Michal Bockowski

III-V nitrides : thermodynamics and crystal growth at high N2 pressure

Abstract In this paper, thermodynamical properties of AIN, GaN and InN, melting, thermal stability and solubility in liquid Al, Ga and In at N 2 pressures up to 20 kbar are considered. It is shown that significant differences in the thermodynamical properties of AlN, GaN and InN are caused mainly by different bonding energy in the solid phase. These differences lead to different results in the crystal growth of AlN, GaN and InN from the solutions in liquid Al, Ga and In, at high nitrogen pressure. High quality, 1-mm single crystals of GaN can be grown in a 5–24 h processes. The crystallization of AlN is less efficient due to the relatively low solubility of AlN in liquid Al, in the experimentally accessible temperature range. Possibility for the growth of InN crystals is strongly limited since this compound loses its stability at T > 600 °C, even at 20 kbar N 2 pressure. The mechanisms of nucleation and growth of GaN crystals is discussed on the basis of the experimental results. The quality of the 1-mm and 1-cm GaN single crystals is compared and discussed in terms of growth stability, which is the necessary condition for obtaining high quality, large single crystals of GaN. The physical properties of pressure grown crystals are reviewed briefly.

Technology of gallium nitride crystal growth

Market for Bulk GaN Crystals.- Development of the Bulk GaN Substrate Market.- Vapor Phase Growth Technology.- Hydride Vapor Phase Epitaxy of GaN.- Growth of Bulk GaN Crystals by HVPE on Single Crystalline GaN Seeds.- Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy Using Void-Assisted Separation Technology.- Nonpolar and Semipolar GaN Growth by HVPE.- High Growth Rate MOVPE.- Solution Growth Technology.- Ammonothermal Growth of GaN Under Ammono-Basic Conditions.- A Pathway Toward Bulk Growth of GaN by the Ammonothermal Method.- Acidic Ammonothermal Growth Technology for GaN.- Flux Growth Technology.- High Pressure Solution Growth of Gallium Nitride.- A Brief Review on the Na-Flux Method Toward Growth of Large-Size GaN Crystal.- Low Pressure Solution Growth of Gallium Nitride.- Characterization of GaN Crystals.- Optical Properties of GaN Substrates.- Point Defects and Impurities in Bulk GaN Studied by Positron Annihilation Spectroscopy.

Crystal growth of III-N compounds under high nitrogen pressure

Abstract The thermodynamical properties of AlN, GaN and InN are analysed in order to estimate the possibility of the synthesis and crystal growth of these nitrides using the high gas pressure technique. The experimental results of the synthesis and crystal growth of AlN, GaN and InN are presented.

Preparation of Free-Standing GaN Substrates from Thick GaN Layers Crystallized by Hydride Vapor Phase Epitaxy on Ammonothermally Grown GaN Seeds

Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is described. The initial growth conditions for HVPE are determined and applied for further bulk growth. Smooth GaN layers up to 1.1 mm thick and of excellent crystalline quality, without cracks, and with low dislocation density are obtained. Preparation of the free-standing HVPE-GaN crystal by slicing and structural and optical quality of the resulting wafer are presented.

Application of a composite plasmonic substrate for the suppression of an electromagnetic mode leakage in InGaN laser diodes

We demonstrate an InGaN laser diode, in which the waveguiding quality of the device is improved by the introduction of highly doped (plasmonic) layer constituting an upper part of the GaN substrate. Thanks to this, we were able to suppress the electromagnetic mode leakage into the substrate without generating additional strain in the structure, in contrast to the typical design relying on thick AlGaN claddings. The plasmonic substrate is built as a stack of gallium nitride layers of various electron concentrations deposited by a combination of hydride epitaxy and high-pressure solution method. The mentioned improvements led to the reduction of the threshold current density of our devices down to 2 kA/cm2 and to the optimization of the near and far field pattern.

