United States

Abstract Body: A molecular beam epitaxy (MBE) approach to selective area epitaxy (SAE) of III-V semiconductors could enable the seamless integration of metals, dielectrics, and high-quality crystalline semiconductors. This technique could advance novel optoelectronic structures, such as high-contrast photonics, site-controlled quantum emitters, stacked pixel detectors, and photonic integrated circuits. Using patterned dielectrics and metals to define crystal seeding regions, SAE can be performed to embed microstructures into crystalline semiconductors1. While SAE by metal organic chemical vapor deposition has been widely successful due to its high material deposition selectivity, a SAE MBE method could enable further advances through its high layer precision and access to non-equilibrium growth conditions1,2. SAE is difficult to achieve with conventional (continuous) MBE growth due to III-V polycrystalline nucleation on the mask, even at growth temperatures as high as 700°C. As a result, Allegretti et al. developed periodic supply epitaxy (PSE), a method to inhibit polycrystalline deposition by cycling group III deposition under a constant group V flux2,3. SAE with PSE was achieved using high growth temperatures and low growth rates3-4. This mitigates nucleation of poly-GaAs through a combination of increased Ga adatom diffusion to seeding windows and desorption of remaining Ga adatoms3-4, enabling high-quality planar encapsulation of the mask1. While an all-MBE approach has been demonstrated to achieve selective growth of features ~2µm in width and ~300nm in height, applications requiring larger features are limited by nucleation of polycrystalline semiconductor on the amorphous surface due to poor deposition selectivity and low adatom surface diffusion1,3. We present experimental results of GaAs growth on SiO2 films that demonstrate decreased poly-GaAs formation at higher temperatures and lower As4/Ga flux ratios, enabling SAE of larger patterned features. However, this improvement in selectivity has yet to be explained quantitatively. In order to expand the applicability of all-MBE SAE, we developed a PSE model that (1) identifies selective growth regimes to clarify their underlying mechanisms and (2) allows optimized SAE growth conditions to be determined for a given SiO2 pattern. A numerical 1D model was developed to describe PSE selectivity by fitting adsorption, desorption, and diffusion constants to GaAs growth on SiO2 films at 600°C with a growth rate of 0.7 Å/s and 65x As4/Ga ratio5-8. Two growth regimes have been identified: desorption- and diffusion-limited. The desorption-limited regime depends only on thermal desorption from the mask surface to achieve selectivity and as a result, is feature-independent. This model has identified the desorption-based selectivity limit to be an 18.7% PSE duty cycle and was demonstrated by observing no polycrystal formation after growth of 100nm GaAs with 10% PSE duty cycle on an SiO2 film. The remainder of the selective growth space is diffusion-limited and sufficient III adatom diffusion off the mask is required to achieve selectivity. Experiments to fully investigate the influence of growth temperature and V/III ratio on selectivity are underway and will be reported at the conference. [1] D. J. Ironside et al., ACS J. Crystal Growth and Design (2019). [2] A. M. Skipper et al., 2019 MRS Electronic Materials Conf. [3] F. E. Allegretti et al., Journal of Crystal Growth (1995). [4] S.C. Lee et al. Journal of Applied Physics (2002). [5] S. Shankar. Diffusion in 1D and 2D, MATLAB. Retrieved April 11, 2020. [6] Aseev et al. Nano. Lett. (2019). [7] S. C. Lee et al. Cryst. Growth Des. 2016. [8] E.M. Gibson et al. Appl. Phys. Lett. (1990). This research was partly done at the Texas Nanofabrication Facility supported by NSF grant NNCI-2025227 and was supported by Lockheed Martin and NSF through the UT CDCM: an NSF MRSEC under Cooperative Agreement No. DMR-1720595, as well as CCF-1838435 and DMR-1839175.

