Myung Yoon Um

The protection of MgO film against hydration by using Al2O3 capping layer deposited by magnetron sputtering method

Al 2 O 3 capping layer was deposited by DC reactive sputtering method on MgO protective layer deposited by RF magnetron sputtering method. Thickness of capping layer, Al 2 O 3 , was varied from 1 to 15 nm. Deposited MgO thin films were hydrated in the ambience of 80% humidity and at room temperature. Surface morphology and rms roughness were observed by SEM and AMF, respectively. Chemical shift of electron binding energy was observed by XPS. And the composition and concentration of hydrogen were observed by RBS and ERD. From these analyses, it was found that Mg atoms diffused into Al 2 O 3 layer and reacted with moisture at surface, forming Mg(OH) 2 phase during hydration reaction. At the thin capping layer region (1-5 nm thickness), as thickness of Al 2 O 3 increased, total amount of hydration decreased. But, beyond 5 nm thickness, the tendency became reverse. Although Al 2 O 3 -capped MgO thin film was hydrated to some extent, total amount of hydration is much less than uncapped MgO single layer. Therefore, it can be concluded that Al 2 O 3 might be suitable for protecting MgO layer against hydration.

Characterization of Undoped and Nitrogen-Doped 4H-SiC Thin Films by CVD from Bis(trimethylsilylmethane) Precursor

High-quality 4H-SiC epi layers were grown on 8 off-axis (0001) 4H-SiC substrates by chemical vapor deposition (CVD) using a single precursor, bis(trimethylsilylmethane) at a substrate temperature of 1380°C. The background doping level of the undoped epi layers was reduced by adjusting of the CVD chamber pressure and Si/C ratio. As the chamber pressure decreased from 360 to 180 Torr, the carrier concentration of the undoped epi layers decreased from 6.8 × 10 16 to 2.0 x 10 16 cm -3 . Moreover, CH 4 addition of 10 standard cubic centimeters per minute in the chamber at 180 Torr resulted in the reduction of the carrier concentration to 2 × 10 15 cm -3 , which can be explained by the well-known site-competition effect. The impurity incorporation effect on macrostep bunching was also discussed based on the strong correlation of atomic force microscopy topologies and impurity concentration. As the flow rate of nitrogen gas (N 2 ) increased, the electron concentration of the epi layers linearly increased. With variation of the N 2 flow rate, a total n-doping range from 2.0 × 10 17 to 1.0 X 10 21 cm -3 was achieved.

Effect of annealing on electrical properties of Pt/β-SiC contact

Abstract The property of contacts on semiconductors, which are deposited and annealed in high temperatures, is significantly affected by the annealing condition. SiC is one of the most attractive semiconductors applied in high temperature devices. In this study, the possibility of Pt being used as Schottky contact on SiC was examined by investigating the effects of annealing on the electrical properties of Pt/β-SiC contact. The as-deposited Pt/n-type β-SiC contacts showed ohmic property, which is attributed to the donor-like traps at the interface due to the sputtering damage. On the other hand, after annealing the contacts showed Schottky property, which seems to be originated from the annealing of traps and the movement of the junction into the defect-free SiC film during annealing. The barrier height increased with increasing annealing temperature, showing 1.37 eV at the annealing temperature of 900°C.

Morphological and structural characteristics of homoepitaxial 4H-SiC thin films by chemical vapor deposition using bis-trimethylsilylmethane precursor

High quality 4H-SiC monocrystalline films were homoepitaxially grown on 8.0° off-oriented (0001) 4H-SiC at a low temperature (1643 K) by metal-organic chemical vapor deposition (MOCVD). The correlation between the structural properties of the films and the growth parameters, in particular, the substrate temperature, and the flow rate of source material BTMSM, was investigated to elucidate the possible benefits of a single precursor on low-temperature thin film growth. The films were examined by optical microscopy, scanning electron microscopy (SEM), triple crystal X-ray diffractometry (TCD), and photoluminescence. Reciprocal space mapping results of the (0004) Bragg spot showed that the full width at half maximum (FWHM) of the rocking curve of the epilayer grown at 1643 K was 9.3 arcsec, which is extremely low as compared with the lowest value reported (13 arcsec). The high film quality was confirmed by low-temperature photoluminescence. The homoepitaxial growth condition for high quality monocrystalline 4H-SiC was also suggested in terms of the growth temperature and supersaturation of the source material.

