Question

What would be system pressure of Si-MBE system in order to achieve 5 × 10⁷ cm mean free path(L) of atoms ?
1. 10^-7 torr
2. 10^-8 torr
3. 10^-9 torr
4. 10^-10 torr

Answer 1

Correct Answer - Option 4 : 10^-10 torr

Concept:

Generalized MBE operating condition is:

\(MFP\left( {cm} \right) = \frac{{5 \;\times \;{{10}^{ - 3}}}}{{\rho \left( {torr} \right)}}\) 

MFP = Mean free path

Calculation:

Given:

Mean free path = 5 × 10⁷ cm, i.e.

\(5 \times {10^7} = \frac{{5\; \times \;{{10}^{ - 3}}}}{{\rho \left( {torr} \right)}}\) 

\(\rho \left( {torr} \right) = \frac{{5\; \times \;{{10}^{ - 3}}}}{{5\; \times\; {{10}^7}}}\) 

ρ = 10^-10 torr