Anand P. Pathak

Subcellular S-factors for low-energy electrons: a comparison of Monte Carlo simulations and continuous-slowing-down calculations.

Purpose: To study the energy deposition by low-energy electrons in submicron tissue-equivalent targets by comparing two widely used methodologies, namely, the continuous-slowing-down-approximation (CSDA) convolution integral and the Monte Carlo (MC) simulation. Methods: An MC track-structure code that simulates collision-by-collision the complete slowing down process is used to calculate the energy deposition in spherical volumes of unit density water medium. Comparisons are made with calculations based on the CSDA convolution integral using both empirical and MC-based range-energy analytic formulae. Results: We present self-irradiation absorbed fractions and S-factors for monoenergetic electrons of initial energies from 0.1–10 keV distributed uniformly in spheres of 5, 10, 50, 100, 500, and 1000 nm radius. The MC and CSDA results were found, in some cases, to differ by a factor of 2 or more; differences generally increase with decreasing sphere size. Contrary to high energies, the uncertainties associated with the straight-ahead approximation implicit in the CSDA calculations are of the same order as those related to straggling and δ-ray effects. Conclusion: The use of the CSDA methodology may be unsuitable for the sub-micron scale where a more realistic description of electron transport becomes important.

The ground state of two-hole centres in oxides

The nature of the ground state of the V centre in ionic oxides is discussed, where two holes are trapped by a cation vacancy. Experiment appears to indicate an S=1 ground state for BeO and Al2O3 and possibly ZnO, with S=0 in MgO and CaO. Simple theoretical arguments suggest S=0. It is shown that a weak mixture of (O0O2-) configurations into the dominant (O-O-) configuration can explain the observations. Quantitative calculations are given for MgO, and suggest the singlet state is lowest by about 30 cm-1.

Energy loss of MeV heavy ions in carbon

Abstract A novel technique, using secondary forward recoil ions generated in thin standard targets by energetic very heavy primany ion beam, has been employed to measure energy loss of several ion species (Z1 = 8 to 29) covering energies between 0.1 and 1.0 MeV/u. Energy variation was simp y affected by changing the detector angle between 35° to 70°, avording the dominant elastic scattering of the primary beam. A twin detector permitted simultaneous measurement of both unabsorted and absorbed spectra. The experimental data on stopping power are compared with evaluated values using LSS theory and TRIM. A better understanding of the validity of available theoretical evaluations has become possible, with a need for higher precision in experimental data in some cases.

Electronic structure of the V- centre in MgO

The authors have investigated in detail the two models of the V- centre, where a hole is trapped at a cation vacancy. These are the model of Bartram, Swenberg and Fourn (1965), where optical transitions occur within an O- ion, and the model of Schirmer, Koidl and Reik (1974), which involves a hole hopping from one oxygen ion to another. The calculations show (i) that the hole should be self-trapped on a single oxygen, as observed, (ii) that the predictions of ground-state ionization energy and of elastic and electrical dipole moments (apart from some uncertainties in local-field corrections) agree adequately with measurements, (iii) that the transitions within an O- ion appear to account for observed structure at around 1.5 eV, both in energy and oscillator strength, and (iv) that the observed 2.3 eV band corresponds to the transition suggested by Schirmer et al. Calculations are also given for some centres related to the V- centre, V0, V(Al)- and (Na)0, again predicting optical absorption in good agreement with experiment.

Inelastic cross-sections of energetic protons in liquid water calculated by model dielectric functions and optical data

Abstract Total and differential inelastic inverse-mean-free-paths for the energy loss of protons in liquid water are calculated on the basis of the dielectric approach. Energetic protons, much above the Bragg peak, are considered where the first-Born-approximation is justified (∼0.3–10 MeV). A model dielectric-response-function for the valence electrons of liquid water is constructed in accordance with the general properties of the Bethe-surface. A modified Drude-expansion model developed earlier provided the long-wavelength response, whereas several dispersion schemes were examined for introducing the momentum-transfer dependence. In particular, we have examined the δ-oscillator dispersion models of Ashley and Liljequist, and Ritchie’s extended-Drude models. A comparison with optical dielectric models will also be presented for assessing the influence of the dispersion. In view of the limited experimental information for the inelastic properties of liquid water, the present work provides a comparative study of various – computationally tractable – schemes that may be used in analog Monte Carlo transport codes for protons and heavier ions in liquid water.

Development of a large area two-dimensional position sensitive ΔE−E detector telescope for material analysis

A large area two-dimensional position sensitive detector telescope has been designed and fabricated at NSC, New Delhi for performing ERDA based material characterization experiments. This gas ionization detector is working well with position resolution better than 2 mm and good z-separation in both high and low mass regions. Large solid angle is a pre-request to minimize the possible unwanted radiation damage during the measurement. The consequent kinematic broadening has been corrected by recording the position information. Two-dimensional position sensitivity has also been obtained. Possibility of controlled modification is shown in this work.

Interatomic potential and channelling

Abstract An interatomic potential proposed recently in connection with some planar channelling calculations has been examined with regards to its predictions for the quantities of interest in axial channeling, such as string potential and critical angles. The simplicity of this potential has been found to be of great use in connection with the calculations on effects of dislocations on the energy loss of channelled ions. The quantities of interest in dechannelling, both axial and planar, have also been calculated and compared with those obtained using Lindhard′s potential.

Ohmic contacts to pseudomorphic HEMTs with low contact resistance due to enhanced Ge penetration through AlGaAs layers

AuGe/Ni ohmic contacts are used as source and drain electrodes of pseudomorphic HEMTs (pHEMTs). High alloying temperatures are generally believed to be necessary to enhance penetration of the alloy materials through the AlGaAs layers in order to establish a very low resistance path for the source–drain currents to access the two-dimensional electron gas (2DEG) layer. Here we have performed alloying experiments in the temperature range of 390–450 °C, and the contact resistance was determined using transfer length method measurements. Germanium diffusion was studied using backside secondary ion mass spectrometry. During our study, we have observed that doping of the channel by germanium is possible even at lower temperatures. But alloying at lower temperatures does not appreciably enhance the concentration throughout the different device layers below the contact pads. Hence, unlike MESFET alloying, higher alloying temperatures are essential for increasing the doping concentration so as to reduce the contact resistance and overcome the resistance of the AlGaAs layers.

Ion beam characterization and engineering of strain in semiconductor multi-layers

Abstract The objective of this work is to measure and engineer the strain in III–V compound semiconductor multi-layers using ion beams, which leads to spatial band-gap tuning for the integration of optoelectronics devices. Strained layer superlattices have been of unique interest due to their ability to tune the band gap, which depends on the strain at the interface. Therefore the strain measurements at the interface are of great interest. A large area two-dimensional positions sensitive Δ E − E detector telescope and a fully automated high energy channeling facility have been developed at NSC, New Delhi. An overview of the series of ERDA, RBS, Channeling, HRXRD and ion beam mixing experiments performed in this direction will be discussed in this paper.

Interatomic potential for atomic collisions in real solids

Abstract An extremely simplified inter-atomic potential proposed recently has been modified so that it satisfied the basic properties like charge neutrality and unit screening function at origin (φ(0)= 1). The simplicity is retained to a great extent so that it can still be used for calculations on charged particle propagation in real solids with defects.