The precise $Al_xGa_{1-x}As$ Bragg reflectors are most important parts of VCSEL (Vertical-Cavity Surface-Emitting Laser). To achieve lasing in VCSEL, we need an accurate thickness control, which is difficult for MOCVD (Metal-Organic Chemical Vapour Deposition). This paper presents an in-situ thickness monitoring method by laser reflectometry.
In general, the intensity of reflected laser beam from a thin film oscillates regularly with film thickness increase under a constant growth condition. Therefore, by measuring the period of oscillation, the growth rate of the thin film can be found. For accurate estimation of growth rate, reasonably thick buffer layers are grown in advance before the actual growth of the Bragg reflector. Then we can use the growth rates for the growth of the Bragg reflector which will be grown on top of the buffer layers, without changing the wafer.
We calibrated the complex refractive index of $Al_xGa_{1-x}As$ at the MOCVD growth temperature of 650℃. Since the laser reflectance depends on the optical thickness (n*d) of the thin film, we cannot know the film thickness without the refractive index. A new method is introduced for index calibration in this paper. The new method uses the real time reflectance signal and the reflection spectroscopy of the Bragg reflector after growth. The result shows that the real and imaginary parts of refractive index for $Al_xGa_{1-x}As$ are
$n_x$=4.08-0.688×- 0.125 $x^2$
$k_x$=0.31-0.18×-0.414 $x^ 2(0≤x≤0.6)
respectively, at 650℃.
Using these index data, we achieve in-situ thickness monitoring and MOCVD system control during the growth of Bragg reflectors. The Bragg reflectors with accuracy better than 2% are successfully grown.
