Showing papers by "Ali Javey published in 2009"

Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates

Abstract: Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

Diameter-dependent electron mobility of InAs nanowires.

Abstract: Temperature-dependent I−V and C−V spectroscopy of single InAs nanowire field-effect transistors were utilized to directly shed light on the intrinsic electron transport properties as a function of nanowire radius. From C−V characterizations, the densities of thermally activated fixed charges and trap states on the surface of untreated (i.e., without any surface functionalization) nanowires are investigated while enabling the accurate measurement of the gate oxide capacitance, therefore leading to the direct assessment of the field-effect mobility for electrons. The field-effect mobility is found to monotonically decrease as the radius is reduced to <10 nm, with the low temperature transport data clearly highlighting the drastic impact of the surface roughness scattering on the mobility degradation for miniaturized nanowires. More generally, the approach presented here may serve as a versatile and powerful platform for in-depth characterization of nanoscale, electronic materials.

Toward the Development of Printable Nanowire Electronics and Sensors

Abstract: In recent years, there has been tremendous progress in the research and development of printable electronics on mechanically flexible substrates based on inorganic active components, which provide high performances and stable device operations at low cost. In this regard, various approaches have been developed for the direct transfer or printing of micro- and nanoscale, inorganic semiconductors on substrates. In this review article, we focus on the recent advancements in the large-scale integration of single crystalline, inorganic-nanowire (NW) arrays for electronic and sensor applications, specifically involving the contact printing of NWs at defined locations. We discuss the advantages, limitations, and the state-of-the-art of this technol- ogy, and present an integration platform for future printable, heteroge- neous-sensor circuitry based on NW parallel arrays.

Challenges and Prospects of Nanopillar-Based Solar Cells

Abstract: Materials and device architecture innovations are essential for further enhancing the performance of solar cells while potentially enabling their large-scale integration as a viable source of alternative energy. In this regard, tremendous research has been devoted in recent years with continuous progress in the field. In this article, we review the recent advancements in nanopillar-based photovoltaics while discussing the future challenges and prospects. Nanopillar arrays provide unique advantages over thin films in the areas of optical properties and carrier collection, arising from their three-dimensional geometry. The choice of the material system, however, is essential in order to gain the advantage of the large surface/interface area associated with nanopillars with the constraints different from those of the thin film devices.

Wafer-scale, sub-5 nm junction formation by monolayer doping and conventional spike annealing.

Abstract: We report the formation of sub-5 nm ultrashallow junctions in 4 in. Si wafers enabled by the molecular monolayer doping of phosphorus and boron atoms and the use of conventional spike annealing. The junctions are characterized by secondary ion mass spectrometry and noncontact sheet resistance measurements. It is found that the majority ( approximately 70%) of the incorporated dopants are electrically active, therefore enabling a low sheet resistance for a given dopant areal dose. The wafer-scale uniformity is investigated and found to be limited by the temperature homogeneity of the spike anneal tool used in the experiments. Notably, minimal junction leakage currents (<1 microA/cm(2)) are observed that highlights the quality of the junctions formed by this process. The results clearly demonstrate the versatility and potency of the monolayer doping approach for enabling controlled, molecular-scale ultrashallow junction formation without introducing defects in the semiconductor.

Nanoscale doping of InAs via sulfur monolayers

Abstract: One of the challenges for the nanoscale device fabrication of III-V semiconductors is controllable postdeposition doping techniques to create ultrashallow junctions. Here, we demonstrate nanoscale, sulfur doping of InAs planar substrates with high dopant areal dose and uniformity by using a self-limiting monolayer doping approach. From transmission electron microscopy and secondary ion mass spectrometry, a dopant profile abruptness of ∼3.5 nm/decade is observed without significant defect density. The n+/p+ junctions fabricated by using this doping scheme exhibit negative differential resistance characteristics, further demonstrating the utility of this approach for device fabrication with high electrically active sulfur concentrations of ∼8×1018 cm−3.

Monolayer resist for patterned contact printing of aligned nanowire arrays.

Abstract: Large-area, patterned printing of nanowires by using fluorinated self-assembled monolayers as the resist layer is demonstrated. By projecting a light pattern on the surface of the monolayer resist in an oxygen-rich environment, sticky and nonsticky regions on the surface are directly defined in a single-step process which then enables the highly specific and patterned transfer of the nanowires by the contact printing process, without the need for a subsequent lift-off step. This work demonstrates a simple route toward scalable, patterned printing of nanowires on substrates by utilizing light-tunable, nanoscale chemical interactions and demonstrates the versatility of molecular monolayers for use as a resist layer.

