C.-H. Shen

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Valence band offset of wurtzite InN∕AlN heterojunction determined by photoelectron spectroscopy

The valence band offset (VBO) at the wurtzite-type, nitrogen-polarity InN∕AlN(0001¯) heterojunction has been determined by photoelectron spectroscopy to be 3.10±0.04eV. The heterojunction samples used for this study have an atomically abrupt 8:9 commensurate interface, at which every eight-unit cell of InN aligns exactly with every nine-unit cell of AlN. The commensurately matched InN∕AlN heterojunction system grown on Si(111) is particularly suitable for the determination of VBO since both InN and AlN epitaxial layers are completely relaxed and the strain-induced piezoelectric fields, which are difficult to be quantitatively determined, have a negligible effect.

Spectroscopic ellipsometry study of wurtzite InN epitaxial films on Si(111) with varied carrier concentrations

Spectroscopic ellipsometry (SE) has been performed to determine the optical properties of the InN epitaxial films grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. In addition to SE, cross-sectional transmission electron microscopy and x-ray diffraction reveal that epitaxially grown InN epilayer is homogeneous with high crystalline quality and does not include any metallic In. SE results analyzed by the Adachi’s model for the dielectric function of InN show that the optical absorption edge of InN varies in the range of 0.76–0.83 eV depending on the carrier concentration, which is determined by the thickness of the AlN buffer layer.

Direct evidence of 8:9 commensurate heterojunction formed between InN and AlN on c plane

We show that, despite a large difference in lattice constants, high-quality InN∕AlN heterostructures can be formed on Si(111) due to the existence of “magic” ratios between the lattice constants of comprising material pairs: 2:1 (Si∕Si3N4), 5:4 (AlN∕Si), and 8:9 (InN∕AlN). For InN growth on AlN with nitrogen polarity, by using reflection high-energy electron diffraction and cross-sectional transmission electron microscopy, we have found that the pseudomorphic to commensurate lattice transition occurs within the first monolayer of growth, resulting in an abrupt heterojunction at the atomic scale. This new route of lattice match allows the formation of commensurate and nearly strain-free interface with a common two-dimensional superlattice.

Determination of the electron effective mass of wurtzite InN by coherent upper-branch A1(LO) phonon-plasmon coupling mode

Coherent A1(LO) phonon and its coupling with photoexcited plasmon in wurtzite InN were generated and detected with time-resolved second-harmonic generation. The experimental results directly reveal that the plasma damping time constant is about 60∼120fs, which depends on the photoexcited plasma density in InN. The frequency of the upper-branch A1(LO) phonon-plasmon coupling mode shifts as a function of the photoexcited plasma density. This frequency shift can be fitted consistently with different InN films by solving the InN dielectric response function and leads to the determination of the electron effective mass m‖*=(0.033±0.003)me, parallel to the c axis of wurtzite InN.

Identification of surface optical phonon in wurtzite InN epitaxial thin films by coherent phonon spectroscopy

Coherent phonon spectroscopy of wurtzite InN epitaxial thin films was carried out with time-resolved second-harmonic generation technique. In addition to the bulk A1(LO) phonon and A1(LO)–plasmon coupling modes, a phonon mode at 543cm−1 was identified and characterized. It is found that this phonon mode is localized in the topmost few atomic layers and sensitive to surface modification. This vibration mode is described as the opposite motion between the In and N atoms along the c axis. This phonon mode is assigned to the surface optical phonon of InN.

Uniaxial to unidirectional transition of perpendicular interlayer coupling in IrMn/CoFe/NiFeO/CoFe quadrilayers

We studied the interlayer coupling in the quadrilayer consisting of IrMn∕CoFe (bottom layer)/NiFeOx∕CoFe (top layer). An in-plane perpendicular interlayer coupling is observed between CoFe layers at room temperature. An anisotropy transition from uniaxial to unidirectional in a perpendicular direction is observed around Tt=55K. The nano-oxide layer NiFeOx shows no distinguishable ferromagnetic signal in the high-temperature (uniaxial) phase, while a strong signal appeared in the low-temperature (unidirectional) phase. A possible two-component scenario, in which the nano-oxide layer may contain both amorphous short-range antiferromagnetic domains and superparamagneitc clusters, is proposed to explain the phase transition.

Co-existence of biquadratic and unidirectional anisotropy in IrMn/CoFe/FeO/sub x//CoFe films

We observed the coexistence of biquadratic and unidirectional anisotropy in the CoFe layer by inserting the nano-oxide-layer FeO/sub x/ in the IrMn/CoFe bilayers. Analyses of X-ray magnetic circular dichroism suggested that the FeO/sub x/ possessed ferromagnetic components. Mixed valence states of Fe, Fe/sup 2+/ and Fe/sup 3+/ were observed in the FeO/sub x/ layer, which implied that both antiferromagnetic and ferromagnetic components might co-exist in the FeO/sub x/ layer. The uniaxial anisotropy in CoFe at 90/spl deg/ may originate from the interaction of CoFe with the antiferromagnetic components of the FeO/sub x/ layer in the presence of interfacial magnetic roughness. The unidirectional anisotropy at 0/spl deg/ may result from the exchange biasing from the IrMn layer through the ferromagnetic components in the FeO/sub x/layer.

Asymmetric Magnetization Reversal in IrMn/CoFe/FeO

In this work, we investigate the mechanism of magnetization reversal in nano-oxide layers and explain the observed asymmetric hysteresis loops in the IrMn/CoFe/FeOx/CoFe system.We analyzed anisotropy in IrMn/CoFe/FeOX/CoFe multilayers with angular remanence measurements. The results showed the coexistence of biquadratic and unidirectional anisotropies. A significantly asymmetric magnetization reversal had been observed. We studied the reversal mechanism by using vector coils and suggested that the switching in the decreasing branch comprised both coherent rotation and 90deg domain wall motion; however, the switching in the increasing branch showed complicated multidomain rotation