Mitsuhisa Ikeda

Photoluminescence study of high density Si quantum dots with Ge core

Si quantum dots (Si-QDs) with Ge core were self-assembled on thermally grown SiO2 from alternate thermal decomposition of pure SiH4 and GeH4 diluted with He. When the sample was excited by the 979 nm line of a semiconductor laser, fairly broad photoluminescence (PL) spectra in the region of 0.6–0.8 eV were observed at room temperature. The observed PL spectra suggested that radiative recombination of photo-generated carriers through quantized states of Ge core is the dominant pathway for the emission from the dots, reflecting the type II energy band discontinuity between the Si clad and Ge core. We also found that P-δ doping to Ge core plays an important role in recombination through the quantized states in the valence band of Ge core and P donor levels.

Nano spin-diodes using FePt-NDs with huge on/off current ratio at room temperature.

Spin transistors have attracted tremendous interest as new functional devices. However, few studies have investigated enhancements of the ON/OFF current ratio as a function of the electron spin behavior. Here, we found a significantly high spin-dependent current ratio—more than 102 at 1.5 V—when changing the relative direction of the magnetizations between FePt nanodots (NDs) and the CoPtCr-coated atomic force microscope (AFM) probe at room temperature. This means that ON and OFF states were achieved by switching the magnetization of the FePt NDs, which can be regarded as spin-diodes. The FePt magnetic NDs were fabricated by exposing a bi-layer metal stack to a remote H2 plasma (H2-RP) on ~1.7 nm SiO2/Si(100) substrates. The ultrathin bi-layers with a uniform surface coverage are changed drastically to NDs with an areal density as high as ~5 × 1011 cm−2. The FePt NDs exhibit a large perpendicular anisotropy with an out-of-plane coercivity of ~4.8 kOe, reflecting the magneto-crystalline anisotropy of (001) oriented L10 phase FePt. We also designed and fabricated double-stacked FePt-NDs with low and high coercivities sandwiched between an ultra-thin Si-oxide interlayer, and confirmed a high ON/OFF current ratio when switching the relative magnetization directions of the low and high coercivity FePt NDs.

Formation and characterization of high-density FeSi nanodots on SiO2 induced by remote H2 plasma

We demonstrated the formation of high-density iron silicide nanodots (NDs) on thermally grown SiO2 by exposing an electron-beam-evaporated Fe/amorphous-Si/Fe (Fe/a-Si/Fe) trilayer stack to remote H2 plasma without any external heating and characterized their silicidation state and crystalline phase. After the remote H2 plasma exposure, the formation of NDs with an areal density of ~4.3 × 1011 cm−2 and an average height of ~7.1 nm was confirmed. X-ray photoelectron spectroscopy (XPS) analyses indicate silicidation reaction induced by the remote H2 plasma exposure, which was accompanied by the agglomeration of Fe and Si atoms on the SiO2 surface. The formation of a crystalline β-FeSi2 phase was confirmed by Raman scattering spectroscopy and XRD pattern measurements. The electrical separation among the β-FeSi2 NDs was confirmed from changes in surface potential due to charging of the dots. The surface potential of the NDs changed in a stepwise manner with respect to the tip voltage because of multistep electron injection into and extraction from the semiconductor β-FeSi2 NDs.

Segregated SiGe ultrathin layer formation and surface planarization on epitaxial Ag(111) by annealing of Ag/SiGe(111) with different Ge/(Si + Ge) compositions

Two-dimensional (2D) crystals of Si and Ge atoms such as silicene and germanene are currently receiving much attention because of their exceptional electronic properties. We have studied the growth of ultrathin Si and Ge layers by the segregation of Si and Ge atoms on the Ag surface from the substrate by annealing the Ag/Si(111), Ag/Ge(111), and Ag/Si0.5Ge0.5(111) structures. After the epitaxial growth of the Ag(111) layer on the substrate by thermal evaporation, the stability of the Ag surface was also investigated for use as the template of ultrathin Si and Ge crystals. After annealing a ~90-nm-thick Ag(111)/Ge(111) structure at 450 °C in N2 ambience for 2 h, the surface segregation of Ge and a flat Ag surface were demonstrated.

Interface properties of SiO2/GaN structures formed by chemical vapor deposition with remote oxygen plasma mixed with Ar or He

The impacts of noble gas species (Ar and He) on the formation of a SiO2/GaN structure formed by a remote oxygen plasma-enhanced chemical vapor deposition (ROPE-CVD) method were systematically investigated. Atomic force microscopy revealed that ROPE-CVD with He leads to a smooth SiO2 surface compared with the case of Ar. We found that no obvious oxidations of the GaN surfaces after the SiO2 depositions with the both Ar and He cases were observed. The capacitance–voltage (C–V) curves of the GaN MOS capacitors formed by ROPE-CVD with the Ar and He dilutions show good interface properties with no hysteresis and good agreement with the ideal C–V curves even after post deposition annealing at 800 °C. Besides, we found that the current density–oxide electric field characteristics shows a gate leakage current for the Ar case lower than the He case.

High thermal stability of abrupt SiO2/GaN interface with low interface state density

The effects of postdeposition annealing (PDA) on the interface properties of a SiO2/GaN structure formed by remote oxygen plasma-enhanced chemical vapor deposition (RP-CVD) were systematically investigated. X-ray photoelectron spectroscopy clarified that PDA in the temperature range from 600 to 800 °C has almost no effects on the chemical bonding features at the SiO2/GaN interface, and that positive charges exist at the interface, the density of which can be reduced by PDA at 800 °C. The capacitance–voltage (C–V) and current density–SiO2 electric field characteristics of the GaN MOS capacitors also confirmed the reduction in interface state density (D it) and the improvement in the breakdown property of the SiO2 film after PDA at 800 °C. Consequently, a high thermal stability of the SiO2/GaN structure with a low fixed charge density and a low D it formed by RP-CVD was demonstrated. This is quite informative for realizing highly robust GaN power devices.

