Abstract
The relation between the film thickness and the growth conditions in the liquid phase epitaxy (LPE) process is discussed. A convolution approach for the thickness is developed on the assumption that the growth rate is determined by the solute diffusion process. Using this convolution expression, the relations between thickness, growth time and cooling rate can be obtained for various LPE techniques. Moreover, the convolution algorithm can also be used to deal with some complex growth conditions, such as nonuniform cooling rate, nonlinearity of the liquidus curve and the finite growth solution.
Keywords
As a mature technique for the growth of semiconductor films, Liquid Phase Epitaxy (LPE) has been widely used to grow III-V and II-VI semiconductor materials. In the LPE process, a substrate is inserted into the saturated solution and then the temperature of the system decreased while the solute crystallizes and deposits on the substrate to form a film. The thickness is the fundamental parameter of the film. Compared to Vapor epitaxy methods (MBE, MOCVD, etc.), it is usually difficult to monitor the thickness directly in the LPE process because the temperature of the growth solution is quite high and the growth crucible is opaque. So, it is essential to control the growth conditions carefully to obtain a designed film thickness.
Some factors that affect the epilayer thickness include the growth temperature, the growth time, the cooling rate, the Solid-Liquid phase diagrams, and so on. It is important to reveal the relation between the film properties and these factors. In the early study
These studies were mostly based on the diffusion-limited model. The driving force of growth is the constitution gradient caused by the solute deposition in the cooling process. A constant cooling rate was mostly adopted in the theoretical models
Assuming the thickness of the solution is , the origin locates in the solid-liquid interface and the positive x direction points to the liquid, the LPE process could be described with the following diffusion equation, boundary, and initial conditions
Diffusion equation:
. | (1) |
Initial condition:
. | (2) |
Boundary condition:
. | (3) |
With the semi-infinite growth solution, ,the boundary condition Eq.3 changes to
Boundary condition:
. | (4) |
Referring to

Fig.1 The schematic phase-diagram used in the LPE growth model
图1 液相外延生长模型中所采用的相图的示意图
In appendix A, the relation between the epilayer thickness and the growth time in the semi-infinite solution condition is given by
. | (5) |
The symbol * in Eq.5 is the convolution operator. So, the epilayer thickness is proportional to the convolution of the reciprocal of the square root of time and the “constitutional supercooling”. For different growth process, “constitutional supercooling” may have different form. J. J. Hsieh
For the Equilibrium-cooling technique, the cooling rate is a constant. Assuming the slope of liquidus is also a constant , it is obvious that . So,
. | (6) |
For the Step-cooling technique,the degree of the supercooling is a constant and . So,
. | (7) |
For the supercooling technique, . So,
. | (8) |
Equations
As for the two-phase-solution technique, the process deviates from equilibrium and the deposition will occur on both the substrate and the precipitate
Moreover, in appendix A, the epilayer thickness on the bounded solution condition is also obtained. On such conditions, the epilayer thickness is a sum of infinite series as described by Eq. A19. Supposing , then the epilayer thickness can be calculated,
. | (9) |
This result consists with that of obtained in Ref.[
As mentioned above, it is difficult to maintain a constant cooling-rate in a real growth process. The cooling-rate will change slightly during the cooling process. According to Eq. 6, we should get a straight line passing through the origin if we make a curve of versus . However, the line did not always pass through the origin in the experiments. R. L. Moon etc.
. | (10) |
Numerically, the growth time is divided into N equal parts and the interval is for each part. The midpoint of each time interval is . The degree of supercooling at moment is . According to Eq. 10 and the definition of convolution, we get,
. | (11) |
In the above expression, and are both measurable values. So, for this non-uniform cooling process, the numerical solution of the partial differential equation (PDE) is simplified to an algebraic sum.
As an example, the relationship between the temperature and the growth time in a HgCdTe LPE is shown in
. | (12) |

Fig. 2 The relation between temperature and the time in a LPE process
图2 液相外延生长过程中温度随时间的变化关系
The above result means the real thickness will be smaller than the value calculated by Eq.6, the derivation of the thickness is about 7%. In
In the LPE process illustrated in

Fig. 3 LPE layer thickness versus 3/2 power of growth time.
图3 液相外延层厚度与生长时间的3/2次方的关系。
In the more general situation, the slope of the liquidus is not a constant. may be a function of temperature , . If the relation between temperature and time is , can be expressed as a function of time , . For instance, Henry T. Minden
, | (13) |
where is the Dawson function, .
The liquid phase epitaxy (LPE) process can be described by the diffusion-limited model. The convolution expression deduced in this study can deal with the three LPE techniques, namely step-cooling, equilibrium-cooling and supercooling. For the nonuniform cooling process, we compared the difference between the convolution calculation and the simply-model (uniform cooling-rate) calculation. The result shows there is a quite difference between these two methods which should be considered in the real LPE process. Moreover, by adopting the phase diagram data, the epilayer thickness could be predicted which is helpful to the control of LPE process.
Appendix
The control equation for the one-dimension diffusion process in a finite solution can be written as:
Diffusion equation:
. | (A1) |
Initial condition:
. | (A2) |
Boundary condition:
, | (A3) |
where is the solute concentration at the growing interface and is the thickness of the solution.
Let , the Eqs. A1-A3 become
, | (A4) |
, | (A5) |
. | (A6) |
The Laplace transform method is used to solve (A4)-(A6).
The Laplace transform of is , Eq. A4 becomes
. | (A7) |
Boundary condition (A6) becomes
. | (A8) |
Solving the differential Eqs. (A7)-(A8) leads to
. | (A9) |
Taking the inverse Laplace transform of (A9) leads to
, | (A10) |
where * represents the convolution operator. The convolution between two functions u(t) and v(t) is defined as:
. | (A11) |
So, the differential of C(x,t) at x=0 is
[C-] . | (A12) |
The thickness of the film can be expressed as
, | (A13) |
where is concentration in the solid.
Substituting (A12) into (A13) yields
. | (A14) |
Taking the Laplace transform of (A14) leads to
. | (A15) |
Then taking the inverse Laplace transform of (A15), we can obtain
. | (A16) |
If the solution is semi-infinite,L→∞,tanh()→1,eq. (A16) becomes
. | (A17) |
If the solution is bounded,we need simplify the inverse Laplace transform in (A16) which is
. | (A18) |
The residue theorem is used to calculate (A18).The residue of the pole at p=0 is just zero.The poles due to the hyperbolic tangent function in (A18) is (n+
So, the thickness of the film in bounded solution is
. | (A19) |
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