Correlation between transient decays in wide gap semiconductors: Photoluminescence vs. photocurrent

Recently, we introduced the Thermalization–Recombination (TR) Model to explain the ubiquitous broad subgap photoluminescence bands in wide bandgap semiconductors [M. Niehus, R. Schwarz, Phys. Status Solidi C 3 (2006) 1637]. The model describes the competition between the thermalization and recombination dynamics of excess carriers trapped in localized states distributed exponentially in energy.In this contribution, we confronted the theoretical and qualitative predictions of the TR Model with experimental results of transient photoluminescence (TPL) of pulsed laser deposited (PLD) polycrystalline gallium nitride (GaN) and polycrystalline zinc oxide (ZnO). The TPL results are compared with transient photocurrent (TPC) measurements in order to highlight the relation between TPL and TPC, as well as similarities in the relaxation dynamics of GaN and ZnO.The general features of the transient decays for both materials can be explained within the framework of the TR model, which is shown to offer significant inside into the relaxation dynamics of wide gap semiconductor materials.     

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional(2D) layered materials have potential applications in nanoelectronics, nanophotonics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes(LEDs), and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.     

Strain engineering of topological properties in lead‐salt semiconductors

Rock‐salt chalcogenide SnTe represents the simplest realization of a topological insulator where a crystal symmetry allows for the appearance of surface metallic states. Here, we theoretically predict that strain, as realized in thin films grown on (001) substrates, may induce a transition to a topological crystalline insulating phase in related lead‐salt chalcogenides. Furthermore, relevant topological properties of the surface states, such as the location of the Dirac cones on the surface Brillouin zone or the decay length of edge states, appear to be tunable with strain, with potential implications for technological devices benefiting from those additional degrees of freedom. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase shift analysis of modulated photocurrent: Determination of the energy scale

For the determination of the density of states in the mobility gap of amorphous semiconductors using the phase shift analysis of modulated photocurrent, this paper suggests that making use of the magnitude of the induced photocurrent helps to remove arbitrariness in the energy scale. The working equations for the density of states and the corresponding energy position are expressed in terms of the intensity of the photocurrent. A simulation is made for a specific distribution, to investigate the validity of the procedure. The results show that the profile of the energetic distribution of localized states and the exact energy position of each state are consistent with the original distribution considered.     

Enhancement of ferromagnetism in polycrystalline Si0.965Mn0.035:B films by boron plasma treatment

This paper reports that the polycrystalline Si0.965Mn0.035:B films have been prepared by cosputtering deposition followed by rapid thermal annealing for crystallization. The polycrystalline thin films consist of two ferromagnetic phases. The low temperature ferromagnetic phase with Curie temperature (Tc) of about 50 K is due to the Mn4Si7 phase in the films, while the high temperature one (Tc～250 K) is resulted from the incorporation of Mn into silicon. The films are treated by boron plasma excited with the approach of microwave plasma enhanced chemical vapor deposition for 40 minutes. After plasma treatment, it is observed that no extra magnetic phases or magnetic complexes exist in the films, while both the high temperature saturation magnetization and the hole concentration in the films increase. The obvious correlation between the magnetic properties and the electrical properties of the polycrystalline Si0.965Mn0.035:B films suggests that the hole carriers play an important role in Si:Mn diluted magnetic semiconductors.     

Structure of van der Waals bound hybrids of organic semiconductors and transition metal dichalcogenides: the case of acene films on MoS2

Transition metal dichalcogenides (TMDC) are important representatives in the emerging field of two‐dimensional materials. At present their combination with molecular films is discussed as it enables the realization of van der Waals bound organic/inorganic hybrids which are of interest in future device architectures. Here, we discuss the potential use of molybdenum disulfide (MoS₂) as supporting substrate for the growth of well‐defined, crystalline organic adlayers. By this means, hybrid systems between the TMDC surface and organic compounds can be prepared, allowing for the profound investigation of mutual optical and electronic coupling mechanisms. As model system, we choose pentacene and perfluoropentacene as prototypical organic semiconductors and analyze their film formation on MoS₂(001) surfaces. In both cases, we observe smooth, crystalline film growth in lying molecular configuration, hence enabling the preparation of well‐defined hybrid systems. By contrast, on defective MoS₂ surfaces both materials adopt an upright molecular orientation and exhibit distinctly different film morphologies. This emphasizes the importance of highly ordered TMDC surfaces with low defect density for the fabrication of well‐defined hybrid systems.     

