Browsing by Subject "Semiconductor doping"

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Open Access
Co doping induced structural and optical properties of sol-gel prepared ZnO thin films
(Elsevier BV, 2014-11) Gungor, E.; Gungor, T.; Caliskan, D.; Ceylan, A.; Özbay, Ekmel
The preparation conditions for Co doping process into the ZnO structure were studied by the ultrasonic spray pyrolysis technique. Structural and optical properties of the Co:ZnO thin films as a function of Co concentrations were examined. It was observed that hexagonal wurtzite structure of ZnO is dominant up to the critical value, and after the value, the cubic structural phase of the cobalt oxide appears in the X-ray diffraction patterns. Every band-edge of Co:ZnO films shifts to the lower energies and all are confirmed with the PL measurements. Co substitution in ZnO lattice has been proved by the optical transmittance measurement which is observed as the loss of transmission appearing in specific region due to Co2+ characteristic transitions. © 2014 Elsevier B.V. All rights reserved.
Open Access
Electrical properties from photoinduced charging on Cd-doped (100) surfaces of CuInSe2 epitaxial thin films
(AVS Science and Technology Society, 2016) Johnson, N.; Aydogan, P.; Süzer, Şefik; Rockett, A.
The photoresponse of Cd-doped CuInSe2 (CIS) epitaxial thin films on GaAs(100) was studied using x-ray photoelectron spectroscopy under illumination from a 532 nm laser between sample temperatures of 28-260 °C. The initial, air-exposed surface shows little to no photoresponse in the photoelectron binding energies, the Auger electron kinetic energies or peak shapes. Heating between 50 and 130 °C in the analysis chamber results in enhanced n-type doping at the surface and an increased light-induced binding energy shift, the magnitude of which persists when the samples are cooled to room temperature from 130 °C but which disappears when cooling from 260 °C. Extra negative charge trapped on the Cu and Se atoms indicates deep trap states that dissociate after cooling from 260 °C. Analysis of the Cd modified Auger parameter under illumination gives experimental verification of electron charging on Cd atoms thought to be shallow donors in CIS. The electron charging under illumination disappears at 130 °C but occurs again when the sample is cooled to room temperature.
Open Access
Electronic structure of conducting organic polymers: insights from time-dependent density functional theory
(John Wiley & Sons Ltd., 2014) Salzner, U.
Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e.g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e.g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs.
Open Access
Exchange-correlation effects in the impurity-limited mobility of GaAs quantum wires
(Sci Tech Res Counc Turkey, Ankara, Turkey, 1999) Tanatar, Bilal
We study the many-body effects described by the local-field corrections on the mobility of quasi-one dimensional electron systems. The low temperature mobility due to remote-impurity doping and interface-roughness scattering is calculated within the relaxation time approximation. We find that correlation effects significantly reduce the mobility at low density.
Open Access
Experimental and theoretical investigation of phosphorus in-situ doping of germanium epitaxial layers
(Elsevier, 2013) Yu, H. -Y.; Battal, E.; Okyay, Ali Kemal; Shim, J.; Park J. -H.; Baek, J. W.; Saraswat, K. C.
We investigate phosphorus in-situ doping characteristics in germanium (Ge) during epitaxial growth by spreading resistance profiling analysis. In addition, we present an accurate model for the kinetics of the diffusion in the in-situ process, modeling combined growth and diffusion events. The activation energy and pre-exponential factor for phosphorus (P) diffusion are determined to be 1.91 eV and 3.75 × 10-5 cm2/s. These results show that P in-situ doping diffusivity is low enough to form shallow junctions for high performance Ge devices.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).
Open Access
Gunn oscillations in GaN channels
(IOP, 2004) Sevik, Cem; Bulutay, Ceyhun
Gallium nitride with its high negative differential mobility threshold is an appealing material for high power millimetre-wave oscillators as a Gunn diode. By means of extensive ensemble Monte Carlo simulations, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz is studied in detail. Their operations are checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations cease. The degeneracy effects due to the Pauli exclusion principle are also considered, but their effects are seen to be negligible within the realistic bounds of the Gunn diode operation.
Open Access
Hot electron effects in unipolar n-type submicron structures based on GaN, AlN and their ternary alloys
(The Institution of Engineering and Technology, 2003) Sevik, C.; Bulutay, C.
The authors present an analysis of impact ionisation (II) and related hot electron effects in submicron sized GaN, AlN and their ternary alloys, all of which can support very high field regimes, reaching a few megavolts per centimetre (MV/cm). The proposed high field transport methodology is based on the ensemble Monte Carlo technique, with all major scattering mechanisms incorporated. As a test-bed for understanding II and hot electron effects, an n+-n-n+ channel device is employed having a 0.1 μm thick n-region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n+-region. For AlxGa1-xN-based systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Finally, at very high fields II is shown to introduce a substantial energy loss mechanism for the energetic carriers that have just traversed the unintentionally doped n-region.
