Photomodulated Reflectance Spectra Research Articles

Determination of electronic band structure of quaternary ferromagnetic Ga0.97-yMn0.03CryAs epitaxial layers

Introducing transition metals to conventional III-V semiconductors anomalously changes their fundamental characteristics, such as electronic, magnetic, and structural properties. In this study, we show that the valence band anti-crossing (VBAC) model can be exploited to calculate the electronic band structure of the quaternary Ga0.97-xMn0.03CrxAs epitaxial layers. In this model, the localized Mn and Cr defect states interact with the valence band states (VB), reconstructing VBs and splitting each VB state. The splitting top of the VB state forms an impurity band (IB) and fundamental VB edge. Photomodulated reflectance (PR) spectroscopy is exploited to determine optical transition energies at room temperature. PR spectra were analyzed with the third derivative functional form (TDFF) signal's line shape. The experimental optical transition energies, including band-to-band and spin split-off band transitions, match the calculated optical transition energies by the VBAC model. In the calculation, the interaction energy between localized Mn/Cr-energy level and valence band edges is experimentally determined as 0.7 eV.

Complete-series excitonic dipole emissions in few layer ReS2 and ReSe2 observed by polarized photoluminescence spectroscopy

Excitonic emission is a key factor for a two-dimensional (2D) semiconductor to be fabricated in light-emitting device for optoelectronics application. Especially when the 2D material has a specific in-plane optical anisotropy, its excitons would become polarized dipole emitters. In this study, the whole series excitonic emissions of few-layer ReS2 and ReSe2 have been detailed characterized using polarized micro-photoluminescence (μPL) measurement. There are totally five excitonic emissions denoted as E1ex, E2ex, E3ex, ES1ex and ES2ex can be detected by the μPL experiment for both the few-layer ReS2 and ReSe2. The angular dependencies of the polarized excitonic emissions with respect to b axis (Re4 diamond-chain direction) are determined. The emission feature E1ex is the band-edge exciton while it shows orthogonal polarization with respect to both E2ex and E3ex in layered ReX2 (X = S, Se). The formation of Re4 diamond chains (DCs) in the 1 T′-phase ReS2 and ReSe2 will also assist the development of in-plane built-in electric field and forming dipoles around the DC regions. The built-in electric field effect greatly enhances the higher ES1ex and ES2ex excitonic features as evident in the photo-modulated reflectance spectra of the layered ReX2 with in-plane electrical field modulation. The binding energy and transition origin of all the excitons are also analyzed and discussed.

Optical properties of GaAs1−xBix/GaAs quantum well structures grown by molecular beam epitaxy on (100) and (311)B GaAs substrates

In this work, the electronic bandstructure of GaAs1−xBix/GaAs single quantum well (QW) samples grown by molecular beam epitaxy is investigated by photomodulated reflectance (PR) measurements as a function of Bi content (0.0065 ≤ x ≤ 0.0215) and substrate orientation. The Bi composition is determined via simulation of high-resolution x-ray diffraction measurement and is found to be maximized in the 2.15%Bi and 2.1%Bi samples grown on (100) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi QW layer but extends out of the GaAsBi QW towards the GaAs barrier and forms a GaAsBi epilayer. PR spectra are fitted with the third derivative function form (TDFF) to identify the optical transition energies. We analyze the TDFF results by considering strain-induced modification on the conduction band (CB) and splitting of the valence band (VB) due to its interaction with the localized Bi level and VB interaction. The PR measurements confirm the existence of a GaAsBi epilayer via observed optical transitions that belong to GaAsBi layers with various Bi compositions. It is found that both Bi composition and substrate orientation have strong effects on the PR signal. Comparison between TDFF and calculated optical transition energies provides a bandgap reduction of 92 meV/%Bi and 36 meV/%Bi and an interaction strength of the isolated Bi atoms with host GaAs valence band (CBiM ) of 1.7 eV and 0.9 eV for (100) and (311)B GaAs substrates, respectively.

Thermal annealing effects on optical and structural properties of GaBiAs epilayers: Origin of the thermal annealing-induced redshift in GaBiAs

