Structural and optical properties of vacuum evaporated CdxZn1−xS thin films

II–VI semiconductors are of great importance due to their applications in various electro-optic devices. Sulphides of zinc and cadmium have been utilized effectively in various opto-electronic devices. We have prepared vacuumed CdZnS films by the vacuum evaporation method. Wide band gap binary films have wide application in solar cells. The structural and optical properties of these films have been studied. The band gap of these films is studied by absorption spectra in the wavelength range 400–650 nm. The films have a direct band gap, which varies from 3.50 eV for zinc sulphide to 2.44 eV for cadmium sulphide. The X-ray diffraction pattern of these films for structural analysis is also reported.     

Oxid-Halbleiter mit ultrabreiter Bandlücke

Oxide semiconductors with ultra-wide band gap Pulsed laser deposition (PLD) is a comparatively simple physical, vacuum-based method for producing thin films. Here, different aspects of the deposition of oxide alblides are investigated, especially for the (Al,Ga,In)₂O₃ system with a very large band gap. The method of combinatorial PLD for a lateral concentration gradient (CCS-PLD, continuous composition spread PLD) allows a simple representation of alloy series. In combination with subsequent spatially resolved methods for structural, electrical and optical characterization, an efficient and systematic investigation of the physical properties of the alloy semiconductors results.     

Reflection and absorption spectra of β-FeSi2 under pressure

High-pressure optical reflection and absorption study of β-FeSi₂ were carried out up to 5.0 and 4.8 GPa, respectively at room temperature. By the absorption experiment, the pressure coefficient of the direct band gap was determined to be 15.9 meV/GPa. On the other hand, the pressure dependence of dielectric functions were obtained by using Kramers-Kronig relations for the reflection spectra and the pressure coefficients of three different higher energy gap were tentatively evaluated to be 8.73, 8.63 and 16.7 meV/GPa, respectively. These pressure coefficients are quite smaller than those of II-VI, III-V and group IV common semiconductors. We propose that the small pressure coefficient is caused by not only the larger bulk modulus compared with common semiconductors but also the shift of the valence band maximum of β-FeSi₂ to higher energy with increasing pressure.     

Band Structures of Narrow Gap Semiconductors in Terms of Coulombic Polaron Effect

We consider the formation of a polaron in an interacting Fermi gas due to Coulomb polarization of electron gas by a single electron. We show that the polarization effect due to the Coulomb field in an interacting Fermi gas leads to the Coulomb polaron caused by the exchange-correlation hole and the electron spectrum shows an insulating behavior with the dispersion law of quasiparticles similar to the Kane kp-model of a band structure for narrow gap semiconductors. We apply the polaronic model to the case of inhomogeneous periodic and interacting electron Fermi gas of crystals. We show that the formation of the Coulomb polaron leads to the transition to the semiconductor with the energy bands similar to the well-known semiempirical Kane kp-model of the band structure of narrow gap semiconductors. We also discuss the impact of the results on band structure calculations.     

Derivation of ineffective thickness method for investigation of the exact behavior of the optical transitions in nanostructured thin films

Optical band gap determination in nanostructured semiconductor thin films is to some extent difficult. Procedure of the extrapolating the linear part of the graphs in optical band gap determination in Tauc’s model for nanostructures has shortage because the edges of the tail states complicate the definition of the accurate optical band gap (due to the surface atoms). A new revised method (named as DITM: Derivation of Ineffective Thickness Method) is proposed for the exact determination of the optical band gap in addition to the treatment of optical transitions in nanostructure semiconductors. In Tauc model and ineffective thickness method (ITM), one has to determine the kind of optical transition before the determination of the optical band gap, but in DITM, not any supposition of the treatment of optical transition. DITM method was employed on CdSe thin films semiconductor alloy in order to confirm the validity of this new method.     

Overcoming doping bottleneck by using surfactant and strain

Overcoming the doping bottleneck in semiconductors, especially in wide band gap semiconductors, has been a challenge in semiconductor physics for many years. In this paper, we review some recent progresses in enhancing doping by surfactant and strain. We show that surfactant and strain are two effective approaches to enhance dopant solubility in epitaxial growth. The surfactant can introduce an energy level deep inside the band gap, making the host compound less stable, thus lower the formation energy of the intentional dopant. The strain enhanced doping is based on the observation that dopant induces volume change in the host. If the external strain is in the same direction as the dopant induced volume change, the formation energy of the dopant is reduced. This effect can be used to tune doping sites, thus doping type, in a host. A hybrid method to both include strain and surfactant is proposed, which can be a promising general method to further enhance doping.     

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.     

