GaP Thin Films Research Articles

P‐7.22: Pulsed laser deposition of wide‐band gap and low leakage current density hafnium oxide thin films

P‐7.22: Pulsed laser deposition of wide‐band gap and low leakage current density hafnium oxide thin films

Gallium phosphide conformal film growth on in-situ tri-TBP dry-cleaned InGaP/GaAs using atomic hydrogen ALD

Nearly stoichiometric (high P/Ga ratio and low oxygen content) ∼ 20 nm GaP epi-layers were conformally deposited on the side-walls of InGaP layers using an atomic H ALD process with triethyl gallium (TEGa) and tri-tertiary butyl phosphine (tri-TBP). The key for uniform nucleation is an in-situ tri-TBP dry-clean in 40 mTorr Ar and 160 mTorr H2 without acidic wet-clean to generate a particle-free and damage-free InGaP surface which induces local epitaxy growth of homogeneous GaP thin films. The in-situ tri-TBP dry-clean removed the native oxide of InGaP, surface particles, and the residual impurities simultaneously. It is hypothesized that the key to the process is in-situ formation of PHx(C4H9)y surface adsorbates from atomic H and tri-TBP surface reactions. In-situ AES, cross-sectional STEM image, ex-situ XPS, AFM, and EDS confirmed the formation of high-quality GaP thin films on InGaP/GaAs with a high-quality interface and smooth surface morphology.

Comparison of Harmonic Generation from Crystalline and Amorphous Gallium Phosphide Nanofilms

AbstractGallium phosphide (GaP) is a promising material for nanophotonics, given its large refractive index and a transparency over most of the visible spectrum. However, since easy phase‐matching is not possible with bulk GaP, a comprehensive study of its nonlinear optical properties for harmonic generation, especially when grown as thin films, is still missing. Here, second harmonic generation is studied from epitaxially grown GaP thin films, demonstrating that the absolute conversion efficiencies are comparable to a bulk wafer over the pump wavelength range from 1060 to 1370 nm. Furthermore, the results are compared to nonlinear simulations, and the second order nonlinear susceptibility is extracted, showing a similar dispersion and magnitude to that of the bulk material. Furthermore, the third order nonlinear susceptibility of amorphous GaP thin films is extracted from third harmonic generation to be more than one order of magnitude larger than that of the crystalline material, and generation of up to the fifth harmonic is reported. The results show the potential of crystalline and amorphous thin films for nonlinear optics with nanoantennas and metasurfaces, particularly in the visible to near infrared part of the spectrum.

Emergent Insulator-Metal Transition with Tunable Optical and Electrical Gap in Thin Films of a Molecular Conducting Composite.

Composites exhibit unique synergistic properties emerging when components with different properties are combined. The tuning of the energy bandgap in the electronic structure of the material allows designing tailor-made systems with desirable mechanical, electrical, optical, and/or thermal properties. Here, we study an emergent insulator–metal transition at room temperature in bilayered (BL) thin-films comprised of polycarbonate/molecular-metal composites. Temperature-dependent resistance measurements allow monitoring of the electrical bandgap, which is in agreement with the optical bandgap extracted by optical absorption spectroscopy. The semiconductor-like properties of BL films, made with bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF or ET) α-ET2I3 (nano)microcrystals as two-dimensional molecular conductor on one side and insulator polycarbonate as a second ingredient, are attributed to an emergent phenomenon equivalent to the transition from an insulator to a metal. This made it possible to obtain semiconducting BL films with tunable electrical/optical bandgaps ranging from 0 to 2.9 eV. A remarkable aspect is the similarity close to room temperature of the thermal and mechanical properties of both composite components, making these materials ideal candidates to fabricate flexible and soft sensors for stress, pressure, and temperature aiming at applications in wearable human health care and bioelectronics.

