Contactless Electroreflectance Research Articles

Contactless and soft contact electroreflectance of AlGaN/GaN and GaN/AlGaN/GaN field effect transistor heterostructures

AbstractAlGaN/GaN field effect transistor (FET) structures with a two dimensional electron gas (2DEG) at the interface were investigated by electromodulation spectroscopy [contactless electroreflectance (CER), and soft contact electroreflectance (SCER)]. It has been clearly shown that in contactless mode of electroreflectance (i.e., CER spectroscopy) the 2DEG screens the band bending modulation in GaN layer from the external electromodulation which is generated by the capacitor‐like system. In a result only the optical transition in AlGaN layer and no signal, which could be associated with the optical transition in GaN buffer layer, is observed in CER spectra. This screening is not present in soft contact mode of electroreflectance (i.e., SCER spectroscopy). It has been clearly shown that the soft contact mode of electrorefletance works very well for AlGaN/GaN FET structures with a 2 nm thick GaN cap layer. In this mode of electroreflectance measurements the signal from GaN buffer layer was clearly observed whereas in CER spectra this signal was not detected. In CER spectra only the AlGaN‐related transition and the GaN cap‐related transition were observed. The last transition was observed at the energy of ∼3.65 eV because of quantum confinement in GaN cap layer, i.e., a surface quantum well in fact. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of As/III pressure ratio on nitrogen nearest-neighbor environments in as-grown GaInNAs quantum wells

The energy fine structure, corresponding to different nitrogen nearest-neighbor environments, was observed in contactless electroreflectance (CER) spectra of as-grown GaInNAs quantum wells (QWs) obtained at various As/III pressure ratios. In the spectral range of the fundamental transition, two CER resonances were detected for samples grown at low As pressures whereas only one CER resonance was observed for samples obtained at higher As pressures. This resonance corresponds to the most favorable nitrogen nearest-neighbor environment in terms of the total crystal energy. It means that the nitrogen nearest-neighbor environment in GaInNAs QWs can be controlled in molecular beam epitaxy process by As/III pressure ratio.

Contactless electroreflectance of InGaN layers with indium content ≤36%: The surface band bending, band gap bowing, and Stokes shift issues

Contactless electroreflectance (CER) supported by photoluminescence (PL) has been applied to study (i) the surface band bending, (ii) the band gap bowing, and (iii) the Stokes shift for InGaN layers grown by molecular beam epitaxy with 0.14≤In≤0.36. The type of surface band bending has been investigated on the basis of the shape of CER resonance. It has been found that the surface band bending changes from n-type for layers with low indium content (In<27%) to flatband (or weak p-type band) for layers with In∼35%. The band gap bowing has been determined to be 1.4±0.2 and 2.1±0.3 eV for CER data with and without strain corrections, respectively. From this analysis it has been concluded that the reliable value of the bowing parameter for unstrained InGaN should be between 1.4 and 2.1 eV. Comparing CER with PL data it has been found that the Stokes shift rises from 20 to 120 meV when the indium concentration increased from 14% to 36%. In addition, it has been observed that the intensity of PL from InGaN layers decreased exponentially with the increase in the indium content. The last two findings are attributed to an easier formation of native point defects and stronger indium segregation in InGaN alloys with higher indium concentrations.

Optical investigation of band-edge structure and built-in electric field of AlGaN/GaN heterostructures by means of thermoreflectance, photoluminescence, and contactless electroreflectance spectroscopy

The band-edge property and built-in electric fields of two different Al(x)Ga(1-x)N/GaN(AlGaN/GaN) heterostructures (HSs) with and without an additional AlGaN inserted layer were studied by thermoreflectance (TR), photoluminescence (PL), and contactless electroreflectance (CER) techniques. The PL spectra characterize the band-edge luminescence property of GaN. Free exciton transitions of AlGaN and GaN were probed experimentally by TR. Prominent Franz-Keldysh oscillations (FKOs) of GaN and the opposite FKO phase of AlGaN were simultaneously detected by the additional AlGaN inserted sample with CER owing to the enhancement effect of built-in electric fields of GaN and AlGaN with the same polarity direction. Optoelectronics properties of the two HSs were characterized by the experimental analyses.

