Energy Position Of Peak Research Articles

Optical Absorption of Nitrogen Vacancy in Proton Irradiated AlxGa1-xN thin Films

AbstractOptical absorption measurements were used to investigate deep defects in proton irradiated doped and undoped AlGaN thin films grown on sapphire substrates. Several samples were proton irradiated with energies ranging between 10 keV and 1 MeV. In certain samples, multiple-energy ion implantation was found necessary to produce a defect, which is responsible for the absorption band observed at 4.61 eV with a shoulder at around 4.10 eV in Al0.6Ga0.4N. Furnace thermal annealing of the irradiated samples show that this absorption band starts to anneal out at temperature as low as 200 oC. A combined isochronal and isothermal annealing in the temperature range of 200- 350°C shows that the activation energy (enthalpy associated with the migration process) of this defect is approximately 0.41 eV. This leads us to conclude that this absorption band is due to a N-vacancy related defect. It is observed that the peak position energy of the absorption band due to this defect is shifted depending on the Al mole fraction in good agreement with the theoretical predictions.

Study of gain mechanisms in AlGaN in the temperature range of 30–300 K

We report the results of an experimental study of the stimulated emission (SE) properties of AlGaN epilayers grown by metalorganic chemical vapor deposition under high optical excitation conditions in the temperature range of 30–300 K. The band gap and energy position of spontaneous and SE peaks were measured over the entire temperature range studied. Through an analysis of the temperature dependence of the relative energy positions and the SE threshold, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density at threshold to be ∼1019 cm−3 throughout the temperature range studied. Such a high carrier density indicates that an electron-hole plasma is responsible for the generation of gain in this material system from 30 to 300 K. Issues related to the development of short-wavelength AlGaN-based light emitting devices are discussed.

Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy

Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.

Thermal annealing recovery of intersubband transitions in proton-irradiated GaAs/AlGaAs multiple quantum wells

Intersubband transitions in 1 MeV proton-irradiated GaAs/AlGaAs multiple quantum wells were studied using an optical absorption technique and isochronal thermal annealing. The intersubband transitions were completely depleted in samples irradiated with doses as low as 4×1014 cm−2. More than 80% recovery of these depleted transitions was achieved after the samples were thermally annealed at temperatures less than 650 °C. The total integrated areas and peak position energies of the intersubband transitions in irradiated and unirradiated samples were monitored as a function of annealing temperature. It was noted that the recovery of the depleted intersubband transitions in irradiated samples depend on the irradiation dose and thermal annealing temperature.

Strain distribution in self-assembled InP/GaInP quantum dots

Local strain on an atomic scale, as well as the size and the shape of InP quantum dots (QDs) embedded in GaInP, have been measured directly from high-resolution electron microscopy images. There is a strong spatial variation in strain within a QD. A large part of the misfit strain is distributed in the GaInP matrix surrounding the QDs, especially in the case of small spacing between the QD layers. The strain distribution varies significantly with the thickness of the GaInP spacer layers between QD layers. The compressive strain within the QDs decreases with decreasing spacing between the QD layers. This contributes to an increasing red shift of photoluminescence energy peak positions of QDs of multi-layers with decreasing thickness of the spacer layers between the QD layers in comparison with that of single layer QDs.

Peak position of the intersubband absorption spectrum of quantum wells with controlled electron concentrations

We investigated the influence of electron concentration ${N}_{s}$ on the peak position energy of intersubband absorption spectra in single modulation doped $\mathrm{GaAs}\ensuremath{-}{\mathrm{Al}}_{0.4}{\mathrm{Ga}}_{0.6}\mathrm{As}$ quantum well structures of different thicknesses and doping profiles. We found experimentally very small shifts of peak position as a function of ${N}_{s}.$ Also the sign of the energy shift is dependent on the well thickness. Findings were well explained by a compensation between depolarization shift and shift due to static Coulomb interaction of the electrons. The effect of the well width and the doping profile on the peak shift were also discussed.

