Photomodulated Reflectance Research Articles

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.

Two-photon excitation in an intermediate band solar cell structure

We present evidence for the production of photocurrent due to two-photon excitation in an intermediate band solar cell structure. The structure consists of an n-GaNAs intermediate band layer sandwiched between a p-AlGaAs emitter and an n-AlGaAs barrier layer with suitable doping level to block electron escaping from the intermediate band to the bottom n-GaAs substrate. Multi-band transitions observed in two-photon excitation experiments are explained using photo-modulated reflectance spectrum, and further support for intermediate band solar cell operation of this structure is given by current-voltage measurements.

Demonstration of ZnTe1-xOx Intermediate Band Solar Cell

We report the optical properties of ZnTe1-xOx (ZnTeO) and evidence for the photovoltaic (PV) activity of a ZnTeO intermediate band solar cell (IBSC). By photomodulated reflectance measurements, electron transitions from the intermediate band to the conduction band were demonstrated. The optical absorption coefficients for the electron transition from the valence band to the intermediate band exceeds 2×104 cm-1, suitable for thin-film PV device applications. The ZnTeO IBSC exhibits an enhanced spectral response below the band edge of ZnTe, and all results are consistent with the proposed conversion mechanism of IBSC.

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

Effects of ultra-low Al alloying In(Al)As layer on the formation and evolution of InAs/GaAs quantum dots

We have introduced ultra-low Al composition at the two-dimensional to three-dimensional transition stage of InAs/GaAs quantum dots (QDs) formation. Two main effects of AlAs on the QDs are revealed: one is to lower the nucleation barrier so as to reduce the critical nucleation thickness of QDs, which is demonstrated by a surface kinetic nucleation model. The other is to facilitate the In atoms migration from wetting layer (WL) to QDs, which holds some signatures such as both increased QDs density and size with increasing AlAs composition, as well as the peak energy red-shift of photoluminescence spectra. The enhanced In atoms migration from WL to QDs is further confirmed via photo-modulated reflectance experiments and energy band calculation, which both demonstrate the reduction of effective WL thickness after AlAs insertion. The observed effects of AlAs on QDs formation and growth evolution could be explained by the Al-alloying effects of InAs wetting layer.

Optical identification of electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure

We have studied the electronic state levels of an asymmetric InAs/InGaAs/GaAs dot-in-well structure, i.e., with an In0.15Ga0.85As quantum well (QW) as capping layer above InAs quantum dots (QDs), via temperature-dependent photoluminescence, photo-modulated reflectance, and rapid thermal annealing (RTA) treatments. It is shown that the carrier transfer via wetting layer (WL) is impeded according to the results of temperature dependent peak energy and line width variation of both the ground states (GS) and excited states (ES) of QDs. The quenching of integrated intensity is ascribed to the thermal escape of electron from the dots to the complex In0.15Ga0.85As QW + InAs WL structure. Additionally, as the RTA temperature increases, the peak of PL blue shifts and the full width at half maximum shrinks. Especially, the intensity ratio of GS to ES reaches the maximum when the energy difference approaches the energy of one or two LO phonon(s) of InAs bulk material, which could be explained by phonon-enhanced inter-sublevels carrier relaxation in such asymmetric dot-in-well structure.PACS: 73.63.Kv; 73.61.Ey; 78.67.Hc; 81.16.Dn

Nondestructive extraction of junction depths of active doping profiles from photomodulated optical reflectance offset curves

The ITRS Roadmap highlights the electrical characterization of the source and drain extension regions as a key challenge for future complimentary-metal-oxide-semiconductor technology. Presently, an accurate determination of the depth of ultrashallow junctions can routinely only be performed by time-consuming and destructive techniques such as secondary ion mass spectrometry (SIMS). In this work, the authors propose to use the fast and nondestructive photomodulated optical reflectance (PMOR) technique , as implemented in the Therma-Probe® (TP) dopant metrology system, for these purposes. PMOR is a pump-probe technique based on the measurement of the pump-induced modulated change in probe reflectance, i.e., the so-called (photo) modulated reflectance. In this article, the authors demonstrate that the absolute junction depths of boxlike active dopant structures can be extracted in a very simple and straightforward way from the TP offset curves, which represent the behavior of the modulated reflectance as a function of the pump-probe beam spacing. Although the procedure is based on the insights into the physical behavior of the offset curves, no modeling is involved in the actual extraction process itself. The extracted junction depths are in good correlation with the corresponding junction depths as measured by means of SIMS. The technique has a subnanometer depth sensitivity for depths ranging from 10to35nm with the present Therma-Probe® 630XP system. The extension of the proposed procedure to the general ultrashallow profiles is also explored and discussed.

