Urbach Edge Research Articles

Excitation of rare earth emission in chalcogenide glasses by broadband Urbach edge absorption

Studies of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy in rare earth (RE)-doped chalcogenide glasses have shown that there are interactions between the fundamental optical absorption and emission processes of the chalcogenide host glasses such as Ge 33As 12Se 55 and the spectrally overlapping intra 4f-shell transitions of Er, Pr and Dy dopants. A broad (∼0.5 eV fwhm), near-band edge PLE band is observed in the excitation spectra of RE emission bands in chalcogenide glasses, superimposed on the expected 4 f-RE PLE peaks. The results reviewed here demonstrate that the broad band PLE of the RE emission is due to the absorption of light in the Urbach edge of the host glass followed by non-radiative transfer of the excitation energy to the RE emitters. On the basis of several unique properties of the broad band PLE, we suggest a model for the energy transfer process from the host glass to the REs mediated by intrinsic defect states in the glass.

OPTICAL ABSORPTION EDGE INVESTIGATIONS IN Sn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> AND SnP<sub>2</sub>S<sub>6</sub> CRYSTALS

Temperature variation of optical absorption edge of Sn 2 P 2 S 6 ferroelectric with “three-dimensional” (3D) crystal structure and of SnP 2 S 6 crystal with layered (2D) structure, was studied. The anisotropy of Urbach edge parameters and their temperature behaviour in the range of phase transition in Sn 2 P 2 S 6 crystal, was investigated. The effect of various types of disordering, as well as 3D→2D transition, upon the energy position and shape of absorption edge in Sn 2 P 2 S 6 and SnP 2 S 6 crystals, was analyzed.

The Urbach rule for the PbO-SiO2 glasses

An analysis is made of the behavior of optical spectra of lead-silicate glasses, with variable lead content near the UV absorption edge, and within the 80–470-K range. A generalized formulation of the modified Urbach rule, applicable to glassy materials within a broad temperature range, is proposed for the interpretation of experimental spectral relations. Within this approach, the effective energies of the phonons responsible for the temperature-induced shift of the Urbach edge have been calculated. It is shown that the spectral and temperature parameters of the modified Urbach rule are structure-sensitive, and that their concentration behavior reflects the change of the type of short-range order in the glassy matrix.

A photoacoustic spectroscopic investigation of the optical energy gap in Zn–Mn–Se type dilute magnetic semiconductors

Using photoacoustic spectroscopy (PAS) we have measured the composition dependent optical absorption coefficient and band gap of the Zn l− x Mn x Se (0≤ x≤0.5) diluted magnetic semiconductors at room temperature. The band gap E 0 estimated from the PAS spectra varies nonlinearly, showing a downwards bowing with the Mn-concentration. The exchange interaction of electrons in conduction and valence bands with the d-electrons of Mn ion interactions, effects of polytypes, micro-structures and mixed crystallization (zinc-blende and wurtzite structures) are considered for the analysis of the data. The observed exponential edge (Urbach's edge) can be considered as an internal Franz–Keldish effect arising from the charged impurity-generated and “frozen-in” optical phonon-generated fields. This can be described in the framework of the Tauc and Dow–Redfield model. The phonon-assisted indirect transition at the band tail regions for some samples are also observed from the present studies.

Development of a wide-temperature range VUV and UV spectrophotometer and its applications to silica glass

A unique ultraviolet and vacuum ultraviolet spectrophotometer was developed, which enables us to make measurements from 4 to 9.5 eV as photon energy, and over temperatures ranging from liquid helium to 2000°C. Elimination of thermal radiation, which is indispensable for the absorption measurement in the high temperature region, was achieved, and was demonstrated with a fused silica glass and 15.7GeO 2·84.3SiO 2 glass samples. Temperature dependence of the Urbach edge in both samples was measured. In addition a thermal equilibrium reaction between the 4.9 and 5.1 eV bands in Ge-doped silica glass was observed. Radiation damage of silica glass by a D 2 lamp as a light source for the spectrometer was investigated, and defect creation by its vacuum ultraviolet component during measurement was found to occur in some types of silica glass. Some measures to reduce the radiation damage were taken in this apparatus. Furthermore, an absorption band at 6.3 eV was detected.

Disorder, defects, and optical absorption ina−Sianda−Si:H

In this paper we present the optical properties of various structural models for $a\ensuremath{-}\mathrm{Si}$ and $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ We discuss how topological disorder, hydrogen content, and different types of defects (dangling bonds and floating bonds) influence the shape of the optical-absorption spectrum and the position of the Urbach edge. The absorption behavior is characterized by the joint density of states. The band gaps are obtained via the Tauc plot. The principal structure of the optical absorption spectrum is determined mainly by the degree of topological disorder and the amount of hydrogen. The presence of defects gives rise to tail absorption. Dangling bonds affect the optical properties more than floating bonds do.