We report on an investigation of optical and structural properties of as-grown and thermal annealed GaBixAs1−x (x = 0.012, 0.018 and 0.03) epilayers by photomodulated reflectance (PR) spectroscopy, atomic force microscopy (AFM) and micro-Raman spectroscopy. PR spectra are analyzed by third derivative functional form (TDFF) lineshape. Optical transition energies are calculated using strain-included valence band anticrossing (VBAC) model. According to the results, optical transitions are from conduction bandedge to heavy hole bandedge and to light hole bandedge, which are affected by compressive strain and effect of strain is clearly seen when Bi concentration exceeds 2%. We also determine the band-anticrossing interaction parameter CBiM as 1.33 eV. TDFF analysis of as-grown and thermal annealed samples reveal that thermal annealing causes an unexpectedly large redshift of ∼22 meV/Bi% in the bandgap that corresponds to the increase of Bi composition by ∼25% in the GaAs lattice. The origin of this redshift is analyzed by AFM and Raman spectroscopy. Surface morphology of the GaBiAs samples show that droplets appear on the GaBiAs surface and their sizes increase with increasing Bi concentration, but tend to decrease following thermal annealing process and some surface droplets leave pits in their locations. Raman spectroscopy and AFM results give hints of diffusion of some Bi atoms from surface into the epilayer that causes an increase in Bi concentration of the alloy. This increase explains the observed annealing-induced redshift of the bandgap in PR measurement.

Optical properties of GaBiAs single quantum well structures grown by MBE

In this study, molecular beam epitaxial-grown GaAs/GaBiAs single quantum well systems with two different Bi contents were investigated. Spectral dependence of room temperature photomodulated reflectance (PR) and photoluminescence (PL) measurements in the temperature range of 35–300 K were employed. PR spectra indicate that increasing Bi concentration promotes a tendency to approach quantized higher energy levels in the heavy and light holes’ bands due to the different effects of compressive strain, which depends on Bi concentrations. In addition, a defect level is identified at 0.71 eV at room temperature PR spectra and is attributed to a AsGa antisite defect in GaAs barrier layers caused by the low temperature growth process. From the analysis of the temperature dependence of emission energy and amplitude in the PL spectra, localized states are determined in the range of 8 to 45 meV and attributed to the different bonding configuration of Bi clusters. Low temperature PL results imply that Bi cluster states tend to move into the valance band when Bi content increases from 2.4 to 7.0% in the GaBiAs system.

A study of photomodulated reflectance on staircase-like, n-doped GaAs/Al x Ga1−xAs quantum well structures

In this study, photomodulated reflectance (PR) technique was employed on two different quantum well infrared photodetector (QWIP) structures, which consist of n-doped GaAs quantum wells (QWs) between undoped AlxGa1−xAs barriers with three different x compositions. Therefore, the barrier profile is in the form of a staircase-like barrier. The main difference between the two structures is the doping profile and the doping concentration of the QWs. PR spectra were taken at room temperature using a He-Ne laser as a modulation source and a broadband tungsten halogen lamp as a probe light. The PR spectra were analyzed using Aspnes’ third derivative functional form.Since the barriers are staircase-like, the structure has different ground state energies; therefore, several optical transitions take place in the spectrum which cannot be resolved in a conventional photoluminescence technique at room temperature. To analyze the experimental results, all energy levels in the conduction and in the valance band were calculated using transfer matrix technique, taking into account the effective mass and the parabolic band approximations. A comparison of the PR results with the calculated optical transition energies showed an excellent agreement. Several optical transition energies of the QWIP structures were resolved from PR measurements. It is concluded that PR spectroscopy is a very useful experimental tool to characterize complicated structures with a high accuracy at room temperature.

Effect of localized boron impurities on the line shape of the fundamental band gap transition in photomodulated reflectance spectra of (B,Ga,In)As

Effect of localized boron impurities on the line shape of the fundamental band gap transition in photomodulated reflectance spectra of (B,Ga,In)As

Band anticrossing in diluted AlxGa1−xAs1−yNy (x⩽0.37,y⩽0.04)

We show that the conduction band structure of dilute AlxGa1−xAs1−yNy with x⩽0.37 and y⩽0.04 can be described consistently by the experimentally motivated band anticrossing model. The interband transition energies E−, E−+Δ0, and E+ have been derived from a full line shape fit to photomodulated reflectance (PR) spectra recorded at room temperature. The PR data were taken (a) from a series of Al0.06Ga0.94As1−yNy samples with y⩽0.04 and (b) from a set of AlxGa1−xAs0.99N0.01 layers with x⩽0.37. The latter series covers the range of Al concentrations where the AlxGa1−xAs band gap energy EM is expected to cross the nitrogen-induced energy level EN. The resulting nitrogen- and Al-concentration dependent interband transition energies are described by the band anticrossing model using a matrix element for the coupling between the nitrogen-induced states and the extend lowest conduction band states of CMN=2.32eV and a nitrogen level energy EN=(1.625+0.069x)eV, the latter measured with respect to the GaAs valence band edge.

Effects of rapid thermal annealing on the properties of GaNxAs1−x

We report the effect of rapid thermal annealing on the valence-band splitting behavior of GaNxAs1−x films grown by molecular beam epitaxy. The light- and heavy-hole valence-band splitting induced by the elastic strain is observed in both photomodulated reflectance and photoluminescence spectra for as-grown GaNxAs1−x epilayers. The valence-band splitting energy increases with the nitrogen composition. This splitting is decreased with the increase of annealing temperature by the rapid thermal annealing temperature process. These properties have been well explained by strain relaxation model including both in-plane strain at GaAs/GaNAs interface and internal strain in GaNAs epilayer. These strain effects have been confirmed by x-ray diffraction and Raman measurements.

