Department of Physics
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Item Two-step growth of InGaN quantum dots and application to light emitters(Wiley, 2007-06) Gangopadhyay, SubhashisA two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an InxGa1–xN nucleation layer (NL) without island structures and an InyGa1–yN formation layer (FL) with an indium content lower than that of the InxGa1–xN NL. The realization of QDs was confirmed by micro-photoluminescence (μ-PL) measurements only for the sample with both the InxGa1–xN NL and the InyGa1–yN FL. The spectral position of the QD ensemble recombination was controlled mainly by the deposition time of the InxGa1–xN NL. Green (∼520 nm) and amber (∼600 nm) LEDs with the QD layers grown by the two-step growth method as the active region were also fabricated and compared with that having InGaN QW layers, reported previously. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Item Purpose-led Publishing logo. Evolution of Ge nanoislands on Si(110)-'16 × 2' surface under thermal annealing studied using STM(IOP, 2009-10) Gangopadhyay, SubhashisThe initial nucleation of Ge nanoclusters on Si(110) at room temperature (RT), annealing-induced surface roughening and the evolution of three-dimensional Ge nanoislands have been investigated using scanning tunneling microscopy (STM). A few monolayers (ML) of Ge deposited at room temperature lead to the formation of Ge clusters which are homogeneously distributed across the surface. The stripe-like patterns, characteristic of the Si(110)-'16 × 2' surface reconstruction are also retained. Increasing annealing temperatures, however, lead to significant surface diffusion and thus, disruption of the underlying '16 × 2' reconstruction. The annealing-induced removal of the stripe structures (originated from '16 × 2' reconstruction) starts at approximately 300 °C, whereas the terrace structures of Si(110) are thermally stable up to 500 °C. At approximately 650 °C, shallow Ge islands of pyramidal shape with (15,17,1) side facets start to form. Annealing at even higher temperatures enhances Ge island formation. Our findings are explained in terms of partial dewetting of the metastable Ge wetting layer (WL) (formed at room temperature) as well as partial relaxation of lattice strain through three-dimensional (3D) island growth.Item Formation and morphology of InGaN nanoislands on GaN(0001)(AIP, 2007-04) Gangopadhyay, SubhashisThe morphology and density of InGaN nanoislands can be controlled by the choice of proper growth conditions for metal organic vapor phase epitaxy. Scanning tunneling microscopy has been used to investigate the dependence of InGaN island morphology on the growth parameters. A heterogeneous nucleation of large InGaN islands with a complex structure is observed after growth at in conjunction with a high In partial pressure. For and low In partial pressure, however, the homogeneous nucleation of small islands of sizes suitable for three-dimensional quantum confinement is found, with very high densities of . The influence of the growth temperature and the In partial pressure is discussed in terms of thermally activated diffusion and surface mobility.Item Growth and formation of InGaN and GaN nano-structures studied by STM(JSTAGE, 2006) Gangopadhyay, SubhashisGrowth and morphology of metal organic vapour phase epitaxy (MOVPE) deposited InGaN nano-islands and molecular beam epitaxy (MBE) grown GaN films on GaN(0001) template layers on sapphire substrates have been investigated using scanning tunneling microscopy. For MOVPE InGaN growth, the nucleation of self-organized nano-structures can be achieved by a careful choice of the growth temperature, the In partial pressure, the growth rate and V/III flux ratio. For growth at 650°C, large spiral disc-like islands are found, preferentially nucleating at GaN substrate defects. At 600°C, islands of smaller average size are observed. Lowering the In flux at this temperature, a homogeneous nucleation of small quantum dot like islands with a density of 1012/cm2 is found. For homoepitaxial MBE growth of thin GaN layers on GaN templates, a layer-by-layer growth mode is observed for Ga rich growth conditions. For growth at 750°C, an atomically resolved 4×4 surface reconstruction with a high defect density is found in the initial growth stage. However, subsequent growth at 790°C leads to the formation of one dimensional nanoclusters of about 3 nm lateral spacing. For GaN growth at a lower Ga-flux, a rougher surface morphology and three dimensional growth is observed. Independent on the Ga flux, one-dimensional nanostructures appear after prolonged growth at higher temperature, which are attribute to the impact of ions emerging from the N-plasma.Item Influence of substrate domain boundaries on surface reconstructions of Ga/Si(1 1 1)(Elsevier, 2004-03) Gangopadhyay, SubhashisThe temperature induced phase transition of the Ga/Si(1 1 1) surface for submonolayer Ga coverages