S-shaped Temperature Dependence Research Articles

Localization and Hopping of Excitons in Quaternary AlInGaN

Evolution of photoluminescence spectra in quaternary AlInGaN alloys is studied as a function of indium content. An emergence of an S-shaped temperature dependence of the luminescence peak position and a W-shaped temperature dependence of the line width was observed with a gradual increase of the In molar fraction. This infers that the incorporation of indium into AlGaN results in formation of a network of states that facilitate exciton localization and hopping typical of ternary InGaN.

The effects of concomitant In and N incorporation on the photoluminescence of GaInNAs

We investigated the effects of concomitant In- and N-incorporation on the photoluminescence (PL) of GaInNAs grown by molecular beam epitaxy. In comparison with the N-free GaInAs epilayer, the PL spectra of the GaInNAs epilayer exhibit an anomalous S-shape temperature dependence of dominant luminescence peak. Through further careful inspection, two PL peaks are clearly discerned and are associated with the interband excitonic recombinations and excitons bound to N-induced isoelectronic impurity states, respectively. By comparing the PL spectra of GaInNAs/GaAs quantum wells (QWs) with those of In-free GaNAs/GaAs QWs grown under similar conditions, it is found that the concomitant In- and N-incorporation reduces the density of impurities and has an effect to improve the intrinsic optical transition of GaInNAs, but also enhance the N-induced clustering effects. At last, we found that rapid thermal annealing can significantly reduce the density of N-induced impurities.

Optical investigations of GaInNAs/GaAs multi-quantum wells with low nitrogen content

The optical properties of GaInNAs/GaAs multi-quantum wells were investigated by photoluminescence excitation (PLE) spectroscopy, as well as by photoluminescence (PL), under various excitation intensities and at various temperatures. The PLE spectra demonstrated pronounced excitonic features and the corresponding transitions were identified. At low temperatures the PL spectra were sensitive to the excitation intensity. Under fixed excitation intensity, both the peak energy and the linewidth of photoluminescence showed anomalous temperature dependence, specifically an S-shaped temperature dependence of the peak energy and a N-shaped temperature dependence of linewidth in the PL spectra. The observed results are explained consistently in terms of the exciton localization effect due to the local fluctuations of nitrogen concentration.

Molecular beam epitaxy growth of GaAsN layers with high luminescence efficiency

(In)GaAsN bulk layers and quantum wells usually demonstrate lower photoluminescence intensity than the nitrogen-free compositions. In the present work we have carefully optimized both conductance and operation of a nitrogen plasma source as well as growth parameters of GaAsN layers. We found conditions when incorporation of nitrogen did not lead to formation of additional nonradiative recombination. There is some minimum growth rate to obtain good crystal and optical quality of GaAsN. At growth rates below this value the pattern of reflection high energy electron diffraction turns spotty and the growth proceeds in a three-dimensional mode. This leads to a steep decrease in luminescence efficiency of the grown layer. The minimum value of growth rate depends on nitrogen content and growth temperature. Defects caused by low temperature growth are removed by post-growth annealing. We achieved the same radiative efficiency of GaAsN samples with nitrogen content up to about 1.5% grown at 520 °C as that of a reference layer of GaAs grown at 600 °C. Compositional fluctuation in the GaAsN layers leads to the S-shape temperature dependence of photoluminescence peak position. Post-growth annealing reduces compositional fluctuation.

Strong localization in InGaN layers with high In content grown by molecular-beam epitaxy

The effect of the III/V ratio and growth temperature on the In incorporation has been studied in thick (>300 nm) InGaN layers, with In mole fractions from 19% to 37%, grown by molecular-beam epitaxy on sapphire and on GaN templates. Significant desorption of In occurs at growth temperatures above 550 °C. Symmetric and asymmetric reflections from high resolution X-ray diffraction reveals that the layers are not fully relaxed. A bowing parameter of 3.6 eV is calculated from optical absorption data, once corrected for strain-free band gap values. The increase of both, the absorption band-edge broadening and the photoluminescence full width at half maximum at room temperature with the In content, is discussed in terms of a strong In localization effect. This localization effect is further evidenced by the S-shaped temperature dependence of the emission energy.

Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots

The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions.

