Vibrational properties of $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ under hydrostatic pressures up to $20\mathrm{GPa}$

We investigated a $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ single crystal under hydrostatic pressures up to $20\mathrm{GPa}$ by Raman spectroscopy. The zinc blende optical phonons show great similarities to those of binary GaAs under the same conditions demonstrating that nitrogen incorporation has no major influence on the GaAs-I to GaAs-II phase transition and its partial reversibility upon decompression. Frequency shifts of the nitrogen local vibrational mode under hydrostatic pressure are very different from those in binary GaN because of the different compressibilities of the two materials and the overstretched character of the $\mathrm{Ga}\mathrm{N}$ bond in Ga(As,N). This is also reflected by the anharmonicity of the $\mathrm{Ga}\mathrm{N}$ bond potential in Ga(As,N).

Figure 1

Typical Raman spectra of epitaxial GaAs (lower curves) and $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ (upper curves) single crystals taken at room temperature and ambient pressure in LO-allowed backscattering geometry $x(y^{\prime },y^{\prime })\overline{x}$ with $514.5\mathrm{nm}$ excitation wavelength. The spectral range higher than $300{\mathrm{cm}}^{-1}$ has been enlarged by a factor of 30 with respect to intensity to show the relatively weak nitrogen LVM and second-order features. Peak assignments are given according to Refs. 8, 22.

Figure 2

Room-temperature Raman spectra of $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ under increasing hydrostatic pressure in the spectral ranges of the GaAs-like phonons (a) and of the GaN-like LVM (b). In (b) the fits of the LVM to a Gaussian line shape are also shown for pressures between 0.77 and $15.5\mathrm{GPa}$.

Figure 3

Pressure dependences of the mode frequencies on the pressure upstroke in $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ obtained from Raman spectra. (a) the GaAs-like LO and TO frequencies are represented by circles and squares, respectively. Open and closed symbols correspond to two measurement series. The two black solid lines show the quadratic fits to the experimental frequency shifts of both phonons as an average between the two series. For comparison, the experimental phonon frequencies (crosshairs) and the respective quadratic curve fits (grey lines) of GaAs are shown. (b) The shift of the GaN-like LVM in Ga(As,N) and a quadratic fit to the data points (black line) are plotted where the point at $15.5\mathrm{GPa}$ has been neglected in the curve fit. Exemplarily an error bar is given. As an example for a GaN mode, the pressure dependence of the $A_1\left(\mathrm{TO}\right)$ phonon in wurtzite-GaN is plotted (crosshairs). The grey line corresponds to a quadratic fit to these data.

Figure 4

Selected Raman spectra of $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$ from the second series taken on pressure upstroke (1 and 2) followed by pressure downstroke (3 and 4) after pressurization up to approximately $20\mathrm{GPa}$. Spectrum (4) was taken at ambient pressure outside the pressure cell in $x(y^{\prime },y^{\prime })\overline{x}$ backscattering geometry. Raman shifts are given relatively to the one of the LO phonon peak. Enlarging factors for spectra (3) and (4) are denoted on the right side of the respective curves.

Figure 5

Comparison of the force constant versus bond length calculated using Murnaghan’s equation of state for the LO phonon of wurtzite GaN and the LVM of $\mathrm{Ga}{\mathrm{As}}_{0.915}{\mathrm{N}}_{0.085}$.