Coupled plasmon-LO Phonon Modes Research Articles

Composition, Optical Resonances, and Doping of InP/InGaP Nanowires for Tandem Solar Cells: a Micro-Raman Analysis.

We present a micro-Raman study of InP/InGaP tandem junction photovoltaic nanowires. These nanowires render possible InGaP compositions that cannot be made in thin films due to strain. The micro-Raman spectra acquired along the nanowires reveal the existence of compositional changes in the InGaP alloy associated with the doping sequence. The heavily Zn-doped InxGa1-xP (x is the In molar fraction) side of the tunnel diode is Ga rich, x = 0.25, with respect to the n-type and intrinsic segments of the top cell, which are close to the nominal composition of the NWs (x = 0.35). The p-type end segment is still Ga-rich. Electromagnetic resonances are observed in the tunnel diode. The Raman signal arising from the InGaP side of the tunnel diode is significantly enhanced. This enhancement permits the observation of a Raman mode that can be associated with an LO phonon plasmon coupled mode (LOPCM). This mode has not been previously reported in the literature of InGaP, and it permits the Raman characterization of the tunnel diode. The analysis of this mode and its relation to the LO phonon modes of the alloy, InP-like and GaP-like, allows to establish an apparent one-mode behavior for the phonon plasmon coupling. It indicates that hole plasma couples to the GaP-like LO mode. The LOPCMs are modeled using the Lindhard Mermin formalism for the dielectric function.

Raman scattering study on dilute nitride-bismide GaNAsBi alloys: behavior of photo-excited LO phonon-plasmon coupled mode

We present a Raman scattering study of as-grown and annealed GaAsBi, GaNAs, and GaNAsBi epilayers grown by plasma-assisted molecular beam epitaxy. In particular, photo-excited longitudinal optical phonon-plasmon-coupled (LOPC) modes were investigated using excitation power-dependent measurements. The optical properties of the samples were characterized by photoluminescence and photoreflectance measurements. From Raman scattering, local vibrational modes and disordered activated modes originating from N- and Bi-alloying were confirmed. The strong photo-excited LOPC modes observed in GaAsBi can be attributed to hole trapping caused by the Bi-induced shallow localized states. The weak photo-excited LOPC modes observed in the as-grown GaNAs and GaNAsBi indicate rapid carrier trapping owing to the high defect density in these dilute nitride alloys. After thermal annealing of GaNAs and GaNAsBi, the heavily damped photo-excited LOPC modes were observed, which can be explained by the low electron mobilities related to the N-induced shallow localized states.

Probing carrier concentration of doped GaN single crystals from LO phonon-plasmon coupled modes

Raman and infrared reflectance spectroscopy were used to study GaN single crystals with different doping types and carrier concentrations. In different samples, the characteristic spectra related to LO phonon-plasmon coupled (LOPC) mode are observed as changing regularly with carrier concentration. Furthermore, to obtain the carrier concentration and mobility of GaN crystals, Raman and infrared reflectance spectra with LOPC modes were fitted. The carrier concentration obtained agrees well with that obtained from the Hall measurement, which confirms that these non-contact and non-destructive spectroscopy methods with high spatial resolution are reliable to probe the carrier concentration of doped GaN single crystals.

Scattering of Rydberg excitons by phonon-plasmon modes

We propose a new scattering mechanism of Rydberg excitons, i.e., those with high principal quantum numbers, namely scattering by coupled LO phonon-plasmon modes, which becomes possible due to small differences in energies of the states due to different quantum defects. Already in very low-density electron–hole plasmas these provide a substantial contribution to the excitonic linewidth. This effect should allow determining plasma densities by a simple line shape analysis. Whenever one expects that low-density electron–hole plasma is present the plasmon induced broadening is of high significance and must be taken into account in the interpretation.

Terahertz electromagnetic waves radiated from coherent longitudinal optical (LO) phonons and LO-phonon plasmon coupled modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals

We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This finding indicates that the coupling of the LO phonon with the plasmon is not influenced by the surface orientations of the three samples.

