Carrier Localization Energy Research Articles

2D-GaN/AlN Multiple Quantum Disks/Quantum Well Heterostructures for High-Power Electron-Beam Pumped UVC Emitters.

This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

Enhancing the thermoelectric power factor of nanostructured SnO2 via Bi substitution

Tin dioxide (SnO2) has recently proved to be a promising material for thermoelectric applications. We have investigated the influence of highly valence Bi doping as an electron donor in oxygenated SnO2 materials on their thermoelectric properties. We have synthesized the pure and Bi doped SnO2 nanoparticles (x = 0%, 5%, 10%, and 15%) through a simple hydrothermal approach. The Seebeck coefficient and Hall measurements have been used to determine thermoelectric behaviour. The measured value of the Seebeck coefficient increases from - 56 to - 83 μV/°C as the Bi content increases. This improvement in the Seebeck coefficient has been attributed to the charge carrier localization (energy filtering effect) caused by the inclusion of the bismuth atoms and the presence of secondary phases based on BiO2. However, the electrical conductivity measurements show an inverse relation with the Bi doping, increasing the impurities. The Sn1-xBixO2 sample with x = 15 has achieved the maximum Seebeck value, resulting in the upward trend in power factor of up to 1.97 × 10−4 Wm−1C−2. Further, we have used X-ray diffraction and scanning electron microscopy to determine the effect of Bi on the SnO2 crystal structure and surface morphology. Which also demonstrates the presence of composites with mixed phases.

Tuning the magneto-electrical properties of multiferroic multilayers through interface strain and disorder

Artificially engineered superlattices were designed and fabricated to induce different growth mechanisms and structural characteristics. DC sputtering was used to grow ferromagnetic (La0.8Ba0.2MnO3)/ferroelectric (Ba0.25Sr0.75TiO3 or BaTiO3) superlattices. We systematically modified the thickness of the ferromagnetic layer to analyze dimensional and structural disorder effects on the superlattices with different structural characteristics. The crystalline structure was characterized by X-ray diffraction and transmission electron microscopy. The magnetic and electronic properties were investigated by SQUID magnetometry and resistance measurements. The results show that both strain and structural disorder can significantly affect the physical properties of the systems. Ba0.25Sr0.75TiO3 based superlattices with a low thickness of the ferromagnetic layers (4 nm) present compressive strain that decreases the ferromagnetic transition temperature from 250 K corresponding to the unstressed samples to 230 K. In these samples, the localization energy of the charge carrier through the electron-phonon interaction decreases at low temperatures (∼100 meV). Ba0.25Sr0.75TiO3 based superlattices with thicknesses of the ferromagnetic layers higher than 12 nm present tensile strain that reduces the charge carrier localization energy at low temperatures (∼1 meV), increasing the ferromagnetic transition temperature (Tc∼265 K). Structural defects in BaTiO3 based superlattices have a stronger influence on the magnetic properties than on the transport properties. Nevertheless, disorder blocks the ferromagnetic transition for highly disordered samples (thickness of the ferromagnetic layer < 3 nm). These results help to further understand the role of strain and interface effects in the magnetic and transport properties of manganite based multiferroic systems.

Trade-off between morphology, extended defects, and compositional fluctuation induced carrier localization in high In-content InGaN films

We elucidate the role of growth parameters (III/N flux ratio, temperature TG) on the morphological and structural properties, as well as compositional homogeneity and carrier localization effects of high In-content (x(In) &amp;gt; 0.75) In–polar InGaN films grown by plasma–assisted molecular beam epitaxy (PAMBE). Variations in III/N flux ratio evidence that higher excess of In yields higher threading dislocation densities as well as larger compositional inhomogeneity as measured by x-ray diffraction. Most interestingly, by variation of growth temperature TG we find a significant trade-off between improved morphological quality and compositional homogeneity at low–TG (∼450–550 °C) versus improved threading dislocation densities at high–TG (∼600–630 °C), as exemplified for InGaN films with x(In) = 0.9. The enhanced compositional homogeneity mediated by low–TG growth is confirmed by systematic temperature-dependent photoluminescence (PL) spectroscopy data, such as lower PL peakwidths, &amp;gt;5× higher PL efficiency (less temperature-induced quenching) and a distinctly different temperature-dependent S-shape behavior of the PL peak energy. From these, we find that the carrier localization energy is as low as ∼20 meV for low–TG grown films (TG = 550 °C), while it rises to ∼70 meV for high–TG grown films (TG = 630 °C) right below the onset of In–N dissociation. These findings point out that for the kinetically limited metal-rich PAMBE growth of high In-content InGaN a III/N flux ratio of ∼1 and low-to-intermediate TG are required to realize optically more efficient materials.

