Intervalence Band Transitions Research Articles

Probing Intervalence Band Coupling via High‐Harmonic Generation in Binary Zinc‐Blende Semiconductors

A systematic microscopic theory is presented for high‐harmonic generation in III–V semiconductors that are excited by strongly detuned high‐intensity electromagnetic pulses. As a mechanism for the appearance of even harmonic orders, the quantum interference of different transition paths is analyzed. For the binary zinc‐blende semiconductors, InAs, InP, and GaAs, the intensity ratio between even‐ and odd‐order harmonic emission depends on the strength of the respective intervalence band dipole coupling. However, the large intervalence band dipole in InP leads to even and odd harmonics of similar intensity, no or only very minor even‐order emission due to quantum interference is predicted for InAs where the fundamental transition is strongly favored over intervalence band transitions.

Relationship between the Fano asymmetry parameter and time-domain spectra studied by time-resolved measurement of carriers and phonons in p -type Si

We performed time-resolved reflectivity measurements in $p$-type Si to study the Fano asymmetry parameter as a function of time-domain parameters. We observed a fast decay of intervalence band transitions, and slow decay of optical phonons, which respectively correspond to the continuum and discrete states. The experimental results revealed that the initial phase of the discrete state and the ratio of the relative transition amplitude between the discrete and continuum states affect the Fano asymmetry parameter. We derived an equation for the Fano asymmetry parameter as a function of the time-domain parameters that explained the experimental results. The present study indicates how the Fano asymmetry parameter is controlled by the time-domain parameters.

On the infrared absorption coefficient measurement of thick heavily Zn doped GaAs using spectrally resolved modulated photothermal infrared radiometry

In this paper, we report on measurements of the infrared absorption coefficient in the mid-infrared range of a heavily Zn-doped GaAs wafer using spectrally resolved modulated photothermal infrared radiometry (PTR). The method allows us to measure the infrared absorption coefficient of (i) much thicker samples as compared to the one used in Fourier Transform Infrared (FTIR) spectroscopy in transmission configuration and (ii) with non-mirror-like surfaces as would be required for measurements in the reflection configuration. From the best fits of the theoretical model to the PTR results, the values of the infrared absorption coefficient and thermal diffusivity of GaAs wafer are obtained. These values of infrared absorption coefficients are compared both with the literature values on very thin, similarly doped GaAs:Be sample and with infrared absorption coefficients calculated from FTIR specular reflectance measurements on the same sample. FTIR reflectance measurements demand additional assumptions for the evaluation of absorption coefficient and mirror-like surfaces. The results obtained from both experimental methods yield the same order of the infrared absorption coefficients. It is observed that the infrared absorption coefficient decreases with increasing wavelength because of inter-valence band transitions. However, only the infrared spectrum estimated using PTR exhibits free carrier absorption effect at a shorter wavelength as observed in previous works on very thin Be-doped GaAs samples. It is worth mentioning that the presented method is not limited to semiconductors, but can be used for other highly infrared absorbing samples. In addition, the spectrally resolved PTR measurements simultaneously provide the same values of thermal diffusivity of the GaAs wafer within estimation uncertainties thus demonstrating the reliability of the PTR method in the measurement of thermal diffusivity of such samples.

Electrodeless measurement method of conduction type based on the free carrier absorption effect in CdZnTe

In this paper, we review theoretical and experimental studies on the infrared attenuation spectrum between 400 and 4000cm−1, which is principally due to the free carrier absorption (FCA). The free carrier absorption is possible by means of inter-conduction band transitions which occur in n-CdZnTe, or by means of inter-valence band transitions which occur in p-CdZnTe. Based on the power law fit results of absorption spectra determined by FCA characteristics, we present an optical measurement technique, which can help us to determine the conduction type of CdZnTe in another way.

Raman study of the quantum interference of multiple discrete states and a continuum of states in the phonon energy region of semiconductors: examples of p-type Ga0.5In0.5P films

Although the Fano quantum interference of one longitudinal optical (LO) phonon mode and one electronic continuum of states due to inter-valence band transition has been investigated by the Raman spectra, there is no observation of the interference of multiple phonon modes despite its potential for electromagnetically induced transparency in the far infrared region. In this study, p-type Ga0.5In0.5P films with two main LO modes in the same plane of atomic vibration and one more mode due to CuPt-type ordering are investigated. Asymmetric line profiles, peak energy shifts and peak broadenings in the Raman spectra are analyzed, and the destructive quantum interference of these LO-continuum systems are observed. Discussion with the reference data of p-type GaAs and Si for the one-LO mode system reveals that the asymmetry parameter features the derivatives of the density of the continuum of states in the vicinity of the LO phonon energies and the degree of the electric polarization. It is found experimentally that the interaction between the two main LO modes through the interactions with the continuum states takes place. A1(LO) mode due to the ordering affects the spectrum, however the interaction with the main two LO modes through the continuum is small. On the basis of the experimental result, the spectrum features for this alloy with greater hole density are estimated, where the various controls of the absorption spectrum in the phonon energy region are predicted.

