Thomas Straubinger

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.

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

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.

On the preparation of semi-insulating SiC bulk crystals by the PVT technique

Abstract Vanadium in SiC can act as a deep acceptor compensating residual nitrogen ( Δ E≈0.8 eV ) or as a deep donor compensating p-type (Al/B) impurities ( Δ E≈1.6 eV ) leading to semi-insulating behavior. In any case, doping homogeneity is crucial. Additionally, the V solubility limit in SiC must not be exceeded. To determine impurity incorporation, growth of nominally undoped crystals was performed. Here, nitrogen is the residual impurity and the charge carrier concentration n is decreasing exponentially with growth time. Wafers with remaining n=8×10 15 cm −3 were obtained. During B-doped growth, the hole concentration increases with growth time. The influence of the compensation by nitrogen and the loss of boron content in the source is discussed. While effective boron segregation on the facet during growth is determined to be close to unity, this value is not reached due to evaporation of the B source. V incorporation is related to the partial pressure of the V species in the crucible. Several regions were found in V-doped crystals. V exceeding the solubility limit leads to the formation of V-rich precipitates, V source depletion is indicated by a change to n-type conducting behavior because of residual nitrogen. With temperature-dependent Hall effect measurements, the specific resistivity at room temperature is determined to ρ 293 K =2–5×10 11 Ω cm , while resistivity mappings show dopant inhomogeneities. By reducing the V species evaporation rate, bulk SiC crystals exhibiting precipitate-free, semi-insulating behavior were obtained.

Preparation of Semi-Insulating Silicon Carbide by Vanadium Doping during PVT Bulk Crystal Growth

To fabricate semi-insulating SiC bulk crystals, vanadium doping was performed by adding vanadium as a solid source to the SiC starting material. El ectrical and optical properties of the PVT grown crystals were investigated. The nitrogen concentrat io up to about 2 × 10 cm in the crystal areas near the seed as well as the maximum sol bility of vanadium in SiC of about 5 × 10 cm are limiting yield and electrical homogeneity of vanadium doped SiC bulk crystals. Depletion of vanadium during growth can be prevented by lowering the growth temperature or using an inner container filled with a SiC/VC-mixture leading to a homogeneous vanadium incorporation in the crystals. Co-doping of vanadium and boron was successfully performed to attain SiC crystals with a fermi level close to mid-gap, leadi ng to thermal activation energies up to about 1,7 eV. The V 3+ and V charge states of vanadium can be detected separately using optica l absorption or electron spin resonance (ESR). With these techniques the e lectrical domination of the V/V acceptor or the V /V donor level in V doped samples can be determined. Introduction. Since in 1990 Schneider et al. found deep levels in SiC caused by vanadium i mp rities [1], vanadium doping techniques to obtain semi-insulating (s.i.) Si C were developed [2] resulting in the commercial production of 2 inch s.i.-SiC wafers in r ecent years. Additionally, the electrical behavior of vanadium doped SiC was investigated several t imes in the past [3,4]. But the technique of homogeneous vanadium incorporation in SiC bulk crystals during growth leading to homogeneous electrical properties still needs improvement [5-7] . Important issues limiting both yield and electrical properties of semi-insulating wafers c ut from vanadium doped PVTgrown crystals as well as workarounds for these issues will be discussed in this pa per. Experimental. Several 6H-SiC bulk crystals with 35...40 mm in diameter were grown in our laboratory by the Modified Lely method using high purity conditions [5]. Va nadium was added as a solid source in various amounts to the starting material. In this case, donors are compensated as nitrogen is the main impurity present in the growth system. To inves tigate the compensation of acceptors as well, boron/vanadium co-doping was performed. Additionally, res ults obtained from nominally undoped and boron doped SiC bulk crystals were taken as a referenc . El ctrical properties of the SiC wafers were determined by Hall effect m easurements performed at elevated temperatures (T > 473 K) and scanning capacitance mapping. The compensation mechanism of the samples was investigated using optical absorption and electron spin resonance (ESR) . Results and Discussion. Nitrogen incorporation To successfully compensate all shallow impurities in SiC, vanadium m st predominate in the grown crystals, i.e. N V > |ND – NA|. GDMS analysis at the interface between areas with and wit hout precipitation (see below) prove the V solubility limit to be at about 5 × 10 cm in correspondence with literature data [4]. Nitrogen introduced by loading/unloading the rowth reactor in air and desorbing from porous graphite parts during growth [5] is pres ent up to about 2 × 10 Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 51-54 doi:10.4028/www.scientific.net/MSF.433-436.51

Optical quantitative determination of doping levels and their distribution in SiC

Abstract We report the development of an absorption measurement-based characterization tool for the quantitative determination of doping levels and their lateral distribution in silicon carbide wafers. Calibration plots for the technologically important silicon carbide polytypes 4H–SiC (n-/p-type), 6H–SiC (n-/p-type) and 15R–SiC (n-type) are presented. A review of the underlying physical effects of the measurement procedure as well as a description of the experimental setup is given. The applicability of the characterization tool as a production friendly non-contact wafer quality test is demonstrated by showing several mappings of the lateral doping level distribution. The accuracy of the described measurement procedure is typically 15–20% and is of the same order as its electrical Hall measurement counterpart.

Impact of source material on silicon carbide vapor transport growth process

We have studied the impact of morphological changes of the source material during physical vapor transport growth of silicon carbide (SiC). Digital X-ray imaging (P.J. Wellmann et al., Mat. Res. Soc. Symp. Proc. 572 (1999) 259) was carried out to visualize the ongoing processes inside the SiC source material and numerical modeling was performed in order to study the impact on the crystal growth process. According to numerical modeling there is a large impact of the SiC powder compression and morphology on the global heat transfer and mass transfer inside the growth cell. Two different SiC sources containing microscopic SiC powder and macroscopic SiC pieces, respectively, were investigated. Although the SiC source material undergoes fundamental transitions during growth (i.e. evolution from powder to compressed SiC block) it was found that self-stabilizing of the growth process occurred by formation of a disk-like structure on the top of the source material, independent of the initial source morphology. The experimental results were confirmed by the numerical simulation of the global growth process.

Numerical simulation of global heat transfer in reactors for SiC bulk crystal growth by physical vapor transport

A modeling approach for the numerical simulation of heat transfer during SiC sublimation growth in inductively heated PVT-reactors is introduced. The physical model takes into account the volume heat sources induced by the electromagnetic field and the main processes contributing to the conservation equations for mass, momentum and energy. Results of calculations are compared to experimental observations to discuss the validity of the modeling approach. The computed isotherms and their temporal evolution agree with the shape of grown SiC crystals during a growth run.

Absorption mapping of doping level distribution in n-type and p-type 4H-SiC and 6H-SiC

An optical characterization method for determination of spatial doping level concentration in n-type 4H-SiC and p-type 6H-SiC is discussed. The absorption bands of free charge carriers at 460 nm in n-type 4H-SiC are used to determine its doping concentration. In p-type 6H-SiC, the band edge related absorption at 410 nm is a measure for the doping concentration. In both cases, Hall measurements are performed for calibration. Various examples of SiC-wafer mappings are shown and the relationships to crystal growth conditions, i.e. control of doping level and distribution, are investigated.