Growth Of Semiconductor Films Research Articles

Newly Synthesized High‐k Polymeric Dielectrics with Cyclic Carbonate Functionality for Highly Stability Organic Field‐Effect Transistor Applications

AbstractOrganic field‐effect transistors (OFETs) with low‐voltage‐operating high‐stability are regarded as one of the key components of future electronics. However, it remains a challenge to enhance bias–stress stability, mechanical durability and environmental adaptability while reducing the operating voltage of the flexible OFETs. In this study, a new strategy of introducing high‐dipole‐moment groups into polymer side chains to enhance the intensity of polarization was proposed. This strategy can redirect cyclic carbonate side chains of high‐dipole groups under the action of electric fields and realize stable operation and efficient charge transfer. The experiments showed that high‐performance flexible OFETs were mainly attributed to the synthesized polymers through molecular structure designing which not only have high dielectric constant (k > 5) and high electrical insulating property but also favor the growth of organic semiconductor films. The flexible OFETs still showed excellent mechanical flexibility, high electrical, thermal and humidity stability. In addition, highly OFETs were applied into a floating‐gate memory with fullerene (C60) embedded charge memory layer and an integrated one‐transistor‐one‐transistor memory cell. They exhibited excellent memory performance with a large memory window (8.5 V), current ratio (103), stable retention (2 × 104 s), cyclic endurance (200 cycles), multi‐level memory (over 4 levels) and non‐destructivity.

Epitaxial growth of large area ZrS2 2D semiconductor films on sapphire for optoelectronics

Epitaxial growth of large area ZrS2 2D semiconductor films on sapphire for optoelectronics

Multiphoton Absorption Simulation of Sapphire Substrate under the Action of Femtosecond Laser for Larger Density of Pattern-Related Process Windows.

It is essential to develop pattern-related process windows on substrate surface for reducing the dislocation density of wide bandgap semiconductor film growth. For extremely high instantaneous intensity and excellent photon absorption rate, femtosecond lasers are currently being increasingly adopted. However, the mechanism of the femtosecond laser developing pattern-related process windows on the substrate remains to be further revealed. In this paper, a model is established based on the Fokker–Planck equation and the two-temperature model (TTM) equation to simulate the ablation of a sapphire substrate under the action of a femtosecond laser. The transient nonlinear evolutions such as free electron density, absorption coefficient, and electron–lattice temperature are obtained. This paper focuses on simulating the multiphoton absorption of sapphire under femtosecond lasers of different wavelengths. The results show that within the range of 400 to 1030 nm, when the wavelength is large, the number of multiphoton required for ionization is larger, and wider and shallower ablation pits can be obtained. When the wavelength is smaller, the number of multiphoton is smaller, narrower and deeper ablation pits can be obtained. Under the simulation conditions presented in this paper, the minimum ablation pit depth can reach 0.11 μm and the minimum radius can reach 0.6 μm. In the range of 400 to 1030 nm, selecting a laser with a shorter wavelength can achieve pattern-related process windows with a smaller diameter, which is beneficial to increase the density of pattern-related process windows on the substrate surface. The simulation is consistent with existing theories and experimental results, and further reveals the transient nonlinear mechanism of the femtosecond laser developing the pattern-related process windows on the sapphire substrate.

Collective orientation barrier in growth of organic thin films revealed by pMAIRS

We used $p$-polarized multiple-angle incidence resolution spectrometry (pMAIRS) to investigate a collective orientation barrier (COB) in the growth of organic semiconductor (OSC) films. We demonstrate a temperature-dependent variation in the growth of pentacene (PEN) on $\mathrm{Si}{\mathrm{O}}_{2}$ films as a model system. The molecular orientation varied from lying to standing as the growth temperature increased. This change suggests that the formation of a standing orientation is thermally activated compared with the lying state. The nucleation of standing-oriented islands occurs by molecular self-assembly at sufficiently high temperatures. Conversely, molecules deposit in a flat-lying state at low temperatures owing to hindrance of the kinetic barrier to reorientation from lying to standing states. The COB is defined as a collective energy barrier for the reorientation processes throughout the growth. The COB is quantitatively estimated from an Arrhenius plot of the probability to form a standing orientation, derived from the dichroic ratio measured by pMAIRS. We found that the COB in the growth of PEN on the $\mathrm{Si}{\mathrm{O}}_{2}$ system was approximately 0.02 eV, which is a key parameter for determining the molecular orientation. A quantitative evaluation of the COB will be applicable to other systems and enable effective control over the molecular orientation of OSC films.

