Photoreflectance Measurements Research Articles

Signature of optical absorption in highly strained and partially relaxed InP/GaAs type-II quantum well superlattice structures

Signature of optical absorption in highly strained and partially relaxed InP/GaAs type-II quantum well superlattice structures is clearly observed at room temperature. Strong coupling effects between the wavefunctions of confined carriers in the neighbouring quantum wells of InP/GaAs type-II superlattice structures are observed in electroreflectance (ER) spectra. Only ER measurements provide clear signatures of the superlattice feature for partially relaxed superlattice structures. However, both ER and photoreflectance (PR) measurements provide the signature of superlattice features for highly strained superlattice structures. A possible explanation is given in terms of the trapping of photogenerated carriers at defect/trap levels generated by the partial relaxation or due to the presence of oscillatory features in PR spectra. ER being a direct modulation technique, where one applies an external electric field to modulate the built-in electric field, is therefore preferred over PR. Increase in the surface photovoltage magnitude for energies lower than the band edge position of GaAs buffer layer confirms that the absorption takes place between the electronic minibands formed in the type-II superlattice structures.

A novel approach for normalizing the photoreflectance spectrum by using polymer-dispersed liquid crystal

This study developed a novel type of normalization procedure for modulation reflectance spectroscopy experiments to obtain the relative change in the reflectance spectrum, ΔR/R. This technique uses a polymer-dispersed liquid crystal to ensure that the dc component of the signal from the detector remained constant by varying the intensity of the light striking the sample. This method is particularly useful for photoreflectance measurement, which may encounter background problems because of scattered pump light and/or photoluminescence. It does not require a change in the gain of the detector or the use of a variable neutral density filter mounted on a servo-motor.

Laser photo-reflectance characterization of resonant nonlinear electro-refraction in thin semiconductor films

Photo-reflectance (PR) measurements provide a non-contact means for the precise characterization of semiconductor electronic properties. In this paper, we investigate the use of a laser beam as the probe beam in the PR setup. In this case it is seen that the nonlinear refraction is responsible for the amplitude change of the reflected probe field, whereas the phase change is due to nonlinear absorption. The open aperture condition may then be used to eliminate the spatial phase at the detector, thereby isolating the electro-refractive contribution to the PR signal. This greatly simplifies the PR analysis and allows absolute measurements of electro-refraction in thin semiconductor films. We report the application of the laser PR technique to characterize physical strain in thin silicon on silicon-germanium films.

Dynamics of localized excitons in Ga0.69In0.31N0.015As0.985/GaAs quantum well: Experimental studies and Monte-Carlo simulations

Time resolved photoluminescence (PL) spectroscopy has been applied to study dynamics of localized excitons in a single Ga0.7In0.3N0.015As0.985/GaAs quantum well (QW). The decay time constant, τPL, has been determined for different PL peak energies at various temperatures. An increase in temperature produced two effects: (i) a reduction of τPL and (ii) changes in the τPL dispersion. These two experimental observations as well as the shape of PL decay curves were very well reproduced by Monte-Carlo simulations of hopping excitons with parameters derived from PL and photoreflectance measurements for this QW.

Note: Improved sensitivity of photoreflectance measurements with a combination of dual detection and electronic compensation

A setup is described for performing photoreflectance (PR) measurements using two photodetectors, wherein the photodetectors need not be identical. The second detector monitors the photoluminescence and scattered pump laser background signal in real time. It is then eliminated from the measured PR signal using electronic circuits that compensate for amplitude and phase differences between the background signals from the two detectors. The technique overcomes the adverse effect of short-term fluctuations in the pump laser intensity. The signal-to-noise ratio is shown to improve significantly, enabling measurement of weak PR signals.

