The objective of the research in this thesis was to develop high-efficiency n-type silicon solar cells at low-cost to reach grid parity. This was accomplished by reducing the electrical and optical losses in solar cells through understanding of fundamental physics and loss mechanisms, development of process technologies, cell design, and modeling. All these technology enhancements provided a 3.44% absolute increase in efficiency over the 17.4% efficient n-type PERT solar cell. Finally, 20.84% efficient n-type PERT (passivated emitter and rear totally diffused) solar cells were achieved on commercial grade 239cm2 n-type Cz silicon wafers with optimized front boron emitter without boron-rich layer and phosphorus back surface field, silicon dioxide/silicon nitride stack for surface passivation, optimized front grid pattern with screen printed 5 busbars and 100 gridlines, and improved rear contact with laser opening and physical vapor deposition aluminum. This thesis also suggested research directions to improve cell efficiency further and attain ≥21% efficient n-type solar cells which involves three additional technology developments including the use of floating busbars, selective emitters, and negatively charged aluminum oxide (Al2O3) film for boron emitter surface passivation.

<p> The integration of silicon germanium (SiGe) in the base of heterojunction bipolar transistors (HBTs) has recently put the alloy into prominence to produce fast-switching transistors. However, the thin highly doped SiGe base makes the transistor susceptible to base dopant outdiffusion during device processing, which results in device performance degradation. Adding carbon to the base was shown to significantly suppress boron outdiffusion and help retain the narrow as-grown profile. Dopant behavior in the presence of various species needs to be well understood and modeled for two reasons: (1) to have accurate and predictive process simulators; and (2) to obtain insight into process development. </p> <p> Modeling carbon diffusion and its role in suppressing boron diffusion in silicon and SiGe has been studied by several groups. While boron diffusion is well-established, different modeling regimes have been developed for carbon diffusion. Each of the existing studies has focused on subsets of the available experimental data. We present a consistent and complete model that accounts for carbon and boron diffusion in silicon and SiGe, under equilibrium and non-equilibrium conditions. In our regime, carbon diffusion is modeled according to the kick-out and Frank-Tumbull mechanisms for diffusion; in addition, we incorporate the carbon clustering phenomenon. To completely model boron diffusion, we account for the boron-interstitial clustering (BICs) effect and the { 311} defects that are associated with boron transient enhanced diffusion (TED). In the developed model we make use of the well-established literature data for carbon diffusion, as well as boron diffusion and Si self-diffusion. The model was verified by simulating experiments that involve boron and/or carbon diffusion in silicon and SiGe and cover the complete temperature range of 750 - 1070 °C. The test structures include published experiments in addition to recent experimental results obtained through collaboration, and feature diffusion in inert and oxidizing ambients, under rapid thermal annealing (RTA) conditions, as well as in the presence of implant damage. We also investigated the validation of the model without the inclusion of either the clustering or the Frank-Turnbull reactions. </p> / Thesis / Master of Applied Science (MASc)

This thesis undertakes the development, characterization and optimization of boron diffusion for silicon solar cells. Heavy diffusions (sheet resistance < 40 Ohm/square) to form a back surface field, and light diffusions (sheet resistance > 100 Ohm/square) to form oxide-passivated emitters were developed. Test structures and solar cells were fabricated to assess uniformity, lifetime and recombination effects due to the light and heavy boron diffusions. It was found that the growth of a thin ~200 ??, thermal oxide, during stabilization ??? immediately prior to the boron diffusion - was required to maintain high lifetime and reduce surface recombination (reducing the emitter saturation current density) for all boron diffusions. The heavy boron diffusion process was incorporated into the single side buried contact solar cell processing sequence. The solar cells fabricated had both boron diffused and Al/Si alloyed P+ regions for comparison. This research conclusively showed that boron diffused solar cells had significantly higher open circuit voltage compared to Al/Si alloyed devices. Fabrication of n-type solar cells, and their subsequent characterization by overlayed secondary electron image and the electron beam induced current map showed that the Al/Si alloy varied in depth from 5 to 25 micrometers deep. Methodology and characterization for light, oxide-passivated boron diffusions are also presented. This study yielded boron diffused emitters (sheet resistance > 100 Ohm/square) with low emitter saturation current. It was observed that this was possible only when the thermal oxidation after the boron diffusion was minimal, less than 1,000 ??. This was due to the segregation effect of boron with oxide, decreasing the surface concentration that in turn decreased the electric field repulsion of electrons from the surface. Device modelling of n-type solar cells is presented where the parameters of the modelling include the results of the light, oxide-passivated boron diffusions. This modelling shows n-type-base material with light oxide-passivated boron diffusion has higher potential conversion efficiency than forming a solar cell from phosphorous diffused p-type material.

