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- All Subjects: Silicon
- Creators: Bertoni, Mariana
- Creators: Kouvetakis, John
Description
Carrier lifetime is one of the few parameters which can give information about the low defect densities in today's semiconductors. In principle there is no lower limit to the defect density determined by lifetime measurements. No other technique can easily detect defect densities as low as 10-9 - 10-10 cm-3 in a simple, contactless room temperature measurement. However in practice, recombination lifetime τr measurements such as photoconductance decay (PCD) and surface photovoltage (SPV) that are widely used for characterization of bulk wafers face serious limitations when applied to thin epitaxial layers, where the layer thickness is smaller than the minority carrier diffusion length Ln. Other methods such as microwave photoconductance decay (µ-PCD), photoluminescence (PL), and frequency-dependent SPV, where the generated excess carriers are confined to the epitaxial layer width by using short excitation wavelengths, require complicated configuration and extensive surface passivation processes that make them time-consuming and not suitable for process screening purposes. Generation lifetime τg, typically measured with pulsed MOS capacitors (MOS-C) as test structures, has been shown to be an eminently suitable technique for characterization of thin epitaxial layers. It is for these reasons that the IC community, largely concerned with unipolar MOS devices, uses lifetime measurements as a "process cleanliness monitor." However when dealing with ultraclean epitaxial wafers, the classic MOS-C technique measures an effective generation lifetime τg eff which is dominated by the surface generation and hence cannot be used for screening impurity densities. I have developed a modified pulsed MOS technique for measuring generation lifetime in ultraclean thin p/p+ epitaxial layers which can be used to detect metallic impurities with densities as low as 10-10 cm-3. The widely used classic version has been shown to be unable to effectively detect such low impurity densities due to the domination of surface generation; whereas, the modified version can be used suitably as a metallic impurity density monitoring tool for such cases.
ContributorsElhami Khorasani, Arash (Author) / Alford, Terry (Thesis advisor) / Goryll, Michael (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2013
Description
The thesis studies new methods to fabricate optoelectronic Ge1-ySny/Si(100) alloys and investigate their photoluminescence (PL) properties for possible applications in Si-based photonics including IR lasers. The work initially investigated the origin of the difference between the PL spectrum of bulk Ge, dominated by indirect gap emission, and the PL spectrum of Ge-on-Si films, dominated by direct gap emission. It was found that the difference is due to the supression of self-absorption effects in Ge films, combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a model suggesting that the deviation is caused by the shorter recombination lifetime in the films relative to bulk Ge. The knowledge acquired from this work was then utilized to study the PL properties of n-type Ge1-ySny/Si (y=0.004-0.04) samples grown via chemical vapor deposition of Ge2H6/SnD4/P(GeH3)3. It was found that the emission intensity (I) of these samples is at least 10x stronger than observed in un-doped counterparts and that the Idir/Iind ratio of direct over indirect gap emission increases for high-Sn contents due to the reduced gamma-L valley separation, as expected. Next the PL investigation was expanded to samples with y=0.05-0.09 grown via a new method using the more reactive Ge3H8 in place of Ge2H6. Optical quality, 1-um thick Ge1-ySny/Si(100) layers were produced using Ge3H10/SnD4 and found to exhibit strong, tunable PL near the threshold of the direct-indirect bandgap crossover. A byproduct of this study was the development of an enhanced process to produce Ge3H8, Ge4H10, and Ge5H12 analogs for application in ultra-low temperature deposition of Group-IV semiconductors. The thesis also studies synthesis routes of an entirely new class of semiconductor compounds and alloys described by Si5-2y(III-V)y (III=Al, V= As, P) comprising of specifically designed diamond-like structures based on a Si parent lattice incorporating isolated III-V units. The common theme of the two thesis topics is the development of new mono-crystalline materials on ubiquitous silicon platforms with the objective of enhancing the optoelectronic performance of Si and Ge semiconductors, potentially leading to the design of next generation optical devices including lasers, detectors and solar cells.
ContributorsGrzybowski, Gordon (Author) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2013
Description
The metallization and interconnection of Si photovoltaic (PV) devices are among some of the most critically important aspects to ensure the PV cells and modules are cost-effective, highly-efficient, and robust through environmental stresses. The aim of this work is to contribute to the development of these innovations to move them closer to commercialization.Shingled PV modules and laser-welded foil-interconnected modules present an alternative to traditional soldered ribbons that can improve module power densities in a cost-effective manner. These two interconnection methods present new technical challenges for the PV industry. This work presents x-ray imaging methods to aid in the process-optimization of the application and curing of the adhesive material used in shingled modules. Further, detailed characterization of laser welds, their adhesion, and their effect on module performances is conducted. A strong correlation is found between the laser-weld adhesion and the modules’ durability through thermocycling. A minimum laser weld adhesion of 0.8 mJ is recommended to ensure a robust interconnection is formed.
