Dec 10, 2018

Comparative study on stress in AlGaN/GaN HEMT structures grown on 6H-SiC, Si and on composite substrates of the 6H-SiC/poly-SiC and Si/poly-SiC


The stresses in GaN-based HEMT structures grown on both single crystal 6H SiC and Si have been compared to these in the HEMT structures grown on new composite substrates engendered as a thin monocrystalline film attached to polycrystalline 3C-SiC substrate. By using HRXRD technique and wafer curvature method we show that stress of monocrystalline layer in composite substrates of the type mono-Si/poly-SiC is lower than 100 MPa and residual stress of epitaxial GaN buffer grown on the composite substrate does not exceed 0.31 GPa, but in the cases of single crystal SiC or Si substrates the GaN buffer stress is compressive in the range of -0.5 ÷ -0.75 GPa. The total stress of the HEMT structure calculated from strains is consistent with the averaged stress of the multilayers stack measured by wafer curvature method. The averaged stress of HEMT structure grown on single crystals is higher than those in structures grown on composites substrates.



Source:IOPscience

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Nov 27, 2018

Current status of self-organized epitaxial graphene ribbons on the C face of 6H–SiC substrates


The current status of long, self-organized, epitaxial graphene ribbons grown on the (0 0 0 −1) face of 6H–SiC substrates is reviewed. First, starting from the early stage of growth it is shown that on the C face of 6H–SiC substrates the sublimation process is not homogeneous. Most of the time it starts from defective sites, dislocations or point defects, that define nearly circular flakes surrounded by bare SiC. These flakes have a volcano-like shape with a graphite chimney at the centre, where the original defect was located. At higher temperatures a complete conversion occurs, which is not yet homogeneous on the whole sample. This growth process can be modified by covering the sample with a graphite cap. It changes the physics of the surface reconstruction during the Si-sublimation process and, on the C face, makes more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of strongly step-bunched areas with, in between, long and large reconstructed terraces covered by graphitic material. Despite the low intrinsic optical absorption of a few graphene layers on SiC, micro-transmission experiments, complemented by micro-Raman spectroscopy, demonstrate that most of this graphitic coverage is made of one or two homogeneous graphene layers. We show also that most of the thermal stress between the graphene layer and the 6H–SiC substrate is relaxed by pleats or wrinkles which are clearly visible on the AFM images. Finally, the results of transport experiments performed on the graphitic ribbons reveal the p-type character of the ribbons.


Source:IOPscience

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Nov 13, 2018

Characterization of Nonuniformity of 6H-SiC Wafers by Photoluminescence Mapping at Room Temperature


Nonuniformity has been demonstrated of commercially available p-type 6H SiC bulk and epitaxial wafers by using photoluminescence (PL) mapping at room temperature. Three deep-level PL bands with peaks at 1.95, 1.35 and 1.05 eV have been detected for the first time besides the near band-edge emission associated with Al acceptors. The intensity variations of these bands on wafers differ depending on the sample. A circular pattern appeared corresponding to the dark core region of the wafer. A one-sided intensity pattern was observed in an epitaxial layer, which is speculated to reflect inhomogeneous gas flow and/or temperature gradient in the reactor. We believe that the ability of the present technique for the characterization of nonuniformity contributes greatly to the improvement of the crystalline quality of SiC.




Source:IOPscience

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Aug 16, 2018

Characterization of defect levels in semi-insulating 6H-SiC by means of photoinduced transient spectroscopy and modulated photocurrent technique

Parameters of electrically active defect centres in vanadium-doped 6H silicon carbide (6H-SiC:V) were investigated by means of the photoinduced transient spectroscopy (PITS) and modulated photocurrent (MPC) method. After a short description of the two techniques, experimental results are presented and briefly compared. Our aim is mainly to understand and explain these experimental results. In particular, in the PITS technique a shallow level seems to be at the origin of negative photoconductivity. Besides, in the same temperature range hole and electron levels can be detected at the same time. Finally, the detection of a given level seems to depend on the photon flux used to perform the PITS experiment. As far as the MPC experiment is concerned, it has put into evidence a very efficient shallow level. A numerical calculation was developed to simulate both experiments in order to understand the experimental results. By means of this simulation, we have explained all the phenomena observed experimentally in each technique and we propose a simple model for the distribution of electrically active defect centres in 6H-SiC:V crystals.


Source:IOPscience

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Aug 3, 2018

Temperature-dependent interface reactions and electrical contact properties of titanium on 6H-SiC

In this study the system Ti-Si-C was investigated in terms of two aspects, metallurgical and electrical, in order to understand the formation and the properties of Ti electrical contacts on n-type 6H-SiC. For the metallurgical investigation bulk diffusion couples were prepared from monocrystalline 6H-SiC and Ti and annealed between 700 and 1200 degrees C for different lengths of time. The reaction zones were investigated using a SEM (secondary electron and backscattered electron images as well as energy-dispersive X-ray analysis). For the investigation of the electrical properties Ti contacts were sputter-deposited onto 6H-SiC wafer stripes and annealed at similar temperatures. The contact properties were measured in terms of current-voltage characteristics. We discovered that, over the whole temperature range investigated, the reaction layer growth follows a parabolic growth law which is thermally activated. Above 1200 degrees C the diffusion path from SiC to Ti is SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti5Si3/Ti. The contacts show ohmic behaviour. Between 1000 and 800 degrees C the diffusion path is: SiC/Ti3SiC2/Ti5Si3/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are also ohmic. Below 700 degrees C the diffusion path is SiC/TiC1-y/two-phase Ti5Si3+TiC1-y/Ti3Si/Ti. The contacts are of Schottky type. It is concluded that the phase in contact with the SiC determines the electrical properties of the junction and the possibility of manufacturing a complete Schottky diode using n-type 6H-SiC and Ti only is demonstrated.


Source:IOPscience

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Jul 18, 2018

Reduction of Hollow Defects in 6H-SiC Single Crystals Grown by Sublimation Boule Growth Technique on (1120) 6H-SiC Substrates

Sublimation boule growth of 6H-SiC on (1120) 6H-SiC substrates was investigated. Hollow defects were generated in the crystals grown on (1120) substrates, and they penetrated along the growth direction to the as-grown surface. A new growth model of SiC on (1120) SiC substrates that distinguishes between two-dimensional growth and three-dimensional growth is proposed. The generation of both the stacking faults and the hollow defects on (1120) substrates was attributable to the three-dimensional growth. The growth conditions were modified and applied to suppress the generation of hollow defects. These conditions included the reduction of the growth temperature in the initial stage of the growth, and the increase of the nitrogen concentration in the vapor phase. Due to the changes in the growth conditions, the density of the hollow defects on (1120) substrates was reduced by one order of magnitude to approximately 103 cm-2. An atomically flat as-grown surface was obtained by the sublimation boule growth technique in nitrogen ambient.

Source:IOPscience

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Jul 1, 2018

Fabrication of Ti ohmic contact to n-type 6H-SiC without high-temperature annealing

The effect of surface morphology of 6H-SiC substrate on the ohmic contact properties of Ti/6H-SiC structure is studied. The H-terminated surface on Si-face 6H-SiC is obtained by both dipping SiC into HF acid solution for 15 s and thermal heating SiC in hydrogen atmosphere at 1100 °C for 10 min, while the H-terminated surface on C-face 6H-SiC could be obtained only by the latter method. Ti is deposited on Si-face and C-face SiC substrates with H-terminated surfaces and ohmic contact is obtained without high-temperature annealing.


Source:IOPscience

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