AVS2018 Session EM-ThP: Electronic Materials and Photonics Division Poster Session

Tellurium oxide (TeO₂) based glasses are widely used in many applications such as fibre optic, waveguide devices and Raman gain, and are now being considered for use in optical waveguide amplifiers. These materials exhibit excellent transmission in the visible and near IR wavelength range (up to 2.0 µm), low phonon energy, and high rare-earth solubility. Siloxane polymer materials on the other hand, have remarkable thermal, mechanical and optical properties and allow the fabrication of low-loss optical waveguides directly on printed circuit boards. In recent years, various low-cost optical backplanes have been demonstrated using this technology.However, all polymer optical circuits used in such applications are currently passive, requiring therefore amplification of the data signals in the electrical domain to extend the transmission distance beyond their attenuation limit. The combination of the TeO₂ and siloxane technologies can enable the formation of low-cost optical waveguide amplifiers that can be deployed in board-level communications. However, there are significant technical challenges associated with the integration of these two dissimilar materials mainly due to the difference in their thermal expansion coefficients.

In this paper therefore, we propose a new approach for incorporating erbium (Er³⁺)-doped tellurium-oxide glass nanoparticles into siloxane polymer thin films using femtosecond pulsed laser deposition (fs-PLD). . Erbium-doped sodium zinc tellurite (Er-TZN) nanoparticles (NPs) are embedded into siloxane polymer films spin-coated on silica substrates using fs-PLD at low temperatures (100 °C) and under two different pulse energies. The surface morphology, and the compositional and structural characteristics of the samples fabricated are evaluated using scanning and transmission electron microscopy (SEM and TEM), TEM- energy-dispersive X-ray spectrometry, X-ray diffraction (XRD), and Raman spectroscopy. Additionally, photoluminescence (PL) and lifetime measurements are carried out at 1534 nm at room temperature under a 980 nm laser diode excitation. The SEM results show that average particle size of the Er-TZN NPs incorporated into polymer layer decreases with decreasing fs-pulse laser energy, while the PL measurements reveal a full-width at half-maximum (FWHM) of about 39 nm. The respective FHWM value obtained from the bulk glass is 51 nm. The obtained luminescence lifetime of the samples is in the range 3.52 to 4.18 ms, which is slightly lower than the value obtained from the bulk glass target of 4.37 ms.

Devices based on III-V compounds for optical and electronic applications have various heterojunctions designed in their structures, such as, two-dimensional electron gas (2DEG), dielectric/semiconductor and metal/semiconductor interfaces. Different to Si devices, most of these heterojunctions are formed by epitaxial growth or ex-situ deposition, therefore, the properties of interfaces in the devices are very critical to device performance, and also impacted by many hardly controlled factors. We noticed that interfacial mixture at AlGaN/GaN interface leads its band offset uncertain if using MOCVD to prepare the junction, and also defects/impurities at metal/GaN interfaces modify the contact properties obviously. However, to investigate thoroughly the intrinsic properties and structures of interfaces in III-V compound devices is limited so far by the uncontrollable environments during experiments. To avoid unfavorable effects of contaminations on surfaces or interfaces introduced from atmospheric contacts, Ultra-high vacuum ambits are normally used to prepare and analyze sensitive materials and devices. Currently an ultra-high-vacuum (UHV) connected research facility (Nano-X) is being constructed at the SuZhou Institute of Nano-tech and Nano-bionics, CAS, by which it is possible to integrate all major functions of tools for material growth, device processing, and characterization under one UHV environment. With this facility, 30+ tools connected to a 100 meter UHV tube, wafers/samples could be transferred from one to another without exposure to air. Therefore, we were able to explore the intrinsic properties of heterojunctions during the formation of interfaces, as well as improve the device performance through interface controlling and modification. It has been found that surface states have large effects on the measurements of the band offsets at the GaN/AlGaN interfaces, which could be prepared by Molecular Beam Epitaxy (MBE), and precisely determined within an uncertainty of 45meV by combining in-situ Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS) and theoretical calculations of the total DOS in the valence bands of GaN and AlGaN.

