Suparna Pal

A comparative study on nanotextured high density Mg-doped and undoped GaN

Nanotextured high density Mg-doped and undoped GaN were obtained using photoelectrochemical etching. Interesting features are observed in the temperature dependent photoluminescence (PL) studies of these nanotextured materials. First, the PL intensity of the excitonic emissions shows more than three orders of enhancement. At low temperature, the peak energy shows a blueshift with temperature. This phenomenon is attributed to the formation of excitonic band-tail states. Second, the excitonic emissions in the nanotextured samples are redshifted compared to the as-grown GaN suggesting strain relaxation. Third, the blue luminescence band (2.7–2.9eV in Mg-doped GaN) shows a large redshift, which is not consistent with strain relaxation calculated from excitonic band. Furthermore, temperature dependence of the blue luminescence band energy shows an asymmetric S-shaped behavior in nanotextured GaN. All these observations are explained by invoking an increase in carrier localization due to an increase in potentia...

Time evolution studies of laser induced chemical changes in InAs nanowire using Raman spectroscopy

We report the study of time evolution of chemical changes on the surface of an InAs nanowire (NW) on laser irradiation in different power density regime, using Raman spectroscopy for a time span of 8–16 min. Mixture of metastable oxides like InAsO4, As2O3 are formed upon oxidation, which are reflected as sharp Raman peaks at ∼240–254 and 180–200 cm−1. Evidence of removal of arsenic layer by layer is also observed at higher power density. Position controlled laser induced chemical modification on a nanometer scale, without changing the core of the NW, can be useful for NW based device fabrication.

Quantum beats from the coherent interaction of hole states with surface state in near-surface quantum well

We report tunneling assisted beating of carriers in a near-surface single GaAsP/AlGaAs quantum well using transient reflectivity measurement. The observed damped oscillating signal has a period of 120 ± 6 fs which corresponds to the energy difference between lh1 and hh2 hole states in the quantum well. Comparing the transient reflectivity signal at different photon energies and with a buried quantum well sample, we show that the beating is caused by the coherent coupling between surface state and the hole states (lh1 and hh2) in the near-surface quantum well. The dependence of decay of coherence of these tunneling carriers on the excitation fluence is also reported. This observation on the coherent tunneling of carrier is important for future quantum device applications.

Effect of light-hole tunnelling on the excitonic properties of GaAsP/AlGaAs near-surface quantum wells

Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ~500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

Intersubband plasmon-phonon coupling in GaAsP/AlGaAs near surface quantum well

The investigation of electron-phonon coupling in near surface GaAs1−xPx/AlyGa1−yAs quantum well structures using wavelength and intensity dependent Raman spectroscopy shows that in the near surface quantum well case, coupled modes are situated at the frequency between longitudinal optical (LO) and transverse optical phonons of GaAsP, which is forbidden for the coupled electron-phonon modes in the bulk ternary alloy. The observed “GaAs like” and “GaP like” LO phonon-intersubband plasmon coupled mode frequencies decrease with increase in carrier density. These results corroborate well with the theoretical calculation for variation of two dimensional electron gas-phonon coupling with carrier density in ternary alloys.

Intersubband Plasmon ‐ Phonon Coupling in GaAsP/AlGaAs Single Quantum Well: A Raman Spectroscopy Study

A Raman scattering study of GaAs1−yPy (y = 0.14)/AlxGa1−xAs (x = 0.7) single quantum wells has shown two mode behavior for both barrier and quantum well alloys. In this report we present investigation of origin of an additional mode ∼284 cm−1. From wavelength and intensity dependent Raman measurements, it was concluded that the mode is intersubband plasmom‐phonon coupled mode due to photoexcited carriers. The coupled mode frequency dependence on free carrier density as predicted by theory is qualitatively in agreement with observed coupled mode frequencies in intensity dependent Raman measurements.