Influence of the Coulomb gap on the impurity absorption in AIIIBV semiconductors

Using the interpolation formula, which adequately describes the impurity band structure in a broad energy range regardless of the degree of compensation, an explicit expression is obtained for the light absorption coefficient related to the transitions from the impurity band to the conduction band. It is shown that at low temperatures the absorption coefficient has a clearly expressed absorption threshold. The explicit frequency dependence of the absorption coefficient is derived for a wide frequency range of the incident radiation. The absorption coefficient dependence on the degree of doping of a semiconductor is also studied.     

Surface order dependent magnetic thin film growth: Fe on GaN(0 0 0 1)

X-ray photoemission spectroscopy and X-ray magnetic circular dichroism have been used to study the growth process, chemical composition and magnetic character of iron deposited on ordered and disordered GaN(0 0 0 1) surfaces. On the (1 × 1) ordered surface the Fe grows uniformly but with disruption to the substrate surface, subsequently nitrogen desorbs from the surface, some of which diffuses into the Fe overlayer. The film is magnetically fractured, with high magnetic coercivity and broad switching fields. Conversely, the gallium rich disordered surface protects the underlying substrate from any disruption and initially induces non-uniform growth, the Fe clusters coalesce at ∼12 Å, to produce a uniform film with desirable magnetic characteristics. Films beyond this point (>12 Å) indicate sharp hysteresis loops with low coercivities. For the resultant film (36 Å) we measure a magnetic moment of 2.02 μ_B, in agreement with bulk bcc iron (2.068 μ_B.)     

A simple analysis of the subband energies in quantum-confined non-parabolic semiconductors in the presence of a parallel magnetic field

Summary We study the subband energies in quantum wells and quantum wires in the presence of a parallel magnetic field in non-parabolic semiconductors, on the basis of a generalized dispersion relation considering all types of anisotropies of the energy-band parameters within the framework ofk·p formalism, by formulating the respective electron energy spectra. It is found, by takingn-Cd₃As₂ as an example, that the subband energies are greater for quantum wires and smaller for quantum wells, respectively. The magnetic field diminishes the above values and the corresponding well-known results for quantum-confined parabolic semiconductors have also been obtained from our generalized expressions under certain limiting conditions.     

The effects of post-thermal annealing on the optical parameters of indium-doped ZnO thin films

Indium-doped ZnO thin films are deposited on quartz glass slides by RF magnetron sputtering at ambient temper- ature. The as-deposited films are annealed at different temperatures from 400 C to 800 C in air for 1 h. Transmittance spectra are used to determine the optical parameters and the thicknesses of the films before and after annealing using a nonlinear programming method, and the effects of the annealing temperatures on the optical parameters and the thickness are investigated. The optical band gap is determined from the absorption coefficient. The calculated results show that the film thickness and optical parameters both increase first and then decrease with increasing annealing temperature from 400 C to 800 C. The band gap of the as-deposited ZnO:In thin film is 3.28 eV, and it decreases to 3.17 eV after annealing at 400 C. Then the band gap increases from 3.17 eV to 3.23 eV with increasing annealing temperature from 400 C to 800 C.     

Ion tracks in ultrathin polymer films: The role of the substrate

Surface damage produced by single MeV-GeV heavy ions impacting ultrathin polymer films has been shown to be weaker than those observed under bulk (thick film) conditions. The decrease in damage efficiency has been attributed to the suppression of long-range effects arising from excited atoms lying deeply in the solid. This raises the possibility that the substrate of the films itself is relevant to the radiation effects seen at the top surface. Here, the role of the substrate on cratering induced by individual 1.1 GeV Au ions in ultrathin poly(methyl methacrylate) (PMMA) layers is investigated. Materials of different thermal and electrical properties (Si, SiO₂, and Au) are used as substrates to deposit PMMA thin films of various thicknesses from ∼1 to ∼300 nm. We show that in films thinner than ∼40 nm craters are modulated by the underlying substrate to a degree that depends on the transport properties of the medium. Crater size in ultrathin films deposited on the insulating SiO₂ is larger than in similar films deposited on the conducting Au layer. This is consistent with an inefficient coupling of the electronic excitation energy to the atomic cores in metals. On the other hand, the damage on films deposited on SiO₂ is not very different from the Si substrate with a native oxide layer, suggesting, in addition, poor energy transmission across the film/substrate interface. The experimental observations are also compared to calculations from an analytical model based on energy addition and transport from the excited ion track, which describe only partially the results.     

