= Solution
Grow the <semiconductor> stack epitaxially by \b[<molecular beam epitaxy> or metalorganic vapor-phase <epitaxy>] on a suitable crystalline GaAs substrate. Sequential changes of source flux or precursor supply set the GaAs/AlGaAs compositions, while dopant supply sets the n-type and p-type regions. Growth time and calibrated rate control the thicknesses of the InGaAs <quantum well> and the alternating reflector layers.
<epitaxy> preserves a common crystal orientation and clean, abrupt <semiconductor> interfaces. These are important both for carrier confinement in the <quantum well> and for low-loss optical reflection. For a <DBR> centered at free-space <wavelength> $\lambda_0$, the approximate quarter-wave condition is $n_jd_j=\lambda_0/4$ in each layer. Ordinary deposition of a polycrystalline coating would not provide the required active <semiconductor heterostructure>.
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