Practical design of high-voltage SiC Schottky rectifiers requires an understanding of the device physics that affect the key performance parameters. Forward characteristics of SiC Schottky rectifiers follow thermionic emission theory and are relatively well understood. However, the reverse characteristics are not well understood and have not been experimentally investigated in-depth. In this paper we report the analysis and experimental results of both the forward and reverse characteristics of high-voltage SiC Schottky rectifiers. Ti and Ni Schottky rectifiers with boron implant edge termination were fabricated on n-type 4H SiC samples. Ni Schottky rectifiers fabricated on a 13-/spl mu/m thick 3.5/spl times/10/sup 15/ cm/sup -3/ epilayer have a current density of 100 A/cm/sup 2/ at approximately 2 V forward bias and a reverse leakage current density of less than 0.1 A/cm/sup 2/ at a reverse bias of 1720 V. The reverse leakage current is observed to depend on device area, Schottky barrier height, electric field at the metal-semiconductor interface, and temperature (a decreasing temperature dependence with increasing reverse bias). In addition. the reverse leakage current magnitude is larger and the electric field dependence is stronger than predicted by thermionic emission and image-force barrier height lowering. This suggests the reverse leakage current is due to a combination of thermionic field emission and field emission.

Design considerations and experimental analysis of high-voltage SiC Schottky barrier rectifiers

In a method for manufacturing a silicon carbide semiconductor device, preliminary heat treatment is conducted after implanting impurity ions into a silicon carbide substrate, such that the silicon carbide substrate is heated at a temperature in a range of, for example, 800 to 1200° C, in a hydrogen atmosphere or a mixed gas ambient comprising hydrogen and inert gas After the preliminary heat treatment, the silicon carbide substrate may be annealed at a high temperature

Method for manufacturing silicon carbide semiconductor device

Silicon carbide (SiC) devices have the potential to yield new components with functional capabilities that far exceed components based on silicon devices. Selective doping of SiC by ion implantation is an important fabrication technology that must be completely understood if SiC devices are to achieve their potential. One major problem with ion implantation into SiC is the surface roughening that results from annealing SiC at the high temperatures which are needed to activate implanted acceptor ions, boron or aluminum. This paper examines the causes and possible solutions to surface roughening of implanted and annealed 4H-SiC. Samples consisting of n-type epilayers (5 × 1015 cm−3, 4 µm thick) on 4H-SiC substrates were implanted with B or Al to a total dose of 4 × 1014 cm−2 or 2 × 1015 cm−2, respectively. Roughness measurements were made using atomic force microscopy. From the variation of root mean square (rms) roughness with annealing temperature, apparent activation energies for roughening following implantation with Al and B were 1.1 and 2.2 eV, respectively, when annealed in argon. Time-dependent activation and surface morphology analyses show a sublinear dependence of implant activation on time; activation percentages after a 5 min anneal following boron implantation are about a factor of two less than after a 40 min anneal. The rms surface roughness remained relatively constant with time for anneals in argon at 1750°C. Roughness values at this temperature were approximately 8.0 nm. Annealing experiments performed in different ambients demonstrated the benefits of using silane to maintain good surface morphology. Roughnesses were 1.0 nm (rms) when boron or aluminum implants were annealed in silane at 1700°C, but were about 8 and 11 nm for B and Al, respectively, when annealed in argon at the same temperature.

Surface roughening in ion implanted 4H-silicon carbide

The present status of the silicon carbide and gallium nitride bipolar power semiconductor devices is reviewed. Several unipolar and bipolar figures of merit have been examined to demonstrate the potential performance gain to be obtained from silicon carbide and gallium nitride based power devices. Several conventional as well as novel device structures have been examined, some of which have already been demonstrated and others are in their early stages of development. Conventional silicon theory has often been found to be inadequate to explain the characteristics of silicon carbide. Appropriate modifications have been applied to investigate more complicated characteristics of silicon carbide devices.

SiC and GaN bipolar power devices

An optimized multiple floating guard ring structure is investigated for the first time as an edge termination method for high voltage 4H-SiC planar devices. Simulations were performed to investigate SiC guard ring termination, and determine the optimum guard ring spacing for planar diodes with up to four floating rings. Simulated optimized designs predicted breakdown values from 40% of the ideal breakdown with a single ring, to 84% of the ideal value for diodes with four rings. Implanted 4H-SiC pn diodes with optimized guard ring designs were fabricated and results correlated to simulation. Experimental breakdown values of 1.2–1.3 kV for guard ring structure with four rings were in good agreement with simulated results.

Design and fabrication of planar guard ring termination for high-voltage SiC diodes

Aluminum (Al) and boron (B) ion implantations at room temperature into n-type 6H-SiC epilayers have been investigated. Rutherford backscattering spectroscopy (RBS) channeling measurements revealed larger lattice damage in Al+ implantation at a given total implantation dose. A nearly perfect electrical activation ratio (>90%) could be attained by high-temperature annealing at 1600°C for Al+ and 1700°C for B+ implantations. Mesa pn junction diodes formed by either Al+ or B+ implantation with a 1×1014 cm−2 dose exhibited high blocking voltages of 950∼1070 V, which are 80∼90% of the ideal value predicted for the diode structure. The forward current can clearly be divided into two components of diffusion and recombination currents. B+-implanted diodes showed higher breakdown voltage on average but poor forward conduction. Comparison of the performance of Al+ and B+-implanted diodes is discussed.

O. Takemura

Papers

Al + and B + implantations into 6H-SiC epilayers and application to pn junction diodes

Conductivity Control of SiC by In-Situ Doping and Ion Implantation

Implantation of Al and B Acceptors into Alpha-SiC and pn Junction Diodes

Barrier height analysis of metal/4H-SiC Schottky contacts.