Varshni Equation Research Articles

Benzo[ghi]perylene monoimide based photosensitive lamellar Cd-doped ZnO nanohybrids

Lamellar hybrid inorganic/organic nanostructures comprising of alternating layers of benzo[ghi]perylene monoimide and Cd-doped ZnO called BPyM/CZO nanohybrids have been synthesized electrochemically on Ga-doped ZnO/Si. In order to study the effect of organic surfactant, inorganic CZO nanorods have also been electrochemically synthesized. The role of annealing and organic surfactant on morphology, structure, composition and opto-electronic properties is analyzed. The nanohybrids and nanorods are c-axis oriented preferentially with their (002) direction perpendicular to the substrate surface. Annealed BPyM/CZO nanohybrids show visible orange photoluminescence emission while unannealed nanohybrids and inorganic CZO nanorods emit white emission at room temperature. The annealed nanohybrids and inorganic nanorods tend to follow the Varshni equation with temperature variation from 80 K to 300 K. Spectroscopic ellipsometry reveals the thickness, refractive index and band gap of the nanostructures. Annealing tends to expand the band gap by 50 meV in the case of inorganic CZO nanorods, but shrinks the band gap by 20 meV for BPyM/CZO nanohybrids. Temperature dependent photosensitivity measurements reveal that BPyM/CZO nanohybrids are highly photosensitive, ∼8 fold and ∼4 fold higher than inorganic CZO nanorods at 80 K and 300 K, respectively. The studies indicate that BPyM/CZO nanohybrids hold potential as photosensors.

Low temperature studies of the photoluminescence from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin

This article describes the results from low temperature studies of the photoluminescence (PL) from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin. The synthesized nanocrystals were taken from the reaction mixture at different time intervals, their sizes were all within the quantum confinement range, and showed the typical size-dependent UV–vis absorption, PL and Raman spectral characteristics. The PL spectra and their temperature dependences were measured from room temperature (295K) down to temperatures as low as 15K, demonstrating that the exciton PL transitions followed the Varshni equation. The thermal quenching activation energy below 100K was relatively low, while the luminescence was strongly quenched as the temperature was increased above 100K, probably pointing at change in the degree of carrier localization. The smallest (2.6nm in size) CdSe nanocrystals isolated in the beginning of the nanocrystal growth had a relatively symmetric size distribution and showed a single exciton emission band, while the samples isolated at later stages of the growth process showed negatively skewed asymmetric size distribution, indicating that the growth mechanism of nanocrystals could be explained by the model of Ostwald ripening. The obtained results are expected to be helpful for better understanding of the PL properties of quantum dots and their formation and growth in non-polar organic solvents.

Growth and Electrical Properties of ZnAl2Se4 Single Crystal Thin Film by Hot Wall Epitaxy

A stoichiometric mixture of evaporating materials for <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin films was prepared in a horizontal electric furnace. These <TEX>$ZnAl_2Se_4$</TEX> polycrystals had a defect chalcopyrite structure, and its lattice constants were <TEX>$a_0=5.5563{\AA}$</TEX> and <TEX>$c_0=10.8897{\AA}$</TEX>.To obtain a single-crystal thin film, mixed <TEX>$ZnAl_2Se_4$</TEX> crystal was deposited on the thoroughly etched semi-insulating GaAs(100) substrate by a hot wall epitaxy (HWE) system. The source and the substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$400^{\circ}C$</TEX>, respectively. The crystalline structure of the single-crystal thin film was investigated by using a double crystal X-ray rocking curve and X-ray diffraction <TEX>${\omega}-2{\theta}$</TEX> scans. The carrier density and mobility of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film were <TEX>$8.23{\times}10^{16}cm^{-3}$</TEX> and <TEX>$287m^2/vs$</TEX> at 293 K, respectively. To identify the band gap energy, the optical absorption spectra of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film was investigated in the temperature region of 10-293 K. The temperature dependence of the direct optical energy gap is well presented by Varshni's relation: <TEX>$E_g(T)=E_g(0)-({\alpha}T^2/T+{\beta})$</TEX>. The constants of Varshni's equation had the values of <TEX>$E_g(0)=3.5269eV$</TEX>, <TEX>${\alpha}=2.03{\times}10^{-3}eV/K$</TEX> and <TEX>${\beta}=501.9K$</TEX> for the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$ZnAl_2Se_4$</TEX> were estimated to be 109.5 meV and 124.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$ZnAl_2Se_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{21}$</TEX>-exciton peaks for n = 21.

