Gap Shrinkage Research Articles

Fabrication, structure, and photocatalytic activities of boron-doped ZnO nanorods hydrothermally grown on CVD diamond film

The boron (B)-doped zinc oxide nanorods (ZnO NRs) have been fabricated on chemical vapor deposited (CVD) diamond film by hydrothermal technique. The mean diameter of the B-doped ZnO NRs is larger than that of the undoped NRs. The results of X-ray diffraction and first-principles calculations demonstrate that the boron atoms prefer to occupy the octahedral interstice positions of ZnO, which lead to the lattice expansion and band gap shrinkage. The rectifying B-doped ZnO NRs/p-diamond heterojunction structure has been performed to degrade reactive yellow 15 solution, showing a high performance photocatalysis. The photocatalysis mechanism based on the pn heterojunction is discussed.

Challenges toward higher temperature operation of LiFePO4

Large-scale lithium-ion batteries operating at higher temperature may provide additional advantageous aspects such as higher power and use of new materials which are inactive at ambient temperatures. As a first step for this direction, we applied LiFePO4 cathode to middle-temperature region of 60 °C–115 °C and investigated temperature-dependent charge–discharge properties. By selecting suitable electrolyte and battery components tolerant to the elevated temperature, stable operation was attained for no less than 50 cycles below 115 °C. High-rate performance was significantly improved as operating temperature increased up to 100 °C, but suffered from abrupt increase in polarization above 100 °C, where the corresponding impedance signal emerged, which might be assigned to some side reactions occurring at the surface of LiFePO4 particles. At 100 °C, the discharge capacity over 100 mAh g−1 was achieved at 200C rate, even with 10wt% of carbon in the electrode composite of LiFePO4. Shrinkage of miscibility gap was not confirmed for samples larger than 40 nm at < 115 °C.

Increased Interfacial Strength at Microscale Silicon–Polymer Interface by Nanowires Assisted Micro-Sandglass Shaped Interlocks

We present an interfacial strengthening technique for microsystem packaging, particularly at the interface of stiff and soft materials. Micro-sandglass shaped interlocks (MSIs) were first demonstrated at the interface of silicon and Nafion® and exhibited a strong interfacial strength of 1 MPa; moreover, nanowired micro-sandglass shaped interlocks (NW-MSIs) were employed to further enhance the interfacial strength, and a value of 3.4 MPa was achieved. Theoretical predictions indicated the enhancement factors include the gap shrinkage ratio, overhanging angle, contact area, and friction coefficient. The geometry of the MSIs is strongly dependent on the KOH etching time while the friction can be enhanced by the employment of nanowires on the sidewalls of MSIs. Based on the NW-MSIs, the highest interfacial strength obtained is 3.4-fold higher than that of MSIs without nanowires.

Study the Effect of Junction Temperature on the Peak Wavelength in GaN-Based High-Power Green Light Emitting Diodes

The junction temperature of GaN-based high-power green light emitting diodes is measured using the temperature coefficients of the diode forward voltage, from changes in temperature and changes in drive current to measure the LED junction temperature and the corresponding spectral, Respectively. Experiments show that, junction temperature due to environmental temperature increased, and the red shift of the spectral peak wavelength. When low temperature or less then the rated current range, the drive current increased in junction temperature rise due to the spectral peak wavelength blue shift . When the current is increased in the range of close to or greater than the rated current, leading to the junction temperature rise will cause spectral red shift . The peak wavelengths’ shift degree of 0.0579nm / k, 0.0751 nm / k and-0.1974nm / k, -0.0915 nm / k are calculated in both cases. The phenomenon is due to the LED junction temperature increases lead to band gap shrinkage, and the result of the role of spontaneous polarization and piezoelectric polarization in Ⅲ-nitride semiconductor materials.

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage ( I– V) curves and luminescence intensity–current ( L– I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of −3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of −0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.

Temperature induced band gap shrinkage in Cu 2GeSe 3: Role of electron–phonon interaction

The ternary semiconducting compound Cu 2GeSe 3 has been investigated for optical properties with photoacoustic spectroscopy. Optical absorption spectra of Cu 2GeSe 3 is obtained in the range of 0.76–0.81 eV photon-energy at temperatures between 80 and 300 K. The thermal variation of band gap energy has been examined from the optical absorption spectra at different temperatures. The temperature induced band gap shrinkage has been explained on the basis of electron–phonon interaction. Varshni's empirical relation in conjunction with Vina and Passler model is taken into consideration for data fitting. The Debye temperature was calculated approximately as 240 K. The acoustic phonons with a characteristic temperature as 160 K corresponding to effective mean frequency have been attributed to the thermal variation of the energy gap.