Optical characterization of InAlAs/InGaAs metamorphic high-electron mobility transistor structures with tensile and compressive strain

We have measured surface photovoltage (SPV), photoreflectance (PR), and photoluminescence (PL) spectra of two InAlAs/InxGa1−xAs/InAlAs metamorphic high-electron mobility transistor (MHEMT) structures. One possesses a V-shaped InxGa1−xAs (x=0.3–0.5–0.3) tensile-strained channel in In0.5Al0.5As/InxGa1−xAs/In0.5Al0.5As heterostructures, and the other is an In0.42Al0.58As/In0.53Ga0.47As/In0.42Al0.58As MHEMT structures with InxGa1−xAs (x=0.53) compressively-strained channel grown on GaAs by molecular beam epitaxy. The comparison of SPV, PR, and PL spectra facilitates the identification of channel-well transitions in the MHEMT structures with different InxGa1−xAs channels. Inter-subband transitions, Fermi-level energies, and built-in electric field of the two MHEMT structures with dissimilar InxGa1−xAs channel are evaluated and discussed from the experimental analyses of SPV, PR and PL measurements. The results showed that the design of tensile-strained MHEMT structure enhances sheet-carrier density and avoids surface-roughness scattering by increasing V-shape electric field between the two channel interfaces. The electron mobility of the tensile-strained MHEMT structure is hence being promoted.

Electromodulation spectroscopy of optical transitions and electric field distribution in GaN/AlGaN/GaN transistor heterostructures with various AlGaN layer thicknesses

AbstractGaN/AlGaN/GaN field effect transistor (FET) heterostructures with various thicknesses of AlGaN layer (10, 20, and 30nm) were investigated by electromodulation spectroscopy [i.e. contactless electroreflectance (CER) and photoreflectance (PR)]. In PR spectra optical transitions in GaN cap, AlGaN and GaN buffer layers were clearly observed whereas in CER spectra the optical transition in GaN buffer layer was not observed due to the screening of band bending modulation in CER spectroscopy by the two dimensional electron gas at the AlGaN/GaN interface. The transition related to GaN‐cap was observed ∼0.1 eV above the energy gap of GaN due to the quantum confinement in the surface GaN quantum well. The built‐in electric field in AlGaN layer was determined from the analysis of AlGaN‐related Franz‐Keldysh oscillation. For the sample with 10, 20, and 30nm thick AlGaN layer the electric field has been determined from CER(PR) measurements to be 0.30(0.27), 0.46(0.39) and 0.78(0.63) MV/cm, respectively. The smaller electric field, which is obtained from PR measurements, results from the photovoltaic effect which is eliminated in CER spectroscopy. The electric field distribution in the full heterostructure (GaN(cap) layer, AlGaN layer, and near the AlGaN/GaN interface in GaN(buffer) layer) has been found comparing theoretical calculations for the full heterostructure with the electric field for AlGaN layer obtained from CER measurements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth condition dependence of direct bandgap in β-FeSi2 epitaxial films grown by molecular beam epitaxy

Direct bandgap energy (Eg) and lattice deformations were investigated in β-FeSi2 epitaxial films grown by molecular beam epitaxy (MBE) with different growth condition. As Si/Fe flux ratio during the MBE growth became smaller than Si/Fe = 2.0, the lattice constants deviated from those of β-FeSi2 single crystal, which indicated an enhanced lattice deformation at the lower Si/Fe ratio. In photoreflectance (PR) measurements, the PR spectra shifted to lower photon energy with the enhanced lattice deformation. These results revealed that the Eg of β-FeSi2 epitaxial film was modified by the lattice deformation depending on the growth condition.

Effect of built-in electric field on the temperature dependence of transition energy for InP/GaAs type-II superlattices

Built-in electric field in InP/GaAs type-II superlattice structures considerably modifies the temperature dependence of the ground state (GS) transition energy in photoreflectance measurements. For moderate electric fields, the temperature dependence of the GS transition energy follows the bandgap of the GaAs barrier layer, and it decreases at a faster rate than that of the GaAs material in the case of larger values of built-in electric field. The GS excitonic feature red shifts with quantum well thickness, confirming that it originates from the superlattice structure. Further, the variation of the broadening parameter with temperature is governed by the scattering of electrons with longitudinal optical phonons.