EMC 2021

Experimentally-Calibrated Modeling of Molecular Beam Epitaxy Selective Area Regrowth

iii-v semiconductor

selective area epitaxy

molecular beam epitaxy

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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technical paper

Abstract Body: III-Nitride (AlInGaN) avalanche photodiodes (APDs) have merits such as high breakdown field, high electron drift velocity, high thermal conductivity, and low dark current. The III-nitride material bandgap is tunable from 0.7 (1772nm) to 6.2eV (200nm), offering good UV sensitivity in both visible blind and solar blind UV regions. The Geiger-mode operation of III-Nitride APDs have numerous applications in UV single-photon detection, such as quantum-key distribution, optical time-domain reflectometry, positron-emission tomography, biomedical research, and UV communications. In this work, we report low-temperature Geiger mode measurements of GaN p-i-n APDs by using a thermoelectric cooling system. The GaN p-i-n APD structures were grown on c-plane hydride-vapor-phase-epitaxy (HVPE) free-standing (FS) GaN or ammonothermal bulk GaN. The epitaxially grown structure consists of a 0.6 mm unintentionally doped GaN, followed by a 2.3mm thick Si-doped GaN n-layer with a free-electron concentration of 4×1018 cm-3. The i-layer of 280nm thickness is grown on top of the n-layer, with a background Si doping of 2×1016 cm-3, targeting breakdown voltage of 96V. The p-layer stack consists of two layers of 100nm Mg-doped GaN and 15nm of highly Mg-doped GaN as the contact layer. The Mg-doped p-layer has a free-hole concentration of 1×1018 cm-3, and the highly Mg-doped p-layer has Mg concentration of 1×1020 cm-3. The fabrication of APD was performed in the following sequence: 1) mesa etching, 2) n-type metal deposition, 3) p-type metal deposition, 4) passivation and via-hole, and 5) metal interconnect deposition. A gated quenching circuit was used in the Geiger-mode measurement. In detail, a noninverting amplifier with gain of 2 and a transimpedance amplifier are inserted before and after the GaN APD in order to isolate the GaN APD and prevent reflection. An oscilloscope measures and collects the avalanche signals. A computer is utilized to perform signal data processing. For spectral response evaluation, the APD device is illuminated with the UV light from a UV LED with the peak wavelength of 375nm. The UV light is split by a bifurcated fiber: one end is fed to GaN APD, and the other end goes to an attenuator before being fed to a silicon photon counter for reference. For the low-temperature control, the thermoelectric cooler is installed directly under the GaN APD. The thermoelectric cooler is installed in an aluminum cage which serves as water-cooled heat sink. To prevent icing, the aluminum cage is sealed and filled with nitrogen. The Geiger-mode setup is installed in a optically isolated dark box. The method of measurement employed is the Δt method based on Poisson statistics. From the collected avalanche signal, the computer determines time interval between the rising edge of each pulse and its corresponding avalanche signal for 10,000 pulse events, then processes these time interval data into one histogram. Finally, by fitting the exponential function to the histogram, the dark count rate R is obtained. With this low-temperature Geiger-mode measurement system, the breakdown voltage was measured at various temperature from -10°C to 10°C with an increment step of 5°C. The breakdown voltage is proportional to the temperature, monotonically varying from 96.9V to 97.25V when the temperature increases from -10°C to 10°C. From this data, we obtained a temperature coefficient of the breakdown voltage of 0.0177±0.0005V/K. Based on the breakdown voltage vs. temperature, the dark count rate of various temperatures at 1V overvoltage above breakdown was measured. The dark count rate decreases as the temperature decreases from 19MHz to 11MHz when temperature decreases from 10°C to -10°C. By fitting Arrhenius equation, the obtained activation energy is . More detailed description will be presented in the conference. This work is funded by Support from ARO, DoE, NASA, Georgia Tech IEN, and the NSF National Nanotechnology Coordinated Infrastructure (NNCI)

Low Temperature Geiger-Mode Measurement of Gallium Nitride p-i-n Avalanche Photodiode