Facet dependent surface energy gap on magnetic topological insulators

Magnetic topological insulators $({\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}){({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{n}$ ($n=0,1,2,3$) are promising to realize exotic topological states such as the quantum anomalous Hall effect (QAHE) and axion insulator (AI), where the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ layer introduces versatility to engineer electronic and magnetic properties. However, whether surface states on the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ terminated facet are gapless or gapped is debated, and its consequences in thin-film properties are rarely discussed. In this work, we find that the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ terminated facets are gapless for $n\ensuremath{\ge}1$ compounds by calculations. Although the surface ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ (one layer or more) and underlying ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ layers hybridize and give rise to a gap, such a hybridization gap may overlap with bulk valence bands, leading to a gapless surface after all. Such a metallic surface poses a fundamental challenge to realize QAHE or AI, which requires an insulating gap in thin films with at least one ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ surface. In theory, the insulating phase can still be realized in a film if both surfaces are ${\mathrm{MnBi}}_{2}{\mathrm{Te}}_{4}$ layers. Otherwise, it requires that the film thickness be less than $10--20\phantom{\rule{4pt}{0ex}}\mathrm{nm}$ to push down bulk valence bands via the size effect. Our work paves the way to understand surface states and design bulk-insulating quantum devices in magnetic topological materials.

Preparation and characterization of dip coated cobalt oxide thin films

In this work, cobalt oxide thin films were prepared by sol–gel dip coating technique by varying the withdrawal speed of the substrate in the sol and their optical properties were analyzed, using the UV–vis spectrophotometer. The optical constants (n and k) for these films, in the 300–900 nm range and band gap of thin films are evaluated. FTIR confirmed formation of Co3O4 thin films. X-ray diffraction showed polycrystalline films consisting of the Co3O4 phase. Magnetic properties, such as saturation magnetization, remanence, coercivity and squareness ratio were evaluated from the hysteresis loop. Saturation magnetization was in the range of 16·26–114·30 emu/cm3 and coercivity range was from 143·73–6·03 Oe.

Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction

Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.

Fabrication of Single Crystal Gallium Phosphide Thin Films on Glass

Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material for photonic structures targeted at the visible wavelengths. However, these properties are only realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic applications. In this work, we report the fabrication of single crystal gallium phosphide thin films on transparent glass substrates via transfer bonding. GaP thin films on Si (001) and (112) grown by MOCVD are bonded to glass, and then the growth substrate is removed with a XeF2 vapor etch. The resulting GaP films have surface roughnesses below 1 nm RMS and exhibit room temperature band edge photoluminescence. Magnesium doping yielded p-type films with a carrier density of 1.6 × 1017 cm−3 that exhibited mobilities as high as 16 cm2V−1s−1. Due to their unique optical properties, these films hold much promise for use in advanced optical devices.

Dielectric Anisotropy of the GaP/Si(001) Interface from First-Principles Theory.

First-principles calculations of the dielectric anisotropy of the GaP/Si(001) interface are compared to the anisotropy extracted from reflectance measurements on GaP thin films on Si(001) [O. Supplie et al., Phys. Rev. B 86, 035308 (2012)PRBMDO1098-012110.1103/PhysRevB.86.035308]. Optical excitations from two states localized in several Si layers adjacent to the interface result in the observed anisotropy of the interface. The calculations show excellent agreement with experiment only for a gapped interface with a P layer in contact with Si and show that a combination of theory and experiment can reveal localized electronic states and the atomic structure at buried interfaces.

Pulsed laser deposition of Al x Ga1–x As and GaP thin films onto Si substrates for photoelectric converters

Pulsed laser deposition is used to produce AlGaAs and GaP thin films (with a thickness of less than 1 μm) on Si substrates. Methods for reducing the number of structural defects in the films are analyzed, and the effect of strains upon AlGaAs/Si and GaP/Si heterostructures is established by Raman spectroscopy. The application of Al0.3Ga0.7As and GaP films as wide-gap windows of silicon photoelectric converters is examined. The spectral characteristics of photocells based on Al0.3Ga0.7As/Si and GaP/Si heterostructures are studied. The heterostructures can be used as the first p–n junction of a Si-based multijunction photoelectric converter.