Thermal oxidized of Ge films under high pressure have been investigated to examine the possibility for the gate oxide. Ge oxides were grown either Ge wafers or Ge epitaxial films grown on Si wafer. The temperature range was from 450 °C to 550 °C, and three different pressures such 10, 30, and 50 atm were chosen for a high pressure dry oxidation. The physical property of GeO₂ films were analyzed using the transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy(XPS). Additionally, Au/GeO₂/Ge MOS capacitors were characterized using C-V and I-V measurement. The hysteresis behavior in C-V characteristics and the interface trap density (D_it) are significantly reduced through the high pressure oxidation. Consequently, the properties of both GeO₂ film and GeO₂/Ge interface are successfully improved by suppressing GeO volatilization utilizing high pressure.

Junctionless-FET (JL-FET) devices were fabricated on SOI substrate using NH₄OH solution silicon etching as means to thin the channel substrate. The devices gate dielectric was silicon oxynitride grown using O₂/N₂ ECR (Electron-Cyclotron-Resonance) plasma, and its gate metal was TiN defined through lift-off and deposited using reactive sputtering. The electric contacts were fabricated with sputtered aluminum defined through lift-off and annealed on a conventional oven. Samples were characterized during the fabrication processes using optical microscopy and scanning electron microscopy (SEM). The device electrical performance was measured using a probestation and then cross-section SEM images were extracted using Ga+ Focused Ion Beam milling.

The final channel thickness was 65nm measured in the cross-section images, which also showed the angled sidewalls characteristic of the NH₄OH solution wet etching. The channel dopant concentration was estimated at approximately 10¹⁷ atoms/cm³ through Pseudo-MOS electrical measurements, this was the doping concentration that was planned according to the simulation steps to ensure transistor behavior by sacrificing electrical contact quality. Electrical measurements showed transistor behavior and low leakage currents, despite the negative threshold voltage and poor electrical contacts, which distorted the I-V measurements due to their Schottky-like behavior. These results are as expected due to the measured channel thickness and the estimated channel dopant concentration and point favorably towards the silicon etching in NH₄OH solution being a viable technique to fabricate JL-FET devices.

In the future, Atomic Force Microscopy (AFM) measurements will be used to measure the surface roughness after the silicon wet etching in NH₄OH solution. With a more accurate etching rate, new samples will be fabricated with thinner channel thicknesses and higher dopant concentration. The enhanced fabrication process is expected to result in JL-FET devices that rival the performance of state-of-the-art MOSFET devices.

References

[1] J.P. Colinge et al., “Nanowire transistors without junctions”, Nature Nanotechnol. 5, p. 225-229 (2010).

[2] S. Migita, Y. Morita, T. Matsukawa, M. Masahara, and H. Ota, “Experimental Demonstration of Ultrashort-Channel (3 nm) Junctionless FETs Utilizing Atomically Sharp V-Grooves on SOI”, IEEE Trans. on Nanotechnol., vol. 13, no. 2, p. 208-215 (2014).

[2] A. R. Silva, J. Miyoshi, J. A. Diniz, I. Doi and J. Godoy, “The Surface Texturing of Monocrystalline Silicon with NH4OH and Ion Implantation for Applications in Solar Cells Compatible with CMOS Technology.”, Energy Procedia, v. 44, p. 132-137 (2014).

We report the growth of nanoscale multilayered thermoelectric thin films and fabrication of integrated thermoelectric devices for high-efficiency energy conversion and solid-state cooling. Nanoscale multilayered thin films such as Sb/Sb₂Te₃ and Te/Bi₂Te₃ thin films were grown using the e-beam evaporation. Integrated thermoelectric devices were fabricated with the nanoscale multilayered thin films using the clean room-based microfabrication techniques such as UV lithography. X-ray diffraction and reflection and high-resolution tunneling electron micrograph (HR-TEM) were used to analyzed the e-beam-grown nanoscale multilayered thin films. SEM was used to image and analyze the fabricated devices. The thermoelectric characteristics of the fabricated devices were measured and analyzed, and highly-efficient thermoelectric thin-film materials and integrated devices will be demonstrated and reported.