Abstract Body: Energy-saving photodetectors have recently attracted intensive research interest to meet the great demands of future photonic systems. Particularly, according to the large photocurrent, high sensitivity, environment-sensitivity, and lithography-free features, self-powered photoelectrochemical-type photodetectors (PEC PDs) have attracted tremendous attention in the optoelectronics community. However, owing to their poor crystal quality and low carrier separation efficiency of chemically synthesized semiconductor materials, PEC PDs based on such materials demonstrate slow response and low responsivity. In our work, we construct PEC PD using defect-free AlGaN nanowires photoelectrode which synthesized through molecular beam epitaxy (MBE)[1][2]. After decorating AlGaN nanowires with noble metal species (Pt or Ru), our PEC PDs feature the combined benefits of effective carrier separation and fast interfacial kinetics, resulting in superior performance compared to previous reported solar-blind PEC PDs (Responsivity of 45 mA/W and response/recovery time of 47/20 ms for Pt decorated p-type AlGaN nanowires; Responsivity of 48.8 mA/W and response/recovery time of 83/19 ms for Ru decorated n-type AlGaN nanowires;). Besides, according to the attractive surrounding sensitivity of PEC PDs, their photoresponse behaviors can be effectively modulated by varying the applied bias potential, light intensity, and the concentration of the electrolyte further promotes its adaptability in different working conditions. Our work suggests that embracement of defect-free wide-bandgap semiconductor nanowires with appropriate surface decoration offers an unprecedented opportunity for designing future energy-efficient and large-scale optoelectronic systems on the silicon platform. Reference: [1] D. Wang, X. Liu, S. Fang, C. Huang, Y. Kang, H. Yu, Z. Liu, H. Zhang, R. Long, Y. Xiong, Y. Lin, Y. Yue, B. Ge, T. K. Ng, B. S. Ooi, Z.Mi, J. H. He, and H. Sun “Pt/AlGaN Nanoarchitecture: Toward High Responsivity, Self-Powered Ultraviolet-Sensitive Photodetection,” Nano Lett., 2021, 21, 1, 120–129. [2] D. Wang, C. Huang, X. Liu, H. Zhang, H. Yu, S. Fang, B. S. Ooi, Z. Mi, J. H. He, and H. Sun, “Highly Uniform, Self-Assembled AlGaN Nanowires for Self-Powered Solar-Blind Photodetector with Fast-Response Speed and High Responsivity,” Adv. Optical Mater., 2020:202000893.

AlGaN-Based Nanowires for Self-Powered Ultraviolet Photodetectors with High Responsivity

