Semiconductor Thin Films Research Articles

Molecular single crystals induce chain alignment in a semiconducting polymer

The blending of π-conjugated molecules with polymeric semiconductors into a composite system is an effective strategy to promote charge carrier mobility because of the transmission path by the conductive polymers through electrical bridge connection of the small organic molecule crystalline domain. In this work, pentacene single crystal was prepared to induce the conjugated polymer chain alignment of polymeric semiconductor PDPP2T-TT-OD to form the semiconducting composite, which led to enhanced field-effect mobility of the organic thin-film transistor (OTFT) by improving the crystallinity due to nucleation and growth phase separation. Besides, with the addition of anti-solvents, the crystallization of the blend film was further improved, 27 times higher than that of a pure polymer semiconductor–based OTFT. That was because the pentacene nuclei induced polymer crystallization through π-π interactions and the addition of anti-solvent promoted the aggregation of polymer chains in solution, enabling the molecular chains packed more closely in solid films. Therefore, the chain arrangement of polymers induced via small molecular single crystals provides a new idea to improve mobility in composite semiconductor thin films for the construction of novel organic optoelectronic devices.

Unusual Spectrally Reproducible and High Q-Factor Random Lasing in Polycrystalline Tin Perovskite Films.

An unusual spectrally reproducible near-IR random lasing (RL) with no fluctuation of lasing peak wavelength is disclosed in polycrystalline films of formamidinium tin triiodide perovskite, which have been chemically stabilized against Sn2+ to Sn4+ oxidation. Remarkably, a quality Q-factor as high as ≈104 with an amplified spontaneous emission (ASE) threshold as low as 2µJcm-2 (both at 20K) are achieved. The observed spectral reproducibility is unprecedented for semiconductor thin film RL systems and cannot be explained by the strong spatial localization of lasing modes. Instead, it is suggested that the spectral stability is a result of such an unique property of Sn-based perovskites as a large inhomogeneous broadening of the emitting centers, which is a consequence of an intrinsic structural inhomogeneity of the material. Due to this, lasing can occur simultaneously in modes that are spatially strongly overlapped, as long as the spectral separation between the modes is larger than the homogeneous linewidth of the emitting centers. The discovered mechanism of RL spectral stability in semiconductor materials, possessing inhomogeneous broadening, opens up prospects for their practical use as cheap sources of narrow laser lines.

Colloidal Synthesis of Semiconductor Films for Efficient Photoelectrochemical Hydrogen Generation

The semiconductor-based photoanodes have shown great potential on photoelectrochemical (PEC) hydrogen generation. Compared to the pristine semiconductor, photoanodes fabricated with doped semiconductors exhibit modulated bandgap structure and enhanced charge separation efficiency, demonstrating improved optoelectronic properties. In this work, we develop a colloidal cation exchange (CE) strategy on versatile synthesis of heterovalent doped chalcogenide semiconductor thin films with high surface roughness. Using Ag-doped CdSe (CdSe:Ag) thin films as an example, the organized centimeter-scale CdSe:Ag films with nanometer-scale thickness (thickness around 80 nm, length × width around 1.5 cm × 1.2 cm) exhibit enhanced optical absorbance ability and charge carrier density by tuning the energy levels of conduction and valence bands as well as improved electrical conductivity by Ag dopants compared to the pristine CdSe film obtained by the vapor-phase vacuum deposition strategy. In the meantime, the surface roughness of the as-prepared semiconductor thin films is also increased with abundantly exposed active sites to facilitate accessibility to water for hydrogen generation and suppress photogenerated carrier recombination. The CdSe:Ag film photoanodes exhibit superb PEC hydrogen generation performance with a photocurrent density of 0.56 mA/cm 2 at 1.23 V versus reversible hydrogen electrode, which is nearly 3 times higher than the pristine CdSe film. This work provides a new strategy on colloidal synthesis of photoelectrodes with modulated heterovalent doping and surface roughness for PEC applications.