Mixed alkaline earth effect in the compressibility of aluminosilicate glasses

The mixed modifier effect (MME) in oxide glasses manifests itself as a non-additive variation in certain properties when one modifier oxide species is substituted by another one at constant total modifier content. However, the structural and topological origins of the MME are still under debate. This study provides new insights into the MME by investigating the effect of isostatic compression on density and hardness of mixed MgO/CaO sodium aluminosilicate glasses. This is done using a specially designed setup allowing isostatic compression of bulk glass samples up to 1 GPa at elevated temperature. A mixed alkaline earth effect is found in the compressibility and relative change of hardness, viz., a local maximum of density as a function of Mg/Ca ratio appears following compression, whereas a local minimum of hardness in the uncompressed glasses nearly disappears after compression. Moreover, the densification of these glasses is found to occur at temperatures much below the glass transition temperature, indicating that a non-viscous mechanism is at play. This is further supported by the fact that density relaxes in a stretched exponential manner upon subsequent annealing at ambient pressure with an exponent of ∼0.62. This is close to the Phillips value of 3/5 for relaxation in three dimensions when both short- and long-range interactions are activated.

Homoepitaxial growth of GaN using molecular beam epitaxy

In this article, experimental results are presented for the homoepitaxial deposition of a GaN overlayer onto a bulk single‐crystal GaN substrate using molecular beam epitaxy. Transmission electron microscopy shows a superior structural quality of the deposited GaN overlayer when compared to heteroepitaxially grown layers. Photoluminescence shows narrow excitonic emission (3.467 eV) and the very weak yellow luminescence, whereas the bulk substrate luminescence is dominated by this deep level emission. These results show that homoepitaxy of GaN can be used to establish benchmark values for the optoelectronic properties of GaN thin films.

Unique effects of thermal and pressure histories on glass hardness: Structural and topological origin

The properties of glass are determined not only by temperature, pressure, and composition, but also by their complete thermal and pressure histories. Here, we show that glasses of identical composition produced through thermal annealing and through quenching from elevated pressure can result in samples with identical density and mean interatomic distances, yet different bond angle distributions, medium-range structures, and, thus, macroscopic properties. We demonstrate that hardness is higher when the density increase is obtained through thermal annealing rather than through pressure-quenching. Molecular dynamics simulations reveal that this arises because pressure-quenching has a larger effect on medium-range order, while annealing has a larger effect on short-range structures (sharper bond angle distribution), which ultimately determine hardness according to bond constraint theory. Our work could open a new avenue towards industrially useful glasses that are identical in terms of composition and density, but with differences in thermodynamic, mechanical, and rheological properties due to unique structural characteristics.

Pressure-Induced Changes in Interdiffusivity and Compressive Stress in Chemically Strengthened Glass

Glass exhibits a significant change in properties when subjected to high pressure because the short- and intermediate-range atomic structures of glass are tunable through compression. Understanding the link between the atomic structure and macroscopic properties of glass under high pressure is an important scientific problem because the glass structures obtained via quenching from elevated pressure may give rise to properties unattainable under standard ambient pressure conditions. In particular, the chemical strengthening of glass through K(+)-for-Na(+) ion exchange is currently receiving significant interest due to the increasing demand for stronger and more damage-resistant glass. However, the interplay among isostatic compression, pressure-induced changes in alkali diffusivity, compressive stress generated through ion exchange, and the resulting mechanical properties are poorly understood. In this work, we employ a specially designed gas pressure chamber to compress bulk glass samples isostatically up to 1 GPa at elevated temperature before or after the ion exchange treatment of a commercial sodium-magnesium aluminosilicate glass. Compression of the samples prior to ion exchange leads to a decreased Na(+)-K(+) interdiffusivity, increased compressive stress, and slightly increased hardness. Compression after the ion exchange treatment changes the shape of the potassium-sodium diffusion profiles and significantly increases glass hardness. We discuss these results in terms of the underlying structural changes in network-modifier environments and overall network densification.

Temperature dependence of superluminescence in InGaN-based superluminescent light emitting diode structures

We have studied the temperature dependence of electroluminescence in superluminescent light emitting diode InGaN structures emitting light at 405 nm. Devices were fabricated in the “tilted ridge” geometry. We measured the superluminescence emission as a function of temperature from 263 to 295 K and observed a very pronounced power sensitivity with temperature. Simple modeling of the optical intensity reveals that the main temperature dependence is related to the spontaneous emission factor in the amplified spontaneous emission and the temperature dependence of gain is of secondary importance. This result strongly suggests the need for reducing nonradiative recombination in superluminescent devices.