Growth and characterization of GaInNAs/GaAs by plasma-assisted molecular beam epitaxy

We have studied the growth of GaInNAs/GaAs quantum well (QW) by molecular beam epitaxy using a DC plasma as the N source. The N concentration was independent of the As pressure and the In concentration, but inversely proportional to the growth rate. It was almost independent of T g over the range of 400–500°C, but dropped rapidly when T g exceeded 500°C. Thermally-activated N surface segregation is considered to account for the strong falloff of the N concentration. As increasing N concentration, the steep absorption edge of the photovoltage spectra of GaInNAs/GaAs QW became gentle, the full-width at half-maximum of the photoluminescence (PL) peak increased rapidly, and a so-called S-shaped temperature dependence of PL peak energy showed up. All these were attributed to the increasing localized state as N concentration. Ion-induced damage was one of the origins of the localized state. A rapid thermal annealing procedure could effectively remove the localized state.

The detailed transport property of the underdoped Bi-2212 system in the pseudogap state

The detailed character of transport properties in underdoped Bi-2212 systems are analysed in the pseudogap state. In a certain underdoped regime, in-plane resistivity ρ ab ( T) shows a typical S-shaped temperature dependence and a perfect ρ ab(T)=ρ 0 *+β exp (−Δ/T) fit is observed over a wide temperature range from slight above T c up to T * where the pseudogap begins opening. The underdoped crystal, in contrast to the optimum crystal where no pseudogap is observed, in-plane transverse magnetoresistance shows a slower temperature decreasing rate with temperature increasing in the pseudogap state, and a larger value around T *. About the temperature dependence of thermopower S( T), all underdoped samples completely fall into a scaling curve S/ S * vs. T/ T * above T *. Whereas, S( T) below T * deviates from this scaling law. The above transport properties should be intimately related with the detailed prospects of Fermi surface destruction in the pseudogap state below T *.

La substitution induced linear temperature dependence of electrical resistivity and Kondo behavior in the alloys, Ce 2− xLa xCoSi 3

Results of electrical resistivity ( ρ), heat-capacity ( C) and magnetic susceptibility ( χ) measurements (above 1.5 K) for the alloys of the type, Ce 2− x La x CoSi 3, are reported. We find that the S-shaped temperature dependence of ρ for the mixed-valent x=0.0 alloy is not a single-ion property; the most interesting observation is that, for an intermediate concentration, ( x=0.25), a linear temperature dependence of ρ below 30 K is found, an observation of relevance to the current trends in the topic of non-Fermi-liquid behavior. Other observations are: (i) Ce exhibits single-ionic heavy-fermion behavior with moderately enhanced electronic contribution to C; and (ii) the strength of the Kondo interaction, as measured by the Curie–Weiss parameter gets reduced for larger substitutions ( x>0.5) of La for Ce.

Carrier Recombination Dynamics of AlxGa1−xN Epilayers Grown by MOCVD

ABSTRACTWe present a comprehensive study of the optical characteristics of AlxGa1−xN epilayers by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. All AlxGa1−xN epilayers were grown by metalorganic chemical vapor deposition and the Al mole fraction (x) was varied from 0 to 0.6. We observed that (i) the full width at half maximum of the PL emission, (ii) the energy difference between the PL emission peak energy and the PLE absorption edge, and (iii) the effective lifetime increase with increasing x. These facts indicate that degree of band-gap fluctuation due to a spatially inhomogeneous Al alloy content distribution increases with increasing x. We observed anomalous temperature-induced emission shift behavior for AlxGa1−xN epilayers, specifically, an S-shaped (decrease-increase-decrease) temperature dependence of the peak energy with increasing temperature. This anomalous temperature-dependent emission behavior was enhanced as the Al mole fraction was increased. Since the band-gap fluctuation in AlxGa1−xN epilayers due to inhomogeneous spatial variations of the Al content increases with increasing Al content, we believe that band-gap fluctuation causes the PL peak energy to deviate from the typical temperature dependence of the energy gap shrinkage. Therefore, the anomalous temperature-induced emission shift can be attributed to energy tail states due to alloy potential inhomogeneities in the AlxGa1−xN epilayers with large Al content.

