S. Dhamodaran

Simulation and experimental realization of μ-channels using a μED-milling process

Machining of microchannels (µ-channels) on hard conducting materials can be performed effectively using a micro electric discharge milling (µED-milling) process. The method of bulk machining is considered in this work, as it takes less machining time than conventional layer-by-layer machining. However, the tool wear in this method occurs both in the side and bottom of the tool, which causes dimensional inaccuracy. Hence, to achieve the desired µ-channel shape, the technique of multiple-pass machining is introduced, wherein a dressed tool is used in each pass to compensate tool wear. The basic idea behind the work is to compute the number of passes and machining time required in achieving the shape. A computer-aided design-based algorithm is used to simulate the shape of the µ-channel and tool for a single pass and for further multiple passes. At the end of each pass, the volume of material removal and tool wear are estimated quantitatively. The desired channel shape is considered to be achieved when the volume of the material left-out is less than or equal to 5 per cent of the ideal volume to be removed. This paper elaborates on the procedure for single- and multiple-pass simulation to determine the number of passes and machining time. The results are validated with experiments for the desired µ-channel shape.

Swift heavy-ion modification of semiconductor heterostructures

Ion-beam modification of material properties is of great interest from a research and industrial application point of view. In particular, modifications and characterizations of semiconductor heterostructures of nanoscale thickness have been investigated in great detail due to their potential applications in electronic and optoelectronic device applications. In this article, we review recent work in this field and present some of our results on irradiation studies of partially relaxed InGaAs/GaAs heterostructures, both thickness and composition dependence. Some of the recent results on GaN/Sapphire samples have also been discussed. We characterized these structures using high-resolution X-ray diffraction (XRD), Raman spectroscopy and RBS/Channelling. The defects densities have been obtained from high-resolution XRD and RBS/Channelling studies which are complemented by Raman results. The in-plane strain measured from XRD has been utilized to obtain the defects densities, which are confirmed by energy dependence of the dechannelling parameter in RBS/Channelling.

Application of network theory for the description of nanocluster distributions in ion track electronics

The recently developed ion track-based electronic structures of the TEMPOS type have often been filled up with (semi)conducting nanoclusters (NCs) to optimize their operation. Network theory is applied here for the first time to describe the distribution of the ion tracks and the NC, and of the current paths followed by the latter ones. The presence of ion tracks makes the NC distribution deviate from the purely random distribution. The theory is used to calculate the electrical surface currents around a given contact. A radius of influence (ROI) around such a contact can be defined, beyond which the surface currents become negligible. ROI depends on track density, NC density and on the working point of the electronic device.

BLOCK EDG: ISSUES AND APPLICABILITY IN MULTIPLE PASS µED-MILLING

Fabrication of micro-features by a Micro ED-milling (μED-milling) process requires machining of tool electrode and applying them in multi-pass trajectory. A real concern occurs when the tool has to be prepared before each pass and put into trajectory without disturbing the settings. This issue is attested to here by combining the process setup ofμED-milling, and Block Electric Discharge Grinding (Block EDG) without hindering the functionality of individual processes. But the Block EDG process itself suffers with a primary setback such as inability to predict the change in tool diameter during machining. The ability of Block EDG process is improved in this work by formulating regression model and tool quality loss characteristics evaluation. An empirical model for tool diameter as a function of energy and machining time is developed and validated to predict the diameter online at any point of machining. A data chart is prepared to show the typicalμ-tool defects during the fabrication process and possible remedies. Finally, the article concludes with employing the fabricated tools inμED-milling process to produce complex shapes through multiple pass machining technique.

Strain modification of AlGaN layers using swift heavy ions

Epitaxial AlGaN/GaN layers grown by molecular beam epitaxy (MBE) on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×1012 ions/cm2. The samples used in this study are 50 nm Al0.2Ga0.8N/1 nm AlN/1 μ m GaN/0.1 μ m AlN grown on SI 4H-SiC. Rutherford backscattering spectrometry/channeling strain measurements were carried out on off-normal axis of irradiated and unirradiated samples. In an as-grown sample, AlGaN layer is partially relaxed with a small tensile strain. After irradiation, this strain increases by 0.22% in AlGaN layer. Incident ion energy dependence of dechanneling parameter shows E 1/2 dependence, which corresponds to the dislocations. Defect densities were calculated from the E 1/2 graph. As a result of irradiation, the defect density increased on both GaN and AlGaN layers. The effect of irradiation induced-damages are analyzed as a function of material properties. Observed results from different characterization techniques such as RBS/channeling, high-resolution XRD and AFM are compared and complemented with each other to deduce the information. Possible mechanisms responsible for the observations have been discussed in detail.

