AVS 66 Session EM-ThP: Electronic Materials and Photonics Poster Session

Beryllium oxide (BeO) is a large band gap (> 8 eV) dielectric with extreme properties that makes it ideal for pairing with other wide bandgap semiconductors with similar extreme properties for various high-power, -temperature, and -frequency device applications. For such devices to be succesful, large (> 1 eV) valence and conduction band offsets are needed at the interface between BeO and the wide bandgap semiconductor. However, relatively little is known regarding the band alignment between BeO and other materials. In this regard, we have utilized x-ray photoemission spectroscopy (XPS) to determine the valence band offset (VBO) between atomic layer deposited (ALD) BeO and epilayers of diamond and the cubic form of silicon carbide (3C-SiC) grown on silicon substrates. Using the valence band alignment rules of transitivity and commutativity, we are able to combine these results with previously reported values for the band alignment of BN, AlN, GaN, and InN to diamond and SiC to further deduce the alignment of these wide bandgap semiconductors to BeO. We will show that all BeO/wide band semiconductor combinations examined exhibit a type I band alignment with > 1 eV valence and conduction band offsets that are ideally suited for high-power, -temperature, and -frequency device applications.

We have investigated the influence of growth conditions, post deposition curing, and nano-porosity on the thermal conductivity for a series of organo-silicate (SiOCH) low-k dielectric thin films. Time-domain thermoreflectance (TDTR) was specifically utilized to meaure thermal conductivity while the influence of growth conditions and post deposition curing on mass density, network bond structure, percent porosity, pore size and pore interconnectivity were examined using techniques including nuclear reaction analysis (NRA), Rutherford backscattering spectroscopy (RBS), transmission Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), ellipsometric porosimetry (EP), and positronim annihilation lifetime spectroscopy (PALS). Analytical models describing the dependence of thermal conductivity on mass density and volume % porosity were found to generally over-predict the experimentally measured thermal conductivity, but improved agreement was obtained when considering only the density of heat carrying network bonds experimentally measured by FTIR. Ashby’s semi-empirical relation, which assumes that only 1/3 of the heat carrying bonds are aligned to the heat transport direction, was also found to reasonably describe the observed trends relating thermal conductivity and mass density. However, the thermal conductivity results were found to be best described via a model proposed by Sumirat (J. Porous Mater. 9, 439 (2006)) which considers the effect of both the volume percent porosity and phonon scattering by nanometer sized pores.

The synthesis of high-quality transition metal dichalcogenides films is of significant interest for potential applications in nanoelectronic and thermoelectric devices. Molecular beam epitaxy is a promising route towards this aim, providing fine control over growth conditions. To further the present understanding of growth conditions on the quality of transition metal dichalcogenide thin films, we study the effect of growth temperature, chalcogen to metal flux ratio, and the use of a ripening step on the stoichiometry and surface morphology of grown WSe₂ thin films. In-situ X-ray photoelectron spectroscopy is performed to analyze the intrinsic chemical composition of the grown material prior to atmospheric exposure, and ex-situ atomic force microscopy is employed to study the surface morphology of grown, sub-monolayer films. We find that both low and high growth temperature ranges can be detrimental to the chemical makeup of the grown material and that these results are echoed in the resulting grain morphology. An intermediate growth temperature produced chemically superior films over a wide range of chalcogen to metal flux ratios. The chalcogen to metal flux ratio was seen to provide some control of the film morphology, with high fluxes producing films with cleaner grain boundaries. Lastly, we show that the use of a ripening step in the early stages of growth results in a chemically superior material. This ripening step has the added benefit of producing films which are chemically more consistent than those grown in the absence of this step. There is also evidence to suggest that utilizing a ripening step may expand the processing window for film growth, allowing the use of higher processing temperatures and consequently better control over film quality.