This thesis undertakes the development, characterization and optimization of boron diffusion for silicon solar cells. Heavy diffusions (sheet resistance < 40 Ohm/square) to form a back surface field, and light diffusions (sheet resistance > 100 Ohm/square) to form oxide-passivated emitters were developed. Test structures and solar cells were fabricated to assess uniformity, lifetime and recombination effects due to the light and heavy boron diffusions. It was found that the growth of a thin ~200 ??, thermal oxide, during stabilization ??? immediately prior to the boron diffusion - was required to maintain high lifetime and reduce surface recombination (reducing the emitter saturation current density) for all boron diffusions. The heavy boron diffusion process was incorporated into the single side buried contact solar cell processing sequence. The solar cells fabricated had both boron diffused and Al/Si alloyed P+ regions for comparison. This research conclusively showed that boron diffused solar cells had significantly higher open circuit voltage compared to Al/Si alloyed devices. Fabrication of n-type solar cells, and their subsequent characterization by overlayed secondary electron image and the electron beam induced current map showed that the Al/Si alloy varied in depth from 5 to 25 micrometers deep. Methodology and characterization for light, oxide-passivated boron diffusions are also presented. This study yielded boron diffused emitters (sheet resistance > 100 Ohm/square) with low emitter saturation current. It was observed that this was possible only when the thermal oxidation after the boron diffusion was minimal, less than 1,000 ??. This was due to the segregation effect of boron with oxide, decreasing the surface concentration that in turn decreased the electric field repulsion of electrons from the surface. Device modelling of n-type solar cells is presented where the parameters of the modelling include the results of the light, oxide-passivated boron diffusions. This modelling shows n-type-base material with light oxide-passivated boron diffusion has higher potential conversion efficiency than forming a solar cell from phosphorous diffused p-type material.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 91-94). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / Typescript. Vita. Includes bibliographical references (leaves 91-94). Also available on the Internet.

The objective of the proposed research is to develop low-cost, screen-printed 20% efficient silicon solar cells. In the first part of this thesis, a ~19% efficient, screen-printed cell was fabricated using the commercially-dominant aluminum back surface field (Al-BSF) cell structure. Device modeling was then used to determine that increasing the efficiency to 20% required improvements in both back surface passivation and rear reflectance. In the second part of this thesis, a passivated, transparent boron BSF (B-BSF) structure was proposed as a high-throughput method for realizing these improvements. The first step in fabricating the proposed B-BSF cell involved the successful development of a water-based, spin-on solution of boric acid as a low-cost, non-toxic and non-pyrophoric alternative to common boron diffusion sources such as boron tribromide. A review of the literature shows that a common problem with boron diffusion is severe bulk lifetime degradation, with Fe contamination being commonly speculated as the cause. An experimental study was therefore devised in which the impact of boron diffusion and subsequent cell process steps on the bulk lifetime and bulk iron contamination was tracked. From this study, a model for boron diffusion-induced Fe contamination was developed along with methods for gettering Fe from the substrate. A key achievement of this thesis was the discovery of a novel, negatively charged, aluminum-doped spin-on glass (SOG) which can, in a short thermal step, simultaneously getter Fe and provide stable, high-quality passivation of planar, boron-diffused Si surfaces. Since past attempts at achieving low-cost, high-efficiency, boron-diffused cells have suffered from bulk lifetime degradation and difficulties with passivating a boron-diffused Si surface, the Al-doped SOG provides a solution to both challenges. Since a high rear reflectance is important for achieving high-efficiencies, an experimental study of various reflectors was undertaken and a silver colloid material was found which exhibits both high electrical conductivity and Lambertian reflectance >95%. The work on boric acid diffusion, iron gettering, surface passivation and rear reflectors was successfully integrated into a 20.2% efficient, screen-printed, B-BSF cell fabricated on 300 µm thick, p-type float-zone (FZ) Si wafers. Both device theory and modeling was used to show that, due to its well-passivated surfaces, this cell would suffer a large loss in efficiency due to light-induced degradation (LID) if it were fabricated on commercial p-type Czochralski (Cz) Si substrates. Since n-type Si substrates do not suffer from LID, the p-type process was slightly tweaked and applied to n-type FZ wafers, resulting in 20.3% efficient cells on 190 µm thick wafers. Computer modeling shows that both the p-type and n-type cells can maintain efficiencies of 20% for wafers as thin as 100 µm.

