Hung-Chin Chung

Defect-induced negative differential resistance of GaN nanowires measured by conductive atomic force microscopy

We report on negative differential resistance (NDR) from individual GaN nanowires prepared without catalysts by thermal chemical vapor deposition. Conductive atomic force microscopy was used to characterize the electron transport behavior and transmission electron microscopy was employed to characterize the microstructure of the GaN nanowires. The current-voltage curve exhibits two clear NDR regions in the forward bias. The defect assisted inelastic tunneling process resulting in the NDR behavior and the related mechanism for energy band diagram is proposed and discussed.

Electron transport through individual Ge self-assembled quantum dots on Si

Electrical properties of self-assembled quantum dots have been the subject of intensive research due to quantum confinement. Here the authors report on the fabrication of Ge quantum dots (QDs) onto Si (100) by ultrahigh-vacuum ion beam sputtering and the electrical properties of individual QDs. Transmission electron microscopy images show that samples with completely incoherent or coherent semispherical islands can be produced under different ion energies. The current-voltage (I-V) characteristics with conductive atomic force microscopy at room temperature. exhibit linear behavior at low bias and nonlinear behavior at large bias from coherent islands, whereas the staircase structures are clearly observed in the I-V curve from incoherent islands, which are attributed to electron tunneling through the quantized energy levels of a single Ge QD.

Correlation of In–Ga intermixing with band-tail states in InAs∕GaAs quantum dots

We have investigated the shape and composition profiles of buried and surface InAs∕GaAs Stranski–Krastanov quantum dots (QDs) by using the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In–Ga intermixing in the vicinity of the buried QDs.

Anomalous formation of InGaN/GaN multiple-quantum-well nanopillar arrays by focused ion beam milling

A GaN swelling?milling competition phenomenon was found under focused ion beam milling. Tuning the beam dwell time or introducing dopants into GaN is able to control the degree of GaN swelling, which makes feasible the writing of large-area nanopillar arrays. High-aspect-ratio InGaN/GaN MQW nanopillars were accordingly fabricated by a self-masking process where the tops of the p-GaN tips were swollen during milling. The shadowing effect from the swollen tops induced only minimal damage to the MQWs, with the cathodoluminescence (CL) emission peak of the InGaN/GaN MQWs showing a blue shift of about 35?meV compared with that of the as-grown wafer.

Structural and optical properties of cubic-InN quantum dots prepared by ion implantation in Si (100) substrate

The authors have synthesized InN quantum dots by ion implantation into a Si (100) substrate followed by a postannealing process. X-ray photoemission spectroscopy data verified the formation of In–N bonding in both as-implanted and postannealed samples. Diffraction patterns from transmission electron microscopy (TEM) confirm that the dots are of cubic crystal (zinc-blende phase) with no presence of wurtzite InN. The silicon matrix provides a constraint for the formation of the InN cubic metastable phase. However, dislocations were revealed by high resolution TEM at the interfaces between the dots and the silicon. In addition, the authors found that as the annealing temperature or time increases, dot size increases and dot density decreases. Furthermore, they demonstrate that the main emission energy of zinc-blende InN dots is about 0.736eV.

Mechanism for persistent hexagonal island formation in AlN buffer layer during growth on Si (111) by plasma-assisted molecular beam epitaxy

The characteristics of structure and morphology of AlN grown by a growth interruption method on Si (111) with plasma-assisted molecular beam epitaxy are investigated. It is found that the growth interruption method would improve the surface flatness of the AlN layer without the formation of Al droplets. However, AlN hexagonal islands were present and persistent throughout the entire growth owing to effective strain relaxation and Eherlich-Schowebel barrier effect of preexistent surface islands grown on higher terraces of the Si substrate. The density of threading dislocations underneath the hexagonal islands is much less than elsewhere in the film, which is presumably due to dislocation annihilation during the island growth process.

Comparison of the Microstructure and Optical Properties of InAs Quantum Dots Grown with/without an AlAs Insertion Layer

Be microstructure and optical properties of InAs quantum dots (QDs) grown on a GaAs buffer with a 30 nm thick AlAs insertion layer are investigated and compared with those grown on a plain GaAs buffer by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The former InAs QDs exhibit larger dot sizes of 20 nm and higher aspect ratios of 0.4, compared to 15 nm and 0.2, respectively, for the latter. Temperature-dependent PL spectra of the larger dots show that the main emission is dominated by band-tail state transitions at low temperatures and ground state transitions at high temperatures. The ground state transition energy in such quantum dots is significantly red-shifted compared to the smaller InAs QDs. Lower thermal activation energy is also observed for the larger QDs with an AlAs layer. All of the phenomena are caused by different In-Ga intermixing behavior occurring during capping, which is discussed in detail.

Self-clustering phenomenon of epitaxial FeSi nanoislands on Si(001)

This study investigates the evolution in the growth of FeSi nanoislands on Si(001). Under proper growth conditions, nanoislands spontaneously cluster into groups on rectangular FeSi terraces depending on both substrate temperature and deposition coverage. This study discussed the self-clustering mechanism in the context of strain relaxation and mass transportation between nanoislands and terraces.

Molecular beam epitaxy growth of wurtzite AlN nanotips

The characteristics of structure and morphology of AlN nanotips grown under higher V/III ratio on Si (111) with plasma-assisted molecular beam epitaxy are herein investigated. We found that the AlN nanotips were single crystalline with {1-211} inclined facets and embedded in pitlike defects of N-polarity. The density and size of the AlN nanotips can be controlled by the growth conditions. The AlN nanotip growth mechanism can be rationalized as the c-type dislocations generated between two adjacent grains due to the formation of higher strain area in the early stages of growth. The c-type dislocation would reverse the stacking sequence of the following adatoms, leading to the AlN nanotip growth with inverse polarity and higher growth rate compared to the surrounding matrix. These nanotips might serve as the ideal templates for further growth of nanostructure devices.