Investigation of heavily nitrogen-doped n⁺ 4H–SiC crystals grown by physical vapor transport

Abstract

Heavily nitrogen-doped n⁺ 4H–SiC single crystals were grown by the physical vapor transport (PVT) method. The nitrogen incorporation kinetics in a heavily doped regime was studied in terms of growth temperature dependence, and it was revealed that the growth temperature substantially influenced the amount of nitrogen incorporated into the crystals and their surface step structures on the (0 0 0 1¯)C facet plane. The structural quality of heavily nitrogen-doped 4H–SiC crystals was examined by X-ray rocking curve measurements and defect selective etching by molten KOH at around 500 °C. The crystals contained an extremely low density of 3C–SiC inclusions and stacking faults and showed a comparable crystalline quality to conventionally doped 4H–SiC substrates. Furthermore the structural stability of the heavily nitrogen-doped 4H–SiC substrates during high-temperature treatments has been investigated. The substrates with a large {0 0 0 1} surface roughness showed a resistivity increase after annealing at 1100 °C for 2 h, which was confirmed to be caused by the formation and expansion of double Shockley-type basal plane stacking faults in the substrates. The occurrence of the stacking faults largely depended on the surface preparation conditions of substrates, which indicate that the primary nucleation sites of stacking faults exist in the near-surface regions of substrates.

Introduction

Silicon carbide (SiC), particularly 4H–SiC, has in recent years been the focus for research and applications due to its excellent physical and electronic properties [1]. One of the proposed main applications is SiC power diodes and transistors, used in high efficiency power systems, such as DC/AC and DC/DC converters, where the electrical properties of SiC substrates are important parameters which will affect the performance of SiC devices fabricated on them.

From a substrate perspective for high power device applications, the goal is to obtain a sufficiently low, uniform electrical resistivity to prevent unnecessary substrate resistance from impacting the device performance as well as to achieve extremely low ohmic contact resistivities. Nitrogen, which substitutes for carbon atoms in SiC crystals, is the most important shallow donor impurity for SiC, and it can be easily introduced into the crystals during physical vapor transport (PVT) growth using a controlled nitrogen gas flow [2], [3], [4], [5], [6]. Nevertheless, there are still some issues to be addressed in nitrogen doping during PVT growth of SiC. One of them is the achievable minimum resistivity of SiC single crystals. The solubility limit of nitrogen atoms in SiC crystals has been reported to be in the mid 10²⁰ cm⁻³ range [7]; however the actual achievable nitrogen concentration has appeared significantly lower. Even when the crystals were grown under 100% nitrogen atmosphere, the nitrogen concentration of the crystals did not exceed 1×10²⁰ cm⁻³ for PVT-grown SiC crystals [5].

The crystallinity of heavily nitrogen-doped SiC crystals is another vital issue particularly in the viewpoint of their device applications. Generally, heavy impurity doping in semiconductor materials tends to deteriorate the crystal quality of the materials. In the case of SiC, the quality degradation could occur through the formation of foreign polytype inclusions. The polytype instability due to nitrogen doping has so far been discussed with respect to the following two aspects. Firstly, the smaller atomic size of substitutional nitrogen donor could structurally influence the polytype stability via change of the c/na lattice constant ratio of SiC crystals, where c is the lattice constant along the c-axis, a along the a-axis, and n is the number of stacked Si–C bilayers in the unit cell. It was shown [8] that the increase of nitrogen concentration in SiC crystals extends the a lattice parameter while c remains constant, though the doped nitrogen atoms have a smaller ionic radius compared to the substituted carbon atoms. These lattice parameter changes result in the decrease of c/na ratio with the nitrogen doping [8]. The c/na ratio has been reported to be well-correlated with the hexagonality of SiC crystals, and the reduced c/na ratio favors SiC polytypes with a smaller hexagonality [9]. A contradictory result, however, has been reported that nitrogen doping in SiC crystals prefers polytypes with a larger hexagonality. It was shown [10], [11], [12] that nitrogen doping during PVT growth of SiC stabilizes 4H polytype rather than 6H and 15R polytypes; the observed 4H preference has been attributed to a relative enrichment at the growing surface with atoms occupying the carbon sites of SiC crystals [11].

The second argument is regarding the electronically driven polytype transformation. In the pioneering paper in early the 60's by Knippenberg [13], it has already been suggested that nitrogen doping in hexagonal SiC crystals prefers 3C–SiC transformation. He claimed that the large difference in band gap between cubic and hexagonal polytypes gives rise to the energy difference of electrons (lower electron energy for cubic polytype), and doping of nitrogen donors has a preference for the formation of cubic polytype, since electrons moving from hexagonal to cubic polytype crystal bring about an energy gain to the system. According to this argument, 4H–SiC crystals grown in a heavily nitrogen-doped regime are likely to contain 3C–SiC inclusions, in contrast to the above-mentioned 4H preference due to nitrogen doping.

As for the 3C transformation of hexagonal polytypes, another issue of paramount importance is the structural stability of heavily nitrogen-doped hexagonal SiC crystals during high-temperature treatments. It was reported that heavily nitrogen-doped 4H–SiC substrates suffer structural changes during oxidation or Ar annealing at more than 1000 °C [14], [15]. The structural changes have been attributed to a 4H to 3C polytypic transformation that is induced by the formation and expansion of double layer Shockley-type stacking faults during the high-temperature treatments. The driving force for the fault expansion is basically the same as that proposed by Knippenberg, which is the so-called quantum well action (QWA) mechanism, in which electrons in heavily nitrogen-doped 4H–SiC crystals entering stacking fault-induced quantum wells lower the system energy [16], [17]. The 4H to 3C polytypic transformation not only influences the electric properties of substrates, but also severely degrades their geometrical parameters such as substrate flatness [18].

This paper reports a study of some important aspects of heavily nitrogen-doped (N>2×10¹⁹ cm⁻³) low-resistivity (ρ<0.01 Ω cm) 4H–SiC crystals. The topics include the nitrogen incorporation kinetics in heavily nitrogen-doped 4H–SiC crystals, and their structural quality and stability. We present the growth temperature dependence of nitrogen incorporation in heavily nitrogen-doped 4H–SiC crystals, and also results of the structural characterization of the crystals, which contained nitrogen donors of 2×10¹⁹–1.3×10²⁰ cm⁻³. Additionally, we report on the stacking fault formation after annealing in heavily nitrogen-doped 4H–SiC substrates with various {0 0 0 1} surface roughnesses, and the result indicates that double layer Shockley stacking faults are formed via heterogeneous nucleation at the substrate surface.