Temperature Coefficient Of Band Gap Research Articles

Temperature Dependence of the CdS Bandgap in the Extreme Confinement Regime.

A non-empirical equation describing the effect of size on the temperature dependence of the optical bandgap of CdS (dEg/dT) is obtained on the basis of the Brus equation. Intriguingly, we find that dEg/dT diverges strongly from bulk values only within the "extreme confinement" (EC) regime. We conducted both experimental and theoretical investigations of the absorption spectra of CdS clusters and quantum dots as a function of temperature above room temperature. Our results show that the value of dEg/dT obtained from absorption spectra in the EC regime is 2.5 times higher than in the strong confinement regime. Notable ligand sensitivities are also observed for dEg/dT in the case of CdS clusters. Ab initio molecular dynamics simulations and density functional theory calculations reveal that thermal fluctuations are the crucial factor influencing the bandgap temperature coefficient. Our results help resolve some long-standing debates regarding the dEg/dT behavior of semiconductor quantum dots.

First-order Bandgap Reference Source Design with High Power Supply Rejection Ratio

In this paper, a first-order bandgap reference source with a high power supply rejection ratio (PSRR) is designed. To obtain a low-temperature coefficient, a first-order bandgap temperature coefficient compensation method is used by adjusting the resistor resistance of the positive and negative temperature coefficients, and the gate-source voltage of the current mirror transistor is kept constant to increase the PSRR value in the low-frequency band. A self-start circuit is added to make the circuit start up quickly out of the simple bias point. The simulation results show that the bandgap reference voltage source has a constant 1.25V reference voltage with a temperature coefficient of 8.55ppm/°C in the temperature range of -40 to 125°C. The circuit is simulated using Cadence software. The circuit can be applied to OLED constant current source circuits and power management circuits.

The illumination effects on the current conduction mechanisms of the Au/(Er2O3:PVC)/n‐Si (MPS) Schottky diodes

AbstractPhotoresponse of the Au/(Er2O3:PVC)/n‐Si diode was executed in a wide range of illumination intensity (P). Its basic electrical parameters such as ideality factor (n), barrier height (ΦB0), series and shunt resistances (Rs, Rsh), photocurrent (Iph), energy‐dependence of interface states (Nss), and photosensitivity (S) were calculated from the results of the current–voltage (I–V) measurements. At the negative‐bias region, an increase in Iph is observed due to the e−‐h+ pairs that occur as a result of increasing P and their drift in opposite directions under the internal electric field. The ln(Iph)–ln(P) plot shows a good linear behavior under various applied voltages. Their slopes vary from 1.47 to 1.64, indicating that empty trap levels, that is, levels not occupied by charges, are of low density. The values n, ΦB0, and Rs were obtained using the thermionic emission theory, Cheung, and Norde functions. The value of ΦB0 decreases with increasing n and P as linearly with ΦB0(n) = (−0.143n + 1.118) eV and ΦB0(P) = (−2 × 10−4P + 0.831) eV, respectively. This illumination coefficient of ΦB0 (=−2 × 10−4 eV/[mW/cm2]) is close to the temperature coefficient bandgap of Si (=−4.73 × 10−4 eV/K). These results show that the performed Au/(Er2O3:PVC)/n‐Si diode has excellent photo‐response and can be successfully used in photovoltaic applications instead of conventional metal–semiconductor diodes.

Ternary Lead Chalcogenide Alloys for Mid-Infrared Detectors

We demonstrate thin films of PbSe1−xSx and PbSe1−xTex lead chalcogenide ternary alloys as infrared detectors. The films were deposited on single crystal BaF2 substrates using physical vapor deposition. Detectivity in the wavelength range from 1 μm to 5 μm was measured at −40°C, and all films showed photoresponse signals more than an order of magnitude larger than their noise. The detectivity spectra were used to assess the tunability of the band gap from mixing the lead chalcogenide binaries. The PbSe1−xSx system showed tunability that followed Vegard’s law, while the PbSe1−xTex system showed tunability with a bowing parameter of −0.096 eV. Comparisons to measurements from the literature taken at room temperature suggest that the bowing parameter decreases with decreasing temperature, and the band gap temperature coefficient with respect to composition also shows bowing.

