Ion beam synthesis of nanoclusters is studied via both kinetic Monte Carlo simulations and the self-consistent mean-field solution to a set of coupled rate equations. Both approaches predict a steady-state shape for the cluster size distribution that depends only on a characteristic length determined by the ratio of the effective diffusion coefficient times the effective solubility to the ion flux. The average cluster size in the steady state regime is determined by the implanted species/matrix interface energy.

GeSn alloy nanocrystals were formed by implantation of Ge and Sn ions into an amorphous SiO2 matrix and subsequent thermal annealing. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with a high angle annular dark field (HAADF) detector were used to show that phase-segregated crystalline bi-lobe nanocrystals were formed. Rapid melting and solidification using a single excimer laser pulse transformed the bi-lobe structure into a homogeneously mixed amorphous structure. Raman spectroscopy was used to monitor the crystalline nature and approximate grain size of the Ge portion of the nanocrystals after each heat treatment, and the Raman spectra were compared with the TEM images.

The following article is an edited transcript based on the David Turnbull Lecture given by Eugene E. Haller (University of California, Berkeley) at the 2005 Materials Research Society Fall Meeting in Boston on November 29, 2005. The David Turnbull Lectureship is awarded to recognize the career of a scientist who has made outstanding contributions to understanding materials phenomena and properties through research, writing, and lecturing, as exemplified by the life work of David Turnbull. Haller was named the 2005 David Turnbull Lecturer for his “pioneering achievements and leadership in establishing the field of isotopically engineered semiconductors; for outstanding contributions to materials growth, doping and diffusion; and for excellence in lecturing, writing, and fostering international collaborations.”

The scientific interest, increased availability, and technological promise of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies of isotopically controlled semiconductor crystals. This article reviews results obtained with isotopically controlled semiconductor bulk and thin–film heterostructures. Isotopic composition affects several properties, such as phonon energies, band structure, and lattice constant, in subtle, yet—for their physical understanding–significant ways. Large isotope-related effects are observed for thermal conductivity in local vibrational modes of impurities and after neutron transmutation doping. Spectacularly sharp photo-luminescence lines have been observed in ultrapure, isotopically enriched silicon crystals. Isotope multilayer structures are especially well suited for studies of simultaneous self-and dopant-diffusion. The absence of any chemical, mechanical, or electrical driving forces makes possible the study of an ideal random-walk problem, in which moving atoms go in random directions at random intervals. Isotopically controlled semiconductors may find applications in quantum computing, nanoscience, and spintronics.

High-energy particle irradiation has been used to control the free electron concentration and electron mobility in InN by introducing native point defects that act as donors. A direct comparison between theoretical calculations and the experimental electron mobility suggests that scattering by triply-charged donor defects limits the mobility in irradiated samples across the entire range of electron concentrations studied. Thermal annealing of irradiated films in the temperature range 425°C to 475°C results in large increases in the electron mobility that approach the values predicted for singly-ionized donor defect scattering. It is suggested that the radiation-induced donor defects are stable, singly-charged nitrogen vacancies, and triply-charged, relaxed indium vacancy complexes that are removed by the annealing.

We present experimental results of impurity and self-diffusion in an isotopically controlled silicon heterostructure extrinsically doped with phosphorus. As a consequence of extrinsic doping, the concentration of singly negatively charged native defects is enhanced and the role of these native defect charge states in the simultaneous phosphorus and Si self-diffusion can be determined. Multilayers of isotopically controlled 28Si and natural silicon enable simultaneous analysis of 30Si self-diffusion into the 28Si enriched layers and phosphorus diffusion throughout the multilayer structure. An amorphous 260 nm thick Si cap layer was deposited on top of the Si isotope heterostructure. The phosphorus ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer, preventing the generation of excess native defects and enabling the determination of the Si self-diffusion coefficient and the phosphorus diffusivity under equilibrium conditions. These samples were annealed at temperatures between 950 and 1100°C to study the diffusion. Detailed analysis of the diffusion process was performed on the basis of a P diffusion model which involves neutral and positively charged mobile P species and neutral and singly negatively charged self-interstitial.

Isotopically controlled silicon multilayer structures were used to measure the enhancement of self- and dopant diffusion in extrinsic boron doped silicon. 30Si was used as a tracer through a multilayer structure of alternating natural Si and enriched 28Si layers. Low energy, high resolution secondary ion mass spectrometry (SIMS) allowed for simultaneous measurement of self- and dopant diffusion profiles of samples annealed at temperatures between 850°C and 1100°C. A specially designed ion-implanted amorphous Si surface layer was used as a dopant source to suppress excess defects in the multilayer structure, thereby eliminating transient enhanced diffusion (TED) behavior. Self- and dopant diffusion coefficients, diffusion mechanisms, and native defect charge states were determined from computer-aided modeling, based on differential equations describing the diffusion processes. We present a quantitative description of B diffusion enhanced self-diffusion in silicon and conclude that the diffusion of both B and Si is mainly mediated by neutral and singly positively charged self-interstitials under p-type doping. No significant contribution of vacancies to either B or Si diffusion is observed.

We present experimental results of dopant- and self-diffusion in extrinsic silicon doped with As. Multilayers of isotopically controlled 28Si and natural silicon enable simultaneous analysis of 30Si diffusion into the 28Si enriched layers and dopant diffusion throughout the multilayer structure. In order to suppress transient enhanced self- and dopant diffusion caused by ion implantation, we adopted a special approach to dopant introduction. First, an amorphous 250-nm thick Si layer was deposited on top of the Si isotope heterostructure. Then the dopant ions were implanted to a depth such that all the radiation damage resided inside this amorphous cap layer. These samples were annealed for various times and temperatures to study the impact of As diffusion and doping on Si self-diffusion. The Si self-diffusion coefficient and the dopant diffusivity for various extrinsic n-type conditions were determined over a wide temperature range. We observed increased diffusivities that we attribute to the increase in the concentration of the native defect promoting the diffusion.

