model is proposed to describe the observed transport data. Photoluminescence properties were studied in un-doped films. It is proposed that the photoluminescence is due to recombination at structural defects similar to those observed in crystalline silicon.

Effects of substrate temperature on the structure and properties of reactive pulsed laser deposited CN x films

The effects of substrate temperature (25 ≤ T s ≤ 437°C) on the structure and properties of reactive pulsed laser deposited (RPLD) carbon nitride (CN x ) films prepared at a fixed nitrogen ambient pressure of 200 mTorr were investigated. The structure and composition of the films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and IR absorption, while the electrical and optical properties were studied by photothermal deflection spectroscopy, optical transmission and electrical conductivity measurements. The results show that the films contain graphitic rings, in which some of the carbon atoms are replaced by nitrogen atoms. The atomic fraction of nitrogen drops from 0.27 to 0.13 as T s increases from 25 to 437°C, indicating that the incorporation of nitrogen is hindered by a rise in T s. As a consequence, the optical band gap and the width of electron tail states become narrower, and the room temperature electrical conductivity increases greatly such that the properties of the films deposited at high temperature approach those of graphite.