ZnS is one of the attractive II-VI semiconductors because of their potential applications in the novel electronics and optoelectronics devices. ZnS is an n-type semiconductor with relatively high transparency, large Bohr exciton radius (2.5 nm), large exciton binding energy (40 meV), high index of refraction (2.27) [1], and wide bandgap showing different bandgaps of 3.68 eV and 3.91 eV for cubic zinc blende (ZB) phase and hexagonal wurtzite (WZ) phase, respectively [2]. ZnS is considered one of the prospective candidates for the CIGS photovoltaic (PV) applications, compared to CdS, it has non-toxic handling, wide bandgap, and better lattice matching to CIGS absorber with bandgaps of 1.3–1.5 eV [2]. Some dopant metals, such as Al, Cu, Ag, Mn, and Tb, are widely doped into ZnS lattice. Some researchers have studied the effect of Cu doping on the emission of light in ZnS, in this study, ZnS:Cu thin films were deposited by using a co-puttering method for photovoltaic applications. Effect of doping content on morphological, optical and electrical properties of ZnS thin films after rapid thermal annealing (RTA) treatment was investigated‎ with the structural properties of the different phases of ZB, WZ, and the mixture of them in X-ray diffraction studies . Optical and electrical characteristics of the thin films were analyzed by using an UV-Visible spectrophotometer and a Hall effect measurement system for optical transmittance, bandgap, resistivity, and carrier concentration. Acknowledgement: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20184010201650). [1] Sanjeev Kumar, C.L. Chen, C.L. Dong, Y.K. Ho, J.F. Lee, T.S. Chan, R. Thangavel, T.K. Chen, B.H. Mok, S.M. Rao, M.K. Wub, Room temperature ferromagnetism in Ni doped ZnS nanoparticles, J. Alloy Compd. 554, 357 (2013). [2] Md. Anisuzzaman Shakil, Sangita Das, Md. Ashiqur Rahman, Umma Salma Akther, Md. Kamrul Hassan Majumdar, Md. Khalilur Rahman, A Review on Zinc Sulphide Thin Film Fabrication for Various Applications Based on Doping Elements, Mater. Sci. Appl. 9, 751 (2018).

In topological crystalline insulators, the topological conducting surface states are protected by crystal symmetry. Here, we show using scanning tunneling microscopy/spectroscopy that defects that break local mirror symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations [1]. The results reveal the influence of various defects on the electronic properties, including screw dislocations, point defects, and tilt boundaries that lead to dislocation arrays that serve as periodic nucleation sites for pits grown on SrTiOinsulators the topological conducting surface states are protected by crystal symmetry. Here, we show using scanning tunneling microscopy/spectroscopy that defects that break local mirror symmetry of SnTe suppress electron tunneling over an energy range as large as the bulk band gap, an order of magnitude larger than that produced globally via magnetic fields or uniform structural perturbations [1]. The results reveal the influence of various defects on the electronic properties, including screw dislocations, point defects, and tilt boundaries that lead to dislocation arrays that serve as periodic nucleation sites for pits grown on SrTiO₃ [2,3]. Complementary ab initio calculations show how local symmetry breaking obstructs topological surface states as shown by a threefold reduction of the spectral weight of the topological surface states. The findings highlight the potential benefits of manipulating the surface morphology to create devices that take advantage of the unique properties of surface states and can operate at practical temperatures.

[1] O.E. Dagdeviren et al., Physical Review Materials 2, 114205 (2018).

[2] O.E. Dagdeviren et al., Advanced Materials and Interfaces 4, 1601011 (2017).

[3] O.E. Dagdeviren et al., Physical Review B 93, 195303 (2016).

When launching a satellite into orbit, every gram reduced from its total weight counts toward cheaper missions, with this in mind and inspired by the wide range of applications allowed by flexible electronics, this work presents the study and simulation of a leaf-like coplanar microstrip antenna on an one mil thick Kapton substrate centered in the 915MHz frequency to be used with a LoRa communication module in Low Earth Orbit (LEO) CubeSats.

Ring resonators and coplanar transmission lines (CPW) were also simulated to be used in the substrate's material characterization and help understand the various challenges poused by the thin thickness. Comparing the simulations of the CPW and characteristic impedance equations found in the literature, it was possible to notice divergences between the simulated model impedance and the theoretical calculated value when dealing with the thin substrate, witch indicates that the equation's models may not consider effects that appear with the reduced thickness, making it difficult to obtain a good impedance matching.

The designed antenna is presented alongside a impedance matching semi flexible circuit, a coplanar waveguide, ring resonator and the study of the impedance matching hardships when using thin substrates for radio frequency applications.

