Partha S. Dutta

The physics and technology of gallium antimonide: An emerging optoelectronic material

Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 1.55 mu m. The III-V ternaries and quaternaries (AlGaIn)(AsSb) lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices. Consequently, there has been tremendous upthrust in research activities of GaSb-based systems. As a matter of fact, this compound has proved to be an interesting material for both basic and applied research. At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication. This article presents an up to date comprehensive account of research carried out hitherto. It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility. An overview of the lattice, electronic, transport, optical and device related properties is presented. Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed. These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc. Several avenues where further work is required in order to upgrade this III-V compound for optoelectronic devices are listed. It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication

Low temperature deposition of nanocrystalline silicon carbide films by plasma enhanced chemical vapor deposition and their structural and optical characterization

Nanocrystalline silicon carbide (SiC) thin films were deposited by plasma enhanced chemical vapor deposition technique at different deposition temperatures (Td) ranging from 80 to 575 °C and different gas flow ratios (GFRs). While diethylsilane was used as the source for the preparation of SiC films, hydrogen, argon and helium were used as dilution gases in different concentrations. The effects of Td, GFR and dilution gases on the structural and optical properties of these films were investigated using high resolution transmission electron microscope (HRTEM), micro-Raman, Fourier transform infrared (FTIR) and ultraviolet-visible optical absorption techniques. Detailed analysis of the FTIR spectra indicates the onset of formation of SiC nanocrystals embedded in the amorphous matrix of the films deposited at a temperature of 300 °C. The degree of crystallization increases with increasing Td and the crystalline fraction (fc) is 65%±2.2% at 575 °C. The fc is the highest for the films deposited with hydrogen d...

Free-space-optical mobile ad hoc networks: Auto-configurable building blocks

Existence of line of sight (LOS) and alignment between the communicating antennas is one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new FSO node design that uses spherical surfaces covered with transmitter and receiver modules for maintaining optical links even when nodes are in relative motion. The spherical FSO node provides angular diversity in 3-dimensions, and hence provides an LOS at any orientation as long as there are no obstacles in between the communicating nodes. For proof-of-concept, we designed and tested an auto-configurable circuit, integrated with light sources and detectors placed on spherical surfaces. We demonstrated communication between a stationary and a mobile node using these initial prototypes of such FSO structures. We also performed the necessary theoretical analysis to demonstrate scalability of our FSO node designs to longer distances as well as feasibility of denser packaging of transceivers on such nodes.

Building blocks for mobile free-space-optical networks

Existence of line of sight (LOS) and alignment between the communicating antennas are one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new optical antenna design that employs spherical antennas covered with transmitter and receiver modules for maintaining optical links even when antennas are in relative motion. In this paper, for proof-of-concept, we design and test an auto-configurable circuit integrated with light sources and detectors placed on spherical surfaces. We also perform simulation-based analysis of these multi-element FSO modules that can enable mobility and high bandwidth in wireless, particularly indoor, networks. Broader impact of our work is to make FSO communication technology widely applicable in mobile, ad-hoc, and multi-hop wireless networks.

Nature of compensating luminescence centers in Te-diffused and -doped GaSb

Diffusion of tellurium in undoped p‐GaSb has been carried out. Using the cathodoluminescence and photoluminescence techniques, the luminescence centers in Te‐diffused samples have been identified and compared with the Te‐doped bulk GaSb. Fundamental differences in the radiative levels are observed between the diffused and the as‐grown doped samples. Evidence for self‐compensating acceptor complexes are seen in diffused samples. With short and moderate diffusion times, a compensating acceptor complex VGaGaSbTeSb is observed. For long diffusion times, the dominant acceptor center has been attributed to the antisite defect GaSb or related complex. The reasons for the formation of various acceptor centers have been discussed.

Sulphur passivation of gallium antimonide surfaces

Improvement in optical and electrical properties were observed after sulphur passivation of gallium antimonide surface. Enhancement of photoluminescence intensity up to 60 times, reduction in surface state density by two orders of magnitude, and reverse leakage currents by a factor of 20-30 were obtained as a result of surface passivation. While the reduction of surface recombination is attained, the surface is not unpinned.

ZnO nanoparticle surface acoustic wave UV sensor

The response to ultraviolet illumination of ZnO nanoparticles deposited on LiNbO3 substrate was investigated using surface acoustic waves (SAWs) in the wide range of UV wavelengths from 280 to 375 nm. Deposition of ZnO nanoparticles caused a SAW transmission loss of 27 dB at 64 MHz due to the acoustoelectric attenuation. Acoustoelectric change in the SAW velocity by 3.78×10−4 under 375 nm illumination led to downshift in transmitted SAW phase by 5.5° at UV power density of 691 μW/cm2. The spectral measurements show the peak response at 345 nm with corresponding sensitivity on the order of 2.8 ppm/(μW/cm2).

Growth of gallium antimonide by vertical Bridgman technique with planar crystal-melt interface

High quality single crystals of GaSb were grown using vertical Bridgman technique with a planar melt-solid interface. Various factors affecting the interface shape during growth were investigated. In general, the shape of the freezing isotherm was found to depend on the furnace temperature profile near the melt-solid interface, the ampoule lowering rate, the ampoule geometry, the mode of heat extraction from the tip of the ampoule and the extent of lateral heat loss from the side walls of the ampoule. A critical ratio of temperature gradient of the furnace at the melting point to ampoule lowering rate was found to be necessary for planar interface shape during the growth. The sensitivity of the interface shape was found to decrease with increasing temperature gradient of the furnace and ampoule diameter. Crystals grown by employing the flat melt-solid interface exhibited superior quality than those with non-planar interfaces

Suppression of cracks in InxGa1−xSb crystals through forced convection in the melt

It was demonstrated that forced convection or mixing in the melt during directional solidification of bulk InxGa1−xSb (0<x<0.1) ternary alloys significantly reduces cracks in the crystals. In this study, the enhanced mixing in the melt was generated by a rotating submerged baffle. The resultant improvement in spatial compositional homogeneity lowers the strain gradient or chemical stresses; thus eliminating cracks. The results presented are generally beneficial and should also improve the crystalline quality of other mixed alloys.

Band gap and lattice constant of GaxIn1−xAsySb1−y

The energy band gap and lattice constant of the quaternary alloy GaxIn1−xAsySb1−y were determined over the entire composition space (x, y) using a correlated function expansion (CFE) technique to interpolate from observed ternary compound data. Considerable anomalous band gap behavior (i.e., deep bowing) was found, which produced band gap minima as a function of composition. This deep bowing effect was experimentally observed in a quasibinary alloy (GaSb)1−z(InAs)z at low values of z. In addition, the CFE lattice matching relations of the alloy grown on GaSb and InAs were obtained as a function of composition, and the corresponding band gaps were estimated. The CFE estimates were in good agreement with existing experimental data.