Ultrafast dynamics of pure many-body effect and its competition with bandgap widening via electron–phonon coupling in PbTe thin films

Bandgap renormalization (BGR), or bandgap narrowing effect, is one of the many-body effects predicted by many-body theory in semiconductor physics. However, pure BGR effect has never been observed experimentally although the mixed effect of BGR with lattice-heating induced bandgap narrowing was reported in GaAs and CdTe semiconductors. PbTe has a positive temperature coefficient of bandgap. Its lattice-heating induced bandgap narrowing can be eliminated so that the pure BGR effect is possibly observed. However, that has never been reported so far because the BGR effect is weak in PbTe due to its huge dielectric screening. We study the ultrafast dynamics of photo-excited carriers in PbTe films using near-infrared femtosecond-resolved transient differential transmission (TDT) spectroscopy. A complex oscillation-like damped decay dynamics is observed. A negative dip, a fingerprint signature of BGR effect, occurs immediately after the thermalization process of the photo-excited carriers. As such, we have observed a pure BGR effect in PbTe for the first time, strongly supporting the many-body theory. Meanwhile, we have also observed a dynamic transition from a transient absorption enhancement to absorption saturation, revealing the emergence of a slow lattice-heating induced bandgap widening via carrier-phonon couplings. Based on a two-temperature model, a simulation computation is carried out, supporting the lattice-heating induced bandgap widening leads to the dynamic transition. A phenomenological dynamic model is developed, including three dynamic components: the thermalization of the photo-excited carriers, recombination of the thermalized carriers and lattice-heating induced bandgap widening, and used to best fit the excitation-power dependent TDT dynamics. Time constants and strengths of the three dynamic components are obtained for different excitation powers. Time constant variation with the excitation power reveals different roles of the carrier–carrier and carrier-phonon scattering in the thermalizing process, as well as of free carrier and exciton recombination in the decaying process of hot carriers.

Atypically small temperature-dependence of the direct band gap in the metastable semiconductor copper nitrideCu3N

The temperature-dependence of the direct band gap and thermal expansion in the metastable anti-${\mathrm{ReO}}_{3}$ semiconductor ${\mathrm{Cu}}_{3}\mathrm{N}$ are investigated between 4.2 and 300 K by Fourier-transform infrared spectroscopy and x-ray diffraction. Complementary refractive index spectra are determined by spectroscopic ellipsometry at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$. A direct gap of $1.68\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ is associated with the absorption onset at $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$, which strengthens continuously and reaches a magnitude of $3.5\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at $2.7\phantom{\rule{4pt}{0ex}}\mathrm{eV}$, suggesting potential for photovoltaic applications. Notably, the direct gap redshifts by just $24\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ between 4.2 and $300\phantom{\rule{4pt}{0ex}}\mathrm{K}$, giving an atypically small band-gap temperature coefficient $d{E}_{\text{g}}/dT$ of $\ensuremath{-}0.082\phantom{\rule{4pt}{0ex}}\mathrm{meV}/\mathrm{K}$. Additionally, the band structure, dielectric function, phonon dispersion, linear expansion, and heat capacity are calculated using density functional theory; remarkable similarities between the experimental and calculated refractive index spectra support the accuracy of these calculations, which indicate beneficially low hole effective masses and potential negative thermal expansion below $50\phantom{\rule{4pt}{0ex}}\mathrm{K}$. To assess the lattice expansion contribution to the band-gap temperature-dependence, a quasiharmonic model fit to the observed lattice contraction finds a monotonically decreasing linear expansion (descending past ${10}^{\ensuremath{-}6}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ below $80\phantom{\rule{4pt}{0ex}}\mathrm{K}$), while estimating the Debye temperature, lattice heat capacity, and Gr\uneisen parameter. Accounting for lattice and electron-phonon contributions to the observed band-gap evolution suggests average phonon energies that are qualitatively consistent with predicted maxima in the phonon density of states. As band-edge temperature-dependence has significant consequences for device performance, copper nitride should be well suited for applications that require a largely temperature-invariant band gap.

