Hung-Tai Chang

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si self-assembled quantum dots

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si quantum-dot light-emitting diodes is reported. The devices were fabricated in a mesa-type structure, with a silicon oxide layer on the top for surface/sidewall passivation. Different passivation processes were employed. We found that the integrated electroluminescence intensities were relatively less sensitive to temperature, persisting at nearly the same intensity up to RT. The fabricated device shows an internal quantum efficiency of about 0.015% at RT. The improved emission property is attributed to the reduced nonradiative recombination centers due to the surface passivation and thermal treatment.

Catalyst-free growth of indium nitride nanorods by chemical-beam epitaxy

We demonstrate the growth of indium nitride (InN) nanorods on sapphire by chemical-beam epitaxy without a catalyst. The nanorods are synthesized nearly unidirectionally along the ⟨001⟩ direction and the diameters varied in the range of 20–40nm with In∕N flow ratio. Single-crystalline wurtzite structure is verified by x-ray diffraction and transmission electron microscopy. Raman measurements show that these wurtzite InN nanorods have sharp peaks E2 (high) at 491cm−1 and A1 (LO) at 593cm−1.

Electroreflectance studies of InAs quantum dots with InxGa1−xAs capping layer grown by metalorganic chemical vapor deposition

Electroreflectance spectroscopy was used to study the effect of InxGa1−xAs capping layer on InAs quantum dots grown by metalorganic chemical vapor deposition. The optical transitions of the quantum dots and the InxGa1−xAs capping layer were well resolved. The energy shifts in the InxGa1−xAs capping layer show a different trend as compared to a series of referent InxGa1−xAs quantum wells. These results support the concept of strain-driven alloy decomposition during the InxGa1−xAs layer overgrowth.

High quality multifold Ge/Si/Ge composite quantum dots for thermoelectric materials

We present an effective approach to grow high-quality thin film of composite quantum dots (CQDs) as a building block for thermoelectric materials, in which 3 times the usual Ge deposition can be incorporated within a 3-fold CQD. Selective chemical etching experiments reveal that a thin Si inserted layer in the CQDs modifies the growth mechanism through surface-mediated diffusion and SiGe alloying. Such thin-film-like CQD materials are demonstrated to exhibit reduced thermal conductivity κ⊥ with respect to the conventional QDs, perhaps as a consequence of enhanced diffusive phonon scattering from the high Si/Ge interface density and enhanced local alloying effect.

Enhanced light emission from InAs quantum dots in single-defect photonic crystal microcavities at room temperature

The optical properties of InAs quantum dots with photonic crystal microcavity emitting near 1.3μm were investigated at room temperature. The photoluminescence (PL) intensity for quantum dots in cavity was enhanced by two orders of magnitude. The large PL enhancement was attributed to the effects combining the improved extraction efficiency and the enhanced spontaneous emission rate due to the Purcell effect. A threefold Purcell enhancement is observed at room temperature, which is predominantly achieved by the very small mode volume of the photonic crystal microcavity.

Uniform SiGe/Si quantum well nanorod and nanodot arrays fabricated using nanosphere lithography

This study fabricates the optically active uniform SiGe/Si multiple quantum well (MQW) nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using nanosphere lithography (NSL) combined with the reactive ion etching (RIE) process. Compared to the as-grown sample, we observe an obvious blueshift in photoluminescence (PL) spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper multiple quantum dot-like SiGe layers to the lower MQWs. A possible mechanism associated with carrier localization is also proposed for the PL enhancement. In addition, the SiGe/Si MQW nanorod arrays are shown to exhibit excellent antireflective characteristics over a wide wavelength range. These results indicate that SiGe/Si MQW nanorod arrays fabricated using NSL combined with RIE would be potentially useful as an optoelectronic material operating in the telecommunication range.

Temperature stability of single-photon emission from InGaAs quantum dots in photonic crystal nanocavities

This study demonstrates the feasibility to 60K operation of photonic crystal (PC) nanocavities for self-assembled InGaAs quantum dots (QDs) in single-photon applications. With the proper quality factor and small mode volume, this PC nanocavity exhibits excellent spontaneous emission enhancement and high thermal stability. Measuring the second-order correlation function of single QD emission yields clear photon antibunching with a small timing jitter of ∼1ns, which is maintained from T=7to60K. These results demonstrate that PC nanocavities with an appropriate quality factor and mode volume are well suitable for developing thermal-stable single-photon sources.

Optical emission from individual InGaAs quantum dots in single-defect photonic crystal nanocavity

Collective and individual emissions from In0.5Ga0.5As quantum dots (QDs) in single-defect photonic crystal nanocavities are investigated. When the cavity mode is collectively excited by the QD ensemble, a pair of dipolelike modes with definite linear polarizations is shown. Under low excitations, single-exciton emission lines are resolved. The power dependence of individual QD emissions reveals a nearly tenfold light enhancement for on-resonance QDs. The polarization state of each individual QD is also investigated. It is found that an individual dot could excite either a pure dipole mode or a mixture of both dipole modes. This behavior can be attributed to the random distribution of QDs in the nanocavity.

Fabrication of Nanometer-Scale Si Field Emitters Using Self-Assembled Ge Nanomasks

Large-area, nanometer-scale Si field emitters have been fabricated by selective chemical etching of self-assembled Ge islands on Si. Taking advantage of the relatively low etching rate, uniform Ge islands act as virtual nanomasks for the underlying Si substrate. During selective chemical etching, Ge nanomasks shrink into small Ge-core islands, which determine the apex sharpness of the resulting Si pyramidal tips. The results demonstrate that Si pyramidal tips exhibited improved antireflective and electron field emission characteristics compared to as-grown Ge islands. The high field enhancement factor can be attributed to high tip density, nanoscale apex, and well-controlled spacing between Si pyramidal tips. This work offers a low cost alternative for designing and fabricating high efficiency Si-based field emitters or nanodevices.

Boron-Induced Strain Relaxation in Hydrogen-Implanted SiGe/Si Heterostructures

The strain relaxation behavior of H-implanted Si 0.8 Ge 0.2 /Si heterostructures containing a B-doped Si buffer layer was investigated. The annealed H-implanted SiGe/Si samples with a B-doped Si buffer layer exhibit an additional relaxation compared to those with an undoped Si buffer layer. At an annealing temperature of 900°C, relaxation ratios of the H-implanted samples with and without a B-doped Si buffer layer were determined to be 79 and 53%, respectively. The increased relaxation can be attributed to the formation of the larger H-filled bubbles along the interface on both sides of the B-doped Si region. Such an annealed H-implanted SiGe/B-doped Si heterostructure was further demonstrated to have a threading dislocation density of 4.7 X 10 5 cm -2 with a root-mean-square roughness of only 0.48 nm. This work offers an effective approach to fabricate high quality strain-relaxed thin SiGe epilayers for high mobility device applications.