Photoreflectance Spectra Research Articles

Blueshift in sulfur treated GaAsP/AlGaAs near surface quantum well

Large blueshift was observed in a near-surface GaAs0.86P0.14/Al0.7Ga0.3As quantum well upon treatment with Na2S·xH2O solution. Very slow etching with simultaneous surface passivation of the quantum well was obtained using this chemical treatment. Photoreflectance (PR) spectra exhibit maximum blueshift of 28 meV after treating the quantum well surface with Na2S·xH2O solution for 30 min (top layer thickness reduced to 10 Å). The blueshift is attributed to an increase in the confinement and/or an image charge effect due to the penetration of the wave function into vacuum. The blueshift is accompanied by a significant reduction in the broadening parameter of the observed e1-lh1 transition in PR spectra indicating effective passivation along with an increase in the confinement.

Evidence of Nitrogen Reorganization in GaAsSbN Alloys

In this report, the effects of thermal annealing properties on dilute GaAsSbN alloys grown on GaAs substrates using gas-source molecular beam epitaxy were investigated by photoreflectance (PR) and Raman spectroscopy. The PR spectra of the annealed GaAsSbN showed the up shift of dominant near band edge transition as compared to the as-grown samples. The observed shift can be attributed to the chemical change of the second-nearest-neighbor atoms to the N atoms. Raman spectroscopy analysis yielded further insight on the atomic configuration of post annealed GaAsSbN. The plausible physical mechanisms induced by thermal reorganization in GaAsSbN alloys were presented and discussed.

Photoreflectance study of direct-gap interband transitions in Ge/SiGe multiple quantum wells with Ge-rich barriers

Photoreflectance (PR) was used to study the direct-gap interband transitions in strain-compensated Ge/SiGe multiple quantum well (MQW) structures with Ge-rich SiGe barriers grown by low-energy plasma-enhanced chemical vapor deposition. The PR spectra revealed a wide range of possible optical transitions in the MQW structure. Detailed lineshape fits to the PR spectra and comparison to a theoretical calculation based on the envelope-function approximation with conduction band-offset and stain compensation factor as adjust parameters led to the identification of various interband transitions. The results demonstrated that PR is a powerful technique for nondestructive optical characterization of Ge/SiGe MQW structures.

Contactless electroreflectance spectroscopy of optical transitions in low dimensional semiconductor structures

AbstractThe authors present the application of contactless electroreflectance (CER) spectroscopy to study optical transitions in low dimensional semiconductor structures including quantum wells (QWs), step-like QWs, quantum dots (QDs), quantum dashes (QDashes), QDs and QDashes embedded in a QW, and QDashes coupled with a QW. For QWs optical transitions between the ground and excited states as well as optical transitions in QW barriers and step-like barriers have been clearly observed in CER spectra. Energies of these transitions have been compared with theoretical calculations and in this way the band structure has been determined for the investigated QWs. For QD and QDash structures optical transitions in QDs and QDashes as well as optical transitions in the wetting layer have been identified. For QDs and QDashes surrounded by a QW, in addition to energies of QD and QDash transitions, energies of optical transitions in the surrounded QW have been measured and the band structure has been determined for the surrounded QW. Finally some differences, which can be observed in CER and photo-reflectance spectra, have been presented and discussed for selected QW and QD structures.

Characteristics of SiN/GaAs interface under exposure to high-temperature and high-humidity conditions measured by photoreflectance spectroscopy

We investigated the changes in the electrical properties of a SiN/GaAs interface under high-temperature and high-humidity conditions, using photoreflectance (PR) spectroscopy and the electrical device characteristics. The PR spectra show the Franz-Keldysh oscillation (FKO); these spectra show that the period decreases after the sample is exposed to humidity. The electric field strength obtained from the FKO period indicates that the initial high electric field decreases with humidity exposure. Decomposed water molecules are supposed to diffuse into the SiN layer and react with the SiN/GaAs interface, causing a decrease in the interface states.

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.

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)

Optical Transitions between Confined and Unconfined States in p-Type Asymmetric GaAs/InGaAs/AlGaAs QW Structures

We present contactless surface photovoltage spectroscopy and photoreflectance studies of 10 nm wide, p-type doped asymmetric GaAs/InGaAs/AlGaAs quantum well structures. The MBE grown structures differ in spacer thickness between the quantum well and the reservoir of holes. The doping causes that quantum well is placed in electric field. The surface photovoltage spectroscopy measurements gave us detailed information about the optical transitions between confined states and between confined and unconfined states. The comparison of experimental and numerical analysis allows us to identify all features present in the surface photovoltage spectroscopy and photoreflectance spectra. It has been found that spacer layer thickness has significant influence on surface photovoltage spectroscopy spectra.