Photogenerated charge transfer in Dion-Jacobson type layered perovskite based on naphthalene diimide.

Incorporating organic semiconducting spacer cations into layered lead halide perovskite structures provides a powerful approach to mitigate the typical strong dielectric and quantum confinement effects by inducing charge-transfer between the organic and inorganic layers. Herein we report the synthesis and characterization of thin films of novel DJ-phase organic-inorganic layered perovskite semiconductors using a naphthalene diimide (NDI) based divalent spacer cation, which is shown to accept photogenerated electrons from the inorganic layer. With alkyl chain lengths of 6 carbons, an NDI-based thin film exhibited electron mobility (based on space charge-limited current for quasi-layered 〈n〉 = 5 material) was found to be as high as 0.03 cm2 V-1 s-1 with no observable trap-filling region suggesting trap passivation by the NDI spacer cation.

The impact of film deposition and annealing on the nanostructure and dielectric constant of organic semiconductor thin films.

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2'-[(4,4,4',4'-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4'H-{2,2'-bi[cyclo-penta[2,1-b:3,4-b']dithiophene]}-6,6'-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing.

Synthesis and high-temperature ferromagnetism of Fe-doped SiGe diluted magnetic semiconductor thin films.

Fe-doped SiGe (Si0.25Ge0.75:Fex, x = 0.01, 0.025, and 0.05) thin films were prepared by radio frequency magnetron sputtering and subsequent rapid thermal annealing on a Ge (100) substrate and their structural, magnetic and magneto-transport properties were investigated. Structural characterization using AFM, SEM, XRD, and HRTEM shows that the obtained samples are polycrystalline and their lattice constants increase with the Fe concentration. Analysis of their electronic and spintronic states using XPS and XMCD reveals that Fe dopants mainly exist as substitutional Fe2+ ions in the SiGe lattice, providing both local magnetic moments and hole carriers. Furthermore, magnetization measurements indicate that all the samples exhibit ferromagnetism, and their Curie temperature increases with the Fe concentration up to 294 K; meanwhile, magneto-transport measurements reveal a giant magnetoresistance (GMR) effect of over 800% and an anomalous Hall effect (AHE), as well as semiconducting behaviors, in the samples. Further analysis suggests that the ferromagnetism comes from a hole-mediated process originating from substitutional Fe dopants in the SiGe matrix and this is enhanced by the tensile strain in the films. The synthesis and high-temperature ferromagnetism of Fe-doped SiGe thin films may play a key role in group IV-based spintronic applications.

Non-toxic solution processed Cu2ZnSn(SSe)4 thin films for photovoltaic Applications: A review

Due to its earth-abundant and environmentally safe constituents, as well as suitable optical properties like a high absorption coefficient > 104 cm−1 and a direct bandgap of 1.5 eV Kesterite Cu2ZnSnS4 (CZTS) has emerged as a celebrated successor to CuInGaS2 (CIGS) as the absorber layer in thin film solar cells. The binary chalcogenides are first synthesized, and then they are dispersed in hydrazine, yielding the solution process, which has so far the highest conversion efficiency of 12.6% for any CZTSSe fabrication process. In light of hydrazine’s explosive, heptotoxic and carcinogenic properties, further study has been done on the simple synthesis of CZTS films from non-toxic solutions. Numerous non-hydrazine solutions, which are significantly safer to use, have been researched. The formation of single kesterite phase with better control over secondary phases has been the key to its success in photovoltaic applications. In all solution-based approaches for semiconductor thin films, the film growth proceeds from the release of cations and anions from the precursor salts, and their subsequent reactions.