Impact of substrate nitridation on the photoluminescence and photovoltaic characteristics of GaN grown on p-Si (100) by molecular beam epitaxy

This report focuses on the structural and optical properties of the GaN films grown on p-Si (100) substrates along with photovoltaic characteristics of GaN/p-Si heterojunctions fabricated with substrate nitridation and in absence of substrate nitridation. The high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopic studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN epifilms grown with silicon nitride buffer layer when compared with the sample grown without silicon nitride buffer layer. The low temperature PL shows a free excitonic (FX) emission peak at 3.51 eV at the temperature of 5 K with a very narrow line width of 35 meV. Temperature dependent PL spectra follow the Varshni equation well and peak energy blue shifts by ~63 meV from 300 to 5 K. Raman data confirms the strain free nature and reasonably good crystallinity of the films. The GaN/p-Si heterojunctions fabricated without substrate nitridation show a superior photovoltaic performance compared to the devices fabricated in presence of substrate nitridation. The discussions have been carried out on the junction properties. Such single junction devices exhibit a promising fill factor and conversion efficiency of 23.36 and 0.12 %, respectively, under concentrated AM1.5 illumination.

Characteristics of GaN Nanowires Produced Using VLS Method on the Growth Temperatures

One-dimensional GaN NWs (nanowires) were grown by the VLS (Vapor-liquid-solid) method using MOCVD (Metal organic chemical vapor deposition) equipment on growth temperatures in order to evaluate its characteristics. The growth temperatures ranged from 650¢J to 800¢J at an interval of 50¢J. GaN NWs grown at a temperature of 700¢J was approx. 5 μm in length and 200 nm in a diameter, showing relatively uniform size and highly density. The X-ray diffraction analysis showed that NWs grew as wurtzite GaN. In PL (photoluminescence) the spectrum measured at 4 K, at 3.47 eV, BE (band-edge) emissions were identified and the YL (yellow luminescence) was almost not shown. As a temperature dependant PL, the band structure of n-type GaN NWs was identified and E(0) values of FX and DX were calculated using Varshni's equation of 3.476 eV and 3.470 eV, respectively. The result of calculation by quenching equation showed that the activation energy value of FX was 12.8 meV and the activation energy value of DX was 19.1 meV.

Low Temperature Elastic Constants and Piezoelectric Coefficients of LiNbO3 and LiTaO3: Resonant Ultrasound Spectroscopy Measurement and Lattice Dynamics Analysis

The complete sets of elastic constants Cij and piezoelectric coefficients eij for LiNbO3 and LiTaO3 single crystals have been determined by resonant ultrasound spectroscopy (RUS) from ambient temperature to 6 K. Both Cij(T) and eij(T) of the two crystals monotonically increased as the temperature decreased. The Einstein temperature estimated from Varshni's equation revealed that e15 and e22 of LiNbO3 have remarkably low values compared with the acoustic Debye temperature. In addition, the lattice anharmonicity of these piezoelectric coefficients was also extraordinarily low. An analysis based on the group theory and lattice dynamics revealed that both LiNbO3 and LiTaO3 crystals have three types of internal displacement modes: A1, A2, and E, and only the E mode affects e15 and e22. Therefore, it is reasonable to suppose that the E mode internal displacement is responsible for the unusual behaviors of the tow piezoelectric coefficients.

Low Temperature Elastic Constants and Piezoelectric Coefficients of LiNbO3and LiTaO3: Resonant Ultrasound Spectroscopy Measurement and Lattice Dynamics Analysis

The complete sets of elastic constants Cij and piezoelectric coefficients eij for LiNbO3 and LiTaO3 single crystals have been determined by resonant ultrasound spectroscopy (RUS) from ambient temperature to 6 K. Both Cij(T) and eij(T) of the two crystals monotonically increased as the temperature decreased. The Einstein temperature estimated from Varshni's equation revealed that e15 and e22 of LiNbO3 have remarkably low values compared with the acoustic Debye temperature. In addition, the lattice anharmonicity of these piezoelectric coefficients was also extraordinarily low. An analysis based on the group theory and lattice dynamics revealed that both LiNbO3 and LiTaO3 crystals have three types of internal displacement modes: A1, A2, and E, and only the E mode affects e15 and e22. Therefore, it is reasonable to suppose that the E mode internal displacement is responsible for the unusual behaviors of the tow piezoelectric coefficients.