Influence of Hydrogen Plasma Treatment and Post-Annealing on Defects in 4H-SiC

In this work, we have investigated the influence of hydrogen plasma treatment on defects in 4H-SiC. Hydrogen was incorporated in 4H-SiC single crystals by plasma treatment, and its effects were characterized by current–voltage measurements and deep-level transient spectroscopy. The reverse leakage current level reduction and the Schottky barrier height increase after hydrogen plasma treatment could be explained by Fermi level (FL) pinning and interface states passivation. Plasma-induced defects were formed by the bombardment of energetic particles on the crystal lattice, but they were annealed out at low annealing temperatures of 400–500°C. Considering the annealing temperature, it can be concluded that these defect levels are related to the carbon vacancy defects. It was also revealed by I–V measurement that the hydrogen passivation of defects was maintained up to 500°C.

Fabrication and Characterization of 4H-SiC Planar MESFET Using Ion- Implantation

4H-SiC planar MESFETs having submicron-gate length were fabricated using ion-implantation and their DC and RF performances were characterized. The ion-implantation process which is essential to fabricate a planar device was investigated. Activation annealing after ion-implantation was performed in induction heating system under Ar atmosphere and the annealing condition was optimized. The fabricated MESFET showed good contact properties and pinch-off characteristics. The possibility of application of planar MESFETs in high voltage operation was suggested. Introduction SiC is expected to be an attractive candidate for the application of high-frequency and high-power devices due to its superior electrical, chemical and thermal properties. 4H-SiC MESFET, especially, has many advantages benefiting from the merits of 4H-SiC [1,2]. However, there is much to be desired in applying typical recess-etched gate structure to SiC because recess dry etching of the gate region degrades the Schottky characteristics [3]. To improve the device performances, planar MESFET having good ohmic and Schottky contact properties was fabricated without recess gate etching [4]. Ion-implantation was used as an essential process to fabricate a planar device [5]. In this work, 4H-SiC planar MESFETs having submicron-gate length were fabricated and the DC and RF performances were characterized. The surface morphology and electrical properties of ion-implanted region were investigated by AFM and Hall measurements. Experimental Fig. 1 shows the schematic cross-sectional structure and microscope image of a fabricated MESFET. The gate-to-source spacing, gate length and gate-to-drain spacing were 1.5 μm, 0.5 μm and 2.0 μm, respectively. The substrate was an n-type 4H-SiC purchased from Cree, Inc., with a lightly doped, 5 μm-thick p-type buffer layer and a 0.2 μm-thick n-type channel layer doped at 1.9 × 10 cm. The fabrication process included mesa isolation, ion-implantation and activation anneal, field oxide formation, ohmic contact formation, gate definition, and pad metallization. The cleaning of substrate was performed very carefully before each procedure. Mesa isolation was performed by reactive ion etching (RIE) using a SiO2 mask. To form highly doped source and drain regions, high-temperature and multiple-energy ion-implantation with phosphorous was carried out. Induction heating system was used to activate the implanted ions. All metals were deposited by e-beam evaporation. Evaporated Ni obtained using post-deposition annealing (PDA) at 1000oC for 2 min in Ar atmosphere was used for ohmic contacts. Gate contact was formed using Ni followed by the deposition of Pt Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1181-1184 doi:10.4028/www.scientific.net/MSF.457-460.1181

Improvements in the Reverse Characteristics of 4H-SiC Schottky Barrier Diodes by Hydrogen Treatments

We fabricated 4H-SiC Schottky barrier diodes with various metals such as titanium, nickel and platinum. Density of interface states Dit and neutral level φo (which is the position that the Fermi level must assume if the surface is electrically neutral) were calculated 1.8×10 12 cm -2 eV -1 and 1.76 eV, respectively. In order to reduce reverse leakage current, hydrogen annealing and hydrogen plasma treatment were performed after Schottky contact metallization. The reverse leakage current measured at -100 V was reduced by one or two order of magnitude. An improvement of reverse leakage current was observed in case of both hydrogen annealing and hydrogen plasma treatment.