Contactless electroreflectance and photoluminescence characterization of Zn0.68Be0.06Mg0.26Se crystalline alloys

Abstract In this report, results of optical characterization of Bridgman-grown Zn 0.68 Be 0.06 Mg 0.26 Se crystalline alloys in the near-band-edge interband transitions region using temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) in the temperature range 10–300 K and time-resolved PL measurements at 10 K are presented. Two features with separation energy of 40 meV are observed in the near-band-edge CER spectra. The origins of these two features are attributed to the fundamental band-edge excitonic transitions of the coexisted different crystal phases in accordance with the X-ray diffraction and transmission electron microscope results. The increase of the broadening parameter of the excitonic feature of the PL spectra with increasing temperature has been attributed to the electron–longitudinal optical phonon interactions. In addition, the transient PL spectrum at 10 K in the excitonic emission region exhibits the asymmetric lineshape and can be deconvoluted into two Gaussian features with one order of magnitude difference in the decay time. These two components are assigned to the recombination of free excitons and localized excitons in this quaternary compound.

Evidence for Fermi level shift in GaInAs/GaAs quantum wells upon nitrogen incorporation

GaInAs/GaAs and GaInNAs/GaAs quantum well (QW) structures grown by metalorganic vapor phase epitaxy have been studied by contactless electroreflectance spectroscopy. In addition to the N-related redshift of QW transitions, an increase in the electric field in the GaAs cap layer has been observed after the incorporation of nitrogen atoms into the GaInAs QW. This observation is associated with the tendency of the Fermi level shift to a given energy in the GaInNAs QW region due to N-related defects.

Nonpolar GaN substrates grown by ammonothermal method

In this letter, the authors demonstrate large size m-plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below 5×104 cm−2 and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material.

Photoreflectance and contactless electroreflectance measurements of semiconductor structures by using bright and dark configurations

Experimental setup for measurements of photoreflectance (PR) and contactless electroreflectance (CER) spectra in bright and dark configurations is described in this work and applied to study various semiconductor structures. The innovative solution in this setup is the possibility to measure PR and CER spectra in both experimental configurations with the same halogen lamp, monochromator, detector, and only very small modification in the optical path. In this setup the measurement conditions for the two experimental configurations are very similar, and the obtained PR and CER spectra can be compared and discussed in the context of the unwanted constant photovoltaic (PV) effect, which appears in the bright configuration when the sample is illuminated by the spectrum of white light instead of the monochromatic light. It has been clearly shown that for (i) epitaxial layers, (ii) quantum wells, and (iii) quantum dots, exactly the same spectral features are observed in both configurations at room temperature. It means that from the viewpoint of the detection of optical transitions, it is not important what configuration is used since the white light-induced PV effect does not influence the energy of optical transitions in these structures.

Characterization of a ZnxCd1−xSe/Znx′Cdy′Mg1−x′−y′Se multiple quantum well structure for mid-infrared device applications by contactless electroreflectance and Fourier transform infrared spectroscopy

Contactless electroreflectance (CER) and Fourier transform infrared (FTIR) spectroscopy were used to study the intersubband transitions of a ZnxCd1−xSe/Znx′Cdy′Mg1−x′−y′Se multiple quantum well (MQW) structure grown by molecular beam epitaxy for mid-infrared device applications. The CER spectrum revealed a wide range of possible optical transitions in the MQW structure. The ground state transition was assigned by comparison with the photoluminescence emission signal taken from the same structure. A comprehensive analysis of the CER spectrum led to the identification of various interband transitions. The intersubband transitions were estimated and confirmed by FTIR measurements. The results demonstrate the potential of using CER as a complementary technique for the contactless and nondestructive characterization of the wide band gap II–VI MQW structures for mid-IR intersubband device applications.

Optical characterization of thin epitaxial GaAs films on Ge substrates

Two thin epitaxial GaAs films on Ge substrates were characterized by photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR) techniques. The GaAs films containing antiphase domains (APDs) and those APD free were grown at 650 °C by metal organic vapor phase epitaxy. The PL intensity of the APD-containing film is four orders of magnitude lower than that of the APD-free sample. The reduction in intensity is due to the electrically active Ga–Ga and As–As bonds at the boundaries between the different APDs. The electric fields deduced from Franz–Keldysh oscillations of the CER spectra were used to determine the net carrier concentration of the GaAs films. In addition, the conduction to light-hole band transitions and the conduction to heavy-hole band transitions that originated from strain induced valence band splitting in GaAs thin films were observed and identified through a comparison of the relative intensities of light- and heavy-hole related features in the PzR and CER spectra. The results demonstrate that PL, CER, and PzR are useful nondestructive tools to characterize thin epitaxial GaAs films on Ge substrates.