Narrow Band Gap Ternary Absorber Layer for Solar Cell for Small Business Enterprises in Equatorial Africa

According to recent studies, on the Daily nation newspaper, in Kenya of 6 th January 2017 featured the story, the number of people doing the same thing in surrounding villages was so high that many of them had to leave their phones behind at the charging centre (for up to three days) before returning to pick them. To fight darkness, up to 80 percent of Africans depend on personal generators, candles and kerosene (paraffin) lamps to provide basic lighting. Let’s not even talk about all the other things we desperately need electricity for – iron clothes, pump water, charge mobile phones etc. In this article, we shall focus on the types of solar cell materials that can be used to fabricate solar cell to be installed on roof tops to generate electricity used by individuals and households. Photovoltaic devices are used to convert solar radiation energy into electrical forms of energy for various uses. Current research is geared towards using thin films with wide band gaps to allow optimum penetration of radiation within the VIS - NIR region. Inorganic wide band gap, Cd x Zn 1- x S thin films and narrow band gap, PbS thin films were optimized through chemical solution technique in this study. Wide band gap thin films of Cd x Zn 1- x S were grown at about 82 0 C while those of narrow band gap thin films of PbS were grown at room temperature utilizing aqueous conditions. Their optical constants were investigated and found to be suitable for solar cells applications.

Localised nanoscale resistive switching in GaP thin films with low power consumption

Resistive switching behaviour is observed for GaP thin films. Conductive AFM and FORC-IV measurements show that the current is localised at grain boundaries. The switching mechanism is driven by Ga migration along the grain boundaries.

Narrow Band Gap Ternary Absorber Layer for Solar Cell for Small Business Enterprises in Equatorial Africa

According to recent studies, on the Daily nation newspaper, in Kenya of 6th January 2017 featured the story, the number of people doing the same thing in surrounding villages was so high that many of them had to leave their phones behind at the charging centre (for up to three days) before returning to pick them. To fight darkness, up to 80 percent of Africans depend on personal generators, candles and kerosene (paraffin) lamps to provide basic lighting. Let’s not even talk about all the other things we desperately need electricity for – iron clothes, pump water, charge mobile phones etc. In this article, we shall focus on the types of solar cell materials that can be used to fabricate solar cell to be installed on roof tops to generate electricity used by individuals and households. Photovoltaic devices are used to convert solar radiation energy into electrical forms of energy for various uses. Current research is geared towards using thin films with wide band gaps to allow optimum penetration of radiation within the VIS - NIR region. Inorganic wide band gap, CdxZn1-xS thin films and narrow band gap, PbS thin films were optimized through chemical solution technique in this study. Wide band gap thin films of CdxZn1-xS were grown at about 82°C while those of narrow band gap thin films of PbS were grown at room temperature utilizing aqueous conditions. Their optical constants were investigated and found to be suitable for solar cells applications.

Topological phase transition and quantum spin Hall state in TlBiS2

We have investigated the bulk and surface electronic structures and band topology of TlBiS2 as a function of strain and electric field using ab-initio calculations. In its pristine form, TlBiS2 is a normal insulator, which does not support any non-trivial surface states. We show however that a compressive strain along the (111) direction induces a single band inversion with Z2 = (1;000), resulting in a Dirac cone surface state with a large in-plane spin polarization. Our analysis shows that a critical point lies between the normal and topological phases where the dispersion of the 3D bulk Dirac cone at the Γ-point becomes nearly linear. The band gap in thin films of TlBiS2 can be tuned through an out-of-the-plane electric field to realize a topological phase transition from a trivial insulator to a quantum spin Hall state. An effective k·p model Hamiltonian is presented to simulate our first-principles results on TlBiS2.