Photoreflectance study of InGaAs/AlAsSb coupled double quantum wells

AbstractSixty periods of undoped InxGa1–xAs/AlAs0.56Sb0.44 CDQWs with extremely thin Al(Ga)As coupling barriers between the InGaAs wells were grown by molecular beam epitaxy. Photoreflectance (PR) spectra show interband transitions related to the heavy and light holes. After taking into account the strain‐induced and quantum confinement effects, the theoretical calculations are in fairly good agreement with the PR measurement results. The interband transition energy between ground subbands was redshifted as the In composition increased from 0.53 to 0.8, and reducing the AlAs coupling barrier thickness from three mono‐layers (MLs) to two MLs. Further shift of the CDQWs interband absorption edge energy toward a communication wavelength of 1.55 µm, while maintaining the intersubband transition energy at around 0.8 eV, was achieved using 2 MLs of Al0.4Ga0.6As coupling barriers and ln0.8Ga0.2As wells. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transport and magnetic properties of Fe/GaAs Schottky junctions for spin polarimetry applications

The electrical, magnetic and spin-filtering properties of Pd/Fe/GaAs(001) junctions are investigated. The Pd/Fe thin layers are deposited on GaAs(001) surfaces both clean and passivated by a thin oxide layer. The surface composition, structure and electronic properties of the starting surfaces are studied by means of XPS, LEED, EELS and photoreflectance. The Fe layer magnetic properties are characterized by magneto-optical Kerr effect (MOKE) measurements, while the electronic properties of the junctions are characterized by current-voltage (I-V) and photoreflectance measurements. For both types of substrate surfaces, the magnetization of the Fe layers is found close to that of a bulk Fe slice of equivalent thickness. For the oxide interface, the I-V curve exhibits almost an ideal Schottkylike behavior, since it can be very well interpreted by the thermoionic equation, using the ideality factor of n = 1.02 and surface barrier ϕb = 0.7 eV. For junctions prepared on the reconstructed GaAs(001) surface, the ideality factor and barrier heights are found in the range of 1.06–1.2 and 0.60–0.75 eV, respectively. Both structures give rise to similar spin filter effects under injection of spin-polarized electrons.

Determination of Exciton Binding Energy of GaInNAs Quantum Well Structures by Piezoelectric Photothermal Spectroscopy Compared with Photoreflectance Measurements

Determination of Exciton Binding Energy of GaInNAs Quantum Well Structures by Piezoelectric Photothermal Spectroscopy Compared with Photoreflectance Measurements

Determination of Exciton Binding Energy of GaInNAs Quantum Well Structures by Piezoelectric Photothermal Spectroscopy Compared with Photoreflectance Measurements

The exciton binding energies (Exb) of a dilute nitride Ga1-yInyN0.012As0.988 layer (y = 0.0 to 4.5%) with the thickness of 100 nm were determined by both piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies. Curve-fitting analyses were carried out using a three-dimensional direct allowed-transition model with the Voigt function as a convolution integral for PPT and with Aspnes' formula for PR. The observed estimation error for PR was about two times larger than that of PPT. Therefore, we conclude that PPT is a novel methodology for determining Exb when it is small and two expected critical energies exist in the narrow energy region.

Bandgap modifications by lattice deformations in β-FeSi 2 epitaxial films

The modifications of direct transition energies by lattice deformations were investigated in β-FeSi 2 epitaxial films, polycrystal films and single crystal, systematically. The lattice deformations depending on thermal annealing temperature ( T a ) were observed in β-FeSi 2 epitaxial films. In photoreflectance (PR) measurements, the direct transition energies of the epitaxial films shifted to lower energies as the T a increased. The polycrystal films did not show the lattice deformation and the shift of direct transition energies. These results show that the direct bandgap is modified by the lattice deformation originating from the lattice mismatch at the hetero-interface of β-FeSi 2/Si.

Spectral Resolution Effects on the Lineshape of Photoreflectance

Spectral resolution effects on the lineshape of photoreflectance (PR) spectroscopy is experimentally investigated. PR measurements are performed on HgCdTe epilayer and InAs/GaAs quantum dot (QD) low-dimensional samples at low temperatures in a spectral resolution range from 8 to 0.5 meV. The results indicate that the resolution affects not only the identification of narrow PR features, but also the determination of critical-point energies of identified PR features, and a spectral resolution of as high as 0.5 meV may be necessary for low-dimensional semiconductors. The spectral resolution is indeed a crucial parameter, for which the step-scan Fourier transform infrared spectrometer-based PR technique is preferable.

