Photomodulated Reflectance Measurements Research Articles

Monitoring of Dose Dependent Damage in MeV Energy Hydrogen Implanted Silicon by Photo-Modulated Reflectance Measurements

High-energy low-mass proton implantation achieved considerable interest in semiconductor technology, due to much deeper penetration of hydrogen ions into silicon as compared to common dopants, boron, phosphorous, and arsenic. Accordingly, monitoring the accumulation kinetics and stability of proton-implantation induced defects and their influence on the optical and electrical properties of Si achieved increased attention both in technological process control and scientific research. We show that photo-modulated-reflectance (PMR) is effective technique to measure very low defect concentrations in the ppb-ppm range in high energy proton implanted silicon. After ion irradiation, the as-implanted dilute damage structure may lead to long term changes of the defect distribution and the formation of defect compounds due to mobility of point defects at room temperature. Moreover, low-mass hydrogen atoms may move significantly faster at room temperature compared to heavier dopants. We show that PMR is capable to detect differences in implanted proton dose with high sensitivity in a wide dose range, and, on a longer time scale, allows to follow changes in free carrier generation and recombination processes through the measurement of the long term decay of the PMR signal which is related to the sample response based on electro-optic and thermo-optic effects. Our experiments may pave the way toward high precision process control of device structure fabrication which utilizes the high-energy hydrogen implantation step. Also, our aim is to gain insight into the main processes underlying the dose dependent change and long-term decay of the PMR signal in high energy proton implanted Si.

Investigating the physics of higher-order optical transitions in InAs/GaSb superlattices

We present an optical spectroscopy analysis of the molecular beam epitaxy-grown InAs/GaSb quantum systems to study the higher-order optical transitions in InAs/GaSb superlattices (SLs). We have investigated two type-II SL structures with different layer widths: $\mathrm{In}{\mathrm{As}}_{8\mathrm{ML}}/\mathrm{Ga}{\mathrm{Sb}}_{8\mathrm{ML}}$ and $\mathrm{In}{\mathrm{As}}_{10\mathrm{ML}}/\mathrm{Ga}{\mathrm{Sb}}_{10\mathrm{ML}}$. To experimentally determine the spectral distribution of the higher-order transitions, the Fourier-transformed photoluminescence and Fourier-transformed photomodulated reflectance measurements were used. These measurements were undertaken vs temperature and incident polarization of the probing beam. Complementary theoretical calculations based on the eight $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ formalism were also performed to identify the electron and heavy and light hole energy band ladder. Polarization-dependent measurements indicated the interchanging nature of the light and heavy hole states for the second and third valence band vs the SL wave vector $({k}_{z})$ along the growth direction. The results were compared with other calculations based on different models and existing experimental results, then summarized and reviewed for possible numerical parameters application ranges in the interface modeling.

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.

Impact of alloy disorder on the band structure of compressively strained GaBixAs1−x

The incorporation of bismuth (Bi) in GaAs results in a large reduction of the band gap energy (E$_g$) accompanied with a large increase in the spin-orbit splitting energy ($\bigtriangleup_{SO}$), leading to the condition that $\bigtriangleup_{SO} > E_g$ which is anticipated to reduce so-called CHSH Auger recombination losses whereby the energy and momentum of a recombining electron-hole pair is given to a second hole which is excited into the spin-orbit band. We theoretically investigate the electronic structure of experimentally grown GaBi$_x$As$_{1-x}$ samples on (100) GaAs substrates by directly comparing our data with room temperature photo-modulated reflectance (PR) measurements. Our atomistic theoretical calculations, in agreement with the PR measurements, confirm that E$_g$ is equal to $\bigtriangleup_{SO}$ for $\textit{x} \approx$ 9$%$. We then theoretically probe the inhomogeneous broadening of the interband transition energies as a function of the alloy disorder. The broadening associated with spin-split-off transitions arises from conventional alloy effects, while the behaviour of the heavy-hole transitions can be well described using a valence band-anticrossing model. We show that for the samples containing 8.5% and 10.4% Bi the difficulty in identifying a clear light-hole-related transition energy from the measured PR data is due to the significant broadening of the host matrix light-hole states as a result of the presence of a large number of Bi resonant states in the same energy range and disorder in the alloy. We further provide quantitative estimates of the impact of supercell size and the assumed random distribution of Bi atoms on the interband transition energies in GaBi$_{x}$As$_{1-x}$. Our calculations support a type-I band alignment at the GaBi$_x$As$_{1-x}$/GaAs interface, consistent with recent experimental findings.

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.

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.

Optical properties of GaNAs and GaInAsN quantum wells

We present an overview of our optical characterization work on dilute nitride quantumwell (QW) samples. A simple model for calculating interband transition energies isconstructed, tested against published results and used to model experimental data. Steady state photoluminescence (PL),time-resolved PL and photomodulated reflectance measurements are utilized to characterizeGaNAs/GaAs, GaInNAs/GaAs and InGaAs/GaAs QWs. The effects of carrierlocalization, hot-carrier relaxation, non-radiative recombination and the reducedbandgap temperature dependence of dilute nitrides are investigated. Emissionfrom recombining hot carriers in a GaInNAs/GaAs QW is recorded and used toestimate the LO-phonon scattering energy. The addition of small fractions of N isfound to have little effect on phonon energy, which is found to be meV.