Band‐Gap Engineering at a Semiconductor–Crystalline Oxide Interface

The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is to control the manner in which their bands align at interfaces. Here, principles of band‐gap engineering traditionally used at heterojunctions between conventional semiconductors are applied to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZr_xTi_1−xO₃ and Ge, in which the band‐gap of the former is enhanced with Zr content x. Structural and electrical characterization of SrZr_xTi_1−xO₃‐Ge heterojunctions for x = 0.2 to 0.75 are presented and it is demonstrated that the band offset can be tuned from type‐II to type‐I, with the latter being verified using photoemission measurements. The type‐I band offset provides a platform to integrate the dielectric, ferroelectric, and ferromagnetic functionalities of oxides with semiconducting devices.     

Au-N共掺杂ZnO电子结构和光学性质的第一性原理研究

采用基于密度泛函理论的第一性原理平面波超软赝势法,结合广义梯度近似(GGA)研究了Au、N共掺杂纤锌矿ZnO的能带结构、电子态密度,以及复介电函数、光学吸收等光学性质,并与本征ZnO和N掺杂ZnO情况进行了对比.计算结果表明Au、N共掺杂ZnO仍是直接带隙半导体材料,掺杂后ZnO的带隙收缩,价带展宽.在价带顶电子密度分布较N掺杂ZnO情况的局域性减弱,更有利于获得p型ZnO.与未掺杂ZnO相比,介电函数和吸收系数在可见和紫外区域得到显著增强.     

An enhancement of spin polarization by multiphoton pumping in semiconductors

A pump–probe spectroscopic study has been carried out in zinc-blende bulk semiconductors. In the semiconductor samples, a spin-polarized carrier population is produced by the absorption of a monochromatic circularly polarized light beam with two-photon energy above the direct band gap in bulk semiconductors. The production of a carrier population with a net spin is a consequence of the optical selection rules for the heavy-hole and light-hole valence-to-conduction band transitions. This production is probed by the spin-dependent transmission of the samples in the time domain. The spin polarization of the conduction-band-electrons in dependences of delay of the probe beam as well as of pumping photon energy is estimated. The spin polarization is found to depolarize rapidly for pumping energy larger than the energy gap of the split-off band to the conduction band. From the polarization decays, the spin relaxation times are also estimated. Compared to one-photon pumping, the results, however, show that an enhancement of the spin-polarization is achieved by multiphoton excitation of the samples. The experimental results are compared with those obtained in calculations using second order perturbation theory of the spin transport model. A good agreement between experiment and theory is obtained. The observed results are discussed in details.     

Semiconducting glass design for device applications

A method is presented by which stable amorphous semiconductors can be chosen for switching device applications. This method is based on the empirical relationship observed between the glass transition temperature, the band gap and the mean coordination number of covalent amorphous semiconductors. In particular, it is shown that tetrahedrally coordinated glasses with band gaps in the range 0.6–1.2 eV are excellent candidates for high reliability materials. One such material, a-CdAs₂, was studied in some detail and was found to be very stable and very easy to fabricate in thin film form. Both negative resistance and threshold switching devices were successfully fabricated with this material, and preliminary results from accelerated life testing are promising.     

Wurtzite-derived ternary I–III–O2 semiconductors

Abstract

Ternary zincblende-derived I–III–VI₂ chalcogenide and II–IV–V₂ pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I–III–O₂ oxide semiconductors with a wurtzite-derived β-NaFeO₂ structure are limited. Wurtzite-derived β-LiGaO₂ and β-AgGaO₂ form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. β-CuGaO₂, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I–III–O₂ semiconductors is still limited. In this paper we review previous studies on β-LiGaO₂, β-AgGaO₂ and β-CuGaO₂ to determine guiding principles for the development of wurtzite-derived I–III–O₂ semiconductors.     

Wide-gap layered oxychalcogenide semiconductors: Materials, electronic structures and optoelectronic properties

Applying the concept of materials design for transparent conductive oxides to layered oxychalcogenides, several p-type and n-type layered oxychalcogenides were proposed as wide-gap semiconductors and their basic optical and electrical properties were examined. The layered oxychalcogenides are composed of ionic oxide layers and covalent chalcogenide layers, which bring wide-gap and conductive properties to these materials, respectively. The electronic structures of the materials were examined by normal/inverse photoemission spectroscopy and energy band calculations. The results of the examinations suggested that these materials possess unique features more than simple wide-gap semiconductors. Namely, the layered oxychalcogenides are considered to be extremely thin quantum wells composed of the oxide and chalcogenide layers or 2D chalcogenide crystals/molecules embedded in an oxide matrix. Observation of step-like absorption edges, large band gap energy and large exciton binding energy demonstrated these features originating from 2D density of states and quantum size effects in these layered materials.     