Modification of carrier localization in basal‐plane stacking faults: The effect of Si‐doping in a‐plane GaN

AbstractThe optical properties of Si‐doped a‐plane GaN epilayers grown on r‐plane sapphire are studied. The low temperature emission is dominated by basal‐plane stacking fault (BSF) recombination throughout the investigated doping range (1 × 1017 to 5 × 1019 cm−3). From temperature dependent photoluminescence (PL) measurements in conjunction with PL excitation studies, the carrier localization energy within the BSF is inferred to decrease from 17 meV to a negligible level as the doping density increases from 1 × 1017 to 5 × 1018 cm−3. It is proposed that electrons, ionized from the Si‐donor atoms at the growth temperature, are able to transfer to the BSFs, where they progressively fill the available density of localized states. For doping levels in excess of 1 × 1018 cm−3, the luminescence linewidth broadens significantly and the luminescence transients decay with a single exponential time constant. This behaviour is attributed to the onset of band‐filling which causes a marked increase in the free electron density within the BSFs.

Infrared photoluminescence of high In‐content InN/InGaN multiple‐quantum‐wells

AbstractWe report on the thermal evolution of the photoluminescence (PL) from high In‐content InN/In0.9Ga0.1N multiple‐quantum wells (MQWs) synthesized by plasma‐assisted molecular‐beam epitaxy on GaN‐on‐sapphire templates. The structural quality and the well/barrier thickness uniformity in the MQW structure are assessed by X‐ray diffraction and transmission electron microscopy measurements. PL results are compared with the luminescence from a 1‐µm‐thick InN reference sample. In both cases, the dominant low‐temperature (5 K) PL emission peaks at ∼0.73 eV with a full width at half maximum of ∼86 meV. The InN layer displays an S‐shape evolution of the emission peak energy with temperature, explained in terms of carrier localization. A carrier localization energy of ∼12 meV is estimated for the InN layer, in good agreement with the expected carrier concentration. In the case of the MQW structure, an enhancement of the carrier localization associated to the piezoelectric field results in an improved thermal stability of the PL intensity, reaching an internal quantum efficiency of ∼16%.

Carrier recombination dynamics in Si doped InN thin films

Time-integrated and time-resolved photoluminescence (PL) of InN thin films of different background carrier concentrations are investigated. The PL formation mechanism is attributed to the “free-to-bound” transition by analyzing the time-integrated PL spectra at different pump fluences. The dependence of the PL decay time with emission energy is investigated using a theoretical model which speculates upon the carrier localization in InN thin films. The radiative lifetime, mobility edge, and carrier localization energy are obtained from the dependence of the PL decay time on emission energy and are studied at different background carrier concentrations. The effect of intervalley scattering between the Γ1 and Γ3 valley on the radiative lifetime, mobility edge, and carrier localization energy is discussed. The longer radiative lifetime and smaller values of the mobility edge and localization energy for 3.06 eV excitation are observed than that for the 1.53 eV excitation due to the intervalley scattering process.

Impact of carrier localization on the photoluminescence characteristics of (Ga,In)(N,As) and (Ga,In)(N,As,Sb) quantum wells

Using photoluminescence (PL) spectroscopy, we carry out a comparative study of the optical properties of (Ga,In)(N,As) and (Ga,In)(N,As,Sb) quantum wells. The incorporation of Sb into (Ga,In)(N,As) results in a reduced quantum efficiency at low temperatures but an improved one at room temperature (RT). A PL line shape analysis as well as the temperature dependence of the PL peak energy reveals the existence of band-tail localized states in both material systems. The carrier localization energy is larger for (Ga,In)(N,As,Sb) than for (Ga,In)(N,As), leading to a longer radiative lifetime and thus a reduced quantum efficiency at low temperatures for the former material. The thermal quenching of the quantum efficiency is analyzed by a rate equation model, which shows that the density of nonradiative centers is reduced in (Ga,In)(N,As,Sb) resulting in an enhanced quantum efficiency at RT.

HIGH TEMPERATURE STABILITY OF OPTICAL PROPERTIES OF InAs QUANTUM DOTS REALIZED BY CONTROLLING OF QUANTUM DOTS ELECTRONIC SPECTRUM

The optical properties of InAs /( Al ) GaAs quantum dots (QDs) overgrowth by thin AlAs / InAlAs layers are studied as a function of temperature from 10 to 500 K. The QDs emit at 1.27 μm at room temperature. It is shown that the QD energetic spectrum can be tuned by overgrowth of AlAs / InAlAs to provide high temperature stability of the QDs optical properties. Transport of carriers between neighboring QDs is absent, and the carrier distribution remains nonthermal up to room temperature. It is shown that suppression of the thermal escaping of the carriers from QDs is conditioned by high energy separation between ground and excited states, absence of wetting layer level, and increase of carrier localization energy in QDs in case of the Al 0.3 Ga 0.7 As matrix.