Photoluminescence Properties of Core/Shell CdSe/ZnS Quantum Dots Encapsulated with Transparent layers for Third Generation Photovoltaics

ABSTRACTIn this work hydrophobicaly ligated cadmium selenide/zinc sulfide CdSe/ZnS quantum dots (QDs) were incorporated in transparent matrices by formation of CdSe/ZnS/SiO2 core/shell/shell structure using microemolsion synthesis method. The optical properties of the QDs encapsulated with a chemically grown oxide layers were studied. Intense luminescence properties of the QD/silica nanoparticles (NPs) were observed using steady state photoluminescence (PL) measurements. Confocal microscopy demonstrates fluorescence of the single core/shell/shell nanoparticles. The obtained results along with the Secondary Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) images provide information on the geometry of the QDs. The excitonic emission of nanoparticles was also mapped using a liquid nitrogen cryostat the 77K - 300K range. The temperature dependent PL spectra of the film demonstrate the temperature-dependent band gap shrinkage of the QDs. PL lifetime measurements were performed on the ensemble of NPs. Experimental data was fitted to the numerical model with lifetime constants in nanoseconds range. We demonstrate that the main nonradiative processes that limit the quantum yield (QY) of the QDs at room temperature are the carrier trapping at the interface of QD/silica and the exciton-phonon coupling. These studies give us insight to exploit the QD layers for photon down shifting and multiple exciton generation for application in photovoltaics.

Effect of processing method on physical properties of Nb 2O 5

Samples of Nb 2O 5 were prepared by laser floating zone (LFZ) technique and by solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. Single crystals fibres were obtained by LFZ, while a structural orthorhombic to monoclinic phase transition was observed in samples sintered at temperature higher than 800 °C. Transmission optical spectroscopy and photoconductivity measurements allowed identifying a ∼3.2 eV bandgap energy for the H-Nb 2O 5 monoclinic crystalline phase. Band gap shrinkage of ∼100 meV was observed from 14 K to RT. For the orthorhombic phase (T-Nb 2O 5), the photoconductivity measurements evidence a higher energy bandgap. The sintered samples have shown a broad recombination luminescence band at the orange/red spectral region while no luminescence was detected from the LFZ grown fibres. A dielectric constant of ∼40 was found for the 800 °C and 1200 °C sintered pellets while that of 1000 °C reached four times that value.

Band gap engineering with sub-monolayer nitrogen insertion into InGaAs/GaAs quantum well

We investigate the band gap engineering with sub-monolayer nitrogen introduction (: N δ-doping) at the middle of InGaAs/GaAs quantum well. Using plasma-assisted molecular beam epitaxy, we prepare samples varying the introduced nitrogen between 0 and 0.4 monolayer. The growths are carried out at substrate temperature of 520∘C, comparatively higher than conventional substrate temperature for dilute nitride compounds. This induces the roughening of the growth front, slightly enhancing the nitrogen incorporation within the crystal. The as-grown and annealed samples show clear room temperature photoluminescence, indicating the band gap shrinkage depending on the amount of nitrogen.

Characterization of ZnO thin film synthesized on alumina-rich spinel substrate by magnetron sputtering

Zinc oxide (ZnO) thin film has been epitaxially grown on (1 1 1) Mg 0.4Al 2.4O 4 substrate by RF-magnetron sputtering. In resonant Raman scattering, higher-order longitudinal optical phonon modes were clearly observed, revealing high optical quality of the ZnO film. Optical absorption indicated a visible exciton absorption at room temperature. The near band edge emission showed a red shift due to the shrinkage of the band gap with increasing the temperature.

