Michael Laube

Diffusion of boron in silicon carbide: Evidence for the kick-out mechanism

We report diffusion experiments of implanted boron (B) in nitrogen-doped 4H- and aluminum-doped 6H-SiC which were performed at temperatures between 1700 and 1800 °C. Transient enhanced B diffusion caused by implantation damage was effectively suppressed by annealing of the B-implanted samples at 900 °C prior to the diffusion anneal. Concentration profiles of B measured with secondary ion mass spectrometry are accurately described on the basis of the kick-out mechanism. This provides strong evidence that Si self-interstitials mainly mediate B diffusion.

Suppressed diffusion of implanted boron in 4H–SiC

Transient-enhanced diffusion of boron (B) during anneals at 1700 °C is experimentally observed in B-implanted 4H–SiC samples. This enhanced diffusion can strongly be suppressed by coimplantation of carbon or by a preanneal at 900 °C. It is proposed that B in 4H–SiC diffuses via the kick-out mechanism with the assistance of silicon interstitials in analogy to the B diffusion in Si. From the Fickian diffusion tail into the undamaged bulk, the preexponential factor D0 and the activation energy EA of the B diffusion coefficient D(B,T) are determined.

Electrical activation of implanted phosphorus ions in [0001]- and [11–20]-oriented 4H-SiC

Aluminum-doped 4H-SiC epilayers with [0001]- or [11–20]-oriented faces were implanted with phosphorus and subsequently annealed in a temperature range of 1550–1700 °C. The electrical activation of phosphorus ions was studied by Hall effect investigations. Identical free electron concentrations are observed at high temperatures in both types of SiC samples indicating that the electrical activation of implanted phosphorus ions is independent of the orientation of the wafers. The compensation generated by the phosphorus implantation is greater in 4H-SiC samples with (0001) face. Phosphorus donor concentrations above 1020 cm−3 could be activated and an extremely low sheet resistance of 29 Ω/□ was determined in the implanted 4H-SiC layer.

Hall Scattering Factor for Electrons and Holes in SiC

The analysis of Hall effect data taken on n-and p-type 4H-/6H-SiC is briefly described and the effect of excited states is demonstrated. The determination of the Hall scattering factor for electrons r H,e(T, B) at low and high magnetic fields is introduced and temperature-dependent values taken from the literature are provided. The low field Hall scattering factor for holes r H,h(T, B) is obtained from a comparison of the free hole concentration p(T) either determined by Hall effect measurement or calculated on the basis of the neutrality equation with defect parameters determined by SIMS or C-V. The temperature dependence of r H,h(T, B) in the range from 100 K to 800 K is determined and tested for a series of Al-doped 4H-SiC layers.

Self-diffusion in isotopically enriched silicon carbide and its correlation with dopant diffusion

Diffusion of C13 and Si30 in silicon carbide was performed with isotopically enriched 4H-Si28C12∕natSiC heterostructures which were grown by chemical vapor phase epitaxy. After diffusion annealing at temperatures between 2000°C and 2200°C the Si30 and C13 profiles were measured by means of secondary ion mass spectrometry. We found that the Si and C diffusivity is of the same order of magnitude but several orders of magnitude lower than earlier data reported in the literature. Both Si and C tracer diffusion coefficients are in satisfactory agreement with the native point defect contribution to self-diffusion deduced from B diffusion in SiC. This reveals that the native defect which mediates B diffusion also controls self-diffusion. Assuming that B atoms within the extended tail region of B profiles are mainly dissolved on C sites, we propose that B diffuses via the kick-out mechanism involving C interstitials. Accordingly, C diffusion should proceed mainly via C interstitials. The mechanism of Si diffusion...

Electrical and Optical Characterization of SiC

Three topics are reported in this paper: (a) the determination of a temperaturedependent Hall scattering factor for holes r H,h(T) in 4H-SiC, (b) the detection of shallow Al-related defect centers in Al-doped, p-type 6H-/3C-SiC; these defects are generated either by i mplantation of any ion species or by an oxidation process and (c) the observation of absorption lines in infra red (IR) spectra, which are due to phosphorus donors in 6H-SiC.

A P-Channel MOSFET on 4H-SiC

4H-SiC based P-channel MOSFET with nitrided gate oxide have been fabricated and analysed. The threshold voltage and the peak channel-carrier mobility, obtained from the transfer characteristic in the linear regime, are -4.8 V and 6 cm2/Vs. The drain current for zero gate bias (off current) is extremely low, which opens new application possibilities such as nonvolatile randomaccess memories.

Diffusion of boron in silicon carbide

We have performed diffusion experiments with B-implanted 6H-SiC at temperatures between 1700°C and 2100°C. After diffusion, B concentration profiles were measured by means of secondary ion mass spectrometry. Accurate modeling of B diffusion is achieved on the basis of a kick-out diffusion mechanism which only involves point defects on the Si sublattice. Fitting of our experimental profiles yields data for the B interstitial-controlled B diffusion coefficient which are in good agreement with literature data. Moreover, the analysis of B diffusion provides data for the Si interstitial-controlled B diffusion coefficient which is interrelated with the Si self-interstitial contribution to Si diffusion in SiC. This contribution is found to be two orders of magnitude lower than an extrapolation of Si diffusion data reported in the literature.

Phosphorus-Related Centers in SiC

Phosphorus (P) is considered to serve alternatively to nitrogen (N) as a shallow donor in SiC. It is the aim of this paper to report the present status of our knowledge on the optical and electrical properties of P donors. We will not report on results obtained from magnetic resonance experiments in this article and refer to the relevant literature [1]–[4]. Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) investigations are predominantly taken on SiC samples, which are P-doped by neutron transmutation. These magnetic resonance experiments and also a theoretical prediction by A. Gali et al. [5] support that P donors are incorporated into the silicon (Si) sublattice. However with respect to the microscopic structure of P-related centers, there exists still a controversial discussion [1]–[4].

Comparison of the Electrical Channel Properties between Dry- and Wet- Oxidized 6H-SiC MOSFETs Investigated by Hall Effect

The density and Hall mobility of free electrons in the channel of either dryor wetoxidized MOSFETs have been investigated by Hall effect. The wet-oxidized sample reveals a higher DIT close to the conduction band edge than the dry-oxidized one, which leads to a reduction of the free electron density in the channel of the wet-oxidized MOSFET. The electron Hall mobility is nearly independent of the employed oxidation process. The observed change of the sign of the Hall coefficient RH at small gate voltages is attributed to an inhomogeneous distribution of the drain current.