Processing and Characterization of Silicon Carbide (6H-and 4H-SiC) Contacts for High Power and High Temperature Device Applications

다국어 초록 (Multilingual Abstract)

Silicon carbide is a promising wide bandgap semiconductor material for high-temperature, high-power, and high-frequency device applications. However, there are still a number of factors that are limiting the device performance. Among them, one of the most important and critical factors is the formation of low resistivity Ohmic contacts and high-temperature stable Schottky diodes on silicon carbide.
In this thesis, different metals (TiW, Ti, TiC, Al, and Ni) and different deposition techniques (sputtering and evaporation) were suggested and investigated for this purpose. Both electrical and material characterizations were performed using various techniques, such as I-V, C-V, RBS, XRD, XPS, LEED, SEM, AFM, and SIMS.
For the Schottky contacts to n- and p-type 4H-SiC, sputtered TiW Schottky contacts had excellent rectifying behavior after annealing at 500˚C in vacuum with a thermally stable ideality factor of 1.06 and 1.08 for n- and p-type, respectively. It was also observed that the SBH for p-type SiC (ΦBp) strongly depends on the choice the metal with a linear relationship ΦBp = 4.51 - 0.58Φm, indicating no strong Fermi-level pinning. Finally, the behavior of Schottky diodes was investigated by incorporation of size-selected Au nano-particles in Ti Schottky contacts on silicon carbide. The reduction of the SBH is explained by using a simple dipole layer approach, with enhanced electric field at the interface due to the small size of the circular patch (Au nano-particles) and large difference of the barrier height between two metals (Ti and Au) on both n- and p- SiC.
For the Ohmic contacts, titanium carbide (TiC) was used as contacts to both n- and p-type 4H-SiC epilayers as well as on A1 implanted layers. The TiC contacts were epitaxially deposited using a co-evaporation method with an e-beam Ti source and a Knudsen cell for C60, in a UHV system at low substrate temperature (500˚C). In addition, we extensively investigated sputtered TiW (weight ratio 30:70) as well as evaporated Ni Ohmic contacts on both n- and p-type epilayers of SiC. The best Ohmic contacts to n-type SiC are annealed Ni (> 950˚C) with the specific contact resistance of = 8×10-6 Ωcm2 with doping concentration of 1.1×10-19 cm-3 while annealed TiW and TiC contacts are the preferred contacts to p-type SiC. From long-term reliability tests at high temperature (500˚C or 600˚C) in vacuum and oxidizing (20% O2/N2) ambient, TiW contacts with a platinum capping layer (Pt/Ti/TiW) had stable specific contact resistances for > 300 hours.

목차 (Table of Contents)

• Appended Papers = iii
• Summary of appended papers = vii
• Acknowledgements = ix
• Used Acronyms = xii
• 1. INTRODUCTION = 1
• 2. PROPERTIES OF SIC = 5
• 2.1 SIC MATERIAL PROPERTIES = 5
• 2.1.1 Crystal structure = 5
• 2.1.2 Polytypes of SiC = 5
• 2.2 SIC ELECTRONIC PROPERTIES = 7
• 2.2.1 Density of States (DOS) and Energy bandgap (Eg) = 7
• 2.2.2 Bandgap narrowing = 8
• 2.2.3 Incomplete ionization = 9
• 2.2.4 Carrier recombination = 10
• 2.2.5 Impact Ionization = 10
• 2.2.6 Mobility = 11
• 3. METAL-SEMICONDUCTOR JUNCTIONS = 15
• 3.1 CURRENT TRANSPORT MECHANISM = 15
• 3.1.1 Schottky barrier formation = 15
• 3.1.2 Current transport mechanism = l6
• 3.2 OHMIC CONTACTS = 18
• 3.3 SCHOTTKY DIODE PERFORMANCE = 20
• 3.3.1 Specific on-resistance = 20
• 3.3.2 Forward voltage drop = 21
• 3.3.3 Breakdown voltage and reverse leakage current = 21
• 3.3.4 Edge termination for high breakdown voltage = 23
• 3.3.5 Schottky barrier lowering = 25
• 3.4 OTHER RECTIFIERS = 26
• 3.4.1 Junction barrier Schottky (JBS) diodes = 26
• 3.4.2 Merged P-i-N/Schottky (MPS) diodes = 27
• 3.4.3 DMT (Dual metal trench) diodes = 28
• 3.4.4 TMBS (Trench MOS Barrier Schottky) diodes = 28
• 4. FABRICATION PROCESS = 29
• 4.1 PROCESS DESCRIPTION = 29
• 4.1.1 Wafer preparation and surface cleaning = 29
• 4.1.2 Etching process = 30
• 4.1.3 Deposition Techniques = 32
• 4.1.4 Ion implantation = 35
• 4.1.5 Annealing = 36
• 4.2 TEST STRUCTURES FOR OHMIC CONTACTS = 37
• 4.2.1 Kuphal structure = 37
• 4.2.2 Two-terminal contact resistance methods = 38
• 4.2.3 3-contacts, two-terminal methods = 39
• 4.2.4 Linear transmission line method (LTLM) = 40
• 4.2.5 Circular transmission line method (CTLM) = 42
• 4.2.6 Four-terminal contact resistance method = 43
• 4.2.7 Six-terminal contact resistance method = 44
• 4.2.8 Comparison of each measurement technique = 45
• 5. CHARACTERIZATION AND RESULTS = 47
• 5.1 MATERIAL CHARACTERIZATION = 47
• 5.1.1 X-ray Diffraction (XRD) = 48
• 5.l.2 Secondary Ion Mass Spectrometry (SIMS) = 52
• 5.1.3 Rutherford Backscattering Spectrometry (RBS) = 53
• 5.1.4 Transmission Electron Microscopy (TEM) = 55
• 5.1.5 Atomic Force Microscopy (AFM) & Optical microscopy = 55
• 5.2 ELECTRICAL CHARACTERIZATION OF SCHOTTKY CONTACTS = 57
• 5.2.1 Measurement techniques = 57
• 5.2.2 A review of the Schottky contacts (Paper I, IV, VIII) = 61
• 5.2.3 The relationship between metal work, function and barrier height (Paper IV) = 62
• 5.2.4 Reduction of the Schottky barrier height (Paper VIII) = 65
• 5.3 SPECIFIC CONTACT RESISTANCE MEASUREMENTS = 70
• 5.3.1 TiC and Ti on n- and p-SiC (Paper II, III, V) = 71
• 5.3.2 Ni and TiW (30:70) contacts on n- and p-SiC (Paper V, VI, VII) = 74
• 5.3.3 Microscopic mapping of specific contact resistance (Paper VI, VII) = 76
• 5.4 LONG-TERM RELIABILITY TESTS AT HIGH TEMPERATURE = 80
• 5.4.1 In vacuum = 80
• 5.4.2 In oxidizing ambient = 81
• 6. CONCLUSIONS AND FUTURE WORK = 83
• REFERENCES = 85