Low-energy Photoluminescence Band Research Articles

Photoluminescence induced by twinning interface in CdS nanocrystals

The photoluminescence (PL) spectra acquired from CdS nanocrystals encapsulated with oleic acid synthesized by a two-phase approach exhibit two fine features, including a narrow peak arising from near-band edge emission and a broader one composed of two subpeaks at slightly lower energy. Solvent effects suggest that the surface defect states on the nanocrystals are not the origin of this broad PL band. High-resolution transmission electron microscopy examinations and density function theory calculation reveal that the broad low-energy PL band stems from twinning interfaces in the CdS nanocrystals.

Relation between conductance, photoluminescence bands and structure of ITO nanoparticles prepared by various chemical methods

To study the relation between conductance, photoluminescence bands and structure of indium tin oxide (ITO), nano- and microparticles of ITO were synthesised using various methods, including hydrothermal, oxalate precursor decomposition and Pechini type sol–gel combustion methods. The resultant powders were analysed by X-ray diffraction (XRD), resistance measurement, photoluminescence (PL) spectroscopy and scanning electron microscopy techniques. The crystallographic and microstructural parameters of samples were refined from the XRD patterns utilising Rietveld method. Two PL bands were observed in 475 and 560 nm and it was found that the low-energy PL band intensity presents direct relation with conductivity of samples while the other one shows inverse relation. Pechini samples demonstrated the highest conductivity and low-energy PL band intensity, while hydrothermal sample has low conductivity. It was suggested that the low-energy PL band is the characteristics of excitons trapped in oxygen vacancy while the high-energy PL band is attributed to the excitons trapped in other defects centre. The colour of samples and data resulted from XRD pattern refinement were also validated. According to the refinement results and Popa's model, all samples show anisotropic micro-strain and crystallite size. It was also deduced that hydrothermal sample was textured in the 〈111〉 direction.

Localization dynamics of exciton luminescence in InxGa1−xN epitaxial films

Picosecond time resolved photoluminescence (PL) spectroscopy of excitonic processes in MOCVD grown InxG1−xN mixed films with the In concentration in range from x=0.1 to 0.18 under the band-to-band excitation are considered. It is stated that by an In content in alloy up to 12% the band-band photo excitation at 8 K results in creating of localized excitons and biexcitons represented by close overlapping Gaussian shape luminescence bands having FWHM 27 and 8.7 meV, respectively. PL decay kinetics of both bands involves two exponential decay stages. Excitons and biexcitons in unperturbed lattice positions causes fast decay with τ∼ 10 ps, whereas their transfer to the metastable state due to relaxation of In – Ga local configuration causes slow decay τ ∼ 90 ps At increased In content up to 19% both the localized excitons and biexcitons are represented by Gaussian type luminescence bands being non-uniform broadened to the high energy side. The continuous distribution of excitons and biexcitons in transition energies is stated by analysis of the red-shift of luminescence bands during the spectra decay. An additional new narrow low energy PL band arises at low energy side expected to be caused by excitons at InN clusters.

Photoluminescence in microcrystalline silicon films grown from argon diluted silane

The low-energy photoluminescence (PL) of microcrystalline silicon films grown by plasma-enhanced chemical vapor deposition using argon (Ar) diluted silane has been investigated. For the samples with high crystalline volume fraction ( X c) of more than 80%, it is observed that the luminescence peak appears near 1.0 eV, which is much higher than the value of 0.9 eV observed for the samples with a similar X c grown from hydrogen dilution. Also, the low-energy PL peak shifts to the higher energy with decreasing deposition power, but it is nearly independent of Ar dilution ratio. The low-energy PL band is suggested to basically arise from a superposition of at least three subbands. From the results it is suggested that the PL band centered at 1.0 eV originates from radiative band tail-to-tail transitions in the crystallites.

Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased, (c) the low-energy optical absorption at photon energy <1.4 eV increased one to two orders of magnitude, (d) the low-energy PL band at ∼1.0 eV emerged with a decrease of total PL intensity, and (e) the conductivity activation energy decreased. The aforementioned changes correlated well with the crystallinity of the material. We attribute the observations mainly to the formation of the c-Si gain boundaries during crystallization.

Origin of the Low-Energy Photoluminescence in Microcrystalline Silicon Films

AbstractIn this work we have investigated the low-energy photoluminescence (PL) band with a peak between 0.8 eV and 1.0 eV for microcrystalline silicon films (μc-Si:H) grown under various growth conditions. At least four subbands are observed, the peaks of which are located near 0.80 eV, 0.87 eV, 0.92 eV, and 0.97 eV, respectively. It is suggested that the low-energy PL band basically arises from a superposition of these subbands, whose intensities strongly depend on deposition conditions, and thus its peak is determined by the sum of these subband intensities. From the results, it is suggested that the subband centered at 0.92 eV originates from defect-related radiative recombination in the amorphous phase rather than radiative band tail-to-tail transitions in the grain boundaries.

