Room Temperature Band Gap Research Articles

Structural and photoluminescent properties of Gd3+ doped barium aluminate phosphor

Gd3+-doped barium aluminate was synthesized using a conventional solid-state technique and annealed at 1200 °C. The X-ray powder diffraction results confirmed the formation of a hexagonal structure with the space group of P6322 for the studied phosphors. The average crystallite size of 75 nm was calculated using Scherrer's equation based on the X-ray line broadening of all the major diffraction peaks. The shape, distribution, and level of agglomeration of the synthesized samples were confirmed using a field emission scanning electron microscope and energy-dispersive X-ray spectroscopy. The distribution of various ions in the host and doped sample was confirmed using time-of-flight secondary ion mass spectroscopy equipped with a Bi+ ion gun. UV–Visible spectroscopy was employed to record the reflectance of the studied samples, and bandgaps were calculated using the Kubelka-Munk function. The bandgaps were found to be in the range of 3.96–4.75 eV, confirming the formation of a room-temperature wide bandgap semiconductor. Photoluminescence spectroscopy was used to record the excitation and emission spectra of all the samples, which displayed an intense peak at 314 nm attributed to the 6P7/2 → 8S7/2 transition. Moreover, all samples displayed an additional peak in the emission spectrum at a wavelength of 627 nm, which was subsequently confirmed to be generated by second-order diffraction. The decay time for all the Gd3+ doped samples was calculated for λex of 274 nm and λem of 314 nm and was observed to be a few microseconds. The strong emission peak in the wavelength range of Ultraviolet-B has the potential to be utilized for phototherapy to treat skin problems that respond to photons in the range of 312–315 nm.

Unpaired electrons-induced geochemical activity of native sulfur in energy-triggered ring-opening process

Native sulfur (denoted as S0) has been regarded as the crucial species in the biogeochemical cycle of sulfur, and the opening of its eight-membered rings is critical to its activation. However, due to the extreme difficulty in detecting transient ring-opening species, the activation process and inducing factors have yet to be revealed. This study investigates the external (incident optical energy) and internal factors (impurity elements) that trigger the opening of S0 ring, and the underlying activation mechanisms by combining optical measurements, theoretical calculations, and photochemical experiments. Synchrotron radiation in situ X-ray absorption near edge spectra confirms that S0 can undergo ring-opening reactions under light irradiation below ∼575 nm, which results from electron transition via indirect bandgap. Density functional theory calculations and ab initio nonadiabatic molecular dynamics reveal that photo-assisted electron transition evokes ring opening in the femtosecond scale, resulting in a new chain-like molecular configuration with changed charge density and electronic structure. Compared with the ring-like structure, the ring cleavage can provide two unpaired electrons on the terminal S atoms as reactive sites for reducing bicarbonate to formate. The room-temperature bandgap of four S0 samples with different substituting concentrations of impurity elements (mainly As and Se, up to 282.0 and 19.3 μg/g, respectively) decreases linearly with the increase of doping amount. It confirms that As and Se can add 4p orbitals into the S 3p-dominated valence and conduction band of S0, thus reducing the energy required for electron transition and promoting the ring-opening reaction. S0 sample with 282.0 μg/g of As would produce much less HCOOH (3.3 nM·g·h−1·m−2) from carbonates compared with the few-As sample (34.0 nM·g·h−1·m−2), attributed to the new three bonds of As with only one active unpaired electron left after ring opening. Uncovering the critical roles of ring-opening reactions in improving the chemical activities of S0 under the impact of natural light and impurity elements can significantly deepen the understanding of molecular crystalline mineral S0 and the involved biogeochemical sulfur cycle.

