Michael A. Slocum

25 nm immersion lithography at 193 nm wavelength

The physical limitations of lithographic imaging are ultimately imposed by the refractive indices of the materials involved. At oblique collection angles, the numerical aperture of an optical system is determined by nsin(θ) , where n is the lowest material refractive index (in the absence of any refractive power through curvature). For 193nm water immersion lithography, the fluid is the limiting material, with a refractive index of near 1.44, followed by the lens material (if planar) with a refractive index near 1.56, and the photoresist, with a refractive index near 1.75. A critical goal for immersion imaging improvement is to first increase the refractive indices of the weakest link, namely the fluid or the lens material. This paper will present an approach to immersion lithography that will allow for the exploration into the extreme limits of immersion lithography by eliminating the fluid altogether. By using a solid immersion lithography (SIL) approach, we have developed a method to contact the last element of an imaging system directly to the photoresist. Furthermore, by fabricating this last element as an aluminum oxide (sapphire) prism, we can increase its refractive index to a value near 1.92. The photoresist becomes the material with the lowest refractive index and imaging becomes possible down to 28nm for a resist index of 1.75 (and 25nm for a photoresist with a refractive index of 1.93). Imaging is based on two-beam Talbot interference of a phase grating mask, illuminated with highly polarized 193nm ArF radiation. Additionally, a roadmap is presented to show the possible extension of 193nm lithography to the year 2020.

Effect of electric field on carrier escape mechanisms in quantum dot intermediate band solar cells

Carrier escape and recombination from quantum dot (QD) states reduce the probability of two-step photon absorption (TSPA) by decreasing the available carrier population in the intermediate band (IB). In order to optimize the second photon absorption for future designs of quantum dot embedded intermediate band solar cells, the presented study combined the results of simulations and experiments to quantify the effect of electric field on the barrier height and the carrier escape from the QDs in InAs/GaAs quantum dot solar cells with five-layer QD superlattices. The electric field dependent effective barrier heights for ground state electrons were calculated using eight band k·p theory at short circuit conditions. With an increase in electric field surrounding the QDs from 5 kV/cm to 50 kV/cm, the effective barrier height of the ground state electrons was reduced from 147 meV to 136 meV, respectively. Thus, the increasing electric field not only exponentially enhances the ground state electron tunneling rate...

Simulation of nipi photovoltaic devices

The simulation and characterization of multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping structure solar cells has been demonstrated. The nipi device depends almost exclusively on drift rather than diffusion currents to collect the carriers. This architecture has been proposed to increase the radiation hardness of a device due to a decreased dependence upon diffusion length. This doping superlattice will allow photo generated carriers to be rapidly transported through the junction by drift. Converting them to majority carriers, and subsequently conducted laterally to selective contacts positioned at opposite sides of etched V-groove channels in the device. The result is a parallel connected multiperiod solar cell, which has been evaluated extensively under simulation. The nipi solar cells have been simulated, giving a greater understanding of the physical mechanisms at work in the device. Design variables such as finger spacing, doping concentration, nipi stack thickness, and the doped to intrinsic thickness ratio are varied to optimize the device. These results show the nipi device has great promise for development as a high efficiency solar cell, with the potential to be used in applications where radiation hardness is required, such as satellite power systems or radioisotope batteries.

Subbandgap current collection through the implementation of a doping superlattice solar cell

The simulation and fabrication of a multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping superlattice solar cell have been demonstrated. Devices have been fabricated and characterized, demonstrating a proof of concept for a nipi device contacted via epitaxial regrowth. Current-voltage measurements in the dark and under one sun illumination were simulated and measured experimentally. Efficient current collection was demonstrated with an integrated short circuit current of 23.24 mA/cm2 measured through spectral response. Efficiency improvements from prior results have been achieved, with a maximum one sun AM0 efficiency of 3.42%. Sub-bandgap spectral response shows a 3.3% increase over the bulk response.