Micro-photoluminescence from CdSe quantum dots

We have investigated CdSe quantum dots (QDs) by micro photoluminescence (PL). To reduce the number of QDs to be observed, Al masks with sub-micron size apertures are deposited on the sample surfaces, which make it possible to repeatedly trace the same excitation position in taking temperature dependent micro-PL spectra. Resolved sharp lines having linewidth ∼500 μeV are observed. The results of temperature dependence reveal that the linewidths of QDs broaden with increasing temperature, which is considered to come from lifetime broadening of ground state by absorption of phonons. The linewidth broadening depends on the peak energy position of the QDs. The linewidths of lower-energy lines corresponding to larger size QDs, are more temperature-dependent than those of higher-energy lines corresponding to smaller ones. The different linewidth broadening can be qualitatively explained by taking into consideration of different energy level spacing between discrete states of the QDs.

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 1010 cm−2. The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along $$[1\bar 10]$$ direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.

Damage Due to Nitrogen Molecular Ions of GaN Heteroepitaxial Layers Grown on Si(001) Substrates by Molecular Beam Epitaxy Assisted by Electron Cyclotron Resonance

We have observed that the intensity of plasma emission at 391 nm from nitrogen molecular ions in nitrogen plasma is closely related to the crystalline quality and the surface morphology of GaN heteroepitaxial layers grown on Si(001). When plasma emission intensity is increased, the surface morphology is degraded, the photoluminescence (PL) intensities of two donor bound exciton (D0X) emissions from mixed crystal grains of wurtzite-GaN (α-GaN) and zincblende-GaN (β-GaN) and of yellow emissions are abruptly decreased, and the full-width at half maximum of the D0X is broadened. These reflect the influences of damage due to nitrogen molecular ions. The damage generates nonradiative centers. A small number of (001)- and (111)-oriented β-GaN crystal grains exist in the layers, together with a large number of (0001)-oriented GaN. PL efficiency from β-GaN is markedly higher than that from α-GaN, probably because the majority of the carriers accumulate in the β-GaN side at the interface between α- and β-GaN. The broad PL emissions at 3.10 and 3.29 eV with weak intensities are not changed by the damage. The peak energy position of the 3.29 eV emission almost coincides with that of D0X(β-GaN). The damage is not easily eliminated even by high-temperature growth at 900°C.

Presentation and experimental validation of a single-band, constant-potential model for self-assembled InAs/GaAs quantum dots

A single-band, constant-confining-potential model is applied to self-assembled InAs/GaAs pyramidal dots in order to determine their electronic structure. The calculated energy eigenvalues and transition energies agree well with those of more sophisticated treatments which take into account the microscopic effects of the strain distribution on band mixing, confining potentials, and effective masses. The predictions of the model are compared with several spectra reported in the literature by different authors. Very good agreement with both energy position and number of peaks in such spectra is found. The hole energy splitting between ground and first excited states deduced from capacitance and photoluminescence measurements is in excellent agreement with our calculated values. The simplicity and versatility of the model, together with its modest computational demands, make it ideally suited to a routine interpretation and analysis of experimental data.

Energy loss of 18 keV electrons in gaseous T and quench condensed D films

Measurements of the energy loss of fast electrons at an energy of 18 keV have been performed on molecules of hydrogen isotopes, gaseous T 2 and frozen D 2 . Whereas in the case of gaseous T 2 the values of total inelastic cross-section ( cm2 for E = 18.6 keV), average energy loss ( eV) and peak position of the energy loss spectra ( eV) agree well with the expectations, the corresponding values for quench condensed D 2 differ significantly from the ones for gaseous T 2 . We observe a significant lower total inelastic cross-section ( cm2, for E = 18.6 keV) larger average energy loss ( eV) and higher peak position ( eV). These differences may be interpreted in terms of changes of the final state spectrum. A CI calculation for a D 2 cluster shows indeed a clear shift of the excited states in agreement with the observation.

