Thermally stable electrical contacts to 6H silicon carbide

Abstract: Silicon Carbide (SiC) are at present being developed for use in high temperature (≥ 500◦C) and high-power environments under which conventional semiconductors can hardly perform.  Thermally stable electrical contacts are essential to the ability of SiC to function under such extreme conditions.  In this thesis work the effect of high temperature heat treatments of electrical contacts to 6H-SiC have been investigated.  Thermally induced reactions between thin films of Co, Co and Si, W and WC, and silicon face single-crystal 6H-SiC in vacuum at temperatures ranging from 300 to 1100◦C for 1 to 5 hours are investigated by Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), auger electron spectrometry (AES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).  The electrical characteristics of the contacts are characterized using current-voltage (I-V) and capacitance voltage (C-V) measurements.RBS showed that Co reacts with 6H-SiC after 1 hour at 600◦C forming a new phase identified by XRD as Co2Si.  CoSi formation started during anneal at 900◦C and that reaction was completed after 1 hour at 1000◦C.  CoSi2 was not observed to form even at 1350◦C.  Co does not form carbides; thus, the carbon must precipitate somewhere.  A depth profile of carbon revealed a homogenous carbon distribution through the film with a distinct accumulation at the surface.  TEM investigation of the CoSi film revealed a rough morphology and a very jagged interface.  I-V measurements carried out on n-type Schottky contacts after annealing showed an increase in the ideality factor and the leakage current upon reaction.  The estimated barrier heights for the contacts were 1.10 eV (n-type) and 1.70 eV (p-type) before reaction and roughly 1.3 – 1.5 ev for both types after Co2Si formation.  The specific contact resistivity (rC) of n-epilayers with carrier concentration of 7x1018cm-3 was ≤ 2.5x10-3 Wcm2 after CoSi formation.  Corresponding rC values for p-type were 2x1019 cm-3 and 1.05x10-3 Wcm2.To fabricate CoSi2 /SiC contacts, stochiometric deposited films of Co and Si were annealed at 500, 700 and 900◦C, for 5, 2 and 2 hours, respectively.  During the 500◦C annealing CoSi2 was formed via a diffusion-controlled reaction yielding a mirror-like surface.  The annealings at 700◦C and 900◦C had 2 purposes: a) enabling the silicon deficient CoSi2 film to react with the SiC substrate leading to improved adhesion and b) to test the temperature stability of the contacts.  Moreover, TEM investigation of the CoSi2 film revealed a smooth morphology and islands of an amorphous interface layer.  I-V measurements after 900◦C anneal revealed an increase in the ideality factor and the leakage current density.  On low doped epilayers of SiC the obtained barrier height and ideality factors were 1.05 eV, 1.15 (n-type) and 1.90 eV, 1.38 (p-type) and on highly doped epilayers 1.4x1019 cm-3 and 2x1019 cm-3, rC was 3.0±0.4x10-5 Wcm2 and 4.0±0.7x10-6 Wcm2 for n-type and p-type, respectively.Schottky contacts to low-doped 6H-SiC were fabricated via chemical vapor deposition (CVD) at 400◦C followed by reactive ion etch to pattern the contacts. RBS and TEM verified a distinct W/SiW interface after a two-hour vacuum annealing at 800◦C.  I-V and C-V measurements revealed a low n-type barrier height = 0.79 eV, promising for low ohmic applications on high-doped n-type epi-layers.  On p-type a high FBp (1.89 eV at 500◦C) was observed, suitable for rectifying applications since it displayed a current rectification ratio at 200◦C of 107 at ±10 V.WC films of 200nm thickness were deposited by CVD from a WF6/C3H8/H2 mixture at 900◦C onto low-doped epi-layers of 6H-SiC.  TEM cross sections revealed the film to be poly crystalline with small grains yielding a high electrical resistivity.  The interface was clear and unreacted.  The barrier height and ideality factor were 0.79 eV, 1.05 (n-type) and 1.81 eV, 1.13 (p-type) at room temperature.  For the p-type contact a low reverse current density was found after six hours at 500◦C in room atmosphere, and no signs of contact delamination or contact oxidation were observed.The results of this thesis suggest that, by stoichiometric deposition of refractory silicides and carbides, smooth metallic films to SiC are obtained.  The films are thermally stable up to at least 500◦C and reveal useful electrical characteristics.  Hence, they are suitable as contacting layers in device mentalizations to SiC intended for high temperature and high-power operation.I-V (current -voltage), C-V (capacitance-voltage), rectifying, ohmic, interface, silicide, carbide, epilayer, Schottky barrier height (FB), current rectification ratio (CRR), specific contact resistivity, transmission line model (TLM), contact resistance RC), transfer length (LT).