State of the art InGaP2/GaAs/In0.28Ga0.72As inverted metamorphic (IMM) solar cells have achieved impressive results, however, the thick metamorphic buffer needed between the lattice matched GaAs and lattice mismatched InGaAs requires significant effort and time to grow and retains a fairly high defect density. One approach to this problem is to replace the bottom InGaAs junction with an Sb-based material such as 0.73 eV GaSb or ~1.0 eV Al0.2Ga0.8Sb. By using interfacial misfit (IMF) arrays, the high degree of strain (7.8%) between GaAs and GaSb can be relaxed solely by laterally propagating 90° misfit dislocations that are confined to the GaAs-GaSb interface layer.
We have used molecular beam epitaxy to grow GaSb single junction solar cells homoepitaxially on GaSb and heteroepitaxially on GaAs using IMF. Under 15-sun AM1.5 illumination, the control cell achieved 5% efficiency with a WOC of 366 mV, while the IMF cell was able to reach 2.1% with WOC of 546 mV. Shunting and high non-radiative dark current were main cause of FF and efficiency loss in the IMF devices. Threading dislocations or point defects were the expected source behind the losses, leading to minority carrier lifetimes less than 1ns. Deep level transient spectroscopy (DLTS) was used to search for defects electrically and two traps were found in IMF material that were not detected in the homoepitaxial GaSb device. One of these traps had a trap density of 7 × 1015 cm-3, about one order of magnitude higher than the control cell defect at 4 × 1016 cm-3.
Significant development work has been completed in recent years to improve experimental results reaching a record efficiency of 9.14% under one sun AM0 conditions with no anti-reflection coating. The <i>nipi</i> solar cell utilizes epitaxial regrowth contacts to ensure carrier selective contacts to the alternating <i>n</i> and <i>p-type </i>doped layers, forming selectively ohmic and rectifying contacts. Defects or traps formed in the rectifying contact during the epitaxial regrowth process result in injected current that contributes directly to dark current. As a result detailed characterization of the epitaxial regrowth interface is required to understand and minimize the formation of interface traps. Concentration measurements have been completed to characterize the trap states impact on efficiency as higher concentration results in state filling and a recovery in open circuit voltage. A model has been developed to gain further understanding of the measurements under concentration.
Solar cells utilizing doping superlattices in the active region of the device have been proposed as an alternative design to increase radiation hardness. Multiple diodes are connected together in parallel, where each diode can be as thin or thick as the design requires. Thinning the doped layers reduces the diffusion length requirements ensuring efficient carrier collection and maintenance of short circuit current. Experimental comparisons between nipi and a conventional pin solar cells that were irradiated with 1 MeV electrons at fluences from 4x10<sup>14</sup> to 2x10<sup>15</sup> e<sup>−</sup>/cm<sup>2</sup> show much more efficient maintenance of efficiency for the nipi design, maintaining nearly 100% efficiency up to a final dose of 2x10<sup>15</sup> e<sup>−</sup>/cm<sup>2</sup>. Further simulations have indicated that the efficient maintenance of voltage and fill factor are likely due to traps created in the nipi solar cell during the fabrication process. Beginning of life voltage and efficiency values can be improved significantly by limiting the trap density, while this has a minor impact on the efficiency comparison between a nipi and conventional device with respect to radiation.
Doping superlattice devices have been pursued in part because of their inherent radiation hardness which results from long lifetimes and minimum diffusion length requirements in the range of nanometers. Diffusion length requirements are reduced because of the multiple closely spaced doped layers in the superlattice. Higher doping levels in conjunction with close superlattice spacing result in large electric fields in the range of 5x10<sup>5</sup> V/cm that quickly collect carriers into the majority doped layers. The effect of the alternative solar cell structure will be studied by irradiating multiple device structures with 5.057 MeV alpha particles. Comparisons will be made between doping superlattice devices and single junction <i>pin</i> structures. Previous work developing a simulation routine to characterize the radiation response for these devices will be extended to confirm the predictive model developed. This work signifies a step forward in understanding the radiation effects of doping superlattice devices, and their potential for high radiation environments.
