In this study the precipitation of $Zn_3P_2$ during Zn diffusion in a lattice matched GalnAsP/InP heterostructure grown by liquid phase epitaxy has been investigated by high resolution TEM. The composition of quaternary GalnAsP layer is $Ga_{0.28}In_{0.72}As_{0.61}P_{0.39}$. This GalnAsP layer was grown by liquid phase epitaxy on S-doped ($6 \times 10^{18}cm^{-3}$) (0 0 1) InP substrate. Prior to the growth of the quaternary layer (1㎛ thickness), an undoped InP buffer layer (3㎛ thickness) was grown to improve the crystalline quality and also to reduce the influence of predoped S in the substrate upon alloy mixing.
Zn diffusion was carried out in an evacuated silica ampoule employing a sintered mixture of $Zn_3P_2$. InP, and GaAs powders as diffusion sources. Diffusion times were 30 min, 45 min, 1 h, 4 h, respectively, at a temperature of $600\circ \!C$. Experiment on Zn out-diffusion was also carried out i.e. after 4 h Zn diffusion, 16 h Zn out-diffusion was carried out in an evacuated silica ampoule employing a sintered mixture of InP, and GaAs powders as diffusion sources at a temperature of $600\circ \!C$. After 16 h out diffusion, 4 h Zn diffusion was carried out again at a temperature of $600\circ \!C$.
Zn diffusion induced the interdiffusions of Ga, In, As, and P across the interface but interdiffusions of Ga and In was occured exclusively. Therefore special alloy composition regions of $Ga_{x'}In_{1-x'}As_{0.61}P_{0.39}$ and $Ga_xIn_{1-x}P$ were produced across the interface. The $Zn_3P_2$ precipitates nucleated homogeneously or heterogeneously at the tip of the stacking faults in the Ga-mixed $InP(Ga_xIn_{1-x}P)$. The $Zn_3P_2$ precipitates grow to form eptaxial layers to a certain depth of the intermixed $Ga_xIn_{1-x}P$ layer, where the $Zn_3P_2$ crystal lattice coherently matches with the matrix crystal latice. The $Zn_3P_2$ compounds precipitated with special orientation relationship between $Ga_xIn_{1-x}P$ matrix and $Zn_3P_2$ precipitates i.e $[1 1 0]_G$ // $[0 1 0]_Z$. $(0 0 1)_G$ // $(0 0 1)_Z$. Here subscript G means $Ga_xIn_{1-x}P$ and Z means $Zn_3P_2$. The precipitation reaction of $Zn_3P_2$ is explained using the kick-out mechanism and atomic model of $Zn_3P_2$ formation from Ga-mixed InP matrix is proposed.
16 h Zn out-diffusion induced the decomposition of $Zn_3P_2$ and produced the voids in $Zn_3P_2$ layers. 4 h Zn diffusion after 16 h Zn out-diffusion did not occupy the voids with the $Zn_3P_2$ but precipitated $Zn_3P_2$ compounds again in Ga-mixed InP matrix region.
In the In-mixed GaInAsP quaternary layer($Ga_{x'}ln_{1-x'}As_{0.61}P_{0.39}$) the alloy composition did not satisfy the conditions of coherent precipitation of $Zn_3P_2$ and could not find any precipitates in fact.
