IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Optical properties and interdot transfer dynamics of trapped carriers in InGaAs quantum dots (QDs) are investigated. Time resolved photoluminescence (PL) was measured for time-delayed interband and intraband excitations. Terahertz activated luminescence (TAL) from trapped carriers having lifetimes of ∼250 ns at 8 K, was observed. Spectral shift of the TAL with respect to the PL showed the trionic nature of the PL in the n-doped QDs. With increasing terahertz excitation intensity, the TAL increased and reached saturation. The activation energy associated with the trapped carrier decay was quite close to the intersublevel transition energy (∼20 meV) indicating trapping in the QDs. © 2010 American Institute of Physics.

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Applied Physics Letters

Terahertz activated luminescence of trapped carriers in InGaAs/GaAs quantum dots

The authors present results of a perturbation theory study of the effect of strain on the oscillator strengths of interband transitions in wurtzite group-III-nitride films suitable for ultraviolet light emission applications. Ternary alloy films are investigated, which can be pseudomorphically grown on GaN and AlN substrates with nonpolar M-plane (1 100) and A-plane (1120) orientations. Valence band mixing, induced by the anisotropic in-plane strain that arises in these films, can have a dramatic influence on the optical polarization properties of the transitions. An increased efficiency of light emission in the 0.21 μm to 0.25 μm spectral range is best achieved using AlN substrates, on which both Al1-xGaxN and Al1-xInxN films experience compressive strain. On GaN substrates, Al1-xGaxN films experience tensile strain and will exhibit poor light emission efficiency. However Al1-xInxN films on GaN substrates can emit light efficiently in the 0.3 μm to 0.36 μm spectral range. © 2009 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

Physica Status Solidi (B) Basic Research

Are AlN and GaN substrates useful for the growth of non-polar nitride films for UV emission? The oscillator strength perspective

The authors report the observation of strongly polarization-sensitive resonances in the electroreflectance (ER) spectra of multilayered semiconductor structures. The measurements were done on vertical-cavity surface-emitting laser structures grown on GaAs(001) substrates. The ER spectral features undergo a 180° phase shift as the polarization vector E of the probe beam is rotated by 90° from E∥[1 1̄ 0] to E∥[110] in the (001) plane. Through experiments and simulations, these resonances are shown to arise due to the linear electro-optic effect (LEOE). The influence of LEOE at energies below the band gap becomes prominent mainly because of optical-interference induced enhancement of the Seraphin coefficients in a multilayered structure. Implications for the analysis of ER spectral lineshapes of multilayered semiconductor structures are highlighted. © 2008 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Electroreflectance lineshapes in multilayered semiconductor structures: Influence of the linear electro-optic effect

The authors present results of a perturbation theory study of the combined effects of composition and anisotropic in-plane strain on the optical polarization properties of the three interband transitions in the vicinity of the fundamental energy gap of wurtzite group-III-nitride alloy films, pseudomorphically grown on GaN substrates with nonpolar orientation such as M -plane GaN (1 1- 00). Valence band mixing induced by the anisotropic in-plane strain is shown to have a dramatic influence on the optical polarization properties. The results indicate that an increased efficiency of light emission in the visible spectral range can be achieved with compressively strained Inx Ga1-x N active layers. While Alx Ga1-x N layers under tensile strain will exhibit a very poor light emission efficiency in the ultraviolet (UV) spectral range, efficient emission in the UV range can instead be achieved with Inx Al1-x N films. These results also hold for alloy films on A -plane GaN (11 2- 0) substrates. © 2008 American Institute of Physics.

Optical polarization properties of interband transitions in strained group-III-nitride alloy films on GaN substrates with nonpolar orientation

The authors report room temperature reflectance measurements on wurtzite C-plane GaN films grown on sapphire substrates. The spectra clearly show exciton related features near the fundamental bandgap of GaN. The Lorentz oscillator model was used to analyze the excitonic contribution to the spectral lineshape and to determine the transition energies. The results are explained by comparison with electronic band structure calculations which include the effect of strain. The analysis gives an independent estimate of the residual biaxial strain in the films. © 2007 IEEE.

