Gap Shrinkage Research Articles

Direct Observation of an Ordered Phase in (1120) Plane InGaN Alloy

A polar-dependent phase with spontaneous atomic ordering is found in a nonpolar (1120) plane In0.08Ga0.92N film grown by metalorganic vapor-phase epitaxy. An atomic arrangement is a periodic sequence of group-III sublattices, such as In0.04Ga0.96N/In0.12Ga0.88N, that is only observed in a nitrogen polar region through systematic transmission electron microscopy investigation. Cathodoluminescence (CL) in the nitrogen polar region, i.e., spontaneously ordered atomic structure of InGaN, reveals anomalous emission behavior, specifically an S-shape-like (increase–decrease) temperature dependence of CL peak energy. It is suggested that the spontaneous ordered atomic structure of InGaN plays the role of a localized center owing to band gap shrinkage, which has been reported in other III–V alloy systems.

Effect of thallium doping on the electrical and optical properties of CdO thin films

AbstractA series of lightly Tl‐doped CdO thin films (1%, 1.5%, 2%, 2.5%, and 3%) have been prepared by a vacuum evaporation method on glass and Si wafer substrates. The prepared films were subjected to structural study by X‐ray diffraction, optical characterisation by UV–VIS–NIR absorption spectroscopy, and dc‐electrical measurements. Experimental data indicate that Tl3+ doping slightly stretching stresses the CdO crystalline structure and changes the optical and electrical properties. It was observed that Tl doping widens the energy gap of CdO from 2.22 eV to 2.83 eV via a Burstein–Moss energy level shift. The band gap shrinkage was also observed for carrier concentrations Nel > 1.13 × 1020 cm–3, which was explained by merging of the impurity band with the conduction band. The optical properties were explained by using Hamberg et al.'s band‐to‐band transitions and classical Drude theory. The electrical behaviour of the samples shows that they are degenerate semiconductors. The 2% Tl‐doped CdO sample shows an increase in its mobility by about 1.4 times, conductivity by 5 times, and carrier concentration by 3.6 times relative to the undoped CdO film. From the transparent‐conducting‐oxide point of view, Tl is sufficiently effective for CdO doping but does not emulate other dopants like In, Sn, Sc, and Y. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence from ZnO nanoparticles in vapor phase

Photoluminescence (PL) is observed from ZnO nanoclusters formed in ZnO plasma due to cooling of plasma species by passing a collimated beam of fourth harmonic (266 nm) of Nd:YAG (yttrium aluminum garnet) laser through ZnO plasma formed by third harmonic (355 nm) of Nd:YAG laser. A shift of 42 meV in peak position of PL profile of ZnO clusters from that observed in bulk ZnO is seen, which shows the effect of quantum confinement. The behaviors of PL profiles are studied at varying ablating intensities with fixed probe intensity. A superlinear increase in PL peak intensity with narrowing of emission linewidth above a particular ablating intensity is observed. The defect related emission band, usually occurring due to oxygen vacancy, is not observed from the ZnO clusters formed due to cooling of laser induced ZnO plasma. The observed redshift of PL peak positions with increasing ablating intensities could be due to temperature-induced band gap shrinkage arising due to enhancement of electron temperature.

Perveance of a planar diode with a multipoint cathode

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 TEU-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.

Effect of STI-induced mechanical stress on leakage current in deep submicron CMOS devices

The shallow trench isolation (STI) induced mechanical stress significantly affects the CMOS device off-state leakage behaviour. In this paper, we designed two types of devices to investigate this effect, and all leakage components, including sub-threshold leakage (Isub), gate-induced-drain-leakage (IGIDL), gate edge-direct-tunnelling leakage (IEDT) and band-to-band-tunnelling leakage (IBTBT) were analysed. For NMOS, Isub can be reduced due to the mechanical stress induced higher boron concentration in well region. However, the GIDL component increases simultaneously as a result of the high well concentration induced drain-to-well depletion layer narrowing as well as the shrinkage of the energy gap. For PMOS, the only mechanical stress effect on leakage current is the energy gap narrowing induced GIDL increase.

Photoluminescence analysis of p-doped GaAs using the Roosbroeck–Shockley relation

Linking absorption with emission, the Roosbroeck–Shockley relation (RSR) expresses a fundamental principle of semiconductor optics. Despite its elementary character, the RSR is hardly advocated since it is commonly understood that the relation holds for intrinsic materials only. However, we demonstrate that the RSR reproduces very well the photoluminescence of p-doped GaAs over the temperature range of 5–300 K. The fitting parameters used, such as energy position and doping-induced band gap shrinkage, satisfactorily coincide with the literature. The presented results show that the RSR can have a much broader impact in semiconductor analysis than generally presumed.The paper is dedicated to our friend and mentor Rand R Biggers (1946–2006)

Optical investigations and exciton localization in high quality Zn1−xMgxO–ZnO single quantum wells

This work investigates the photoluminescence properties of Zn1−xMgxO–ZnO single quantum wells, which have been fabricated by molecular-beam epitaxy. With increasing temperature from 13to300K the single quantum well-related emission peaks exhibit an irregular S-shaped (redshift-blueshift-redshift) behavior, which is in contrast with that ascribed to band gap shrinkage (redshift). In order to clarify the origin of this behavior, the temperature dependence of the integral photoluminescence intensity of the quantum well emission was studied and the relevant activation energies were calculated and correlated to its full width at half maximum, band offsets, and monolayer fluctuations.