Biexciton photoluminescence in cubic ZnS single crystals

The near-band-edge emission of cubic zinc sulfide single crystals has been studied in detail by steady-state photoluminescence (PL) and time-resolved photoluminescence. Based on the peak energy positions, excitation power density and temperature dependencies of the PL spectra, we interpret the emission as recombination of the free and bound excitons and of the biexcitons. We observed the biexciton emission at 3.791 eV in cubic ZnS single crystals with very low excitation power density. Its binding energy and radiative lifetime were determined to be about 9 meV and 52 ps at 12 K, respectively.

Photoemission study of the bulk and surface electronic structure of Bi(111)

Angle-resolved energy distributions for photoelectrons emitted from the Bi(111) single crystal face are presented at 16.85 eV photon energy. The variation of peak-energy positions as a function of the electron emission angle shows four main dispersing features located between the Fermi level and 5 eV. The bulk electronic structure is found in qualitative agreement with the pseudopotential calculation performed by Golin along the TMU and TQW symmetry lines of the Brillouin zone. The experimental features located at around 0.3 and 3 eV below the Fermi level are sampled in the two-dimensional Brillouin zone along the and symmetry directions (dispersion about 1 eV) and are assigned to electronic surface states.

Proton implantation and rapid thermal annealing effects on GaAs/AlGaAs quantum well infrared photodetectors

We report the use of intermixing techniques to modify GaAs/AlGaAs multiple quantum wells (MQWs). A large shift in the response wavelength of the GaAs/AlGaAs MQW-based infrared photodetector is obtained by proton implantation and then a standard annealing procedure ( 950∘C for 30 s). The photoluminescence (PL) and photoresponse spectra were measured as functions of ion dose in the range from 5 × 1014to 2.5 × 1015cm−3. The peak photoresponse wavelength was tunable between 8.2 and 9.8μ m for the infrared radiation and the energy position of the PL peak from the MQW material changed from 1.62 to 1.645 eV. The effects of the ion implantation and thermal annealing on the device performance have been well characterized theoretically by the inter-diffusion of Al atoms across the GaAs/AlGaAs heterointerfaces and the relaxation energy of free carriers.

Temperature dependence of exciton peak energies in ZnS, ZnSe, and ZnTe epitaxial films

High-quality ZnS, ZnSe, and ZnTe epitaxial films were grown on (001)-GaAs-substrates by molecular beam epitaxy. The 1s-exciton peak energy positions have been determined by absorption measurements from 2 K up to about room temperature. For ZnS and ZnSe additional high-temperature 1s-exciton energy data were obtained by reflectance measurements performed from 300 up to about 550 K. These complete E1s(T) data sets are fitted using a recently developed analytical model. The high-temperature slopes of the individual E1s(T) curves and the effective phonon temperatures of ZnS, ZnSe, and ZnTe are found to scale almost linearly with the corresponding zero-temperature energy gaps and the Debye temperatures, respectively. Various ad hoc formulas of Varshni type, which have been invoked in recent articles for numerical simulations of restricted E1s(T) data sets for cubic ZnS, are discussed.

Properties of photoconductive In 2Se 3 thin films, crystallized by post-deposition heat treatment in nitrogen atmosphere

Textured thin films of γ-In 2Se 3 were grown on glass substrates by sequential thermal evaporation of In and Se. The crystallization was achieved by annealing the as-deposited films in flowing nitrogen. It is shown by X-ray diffraction (XRD) measurements that single phase γ-In 2Se 3 films are obtained by annealing half an hour at 673 K. The films obtained on bare glass substrates and on SnO 2 coated glass substrates are textured along the (001) direction while those obtained on mica sheet and gold coated glass are not. The films are stoichiometric. Films deposited on glass substrates have been optically characterized. At room temperature, the measured band gap is 1.8 eV. In the case of thick films (1 μm), the surface of the films becomes rough, which can explain the higher photoconductivity of the films when the light beam is incident upon the substrate. Effectively, at the interface glass/film there is not any rugosity and the trap density is smaller than at the interface film surface/air. Photoluminescence spectra are obtained in the temperature range 78–200 K. Two peaks are observed. The width and the energy position of the main peak are temperature dependent. It is situated in the gap energy range and can be attributed to exciton recombination, while the other small one can be attributed to some intrinsic defects.

