Heikki Helava

Growth of Low-Defect SiC and AlN Crystals in Refractory Metal Crucibles

Recently the wide bandgap semiconductors, silicon carbide (SiC) and aluminum nitride (AlN), have acquired increased importance due to the unique properties that make them applicable to a variety of rapidly-emerging, diverse technologies. In order to meet the challenges posed by these applications the materials need to be manufactured with the highest possible quality, both structural and chemical, at increasingly lower cost. This requirement places rather extreme constraints on the crystal growth as the simultaneous goals of high quality and low cost are generally incompatible. Refractory metal carbide technology, particularly, tantalum carbide (TaC), was originally developed for application in highly corrosive and reactive environments. The SiC group of Prof Yuri A Vodakov (for example, [1]) at Karmon Ltd in St Petersburg, Russia was the first to study and utilize the properties of refractory metal carbides, first for the growth of SiC and later for the growth of AlN. We discuss how the refractory metal carbides can answer many of the problems of growing SiC and AlN in a relatively simple and low cost manner.

Experimental and Theoretical Analysis of Sublimation Growth of Bulk AlN Crystals

Sublimation seeded growth of bulk AlN single crystals in a resistance-heated furnace has been studied experimentally and theoretically. The growth temperature, the temperature drop between the AlN powder source and single-crystal seed, and the ambient nitrogen pressure are varied to find the optimal growth conditions. Ambiguous effect of the process parameters on the crystal growth rate is revealed; it is found, in particular, that the growth sharply slows down and even fails at a higher temperature and a lower source-seed temperature drop. Simulation of the process suggests that the effect is associated with certain evaporation of the seed under unfavorable growth conditions. The optimal growth conditions providing a stable growth of single-crystal AlN boules of about 10 mm diameter and 10 mm long at a rate of 0.3-0.5 mm/hr are found. The crystals grown at a near-atmospheric pressure have pronounced hexagonal facet shape, decrease of the pressure favors growth of more rounded crystals with a distinct stepwise surface and few growth centers.

Structural defects responsible for excessive leakage current in Schottky diodes prepared on undoped n-GaN films grown by hydride vapor phase epitaxy

Schottky diodes fabricated on undoped n-GaN films grown by hydride vapor phase epitaxy showed more than two orders of magnitude higher reverse current if the films contained open core defects. The open core defects were revealed by scanning electron microscope observation in secondary electrons, microcathodoluminescence (MCL), and electron beam induced current (EBIC) modes. Plan-view EBIC imaging showed that such films contained a relatively high density of large (∼10 μm in diameter) dark defects that were absent in good films with low leakage current. In plan-view scanning electron microscope images, pits with the density similar to the density of dark defects were observed. Cross-sectional MCL observation showed that the pits terminated the vertical micropipes starting near the interface with the substrate. Some of the micropipes closed approximately halfway through the grown thickness. The regions of micropipes, either closed or not, showed a higher intensity of bandedge and defect MCL bands. Possible ...

Sublimation Growth of Bulk AlN Crystals on SiC Seeds

AlN bulk crystals were grown by the sublimation “sandwich method” on the SiC substrates. Two types of containers were used: (i) Ta container with a surface layer of TaC created by the special annealing in contact with carbon, (ii) TaC container created by pressing of TaC powder. Cryptocrystalline AlN wafers grown by oversublimation of the original industrial high purity AlN powder were used as a vapor source. So a considerable decrease of oxygen concentration in the source (10 – 30 times) was achieved. 4H and 6H SiC bulk crystals grown by Nitride Crystals, Ltd., which were used as wafers, were crack-free, micropipe-free and have a low dislocation density (1- 4.103cm-2). The method allowed to grow thick AlN bulk crystals up to 5mm height and up to two inches in diameter with smooth mirror-like surface. X-ray diffractometry and topography of the grown AlN layers show that FWHMs of the rocking curves in ω-scan lie in the range of 60-120 arcsec.

Status of 3" 6H SiC Bulk Crystal Growth

In this paper, we report on the current status of our technology for the commercial production of 3” 6H-SiC substrates, including PVT growth [1] of more than 3” diameter and up to 20 mm long 6H-SiC boules, post-growth processing of the boules, and characterization of the produced wafers. We discuss the preparation of SiC sources and seeds, the initial transient stage of the growth, the distribution of temperature in the growth crucible, and the Si/C ratio in the vapor. Special attention is given to the rise of the process stability and the reduction of crystallographic defects, including micropipes (open core screw dislocations), low-angle grain boundaries, foreign polytype inclusions, and graphite inclusions [2,3].

Faceted Growth of SiC Bulk Crystals

In this paper, we suggest a model of facet formation during bulk SiC growth by Physical Vapor Transport (PVT). The model considers the step-flow growth, with the step density dependent on the local orientation of the crystallization front with respect to the close-packed crystal planes. The growth kinetics employs the Burton-Cabrera-Frank approach extended to binary compounds and a multi-component vapor. Being implemented into a 2D simulator, the model is applied to analysis of faceting in free-spreading bulk SiC growth. The computations predict the crystal shapes very similar to those observed experimentally. The faceting influence on the overall crystal growth rate is discussed.

HVPE GaN Growth on 4H SiC and Die Dicing

Hydride Vapor Phase Epitaxy (HVPE) was used to grow 1-4 μm thick undoped GaN layers on 4H-SiC and sapphire substrates. To adjust mechanical strain and crack formation in the GaN/SiC samples, the AlGaN-based buffer layer was grown at low temperature (920-980°C) and the GaN layer was grown at a higher temperature (1000-1040°C). Laser scribing through the GaN layer or the SiC substrate was applied to fabricate dies from the GaN/SiC and GaN/sapphire samples. The laser irradiation passing through the GaN layer to the sapphire substrate or through the SiC substrate to the GaN layer, along two orthogonal directions created a net of micro-cavities in sapphire and melted grooves in SiC that promote easy breakage of the sample into rectangular dies.

Highly Sensitive NO2 Graphene Sensor Made on SiC Grown in Ta Crucible

Graphene films were grown on SiC substrates by annealing in vacuum or in Ar flow. Gas sensors based on graphene films were made and tested on response to nitrogen dioxide. Graphene film is used in the sensor. The graphene film grown by annealing in Ar flow shows superior sensitivity compared to that annealed in vacuum. Both sensors exhibit good potential for environmental research and monitoring

Investigation of direct water photoelectrolysis process using III-N structures

GaN, GaN/AlGaN and GaN/InGaN-based structures were used to study water photoelectrolysis in KOH-based electrolyte, measurement of current-potential characteristics, investigation of electrode corrosion and for hydrogen generation. The corrosion process of p-n AlGaN/GaN structure starts in the p-layers, spreads via vertical channels associated with threading defects, and continues laterally along the n-layers, where large local hollows and voids were observed. The H2 production rate of 0.3-0.6 ml/cm2×h was measured for n-GaN structure.

AlN Substrates and Epitaxy Results

AlN substrates are produced by Physical Vapor Transport (PVT) growth of AlN bulk single crystals followed by post growth processing of the crystals (calibration, slicing, lapping, and polishing). The AlN substrates are suitable for epitaxial growth. The substrates may be used for development of devices such as ultra violet (UV) light emitting diodes (LEDs) and laser diodes (LDs), Piezo-Electric Transducers, SAW devices, RF Transistors, etc.