Elke Meissner

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.

Synchrotron radiation x-ray topography and defect selective etching analysis of threading dislocations in GaN

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

Analytical stress characterization after different chip separation methods

Abstract Synchrotron white beam X-ray topography (SXRT) and photoelastic stress measurements were used to characterize resulting strain fields after mechanical dicing and laser grooving of bare silicon wafers. The distribution and propagation of the strain fields can be characterized by both methods. In contrast to mechanical dicing, the laser grooving process creates an inhomogeneous strain field. The influenced area is three times larger compared to mechanical dicing. The effect of the dicing procedure on the resulting mechanical fracture strength of the silicon chips was investigated by 3-point bending tests. The fracture strength of samples with an additional laser grooving process was significantly reduced under tensile load. The fracture pattern of the samples indicated that the strain field generated by the separation process causes initial points for μ-cracks propagation under mechanical load. This analysis can help to optimize dicing processes in order to attain a better reliability of chips with regard to process yields.

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

Abstract In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.

Assessment of dicing induced damage and residual stress on the mechanical and electrical behavior of chips

Due to increased technological and economic requirements during wafer production several materials like metal structures or brittle low-k materials are placed within the dicing street. These materials have a negative effect on the quality and reliability results when using conventional separation methods like mechanical dicing. For this case a laser grooving step, prior mechanical dicing is used to remove these materials out of the dicing street. It is known that each of this separation method creates residual stress in the adjacent material in terms of specific strain fields. In the present study the influence of the strain fields on the mechanical and electrical behavior of chips was investigated. For this reason several laser grooving processes and a mechanical dicing process was compared using a dedicated dicing test chip with sensors to measure the electrical behavior. The mechanical fracture strength of the chips was investigated by 3-point bending tests. Photoelastic stress measurements were used to characterize resulting strain fields. The more extensive the strain fields, the lower the fracture strength, the greater the number of electrically conspicuous chips. The combination of strain field, electrical and mechanical strength characterization can help to optimize the process parameters get better reliability and quality of chips.

A Brief Review on the Na-Flux Method Toward Growth of Large-Size GaN Crystal

The growth of bulk GaN by the Na-flux method is reviewed. The largest GaN crystal thus far produced is two inches in size with thickness of a few millimeters along (0001) axis. The historical development of the method, experimental conditions, and quality of the grown GaN crystals are summarized.

Crystal growth of compound semiconductors with low dislocation densities

This paper will highlight some technological developments in the field of Vertical Gradient Freeze growth of InP and GaAs for providing substrates with low dislocation densities. Furthermore, the role of micropipes and basal plane dislocations during sublimation and epitaxial growth of SiC will be addressed. Finally, different strategies will be illustrated to achieve GaN with high structural perfection.

Taking the Next Step in GaN: Bulk GaN Substrates and GaN-on-Si Epitaxy for Electronics

One of the major factors in determining the quality of GaN technology is the epitaxial step. This chapter reviews two different approaches: the use of bulk GaN substrates and GaN-on-Si epitaxy.

A Practical Example of GaN-LED Failure Cause Analysis by Application of Combined Electron Microscopy Techniques

In this paper, we report a failure case of blue LEDs returned from a field application, and propose a practical way to identify the physical and structural reasons for the observed malfunction by a combination of different electron microscope techniques. Cathodoluminescence imaging and electron beam induced current (EBIC) imaging are employed in order to visualize conductive paths through the device in conjunction with subsequent energy dispersive x-ray analysis (EDS), revealing a metal deposition along cracks in the semiconductor layer which short-circuit the device. We demonstrate that the electron beam induced current imaging, in conjunction with other microscopic and analytical techniques at µm scale, is a powerful combination for clearly resolving and visualizing the cause of failure in the GaN LED chip. However, this represents a case study of a real application, which may not have been generally observed in laboratory testing environment.

Investigations of Critical Structural Defects in Active Layers of GaN-on-Si for Power Electronic Devices

The influence of structural defects in the active layer of GaN-on-Si substrates on the vertical leakage current was studied. The structural defects were analyzed by analytical scanning electron microscopy by means of cathodoluminescence (CL). The leakage current was determined by vertical I-V measurements.Two possibilities were found, which give potential explanations for the variations of the vertical leakage current: i) Threading dislocations, which may partially form leakage paths, were detected by CL imaging. ii) Variations of the carbon doping, which is used to tune GaN to a semi insulating material were revealed by CL spectroscopy.