EELS peak intensity dependence on primary electron energy for the Si(111)7×7 and Si(111)-Cr surface structures

The intensity of elastic and bulk plasma loss peaks and also the energy position of the bulk plasma loss peak in the electron energy loss spectra of the Si(111)7×7, Si(111)1×1-Cr and ( 3 × 3 )R30-Cr surface structures were studied as a function of the primary electron energy, E o, in the 200–3000 eV range. It was found that the intensity of elastic and bulk plasma loss peaks had oscillations with periods of 120–140 and 750–900 eV. These oscillations can be caused by the effects of electron diffraction from lattice planes and variation of the electron backscattering coefficient with depth, respectively. During formation of the Si(111)1×1-Cr and α( 3 × 3 )R30-Cr structures the oscillations with periods 120–140 eV were partly retained, showing the existence of an Si(111) lattice framework in subsurface region of the substrate. However, under formation of the Cr-rich phase with β( 3 × 3 )R30-Cr structure they disappeared, showing reconstruction of the Si(111) lattice in the subsurface region. The evolution with E o of the intensity of elastic and bulk loss peaks is correlated with the change in composition and thickness of the different structures of the Si(111)-Cr surface layer. It has been estimated that the thickness of the Cr-rich phase with β( 3 × 3 )R30-Cr structure is between 8 and 15 Å.

X-ray absorption in relation to valency of iridium in sputtered iridium oxide films

Abstract Electronic and structural changes induced by the charge storage reaction due to proton insertion in sputtered iridium oxide films (SIROFs) have been investigated by in situ X-ray absorption spectroscopy at the L3 edge of iridium atoms in 1 M H2SO4. The iridium valency is shown to increase from 3 to 3.85 when the potential varies from −0.2 to +1 V(SCE). In XANES spectra, the white line peak height and energy position decrease with insertion. The fine structures of the spectra have been analyzed and simulated in view of structural parameter extraction. A correspondence curve is established between the interatomic Ir–O distance in the first shell and the iridium valency. A strong decrease of this distance is observed with the oxidation state of iridium accompanied by a conspicuous decrease of the Debye–Waller factor.

Photoluminescence Measurements in Interband Transition in Fast Neutron Irradiated In0.07Ga0.93As/A10.4Ga0.6 As Multiple Quantum Wells

AbstractInterband transitions in n-type InGaAs/AlGaAs multiple quantum wells were studied using a photoluminescence (PL) technique after the samples were irradiated with fast neutrons. It was observed that the PL intensity of the interband transition is reduced as the irradiation dose is increased. In Addition, the peak position energy of the interband transition was increased and then decreased as a function of irradiation dose. The results are explained in terms of electrons trapping, many-body effects, and irradiation-induced damages at the interfaces as well as in the well and barrier regions.

Energy loss in large-angle scattering of slow, highly charged Ar ions from a Au surface

Energy loss of slow ($v\ensuremath{\approx}0.06 \mathrm{a}.\mathrm{u}.$), highly charged ${\mathrm{Ar}}^{9+}$ ions scattered from a Au(111) single-crystal surface has been investigated for different incident angles ($\ensuremath{\psi}=5\ifmmode^\circ\else\textdegree\fi{}$ and $37.5\ifmmode^\circ\else\textdegree\fi{}$) and a fixed scattering angle of $75\ifmmode^\circ\else\textdegree\fi{}.$ The energy position of the elastic peak agrees well with the value expected for the binary elastic scattering. At small incidence angle $(5\ifmmode^\circ\else\textdegree\fi{})$ an additional inelastic energy loss was identified in spite of the large scattering angle. This additional energy loss is found to compare well with a combination of a modified Firsov model with processes of electron capture, recapture, Auger transitions, and image charge acceleration during the ion-surface interactions.