From the viewpoint of power electronics application, widegap semiconductors such as SiC and GaN have been recently attracted much attention. These materials are expected as electronic materials for high-power switching devices owing to their superior material properties. For the innovation of power electronics contributing to the global warming problem, crystal growth technology of these materials are quite essential. In this report, I will briefly introduce the importance of power electronics and related widegap semiconductor crystal growth technology.
Solving the global warming problem is a great challenge before us, and energy conservation with more efficient use of electricity is the definite way to mitigate the problem since it is the most widely used energy source at home and in the industry. Hence, power electronics is now expected to play a vital role in the global energy conservation scenario. Present-day Si power electronics, however, suffer from the performance limitations due to their material properties. Wide band gap semiconductor silicon carbide (SiC) is a possible solution to this problem and has attracted considerable attention in recent years. This paper reviews the present status and future prospects of the SiC single crystal substrate manufacturing technology. Recent developments of the technology include the attainment of crystal diameters up to 100 mm and the significant reduction in densities of crystallographic defects. Owing to the availability of large high-quality substrates, SiC power devices have begun to show performance levels that largely exceed those produced from Si.
Although the performance of existing SiC power devices is superior to that of Si power devices, SiC power devices are not in general use. This is because their production costs are so high that the devices have not been accepted in the power-electronics market. Because the costs of homoepitaxial growth processes involving chemical vapor deposition (CVD) make up the largest proportion of the total cost of producing SiC wafers, an improvement in the growth rate is highly desirable. In this review, I explain three factors that prevent epitaxial growth and show that homogeneous nucleation in the gas phase is the main factor that prevents high-rate growth. And then I introduce recent developments in the high-rate growth of SiC epitaxial layers by CVD. I focus on three topics that realize high-rate growth, namely partial pressure control method, halide method and high efficient gas supply method.
The formation of extended defects in 4H-SiC epilayers has been surveyed. Generation, conversion and propagation of the extended defects, which are basal plane dislocations, threading dislocations, basal plane Frank-type defects, carrot defects and polytype inclusions, in 4H-SiC epitaxial growth are tracked by performing topography before and after the growth procedure. We also have made collation between the detailed feature of topography contrast and the microscopic structure for the extended defects in a combination of X-ray topography, transmission electron microscopy and KOH defect selective etching analysis, and the formation mechanism of each type of defects is discussed.
Recent progress in fast epitaxial growth and defect control of silicon carbide (SiC) toward development of high-voltage power devices is reviewed. In chemical vapor deposition of 4H-SiC on off-axis (0001), a high growth rate of 85 µm/h and a low background doping of 1×1013 cm−3 are achieved. Conversion of basal-plane dislocations to threading edge dislocations and generation of stacking faults during epitaxial growth are discussed. Deep levels in as-grown n-type and p-type 4H-SiC epitaxial layers have been investigated. A lifetime-killing defect, Z1/2 center, can be almost eliminated by thermal oxidation, which leads to significant increase in carrier lifetimes. The obtained carrier lifetimes are long enough to fabricate 10 kV-class bipolar devices. Control of carrier lifetimes by low-energy electron irradiation is demonstrated.
Hydride vapor phase epitaxy (HVPE) is currently used as a practical method for preparing GaN substrates. However, the reduction of dislocation density and the minimization of wafer curvature are dispensable from device application point of view. To reduce the dislocation density, the authors proposed facet -initiated epitaxial lateral overgrowth (FIELO) method. This method makes it possible to bend and reduce threading dislocations in GaN by forming GaN facet structures near the initial growth stage on foreign substrate, such as sapphire. In addition to the use of stripe-type mask pattern in the FIELO method, a novel FIELO method starting from random-islands having facet sidewalls of GaN formed at relatively low temperature is studied. It is shown that this method is superior to the conventional FIELO for reducing both of dislocations at the hetero-interface and the curvature of freestanding GaN crystal.
Impact of growth rate on the productivity of GaN on Si is described as well as the requirements for the epitaxial growth equipment of Metal Organic Vapor Phase Epitaxy (MOVPE). In order to compensate the large thermal mismatch between GaN and silicon substrate multi-layer buffer structure composed of high aluminum containing AlGaN and GaN is used. Because a parasitic reaction between trimethyl-aluminum and ammonia takes place to bring particulates, it is difficult to employ high growth rate for AlGaN. However, total process time should be less than 4 hours for the growth of 5 μm thick layers in order to make the total epitaxial cost less than an expected commercial price. Therefore, we must optimize the buffer layer structure so that the buffer layer works well as a strain compensating layers as well as the growth time is within an acceptable time limit.
We have achieved a thick AlGaN/GaN HEMT on Si substrate using GaN/AlN multilayer. The multilayer is effective in relaxing the stress in the upper GaN layer. The vertical and horizontal breakdown voltages increased with the increase of the epitaxial layer thickness. A breakdown field of 1.8×106 V/cm was estimated from the vertical breakdown voltage. The horizontal breakdown voltage as high as 1813 V was obtained across 10 μm ohmic gap. We also reported on the influence of deep pits on breakdown of AlGaN/GaN HEMTs on Si. For devices with deep pits, the breakdown was greatly affected by large leakage through buffer and substrate. Cross-sectional transmission electron microscopy image revealed that deep pits originate from Si because of Ga etching Si substrate at thermal cleaning. The three terminal-off breakdown decreased rapidly as the increase of density of deep pits. Both thick and pit- free epitaxial layer are important for the fabrication of device with high- breakdown.
In this paper, GaN-based power transistors for a switching application were reported. In order to realize low loss and high power devices, GaN HFET structure on a Si substrate is a significant configuration as well as one of the cost-effective solutions. Furthermore, attempts for normally-off GaN-FETs were examined. A hybrid MOS-HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).
This paper reviews novel design of epitaxial structure for GaN-based power devices taking advantages of the material's unique polarization. This can be called as polarization engineering which enables low on-state resistances and high breakdown voltages. AlGaN/GaN superlattice and polarization-matched InAlGaN quaternary alloy capping layers effectively reduce the series resistance of AlGaN/GaN heterojunction field effect transistors (HFETs) by reducing the potential barriers above the heterojunction. Natural super junction (NSJ) model is proposed, which well explains the limitless increase of the breakdown voltages of GaN transistor by the extension of the gate-drain spacing. This model is applied to diodes with multi channels of AlGaN/GaN resulting in low on-state resistances and high breakdown voltages. The presented polarization engineering is very promising for future GaN power devices with the improved performances.