Abstract Body: Gallium nitride (GaN) with bandgap energy of 3.4 eV with a high breakdown electric field (~3.3 MV/cm) is useful for ultraviolet (UV) optical detectors. Since GaN avalanche photodiodes (APDs) have a high sensitivity and capability of operation in the visible-blind region with high optical gain with low dark current, UV-APD arrays are considerably well-suited for many applications in UV imaging systems, astronomy, military systems, and bio-aerosol detection to improve the collection efficiency and sensitivity for low-level light detection. Achieving the high crystalline quality of epitaxial layer structures with reduced-defect densities as well as optimizing the fabrication processes will significantly improve GaN APD arrays to achieve a high pixel yield and uniform breakdown voltage with low leakage current. Evaluation of these 6x6 arrays will help to confirm the uniformity of the epitaxial materials and device processing as well. In this work, visible-blind front-illuminated GaN p-i-n APDs were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® n-GaN substrates having a low etch pit density (EPD) less than 5 × 104 [cm-2] and processed into 6x6 APD arrays. The MOCVD growth was performed in a close-coupled-showerhead AIXTRON 6x2” MOCVD system and employed trimethylgallium and NH3 as primary precursors and SiH4, and Cp2Mg as dopants. X-ray diffraction (XRD) was used for the characterization of the crystallographic quality of the APD structure. The full widths at half maximum of the rocking curve derived from (0.02) and (1.02) diffraction exhibit values of 73 and 41 arc-sec, respectively, primarily determined by the properties of the GaN substrate. Hall-effect characterization was used for the measurement of the electrical properties of the doped layers including the free-carrier concentration, mobility, and resistivity. The epitaxial layers grown on the GaN substrates include a 1.0 μm thick n-GaN:Si layer (n~7×1018 cm-3), 0.2 μm thick u-GaN undoped layer, 0.1 μm thick p-GaN:Mg layer (p~6x1017 cm-3) and 0.02 μm thick p++-GaN:Mg layer for a target breakdown voltage of 80 V. The surface morphology of the GaN p-i-n UV-APD epitaxial structures were studied by atomic force microscopy (AFM). The corresponding root-mean-square (RMS) values for scanning area of 5×5 μm2 for GaN/sapphire u-GaN template and n-GaN substrate are 0.15 nm and 0.07 nm respectively for the typical case. To confirm the doping profile of the layers, secondary ion mass spectrometry (SIMS) measurements were performed. The as-grown wafers were processed using ion-implantation isolation into 6x6 APD arrays having 60 µm-dia. pixels. The reverse-biased J-V characteristics of the GaN APD arrays under dark and 355 nm illumination conditions have been measured at 300K. The dark current density is 10-9 A/cm2 at reverse bias up to -20 V and breakdown voltage of 80 ± 1 V for all 36 devices which confirms a high uniformity of growth and fabrication processes. The maximum avalanche gain reached 3 × 105 at the breakdown. The details of growth, characterization, and fabrication processes as well as further device data will be discussed in the presentation. Work supported in part by US Army Research Office under W911NF-15-1-0489, administered by M. Gerhold, by the US Department of Energy under DESC0019133, and NASA under 80NSSC18C0093. This work was performed in part in the facilities of the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-154217). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either express or implied, of DoE, ARO, or the U.S. Government.

Growth and Characterization of p-i-n GaN UV 6 × 6 Avalanche Photodiodes Arrays with Large Detection Area

Abstract Body: RHEED spectra taking during molecular beam epitaxy of GaN and related alloys on sapphire substrates were analyzed using a convolutional neural network. An existing image-classifying convolution neural network (CNN)— AlexNet— was employed for the analysis. The RHEED images during various stages of growth were manually categorized to train the CNN. The CNN only works on images, so video frames of RHEED during growth had to first be extracted from the video. These images were then separated into a training set and a validation set for use in training the network and in validating it. Mobility, deposition temperature, X-ray diffraction, atomic force microscopy, and Hall measurements were taken on the resulting films. After training on the manually categorized images and videos, the CNN achieved a proper categorization rate of 97% on the validation data and employing mobility and various growth stages as categorical outputs. Other machine learning techniques were applied as well to analyze RHEED spectra, such as principal components analysis and k-means clustering. Initial results from growth of ternary (AlxIn1-xN) and quaternary (AlGaInN) alloys will also be presented at the conference.

Machine Learning for Evaluation of RHEED Spectra on III-Nitride Films Grown Using Molecular Beam Epitaxy