Heteroepitaxially grown semiconductors on large-scale 2D nanomaterials for optoelectronics devices

Semiconductor nanostructures or thin films are vital components of modern optoelectronic devices, such as light-emitting diodes, sensors, or transistors. While single crystalline wafers are used as heteroepitaxial templates for them, increasing demands on flexibility or transferability require separation of the grown semiconductor structures on such substrates, which is technically challenging and expensive. Recent research suggests that large-scale 2D nanomaterials can serve as heteroepitaxial templates and provide additional functionalities such as transferability to foreign substrates or mechanical flexibility. In this paper, growth, structural properties, and optoelectronic device applications of semiconductor nanostructures or thin films which are heteroepitaxially grown on large-scale 2D nanomaterials are reviewed.

МЕТОДИЧНИЙ АСПЕКТ ПРОВЕДЕННЯ СПОСТЕРЕЖЕНЬ ЗА ЗМІНОЮ ОБ’ЄМНОЇ КОНЦЕНТРАЦІЇ РАДОНУ В ЗАЛЕЖНОСТІ ВІД ГЕОГРАФІЧНО-СЕЙСМІЧНИХ ПАРАМЕТРІВ У РЕГІОНІ

The paper describes one of the methods of investigation of galvanomagnetic phenomena compound thin film semiconductor system AIVBVI. The resulting current-voltage characteristics of thin films PbTe: Sb and analyzed the basic parameters for thin films of different thicknesses using the method of Hall. The dependence of the film thickness PbTe:Sb on the mobility and concentration of major carriers.

ОСОБЛИВОСТІ ДОСЛІДЖЕННЯ ГАЛЬВАНОМАГНІТНИХ ХАРАКТЕРИСТИК НАПІВПРОВІДНИКОВИХ ТОНКИХ ПЛІВОК

The paper describes one of the methods of investigation of galvanomagnetic phenomena compound thin film semiconductor system AIVBVI. The resulting current-voltage characteristics of thin films PbTe: Sb and analyzed the basic parameters for thin films of different thicknesses using the method of Hall. The dependence of the film thickness PbTe:Sb on the mobility and concentration of major carriers.

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta3N5 thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta3N5 as a leading performer among visible‐light‐responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Characterization of Zinc Oxide Semiconductor Thin Film Treated with Cold Plasma

In this study, the photovoltaic properties of zinc oxides are investigated. Different stoichiometries of zinc oxide can be made using vacuum deposition. Thermally evaporated zinc produces thin films of ZnO composed of orthorhombic and tetragonal phases that define the material's characteristics and performance. On-thermal plasma has removed a very thin layer from the surface of the thin film, known as non-thermal plasma, and made a surface cleanup. Measurements of the Atomic Force Microscope (AFM) show that the value of the granular volume rate increases when the duration of exposure to non-heat plasma increases. Measurements of the Atomic Force Microscope (AFM) show that there is an increase in thin film surface roughness values (Roughness, Sq), as well as the square root of the average roughness of the thin film surface (RMS), and this increase in roughness appears as the duration of exposure to non-thermal plasma increases.

Reducing Trap Density in Organic Solar Cells via Extending the Fused Ring Donor Unit of an A-D-A-Type Nonfullerene Acceptor for Over 17% Efficiency.

The high trap density (generally 1016 to 1018 cm-3 ) in thin films of organic semiconductors is the primary reason for the inferior charge-carrier mobility and large nonradiative recombination energy loss (ΔEnr ) in organic solar cells (OSCs), limiting improvement in power conversion efficiencies (PCEs). In this study, the trap density in OSCs is efficiently reduced via extending the donor core of nonfullerene acceptors (NFAs) from a heptacyclic unit to a nonacyclic unit. TTPIC-4F with a nonacyclic unit has stronger intramolecular and intermolecular interactions, affording higher crystallinity in thin films relative to its counterpart BTPIC-4F. Thus, the D18:TTPIC-4F-based device achieves a lower trap density of 4.02×1015 cm-3 , comparable to some typical high-performance inorganic/hybrid semiconductors, with higher mobility and inhibited charge-carrier recombination in devices. Therefore, the D18:TTPIC-4F-based OSC exhibits an impressive PCE of 17.1% with a low ΔEnr of 0.208eV, which is the best known value for A-D-A-type NFAs. Therefore, extending the donor core of NFAs is an efficient method for suppressing trap states in OSCs for high PCEs.