Temperature dependence of the excitonic energy band gap in In(Ga)As nanostructures

We analyzed the temperature-dependent variation of the peak energies in the photoluminescence of InAs/GaAs quantum dots and InAs/In0.2Ga0.8As/GaAs quantum dots in an asymmetric well. The InAs dots were grown by using migration-enhanced molecular beam epitaxy. The peak emission energies decreased monotonously with increasing temperature over the range of measurements from 13 K to 225 K. The temperature dependence of the peak energy was successfully fitted with two analytic models, namely, the phenomenological Varshni equation and the semi-empirical Fan equation based on electron-phonon statistics, respectively. The physical meaning of the Varshni coefficients is discussed in terms of the phonon energy and the strain in the nanostructures.

Chirped InGaAs quantum dot molecules for broadband applications.

Lateral InGaAs quantum dot molecules (QDMs) formed by partial-cap and regrowth technique exhibit two ground-state (GS) peaks controllable via the thicknesses of InAs seed quantum dots (x), GaAs cap (y), and InAs regrowth (z). By adjusting x/y/z in a stacked QDM bilayer, the GS peaks from the two layers can be offset to straddle, stagger, or join up with each other, resulting in multi-GS or broadband spectra. A non-optimized QDM bilayer with a 170-meV full-width at half-maximum is demonstrated. The temperature dependencies of the emission peak energies and intensities from the chirped QDM bilayers are well explained by Varshni's equation and thermal activation of carriers out of constituent quantum dots.

Pulsed laser excitation power dependence of photoluminescence peak energies in bulk ZnO

Photoluminescence (PL) spectra of hydrothermal bulk ZnO were measured in the temperature range from 5 to 298 K. The sample was excited by means of the 266-nm line of an Nd3+: YAG Q-switched pulsed laser with numerous average excitation powers in the range from 0.33 to 7.50 mW. At constant temperatures, the most intense PL peak red-shifts with average excitation power, whereas positions of other near-band-edge peaks remain unchanged. It was experimentally proven that the red-shift is not due to local heating at the excited spot. Rather, it is due to relaxation of photoexcited carriers to lower energy transitions as the most intense transition is saturated by high excitation photon density. Furthermore, the temperature dependence of energy of the most intense PL peak was fitted with the Varshni equation. The Varshni coefficients α and β decrease with increasing pulsed laser excitation power.

