Urbach Band Tail Research Articles

Microstructural and optical investigations of sol–gel derived ferroelectric BaTiO 3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO 3 nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO 2/Si substrates by a modified sol–gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5–5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO 3/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi–Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n ( 0 ) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262 ± 2 and 268 ± 1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.

Temperature-dependent optical properties of Pb1−xMnxSe films epitaxially grown on Zn0.04Cd0.96Te substrates

We report temperature-dependent optical transmission studies on Pb1−xMnxSe films grown on Zn0.04Cd0.96Te substrates by molecular beam epitaxy. The measurements were carried out from liquid nitrogen temperature to room temperature and in the frequency range of 400–4000 cm−1. The transition energy Eg of 0.215 eV, damping parameter Γ of 4.1 meV, refractive index of 5.0, and Urbach bandtail parameter Eu of ∼8 meV were determined by fitting the experimental transmission spectra for Pb1−xMnxSe at, for instance, x = 0.038 and 85 K. Other parameters, such as absorption coefficient α, the oscillator strength A, and the background dielectric constant ɛ∞ were also obtained. Temperature dependences of Γ and Eu indicate stronger carrier–phonon interactions in the Pb1−xMnxSe films.

Electrical and optical properties of ZnO thin films prepared by magnetron rf sputtering—influence of Al, Er and H

Abstract The influence of Al, Er and H in ZnO thin films (ZnO:Al, ZnO:Er and ZnO:H) deposited by magnetron sputtering at different substrate temperatures, T s , on their optical, structural and electrical properties was investigated. X-ray diffraction (XRD) analyses show an improvement of the crystalline structure with increasing T s . The optical band gap, E opt , of the films, from transmission and reflection spectra, ranged from 3.27 to 3.41 eV. The Urbach band tail width was also calculated. Incorporation of Al and Er resulted in a reduced and an increased resistivity, ρ , respectively, and an increase in the Urbach tail width in both cases. However, sputtering in an Ar+H2 gas mixture led to an increase in ρ and an improvement in the structural order of the films. A discussion of the influence of T s and of Al, Er and H on the properties is presented.

Temperature dependence of the optical properties in GaMnN

Temperature-dependent transmission measurements are carried out on ion-implanted GaMnN with different Mn-implantation doses. With a detailed procedure developed for analyzing the transmission spectra, we obtain the temperature effects in optical properties of GaMnN (including GaN), such as absorption coefficient, band gap, Urbach band tail characteristics, refractive index, and extinction coefficient. Two sets of temperature- and photon energy-dependent empirical formulas are established, which not only unify the various experimental data reported in the literature, but also provide an experimental database of optical properties in GaMnN. Furthermore, we reveal that the dependence of these optical properties on the Mn-implantation doses is closely related to the impact of Mn on the crystal structure.

Critical point transitions of wurtzite indium nitride

The optical transmission, photoluminescence, and reflection spectra have been measured on a high-quality wurtzite indium nitride (InN) single crystal in the range of 0.5–20.0 eV. The fundamental bandgap of intrinsic InN has been extracted by taking into account the Burstein–Moss shift, bandgap renormalization and Urbach band tail effects, and found to be very close to the recent strongly re-established value of ∼1.2 eV. With the aid of Adachi's dielectric function model for the vacuum ultraviolet reflection spectra and the empirical pseudopotential method approach for the electron band-structure, we are able to identify up to nine electronic transitions, showing clear picture for the critical point transitions in InN. The temperature dependence of these interband transitions has also been revealed.

A proposal for complete interband absorption in indirect semiconductors

Based on the absorption nature of semiconductors, we propose an improved approach, together with the normal intrinsic and Urbach band tail transitions, for the completeness of interband absorption in indirect semiconductors. Behind this modification lies more meticulous analysis of the transitions where electrons are permitted to transit to both the parabolic band edge with a phonon absorbed and the band tail edge with a phonon emitted. The calculated transmissivity by this novel approach has turned out to be in much better agreement with the experimental results of silicon samples, which paves the way for preferable analysis of indirect semiconductors.