Detailed characterization and modelling are demonstrated on a 21% efficient double-sided tunnel-oxide passivating contact (DS-TOPCon) cell. This technology uses a novel approach that uses the front-metal grid to etch-away the parasitically-absorbing poly-Si material everywhere except for underneath the grid fingers. The modelling yielded a match to the experimental device within 0.06% absolute of its efficiency. This DS-TOPCon device could be improved to a 23.45%-efficient device by improving the optical performance, n-type contact resistivity, and grid finger aspect ratio.
Finally, a modelling approach is explored for simulating Si thermophotovoltaic (TPV) devices. Experimentally fabricated diffused-junction devices are used to validate the optical and electrical aspects of the model. A peak TPV efficiency of 6.8% is predicted for the fabricated devices, but a pathway to 32.5% is explained by reducing the parasitic absorption of the contacts and reducing the wafer thickness. Additionally, the DS-TOPCon technology shows the potential for a 33.7% efficient TPV device.
ContributorsHartweg, Barry (Author) / Holman, Zachary (Thesis advisor) / Chan, Candace (Committee member) / Bertoni, Mariana (Committee member) / Yu, Zhengshan (Committee member) / Arizona State University (Publisher)
Created2023
Description
High photoluminescence (PL) quantum yields reported from amorphous (a-Si) and crystalline (c-Si) nanoparticles have opened up lots of possibilities for use of silicon in optical applications such as light emitting diodes (LEDs), photonics and solar cells with added processing and cost benefits. However, the PL response and the mechanisms behind it are highly dependent on the matrix in which the nanoparticles are grown and on the growth method. While, the bottom-up approach for deposition of free standing nanoparticles seem to be perfectly suited for large area deposition for LED and solar cell applications, the dominant growth techniques (laser ablation and pyrolysis) have been shown to suffer from limitations in control over size distribution of nanoparticles and the requirement of equipment capable of withstanding high temperature. This led to the exploration of plasma based synthesis methods in this work.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
Towards this effort, the development and automation of a novel tool “Anny” for synthesis of silicon nanoparticles using non-thermal plasma chamber is reported. These nanoparticles are then accelerated due to choked flow through a nozzle leading to substrate independent deposition. The nanoparticle properties are characterized against precursor gas flow rates and RF power to identify the optimum growth conditions for a stable, continuous deposition. It is found that amorphous nanoparticles offer a wide variety of chamber conditions for growth with a high throughput, stable plasma for continuous, long term operations.
The quantum confinement model for crystalline and spatial confinement models for amorphous nanoparticles in our size regime (6-8nm) are suggested for free standing nanoparticles and we report a high PL output from well passivated amorphous nanoparticles.
The PL output and its dependence on stability of surface hydrogen passivation is explored using Fourier Transform Infrared spectroscopy (FTIR). It is shown that the amorphous nanoparticles have a better and more stable passivation compared to crystalline nanoparticles grown under similar conditions. Hence, we show a-Si nanoparticles as exciting alternatives for optical applications to c-Si nanoparticles.
ContributorsGarg, Prateek (Author) / Holman, Zachary C (Thesis advisor) / Zhang, Yong H (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2015
Description
Layers of intrinsic hydrogenated amorphous silicon and amorphous silicon carbide
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
were prepared on a polished, intrinsic crystalline silicon substrate via plasma-enhanced chemical vapor deposition to simulate heterojunction device relevant stacks of various materials. The minority carrier lifetime, optical band gap and FTIR spectra were observed at incremental stages of thermal annealing. By observing the changes in the lifetimes the sample structure responsible for the most thermally robust surface passivation could be determined. These results were correlated to the optical band gap and the position and relative area of peaks in the FTIR spectra related to to silicon-hydrogen bonds in the layers. It was found that due to an increased presence of hydrogen bonded to silicon at voids within the passivating layer, hydrogenated amorphous silicon carbide at the interface of the substrate coupled with a hydrogenated amorphous silicon top layer provides better passivation after high temperature annealing than other device structures.