Band Gap Tuning in Transition Metal and Rare-Earth-Ion-Doped TiO2, CeO2, and SnO2 Nanoparticles

The energy gap Eg between the valence and conduction bands is a key characteristic of semiconductors. Semiconductors, such as TiO2, SnO2, and CeO2 have a relatively wide band gap Eg that only allows the material to absorb UV light. Using the s-d microscopic model and the Green's function method, we have shown two possibilities to reduce the band-gap energy Eg-reducing the NP size and/or ion doping with transition metals (Co, Fe, Mn, and Cu) or rare earth (Sm, Tb, and Er) ions. Different strains appear that lead to changes in the exchange-interaction constants, and thus to a decrease in Eg. Moreover, the importance of the s-d interaction, which causes room-temperature ferromagnetism and band-gap energy tuning in dilute magnetic semiconductors, is shown. We tried to clarify some discrepancies in the experimental data.

Theoretical exploration of the optoelectronic properties of InAsNBi/InAs heterostructures for infrared applications: A multi-band k·p approach

We have investigated numerous electronic and optical aspects of the InAsNBi material, considering a multi-band k·p Hamiltonian. We have scrutinised electronic band dispersion, changes in spin–orbit splitting energy (so), charge carrier effective mass, band offset, and optical gain variations of InAsNBi and Quantum Well (QW) structures lattice-matched to InAs. The band gap of InAs0.9209N0.0291Bi0.05 is observed to be 131 meV under lattice-matched conditions, and the operating wavelength equivalent to this bandgap is 9.46 μm, which corresponds to the atmospheric Long Wavelength Infrared (LWIR) communication window of 8–12 μm. In order to tune the bandgap of a quantum confined heterostructure over a wide range, strain analysis plays a pivotal role. While compressive strain of 0.7% offers a band gap of 96 meV, tensile strain of 1.16% generates a band gap to 178 meV, which is almost equivalent to the room temperature bandgap of InSb. Under the influence of strain, spin-orbit splitting energy (Δso) spans the range from 0.31 eV to 0.45 eV. In addition to this, the inclusion of N and Bi into InAs reduces the effective mass of electrons to 0.01343 m0, which is ∼0.5 times of InAs. We have also computed the band structure of the InAsNBi/InAs QW heterostructure and found that under the influence of tensile strain with N = 2.5% and Bi = 1%, there is a changeover from Type-I to Type-II heterostructure. Indeed, InAsNBi can be a potential material for realizing various optoelectronic applications such as atmospheric LWIR communication, intermediate band solar cells, and night vision cameras.

(Digital Presentation) Calculation of the Energy Band Diagram and Estimation of Electronic Transport Parameters of Metastable Amorphous Ge2Sb2Te5

We calculate critical electronic conduction parameters of the amorphous phase of Ge2Sb2Te5 (GST), a common material used in phase change memory. We estimate the room temperature bandgap of metastable amorphous GST to be Eg (300K) = 1.84 eV based on a temperature dependent energy band model. We estimate the free carrier concentration at the melting temperature utilizing the latent heat of fusion to be 1.47 x 1022 cm-3. Using the thin film melt resistivity, we calculate the carrier mobility at melting point as 0.187 cm2/V-s. Assuming that metastable amorphous GST is a supercooled liquid with bipolar conduction, we compute the total carrier concentration as a function of temperature and estimate the room temperature free carrier concentration as p(300K) ≈ n(300K) = 1.69×1017 cm-3. Free electrons and holes are expected to recombine over time and the stable (drifted) amorphous GST is estimated to have p-type conduction with p(300K) ≈ 6×1016 cm-3.

Temperature-tuned optical bandgap of Al-doped ZnO spin coated nanostructured thin films

Al-doped ZnO (AZO) nanostructured thin films were produced by spin coating of AZO ink. The structural characteristics were determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD plot showed well-defined and intensive diffraction peaks belonging to hexagonal crystal structure. AZO thin films were observed in the form of nanostructure with size varying generally between 20 and 30 nm in the SEM image. The room temperature bandgap energies of undoped and Al-doped ZnO nanostructured films were obtained as 3.32(7) and 3.35(3) eV, respectively. Temperature-tuned bandgap energy characteristics of AZO films were revealed applying transmission experiments by varying the sample temperature. The temperature-bandgap energy dependency was studied by Varshni and Bose-Einstein expressions and optical parameters of AZO films were revealed.