Effects of pre-hydrogenation on the relaxation and bandgap variation of thermally nanocrystallized silicon layers

Abstract The effects of pre-hydrogenation on the variation in the optical bandgap E 0 in nanocrystallized silicon layers have been analysed in terms involving the relaxation of the structure after thermal annealing. This is suggested by the decrease in the internal stress Σ for increasingly higher hydrogen content in the precursor film, that is before annealing. The striking resemblance noticed between the evolution of Σ and that of the Urbach edge energy E u seems to suggest that, following the case of amorphous silicon, for example. E u would reflect the disorder in our mixed-phase material and then govern the observed variation in the bandgap. The close correlation found between the widening of E 0 and the increase in the hydrogen content in the basic hydrogenated amorphous silicon layer (a-Si:H), combined with the concomitant decrease in Σ or E u, has been interpreted on the basis of the beneficial role of hydride species, such as SiH2, in the relaxation of the structure during the thermal crystallization. These features are less evident for samples partially crystallized at the origin. The impact of the various degree of disorder on the extent of the temperature range of validity corresponding to each of the conduction mechanisms, the hopping and the thermally activated, has also been evidenced.

Excitation of Er3+ emission by host glass absorption in sputtered films of Er-doped Ge10As40Se25S25 glass

Photoluminescence and photoluminescence excitation (PLE) spectroscopy have been carried out on the ∼1550 nm I413/2→I415/2Er3+ emission from thin films of Er-doped As40Ge10Se25S25 glass deposited on silicon substrates by radio frequency sputtering. The PLE spectroscopy shows that the Er3+ emission is excited in the 1-μm-thick film by the Urbach absorption edge of the host glass rather than direct absorption by the Er3+ intra-4f shell transitions. This enables the use of a broad range of pump wavelengths and novel pumping geometries. Comparison of the PLE spectra of the Er emission obtained before and after a rapid thermal anneal (RTA) clearly manifests a blue shift in the band gap induced by the RTA process. These results reveal that the broad band Er PLE mechanism discovered recently in Er-doped bulk chalcogenide glasses and attributed to host glass Urbach edge optical absorption and energy transfer mediated by native defects also occurs in sputtered films of Er-doped chalcogenide glass.

Exponential absorption edge and disorder in Column IV amorphous semiconductors

We discuss the likely origin of the exponential absorption tail, or Urbach edge, of fourfold coordinated amorphous (a-)semiconductors. The present analysis is based on a compilation of a considerable amount of experimental data originating from a great variety of samples, alloys, and authors, and obtained with quite different spectroscopic techniques. An attempt is made to correlate the measured Urbach edge with the structural and optical properties of the samples. The present analysis indicates that the Urbach edge may not only reflect the shape of the joint density of states of the valence and conduction band tails, but may also have important contributions from short-range order potential fluctuations produced by charged defects or impurities.

Cathodoluminescence, Photoluminescence, and Optical Absorbance Spectroscopy of Aluminum Gallium Nitride (AlxGa1−xN) Films

Aluminum gallium nitride (AlxGa1−xN) films, grown by metalorganic chemical vapor deposition on sapphire, were characterized by low-temperature cathodoluminescence (CL) and photoluminescence (PL), and room-temperature optical absorbance. The aluminum fractions are estimated to range from x = 0 to x = 0.444. Most films were silicon-doped. The absorption spectra have a Urbach (exponential) form below the bandgap. The width of the Urbach edge, EU, increases with Al fraction, x, as EU = (0.045 +1 0.104x) eV. The luminescence (CL or PL) spectra show a relatively narrow band-edge peak and a broad deep-level peak. The full-widths at half-maximum of the band-edge CL peaks (measured at T = 15 K) are remarkably similar to the Urbach absorption widths, EU (measured at T = 300 K). PL spectra were obtained from the top surfaces and the film-substrate interfaces of several films. The interface PL spectra of some films show an extra peak 0.15 eV to 0.45 eV below the bandgap, which is ascribed to structural defects or impurity phases localized near the interface. The energy of the band-edge luminescence peak shifts with excitation mode (CL, top-surface PL, or interface PL). This effect is attributed to the variation of the excitation depth, between the top surface and film-substrate interface, with excitation mode, together with the depth variation of film properties such as residual stress or aluminum fraction.