A 1.58 nW power consumption and 34.45 ppm/°C temperature coefficient bandgap reference (BGR) for subblocks of RFID tag

In this paper a bandgap reference (BGR) circuit irrespective of the temperature and the supply voltage variation with very low power consumption is proposed. The proportional to absolute temperature (PTAT) and complementary to absolute temperature (CTAT) generators of the proposed BGR, which has four cores cascaded with each other, are used in order to increase not only the output voltage, but also the output control ability for the temperature and the voltage insensitivity. To combine produced voltage from PTAT and CTAT generator, a weight combination circuit, which uses internal capacitors of transistors, is applied. Due to the fact that all of the transistors in such a topology are worked in sub-threshold region, the power consumption is significantly diminished to 1.58nW. Also the variation of the temperature from −25°C to 150°C, leads to the temperature coefficient about 34.45ppm/°C. The design simulation is done at 960MHz frequency in TSMC 0.18µm CMOS technology with the help of Cadence software. Also the post layout simulation result and the layout of the proposed circuit are presented. The output and the chip area of this BGR are 141.5mV and 1387µm2 respectively.

A Sub-1V High Precision CMOS Bandgap Reference

In this paper, a low temperature coefficient bandgap voltage (BGR) is designed for A/D converter by adopting piecewise-linear compensation technique. The designed BGR is analyzed and simulated in SMIC 0.18μm CMOS process. Simulation results show that the PSRR of the designed BGR achieves-72.51dB, -72.49dB, and-70.58dB at 10Hz, 100Hz and 1kHz respectively. The designed BGR achieve the temperature coefficient of 1.57 ppm/°C when temperature is in the range from-35°C to 125°C. When power supply voltage VDD changes from 1V to 7V, the deviation of the designed BGR output voltage VREF is only 4.465μV.

Temperature dependent current-transport mechanism in Au/(Zn-doped)PVA/n-GaAs Schottky barrier diodes (SBDs)

In order to obtain detailed information about the current-transport mechanisms (CTMs) in the Au/(Zn-doped) PVA/n-GaAs SBDs, the forward and reverse bias current–voltage (I–V) characteristics were investigated in the temperature range of 80–350K by the steps of 30K. The ideality factor (n) decreases from 12.850 to 2.805, while the zero-bias barrier height (ΦBo) increases from 0.145eV to 0.606eV with increasing temperature from 80K to 350K. While the n decreases, ΦBo increases with increasing temperature. Such positive temperature coefficient (α) of ΦBo is not in agreement with the negative temperature coefficient of band gap GaAs or barrier height (BH) of ideal diode. On the other hand, the value of modified barrier height (=nΦBo) decreases almost linearly with the increasing temperature as ΦB(T)=(1.909–5.852×10−4T)eV. It is clear that this value of the BH is in good agreement with the negative temperature coefficient of band gap of GaAs (−5.4×10−4eVK−1). In addition, the semi-logarithmic lnI–V plots at low bias voltages are almost parallel for each temperature. As a result of that, its inverse slope (Eo=nkT/q=87meV) remained almost constant, indicating it is independent of temperature. Such behavior of BH can be explained by the field emission (FE) theory especially at low temperatures rather than thermionic emission (TE) and thermionic field emission (TFE) theories. Therefore, the non-ideal behavior of the forward-bias I–V characteristics in Au/(Zn-doped)-PVA/n-GaAs SBD was successfully explained in terms of the TE mechanism with a double GD of BHs.