Photonic structures in biological systems typically exhibit an appreciable degree of disorder within their periodic structures. Such disorder contributes to unique optical properties but has not been fully understood. Towards the goal of improving this understanding, we have investigated Langmuir-Blodgett (LB) assembly of silica microspheres to controllably introduce randomness to photonic structures. We theoretically modeled the LB assembly process and determined a condition for surface pressure and substrate pulling speed that results in maximum structural order. For each surface pressure, there is an optimum pulling speed, and vice versa. Photonic structures fabricated at various conditions were characterized by scanning electron microscopy and light scattering analysis, which confirms the modeled optimum condition. However, along the trajectory defined by the optimum condition, the structural order decreases moderately as the pulling speed increases. This moderate decrease in structural order would be useful for controlled introduction of randomness into the periodic structures. Departing from the trajectory, our experiment reveals that a small change in pulling speed at a given surface pressure can significantly disrupt the structural order. According to these observations, mechanism of forming structural order in LB assembly is proposed. Additionally we also find that, for multilayer LB assembly at a fixed pulling speed, the surface pressure should increase as the number of layers increases to achieve maximum structural order. In summary, this work quantitatively presents the optimum trajectories for n^th layer assembly relating surface pressure and pulling speed.

In contrast to manipulating magnetization with applied current, using an applied electric field can significantly reduce the required energy and result in less heat generation, leading to increased ene rgy density. This can be accomplished using the voltage-controlled magnetic anisotropy (VCMA) effect, which forms the basis of next-generation magnetoelectric MRAM devices. Specifically, applying an electric field across a CoFeB/MgO interface can decrease the perpendicular magnetic anisotropy field as a result of the altered electron density at the interface, thus destabilizing the magnetization state and allowing for its efficient and deterministic reorientation with a small applied magnetic field. This op eration principle stands in contrast to that of STT-RAM, which uses upwards of 100 fJ to write a single bit (300,000 times more energy than the actual energy barrier to switching).

Previous research on CoFeB/oxide interfaces has shown that increasing the dielectric constant of the oxide layer also increases the sensitivity of the interfacial magnetic anisotropy energy to an applied electric field (Kita et al., 2012). Our previous work involving MgO/PZT/MgO composite tunneling barriers showed a 40% increase in the VCMA effect upon addition of PZT to the tunneling barrier. However, the ferroelectric order of PZT is very weak at such small dimensions, and the leakage current and high annealing temperatures required of PZT prevent this technology from being industrially relevant. On the other hand, the orthorhombic phase of HfO₂ has been shown to possess desirable ferroelectric properties even in ultrathin films. In addition, ferroelectric HfO₂ boasts superior compatibility with CMOS technology as well as desirable electrical properties for device integration.

In this work, a method for depositing orthorhombic phase HfO₂ (FE-HfO₂)-based thin films via a radical-enhanced atomic layer deposition process is described. These ferroelectric thin films were subsequently incorporated into the tunneling junctions of CoFeB/MgO-based magnetic tunnel junction in an effort to enhance the VCMA effect and introduce ferroelectric functionality into magnetoelectric random-access memory devices.