Determining of Infrared Transition of InN Film Grown on C-Plane Sapphire by Photoreflectance

An InN film was grown on sapphire (c-plane) by plasma-assisted molecular beam epitaxy, and its photoluminescence at 10 K and photoreflectance (PR) spectra from 10 K to 110 K were measured. Some prominent features in the PR spectra were observed in the infrared region below 120 K. The signals become too weak to observable for temperature above 110K. Furthermore, the binding energy of InN exciton was estimated to be 9.43 meV, which is equal to kBT at 109K. Therefore, the features in the PR spectra were assigned to the A, B, and C excitonic transitions associated with the direct gap of wurtzite InN. The thus obtained energies of the A, B, and C excitonic transitions versus temperature were fitted well by Varshini’s equation. The energies of the A, B, and C excitonic transitions at room temperature obtained by the best fit of Varshni’s equation are 0.738, 0.746, and 0.764 eV, respectively.

Temperature Dependence of Contactless Electroreflance Spectroscopy of C-Plane ZnO Bulk

Contactless electroreflectance（CER）spectra of c-plane ZnO bulk have been measured from 20 to 300 K. Photoreflectance（PR）spectrum of ZnO bulk has also been measured at 300 K. The CER spectrum is red-shifted with respect to the PR spectrum at 300 K. It is known that the average field experienced in the CER is larger than that of the PR. In addition, the features of PR or CER is blue-shifted when the field becomes larger if they belong to band-to-band transitions. Hence, the observed features are excluded from band-to-band transitions and are attributed to excitonic transitions. The CER spectra were then fitted by Lorentzian profiles. The energies of the A（B）, B（A）, and C excitonic transitions at 0K, which were obtained by the best fit of Varshni’s equation, are 3.419, 3.436, and 3.482 eV, respectively.

Advanced Contactless Analysis of Plasma-Induced Damage on Si by Temperature-Controlled Photoreflectance Spectroscopy

We propose an advanced photoreflectance (PR) spectroscopy technique, for use as a method for the contactless analysis of plasma-induced damage (physical damage) on Si. Si wafers damaged by Ar plasma were placed on a measurement stage capable of sample cooling by liquid nitrogen. In comparison to the spectra at 300 K, the signal intensity at 90 K was increased by cooling. A spectral peak of a heavily-damaged sample was identified at 90 K, which was difficult at 300 K. Changes in the surface potential induced by plasma exposure (ΔVs) were calculated by analyzing the spectral parameters. Areal densities of trapped charges were estimated from ΔVs. The temperature dependence of the PR spectra was discussed, and the primary cause of the increase in signal intensity was attributed to the reduced electron–phonon interactions in a lower temperature range. The proposed technique expands the applicable range of PR-based damage analysis.

Advanced Contactless Analysis of Plasma-Induced Damage on Si by Temperature-Controlled Photoreflectance Spectroscopy

We propose an advanced photoreflectance (PR) spectroscopy technique, for use as a method for the contactless analysis of plasma-induced damage (physical damage) on Si. Si wafers damaged by Ar plasma were placed on a measurement stage capable of sample cooling by liquid nitrogen. In comparison to the spectra at 300 K, the signal intensity at 90 K was increased by cooling. A spectral peak of a heavily-damaged sample was identified at 90 K, which was difficult at 300 K. Changes in the surface potential induced by plasma exposure (ΔVs) were calculated by analyzing the spectral parameters. Areal densities of trapped charges were estimated from ΔVs. The temperature dependence of the PR spectra was discussed, and the primary cause of the increase in signal intensity was attributed to the reduced electron–phonon interactions in a lower temperature range. The proposed technique expands the applicable range of PR-based damage analysis.

Photoreflectance study of the fundamental optical properties of (Ga,Mn)As epitaxial films

Fundamental optical properties of thin films of (Ga,Mn)As diluted ferromagnetic semiconductor with a low (1%) and high (6%) Mn content and of a reference GaAs film, grown by low-temperature molecular-beam epitaxy, have been investigated by photoreflectance (PR) spectroscopy. In addition, the films were subjected to complementary characterization by means of superconducting quantum interference device (SQUID) magnetometry, Raman spectroscopy, and high resolution X-ray diffractometry. Thorough full-line-shape analysis of the PR spectra, which enabled determination of the E0 electronic transition in (Ga,Mn)As, revealed significant differences between the energy band structures in vicinity of the {\Gamma} point of the Brillouin zone for the two (Ga,Mn)As films. In view of the obtained experimental results the evolution of the valence band structure in (Ga,Mn)As with increasing Mn content is discussed, pointing to a merging the Mn-related impurity band with the host GaAs valence band for high Mn content.