Nitrogenous Interlayers for ITO S/D Electrodes in N‐Type Organic Thin Film Transistors

AbstractThe commercialization of organic thin film transistors (OTFTs) is partly hindered by the high cost and stubborn work function (WF) of common Au source/drain (S/D) electrodes. In this work, indium tin oxide (ITO) S/D electrodes modified with three different nitrogenous interlayers including 1,4‐bis(2‐hydroxyethyl) piperazine (HEP), polyethylenimine ethoxylated (PEIE), and branched polyethylenimine (PEI) are adopted to fabricate n‐type OTFTs. The WF of bare ITO is 4.7 eV, whereas ITO modified with nitrogenous interlayers exhibits WFs ranging from 4.1 to 3.6 eV. By the first‐principle calculation and element analysis, the protonated amines are critical for the reduction of ITO surface WF. Besides, the interlayers with higher nitrogen contents are more likely to improve the wettability of octadecyltrichlorosilane (OTS) modified ITO substrates, and help to gain the continuous and oriented poly[2,5‐bis(4‐tetradecyloctadecyl)pyrrolo[3,4‐c]pyrrole‐1,4(2H,5H)‐dione‐alt‐5,5′‐di(thiophen‐2‐yl)‐2,2′‐(E)‐1,2‐bis(3,4‐difluorothien‐2‐yl)‐ethene] (P4FTVT‐C32) semiconductor thin films. Therefore, P4FTVT‐C32 based OTFTs with maximum electron field‐effect mobility of 1.95 cm2 V−1 s−1 are achieved by PEI treatments. These results indicate that the ITO S/D electrodes modified by nitrogenous interlayers can greatly slash costs and promote commercialization of OTFTs.

Large-Scale Silver Sulfide Nanomesh Membranes with Ultrahigh Flexibility.

The growth of flexible semiconductor thin films and membranes is highly desirable for the fabrication of next-generation wearable devices. In this work, we have developed a one-step, surface tension-driven method for facile and scalable growth of silver sulfide (Ag2S) membranes with a nanomesh structure. The nanomesh membrane can in principle reach infinite size but only limited by the reactor size, while the thickness is self-limited to ca. 50 nm. In particular, the membrane can be continuously regenerated at the water surface after being transferred for mechanical and electronic tests. The free-standing membrane demonstrates exceptional flexibility and strength, resulting from the nanomesh structure and the intrinsic plasticity of the Ag2S ligaments, as revealed by robust manipulation, nanoindentation tests and a pseudo-in situ tensile test under scanning electron microscope. Bendable electronic resistance-switching devices are fabricated based on the nanomesh membrane.

Van der Waals Epitaxy of HgCdTe Thin Films for Flexible Infrared Optoelectronics

AbstractVan der Waals epitaxial (vdW) growth of semiconductor thin films on 2D layered substrates has recently attracted considerable attention since it provides a potential pathway for realizing monolithically integrated devices and flexible devices. In this work, direct growth of epitaxial HgCdTe (111) thin films on 2D layered transparent mica substrates is achieved via molecular beam epitaxy. The full width at half maximum of the ω‐mode X‐ray diffraction peak is measured to be around 306 arc sec. Mid‐wave infrared photoconductors based on the as‐grown HgCdTe thin films have been demonstrated and the self‐heating effect has been evaluated. A peak responsivity at the wavelength of around 3500 nm is measured to be about 110 V W−1 at 80 K and 8 V W−1 at room temperature under a bias of 25 V cm−1. Twinning defects are observed, limiting the crystallinity and mobility‐lifetime product in HgCdTe/mica. Benefiting from the vdW epitaxial growth, an etch‐free layer transfer process for lifting off the HgCdTe from the mica substrate has been demonstrated, leading to large area free‐standing HgCdTe thin films.