Optical characterization ofZn0.95−xBexMn0.05Semixed crystals

A systematic study of a series of ${\text{Zn}}_{0.95\ensuremath{-}x}{\text{Be}}_{x}{\text{Mn}}_{0.05}\text{Se}$ mixed crystals with different Be content $(0.05\ensuremath{\le}X\ensuremath{\le}0.20)$ grown by the modified high-pressure Bridgman method were carried out by room-temperature surface photovoltage spectroscopy (SPS), temperature-dependent photoluminescence (PL), and contactless electroreflectance (CER). A typical PL spectrum at low temperature consists of free exciton line, an edge emission due to recombination of shallow donor-acceptor pairs, and the ${\text{Mn}}^{2+}$-related intraionic emission. The near band-edge transition energies determined by analyzing the CER and SPS spectra showed a blueshift with the increase in Be content. The peak positions of band-edge exciton features in the PL spectra shifted slightly toward lower energies as compared to the corresponding transition energies obtained from CER and SPS data. The observed increases in the CER-PL shift with the increasing of Be content are explained by the increasing compositional disorder causing the smearing of the band-edge energies. The excitonic line broadening for the samples with larger Be/Zn ratio are attributed in part to the alloy-scattering effects and also to the poorer crystalline quality of the samples with higher content of Be. 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.

Electromodulation spectroscopy of In 0.53Ga 0.47As/In 0.53Ga 0.23Al 0.24As quantum wells

In 0.53Ga 0.47As/In 0.53Ga 0.23Al 0.24As quantum wells (QWs) of various widths have been grown by molecular beam epitaxy on the InP substrate and investigated by electromodulation spectroscopy, i.e. photoreflectance (PR) and contactless electroreflectance (CER). The optical transitions related to the QW barrier and the QW ground and excited states have been clearly observed in PR and CER spectra. The experimental QW transition energies have been compared with theoretical predictions based on an effective mass formalism model. A good agreement between experimental data and theoretical calculations has been observed when the conduction band offset for the In 0.53Ga 0.47As/In 0.53Ga 0.23Al 0.24As interface equals ∼60%. In addition, it has been concluded that the conduction band offset for the In 0.53Ga 0.23Al 0.24As/InP interface is close to zero. The obtained results show that InGa(Al)As alloys are very promising materials in the band gap engineering for structures grown on InP substrate.

Preface: Phys. Status Solidi A 206/5

AbstractThe 3rd International Workshop on Modulation Spectroscopy of Semiconductor Structures – 3rd MS3 Workshop took place at the Wrocław University of Technology (Wrocław, Poland) between 3rd and 5th July 2008.For a third time, the MS3 Workshop became the forum for discussing the advances of photoreflectance, electroreflectance, contactless electroreflectance, thermoreflectance, differential reflectance and wavelength‐modulated surface photovoltage spectroscopy.In this special issue of physica status solidi (a), it is our great pleasure to present the most interesting of the 3rd MS3 Workshop contributions.The organizers acknowledge Wrocław University of Technology and Foundation for Polish Science for the financial support of the Workshop.A very sad news reached all scientists during the Workshop. Professor Fred H. Pollak, great scientist and Honorary Chairman of the 1st MS3 Workshop, passed away on June 19, 2008 at the age of seventy‐three. Wishing to honor his memory, we include in this issue a short note by M. Muñoz and Y.‐S. Huang, his former students and friends (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Use of contactless electroreflectance in the development of quantum cascade lasers from ZnCdSe/ZnCdMgSe

AbstractElectro‐modulation techniques have been used extensively to analyze interband transitions in materials and devices, however little has been done to use these techniques for the analysis of intersubband transitions in materials and devices. In this work, contactless electroreflectance has been used to determine the conduction band offset of ZnCdSe/ZnCdMgSe quantum well structures. Toward the development of quantum cascade lasers, multi‐quantum well structures with intersubband absorption at 178 meV were analyzed using contactless electro‐reflectance. Finally, quantum cascade laser structures were designed for emission at 4.8 μm. The multi‐quantum well structures presented an excellent agreement between the designed and measured absorption and emission, respectively. This work demonstrates that contactless electro‐reflectance can accurately determine the conduction band offset and is useful in the analysis of intersubband transitions. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Contactless electroreflectance studies of ultra-dilute GaAs1−xBix alloys

In this work we report a large effect due to relativistic corrections in the electronic structure of very dilute GaAs1−xBix (x < 0.0025) thick epitaxial layers. The variation of the spin–orbit split-off band for x as small as 0.0001 is reported. Very thick (2–3 µm) epilayers were grown by molecular-beam epitaxy to isolate the transitions between the conduction band and the spin–orbit split-off band from the epilayer and the substrate, using contactless electroreflectance. Thick epitaxial quality samples with precise control of the spin–orbit splitting are interesting for applications in spin-based electronics.