Impact of inactive dopants in chemical vapor deposition layers on photomodulated optical reflectance

The ITRS roadmap stresses the electrical characterization of the active doping profiles for sub-32 nm structures as a key challenge for future CMOS technology. Earlier work has shown the promising capabilities of non-destructive photomodulated optical reflectance (PMOR) techniques, based on the localized detection of variations in the reflectivity of the sample, due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as the Therma-Probe ® (TP) system. In this work, we study the impact of the inactive dopant atoms in chemical vapor deposition (CVD) layers both on PMOR and DC reflectance signals. In particular, with the help of numerical simulations, this work shows that, by taking into account the combined degradation of the thermal diffusivity and the recombination lifetime as well as the enhancement of the absorption coefficient, the theory may be rendered semi-quantitative.

Determination of the fundamental and spin-orbit-splitting band gap energies of InAsSb-based ternary and pentenary alloys using mid-infrared photoreflectance

In narrow-gap semiconductors, non-radiative Auger recombination losses (via conduction, heavy-hole, spin split-off hole, heavy-hole processes) are sensitive to how the band gap energy, E 0, compares to the spin-orbit splitting, Δ 0. However, narrow-gap band structures have often not been very well characterised. We report on the mid-infrared photo-modulated reflectance of E 0 and E 0 + Δ 0 transitions in high-quality, InAs-rich InAsSb and GaInAsPSb samples as functions of both temperature ( T = 10–300 K) and antimony content ≤ 22.5%, for wavelengths up to ~ 4.75 μm. The measured T-dependence of E 0 is generally consistent with that accepted in the literature and we confirm that Δ 0 is T-independent. As a function of Sb fraction, E 0 is consistent with the positive bowing parameter of + 670 meV quoted in the literature. However, Δ 0 does not exhibit the currently accepted positive bowing parameter of + 1170 meV: rather, a best fit to our data tentatively suggests a negative bowing of ~ − 165 meV. Due to the importance of Δ 0 in predicting and interpreting the performance of InAsSb-based devices, this result is expected to have a significant impact.

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.

Advances in optical carrier profiling through high-frequency modulated optical reflectance

As indicated by the ITRS roadmap, obtaining accurate information on the electrically active dopant profile for sub-30-nm structures is a key issue. Presently, however, there is no conventional, probe-based (destructive) technique available satisfying the ITRS targeted depth (3%) and carrier level (5%−10%) reproducibility and accuracy. In this work, the authors explore the promising capabilities of nondestructive photomodulated optical reflectance (PMOR) techniques, based on the localized (micrometer beam size) detection of variations in the reflectivity of the sample, due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as the Therma-Probe® (TP) system. Earlier and more recent work using low modulation (1 kHz) frequencies has shown that it is possible, but rather tedious, to extract the electric junction depth (at about 1018 cm−3) and carrier concentration of chemical vapor deposition grown (CVD) (boxlike) structures based on so-called power curves (where the reflected power of the probe laser is plotted versus the power of the pump laser). In this work the authors focus on high-frequency (1 MHz) PMOR, which gives two (instead of one) independent signals, i.e., the amplitude (A) and phase angle (ϕ) of the reflected probe beam. It has been proposed earlier and is confirmed in this work that a single simple measurement allows for the direct and easy extraction of the junction depth (Xj) and carrier concentration (N) of boxlike profiles. Furthermore, the shape of the so-called three-dimensional PMOR offset curves (A and ϕ versus offset), where the distance of the pump relative to the probe beam is varied over several micrometers, might help to obtain information on more complex profiles. The principles allowing for the extraction of arbitrary carrier profiles, with nanometer depth resolution and carrier concentrations between 1018 and 1021 cm−3, from offset curves will be discussed and evidence for the proposed ideas will be given for homogeneously doped material and CVD boxlike structures based on FSEM device simulations.

Theoretical and experimental study of photo-modulated reflectivity detections for transparent film/opaque substrate structures

A photo-modulated reflectivity method is used to evaluate thermal properties of bilayered structures composed of transparent films deposited on opaque substrates. In order to calculate the photo-modulated reflectivity variations of the bilayered structures induced by focused modulated laser irradiations, a 3-D theoretical model is presented. Due to the complexity of the 3-D theory, a finite element method is used to simulate the laser-induced temperature fields, thermoelastic displacement fields and dielectric constant variations, and finally the photo-modulated reflectivities. Then a quantitative investigation of the thermal properties of the transparent films deposited on opaque substrates can be performed by the experimental measurements and theoretical simulations. As examples, the thermal conductivities of ZnO and diamond films deposited on silicon substrates are evaluated and discussed.

