Optical characterization of the structural and electrical properties of the $n$-GaP nanolayers grown on conductive (001) $n$-GaP substrates

The results of investigations of the structural and electrical properties of a homoepitaxial nanoscale (001) $n$-GaP layer of 70 nm in thickness grown by metalorganic vapor phase epitaxy on a conductive highly doped substrate of an $n$-GaP crystal oriented along (001) axis are reported. In the Raman spectrum of such an $n$-GaP nanolayer in the $n$-GaP/(001) $n$-GaP sample, as compared to the spectrum of the high-ohmic crystalline (001) $si$-GaP sample, we have managed to discover two quite narrow lines attributed to vibrations of transverse TO($\Gamma$)$_-$ phonons and high-frequency longitudinal coupled plasmon–phonon LO($\Gamma$)$_+$ vibrations. It is established that the spectral parameters of the LO($\Gamma$)$_+$ vibrations in both the $n$-GaP nanolayer and the (001) $n$-GaP substrate are significantly different from each other and from the spectral parameters of the line of longitudinal optical LO($\Gamma$)$_-$ phonons. Analysis of the revealed strict quantitative features of the spectral parameters has made it possible to obtain valuable data on the degree of perfection of the crystal structure for the nanoscale homoepitaxial (001) $n$-GaP layer. In addition, it is shown that numerical calculations based on the microscopic model of light scattering by LO($\Gamma$)$_+$ vibrations, caused by the mechanisms of the deformation potential and electro-optical scattering, has enabled noncontact and nondestructive determination of the concentration $n$ and mobility $\mu$ of free charge carriers in the nanoscale-thick homoepitaxial layer and the conductive highly doped substrate. The obtained values have proved to be as follows: the concentration $n_{\operatorname{hepi}}$=(3.25 $\pm$ 0.1)$\cdot$10$^{17}$ cm$^{-3}$ for and the mobility $\mu_{\operatorname{hepi}}$=(40.0 $\pm$ 0.1) cm$^{2}\cdot$ V$^{-1}\cdot s^{-1}$ for the homoepitaxial (001) $n$-GaP layer, against с $n_{\operatorname{subs}}$=(2.52 $\pm$ 0.1)$\cdot$10$^{17}$ cm$^{-3}$ and $\mu_{\operatorname{subs}}$=(51.0 $\pm$ 0.1) cm$^{2}\cdot$ V$^{-1}\cdot s^{-1}$ for the (001) $n$-GaP substrate of the $n$-GaP/(001) $n$-GaP sample.