High recovery NO2 sensors of α-6T organic thin film transistors based on interface inducing growth

In order to achieve high recovery for low concentration NO2 gas detection, the α-6T OTFT gas sensors were fabricated by inducing growth method. The effect of interface inducing layer and active layer thickness on the sensor performance was studied. It was found that the p-6P inducing layer improved the surface properties of SiO2 substrates and the α-6T films were manipulated to favorable film formation on p-6P interface layer. The α-6T crystal grains can be fused well and the large-area continuous crystal films were obtained. By the thickness modulation of α-6T active films, the sensors can achieve the detection to the low concentration of 1 ppm to NO2 gas and exhibit the rapid recovery of 28 s. And the sensitivity was as high as 998% / ppm and the responsivity was up to 2645% for 1 ppm NO2 gas. The formation of ultra-thin films on surface substrates is an effective method, which can induce the growth of organic semiconductor films and obtain the high-property devices.

Advances in E-ALD By Slrr for the Growth of Ultra Thin Films of Metal and Alloys - in Recognition to John Stickney's Contribution to Electrodeposition

The idea realization progress of electrochemical atomic layer deposition by surface limited redox replacement (E-ALD by SLRR) is among the brightest and most significant contributions of Professor John Stickney. The approach proposed by him in early nineties of last century for epitaxial growth of compound semiconductor films was extended in the beginning of 21st century with his active participation and contribution to the deposition of ultra thin coatings of metals and alloys with a variety of applications thus establishing foundation for the development of one of the most useful and versatile thin film growth methods. In the beginning of this presentation an overview of existing approaches for epitaxial metal thin film growth along with E-ALD by SLRR will first be presented. A comparison of the kinetics manipulation approaches will be made with emphasis on the ability of those methods to produce epitaxial, flat and uniform deposits. In the course of this comparison illustrations will be provided with results, illustrating the advantages and limitations of these deposition approaches in the growth of Ag and Cu thin films on Au (111) substrates. To support the discussion, on the approaches employed, results of electrochemical, in-situ scanning tunneling microscopy (STM), X-ray Photoelectron Spectroscopy characterization experiments will be presented and compared. In the second part, the applicability of SLRR and electroless SLRR in the growth of Pt and Pd on Au (111) substrate will be discussed with special emphasis on the contribution and achievements of John Stickney’s research team. Experimentally demonstrated will be the SLRR growth of Pt and Pd layers with controllable thickness taking place in “shuttling” chamber, flow cell, and “one-pot” configuration. Open circuit potentiometry during the replacement reaction will be shown as a way to control the completion of each deposition event. Anodic stripping of the entire multilayer will be used for determining the overall film thickness. Cyclic Voltammetry and STM will be employed to characterize the growth of Pt and Pd layers with up to 10 nm thickness. High-resolution STM results and modelling concepts underlying the foundation of the proposed approaches will shed light on the mechanistic aspects associated with the nature of growth by redox exchange at a monolayer level. In the third component of this presentation an overview of recent work on the design of potent catalysts with application in energy production and storage will be presented. This will include the use of SLRR approach for coating nanoparticles and continuous nanoporous metal layers with ultra thin films of metal / alloy with specific functionality. Successful outcome of this coating renders the final product catalytically active. In this part, a brief description will be provided also on the direct use of SLRR based protocols for the smart design of bimetallic alloy and/or core shell clusters with application in the most commonly used reactions in the field of fuel-cell catalysis. The discussion on the synthetic component of all relevant activities will be concluded with demonstration of results of testing of accordingly developed catalysts. Finally, a glimpse into the future of this rapidly developing field will be also provided.