Influence of Alloying on the Optical Properties of IV–VI Nanorods

The synthesis and structural and optical characterization of PbSexS1–x and PbSe/PbSexS1–x nanorods with a diameter between 2 and 4.5 nm and a length of 10 to 38 nm is reported. The energy band gap of the nanorods exhibits a pronounced variation upon the change in diameter and composition, with a minor influence on lengths beyond 10 nm. The photoluminescence spectrum of the nanorods is composed of a dominant band, accompanied by a satellite band at elevated temperatures. The dominant band shows an exceptionally small band gap temperature coefficient and negligible extension of the radiative lifetime at cryogenic temperatures compared with the photoluminescence processes in PbSe nanorods and in PbSexS1–x quantum dots with similar band gap energy. A theoretical model suggests the occurrence of independent transitions from a pair of band-edge valleys, located at the L points of Brillouin zone, related to the dominant and satellite emission processes. Each valley is four-fold degenerate and possesses a relatively small electron–hole exchange interaction.

Correlation between the thermal expansion, isothermal compressibility, and photoconductivity of the (TlGaSe2)1 − x (TlInS2) x (x = 0.1, 0.2) solid solutions

The thermal expansion coefficient, isothermal compressibility, and photoconductivity of the (TlGaSe2)0.9(TlInS2)0.1 and (TlGaSe2)0.8(TlInS2)0.2 solid solutions have been measured as a function of temperature. The measurement results have been used to evaluate the Debye characteristic temperature and band gap temperature coefficient of the solid solutions. It is shown that an increase in their thermal expansion coefficient is accompanied by a decrease in their band gap temperature coefficient.

Optical Properties of Alloyed PbSexS1-x Nanorods

ABSTRACTWe report the synthesis, structural and optical characterization of PbSexS1-x nanorods with diameter between 2 nm to 4.5 nm and length of 12 nm to 24 nm. Their typical photoluminescence spectra exhibit a split of the band-edge exciton band. The temperature dependence photoluminescence of these nanorods revealed a relatively small band-gap temperature coefficient and a mild extension of the radiative lifetime at cryogenic temperatures - all in comparison with photoluminescence processes in PbSe nanorods, as well as in PbSexS1-x quantum dots, with similar absorption band-edge energy. A theoretical model associates the experimental observations to the occurrence of independent transitions from either degenerate or non-degenerate band-edge valleys in PbSexS1-x nanorods, each of which possessing a relatively small electron-hole exchange interaction.

Size and Temperature Dependence of Band-Edge Excitons in PbSe Nanowires

We report the attenuance and temperature-dependent photoluminescence spectra of PbSe nanowires with diameters between 5.6 and 26.4 nm (12−23% relative standard deviation) and lengths greater than 1 μm. The nanowire first exciton energy varies between 0.3 and 0.6 eV as the diameter decreases from 26.4 to 5.6 nm, respectively. Compared to spherical PbSe nanocrystals, PbSe nanowires show less quantum confinement and larger Stokes shifts. The band gap temperature coefficient (dEg/dT) decreases as the nanowire diameter decreases, consistent with previous results for PbSe spherical nanocrystals.

The mechanisms of current transport and properties of a-SiC:H/c-Si heterostructures

The properties of a-SiC:H/c-Si heterostructures formed at various alloy carbon contents are studied. A dominant mechanism of carrier transport in the heterostructures is established. Equivalent electric circuits, which make it possible to describe the current-voltage characteristics of heterostructures over entire bias range under study, are suggested. The band-gap temperature coefficient and electron affinity for a-SiC:H are estimated.

Photoelectric properties of Se-doped Cd1.23Zn1.77As2 crystals

The spectral response of the photocurrent through Se-dopedCd 1.23 Zn 1.77 As 2 single crystals was studied. The results were used to determine the band gap at absolute zero (0.540 eV) and the temperature coefficient of band gap (-0.39 meV/K). The photocurrent spectra were found to contain a photovoltaic peak attributable to Schottky barriers at the interface between the semiconductor and the metallic contact. The heights of the barriers responsible for residual conductivity were evaluated using a two-barrier model. Both surface and bulk inhomogeneities were shown to play a significant part in residual conductivity.