Temperature dependent photoluminescence from ZnO/MgZnO multiple quantum wells grown by pulsed laser deposition

Abstract We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range ∼1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of ∼3.35–∼3.68 eV with decreasing thickness in agreement with the calculated values.

Temperature dependence of excitonic emission in cubic CdSe thin film

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton–phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16±1.5 meV was determined from the Arrhenius analysis.

Direct correlation between ferrite microstructure and electrical resistivity

Variations in the composition and microstructure of Mn-Zn soft ferrites have been directly correlated with spatial variations in electrical resistivity, which were both observed to occur on a length scale of approximately 500 μm. Tomographic energy dispersive diffraction imaging (TEDDI) was used to determine the nonsystematic change in the lattice parameter across the sample volume (8.48±0.05 Å) at a spatial resolution of 50 μm. We have used a microprobe contact technique to measure the local electrical resistivity (∼35 Ω cm) and density functional theory to model the band structure. The band structure calculations directly utilized the experimentally measured lattice parameters from the TEDDI measurements and were in good agreement with the measured resistivity. The mean band gap shrinkage was found to be 0.02 eV. This value for Eg was found to account well for the observed 10−20 Ω cm resistivity variations.

Optical measurement of carrier concentration profile in n‐type semiconducting GaAs substrate

AbstractThe light absorption below the fundamental energy gap in n‐type GaAs has been investigated precisely. It is experimentally confirmed that there are the three different mechanisms: (a) the enhancement of fundamental absoption due to the shrinkage of energy gap and band filling by doping Si atoms, (b) deep‐level (EL2) absorption, and (c) free‐carrier absorption. It is found that the transmittance measurement at the wavelength longer than about 1000 nm in the free‐carrier absorption region is more useful and direct, compared with that near the fundamental absorption region from 900 nm to 950 nm, precisely to determine the carrier concentration in n‐type GaAs. The two‐dimensional profile of carrier concentration in a VB‐grown 3 ϕ GaAs substrate is measured in high spatial resolution with the short time less than 5 seconds by using a sophisticated near‐infrared transmittance‐measuring equipment. The absolute amount and homogeneity of carrier concentration can be assessed in the range from 0.3 to 3 × 1018 cm–3. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Intraband and interband optical deformation potentials in femtosecond-laser-excitedα−Te

We calculated the intraband and interband optical deformation potentials for semiconducting $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Te}$ from experimental data of the band gap shrinkage and softening of the ${A}_{1}$-optical phonon mode in response to femtosecond laser excitation. These potentials were obtained by applying first- and second-order perturbation theory to the Fr\olich Hamiltonian describing the carrier-phonon interaction. The intraband optical deformation potential is considerably smaller than previously estimated values; there are no previously reported values for the interband optical deformation potential.

Band-gap renormalization in carbon nanotubes: Origin of the ideal diode behavior in carbon nanotubep−nstructures

We demonstrate dramatic modifications to the electronic structure in single-walled carbon nanotubes due to band gap renormalization, the many-body induced shrinkage of the fundamental band gap. This is examined within the framework of ideal $p\text{\ensuremath{-}}n$ diodes formed along individual single-walled carbon nanotubes. A combination of photocurrent spectroscopy with detailed transport measurements provides a complete set of energy levels of the nanotube $p\text{\ensuremath{-}}n$ structure. These energy levels confirm the large band gap shrinkage, consistent with enhanced many-body correction in one-dimensional confinement, and result in fundamental changes to the nanotubes diode transport properties as compared to their bulk counterparts. We show that the ideal diode behavior is a direct consequence of significant renormalization of the band gap at the doped $p$ and $n$ regions, resulting in formations of heterointerfaces, in stark contrast to a uniform band gap expected of a homogeneous material.

Photoluminescence study of an ultrathin strained silicon on insulator layer

Low temperature photoluminescence has been performed in order to analyze the strain effect on the Si band structure for an 8 nm thick tensile strained silicon layer on insulator. The authors show three phonon assisted optical transitions related to the strained silicon top layer at 0.923, 0.983, and 1.022eV. The 0.983eV line of the TO phonon assisted transition corresponds to a 115meV strain induced band gap shrinkage at Δ point which agrees with the calculated values of strained silicon band gap.