Temperature effects on optical properties of InN thin films

Transmission measurements have been carried out on InN thin films grown by radio frequency magnetron sputtering on a sapphire (0001) substrate at 10–300 K. With the aid of a novel procedure developed for analyzing the transmission spectra, the effect of temperature on optical properties, such as the absorption coefficient, band-gap, Urbach bandtail characteristics, refractive index and extinction coefficient, of InN thin films has been determined. The wavelength and temperature dependence of the absorption coefficient in both the Urbach and intrinsic absorption regions has been described by a series of empirical formulae. The temperature dependence of the refractive index dispersion below the band-gap is also found to follow a Sellmeier equation. These formulae are very useful for the characterization and device design of InN films. The free-electron concentration in the InN thin film determined here is also found to be in good agreement with that obtained from infrared reflection measurements.

Optical band gap studies and estimation of two photon absorption coefficient in alkali bismuth borate glasses

The optical absorption spectra of the glasses with composition xBi2O3·(30 − x)R2O·70B2O3 (R˭Li, Na, K) and xBi2O3·(70 − x)B2O3·30Li2O (0 ≤ x ≤ 20) have been recorded in the wavelength range 350–650 nm. The glass samples were prepared by the normal melt–quench technique. The fundamental absorption edge for all the series of glasses is analyzed using the theory of Davis and Mott. The position of absorption edges and the values of optical band gap are dependent on the mol% of Bi2O3. The absorption in these glasses is associated with indirect transitions. The values of Urbach's energy and band tailing parameters are reported. The two photon absorption coefficient, β, in these glasses has also been estimated from the optical band gap and its value ranges from 1.3 to 11.6 cm/GW. The relationship between β and glass composition has also been discussed in terms of the electronic structure of the glass system.

Band-tail characteristics in polysilicon

The temperature- and growth-dependent Urbach band-tail characteristics in polysilicon wafers have been investigated both experimentally and theoretically for the improvement of solar cells. The band-tail parameter in polysilicon has been obtained by temperature-dependent transmission spectra, with the aid of a detailed calculation of the transmission profile. The band-tail parameter is in the range of 30–90 meV in polysilicon. A band-tail model based on the calculation of density of occupied states and the carrier–phonon interaction has been employed to analyze the temperature-dependent band-tail characteristics. It is found that the yielded structural information can be well explained by Raman measurements. We have correlated various aspects of the observed structural properties, such as the band-tail parameter, trap concentration, grain–surface charge density, and correlation length, with the conversion efficiency of the polysilicon solar cells.

Bandtail characteristics in InN thin films

The Urbach bandtail characteristics in InN thin films grown by radio-frequency magnetron sputtering on sapphire (0001) substrates have been investigated both theoretically and experimentally. The bandtail parameter in InN thin films has been obtained by temperature-dependent transmission spectra, with the aid of a detailed calculation of the transmission profile. A bandtail model based on the calculation of density of occupied states and the carrier–phonon interaction has been employed to analyze the temperature-dependent bandtail characteristics. The bandtail parameter is in the range of 90–120 meV in the InN thin film. It is found that the carrier–phonon interaction in InN is weak and the structural disorder contribution (∼90 meV) dominates over the interactive terms. The high structural disorder in InN thin films may relate to the high nonradiative recombination centers.

Deposition of polymeric nitrogenated amorphous carbon films (a-C:H:N) using electron cyclotron resonance CVD

In this paper the deposition of polymeric nitrogenated amorphous carbon (a-C:H:N) films from a mixture of hydrogen, methane and nitrogen, using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system is reported. Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA) measurements were used to determine the actual amount of nitrogen, carbon and hydrogen in the films. The results showed that there is insignificant change in the atomic concentration of C but a slight decrease in the atomic concentration of H with increasing N incorporation. A slight decrease in the amount of bonded H and an increase in the CN bond was also observed as deduced from infrared (IR) absorption measurements. The optical gap was found to decrease and the Urbach band tail width increase at larger N 2 flow ratio. The conductivity increased by three orders of magnitude compared to that of films prepared in the absence of N 2. The results suggest that the observed CN double bond effectively bridges the aromatic rings resulting in a delocalization of carriers.