Characterization of InAlAs solar cells grown by MOVPE

Epitaxial layers of InAlAs are prime candidates for the top cell in triple-junction photovoltaics (PV). Growth conditions during metalorganic vapor phase epitaxy (MOVPE) of InAlAs affect the material properties and subsequently the device characteristics of the epilayers. Impurity concentrations in InAlAs epilayers grown under various conditions are analyzed by secondary-ion mass spectrometry (SIMS) in order to assess impurity incorporation. Deep-level transient spectroscopy (DLTS) is used to assess the energy level and concentration of carrier traps. The effect of defects (traps) on the device characteristics are modeled with a Sentaurus simulation. Devices were fabricated and tested in a solar simulator before and after contact etch. Spectral response (SR) and electroluminescence (EL) are also measured. Final experimental results showed an efficiency of 9.74% without an antireflective coating.

GaSb solar cells grown on GaAs via interfacial misfit arrays for use in the III-Sb multi-junction cell

Growth of GaSb with low threading dislocation density directly on GaAs may be possible with the strategic strain relaxation of interfacial misfit arrays. This creates an opportunity for a multi-junction solar cell with access to a wide range of well-developed direct bandgap materials. Multi-junction cells with a single layer of GaSb/GaAs interfacial misfit arrays could achieve higher efficiency than state-of-the-art inverted metamorphic multi-junction cells while forgoing the need for costly compositionally graded buffer layers. To develop this technology, GaSb single junction cells were grown via molecular beam epitaxy on both GaSb and GaAs substrates to compare homoepitaxial and heteroepitaxial GaSb device results. The GaSb-on-GaSb cell had an AM1.5g efficiency of 5.5% and a 44-sun AM1.5d efficiency of 8.9%. The GaSb-on-GaAs cell was 1.0% efficient under AM1.5g and 4.5% at 44 suns. The lower performance of the heteroepitaxial cell was due to low minority carrier Shockley-Read-Hall lifetimes and bulk shu...

Optimization in wide-band-gap quantum dot solar cells

Quantum dots (QDs) have been under extensive study as a promising material to realize the concept of the intermediate band solar cell (IBSC). Because thermal escape is the dominant mechanism of carrier escape at room temperature, wide-band-gap (WBG) semiconductor can be used to suppress thermal escape by increasing barrier height for InAs quantum dots. Embedded InAs QD in the wide-bandgap matrix (InGaP and AlGaAs) is demonstrated with increased sub-band-gap carrier collection. The deeper confinement and larger activation energy is a move towards realizing an IBSC Additionally, activation energy extracted from temperature dependent external quantum efficiency (TDEQE) of InAs/AlGaAs is 324 meV, which is closer to the transition between IB to CB in an ideal IBSC.

Modeling the effects of using polycrystalline substrates for low cost III–V photovoltaics

Recent interest in growing high efficiency solar cells on polycrystalline “virtual” recrystallized substrates requires an understanding of the impact of substrate quality and growth nucleation characteristics. This work provides a study on the effects of substrate material, roughness, and crystallinity on efficiency of a GaAs solar cell. A variety of devices were grown on both monocrystalline and polycrystalline GaAs and Ge substrates, and the results were used to predict minority carrier diffusion length as a function of crystal grain size, material quality, and nucleation defect densities.

Correlation between quantum dot morphology and photovoltaic performance

The use of nanostructures, such as quantum dots (QD) or quantum wells within photovoltaic (PV) devices has demonstrated enhanced current generation, but often at the expense of open-circuit voltage. QD morphology and optical quality have a direct impact on PV performance and optimizing the epitaxial growth of QDs is critical to achieve the desired benefits of QD-enhanced PV. The spatial uniformity of QD epitaxy can determine the PV performance across a large area wafer. In this paper, we demonstrate a correlation between the spatial uniformity of QD size distribution and photovoltaic conversion efficiency. A spatially varying Voc measured on QD-enhanced GaAs solar cells correlates with the presence of coalesced QDs. The results suggest the presence of large, coalesced QDs is a significant cause for a reduced Voc in QD-enhanced GaAs p-i-n solar cells.

Characterization of a quantum dot nipi photovoltaic device

Quantum dot nipi test structures consisting of n-type / intrinsic / p-type / intrinsic layers have been developed to probe the effects of placing quantum dots within a doping superlattice. Coupling between the doping superlattice states and quantum dots that can be grown within the superlattice results in increases in carrier lifetimes within the quantum dots, and modifies the absorption spectrum. With the addition of dots within a nipi device, the benefits have been quantified by measuring absorption, activation energy, photoluminescence, and lifetime. By characterizing the modified quantum dot properties, it is hopeful that this will demonstrate the potential of the nipi structure as an intermediate band solar cell candidate.