Introduction:
Polysilicon gate layers are commonly used in semiconductor devices such as transistors. The growth of the polysilicon gate layer is typically achieved using a process called chemical vapor deposition (CVD). This process involves the deposition of a thin film of material onto a substrate by exposing the substrate to a vaporized precursor chemical.

Explanation:
The chemical used for chemical vapor deposition (CVD) to grow the polysilicon gate layer is silane gas (SiH4). Silane gas is a silicon-containing compound that is commonly used as a precursor in the CVD process.

Chemical Vapor Deposition (CVD):
Chemical vapor deposition (CVD) is a process used in the semiconductor industry to deposit thin films of various materials onto a substrate. In the case of growing the polysilicon gate layer, CVD is used to deposit silicon atoms onto the substrate.

Silane Gas (SiH4):
Silane gas (SiH4) is a colorless and flammable gas that consists of silicon and hydrogen atoms. It is commonly used as a precursor in the CVD process for growing silicon-based thin films. Silane gas is highly reactive and decomposes at high temperatures, releasing silicon atoms that can then deposit onto the substrate.

Growth of Polysilicon Gate Layer:
During the CVD process for growing the polysilicon gate layer, silane gas is introduced into a reaction chamber along with a carrier gas. The reaction chamber is typically heated to high temperatures, which causes the silane gas to decompose.

The decomposition of silane gas results in the release of silicon atoms, which then react with the substrate surface to form a thin film of polysilicon. The polysilicon layer can be grown to a desired thickness by controlling the deposition time and the flow rate of the silane gas.

Conclusion:
In conclusion, the chemical used for chemical vapor deposition (CVD) to grow the polysilicon gate layer is silane gas (SiH4). Silane gas is a silicon-containing compound that decomposes at high temperatures, releasing silicon atoms that can then deposit onto the substrate to form a thin film of polysilicon.