Mihai Lazar

Barrier height determination of SiC Schottky diodes by capacitance and current-voltage measurements

Extractions of barrier heights of 6H and 4H-SiC Schottky diodes have been performed on structures with various gate metallization, using both capacitance-voltage (C-V) and current-voltage (I-V) measurements. The sum of the two barriers extracted by C-V measurements on both n-type and p-type materials is found to be higher than the band gap energy E-G, whereas the one extracted by I-V is less than E-G. However, above room temperature, temperature variations of barrier heights are in agreement with the variations of E-G. We have also computed theoretical I-V characteristics using a two-barrier height model. By taking account of temperature variations of a large number of parameters, e.g., the carrier mobility, free carrier concentration, and barrier height, we have achieved a good fit with experimental data. The model is shown to be valid for n-type Schottky diodes over a wide range of temperatures (from 100 to 500 K).

Effect of ion implantation parameters on Al dopant redistribution in SiC after annealing: Defect recovery and electrical properties of p-type layers

Epilayers of 6H and 4H–SiC were Al implanted with various doses to form p-type layers after a postimplantation annealing performed at 1700 °C/30 min. Rutherford backscattering spectrometry in the channeling mode analyses carried out before and after annealing show virgin nonimplanted equivalent spectra if the implanted layers are not amorphized. The amorphous layers are recrystallized after annealing with a residual damage level of the lattice relative to the quantity of the dopant implanted. Secondary ion mass spectrometry measurements performed on the implanted samples before and after annealing illustrate a good superposition of the profiles obtained before and after the annealing on nonamorphized samples. Dopant redistribution occurs after annealing, only on amorphized layers, with an intensity that increases with the implanted dose. Deduced from sheet resistance measurements, the dopant activation increases with the implanted dose. Activation of 80%–90% is obtained from capacitance–voltage measuremen...

The role of nickel and titanium in the formation of ohmic contacts on p-type 4H?SiC

The formation of low resistivity ohmic contacts to p-type 4H–SiC is achieved. Transfer length method (TLM)-based structures were fabricated on 0.8 µm thick epitaxial p-type silicon carbide (4H–SiC) layers. TLM metal patterns were obtained by a lift-off procedure and electron beam deposition of Ni, Ti and Al. The electrical properties of the contacts were examined using current/voltage measurements. Contact resistivity as a function of annealing was investigated over the temperature range from 700 to 1000 °C. The lowest contact resistivity of 1.5 × 10−5 Ω cm2 was obtained for the Ni/Ti/Al/Ni contact after annealing at 800 °C for 90 s. Using secondary ion mass spectrometry, energy-dispersive x-ray spectroscopy and x-ray diffraction measurements, we quantitatively and qualitatively determined the formation and the nature of the ohmic contact to p-type SiC.

SiC power semiconductor devices for new applications in power electronics

This paper addresses the benefits of SiC semiconductor, owning excellent physical properties able to fulfill new scope of applications in terms of high temperature, high voltage and for more specific applications. Devices and applications developed at Ampere laboratory are detailed.

Measurements of Charge Collection Efficiency of p+/n Junction SiC Detectors

Silicon carbide is a promising wide-gap material because of its excellent electrical and physical properties, which are very relevant to technological applications. In particular, silicon carbide can represent a good alternative to Si in applications like the inner tracking detectors of particle physics experiments [1]. In this work p+/n SiC diodes realized on a medium doped (1×1015 cm -3), 40 µm thick epitaxial layer are exploited as detectors and measurements of their charge collection properties under beta particle radiation from Sr90 source are presented. Preliminary results till 900 V reverse voltage show a good collection efficiency of 1700 e- and a collection length (ratio between collected charges and generated e-h pairs/µm) equal to the estimated width of the depleted region.

Characteristics of aluminum-implanted 6H-SiC samples after different thermal treatments