An enhanced model for the modulated reflectance spectra of vertical‐cavity surface‐emitting laser structures

AbstractModulation spectroscopy has proved to be unique in being able to probe the vital active‐layer quantum well (QW) transitions in vertical‐cavity surface‐emitting lasers (VCSELs). We have been characterising an all‐AlGaAs‐based VCSEL structure designed to operate at high temperatures, using photo‐modulated reflectance (PR) as a function of temperature up to 180 °C. We previously developed an empirical model to fit VCSEL PR spectra when the features from the QW transitions and the Fabry–Perot cavity mode (CM) overlap and interact, and this fitting function has, so far, proved to be very successful. However, the present VCSEL has a particularly high finesse cavity and our established model is unable to fit the unusually detailed interacting CM/QW PR spectra. Here, we perform new reflectivity calculations for this VCSEL structure. These provide new insight into the Seraphin coefficients of the CM, and their inter‐relationship, when they lie near the QW transitions. This deepens our understanding of VCSEL PR lineshapes, yielding an enhanced model which we successfully fit to the temperature‐dependent PR spectra of the present VCSEL. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of rapid thermal annealing on the optical properties of MBE growth GaNAs films

We report the effect of rapid thermal annealing (RTA) on the optical properties of GaN 0.01As 0.99 samples grown by molecular beam epitaxy. In particular, a blueshift of the PL peak energy is observed when annealing the samples at 650–900 °C. Samples annealed showed pronounced enhancement in PL intensity as compared to the as-grown sample, and 850 °C is proposed as the optimum RTA temperature. The results are examined as a consequence of RTA-induced nitrogen diffusion inside the GaN 0.01As 0.99 material rather than diffusion out of the alloy, which homogenizes initial nitrogen composition fluctuations. In addition, for photo-modulated reflectance (PR) spectra of RTA samples, the fundamental band gap transition ( E 0) and the transition from the spin–orbit split-off valence band ( E 0 + Δ 0 ) are observed. Both of the two transitions increase with increasing annealing temperature.

Advances in the application of modulation spectroscopy to vertical cavity structures

Modulated reflectance is a straightforward technique in which the reflectivity spectrum of a semiconductor is perturbed by an external periodic influence—most usefully a chopped laser beam, i.e. photo-modulated reflectance (PR). Since samples need no special mounting, can be studied in air at room-temperature and can be full-sized pre-fabrication wafers, PR is truly non-destructive. In contrast to conventional photo-luminescence, PR spectra are often replete with sharp, detailed derivative-like features arising from ground-state, and other possible higher-energy optical transitions within a sample, from which can be extracted material parameters crucial to successful and efficient device operation, such as compositions, layer thicknesses, built-in electric fields and band line-ups. The paper discusses the advances made by our group in the application and interpretation of modulation spectroscopy of vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes, which so far have defied analysis by the more conventional non-destructive techniques.

Characterisation of VCSEL and RCLED Structures Using Modulation Spectroscopy

Photomodulated reflectance (PR) is a straightforward technique in which the semiconductor's reflectivity is perturbed by a chopped laser beam. In contrast to photo-luminescence, PR spectra are replete with sharp, detailed derivative-like features from ground-state, and other higher-energy transitions, from which can be extracted material parameters crucial to device operation, such as compositions, layer thicknesses, built-in electric fields and band line-ups. Since samples need no special mounting, can be studied in air at 300K, and can be full-sized prefabrication wafers, PR is truly non-destructive. We discuss the advances in the application and interpretation of PR spectra of vertical-cavity surface-emitting lasers and resonant-cavity light-emitting diodes, which are difficult to characterise using the more conventional non-destructive techniques.

Global changes of the band structure and the crystal lattice of Ga(N,As) due to hydrogenation

The effect of hydrogenation on five ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ epitaxial layers $(0.00043<~x<~0.019)$ grown by metal-organic vapor-phase epitaxy was investigated. Photomodulated reflectance (PR) and photoluminescence spectroscopy were used to study the electronic band structure, and x-ray diffraction (XRD) and Raman spectroscopy to probe, respectively, the static and dynamic properties of crystal lattice before and after hydrogenation. Hydrogen almost completely neutralizes the effect of N on the band structure of the GaAs host. The direct band gap ${E}_{\ensuremath{-}}$ and the spin-orbit split-off band ${E}_{\ensuremath{-}}+{\ensuremath{\Delta}}_{0}$ blueshift toward the corresponding energies in GaAs and the ${E}_{+}$ band disappears after hydrogenation. The PR spectra of hydrogenated ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ resemble broad GaAs-like spectra. The XRD traces reveal that hydrogenation removes the tensile strain in ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ layers and even induces compressive strain. After hydrogenation the GaAs-like features in the Raman spectra persist whereas the local vibrational mode due to N disappears. Three H-related modes can be distinguished in the Raman spectra.

