This paper presents the method of box diffusion with fixed pressure using ZnP

2

as a diffusion source to get high surface concentration in the LPE layer of GaP. It is econmic and technologically simple as comparing with the ordinary closed tube diffusion method. The quartz box with a polished cover was designed

which enables the Zn

P

4

vapor to diffuse into GaP surface layer at 810℃. The movements of the Zn vapor atoms and P

4

vapor molecules were analysed

the relation between the escape rate of the Zn atoms (or P

4

molecules) and the box capacity

the structure factor of the gap was calculated.The formula (9) is the theoretical expression of escape rate of the Zn atoms through the gap between the cover and the box

where D is diffusion coefficient

V is the effective volume of the box

L

Δ

l

is parameters of the gap. We define G=LΔ/l where G represents the structure factor of the gap. Then

the expression of escape rate is:dN/dt=-GDN(t)/V.