Open Access
Mg-doped AlGaN grown on an AlN/sapphire template by metalorganic chemical vapour deposition
(2006) Yu, H.; Strupinski, W.; Butun, S.; Özbay, Ekmel
The growth of high-performance Mg-doped p-type Al xGa 1-xN (x = 0.35) layers using low-pressure metal-organic chemical vapour deposition on an AlN/sapphire template is reported. The influence of growth conditions on the p-type conductivity of the Al xGa 1-xN (x = 0.35) alloy was investigated. It was found that the p-type resistivity of the AlGaN alloy demonstrates a marked dependence on the Mg concentration, V/III ratio and group III element flow rate. A minimum p-type resistivity of 3.5 Ω cm for Al xGa 1-xN (x = 0.35) epilayers was achieved. A Ni/Au (10 nm/100 nm) ohmic contact was also fabricated and a specific contact resistivity of 8.1 × 10 -2 Ω cm 2 was measured. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.
Open Access
Nano-structured organically modified silica thin films for functional surfaces
(2011) Bayındır, Mehmet; Yıldırım, Adem; Budunoglu, Hülya; Yaman, Mecit; Deniz, Hakan; Güler, Mustafa O.
We report a template-free sol-gel method for preparation of nanoporous ormosil thin films at ambient conditions. The thin films are coated to the surfaces by using colloidal suspensions of ormosil gels. Gels are synthesized by using a trifunctional organosilane monomer, methyltrimethoxysilane (MTMS), with a two-step acid base reaction. We prepared several ormosil thin films on glass, metal, plastic and paper surfaces with different functionalities like superhydrophobic, antireflective, antifogging and ice retarding properties, from gels prepared in different conditions. Also films on flexible substrates exhibits durable surface properties after several bending cycles. In addition, we also demonstrate that these thin films can be used for fluorescent sensing of explosives by doping them with fluorescent dyes.
Open Access
Near-unity emitting copper-doped colloidal semiconductor quantum wells for luminescent solar concentrators
(Wiley-VCH Verlag, 2017) Sharma, M.; Gungor K.; Yeltik A.; Olutas M.; Guzelturk, B.; Kelestemur Y.; Erdem, T.; Delikanli S.; McBride, J. R.; Demir, Hilmi Volkan
Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.
Open Access
Raman studies of doped polycrystalline silicon from laser-annealed, doped a-Si:H
(Pergamon Press, 1994) Compaan, A.; Savage, M. E.; Aydınlı, Atilla; Azfar, T.
We have used Raman scattering to follow the progress of multiple-pulse (sub-melt-threshold) laser annealing in doped hydrogenated amorphous silicon films (a-Si:H) on glass. In phosphorous-doped a-Si:H the Raman signal shows that recrystallization begins with the first laser pulse but the multiple pulses are needed to generate the highest hole concentrations of ∼6×1020 cm-3. In boron-doped a-Si:H the electron concentration reaches ∼1×1021 cm-3 after laser anneal which produces a dip rather than a peak near the phonon line as a consequence of a negative Fano-interference parameter, q. The results show that Raman scattering can be used to obtain carrier concentrations in poly-silicon provided that wavelength-dependent Fano interference effects are properly included. © 1994.
Open Access
Surfactant-mediated growth of semiconductor materials
(Institute of Physics Publishing, 2002) Fong, C. Y.; Watson, M. D.; Yang, L. H.; Çıracı, Salim
During epitaxial growth of semiconducting materials using either molecular beam epitaxy or organometallic vapour deposition, the addition of a surfactant can enhance two-dimensional layer-by-layer growth. This modified growth process is now called the surfactant-mediated growth (SMG) method. It has had an important impact on the development of technologically important materials in device applications, such as heterostructures used for laser applications. Recent developments that use surfactants to improve doping profiles in semiconducting systems and antisurfactants (ASMG) to grow quantum dots further ensure that SMG/ASMG will play a major role in the future development of optoelectronic materials and nanoparticles. In this paper, we review important earlier experimental work involving the SMG method as well as some recent developments. Theoretical work involving first-principles methods and kinetic Monte Carlo simulations are discussed but confined only to the surfactant effect.
Open Access
Tuning electronic properties of monolayer hexagonal boron phosphide with group III-IV-V dopants
(American Chemical Society, 2017-02) Onat, B.; Hallioglu, L.; Ipek, S.; Durgun, Engin
An extensive study on doping of two-dimensional (2D) hexagonal boron phosphide (h-BP) which is a direct band gap semiconductor was performed by using ab initio methods based on spin-polarized density functional theory. The interaction of group III-IV-V elements with h-BP is explored, considering both adsorption and substitution cases, and the resulting structural and electronic properties are examined. The variation of adsorption (substitution) energies and band gap values are systematically analyzed and trends are identified. Upon adsorption, the most of the elements bound on top of P atom forming dumbbell geometry which generates characteristic spin-polarized impurity states. The substitution of B or P by group III-IV-V elements can produce extra electrons/holes which lead to n-type and p-type doping for adequate cases. Additionally, doping can further generate impurity resonant states. Functionalization of h-BP with adatoms can tune the electronic structure and would be useful for nanoelectronic applications in low-dimensions.