Yeonjoon Park

Miniaturization of optical spectroscopes into Fresnel microspectrometers

Miniaturized optical instruments have become very important in industry as smart phones and tablet PCs increase in popularity. A chronology of spectrometer development shows that a simple numerical point of view affords important insights. A tiny spectrometer, which is smaller than a few millimeters size, cannot easily rely on the conventional Fraunhofer diffraction due to its optical criterion limit. As an alternate solution to build smaller spectrometers, a Fresnel spectrometer chip with a gradient line grating is attractive. The fabricated Fresnel spectrometers have optical path volumes of about 1 mm3 and spectral resolutions of 10 to 23 nm.

Defect-engineered Si1−xGex alloy under electron beam irradiation for thermoelectrics

We report the development of a defect-engineered thermoelectric material using Si1−xGex alloys grown on a c-plane sapphire substrate via electron beam (E-beam) irradiation. This paper outlines the idea of growing the Si1−xGex film at relatively high temperatures to obtain good crystalline properties, then controlling the amount of twins or dislocations through ex situ electron-beam irradiation. The current work suggests that structure reconstruction by bond rearrangement through E-beam irradiation may be used for tailoring thermoelectric properties.

Versatile smart optical material characterizer system

A versatile optical characterization system is fabricated to measure various optical properties of materials and devices. The optical system is based on Michelson interferometer with advanced software algorithm to measure the intensity, phase angle, polarization state, and coherence of transmitted or reflected light from the materials and devices under test. Innovative contour map of phase/intensity vs. time/physical-quantity relation shows the dynamic evolution of interference patterns of multiple points in the analysis area. Advanced software semi-automatically calculates change of photon intensity, phase angle, polarization, and coherence which are results of various applied physical quantities such as voltage, electric field, current, temperature, pressure, chemical density, and reaction time. The measured optical property changes are converted by software to the changes of intrinsic and extrinsic properties of materials and devices under test. The system is designed for multi-point measurements which are suitable for 2D-array-pixel type devices. Therefore, this versatile optical measurement system can accelerate the development of advanced adaptive optics elements and phase control elements.

X-ray diffraction wafer mapping method for SiGe twin defects characterization

Group IV semiconductors, silicon, germanium, and carbon are todays most important cubic diamond structure forming semiconductors. A recently developed rhombohedral super-hetero epitaxy technology has enabled the single-crystal growth of cubic diamond semiconductors on the basal plane of selected trigonal crystals. This kind of hetero-crystal-structure epitaxy was previously thought to be impossible or very difficult to grow. We found this apparent lacuna in the earlier studies to be stemming from the lack of a proper characterization tool and a deficit in the knowledge of growth parameters employed. Here, we present X-ray diffraction (XRD) methods for characterizing twin crystal defects in the rhombohedral-trigonal epitaxy scheme. These XRD methods not only measure the total density of the twin defect crystals but also map their distribution on the wafer with high sensitivity and spatial resolution.

Towards rhombohedral SiGe epitaxy on 150mm c-plane sapphire substrates

Previous work demonstrated for the first time the ability to epitaxially grow uniform single crystal diamond cubic SiGe (111) films on trigonal sapphire (0001) substrates. While SiGe (111) forms two possible crystallographic twins on sapphire (0001), films consisting primarily of one twin were produced on up to 99.95% of the total wafer area. This permits new bandgap engineering possibilities and improved group IV based devices that can exploit the higher carrier mobility in Ge compared to Si. Models are proposed on the epitaxy of such dissimilar crystal structures based on the energetic favorability of crystallographic twins and surface reconstructions. This new method permits Ge (111) on sapphire (0001) epitaxy, rendering Ge an economically feasible replacement for Si in some applications, including higher efficiency Si/Ge/Si quantum well solar cells. Epitaxial SiGe films on sapphire showed a 280% increase in electron mobility and a 500% increase in hole mobility over single crystal Si. Moreover, Ge possesses a wider bandgap for solar spectrum conversion than Si, while the transparent sapphire substrate permits an inverted device structure, increasing the total efficiency to an estimated 30-40%, much higher than traditional Si solar cells. Hall Effect mobility measurements of the Ge layer in the Si/Ge/Si quantum well structure were performed to demonstrate the advantage in carrier mobility over a pure Si solar cell. Another application comes in the use of microelectromechanical devices technology, where high-resistivity Si is currently used as a substrate. Sapphire is a more resistive substrate and offers better performance via lower parasitic capacitance and higher film carrier mobility over the current Si-based technology.

