Temperature-dependent Photoreflectance Research Articles

Influence of interface states on built-in electric field and diamagnetic-Landau energy shifts in asymmetric modulation-doped InGaAs/GaAs QWs

The impact of interface defect states on the recombination and transport properties of charges in asymmetric modulation-doped InGaAs/GaAs quantum wells (QWs) is investigated. Three sets of high-mobility InGaAs QW structures are systematically designed and grown by the metal-organic vapor phase epitaxy technique to probe the effect of carrier localization on the electro-optical processes. In these structures, a built-in electric field drifts electrons and holes towards the opposite hetero-junctions of the QW, where their capture/recapture processes are assessed by temperature-dependent photoreflectance, photoluminescence, and photoconductivity measurements. The strength of the electric field in the structures is estimated from the Franz Keldysh oscillations observed in the photoreflectance spectra. The effects of the charge carrier localization at the interfaces lead to a reduction of the net electric field at a low temperature. Given this, the magnetic field is used to re-distribute the charge carriers and help in suppressing the effect of interface defect states, which results in a simultaneous increase in luminescence and photoconductivity signals. The in-plane confinement of charge carriers in QW by the applied magnetic field is therefore used to compensate the localization effects caused due to the built-in electric field. Subsequently, it is proposed that under the presence of large interface defect states, a magnetic field-driven diamagnetic-Landau shift can be used to estimate the fundamental parameters of charge carriers from the magneto-photoconductivity spectra instead of magneto-photoluminescence spectra. The present investigation would be beneficial for the development of high mobility optoelectronic and spin photonic devices in the field of nano-technology.

Investigation of hydrogen inductively coupled plasma treatment effect for Ge0.938Sn0.062/Ge/Si film using photoreflectance spectroscopy

Temperature-dependent photoreflectance (PR) measurements for Ge0.938Sn0.062/Ge/Si films treated with a hydrogen inductively coupled plasma (H2-ICP) have been performed. The Ge0.938Sn0.062 film is grown on Ge-buffered Si substrate by ultra-high vacuum chemical vapor deposition method, and the H2-ICP treatment was carried out for 5min for the Ge0.938Sn0.062 epitaxial layer. The high-resolution X-ray diffraction results show that the compressive strain of the Ge0.938Sn0.062 layer decreases and the tensile strain of the Ge buffer layer increases after H2-ICP treatment. The PR spectrum of as-grown Ge0.938Sn0.062/Ge/Si film at 300K consists of three signals at 0.603, 0.782 and 0.814eV, which are assigned to the direct transitions from conduction Γ valley to the valence bands related to the surface of Ge0.938Sn0.062, the Ge/Si and GeSn/Ge interfaces, respectively. After H2-ICP treatment, two PR signals of 0.604 and 0.781eV were obtained at 300K and they are attributed to the direct transition energies of the Ge0.938Sn0.062 and the Ge buffer layer, respectively. As temperature decreases, new weak PR signals appear in the lower energy regions of both PR signals from the H2-ICP treated GeSn and Ge layers at 210K and 130K, respectively, due to the increase of tensile strain in Ge layer while no new signal appears for the as-grown sample. The PR spectrum of the H2-ICP treated sample at 10K shows four signals, and these signals are assigned to the surface of GeSn and GeSn at the interface between GeSn and Ge buffer layers, the Ge at the interface between GeSn and Ge buffer layers, and the Ge at the interface between Ge buffer layer and Si substrate, respectively. These PR results indicate that H2-ICP treatment significantly affects the strain and defects near the interface between GeSn and Ge layers, and thus optical properties of GeSn layer are also altered by H2-ICP treatment.

Temperature-dependent photoreflectance of SnS crystals

The optical properties of single-crystal SnS were studied by photoreflectance (PR) spectroscopy. Temperature-dependent PR spectra were measured in the range 20–200K. A room-temperature bandgap energy value of Eg=1.317eV was estimated by fitting the temperature dependence of the bandgap energy obtained from the PR spectra. The vibrational properties of orthorhombic SnS were studied using Raman spectroscopy. Four vibrational modes were detected at 95, 163, 191, and 218cm–1.

Optical absorption and photoreflectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe2

Optical absorption and photoreflectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe2 for light polarization perpendicular (E ⊥ c) and parallel to the c-axis (E ‖ c) at T = 15–300 K. Optical absorption measurements suggest that AgGaSe2 is a direct-gap semiconductor having an optical band gap of E0 ∼ 1.8 eV at T = 15–300 K. The temperature-dependent PR spectra are obtained at T = 20–300 K in the 1.8–2.5 eV spectral ranges. The lowest band-gap energy E0 of AgGaSe2 shows unusual temperature dependence at T ≤ 80 K. The resultant temperature coefficients dE0/dT are positive at T ≤ 70 K and negative above 70 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Δso = 327 meV and Δcr = −288 meV at T = 20 K, respectively.

