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Forward and reverse current – voltage (I–V) characteristics of N and P-type Schottky diodes on 6H-SiC are compared in a temperature range of room temperature to 550K. While the room temperature I–V characteristics of the N-type Schottky diode after turn-on is more or less linear up to ∼ 100 A/cm2, the I–V characteristics of the P-type Schottky diode shows a non-linear behavior even after turn-on, indicating a variation in the on-state resistance with increase in forward current. For the first time it is shown that at high current densities (> 210 A/cm2) the forward voltage drop across P type Schottky diodes is lower than that across N type Schottky diodes on 6H-SiC. High temperature measurements indicate that while the on-state resistance of N type Schottky diodes increases with increase in temperature, the on-state resistance of P type Schottky diodes decreases with increase in temperature until a certain temperature. While the N-type diodes seem to have soft breakdown characteristics, the P-type diodes exhibit more or less abrupt breakdown characteristics.
The electrical properties of thick oxide layers on n and p-type 6H-SiC obtained by a depoconversion technique are presented. High frequency capacitance-voltage measurements on MOS capacitors with a ∼ 3000 Å thick oxide indicates an effective charge density comparable to that of MOS capacitors with thermal oxide. The breakdown field of the depo-converted oxide obtained using a ramp response technique indicates a good quality oxide with average values in excess of 6 MV/cm on p-type SiC and 9 MV/cm on n-type SiC. The oxide breakdown field was observed to decrease with increase in MOS capacitor diameter.
P-type 6H SiC Schottky barrier diodes with good rectifying characteristics upto breakdown voltage as high as 1000V have been successfully fabricated using metal-overlap over a thick oxide layer (∼ 6000 Å) as edge termination and Al as the barrier metal. The influence of the oxide layer edge termination in improving the reverse breakdown voltage as well as the forward current – voltage characteristics is presented. The terminated Schottky diodes indicate a factor of two higher breakdown voltage and 2–3 times larger forward current densities than those without edge termination. The specific series resistance of the unterminated diodes was ∼228 mΩ-cm2, while that of the terminated diodes was ∼84 mΩ-cm2.
A correlation between gate oxide breakdown in MOS capacitor structures and structural defects in SiC wafers is reported. The oxide breakdown under high applied fields, in the accumulation regime of the MOS capacitor structure, is observed to occur at locations corresponding to the edge of bulk structural defects in the SiC wafer such as polytype inclusions, regions of crystallographic mis-orientation, or different doping concentration. Breakdown measurements on more than 50 different MOS structures did not indicate any failure of the oxide exactly above a micropipe. The scatter in the oxide breakdown field across a 10mm × 10mm square area was about 50%, and the highest breakdown field obtained was close to 8 MV/cm.
Using a fast ramp response technique, the high field characteristics, specifically the breakdown strength, of thermally grown silicon-dioxide (SiO2) and MOCVD grown aluminum-nitride (AIN), on n-type 6H-SiC epilayers is obtained as a function of three different processing conditions for the insulator growth. Significant improvement in the breakdown strength of thermally grown SiO2 after a 30 minute post annealing at 400°C in nitrogen ambient is reported. Further, the influence of temperature profile during the AIN growth on the breakdown strength is reported.
Edge termination is an important aspect in the design of high power p-n junction devices. In this paper, we compare the breakdown characteristics of 4H-SiC p+-n diodes with oxide passivation and with edge termination using either low or high energy ion implantations. N- and p-type epilayers of 4H-SiC were grown by chemical vapor deposition on n+ 4H-SiC wafers. Circular mesa structures of different diameters were patterned and isolated by reactive ion etching. Four types of samples were fabricated. The first group was not implanted or passivated and was left for control. The second type consisted of oxide-passivated diode structures while the third and fourth types were ion implanted with 30 keV Ar+ and 2.2 MeV He+ ions, respectively. The time dependent breakdown characteristics were determined using a fast voltage ramp technique. The reverse bias breakdown voltages and leakage currents of these diodes were different for the different types of the edge termination. Diodes terminated using 2.2 MeV ion implantation yielded the best breakdown characteristics. A majority of the diodes exhibited abrupt breakdown.
In this paper we demonstrate the growth of thick SiC epitaxial layers (≥100 μm) of good structural quality at a high growth rate (>100 μm/hr) by controlling the vapor dynamics during conventional physical vapor transport (PVT) process. We propose that our PVT technique be used to ‘repair’ or ‘heal’ commercially available substrates dominated by micropipes, by ‘filling up’ the micropipes through crystal growth inside the micropipe. Extensive experiments performed on thick SiC epitaxial layers grown on Lely substrates indicate that the thick epitaxial layers are of single polytype of high structural quality, with a single peak X-ray rocking curve of less than 12 arcsecs FWHM.
The possibility of single crystal SiC expitaxial growth on freestanding amorphous carbon films (500–1000 Å) as well as thin amorphous carbon layers deposited on mono-crystalline SiC seeds, by conventional physical vapor transport (PVT) technique, is demonstrated. Preliminary experiments indicate that under certain specific growth conditions, 3D SiC single crystals (100 – 600 Å) of different polytypes can be grown on freestanding amorphous carbon layers, with more or less equal probability of formation for each polytype. On the other hand, under low axial temperature gradients (< 30°C/cm), the SiC epitaxial growth on carbon is amorphous in nature. Also, experimental results that demonstrate two-dimensional single crystal SiC epitaxial growth on an amorphous carbon film deposited on mono-crystalline 6H-SiC wafer, is presented. Experiments performed in our laboratory indicate that monocrystalline SiC growth is possible on amorphous carbon layers upto 0.1 μm thickness.
We have characterized the high electric field breakdown process of several epitaxial 4H-SiC p-n structures with oxide passivation. The breakdown voltage was found to be dependent on the size of the diode structures as well as their proximity to any structural defects. The time dependence of the breakdown process was also measured to determine the characteristics of the breakdown mechanism. This time dependence measurement provides an indication of the quality of the diode structures. Both soft and abrupt breakdown mechanisms were observed showing the influence of defects on the high field behavior of the diode structures. Measurements done with and without the use of Fluorinert fluid did not show any difference in the breakdown voltage indicating that surface flashover breakdown mechanism did not play a major role in the avalanche breakdown process.
The current vs voltage characteristics of 4H-SiC MOS capacitors under deep depletion are obtained by a fast ramp response technique so as to obtain the maximum field that can be applied to the MOS structure without the failure of either the semiconductor or the oxide layer. The experiments on n-type 4H SiC wafers having a 5 μm thick epilayer of 1015 – 1016 cm−3 doping concentration and an oxide layer 1200Å – 1500Å thick, indicate the significant influence of the oxide quality and defects in the semiconductor on the nature of the current response during accumulation and deep depletion measurements. The effect of the conductivity of the oxide layer is reflected clearly in the current response, even though classical C-V measurements do not indicate any abnormality. Apart from obtaining the maximum breakdown fields of the semiconductor and the oxide, the fast-ramp response technique provides useful information about the generation processes associated with defects in the MOS structure.
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