Showing papers by "Rangarajan Muralidharan published in 2007"

HRXRD, AFM and optical study of damage created by swift heavy ion irradiation in GaN epitaxial layers

Abstract: Epitaxial GaN layers grown by MOCVD on c -plane sapphire substrates are irradiated with 150 MeV Ag ions at a fluence of 5 × 10 12 ions/cm 2 . Samples used in this study are 2 μm thick GaN layers, with and without a thin AlN cap-layer. Surface morphology is studied using contact mode atomic force microscopy (AFM). Irradiated samples show qualitatively different morphologies as well as quantitative changes. Different kinds of morphology are attributed to specific type of dislocations using the existing models available in the literature. The residual strain and sample quality have been analysed before and after irradiation using high resolution X-ray diffraction (HRXRD). The Lorentzian shape analyses of the experimental scans complement the AFM results. Optical properties are studied by spectrophotometer used in the transmission mode. A sharp band-edge in the as grown samples was observed at ∼3.4 eV. The band-edge absorption broadened due to irradiation and these results have been discussed in view of the damage created by the incident ions which compliment HRXRD results. In general the effect of irradiation induced-damages are analysed as a function of material properties. A possible mechanism responsible for the observations has been discussed.

High-resolution XRD analysis of swift heavy ion irradiated InGaAs/gaas heterostructures

Abstract: Strain relaxation and swift heavy ion irradiation effects on molecular beam epitaxy grown InGaAs/GaAs heterostructures have been studied by high-resolution X-ray diffraction. Symmetric and asymmetric scans were analyzed to understand the irradiation effects on strain and defects. The broad layer peak of the experimental scans in the samples was observed. The excess width of the layer peak than the expected one has been attributed to the strain relaxation induced defects. Irradiated samples show an additional peak close to the substrate peak of compressive nature, which has been attributed to radiation damage in the substrate due to the heavy ions. Dynamical scattering theory based simulations have been compared with the experimental scans. It has also been used to deconvolute the additional peak. The simulation was comparable with the experimental scan and a satisfactory fit was obtained. The dislocation density has been reduced upon irradiation. The observed decrease in the defect density is attributed to the high electronic energy loss of the ions near the interface region. Also, the damage created by the heavy ions near the interface would enhance the diffusion of indium across the interface and hence reduce the relative mismatch at the interface.

Process development of GaAs based RF MEMS

Abstract: GaAs MMIC compatible process for RF MEMS Switch using 7-layered mask set has been developed. Semi- Insulating GaAs is used as substrate. E-beam evaporated 7000 A thick Ti/Pt/Au is used as bottom metal followed by 2.5 mu thick gold plating is used for CPW structure. PECVD nitride is used as field dielectric layer. Sputtered Au metal is used as a beam layer. Different sacrificial layers are tried and Polyimide was found to be good sacrificial layer to realise the air bridge structures. Different sizes of air bridge structures up to 320 microns length and 100 microns width were realised. SEM was extensively used for analyzing beam structure of different dimensions.

Surface modification of InGaAs/GaAs heterostructures by swift heavy ion irradiation

Abstract: We investigate the surface morphology of molecular beam epitaxy (MBE) grown InGaAs/GaAs(0 0 1) heterostructures using atomic force microscope (AFM) before and after irradiation. Samples with layer thicknesses below critical layer thickness (i.e. fully strained) have smooth surface where as, the samples grown beyond critical layer thickness have cross hatch patterns at the surface. The transition from smooth to cross-hatch pattern may be used to identify the onset of strain relaxation. The samples were subjected to swift heavy ion (SHI) irradiation using 150 MeV Ag12+ ions with a fixed fluence of 1 × 1013 ions/cm2. The morphology of the strained samples was almost similar before and after irradiation where as, the partially relaxed samples were observed to have variations. The electronic energy loss of the incident ions which is dominant compared to the nuclear energy loss is effective to modify with the fluence used in the present study for partially relaxed samples. The relaxation of excited electron subsystem of the target results in the melting and re-growth which reflects in the surface morphology. The observed modifications at the surface may be attributed to (i) irradiation induced surface mass transport and (ii) interface modifications, where both these factors determine the surface morphology of heterostructures. The effects of irradiation on the surface and interface of the samples have been realized by AFM studies.

Reliability studies of AuGe/Ni/Au ohmic contacts to MESFETs by accelerated thermal aging tests

Abstract: Optimally alloyed AuGe/Ni/Au source-drain ohmic contacts to MESFETs with very low contact resistance (Rc) of the order 0.05 - 0.07 Omega-mm were obtained after rapid thermal alloying at 400degC. We have studied the degradation of ohmic contacts at elevated temperatures for 4000 hours. Ohmic contact test structures were subjected to accelerated life test temperatures at 185degC, 200degC and 230degC. We found that the drifts in Rc of the optimally alloyed contacts after thermal aging were as low as +13%, which are possibly one of the lowest drifts reported hitherto. This low drift may be due to the formation of thermally stable compounds and their nature at the ohmic contact interfaces, and the access regions during optimum alloying.