Structural characterization of the grown crystal/seed interface of physical vapor transport grown 4H-SiC crystals using Raman microscopy and x-ray topography

The defect structure at and near the grown crystal/seed interface of 4H-SiC single crystals grown by the physical vapor transport (PVT) growth method has been investigated using Raman microscopy and x-ray topography. To examine the detailed distribution of crystallographic defects across the interface, 4H-SiC wafers containing a beveled growth interface were prepared from nitrogen-doped 4H-SiC single crystal boules; the beveled interface allows more spatially resolved observations of the defect structure near the interface compared to the interface on vertically sliced [e.g., (112¯0) and (11¯00)] wafers. Raman microscopy imaging using the longitudinal optical phonon-plasmon coupled (LOPC) mode determined the position of the growth interface and revealed several characteristic features of the interface through the analyses of the scattering intensity, peak position, and width of the LOPC mode peak. X-ray topography revealed that there existed networks of basal plane dislocations at the growth interface, and they extended deep inside the seed crystal. Based on these observations, we discussed the defect formation process at the initial stage of PVT growth and suggested an important role of vacancy injection during PVT growth of SiC.

Micro-Raman imaging on 4H-SiC in contact with the electrode at room temperature

Raman images (30μm×30μm×180μm) of a bulk 4H-SiC wafer in contact with a Ni/Au electrode film in 100nm/200nm thick were measured with Micro-Raman spectroscopy at room temperature. As the imaging area approached the interface between the SiC and electrode, the center frequency of the E2(TO) mode (778cm−1) immediately declined; in the Raman imaging, relative distribution of compressive residual stress around residual tensile stress, and linewidth were broadened due to crystal distortion. For LOPC (LO-phonon-plasmon-coupled) mode (970cm−1), center frequency showed variation right next to the interface, while linewidth decreased slowly as the imaging area approached the interface. We evaluated the temperature dependence of the line broadening and the center frequency of the LOPC mode in 4H-SiC in a high-temperature region. Free carrier concentration increased with temperature, and remained almost constant in the center frequency after impurities were ionized completely.

Effects of photogenerated carrier scattering on the decay process of coherent longitudinal optical phonons in an undoped GaAs/n-type GaAs epitaxial structure investigated by terahertz time-domain spectroscopy

The authors investigated the scattering effect of photogenerated carriers on the decay of the coherent longitudinal optical (LO) phonons in an undoped GaAs/n-type GaAs epitaxial structure using a terahertz time-domain spectroscopic technique. The terahertz wave from the coherent LO phonon was observed together with those emitted both by the ultrafast photocurrent and by the coherent LO-phonon-plasmon coupled (LOPC) mode. The simultaneous observation of the coherent LO phonon and coherent LOPC mode originates from the fact that the photogenerated carrier density laterally distributes around the surface of the undoped GaAs layer owing the Gaussian profile of the pump beam. The authors found that the terahertz wave from the coherent LO phonon lives up to 5.0 ps, whereas those from the coherent LOPC mode and ultrafast photocurrent disappear within 1.0 ps. The decay time of the coherent LO phonon monotonically decreases with an increase in the pump power. This finding indicates that the dephasing of the terahertz wave from the coherent LO phonon is dominated by the photogenerated carrier scattering in the time range shorter than 1.0 ps in which photogenerated carriers remain.

Longitudinal Optical Phonon-Plasmon Coupled Mode in Undoped GaAs/n-Type GaAs Epitaxial Structures Observed by Raman Scattering and Terahertz Time-Domain Spectroscopic Measurements: Difference in Observed Modes and Initial Polarization Effects

We investigated longitudinal optical (LO) phonon-plasmon coupled (LOPC) mode in undoped GaAs/n -type GaAs epitaxial structures using Raman scattering and terahertz time-domain spectroscopic measurements. In the Raman measurement, the LOPC mode, the frequency of which is determined by the doped electron density in the n -type layer, was observed in addition to the LO phonon in the undoped layer. Contrastingly, in terahertz time-domain measurements, the frequency of the observed coherent LOPC mode is dominated by a photogenerated electron density: frequency-tunable LOPC mode. This indicates that the LOPC mode is generated in the undoped layer. Thus, the probing mechanisms for the LOPC mode are different in the Raman and terahertz time-domain measurements. The built-in electric field exists in the undoped layer. This field enlarges the initial polarization of the LO phonon and, as a result, enables to observe the coherent LOPC mode generated in the undoped layer with the use of the terahertz time-domain spectroscopy.

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

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 $e\text{\ensuremath{-}}h$ 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 multicomponent plasma. For the 400-nm excitation, the majority of the photoexcited electrons are scattered from the $\mathrm{\ensuremath{\Gamma}}$ 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 $\ensuremath{\gtrsim}5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. 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.

Electron and phonon coupling dynamics in low-gap semiconductor: quantum versus classical scale.

