Condensed Matter Physics Page(experimental)

Experimental Condensed Matter Group

D. P. Mahapatra
B. N. Dev
S. N. Sahu
S. B. Ota
Shikha Varma
B. R. Sekhar
P. V. Satyam
T. Som
G. V. Raviprasad

The microbeam facility of the IOP which was setup jointly by IOP and SUNY at Albany is ready for routine use. Data have already been taken on some samples involving self assembled island growths of metals like Au, Ag etc on Si. Results are quite interesting and are going to be presented in IBA-14/ECAART-6, Dresden, Germany. The spatial resolution is about 5 microns where it is possible to get both x-ray and charged particle spectra simultaneously.

On the AMS side the machine augmention is complete. The new system has a 40 cathode MC-SNICS source along with a 45⁰ spherical electrostatic analyser. The main idea behind using this analyser is to reduce the high and the low energy tails in the sputter ion beam. However, with rotatable electrodes it will also be used to inject beams from our old alpha source for routine RBS,Channeling and PIXE works. The injection magnet is replaced by a 90⁰ double focussing injection magnet with all the facilities for a fast isotope switching as may be necessary for AMS work using 14C and 10Be beams. The high energy post acceleration quadruple magnetic doublet lens has been replaced by a suitable electrostatic quadruple triplet lens. Provision has been made to use an offset Faraday cup for 13C(or 9Be) while carrying out 14C (or 10Be) measurements. A new AMS beamline with a +15⁰ cylindrical electrostatic analyser has been erected along the -15deg line of the switching magnet. Last year the new ionsource was erected as a standalone system with the 45⁰ ESA and the 90⁰ magnet in a different room and was thoroughly tested. At the moment the MC source is operating at 35 kV only and the transmission is not optimal.

Arrangements are being made to increase the injection energy to about 50 keV where the transmission is expected to be good. The computer control of the sequential injection of isotopes is being tested.

B. N. Dev

An energetic ion interacting with a solid loses energy through electronic excitation in target atoms and scattering by target atomic nuclei. If an energetic ion containing two atoms (dimer) enter into a solid they can travel a short distance without being completely separated. Within this distance in the solid, the closeness of the two components of the dimer ion can cause modifications of the response of the electrons in the solid, in comparison to the passage of monomer ions. We indirectly observed this coherent dynamic response in C⁺₂ implantation into a semi-insulating GaAs crystal. In a semi-insulating material electronic excitation can cause damage. We observed enhanced damage production in the GaAs crystal surface region for C⁺₂ implantation compared to C⁺ implantation, where in both cases we used the same energy/mass of the ions and the same atomic fluence. The near-surface modified layer has been studied by X-ray reflectometry, Rutherford backscattering/ Channeling spectrometry and transmission electron microscopy.Transfer of energy from the projectile ions to the atoms in the solid usually causes damage to a crystalline solid. However, it can also bring order in a disordered solid in a process like annealing. We have observed this phenomenon in a grainy thin (~ 1000� epitaxial Ag(111) layer grown on a Br-passivated Si(111) substrate. First we have demonstrated that it is possible to grow an epitaxial Ag(111) layer under high vacuum conditions on a Br-passivated Si(111) substrate. High resolution X-ray diffraction and ion channeling experiments show that the Ag layer is epitaxial with (111) orientation. However the crystalline quality is poor. Transmission electron microscopy reveals that the layer is grainy instead of a continuous layer. Thermal annealing upto 500⁰C improved the crystalline quality. This happens through grain boundary melting and consequent growth of oriented grains and the shrinkage of slightly misoriented grains. Exposure of the Ag(111)/Si(111) sample to 1 MeV Si⁺ ion beam caused similar effect as in thermal annealing. The displacement/ movement of Ag atoms, having received energy from the incident Si⁺ ions, would be dominant at the grain boundaries enhancing the growth of larger oriented grains. This essentially improves the crystalline quality, which we quantitatively measured by Rutherford backscattering and channeling experiments using an MeV He⁺ ion beam.