Abstract Body: III-Nitride materials have long been promising candidates for the development of infrared optoelectronic devices. However, design of multiple quantum well (MQW) structures on conventional c-plane (0001)-oriented GaN substrates is hindered by the spontaneous and piezoelectric polarization fields associated with lack of inversion symmetry. Low-defect density free-standing m-plane (10-10) GaN substrates have enabled the growth of non-polar MQW structures without strong built-in polarization fields. We have previously demonstrated the potential of m-plane Al0.16Ga0.84N/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy (PAMBE) for terahertz devices [1]. To support higher energy near-infrared optical transitions, though, a larger conduction band offset (CBO) is required. This can be achieved through higher Al composition in the barriers. However, m-plane AlGaN layers with aluminum fractions above 0.2 grown under conventional gallium-rich PAMBE conditions exhibit severe material inhomogeneities [2]. Recently, we demonstrated that growth of m-plane AlGaN by indium surfactant assisted epitaxy (ISAE) at low temperature significantly reduces composition inhomogeneities [3]. Here, we present an in-depth comparison of the PAMBE growth of m-plane AlGaN with and without indium surfactant at 565°C. We investigated the compositions and growth rates of low temperature m-plane AlGaN layers grown by conventional gallium-rich epitaxy and by ISAE across a range of aluminum fluxes. It is expected that under nitrogen-limited growth conditions the growth rate would be independent of metal flux. We find the opposite to be true: as aluminum flux increases, the growth rate of AlGaN decreases rapidly. We attribute this phenomenon to nitrogen loss during PAMBE of m-plane AlGaN that increases super-linearly with increasing aluminum flux. These films also show a super-linear increase in aluminum content with increasing aluminum flux. Transmission Electron Microscopy (TEM) images indicate that these effects are correlated with high aluminum containing defects within the material. Our results indicate that ISAE mitigates this effect almost entirely. Films grown by ISAE with identical aluminum fluxes to the gallium-rich layers do not exhibit a significant decrease in growth rate with increasing aluminum flux, and their aluminum compositions increase linearly with aluminum flux. TEM images show the ISAE layers are thicker, and exhibit a higher degree of material uniformity up to 0.30 aluminum metal fraction. X-ray diffraction (XRD) measurements suggest minimal indium incorporation in the ISAE AlGaN layers. The CBO of MQWs can be further increased by using InGaN as the quantum well (QW) material. We have recently demonstrated the growth of high-quality m-plane InGaN with up to 0.16 indium metal fraction [4], enabling the growth of In0.055Al0.19Ga0.755N/In0.16Ga0.84N MQW structures using the ISAE technique [3]. We will also present results of structural and optical characterization of m-plane (In)Al0.3Ga0.7N/In0.16Ga0.84N MQW structures. Due to the anisotropic nature of strain in m-plane structures, AlGaN/InGaN structures cannot be perfectly strain-balanced. We will discuss the method used to control strain accumulation along the (0001) and (11-20) directions below the level for defect formation in both the InGaN QWs and AlGaN barriers. [1] C. Edmunds et al., Appl. Phys. Lett. 105, 021109 (2014). [2] M. Shirazi-HD et al., J. Appl. Phys. 123, 161581 (2018). [3] B. Dzuba et al., J. Appl. Phys. 128, 115701 (2020). [4] A. Senichev et al., APL Mater. 7, 121109 (2019).

Anomalous Super-Linear Growth Rate Reduction with Increasing Aluminum Flux in M-Plane AlGaN Grown by PAMBE

Abstract Body: A parametric analysis of growth conditions on the structural properties of N-Polar InGaN films was studied. Three series of N-polar InGaN films were grown on bulk GaN substrates by plasma-assisted molecular beam epitaxy (PAMBE) to study the effect of substrate temperature, Ga, and In fluxes. For all series, the surface morphology was measured by atomic force microscopy (AFM), and the composition and strain were determined by high resolution X-ray diffraction (HRXRD) and XRD reciprocal space mapping (RSM). All the samples in this study were 350 nm thick. First, a series of samples was grown at temperatures varying from 620 oC to 690 oC. The Ga flux for all samples in this series was kept at 3.8E-7 Torr and the In flux was increased by temperature to ensure metal-rich growth conditions. The surface morphology improved significantly by increasing the temperature, and the indium content was reduced as expected. The samples grown at 670 oC and 690 oC showed surprisingly smooth surface morphologies with surface roughness <0.7 nm which has never been reported before on such thick InGaN films. An In content of 16.5% and 13.6% was measured on samples grown at 670 oC and 690 oC, respectively. These temperatures are significantly higher than growth temperatures (<600 oC) typically used for the growth of InGaN films on Ga-polar GaN. In the next step, samples were grown with In fluxes of 3E-7 Torr and 4E-7 Torr. The Ga flux was kept at 3.8E-7 Torr and both samples were grown at a substrate temperature of 670 oC. Both samples showed step flow growth and low surface roughness despite the large difference in In flux. Nonetheless, increasing the In flux from 3E-7 Torr to 4E-7 Torr resulted in an improved surface morphology, and increased In composition and strain relaxation. These results indicate that the In flux can be tuned within an ample window to obtain specific sample properties. For the last series, three samples were grown at 660 oC and with In flux of 4E-7 Torr, while the Ga flux was varied from 3.2E-7 Torr to 3.6E-7 Torr. The surface morphology degraded with decreasing Ga flux. The Indium composition simultaneously increased due to the limited availability of Ga during growth. This indicates that the strain caused by increased In incorporation is relaxed by generating dislocations which in turn results in further increase in In incorporation as relaxation proceeds (the In pulling effect).

