Jong Hyeok Park

High carrier mobility in orientation-controlled large-grain (≥50 μm) Ge directly formed on flexible plastic by nucleation-controlled gold-induced-crystallization

High-carrier-mobility semiconductors on flexible-plastic are essential to realize flexible electronics. For this purpose, electrical properties of orientation-controlled large-grain Ge crystals on flexible-plastic directly formed by nucleation-controlled gold-induced-crystallization (GIC) are examined, and compared with those obtained by aluminum-induced-crystallization (AIC). The Ge crystals show p-type conductions. Here, hole concentrations are 2.2 × 1017 and 5.8 × 1020 cm−3 for GIC-Ge and AIC-Ge, respectively, which are explained on the basis of the solubility of Au and Al in Ge. Thanks to the low hole concentration, GIC-Ge shows high hole mobility (160 cm2 V−1 s−1) compared with AIC-Ge (37 cm2 V−1 s−1). These demonstrate significant advantage of GIC to realize high-performance flexible-electronics.

A pseudo-single-crystalline germanium film for flexible electronics

We demonstrate large-area (∼600 μm), (111)-oriented, and high-crystallinity, i.e., pseudo-single-crystalline, germanium (Ge) films at 275 °C, where the temperature is lower than the softening temperature of a flexible substrate. A modulated gold-induced layer exchange crystallization method with an atomic-layer deposited Al2O3 barrier and amorphous-Ge/Au multilayers is established. From the Raman measurements, we can judge that the crystallinity of the obtained Ge films is higher than those grown by aluminum-induced-crystallization methods. Even on a flexible substrate, the pseudo-single-crystalline Ge films for the circuit with thin-film transistor arrays can be achieved, leading to high-performance flexible electronics based on an inorganic-semiconductor channel.

(111)-oriented large-grain (≥50 µm) Ge crystals directly formed on flexible plastic substrate by gold-induced layer-exchange crystallization

Orientation-controlled large-grain Ge crystals grown on plastic substrates (softening temperature: ~300 °C) are desired to realize advanced flexible electronics, where various functional devices are integrated on flexible substrates. To achieve this, gold-induced crystallization (annealing temperature: 250 °C) using a-Ge/Au stacked structures is developed on plastic substrates, where thin-Al2O3 layers (thickness: ~7 nm) are introduced at a-Ge/Au interfaces. Interestingly, (111)-oriented nucleation at the Au/plastic interface dominates over random bulk nucleation in Au layers. As a result, the formation of (111)-oriented large-grain (≥50 µm) Ge crystals directly on flexible plastic substrates is realized. This technique will pave the way for advanced flexible electronics.

Crystallization of Electrodeposited Germanium Thin Film on Silicon (100)

We report the crystallization of electrodeposited germanium (Ge) thin films on n-silicon (Si) (100) by rapid melting process. The electrodeposition was carried out in germanium (IV) chloride: propylene glycol (GeCl4:C3H8O2) electrolyte with constant current of 50 mA for 30 min. The measured Raman spectra and electron backscattering diffraction (EBSD) images show that the as-deposited Ge thin film was amorphous. The crystallization of deposited Ge was achieved by rapid thermal annealing (RTA) at 980 °C for 1 s. The EBSD images confirm that the orientations of the annealed Ge are similar to that of the Si substrate. The highly intense peak of Raman spectra at 300 cm−1 corresponding to Ge-Ge vibration mode was observed, indicating good crystal quality of Ge. An additional sub peak near to 390 cm−1 corresponding to the Si-Ge vibration mode was also observed, indicating the Ge-Si mixing at Ge/Si interface. Auger electron spectroscopy (AES) reveals that the intermixing depth was around 60 nm. The calculated Si fraction from Raman spectra was found to be in good agreement with the value estimated from Ge-Si equilibrium phase diagram. The proposed technique is expected to be an effective way to crystallize Ge films for various device applications as well as to create strain at the Ge-Si interface for enhancement of mobility.

