Linus C. Chuang

Nanolasers grown on silicon

Based on a CMOS-compatible growth process, researchers successfully demonstrate the bottom-up integration of InGaAs nanopillar lasers onto silicon chips. The resulting nanolaser offers tiny footprints and scalability, making it particularly suited to high-density optoelectronics.

Critical diameter for III-V nanowires grown on lattice-mismatched substrates

The authors report the experimental observation of a critical diameter (CD) of III-V compound semiconductor epitaxial nanowires (NWs) grown on lattice-mismatched substrates using Au-catalyzed vapor-liquid-solid growth. The CD is found to be inversely proportional to the lattice mismatch. NWs with well-aligned orientation are synthesized with catalysts smaller than the CD. Well-aligned InP NWs grown on a Si substrate exhibit a record low photoluminescence linewidth (5.1meV) and a large blueshift (173meV) from the InP band gap energy due to quantization. Well-aligned InAs NWs grown on a Si substrate are also demonstrated.

Atomically sharp catalyst-free wurtzite GaAs∕AlGaAs nanoneedles grown on silicon

We report a catalyst-free, self-assembled growth mode generating single-crystal wurtzite phase ultrasharp GaAs∕AlGaAs nanoneedles on both GaAs and Si substrates via low-temperature metal-organic chemical vapor deposition. The needles exhibit record-narrow tip diameters of 2–4nm wide and sharp 6°–9° taper angles. The length is dependent on growth time and up to 3–4μm nanoneedles are attained. The structures do not exhibit twinning defects, contrary to typical GaAs nanowires grown by vapor-liquid-solid catalyzed growth. AlGaAs layered nanoneedle structures are also demonstrated.

GaAs-Based Nanoneedle Light Emitting Diode and Avalanche Photodiode Monolithically Integrated on a Silicon Substrate

Monolithic integration of III-V compound semiconductor devices with silicon CMOS integrated circuits has been hindered by large lattice mismatches and incompatible processing due to high III-V epitaxy temperatures. We report the first GaAs-based avalanche photodiodes (APDs) and light emitting diodes, directly grown on silicon at a very low, CMOS-compatible temperature and fabricated using conventional microfabrication techniques. The APDs exhibit an extraordinarily large multiplication factor at low voltage resulting from the unique needle shape and growth mode.

Growth mechanisms and crystallographic structure of InP nanowires on lattice-mismatched substrates

We present a growth model that predicts the growth phase and mechanism of InP nanowires (NWs) and the experimental verifications of the model. The NWs were grown on lattice-mismatched GaAs substrates using metal-organic chemical vapor deposition via Au nanodrop-assisted vapor-liquid-solid growth. Nanodrops with larger diameters are shown to grow longer NWs because growth is governed mainly by direct precursor impingement on the nanodrop surface. The theoretical and experimental results also show that growth phase is dependent on NW diameter. We show that InP NWs with a diameter less than a certain value exhibit coherent growth of a single crystalline wurtzite (WZ) phase, whereas larger diameter InP NWs often contain sequences of WZ and zincblende phases and stacking faults. These findings allow one to achieve coherent NW growth and WZ phases free from twinning if the NW diameter is below certain material-dependent critical diameters.

Second-harmonic generation from a single wurtzite GaAs nanoneedle

We demonstrate second harmonic generation from a single GaAs nanoneedle with a wurtzite crystal structure. The optical anisotropy of the polar crystal results in strong nonlinear optical conversion compared to normal zincblende GaAs.

Core-shell InGaAs/GaAs quantum well nanoneedles grown on silicon with silicon-transparent emission

In(x)Ga(1-x)As wurtzite nanoneedles are grown without catalysts on silicon substrates with x ranging from zero to 0.15 using low-temperature metalorganic chemical vapor deposition. The nanoneedles assume a 6 degrees - 9 degrees tapered shape, have sharp 2-5 nm tips, are 4 microm in length and 600 nm wide at the base. The micro-photoluminescence peaks exhibit redshifts corresponding to their increased indium incorporation. Core-shell InGaAs/GaAs layered quantum well structures are grown which exhibit quantum confinement of carriers, and emission below the silicon bandgap.

Optical properties of InP nanowires on Si substrates with varied synthesis parameters

We report the effect of synthesis parameters on the physical appearance and optical properties of InP nanowires (NWs) grown on Si substrates by metal-organic chemical vapor deposition. A strong dependence on the group V to III precursor ratio is observed on the NW shape and, consequently, its photoluminescence (PL). Narrow, uniform-diameter NWs are achieved with an optimized V/III ratio. The uniform NWs exhibit PL widths as low as 1.4meV. Their peak wavelength does not vary much with excitation, which is important for NW lasers on Si. These characteristics are attributed to the one-dimensional density of states in uniform-diameter NWs.

GaAs nanoneedles grown on sapphire

Heterogeneous integration of dissimilar single crystals is of intense research interests. Lattice mismatch has been the most challenging bottleneck which limits the growth of sufficient active volume for functional devices. Here, we report self-assembled, catalyst-free, single crystalline GaAs nanoneedles grown on sapphire substrates with 46% lattice mismatch. The GaAs nanoneedles have a 2–3 nm tip, single wurtzite phase, excellent optical quality, and dimensions scalable with growth time. The needles have the same sharp, hexagonal pyramid shape from ∼100 nm (1.5 min growth) to ∼9 μm length (3 h growth).

Photoluminescence properties of InAs nanowires grown on GaAs and Si substrates.

We report the first photoluminescence (PL) characterization of InAs nanowires (NWs). The InAs NWs were grown on GaAs(111) B and Si(111) substrates using the Au-assisted molecular beam epitaxy (MBE) growth technique or metal-organic chemical vapor deposition (MOCVD). We compared the PL response of four samples grown under different conditions using MBE or MOCVD. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) patterns were utilized to determine the crystal structure and growth directions of the NWs to relate PL features to NW structural parameters. We observed mainly three PL peaks which were below, near and above InAs bandgaps, respectively. Temperature and excitation intensity dependence PL measurements were also performed to help elucidate the origins of the PL peaks of NWs. Of particular interest was a band-edge emission peak that was blue-shifted due to quantization effects of the InAs NWs, as confirmed by our calculation.