Peter J. Wellmann

Global numerical simulation of heat and mass transfer for SiC bulk crystal growth by PVT

Abstract A modeling approach for the numerical simulation of heat and mass transfer during SiC sublimation growth in inductively heated physical vapor transport (PVT) reactors is introduced. The physical model is based on the two-dimensional solution of the coupled differential equations describing mass conservation, momentum conservation, conjugate heat transfer including surface to surface radiation, multicomponent chemical species mass transfer and advective flow. The model also includes the Joule volume heat sources induced by the electromagnetic field. The evolution of the temperature profiles inside the crucible and of the crystallization front is studied. The radial temperature gradient at the crystal/gas interface causes strong radial non-uniformity of the growth rate and, in turn, influences the shape of the growing crystal. Results of calculations are compared to experimental observations to analyse the validity of the modeling approach. Both the computed growth rates, their temporal evolution and the shape of the growing crystal agree with experimental data.

Analysis on defect generation during the SiC bulk growth process

Abstract SiC crystals (1.2–1.5′′ diameter) were grown by the modified Lely technique on seeds with different micropipe densities in order to study the defect generation during seeding and subsequent bulk growth. The micropipe generation is found to be strongly correlated with the occurrence of second phases in SiC like carbon inclusion formation. Model approaches for stable SiC growth conditions, i.e. without inclusions, are discussed. Numerical modeling was performed to reveal the radial and axial temperature gradients of our crucible set-up. Stress formation and micropipe generation are determined to be enhanced in the presence of a large axial temperature gradient.

Determination of charge carrier concentration in n- and p-doped SiC based on optical absorption measurements

We have investigated the effect of doping on absorption for various SiC polytypes, i.e., n-type (N) 6H–SiC, 4H–SiC, and 15R–SiC, p-type (Al) 6H–SiC, and 4H–SiC, and p-type (B) 6H–SiC. For these polytypes the band-gap narrowing with higher doping concentration is observed. In addition, for n-type doping below band-gap absorption bands at 464 nm for 4H–SiC, at 623 nm for 6H–SiC, and at 422 and 734 nm for 15R–SiC are observed. The peak intensities of these absorption bands show a linear relation to the charge carrier concentration obtained from Hall measurements. The corresponding calibration factors are given. As an application a purely optical wafer mapping of the spatial variation of the charge carrier concentration is demonstrated.

Aluminum p-type doping of silicon carbide crystals using a modified physical vapor transport growth method

Abstract We report the development of a modified physical vapor transport (PVT) growth setup for the improved aluminum p-type doping of silicon carbide (SiC) single crystals. Usually aluminum doping of SiC is carried out by adding the dopant to the SiC powder source material. However, due to aluminum source depletion a strong exponential decrease of the dopant incorporation with increasing process time is observed. In addition, often defect generation takes place due to a high initial aluminum sublimation rate. In order to improve the aluminum supply we have installed an additional gas pipe which provides a continuous flux of aluminum atoms out of an external reservoir into the growth cell. We will discuss the influence of the additional gas flow on the thermal field and mass transport inside the growth cell. Technological steps will be pointed out which were necessary to establish crystal growth with structural properties comparable to the conventional PVT process. With the modified PVT method high quality SiC single crystals with an improved axial and lateral aluminum doping homogeneity were grown (4H-SiC: 2×10 16 cm −3 16 cm −3 , Δp/p 8×10 16 cm −3 10 17 cm −3 , Δp/p

On the lattice parameters of silicon carbide

The thermal expansion coefficients of the hexagonal SiC polytypes 4H and 6H and with Al and N dopants have been determined for temperatures between 300 and 1770 K. Further, a set of the room temperature lattice parameters in dependence on doping with N, Al, and B has been obtained. Data for the thermal expansion were taken on a triple axis diffractometer for high energy x rays with a photon energy of 60 keV, which allows the use of large single crystals with a volume of at least 6×6×6 mm3 without the need to consider absorption. The room temperature measurements for samples with different dopants have been performed on a four-circle diffractometer. The thermal expansion coefficients along the a- and c-directions, α11 and α33, increase from 3×10−6 K−1 at 300 K to 6×10−6 K−1 at 1750 K. It is found that α11 and α33 are isotropic within 107 K−1. At high temperatures both coefficients for doped samples are ∼0.2×10−6 and 0.3×10−6 K−1 lower than for the undoped material.

In situ visualization and analysis of silicon carbide physical vapor transport growth using digital X-ray imaging

Using digital X-ray imaging we have investigated the on-going processes during physical vapor transport growth of SiC. A high-resolution and high-speed X-ray detector based on image plates and digital recording has been used to monitor SiC bulk crystal growth as well as SiC source material degradation on-line during growth. We have analyzed the shape of the growth interface and the evolution of the SiC source morphology. The crystal growth process will be discussed in terms of growth rate and limitations of the physical vapor transport of SiC gas species from the source to the growth interface.

Status of SiC bulk growth processes

The present paper gives an overview of the different routes to grow SiC single crystals. The focus is put on the new emerging processes compared with the well established ones. A review of the process engineering modelling is given. Finally, some selected results are pointed out as they should be considered for the future development of SiC material.

Fluorescent SiC as a new material for white LEDs

Current III–V-based white light-emitting diodes (LEDs) are available. However, their yellow phosphor converter is not efficient at high currents and includes rare-earth metals, which are becoming scarce. In this paper, we present the growth of a fluorescent silicon carbide material that is obtained by nitrogen and boron doping and that acts as a converter using a semiconductor. The luminescence is obtained at room temperature, and shows a broad luminescence band characteristic of donor-to-acceptor pair recombination. Photoluminescence intensities and carrier lifetimes reflect a sensitivity to nitrogen and boron concentrations. For an LED device, the growth needs to apply low-off-axis substrates. We show by ultra-high-resolution analytical transmission electron microscopy using aberration-corrected electrons that the growth mechanism can be stable and that there is a perfect epitaxial relation from the low-off-axis substrate and the doped layer even when there is step-bunching.

Broadband and omnidirectional light harvesting enhancement of fluorescent SiC

In the present work, antireflective sub-wavelength structures have been fabricated on fluorescent 6H-SiC to enhance the white light extraction efficiency by using the reactive-ion etching method. Broadband and omnidirectional antireflection characteristics show that 6H-SiC with antireflective sub-wavelength structures suppress the average surface reflection significantly from 20.5 % to 1.01 % over a wide spectral range of 390-784 nm. The luminescence intensity of the fluorescent 6H-SiC could be enhanced in the whole emission angle range. It maintains an enhancement larger than 91 % up to the incident angle of 70 degrees, while the largest enhancement of 115.4 % could be obtained at 16 degrees. The antireflective sub-wavelength structures on fluorescent 6H-SiC could also preserve the luminescence spectral profile at a large emission angle by eliminating the Fabry-Pérot microcavity interference effect.

Evaluation of n-type doping of 4H-SiC and n-/p-type doping of 6H-SiC using absorption measurements

Abstract A non-destructive, absorption measurement based optical method has been developed in order to determine doping type (n- or p-type), doping level and doping level distribution in 4H and 6H silicon carbide (SiC) wafers. The bandgap absorption has been calculated numerically taking into account band filling, band shrinkage and band tailing effects which are a function of donor and acceptor concentration N D and N A , respectively. The numerical results are compared with experimental data. A calibration plot of the doping dependence of the absorption of n-type 6H SiC is presented and the application for mapping of the SiC wafer doping level distribution is demonstrated.