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II-VI Aerospace & Defense currently supports the defense industry on several aerospace platforms by providing high quality, large, single crystal sapphire substrates and windows. There continues to be a rapidly increasing interest in large substrates capable of use in high energy laser (HEL) systems. Such applications require materials with high thermal conductivity and high thermal shock resistance, high strength, and near-zero defect levels. Laser damage-inducing defects include those which are incorporated in the substrate during crystal growth, as well as defects incurred during routine optical fabrication and finishing processes. In the following paper, II-VI A&D presents data for a-plane and cplane sapphire windows of sufficient size for projected HEL applications exhibiting laser-grade surface quality and surface roughness, while still maintaining low rms transmitted wavefront error. Included is a discussion evaluating the benefits of sapphire for envisioned HEL windows, as well as engineering challenges that should be considered.

Clostridial myonecrosis is a medical and surgical emergency which requires early and aggressive intervention to reduce mortality. We report a rare case of Clostridium septicum myonecrosis that disseminated hematogenously from a gastric perforation. The patient was afebrile and hemodynamically stable upon admission. He rapidly developed spontaneous clostridial myonecrosis and succumbed to septic shock 36 hours after presentation. In our extensive literature review this is the only case with blood cultures confirming Clostridium septicum bacteremia with a surgically confirmed gastric perforation source in the setting of spontaneous clostridial myonecrosis.

In this work, we are showing that iron (Fe) related defects in mono-silicon have very different recombination characteristics depending on the doping element employed. While the defect characteristics of the Fe in its dissociated state is comparably the same in the materials of investigation, the defect characteristics of the associated state vary considerably. By using, defect parameter contour mapping (DPCM), a newly developed method for analyzing temperature and injection dependent lifetime data, we have for the first time, been able to show that in the case of gallium doping it is the orthorhombic state of the Fe-acceptor complex that is dominating the lifetime.

There is an increasing industry need for high quality materials which can be used in high energy laser (HEL) applications. Currently, II-VI OS is growing large sapphire substrates for use in the defense industry on aerospace applications that operate in the visible and mid-wave infrared regions. II-VI OS is capable of producing a-plane and cplane sapphire substrates of sufficient size for envisioned HEL applications, and in the following paper will present optical and material data on properties of interest. Specifically, data will be presented on BTDF and BRDF at 1.06μm in both s- and p-polarizations at angles of incidence (AOI) ranging from 0 to 60 degrees. These measurements are performed on a-plane and c-plane windows grown, fabricated and polished at II-VI OS Additionally, other critical properties evaluated include absorption at 1.06μm, as well as thermal expansion coefficient and Knoop hardness of cplane sapphire.

Single crystal production of silicon for solar cell substrates has relied on the Dash neck technique developed more than 50 years ago. The technique is simple and repeatable and enables truly dislocation free crystal growth. It does have drawbacks, however, including limits on throughput and some structural difficulties. It has long been assumed that dislocation-free growth is not possible by any other method. In the ‘quasi-mono’ crystal growth technique, one of the key elements is the use of large area single crystal seeds. By melting the seeds at near-equilibrium conditions, it is feasible to avoid the production of dislocations during melting. We will review the dislocation relevant details of the large area seeding process and present best case results for dislocation density, including measured minority carrier lifetimes in excess of 1 ms on p-type material. We will focus on dislocation density exclusive of seed boundaries, but we will also present a potential best-case limit for the technique.

Spalling is a promising kerfless method for cutting thin silicon wafers while doubling the yield of a silicon ingot. The main obstacle in this technology is the high total thickness variation of the spalled wafers, often as high as 100% of the wafer thickness. It has been suggested before that a strong correlation exists between low crack velocities and a smooth surface, but this correlation has never been shown during a spalling process in silicon. The reason lies in the challenge associated to measuring such velocities. In this contribution, we present a new approach to assess, in real time, the crack velocity as it propagates during a low temperature spalling process. Understanding the relationship between crack velocity and surface roughness during spalling can pave the way to attain full control on the surface quality of the spalled wafer.

Single crystal growth by the edge-defined film-fed growth method (EFG) has been commercially practiced for over 40 years. State of the art sapphire crystals can be over 300 mm wide, 10 mm thick and 1 m long. In addition to well-known defects such as bubble inclusions and bulk structure loss, we describe a crystalline defect occurring uniquely at the outer surfaces of the sapphire panel. Termed ‘surface crystals’, they penetrate less than 1 mm into the depth of the A-plane panel and can originate and terminate multiple times in a single panel. We report correlations with both temperature and ambient conditions and present micro-scale characterization to elucidate the nature of these defects.

Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determined to be 220.

In this work we show that the high minority carrier lifetime in as-grown Ga-Si wafers is dominated by low levels of iron contamination incorporated during silicon growth. Upon phosphorous diffusion iron is however effectively removed, increasing the bulk carrier lifetime from a few hundred microseconds to well above one milli-second. Lifetime spectroscopy in combination with Shockley Read Hall theory was used to determine the concentrations of Fe i and FeGa complexes in the course of the FeGa association. Finally, we use the estimated concentrations of FeGa as a function of time of storage in the dark to validate that FeGa association follows the laws of coulombic attraction similar to FeB.

Cast multicrystalline silicon ingots are widely used in photovoltaic manufacturing. A key issue to achieving high solar cell efficiencies is attaining an optimized temperature profile during ingot fabrication in order to control the formation of extended defects such as grain boundaries, sub-grain boundaries and dislocations. Numerical simulation is a powerful tool to help understand and depict the temperature distribution inside the growth furnace and to allow tailoring of the growth conditions to achieve high quality with a minimum of defects. Two major types of casting furnaces, direct solidification system and heat exchange method system, will be compared from the aspects of thermal field, crystal quality and related numerical simulations.