Forward tunneling current in GaN-based blue light-emitting diodes

Forward tunneling current in InGaN/GaN multiquantum-well blue light-emitting diodes grown on sapphire substrate was studied by temperature-variable current-voltage (I-V) measurement. All semilog I-V curves obtained in the temperature range from 100 to 300 K exhibit two successive linearly dependent regions at low forward bias. The corresponding slopes appear to be insensitive to temperature, which indicates a dominant defect-assisted tunneling process. It is found that the tunneling current varies approximately as a function of ∼exp(−βEg+λeV), where β and λ are constants independent of temperature and voltage. The temperature- and voltage-dependence of forward tunneling current are explained by thermally induced band gap shrinkage and bias-induced route change of diagonal tunneling, respectively. The likely tunneling entities involved in the forward tunneling process are also discussed.

Optical properties of ZnO/Zn0.9Mg0.1O multiple quantum wells grown on (111) Si using buffer assisted pulsed-laser deposition

This work investigates the photoluminescence properties of ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs), which have been fabricated by pulsed-laser deposition on (111) Si substrates using intervening epitaxial Lu2O3 buffer layers. In ZnO/Zn0.9Mg0.1O MQWs, the luminescence is dominated by localized exciton (LE) emission throughout the whole temperature range studied. With increasing temperature from 10 to 300 K, the LE emission redshifts by 38 meV. This redshift is believed to be due to the thermalized excitons occupying higher-lying localized states where they emit higher energy radiation and temperature-induced band gap shrinkage. Moreover, the LE emission from the MQWs decays more slowly than exciton emission from ZnO. In addition, the LE emission in the MQWs shows a systematic blueshift with decreasing well width, which is consistent with a quantum size effect.

10.1007/s11454-008-3013-y

The I–V characteristics of a planar diode with a multipoint explosive emission cathode are studied under the conditions when the impedance of the diode is matched to the output resistance of the nanosecond generator. The experiments are carried out on the TÉU-500 (350–500 keV, 60 ns, 250 J per pulse) high-current electron accelerator. The perveances of the diodes with a spike (140 spikes) cathode, multipoint copper cathode, and cathode made of carbon felt are measured in the course of electron beam generation. Comparison is performed between the I–V characteristics of the diodes with multipoint and continuous cathodes made of the same material. It is found that the electron current of the diode is described well by Child’s law with regard to the shrinkage of the anode-cathode gap and an increase in the emissive surface due to the expanding plasma (formfactor). When the cathode is made of carbon felt or is multipoint, the space-charge-limited electron current mode sets in even 5–10 ns after the voltage is applied.

Effects of gap size and surrounding trees on light patterns and aspen branch growth in the western boreal forest

Vertical and horizontal patterns of light within gaps and expansion of trees into gaps were examined in young stands dominated by trembling aspen ( Populus tremuloides Michx.) in northeastern British Columbia and northwestern Alberta. Lateral growth of branches of aspen edge trees averaged 15.2 cm/year and was similar to crown expansion rates reported for edge trees in eastern North America. Branches growing into gaps were significantly longer than those growing away from gaps and are likely to lead to gap shrinkage and reductions in light within gaps. However, height growth of edge trees was not significantly different from that of trees within the surrounding stands. Models describing relationships between diffuse transmittance and locations within gaps and surrounding stand height were developed in this paper. Sky view angle (SVA) was found to be effective for predicting transmittance to different locations within gaps. When SVA values exceed 1.2 (at the B.C. sites), white spruce ( Picea glauca (Moench) Voss) will receive &lt;40% of full sunlight, which is less than optimal for spruce height growth.

Relation between photocurrent and DLTS signals observed for quantum dot systems

The electro-optical properties of InAs/GaAs quantum dots (QD) are presented. It is shown that they can contribute to the photocurrent at temperatures where photo-generated excitons can split. This happens if the carrier binding energies are not too large to prevent the carrier emission process. At lower temperatures, the exciton recombination process can effectively compete with the carrier emission processes and no photocurrent signal is observed. The Laplace DLTS technique has been used in a wide temperature range to analyze the electron and hole emission separately and combine them with the observed temperature dependence of the photocurrent. The photocurrent measurements also showed that with an increase in temperature the dot-related photocurrent peaks shift to lower energies with a rate similar to GaAs band gap shrinkage.