Class II restorations: influence of a liner with rubbery qualities on the occurrence and size of cervical gaps

The aim of this study was to evaluate the ability of new rubbery liners, used as a cervical increment, to relieve contraction stress and thereby reduce the formation of cervical gaps in class II composite restorations. The investigated liners were made of polyester-acrylate (PE(1), PE(2) or PE(3)) or silicone-acrylate (S), mixed with UDMA, without (A, B, C, D) or with HEMA (AH, BH, CH, DH). A silanized filler was added to the mixture, DH, to give composites with 20, 40, 60, and 70% (w/w) of filler (DHF20, DHF40, DHF60, DHF70, respectively). The presence and width of cervical gaps were determined using a light microscope. Statistical analysis showed that six of the 12 rubbery liners (AH-DH, DHF20-DHF40) significantly decreased gap formation in comparison with the control group. In addition, the polymerization shrinkage, flow, and strain capacity of these liners were measured and the influence of these factors on gap formation was examined. Two- and three-dimensional regression analyses showed significantly negative linear correlations between gap formation and strain capacity, and between gap formation and flow, and a significantly positive linear correlation between gap formation and shrinkage.

The influence of phonons on the optical properties of GaN

We comprehensively examine the importance of phonons on the optical properties of GaN. Using Raman and photoluminescence (PL) spectroscopies, the energies and linewidths of optic phonons, excitons, and discrete phonon sidebands (PSBs) are studied between 20 and 325 K. The temperature dependence of the A1 (LO) phonon energy and linewidth are described by a combined two- and three-phonon decay process. The narrow E22 phonon decays by the three-phonon emission process. Three band-edge excitons are observed in PL with linewidths between 2.8 and 5.3 meV at temperature 22 K. The energy gap shrinkage and exciton linewidths are completely interpreted based on electron-phonon interactions. The shift, broadening, and asymmetry of the PSBs are explained by incorporating the decay mechanism of A1 (LO) phonon and the exciton broadening from electron-phonon interactions.

Effect of Ge doping on the creation of luminescent radiation defects in MBE Si

We report on the observation of new luminescence centres in irradiated Ge-doped Si (Si:Ge). Molecular beam epitaxy grown Si:Ge samples with Ge content in the range 0–1.25 at.% were irradiated with protons at room temperature and subjected to post-irradiation heat treatments at temperatures from 100 to 650 °C. For each annealing step, the samples were investigated by means of photoluminescence (PL) spectroscopy at temperatures between 4 and 300 K. We have found that the PL spectra of Ge-containing samples are different from those measured in Ge-free samples and from those previously reported in irradiated bulk Si:Ge. Several new lines have been found in the near band edge (NBE) region of the spectrum, whereas many lines typically measured in Ge-free irradiated Si have not been observed. The dependence of the intensity of the new NBE lines on the measurement temperature has been studied. With increasing Ge content, these lines shift to lower energies following the band gap shrinkage. However, they are broader than expected, when only a purely statistical variation of the band gap energy produced by local fluctuations in the alloy composition is taken into account.

Pressure-Induced Magnetic Quantum Phase Transition in Gapped Spin System KCuCl3

Magnetization and neutron elastic scattering measurements under a hydrostatic pressure were performed on KCuCl3, which is a three-dimensionally coupled spin dimer system with a gapped ground state. It was found that an intradimer interaction decreases with increasing pressure, while the sum of interdimer interactions increases. This leads to the shrinkage of spin gap. A quantum phase transition from a gapped state to an antiferromagnetic state occurs at Pc ? 8.2 kbar. For P > P c, magnetic Bragg reflections were observed at reciprocal lattice points equivalent to those for the lowest magnetic excitation at zero pressure. This confirms that the spin gap decreases and closes under applied pressure.

Impact of band gap shrinkage on simulated bifurcation routes in directly modulated semiconductor lasers

A carrier heating model is derived by taking into account the various temperature processes in the active layer of laser diodes. This model is then used to simulate the static and dynamic characteristics of a directly modulated 1.55μm distributed-feedback laser diode. The calculated results are compared with the measured results of this device as obtained in an earlier work [H. F. Liu and W. F. Ngal, IEEE J. Quantum Electron. 29, 1668 (1993)], and this reveals the significant impact of band gap shrinkage on simulated results. This study also shows that the carrier heating model is a self-consistent model that naturally describes the gain suppression phenomena in directly modulated laser diodes.

The temperature dependence of the band gap shrinkage due to the electron–phonon interaction inAlxGa1−xAs

The photoluminescence spectrum of band edge transitions inAlxGa1−xAs is studied as a function of temperature and electron concentration. The parameters thatdescribe the temperature dependence redshift of the band edge transition energy areevaluated using different models. We find that a semi-empirical relation based on a phonondispersion related spectral function leads to an excellent fit to the experimentaldata.