P. Rajaram

Advancement in nanoscale CMOS device design en route to ultra-low-power applications

In recent years, the demand for power sensitive designs has grown significantly due to the fast growth of battery-operated portable applications. As the technology scaling continues unabated, subthreshold device design has gained a lot of attention due to the lowpower and ultra-low-power consumption in various applications. Design of low-power high-performance submicron and deep submicron CMOS devices and circuits is a big challenge. Short-channel effect is a major challenge for scaling the gate length down and below 0.1 µm. Detailed review and potential solutions for prolonging CMOS as the leading information technology proposed by various researchers in the past two decades are presented in this paper. This paper attempts to categorize the challenges and solutions for low-power and low-voltage application and thus provides a roadmap for device designers working in the submicron and deep submicron region of CMOS devices separately.

Synthesis of SnO2 nanostructures by ultrasonic-assisted sol-gel method

Fluorine doped SnO2 nanostructures were grown using ultrasonic assisted sol–gel method. The gel was obtained by dissolving stannous chloride in methanol with ammonium fluoride as dopant followed by irradiation with ultrasonic vibrations. Obtained samples were characterized by structural, morphological and optical studies. All the peaks in the X-ray diffractograms are identified and indexed as tetragonal cassiterite structure. Negative slope of Williamson–Hall plots indicates compressive strain. Particle size of SnO2 nanostructures is decreases with increases in concentration of fluorine doping. Atomic force microscopy, scanning electron microscopy and transmission electron microscopy studies confirm the formation of ring like porous structures and then hollow tube like growth with increase in the fluorine concentration. Peaks in Raman spectra also indicate strong confinement in SnO2 particles. Distinct peaks in the PL spectra make the structure suitable for photovoltaic applications.

Growth and characterization of pulse electrodeposited CuInSe2 thin films

CuInSe2 thin films were grown on SnO2-coated glass substrates using the pulse electrodeposition (PED) technique. The CuInSe2 films were grown at different potentials using pulses of different frequencies ranging from 1 KHz to 1 MHz. The electrodeposited films were annealed in vacuum at 300°C, for 1 hr, to improve the crystallinity. The annealed films were characterized by x-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Analysis of x-rays (EDAX), and UV-VIS-NIR spectra. The results show that the electrodeposited CuInSe2 films have good stoichiometry and are single phase with an optical gap around 1 eV. SEM studies show that the films grown using pulses of frequency 1 KHz contain nanoparticles of size 50–60 nm, whereas those grown using pulses of frequency 100 KHz and 1 MHz contain micronsized particles.

Structural, morphological and optical properties of sprayed nanocrystalline thin films of Cd1-xZnxS solid solution

A series of nanocrystalline thin films of cadmium zinc sulphide (Cd1-xZnxS) solid solution were deposited on glass substrates using spray pyrolysis. Cadmium chloride (CdCl2), zinc chloride (ZnCl2) and thiourea (NH2CSNH2) were used as the sources of Cd, Zn and S respectively. The films were characterized using structural, morphological and optical techniques. X-ray diffraction (XRD) studies show that the films of Cd1-xZnxS are polycrystalline and single phase having the wurtzite structure. The crystallites in the thin films of pure CdS possess preferred crystallographic orientation along the <002> direction. The preferred orientation of crystallites in the Cd1-xZnxS films changes from <002> to <101> with increase in Zn concentration. The lattice parameters of Cd1-xZnxS decrease with increase in Zn concentration. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that the surfaces of the films are smooth and are uniformly covered with nanoparticles. Energy dispersive analysis of xrays (EDAX) results reveal that the grown films have good stoichiometry. Optical transmission spectra confirm the good quality of the Cd1-xZnxS films.

Enhanced I ON/I OFF in ultra deep submicron CMOS devices using grooved nMOSFETs in comparison to planar nMOSFET

To manage the increasing static leakage in low power applications and reducing ON‐OFF current ratio due to scaling limitations, solutions for leakage reduction as well as improving the current drive of the device are sought at the device design and process technology levels. At the device design level, the important low power variables are the threshold voltage, the gate leakage current, the subthreshold leakage current and the device size. Grooved‐gate MOS devices are considered as the most promising candidates for use in submicron and deep submicron regions as they can overcome the short‐channel effects effectively. By varying the corner angle and adjusting other structural parameters such as junction depth, channel doping concentration, negative junction depth and oxide thickness, leakage current in nMOS devices can be minimised. In this article, 90, 80, 70, 60 and 50 nm devices are simulated using Devedit and Deckbuild module of Silvaco device simulator. The simulated results show that by changing the structural parameters, ON‐OFF current ratio is improved and maintained constant even in the deep submicron region. This study can be helpful for low power applications as the static leakage is drastically reduced, as well as applicable to high speed devices as the ON current is maintained at a constant value. The results are compared with those of corresponding conventional planar devices to bring out the achievements of this study.