ContributorsJackson, Alec James (Author) / Holman, Zachary (Thesis advisor) / Bertoni, Mariana (Committee member) / Kozicki, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
Modern semiconductor technologies have been dominated by group-IV materials and III-V analogues. The development of hybrid derivatives combining appropriate members of these systems has been of interest for the purpose of extending the optoelectronic capabilities of the state-of-the-art. Early work on pseudo-binary (III-V)-IV alloys, described with the general formula (III-V)1-x(IV2)x,
showed limited progress due to phase segregation, auto-doping and compositional inhomogeneities. Recently, new techniques were introduced for synthesizing new classes of (III-V)-IV hybrid materials using reactions of V(IVH3)3 molecules [V = N, P, As and IV = Si, Ge] with group-III elements (B, Al, Ga, In). The reactions produce (III-V)-IV3 building blocks that interlink to form diamond-like
frameworks in which the III-V pairs incorporate as isolated units within the group-IV lattice. This approach not only precludes phase segregation, but also provides access to structures and compositions unattainable by conventional means. Entire new families of crystalline (III-V)-IV3 and (III-V)y(IV)5-y alloys with tunable IV-rich compositions, different from conventional (III-V)1-x(IV2)x systems, have been grown on Si(100) and GaP(100) wafers as well as Si1-xGex and Ge buffer layers which, in most cases, provide lattice matched templates for Si integration.
In this work, materials in the In-P-Ge, Ga-As-Ge and Ga-P-Si systems that would exhibit direct-gap behavior were targeted. A series of (InP)yGe5-2y alloys with tunable Ge contents above 60% were synthesized by reactions of P(GeH3)3 and indium atoms and were studied for bonding, structure, and optical response. (GaAs)yGe5-2y analogues were also grown and exhibited strong photoluminescence for applications in mid-IR photonics. The GaPSi3 alloy and Si-rich derivatives were produced via reactions of P(SiH3)3 and [H2GaNMe2]2 and exhibit enhanced absorption in the visible range. Quaternary analogues in the Al1-xBxPSi3 system were grown on Si via reactions of Al(BH4)3 and P(SiH3)3 leading to the formation crystalline materials with extended absorption relative to Si. This makes them imminently suitable for applications in Si-based photovoltaics. The work emphasized use of quantum-chemical simulations to elucidate structural, thermodynamic, and mechanical properties of the synthesized systems. The theory also included simulations of new synthetic targets such as BNC3, BNSi3, BPC3, and BPSi3 with interesting mechanical properties and strong covalent bonding.
showed limited progress due to phase segregation, auto-doping and compositional inhomogeneities. Recently, new techniques were introduced for synthesizing new classes of (III-V)-IV hybrid materials using reactions of V(IVH3)3 molecules [V = N, P, As and IV = Si, Ge] with group-III elements (B, Al, Ga, In). The reactions produce (III-V)-IV3 building blocks that interlink to form diamond-like
frameworks in which the III-V pairs incorporate as isolated units within the group-IV lattice. This approach not only precludes phase segregation, but also provides access to structures and compositions unattainable by conventional means. Entire new families of crystalline (III-V)-IV3 and (III-V)y(IV)5-y alloys with tunable IV-rich compositions, different from conventional (III-V)1-x(IV2)x systems, have been grown on Si(100) and GaP(100) wafers as well as Si1-xGex and Ge buffer layers which, in most cases, provide lattice matched templates for Si integration.
In this work, materials in the In-P-Ge, Ga-As-Ge and Ga-P-Si systems that would exhibit direct-gap behavior were targeted. A series of (InP)yGe5-2y alloys with tunable Ge contents above 60% were synthesized by reactions of P(GeH3)3 and indium atoms and were studied for bonding, structure, and optical response. (GaAs)yGe5-2y analogues were also grown and exhibited strong photoluminescence for applications in mid-IR photonics. The GaPSi3 alloy and Si-rich derivatives were produced via reactions of P(SiH3)3 and [H2GaNMe2]2 and exhibit enhanced absorption in the visible range. Quaternary analogues in the Al1-xBxPSi3 system were grown on Si via reactions of Al(BH4)3 and P(SiH3)3 leading to the formation crystalline materials with extended absorption relative to Si. This makes them imminently suitable for applications in Si-based photovoltaics. The work emphasized use of quantum-chemical simulations to elucidate structural, thermodynamic, and mechanical properties of the synthesized systems. The theory also included simulations of new synthetic targets such as BNC3, BNSi3, BPC3, and BPSi3 with interesting mechanical properties and strong covalent bonding.
ContributorsSims, Patrick Edward (Author) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew V. G. (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2017