Govindhan Dhanaraj

Silicon Carbide Crystals — Part I: Growth and Characterization

Publisher Summary This chapter reviews the growth and characterization of Silicon Carbide (SiC) Crystals. Recent developments in SiC bulk growth and epitaxial film technology have greatly advanced the SiC-based device technology. The modified Lely method has become a standard process for industrial production of SiC boules. This has been made possible by optimizing the crystal growth technology in conjunction with modeling and computer simulation. The defects, particularly micropipes, have been significantly reduced by improving the growth technique, optimizing the process parameters, and developing better understanding of the defect generation and propagation. Defects present in the SiC crystals have been characterized using X-ray topography, and microscopy-based techniques such as chemical etching, AFM, SEM, TEM, reflection and transmission optical microscopy. Even though many of these techniques are used in a complementary manner to obtain detailed information on defects present in the crystal, X-ray topography, particularly SWBXT, is quite superior to other methods in revealing defects present in the SiC crystals.

Sense determination of micropipes via grazing-incidence synchrotron white beam x-ray topography in 4H silicon carbide

Computer modeling using the ray-tracing method has been used to simulate the grazing-incidence x-ray topographic images of micropipes in 4H silicon carbide recorded using the pyramidal (11−28) reflection. Simulation results indicate that the images of micropipes appear as white features of roughly elliptical shape, canted to one side or other of the g vector depending on the dislocation sense. Observed images compare well with the simulations, demonstrating that the direction of cant provides a simple, nondestructive, and reliable way to reveal the senses of micropipes. Sense assignment has been validated using back-reflection reticulography.

Behavior of Basal Plane Dislocations and Low Angle Grain Boundary Formation in Hexagonal Silicon Carbide

The interactions between basal plane dislocations (BPDs) and threading screw and edge dislocations (TSDs and TEDs) in hexagonal SiC have been studied using synchrotron white beam x-ray topography (SWBXT). TSDs are shown to strongly interact with advancing basal plane dislocations (BPDs) while TEDs do not. A BPD can cut through an individual TED without the formation of jogs or kinks. The BPDs were observed to be pinned by TSDs creating trailing dislocation dipoles. If these dipoles are in screw orientation segments can cross-slip and annihilate also potentially leaving isolated trailing loops. The three-dimensional (3D) distribution of BPDs can lead to aggregation of opposite sign edge segments leading to the creation of low angle grain boundaries (LAGBs) characterized by pure basal plane tilt of magnitude determined by the net difference in densities of the opposite sign dislocations. Similar aggregation can also occur against pre-existing prismatic tilt boundaries made up of TED walls with the net difference in densities of the opposite sign dislocations contributing some basal plane tilt character to the LAGB.

Porous SiC for HT Chemical Sensing Devices: an Assessment of its Thermal Stability

Porous silicon carbide substrates produced by anodization that are intended for use in chemical sensing solid state devices that will operate at high temperatures (at or above 900°C) have been studied with a view to assessing the structural stability/pore modification processes occurring during thermal processing. Annealing experiments were performed using n-type porous 4H-SiC (PSiC) in the temperature range of 900°C-1700°C in Ar atmosphere. The samples were kept at the desired temperatures for 30 minutes. The morphology of the as-received samples consisted of welloriented pores along the <041> direction. Signinficant pore modification occurred at 1400°C. The thickness of the porous layer was reduced following heating at 1700°C. The suitability of PSiC as a chemical sensing element is discussed.

Design and fabrication of physical vapor transport system for the growth of SiC crystals

A physical vapor transport (PVT) system has been designed and fabricated for growing SiC single crystals. Novel multisegmented graphite insulation has been used for improved heat containment in the hotzone. Numerical modeling was applied to obtain the temperature field inside the hotzone, which also helped in predicting various growth parameters. Single crystals of 6H SiC were grown by the modified Lely method using the PVT system developed in the laboratory. The grown crystals were subjected to preliminary characterization.

PVT Growth of 6H SiC Crystals and Defect Characterization

SiC single crystals have been grown by seeded sublimation method using physical vapor transport (PVT) system designed and fabricated in our laboratory. A novel multi-segmented graphite insulation has been used for improved heat containment in the hot-zone. Numerical modeling was used to obtain the temperature field and predict various growth parameters. The grown crystals were characterized using AFM, SWBXT and chemical etching.

X-Ray Topography Techniques for Defect Characterization of Crystals

X-ray topography is the general term for a family of x-ray diffraction imaging techniques capable of providing information on the nature and distribution of structural defects such as dislocations, inclusions/precipitates, stacking faults, growth sector boundaries, twins, and low-angle grain boundaries in single-crystal materials. From the first x-ray diffraction image, recorded by Berg in 1931, to the double-crystal technique developed by Bond and Andrus in 1952 and the transmission technique developed by Lang in 1958 through to present-day synchrotron-radiation-based techniques, x-ray topography has evolved into a powerful, nondestructive method for the rapid characterization of large single crystals of a wide range of chemical compositions and physical properties, such as semiconductors, oxides, metals, and organic materials. Different defects are readily identified through interpretation of contrast using well-established kinematical and dynamical theories of x-ray diffraction. This method is capable of imaging extended defects in the entire volume of the crystal and in some cases in wafers with devices fabricated on them. It is well established as an indispensable tool for the development of growth techniques for highly perfect crystals (for, e.g., Czochralski growth of silicon) for semiconductor and electronic applications. The capability of in situ characterization during crystal growth, heat treatment, stress application, device operation, etc. to study the generation, interaction, and propagation of defects makes it a versatile technique to study many materials processes.

Growth and Characterization of Silicon Carbide Crystals

Silicon carbide is a semiconductor that is highly suitable for various high-temperature and high-power electronic technologies due to its large energy bandgap, thermal conductivity, and breakdown voltage, among other outstanding properties. Large-area high-quality single-crystal wafers are the chief requirement to realize the potential of silicon carbide for these applications. Over the past 20 years, considerable advances have been made in silicon carbide single-crystal growth technology through understanding of growth mechanisms and defect nucleation. Wafer sizes have been greatly improved from wafer diameters of a few millimeters to 100 mm, with overall dislocation densities steadily reducing over the years. Device-killing micropipe defects have almost been eliminated, and the reduction in defect densities has facilitated enhanced understanding of various defect configurations in bulk and homoepitaxial layers. Silicon carbide electronics is expected to continue to grow and steadily replace silicon, particularly for applications under extreme conditions, as higher-quality, lower-priced large wafers become readily available.

Crystal Growth Techniques and Characterization: An Overview

A brief overview of crystal growth techniques and crystal analysis and characterization methods is presented here. This is a prelude to the details in subsequent chapters on fundamentals of growth phenomena, details of growth processes, types of defects, mechanisms of defect formation and distribution, and modeling and characterization tools that are being employed to study as-grown crystals and bring about process improvements for better-quality and large-size crystals.

Growth and surface morphologies of 6H SiC bulk and epitaxial crystals

Bulk crystals and epitaxial layers of 6H SiC have been grown and their surface morphologies have been investigated. Seeded sublimation has been employed to obtain bulk 6H SiC crystals whereas a silicon tetrachloride-propane based chemical vapor deposition (CVD) was used for growing epitaxial layers. The hot-zones were designed using numerical simulation. Growth rates up to 200 μm/hr could be achieved in the CVD process. A new growth-assisted hydrogen etching was developed to reveal the distribution of the micropipes present in the substrate. Morphological features were studied using Nomarski, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and the structural quality was evaluated using synchrotron X-ray topography.