Hybrid core-shell nanowire forests as self-selective chemical connectors.

Abstract: Conventional connectors utilize mechanical, magnetic, or electrostatic interactions to enable highly specific and reversible binding of the components (i.e., mates) for a wide range of applications. As the connectors are miniaturized to small scales, a number of shortcomings, including low binding strength, high engagement/disengagement energies, difficulties with the engagement, fabrication challenges, and the lack of reliability are presented that limit their successful operation. Here, we report unisex, chemical connectors based on hybrid, inorganic/ organic nanowire (NW) forests that utilize weak van der Waals bonding that is amplified by the high aspect ratio geometric configuration of the NWs to enable highly specific and versatile binding of the components. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (∼163 N/cm 2 ) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio ∼25), reusability (∼27 attach/detach cycles), and efficient binding for both micro- and macroscale dimensions.

Hybrid core-multishell nanowire forests for electrical connector applications

Abstract: Electrical connectors based on hybrid core-multishell nanowire forests that require low engagement forces are demonstrated. The physical binding and electrical connectivity of the nanowire electrical connectors arise from the van der Waals interactions between the conductive metallic shells of the engaged nanowire forests. Specifically, the nanofibrillar structure of the connectors causes an amplification of the contact area between the interpenetrating nanowire arrays, resulting in strong adhesion with relatively low interfacial resistance. The nanowire electrical connectors may enable the exploration of a wide range of applications involving reversible assembly of micro- and macroscale components with built-in electrical interfacing.

Wet and Dry Adhesion Properties of Self‐Selective Nanowire Connectors

Abstract: Here, the wet and dry adhesion properties of hybrid Ge/parylene nanowire (NW) connectors are examined. The ability of the NW connectors to bind strongly even under lubricating conditions, such as mineral oil, sheds light on the dominant role of van der Waals interactions in the observed adhesion. The superhydrophobic surface of the NW connectors enables the wet, self-cleaning of contaminant particles from the surface, similar to the lotus effect. In addition, the effect of NW length on the shear adhesion strength, repeated usability, and robustness of the connectors, all critical properties for applications that require reversible binding of components, is examined.

Wafer-Scale, Sub-5 nm Junction Formation by Monolayer Doping and Conventional Spike Annealing

Abstract: We report the formation of sub-5 nm ultrashallow junctions in 4 inch Si wafers enabled by the molecular monolayer doping of phosphorous and boron atoms and the use of conventional spike annealing. The junctions are characterized by secondary ion mass spectrometry and non-contact sheet resistance measurements. It is found that the majority (~70%) of the incorporated dopants are electrically active, therefore, enabling a low sheet resistance for a given dopant areal dose. The wafer-scale uniformity is investigated and found to be limited by the temperature homogeneity of the spike anneal tool used in the experiments. Notably, minimal junction leakage currents (<1 uA/cm2) are observed which highlights the quality of the junctions formed by this process. The results clearly demonstrate the versatility and potency of the monolayer doping approach for enabling controlled, molecular-scale ultrashallow junction formation without introducing defects in the semiconductor.

Optofluidic assembly of red/blue/green semiconductor nanowires

Abstract: A full-color pixel consisting of CdSe, ZnO, and CdS nanowires has been heterogeneously integrated on a substrate with lithographic accuracy using lateral optoelectronic tweezers (LOET). Red (CdSe, 685nm), blue (CdS, 496nm), and green (ZnO, 518nm) light emissions are obtained by selectively pumping the corresponding nanowires. Potentially the nanowires can be directly assembled on CMOS driver circuit for a low-power display.

Monolayer doping and diameter-dependent electron mobility assessment of nanowires

Abstract: Sub-5nm ultrashallow junctions in planar and non-planar semiconductors are formed by use of a molecular monolayer doping method and conventional spike annealing. ~70% of the dopants are found to be electrically active, allowing for a low sheet resistance for a given dopant areal dose, and minimal junction leakage currents (≪1 µA/cm2) are observed. This indicates the high-quality of the ultrashallow junctions formed by this monolayer doping method. In addition, temperature-dependent current-voltage (I–V) behavior of individual InAs nanowire field-effect transistors is used to study the field-effect mobility as a function of nanowire radius. The field-effect mobility is observed to decrease with decreasing radius. The low-temperature transport behavior reveals the significant impact of surface roughness scattering on mobility degradation in smaller radius nanowires. The successful demonstration of a monolayer doping technique that does not introduce defects into the substrate, combined with a better understanding of diameter-dependent electron mobility in nanowires, contributes toward the advancement of nanoscale, electronic materials.