We have studied the characteristics of longitudinal-optical-phonon-plasmon coupled (LOPC) mode by using the ultrashort pulsed laser with 45 THz bandwidth as a function of thickness in InAs epilayers, ranging from 10 to 900 nm. We have observed the LOPC modes split into the upper (L(+) mode) and the lower (L(-) mode) branches only in the classical scale, but the longitudinal-optical (LO) phonon peak was persistently observed. The shorter decay time of the plasmon-like L(+) modes rather than the phonon-like L(-) modes should be associated with carrier-carrier scattering which is further considered with diffusion properties in the low-gap semiconductors. This result leads to that the absence of the LOPC modes in a scale less than exciton Bohr radius manifests the role of electron diffusion rather than the carrier screening via drift motion in surface depletion region.

Raman spectroscopy from buried semiconductor interfaces: Structural and electronic properties

This review focuses on the analysis of buried semiconductor interfaces by Raman spectroscopy, i.e. inelastic light scattering. Simultaneous access to structural and electronic properties can be obtained by employing phonon Raman scattering, induced by deformation potential, or alternatively by the electric-field induced Fröhlich mechanism, as well as Raman scattering from coupled plasmon–LO-phonon modes. Structural information may comprise intermixing, reactivity, strain and interfacial atomic bond sequences. The main electronic properties derived from Raman spectroscopy are the doping levels of the constituent layers, the interfacial electric field strengths and the free-carrier depletion layer widths. Attention is paid also to the possibility of in situ growth monitoring by Raman spectroscopy. Examples of semiconductor interface analysis are presented for heterovalent interfaces between II–VI and III–V semiconductors, especially ZnSe/GaAs(001) and CdTe/InSb(001), as well as for heavily strained CdTe monolayers, embedded in a BeTe epilayer on GaAs(001).

Terahertz radiation from the coherent longitudinal optical phonon-plasmon coupled mode in an i-GaAs/n-GaAs epitaxial structure

We have investigated the time-domain terahertz radiation from an undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structure using an optical gating method with a femtosecond pulse laser. We have found from the Fourier power spectra of the terahertz waveforms that two terahertz bands, the frequencies of which obviously depend on the pump power, appear in a high density excitation regime in addition to a broad band due to a surge current and a sharp band due to the coherent longitudinal optical (LO) phonon. Based on a model for the LO phonon-plasmon coupled (LOPC) mode, we reasonably explain the pump-power (photogenerated-carrier-density) dependence of the frequencies of the two bands peculiar to the high density excitation. This fact demonstrates that the two terahertz bands are assigned to the lower and upper branches of the LOPC mode. The frequencies of the LOPC mode are determined only by the pump power. Thus, the LOPC mode originates from the i-GaAs layer; namely, the plasmon component of the surge current flowing through the i-GaAs layer couples with the coherent LO phonon.

Delayed formation of coherent LO phonon-plasmon coupled modes inn- andp-type GaAs measured using a femtosecond coherent control technique

Coherent control experiments using a pair of collinear femtosecond laser pulses have been carried out to manipulate longitudinal optical (LO) phonon-plasmon coupled (LOPC) modes in both $p$- and $n$-type GaAs. By tuning the interpulse separation, remarkably distinct responses have been observed in the two samples. To understand the results obtained a phenomenological model taking the delayed formation of coherent LOPC modes into account is proposed. The model suggests that the lifetime of coherent LOPC modes plays a key role and the interference of the coherent LO phonons excited successively by two pump pulses strongly affects the manipulation of coherent LOPC modes.

Longitudinal optical phonon-plasmon coupled modes of degenerate Al-doped ZnO films

We have investigated the interaction between carriers and polar phonons by using Raman scattering spectroscopy in highly conductive Al-doped ZnO films grown by metalorganic chemical vapor deposition. Different from the longitudinal optical phonon-plasmon coupled modes (LOPPCM) observed in nondegenerate ZnO, an A1(LO)-like mode appears at the low frequency side of the uncoupled A1(LO) mode, and it monotonically shifts to higher frequencies and approaches to the uncoupled A1(LO) mode as Al composition increases. Based on line shape calculations, the A1(LO)-like mode is assigned to the large wave-vector LOPPCM arising from nonconserving scattering dominated by the Al impurity-induced Fröhlich mechanism. Benefiting from the nonmonotonic Al composition dependence of the electron density, it is revealed that the LOPPCM depends mainly on the doping level but not the carrier concentration.