Effect of rare earth elements (Er, Ho) on semi-metallic materials (ScN) in an applied electric field

The development of materials and fabrication technology for field-controlled spectrally active optics is essential for applications such as membrane optics, filters for LIDARs, windows for sensors, telescopes, spectroscopes, cameras and flat-panel displays. The dopants of rare earth elements, in a host of optical systems, create a number of absorption and emission band structures and can easily be incorporated into many high quality crystalline and amorphous hosts. In wide band-gap semiconductors like ScN, the existing deep levels can capture or emit the mobile charges, and can be ionized with the loss or capture of the carriers which are the fundamental basis of concept for smart optic materials. The band gap shrinkage or splitting with dopants supports the possibility of this concept. In the present work, a semi-metallic material (ScN) was doped with rare earth elements (Er, Ho) and tested under an applied electric field to characterize spectral and refractive index shifts by either Stark or Zeeman Effect. These effects can be verified using the UV-Vis spectroscopy, the Hall Effect measurement and the ellipsometric spectroscopy. The optical band gaps of ScN doped with Er and doped with Ho were experimentally estimated as 2.33eV and 2.24eV (±0.2eV) respectively. This is less than that of undoped ScN (2.5±0.2eV). The red-shifted absorption onset is a direct evidence for the decrease of band gap energy (Eg), and the broadening of valence band states is attributable to the doping cases. A decrease in refractive index with an applied field was observed as a small shift in absorption coefficient using a variable angle spectroscopic ellipsometer. In the presence of an electric field, mobile carriers are redistributed within the space charge region (SCR) to produce this electro-refractive effect. The shift in refractive index is also affected by the density and location of deep potential wells within the SCR. In addition, the microstructure change was observed by a TEM analysis. These results give an insight for future applications for the field-controlled spectrally active material

Mathematical simulation for integrated linear Fresnel spectrometer chip

A miniaturized solid-state optical spectrometer chip was designed with a linear gradient-gap Fresnel grating which was mounted perpendicularly to a sensor array surface and simulated for its performance and functionality. Unlike common spectrometers which are based on Fraunhoffer diffraction with a regular periodic line grating, the new linear gradient grating Fresnel spectrometer chip can be miniaturized to a much smaller form-factor into the Fresnel regime exceeding the limit of conventional spectrometers. This mathematical calculation shows that building a tiny motionless multi-pixel microspectrometer chip which is smaller than 1mm3 of optical path volume is possible. The new Fresnel spectrometer chip is proportional to the energy scale (hc/λ), while the conventional spectrometers are proportional to the wavelength scale (λ). We report the theoretical optical working principle and new data collection algorithm of the new Fresnel spectrometer to build a compact integrated optical chip.

Optical performance of circular Fresnel spectrometer

Most of todays spectrometers are based on Fraunhofer diffraction with a periodic regular line grating. We demonstrate a new type of a spectrometer which is based on Fresnel diffraction that can be miniaturized smaller than Fraunhofer diffraction limit, a2/λ where a is the aperture size, and λ is the wavelength of the light. The theory, fabrication, and optical performance of the miniaturized Fresnel spectrometer with a circular Fresnel grating, i.e. zone-plate will be presented. The theoretical calculation shows that the spectral resolution of Fresnel spectrometer is not fundamentally determined by the size of the grating but it is determined by the total number of rings. The miniaturized Fresnel spectrometer has a circular grating of 750 micrometer diameter and the volume of the optical path between the grating and the aperture slit is only 1mm3. In spite of this small dimension, it achieved a spectral resolution of 22nm which is similar to the typical value of a color filter.

Development of Neural Probe With Wireless Power Feed

New medical device technology is essential for diagnosing, monitoring, and curing wide spectrum of diseases, anomalies and inflictions. For neural applications, no matter whether non-intrusive or not, currently available devices are generally limited to either a curing or a probing function. In this paper we review the technology requirements for new neural probe and cure device technology. Recent advances in micro and nano-scale devices engineering and wireless power technology offer a great potential to revolutionize many health care systems. The integration of wireless power technology into smart microsensor and probe systems greatly simplifies the healthcare devices and systems and also offers additional device functions for even complex jobs. The wireless power feed technology eliminates the pains and irritations associated with implanted power devices and wires. Neural electronics interfaces (NEI) can be coupled and integrated with the wireless power receiver (WPR). The implantable probe-pin devices (PPD) that include the NEI and WPR allow real-time measurement and control/feedback possible for remedial process of neural anomaly from normal functions. Such a system like a PPD should have an embedded expert system that performs semi-autonomous functions through a routine of sensing, judging, and controlling the neural anomaly.Copyright