Lifan Yan

Formation and Nature of InGaN Quantum Dots in GaN Nanowires

InGaN/GaN disk-in-nanowire heterostructures on silicon substrates have emerged as important gain media for the realization of visible light sources. The nature of quantum confinement in the disks is largely unknown. From the unique nature of the measured temperature dependence of the radiative lifetime and direct transmission electron microscopy, it is evident that such self-organized islands (disks) behave as quantum dots. This is confirmed by the observation of single photon emission from a single disk-in-nanowire and the presence of a sharp minimum in the line width enhancement factor of edge emitting lasers having the InGaN disks as the gain media.

Small signal modulation characteristics of red-emitting (λ = 610 nm) III-nitride nanowire array lasers on (001) silicon

The small signal modulation characteristics of an InGaN/GaN nanowire array edge- emitting laser on (001) silicon are reported. The emission wavelength is 610 nm. Lattice matched InAlN cladding layers were incorporated in the laser heterostructure for better mode confinement. The suitability of the nanowire lasers for use in plastic fiber communication systems with direct modulation is demonstrated through their modulation bandwidth of f-3dB,max = 3.1 GHz, very low values of chirp (0.8 A) and α-parameter, and large differential gain (3.1 × 10−17 cm2).

High performance red-emitting multiple layer InGaN/GaN quantum dot lasers

InGaN/GaN self-organized quantum dots can provide useful advantages over quantum wells for the realization of long-wavelength visible light sources because the dots are formed by strain relaxation. A III–nitride based laser emitting in the red (λ ~ 630 nm), which has not been demonstrated with quantum wells, would be useful for a host of applications. We have investigated the epitaxy and characteristics of self-organized InGaN/GaN multiple layer quantum dots grown by plasma-assisted molecular beam epitaxy and have optimized their properties by tuning the growth parameters. Red-emitting (λ ~ 630 nm) quantum dots have radiative lifetime ~2.5 ns and internal quantum efficiency greater than 50%. Edge-emitting red-lasers with multi-dot layers in the active region exhibit an extremely low threshold current density of 1.6 kA/cm2, a high temperature coefficient T0 = 240 K, and a large differential gain dg/dn = 9 × 10−17 cm2.

Droplet induced compositional inhomogeneities in GaAsBi

Compositional inhomogeneities in III-V alloys heavily influence the device performance. This work presents evidence for Ga droplets inducing inhomogeneities in the Bi composition, which we propose is due to a variation in the Ga flux across the surface. These inhomogeneities may be manipulated through the use of growth interrupts, which eliminate the buildup of Ga at the growth front.

An InN/InGaN/GaN nanowire array guided wave photodiode on silicon

The III-nitride nanowire heterostructure arrays with multiple InN disk light absorbing regions have been grown by plasma-assisted molecular beam epitaxy on (001)Si substrates, and guided wave photodiodes have been fabricated and characterized. The spectral photocurrent of the devices has been measured under reverse bias, and the data exhibit distinct shoulders in the range of 0.69–3.2 eV (0.39–1.8 μm). The estimated responsivity at a wavelength of 1.3 μm is 0.2 A/W. The nanowire photodiode response was also measured with an excitation at one facet provided by an edge-emitting laser fabricated with the same nanowire array and emitting at 1.3 μm.

Structural differences between capped GaSb nanostructures grown by Stranski-Krastanov and droplet epitaxy growth modes

Droplet epitaxy (DE) has emerged as an alternative to Stranski-Krastanov (SK) as a method for epitaxial nanostructure formation. We find significant structural differences of similar sized nanostructures embedded in GaAs between the two methods. Atomic force microscopy and atom probe tomography measurements reveal that uncapped and capped SK structures resemble each other. However, the DE nanostructures appear as rings topographically but are quantum dots compositionally. A GaSb wetting layer is present regardless of the growth method and shares a nearly identical Sb concentration profile. DE nanostructures are shown to have a lower Sb concentration, and transmission electron microscopy measurements reveal that they produce less strain on the capping layer. Despite significant structural differences, SK and DE nanostructures exhibit the same photoluminescence response, suggesting that the emission is from a shared feature such as the wetting layer, rather than the nanostructures.