The properties of GaMnN films grown by metalorganic chemical vapour deposition using Raman spectroscopy

An investigation of room-temperature Raman scattering is carried out on ferromagnetic semiconductor GaMnN films grown by metalorganic chemical vapour deposition with different Mn content values. New bands around 300 and 669 cm−1, that are not observed in undoped GaN, are found. They are assigned to disorder-activated mode and local vibration mode (LVM), respectively. After annealing, the intensity ratio between the LVM and E2(high) mode, i.e., ILVM/IE2(high), increases. The LO phonon-plasmon coupled (LOPC) mode is found in GaMnN, and the frequency of the LOPC mode of GaMnN shifting toward higher side is observed with the increase in the Mn doping in GaN. The ferromagnetic character and the carrier density of our GaMnN sample are discussed.

Orientation dependence of polarized Raman spectroscopy for nonpolar, semi-polar, and polar bulk GaN substrates

Polarized Raman spectroscopy has been performed on high-quality bulk gallium nitride substrates with various crystal orientations. The transverse-optic (TO) and longitudinal-optic (LO) phonons shifted to a lower frequency upon changing the observed crystal plane from polar to semi-polar to nonpolar. This result is well explained by the mixing of A1 and E1 optical phonons. In addition, we analyzed in detail the frequency of LO-phonon-plasmon-coupled (LOPC) modes by calculating the peak frequency as a function of carrier concentration and the propagation direction of the LOPC mode. Carrier densities deduced by the analysis showed excellent agreement with the results of Hall measurements. We provide a simple, quick, and nondestructive procedure for identifying nonpolar GaN planes as well as obtaining the carrier concentration.

Terahertz spectroscopy of dynamics of coupling between the coherent longitudinal optical phonon and plasmon in the surge current of instantaneously photogenerated carriers flowing through the i-GaAs layer of an i-GaAs/n-GaAs epitaxial structure

We demonstrate the dynamics of coupling between the coherent longitudinal optical (LO) phonon and plasmon of instantaneously photogenerated electrons in an undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structure using time-domain terahertz spectroscopy. Initially, we experimentally and numerically clarify the presence of the built-in electric field in the i-GaAs layer of the i-GaAs/n-GaAs epitaxial layer. Next, we performed the terahertz-wave measurements of the i-GaAs/n-GaAs epitaxial structure at various excitation conditions from a low density excitation regime to a high excitation regime. The LO-phonon-plasmon coupled (LOPC) mode has been confirmed from the terahertz-wave measurement. It is found that the frequency of the LOPC mode is determined by the pump-beam power. This fact demonstrates that the LOPC mode is formed in the i-GaAs layer. In addition, we performed the time-partitioning Fourier transform in order to reveal the dynamical change in the LOPC mode as a function of time delay. Using this analysis, we have observed that the disappearance of the LOPC mode immediately occurs within the time delay of 0.6 ps. Following the disappearance of the LOPC mode, only the bare coherent GaAs LO phonon dominates the terahertz waves.

Control of carrier transport in GaAs by longitudinal-optical phonon-carrier scattering using a pair of laser pump pulses

We demonstrate optical control of the LO phonon-plasmon coupled (LOPC) modes in GaAs by using a femtosecond pump-pulse pair. The relaxation time of the plasmonlike LOPC mode significantly depends on the separation time (Δt) of the pump-pulse pair. Especially it is maximized when Δt becomes simultaneously comparable to the half period of the longitudinal optical (LO) phonon oscillation and resonant to the 3/4 period of the plasmonlike LOPC oscillation. We attribute these observations to the modification of carrier-LO phonon scattering and ballistic motion of the plasmonlike LOPC mode.

Photomodulation raman spectroscopy as a pump-probe technique for surfaces and interfaces of semiconductors

Photomodulation Raman spectroscopy (PM-RS) has been employed to study the surface depletion electric field Es and to monitor the change in surface charge density in n-type GaAs, using the forbidden LO phonon scattering for low doping samples and coupled plasmon–LO phonon modes for high doping samples. In PM-RS, the photomodulating pumping beam (PB) is incident on the sample while the Raman measurements are in progress hence PM-RS can be viewed as a pump-probe technique. The photogenerated carriers partly neutralize the surface charges. Two different GaAs surfaces (011) with low and moderate doping density and (001) with high doping density were used. The total surface charge density has been obtained as a function of the PB intensity considering a constant depletion electric field for the lower doping sample of (011) surface and using the dependence of the unscreened LO phonon on the depletion width for the higher doping samples of (001) surface. The minority carrier's lifetime was also determined through dynamical measurements for the PM-RS of the low doping sample as ≈ 21 s, in a good agreement with other techniques