Christian Kisielowski

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Mergers and Acquisitions The crystallization of small molecules or polymers is often described in terms of a nucleation stage, where initial clusters form, followed by a distinct growth stage. Growth can come from the addition of unbound molecules, or through “Ostwald ripening” where larger crystals grow at the expense of smaller ones due to thermodynamic effects. Zheng et al. (p. 1309) studied the growth of platinum nanocrystals inside a transmission electron microscope using a special liquid cell, allowing observation of crystal growth in situ. Both monomer addition to growing particles and the coalescence of two particles were observed. The specific growth mechanism appeared to be governed by the size of each of the particles. The combination of growth processes makes it possible for an initially broad distribution of particles to narrow into an almost uniform one. Transmission electron microscopy provides details of the growth mechanisms of platinum nanocrystals in solution. Understanding of colloidal nanocrystal growth mechanisms is essential for the syntheses of nanocrystals with desired physical properties. The classical model for the growth of monodisperse nanocrystals assumes a discrete nucleation stage followed by growth via monomer attachment, but has overlooked particle-particle interactions. Recent studies have suggested that interactions between particles play an important role. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution or by particle coalescence. Through the combination of these two processes, an initially broad size distribution can spontaneously narrow into a nearly monodisperse distribution. We suggest that colloidal nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies.

Effect of Si doping on the dislocation structure of GaN grown on the A‐face of sapphire

Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.

Atomic scale indium distribution in a GaN/In0.43Ga0.57N/Al0.1Ga0.9N quantum well structure

Quantitative high resolution electron microscopy (HRTEM) is used to map the indium distribution in a GaN/In0.43Ga0.57N/Al0.1Ga0.9N quantum well at the atomic scale. Samples with atomically flat surfaces were prepared for microscopy by anisotropic chemical etching. The developed preparation procedure minimizes a possible confusion of thickness variations with local compositional fluctuations in the lattice images. An irregular distribution of indium is observed that is attributed to the formation of clusters with estimated diameters of 1–2 nm. The indium concentration gradient across GaN/In0.43Ga0.57N interfaces is measured to extend typically over a distance of 1nm. It is more than twice as large across the In0.43Ga0.57N/Al0.1Ga0.9N interface. Indium segregation into the Al0.1Ga0.9N layer during crystal growth is likely to cause this unusual large width of the In0.43Ga0.57N/Al0.1Ga0.9N interfaces. This introduces an asymmetric In distribution across the quantum well with respect to the growth direction.

Quantum shift of band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes

We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.

INGAN/GAN QUANTUM WELLS STUDIED BY HIGH PRESSURE, VARIABLE TEMPERATURE, AND EXCITATION POWER SPECTROSCOPY

The energies of photo- and electroluminescence transitions in InxGa1−xN quantum wells exhibit a characteristic “blueshift” with increasing pumping power. This effect has been attributed either to band-tail filling, or to screening of piezoelectric fields. We have studied the pressure and temperature behavior of radiative recombination in InxGa1−xN/GaN quantum wells with x=0.06, 0.10, and 0.15. We find that, although the recombination has primarily a band-to-band character, the excitation-power induced blueshift can be attributed uniquely to piezoelectric screening. Calculations of the piezoelectric field in pseudomorphic InxGa1−xN layers agree very well with the observed Stokes redshift of the photoluminescence. The observed pressure coefficients of the photoluminescence (25–37 meV/GPa) are surprisingly low, and, so far, their magnitude can only be partially explained.

Effect of internal absorption on cathodoluminescence from GaN

We have studied optical properties of GaN grown on sapphire by metalorganic chemical vapor deposition in the near band-edge energy range by cathodoluminescence. A large shift of the band-edge luminescence to lower energies is induced by increasing the beam energy. The free exciton position shifts about 20 meV when the beam energy is increased from 5 keV to 25 keV at room-temperature. The effect is explained by internal absorption caused by an exponential absorption tail at the band-edge. An Urbach parameter of about 30 to 40 meV for the exponential band-tail in our samples is estimated by comparing experimental with simulated spectra.

The effects of indium concentration and well-thickness on the mechanisms of radiative recombination in In x Ga 1−x N quantum wells

A correlation of the local indium concentration measured on an atomic scale with luminescence properties of In x Ga 1−x N quantum wells reveals two different types of recombination mechanisms. A piezoelectric-field based mechanism is shown to dominate in samples with thick wells (L > 3 nm) of low indium concentration (x

CHAPTER 7 – Strain in GaN Thin Films and Heterostructures

This chapter reviews the current understanding of stress- and strain-related phenomena in gallium nitride (GaN) thin films and heterostructures. It discusses their impact on thin-film growth if any physical property of thin films is affected. The chapter presents several examples to study the surface morphology of the GaN films, native defect formation, the incorporation of dopants, and photoluminescence. A growth cycle for thin films and heterostructures comprises three steps: the surface nitridation, the buffer layer, and the main-layer growth. The main-layer deposition introduces strain if its lattice parameter differs from the buffer layer at the growth temperature. Lattice parameters are usually changed by a variation of native defect concentrations or by the incorporation of dopants and impurities. Such point defects introduce hydrostatic strain components. Thus, hydrostatic and biaxial strain components coexist in the films and are physically of different origin.

Effect Of Mg, Zn, Si, And O On The Lattice Constant of Gallium Nitride Thin Films

This study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼10 17 cm -3 ) and Zn (∼3 × 10 20 cm -3 ) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 10 21 cm -3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.

MBE-Growth of Strain Engineered GaN Thin Films Utilizing a Surfactant

GaN films were grown on sapphire substrates at temperatures below 725 °C utilizing a Constricted Glow Discharge plasma source. A three dimensional growth mode is observed at such low growth temperatures resulting in films that are composed of individual but oriented grains. The strain that originates from the growth on the lattice mismatched substrate with a different thermal expansion coefficient is utilized to influence the thin film growth. The strain can be largely altered by the growth of suitable buffer layers. Thereby, optical and structural film properties can be engineered. It is argued that the surface diffusion of Ga ad-atoms is affected by engineering the strain. Alternatively, surface diffusion can be influenced by surfactants. It is demonstrated that the use of bismuth as a surfactant allows to modify the surface morphology of the GaN films that reflects the size of the grains in the films. The results suggest that a substantial increase of the oriented grain sizes in the films is possible while maintaining a low growth temperature.