Quasi-single crystal SiGe on insulator by Au-induced crystallization for flexible electronics

Orientation-controlled large-grain (≥10 µm) crystal, i.e., quasi-single crystal, Ge-rich (≥50%) SiGe on insulator grown at low temperatures (≤300 °C) are desired for realization of high-performance flexible electronics. To achieve this, the Au-induced crystallization technique using a-SiGe/Au stacked structures has been developed. This enables formation of (111)-oriented large-grain (≥10 µm) Si1−xGex (x ≥ 0.5) crystals on insulating substrates at low temperatures (300 °C). The surface layers of the grown SiGe crystals have uniform lateral composition profiles. By using this technique, formation of quasi-single crystal Ge on flexible plastic sheets is demonstrated. This technique will be useful to realize high-performance flexible electronics.

The effects of annealing temperatures on composition and strain in SixGe1-x obtained by melting growth of electrodeposited Ge on Si (100)

The effects of annealing temperatures on composition and strain in SixGe1−x, obtained by rapid melting growth of electrodeposited Ge on Si (100) substrate were investigated. Here, a rapid melting process was performed at temperatures of 1000, 1050 and 1100°C for 1 s. All annealed samples show single crystalline structure in (100) orientation. A significant appearance of Si-Ge vibration mode peak at ~00 cm−1 confirms the existence of Si-Ge intermixing due to out-diffusion of Si into Ge region. On a rapid melting process, Ge melts and reaches the thermal equilibrium in short time. Si at Ge/Si interface begins to dissolve once in contact with the molten Ge to produce Si-Ge intermixing. The Si fraction in Si-Ge intermixing was calculated by taking into account the intensity ratio of Ge-Ge and Si-Ge vibration mode peaks and was found to increase with the annealing temperatures. It is found that the strain turns from tensile to compressive as the annealing temperature increases. The Si fraction dependent thermal expansion coefficient of SixGe1−x is a possible cause to generate such strain behavior. The understanding of compositional and strain characteristics is important in Ge/Si heterostructure as these properties seem to give significant effects in device performance.

Formation of quasi-single-crystal Ge on plastic by nucleation-controlled Au-induced layer-exchange growth for flexible electronics

A low-temperature (≤250oC) formation technique of orientation-controlled large-grain Ge thin films on insulator is desirable for realization of advanced flexible electronics. To achieve this, the Au-induced crystallization technique combined with inter-diffusion-control and interface-energy-modulation techniques has been investigated. Consequently, selective formation of (111)-or (100)-oriented large-grain (>20 μm) Ge crystals on insulator have been realized. Moreover, formation of (111)-oriented large-grain Ge crystals directly on plastic substrates is demonstrated. This technique facilitates realization of future flexible electronics.

Ultralow-temperature catalyst-induced-crystallization of SiGe on plastic for flexible electronics

Development of a low-temperature (≤250°C) formation technique of orientation-controlled large-grain (<;10 μm) SiGe on insulator is essential to realize flexible electronics, where various advanced devices are integrated on flexible plastic substrates (softening temperature: ~300°C). This is because SiGe provides higher carrier mobility and superior optical properties compared with Si, as well as epitaxial template of various functional materials. In line with this, we have been developing metal-induced crystallization of SiGe. This achieves selectively (100)- or (111)-oriented large-grain (≥20 μm) SiGe at low temperatures (~250°C) [1-4]. Present paper reviews our recent progress in this novel growth technique.

Effect of Sn-doped Ge insertion layers on epitaxial growth of ferromagnetic Fe 3 Si films on a flexible substrate

By an insertion of a Ge(Sn) layer and its CMP treatments, we have improved the crystalline and magnetic characteristics of one of the Heusler-compounds, Fe3Si, on (111)-oriented Ge on a flexible substrate. This work is a first step of high-performance flexible spintronics for flexible system-in-display devices.

Orientation-Controlled Large-Grain SiGe on Flexible Substrate by Nucleation-Controlled Gold-Induced Crystallization

Low-temperature gold-induced crystallization of SiGe is investigated. By annealing (~250C) of a-SiGe/Au/insulator stacked structures, positions of SiGe and Au layers are inverted, and SiGe crystals are obtained on insulator. Combined with nucleation control through modulation of SiGe/Au mixing by diffusion barriers and anisotropic interface energy of SiGe/substrates, (111)or (100)-oriented large (≥20 μm) crystals are obtained. Moreover, direct formation onto flexible plastic substrates is realized. This technique will facilitate high-performance flexible electronics.