Study on solid structure of pentacene thin films using Raman imaging

We present a 532‐nm excited Raman imaging study of pentacene thin films (thickness, 2, 5, 10, 20, 50, 100, and 150 nm) prepared on an SiO₂ surface. The structure of the pentacene films has been investigated by images and histograms of the ratio (R) of intensity of the 1596‐cm⁻¹ band (b_3g) to that of the 1533‐cm⁻¹ band (a_g), which can be used as a marker of solid‐state phases: 1.54‐nm and 1.44‐nm phases. The Raman images showed that island‐like 1.44‐nm phase domains are grown on the 1.54‐nm phase layer from 50 nm, and all the surface of the 1.54‐nm phase layer is covered with the 1.44‐nm phase layer from 100 nm. The structural disorders have been discussed on the basis of the full width at half maximum of a band in the histogram of the R values for each film. Copyright © 2012 John Wiley & Sons, Ltd.     

The influence of screening effects on the Auger recombination in semiconductors

In contrast to the usual screening of the Coulomb potential by the dielectric constant ? an improved treatment of screening effects is given which is based on many-electron theory. As a consequence of it the Auger recombination probability is remarkably enhanced, indeed by a factor (?² + 1)/2 which is of the order 10². Discrepancies between theoretical and experimental recombination rates in Si and Ge can be resolved in this way.     

The role of nickel addition and annealing temperature on ion storage performance of nanostructured nickel ferrite thin films

The sol–gel spin-coated nickel ferrite (NF), NiFe₂O₄, thin films were synthesised and the effect of annealing temperature and compositional ratio on different properties of samples were investigated. Electrochemical performance of the films was measured in the presence of KOH and LiClO₄/PC electrolyte. Generally, addition of nickel increases the current density. The NF thin films with molar ratio of 0.5 and annealed at 400 °C have the highest charge density value and the highest capacitance in both electrolytes. Annealing temperature had significant effect on electrochemical properties of NF thin films and the diffusion coefficient enhanced by increasing the annealing temperature. X-ray diffraction patterns of prepared samples showed the rhombohedral structure, hematite phase (α-Fe₂O₃), of iron oxide sample and the presence of inverse spinel structure confirms the formation of NF. Field emission scanning electron microscopy images revealed that the morphology of films changes from larvae shape to granular structure by nickel incorporation and the grain size increased by raising the annealing temperature. The absorption edge of the hematite shift to higher wavelength by annealing and nickel incorporation and band gap narrowing has been occurred.     

温度对来自镶嵌在BaO薄膜中的纳米银粒子的光电发射影响

 研究了温度对来自镶嵌在BaO薄膜中的纳米银粒子的光电发射的影响。在不同温度下计算和讨论了其光电发射谱（入射波长在0.2~0.8 μm）。光电发射谱中温度越高，光电发射峰越大。同时发现镶嵌在BaO薄膜中银纳米的临界尺寸小于3.8 nm。     

Comparative study of the p+ layer created in CdTe films by nitric-phosphoric etching and chemical-deposition methods

The physical properties of the etched CdTe surfaces obtained during the process of formation of a p+ region on CdTe surface films using (i) a nitric/phosphoric acid mixture and (ii) a chemical Te-deposition method involving thermal annealing have been compared in this study. This study suggests the chemical-deposition method as an alternative to the chemical-etching methods for use in back-contact technology to increase the efficiency of CdTe solar cells.     

低能电子束照射接地绝缘薄膜的负带电过程

为揭示低能电子束照射接地绝缘薄膜的负带电过程及其机理,建立了同时考虑电子散射与电子输运的计算模型,综合Monte Carlo方法和有限差分法进行了数值模拟,获得了内部空间电荷、泄漏电流和表面电位随电子束照射的演化规律.结果表明,入射电子因迁移、扩散效应会超越通常的散射区域产生负空间电荷分布,并经过一定的渡越时间后到达接地基板,形成泄漏电流,负带电暂态过程则随着泄漏电流的增加而趋于平衡.在平衡状态下,泄漏电流随电子束能量和电流而增大;薄膜净负电荷量和表面电位随膜厚而增加、随电子迁移率的增大而降低,随着电子束 关键词： 绝缘薄膜 电子束照射 带电效应 数值模拟     

Interference effects in the UV(VUV)‐excited luminescence spectroscopy of thin dielectric films

The problem of exciting UV and VUV light interference affecting experimental photoluminescence excitation spectra is analysed for the case of thin transparent films containing arbitrarily distributed emission centres. A numerical technique and supplied software aimed at modelling the phenomenon and correcting the distorted spectra are proposed. Successful restoration results of the experimental synchrotron data for ion‐implanted silica films show that the suggested method has high potential.