Carrier wavefunction control in a dilute nitride-based quantum well—a quantum dot tunnel injection system for 1.3 µm emission

There are reported results of numerical calculations of energy structure and carrier wavefunctions in a quantum mechanically coupled complex system consisting of an InGaAsN injector quantum well (QW) (with various N contents), separated by a thin GaAs barrier from an InAs/GaAs quantum dot (QD) layer covered with an InGaAs strain reducing layer. The investigated system consists in a tunnel injection structure, whose operation principle is to transfer the carriers captured by the injector QW to QDs by means of tunnelling. The QDs emit light at the 1.3 µm telecom window at room temperature, so in order to match the energy levels of the confined states in the injector QW to those in the QDs, nitrogen has been added to the former. The obtained calculation results are compared with measured photoreflectance spectra and used to explain their features. A detailed discussion on the spatial distribution of carrier wavefunctions over both parts of the tunnelling system versus nitrogen concentration in the injector QW is presented. The importance of the control of wavefunctions for the performance of tunnel-injection-based devices is discussed.

Room-temperature spin photocurrent spectra at interband excitation and comparison with reflectance-difference spectroscopy in InGaAs/AlGaAs quantum wells

Spectra of the interband spin photocurrent due to Rashba and Dresselhaus spin splittings have been experimentally investigated in InGaAs/AlGaAs quantum wells at room temperature. The Rashba- and Dresselhaus-induced circular photogalvanic effect (CPGE) spectra are found to be quite similar in the spectral regions corresponding to the transitions 1e1hh (the first conduction to the first valence sub-band of heavy hole) and 1e2hh. The ratio of Rashba- and Dresselhaus-induced CPGE currents for the transition 1e1hh is estimated to be 4.95. The magnitude of the Rashba-induced CPGE current is up to several tens of nA/W for the transition 1e1hh, which is 1 order of magnitude larger than that obtained in GaN/AlGaN superlattices. Comparing the CPGE spectrum with reflectance-difference and photoreflectance spectra, we find that the large Rashba spin splitting is mainly induced by a large indium atom segregation effect and by the internal field in the quantum wells.

Photoreflectance study of InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs quantum wells

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (100) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron–heavy-hole (Ee–hh) and electron–light-hole (Ee–lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8eV, respectively. By analyzing the Franz–Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.

Photoluminescence and photoreflectance studies in Si/[formula omitted]-[formula omitted]/Si(001) double heterostructure

Photoluminescence (PL) and photoreflectance (PR) properties were investigated in Si/β-FeSi2/Si(001) double heterostructure (DH) samples. The as-grown sample did not show clear PL, but the annealed one showed the 1.54μm PL. In PR spectra, the direct transition energies (Eg) were observed at 0.910–0.935 eV in the samples. The Eg shifted to lower photon energy with the increase of annealing temperature, which indicates a modification of the band structure. These results revealed that the 1.54μm PL was related to the transition at the indirect band gap. The effect of thermal annealing on the 1.54μm PL was discussed.

Photoreflectance study of GaMnAs layers grown by MBE

GaMnAs layers were grown by MBE on GaAs (001) substrates. The structural properties of the epilayers were studied by atomic force microscopy, secondary ion mass spectroscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Photoreflectance spectra at room temperature exhibited Franz–Keldysh oscillations; from an analysis of these oscillations we obtained the built-in internal electric field and the bandgap energy of the GaMnAs epilayers. We studied the variation of these parameters as a function of Mn in the epilayers. In addition the holes concentration in the samples was extracted from the bandgap narrowing value and compared with the carrier density obtained by the Hall measurements.

Photoluminescence and photoreflectance characterization of ZnxCd1−xSe/MgSe multiple quantum wells

Photoluminescence (PL) and photoreflectance (PR) were used to characterize ZnxCd1−xSe/MgSe multiple quantum well (MQW) structures grown on InP substrates by molecular beam epitaxy for mid-infrared (IR) device applications. The PL spectra yielded information of the fundamental excitonic recombination and ZnxCd1−xSe cap/spacer band edge emission of the samples. The PR spectra revealed multitude of possible interband transitions in MQW structures. The ground state transitions were assigned by comparing with the PL emission signals taken from the same structures. A comprehensive analysis of the PR spectra led to the identification of various interband transitions. The intersubband transitions were then estimated and found to be in a good agreement with the previous report of Fourier-transform IR absorption measurements [Li et al., Appl. Phys. Lett. 92, 261104 (2008)]. The results demonstrate the potential of using PL and PR as nondestructive optical techniques for characterization of ZnxCd1−xSe/MgSe MQWs for mid-IR device applications.

Photoreflectance analysis of a GaInP/GaInAs/Ge multijunction solar cell

We have analyzed the photoreflectance spectra of a GaInP/GaInAs/Ge triple junction solar cell. The spectra reveal signatures from the window layer and middle and top subcells included in the stack. Additional contributions from the multilayer buffer introduced between the mismatched bottom and middle cells have been detected. Franz–Keldysh oscillations (FKOs) dominate the spectra above the fundamental bandgaps of the GaInP and GaInAs absorbers. From the FKO analysis, we have estimated the dominant electric fields within each subcell. In light of these results, photoreflectance is proposed as a useful diagnostic tool for quality assessment of multijunction structures prior to completion of the device or at earlier stages during its processing.