Photoresponse of solution-processed transparent heterojunction ultraviolet photodetectors composed of n-type ZTO and p-type NiO-based semiconductor thin films

The n-type zinc-tin oxide (ZTO) and p-type nickel oxide (NiO) semiconductor thin films were prepared by the sol-gel spin coating process. The influence of 4 at% Mg and Cu doping on the microstructural, optical, and electrical characteristics of NiO thin films and physical properties of ZTO thin film were investigated. In addition, bi-layered ZTO/NiO thin films were grown onto Corning glasses to develop a visible light transparent p-n heterojunction ultraviolet (UV) photodetector. Mg doping slightly enhances the visible transparency and enlarges the optical bandgap energy, while Cu doping decreases the visible transparency and narrows the optical bandgap energy. The electrical properties of NiO sol-gel thin films were improved through Mg and Cu doping. According to the current-voltage (I–V) characteristic curves, the n-ZTO/p-NiO:Mg photodetector had the highest rectification ratio of 11.7 and on-to-off current ratio of 18.3 at 5 V bias. Under UV illumination, the three kinds of devices all exhibited a self-powered nature, while the Mg- and Cu-doped NiO devices had better photoresponse characteristics than the undoped NiO device. It was found that the Mg-doped NiO devices displayed the best photoconductivity gain and sensitivity, and acceptable response speed and responsivity, with applied reverse bias of 1 V.

Copper effect on cadmium sulfide thin films for spintronics: microstructures, morphological, photoluminescence and magnetic properties

CdS1-xCux (with 0 ≤ x ≤ 0.10) semiconductor thin films were successfully fabricated using electron beam evaporation. The effect of [Cu]/[S] on the structural properties of thin films was investigated using XRD, EDX, SEM and UV spectroscopy. Furthermore, the effects of different Cu concentrations on the optical parameters of these films are also reported. XRD analysis showed that the CdS1-xCux film was improved, showing a hexagonal polycrystalline structure with an increased Cu doping ratio. Furthermore, the size of the crystallites D decreases, while the microstrain ε-strain increases with increasing copper incorporation into the CdS lattice. The observed shift in the emission band of the photoluminescence spectrum confirms the substitution of Cu by the CdS lattice. Magnetization measurements using a vibrating sample magnetometer revealed hysteresis loops in Cu-doped CdS films and confirmed ferromagnetism at room temperature.

New Approaches to Produce Large-Area Single Crystal Thin Films.

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

The Role of Zn Substitution in Improving the Electrical Properties of CuI Thin Films and Optoelectronic Performance of CuI MSM Photodetectors.

Pure CuI and Zn-substituted CuI (CuI:Zn) semiconductor thin films, and metal-semiconductor-metal (MSM) photodetectors were fabricated on glass substrates by a low-temperature solution process. The influence of Zn substitution concentration (0-12 at%) on the microstructural, optical, and electrical characteristics of CuI thin films and its role in improving the optoelectronic performance of CuI MSM photodetectors were investigated in this study. Incorporation of Zn cation dopant into CuI thin films improved the crystallinity and increased the average crystalline size. XPS analysis revealed that the oxidation state of Cu ions in all the CuI-based thin films was +1, and the estimated values of [Cu]/[I] for the CuI:Zn thin films were lower than 0.9. It was found that the native p-type conductivity of polycrystalline CuI thin film was converted to n-type conductivity after the incorporation of Zn ions into CuI nanocrystals, and the electrical resistivity decreased with increases in Zn concentration. A time-resolved photocurrent study indicated that the improvements in the optoelectronic performance of CuI MSM photodetectors were obtained through the substitution of Zn ions, which provided operational stability to the two-terminal optoelectronic device. The 8 at% Zn-substituted CuI photodetectors exhibited the highest response current, responsivity, and EQE, as well as moderate specific detectivity.