H. R. Chandrasekhar

Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.

Temperature dependent photoluminescence of organic semiconductors with varying backbone conformation

We present photoluminescence studies as a function of temperature from a series of conjugated polymers and a conjugated molecule with distinctly different backbone conformations. The organic materials investigated here are: planar methylated ladder type poly para-phenylene, semi-planar polyfluorene, and non-planar para hexaphenyl. In the longer-chain polymers the photoluminescence transition energies blue shift with increasing temperatures. The conjugated molecules, on the other hand, red shift their transition energies with increasing temperatures. Empirical models that explain the temperature dependence of the band gap energies in inorganic semiconductors can be extended to explain the temperature dependence of the transition energies in conjugated molecules.

Phonons in titanium doped vitreous silica

Abstract In this paper we report the infrared and Raman studies of titanium doped silicate glasses for a number of Ti concentrations. New modes associated with the titanium groups are identified. The observed shifts and broadenings in the spectra of vitreous silica are discussed in the context of random network models. The “anomalous” decrease in density with the addition of titanium is attributed to an increase in the average intertetrahedral angle of the SiO 4 network.

Phonons in TiO2SiO2 glasses

We report the infrared and Raman studies of titanium doped vitreous silica glasses for a number of titanium concentrations. The vibrational modes associated with the randomly oriented chains of SiO4 tetrahedra show broadenings and shifts. The LO-TO splitting of some Raman active modes decreases with increasing titanium concentration. This is attributed to the decrease in long-range coulomb fields associated with the chains of SiO4 tetrahedra which are broken by the titanium atoms. The results are discussed in the context of random network models. An increase in the average intertetrahedral angle of the SiO4 network is calculated from the data. This explains the anomalous decrease in the density of TiO2SiO2 glasses with increasing titanium content. We identify two new modes associated with the distorted titanium tetrahedra. A polarized Raman mode at 1115 cm−1 which is infrared inactive and an unpolarized Raman mode at 945 cm−1 which is infrared active are observed.

Optical studies of strained pseudomorphic semiconductor heterostructures under external pressure

Abstract Epilayers and quantum well heterostructures of group III-V and II-VI binary and ternary compounds offer an exciting array of band gaps and physical properties. The band gaps range from the infrared to the near ultraviolet region (0.1 to 3 eV); continuous tuning of the band gap is achieved by means of alloying, strain and quantum confinement. Pseudomorphic heterostructures with large lattice mismatch and minimal dislocations and defects at the interface have been successfully grown by various techniques. This paper reviews the effects of strain on the optical and electronic properties, determination of strain, tuning of strain, and the structural stability of these metastable structures. Photoreflectance, photoluminescence and Raman scattering studies as a function of temperature and externally applied hydrostatic pressure are discussed. Owing to the different compressibilities and thermal expansion coefficients of the constituent members of a heterostructure, the strain can be continuously tuned ...

Electronic transitions in semiconductor quantum wells and epilayers under pressure

Abstract We review the effects of hydrostatic pressure on the confined transitions in quantum well heterostructures, using lattice matched GaAs/AlxGa1-x As, strained layer narrow band gap GaSb/AlSb and InXGa1-xAs/GaAs, and strained layer wide gap Zn1-xCdxSe/ZnSe as examples. We address the problem of shear in lattice mismatched structures by examining a pseudomorphic epilayer, ZnSe/GaAs, where we tune the lattice mismatch with pressure in a configuration where the effects of quantum confinement are absent.

Temperature dependence of strain in ZnSe(epilayer)/GaAs(epilayer)

A study of biaxial strain as a function of temperature in a ZnSe epilayer grown on a GaAs substrate is presented. The strains are determined by measuring the heavy‐ and light‐hole related excitonic transitions via photomodulated spectroscopy. The strain is found to increase with increasing temperature. The data are compared with a calculation using a previously determined elastic constant and thermal expansion coefficients. The temperature dependence determined here allows a comparison of various other optical measurements performed at different temperatures.

Pressure-induced shifts of the fluorescence spectrum of rhodamine 6G in solution

The effect of hydrostatic pressure on the fluorescence spectrum of rhodamine 6G dye in two different solutions is studied. The peak shifts to longer wavelengths with increasing pressure with a pressure coefficient of -29 and -19 cm(-1)/kbar for ethanol and 4:1 methanol-ethanol solvents, respectively. Possible applications of increasing the tunability of dye lasers by pressure are discussed.

Photoreflectance spectroscopy of GaAsAlxGa1−xAs quantum wells under hydrostatic pressure

Abstract We report the effect of hydrostatic pressure on the photoreflectance spectra derived from the Γ, X and L bands of a GaAsAl x Ga 1−x As heterostructure. The pressure dependence of the quantum well transitions and the valence band-offsets are accurately determined.

Pressure tuning of strain in CdTe/InSb epilayer: a photoluminescence and photomodulated reflectivity study

The heavy‐hole and light‐hole excitons of a CdTe epilayer, pseudomorphically grown on an InSb epilayer by molecular beam epitaxy, are studied with a diamond anvil cell as a function of applied hydrostatic pressure via photoluminescence (PL) and photomodulated reflectivity (PR) spectroscopies. They are compared with the excitonic features in the simultaneously measured PL spectra of a sample of bulk CdTe. Under applied pressure, the lattice mismatch‐induced splitting between the light‐hole and heavy‐hole related transitions increases in a continuous and reversible manner because of the additional pressure‐induced compression due to the difference in the compressibilities of CdTe and InSb. The unusually large strain sustained by the CdTe epilayer under pressure is discussed in the light of various models. The PR signal vanishes after the InSb epilayer goes through a structural phase transition at approximately 20 kbar, while the PL signal persists until it is irreversibly quenched by the CdTe epilayer under...