What is the difference between monocrystalline silicon for photovoltaics and monocrystalline silicon for semiconductors?

Monocrystalline Silicon:

When molten elemental silicon solidifies, silicon atoms arrange into a diamond lattice, forming multiple crystal nuclei. If these nuclei grow into grains with the same crystal orientation, monocrystalline silicon is formed. The purity of silicon for general semiconductor devices is above 99.9999% (six nines), while for large-scale integrated circuits, the purity reaches over 99.9999999% (nine nines). Currently, monocrystalline silicon with a purity of 99.9999999999% (twelve nines) can be produced.

Polycrystalline Silicon:

If the crystal nuclei grow into grains with different crystal orientations, polycrystalline silicon is formed. Polycrystalline silicon is classified based on purity into:

• Metallurgical-grade polycrystalline silicon (MG): 90%–99.8% purity.

• Solar-grade polycrystalline silicon (SG): 99.99%–99.9999% purity (four to six nines).

• Electronic-grade polycrystalline silicon (EG): 99.9999999%–99.999999999% purity (nine to eleven nines).

Amorphous Silicon:

There is also amorphous silicon, where silicon atoms are arranged in a disordered manner. It is mainly used in thin-film solar cells but has much lower efficiency compared to monocrystalline silicon.

Figure 1:Schematic Diagram of Monocrystalline Silicon, Polycrystalline Silicon, and Amorphous Silicon.

Monocrystalline Silicon in Photovoltaics:

For a long time, polycrystalline silicon technology dominated the photovoltaic industry over monocrystalline silicon. However, in recent years, monocrystalline silicon has overtaken polycrystalline silicon in market share.

Main Manufacturing Methods for Monocrystalline Silicon:

1. Czochralski (CZ) Method (Mainstream Process)

Figure 2:A schematic diagram of the Czochralski technique as used for growth of GaAs single crystal bond.Image from Jobilize

(1) Process Overview

The material is placed in a crucible (quartz or graphite) and heated above its melting point. A rotating and vertically movable pull rod is positioned above the crucible, with a seed crystal clamped at the bottom end. The pull rod is lowered until the seed crystal contacts the molten silicon. Then, the rod is slowly pulled upward while rotating, and the heating power is gradually reduced, allowing the seed crystal to grow into a larger monocrystalline silicon ingot.

(2) Process Flow

Preparation before loading → Loading the furnace → Silicon melting → Seed crystal dipping → Necking → Shoulder and transition shoulder growth → Constant diameter growth → Finalizing growth → Furnace shutdown.

(3) Determining the Conductivity Type of the Monocrystalline Silicon Rod

A common method is the thermal probe method, which uses a galvanometer to detect conductivity type. This method relies on the thermoelectric effect of semiconductors:

• A hot and a cold probe contact the silicon rod simultaneously.

• Heat excitation at the hot probe generates a large number of charge carriers, while the cold probe region has fewer carriers.

• Due to the carrier concentration difference, carriers diffuse from the hot probe area to the cold probe area, creating a potential difference.

• If the hot probe region has a higher potential, the majority carriers are electrons, and the silicon rod is N-type.

• If the hot probe region has a lower potential, the majority carriers are holes, and the silicon rod is P-type.

(4) Disadvantages

The direct contact between the molten silicon and the quartz crucible leads to contamination, especially with oxygen impurities. These oxygen contaminants may cause oxygen donors during crystal growth or subsequent thermal processing, leading to micro-defects in the silicon crystal.

2. Float-Zone (FZ) Method

Figure 3:A schematic drawing of the floating zone (FZ) method.

The float-zone method uses a specialized float-zone monocrystalline furnace and is capable of producing high-resistance monocrystalline silicon with superior quality.

The FZ method is also used for impurity removal, known as zone refining. This technique utilizes the difference in impurity solubility between a material’s solid and liquid phases to purify high-purity metals.

Principle of Zone Refining

• When a metal contains impurities, it is first melted and then cooled.

• As the solid phase precipitates, the impurity concentration in the solid is lower than in the original metal.

• By repeating this process multiple times, high-purity metal can be obtained.

Practical Operation

• The metal is processed into a rod and placed in a tube furnace.

• A movable heating coil (typically using high-frequency heating) surrounds the furnace.

• The coil is initially positioned at one end of the rod (e.g., the left end) and heats the area, melting the metal.

• The coil is then slowly moved toward the right end, causing solidification at the left end while pushing impurities toward the right.

• By repeating this process multiple times, ultra-high-purity metal can be obtained.

• This method reduces impurity content to parts per billion (ppb) or even parts per trillion (ppt).