Formation of Horizontally and Vertically Oriented MoS2 and MoSe2 Films Using Cracked Small S- and Se-Molecules and Its Applications for Transparent a-Si:H Thin Film Photovoltaic Devices

Molybdenum dichalcogenide films (MoX2, X=S, Se) were fabricated on 6-inch wafers by a breakthrough technique utilizing cracked small X-molecules. Uniformly deposited Mo-metal films were sulfurized or selenized by reactive chalcogen (Xa, a= 1- 2) small molecules, and MoX2 film was approximately 3 times thicker than the Mo metal precursor. The behavior of reaction chamber pressure according to the cracking temperature indicated that one large molecule could crack into 4 – 5 small particles in experimental condition used in this work. The formation of protrusions was also effectively inhibited to obtain MoX2 films of a device quality. First, I will explain how effectively large chalcogen molecules (normally X8) were cracked, and the cracking effect on the film quality. By controlling the cracking temperature separately, we could lower the growth temperature down to 400 oC or lower. The effect of cracking and X-source evaporation temperatures on the film quality will be also presented. The film quality was dependent on the deposition technique of Mo metal films. The films were characterized using x-ray photoelectron spectroscopy (XPS), Raman scattering, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Cross sectional high resolution TEM image, energy-dispersive x-ray spectroscopy, and angle-resolved XPS analysis of 6 nm-thick MoS2 sulfurized at 570 oC showed that the film has well-aligned layer-by-layer crystalline structure and the composition and chemical states were uniform through depth. Normally, layer-by-layer grown MoX2 films on very flat surfaces are studied for device applications as reported in earlier works. But, practical devices require the growth of semiconductor films on rough and/or patterned surfaces. Then, we studied the formation of a few nm-thick and a few tens nm-thick MoX2 films on rough surfaces, and found that the MoX2 films could be successfully grown on a very rough substrate (root mean square roughness = 38 nm) using the method introduced in this work. When MoX2 films were grown by chalcogenizing Mo, the crystalline orientation of MoX2 films depended on the thickness of Mo films. When the Mo film is thinner than 5 nm, we could obtain layer-by-layer structured, i.e., MoX2 films. We had reported the successful application of the horizontally grown MoX2 film to the field effect transistors in the previous work [1]. Contrarily, when the Mo film was 5 nm or thicker, the formed MoX2 films had vertically oriented crystalline structure. It is because the stress due to 3 times volume expansion should be released by changing the crystalline orientation from planar layer-by-layered to vertically oriented structures. In this work, we utilized the vertically oriented MoX2 films as an interlayer between the semiconductor layer and the electrode. By adopting the MoX2 interlayer in amorphous silicon (a-Si:H) thin film photovoltaic devices having the inverted structure, we could obtain greatly improved cell performances. In the substrate type photovoltaic cells, the shunt resistance increased from 10 kΩ (no MoSe2) to 18 kΩ (with MoSe2) and the open circuit voltage increased from 0.717 V to 0.785 V. The a-Si:H photovoltaic cell having 26 % visible light transmittance on a glass substrate showed the conversion efficiency of 7.7 % under blue light irradiation of 7 mW/cm2. In conclusion, the horizontally and vertically oriented MoX2 films of high crystallinity were formed by chalcogenizing Mo metal films using reactive cracked small X-molecules, and the use of MoX2 film as an interlayer greatly improved the performance of a-Si:H thin film photovoltaic devices. The growth method introduced in this work would be one of the most promising methods to obtain uniform and high quality metal dichalcogenide films on a large substrate. The most important advantage of this method is that it can be easily applied to other metal chalcogenide materials. [1] Jung, K. H.; Yun, S. J.; Choi, Y.; Cho, J. H.; Lim, J. W.; Chai, H.-J.; Cho, D.-H.; Chung, Y.-D.; Kim, G. Nanoscale 2018, 10, 15213-15221.