Effect of Crucible Design on the Shape and the Quality in 6H-SiC Crystals Grown by Physical Vapor Transport

In this study, we newly designed the inner shape of crucible to improve the growth morphology and the quality of crystal grown by physical vapor transport. The effect of crucible design on the shape and the quality of grown crystal was investigated. Modified crucible contained a truncated-cone shaped pedestal at the bottom. Recrystallization severely occurred on the surface of source in case of a crucible without truncated cone. But for the crucible with it, sublimation of source was relatively uniformly achieved and sublimation yield was also increased. Crystal grown in a crucible with a truncated cone showed better morphology, smaller value of FWHM and less dislocation density than crystal without it. And a further enlargement was achieved in the crystal. The geometry of truncated cone was optimized in order to grow a crystal with best growth morphology and the highest quality. The experimental results were also discussed with the result of temperature distribution calculation. Introduction Silicon carbide (SiC) is expected as a new semiconductor to realize high power, high temperature and high frequency electronic devices. The quality of the grown crystals, however, still needs improvement, because the crystals still contain a lot of micropipes and dislocations. And these improvement of the quality and the enlargement of SiC substrate are main problems to be solved for practical use of the SiC power devices and their applications. Large-sized SiC crystals are generally grown by the physical vapor transport [1]. Enlargement of SiC crystal has been investigated, using the modified platform on which the seed crystal was attached [2] or the lateral growth by newly designed crucible [3]. And a crucible with an inner guide-tube to improve the crystal quality was tried to grow single crystal separated from polycrystal [4]. In this paper, we newly designed the inner structure of the crucible in order to improve the growth morphology and the crystal quality. The effect of crucible design on the shape and the quality of grown crystal was investigated. Experimental SiC single crystals were grown by physical vapor transport using a quartz tube reactor with a water-cooled jacket. Modified crucible contained a truncated-cone shaped pedestal at the bottom of the crucible. It was installed in order to improve the sublimation yield of the source material and the growth morphology of crystal. The geometrical parameter of the modified crucible was shown in Fig.1. The seed crystal was fixed to the graphite lid. Abrasive commercial SiC powder for a source material was charged in the graphite crucible. And then the graphite crucible was heated to 2200 o C by RF induction heating with a frequency of 20 kHz. The top of the crucible was kept at the temperature of 2200 o C and the temperature between the top and the bottom of the crucible was about 120 o C. Crystals were grown in argon atmosphere and process pressure was kept at 10 torr during the growth. Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 103-106 doi:10.4028/www.scientific.net/MSF.457-460.103

Homoepitaxial Growth of Al-Doped 4H-SiC Using Bis-Trimethylsilylmethane Precursor

In this paper, we talk about Al doped 4H-SiC epitaxial films grown on 8 off-axis (0001) 4H-SiC substrate by cold-wall horizontal CVD system. In order to lower the deposition temperature, we use the novel organo-silicon precursor material, bis-trimethylsilylmethane (BTMSM, C7H20Si2 ). For the p-type doping, TEA (triethylaluminum (C2H5)3Al) was used as Al source. For the control of Si/C ratio, CH4 gas was also added. The n-type and p-type substrates having the size of 6×6 mm were used for C-V measurement, Hall effects measurement, and X-ray diffraction analysis. X-ray diffraction analysis showed that degradation of the crystal quality of SiC epilayer was relatively small despite increasing Al doping contents. In addition, the crystal quality of SiC epilayer was improved in case of adding small amount of Al contents. The similar tendency was observed in the RMS roughness variation of epilayer. A doping concentration of the most heavily doped SiC epilayer was approximately 10 18 cm -3 .

Growth of GaN Films on Porous 4H-SiC Substrate by Metal-Organic Chemical Vapor Deposition

High quality GaN epitaxial film was successfully grown directly on porous 4H-SiC (PSC) substrate. The difficulty in the nucleation of GaN islands, which is typically observed during metal-organic chemical vapor deposition (MOCVD), was solved by using the concept of facet-controlled epitaxial lateral overgrowth (FACELO) technique. The continuous GaN epitaxial film was obtained at reduced chamber pressure and lower N/Ga ratio. The full-width-half-maximum (FWHM) of (10-13) rocking curve of GaN epitaxial film on PSC substrate at the optimized condition was 220 arcsec under a skew symmetric diffraction geometry and the intense bound exciton lines with FWHM of 10 meV and LO phonon replica lines appeared in GaN layer, which indicates that GaN film on PSC substrate is of very high quality.