Subgap Absorption Research Articles

Defect-related optical transitions in GaN.

Sub-band-gap absorption of GaN grown by metal-organic chemical vapor deposition on sapphire was investigated by photothermal deflection spectroscopy (PDS), transmission measurements, and the constant photocurrent method (CPM). We determine acceptor binding energies in undoped GaN at 220 and about 720 meV. A comparison between absorption and CPM spectra yields the dependence of the quantum efficiency-mobility-lifetime-product ($\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$) versus energy and gives relevant information about the excitation mechanisms. CPM spectra show a significantly smaller absorption (up to a factor of $\frac{1}{10}$) in the range between 3.0 and 3.3 eV as compared to PDS. This indicates that the majority of carriers excited with these photon energies have a relatively small $\ensuremath{\eta}\ensuremath{\mu}\ensuremath{\tau}$ product and thus do not contribute to the externally detected photocurrent. We propose that in this energy range the spectrum is dominated by interband absorption in isolated cubic-phase crystallites in the hexagonal matrix and by excitation of electrons from occupied acceptors into the conduction band of the main hexagonal crystal modification ($h$-GaN). Temperature-dependent photoluminescence measurements, excited with energies below and above the direct band gap of hexagonal GaN, confirm this interpretation and can be correlated with the subgap absorption detected by PDS. Transient photocurrent measurements show a persistent photoconductivity, which can also be explained by the existence of isolated cubic-phase inclusions.

Initial, rapid light-induced changes in hydrogenated amorphous silicon materials and solar cell structures: The effects of charged defects

Large, rapid light induced changes are reported for photoconductivities, electron mobility-lifetime products, and forward bias currents in hydrogenated amorphous silicon (a-Si:H) films and Schottky barrier cell structures. The absence of concurrent changes in subgap absorption and quantum efficiencies are clearly inconsistent with the widely held view that the kinetics of degradation in a-Si:H materials and cells can be quantified solely in terms of neutral dangling bond defects. The self-consistent analysis of all the results was carried out for films and Schottky barrier structures by including charged defects and using a three-Gaussian distribution of donorlike and acceptorlike defect states. Such self-consistent development of ‘‘operational’’ parameters for these gap states offers a method for reliable and quantitative correlations between solar cell performance and stability with the properties of their bulk materials.

A simple method to determine the optical constants and thicknesses of Zn xCd 1−xS thin films

A new method to determine the optical parameters of Zn xCd 1−xS polycrystalline thin films is proposed. Sub-gap absorption, caused by acceptor states, is taken into account in the determination of the refractive index and film thickness. This procedure avoids errors in the results, frequently observed when the absorption is ignored. In the low and medium absorption region the refractive index is parametrized by the Wemple and DiDomenico (W-D) single oscillator model. The W-D parameters are further used in order to extrapolate the refractive index at the absorption edge to compute the absorption coefficient and the optical gap. The transmission spectra of typical CdS, (Zn,Cd)S and ZnS thin films and their corresponding values of n, α and E g are presented.

High resolution electron energy loss spectroscopy: A new probe of subgap absorption in amorphous solids

The use of high resolution electron energy spectroscopy (HREELS) as a new method for studies of subgap absorption in thin films of amorphous semiconductors is demonstrated. For a-Si films, the α(ω) values extracted from the measured loss spectra are in quantitative agreement with previous optical measurements. The method is also applied to both threefold and diamond-like amorphous carbon films, yielding α(ω) down to considerably lower energies (∼50 meV) than previously reported. The HREELS method is shown to be complementary to existing techniques in that it can access the regime of low energies and ultrathin films which is difficult to investigate with the conventional methods.

Origin of characteristic subgap optical absorption in CVD diamond films.

Photothermal deflection spectroscopy (PDS) is used to study subgap optical absorption in chemical vapor deposited (CVD) diamond films. The high PDS sensitivity over a broad investigated spectral range enabled the investigation of the main defect structure in CVD diamond upon changing the deposition conditions. All films exhibit a very characteristic shape of the subgap optical absorption in the infrared and visible spectral regions, which scales with the content of amorphous carbon phase in grain boundaries. In best-quality films the character of the fundamental absorption edge is attained and the subgap absorption reduced. For nanocrystalline approaching films, an absorption spectrum resembling closely the one for amorphous carbon (a-C:H) films is observed. Based on these results a mathematical model for the description of the optical absorption coefficient in the subgap region is developed. A numerical deconvolution procedure is applied to fit the experimental data. \textcopyright{} 1996 The American Physical Society.