(Invited) Isotopically Programmed Group IV Semiconductors: A Versatile Platform for Quantum Technologies

The introduction of stable isotopes as an additional degree of freedom in the growth of semiconductor films and quantum structures provides a wealth of opportunities to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. In this presentation, I will describe the historical context and outline the recent progress in the area of isotopically engineering group IV semiconductors with focus on nanoscale and quantum structures and devices. This ability to isotopically program semiconductor structures has been a powerful paradigm to investigate and manipulate some of the important physical properties of semiconductors and exploit them in innovative device structures [1-13]. Isotopes of an element differ in the number of neutrons in the nucleus. This creates differences between the isotopes in their lattice dynamics and nuclear properties. For instance, the slight difference in zero-point motion leads to a difference in atomic volume between the isotope atoms, which influences the lattice constant [3]. Also, the difference in electron-phonon coupling between crystals of different isotopic composition was found to affect the electronic band gap [4]. The nuclear spin is another significant difference between stable isotopes. For instance, natural silicon (Si) has three stable isotopes: 28Si, 29Si, and 30Si, with isotopic abundances of 92.23%, 4.67%, and 3.10%, respectively. Among these three isotopes, only 29Si has a nuclear spin of ½, whereas 28Si and 30Si are nuclear spin-free. This property has been crucial in the realization of Si-based quantum information devices [5-8]. One of the most drastic isotope related effect in semiconductors is found in phonon properties [9-13]. Mass fluctuation induced by isotope disorder acts as a substitutional defect in a crystal thus affecting the phonon mean free path and consequently the phononic thermal conductivity. Measurements on isotopically pure Ge [9] and Si [10] crystals showed an enhanced thermal conductivity as compared to their natural counterparts. Also, lower thermal conductivity was recently demonstrated in Si isotope superlattices [11]. All the properties of semiconductor stable isotopes have been investigated and exploited in bulk materials or thin films. Herein, we will describe the new opportunities emerging from the combinations of the isotope effects with size-related effects in nanoscale materials [14-18]. More specifically, we will discuss phonon engineering in metal catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane and german precursors 28SiH4, 29SiH4, 30SiH4, 74GeH4 and 76GeH4, with purity better than 99.9%. Isotopically mixed nanowires 28Six 30Si1-x with a composition close to the highest mass disorder (x ~ 0.5) were used as a playground to elucidate the interplay between nanoscale interface phenomena and heat transport [16]. We will show how isotopically engineered nanowire homo-junctions can be introduced to realize innovative phononic devices such as thermal diodes and thermal transistors. Additionally, we will also discuss the use of nuclear spin-full 29Si to engineer novel quantum devices in nuclear spin-free SiGe nanostructures. Finally, atomistic-level investigations of isotopically programmed nanoscale materials will be presented based on laser-assisted atom probe tomography [15,17].

Development of a Long-Wave Infrared Band-Edge (LWIR BE) thermometry instrument.

Accurate measurement of substrate temperature is one of the most critical process control parameters for molecular beam epitaxy (MBE) growth. Band-edge thermometry instruments have proven to be a valuable tool for process control during MBE growth of semiconductor films, providing as high as ±1 °C temperature resolution. The increasing use of InAs, GaSb, and AlSb iii-v materials necessitates a method for accurately measuring the temperature of their closely lattice-matched GaSb substrates. Current-technology instruments typically rely on InGaAs detector materials which have a maximum wavelength λ detection of ∼1.7 μm, but GaSb substrates have a band gap energy corresponding to λ > 2 μm. A band-edge thermometry instrument capable of λ > 2 μm has been developed using an InAs/InGaSb strained-layer superlattice detector sensitive to 2-9.5 μm long-wave IR wavelengths.

Orientation domains in a monoclinic Mg–Al–O phase

Two types of Mg–Al–O structures were successfully synthesized under high temperature (above 1173 K). Transmission electron microscopy and group theory analysis reveal the existence of cubic MgAl2O4 and an unreported monoclinic MgAl x O y phase with four domain variants. The structural relationship between these two phases is discussed in detail. The results shed light on the structural investigation of Mg–Al–O oxides, which are important mineral components of the Earth's lower mantle as well as substrates for the epitaxial growth of semiconductor films. Monoclinic MgAl x O y nanowires with domain boundaries may also provide a possible high-strength candidate for industrial applications.