The <i>nipi</i> photovoltaic device is a doping superlattice-based device, that uses iterative <i>n-type</i> / intrinsic / <i>p-type</i>
/ intrinsically doped GaAs layers to minimize the effect of minority carrier diffusion length. Following photon
absorption, carriers are quickly swept vertically by drift into majority doped layers. Carriers are collected in the
lateral contacts via diffusion through the doped superlattice layers. Epitaxial regrowth is used to form selective
lateral contacts in v-grooves that are etched into the superlattice layers. Testing was completed to improve
the epitaxial regrowth process used, where an improvement in the morphology of the regrown material was
demonstrated by adjusting the growth parameters. Devices have been fabricated, and the effects of varying the
cell size and grid finger spacing have been studied. The competing effects of series resistance which increases
as the grid finger spacing increases and shunt resistance which decreases as the finger spacing decrease have to
be balance to optimize the efficiency for the design. Although an additional shunt path was created between
the contacts, a one sun efficiency of 3.42% was achieved. The development of a fabrication process makes way
for the use of the nipi device to be used in conjunction with quantum dots to increase subband absorption and
potentially realize an intermediate band solar cell.
The simulation and characterization of multi-period GaAs n-type/intrinsic/p-type/intrinsic (nipi) doping structure solar
cells has been demonstrated. The <i>nipi</i> 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 <i>nipi</i> 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, <i>nipi</i> 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
Projection and interference imaging modalities for application to IC microlithography were compared at the 90 nm imaging node. The basis for comparison included simulated two-dimensional image in resist, simulated resist linesize, as well as experimental resist linesize response through a wide range of dose and focus values. Using resist CD as the main response (both in simulation and experimental comparisons), the two imaging modes were found nearly equivalent, as long as a suitable Focus-Modulation conversion is used. A Focus-Modulation lookup table was generated for the 45 nm imaging node, and experimental resist response was measured using an interferometric tool. A process window was constructed to match a hypothetical projection tool, with an estimated error of prediction of 0.6 nm. A demodulated interferometric imaging technique was determined to be a viable method for experimental measurement of process window data. As long as accurate assumptions can be made about the optical performance of such projection tools, the response of photoresist to the delivered image can be studied experimentally using the demodulated interferometric imaging approach.
An approach to measurement of resist CD response to image modulation and dose is presented. An empirical model with just three terms is used to describe this response, allowing for direct calculation of photoresist modulation curves. A thresholded latent image response model has been tested to describe CD response for both 90 nm and 45 nm geometry. An assumption of a linear optical image to photoresist latent image correlation is shown as adequate for the 90 nm case, while the 45 nm case demonstrates significant non-linear behavior. This failure indicates the inadequacy of a "resist blur" as a complete descriptive function and uncovers the need for an additional spread function in OPE-style resist models.
In a photolithographic system, the mask patterns are imaged through a set of lenses on a resist-coated wafer. The image of mask patterns physically can be viewed as the interference of the plane waves of the diffraction spectrum captured by the lens set incident on the wafer plane at a spectrum of angles. Two-beam interference fringe is the simplest format of the image. Consequently, two-beam interferometric lithography is often employed for photolithographic researches. For two-beam interferometric lithography, beam pointing instability of the illumination source can induce fringe displacement, which results in a loss of fringe contrast if it happens during the exposure. Since some extent of beam pointing instability is not avoidable, it is necessary to investigate its effects on the contrast of the interference fringe. In this paper, the effects of beam pointing instability associated with a two-beam interferometric lithography setup are analyzed. Using geometrical ray tracing technique and basic interference theory, the relationship between the beam tilt angle and interference fringe displacement is established. For a beam pointing instability with random distribution, the resulted fringe contrast is directly proportional to the Fourier transform of the pointing distribution evaluated at 1/(2π). The effect of a pointing instability with normal distribution on interference contrast is numerically investigated.
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 <i>nsin(θ)</i> , 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.