IR Laser Assisted Grading of MOCVD GaAsP
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GaAsP is
that they can be affected either thermally or photochemically by the absorption of light from a CO2 TEA laser tuned to an absorption maxima. Our preliminary results show that the laser dramatically alters the morphology and supresses the phosphorus incorporation rate when the laser is tuned to an absorption maxima for the AsH We interpret these results to indicate that, for the conditions used, the the primary effect of the laser is photochemical,
and that the phosphorus incorporation can be increased by exciting PH3 instead of AsH 3' Once the growth mechanisms are better understood and the growth conditions are optimized GaAs 1xP x/GaAs1yPy superlattices with atomically abrupt junctions should be able to be grown simply by turning the laser on and off. Moss et al.
[1] have shown that the room temperature reaction between
Me3 In (Me-methyl) and PH3 or AsH3 can be prevented by forming the Me 3In • Et 3 P (Et-ethyl) adduct. Since Me3 In is a Lewis acid and PEt 3 is a Lewis base, they can bond together to form an adduct pair. Adduct bonds are relatively weak (5-20 kcal/mole) so that they can partially dissociate near the growth temperature.
The Me 3In is then free to react with the AsH3 or PH3 to
form the desired semiconductor. The fact that the PEt 3 can greatly reduce the ability of the hydride to attack the Me3 In suggests that the hydride attacks the metal alkyl through the unfilled orbital which forms part of the adduct bond. One way this can be done is for the hydride to form an adduct, and then the hydrogen atoms attack the methyl groups via a methane elimination reaction as is
il-
lustrated in Fig. 1. For there to be methane elimination reactions the hydride must not yet have decomposed. This would be the case if the reaction is surface catalyzed. Also, the extent of the catalytic effect of the surface on the individual species can affect the composition of the ternary and quaternary film grown on it.
Mat. Res. Soc. Symp. Proc. Vol. 54. 91986 Materials Research Society
374
Fig. 1. Schematic of a methane
CI
elimination reaction.
3
C!!I- M "" X
!!
-
if 3 CIl1 (j113I
M
X
It
+
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Studies have shown that the phosphorus composition increases with the deposition temperature [2,3], and this has been attributed to the less efficient cracking of the PH3 molecule [3].
An alternate explanation is that
the surface catalyzed methane elimination reactions [4] for the formation of GaP are kinetically more difficult than those for the formation of GaAs. The optimum temperature is the lowest temperature at which the methane For the growth of GaP the methane
elimination reactions are complete.
elimination reactions are slower and the minimum temperature at which the methane elimination reactions are complete is higher. Evidence that supports this hypothesis that the deposition reactions are methane elimination reactions and not reactions between species that are prepyrolyzed are (1) different hydrocarbons are formed when Me3 Ga pyrolyzes than when a Me 3Ga and AsH3 mixture is heated; (2)
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