Heterogeneous integration of dissimilar single-crystalline materials, such as III-V compound on Si or sapphire, enables functionalities and performance that cannot be achieved with single material system. Yet, lattice mismatch makes growing high quality thin film on dissimilar substrates difficult. Recently high quality nanowires have been demonstrated on substrates with large lattice mismatches [1]. However, in that case, there exists a critical nanowire diameter for a given mismatch, below which single crystalline can be obtained. This, in turn, presents an upper bound of the substrate lattice mismatch to ~15%, on which nanowires with physically meaningful diameters can be attained with the growth of three-dimensional nanostructures. In this paper, we report a completely new growth mechanism that leads to self-assembled, single crystalline GaAs nanoneedles grown on a sapphire substrate with 46% lattice mismatch. The needles exhibit a single crystalline wurzite phase, have a sharp hexagonal pyramid shape and can be scaled to micron size with growth time. The GaAs nanoneedles were grown on a (0001) sapphire substrate in a commercial MOCVD at 400°C. The growth was spontaneous without any prior substrate surface treatment. At typical growth condition, the nanoneedle shows a hexagonal base, six slanted sidewalls and a sharp tip. With one-hour growth time,the nanoneedle has a base diameter of ~600 nm and a height of ~3 μm. Furthermore, high-resolution TEM shows that the tip is ~ 3 nm wide, and the needle has a taper angle of ~11°. TEM and diffraction analysis reveals that the NN has single-crystalline wurtzite structure as opposed to the normal zinc-blende GaAs structure. The effect of growth condition on nanoneedle was studied by varying the growth time. When the growth time was varied, the base dimension and height of the nanoneedle scales linearly, maintaining the same taper angle, even when the growth time was only 1.5 min. This suggests that the nanoneedle nucleates as a seed and grows with a two-dimensional thin-film deposition process on the six sidewall facets. Despite the large 46% lattice mismatch, the NN shows bright photoluminescence. A 514 nm excitation laser is focused down to a ~1.5 μm spot. With 100 μW excitation power, the NN shows emission peak at 1.519 eV at 4 K. The linewidth of the peak is 18 meV. This narrow linewidth indicates the excellent crystal quality of the NN. These high-quality GaAs NNs open an opportunity for integrating high-performance electronic and optoelectronic devices onto highly lattice-mismatched substrates.

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.

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).

Catalyst-free GaAs nanoneedles are grown on a c-plane sapphire substrate at 400°C using MOCVD. Despite of an extremely large lattice mismatch of 46%, the nanoneedles show single wurtzite-phase and bright room-temperature photoluminescence with narrow linewidths.

We spatially resolve photoluminescence from wurtzite InGaAs/GaAs core-shell nanoneedles and characterize their nonuniform quantum well emission. Polarization measurements reveal anisotropy behavior that is reminiscent of GaN and other wurtzite materials.

Zone-center optical phonons are studied via polarized micro-Raman scattering in wurtzite GaAs crystals grown on sapphire. Polarized spectra reveal the A(1)-E(1) splitting in the Raman spectra of all polar optical modes of the wurtzite crystal. The polar mode splitting can be used to estimate a material birefringence of Delta(n)similar to 0.02.

We report and characterize second-harmonic generation from a single wurtzite GaAs nanoneedle. The wurtzite crystal structure of the nanoneedle relaxes the strict nonlinear selection rules of normal zincblende GaAs while maintaining its strong nonlinear optical coefficients. The ability to grow GaAs nanoneedles without catalysts on (111) Si makes them particularly attractive as nonlinear optoelectronic media compatible with complementary metal-oxide-semiconductor technology.

We report novel single-crystalline wurtzite InGaAs/GaAs core-shell quantum well nanoneedles with photoluminescence below the 1.12 eV silicon bandgap, grown on GaAs or Si. This long wavelength enables integration with silicon waveguides and CMOS devices.

We present a general model for the vapor-liquid-solid nanowire (NW) growth rates which accounts for adatom diffusion from the substrate and sidewalls into the Au catalyst drop as well as the Gibbs-Thomson effect of elevated chemical potential in the drop with a curved surface. The growth model is compared with the experimental length-diameter dependences for InP and Si NWs grown via metal organic chemical vapor deposition (MOCVD) and GaAs nanowires grown via molecular beam epitaxy (MBE). We show that MBE growth is affected mainly by adatom diffusion from the substrate, whereas MOCVD growth is affected mainly by direct Au drop impingement and sidewall diffusion. The Gibbs-Thomson effect is shown to limit growth for smaller diameter nanowires. Fits for diffusion lengths and the Gibbs-Thomson radii are determined which explain the experimental length-diameter dependence observed.

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.

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.

We review recent advances in subwavelength high-index-contrast gratings (HCGs) and a variety of applications in optoelectronic devices, including vertical-cavity surface-emitting lasers (VCSELs), tunable VCSELs, high-Q optical resonators, and low-loss hollow-core waveguides (HWs). HCGs can serve as broadband (Delta lambda/lambda ~ 35%), high-reflectivity (>99%) mirrors for surface-normal incident light, which is useful to replace conventional distributed Bragg reflectors in optical devices. HCGs can also be designed as high-Q resonators with output coupling in the surface-normal direction. Finally, we discuss a novel design of HCG as shallow angle reflectors and HWs.

We report an experimental demonstration of generating electron spin polarization with linearly polarized light in a (110) GaAs quantum well. A detailed frequency-domain pump-probe study shows that the dynamic nuclear spin polarization arising from the oriented electron spins results in a strong dependence of the electron spin splitting on the photon energy and intensity of the linearly polarized excitation laser.

We report the polarization properties of Raman scattering in a single wurtzite GaAs nanoneedle. Micro-Raman measurements are performed with varying incident and scattered light polarizations, affecting the relative intensities of the observed zone-center phonon lines.

P-n junction GaAs nanoneedle photodetectors are monolithically grown on a (111) Si substrate by MOCVD with CMOS compatibility. A linear response of the photocurrent to the irradiance can be obtained under room temperature operation.

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.

We present a novel high-Q resonator using high contrast subwavelength grating. The simulated Q-factor of the resonator can be as high as similar to 500,000. A Q-factor of 14,000 is experimentally measured in fabricated devices. (C) 2008 Optical Society of America

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.

We compare InP nanowires grown by MOCVD with a growth rate model which includes both direct precursor impingement and sidewall diffusion effects. The nanowires growth is limited by direct impingement and the Gibbs-Thomson effect.