Nonequilibrium Room-Temperature Carrier Distribution in InAs Quantum Dots Overgrown with Thin AlAs∕InAlAs Layers

The optical properties of quantum dots (QDs) formed in GaAs or Al0.3Ga0.7As matrices by overgrowth of initial InAs islands formed in the Stranski-Krastanov mode with thin AlAs/InAlAs layers have been studied. It is shown that no transport of carriers between the QDs occurs in the temperature range 10–300 K, so the carrier distribution is of a nonequilibrium nature. The thermal excitation of carriers from the QDs is suppressed by an increase in the energy spacing between the ground and excited states, absence of the level related to the wetting layer, and higher carrier localization energy in the QDs with respect to the continuum states when the Al0.3Ga0.7As matrix is used.

Excitation power dynamics of photoluminescence in InGaN∕GaN quantum wells with enhanced carrier localization

Excitation-power dynamics of near-band-edge photoluminescence (PL) peak position in InxGa1−xN∕GaN multiple quantum wells (x∼0.15) was analyzed as a function of well width. The analysis was based on energy reference provided by photoreflectance (PR) spectra. The difference in spectral position of the PR feature and low-excitation PL band (the Stokes Shift) revealed carrier localization energy, which exhibited a remarkable sensitivity to the well width, increasing from 75meV in 2nm wells to about 250meV in 4nm wells. Meanwhile collating of the PR data with the flat-band model for the optical transition energy in quantum wells rendered a relatively weak (0.5MV∕cm) built-in piezoelectric field. The blueshift of the PL peak position with increasing photoexcitation power density was shown to be in qualitative agreement with the model of filling of the band-tail states with some contribution from screening of built-in field in the thickest (4nm) wells. Increased incident photon energy resulted in an additional blueshift of the PL peak, which was explained by a nonthermalized distribution of localized carriers and/or carrier localization in the interface region. Our results are consistent with a concept of emission from partially relaxed large In-rich regions with internal band potential fluctuations, which are enhanced with increasing the growth time.

Effect of carrier localization on the optical properties of MBE-grown GaAsN/GaAs heterostructures

The optical properties of GaAsN/GaAs heterostructures grown by molecular-beam epitaxy with different nitrogen content in the layers have been studied. The optical properties of GaAsN layers in the growth conditions under study are defined by the carrier recombination via localized states related to a strong composition inhomogeneity in the solid solution. The increasing of the nitrogen content raises the composition inhomogeneity and the carrier localization energy.

Temperature dependence of recombination processes of photogenerated carriers in a quantum-well structure

Time-resolved photoluminescence (PL) measurements are performed on an Al0.53In0.47P/Ga0.52In0.48P/Al0.53In0.47P quantum-well (QW) structure in a temperature range of 5–398 K. The radiative and nonradiative carrier recombination rates are obtained from the PL decay rates and the time-integrated PL intensities. It is found that the radiative recombination rate decreases rapidly as the temperature increases, and becomes constant at temperatures higher than 275 K. The nonradiative recombination rate increases rapidly at temperatures higher than 300 K. The dependence of the PL intensity on the temperature is expressed by a formula, which consists of the terms expressing the nonradiative recombination or recombination via deep levels of carriers which are free from localized states, the carrier transfer from QWs to barriers, and the temperature-dependent radiative recombination. The carrier localization energy is found to be 4–10 meV. The apparent barrier height for the carrier transfer from QWs is found to be 350–600 meV.

Effect of matrix on InAs self-organized quantum dots on InP substrate

InAs self-organized quantum dots in In0.53Ga0.47As and In0.52Al0.48As matrices have been grown on InP substrates by molecular beam epitaxy. The dot size in InGaAs has been found to be 3–4 times larger, but the areal density about an order of magnitude smaller than that in InAlAs. Low-temperature photoluminescence (PL) of the InAs/InGaAs quantum dots is characterized by a narrow (35 meV) PL line as compared to that of InAs/InAlAs quantum dots (170 meV). Quantum dot formation increases the carrier localization energy as compared to quantum well structures with the same InAs thickness in a similar manner for both InAs/InGaAs and InAs/InAlAs structures. The effect of the barrier band gap on the optical transition energy is qualitatively the same for quantum well and quantum dot structures. The results demonstrate a possibility of controlling the quantum dot emission wavelength by varying the matrix composition.

Thermal stability of vertically coupled InAs-GaAs quantum dot arrays

The effect of high-temperature annealing on the optical properties of vertically coupled InAs quantum dots in a GaAs matrix and on the performance of a quantum-dot laser are studied. A strong blue shift of the photoluminescence peak and lasing line, as well as changes in the photoluminescence intensity and temperature dependence of the threshold current density are observed. The reason for this behavior is probably a reduction in the carrier localization energy due to a partial mixing of the In and Ga atoms as well as an improvement in the quality of the low-temperature grown Ga(Al)As layers achieved by high-temperature annealing.