Dislocation properties in InGaN/GaN Quantum Wells and GaN grown on bulk GaN and sapphire substrates by metalorganic chemical vapor deposition (MOCVD) were characterized using cathodoluminescnece (CL), transmission electron microscopy (TEM), atomic force microscopy (AFM) and photoluminescence (PL). It was clearly demonstrated that dislocations act as nonradiative recombination centers in both n-type (undoped and Si-doped) GaN and InGaN layers. Furthermore the very short-minority carrier diffusion length was a key parameter to explain the high light emission efficiency in GaN-based light emitting diodes (LEDs) prepared on sapphire substrates. On the other side band-tail states were detected in the heteroepitaxial InGaN layers only by temperature dependence PL measurement. Additionally InGaN phase separation, which consists of few micron domains, has been produced under growth conditions which favors the spiral growth. These results indicate that the dislocations in the InGaN layers act as triggering centers for the InGaN phase separation which cause both a compositional fluctuation and the formation of few micron phase separated domains. The homoepitaxial InGaN layers showed however quite normal behaviors for all characterizations.

Continuous-wave operation at room-tempera-ture has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on FIELO GaN substrates with a backside n-contact. This was made possible by introducing important new concept of reducing threading dislocations that occur during the growth of the GaN substrates. We found that InGaN active layers grown on FIELO GaN are superior to those grown on conventional sapphire substrates in terms of their growth mode and the resultant In compositional fluctuation. The fabricated laser diode shows the threshold current, the threshold current density and the threshold voltage were 36 mA, 5.4 kA/cm2 and 7.5 V, respectively, with the lasing wavelength of 412 nm and internal quantum efficiency as high as 98%.

In order to get a perspective to the future of GaN materials, theoretical and experimental knowledge of the optoelectronic activities of dislocations in hexagonal GaN have been reviewed. Although the dislocations in GaN have been thought to be not quite harmful, a growing number of evidences have been accumulated for the intrinsic noxiousness of the dislocations. There are some inconsistencies between experimental data reported by different groups or at different dates, which can be reconciled by a proposed simple model that takes into account the trapping of free excitons. A transmission electron microscopic study revealed that some type of dislocations exhibit the recombination enhanced dislocation glide effect, suggesting the non-radiative recombination activity of the fresh dislocations. Such intrinsic activities of dislocations in GaN, in both electronical and mechanical respects, will possibly cause great difficulties in optoelectronic devices based on this material when the crystal quality becomes improved.

Dislocation-free thin silicon layers are created on the three kinds of substrates such as oxide film, synthetic quartz glass and sapphire. They are bonded with silicon wafers using hydrogen bonding at room temperature but without any adhesive, and their bonding are changed into covalent bonding at elevated temperature. Thick (2 µm) silicon layers are first produced by surface grinding and polishing, and then thinned to 0.1 µm by plasma assisted chemical etching (PACE). A multiple repeated process of thinning the silicon layer and annealing the bonded silicon/quartz and silicon/sapphire interface is applied for tight bonding between a silicon wafer and a quartz wafer, and a silicon wafer and a sapphire wafer which have different thermal expansion coefficients. In case of bonding with sapphire, oxide with 200 in thickness plays an important role in the preventions of void formation and diffusion of interface contaminants into the silicon layer.

The threading dislocation density and the structure of SIMOX wafers formed under different implantation conditions have been invenstigated using Secco etching, cross-sectional transmission electron microscopy and Raman spectroscopy. The breakdown voltage of the buried oxide layer has also been studied. The dislocation density is greatly affected by the dose and the wafer temperature during implantation. The SIMOX wafer implanted at 180 keV with a substoichiometric dose of 0.4 1018 O+ cm-2 at 550 and subsequently annealed at 1350 has an extremely low dislocation density on the order of 102 cm-2. The effect of the wafer temperature on the reduction of the dislocation density is discussed.