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)

Photoreflectance determination of the wetting layer thickness in the InxGa1−xAs∕GaAs quantum dot system for a broad indium content range of 0.3–1

We have investigated a set of InxGa1−xAs∕GaAs quantum dot structures grown by solid source molecular beam epitaxy for a wide range of In content ranging from 30% to pure InAs∕GaAs dots. It is well known that in a self-assembled growth mode, the dots are formed on a thin layer of the InxGa1−xAs material, which is called wetting layer (WL). The WL thickness is driven by the strain, i.e., lattice mismatch between the layer and substrate materials. Usually, the WL quantum well is not optically active in emission type of experiments (the whole radiative recombination goes through the dot states) and even if so, the heavy hole ground state transition is probed only. In order to detect all the possible transitions, e.g., transitions related to light hole and possible higher order heavy hole states (including those transitions, which are nominally parity forbidden), we have used modulation spectroscopy in a form of photomodulated reflectivity measurements. This is an absorptionlike method, which has been proven to be highly sensitive to even very low intensity transitions in low-dimensional semiconductor structures. The aim of this work is to determine in a contactless optical manner the thickness of the wetting layer as a function of indium content starting with very low content and low-strain structures (at the limit of self-assembled dot creation) up to typical InAs dots on gallium arsenide with the effective wetting layer as thin as 1.5 ML. The observed optical transitions have been identified based on energy level calculations for thin rectangular wells and using effective mass approximation in an envelope function approach in which the well width has been treated as a fitting parameter to the experimental data.

Three-dimensional analyses of photo-modulated reflectivity for transparent film/opaque substrate structures

For a structure consisting of a transparent film deposited on an opaque substrate radiated by a modulated focused laser beam, a 3-D model is established to calculate the temperature distributions. Then based on 3-D thermoelastic displacements and dielectric constants depended on the temperature of the bilayer structure, the photo-modulated reflectivity of the structure is calculated by using FEM method. As an example, the modulated reflectance variations with the modulation frequency of the optical beam and the thermal conductivity of the film in ZnO/Si bilayer structures are also obtained.

Towards nondestructive carrier depth profiling

As indicated by the International Technology Roadmap for Semiconductors (ITRS) (http://public.itrs.net/), obtaining accurate information on the electrically active dopant profile for sub-30nm structures is a key issue. Presently, however, there is no conventional destructive (probe based) technique available satisfying the ITRS targeted depth (3%) and carrier level (5%–10%) reproducibility and accuracy. In this work, we explore the promising capabilities of a nondestructive photomodulated reflectance technique, based on the local detection of variations in the reflectivity of the sample due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as in the therma-probe (TP) system. Experimental data obtained on chemical vapor deposition grown layers will be discussed with respect to junction depth and active dopant (carrier) level sensitivity. A methodology to decouple the junction depth and peak carrier concentration information for unknown samples based on the conventionally available TP signals will be presented. Also simulations and theoretical insights relating to the methodology will be discussed.

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.

Characterization of vertical‐cavity surface‐emitting laser structures by modulation spectroscopy: A status report

AbstractThe present issue of physica status solidi (a) contains contributions from the International Workshop on Modulation Spectroscopy of Semiconductor Structures (MS3), held in Wrocław, Poland, 1–3 July 2004.Editor's Choice is the article by P. J. Klar et al. [1] in which the advantages of these techniques are used to characterize optoelectronic devices. For a vertical‐cavity surface‐emitting laser structure (shown schematically in the lower part) to operate, the emission wavelength λqw of its active region and the cavity mode at λcav of its resonator structure need to coincide. The photomodulated reflectance spectrum (upper left part, bottom) shows clear features at both wavelengths whereas a feature at λqw cannot be distinguished in the corresponding reflectance spectrum (top) due to its complicated photonic nature.The first author, Peter J. Klar is currently a lecturer of Physics at the Philipps‐University of Marburg, Germany. His research interests include the physics of magnetic semiconductors and hybrids for spintronics, of III–N–V semiconductor structures for optoelectronics, and of novel nanostructures ranging from nanomagnetism to applications in catalysis.

Spectroscopic investigations of GaAsSb/GaAs based structures for 1.3 μm VCSEL applications

A spectroscopic investigation of GaAsSb/GaAs quantum well (QW) structures grown on GaAs substrates by molecular beam epitaxy is presented. Besides studying their temperature-dependent photoluminescence (PL), the low temperature PL of a series of GaAsSb/GaAs/AlGaAs structures, with varying GaAs spacer thickness is investigated. This latter study was undertaken to investigate the GaAsSb/GaAs band alignment. Blue shifts in the PL peak as a function of excitation laser intensity at a temperature of 10 K for the range of spacer thickness variation are studied. It is observed that significant blue shifting occurs only for spacers thicker than /spl sim/2 nm. It is tentatively suggested that this is indicative of a transition from the electrons and holes being predominantly confined in the same layer (the QW) to being more strongly confined in adjacent layers, as the spacer thickness increases. The angle-dependent photomodulated reflectivity (PR) of similar samples is investigated. Here strong low energy interference oscillations (LEIO) are encountered, which tend to obscure any PR signals arising from the QW. The latter are exploited to estimate the refractive index of the layer responsible for the LEIO, and thus identify it. However, a way to avoid the LEIO is shown, by shortening the laser excitation wavelength, which results in measurable PR signals from the QW region, yielding several QW transition wavelengths.