Hot wall epitaxy법에 의한 MgGa2Se4단결정 박막 성장과 광학적 특성

단결정 성장을 위한 <TEX>$MgGa_2Se_4$</TEX> 다결정은 수평 전기로에서 합성하였으며, 결정구조는 rhombohedral이고 격자상수 <TEX>$a_0$</TEX>는 3.953 <TEX>${\AA}$</TEX>, <TEX>$c_0$</TEX>는 38.890 <TEX>${\AA}$</TEX>였다. <TEX>$MgGa_2Se_4$</TEX> 단결정박막은 HWE(Hot Wall Epitaxy) 방법으로 반절연성 GaAs(100)기판에 성장시켰다. 단결정박막의 성장 조건은 증발원의 온도 <TEX>$610^{\circ}C$</TEX>, 기판의 온도 <TEX>$400^{\circ}C$</TEX>에서 진행되었으며 성장 속도는 0.5 <TEX>${\mu}m/h$</TEX>였다. 단결정박막의 결정성은 이중 결정 x-선 회절곡선의 반폭치와 X-선 회절무늬의 <TEX>${\omega}-2{\theta}$</TEX>로부터 구하여 최적 성장 조건을 알 수 있었다. Hall 효과는 van der Pauw 방법에 의해 측정되었으며, 온도에 의존하는 운반자 농도와 이동도는 293 K에서 각각 <TEX>$6.21{\times}10^{18}/cm^3$</TEX>, 248 <TEX>$cm^2/v{\cdot}s$</TEX>였다. <TEX>$MgGa_2Se_4$</TEX>/SI(Semi-Insulated) GaAs(100) 단결정 박막의 광흡수 스펙트럼을 10 K에서 293 K까지 측정하였다. 광흡수 스펙트럼으로부터 구한 에너지 갭 <TEX>$E_g(T)$</TEX>는 varshni 공식 <TEX>$E_g(T)=E_g(0)=({\alpha}T^2/T+{\beta})$</TEX>을 잘 만족함을 알 수 있었다. 여기서 <TEX>$E_g(0)=2.34\;eV$</TEX>, <TEX>${\alpha}=8.81{\times}10^{-4}\;eV/K$</TEX>, <TEX>${\beta}=251\;K$</TEX>였다. A stoichiometric mixture of evaporating materials for <TEX>$MgGa_2Se_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. The crystal structure of these compounds has a rhombohedral structure with lattice constants <TEX>$a_0=3.953\;{\AA}$</TEX>, <TEX>$c_0=38.890\;{\AA}$</TEX>. To obtain the single crystal thin films, <TEX>$MgGa_2Se_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$610^{\circ}C$</TEX> and <TEX>$400^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by the double crystal X-ray rocking curve and X-ray diffraction <TEX>${\omega}-2{\theta}$</TEX> scans. The carrier density and mobility of <TEX>$MgGa_2Se_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method were <TEX>$6.21{\times}10^{18}\;cm^{-3}$</TEX> and 248 <TEX>$cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The optical absorption of <TEX>$MgGa_2Se_4$</TEX> single crystal thin films was investigated in the temperature range from 10 K to 293 K. The temperature dependence of the optical energy gap of the <TEX>$MgGa_2Se_4$</TEX> obtained from the absorption spectra was well described by the Varshni's equation, <TEX>$E_g(T)=E_g(0)-({\alpha}T^2/T+{\beta})$</TEX>. The constants of Varshni's equation had the values of <TEX>$E_g(0)=2.34\;eV$</TEX>, <TEX>${\alpha}=8.81{\times}10^{-4}\;eV/K$</TEX> and <TEX>${\beta}=251\;K$</TEX>, respectively.

STUDIES ON UV PHOTOLUMINESCENCE FROM PULSED LASER DEPOSITED ENSEMBLES OF CAPPED SILICON NANOPARTICLES

Multilayer ensembles of alumina capped, widely dispersed silicon nanoparticles ( Si -nps) with mean diameter in the range of about 1–4 nm were grown using pulsed laser deposition. With photo-excitation at ~3.82 eV, photoluminescence (PL) was found to emanate from these Si -nps mainly in the UV spectral region centered at about 3.37 eV due to the Γ25–Γ15 transitions. It was found that while the bandgap measured from photoabsorption spectra showed blueshift with decreasing mean size of the Si -nps, spectral position of the PL peaks remained almost insensitive to variation in the mean size. The PL peak at about 3.37 eV was observed to vanish at temperatures higher than 70 K and another one at about 3.31 eV attributed to the TO phonon replica disappeared above 100 K. In general FWHM of both these PL peaks was found to increase monotonically from about 6 to 19 meV and the peak positions were found to undergo redshift with increase in the sample temperature from 10 to 100 K. These observations could be explained by applying Bose statistics for a 6 meV confined phonon mode broadening of the Si -nps and using the Varshni equation, respectively. Si -nps grown in oxygen ambient at 600°C showed significant enhancement in the PL intensity with increasing pressure. These findings elicit that light emission from Si -nps is either due to the nanoparticles of about 1 nm and smaller size primarily driven by the quantum confinement or due to an interface state pumped by these Si -nps.

Photoluminescence of PbS quantum dots on semi-insulating GaAs

We studied the emission properties of colloidal PbS quantum dots (QDs) (5.3 nm) dispersed on semi-insulating GaAs in the temperature range of 5–300 K by employing Fourier transform infrared photoluminescence spectroscopy. The results reveal that the PbS QDs alter and notably enhance the emission features of the GaAs substrate itself. The dependence of the QD emission peak position on temperature is modeled equivalently well with the well-known empirical Varshni equation and with a relation based on thermodynamics. The work reveals that emission properties of PbS QDs do not follow predictably general rules but are determined sensitively by the preparation method and substrate used.