Influence of temperature on the optical absorption edge in amorphous Zn–P thin films

The effect of temperature (in the range 8 to 300 K) on the fundamental absorption edge of amorphous Zn32P68 thin films have been determined from transmission and reflectivity measurements. Both the Tauc gap, EG, and the Urbach energy, EU, describing the absorption edge, have been shown to follow the Einstein expression for the total energy of a solid. Below 150 K, these two parameters are almost independent of temperature, while in the range 150 to 300 K, they change approximately linearly, but in opposite directions (EG decreases from 1.43 to 1.28 eV, while EU increases from 109 to 145 meV). The coordinates of the temperature independent Urbach focus and a linear dependence between the Tauc gap and the Urbach edge have been found. The observed changes of the optical gap and Urbach energy are discussed and compared with the structural study of the amorphous Zn–P films.

Optical properties of laser ablated gallium lanthanum sulphide chalcogenide glass thin films prepared at different deposition laser energy densities

This paper describes the optical properties of GLS thin films deposited by laser ablation technique. These results complement the structural and compositional data on the same specimens reported in the preceding papers. A systematic investigation of the energy dependence of the refractive index, optical absorption edge, Urbach edge and optical gap has been carried out as a function of increasing deposition energy density. The optical absorption coefficient as a function of photon energy, deduced from transmission ( T) and reflection ( R) measurements, shows a very large shift of the edge towards lower energies relative to that of bulk glass with increasing deposition energy density. The optical gap, as determined by Tauc extrapolation, and Urbach parameter have been determined as a function of deposition energy density. The changes in optical properties are correlated with the structural data.

Excitation and temperature quenching of Er-induced luminescence ina-Si:H(Er)

Photoluminescence (PL) and light absorption of Er-doped amorphous hydrogenated silicon are studied in the temperature range 77--300 K. In the linear pumping regime the intensity of the Er-induced luminescence decreases by a factor of about 15 in this temperature range. Frequency resolved spectroscopy shows that in the same temperature range the lifetime of the excited erbium ions in the amorphous matrix decreases by a factor of 2.5 only. Excitation spectroscopy proves that the primary step of the excitation mechanism is the absorption and free carrier generation in the amorphous matrix. The emission is excited effectively by subgap light in the range of the Urbach edge and even in the range of defect absorption. Based on these experimental findings we propose a defect-related Auger excitation (DRAE) mechanism of erbium luminescence. Probabilities of the DRAE and the competing radiative defect recombination processes ${(D}^{0}+e\ensuremath{\rightarrow}{D}^{\ensuremath{-}})$ are calculated. It is shown that the probability of the DRAE process is larger by an order of magnitude. In this model the temperature quenching of the erbium luminescence observed above 200 K results from the competition of the DRAE and multiphonon nonradiative recombination at the defects.

Photoacoustic spectroscopic estimation of optical absorption and thermal diffusivity of amorphous Ge15AsxSe85−x

Photoacoustic spectroscopy (PAS) has been applied to measure the composition dependent optical energy gap (Eo), optical absorption coefficient (α), and thermal diffusivity (σs) of glassy Ge15AsxSe85−x (0≤x≤25) alloys. The energy gap is found to decrease with increase of As concentration and shows a threshold behavior around x∼15, which corresponds to the average coordination number 〈r〉∼2.45. This behavior is found to be consistent with the Phillips-Thorpe theory. The variation of the optical gap (Eo) with composition (x) is analyzed on the basis of the Kastner’s model of bond energies. The observed exponential edge (Urbach edge) can be considered as an internal Franz-Keldysh effect arising due to the charged impurity generated, as well as ‘‘frozen-in’’ optical phonon-generated, electric microfields. It could be described in the framework of Tauc and Dow-Redfield model which ascribes the Urbach rule to the ionization of the exciton as an extension of the stark shift. The concentration (x) dependent thermal diffusivity (σs) estimated from the PAS studies also showed a similar critical behavior at the same concentration x (≊15) which arises due to the threshold percolation of rigidity of the system. The measurements of glass transition temperatures (Tg) and the magnetic susceptibilities (χ) of the samples also support this critical behavior around x=15.

Photoluminescence studies of broadband excitation mechanisms for Dy3+ emission in Dy:As12Ge33Se55 glass

