Slective Epitaxy Of GaN In Deep And Submicron Holes And Lateral Growth On Si Substrate

GaN is one of the excellent third generation semiconductor materials,or wide bandgap semiconductor,with 3.4 eV midgap width.It can be used for many advanced semiconductor devices,such as high-performance optoelectronic,high-efficiency power electric,high-frequency electonic devices,etc.GaN semiconductor has wide application prospect and great economic value.However,due to the extreme difficulty in fabricating free-standing single crystal GaN substrate and its extremely high cost,GaN is one of the most difficult semiconductor materials to acquire.This is a very significant bottleneck and has greatly restricted the development of GaN technology.Over the past decades,the most common method to overcome this issue is to grow GaN film on a foreign substrate such as sapphire or SiC substrate,by means of heterogeneous epitaxy technique.Compared with the commonly used substrates such as sapphire and SiC,Si as a substrate to grow GaN has distinct advantages of not only low material cost,but also the easiness to integrate with existing mainstream Si-based CMOS integrated circuit and other Si-based devices and systems.Unfortunately,the large mismatch in lattice constant and thermal expention coefficient between GaN and Si are two big obstacles to grow GaN on Si.On one hand,the typical dislocation density is higher than other commonly used substrates.On the other hand,it can result in cracks on the epitaxial film in severe case,which cannot meet the requirements of manufacturing devices.In order to reduce stress in the Si-based GaN epitaxial film and improve the quality of the GaN crystal,the techniques used in our study are the patterned substrate technology and the confined lateral guided growth.After GaN growth,we study the quality of GaN crystals in these structures.1.The selective growth of GaN in submicron and nano scale size patterned structures is found to be beneficial to reducing the accumulation of stress between GaN and Si substrate heterogeneous interface,the amount of dislocations,and avoiding epitaxial film cracking.Moreover,the selective growth in high aspect ratio patterns can block the threading dislocations.So,we designed the sub-micron and nano-scale patterns,with high aspect ratio greater than 1:1.Combined with the AlN buffer layer,GaN crystals were grown in these small and deep holes.Epitaxy in these small and deep holes achieve nearly dislocation-free high quality GaN crystals,and dislocation filtering effect in these holes on the heteroepitaxy is observed.2.The area of GaN inside those small holes is too small to manufacture the devices whose horizontal size are greater than sub-micron scale.In order to fabricate bigger size GaN semiconductor devices,after growing high quality GaN in aforementioned small holes,this study using two methods to fabricate GaN film with larger lateral dimensions:(1)After the growth of GaN in the holes,epitaxial lateral overgrowth above the mask layer were performed.Furthermore,GaN films from neighboring holes may coalesce to further increase the lateral area of the GaN material;(2)By using the technology of the confined lateral guided growth,the large area of GaN films were grown in cavity structures with 8um to 25 um lateral distance and 300 nm height.Experiment shows that(1)The quality of lateral grown GaN above the holes and over the mask layer is extremely good.But the coalescence of the GaN between adjacent holes generated a lot of stacking faults;(2)GaN crystals were grown in the 8um confined lateral guided growth structures.In summary,by utilizing the advantages of the selective growth of GaN in this study,we designed a variety of deep and sub-micron and nanometer pattern sizes.aspect ratios and a number of confined lateral guided growth structures.Experiment shows that deep and sub-micron hole structures have the effect of reducing dislocations and improving the quality of the GaN crystal.And based on these submicron hole structures,an additional step of the epitaxial lateral overgrowth above the mask layer directly,or the use of confined lateral guided growth,the lateral area of GaN films can be increased significantly,making it suitable for manufacturing of larger size of GaN devices.These epitaxy technologies also provide a new approach for the growth of GaN semiconductor material on other substrates.