Sub-gap optical properties of ion implanted SiC

〈0001〉 6HSiC was implanted with B + ions at room temperature. The relative defect concentration n dat was determined by Rutherford backscattering spectrometry, and the optical properties in the photon energy range 0.7 eV ≤ h ̵ ω ≤ 3.2 eV were analysed using transmission and reflection measurements. For n dat ≲ 0.15 the layers contain mainly point defects and point defect complexes and the occurrence of a refractive index change can be largely excluded. The measured transmission can be simulated assuming an exponential sub-gap absorption tail which is superimposed on a broad absorption band in the range 1.3 eV ≤ h ̵ ω ≤ 2.4 eV . Amorphous SiC is characterized by a refractive index change of 30% with respect to the unimplanted substrate and by an optical gap of 1.3 eV. In the case of medium damage (0.15 < n dat < 1), analysis of the transmission spectra indicates the existence of amorphous zones and of point defects and point defect complexes in the crystalline regions between the amorphous zones in the implanted SiC layers.

Evidence for the principal defect states in CVD diamond films: optical study

Photothermal deflection spectroscopy (PDS) was used to study the subgap optical absorption in CVD diamond films. All films exhibited a very characteristic shape of the absorption in the IR and visible spectral regions, varying in intensity over several orders of magnitude and depending on the film quality. The study of the morphology proved that the films with the highest subgap absorption were nanocrystalline with increased surface area of grain boundaries. The optical absorption spectra of these films resembled the optical absorption in amorphous carbon films which is due to the optical transitions in the π-π ∗ pseudogap. The best quality CVD diamond attained the character of the fundamental indirect absorption edge of diamond with strongly reduced characteristic subgap absorption. From secondary ion mass spectrometry analysis it is shown that the characteristic absorption cannot be due to the extrinsic impurities.

Extended infrared response of silicon solar cells and the impurity photovoltaic effect

Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10 −7 cm −1 have been determined, revealing structure due to 3- and 4-phonon assisted absorption. These values are compared with a more recent determination of the absorption edge based on photoluminescence measurements. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross section for electron photoemission from the indium level, a crucial parameter for modelling indium's IPV effect, is determined.

Electrical transport and optical properties of tetraanilinobenzene Langmuir–Blodgett films

We report a study of the electrical and optical properties of Langmuir–Blodgett films of tetraanilinobenzene. The optical absorption, peaking at 4.1 eV, is blue shifted by 0.2 eV, and photoluminescence, peaking at 3.0 eV, is red shifted by 0.4 eV compared to the material in solutions, indicating H aggregates. p-type doping by iodine gas or protonation in acidic solutions results in two polaronic subgap absorption bands and leads to an increase in conductivity from 10−10 to about 10−4 S/cm. The optical properties and doping mechanisms have been discussed and compared to those of vacuum-evaporated films. The oxidized units as well as polaronic states were found to act as nonradiative recombination centers for excitons. The conductivity σ of the protonation-doped samples has a temperature T dependence log σ∝T−1/2 indicating variable-range hopping in a quasigap, probably due to the Coulomb interactions between localized carriers in polaronic states. The nonohmic region was studied for moderately high electric fields up to 105 V/cm.

Light-induced metastability of ‘stable’ a-Si:H deposited from silane-dichlorosilane mixtures — comprehensive characterization

Subgap near-infrared absorption as well as spin density in light-soaked hydrogenated and lightly chlorinated amorphous silicon films were studied experimentally, and the results were compared with a defect pool model based upon experimentally obtained band gap, Fermi energy position, and annealing energy distribution. It was demonstrated that a shift of Fermi energy level, and change in defect distribution result in the change of charge state of defects, and different gas ratio dependencies in transport and thermal measurements of subgap absorption.

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.

Light-induced metastable changes in defect density and photoconductivity of a-Si:H between 4.2 and 300 K

The creation kinetics of light-induced defects (LIDs) in intrinsic a-Si:H are found to be quite independent of exposure temperature, T e, between 4.2 and 300 K and exposure light intensity. The findings suggest that LID creation is a very robust process. The creation efficiency of LIDs varies by a factor 2.5 with T e and has a broad minimum between 80 and 200 K. 10 5 V/cm electric fields at 4.2 K decrease the creation efficiency by 30%. A spectral relaxation of subgap absorption due to LIDs created below 200 K is found above 200 K prior to the anneal of LIDs.

Photoconductive a-GaN prepared by reactive sputtering

Amorphous and microcrystalline GaN thin films have been made by reactive sputtering. Transparent films of optical gap, E o5 ∼ 3.95 eV, are obtained and these show semiconductor characteristics. The conductivity changes dramatically from ∼ 10 −11 S/cm to ∼ 10 −3 S/cm on microcrystallization. Photoconductivity is observed in a-GaN. The results of electron spin resonance and sub-gap absorption suggest a low mid-gap defect density of states. It is demonstrated that an amorphous semiconducting material with a large optical gap can be obtained.