Room Temperature D0 Ferromagnetism, Band-Gap Reduction, and High Optical Transparency in P-Type K-Doped Zno Compounds for Spintronics Applications

Room Temperature D0 Ferromagnetism, Band-Gap Reduction, and High Optical Transparency in P-Type K-Doped Zno Compounds for Spintronics Applications

Lithium indium diselenide — An advanced material for neutron detection

This paper describes the synthesis, crystal growth, detector fabrication, radiation hardening studies, MCNP modeling, and characterization of lithium indium diselenide or LiInSe2. This newly-developed room-temperature thermal neutron detector has semiconducting and scintillating properties and it is suitable for neutron detection application. LiInSe2 was synthesized starting from elemental Li, In, Se in two steps due to high reactivity of Li. A single crystal of LiInSe2 was grown using the Vertical Bridgman method. The room temperature band gap was found to be 2.8eV using optical absorption measurements. Bulk resistivity was measured at ∼5 × 1011 Ωcm. Photoconductivity measurements of LiInSe 2 wafers identified a peak in the photocurrent around 445 nm. Nuclear radiation detectors were fabricated from single crystal wafer and the responses to alpha particles at various biases were measured. The mobility-lifetime product was estimated. Gamma irradiation studies were performed with calculated absorbed doses ranging from 0.2126 to 21,262 Gy. The characterization of the two wafers for their scintillator performance was conducted after each irradiation. The gamma irradiation produced a reduction of the light yield that translated to a lower channel number for the centroid of alpha detection spectra. It also showed a considerable reduction of the decay time after the first irradiation. These are the first studies on gamma radiation hardening with this material.

Oxygen vacancy mediated room-temperature ferromagnetism and band gap narrowing in DyFe0.5Cr0.5O3 nanoparticles.

We report on the magnetic and optical properties of DyFe0.5Cr0.5O3 nanoparticles synthesized by a sol-gel method. Rietveld refinement of a powder X-ray diffraction (XRD) pattern confirms the formation of an orthorhombic disordered phase with the Pnma space group. The formation of nano-sized particles, with an average size of 42(±12) nm, was approximated by the transmission electron microscopy (TEM) image analysis. X-ray photoelectron spectroscopy (XPS) of this compound reveals the presence of Fe2+/Fe3+ and Cr2+/Cr3+ mixed-valence states as a consequence of oxygen vacancies present at the surface of nanoparticles. The temperature-dependent magnetization (M-T) shows a finite non-zero magnetization up to 300 K and the field-dependent magnetization (M-H) curve exhibits a weak ferromagnetic (WFM) nature at 300 K with a clear hysteresis loop, which is quite appealing compared to that of the previously reported micron-sized DyFe0.5Cr0.5O3. These observations indicate that the large concentration of uncompensated surface spin of nanoparticles could be responsible for the observed room-temperature ferromagnetism. Moreover, DyFe0.5Cr0.5O3 nanoparticles show a significantly narrow band gap (Eg ∼ 2.0 eV). Meanwhile, the oxygen vacancies may generate shallow trap energy levels within the band gap as observed from photoluminescence (PL) spectroscopy. The observed band gap narrowing by Fe doping and the effect of oxygen vacancies on the band gap are consistent with the predictions of density functional theory (DFT) calculations. The evidence of room-temperature ferromagnetism in DyFe0.5Cr0.5O3 nanoparticles compared to their bulk counterparts and the significantly narrow band gap in the visible range manifest the potential of this material in spintronic and optical applications.

