Current Research Projects
1.Understanding the Origins of Defects and Strain in 4H-SiC Bulk Crystals and Epitaxial Layers
This study focuses on defect characterization and strain analysis on 4H-SiC bulk crystals and epitaxial layers using Synchrotron x-ray topography. Other techniques such as HRXRD, SEM, HRTEM, optical microscopy and Raman spectroscopy have also been applied. In collaboration with semiconductor companies including CREE, SK Siltron, Pallidus and SICC, we have managed a large amount of data from crystals with various sizes and growth conditions, which provides insight into further optimization of the crystal growth process and thus improve crystal quality. Our recent developments include estimation of lattice strain variation in heavily N-doped 4H-SiC using synchrotron double-crystal contour mapping method and understanding the relationship between basal plane dislocation (BPD) distribution and local basal plane bending in 4H-SiC substrates. Moreover, we have studied dislocation behavior at the early stage of PVT growth and deduced that deflection of threading dislocations by the overgrown of macro-steps are the major source for BPD generation within the early-grown SiC layers.
2.Defect characterization of GaN materials using synchrotron x-ray topography
The wide bandgap semiconductor GaN has been widely applied in optoelectronic devices such as light emitting diodes and laser diodes, as well as in power electronic devices such as switches and inverters. GaN along with the other wide bandgap semiconductors, especially silicon carbide (SiC), are being developed to replace silicon in power electronic devices because of their large electric breakdown field, higher current density and thermal conductivity, faster switching and lower on-resistance. Realization of GaN power devices requires affordable, high quality GaN wafers for commercialization.
In the ARPA-E sponsored Power Nitride Doping Innovation Offers Devices Enabling SWITCHES (PNDIODES) project, GaN wafers grown by various methods, including hydride vapor phase epitaxy (HVPE) and ammonothermal methods, are characterized by both synchrotron white beam x-ray topography (SWBXT) and synchrotron monochromatic beam x-ray topography (SMBXT). The structure and distribution of dislocations, including basal plane dislocations (BPDs), threading screw dislocations (TSDs), threading edge dislocations (TEDs) and threading mixed dislocations (TMDs) can be determined. By studying the dislocation structure and distribution, relationship between crystal growth process and crystal quality can be established. GaN materials in different stages of power device fabrication are also characterized. Dislocation behaviors affected by device processing are therefore revealed.
Other techniques used are x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Image from iganpower.com
3. Study of AlInGaP for Solid State LEDs Applications
(AlxGa(1-x))0.5In0.5P grown lattice matched on GaAs substrate using OMVPE has been applied in LEDs since late 1980s, where the ratio of Al to Ga can be varied to tune the color of the light emitted. The goal of this project is to design more efficient amber and red LEDs by characterizing and mitigating defects associated with high efficiency epitaxy designs. Though high brightness can be achieved, the reliability might be sacrificed, which are usually caused by the defects. In this project, nondestructive characterization technique synchrotron X-ray topography is applied to study the distribution of the defects across the 6-inch wafer, and the relationship between the defect distribution and reliability is analyzed. The samples might exhibit different density of defects (including dislocations, grain boundaries, misfit dislocations and inclusions) if the growth conditions such as temperature, pressure, V/III ratio and doping concentration are changed. Other characterization techniques like TEM, SEM, AFM and XRD are also applied together with synchrotron X-ray topography to build the relationship between the growth conditions and the formation mechanism of the defects.
4.Investigation of Defect Generation and Propagation in Electrically and Photonically Stressed Silicon Carbide
Optical Transconductance Varistor(OTV) relies on the photoconductive property of semi-insulating silicon carbide, which is normally insulating but when illuminated, charge carriers are pumped to the conduction band and the material becomes conductive in proportion to the light intensity. However, very little is scientifically known about defect creation and propagation in highly stressed photonic driven high voltage devices. The recombination process can release energy in the form of phonons into the crystal lattice which may generate, propagate and/or convert defects, and their response to the simultaneous application of high electric field and intense light has not been studied. In this project, both ex-situ and in-situ experiment (white beam and monochromatic beam synchrotron X-ray topography) will be carried out. All dislocation types (TED, TSD, TMD, BPD) will be characterized and mapped out, and the propagation of the defects will be tracked when high electric field and intense light are applied.
Image from opcondys.com/
5. Heated High Energy Implantation and Activation Annealing Studies of 4H-SiC Epi Wafers
4H-SiC has been widely applied to power electronics, such as switches and inverters, due to its noted advantages of wide bandgap, high breakdown voltage and good thermal stability. During device fabrication, ion implantation and annealing process are involved and may generate lattice strain and defects in the crystal. A novel high energy implantation system has been developed at the Tandem Van de Graaf facility at Brookhaven National Laboratory capable of deep implantation down to 12µm in 4H-SiC wafers using ion energies ranging from 13MeV to 66MeV. In this project, the impact of this implantation process both at room and high temperatures on the microstructures in 4H-SiC epi wafers will be investigated. All types of dislocations, such as TEDs, TSDs, BPDs, in 4H-SiC devices will be characterized by both synchrotron white beam x-ray topography (SWBXT) and synchrotron monochromatic beam x-ray topography (SMBXT). Lattice strain will be characterized by synchrotron X-ray rocking curve topography (SXRCT) and reciprocal space mapping (RSM). Since such defects and lattice strain are known to be detrimental to the device performance, correlation between electrical properties and defects/lattice strain will also be performed.
6. Defect Characterization of AlN Substrate Wafers
As direct band gap semiconductor with the largest band gap, aluminum nitride (AlN) is promising in many applications varying from power electronics to ultraviolet (UV) optoelectronics. This study characterizes defects in AlN substrate wafers that can degrade the device performance, such as threading edge dislocations (TEDs), threading screw dislocations (TSDs), basal plane dislocations (BPDs), and low angle grain boundaries. Synchrotron x-ray topography is the main technique used. Other techniques, such as HRXRD, SEM, HRTEM, optical microscopy and Raman spectroscopy might also be applied. We have conducted analysis on several AlN wafers with various size and growth conditions in collaboration with companies including Crystal IS, Hexatech Inc., and others.