Contactless Electroreflectance Spectra Research Articles

Piezoreflectance and contactless electroreflectance spectra of an optoelectronic material: GaInNP grown on GaAs substrates

We report a detailed study of piezoreflectance (PzR) and contactless electroreflectance (CER) on Ga 0.44In 0.56N x P 1− x epitaxial layers at room temperature. The polarized PzR spectra show anisotropic character along the [1 1 0] and [1 1 ̄ 0] directions. Ordering-induced superlattice-like microstructure shown in high resolution transmission electron microscope (HTEM) images confirms the spontaneous ordering in Ga 0.44In 0.56N x P 1− x epitaxial layers. The obtained PzR and CER spectra are fitted with the theoretical line shape functional form. The valence-band maximum, crystal field/strain splitting and spin–orbit splitting to conduction band transition energies denoted as E g, E g+Δ 12 and E g+Δ 13, respectively, are accurately determined. With nitrogen incorporation, the PzR and CER features red-shifts, indicating band gap reduction. The experimentally observed band gap energy reduction of GaInP incorporation nitrogen is of the same order as for nitrogen in binaries. However, the bowing coefficient b is determined to be −10.5 eV, that is considerable smaller than that of the incorporation of nitrogen in InP ( b=−16 eV) or GaP ( b=−14 eV).

Temperature Dependence of the Band-Edge Transitions of ZnCdBeSe

We have characterized the temperature dependence of band-edge transitions of three (Zn0.38Cd0.62)1-xBexSe II–VI films with different Be concentrations x by using contactless electroreflectance (CER) and piezoreflectance (PzR) in the temperature range of 15 to 450 K. By a careful comparison of the relative intensity of PzR and CER spectra, the identification of light-hole (lh) and heavy-hole (hh) character of the excitonic transitions of the samples has been accomplished. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the energy (including thermal expansion effects) and broadening parameter of the band edge transitions of ZnCdBeSe. The study shows that Be incorporation can effectively reduce the rate of temperature variation of the energy gap.

An analysis of temperature dependent piezoelectric Franz–Keldysh effect in AlGaN

Strong Franz–Keldysh oscillations near the band gap of AlGaN are observed in the contactless electroreflectance (CER) studies of a GaN/InGaN/AlGaN multilayer structure. The line shape analysis of the CER spectra at different temperatures provides an accurate determination of the AlGaN band gap energies and the built-in electric fields. Using the existing data of the thermal expansion coefficients of GaN and sapphire, and the piezoelectric constants of AlGaN, the temperature dependence of the electric field is estimated and is in good agreement with the experimental results between 15 and 300 K. We attribute such electric field to the piezoelectric strain effect.

Spectroscopic study of Ga-In-P based self-organized lateral superlattices

This paper reports polarization and temperature dependent contactless electroreflectance (CER) and photoluminescence (PL) study of strain compensated short period superlattices on GaAs(001) substrates which show self-organized lateral composition modulation (LCM) in the form of alternate In/Ga rich regions along the [110] direction in the (001) plane. The LCM related PL peak position is at a lower energy as compared to the corresponding feature in the CER spectrum and the difference is found to be the same as the activation energy for thermal quenching of the PL signal. These results have been explained by suggesting that the PL signal arises mainly from recombination of carriers localized at potential fluctuations (In rich regions with excessively high In concentration) within the lateral superlattice (LS) structure that is formed due to the LCM while the CER feature arises from transitions between the minibands of the average LS structure. The PL signal is quenched due to thermal excitation of the carriers from these excessively In rich regions into the minibands. The LS band gap energy is accurately determined and its implication for the random alloy model used to describe the electronic band structure of such systems is discussed. The temperature dependences of critical point energies have also been measured and analysed.