A Si_1-xGe_x alloy has an energy band gap which is between the band gap values of Si and Ge. The band gap of the alloy can be tuned by varying the comparison, x. This leads to band gap engineering. If such a layer can be sandwiched between Si or Ge, potential wells or barriers can be built within the material. Ion implantation provides a way to prepare a buried sandwiched alloy layer. We have used 1 MeV Si⁺ implantation at an ion fluence of ~ 8.5 10¹⁷ cm^-2 into a polised Ge(111) crystal to fabricate a buried Ge_0.8Si_0.2 layer. The actual composition varies somewhat depending on the post-implantation annealing temperature. We studied the Si distribution in Ge, the implantation-induced damage and recrystallization by Rutherford backscattering/ Channeling spectrometry using MeV H⁺ and He⁺ ions.

S. N. Sahu

Nanocrystalline semiconductors with crystalline size comparable to Bohr exciton radius show quantum size effect(SQE). For such semiconductors showing QSE the continuum of energy levels broke down into discrete states resulting widening of the band gap. The increased band gap show optical properties far different from their bulk counter part. Apart from the deviation in their optical properties these nanostructure semiconductors also exhibit unusual structural,electrical,surface and intereface properties. In order to confirm the above features, PbS and Porous Si nanocrystalline semiconductors have been synthesized and their properties have been studied. PbS nanocrystalline semiconductors have been grown by electrodeposition technique on conducting substrates from precursers viz.PbSO4 and thiosulphate solution in acidic media. Impurity and compositional analysis indicate stoicheometric deposits and with out any impurities even at ppm level confirmed from RBS and PIXE studies. RBS also predicted the surface roughness and measured film thickness. Structural characterisation by XRD indicate a lattice distortion of 1.5 %. Nanocrystalline sample surface show a branched fractal pattern. Nanocrystalline PbS show QSE for size 18nm(estimated from AFM and XRD) and a blue shift of 2.2 eV has been attained which is confirmed from optical abosorption studies. Photoluminescence (PL) measurements show red shift caused by structural diffects. PL measurements also show temperature insenistivity of nanocrystalline sample indicate molecular nature of PbS.

Porous Si has ben synthesized by anodizing p-Si in ethanol and HF mixture. The growth proces has been characterized by in-situ Current (I)-Voltage (V) and Capacitance (C)- Voltage (V) measurements. The in-situ C-V studies show two capacitance maxima corresponding to two surface state energy levels generated by oxide growth. The contribution of such surface states to capacitance at higher frequencies show to be less dominant as expected. The in-situ I-V studies clearly identify the pore formation, oxide growth and electro polishing regimes. These studies are in progress.

The aging effect of uncapped nanocrystalline semiconductors have been studied by photoluminescence measurements.For the above purpose CdS nanocrystalline semiconductors have been grown by a chemical route and stored for 15 and 30 days respectively and subjected to photoluminescence measurements which has been installed in IOP recently. The PL studies show that the crystalline size grows with time if not capped. The PL peaks are red shifted and further shifted with aging suggests increase in crystalline size.

S. B. Ota

We studied the magnetic properties of commercially available cryogenic silicon diodes temperature sensors from CRYO Industries of America Inc., in the temperature range 5-255K using a SQUID magnetometer. Further analysis of the data was carried out. We conclude that the magnetic properties of such diodes allow convenient background subtraction if used in close proximity of the sample in magnetisation measurements.

Earlier we studied the forward characteristics of cryogenic temperature sensor diodes. We concluded that such sensors can be used for a range of current values with suitable calibration enhancing its applicability as a low temperature sensor. A calorimeter is being constructed using such a cryogenic temperature sensor diode. This setup is being built around a Leybold closed cycle refrigerator. The computer program for automation of the calorimeter setup has been developed. This computer program will control the instruments using GPIB IEEE-488 interface. The software is written using MS-DOS GWBASIC.

Further study of ergodicity of this technique for the 2d q- state Potts model was carried out. Ergodicity does not seem to depend on the value of q. Lack of ergodicity, which occurs in a particular situation, when the system energy is discrete as a function of temperature is reported and discussed. This occurs only for small values of system energy. This is a drawback especially when the system size is small, as it limits the lowest temperature. However, such a situation does not arise in models with continuous symmetry such as the XY-model.