Defect-Induced Optical Absorption in CVD Diamond Films

Photothermal deflection spectroscopy (PDS) was used to study defect-induced optical absorption in CVD diamond films. PDS has shown very high sensitivity, allowing to measure absorbance values down to 10−5 level. Moreover, PDS is significantly less sensitive to elastic light scattering than optical transmission measurements. Based on PDS data, it is shown that undoped polycrystalline and homoepitaxial diamond films exhibit a very characteristic subgap absorption, which is ascribed to electronic transitions between π→* states, appearing in the gap and due to the presence of amorphous carbon. The incorporation of N (and H and O) atoms in diamond films was studied. The optical absorption of N-doped films is compared with the optical absorption due to single substitutional N in Ib synthetic diamond. The incorporation of Li by in-situ doping with organo-metallic compounds during film growth is discussed and the observed transitions are tentatively explained. Finally, a mathematical model for the characteristic subgap absorption is described and the experimentally obtained optical absorption coefficients are deconvoluted to obtain parameters for the description of the density of gap states.

Optical absorption of free-standing porous silicon films

The optical absorptions of anodically etched p+ and n+ porous silicon (PS) films were investigated by photothermal deflection spectroscopy. Si–H stretching overtones and combination bands of Si–F and Si–H were observed. The defect model in hydrogenated amorphous silicon was used to explain the Urbach edge and the subgap absorptions of PS. The dangling bond defect densities in PS were estimated.

Nonlinear Dependence of μeτe on Subgap Absorption in Pulse and cw Light-Soaked a-Si:H

AbstractWe report new results from subgap absorption and transport measurements on pulse and continuous (cw) light-soaked a-Si:H which include that (1) the mobility-lifetime product of electrons (μeτe) shows the same dependence on defect subgap absorption irrespective of whether the sample was soaked by pulse or continuous lights, but that (2) the dependence of μeτe on defect subgap absorption is not linear. These results are compared with annealing energy distribution of light-induced defects, and discussed in terms of (1) annealing energy (Eann) distribution of light-induced defects (LIDs), and of (2) photo-induced structural change.

Defect Transitions in GaN Between 3.0 and 3.4 eV

ABSTRACTWe have studied optical transitions (absorption and luminescence) in nominally undoped and Mg-doped GaN deposited by MOCVD and MBE. In the range between 3.0 and 3.4 eV, a variety of well known low-intensity luminescence lines are observed, whose origin is discussed. In particular, by comparing excitation with subgap versus above-gap laser lines as well as by combining optical subgap absorption with spectrally resolved photoconductivity, we identify localized optical transitions occuring in isolated cubic inclusions in the otherwise hexagonal GaN epitaxial layers. Implications of these strucural defects for photocurrent transients are also presented.

Light Bias CPM Study of the Density of States in N-type Amorphous Silicon

AbstractWe measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.

Photoacoustic spectroscopy use in the analysis of ion-implanted CuInSe2 single crystals

A high-resolution near-infrared photoacoustic spectrometer of the gas–microphone type is used for room-temperature analysis of the defect states in ion-implanted CuInSe2 single crystals. A wide range of ions differing in their masses as well as in their electrical activities in the host crystal (O+, Ne+, Cu+, Xe+, and Li+) have been implanted at various energies and doses to assess the technique sensitivity to detect changes affecting the compound subgap absorption spectrum, which is characteristic of impurities. The detected changes (either by the appearance of new peaks or by a change in the impurity or defect concentrations) are shown to differ from one ion species to another. The depth profiling capability of the technique in analyzing implanted samples is also discussed. The results obtained here are correlated to existing published data. It is shown that photoacoustic spectrometry can be very useful if used to appraise the complex defect structure of ion-implanted CuInSe2.

Nature of optical transitions in conjugated oligomers. I. Theoretical characterization of neutral and doped oligo(phenylenevinylene)s

The nature of the main optical transitions taking place in oligo(phenylenevinylene)s is analyzed in both the neutral and oxidized states. Geometry optimizations are first carried out with the Hartree–Fock semiempirical Austin Model 1(AM1) method; on the basis of the resulting geometries, the transition energies and their intensities are determined by means of the Hartree–Fock semiempirical intermediate neglect of differential overlap (INDO) method combined with a single configuration-interaction (SCI) technique. The major aspect of the results is to show that two subgap absorption peaks are induced by the generation of polarons (radical cations), while a single absorption feature is expected when bipolarons (dications) are formed, in agreement with the experimental absorption spectra of the oligomers; this situation is, however, in marked contrast with that in the polymer. The possible formation of diamagnetic π dimers is also discussed.