High-field nonlinear conductivities of n- and p-type GaAs thin films in the terahertz region

We investigated high-field terahertz (THz) responses and the nonlinear conductivities of n- and p-type GaAs thin films in the THz region. As the THz pulse intensity was increased to 59 μJ/cm2, the THz transmission of the n-type GaAs thin film was significantly increased, more than that of the p-type film. This result is correlated with the conductivity of the electronic system caused by scattering processes. Intervalley scattering was dominant in n-type GaAs, whereas hot–cold hole interactions and intervalence band transitions were the main factors reducing the conductivity in p-type GaAs. In addition, we extracted the frequency-domain conductivity of the n- and p-type GaAs thin films and used the Drude–Smith model to explain non-Drude-like behavior due to high-field excitation.

Electron transfer across the germanium-polyelectrolyte-gold nanoparticle interface: convenient detection and applications in sensing.

Germanium-polyelectrolyte-gold nanoparticle composites were prepared and characterized using FTIR-ATR spectroscopy and scanning electron microscopy. The germanium (Ge) element served as an internal reflection element and the buildup of the layered system was followed in situ. A positively charged polyelectrolyte, poly (allylamine hydrochloride) (PAH) adsorbs spontaneously on negatively charged Ge. Citrate-stabilized gold nanoparticles can then be adsorbed onto the PAH layer. Upon illumination of the device with visible light a prominent absorption over the entire mid infrared region is observed which is due to intervalence band transitions in Ge. The strong infrared signals are evidence for holes in the valence band of the Ge semiconductor, which arise due to electron transfer to the gold nanoparticles (GNPs). The electron transfer, as evidenced by the holes in Ge, is affected by the nature of the gap between the Ge semiconductor and the GNPs. Increasing the gap by adsorbing polyelectrolyte multilayers hinders the electron transfer. Also, heating and vacuum have a pronounced effect. The device is proposed as a sensor, where the sensing event is transduced into an optical signal in the infrared region, as demonstrated for a thiol molecule. The thiol has a large affinity for gold, and therefore affects the germanium-gold nanoparticle gap. This reduces the electron transfer, and therefore the absorption in the infrared region upon illumination with visible light. Removal of the thiol from the solution leads to the recovery of the signal.

(Invited) Strain and Phonon-Carrier Interactions in Ge-Si0.5Ge0.5 Core-Shell Nanowires Probed Using Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is used to characterize nanoscale correlations between strain and nanowire dimensions in Ge-Si0.5Ge0.5 core-shell nanowires, and to elucidate phonon-carrier interactions that are manifested via shifts in Raman peaks originating from Ge-Ge vibrational modes within the quantum-confined hole gas formed at the Ge-Si0.5Ge0.5 interface. Two distinct peaks associated with Ge-Ge vibrational modes are observed – one arising from Ge-Ge vibrations in the nanowire Ge core region, and the other from Ge-Ge vibrations occurring within the quantum-confined hole gas at the Ge-Si0.5Ge0.5 interface. Both peaks exhibit a dependence of Raman shift on local nanowire diameter consistent with coherent strain in the nanowire, demonstrating that the TERS measurements provide the ability to probe strain at the nanoscale. The separation between these peaks is a consequence of coupling between intervalence band transitions and the Ge-Ge phonon mode leading to Fano resonance behavior and a shift in the phonon self-energy.

(Invited) Strain and Phonon-Carrier Interactions in Ge-Si0.5Ge0.5 Core-Shell Nanowires Probed Using Tip-Enhanced Raman Spectroscopy