Light emission in silicon nanostructures

The many and diverse approaches to materials science problems have greatly enhanced our ability in recent times to engineer the physical properties of semiconductors. Silicon, of all semiconductors, underpins nearly all microelectronics today and will continue to do so for some time to come. However, in optoelectronics and, more recently, in photonics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Here we review a number of diverse approaches to engineering efficient light emission in silicon nanostructures. These different approaches are placed in context and their prospects are assessed for applications in silicon-based photonics.     

Electronic and optical properties of <Emphasis Type="Italic">β</Emphasis>-graphyne nanotubes and their BN analogues

The electronic and optical properties of zigzag and armchair β-graphyne nanotubes (β-GNTs) and their BN analogues (labeled as β-BNyne NTs) with different tube diameters are systematically investigated by the first-principles calculations. The calculated results reveal that all zigzag and armchair β-graphyne nanotubes are direct band gap semiconductors. As for zigzag β-BNyne NTs, they are wide direct band gap semiconductors. Nevertheless, armchair β-BNyne NTs are indirect band gap semiconductor. The optical spectra of β-GNTs and BNyne NTs show remarkable anisotropic behavior. Interestingly, the static dielectric constant of β-GNTs is quite high in comparison with carbon NTs, indicating higher conductivity and carrier mobility. In addition, quite broad frequency absorption spectra, extended from the infrared to the ultraviolet (UV) region, are observed for all β-GNTs. However, the photoresponse of β-BNyne NTs is mainly located in the UV region. The β-GNTs exhibit high reflectivity in the infrared region of 0.0 to 1.5 eV for both parallel and perpendicular polarization, but all β-BNyne NTs possess very low reflectivity and are highly sensitive to the UV light. Particularly, all β-GNTs demonstrate no obvious size-dependent optical properties, however, for β-BNyne NTs, all the photoresponse intensity (static dielectric constant, reflectivity and absorption coefficient) decreases monotonically with increasing tube size.     

Atom Probe Tomography and field evaporation of insulators and semiconductors: Theoretical issues

After reviewing the physics and chemistry in high electrostatic fields and summarizing the theoretical results for Atom Probe Tomography of metallic tips, we turn to the new challenges associated with insulators and semiconductors with regard to local fields inside and on the surface of such materials. The recent (theoretical) discovery that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion.     

Intrinsic metallic and semiconducting cubic boron nitride nanofilms

We show by density functional theory calculations with both hybrid and semilocal functionals that cubic boron nitride (111) nanofilms are intrinsically metallic and even turn into semiconductors once the thickness is less than 0.69 nm, which is in sharp contrast to the known insulating nature of boron nitride materials. The exceptional metallic or semiconducting band gap is due to a combined effect of thickness-dependent inbuilt electric polarization and labile near-gap states unique in the polar nanofilms. The band gap and dipole moment of the nanofilms can be further significantly tuned by applying an in-plane strain. These distinguished features of the boron nitride nanofilms are robust to surface passivation and can be enhanced by hybridizing with diamond films, thereby opening an exciting prospect of using the versatile cubic nanofilms in future electronic and piezoelectric devices.     

Composition and thickness dependant optical study of a-Pb–Se–Ge–Sn glassy thin films

Thickness and compositional dependence of optical properties of Pb₉Se₇₁Ge_20?xSn_x (8 ≤ x ≤ 12) glass has been studied. Various optical constants such as refractive index, extinction coefficient and optical band gap have been determined by analyzing optical transmittance data in the wavelength range of 200–3,500 nm. Density of localized states and disorder plays a crucial role in deciding the optical properties of amorphous semiconductors. Refractive index and extinction coefficient increase as Sn content increase in material. With the rise in thickness, there may be increase in order of short range order of the film and continuous random network simultaneously, result in reduced band gap. The isoelectronic substitutions of Ge by Sn in the glassy system also contribute to reduction in optical band gap of the material.     

Junction formation in chalcopyrite solar cells by sputtered wide gap compound semiconductors

In an effort to eliminate the CdS buffer layer and its costly preparation process we are considering sputtered buffer layers. In particular, we report in this contribution on the reactive sputtering of wide gap Zn(O,S) compound semiconductors and their application in solar cells with different types of chalcopyrite absorbers. While we were able to freely adjust the composition through the oxygen partial pressure, the structural and optical properties are superior when the composition is close to the ternary endpoints. Open circuit voltage and short circuit current density as a function of Sulphur content in the buffer show opposite trends. Working cells were achieved with low band gap as well as wide band gap absorbers, however, their performance is so far inferior to that of the standard stacks.