Doping Control in Sublimation SiC Growth

Silicon carbide (SiC) plays an important role in manufacturing power electronics and high-frequency devices due to its excellent electrical and thermal properties. The quality and doping level of SiC crystals directly affect the performance of the device, so precise control of doping is one of the key technologies in the SiC growth process.

1. Effect of impurity doping

In the sublimation growth of SiC, the preferred dopants for n-type and p-type ingot growth are Nitrogen (N) and Aluminum (Al) respectively. However, the purity and background doping concentration of SiC ingots have a significant impact on device performance. The purity of SiC raw materials and graphite components determines the nature and quantity of impurity atoms in the ingot. These impurities include Titanium (Ti), Vanadium (V), Chromium (Cr), Ferrum (Fe), Cobalt (Co), Nickel (Ni) ) and Sulfur (S). The presence of these metal impurities may cause the impurity concentration in the ingot to be 2 to 100 times lower than that in the source, affecting the electrical characteristics of the device.

2. Polar effect and doping concentration control

Polar effects in SiC crystal growth have a significant impact on doping concentration. In SiC ingots grown on the (0001) crystal plane, the nitrogen doping concentration is significantly higher than that grown on the (0001) crystal plane, while aluminum doping shows the opposite trend. This effect originates from surface dynamics and is independent of gas phase composition. The nitrogen atom is bonded to three lower silicon atoms on the (0001) crystal plane, but can only be bonded to one silicon atom on the (0001) crystal plane, resulting in a much lower desorption rate of nitrogen on the (0001) crystal plane. (0001) crystal face.

3. Relationship between doping concentration and C/Si ratio

Impurity doping is also affected by the C/Si ratio, and this space-occupancy competition effect is also observed in CVD growth of SiC. In standard sublimation growth, it is challenging to independently control the C/Si ratio. Changes in growth temperature will affect the effective C/Si ratio and thus the doping concentration. For example, nitrogen doping generally decreases with increasing growth temperature, while aluminum doping increases with increasing growth temperature.

4. Color as an indicator of doping level

The color of SiC crystals becomes darker with increasing doping concentration, so color and color depth become good indicators of doping type and concentration. High-purity 4H-SiC and 6H-SiC are colorless and transparent, while n-type or p-type doping causes carrier absorption in the visible light range, giving the crystal a unique color. For example, n-type 4H-SiC absorbs at 460nm (blue light), while n-type 6H-SiC absorbs at 620nm (red light).

5. Radial doping inhomogeneity

In the central region of a SiC(0001) wafer, the doping concentration is typically higher, manifesting as a darker color, due to enhanced impurity doping during facet growth. During the growth process of the ingot, rapid spiral growth occurs on the 0001 facet, but the growth rate along the <0001> crystal direction is low, resulting in enhanced impurity doping in the 0001 facet region. Therefore, the doping concentration in the central region of the wafer is 20% to 50% higher than that in the peripheral region, pointing out the problem of radial doping non-uniformity in SiC (0001) wafers.

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