Multiple energy aluminum (Al) implantations were performed at room temperature in n-type epitaxial 6H-SiC layers, aiming at amorphizing the material from the surface up to a depth inferior to 0.5 μm. Annealings were then carried out in an induction furnace. The goal of this paper is to optimize the furnace geometrical configuration, in order to reduce the surface degradation and improve the crystal reordering. This optimization was established for one-side amorphized wafers, which need restricting annealing parameters, and is therefore supposed to be valid for less crystal damaging implantations. Two types of geometrical parameters were essentially studied: the internal configuration, which tends to increase the silicon partial pressure inside the reactor, and the position of the sample, which has a direct influence on the recrystallization and on the dopant electrical activation. The annealings are compared for the same thermal parameters: the plateau temperature (1700 °C), the annealing duration (30 min), and the heating rate (60 °C s−1). The surface roughness was evaluated by using atomic force microscopy. Two final configurations were retained, leading to satisfactory results with respect to the as-implanted material: (i) Rutherford backscattering spectrometry in channeling geometry revealed a very good recrystallization in both cases, giving a signal level similar to the virgin crystal one; (ii) secondary ion mass spectrometry showed two distinct results depending on the sample position: one position led to some material etching, especially the SiC part which was amorphized by the implantation, and the second position gave rise to the deposition of a crudely monocrystalline SiC layer on the surface of the sample implanted side. This coating was found to prevent from any dopant loss by exodiffusion or material etching. Electrical measurements (four-point probe at 300 K) proved an Al substitutional ratio of 97 and 78% depending on the configuration, giving room temperature sheet resistances of about 2×104 and 4×104 Ω sq.−1, respectively, for 4×1019 cm−3 Al implanted samples.

A Comparative Study of High-Temperature Aluminum Post-Implantation Annealing in 6H- and 4H-SiC, Non-Uniform Temperature Effects

4H-and 6H-SiC small samples were implanted by keV Al + ions at room temperature and annealed in an induction heating furnace, at the center of the susceptor, for different temperatures and times in the range 1600-1800°C and 5-60 min, respectively. The implanted layers were amorphous but the SiC crystalline structures were recovered after annealing, as measured by Rutherford Back-Scattering analyses in Channeling geometry. Al + electrical activation determined by sheet resistance and Hall effect measurements increases with the annealing temperature or time, on both polytypes. When whole SiC wafers were annealed in the same induction heating furnace, sheet resistance mapping systematically presented a radial gradient from the center to the periphery of the wafer. The measured linear dependence between sheet resistance and temperature allowed us to rebuild the radial temperature gradient at the crucible-susceptor furnace during the annealing process. Introduction Silicon carbide (SiC) is envisaged as a promising semiconductor material for a wide variety of high-temperature, high-power and high-frequency electronic applications. Ion implantation, an indispensable technique to locally dope silicon carbide still presents many problems in particular for p-type zone creation. High ionization energy of dopants imposes to raise the implanted dose above the amorphization threshold for room temperature implantations. Structure recrystallization and electrical activation of dopants, i.e. their incorporation in active SiC atomic sites, require high temperature annealing, about 1700°C in special configuration, with an overpressure of silicon and carbide. In this work p-type 6H and 4H-SiC layers created by Aluminum (Al) ion implantations followed by high temperature annealings are studied in order to realize efficient p +-n junctions for bipolar power diodes. Dopant electrical activation dependence on the post-implantation annealing conditions is discussed considering the non-uniform temperature at the SiC sample surface during this process.

Contact resistivity of Al/Ti ohmic contacts on p-type ion implanted 4H- and 6H-SiC.

Al-Ti alloys with 72 wt% Al were employed for the realisation of ohmic contacts on 4×10 19 cm -3 p-type ion implanted 4H- and 6H-SiC samples. Contact resistivity characterisations by TLM measurements were done at wafer level in the temperature range 28–290°C. Analysis of the TLM measurements took into account current crowding at the metal pads. More than half of the evaluated contact resistivity reached the minimum value detectable by the used TLM devices, that was slightly higher than 1×10 -6 Ωcm 2 . Above this limit value, contact resistivity decreased for increasing temperature and was spread over a few decades. The maximum contact resistivity at 28°C was 2×10 -4 Ωcm 2 , which changed to 5×10 -6 Ωcm 2 at 290°C. The thermal behaviour of these TLM structures featured thermionic-field emission conduction.

Very low specific contact resistance measurements made on a highly p-type doped 4H-SiC layer selectively grown by vapor-liquid-solid transport

This work reports on the performances of ohmic contacts fabricated on highly p-type doped 4H-SiC epitaxial layer selectively grown by vapor-liquid-solid transport. Due to the very high doping level ...

Investigation of Al-implanted 6H– and 4H–SiC layers after fast heating rate annealings

Abstract 6H– and 4H–SiC n-type layers were amorphised by multiple Al implantations at room temperature, in order to study the annealing process. This paper deals with the influence of specific annealing conditions, such as furnace atmosphere and heating rate, on SiC reordering and Al profile. Below a certain deposited nuclear energy, solid phase epitaxy (SPE) is possible and leads to recrystallisation under precise conditions (high heating rate, silicon partial pressure prescribed). Above that, partial reordering is only observed for 4H–SiC layers, and leads to some increasing roughness, even though annealing has proven to be efficient for avoiding surface impairment (due to a specific cleaning process). Some Al oscillations have been observed near the junction in case of SPE, whereas a flat homogeneous profile is formed in amorphised layers.