Thin Ba0.2Sr0.8TiO3 films were pulsed laser deposited at laser pulse repetition rates of 1, 5 and 20 Hz in order to assess the effect of grain size on pyroelectric properties. Scanning electron microscopy reveals an increase in columnar grain size with decreasing laser pulse rate, as expected. Substrate bending measurements show an increase in tensile stress with increasing grain size, suggesting some form of stress relaxation has occurred. X-ray diffraction studies reveal all films to be epitaxial and oriented, while Rutherford Backscattering Spectroscopy finds no significant differences in film composition. The films grown at 5 and 20 Hz display a suppressed dielectric permittivity as a function of temperature, while the film deposited at 1 Hz displays an unusually sharp and large dielectric permittivity peak. A variation on current theory concerning phase transitions of stressed ferroelectric thin films is proposed to explain these results.

We report on increasing the pulse length and repetition rate of p-type germanium lasers through miniaturization, increased electric field uniformity, and improved cooling. We have recently demonstrated that it is possible to improve the electric field uniformity necessary for an efficient laser and at the same time decrease the electrical input power by using a geometry with d/L>>1, where d is the distance between electrical contacts and L is the length in the direction of the Hall field. In order to achieve good heat sinking along with a large d/L ratio, we have developed a new, planar contact geometry. Attaching an undoped, high-resistivity, single-crystal Si heat sink to the base of the Ge planar contact laser increases the duty cycle by a factor of 5.5. In order to further decrease the input power by decreasing the volume of laser crystals in the planar contact geometry, we show as a proof-of-concept the use of polished strontium titanate single crystals as electrically insulating far-infrared mirrors based on restrahl band reflection. The physical phenomena underlying these improvements in this novel geometry will be discussed.

We propose a new approach for modeling of impurity diffusion at semiconductor heterointerfaces. The approach is based on the notion of a common energy reference for highly localized defects. It is shown that in the kick-out process, the segregation of group II acceptors is controlled by the valence band offsets among different constituent layers of the heterostructure. Extensive numerical modeling of the diffusion provides an explanation for the experimentally observed strong segregation of Zn and Be acceptors in the lattice matched InP/InGaAs, InP/InGaAsP and GaAs/AlGaAs heterostructures.

Impurities and defects with masses smaller than the masses of the host semiconductor crystal atoms typically exhibit vibrational frequencies well above the phonon frequency spectrum. These vibrational modes produce sharp spectral absorption features in the infrared. Because of their strong spatial localization these modes are not affected by neighboring impurities and/or defects with concentrations up to 1019 cm−3. This insensitivity is especially advantageous when the free carrier concentration must be reduced through the introduction of electron irradiation defects or when highly doped thin layers must be investigated. LVM spectroscopy with perturbations such as polarization of the probe light, uniaxial and hydrostatic stress, and isotope substitution has been highly successful in identifying the structure and composition of a large number of defect complexes. Hydrogen, in particular, forming a wide variety of complexes in elemental and compound semiconductors has been extensively studied with LVM spectroscopy. For example, it has been shown recently that nitrogen acceptors are hydrogen passivated in MOCVD grown ZnSe. Carbon and oxygen have been investigated in all major semiconductors with LVM spectroscopy. The extreme simplification of the spectrum of bond centered oxygen through isotope enrichment of several Ge crystals has been demonstrated. Additional recent investigations of importance to the currently much studied semiconductors will be reviewed.

The efficiency of far-IR germanium photoconductive detectors can be markedly improved by antireflective coatings. Recently, there has been an effort to develop several micrometer thick, low stress, amorphous carbon films for this purpose. To date, films of no more than 1 to 2 μm have been reported in the literature. In this paper we report the deposition of low stress carbon films which are over 5 μm thick and are effective antireflective coatings at wavelengths of up to Λ = 43 μm. Minimal stress, a requirement for good adhesion, was achieved with a chemical vapor deposition process (CVD) by controlling the hydrocarbon partial pressure (2.2 mTorr) and by doping the carbon film with nitrogen.

We report nuclear magnetic resonance measurements of deuterone diffused into silicon powders. Deuterium spectra show a two-component lineshape corresponding to two distinct hydrogen sites. The component with a narrow Lorentzian lineshape is assigned to D2 molecules. The second component is a quadrupolar doublet powder pattern, and its characteristic splitting provides a value of the electric field gradient at the site of the deuterium atom of 1.45×105 dyn/cm.

The elemental semiconductors silicon and germanium can be purified to electrically active impurity concentrations as low as ∼1010cm−3. Highly sensitive, energy dispersive analytical techniques have been developed to identify and measure the concentration of the residual elemental impurities. The application of these techniques to very pure materials has also resulted in the discovery of a large number of new levels which are due to impurity/defect complexes. Photothermal ionization spectroscopy using uniaxial stress or a magnetic field, electron paramagnetic resonance, and doping experiments using stable and radioactive elements have been used in combination to identify the composition and the structure of some of the new centers.

Silicon nuclear radiation detectors are available today in a large variety of sizes and types. This profusion has been made possible by the ever increasing quality and diameter silicon single crystals, new processing technologies and techniques, and innovative detector design. The salient characteristics of the four basic detector groups, diffused junction, ion implanted, surface barrier, and lithium drift are reviewed along with the silicon crystal requirements. Results of crystal imperfections detected by lithium ion compensation are presented. Processing technologies and techniques are described. Two recent novel position-sensitive detector designs are discussed—one in high-energy particle track reconstruction and the other in x-ray angiography. The unique experimental results obtained with these devices are presented.