Quasi-Layer-by-Layer Growth of Pentacene on HOPG and Au Surfaces

High-resolution scanning tunneling microscopy (STM) is a promising method for characterizing organic semiconductors to obtain a deep understanding of organic semiconductor physics. However, organic films on conductive single-crystal substrates, which are required for STM, usually present different growth behaviors than the films on inert substrates such as SiO2. Here, we reported a simple modification method for modulating the organic semiconductor film growth on the highly oriented pyrolytic graphite (HOPG) and Au(111) substrates and investigated the detailed morphology evolution. Self-assembled monolayers (SAMs) fabricated from vacuum deposition and solution processing were introduced on these conductive substrates. Pentacene, a prototypical organic semiconductor, presented quasi-layer-by-layer growth on HOPG or Au(111) sufaces modified with solution-processed alkane monolayer. The pentacene film resembled the upright packing and terraced morphology but with larger grain size than that of thin-film phas...

Real-time monitoring of 2D semiconductor film growth with optical spectroscopy

Real-time monitoring of the growth is essential for synthesizing high quality two dimensional (2D) transition-metal dichalcogenides with precisely controlled thickness. Here, we report the first real time in situ optical spectroscopic study on the molecular beam epitaxy of atomically thin molybdenum diselenide (MoSe2) films on sapphire substrates using differential reflectance spectroscopy. The characteristic optical spectrum of MoSe2 monolayer is clearly distinct from that of bilayer allowing a precise control of the film thickness during the growth. Furthermore, the evolution of the characteristic differential reflectance spectrum of the MoSe2 thin film as a function of the thickness sheds light on the details of the growth process. Our result demonstrates the importance and the great potential of the real time in situ optical spectroscopy for the realization of controlled growth of 2D semiconductor materials.

Epitaxial Growth of Large-Grain NiSe Films by Solid-State Reaction for High-Responsivity Photodetector Arrays.

Film-based photodetectors have shown superiority for the fabrication of photodetector arrays, which are desired for integrating photodetectors into sensing and imaging systems, such as image sensors. But they usually possess a low responsivity due to low carrier mobility of the film consisting of nanocrystals. Large-grain semiconductor films are expected to fabricate superior-responsivity photodetector arrays. However, the growth of large-grain semiconductor films, normally with a nonlayer structure, is still challenging. Herein, this study introduces a solid-state reaction method, in which the growth rate is supposed to be limited by diffusion and reaction rate, for interface-confined epitaxial growth of nonlayer structured NiSe films with grain size up to micrometer scale on Ni foil. Meanwhile, patterned growth of NiSe films allows the fabrication of NiSe film based photodetector arrays. More importantly, the fabricated photodetector based on as-grown high-quality NiSe films shows a responsivity of 150 A W-1 in contrast to the value of 0.009 A W-1 from the photodetector based on as-deposited NiSe film consisting of nanocrystals, indicating a huge responsivity-enhancement up to four orders of magnitude. It is ascribed to the enhanced charge carrier mobility in as-grown NiSe films by dramatically decreasing the amount of grain boundary leading to scattering of charge carrier.

Surface Polarity and Self-Structured Nanogrooves Collaboratively Oriented Molecular Packing for High Crystallinity toward Efficient Charge Transport.

Efficient charge transport in organic semiconductors is essential for construction of high performance optoelectronic devices. Herein, for the first time, we demonstrate that poly(amic acid) (PAA), a facilely deposited and annealing-free dielectric layer, can tailor the growth of organic semiconductor films with large area and high crystallinity toward efficient charge transport and high mobility in their thin film transistors. Pentacene is used as a model system to demonstrate the concept with mobility up to 30.6 cm2 V-1 s-1, comparable to its high quality single crystal devices. The structure of PAA has corrugations with OH groups pointing out of the surface, and the presence of an amide bond further allows adjacent polymer strands to interact via hydrogen bonding, leading to a self-rippled surface perpendicular to the corrugation. On the other hand, the strong polar groups (-COOH/-CONH) of PAA could provide repulsive forces between PAA and pentacene, which results in the vertical orientation of pentacene on the dielectric surface. Indeed, in comparison with its imidized counterpart polyimide (PI), PAA dielectric significantly enhances the film crystallinity, drastically increases the domain size, and decreases the interface trap density, giving rise to superior device performance with high mobility. This concept can be extended to more organic semiconducting systems, e.g., 2,6-diphenylanthracene (DPA), tetracene, copper phthalocyanine (CuPc), and copper hexadecafluorophthalocyanine (F16CuPc), demonstrating the general applicability. The results show the importance of combining surface nanogrooves with the strong polarity in orienting the molecular arrangement for high crystallinity toward efficient charge transport in organic semiconductors.