Modification of Band Gap of β-SiC by N-Doping

The geometrical and electronic structures of nitrogen-doped β-SiC are investigated by employing the first principles of plane wave ultra-soft pseudo-potential technology based on density functional theory. The structures of SiC1−xNx (x = 0, 1/32, 1/16, 1/8, 1/4) with different doping concentrations are optimized. The results reveal that the band gap of β-SiC transforms from an indirect band gap to a direct band gap with band gap shrinkage after carbon atoms are replaced by nitrogen atoms. The Fermi level shifts from valence band top to conduction band by doping nitrogen in pure β-SiC, and the doped β-SiC becomes metallic. The degree of Fermi levels entering into the conduction band increases with the increment of doping concentration; however, the band gap becomes narrower. This is attributed to defects with negative electricity occurring in surrounding silicon atoms. With the increase of doping concentration, more residual electrons, more easily captured by the 3p orbit in the silicon atom, will be provided by nitrogen atoms to form more defects with negative electricity.

Temperature dependent band gap shrinkage in GaN: Role of electron–phonon interaction

We present an experimental investigation of temperature dependent band-gap shrinkage in GaN using photoluminescence spectroscopy. The near-band-edge transition energy shifts to lower energy with increasing temperature. The parameters that describe the temperature-dependent red-shift of the band-edge transition energy are evaluated using different models. It has been found that the semi-empirical relation based on phonon-dispersion related spectral function leads to an excellent fit to the experimental data.

UV photoreflectance spectroscopy in strained silicon on insulator structures

AbstractThe optical modulation technique of photoreflectance (PR) is applied on tensely‐strained silicon on insulator (sSOI) substrates in order to determine the deformation potential in the strained silicon direct band gap. Raman spectroscopy is used for strain determination. A redshift of the Si direct band gap transition is observed as the strain induced in the layer is increased. We report a band gap shrinkage at Γ‐point down to 60 ± 6 meV, 120 ± 15 meV and 127 ± 13 meV for 0.77 ± 0.02%, 1.22 ± 0.02% and 1.40 ± 0.03% of strain, respectively. The obtained deformation potential in the direct band gap (a = –0.55 eV and b = –2.45 eV) are in good agreement with previously reported results for bulk material. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Enhanced p-type conductivity and band gap narrowing in heavily Al doped NiO thin filmsdeposited by RF magnetron sputtering

Stoichiometric NiO, a Mott–Hubbard insulator at room temperature,shows p-type electrical conduction due to the introduction ofNi2+ vacancies(VNi′′) andself-doping of Ni3+ ions in the presence of excess oxygen. The electrical conductivity of this important materialis low and not sufficient for active device fabrication. Al doped NiO thin films were synthesizedby radio frequency (RF) magnetron sputtering on glass substrates at a substrate temperature of250 °C in anoxygen + argon atmosphere in order to enhance the p-type electrical conductivity. X-ray diffractionstudies confirmed the correct phase formation and also oriented growth of NiO thin films.Al doping was confirmed by x-ray photoelectron spectroscopic studies. The structural,electrical and optical properties of the films were investigated as a function of Al doping(0–4 wt%) in the target. The room temperature electrical conductivity increased from0.01–0.32 S cm −1 for 0–4% Al doping. With increasing Al doping, above the Mott critical carrier density,energy band gap shrinkage was observed. This was explained by the shift of the band edgesdue to the existence of exchange and correlation energies amongst the electron–electron andhole–hole systems and also by the interaction between the impurity quasi-particle system.

Mott transition of excitons in ZnSe studied by phase resolved reflection

AbstractThe Mott transition of excitons in a semiconductor with increasing carrier density is in principle well understood as a consequence of screening of the Coulomb interaction between carriers. While the position of the exciton stays widely unchanged the exciton peak disappears due to band gap shrinkage. However, a more quantitative check of applied theoretical models for the screening and of the role of quantum kinetic effects in this process is still open.We demonstrate that the phase‐resolved reflection in shallow‐confined ZnSSe heterostructures opens the possibility of a detailed study of the Mott transition due to the appearance of pronounced interferences effects of propagating polariton modes.Our theoretical approach for the investigation of the influence of excited carriers on the dielectric susceptibility is based (i) on a quasi‐particle approximation for the carrier energies and damping, and (ii) on the semiconductor Bloch equations including dynamical screening and a quantum kinetic treatment of scattering. These manybody effects lead to drastic changes of both amplitude and phase of the reflected light: Jumps of the phase are steepened, show abrupt changes from +π to –π and are smoothed out, and interference peaks in the amplitude decrease and disappear completely with increasing excitation. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)