A novel crystal growth method has been developed for the production of ingots, bricks and wafers for solar cells. Monocrystallinity is achievable over large volumes with minimal dislocation incorporation. The resulting defect types, densities and interactions are described both microscopically for wafers and macroscopically for the ingot, looking closely at the impact of the defects on minority carrier lifetime. Solar cells of 156 cm2 size have been produced ranging up to 17% in efficiency using industrial screen print processes.

Recently we reported room temperature point defect creation and subsequent extended defect nucleation in nitrogen-doped silicon during 200 kV electron irradiation, while identical irradiation of nitrogen-free silicon produced no effect. In this paper, first principles calculations are combined with new transmission electron microscope (TEM) observations to support a new model for elastic electron-silicon interactions in the TEM, which encompasses both nitrogen doped and nitrogen free silicon. Specifically, the nudged elastic band method was used to study the energetics along the diffusion path during an electron collision event in the vicinity of a nitrogen pair. It was found that the 0 K estimate for the energy barrier of a knock-on event is lowered from ∼ 12 to 6.2 eV. However, this is still inadequate to explain the observations. We therefore propose an increase in the energy barrier for Frenkel pair recombination associated with N2-V bonding. Concerning pure silicon, stacking fault formation near irradiation-induced holes demonstrates the participation of bulk processes. In low oxygen float zone material, 2–5 nm voids were formed, while oxygen precipitation in Czochralski Si has been verified by electron energy-loss spectroscopy. Models of irradiation-induced point defect aggregation are presented and it is concluded that these must be bulk and not surface mediated phenomena.

Single crystal Silicon has been produced using a cast-in-place process usually used for multicrystalline ingot production. This Mono2 ™ material produces higher cell efficiencies compared to multi crystalline Silicon material with the same average minority carrier lifetime. Profiles of lifetime through the height of the brick and across the ingot demonstrate rough equivalency in bulk lifetime between Mono2 ™ and multi material. However, the standard deviation of lifetime in wafers is smaller for the Mono2 ™ material. This quality difference accounts for about half of the potential efficiency gain in Mono2 ™ material, while the rest of the gain, primarily in J sc , is derived from the ability to form pyramidal light trapping texture on the (100) surface. Minority carrier lifetime data is compared with cell electrical data from the same collection of ingot positions. Open circuit voltage and short circuit current correlate well to some degree with bulk lifetime, while fill factor is independent both of bulk lifetime and position in the ingot.

This paper will report on the BP Solar/Solarex efforts on polycrystalline silicon PV modules under the NREL/DOE PV Manufacturing Technology Program. BP Solar (previously Solarex) has been working on polycrystalline silicon PV technology under the NREL sponsored Photovoltaic Manufacturing Technology Program since 1993. This paper will review some of the major achievements of these programs. Results from a recently completed contract will be presented with emphasis on the development of processes and handling procedures for ultra-thin silicon wafers and cells. Finally the paper will conclude with a discussion of the future work planned in the present program.

In this Letter, we present ab initio results identifying a new diffusion path for the nitrogen pair complex in silicon, resulting in an effective diffusivity of 67exp(-2.38 eV/kT) cm{sup 2}/s. This nudged elastic band result is compared with other nitrogen diffusion paths and mechanisms, and is determined to have unmatched agreement with experimental results. It is also shown that careful consideration of total energy corrections and use of a fully temperature-dependent diffusion prefactor have modest but important effects on the calculation of diffusivity for paired and for interstitial nitrogen.

Nitrogen doped silicon samples were irradiated with 200 kV electrons in a transmission electron microscope (TEM). The resulting room temperature point defect creation, bonding and segregation were studied by in situ conventional and Z Contrast TEM imaging and electron energy loss spectroscopy (EELS). Energy loss spectra from areas attributed to be rich in vacancies or silicon self-interstitials are found to be significantly different from the bulk in the near-edge structure of their Si-L2,3 edges. The experimental results are compared with ab initio density of states calculations for electronically excited atoms near relaxed point defect structures and plans are outlined to extend this technique to individual point defect characterization.

A sample of nitrogen-doped, single crystal Czochralski silicon was subjected to several different surface preparations. Secondary ion mass spectrometry depth profiling has shown that prolonged glancing-angle bombardment by 3–5kV Ar+ ions significantly increases the nitrogen concentration in the near surface by up to an order of magnitude over the bulk value. Concentrations are observed to be elevated over the bulk value to a depth up to 25μm. Nitrogen-implanted samples and samples with a 1nm surface nitride did not exhibit nitrogen segregation under the same conditions, but a sample with 100nm of surface nitride did exhibit ion bombardment induced drive-in. In nitride-free samples, the source of the nitrogen is indicated to be a nitrogen-rich layer in the first micron of material. The diffusion behavior of nitrogen in silicon is discussed and the Crowdion mechanism for diffusion is suggested as the enabling mechanism for the enhanced low temperature diffusion.

Samples of Czochralski silicon were observed after irradiation by a convergent electron beam in a transmission electron microscope. In a nitrogen-doped sample, the 200 keV electrons induced a vacancy-rich region containing point-defect clusters, surrounded by a ring rich in self-interstitials. No comparable effect existed in nitrogen-free reference samples. It is proposed that Frenkel pairs, created by electron collisions, are separated and stabilized by nitrogen or related complexes. Some interstitials become free to diffuse while the nitrogen, vacancies and oxygen agglomerate. This study demonstrates that the initial formation of voids and precipitate nuclei from point defects can be observed at low temperatures. © 2003 The Electrochemical Society. All rights reserved.