It has been found that the temperature dependence of the energy exhibits `S'-shaped nature at the first order transition of a finite isolated system in microcanonical Monte Carlo simulations. Further study was carried out on origin of `S'-shaped nature of the coexistence region on the basis of the recently developed understanding. The present results suggest independent study of finite size scaling of coexistence region in microcanonical ensemble. In this context, we have estimated the interphase surface entropy for the 2d Potts model and the extended 2d classical XY-model.

The 2d classical XY-model has been chosen as a model for the real magnetic crystals. However, real magnetic crystals are subjected to symmetry-breaking crystalline fields in addition to an isotropic exchange coupling. In practice, we expect to encounter crystalline anisotropies with 2-4 fold and 6-fold symmetries. However, a detailed Monte Carlo study is lacking. Preliminary, microcanonical Monte Carlo simulations have been carried out for the extended 2d XY-model undergoing first order transition in the presence of a 3-fold symmetry-breaking field. It is observed that the transition is clearly first order in nature and the coexistence region of this first order transition is suppressed as the field strength is increased. Preliminary computer simulations have been carried out on the extended 3d classical XY-model undergoing first order transition.

Shikha Varma

Dose dependent structural modifications in Si(100) due to 1.5~MeV implantation of Sb have been characterized using Raman spectroscopy and Rutherford Backscattering Spectrometry/Channeling (RBS/C) techniques. With increasing fluence, an intensity reduction of the 1^st order Raman peak, characteristic of crystalline Si, is observed. The amorphicity in Si lattice appears at a dose of 1 10¹³ ions/cm² and it increases with dose. For a dose of 5 10¹⁴ ions/cm² the Raman spectrum resembles that of amorphous Si. RBS/C studies also support a fully amorphized lattice at this dose though for smaller doses it suggests lower disorder. For the fluences of 1 10¹³ ions/cm² and 1 10¹⁴ions/cm² a coexistence of undamaged crystalline Si regions and amorphous zones is observed. Consequently phonon confinement is observed. Lattice recovery achieved by subsequent annealing has also been investigated using Raman Spectroscopy. By annealing at 600⁰C, sample crystallinity is fully recovered in all the cases upto the fluence of 5 10¹⁴ ions/cm². For higher doses small amorphicity still remains. Depth dependent measurements of the shifts in the Raman peaks demonstrate a gradient in stress which is of compressive nature near the surface region but is tensile in deeper layers. Maximum stress in the lattice appears for a dose of 1 10¹²ions/cm² which gets relaxed by the incorporation of amorphous zones at higher fluences.

We have also studied the density and nature of defects produced in a sample with heavy high energy ions during implantation, using IOP Pelletron . For this purpose we have carried out Channeling studies of Sb implanted in Si and of Cu implantation in MgO. By removing the dechanneling contributions, we have developed a Multiple Scattering Program or understanding the modifications in density distribution during heavy ion implantations as compared to TRIM model. For this investigation, Sb²⁺ ions were implanted in Si(100) with a dose of 5 10¹⁵ ions/cm² at IOP Pelletron. We investigated the spatial distribution of damage for the implanted sample after annealing as well as after co-implantation with 8 MeV carbon (5 10¹⁶ atoms/cm²) ions at Room Temperature(RT). Carbon irradiation resulted in an increase of sub-surface damage. The increase in damage can be due to Sb and Si recoils produced or due to mobile defects that drift from bulk towards the surface during the carbon-irradiation. After annealing the irradiated sample to 800⁰C a decrease in damage is observed. The spatial distribution of substitutional fraction of Sb in Si has also been studied. In this regard we have also made some SIMS measurements at IGCAR, Kalpakkam.

G. V. Raviprasad

Under the Accelerator Mass Spectrometry project, the existing 3MV tandem Van de Graaff pelletron accelerator will be augmented to do radio carbon (¹⁴C) dating on the samples. We have developed software to control the magnet power supplies for mass analysis and to measure the beam currents through computer. We have successfully tested the slow switching scheme where ¹³C and ¹⁴C beams are alternately injected into the accelerator and their currents are measured periodically every few seconds. The scheme was tested with ¹²C, ¹³C and ¹²C, ¹⁶O systems with satisfactory results. A new multi cathode SNICS source with a 45-deg electrostatic analyser has been tested for fast switching through computer control and has been installed. Trial runs are being taken to measure ¹⁴C/¹²C ratio.