The systematicstudy of the normal-state transport properties of Bi-2212 crystals

The temperature dependences of the in-plane resistivity ab(T ) and out-of-plane resistivity c(T ) in Bi-2212 crystals covering the region from the underdoped to the overdoped regime have been measured. In the underdoped regime, ab(T ) deviates from a linear temperature dependence below a characteristic temperature T *, well above Tc, whose value decreases with increasing hole concentration. For underdoped crystals, ab(T) shows a typical S-shaped temperature dependence and ab = 0*+exp(-/T ) is satisfactorily obeyed over a much wider temperature range from slightly above Tc up to T *. Near the optimal region, the T -linear dependence of ab(T ) is maintained over a wide temperature interval. In contrast, a power law ab~Tn (n = 1.5-1.8) is followed in the overdoped regime. As regards the out-of-plane resistivity, on the other hand, c(T ) for the underdoped Bi2Sr2CaCu2Oy crystals shows a semiconductive behaviour, which is well described by the formula c = (C1/T)exp(C2/T ) + C3T + C4. The difference between the temperature dependences of c(T ) in the overdoped Bi2Sr2CaCu2Oy and Bi1.85Pb0.15Sr2CaCu2O8 + crystals, with basically the same values of Tc and nearly the same power-law temperature dependences of ab(T ) (ab~T 1.4), reveals that the inter-plane disorder in the form of oxygen vacancies and substituted cations acting as an extra blocking layer plays an important role in out-of-plane transport.

“S-shaped” temperature-dependent emission shift and carrier dynamics in InGaN/GaN multiple quantum wells

We report temperature-dependent time-integrated and time-resolved photoluminescence (PL) studies of InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition. We observed anomalous emission behavior, specifically an S-shaped (decrease–increase–decrease) temperature dependence of the peak energy (Ep) for InGaN-related PL with increasing temperature: Ep redshifts in the temperature range of 10–70 K, blueshifts for 70–150 K, and redshifts again for 150–300 K with increasing temperature. In addition, when Ep redshifts, the spectral width is observed to narrow, while when Ep blueshifts, it broadens. From a study of the integrated PL intensity as a function of temperature, it is found that thermionic emission of photocarriers out of local potential minima into higher energy states within the wells is the dominant mechanism leading to the thermal quenching of the InGaN-related PL. We demonstrate that the temperature-induced S-shaped PL shift is caused by a change in the carrier dynamics with increasing temperature due to inhomogeneity and carrier localization in the InGaN/GaN MQWs.

Photoluminescence properties of the Al0.48In0.52As/InP interface and the diffusion of carriers thereto

A study of the photoluminescence (PL) and photoluminescence excitation (PLE) properties of the interface luminescence of the staggered-aligned Al0.48In0.52As/InP is presented, together with a study of the diffusion of carriers to this interface. Two PL peaks originating from the interface were measured at energies higher than commonly reported. This is the result of growth of the Al0.48In0.52As layer directly on the semi-insulating InP substrate, which results in a much sharper triangular well in the conduction band than when grown on an n-type InP buffer layer. Data from PL in a magnetic field and from PLE both showed that the PL transitions are excitonic in character. Furthermore, an inverted S-shape temperature dependence was found for the PL energy, which is characteristic of carrier localization. PLE measurements showed that at 4.2 K both electrons and holes participating in the interface PL are provided by exciton diffusion from the InP, while at 70 K they are provided by exciton diffusion from the Al0.48In0.52As. The difference arises from carrier localization in the Al0.48In0.52As top layer below 50 K. Unexpectedly, for the InP at 4.2 K the PL intensities of both excitonic and donor-to-acceptor transitions were independent of the absorption of laser light in the Al0.48In0.52As top layer.

Mössbauer spectroscopic study of the early crystallization stage of iron(III) hydroxide particles

Mössbauer spectroscopy was used to investigate the early aging stage of iron(III) hydroxide sols prepared by oxidation of Fe(CO) 5 in ethanolic solution, followed by vacuum drying at room temperature. One sample was composed of amorphous particles, while two other samples were partially crystallized, either as a result of solvent change or of spontaneous aging. The main results of Mössbauer measurements in the 80–320 K temperature range are: (a) partially crystallized particles exhibit a strong, S-shaped temperature dependence of the quadrupole splitting, in contrast to a weak and linear variation for amorphous particles; (b) the recoilless fraction temperature dependence is affected by vibration of the particles as a whole, with an effective force constant which is smaller for crystallized particles than for amorphous ones. Furthermore, the former exhibit an f-factor discontinuity near 0°C, which is attributed to melting of a surface layer built up during the crystallization process.

Positron annihilation and vacancy formation in Al

High-precision measurements of the temperature dependence of positron annihilation in Al, over the range 290-930K, have been made via the Doppler-broadening technique and using an internal 68Ge positron source. The results are considered mainly in terms of two lineshape parameters. These are denoted L and W and describe, respectively, the intensities of well defined central and wing portions of a normalised annihilation photopeak area. Both L and W exhibit the characteristic, overall S-shaped temperature-dependence profile associated with the change from free to vacancy-trapped annihilation modes. In detail, however, the temperature dependences of L and W are quite distinct.