Growth of High Quality 350nm-Thick InGaN Films on N-Polar GaN Substrates by Plasma-Assisted Molecular Beam Epitaxy

Abstract Body: Carbon based electronics, also known as organic electronics are facilitating the development of emerging technologies due to their inherent bendability and potential for low cost manufacturing compared to brittle conventional silicon semiconductors. Furthermore, the chemical toolbox available to us enables the fine-tuning of the materials as a way to design and engineer the desired properties. This talk will focus on our groups recent advances in the development of high performing organic thin film transistors through the self-assembly of well-defined poly(ionic liquids) block copolymer based gating materails for n-type polymer based OTFTs. The use of these block copolymers over conventioanl dielectrics lead to drops in threshold voltages from 35V to under 1V while maininting high swtiching speeds and greater device currents. We aim to build structure property relationships between polymer architecture, thin film processing and device performance.

Low Voltage Operation of Flexible Organic Thin-Film Transistors Using Poly(Ionic Liquid) Block Copolymer Based Gating Materials

Abstract Body: This work presents the design, fabrication, and characterization of 3D thin-film metal electrodes for electrochemical bio-sensing (EBS) applications utilizing inkjet printing (IP) on shape memory polymer (SMP) substrates. Inkjet printing electrodes on SMP allows for 3D electrode formation in a single process step at low temperature and atmospheric pressure. In this study, a thermally contracting SMP has been utilized to print EBS electrode patterns using IP silver nanoparticle ink, extending previous work. The silver nanoparticle ink electrode geometry is sintered and shrunk simultaneously by placing it into an oven for 3 minutes. It is observed that the reduction factor of the processed EBS (sintered and shrunk) achieves up to 60% approximately and creates a complex 3D metal topology which is difficult to obtain using conventional electrode fabrication techniques. SMP is a polymer which remembers its initial shape and returns to that shape when heated above its glass transition temperature. Generally, at room temperature SMP stays at martensite phase and achieves austenite phase after heat application (in this study, 180 C). In order to observe the shrinking characteristics of the SMP film used for this study (MagicShrink), 18 different samples (1 cm by 1 cm) were shrunk at six different temperatures (140 C to 190 C) for 3 minutes. It is observed that, the shrinking mechanism is similar and stable for each sample. The reduction factor of the shrunk film is 0.6 for each sample. In this study, nanoparticle silver ink (ANP DGP-40-LT-15-C) is used to IP EBS pattern using Dimatix Materials Printer (Fujifilm, DMP-2831). This nanoparticle ink becomes conductive by curing it at 120 C ~150 C for 10 minutes. But it is perceived that the desired conductivity can be achieved only within 3 minutes at 140 C to 190 C with SMP (simultaneous curing and shrinking mechanisms). Resistivity measurements of the SMP fabricated electrodes have been performed to characterize the process impact on thin-film performance. Scanning electron microscopy is utilized to further characterize the 3D microstructure and topology created by IP on SMP fabrication process for EBS at different process temperatures. Further, electrochemical characterization of the 3D SMP electrodes are performed and compared to a conventional EBS 2D electrode. Fabricating EBS on SMP with IP technology have shown promising potential to improve electrochemical sensing. The inkjet on SMP process demonstrates significant 3D silver electrode using nanoparticle ink for an increased electrode surface area for EBS, which is difficult to fabricate using conventional techniques, with potential applications for other electrode technologies benefitting from increased device surface area. References [1] R. C. Roberts, J. Wu, N.Y. Hau, Y.H. Chang, S.P. Feng, D.C. Li, "Facile 3D Metal Electrode Fabrication for Energy Applications via Inkjet Printing and Shape Memory Polymer," Journal of Physics: Conference Series 557 (2014) 012006. [2] R. C. Roberts, S. Zeng, and N. C. Tien, "Design and Fabrication of Buckled Metal Strain Gauges using Shape Memory Polymer and inkjet Additive microfabrication," Technical Digest, The 14th IEEE Conference on Sensors (Sensors 2015), Busan, Korea, Nov. 1-4, 2015, pp. 1665-1668.