Narrow bandgap group III‐nitride alloys

AbstractHigh‐quality wurtzite In‐rich In1−xGaxN (0 ≤ x ≤ 0.5) and In1−yAlyN films (0 ≤ y ≤ 0.25) were grown on sapphire substrates by molecular‐beam epitaxy. Optical absorption, photoluminescence and photomodulated reflectance measurements demonstrate that the fundamental bandgap for InN is only about 0.7 eV. The free electron effective mass is found to vary with free electron concentration, the consequence of a strongly non‐parabolic conduction band caused by the k · p interaction with the valence bands across the narrow bandgap. The bandgap gradually increases with increasing Ga or Al content in In1−xGaxN or In1−yAlyN alloys. The composition dependencies of the bandgaps are well described by bowing parameters of 1.4 eV for In1−xGaxN and 3.0 eV for In1−yAlyN. The direct gaps of the group III‐nitride alloy system cover a very broad spectral range from the near‐infrared in InN to deep‐ultraviolet in AlN. This offers unique opportunities for the use of these alloys in a wide range of optoelectronic and photovoltaic devices. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of theoretical models for interband transitions in dilute nitrides and experimental measurement

We have developed a model for the rapid calculation of interband transitions in dilute nitride quantum well structures. The model assumes parabolic bands with the conduction band modified using the band anticrossing approach. The model is compared to results from k · p models, which while being more accurate and flexible than our model, require extensive computational time. The model predictions are compared to photoluminescence and photomodulated reflectance measurements of interband transitions in a number of GaNAs quantum well samples.

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.

Determining the band-structure of an InGaNAs/GaAs semiconductor laser structure using non-destructive photomodulated reflectance measurements and k· p studies

We describe micro-photomodulated reflectance (PR) measurements on a representative dilute-N InGaNAs/GaAs-based laser device structure designed to emit at 1.3 μm. The quantum well (QW) transition energies obtained from PR are modeled using a realistic 10-band k· p Hamiltonian that includes tight-binding-based energies and coupling parameters for the N-levels. From this we are able to determine accurately the band structure and thus predict some important device properties for this InGaNAs/GaAs-based laser device.

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.

Nondestructive spectroscopic characterisation of visible resonant cavity light emitting diode structures

Photomodulated reflectance (PR), conventional reflectance (R), and edge emission electroluminescence (EL) studies are performed at room temperature on bare as-grown wafers of GaInP/AlGaInP/AlGaAs resonant cavity light emitting diode (RCLED) structures, designed for red emission (650–670 nm). The interaction between the cavity mode (CM) and the quantum well (QW) excitonic transitions is altered by varying the angle of incidence of the probe beam, which shifts the CM wavelength, but not the QW PR features. Both features are clearly observed in the nondestructive, noncontact, PR measurements, at all angles of incidence, although only the CM feature is visible in the R spectra. A higher order QW transition is also observed in the PR. Close to the resonance between the CM and QW transitions, an enhancement of the PR signal is clearly observed. This could provide a sensitive way of qualifying RCLED wafers prior to fabrication, and estimating device yields. To refine the analysis, the PR spectra are also fitted with simplified version of a line-shape model developed in our previous studies on vertical-cavity surface-emitting laser structures. Finally, an edge emission EL method, which does not need direct electrical contacts, is used as an easy way of confirming the QW ground state transition. The application of these techniques to RCLED characterisation and fabrication is discussed.

Arsenic implantation-induced intermixing effects on AlGaAs/GaAs single QW structures

The effects of intermixing Al 0.54Ga 0.46As/GaAs/Al 0.54Ga 0.46As quantum well (QW) enhanced by arsenic ion implantation and subsequent annealing have been investigated by photoluminescence and photo-modulated reflectance measurements. Comparing with as-grown QW, obvious blueshifts of all the transitions were observed. The H 22 transition was found to be much less sensitive to the implantation doses than that of H 11. The experimental results are different from the theoretical results calculated by using the model of error function profile of Al composition. The results are fruitful for understanding the potential profile after intermixing enhanced by arsenic ion implantation, and also for the application of implantation enhanced intermixing effects on devices, such as QW lasers and photodetectors, opto-nonlinear devices, etc..

Modeling and analysis of photomodulated reflectance and double crystal x-ray diffraction measurements of tensilely strained InGaAs/InGaAsP quantum well structures

Room temperature photomodulated reflectance (PR) and double crystal x-ray diffraction (DCXRD) measurements have been performed on a series of tensilely strained InxGa1−xAs multiple quantum well (QW) laser structures, with In0.80Ga0.20As0.43P0.57 barriers, which are lattice-matched to an InP substrate. Seven samples are studied, with nominal QW In composition varying between x=0.533 and 0.316, corresponding to biaxial tensile strains between 0% and 1.5%, respectively. The DCXRD measurements provide accurate information on composition, strain and layer thickness, while the PR yields the energies of both allowed and forbidden critical point interband QW transitions, and how these vary with strain, particularly the transitions between the ground-state conduction band and heavy/light hole valence band levels. A three-band effective mass formalism is used to model the QW transitions and very good agreement with the PR measurements is obtained once excitonic binding energies, and the quantum confined Stark effect are taken into account.

Photomodulated reflectance of tensilely strained InGaAs/InGaAsP quantum well structures

Room-temperature photomodulated reflectance (PR) has been performed on a series of tensilely strained undoped multiple quantum well (QW) structures with barriers, which were lattice matched to an InP substrate. The nominal QW Ga composition varied from x = 0.467 to 0.684 corresponding to tensile strains between zero and 1.5%, respectively. In this range of strain, the QW heavy-hole (HH) and light-hole (LH) ground-state zone-centre excitonic transitions increase in energy, with the HH varying more rapidly, with the result that it moves from energies below that of the LH, in the unstrained wells, to above the LH, at the highest strain. The variation in the LH and HH energies with strain is measured using PR which is able to resolve these two transitions even when they are almost degenerate, which occurs at % tensile strain. The results from the PR measurements are also compared with room-temperature photoluminescence and photovoltage measurements on samples from the same wafers. The experimental results are shown to compare well with theoretical QW calculations based on an effective mass formalism.