Manoj Kesaria

Nitrogen flux induced GaN nanostructure nucleation at misfit dislocations on Al2O3(0001)

The work demonstrates the dominant role of nitrogen flux rate on GaN nanostructure formation on bare Al2O3(0001). In nitrogen rich conditions, wurtzite c-oriented GaN nanowall honeycomb network is formed as strain relaxation pathway of nucleation at edge dislocations. A specific nitrogen flux rate in a plasma assisted molecular beam epitaxy growth is necessary for fixed Ga flux and substrate temperature to form columnar self assembled nanostructures. It is argued that kinetically hindering diffusion of Ga adatoms and the low sticking coefficient of r and m planes of nanowalls promote 1-dimension nanocolumn formation at screw dislocations formed at the GaN-Sapphire interface.

Superstructure of self-aligned hexagonal GaN nanorods formed on nitrided Si(111) surface

We present here the spontaneous formation of catalyst-free, self-aligned crystalline (wurtzite) nanorods on Si(111) surfaces modified by surface nitridation. Nanorods grown by molecular beam epitaxy on bare Si(111) and non-stoichiometric silicon nitride interface are found to be single crystalline but disoriented. Those grown on single crystalline Si3N4 intermediate layer are highly dense c-oriented hexagonal shaped nanorods. The morphology and the self-assembly of the nanorods shows an ordered epitaxial hexagonal superstructure, suggesting that they are nucleated at screw dislocations at the interface and grow spirally in the c-direction. The aligned nanorod assembly shows high-quality structural and optical emission properties.

High electron mobility through the edge states in random networks of c-axis oriented wedge-shaped GaN nanowalls grown by molecular beam epitaxy

Transport and optical properties of random networks of c-axis oriented wedge-shaped GaN nanowalls grown spontaneously on c-plane sapphire substrates through molecular beam epitaxy are investigated. Our study suggests a one dimensional confinement of carriers at the top edges of these connected nanowalls, which results in a blue shift of the band edge luminescence, a reduction of the exciton-phonon coupling, and an enhancement of the exciton binding energy. Not only that, the yellow luminescence in these samples is found to be completely suppressed even at room temperature. All these changes are highly desirable for the enhancement of the luminescence efficiency of the material. More interestingly, the electron mobility through the network is found to be significantly higher than that is typically observed for GaN epitaxial films. This dramatic improvement is attributed to the transport of electrons through the edge states formed at the top edges of the nanowalls.

Highly-mismatched InAs/InSe heterojunction diodes

We report on heterojunction diodes prepared by exfoliation and direct mechanical transfer of a p-type InSe thin film onto an n-type InAs epilayer. We show that despite the different crystal structures and large lattice mismatch (∼34%) of the component layers, the junctions exhibit rectification behaviour with rectification ratios of ∼104 at room temperature and broad-band photoresponse in the near infrared and visible spectral ranges.

InSb quantum dots for the mid-infrared spectral range grown on GaAs substrates using metamorphic InAs buffer layers

Type II InSb/InAs quantum dots (QDs) were successfully grown on GaAs substrates using three different metamorphic buffer layer (MBL) designs. The structural properties of the resulting metamorphic InAs buffer layers were studied and compared using cross-sectional transmission electron microscopy and high resolution x-ray diffraction measurements. Photoluminescence (PL) originating from the InSb QDs was observed from each of the samples and was found to be comparable to the PL of InSb QDs grown onto homo-epitaxially deposited InAs. The 4 K PL intensity and linewidth of InSb QDs grown onto a 3 µm thick InAs buffer layer directly deposited onto GaAs proved to be superior to that from QDs grown onto an InAs MBL using either AlSb or GaSb interlayers. Light-emitting diode structures containing ten layers of InSb QD in the active region were subsequently fabricated and electroluminescence from the QDs was obtained in the mid-infrared spectral range up to 180 K. This is the first step towards obtaining mid-infrared InSb QD light sources on GaAs substrates.