Characterization of InGaAsSbN layers grown on InP by MBE

AbstractOptical and electrical properties of the InGaAsSbN layers grown by molecular beam epitaxy (MBE) on InP substrates were studied in comparison with InGaAsN layers. It was found that the optical properties of the InGaAsSbN layer characterized by photoreflectance (PR) and photoluminescence (PL) measurements are superior to those of InGaAsN layer. In addition, electrical properties characterized by Hall measurements were also improved by adding Sb atoms. Annealing studies on photoluminescence properties of the InGaAsSbN layer revealed that the PL intensity increases and the PL spectral half width decreases upon annealing up to 700 °C. These results indicate that post‐annealing is still necessary for InGaAsSbN layers to improve crystalline quality (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of direct transition energies in [formula omitted]-[formula omitted] epitaxial films on Si(111) substrate

Temperature dependence of direct transition energies (Eg) was investigated in β-FeSi2 epitaxial films on Si(111) substrate. The lattice volume of the epitaxial films was reduced as the annealing temperature (Ta) increased. In photoreflectance measurements, the samples annealed at higher Ta showed a larger temperature dependence of Eg. These results revealed that the temperature dependence of Eg depended on the lattice deformation by the thermal annealing. The fact supports the band gap modifications by the lattice deformation.

Photoreflectance of AlGaN/GaN Heterostructure Measured by Using Mercury Lamp as Pump Beam

Photoreflectance (PR) spectra of a GaN thin film and an AlGaN/GaN heterostructure were measured by using a HeCd laser or a mercury lamp as a pump beam. The wavelengths (λ) of the HeCd laser and the mercury lamp are 325 nm and 253.7 nm, respectively. The energy of the HeCd laser is lower than band-gap energy of AlxGa1-xN (x > 0.2), so electron-hole pairs cannot be generated in the AlGaN layer. Hence, the PR of the AlGaN was measured by using Argon ion laser (λ =300 nm) or quadrupled Nd:YAG (λ =266 nm) rather than HeCd laser in the previous works. In this work, the mercury lamp was used as the pump beam. Although the mercury lamp is a diffused source, it is not a hindrance to the PR measurements. The signal to noise ratio is improved by using defocused pump and probe beams in the PR measurement.

Frequency-tunable terahertz electromagnetic wave emitters based on undoped GaAs/n-type GaAs epitaxial layer structures utilizing sub-picosecond-range carrier-transport processes

We have investigated sub-picosecond-range carrier-transport processes in undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial layer structures with various i-GaAs-layer thicknesses d ranging from 200 to 2000 nm, focusing on the relation between carrier-transport processes and terahertz electromagnetic wave frequency. Initially, using numerical simulation and photoreflectance measurement, we confirm that a decrease in d enhances the built-in electric field in the i-GaAs layer. In the time-domain terahertz waveform, it is observed that the intense monocycle oscillation induced by the surge current of photogenerated carriers, the so-called first burst, is followed by the oscillation patterns originating from the coherent GaAs longitudinal optical (LO) phonon. From the Fourier power spectra of the terahertz waveforms, it is clarified that the decrease in d causes a high frequency shift of the band of the first burst. Consequently, we conclude that, in the sub-picosecond time range, the photogenerated carriers are monotonously accelerated by the built-in electric field without being affected by intervalley scattering. The present conclusion signifies that the frequency-tunable terahertz emitters are realized by controlling i-GaAs-layer thickness. We also find the intensity of the coherent LO phonon band is enhanced by a decrease in d.

Temperature dependence of energies and broadening parameters of near band edge interband excitonic transitions in wurtzite ZnCdMgSe

An optical characterization of two wurtzite ZnCdMgSe crystalline alloys grown by the modified high-pressure Bridgman method was carried out by temperature-dependent photoluminescence (PL) and contactless electroreflectance (CER) in the temperature range of 10–300 K and photoreflectance (PR) measurements between 300–400 K. Low temperature PL spectra of the investigated samples consisted of the excitonic line, the “edge-emission” due to radiative recombination of shallow donor-acceptor pairs and a broad band related to recombination through deep level defects. Three excitonic features, A, B, and C, in the vicinity of band edge were observed in the CER and PR spectra. The peak positions of band edge excitonic features in the PL spectra were shifted slightly toward lower energies as compared to the lowest corresponding transition energies of A exciton determined from CER and PR data. The increase in the CER-PL shift with the increasing of Mg content in the investigated crystals was explained by the rising of compositional disorder causing the smearing of the band edge energies. In addition, the parameters that describe the temperature dependence of the transition energies and broadening parameters of the band edge excitonic transitions were evaluated and discussed.