Figures

Figure 1

Dispersion of pseudo surface polariton modes at the interface doped / undoped GaAs as a function of the free-carrier concentration n for kx = sin(70°). The two branches (PSP+, PSP-) follow closely those of the longitudinal-optical coupled plasmon-phonon bulk modes (LPP+, LPP-). The horizontal lines indicate the TO and LO frequencies of GaAs. The inset displays the difference between the PSP and LPP modes (PSP+-LPP+: right axis, PSP--LPP-: left axis). Parameters used for calculations are LO =291.7 cm-1, TO =267.8 cm-1, = 11.7, m* = 0.063me. No broadening is assumed here for simplicity.

Figure 2

Experimental data (symbols), and best-fit calculation (solid lines) of from GaAs/GaNxAs1-x-SL heterostructures (See Table 1). Vertical lines indicate the GaAs TO (TO1) and LO (LO1) frequencies. The Berreman-polariton effect above the GaAs LO1 frequency causes distinct resonance features in all samples. The upper-branch PSP+ mode is excited near 306cm-1 between the n-type GaAs substrate and the undoped GaAs buffer layer / SL heterostructure. See also Figure 3. The angle of incidence is a = 70°. Spectra are to scale, but shifted for convenience by 20° each. The PSP+ resonance becomes washed out for higher nitrogen contents due to screening of the incident electromagnetic field components within the GaNxAs1-x-SL sublayers by free carriers, which concentrate within the GaNxAs1-x-SL sublayers. At TO2, the Ga-N sublattice vibration is detected within the IRSE spectra. See also Figure 4.

Figure 3

Index of refraction N and extinction coefficient k ( = [N + ik]2) of the n-type GaAs substrate (dotted lines), the undoped GaAs buffer layer and SL sublayers (solid lines), and GaN0.009As0.991 (dashed line) for example. The position of the PSP+ mode is indicated as the frequency at which the substrate refractive index becomes less than 1. The GaN0.009As0.991 optical spectra reveal the Ga-N sublattice resonance at TO2 (vertical line).

Figure 4

Enlarged section of Figure 2 within the Ga-N sublattice resonance frequency (TO2) range. (Spectrum for x=0.9% is shifted by -1° for convenience.) The feature labeled 2LO1 is assigned as the forbidden but disorder-activated second harmonic of the GaAs LO frequency in sample GaNAs019.

Figure 5

GaNxAs1-x relative TO-LO splitting f and carrier concentration n versus x obtained from IRSE data analysis (See Table 2; Lines are drawn to guide the eye.).

© 2000 The Materials Research Society