has been monitored by variable temperature scanning tunneling microscopy. After room temperature deposition of about 1/3 monolayer of Ga on Si(1 1 1), the Si(1 1 1) surface is mostly covered with Ga-induced magic clusters in a 7 × 7 like arrangement whereas at the domain boundary regions of the former Si(1 1 1)-7 × 7 reconstruction an increased density of excess Ga islands is found. The magic clusters are stable against annealing up to 350 °C. At this temperature, however, the Ga-islands coalesce and a mixture of and 6.3 × 6.3 structures is formed at the domain boundary regions. At an annealing temperature of 400 °C a phase transformation of the structure to 6.3 × 6.3 is found at the domain boundary region opposite to the usual thermal phase sequence. This can be explained in terms of an interplay of surface kinetics and surface stress.Item C60 submonolayers on the Si(1 1 1)-(7 × 7) surface: Does a mixture of physisorbed and chemisorbed states exist?(Elsevier, 200-09) Gangopadhyay, SubhashisWe have carried out a combined X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy(UPS), and scanning tunnelling microscopy (STM) study of the C60-Si(1 1 1) interaction where the XPS/UPS spectrometer and STM are integrated on a single UHV system. This enables a direct comparison of the XPS/UPS spectra with the STM data and eliminates any uncertainty in C60 coverage measurements. X-ray standing wave measurements and density functional theory calculations have been used to support and interpret the results of the XPS/UPS/STM experiments. Our data conclusively rule out models of C60 adsorption which involve a mixture of physisorbed and chemisorbed molecules [K. Sakamoto, et al., Phys. Rev. B 60 (1999) 2579]. Instead, we find that all molecules, up to 1 monolayer coverage, bond to the surface via Si–C bonds which are predominantly of covalent character.Item Spectro-microscopy of Si doped GaN films(Elsevier, 2006-05) Gangopadhyay, SubhashisThe surface segregation of Si used for doping of GaN films grown by metal-organic vapor phase epitaxy has been detected by spectro-microscopy. Facetted cracks with a threefold symmetry have been observed which extend over several micrometers. From local spectra, an enhanced Si segregation is deduced at the facets as compared to the flat surface. A scheme is presented which allows to extract quantitative information about the local surface concentrations for such facetted surface systems. Following to this scheme, Si coverages as high as approximately 4.5 × 1014 Si atoms/cm2 occur at the facets which clearly proves the segregation tendency of Si.Item Initial stage of silicon nitride nucleation on Si(111) by rf plasma-assisted growth(JSTAGE, 2006) Gangopadhyay, SubhashisThe nucleation of silicon nitride films on Si(111) using a radio frequency nitrogen plasma source has been investigated by scanning tunneling microscopy. The initial nucleation of Si3N4 is always observed at the steps, i.e., either at the step-edges of the initial Si(111) surface or at the step edges of vacancy islands (etch pits) formed on the terrace areas. With increasing nitridation temperature the nitrified patches become larger with lower density and show a triangular shape. After post annealing the triangular nucleation patches at the step-edges disappear and free-standing Si3N4 islands are observed with a hexagonal shape. Nitridation at high temperatures or post-annealing improves the crystalline quality of the nitride films and an atomically resolved honeycomb-like ”8×8” surface reconstruction is observed in STM for thin Si3N4 films grown at 850°C.Item qPlus atomic force microscopy of the Si(100) surface: Buckled, split-off, and added dimers(AIP, 2009-08) Gangopadhyay, SubhashisDimer configurations at the Si(100) surface have been studied with noncontact atomic force microscopy in the qPlus mode at 77 K, using both large (10 nm peak to peak) and small (0.5 nm peak to peak) oscillation amplitudes. In addition to the , , and reconstructions of the pristine surface, a variety of defect types including ad-dimers, vacancies, and split-off dimers have been imaged. Our data appear at odds with the currently accepted structural model for split-off dimers. At low oscillation amplitudes the degree of apparent dimer buckling can be “tuned” by varying the frequency shift set point.Item N-plasma assisted MBE grown GaN films on Si(111)(Wiley, 2006-06) Gangopadhyay, SubhashisGaN films were grown by rf-plasma assisted molecular beam epitaxy on nitrified Si(111) surfaces. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the structural and chemical properties of the GaN films. The XPS results show that GaN growth can be initiated only at temperatures below 650 °C. LEED indicates an improvement of the crystalline quality after introduction of a crystalline Si3N4 interface layer. This is confirmed by STM, where an atomically resolved 3 × 3 reconstruction for thin GaN films is observed as well as a smooth growth morphology