Temperature-dependent optical characteristics of sputtered Ga-doped ZnO thin films

The present paper reports structural and optical properties of gallium (Ga) doped ZnO thin films (GZO) grown by magnetron sputtering technique. The crystalline properties were determined from X-ray diffraction measurements and analyses pointed out the crystalline structure as hexagonal, crystalline size as 43 nm and strain as 6.9 × 10−5. Derivative spectroscopy analyses showed that band gap energy of GZO thin films decreases from 3.50 eV (10 K) to 3.45 eV (300 K). Temperature-band gap energy dependency was analyzed using Varshni and O’Donnell-Chen models. The absolute zero band gap energy, the rate of change of band gap energy with temperature and phonon energy were found as 3.50 eV, −2.8 × 10−4 eV/K and 15 meV, respectively. The room temperature band gap and Urbach energies were also determined as 3.43 eV and 102 meV, respectively, from the absorption analysis.

Sn 5s2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation

The effects of Sn $5s$ lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn $5s$ orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn $5s$ state contribution at the top of the valence band in the different Sn-sulphides, SnS, ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$, and ${\mathrm{SnS}}_{2}$, to be clearly identified. SnS and ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$ contain Sn(II) cations and the corresponding Sn $5s$ lone pairs are at the valence band maximum (VBM), leading to $\ensuremath{\sim}1.0$--1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, ${\mathrm{SnS}}_{2}$ only contains Sn(IV) cations, no filled lone pairs, and therefore has a $\ensuremath{\sim}2.3$ eV room-temperature band gap and much lower VBM compared with SnS and ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS, ${\mathrm{Sn}}_{2}{\mathrm{S}}_{3}$, and ${\mathrm{SnS}}_{2}$, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn $5s$ lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with $\mathrm{n}{s}^{2}$ configuration.

GeSe: Optical Spectroscopy and Theoretical Study of a van der Waals Solar Absorber.

The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the optoelectronic and structural properties of GeSe are determined. A fundamental absorption onset of 1.30 eV is found at room temperature, close to the optimum value according to the Shockley–Queisser detailed balance limit, in contrast to previous reports of an indirect fundamental transition of 1.10 eV. The measured absorption spectra and first-principles joint density of states are mutually consistent, both exhibiting an additional distinct onset ∼0.3 eV above the fundamental absorption edge. The band gap values obtained from first-principles calculations converge, as the level of theory and corresponding computational cost increases, to 1.33 eV from the quasiparticle self-consistent GW method, including the solution to the Bethe–Salpeter equation. This agrees with the 0 K value determined from temperature-dependent optical absorption measurements. Relaxed structures based on hybrid functionals reveal a direct fundamental transition in contrast to previous reports. The optoelectronic properties of GeSe are resolved with the system described as a direct semiconductor with a 1.30 eV room temperature band gap. The high level of agreement between experiment and theory encourages the application of this computational methodology to other van der Waals materials.

Characterization of Silver-Doped LiF Crystal Grown by Czochralski Technique for Dark Matter Search Application

In this article, the growth, luminescence, and scintillation properties of a silver-doped LiF crystal grown by the Czochralski technique are studied. The absorption spectrum of the crystal is measured at room temperature and optical energy bandgap of the crystal is calculated as ~5.2 eV. The luminescence and scintillation properties of the crystal are studied at different temperatures (10–550 K) under the excitations with X-ray, 266-nm laser, 280-nm light emitting diode (LED), and 90Sr beta source. The crystal shows a violet emission (~405 nm) under the excitation of a 280-nm LED source. The luminescence decay time of the crystal is measured from room temperature (300 K) to 10 K under the excitation of the 266-nm laser source. The decay time curves are fit with 2-exponential (300–200 K) and 3-exponential (175–10 K) decay functions. The shortest ( $27.2~\mu \text{s}$ ) and the longest decay time ( $42.8~\mu \text{s}$ ) of the crystal are obtained at 300 and 75 K, respectively. Thermoluminescence (TL) measurements were carried out from 10 to 300 K and 325 to 550 K in order to investigate the presence of trap centers. The different kinematic parameters such as order of kinematics, trap depth, and frequency factor are calculated for the observed TL peaks. The scintillation light yield of the crystal is measured with a continuous single photon counting technique using a 90Sr beta source in the temperature range from 300 to 10 K. In this article, we will emphasize the potentiality of a Li-based crystal for dark matter search applications.