Characterization of remote O2-plasma-enhanced CVD SiO2/GaN(0001) structure using photoemission measurements

The control of chemical composition and bonding features at a SiO2/GaN interface is a key to realizing high-performance GaN power devices. In this study, an ~5.2-nm-thick SiO2 film has been deposited on an epitaxial GaN(0001) surface by remote O2-plasma-enhanced chemical vapor deposition (O2-RPCVD) using SiH4 and Ar/O2 mixture gases at a substrate temperature of 500 °C. The depth profile of chemical structures and electronic defects of the O2-RPCVD SiO2/GaN structures has been evaluated from a combination of SiO2 thinning examined by X-ray photoelectron spectroscopy (XPS) and the total photoelectron yield spectroscopy (PYS) measurements. As a highlight, we found that O2-RPCVD is effective for fabricating an abrupt SiO2/GaN interface.

Electroluminescence of superatom-like Ge-core/Si-shell quantum dots by alternate field-effect-induced carrier injection

We have fabricated high-density superatom-like Si–Ge-based quantum dots (Si-QDs with Ge core) and studied their luminescence properties. Electroluminescence was observed from the Si-QDs with Ge core at room temperature in the near-infrared region by the application of square-wave pulsed bias of ±1 V at 500 kHz, which was attributed to radiative recombination between quantized states in the Ge core with deep potential well for holes caused by field-effect-induced alternate electron/hole injection from the substrate. The results lead to the development of Si-based light-emitting devices that are highly compatible with ultra-large-scale integration processing, which was found difficult to realize in silicon photonics.

High-density formation of Ta nanodot induced by remote hydrogen plasma

序>これまでに、金属二層薄膜に H2-RP 照射した場合、合金化反応と共に、原子のマイグレーシ ョン・凝集が同時に促進することに起因して、合金ナノドットが形成できることを報告した[1]。 本研究では、Mn と Ge の合金に着目し、SiO2上の Mn 膜と非結晶 Ge(a-Ge)薄膜の積層構造への H2-RP 処理による表面形状および化学結合状態の変化を調べると共に、Mnジャーマナイドナノド ットの形成を試みた。 実験>p-Si(100)基板を化学溶液洗浄後、乾燥酸素雰囲気中で熱酸化 SiO2膜を形成した。SiO2膜上 に、電子線蒸着により膜厚~10nm の a-Ge 膜を堆積し、膜厚~1.0nm および~10nm の Mn 膜を同一 チャンバ内で連続堆積した。その後、外部非加熱で H2 ガスのリモートプラズマ(H2-RP)処理 (60MHz-ICP: 500W, 13.3Pa)を 10分間行った。 結果及び考察>SiO2 上に形成したMn/a-Ge 積層膜の形成直後における表面形状像は、何れも表面 ラフネスが~0.4nm以下であり、均一膜の形成が確認できる(Figs. 1(a), (b))。一方、H2-RP 処理後で は、Mn の膜厚依らずナノドットの形成が認められ、ドットの面密度は、Mn 膜~1.0nm では~ 1.7× 10 11 cm -2 であるのに対して、膜厚~10nm では~ 1.4×10cmであり、ドット高さは、Mn 単層膜に比 べ 2 倍であった(Figs. 1 (a’), (b’))。形成したナノドットの 化学結合状態を X 線光電子分光法(単色化 AlKα 特性 X 線、hν = 1486.6eV)を用いて評価した結果、Mn 膜~1.0nm では、Mn3p信号が明瞭に観測されるが、Geに起因する 信号が認められない。これに対して、Mn膜~10nmでは、 金属 Ge 成分が明瞭に観測され、Mn-Ge 系ナノドットの 形成が示唆される(Fig. 2)。別途、Mn 箔で覆った熱電対 の H2-RP 処理時における温度変化を調べた結果、600 C まで急激に温度上昇することから、Mn 膜表面での原子 状水素の吸着・再結合に起因した局所温度上昇により Mn原子の表面マイグレーション・凝集と下地 a-Geとの 合金化反応が同時進行すると考えられる。これらの結果 から、厚さ~1.0nm の Mn 膜では、Mn と Ge の合金化反 応とともに、Ge原子がナノドット表面に露出することで 水素原子による Ge のエッチング が進行したと推察される。 結論>Mn/Ge 積層膜への H2-RP 照 射によるMn-Ge系ナノドットの形 成では、上層 Mn 膜厚を制御し、 H2-RP時に生じるGeのエッチング を抑制することが重要であること が分かった。 文 献 > [1] R. Fukuoka et al., Magnetics and Optics Research Int. Symp. (Omiya, 2014), We-P-07 謝辞>試料作成は、名古屋大学 VBL の設備を利用して行った。本 研究の一部は、東北大学電気通信 研究所 共同プロジェクト研究の 支援により行われた。 第 75 回応用物理学会秋季学術講演会 講演予稿集(2014 秋 北海道大学)

Effects of remote hydrogen plasma on chemical bonding features and electronic states of 4H-SiC(0001) surface

We have demonstrated a novel dry cleaning process for the 4H-SiC surface using remote hydrogen plasma (H2-RP). The effects of H2-RP exposure on the chemical structures and electronic states of the wet-cleaned 4H-SiC surface have been evaluated by X-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS). XPS shows that H2-RP exposure is effective in removing contaminants from the 4H-SiC surface. PYS indicates that the defect states at the surface are generated after the H2-RP exposure and such generated defect states are drastically decreased by annealing at 850 °C. The combination of H2-RP exposure with the subsequent annealing is found to be effective in 4H-SiC surface cleaning.