C.-H. Hsu

Cubic HfO2 doped with Y2O3 epitaxial films on GaAs (001) of enhanced dielectric constant

Nanometer thick cubic HfO2 doped with 19at.% Y2O3 (YDH) epitaxial films were grown on GaAs (001) using molecular beam epitaxy. Structural studies determined the epitaxial orientation relationships between the cubic YDH films and GaAs to be (001)GaAs∕∕(001)YDH and [100]GaAs∕∕[100]YDH. The YDH structure is strain relaxed with a lattice constant of 0.5122nm with a small mosaic spread of 0.023° and a twist angle of 2.9°. The YDH/GaAs interface is atomically abrupt without evidence of reacted interfacial layers. From C-V and I-V measurements a 7.7nm thick YDH film has an enhanced dielectric constant κ∼32, an equivalent oxide thickness of ∼0.94nm, an interfacial state density Dit∼7×1012cm−2eV−1, and a low leakage current density of 6×10−5A∕cm2 at 1V gate bias.

Structure of HfO2 films epitaxially grown on GaAs(001)

High-quality HfO2 films of technologically important thickness ranging from 1.8to17nm have been grown epitaxially on GaAs (001) by molecular beam epitaxy. Thorough structural and morphological investigations were carried out by x-ray scattering and high-resolution transmission electron microscopy. The films exhibit an atomically sharp interface with the substrate and are of a monoclinic phase with predominant (001)-plane epitaxy between the HfO2 films and GaAs, in spite of a large lattice mismatch of >8.5%.

Morphology control of silicon nanotips fabricated by electron cyclotron resonance plasma etching

Formation of well-aligned silicon nanotips etched monolithically from a silicon substrate has been demonstrated. The effect of the process temperature on the physicochemical etching of silicon and subsequent fabrication of these nanotips has been investigated. 2.2-μm-long nanotips were formed at the process temperature of 250°C and then decreased in length with increasing process temperature. Above 800°C, the formation of the silicon nanotips was inhibited. Spectroscopic evidence attributes this fact to the efficient formation of silicon carbide thin film at higher process temperatures, instead of discontinuous nanomasks at lower process temperatures that prevent etching of the substrate. Another reason for this inhibited formation of nanotips is the reduced etching rate of the silicon by agents such as atomic H at higher process temperatures.

Structural and compositional investigation of yttrium-doped HfO2 films epitaxially grown on Si (111)

Cubic phase yttrium-doped HfO2 (YDH) ultrathin films were grown on Si (111) substrates by molecular beam epitaxy. Thorough structural and morphological investigations by x-ray scattering and transmission electron microscopy reveal that the YDH thin films are epitaxially grown on the Si substrates with (111)YDH‖(111)Si and [101¯]YDH‖[11¯0]Si. The interface between YDH and Si is atomistic sharp and free of interfacial layer. We have also determined the yttrium content of YDH films to be 19% by using anomalous x-ray diffraction (AXD) across Y k edge and angle resolved x-ray photoelectron spectroscopy (AR-XPS). The agreement between the AXD and AR-XPS results manifests that the incorporated Y atoms homogeneously substitute Hf atoms in the crystalline lattice and form a substitutional solid solution.

Nanometer thick single crystal Y2O3 films epitaxially grown on Si (111) with structures approaching perfection

Cubic phase Y2O3 films 1.6–10nm thick of excellent quality have been epitaxially grown on Si (111) with Y2O3(111)∥Si(111) using electron beam evaporation of Y2O3 in ultrahigh vacuum. Structural and morphological studies were carried out by x-ray scattering and reflectivity and high-resolution transmission electron microscopy, with the growth being in situ monitored by reflection high energy electron diffraction. There are two Y2O3 domains in the initial stage of the oxide growth with equal population, and the B-type domain of Y2O3[21¯1¯]∥Si[112¯] becomes predominating over the A-type domain of Y2O3[21¯1¯]∥Si[21¯1¯] with increasing film thickness. Besides the excellent crystallinity of the films as derived from the small ω-rocking curve width of 0.014°, our results also show atomically sharp smooth surface and interfaces.