Contactless electroreflectance of GaInNAsSb/GaNAs/GaAs quantum wells emitting at 1.5–1.65 μm: Broadening of the fundamental transition

Contactless electroreflectance (CER) has been applied to study the broadening of the fundamental transition for GaInNAsSb/GaNAs/GaAs quantum wells (QWs) obtained at various growth and annealing conditions. It has been observed that CER resonances are about 50% narrower for QWs grown at lower group V fluxes and annealed at lower temperatures (660–720 °C) and longer time (30–60 min) than those previously considered optimal (∼760 °C and ∼60 s). The long annealing can be partially realized in situ during (and/or after) the growth of the upper part of the laser structure instead the ex situ short-time annealing, where the laser structure can unintentionally be overannealed very easily.

Electromodulation spectroscopy in midinfrared spectral region: The band offset study for GaSb-based quantum wells

Electromodulation spectroscopy (photoreflectance and electroreflectance) is an excellent technique to study optical transitions in quantum well (QW) structures but the development of this technique in the midinfrared spectral region is still limited due to various reasons. In this work we report our recent progress in the development of contactless electroreflectance spectroscopy in midinfrared and discuss some aspects and perspectives of this technique in the context of its application to study optical transitions in GaSb-based QWs. Especially, we have focused on the band offset issue in GaInAsSb/GaSb QWs since the accurate study/verification of band gap discontinuities at QW interfaces is a crucial point in the optimization of laser structures grown on GaSb substrate and operating in the 1.8–3.0 μm spectral range.

Electromodulation spectroscopy of the ground and excited state transitions in GaInN/AlInN multi-quantum wells

Contactless electroreflectance (CER) has been applied to investigate energies and intensities of optical transitions in GaInN/AlInN multi-quantum wells (MQWs) with the various QW widths (1.55 and 1.8 nm) and barrier thicknesses (3 and 6 nm). In addition to the ground state transition, optical transitions related to excited states (2H–1C and 1H–2C transitions, where kH– lC denotes a transition between the kth valence and the lth conduction subbands) have been observed in CER spectra. It has been found that the intensities of 2H–1C and 1H–2C transitions (i.e., transitions which are forbidden for non-polar square-like QWs) are comparable with the intensity of the ground state transition. In addition, it has been observed that the relative intensities of QW transitions depend on both the QW width as well as the barrier thickness. This experimental observation has been confirmed by theoretical calculations performed in the framework of the electron effective mass approximation model taking into account polarization effects in wurtzite III-N alloys. The calculations clearly show that the intensity of QW transitions for GaInN/AlInN MQWs strongly varies with the QW width as well as the barrier thickness.

Contactless electroreflectance evidence for reduction in the surface potential barrier in AlGaN/GaN heterostructures passivated by SiN layer

Contactless electroreflectance (CER) has been applied to study the AlGaN potential-barrier height in AlGaN/GaN heterostructures without and with a SiN passivation layer. In the case of an unpassivated structure, an AlGaN band-edge signal with a strong Franz–Keldysh oscillation (FKO) was observed. On the basis of the FKO period, the surface potential barrier has been determined to be ∼1.1 eV. For the SiN passivated structure, a broad CER signal without FKO appears at the AlGaN edge. This observation is associated with a decrease in the height of the surface potential barrier, i.e., a shift in the Fermi level position at the AlGaN surface toward the conduction band.

Temperature dependent electromodulation characterization of Zn1−x−yBexMgySe mixed crystals

We report a detailed investigation of the temperature dependence of the band-edge excitonic transitions of three Zn1−x−yBexMgySe mixed crystals using contactless electroreflectance (CER) and photoreflectance (PR) in the temperature range of 15–400 K. The samples were grown by the modified high pressure Bridgman methods. The fundamental transition energy E0 and broadening parameters are determined via a lineshape function fit to the CER and PR spectra. The parameters that describe the temperature dependence of the band-edge excitonic transition energy and broadening parameters are evaluated and discussed.