Group IV nanowires and nanowire heterostructures are of current interest for a broad range of applications including field-effect transistors with enhanced carrier mobility, and thermoelectric devices with engineered thermal and carrier transport properties. The energy band alignments among Si, Ge, and SixGe1-x alloys enable quantum confinement of either holes or electrons, while compositional variations and structural morphology at the nanoscale can lead to new phonon scattering and confinement effects. Because of the lattice mismatch between Si and Ge, however, the dimensions and alloy compositions in nanowire heterostructures must be carefully controlled and characterized to maintain coherent strain. Here we describe the use of tip-enhanced Raman spectroscopy to characterize nanoscale correlations between strain and nanowire dimensions in Ge-SixGe1-x core-shell nanowires grown by ultrahigh-vacuum chemical vapor deposition, and to elucidate phonon-carrier interactions that are manifested via shifts in Raman peaks originating from Ge-Ge vibrational modes within the quantum-confined hole gas formed at the Ge-SixGe1-x interface. Typical nanowire structures consisted of a Ge core 35-55nm in diameter surrounded by a 5nm thick Si0.5Ge0.5 shell, leading to compressive strain in the Ge core and formation of a quantum-confined hole gas at the Ge-Si0.5Ge0.5 interface. Tip-enhanced Raman spectroscopy (TERS) was performed under ambient atmospheric conditions on nanowires dispersed on an Au surface using an Au-coated atomic force microscope probe tip and 633nm laser excitation in a side illumination geometry. Tip-enhanced Raman spectra exhibit two distinct peaks associated with Ge-Ge vibrational modes – one arising from Ge-Ge vibrations in the nanowire Ge core region, and the other from Ge-Ge vibrations occurring within the quantum-confined hole gas at the Ge-Si0.5Ge0.5 interface. Both peaks exhibit a dependence of Raman shift on local nanowire diameter consistent with coherent strain in the nanowire, demonstrating that the TERS measurements provide the ability to probe strain at the nanoscale. The difference in Raman shift associated with these peaks is shown to be a consequence of coupling between intervalence band transitions and the Ge-Ge phonon mode that leads to Fano resonance behavior and a shift in the phonon self-energy. In addition, positioning of the Au-coated probe tip in closer proximity to the nanowire leads directly to an increase in the separation between these peaks and in the relative amplitude of the peak from the Ge-Si0.5Ge0.5 interface region, confirming that the TERS measurement is sensitive to phenomena within an extremely localized region in the vicinity of the probe tip apex. Computational electromagnetic simulations confirm that proximity to the probe tip leads to a large increase in electromagnetic field amplitude in the nanowire within ~10-20nm of the probe tip surface, and provide insight into the dependence of coupling to intervalence-band carrier transitions on proximity to the probe tip during the TERS measurement. Figure 1

The relevance of dephasing in THz valence band polariton dispersion relations

THz polaritons have strong potential for device applications and are a challenging field of fundamental studies. In this paper, we use a simplified mathematical model based on a nonlinear dielectric constant approach to the optical susceptibility to describe the coupling of TE-polarized THz radiation with an intervalence band transition in $$\hbox {GaAs/Al}_{0.3}\hbox {Ga}_{0.7}\hbox {As}$$ multiple quantum wells embedded in microcavities. The energy dispersions relations leading to THz polaritons are investigated. Here we focus on the interplay of cavity length and size of the active region against the dephasing mechanisms on polariton dispersion relations.

THz intervalence band antipolaritons

THz polaritons and antipolaritons have strong potential for device applications and are a challenging field of fundamental studies. In this paper, we start from a numerically exact nonequilibrium many body solutions and adjust it to a simplified nonlinear dielectric constant approach to the optical susceptibility. The resulting expression is inserted in the wave equation to describe the coupling of TE-polarized THz radiation with an intervalence band transition in GaAs/Al0.3Ga0.7As multiple quantum wells embedded in microcavities. The energy dispersions relations leading to THz polaritons are investigated. Here we focus on the impact of dephasing and scattering processes for different structures and excitation conditions in an inverted medium leading to antipolaritons.

Coupling of THz radiation with intervalence band transitions in microcavities

The strong coupling of THz radiation and material excitations can improve the quantum efficiency of THz emitters. In this paper, we investigate THz polaritons and antipolaritons based on valence band transitions, which allow TE coupling in a simple configuration. The approach can improve the quantum efficiency of THz based devices based on TE mode in the strong coupling regime of THz radiations and intervalence bands transitions in a GaAs/AlGaAs quantum wells. A Nonequilibrium Many Body Approach for the optical response beyond the Hartree-Fock approximation is used as input to the effective dielectric function formalism for the polariton/antipolariton problem. The energy dispersion relations in the THz range are obtained by adjusting the full numerical solutions to simple analytical expressions, which can be used for non specialists in a wide number of new structures and material systems. The combination of manybody and nonparabolicity at high densities leads to dramatic changes in the polariton spectra in a nonequilibrium configuration, which is only possible for intervalence band transitions.

Plasma frequency and dielectric function dependence on doping and temperature for p-type indium phosphide epitaxial films

The optical properties of p-type InP epitaxial films with different doping concentrations are investigated by infrared absorption measurements accompanied by reflection and transmission spectra taken from 25 to 300 K. A complete dielectric function (DF) model, including intervalence band (IVB) transitions, free-carrier and lattice absorption, is used to determine the optical constants with improved accuracy in the spectral range from 2 to 35 μm. The IVB transitions by free holes among the split-off, light-hole, and heavy-hole bands are studied using the DF model under the parabolic-band approximation. A good understanding of IVB transitions and the absorption coefficient is useful for designing high operating temperature and high detectivity infrared detectors and other optoelectronic devices. In addition, refractive index values reported here are useful for optoelectronic device designing, such as implementing p-InP waveguides in semiconductor quantum cascade lasers. The temperature dependence of hole effective mass and plasma frequency is also reported.