Tunnelmagnetowiderstandselemente (MTJ) mit einer kristallinen MgO Tunnelbarriere zwischen amorphen CoFeB-Elektroden haben Aufgrund ihres hohen Tunnelmagnetowiderstandes (TMR) und der guten Integrationsmöglichkeit in bestehende Prozesse viel Aufmerksamkeit bekommen. Dabei zeigten vorherige Berechnungen, dass die strukturellen und chemischen Eigenschaften der Grenzfläche einen signifikanten Einfluss auf den TMR aufweisen, weshalb diese Grenzfläche im Rahmen dieser Arbeit mittels quantitativer, hochauflösender und analytischer Transmissionselektronenmikroskopie analysiert wurde. Um einen hohen TMR in die diesen Systemen zu erzielen ist ein kristalliner Übergang zwischen der Tunnelbarriere und den Elektroden notwendig. Berechnungen zeigten, dass bereits wenige Monolagen kristallinen Materials an der Grenzfläche ausreichen, um einen hohen TMR in diesen Systemen zu erzielen. Ausgehend von diesen Berechnungen wurde die Mikrostruktur auf der Subnanometer-Skala an der kristallin/amorphen Grenzfläche von MgO-CoFeB in dieser Arbeit untersucht. Die experimentellen Daten wurden hierfür mittels aberrationskorrigierter, hochauflösender Transmissionselektronenmikroskopie (HRTEM) an Modellsystemproben erstellt und die vom MgO induzierte kristalline Ordnung an der Grenzfläche zum CoFeB mittels iterativen Bildserienvergleichs mit simulierten Daten quantifiziert. Zur Simulation der HRTEM-Grenzflächenabbildungen wurde die „Averaged-Projected-Potential“-Näherung genutzt, welche im Rahmen dieser Arbeit um die Berücksichtigung von monoatomaren Stufen entlang der Strahlrichtung des Mikroskops erweitert wurde. Es zeigte sich, dass mit dieser Methode die Ordnung an der MgO-CoFeB-Grenzfläche von nicht ausgelagerten Systemen gut beschrieben werden kann. In ausgelagerten Systemen kommt es dagegen zu einer Bor-diffusion aus dem a-CoFeB heraus um damit eine Kristallisation am MgO zu ermöglichen. Im zweiten Teil dieser Arbeit werden die Bordiffusion und die Kristallisation in Abhängigkeit von der Deckschicht als auch der MgO-Depositionsmethode sowohl an Modellsystemproben als auch an funktionsfähigen MTJs untersucht. Elektronen-Energie-Verlustspektroskopie (EELS) an diesen Proben konnten zeigen, dass sowohl die Deckschicht also auch die MgO-Depositionsmethode einen entscheidenden Einfluss auf die Bor-Diffusion in diesen Systemen ausüben.