The annealing effects on ZnCdSe/ZnSe quantum wells and ZnSe/GaAs interfaces

This study investigated the thermally induced interdiffusion in ZnCdSe/ZnSe quantum wells and the outdiffusion of Ga atoms from the GaAs substrate by photoluminescence (PL), secondary ion mass spectrometry (SIMS), and contactless electroreflectance (CER) spectroscopy. The quantum well structures grown by molecular beam epitaxy were thermally annealed at temperatures between 250 and 700 °C. According to our results, the 15 K PL main peak disappeared when the annealing temperature reached 550 °C and above for samples grown with a 0.1 μm ZnSe buffer (sample A). In contrast, the PL main peak disappeared when the annealing temperature reached 600 °C and above for samples grown with 0.85 μm ZnSe buffer (sample C). In addition, for sample A, two extra PL peaks around 2.0 and 2.3 eV were observed when the annealing temperature reached 500 °C and above; those peaks were observed only when the annealing temperature reached 700 °C for sample C. SIMS results indicated the interdiffusion of Cd in the ZnCdSe/ZnSe quantum well regions in both samples. However, only sample A revealed a strong outdiffusion of Ga atoms from the substrate into the epilayer side, but a weaker one in sample C. The main peaks in PL spectra and the intersubband transitions in CER spectra disappeared owing to the Cd interdiffusion in the wells and the defect-related transitions introduced by the Ga outdiffusion onto the epilayer side. Based on the SIMS, PL, and CER spectra results, a thicker ZnSe buffer layer can increase the thermal stability of ZnCdSe/ZnSe quantum wells grown on GaAs substrates because of its effectiveness in mitigating the outdiffusion of Ga atoms into buffer layers and the interdiffusion of quantum well regions.

The structural and optical properties of high quality ZnTe grown on GaAs using ZnSe/ZnTe strained superlattices buffer layer

This work studied the structural and optical properties of ZnTe epilayers grown on GaAs substrates with ZnSe/ZnTe strained superlattices buffer layers. Material properties were characterized using cross-sectional transmission electron microscopy, photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR). PL spectra clearly distinguished the strong free exciton peaks, weak donor-acceptor pair, Y lines, and oxygen-bound exciton peaks, indicating the high quality of the films. In addition, the CER and PzR spectra were compared to ascertain that ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. An attempt was also made to identify the origins of the near-band-edge transitions of ZnTe epilayer in the CER and PzR spectra by comparing these spectra with PL spectra and the second harmonic frequency CER. By doing so, the interference below the band gap of ZnTe could be effectively eliminated. Moreover, the energy splitting between heavy- and light-hole valence bands at 15 K was calculated by utilizing the temperature-dependent elastic constants for ZnTe and the thermal-expansion coefficients for ZnTe and GaAs. The discrepancy between experiment and calculation indicates that the residual mismatch-induced strain and the thermally induced strain in ZnTe epilayers grown on GaAs during cooling must be simultaneously considered.

Structural and Optical Studies of ZnCdSe/ZnSe/ZnMgSSe Separate Confinement Heterostructures with Different Buffer Layers

Detailed structural and optical studies of ZnCdSe/ZnSe/ZnMgSSe separate confinement heterostructures (SCH) grown on ZnSe, ZnSe/ZnSSe strained-layer superlattices (SLS), and GaAs buffer layers at the II–VI/GaAs interface have been carried out by employing transmission electron microscopy, variable temperature photoluminescence (PL), and contactless electroreflectance (CER) measurements. A significant improvement on the defect reduction and the optical quality has been observed by using either the ZnSe/ZnSSe SLS or GaAs as the buffer layers when compared to that of the sample using only ZnSe as the buffer layer. However, the sample grown with the SLS buffer layers reveals a room temperature PL intensity higher than that of the sample grown with a GaAs buffer layer, which may still suffer from the great ionic differences between the II–V and III–V atoms. Using 15 K CER spectra, we have also studied various excitonic transitions originating from strained Zn0.80Cd0.20Se/ZnSe single quantum well in SCH with different buffer layers. An analysis of the CER 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. An excellent agreement between experiments and theoretical calculations based on the envelope function approximation model has been achieved.

Polarization anisotropy in room temperature contactless electroreflectance spectrum of self-organized lateral compositional superlattices

We report the results of room temperature contactless electroreflectance measurements on (GaP)2(InP)2superlattices grown on GaAs (001) substrates which show lateral composition modulation along the [110] direction in the (001) plane. The strong polarization anisotropy of some features in the spectrum has been explained by identifying them as arising from transitions in the lateral superlattice formed due to the composition modulation. The measured optical transition energies are considerably lower than theoretical estimates based on the random alloy model, suggesting the need for a better model to describe the electronic band structure of this system.