Dielectric function model for p-type semiconductor inter-valence band transitions

The contributions of inter-valence band (IVB) transitions to the dielectric function (DF) by free holes among the split-off (so), light-hole (lh) and heavy-hole (hh) bands were investigated. A model was developed to determine the DF of two p-type semiconductors, GaAs and Ge1−ySny with the Zinc-blend and Diamond crystal structures, respectively. The IVB transitions dominate the spectral range between 0.1–1eV with respect to the spin-orbit splittings between so-hh and lh-hh bands. In conjunction with inter-band transitions, free-carrier and lattice absorption, a complete DF model allows the determination of optical constants with improved accuracy in the spectral range covering both ultraviolet and infrared regions. The model should be applicable to most of the group III-V and IV materials since their valence band structures resemble the ones under investigation.

Doping-type dependence of phonon dephasing dynamics in Si

We studied the dephasing dynamics of coherent phonons in n-type, p-type, and intrinsic Si using time-resolved reflectivity measurements with sub-10 fs laser pulses. The dephasing time of the coherent phonons increases (decreases) for n-type (p-type) doping compared with that of intrinsic Si, while the frequencies of the coherent phonons exhibit a redshift for both types of doping. These doping-induced changes in the coherent phonon dynamics are observed when the carrier concentration exceeds 1019 cm−3. The doping-type dependent changes in the dephasing time are attributed to the interconduction and intervalence band transitions in n-type and p-type Si, respectively.

Optical absorption and photoluminescence studies on heavily doped (Ga,Mn)Sb crystals

Ga1-xMnxSb crystals are grown with different Mn doping concentrations by the horizontal Bridgman method (x = 0 – 0.04). Optical absorption and photoluminescence studies are carried out in the temperature range 3–300 K. Optical absorption studies reveal that the inter-valence band transition from the spin-orbit split-off band to the light/heavy hole bands is dominant over the fundamental valence band to conduction band absorption. In higher doped crystals, the fundamental absorption peak is merged with the inter-valence band transition and could not be resolved. Photoluminescence measurements in heavily doped crystals reveal the band gap narrowing and band filling effects due to the Fermi level shifting into the valence band.

Crystal growth and below-bandgap optical absorption studies in InAs for non-linear optic applications

Abstract Indium arsenide (InAs) is a direct bandgap semiconductor ( E g =0.36 eV) and is well suited for non-linear optic (NLO) applications. The NLO applications require high linear optical transmission for below-bandgap energy photons in the material. Due to this stringent criterion, there is a need for understanding the fundamental optical absorption mechanisms in the material as well as correlating them with the presence of native defects and impurities introduced during the crystal growth. In this paper, we will present the experimentally measured and theoretically analyzed infrared absorption in an undoped and Te-doped n-type InAs in the wavelength region of 4–20 μm for horizontal Bridgman grown crystals. Transmission measurements at 300 K were performed on samples of undoped and Te-doped n-type InAs with carrier concentration in the range of 2.6×10 16 –1.7×10 17 cm −3 . The dependences of various absorption mechanisms as a function of wavelength have been presented. Free carrier absorption by electrons is found to be the dominant absorption mechanism at longer wavelengths ( λ >9 μm) for all the samples. At shorter wavelengths ( λ n i =1×10 15 cm −3 ), the inter-valence band transitions by free holes contributes significantly to the total absorption. Unique fitting parameters have been deduced to theoretically explain the infrared absorption mechanism in InAs for the above-mentioned carrier concentration range.

Hole spin relaxation: optical deformation potential scattering

The scattering of hole spin during hole intra- and intervalence band transitions due to optical deformation potential interaction in tetrahedral semiconductors is considered. Matrix elements for transitions mediated by longitudinal and transverse optical phonons are presented. It is shown that for this type of scattering, the spin relaxation time can be found from hole momentum scattering time.

Temperature dependence of polarized absorption bands in p-type CdGeAs2

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.

Mid-IR optical limiter based on type-II quantum wells

We show that strong optically induced intervalence band transitions in type-II InAs/GaSb/AlSb quantum wells lead to reverse saturable absorption, and propose to apply it to optical limiting in the mid-infrared spectral region. A salient feature of the proposed limiter is the flexibility of the design for different wavelengths and threshold powers. We develop a rigorous theoretical model of the proposed limiter, and use it to estimate the relationship between the key figures of merit: insertion loss, threshold, optical bandwidth and dynamic range. We investigate tradeoffs involving the dynamic range, optical bandwidth and thickness of the limiter, and show that one can attain a favorable combination of these.