DC voltage fields generated by RF plasmas and their influence on film growth morphology through static attraction to metal wetting layers: Beyond ion bombardment effects

It is shown that attractive electrostatic interactions between regions of positive charge in RF plasmas and the negative charge of metal wetting layers, present during compound semiconductor film growth, can have a greater influence than substrate temperature on film morphology. Using GaN and InN film growth as examples, the DC field component of a remote RF plasma is demonstrated to electrostatically affect metal wetting layers to the point of actually determining the mode of film growth. Examples of enhanced self-seeded nanopillar growth are provided in the case where the substrate is directly exposed to the DC field generated by the plasma. In another case, we show that electrostatic shielding of the DC field from the substrate can result in the growth of Ga-face GaN layers from gallium metal wetting layers at 490 °C with root-mean-square roughness values as low as 0.6 nm. This study has been carried out using a migration enhanced deposition technique with pulsed delivery of the metal precursor allowing the identification of metal wetting layers versus metal droplets as a function of the quantity of metal source delivered per cycle. It is also shown that electrostatic interactions with the plasma can affect metal rich growth limits, causing metal droplet formation for lower metal flux than would otherwise occur. Accordingly, film growth rates can be increased when shielding the substrate from the positive charge region of the plasma. For the example shown here, growth rates were more than doubled using a shielding grid.

Effect of Pulse Frequency on Structural and Thermoelectric Properties of Bismuth Telluride Thin Films By Electrodeposition

Recently, thin film materials for environmental energy harvesting such as solar cells and thermoelectric generators have been growing the interest. In particular, thin film thermoelectric generators are promising devices with applications in sensor nodes for the internet of things (IoT). To justify a wider range of practical applications for these devices, higher performance thermoelectric materials are needed. Thermoelectric performance is defined by a figure of merit, ZT = s S2T/ k, where S is the Seebeck coefficient, s is the electrical conductivity, and k is the thermal conductivity. The figure of merit depends on the temperature, and its maximum value is different for different kinds of thermoelectric materials. Among thermoelectric materials, bismuth telluride (Bi2Te3) and alloys based on it are especially favorable because they exhibit a high figure of merit near room temperature (RT), and can be produced in both n-type and p-type. Electrodeposition is one of the most important methods of film fabrication, offering low-cost growth of high-quality metal, alloy, and semiconductor films. This is because it involves operation at low temperature with no requirement for vacuum conditions. However, the thin films prepared by electrodeposition generally obtained relatively low thermoelectric performance compared with that by other deposition methods. Therefore, in this study, we performed a pulse-electrodeposition method to improve the thermoelectric performance. We used bismuth telluride (Bi2Te3) as a thermoelectric material because this material exhibits high performance near room temperature (RT). For the electrodeposition, we used a standard three-electrode cell with a potentiostat and a function generator. The working electrode was a stainless steel (SUS304) substrate, chosen because of its excellent corrosion resistance. A platinum-coated titanium mesh on a titanium plate was used as the counter electrode. A Ag/AgCl (saturated KCl) electrode was used as the reference electrode. The pulse frequency was varied from 1 Hz to 10 kHz whereas the mixing ratio (Te/[Te+Bi]) of Bi(NO3)3, and TeO2was constant through all the experiments. To avoid complications in analysis due to the electrical conduction of the stainless steel substrate, the film was fixed on a glass plate using an epoxy resin, followed by thin film removal from the substrate. The structural properties were evaluated by means of scanning electron microscopy (SEM) and XRD (X-ray diffraction) analysis. The thermoelectric properties, which were electrical conductivity, Seebeck coefficient and power factor, were measured at RT. The most apparent changes between the thin films with the pulse-electrodeposition and that with the potential constant electrodeposition were the electrical conductivity. The thin films prepared by the potential constant electrodeposition obtained the electrical conductivity and the other properties as follows; s = 135.6 S/cm, S = −57.3 µV/K, and P.F. = 0.44 µW/(cm·K2). The thin films prepared by the pulse-frequency electrodeposition at 10 Hz obtained the electrical conductivity and the other properties as follows; s = 211.8 S/cm, S = −52.9 µV/K, and P.F. = 0.59 µW/(cm·K2). Therefore, we observed that the thermoelectric properties were improved by applying pulse potential to the electrodeposition. To further investigate the effect of pulse frequency of electrodeposition, we plan to analyze the surface morphology and crystallographic properties.