The structural evolution of InN nanorods to microstructures on Si (111) by molecular beam epitaxy

We report the catalyst free growth of wurtzite InN nanorods (NRs) and microislands on bare Si (111) by plasma-assisted molecular beam epitaxy at various temperatures. The morphological evolution from NRs to three dimensional (3D) islands as a function of growth temperature is investigated. A combination of tapered, non-tapered, and pyramidal InN NRs are observed at 490 °C, whereas the InN evolves to faceted microislands with an increase in growth temperature to 540 °C and further developed to indented and smooth hemispherical structures at extremely high temperatures (630 °C). The evolution from NRs to microislands with increase in growth temperature is attributed to the lowering of the surface free energy of the growing crystals with disproportionate growth velocities along different growth fronts. The preferential adsorption of In atoms on the (0001) c-plane and (10-10) m-plane promotes the growth of NRs at relatively low growth temperature and 3D microislands at higher temperatures. The growth rate imbalance along different planes facilitates the development of facets on 3D microislands. A strong correlation between the morphological and structural properties of the 3D films is established. XRD studies reveal that the NRs and the faceted microislands are crystalline, whereas the hemispherical microislands grown at extremely high growth temperature contain In adlayers. Finally, photoluminescent emissions were observed at ~0.75 eV from the InN NRs.

Transport and optical properties of c-axis oriented wedge shaped GaN nanowall network grown by molecular beam epitaxy

The transport and optical properties of wedge-shaped nanowall network of GaN grown spontaneously on cplane sapphire substrate by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) show interesting behavior. The electron mobility at room temperature in these samples is found to be orders of magnitude higher than that of a continuous film. Our study reveals a strong correlation between the mobility and the band gap in these nanowall network samples. However, it is seen that when the thickness of the tips of the walls increases to an extent such that more than 70% of the film area is covered, it behaves close to a flat sample. In the sample with lower surface coverage (≈40% and ≈60%), it was observed that the conductivity, mobility as well as the band gap increase with the decrease in the average tip width of the walls. Photoluminescence (PL) experiments show a strong and broad band edge emission with a large (as high as ≈ 90 meV) blue shift, compared to that of a continuous film, suggesting a confinement of carriers on the top edges of the nanowalls. The PL peak width remains wide at all temperatures suggesting the existence of a high density of tail states at the band edge, which is further supported by the photoconductivity result. The high conductivity and mobility observed in these samples is believed to be due to a “dissipation less” transport of carriers, which are localized at the top edges (edge states) of the nanowalls.

Resonant Zener tunnelling via zero-dimensional states in a narrow gap diode

Interband tunnelling of carriers through a forbidden energy gap, known as Zener tunnelling, is a phenomenon of fundamental and technological interest. Its experimental observation in the Esaki p-n semiconductor diode has led to the first demonstration and exploitation of quantum tunnelling in a condensed matter system. Here we demonstrate a new type of Zener tunnelling that involves the resonant transmission of electrons through zero-dimensional (0D) states. In our devices, a narrow quantum well of the mid-infrared (MIR) alloy In(AsN) is placed in the intrinsic (i) layer of a p-i-n diode. The incorporation of nitrogen in the quantum well creates 0D states that are localized on nanometer lengthscales. These levels provide intermediate states that act as “stepping stones” for electrons tunnelling across the diode and give rise to a negative differential resistance (NDR) that is weakly dependent on temperature. These electron transport properties have potential for the development of nanometre-scale non-linear components for electronics and MIR photonics.

Room temperature mid-infrared InAsSbN multi-quantum well photodiodes grown by MBE

Room temperature photoresponse in the mid-infrared spectral region is demonstrated from InAsSbN/InAs multi-quantum well photodiodes grown by nitrogen plasma assisted molecular beam epitaxy. The structural quality of the InAsSbN MQWs was ascertained in situ by reflection high energy electron diffraction and ex situ by high resolution x-ray diffraction and photoluminescence measurements. The extended long wavelength photoresponse is identified to originate from the electron–heavy hole (e1–hh1) and electron–light hole (e1–lh1) transitions in the InAsSbN MQW, with a cut off wavelength ~4.20 µm and peak detectivity D *  =  1.25  ×  109 cm Hz1/2 W−1.

Effect of N2* and N on GaN nanocolumns grown on Si(111) by molecular beam epitaxy

The self-induced growth of GaN nanocolumns (NCs) on SixN1−x/Si (111) is investigated as a function of the ratio of molecular to atomic nitrogen species generated via plasma assisted molecular beam epitaxy. Relative concentrations of the molecular and atomic species are calculated using optical emission spectroscopy. The growth rate (GR), diameter, and density of NCs are found to vary with the molecular to atomic nitrogen species relative abundance ratio within the plasma cavity. With increasing ratio, the GR and diameter of NCs increase while the density of NCs seems to be decreasing. The morphologies and the coalescence of GaN NCs exhibit a trend for molecular/atomic ratios up to 11, beyond which they still change but at a lower rate. The detrimental effect of taperedness of the NCs decreases with increasing molecular/atomic ratios. This is possibly because of reduction in radial growth in NCs due to increase in diffusivity of nitrogen species with increasing ratios.