Proceedings of the 14th International Workshop on the Physics of Semiconductor Devices, IWPSD

Reflectance spectroscopy study of epitaxial GaN films at room temperature

We present results of a theoretical study of the electronic band structure of wurtzite InN films under biaxial strain in the C-plane (0001) and in planes that correspond to non-polar orientations such as the A-plane (112̄0) and the M-plane (11̄00). The calculations are performed under the k · p perturbation theory approach using the Bir-Pikus Hamiltonian. The results show that the fundamental bandgap of InN shifts by 30 meV (14 meV) for isotropic tensile (compressive) strain in the C-plane with out-of-plane contraction (dilation) of 0.2%. For films of non-polar orientations, the c-axis lies in the film plane and the strain is expected to be different between directions parallel and perpendicular to c. Such anisotropic strain give rise to valence band mixing which results in dramatic changes in the optical polarization properties as evidenced by the calculated oscillator strengths of the interband transitions. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Physica Status Solidi (A) Applications and Materials Science

Electronic band structure of wurtzite InN around the fundamental gap in the presence of biaxial strain

The authors investigate the in-plane optical polarization properties of [1 1- 00] -oriented (M -plane) GaN films on γ-LiAl O2 (100) substrates by transmittance anisotropy spectroscopy (TAS). This technique is sensitive to the difference in the transmittance between light polarized parallel and perpendicular to the c axis of GaN, which for M -plane GaN lies in the film plane. The TAS spectrum exhibits a clear resonance in the vicinity of the fundamental bandgap. Simulations demonstrate that this resonance directly reflects the polarization-dependent shift of the bandgap. The zero crossings of the differential TAS spectrum are shown to be a measure for the polarization-dependent transition energies. © 2007 American Institute of Physics.

Optical polarization properties of M-plane GaN films investigated by transmittance anisotropy spectroscopy

The authors report the observation of strong polarization anisotropy in the photoluminescence (PL) and the absorption spectra of [11 2- 0] oriented A -plane wurtzite InN films grown on R -plane (1 1- 02) sapphire substrates using molecular beam epitaxy. For A -plane films the c axis lies in the film plane. The PL signal collected along [11 2- 0] with electric vector E⊥c is more than three times larger than for E∥c. Both PL signals peak around 0.67 eV at 10 K. The absorption edge for E∥c is shifted to higher energy by 20 meV relative to E⊥c. Optical polarization anisotropy in wurtzite nitrides originates from their valence band structure which can be significantly modified by strain in the film. The authors explain the observed polarization anisotropy by comparison with electronic band structure calculations that take into account anisotropic in-plane strain in the films. The results suggest that wurtzite InN has a narrow band gap close to 0.7 eV at 10 K. © 2006 American Institute of Physics.

Polarized photoluminescence and absorption in A-plane InN films

We have studied self-assembled InAs quantum dot (QD) ensembles on GaAs(001) substrate using polarized photovoltage spectroscopy. The photovoltage spectrum shows four prominent QD related features whose nature differs for probe light incident along [001] and polarized parallel to [110] and [1 1- 0] directions. The polarization anisotropy suggests that for the lowest energy transition there is only a change in the oscillator strength with change in polarization, while for the higher energy transitions there is also an apparent shift in the transition energy. By comparison with simulations of the absorption spectrum, we show that the main features of the observed polarization anisotropy can be understood on the basis of a model where an anisotropic two dimensional harmonic oscillator potential represents the lateral confinement of the carriers within the QD in the (001) plane. © 2005 American Institute of Physics.

Polarized photovoltage spectroscopy study of InAs/GaAs (001) quantum dot ensembles

We describe two mechanical light chopper blade designs that can be used, with a monochromator and broadband light-source-based spectroscopy setup, to perform wavelength modulation spectroscopy. The left and the right half of a beam emerging from a monochromator would have spectral distributions that are skewed to longer and shorter wavelengths about the central wavelength for positive grating orders. Our two designs, the alternating double-slot blade and the vertical right-angled blade, allow switching between the two halves of the beam front to generate a small periodic variation in the wavelength distribution, enabling wavelength modulation spectroscopy. In contrast to existing methods of wavelength modulation, this technique does not require modification of the monochromator or the light source unit. We analyze the signal waveforms and show how, and under what conditions, wavelength modulation data can be extracted from the alternating signal components measured by the detector. To demonstrate their functioning and applicability, we have performed wavelength-modulated reflectance spectroscopy on epitaxial semiconductor samples using these chopper blades. © 2005 American Institute of Physics.