N. Sathish

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.

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.

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.

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

Investigation of Strain in AlGaN/GaN Multi Quantum Wells by Complementary Techniques

Al0.49Ga0.51N (12 nm)/GaN (13 nm) Multi Quantum Wells of 15 periods are grown on sapphire by MOCVD technique. GaN/AlN, each of thickness 200 nm and 20 nm respectively, are used as buffer layers between substrate and epilayer to incorporate the strain in epilayers. It is a well established technique to engineer the band gap in AlxGa1−xN by adjusting alloy composition. These samples are used in visible & UV light emitters. In the present study, we employ a photoluminescence technique to estimate the composition and luminescence peak positions of AlGaN and GaN. Crystallinity and quality of interfaces have been studied by Rocking curve scan. The Threading Dislocations formed at the GaN buffer layer travel across the entire layers to the surface to form good quality films. Photo‐luminescence results show a very sharp GaN peak at 3.4 eV, as observed and reported by others, which shows that samples are free from point defects.

SHI Effects On Ge+SiO2 Composite Films Prepared By RF Sputtering

Ge+SiO2 composite films were deposited on Silicon substrate using RF magnetron sputtering. The as‐deposited samples were irradiated with 150 MeV Ag+12 ions at a fixed fluence of 3×1013 ions/cm2. These samples were subsequently characterized by X‐ray diffraction (XRD) and Raman spectroscopy to understand the crystallization behavior. Formation of Ge nanocrystal in amorphous silicon dioxide film was studied using transmission electron microscopy (TEM). We also studied the surface morphology of these high energy irradiated samples by Atomic Force Microscopy (AFM). The basic mechanism for ion beam induced crystallization in these films has been discussed.

Optimization of pulse reversal electrodeposition with fine grains and low roughness for GaAs RF MEMS structures

GaAs MESFET-based switches suffer from high insertion losses. As an alternative, GaAs RF MEMS have shown great promise due to high isolation, low insertion losses, and wide bandwidths. Some factors constraining the fabrication have been suitable planarization techniques, quality of metallisation, stress in the beams, and elimination of stiction of beams to the central signal electrode. Quality of metallisation makes pulse reversal plating technique viable for production compared to DC plating. Coplanar waveguide pads, anchors, and beams are formed using this process. This paper discusses the optimization of pulse reversal electrodeposition process to fabricate different stages of RF MEMS switches.

Channeling techniques to study strains and defects in heterostructures and multi quantum wells

The importance and advantages of heterostructures and Quantum Wells (QWs) in device technology has made research challenging due to lack of direct techniques for their characterization. Particularly the characterization of strain and defects at the interfaces has become important due to their dominance in the electrical and optical properties of materials and devices. RBSiC has been used to study variety of defects in single crystalline materials, for nearly four decades now. Channeling based experiments play a crucial role in giving depth information of strain and defects. Ion beams are used for both material characterizations as well as for modifications. Hence it is also possible to monitor the modifications online, which are discussed in detail. In the present work, Swift Heavy Ion (SHI) modification of III-V semiconductor heterostnictures and MQWs and the results of subsequent strain measurements by RBSiC in initially strained as well as lattice matched systems are discussed. We find that the compressive strain decreases due to SHI irradiation and a tensile strain is induced in an initially lattice matched system. The incident ion fluence dependence of strain modifications in the heterostructures will also be discussed. The use of high energy channeling for better sensitivity of strain measurements in low mismatch materials will be discussed in detail. Wherever possible, a comparison of results with those obtained by other techniques like HRXRD is given.