Increasing The Dark Conductivity Activation Energy in Undoped Microcrystalline Silicon by Post-Growth Anneals

ABSTRACTUndoped µc-Si:H film of the strong n-type character with the dark conductivity activation energy of about 0.28 eV was annealed. Annealing was carried out by slowly increasing the temperature from 25 °C to 450 °C at a constant rate of 12 °C/min (one annealing cycle). Annealing effects were monitored by measuring the changes in dark conductivity, oxygen and hydrogen concentrations, and photoluminescence (PL). Dark conductivity activation energy gradually increases with increasing the number of annealing cycles to a saturation value of about 0.6 eV. There is little or no change in the oxygen concentration, but the hydrogen concentration decreases with increasing the number of annealing cycles. The PL band near 1.2 eV disappears with annealing, while the low energy PL band near 0.85 eV dominates rather as the number of annealing cycles increases. A possible explanation will be discussed.

Photoluminescence study of CdTe/ZnTe self-assembled quantum dots

We report on optical properties of CdTe self-assembled quantum dots (SADs) grown by molecular beam epitaxy on ZnTe. Formation of SADs was achieved by deposition of 1.5–2.5 monolayers of CdTe at a substrate temperature of 420 °C and by applying growth interrupts for few seconds in Cd flux. The resulting dots have a typical diameter of 2 nm and a sheet density of 1012 cm−2. At T=2 K the photoluminescence (PL) spectra consist of two emission lines. The high-energy line originates from excitonic recombination in a wetting layer while the low-energy emission PL band is assigned to recombination in SADs. The increase in temperature up to 70 K does not affect the SADs-related emission intensity. It shifts, however, the PL peak energy towards low energies and causes a significant narrowing of the PL linewidth, from 80 meV at 1.9 K to 50 meV at 130 K. The activation energy of the thermal quenching of SADs-related PL emission was found to be equal to 47 meV. This value is three times greater than the one observed in CdTe/ZnTe quantum wells, that is, 12–17 meV.

Spin Dependent Photoinduced Absorption in a-Si:H

ABSTRACTWe have studied photoexcitation dynamics in undoped a-Si:H from 80 K to 300 K by the techniques of photoluminescence (PL), photoinduced absorption (PA) and their respective versions of optically detected magnetic resonance, namely PLDMR and PADMR. Both PL and PA spectra are composed of low and high energy bands, respectively. Using their respective temperature dependences we correlate the low energy PA band (∼ 0.4 eV) to the high energy PL band (∼1.3 eV) and the high energy PA band (∼ 1 eV) to the low energy PL band (∼ 0.8 eV). We also found that the PADMR spectrum is composed of three main contributions: a narrow resonance at g ∼ 2 (FWHM = 15 G), a broad resonance at g ∼ 2 (160 G) and a narrow asymmetric resonance at g ∼ 4 (20 G). By measuring their photon energy spectrum we found that the g ∼ 4 and the broad g ∼ 2 resonances are correlated with each other and therefore are due to trapped electron-hole pairs in the triplet spin configuration.

Plasma damage and acceptor passivation in D2-plasma-treated InP:Zn: A photoluminescence and ellipsometry study.

Deuteration of Zn-doped InP was performed by indirect exposure of the InP surface to a remote ${\mathrm{D}}_{2}$ glow-discharge plasma. The dopant passivation and plasma-induced defects were investigated by photoluminescence (PL) and spectroscopic ellipsometry as a function of temperature, ${\mathit{T}}_{\mathrm{pt}}$, during plasma treatment in the range 30--220 \ifmmode^\circ\else\textdegree\fi{}C. It is found that low ${\mathit{T}}_{\mathrm{pt}}$ causes weak surface alteration whereas high ${\mathit{T}}_{\mathrm{pt}}$ leads to preferential etching of P and the formation of In islands. At an intermediate deuteration temperature ${\mathit{T}}_{\mathrm{pt}}$=90 \ifmmode^\circ\else\textdegree\fi{}C, the surface is less affected by the plasma treatment compared to any other ${\mathit{T}}_{\mathrm{pt}}$. In addition, a broad, low-energy PL band in the range 1.24--1.34 eV shows up. These results are explained by the in-diffusion of P vacancies (${\mathrm{V}}_{\mathrm{P}}$) created at the surface and the subsequent formation of P vacancy-related defects in the bulk of InP. Complexes such as (${\mathrm{V}}_{\mathrm{P}}^{+}$${\mathrm{Zn}}_{\mathrm{In}}^{\mathrm{\ensuremath{-}}}$) are believed to form and yield the PL band, in analogy with earlier PL studies on GaAs:Zn. The results of secondary-ion mass spectrometry and effusion measurements indicate that the diffusion of D proceeds by trapping on the Zn atoms, thus confirming the dopant passivation seen by PL measurements. In addition, the sample treated at 90 \ifmmode^\circ\else\textdegree\fi{}C contains a significant fraction of excess deuterium near the surface, which passivates the plasma-induced recombination centers. Additional centers are observed in InP deuterated between 90 \ifmmode^\circ\else\textdegree\fi{}C and 170 \ifmmode^\circ\else\textdegree\fi{}C. These centers can be partly annealed by laser irradiation at 77 K. The resulting strong enhancement of the low-energy PL band is interpreted as being due to the rearrangement of ${\mathrm{V}}_{\mathrm{P}}$-related recombination centers into ${\mathrm{V}}_{\mathrm{P}}$-related radiative centers assumed to be ${\mathrm{V}}_{\mathrm{P}}^{+}$ ${\mathrm{Zn}}_{\mathrm{In}}^{\mathrm{\ensuremath{-}}}$.