A study of the Franz–Keldysh oscillations in electromodulation reflectance of Si-delta-doped GaAs by a fast Fourier transformation

The line shapes of electromodulation spectra exhibit extended Franz–Keldysh oscillations and interference beats in the presence of a uniform built-in electric field. For this work, we used photoreflectance and contactless electroreflectance to study the Franz–Keldysh oscillations in Si-delta-doped GaAs. The fast Fourier transformation taken to the photoreflectance and contactless electroreflectance spectra, produced more complicated results than were observed in previous studies, when the effect of the modulation field is nonnegligible. This indicates that the interference beats are not only due to different effective heavy-hole and light-hole mass but also to the modulation field. We propose that the Franz–Keldysh oscillations generally contain four frequencies, which correspond to the heavy-hole and light-hole splitting. A comparison between the experiments and some numerical simulations attest to the validity of our proposal. This line shape analysis could then be applied to estimate the strength of a modulation field in contactless electroreflectance and photoreflectance.

Contactless electroreflectance and piezoreflectance studies of temperature-dependent strain in ZnTe/GaAs heterostructures with ZnSe/ZnTe superlattice buffer layers

The temperature-dependent optical properties of ZnTe epilayers grown on GaAs substrates by molecular beam epitaxy with and without ZnSe/ZnTe strained-layer superlattice (SLS) buffer layers have been studied using contactless electroreflectance (CER) and piezoreflectance (PzR). Our ZnTe epilayers of 1.5 μm in thickness grown on GaAs substrates are under a biaxial tensile strain according to the results shown in CER and PzR spectra. Furthermore, the strain induced energy splitting between heavy- and light-hole valence bands in the ZnTe epilayer can be reduced by using the ZnSe/ZnTe SLS buffer layers. We have also justified the temperature-dependent energy splitting between heavy- and light-hole valence bands for ZnTe through theoretical calculations. Discrepancy between experiments and calculations indicates that the residual mismatch-induced strain as well as the thermally induced strain during cooling must be taken into account at the same time.

The study of piezoelectric effect in wurtzite GaN/InGaN/AlGaN multilayer structures

By using photovoltaic spectroscopy and contactless electroreflectance spectroscopy, large electric field was found to exist in wurztite GaN/InGaN/AlGaN multilayer structures. The electric field strengths in the GaN layer and the AlGaN layer were obtained from the analysis of the Franz–Keldysh oscillations in contactless electroreflectance spectra. These electric fields were attributed to be induced by the piezoelectric effect in wurztite nitride systems.

Polarization anisotropy of sub-band gap oscillatory features in contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates

We report the observation of strongly polarization sensitive sub-band gap oscillatory features in the contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates. At a given energy in the sub-band gap region, the peak strength of these oscillatory features decreases from a positive maximum to a negative minimum passing through zero as the polarization of the incident probe beam is rotated by 90° from [11̄0] direction to [110] direction in the (001) plane. The origin of this phenomenon is explained on the basis of optical interference coupled with linear electro-optic effect induced changes in the sub-band gap refractive index of the InxGa1−xP layers. Numerical simulations based on the above mechanism are shown to reproduce the polarization dependent observations quite well.

Near-band-edge optical properties of molecular beam epitaxy grown ZnSe epilayers on GaAs by modulation spectroscopy

A study of near-band-edge optical properties of ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoreflectance spectra to ascertain that our ZnSe epilayers of 1.2 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy-hole and light-hole related transition energies as a function of temperature in the 15–200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy-hole and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal expansion coefficients for ZnSe and GaAs. Both the experimental result and the theoretical calculation have shown a similar trend that the biaxial tensile strains decrease in magnitude with increasing temperatures in the 1.2 μm ZnSe epilayer grown on a GaAs substrate.

Contactless electroreflectance study of strained Zn0.79Cd0.21Se/ZnSe double quantum wells

We have studied various excitonic transitions of strained Zn0.79Cd0.21Se/ZnSe double quantum wells, grown by molecular beam epitaxy on (100) GaAs substrates, using contactless electroreflectance (CER) at 15 and 300 K. A number of intersub-band transitions in the CER spectra from the sample have been observed. An analysis of the CER 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 conduction-band offset Qc is used as an adjustable parameter to study the band offset in the strained Zn0.79Cd0.21Se/ZnSe system. The value of Qc is determined to be 0.67±0.03.