Amorphous gallium oxide sulfide: A highly mismatched alloy

Stoichiometric gallium oxide sulfide Ga2(O1 − xSx)3 thin-film alloys were synthesized by pulsed-laser deposition with x ≤ 0.35. All deposited Ga2(O1 − xSx)3 films were found to be amorphous. Despite the amorphous structure, the films have a well-defined, room-temperature optical bandgap tunable from 5.0 eV down to 3.0 eV. The optical absorption data are interpreted using a modified valence-band anticrossing model that is applicable for highly mismatched alloys. The model provides a quantitative method to more accurately determine the bandgap as well as an insight into how the band edges are changing with composition. The observed large reduction in energy bandgap with a small sulfur ratio arises from the anticrossing interaction between the valence band of Ga2O3 and the localized sulfur level at 1.0 eV above the Ga2O3 valence-band maximum.

(Invited) Hexagonal Boron Nitride Epilayers

As a member of the III-nitride wide bandgap semiconductor family, BN has received much less attention in comparison with other nitride semiconductors. The stable phase of BN synthesized at any temperature under ambient pressure is hexagonal. Due to its wide bandgap (> 6 eV) and layered structure, hexagonal BN (h-BN) is an ideal platform for probing fundamental 2D properties of wide bandgap semiconductors. In this talk, a brief overview of the synthesis and electrical and optical properties of wafer-scale h-BN epilayers will be presented [1-7]. It was shown that the unique 2D structure of h-BN induces exceptionally high density of states, large exciton binding energy and high optical absorption and emission intensity. By growing h-BN under high V/III ratios, epilayers exhibiting pure free exciton emission have been obtained [4]. Photocurrent excitation spectroscopy results directly provided a room temperature bandgap value for h-BN in between 6.4 and 6.5 eV and the free exciton binding energy (Ex) of 0.73 eV [5]. Layer-structured B-rich BGaN alloys, heterostructures, and quantum wells have also been successfully grown and their properties will be discussed. Thermal neutron detectors fabricated from 100% B-10 enriched h-BN epilayers of thicknesses exceeding 50 mm have attained the highest detection efficiency to date among solid-state detectors at about 58% [6, 7]. These solid-state neutron detectors have become increasingly desirable for a wide range of applications from fissile materials sensing to well logging, because 3He gas detectors are inherently bulky, require high pressurization and high voltage application, slow response time, and expensive. It is our belief that h-BN will lead to many potential applications from deep UV optoelectronics, radiation detectors, to novel layered-structured photonic and electronic devices. [1] R. Dahal, J. Li, S. Majety, B.N. Pantha, X. K. Cao, J. Y. Lin, and H.X. Jiang, Appl. Phys. Lett. 98, 211110 (2011). [2] S. Majety, J. Li, X. K. Cao, R. Dahal, B. N. Pantha, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 100, 061121 (2012). [3] H. X. Jiang, and J. Y. Lin, Semicon. Sci. Technol. 29, 084003 (2014). [4] X. Z. Du, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 106, 021110 (2015); 108, 052106 (2016). [5] T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 122101 (2016). [6] A. Maity, T. C. Doan, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 109, 072101 (2016). [7] A. Maity, S. J. Grenadier, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 111, 033507 (2017) and J. Appl. Phys. 123, 044501 (2018).