Al_0.85Ga_0.15N/Al_0.7Ga_0.3N high electron mobility transistors (HEMT) underwent DC characterization across the temperature ranging from room temperature to 500°C. Due to a high Schottky barrier height and low gate leakage current achieved on Al0.85Ga0.15N barrier layer, drain current modulation up to a gate voltage of 10 V was demonstrated at 500°C. The high aluminum content in these devices enables stability at high temperature due to the ultra-wide bandgap of ~ 5.7 eV. Conventional low Al content HEMT devices have previously shown improved elevated temperature operation as compared to their Si or GaAs counterparts, but are unable to operate under extreme temperature tested herein and suffer from high gate leakage current with heating. The drain current on/off ratio of 1011 were obtained with low gate leakage currents. The drain current degraded by ~50% from room temp to 500°C. The subthreshold slope of 80 mV/dec and 230 mV/dec were obtained at room temperature and 500°C, respectively. From the subthreshold slopes, trap densities were calculated to be 2.3 × 10¹¹ cm^-2 at room temperature and 3.3 × 10¹² cm^-2 at 500°C. These novel devices show great promise for application in the power, space, and defense industry where extreme performance is necessary.

In the past ten years, there have been constant attempts to develop high efficient thin film solar cells, which are cost-effective, environmentally benign, and reliable. The most strongest candidate of emerging alternatives is Cu₂ZnSn(S,Se)₄ (or kesterite) solar cells, herein CZT(S,Se). With abundant elements in earth’s crust, CZT(S,Se) shows high absorptions coefficient (>10⁴ cm^-1) and a tunable direct band gap energy, ranging from 1 to 1.4 eV, which makes it an ideal platform for future renewable energy devices . However, the efficiency improvement and understanding of emerging CZT(S,Se) is still in early stage compared to the counterparts such as Cu(In,Ga)Se₂ (CIGS) and CdTe. In recent progress, Germanium (Ge) incorporation into CZT(S,Se) solar cells has received extensive attention to deepen the understanding of this types of devices as Ge-alloyed CZT(S,Se) solar cells have demonstrated improvements in device performance. However, several challenges still remain such as a large Voc-deficit, sever heterojunction interface recombination, and a Schottky-type back contact barrier.

The presence of Schottky barrier near back contact limits the hole movement which influences the current-voltage characteristics and deteriorates the Voc-deficit as well. To investigate the impact of this back contact barrier height, we fabricated and characterized a set of CZTSe solar cells by utilizing DC magnetron sputtering by applying ultra thin Ge nanolayers. We investigated the back contact interface between CZTSe/MoSe₂ and Mo metal contact in an effort to improve a back contact barrier. By incorporating nanoscale Ge bi-layers below and below the absorber, a barrier height is considerably improved. The results indicate that nanoscale Ge bi-layers improves the back contact barrier height by 27 % as compared to CZTSe:Ge monolayer devices (see a supporting document device A). The back contact improvement is possibly caused underlying Ge nanolayer (<2.5 nm) between the absorber and Mo metal contact. The improvement of the efficiency loss caused by the series resistance component is reduced by 50%, which is attributed to the improved Schottky-type back contact barrier. This allows the improved efficiency up to 8.3% by incorporating nanoscale CZTSe: Ge bi-layers.

Gallium nitride-based high electron mobility transistors (HEMTs) utilize a variety of substrates, including those that are optically transparent in the visible and ultraviolet wavelength spectrum such as sapphire and silicon carbide(SiC). Compared to silicon substrates, SiC and sapphire substrates can exhibit a distinct set of photolithography patterning challenges such as backscattering (from the underlying chuck and other areas of the substrate) which can influence the critical dimension (CDs), pattern integrity, and pattern resolution. In this study, computational photolithography and rigorous coupled wave guide analysis are used to calculate the optical reflectivity, the transmission and absorption of a multilayered stack comprised of i-line photoresist and an antireflective coating on sapphire substrates with Al_xGa_1-xN epitaxial layers (with x ranging from 0 to 1). These simulations are used to target optimal resist and arc thickness, and exposure energy needed to reach the target feature with high fidelity. As a proof of concept, fully resolved patterns down to 0.5 µm are experimentally obtained on sapphire substrates using conventional contact photolithography driven by simulated for optimal conditions. Due to the lack of experimental information on photolithography on optically transparent substrates with ultra-wide bandgap heterostructures, this method can provide relevant insight into determining optimal process window for optically transparent substrates.