Interference effects in photoreflectance and contactless electroreflectance spectra of CdTe films grown on Si substrate

We report the observation of oscillatory features (OFs) in the photoreflectance (PR) and contactless electroreflectance (CER) spectra of CdTe films grown on Si substrate, at energies below the band gap of CdTe. The simultaneous observation of OF in the reflectance (R) spectrum having the same period as those in the PR and CER spectra (as also their dependence on film thickness) provides a direct proof for optical interference effects as being the source of these features. However, in the present case the amplitude of the OF gets damped towards shorter wavelengths while remaining nearly wavelength independent in the longer wavelength region indicating a modulation mechanism different from those reported earlier. A series of experiments and simulations performed by us seem to indicate that while in PR the principal mechanism is the pump beam induced periodic temperature changes which in turn modulates the optical path length of the CdTe film, in CER the mechanism is the electric field induced modulation of the subband gap refractive index of the film through the electro-optic effect. The damping of the OF has been explained on the basis of subband gap absorption by the CdTe film.

Temperature dependence of quantized states in an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In0.86Ga0.14As0.3P0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER 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 parameters that describe the temperature dependence of EmnH(L) are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.

Temperature dependence of quantized states in an InGaAs/GaAs strained asymmetric triangular quantum well

Photoreflectance (PR), contactless electroreflectance (CER) and piezoreflectance (PzR) measurements of an InGaAs/GaAs strained asymmetric triangular quantum well (ATQW) heterostructure as a function of temperature in the range of 20 to 300 K have been carried out. The structure was fabricated by molecular beam epitaxy using the digital alloy compositional grading method. A careful analysis of the PR, CER and PzR spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state to the nth heavy(light)-hole band state. Comparison of the observed intersubband transitions with a theoretical calculation based on the envelope function model, including the effects of strain, provide a self-consistent check of the ATQW composition profile. The detailed study of the temperature dependence of the excitonic transition energies indicates that the potential profile of the ATQW varies at different temperatures. The parameters that describe the temperature dependence of are evaluated. The anomalous behaviour of the temperature dependence of the linewidth of 11H, , is compared with recent results for GaAs/AlGaAs and InGaAs/GaAs symmetric rectangular quantum wells of comparable dimensions.

The determination of the band offsets in strained-layer InxGa1-xAs/GaAs quantum wells by low-temperature modulation spectroscopy

We report a determination of the conduction-band offset ratio Oc of the InxGa1-xAs/GaAs strained-layer single quantum wells by low-temperature modulation spectroscopy. Various modulation reflectance measurements include contactless electroreflectance (CER), photoreflectance (PR) and piezoreflectance (PZR) are carried out on four different samples at room temperature and 20 K. The samples have In composition x=0.05 (sample A), 0.11 (sample B), 0.14 (sample C) and 0.21 (sample D) respectively. A number of allowed and forbidden excitonic transitions have been observed. The light-hole transitions were unambiguously identified by comparing the CER spectra with different modulation strengths. The origins of the various spectral features have been assigned by comparison with an envelope-function calculation taking into account the effects of strain. Good agreement between experiments and theory is achieved by taking Qc=0.46+or-0.05, 0.50+or-0.10, 0.54+or-0.07 and 0.63+or-0.05 for samples A, B, C and D respectively. It has been shown that Oc depends linearly with the In composition x between 0.05 and 0.21.

Contactless electromodulation for in situ characterization of semiconductor processing

We present results of a new contactless electroreflectance (CER) mode which has considerable potential for in situ monitoring. This method utilizes a condenser-like system, one electrode consisting of a transparent conductive coating on a transparent substrate which is separated from the sample surface by a thin layer of air (or other ambient). In order to demonstrate the utility of this approach we have measured the CER spectra from (a) bulk Hg 0.8Cd 0.2Te (80 and 300 K) sputtered with Xe ions as well as (b) (001) n- and p-type GaAs structures with large, uniform electric fields. The latter configurations can be used to investigate Fermi level pinning effects and metallization. Studies were carried out as a function of temperature (10 < T < 600 K) yielding a temperature dependence of surface barrier height. The relative merits of CER and photoreflectance will be discussed.

Novel contactless mode of electroreflectance

We report a new contactless mode of electroreflectance which utilizes a condenserlike system. One electrode consists of a transparent conductive coating on a transparent substrate which is separated from the sample surface by a thin layer of air. We have measured the contactless electroreflectance spectra at 300 K from a number of materials including semi-insulating bulk GaAs, bulk narrow gap Hg0.8Cd0.2Te, a GaAs structure with a large, uniform electric field and a GaAs/Ga1−xAlxAs (x≊0.2) coupled double quantum well.