Dynamically coupled plasmon-phonon modes in GaP: An indirect-gap polar semiconductor

Abstract

The ultrafast coupling dynamics of coherent optical phonons and the photoexcited electron-hole plasma in the indirect gap semiconductor GaP are investigated by experiment and theory. For below-gap excitation and probing by 800-nm light, only the bare longitudinal optical (LO) phonons are observed. For above-gap excitation with 400-nm light, the photoexcitation creates a high density, nonequilibrium e−h plasma, which introduces an additional, faster decaying oscillation due to an LO phonon-plasmon coupled (LOPC) mode. The LOPC mode frequency exhibits very similar behavior for both n-and p-doped GaP, downshifting from the LO to the transverse optical (TO) phonon frequency limits with increasing photoexcited carrier density. We assign the LOPC mode to the LO phonons coupled with the photoexcited multi-component plasma. For the 400-nm excitation, the majority of the photoexcited electrons are scattered from Γ valley into the satellite X valley, while the light and spin-split holes are scattered into the heavy hole band, within 30 fs. The resulting mixed plasma is strongly damped, leading to the LOPC frequency appearing in the reststrahlen gap. Due to the large effective masses of the X electrons and heavy holes, the coupled mode appears most distinctly at carrier densities 5 × 10 18 cm −3 . We perform theoretical calculations of the nuclear motions and the electronic polarizations following an excitation with an ultrashort optical pulse to obtain the transient reflectivity responses of the coupled modes. We find that, while the longitudinal diffusion of photoexcited carriers is insignificant, the lateral inhomogeneity of the photoexcited carriers due to the laser intensity profile should be taken into account to reproduce the major features of the observed coupled mode dynamics.

Intrinsic coherent acoustic phonons in the indirect band gap semiconductors Si and GaP

“…For GaAs, the effect of the ultrafast carrier diffusion on the strain pulse shape is significant [22] [45][46][47] and of the steep initial distribution of carriers in the depth direction arising from the small optical penetration depth (α −1 pu =14 nm) of the 3.1-eV pump light. By contrast, we expect a smaller diffusion coefficient, D am = 5−10 cm 2 s −1 for both GaP [33] and Si [48] in the present excitation conditions. Moreover, the initial carrier distribution is less steep than that of GaAs because of the larger optical penetration depth (α ≃100 nm).…”

“…For the first picosecond [ Fig. 3b], the reflectivity traces exhibit a non-oscillatory electronic response, and on top of that, a fast periodic modulation due to the generation of coherent longitudinal optical (LO) phonons at 12 and 15.6 THz for GaP and Si, which have been previously reported elsewhere [33,42].…”

“…Schematic band structures of GaP (a) and Si (b) near the fundamental band gaps [33,34]. Solid and broken arrows indicate the major transitions with 3.1-eV photons and the relaxation pathways discussed in the text.…”

Comparison between optical and electrophysical data on free electron concentration in tellurium doped n-GaAs

“…If ωр and ω LO are close then the independent plasmons and longitudinal optical phonons which existed earlier are replaced by coupled plasmon-phonon modes [13][14][15][16][17][18][19][20][21][22][23][24][25][26] whose frequencies (where ω + is the high-frequency one and ωis the low-frequency one) can easily be calculated using the following formula (plasmon and LO phonon damping is disregarded):…”

Nanomaterial‐Based Plasmon‐Enhanced Infrared Spectroscopy

“…From the perspective of device integration, we need to rely on semiconductors (especially group-IV semiconductors), which additionally can be used to dynamically adjust the resonance frequency through chemical doping or electrical gating. Thus, a wide range of doped semiconductors have been explored as mid-IR plasmonic materials, such as group-IV semiconductors (e.g., Si, [27,101,102] Ge, [28][29][30] and SiC [103] ), group III-V semiconductors (e.g., GaAs, [104,105] InAs, [32,33] InAsSb, [35] GaN, [106] and GaP [107] ), and oxide semiconductors (e.g., indium tin oxide (ITO), [108] aluminum-doped zinc oxide (AZO), [109] and gallium-doped zinc oxide (GZO) [110] ). For example, silicon, the most widely used semiconductor, can be highly doped to achieve a free-carrier concentration as high as 10 20 cm −3 using various approaches (e.g., ion implantation) [27,101] that is promising for IR plasmonics.…”