The semi-empirical tight-binding model for carbon allotropes “between diamond and graphite”

The new carbon allotropes “between diamond and graphite” have come under intensive examination during the last decade due to their numerous technical applications. The modification of energy gap in thin films of these allotropes was studied experimentally using optical methods. The proposed simple model of carbon clusters with variable lengths of chemical bonds allows us to imitate the transfer from diamond and diamond-like to graphite-like structures, as well as the corresponding modification of hybridization sp3/sp2 for diamond-like and spz for graphite-like phases. This enables us to estimate various allotropes parameters, like the gap Eg, energies of valence Ev, and conduction Ec band edges, and the value of electronic affinity, i.e., optical work function X, which are all of practical importance. The obtained estimations correspond to the experimental data.

Wide Band Gap and p-Type Conductive BaCuSeF Thin Films Fabricated by Pulsed Laser Deposition

Wide Band Gap and p-Type Conductive BaCuSeF Thin Films Fabricated by Pulsed Laser Deposition

Wide Band Gap and p-Type Conductive BaCuSeF Thin Films Fabricated by Pulsed Laser Deposition

BaCuSeF films were fabricated on glass substrates for solar cell application. The crystallographic orientation of the films depended on the thickness of the films. The BaCuSeF films with thicknesses of 0.35, 0.6, and 1.0 µm had 102 preferential orientation, and a thicker film of 1.3 µm was polycrystalline. All of the films showed average transmittance of >50% in the visible light region, and the determined band gap energy was 2.8 eV. All of the films showed p-type conductivity of more than 1 S cm-1. The 0.35-µm BaCuSeF film showed the highest p-type conductivity of 19.2 S cm-1. The BaCuSeF is applicable to electrodes for chalcopyrite-based thin film tandem solar cells.

Determination of the Optical GAP in Thin Films of Amorphous Dilithium Phthalocyanine Using the Tauc and Cody Models

Semiconducting thin films were grown on quartz substrates and crystalline silicon wafers, using dilithium phthalocyanine and the organic ligands 2,6-dihydroxyanthraquinone and 2,6-diaminoanthraquinone as the starting compounds. The films, thus obtained, were characterized by Fourier Transform infrared (FTIR), fast atomic bombardment (FAB+) mass and ultraviolet-visible (UV-Vis) spectroscopies. The surface morphology of these films was analyzed by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM). It was found that the temperature-dependent electric current in all cases showed a semiconductor behavior with conductivities on the order of 10−6·S cm−1, whereas the highest value corresponded to the thin film based upon the bidentate amine. The Tauc and Cody optical band gap values of thin films were calculated from the absorption coefficients and were found to be around 1.5 eV, with another strong band between 2.3 and 2.43 eV, arising from non-direct transitions. The curvature in the Tauc plot influencing the determination of the optical gap, the Tauc optical gap corresponding to the thicker film is smaller. The dependence of the Cody optical gap on the film thickness was negligible.

Growth and characterization of GaP layers on silicon substrates by metal‐organic vapour phase epitaxy

AbstractThis paper presents X‐ray diffraction and atomic force microscopy investigation of GaP thin films grown on silicon (100) substrate by metalorganic vapour phase epitaxy. Crystalline GaP has been grown by two‐step growth method, in which first a low temperature nucleation layer is grown at 475 °C, and then, a 100‐nm‐thick layer is grown at 700 °C. The GaP layer is comprised of trenches and smooth terraces which are attributed to anti‐phase domains. Without the two‐step growth method, GaP forms a cross‐hatch pattern of wires. Formation of GaP wires is attributed to anisotropic diffusion, and thus, the GaP wires grow along [011] crystal directions (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Properties of NiO Thin Films: A Potential Material for Optoelectronic Devices

The influence of substrate temperature on the UV-Visible-near-IR optical properties, namely the band gap, the Urbach energy and the refractive index of NiO thin films deposited by RF sputtering has been investigated. The optical band gap of thin films showed the blue-shift in the transmission spectra with increase in the substrate temperature which is related to variation in carrier concentration of the deposited films. Urbach energy (EU) values indicate that the films deposited at 400 oC substrate temperature show least structural disorder. The refractive index of the films is found to decrease continuously with increase in the substrate temperature at all photon energies in the visible and near-IR region, and is attributed to the decreasing packing density of the films. Introduction