We use atom probe tomography (APT) to develop an understanding of the composition of our GaAs_1-x-yN_xBi_y samples. These alloys are of interest because of the significant bandgap narrowing that can be generated by incorporation of dilute concentrations of N and Bi. Notably lattice-matching with a GaAs substrate has also been demonstrated, yielding a bandgap of ~1 eV with a ratio of x_N /y_Bi = 083. The distribution of these alloys in the sample is of marked interest because of the observation of localized states generated by their incorporation, notably in the case of interstitial complexes developing in the As sublattice, which are important contributors to the electronic structure of these materials. We use APT to demonstrate the presence of As cluster states and to evaluate the composition and distribution of impurities of our sample.

This paper presents the fabrication and testing of a low cost, silicon nanowire photovoltaic device. The silicon nanowires are etched into the surface of a silicon wafer, via metal assisted chemical etching (MACE). This method of nanowire fabrication does not require photolithographic patterning, thereby reducing manufacturing complexity and related costs. Vertically aligned nanowire p-n junctions have the potential to increase the optical bandwidth of a silicon photovoltaic device by allowing a greater amount of short wavelength light to reach the depletion region near the junction, resulting in improved conversion efficiency. When compared to a planar analog, the nanowire device produced an order of magnitude higher power in response to blue light (405 nm), attributed to increased collection at the exposed p-n junctions. Power conversion efficiency is eight times better than previously reported with a similar construct.

The high electron saturation velocity, small effective electron mass and high electron mobility of indium nitride (InN) makes it a suitable material for high frequency electronics. The possibility of InN in the existing high electron mobility transistors (HEMTs), currently based on other group III-nitrides. However, InN decomposes to In metal and N₂ gas at around 500°C, making deposition of the InN films challenging with conventional methods such as metal organic chemical vapor deposition (MOCVD) and Molecular Beam Epitaxy (MBE). Nevertheless, Hollow cathode plasma assisted atomic layer epitaxy (HCPA-ALD) is a layer-by-layer crystalline growth technique that is based on a pair of self-terminating and self-limiting gas-surface half-reactions, in which at least one half-reaction involves species from plasma. The inclusion of plasma generally offers the benefit of substantially reduced growth temperatures and greater flexibility in tailoring the gas-phase chemistry to produce varying film characteristics. The benefits of plasma come at the cost of a complex array of process variables that often challenge the ability to predict, a priori, the influence of any one input parameter. This work focuses on a variety of gas input flow fractions (N₂ and N₂/H₂) used in the HCPA-ALD growth of InN films. Changes in plasma parameters are then linked with changes in film characteristics. To evaluate the optical properties of the InN films, we use spectroscopic ellipsometer to measure the dielectric function and a complex refractive index. Data were fitted using a fitting based analysis program, and the results show our films have a bandgap of about 1.4 eV, which is bigger than the previously reported values. The Raman spectra showed two Raman active modes of E₂ and A₁(LO) of the wurtzite InN for all InN samples. For InN, we found out that addition of H₂ plasma with N₂ plasma resulted in InN films with poor crystalline quality showing high level of impurities with significant voids in the films, resulting in low-density films with poor adhesion properties. Our results indicate that higher N₂ plasma exposure time is necessary to obtain InN films with minimum amount of carbon incorporation. The presence of C impurities was observed in all films grown with N₂ plasma only and suggests that the N₂ plasma without H₂ is not efficient in terms of effectively removing the ligands of the chemisorbed organometallic trimethyl-metal precursors.

Among the advanced electronic devices, transparent flexible organic electronic devices with rapid development are the most promising technologies to customers and industries. However, thin-film encapsulation (TFE) and the transparent oxide conductive (TOC) of organic devices still remain a big challenge, because of the difficulty in low temperature and low plasma power fabricating process. Atomic layer deposition (ALD) is increasingly used in the field of organic electronics. However, the deposition of ALD outside the temperature window still cannot be stably implemented. In this study, transient steric hindrance caused by gas-phase molecules at low-temperature (80℃) was investigated. In order to mitigate the effect of this transient hindrance, a process of consecutive short-pulses was adopted in fabricating TFE and TOC. Overall, the proposed idea would help low-temperature ALD for organic electronics become mature and be widely promoted.