Temperature dependence of the energy gap, refractive index and optical‐oscillator parameters of amorphous GaxSe1–x (x = 0.4, 0.5, 0.6) thin films

AbstractThin films of a‐GaxSe1–x (x = 0.4, 0.5, 0.6) are prepared by vacuum evaporation technique onto well‐cleaned glass substrates. The transmittances of the films are measured at various temperature ranges (100 K‐400 K) in the wavelength range of 400‐1100 nm. Optical absorption obeyed the non‐direct transition process. The temperature dependence of the optical energy gaps in that range are fitted with four equations beginning with linear, non linear, Varshni and Bose Einstein equations. The results have got appeared the best fitting is with the Varshni equation. We have found that the energy gap decreases with the increase in Ga percentage in these thin films and increase in energy gap with the decrease of temperature. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The correlation between shear elastic modulus and glass transition temperature of bulk metallic glasses

Based on Varshni equation, the shear elastic modulus at the glass transition temperature [G(Tg)] and the shear elastic modulus at 0 K [G(0)] were calculated from the elastic modulus measured at room temperature for various bulk metallic glasses (BMGs). The G(Tg)/G(0) ratios for various BMGs are almost the same and have a value around 0.85. This unique correlation implies that the glass transition occurs when the shear modulus of a BMG decreases to about 85% of G(0). This correlation between shear modulus and glass transition is of significance in understanding the glass transition of BMGs.

Photoluminescence investigation of ferromagnetic Ga 1− xMn xN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga 1− x Mn x N layers (where x ≈ 0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga 1− x Mn x N layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga 1− x Mn x N layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10 −4, 2.7 × 10 −4, 3.4 × 10 −4 eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga 1− x Mn x N layers increased, the temperature dependence of the band-gap energy was clearly reduced.

Modification of the anomalous optical transitions in multilayer AlGaN-based nanoheterostructure using a nonbonding laser lift-off technique

We present a study of the effect of the strain on the anomalous temperature dependence of the photoluminescence (PL) transition energy of three-nanolayer AlxGa1−xN nanoheterostructur∕GaN (0&amp;lt;x⩽0.29) grown on sapphire substrate by hydride vapor phase epitaxy. We proved that this effect is due to the lattice constant and thermal coefficient mismatches among the various layers, especially between the sapphire and GaN epilayers. We separated the nanoheterostructure from its sapphire substrate using a nonbonding laser lift-off technique which we described in details elsewhere. The separation caused the PL peak positions to shift, which proves that a change in the strain took place after the separation and that the mismatches in the physical parameters between the sapphire and the GaN play an essential role in the anomalous behavior. Furthermore, we observed a much better agreement of the transition energy of the temperature dependent PL with the Varshni equation, after the separation.

Structural and optical properties of CuGaSe 2 layers grown by the hot wall epitaxy method

Copper gallium selenide (CuGaSe 2, CGS) layers were grown by the hot wall epitaxy method. The optimum temperatures of the substrate and source for growth turned out to be 450 and 610 °C, respectively. The CGS layers were epitaxially grown along the 〈1 1 0〉 direction and consisted of Ga-rich components indicating the slight stoichiometric deviations. Based on the absorption measurement, the band-gap variation of CGS was well interpreted by the Varshni's equation. The band-gap energies at low temperatures, however, had a higher value than those of other CGS. It suggests that the band-gap increase is influenced by the slightly Ga-rich composition. From the low-temperature photoluminescence experiment, sharp and intensive free- and bound-exciton peaks were observed. By analyzing these emissions, a band diagram of the observed optical transitions was obtained. From the solar cell measurement, an 11.17% efficiency on the n-CdS/p-CGS junction was achieved.

A simple model to predict the temperature dependence of elastic moduli of bulk metallic glasses

We report a simple model to predict the temperature dependence of elastic moduli of bulk metallic glasses from room temperature measurements, using the Varshni equation and the basic assumption that between absolute zero and the melting point, the shear and bulk moduli change, respectively, by 45% and 22%. This model has been tested using experimental data obtained on a large variety of bulk metallic glasses, and the predicted values are found to be in very good agreement with the experimental results.

Optical and electrical properties of modulation-doped n and p-type Ga x In1-x N y As1-y /GaAs quantum wells for 1.3 μm laser applications

We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga1-x In x N y As1-y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by band-to-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.