Inkjet Printed 3D Metal Electrodes on Shape Memory Polymer Towards Improved Electrochemical Bio-Sensing Performance

Abstract Body: Wearable electronics are increasingly important in healthcare applications for monitoring of the daily physical condition and diagnosis of an early stage of disorders. In particular, the wearable sensors on human skin can be utilized in real-time continuous monitoring systems because they provide comfortable wearing and consume less power. Pulse detection is one of the options and critical for high-risk groups of cardiovascular diseases such as angina, acute coronary syndrome, and myocardial infarction among the real-time monitoring of several physical parameters. Moreover, the pulse is easily detected from specific points on the human body such as carotid, temporal, fingertip, dorsal, and posterior tibial artery using skin-attachable sensor [Chen, et al. High durable, biocompatible and flexible piezoelectric pulse sensor using III-N thin film, Adv. Funct. Mater. 29, 1903162 (2019)]. Accurate sensing of the eyelid and eyeball movement is another new application. Brain disorders (e.g. Alzheimer's, Parkinson’s disease, and stroke) are accompanied by eye-related abnormal motions as early symptoms such as slow movements and the long interval between eye blinking. The diseases can be diagnosed in an early stage from the detection of unusual eye motions before they are seriously progressed. For both applications, the sensors should be light and small to attach on the human body, e.g. temple area. Recently developed thin-film piezoelectric skin-attachable sensor based on lead zirconate titanate (PZT) and ZnO inherently possess weakness for the wearable healthcare sensors. PZT contains lead, which is very harmful to the human body and environment by poisoning. A skin-attachable sensor should be containing toxic-free elements. ZnO sensor shows low sensitivity and resolution from the limitation of material, which demands additional signal processing for amplification of exact signal and minimization of the noise level. The benefit of non-toxicity is canceled by low sensitivity and extra work. As an alternative sensing element, III-N thin films are very promising to detect the physical motion of skin sensing due to their advantages of high sensitivity and durability, rapid response time, non-toxic nature on humans, lightweight, and low power consumption. Especially, single-crystalline gallium nitride (GaN) and aluminum-gallium nitride (AlGaN) piezoelectric thin films have been reported to show excellent output voltage, indicating outstanding sensitivity, rapid response time by a high electromechanical coupling factor, chemical and mechanical long-term stability, high thermal resistance, and excellent biocompatibility [Chen et al. Biocompatible and sustainable power supply for self-powered wearable and implantable electronics using flexible III-nitride thin-film-based piezoelectric generator, Nano Energy 57, 670 (2019)]. In this study, we developed and demonstrated skin-attachable GaN and AlGaN sensors for the detection of eye-lid motion to monitor the eye blinking and eyeball movements. Flexible III-N thin films were designed by layer transfer method from the rigid silicon substrate. They were analyzed by X-ray diffraction to reveal its single-crystalline quality without second phase or defects. The single sensor was applied to the temple area of the face for sensing of eye blinking and simple eyeball motion. Eye blinking is conducted with three different levels. Then, the multiple sensors were utilized with appropriate distance on the identical region to detect the transverse, longitudinal, diagonal, and rotational eyeball motions, which explain the abnormal eye movements. All the results from each sensor are distinguishable, which indicates that the sensors generated different values of output signals from each position with varied stimulates.

