**Faebian Bastiman**

GaAs and InP are two very popular material systems for opto-electronic devices. After measuring and calculating and cross-referencing your Ga, In and Al growth rates on GaAs(100) with a variety of techniques the time has finally come to grow on InP(100). Hurray. However for some reason the growth rates have gone awry! The ML/s growth rate you get on InP is 108% what your very careful and accurate measurements tell you is 1ML/s on GaAs. Worse your µm/h growth rate is 112% out. How can I quote your growth rate error with such startling accuracy?

Well, the fact is there is no error in the growth rate. The discrepancy comes because GaAs and InP have a lattice constant of 5.65338 Å and 5.86860 Å, respectively. When you are talking about 1ML/s on GaAs you are talking about a flux of 6.258 x 10^{14} atoms/cm^{2}/s. In fact you should say *1 monolayer of material with the lattice parameter of GaAs per second* or for short: 1 ML/s_{GaAs}. Since 1ML/s_{InP} is in fact 5.807 x 10^{14} atoms/cm^{2}/s, when you put down 1ML_{GaAs} you are in fact depositing 1.08 ML_{InP}. The ML growth rate of different material systems is proportional to the square of the lattice constant:

The square term comes from the fact you are only worrying about a 2D ML on the (100) plane, and hence you are only concerned with the ratio of area of the face of the zinc blende unit cell. The larger discrepancy on the µm/h growth rate comes from the fact you are now looking at a ratio of the volume of the zinc blende unit cell. Thus the term is proportional to the cube of the lattice constant:

This is a good example why the growth rate should be quoted in terms of the substrate independent unit of atoms/cm^{2}/s.

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