Papers by Keyword: Specific Contact Resistance

Abstract: This study shows that a thin Ni film on Al/Ti/4H-SiC metal pads allows to preserve the pad form factor during a 1000 °C/2 min treatment, provided that the Al and Ti film thicknesses are sufficiently thin. Moreover, by reducing the Al to Ti thickness ratio, droplet formation in the contact area is avoided and a mirror-like appearance is obtained. This optimal contact morphology corresponds to a specific contact resistance of few 10^-4 Ωcm² at room temperature on p-type 4H-SiC with resistivity in the range 0.1 – 1 Ωcm.

Abstract: This study shows that an Al-Ti bilayer with an Al to Ti atomic ratio suitable forohmic contacts on p-type 4H-SiC can be covered by a Ni film during the high temperature alloying process, without altering the ohmic nature, while eliminating a detrimental contact morphology caused by the presence of molten Al-Si during alloying. On 1×10²⁰ cm^-3 Al-implanted 4H-SiC layer, the RT specific contact resistance of this Ni-Al-Ti contact measured by TLM-C method is (3 ± 1)×10^-6 Ωcm².

Abstract: For this study, 4H SiC samples were implanted with aluminum at room temperature, 200°C and 600°C with different energies, ranging from 30 to 380 keV, for a total dose of 4x1015 cm 2, to create a “box-like” profile. To activate dopants, samples were then isochronally annealed from 1650°C to 1850°C during 30min. The lowest specific contact resistance achieved, evaluated to 1.3x10-5 Ω.cm2, has been obtained for the 200°C implanted sample annealed at 1850°C. For this condition, Scanning Capacitance Microscopy study has proved that the dopant activity is quite homogeneous in opposition with the samples implanted at RT and 600°C.

Abstract: The electrical properties of epitaxial graphene (EG) grown on 8° off-axis 4H-SiC (0001) by annealing at 1600 °C in inert gas ambient (Ar) were studied. The sheet resistance of the EG layers (R_sh=740±50Ω/sq) and the specific contact resistance of Ni contacts to EG (ρ_c≈6x10^-5 Ωcm²) were evaluated on micrometer scale by measurements on transmission line model (TLM) structures. Si₃N₄ was evaluated as a gate dielectric, showing excellent coverage to EG and a limited effect on its conductivity. The high n-type doping (~10¹³ cm^-2) of EG, as well as the field effect mobility (μ) dependence on n were determined using top gated field effect transistors (FETs) with Si₃N₄ gate dielectric. Electron mean free path (l_loc) and mobility (μ_loc) were also locally determined, on submicrometer scale, by scanning probe microscopy, showing a broad distribution of μ_loc values, with the most probable value very similar to macroscopic carrier mobility μ.

Abstract: In this paper, we evaluated gallium nitride heteroepitaxially grown on sapphire (GaN/Sa) and grown on silicon (GaN/Si) faced to implantation doping. Si⁺ was implanted on low doped n-type epilayers in order to create a plateau around 10²⁰at.cm^-3. All the samples were capped with a silicon oxide and annealed between 1000°C and 1150°C. The surface quality was evaluated in terms of roughness, pit density and maximum pit diameter using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Finally, the dopant electrical activation was studied with Ti-Al contacts using the circular Transfert Length Method (c-TLM). This study shows that low Specific Contact Resistance (SCR) values of 8x10^-5Ω.cm² and 6x10^-6Ω.cm² are respectively obtained on GaN/Sa sample annealed at 1150°C-2min and on GaN/Si sample annealed at 1150°C-30s, proving that good ohmic contacts are obtained on both materials. Nevertheless, a compromise has to be done between the low SCR values obtained and the GaN surface degradation, observed by AFM and SEM after the different annealing treatments and which could affect the good behaviour of the GaN devices.

Abstract: We report on the influence of titanium thickness on the structural and electrical properties of annealed Ti/Ni ohmic contacts on highly doped n-type 3C-SiC. Electrical analysis by means of circular transfer length method demonstrate that an interlayer of titanium with thickness in the range of 25-150 nm has no significant influence on specific contact resistance. However, from a structural point of view, the formation of nickel silicides as well as Ti₃SiC₂ is severely affected by the titanium thickness. Moreover, the Kirkendall effect due to the reaction between Ni and SiC is influenced by the titanium thickness. In fact, Scanning Electron Microscopy analysis demonstrates that the adjunction of titanium affects the distribution of Kirkendall voids in the contact. Current maps determined by conductive Atomic Force Microscopy reveal significant variation of uniformity according to the titanium thickness.

Abstract: Two electrical characterization methods were used to study 3C-SiC epilayers doped by nitrogen implantation: circular Transfer Length Method (c­TLM) which allows extracting the specific contact resistance and Scanning Spreading Resistance Microscopy (SSRM) used to measure activated doping concentration. 3C-SiC samples were implanted at room temperature with different energies (ranging from 30 to 150keV) and doses (from 1 to 5.4x1015cm-2) in order to obtain a 300nm thick box-like profile at 5x1020cm-3. To activate the dopant, the samples were then annealed from 1150°C to 1350°C for 1h to 4h. Titanium-nickel c-TLM contacts annealed at 1000°C under argon showed the best results in terms of specific contact resistance (8x10-6.cm2) after a 1350°C–1h annealing. For this annealing condition, the activation rate was assessed by SSRM around 13%. This value confirms the difficulty to activate the dopants introduced into the 3C-SiC as the temperature is limited by the silicon substrate. However, this work demonstrates that low resistance values can be achieved on 3C-SiC, using nitrogen implantation at room temperature.

Abstract: This paper reports on the macro- and nanoscale electro-structural evolution, as a function of annealing temperature, of nickel-silicide Ohmic contacts to 3C-SiC, grown on 6H-SiC substrates by a Vapor-Liquid-Solid (VLS) technique. The structural and electrical characterization of the contacts, carried out by combining different techniques, showed a correlation between the annealing temperature and the electrical characteristics in both the macro- and the nanoscale measurements. Increasing the annealing temperature between 600 and 950 °C caused a gradual increase of the uniformity of the nanoscale current-distribution, with an accompanying reduction of the specific contact resistance from 5 x 10-5 to 8.4 x 10-6 Ωcm2. After high temperature annealing (950 °C) the structural composition of the contacts stabilized, as only the Ni2Si phase was detected. A comparison with previous literature findings suggests a superior crystalline quality of the single domain VLS 3C-SiC layers.

Abstract: In this work, ohmic contacts, formed by 100nm Ni layer RTA annealed or not, were investigated on 3C-SiC epilayers exhibiting different nitrogen doping levels. The epilayers were grown on (100) silicon. Doping level (N) and eventual dopant contamination (Al) were analyzed by C-V and/or SIMS. The specific contact resistance was determined by using Transmission Line Model (TLM) patterns for each condition (doping and annealing). Our results clearly evidence that very low specific contact resistance (~10-51.cm²) is obtained on highly doped 3C-SiC epilayers, enlightening the interest of both material and Ni contacts for future devices fabrication.