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of Dy2S3-doped As12Ge33Se55 glasses demonstrate that the broad, below-gap PLE mechanism observed previously for rare-earth emissions in Er- and Pr-doped chalcogenide glasses is a general or universal feature of rare-earth-doped chalcogenide glasses, provided the transition energies of the rare earths are in the correct energy range. The PL spectrum excited in the 815 nm Dy3+ absorption band shows the 1150, 1340, and 1700 nm sequence of Dy3+ transitions expected for Dy-doped chalcogenide glasses. The PLE spectra of the 1340 (6F11/2,6H9/2→6H15/2) and 1700 nm (6H11/2→6H15/2) Dy3+ emissions exhibit broad excitation bands from ∼500 to 1000 nm, upon which the sharp intra F-band absorptions of Dy3+ are superimposed. These broad PLE bands are characterized by an exponentially decreasing slope with decreasing energy in the spectral range below the Urbach edge which is associated with the below-gap, defect- and impurity-induced exponential tails observed in the below-gap absorption spectra of chalcogenide glasses. At high energy, the exponentially rising Urbach absorption edge of the host glass, which leads to competing nonradiative decay mechanisms, eventually dominates the absorption spectrum and imposes an exponentially decreasing slope on the PLE spectra. These features of the broadband PLE have been observed in the PLE spectra of the Er-, Pr- and Dy-doped chalcogenide glasses we have studied. There is a difference in the relative strengths of the broad PLE bands, with the broadband for the 1340 nm PL band being a factor of 3 stronger than that for the 1700 nm PL emission. In addition, there is a shift in the peak energy of the different PLE spectra. Qualitatively speaking, the higher the energy of the luminescence transition, the higher the energy of the corresponding broad PLE peak. Proposed mechanisms for the broadband PLE of rare-earth emissions in chalcogenide glasses are discussed in the context of models for the below-gap, defect- and impurity-induced exponential absorption tails, including the possible role of lattice relaxation associated with charge transfer transitions and the involvement of transition metal impurities.

Photothermal deflection spectra of solid

The optical absorption spectra of single crystal and thin film in the weak-absorption region have been measured by photothermal deflection spectroscopy. The optical energy gaps, corresponding to optically forbidden transitions of weak absorption, were derived from Tauc plots. On comparison with the photothermal deflection spectrum of an amorphous semiconductor, the similarity suggests that the gap region of solid can be described in terms used for amorphous semiconductors such as an Urbach edge and sub-gap defect absorption. The weak-absorption characteristics of solid are discussed.

Optical absorption spectra of C70 thin films

Both optical transmission spectroscopy and photothermal deflection spectroscopy are used to determine the spectra of C70 thin films over a wide energy range (0.6–6.5 eV). Based on a molecular orbital model, the optical transitions for the C70 thin film are analyzed. The weak absorption spectra of C70 thin films are similar to that of an amorphous semiconductor. The optical energy gap is derived by a Tauc plot as 1.66 eV. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge and subgap defect absorption, which are interpreted as a broadening due to disorder or impurities. The effects of the deflection medium on the weak absorption spectra of C70 films are discussed.

Sulfur doping in a-SiS x:H

The group VI element, sulfur, is an inefficient donor in a-Si:H. Unlike phosphorus doping, a portion of the sulfur-related donors is passivated by hydrogen in the annealed state. This passivated portion can be rendered electrically active by optical excitation. The compensation by sulfur donors of the p-type conductivity obtained with diborane in a-Si:H provides additional evidence of the role of sulfur as a donor. By adding equal amounts of diborane and hydrogen sulfide in the plasma, the dark conductivity at room temperature can be reduced by one to two orders of magnitude compared to the corresponding p-type a-Si:H with the same boron concentration. In these partially compensated samples the absorption due to deep defects, as measured by photothermal deflection spectroscopy, is reduced and the Urbach edge becomes steeper and shifts to higher energy. In related experiments, phosphorus doping produces conductive n-type samples in a-SiS x :H for sulfur concentrations as high as 10 at.%.

Photo-thermoelectric power of a-Si as a function of incident wavelength

Measurements have been made at room temperature of the thermoelectric power as a function of illumination wavelength on undoped and boron doped films. During these measurements the intensity of the light was increased as the incident photon energy was decreased in such a way that the photocurrent would have remained constant. Results show that in the photon energy range 1–3 eV photocarrier transport takes place predominantly through one conduction path and the position of the quasi Fermi level relative to that conduction path remains invariant for both undoped and p-type specimens. One of the more important assumptions of the constant photocurrent method is that carrier lifetime is independent of photon energy. The results confirm this assumption for measurements on amorphous silicon films in the photon energy range investigated but bring into question the usual interpretation of the Urbach edge for p-type material.

Optical gap and Urbach edge slope in a-Se

Good correspondence between the temperature shift of the optical gap and the Urbach edge slope has been found for a-Se. Such coherence, however, does not exist for photoinduced changes of the gap and Urbach edge slope measured at 77 K. The kinetics of photoinduced changes of the gap takes the form of first-order process, while the kinetics of photoinduced changes of Urbach edge obeys stretched exponential form. It is supposed that overheating due to illumination is most probably not responsible for photoinduced changes. Rather an increase in density of localized states at band edges, together with formation of self-trapped exciton-like states could explain the photoinduced changes observed in the gap and Urbach edge slope.