Self-catalyst assisted and catalyst-free epitaxial growth of InAs on Ge (111): Role of substrate surface and evolution of polytypism

In the present work, the authors report on the role of a self-catalyst (indium) and a substrate surface in various growth mechanisms and the crystalline structure of InAs nucleating layers on Ge (111) using metal organic vapor phase epitaxy. This study opens up the possibility of direct integration of InAs with Ge (111) substrates for next generation optoelectronic and microelectronic device applications. Different nucleation modes like droplet-epitaxy (DE), Volmer–Weber (VW), and Frank–Van der Merwe (FM) modes are observed in various growth windows. DE and FM modes are the dominating nucleation modes at lower growth temperature (400–470 °C) with the assistance of indium catalysts, while the VW mode dominates at higher growth temperature (470–540 °C). Three dimensional faceted structures are formed with varying densities in the overall growth temperature range of 400–540 °C, which is attributed to various factors like surface strain of the indium adatom covered Ge (111) surface, indium surface diffusion, and twin defect formation. High resolution x-ray diffraction identifies the twin formation at low growth temperature. Raman mapping reveals that InAs nano-/microstructures grow with a zinc blende-wurtzite polytypic crystal structure at higher growth temperature and arsenic-rich conditions. A comparison is made on the growth modes, morphology, and polytypism in InAs nanostructures, which are grown on Ge (111) and Si (111) substrates under identical conditions. Low surface diffusivity of indium on the Ge substrate and InAs/Ge interface energy are proposed as the main factors for the inhibition of anisotropic axial growth of InAs on Ge via the vapor-liquid-solid mode, while they succeed on the Si substrate.In the present work, the authors report on the role of a self-catalyst (indium) and a substrate surface in various growth mechanisms and the crystalline structure of InAs nucleating layers on Ge (111) using metal organic vapor phase epitaxy. This study opens up the possibility of direct integration of InAs with Ge (111) substrates for next generation optoelectronic and microelectronic device applications. Different nucleation modes like droplet-epitaxy (DE), Volmer–Weber (VW), and Frank–Van der Merwe (FM) modes are observed in various growth windows. DE and FM modes are the dominating nucleation modes at lower growth temperature (400–470 °C) with the assistance of indium catalysts, while the VW mode dominates at higher growth temperature (470–540 °C). Three dimensional faceted structures are formed with varying densities in the overall growth temperature range of 400–540 °C, which is attributed to various factors like surface strain of the indium adatom covered Ge (111) surface, indium surface diffusion, ...

MODULATIONS IN LOW-TEMPERATURE TRANSIENT REFLECTIVITY MEASUREMENTS

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/AlGaAs single quantum well is studied. Similar anomalous oscillations are also reported in layered manganite [K. Kouyama et al., J. Phys. Soc. Jpn.76 (2007) 123702(1–3).] We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studies carried out on reflectivity of different materials under identical conditions show that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

Blue shift in GaAs0.86P0.14/Al0.7Ga0.3As Near Surface Quantum Well

Large blue shift is observed in a near surface GaAs0.86P0.14/Al0.7Ga0.3As quantum well with the reduction in top barrier thickness. Very slow etching with simultaneous surface passivation of the quantum well is obtained by treating the sample with Na2S. xH2O solution. Photoreflectance (PR) spectra exhibit maximum blue‐shift of 28 meV after treating the quantum well surface with Na2S.xH2O solution for 30 minutes (top layer thickness reduced to 10 A). The blue‐shift is attributed to increase in confinement and /or image charge effect due to penetration of the wave function into vacuum. The blue‐shift is accompanied by significant reduction in the broadening parameter of the observed lowest transition in PR spectra indicating effective passivation along with increase in confinement.

Comparative studies on As-grown and nanotextured GaN:Mg epilayer

Nanotextured high-density Mg-doped GaN were obtained using photoelectrochemical etching. The blue luminescence bands (2.7 - 2.9 eV) of the as-grown and nanotextured samples show a U-shaped intensity variation with temperature in the range of 10 - 150 K. The variation in intensity of observed three peaks are analysed in the light of variation of their peak position and widths as a function of temperature. The origins of these features are discussed in this paper.