N+ion–target interactions in PPO polymer: A structural characterization

N+ ion beam induced effects on the spin coated amorphous poly(2,6-dimethyl phenylene oxide) (PPO) films in terms of chemical structure and electronic and vibrational properties were investigated using Fourier Transform Infrared spectroscopy (FTIR) and Ultraviolet–Visible (UV–VIS) spectroscopy. Both techniques revealed that the stability of PPO was very weak towards 100 keV N+ ions revealing the threshold fluence to be 1014 ions/cm2 for fragmentation of the polymer. FTIR analysis showed disappearance of all characteristic IR bands at a total fluence of 1014 ions/cm2 except for the band CC at 1608 cm−1 which was found to shift to a lower wave number along with an enhancement in the full width half maximum (FWHM) value with increasing fluence. A new bond appeared due to oxidation as a shoulder at 1680 cm−1 in FTIR spectra indicating the presence of CO type bond as a result of N+ implantation on PPO films. The optical band gap (Eg) deduced from absorption spectra, was observed to decrease from 4.4 to 0.5 eV with fluence. The implantation induced carbonaceous clusters, determined using Robertson’s formula for the optical band gap, were found to consist of ∼160 fused hexagonal aromatic rings at the maximum energy fluence. An enhanced absorption coefficient as a function of fluence indicated incorporation of either much larger concentration of charge carriers or their mobility than that of the pristine sample. Calculated band tail width from Urbach band tail region for the implanted samples pointed the band edge sharpness to be strongly dependent on fluence indicating an increased disorder with increasing fluence.

Low filament temperature deposition of a-Si:H by hot-wire chemical vapor deposition

Hydrogenated amorphous silicon, a-Si:H, is deposited from silane and hydrogen by hot-wire chemical vapor deposition using a tungsten wire filament at a temperature Tfil=1200 °C. Film properties depend on whether the films were deposited using filaments with an accumulated deposition time lower than 90 min (‘‘new’’ filaments) or longer than 90 min (‘‘old’’ filaments). The deposition rate for films deposited with ‘‘new’’ filaments is 4 times higher than that for aged filaments. For ‘‘new’’ filaments, a monotonic increase of the growth rate, rd, with the pressure is observed, as well as a maximum of rd for FH2/FSiH4 close to unity. The optoelectronic properties are controlled by the substrate temperature Tsub, and show different dependences for ‘‘new’’ and ‘‘old’’ filaments. The Urbach band tail energy, Eu, is lower for films deposited with ‘‘new’’ filaments. A kinetic growth model for hot-wire chemical vapor deposition in the Tfil∼1200 °C regime is proposed.

Random walks in an exponential band.

The behavior of a single random walker on a cubic lattice with energetic disorder is simulated. Site energies are selected at random from an exponential distribution, simulating thermally activated hopping in the Urbach band tail of a disordered semiconductor. The apparent fracton dimension d of the system [from S(t)\ensuremath{\propto}${t}^{d/2}$, where S(t) is the number of distinct sites visited] varies linearly with temperature for up to 3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ simulation time steps. A separate examination of site visitation as a function of successful hop attempts and the hopping rate reveals more complex behavior.

Band tails in disordered systems.

The density of electron states, \ensuremath{\rho}(E), in disordered systems in the band-tail region near band edges is investigated. We show that the \ensuremath{\rho}(E) of the Halperin and Lax type derived by Sa-yakanit predicts, in three dimensions, an exponential (Urbach) band tail for the correlation lengths found in amorphous Si and within the energy range observed in optical absorption near band edges. The simple exponential behavior is not universal and may not, for example, be observed in heavily doped semiconductors.