Optoelectronic properties of polycrystalline CdInTe films

Close-spaced vapor transport combined with free evaporation was used for the growth of polycrystalline CdInTe films on glass substrates. The optoelectronic properties of the films were investigated by absorption at 77 and 300 K, and photoluminescence at 77 K with the indium concentration ranged from 0 to 22 at. %. The band-gap energy was found to vary linearly, and the band-gap temperature coefficient increases as the indium concentration increases. It was found that the ionization energy of indium displaced from the cationic sites of the CdTe lattice corresponds to a donor level at ED>32 meV (ED≃36 meV) below the conduction-band bottom. This level and the acceptor level associated with Cd vacancies stay at the same energy distance of their corresponding bands for all the indium concentrations studied. The relation between Eg and the lattice parameter is also shown.

Optical Properties of Amorphous and Polycrystalline Stibnite (Sb2S3) Films

A systematic optical absorption study near the fundamental absorption edge was carried out on both amorphous and polycrystalline films of Stibnite in the temperature range 90-320 K. Films were grown using naturally occurring single crystals of Stibnite. The optical band gap of thermally evaporated films which have the stoichiometry Sb 43 S 57 was determined to be direct and temperature dependant. The band gap of amorphous phase at any temperature is considerably less than that of crystalline ones. The temperature dependence of band gap in both the cases agrees well with Varshni formula. The optical gap at 0 K for amorphous and crystalline films were determined by fitting experimental data to Varshni formula to be 1.95 eV and 2.13 eV with the temperature coefficient of band gap -8.72 x 10 -4 eV/K and -7 x 10 -4 eV/K respectively. We strongly believe that the band gap value depends on the sulphur content in the films. The films of thickness greater than 2000 A show interference pattern in the transmission spectra. The optical constants n and k were determined using the interference fringes in the wavelength range 600 - 900 nm for film thickness 2000 - 8000 A. In both cases n and k show thickness dependence.

Optical constants of ZnSiAs 2 thin films grown by metal-organic chemical vapor deposition

Optical measurements were made on ZnSiAs 2 films that were grown on sapphire substrates by the metal-organic chemical vapor deposition technique. The refractive index and absorption coefficient were determined over the wavelength range 600–1000 nm. The optical band gap was determined over the temperature range from -50°C to +50°C and a band gap temperature coefficient of -5.5 × 10 -4eVK -1 was obtained.

Optical constants and band gap of PbTe from thin film studies between 25 and 300°K

Optical constants of PbTe have been derived in the fundamental gap region between 25 and 300°K. It is shown in this paper by a direct compatibility check through Kramers-Krönig inversion, that properly prepared thin films have the optical properties of homogeneous material. A physical model is used to relate the optical edge and the refractive index signularity to the band gap and a value of the low-temperature dispersion of the refractive index is given, for the low-energy side of the singularity where photoelectronic phenomena take place. The precise shape of the refractive index singularity, and its relation to band tailing are discussed. The values of the optical band-gap temperature coefficient are compared with results from other authors.

Photoconductivity of Gallium Selenide Crystals

Single crystals of GaSe have been prepared by reaction of the elements, followed by gradient freeze crystallization. Crystals as grown were $p$-type, probably because of Cu acceptors, with hole mobility of 15 ${\mathrm{cm}}^{2}$/volt sec, hole concentration of ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and acceptor ionization energy of 0.12 ev. In many ways the properties of GaSe crystals are very similar to those of ZnTe crystals. The absorption edge of GaSe is at 6310 A, corresponding to a band gap of 1.97 ev; the temperature coefficient of band gap is about -4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ev/deg. Insulating and photosensitive GaSe crystals can be prepared by compensating the acceptor impurities by incorporated donors from Groups VII or IV. Rectification tests indicate $p$-type photoconductivity. Thermal quenching of photoconductivity corresponds to an ionization energy of about 0.5 ev, whereas optical quenching corresponds to an ionization energy of about 1.0 ev; thus a large Franck-Condon shift is indicated. Below the temperature at which thermal quenching of photoconductivity occurs, the sensitivity is within the range of sensitive CdS or CdSe crystals.