Analysis of temperature-dependent transmittance spectra of Zn0.5In0.5Se (ZIS) thin films

Temperature-dependent transmission experiments of ZnInSe thin films deposited by thermal evaporation method were performed in the spectral range of 550–950 nm and in temperature range of 10–300 K. Transmission spectra shifted towards higher wavelengths (lower energies) with increasing temperature. Transmission data were analyzed using Tauc relation and derivative spectroscopy. Analysis with Tauc relation was resulted in three different energy levels for the room temperature band gap values of material as 1.594, 1.735 and 1.830 eV. The spectrum of first wavelength derivative of transmittance exhibited two maxima positions at 1.632 and 1.814 eV and one minima around 1.741 eV. The determined energies from both methods were in good agreement with each other. The presence of three band gap energy levels were associated to valence band splitting due to crystal-field and spin–orbit splitting. Temperature dependence of the band gap energies were also analyzed using Varshni relation and gap energy value at absolute zero and the rate of change of gap energy with temperature were determined.

Bandgap of cubic ZnS1-xOx from optical transmission spectroscopy

ZnS1-xOx is a highly mismatched semiconductor alloy with potential light-emitting and solar-cell applications. In this work, optical transmission spectroscopy and a modified derivative method were employed to determine the room-temperature bandgap of cubic (zinc blende) ZnS1-xOx from x = 0.01 to 0.3. The bandgap drops steeply for dilute oxygen concentrations, followed by a more gradual decrease for x > 0.05. This nonlinear behavior is attributed to a transition from isolated oxygen impurities to pairs and larger clusters. Alloying with x = 0.3 causes bandgap to drop from 3.7 to 3.1 eV. Previous work showed that the bandgap of wurtzite ZnS1-xOx shifts from 3.7 to 2.8 eV over the same composition range.

Enhanced efficiency of CdTe Photovoltaic by thermal evaporation Vacuum

CdTe Cadmium telluride is a hopeful material for large scale photovoltaic applications. This paper we study effect annealing temperature( 200, 400) for CdTe heterojunction solar cells with thermal evaporation technique in vacuum with rate deposition (4.32 A°/sec) and thickness ( ≈ 400 ±10 nm), solar cells were realization with conversion efficiencies of ( 6.4% - 10.8%). The main focus in this work was to study the annealing temperature dependent conduct of the expansion parameter and the band gap energy of (CdTe ) thin film in solar cells. Room temperature band gap for ( CdTe) is found ( Eg =1.5 to 2) eV for efficiency solar cells.

Weak ferromagnetism in band-gap engineered α-(Fe2O3)1−X(Cr2O3)X nanoparticles

We report on the tunable band engineering in α-(Fe2O3)1−X(Cr2O3)X nanoparticles. Nanoparticles were prepared by the sol-gel method and stabilised in the hexagonal crystal structure with R-3C space group. At room temperature band-gap studies have shown the narrowing of the band gap from 2.1 eV to 1.6 eV with Cr addition. Compared to antiferromagnetic α-Fe2O3 nanoparticles, doping of Cr2O3 increases the magnetic hysteresis significantly. Impedance spectroscopic studies have revealed Maxwell-Wagner dielectric relaxation near room temperature. Our results improve the understanding of the structural, optical, dielectric and magnetic properties of Cr-doped α-Fe2O3 nanoparticles and illustrates that these nanoparticles have potential candidate in spintronic, optoelectronic, solar cell and photocatalysis applications.

Microstructural, Optical, and Magnetic Properties of BiCoO3-Modified Bi0.5K0.5TiO3

Pb-free ferroelectric solid solution of Bi0.5K0.5TiO3 with BiCoO3 has been fabricated using a sol-gel technique, exhibiting room-temperature ferromagnetism. The optical bandgap of Bi0.5K0.5TiO3 reduced from 3.21 eV to 1.77 eV with increasing amount of BiCoO3 solid solution. The room-temperature ferromagnetism and bandgap reduction occurred due to BiCoO3 diffusion into Bi0.5K0.5TiO3, resulting in formation of solid solution. These results provide a simple approach to realize room-temperature ferromagnetism in Pb-free ferroelectric materials.