CuIn1−XAlXSe2 Thin Films Grown By Electrodeposition

We have grown CuIn1−xAlxSe2 thin films on fluorine doped tin oxide glass at a constant deposition potential of −650 mV by the electrodeposition technique. The electrochemical bath from which the CuIn1−XAlXSe2 (CIAS) thin films were grown was made up of a mixture of aqueous solutions of 0.025 M CuCl2, 0.05 M InCl3, 0.05 M AlCl3 and 0.025 M SeO2. Ethylene‐diamine‐dihydrochloride (EDC) was used as a complexing agent which was found to reduce the particle size of the CIAS thin films. The electrodeposited films were characterized using XRD, SEM and EDAX. The results show that single phase CuIn1−xAlxSe2 films with Al content x around 0.28 and having good stoichiometry can be produced.

Optically Enhanced SnO2/CdS Nanocomposites by Chemical Method and Their Characterization

SnO2/CdS heterostructures nanocomposites were synthesized using newly develop ultrasonic sol-gel method. Stannous chloride, cadmium chloride (CdCL2 · 2H2O) and thiourea were used as Sn, Cd and S precursors respectively and Ethylene glycol was used as a complexing agent. The samples were characterized by XRD, SEM, EDX, optical studies. All the XRD peaks are identified for SnO2 however a slight shift is observed with addition of CdS. EDX confirms the presence of Sn, Cd and S in the samples. AFM and SEM studies also confirm the nanofibers structures with roughness 2.9136 nm and conversion of hollow tubes into nanofibers. The UV–Vis spectrum of the nanostructures displays a new absorption band range lies in the range between 450–530 nm compared with the bare SnO2 hollow tubes. The strong emission peak is observed at 375 nm in UV region for all the samples and intensity of emission become weaker due to incorporation of CdS nanoparticles. Addition of CdS introduces effective charge separation in the heterostructures which controlled the intensity of photo luminescence makes them suitable for optoelectronic applications.

Growth and characterization of pulse electrodeposited CuAlSe2 thin films

In the present work we have grown CuAlSe2 thin films on the fluorine doped tin oxide glass substrates by using Pulse electrodeposition technique. Thin films of CuAlSe2 were co-deposited from an aqueous solution containing CuCl2, AlCl3 and SeO2 adjusted to pH=1.60. Ethylenediamine-di-hydrochloride was used as a complexing agent and films were deposited at the constant deposition potential of −650 mV using pulses of frequency 50 Hz.CuAlSe2 thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy dispersive analysis of X-rays (EDAX) which indicate the good and reliable quality of the films. An interesting results of this work is that complexing agent plays an important role in reducing the grain size, thus all deposits were found to be in the range of 10-20 nm.

Synthesis of nanocrystalline Cu2ZnSnS4 thin films grown by the spray-pyrolysis technique

Spray pyrolysis was used to deposit Cu2ZnSnS4 (CZTS) thin films on soda lime glass substrates at 300 °C. Aqueous solutions of copper chloride, zinc chloride, stannous chloride and thiourea were mixed together to form the spray liquid. The sprayed films were annealed under vacuum at 350 °C, 400 °C and 450 °C. Structural and optical characterization was performed on the CZTS films using X-ray diffraction (XRD) and UV-VIS spectrophotometry. XRD results indicate that the films are single phase nanocrystalline CZTS. Optical studies show that the optical gap values are 1.44 eV for the as-grown film and 1.46 eV, 1.48 eV and 1.49 eV for the films annealed at 350 °C, 400 °C and 450 °C, respectively.

Electrochemical growth and studies of CuInSe2 thin films

Thin films of CuInSe2 were grown on fluorine doped tin oxide (<10 Ω/) coated glass using the electrodeposition technique. The electrodeposition was carried out potentiostatically using an aqueous bath consisting of solutions of CuCl2, InCl3 and SeO2 with ethylenediamine-dihydrochloride (EDC) added for complexation. CuInSe2 films were also deposited without using any complexing agent in the bath. To improve the crystallinity the CuInSe2 films were annealed in vaccum at 300 °C for one hour. The annealed films were analyzed by x-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive analysis of x-rays (EDAX), atomic force microscopy (AFM) and optical spectra. The results obtained in this work show that by adding a suitable complexing agent to the electrochemical bath, nanocrystalline CuInSe2, 20 nm to 30 nm in size, can be grown. The composition of the CuInSe2 films can be controlled by means of the bath composition and stoichiometric films can be obtained for a bath with ionic Cu:In:Se composition close to 1:4:2. AFM micrographs show that the particles are generally oval shaped for near stoichiometric compositions. However for extreme copper rich layers, the morphology is completely different, the particles in this case appearing in the form of nanoflakes. Each flake has a thickness in the nano range, but the surface extends to a length of several microns.