Electron field emission, a phenomenon that electrons emitted from solid surface under strong electrical field, is the basis of cold cathode, which has important applications in modern high-resolution electron microscopy, flat panel e-beam lithography and X-ray source. ZnO nanowires have been regarded as one of the most important cold cathode materials due to their excellent field emission properties, chemical stability and simple synthesis method. To further improve the device performance, the understanding of their electron field emission mechanism is significant. Although many efforts have been devoted to investigating the influence of resistance, geometry structure, doping and back contact with substrate on the field emission of ZnO nanowires, their underlying mechanism is still remained unclear. One important reason causing this difficulty is that many of these works were based on the measurement of ZnO nanowires film, which should induce average effect on the results. Therefore, in-situ field emission measurement of single ZnO nanowire is necessary to obtain its intrinsic field emission properties and physical mechanism.

The electronics that are used in spacecraft are subject to space radiation hazards. The space radiation environment includes trapped electrons and protons of the Van Allen radiation belts, and non-trapped transient solar and galactic cosmic rays and solar flare particles. The protons with the cut-off energy of 100 keV are present above the upstream of the Earth’s bow shock. Since gallium nitride and its alloys are proven to be relatively radiation tolerant, these materials are considered as promising candidates for space electronic applications. Therefore, it is consequential to study the effect of 100 keV protons on the AlGaN/GaN HEMTs if the devices are to be used for space applications. In this research, the effect of 100 keV protons with the fluences 1×10¹⁰, 1×10¹², and 1×10¹⁴ cm^-2 on materials/device characteristics of AlGaN/GaN HEMTs constructed on Si wafers were studied by means of optical and electrical characterization. The electrical characteristics of the devices were analyzed by using conventional transistor I-V and C-V measurements in order to relate the material’s fundamental properties to the device performance. The slight degradation on the electrical characteristics and the shift in the threshold voltage was observed in the irradiated samples. The crystal quality of the epilayer was examined via micro-Raman spectroscopy and no substantial degradation in the crystal quality was observed.The possible introduction and/or the alternation of the defects were probed using the photoluminescence (PL) spectroscopy and spectroscopic photocurrent-voltage techniques. The surface morphology of the samples was studied by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and a slight increase in the surface roughness was observed. The surface analysis was performed using X-ray photoelectron spectroscopy, and the surface elemental composition was not altered after irradiation. It is concluded that the crystal quality of the AlGaN/GaN HEMT layers and the electrical characteristics of the AlGaN/GaN HEMTs were not severely degraded in spite of the exposure to a high fluence of protons with the energy 100 keV.

Gallium oxide (Ga2O3) has recently become recently become a material of great interest due as it is a stable, wide bandgap oxide which is capable of being used as a transparent conducting oxide or dielectric layer in the next generation of electronic devices [2]. While the majority of work has focused on single crystal materials, few results exist regarding the growth and nucleation of polycrystalline Ga2O3 materials [2,3].

The present study utilizes spectroscopic ellipsometery, x-ray diffraction, and four-point resistivity measurements to investigate how the physical properties of Ga2O3 thin-films may be altered by changing the partial pressure of O2 during reactive RF sputter deposition. The results will yield important information regarding how material properties are related to deposition conditions using an industrially applicable fabrication process.

[1] “Guest Editorial: The dawn of gallium oxide microelectronics“,

Masataka Higashiwaki and Gregg H. Jessen, Appl. Phys. Lett. 112, 060401 (2018)

[2] “Chemical vapour deposition and characterization of gallium oxide thin films”,

G.A .Battiston, R. Gerbasi, M. Porchia, R. Bertoncello, and F. Caccavale, Thin Solid Films 279, 115-118, (1996)

[3] “Structure, Morphology, and Optical Properties of Amorphous and Nanocrystalline Gallium Oxide Thin Films”, S.S. Kumar et al., J. Phys. Chem. C, 2013, 117 (8), pp 4194–4200