Transmission and absorption measurements of GaAsP layers grown from the vapor phase

We perform transmission measurements in the 0.3-25 mu m spectral range on thin (0.2-0.4-mm) GaAsP layers grown by Hydride Vapor Phase Epitaxy (HVPE) on plain (100) GaAs substrates. The purpose is to evaluate the evolution of the cut-off wavelengths in the visible ( VIS) and in the mid-IR spectral range. The former is important in order to assess the potential of such mixed ternary quasi-phase-matching structures for pumping by intermediate laser systems (compared to pure GaP and GaAs), such as Cr4+ and Er3+ lasers emitting in the 1.5-1.6 mu m spectral range. The latter is essential to see how the presence of P shortens the mid-IR clear transparency cut-off wavelength of GaAs due to phonon absorption. In all cases the substrates have been completely removed by polishing prior to the measurements. The multiple-reflection effect, which is considerable for such high refractive index materials has been taken into account for the incoherent light source of the spectrophotometer both in the region of clear transparency and in the presence of absorption. The actual sample thickness is derived from the observed interference fringes and the refractive index which is interpolated using available data on GaAs and GaP. With this information we calculate the wavelength dependent absorption coefficient. For some compositions, e.g. 33% P, we observe nice clear transmission plateau with almost vanishing absorption/scatter losses. Further increase of the P content shifts the VIS cut-off limit to shorter wavelengths but has negligible effect on the clear transparency upper limit which is around 13 mu m in the mid-IR.