Ion beam analysis of defects and strain in swift heavy ion irradiated InGaAs/GaAs heterostructures

Abstract: Analysis of defects by Channeling in strain relaxed In 0.18 Ga 0.82 As/GaAs heterostructures before and after swift heavy ion irradiation has been reported. Energy dependence of dechanneling parameter has been analyzed which indicates a thickness dependence of defects, involving dislocations (for 36 and 96 nm) and stacking faults (for 60 nm). The dislocation density reduces upon irradiation and the possibilities for the same have been discussed in detail. The cross-sectional transmission electron microscopy (TEM) analysis indicates the presence of stacking faults in 60 nm and dislocations in 96 nm irradiated samples complementing the dechanneling studies. Angular scans along off-normal axis have been carried out for strain analysis. A strong strain relaxation as a function of thickness is observed from the strain measurements. The strain values are almost same after irradiation compared with unirradiated ones. The flux distribution of channeled ions at smaller thicknesses is discussed in detail.

Etching of 200 ϋm deep GaAs via holes with near vertical wall profile using photoresist mask with inductively coupled plasma

Abstract: Metal semiconductor field effect transistors (MESFETs) and pseudomorphic high electron mobility transistors (pHEMTs) are used in high frequency microwave applications in monolithic microwave integrated circuits (MMICs) Ground via holes with low inductance to the source electrode of these devices is necessary for high frequency applications and also increasing the packing density of MMICs Dry etching of via holes to the depth of 200 mum in GaAs using photoresist mask is particularly challenging due to its poor selectivity with masking material and the constraints of top and bottom dimensions with smooth sidewall profiles The desired sidewall profile promotes easy coverage of the plated metallisation subsequently Via holes of 200 mum depth with the desired profile and good uniformity over 3" GaAs wafers have been realized with inductively coupled plasma (ICP) etching using chlorine chemistry The near vertical sidewall profile has been achieved by tailoring the photoresist mask profile, photoresist hardening process and the GaAs etching parameters Optical and scanning electron microscopy (SEM) were used to characterize the via profiles

Silicon nitride films for passivation of pHEMT based MMIC

Abstract: Passivation of psuedomorphic high electron mobility transistor (pHEMT) based MMIC's by silicon nitride films deposited by PECVD is reported here. These films were first optimized for passivation of MESFET's to get desired performance. No degradation in pHEMT/MESFET characteristics like Idss, gm Vp and Cgs were observed while only nominal degradation was found in Schottky diode ideality factor eta and its breakdown voltage VB. The refractive index of the films was 1.92 with 1200 A0 thickness. The films have high dielectric strength > 5E6 V/cm, low tensile stress < 5E9 dynes/cm2 and dielectric constant 6.9 -7.1. The etch rate of the film is ~ 900 A0/min in BHF and fine patterns can be etched with 1-2 minutes etch time in BHF. The film composition was analyzed by FTIR and SIMS studies.

Study of selective gate recess etching of InGaAs/InAlAs/InGaAs metamorphic HEMT structures using succinic acid based etchant

Abstract: Metamorphic HEMTs on GaAs substrates are promising devices of today as they are operated at even higher frequencies for microwave applications compared to pseudomorphic HEMTs The selective removal of n+ InGaAs ohmic contact layer from the top of the device structure poses a major challenge during fabrication We have studied the influence of temperature on the selectivity of etch rate between the n+ InGaAs and underlying InAlAs layers using succinic acid based etchant The etchant was composed of succinic acid solution mixed with hydrogen peroxide and deionized water The pH of the solution was adjusted to 5 by adding NH4OH The etch rates at different temperatures between 14degC to 30degC were estimated by profiling the etched pattern using atomic force microscopy (AFM) Surface roughness of the etched area also was studied using AFM It was found that the selectivity has improved with temperature This is possibly due to simultaneous occurrence of low etch rates of InAlAs due to presence of aluminum oxide and high etch rates of InGaAs due to increased temperature It was also found that the surface roughness was higher at lower temperatures contrary to the observations made in the case of pseudomorphic HEMTs

MOVPE growth and optimization of GRINSCH single quantum well AlGaAs/GaAs laser diodes

Abstract: In the present paper we report the MOVPE growth of GaAs/AlGaAs graded-index separate confinement heterostructure (GRINSCH) single quantum well (SQW) laser structure at relatively low temperature. Low temperature growth was possible by controlling various growth parameters as well as external sources of oxygen contamination. The grown structures were characterized by photoluminescence (PL) and high resolution X-ray diffraction (HRXRD) technique. Stripe geometry laser diodes were fabricated on the grown structures. The laser parameters such as threshold current, slope efficiency and peak power were measured.

Low- and high-energy irradiations of InGaAs/GaAs heterostructures: a comparison

Abstract: The effects of ion irradiation on InGaAs/GaAs heterostructures have been studied using Raman spectroscopy. The structures consist of molecular beam epitaxy grown InGaAs layers on GaAs(0 0 1), having layer thicknesses of 12, 36, 60 and 96 nm. For irradiation, 3 MeV Ag2+ and 150 MeV Ag12+ have been used at 1×1015 and 1×1013 ions/cm2 fluences, respectively. We refer to 3 and 150 MeV irradiations as low- and high-energy irradiations, respectively. The low-energy irradiation damaged the lattice heavily and no phonon modes were observed in the as-irradiated samples. The samples were then subjected to rapid thermal annealing (RTA) to recover the phonon modes. Though the phonon modes were observed after RTA, the GaAs-type TO mode is more intense than the LO mode indicating residual defects. The TO mode position is found to be the same in all the annealed samples, whereas the LO phonon mode positions do not follow any trend in the low-energy irradiated samples. In contrast, no post-annealing was performed in high-...