Photoreflectance Spectroscopy Characterization of Ge/Si0.16Ge0.84Multiple Quantum Wells on Ge Virtual Substrate

We report a detailed characterization of a Ge/Si0.16Ge0.84multiple quantum well (MQW) structure on Ge-on-Si virtual substrate (VS) grown by ultrahigh vacuum chemical vapor deposition by using temperature-dependent photoreflectance (PR) in the temperature range from 10 to 300 K. The PR spectra revealed a wide range of optical transitions from the MQW region as well as transitions corresponding to the light-hole and heavy-hole splitting energies of Ge-on-Si VS. A detailed comparison of PR spectral line shape fits and theoretical calculation led to the identification of various quantum-confined interband transitions. The temperature-dependent PR spectra of Ge/Si0.16Ge0.84MQW were analyzed using Varshni and Bose-Einstein expressions. The parameters that describe the temperature variations of various quantum-confined interband transition energies were evaluated and discussed.

Optical properties of single-crystalline chalcopyrite semiconductor AgInSe2

The optical properties of single-crystalline AgInSe2 have been studied by optical absorption, spectroscopic ellipsometry (SE), and photoreflectance (PR) spectroscopy. Optical absorption measurements suggest that AgInSe2 is a direct-gap semiconductor having an optical band gap of E0∼1.2 eV at T=15–300 K. The pseudodielectric-function spectra, ε(E)=ε1(E)+iε2(E), of AgInSe2 are determined by SE for light polarization perpendicular (E⊥c) and parallel to the c axis (E∥c) in the E=1.2–5.3 eV spectral region at room temperature. The measured SE spectra reveal distinct structures at energies of the critical points (En with n=0–8) in the Brillouin zone. The temperature-dependent PR spectra are obtained at T=15–300 K in the 1.2–1.7 eV spectral ranges. The lowest band-gap energy E0 of AgInSe2 shows unusual temperature dependence at T≤120 K. The resultant temperature coefficients dE0/dT are positive at T≤120 K and negative above 120 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Δso=321 meV and Δcr=−172 meV, respectively, and are discussed from an aspect of the electronic energy-band structure consequences.

Temperature-dependent photoreflectance and photoluminescence characterization of the subband structure and built-in electric field of GaAs/GaInAs graded-channel high electron mobility transistor structures

We have measured temperature-dependent photoreflectance (PR) and photoluminescence (PL) spectra of three symmetric linearly graded GaAs/GaInAs high electron mobility transistor (gHEMT) structures with opposite channel-band slopes in the temperature range between 30 and 300 K. Analyses of the PL and PR spectra indicate the existence of two-dimensional electron gas (2DEG) in the graded-channel well of the three gHEMTs in the temperature range of 30–300 K. The comparison of PL and PR spectra facilitates the identification of channel-well transitions in the gHEMTs with different channel-band slopes. Inter-subband transitions, Fermi-level energies, 2DEG and the built-in electric field of the three gHEMTs with dissimilar channel-band slopes are evaluated. The well widths for the gHEMTs of different channel-band slopes are estimated. The temperature dependences of the channel inter-subband transition energy and built-in electric field of the three gHEMTs are analyzed and discussed.

Temperature-dependent parameters of band anticrossing in InGaPN alloys

Temperature-dependent photoreflectance (PR) measurements are employed to characterize the conduction band structure of In0.54Ga0.46P1−yNy (y=0 and 0.02) grown on GaAs substrates. The band gap and the upper subband E+ transition are observed in InGaPN as predicted by the band anticrossing (BAC) model. To investigate the energetic positions of the features in the PR spectra, a Kramers–Kronig analysis is proposed. Based on the PR data and the BAC model, we find that the energy EN of isolated nitrogen states shifts significantly to higher energies with decreasing temperature. Simultaneously, the interaction potential V between the nitrogen states and the unperturbed conduction band also rises to higher values. At 293 K, EN=2.054 eV and V=1.513 eV are determined. The thermal shifts of EN and V are dEN/dT≈−0.43 meV/K and dV/dT≈−0.67 meV/K, respectively. The temperature-dependent EN level and interaction potential V are attributed to the lattice distortions, which can be affected by temperature-induced changes in deformation potential. This information is important for overall validity of the BAC model to dilute nitride InGaPN alloys.