Pressure and k · p studies of band parameters in dilute‐N GaInNAs/GaAs multiple quantum wells

AbstractWe report pressure dependent photomodulated reflectance (PR) measurements on a series of dilute‐N InGaNAs/GaAs multiple quantum wells (MQWs). Our experimental results indicate the presence of important N‐related disorder effects due to different nearest‐neighbour N‐cation configurations The quantum well transition energies obtained from the PR spectra are modelled using a realistic 10‐band k · p Hamiltonian that includes tight‐binding‐based energies and coupling parameters for the N‐levels. By matching experiment with theory we are able to determine accurately the band structure and thus predict some important material parameters for dilute‐N InGaAsN alloys.

Monitoring the non-parabolicity of the conduction band in GaN 0.018As 0.982/GaAs quantum wells

Dramatic changes of the electronic band structure occur when incorporating even a small fraction of N into GaAs. One important consequence of the N-incorporation is a strong non-parabolicity of the conduction band of GaN x As 1− x yielding already for x less than 1% a considerable increase in the electron effective mass and a strong variation of the electron effective mass with increasing k-vector. We demonstrate how this N-induced non-parabolic dispersion of the conduction band in Ga(N,As) can be determined by a careful analysis of the interband transitions of Ga(N,As)-based quantum wells as a function of hydrostatic pressure. A series of GaN 0.018As 0.982/GaAs wells of various widths was studied by photomodulated reflectance (PR) at 300 K and hydrostatic pressures up to 20 kbar. The PR spectra were fitted using derivative-like line shapes to extract the energy positions of the interband transitions. The transition energies were compared with theoretical values calculated using a 10-band k.p-model including the effects of nitrogen. The good agreement between experiment and theory allows one to extract the valence band offset as well as the conduction band dispersion and hence the change of the effective mass with pressure and energy.

Band Gap of Hexagonal InN and InGaN Alloys

A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 10 18 and 4 × 10 19 cm -3 . It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich In x Ga 1-x N alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of In x Ga 1-x N alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In 2 O 3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

Electronic structure ofInyGa1−yAs1−xNx/GaAsmultiple quantum wells in the dilute-Nregime from pressure andk⋅pstudies

We report photomodulated reflectance measurements of several intersubband transitions for a series of as-grown ${\mathrm{In}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}/\mathrm{GaAs}$ multiple quantum well samples as functions of hydrostatic pressure (at room temperature) and temperature (at ambient pressure). The experimental results provide support for the effects of disorder due to different nearest-neighbor N-cation configurations. The quantum well transition energies obtained from the photomodulated reflectance spectra are fitted as a function of pressure with a realistic 10 band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian, that includes tight-binding-based energies and coupling parameters for the N levels. The quality of match between theory and experiment confirms the theoretical model and predicts some important material parameters for dilute-N InGaAsN alloys.

Accurate methods for study of light emission from quantum wells confined in a microcavity

Photomodulated reflectance (PR) and conventional reflectance studies have been performed on an InGaP/AlGaInP/Al/sub x1/Ga/sub y1/As/Al/sub x2/Ga/sub y2/As resonant-cavity light emitting diode structure in the red spectral region. The PR spectra show prominent signals from the Fabry-Perot cavity mode and the quantum-well (QW) ground state excitonic transition. This high-precision technique, and its variations as functions of incidence angle and temperature, as reported in this article, allow one to investigate light emission from the QW confined in a microcavity with relation to the Fabry-Perot mode, and is the only known nonconductive, nondestructive method of doing so.

N-Composition and Pressure Dependence of the Inter Band Transitions of Ga(N,As)/GaAs Quantum Wells

A series of GaN x As 1 m x /GaAs quantum well structures with well widths of about 20 nm and x varying between 1% and 3.5% has been grown by metal-organic vapour phase epitaxy. We have studied the evolution of the quantum well states under hydrostatic pressure up to 20 kbar at 300 K by photomodulated reflectance (PR) spectroscopy. The energy positions of the quantum well transitions have been obtained by fitting the PR spectra. The pressure dependence of the allowed heavy-hole transitions e n hh n decreases with increasing n . This directly reflects the strong non-parabolic dispersion of the conduction band originating from the interaction of the N-impurity level with the bands of the GaAs host. The fitted energy positions and their pressure dependence can be well described by a 10 band k.p model. The observed splitting between the lowest light-hole and heavy-hole transitions are in agreement with a type I band alignment.