Elemental Identification of Materials Using Optical Emission Spectra during Electric Discharge Machining

Elemental identification of material is a prime most important in material science field. Electric discharge is used for material elemental identification with optical emission spectroscopy. During electric discharge machining between two electrodes plasma is generated which emits intense radiation in the UV-Visible region. The generated plasma captured by optical emission spectroscopic technique and elements are identified from recorded spectra by matching with standard NIST database. This method is simple, rapid, and inexpensive compared to all other elemental identification method. The elements of the metal, semiconductors, even insulators can be identified without much difficulty. The elemental identification of material has been investigated in macro and micro level.

Effects of concentration and thermal annealing on the optical activation of Er implanted into GaN layers

The wide band gap semiconductor, GaN, has emerged as an important host for rare earth-electroluminescence. The annealing behaviour and lattice site location of Er implanted into GaN were studied with the Rutherford Backscattering Spectrometry (RBS)/channelling and photoluminescence (PL) techniques. Also Er site dependence on the annealing temperature and implantation dose has been studied in detail. The optical properties of the Er-doped GaN system, evidencing their dependence on the parameters adopted during the synthesis procedure (Er implantation dose, annealing temperature) have been discussed. RBS/channelling measurements suggested that mostly Er occupy substitutional site and depends on the Er concentration. The main result is the activation of a typical Er giving rise to PL emission in the 1450–1650 nm range, related to radiative 4 I 13/2→4 I 15/2 transitions. Depending on the Er dose, we observe a specific behaviour linked to variation of the annealing temperature that strongly determines PL emission band. We observed a PL spectral shape with the main peak located at 1542 nm and shoulder peak at 1558 nm (and full width at half maximum (FWHM) of 33 nm) with a series of weaker PL structures at 1519, 1572 and 1591 nm, due to the Stark sub-level splitting.

Single Discharge of Dry µ-EDM on Silicon: Crater and Plasma Temperature Measurement

To improve the performance of the Electric discharge machining (EDM) process it is of interest to characterize the plasma involved. Plasma temperature needs to be measured as an initial phase of plasma characterization. Non-contact optical emission spectroscopy has been used to measure the plasma temperature. The plasma temperature and crater morphology has been investigated for different energy conditions on Silicon in dry µ-EDM condition. The plasma temperature is calculated using line pair method and crater morphology analyzed by scanning electron microscope (SEM) and profilometer.

Synthesis and characterization of nc-Ge embedded in SiO2/Si matrix

We have prepared germanium nanoparticles embedded in SiO2 matrix by atom beam co-sputtering (ABS) of Ge+SiO2 on Si substrate. The as-deposited films were annealed at various temperatures in Ar+H2 atmosphere and irradiated with various energies with fixed fluence. The pristine and irradiated samples were characterized by Raman, X-ray diffraction and atomic force microscopy (AFM). Rutherford back scattering (RBS) was used to quantify the concentration of Ge in the SiO2 matrix and the film thickness. Raman studies of the films indicate the formation of Ge crystallites as a result of swift heavy ion (SHI) irradiation. Moreover, the crystalline nature of Ge improves with an increase in energy. Glancing angle X-ray diffraction and Raman results also confirm the presence of Ge crystallites in the irradiated samples. Similarly, 400 keV Ge+ ions implanted into silicon substrate at higher fluence at 573 K have been irradiated with 100 MeV Au8+ions at RT. These irradiated implanted samples were subsequently characterized by XRD and Raman to understand the crystallization behavior. We also studied the surface morphology of a high-energy irradiated sample by AFM. The irradiation results were compared with those obtained by thermal annealing in ABS. The basic mechanism for crystallization induced by SHI in these films has been investigated.

RBS/Channeling Studies of Swift Heavy Ion Irradiated GaN Layers

Epitaxial GaN layers grown by MOCVD on c‐plane sapphire substrates were irradiated with 150 MeV Ag ions at a fluence of 5×1012 ions/cm2. Samples used in this study are 2 μm thick GaN layers, with and without a thin AlN cap‐layer. Energy dependent RBS/Channeling measurements have been carried out on both irradiated and unirradiated samples for defects characterization. Observed results are compared and correlated with previous HRXRD, AFM and optical studies. The χmin values for unirradiated samples show very high value and the calculated defect densities are of the order of 1010 cm−2 as expected in these samples. Effects of irradiation on these samples are different as initial samples had different defect densities. Epitaxial reconstruction of GaN buffer layer has been attributed to the observed changes, which are generally grown to reduce the strain between GaN and Sapphire