Skin-Attachable Physical Sensors Using Single-Crystalline III-N Piezoelectric Thin Film for Personal Health and Safety Monitoring

Abstract Body: Fabrication of a ZnTe (110) thin film is highly preferable for the application of terahertz wave detection devices utilizing the electro-optical (EO) effect. The preparation of the (110) oriented ZnTe (110) thin film has been tried on a sapphire substrate by molecular beam epitaxy. Sapphire (hetero material) is used as a non EO substrate, and ZnTe is a zincblende structure material and known to exhibit high electro-optical coefficients. However, it is difficult to prepare a ZnTe (110) thin film with high crystallinity. In previous works, the high quality ZnTe layer with the single domain structure of (111) was achieved on the S-plane sapphire substrate. It was also confirmed that high temperature treatments of m-plane sapphire substrates has resulted in nano-facet structures consisting of r- and S-planes. The calculation was performed and revealed that (110) oriented ZnTe layer on the sapphire substrate could be achieved by using a 17.66o off m-plane substrate, and by preferentially nucleating the (111) oriented domain on the S-plane of the nano-facet. The surface coverage of S-plane on the 17.66o off substrate was only about 31 %, which would hinder the preferential nucleation of ZnTe only on the S-plane. In this work, the nucleation process was varied, and the resulting film quality was characterized. The nucleation was initiated by a low temperature deposition of an amorphous layer, and its annealing to form nuclei only on the S-plane of the nano-facet. It was followed by the migration enhanced epitaxy (MEE) of ZnTe to develop the nuclei size. In this work, the MEE sequence was mostly focused, and the substrate temperature along with the MEE cycle were mainly adjusted. It was confirmed from the scanning electron microscopy (SEM) analysis that desorption from r-plane took place at lower temperature and nuclei on the S-plane nano-facet was preferentially remained when the annealing temperature was at around 420oC. When the annealing temperature was 400oC, the selectivity of the nuclei on the nano-facets were disappeared and the nuclei was uniformly observed on the substrate. The MEE was performed at 420oC for 200 cycles and then the substrate temperature was gradually lowered toward 340oC for the film growth. During this temperature lowering process, MEE was also introduced at 400oC with the cycle from 50 to 170. The theta-2theta scan of the film suggested that (110) oriented layer was formed on the substrate surface. Offset for the theta was not required to obtain the 220 diffraction signal. These results suggest that the nucleation of (111) oriented ZnTe preferentially took place on the S-plane of the nano-facet, and the (110) plane was formed parallel to the substrate surface. Although several kinds of additional domains were confirmed from the theta-2theta scan, the dominant domain was (110). When the MEE cycle at 400oC was increased to 170, the 220 X-ray diffraction peak became strong. It is probably associated with the size of nuclei developed during the MEE process. It was developed to much larger size by increasing the MEE cycle at 400 oC, which caused the improvement of the crystallinity. X-ray diffraction pole figure analysis was also performed. The (111) oriented domain formation on the S-plane nano-facet was clearly observed, and four addition domains were observed. Those additional domains were originated from twins formed on {111} planes. Zn molecular beam irradiation was also carried out during the last part of the annealing at 420oC. The pole figure image exhibited pronounced signals originated from the (111) oriented domain on the S-plane nano-facet when the Zn irradiation was employed. The Zn irradiation probably limited desorption of ZnTe from S-plane nano-facets, and increased the density of the nuclei remained on the S-plane nano-facet. This capping of the nuclei has resulted in the improvement of the film quality. This work was supported in part by Waseda University Specific Research Projects.

Experimentally-Calibrated Modeling of Molecular Beam Epitaxy Selective Area Regrowth

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Low Temperature Geiger-Mode Measurement of Gallium Nitride p-i-n Avalanche Photodiode

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