Toward the Atomically Abrupt Interfaces of SiOx/Semiconductor Junctions

A novel simple method to produce a high-quality interface between semiconductor (e.g., Ge or III-V) and dielectric film (i.e., SiO2) is suggested. A nonoxygen element is deposited directly on an oxidized substrate at T ≥ 300 °C, leading to semiconductor reduction and dielectric film growth. As a result, the formed interface is atomically abrupt and has a low density of defect gap states. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

On the Applications of Newly Architectured CdTe Nanostructures from Ionic Liquid Medium

The 2D nanostructures are gaining lot of interest in the field of electronics and solar photovoltaics. We have recently developed new shape and architecture of CdTe nanostructures on low cost F:SnO2 (FTO) substrate by electrodeposition from ionic liquid medium. It was observed that the ionic liquid medium facilitate the fabrication of nanostructured thin film directly on the substrate. By using two different substrates, FTO and CdS coated FTO it was possible to fabricate Schottky and p-n junction solar cells out of developed CdTe nanostructured films. It was interesting to observe that the Schottky junction solar cells of such nanostructured films result in higher Voc of 805 mV which is a quite high value for such solar cells. However, low connectivity between nanostructures resulted in lower Jsc values. On the other hand we found that the growth of semiconductor films from ionic liquid medium is highly substrate dependent. It was found that stoichiometric CdTe films can only be deposited on 50 nm thick film of CdS. Increasing the thickness of CdS layer always lead to non stoichiometric CdTe films which no longer can be utilized in solar cells. The first ever successful deposition of stoichiometric CdTe film on CdS buffer layer resulted in a solar cell with Voc of 150 mV, Jsc of 500 μA/cm2, FF of 27% and efficiency of 0.02%. This initial study will be helpful in understanding the growth of binary alloy on different substrates from ionic liquid medium. Figure 1

Monolayer-Mediated Growth of Organic Semiconductor Films with Improved Device Performance.

Increased interest in wearable and smart electronics is driving numerous research works on organic electronics. The control of film growth and patterning is of great importance when targeting high-performance organic semiconductor devices. In this Feature Article, we summarize our recent work focusing on the growth, crystallization, and device operation of organic semiconductors intermediated by ultrathin organic films (in most cases, only a monolayer). The site-selective growth, modified crystallization and morphology, and improved device performance of organic semiconductor films are demonstrated with the help of the inducing layers, including patterned and uniform Langmuir-Blodgett monolayers, crystalline ultrathin organic films, and self-assembled polymer brush films. The introduction of the inducing layers could dramatically change the diffusion of the organic semiconductors on the surface and the interactions between the active layer with the inducing layer, leading to improved aggregation/crystallization behavior and device performance.

Preparation of ZnO films with variable electric field-assisted atomic layer deposition technique

The ZnO films have been prepared by a variable electric field-assisted atomic layer deposition method (ALD). By applying electric fields during the precursor pulses, we can modulate both the crystal orientation and structure of the obtained ZnO films. The ZnO films with c-axis preferred orientation and the least oxygen vacancy defect were obtained when the holder electric polarities were positive and negative during the DEZn and H2O pulse, respectively. It is supported that when electric field was applied in the chamber, the torque may lead to the precursor molecular alignments along the electric field direction, which could affect the film growth process and then influence their structures and properties. This variable electric field-assisted ALD approach would provide an efficient protocol for the growth of semiconductor films with designed properties.