High-quality nanothick single-crystal Y2O3 films epitaxially grown on Si (111): Growth and structural characteristics

High-quality single-crystal nanothick Y2O3 films have been grown epitaxially on Si (111) despite a lattice mismatch of 2.4%. The films were electron beam evaporated from pure compacted powder Y2O3 target in ultrahigh vacuum. Y2O3 3nm thick exhibited a bright, sharp, streaky reconstructed (4×4) reflection high energy electron diffraction pattern. Structural studies carried out by x-ray diffraction with synchrotron radiation and high-resolution transmission electron microscopy show that the films have the cubic bixbyite phase with a remarkably uniform thickness and high structural perfection. Two Y2O3 domains of B-type Y2O3[21¯1¯]∥Si[112¯] and A-type Y2O3[21¯1¯]∥Si[21¯1¯] coexist in the initial film growth with B type predominating over A type in thicker films as studied using x-ray diffraction. The narrow full width at half maximum of 0.014° in the ω-rocking curve is the characteristic of excellent crystalline films. High-resolution transmission electron microscopy and fast Fourier transform analysis show ...

Single domain m-plane ZnO grown on m-plane sapphire by radio frequency magnetron sputtering.

High-quality m-plane orientated ZnO films have been successfully grown on m-plane sapphire by using radio frequency magnetron sputtering deposition. The introduction of a nanometer-thick, low-temperature-grown ZnO buffer layer effectively eliminates inclusions of other undesirable orientations. The structure characteristics of the ZnO epi-layers were thoroughly studied by synchrotron X-ray scattering and transmission electron microscopy (TEM). The in-plane epitaxial relationship between ZnO and sapphire follows (0002)(ZnO) [parallel] (112[overline]0)(sapphire) and (112[overline]0)(ZnO) [parallel] (0006)(sapphire) and the ZnO/sapphire interface structure can be described by the domain matching epitaxy along the [112[overline]0](ZnO) direction. The vibrational properties of the films were investigated by polarization dependent micro-Raman spectroscopy. Both XRD and micro-Raman results reveal that the obtained m-ZnO layers are under an anisotropic biaxial strain but still retains a hexagonal lattice.

Single-Crystal Y2O3 Epitaxially on GaAs(001) and (111) Using Atomic Layer Deposition

Single-crystal atomic-layer-deposited (ALD) Y2O3 films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 × 6 and GaAs(111)A-2 × 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using in-situ reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y2O3 films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y2O3(110)[001][1¯10]//GaAs(001)[110][11¯0]. On GaAs(111)A, the Y2O3 films are also of a cubic phase with (111) as the film normal, having the orientation relationship of Y2O3(111)[21¯1¯][011¯]//GaAs(111)[2¯11][01¯1]. The relevant orientation for the present/future integrated circuit platform is (001). The ALD-Y2O3/GaAs(001)-4 × 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage (CV) curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of ~1012 cm−2eV−1 as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the Dit are the lowest ever achieved among all the ALD-oxides on GaAs(001).

Growth and structural characteristics of GaN∕AlN/nanothick γ-Al2O3∕Si (111)

The authors report on the growth of GaN by nitrogen plasma-assisted molecular beam epitaxy (MBE) on a 2in. Si (111) substrates with a nanothick (∼4.8nm thick) γ-Al2O3 as a template/buffer. A thin layer of MBE-AlN ∼40nm thick was inserted prior to the growth of GaN. High-resolution transmission electron microscopy (HR-TEM) and high-resolution x-ray diffraction studies indicated that both of the nanothick γ-Al2O3 and AlN are a single crystal. Reflection high-energy electron diffraction, high-resolution x-ray scattering using synchrotron radiation, and cross-sectional HR-TEM measurements indicated an orientation relationship of GaN(0002)∥AlN(0002)∥γ-Al2O3(111)∥Si(111) and GaN[10−10]∥AlN[10−10]∥γ-Al2O3[2−1−1]∥Si[2−1−1]. A dislocation density of 5×(108–109)∕cm2 in the GaN ∼0.5μm thick was determined using cross-sectional TEM images under weak-beam dark-field conditions.

Surface-bound-exciton emission associated with domain interfaces in m-plane ZnO films

Small amount of (101¯3)ZnO domains were found in the m-plane ZnO films grown on m-sapphire by pulsed laser deposition, which provide strain relaxation of the m-ZnO matrix behaving as a low strain layer. Through carefully correlating low-temperature polarized photoluminescence spectra with the x-ray diffraction peak intensity ratio of (101¯3)ZnO/(101¯0)ZnO of the samples grown at different temperature and after thermal treatment, we found that the broad-band emission around 3.17 eV may result from the interface defects trapped excitons at the boundaries between the (101¯3)ZnO domains and the m-ZnO matrix. The more (101¯3)ZnO domains in the m-ZnO layer cause the more surface boundary that makes the stronger surface-bound-exciton emission. And the a-axes of both the (101¯3)ZnO domains and the m-ZnO matrix are aligned with the c-axis of the sapphire (α-Al2O3) substrate. The c-axis of the (101¯3)ZnO domains rotates by about ±59° against the common a-axis of the m-ZnO.