Band anticrossing in InGaPN alloys induced by N-related localized states

Temperature-dependent photoreflectance measurements are employed to characterize the electronic band structure of InGaPN grown on GaAs substrates. In addition to the fundamental band gap, the upper subband E+ is observed as predicted by the band anticrossing (BAC) model. By eliminating the contributions of the epitaxial-strain and atomic-ordering effects in InGaPN and also assigning the localized state energy EN introduced by an isolated N to be 2.040eV at 293K, the interaction potential V is determined as 1.449±0.170eV. The incorporation of a temperature-dependent EN level into the BAC model fits the experimental data better than assuming EN to be a constant. This contrasts with previously published results and so provides a different view of the temperature dependence of the EN level in InGaPN.

Temperature dependent photoreflectance and photoluminescence characterization of GaInNAs∕GaAs single quantum well structures

Ga 0.69 In 0.31 N x As 1 − x ∕ GaAs single quantum well (SQW) structures with three different nitrogen compositions ( x=0%, 0.6%, and 0.9%) have been characterized, as functions of temperature in the range 10–300K, by the techniques of photoreflectance (PR) and photoluminescence (PL). In PR spectra, clear Franz-Keldysh oscillations (FKOs) above the GaAs band edge and the various excitonic transitions originating from the QW region have been observed. The built-in electric field in the SQW has been determined from FKOs and found to increase with N concentration. The PR signal has been found to decrease for nitrogen incorporated samples when the temperature was lowered due to a weakening of the modulation efficiency induced by carrier localization. A careful analysis of PR and PL spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The anomalous temperature dependent 11H transition energy and linewidth observed in the PL spectra have been explained as originating from the localized states as a result of nitrogen incorporation. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the interband transitions.

Temperature dependence of photoreflectance in InAs/GaAs quantum dots

Temperature dependent photoreflectance (PR) and photoluminescence experiments of the InAs/GaAs quantum dot (QD) structures were performed. At 20 K, effective band-gap transitions due to the InAs QDs, wetting layers, and GaAs buffer and cap layers were identified. Transition energies of the ground state and four excited states with nearly equal interlevel spacings (75–80 meV) were observed. The linewidth of the ground-state transition decreased as the temperature increased from 20 K to 100 K while the linewidth became broader at temperatures above 100 K. Energy features of the PR spectra originating from QDs and relating to the in-plane parabolic potentials were discussed.

Surface photovoltage spectroscopy, photoreflectance, and reflectivity characterization of an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser including temperature dependence

We have investigated an InGaAs/GaAs/GaAlAs vertical-cavity surface-emitting laser using surface photovoltage spectroscopy (SPS) in the temperature range of 15 K<T<400 K. For comparison purposes, we have also (a) measured temperature-dependent photoreflectance (PR) and normal-incidence reflectivity (NIR); and (b) calculated the number of photoexcited carriers (PEC), which is related to the SPS signal. The SPS spectra exhibit both the fundamental conduction to heavy-hole (1C–1H) excitonic transition and cavity mode plus a rich interference pattern related to the properties of the mirror stack. The PR data show only the 1C–1H excitonic transition (plus Franz–Keldysh oscillations) while the cavity mode is detected by NIR. The temperature variation of 1C–1H/cavity mode is the same for SPS and PR/SPS and NIR. The SPS interference pattern from the DBRs is in good agreement with the PEC calculation. This experiment demonstrates the considerable potential of SPS for the characterization of these devices.

Electromodulation spectra of a single AlxGa1-xAs/GaAs modulation-doped heterojunction: Experiment and theory.

Electroreflectance spectroscopy is applied to reexamine the theoretical model which has been devised to explicate the temperature-dependent photoreflectance spectra of single ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs modulation-doped heterojunctions. Building on the presumption that the modulation mechanism is mainly due to the electromodulation of the band bending in the buffer layer, we apply the Franz-Keldysh theory to simulate the photoreflectance and electroreflectance spectra at room temperature. As the sample temperature is decreased to 10 K in the photoreflectance experiment, there appears a large number of oscillations extending for more than ten periods. This phenomenon manifests existence of a fairly uniform electric field in the buffer layer.

Excitons and electron-phonon interaction in In 0.52Ga 0.18Al 0.30As layers

Temperature dependent photoreflectance, photoluminescence, and Raman spectroscopy have been used to study the optical properties of MBE-grown InGaAlAs layers lattice matched to InP. A detailed study of the photoreflectance and photoluminescence spectra at low temperatures gives an exciton binding energy of the order of 2 meV. At higher temperatures, a combination of temperature dependent photoluminescence and Raman measurements indicates that InAs-like LO phonons play a dominant role in the electron-phonon interaction of these crystals.