This work was financially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (17J10044) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The fabrication processes were supported by &#34;Nanotechnology Platform&#34; (project No. 12024046), MEXT, Japan. The authors would like to thank Mr. K. Yamashita and Ms. S. Hirata, the University of Tokyo, for their help on TEM observations.

1. Theoretical calculation procedure
<ol>
	<li>Calculate trajectories of TDs. In the calculation, assume the SEG masks to be thin enough to ignore the ART effect on TDD reduction.
	<ol>
		<li>Determine growth surfaces and express them by equation(s). For instance, express the time evolution of a round-shaped cross-section of a SEG Ge layer with the time evolution parameter n = i, SEG Ge heights (hi) and SEG Ge radii (ri), as shown in the Supplemental Video 1a and Eq. (1): 
		<img alt="Equation 4" src="/files/ftp_upload/58897/58897eq4.jpg" style="opacity: 0.9;" /></li>
		<li>Determine normal directions for an arbitrary location on the growth surfaces. For the round-shaped cross-section SEG Ge, describe the normal line at (xi&#160;,&#160;yi) as <img alt="Equation 7" src="/files/ftp_upload/58897/58897eq7v2.jpg" style="opacity: 0.9;" />, shown in Supplemental Video 1b as a red line. Then, obtain the edge of the TD (xi+1&#160;,&#160;yi+1) from the point (xi&#160;,&#160;yi) by solving the following simultaneous equations: 
		<img alt="Equation 10" src="/files/ftp_upload/58897/58897eq10.jpg" style="opacity: 0.9;" /></li>
		<li>Calculate a trajectory of one TD depending on the location of TD generation (x0, 0), as shown in Supplemental Video 1c. In other words, a trajectory for an arbitrary TD can be calculated by the method described above.</li>
		<li>Calculate TDD assuming that TDs penetrate to the bottom surface and contribute to the reduction of TDD (i.e., TDs below the point where SEG Ge layers coalesce are trapped by semicylindrical voids and never appear on the top surface).</li>
	</ol>
	</li>
</ol>
2. Experimental verification procedure
<ol>
	<li>SEG mask preparation
	<ol>
		<li>Prior to the fabrication of SEG masks, define Ge growth areas by preparing a design file. Inthe present work, prepare line-and-space patterns aligned to the [110] direction and square-shaped Si window areas of 4 mm in width using commercial software (e.g., AutoCAD).</li>
		<li>Determine the design of SEG masks(in particular Wwindow and Wmask)using the software.Wwindow is the window width (Si seed width) and Wmask is the SiO2 mask width, such that SEG Ge layers can coalesce with their adjacent ones. Determine Wwindow and Wmask by drawing rectangles by clicking open file &#8594; structure &#8594;&#160;rectangle or polyline. 
		NOTE: The width of the rectangles becomes Wwindow, and the interval of the rectangles becomes Wmask. In the present work, the minimum values of Wwindow and Wmask are 0.5 &#181;m and 0.3 &#181;m, respectively, which are restricted by the resolution in the employed EB lithography system.</li>
		<li>As references, draw square-shaped Si window areas of 4 mm in width D, regarded as the blanket areas. Click open file &#8594;&#160;structure &#8594;&#160;rectangle or polyline to draw the square-shaped Si window. Use the schematics shown in Figure 1 to prepare the line-and-space patterns and the 4 mm square blanket area.</li>
		<li>Prepare B-doped p-Si (001) substrates with the resistivity of 1-100 &#937;&#8729;cm. In the present work, use 4-inch Si substrates. Clean the substrate surfaces with Piranha solution (a mixture of 20 mL of 30% H2O2 and 80 mL of 96% H2SO4) as necessary.</li>
		<li>Open the lid on a tube furnace and load the Si substrates into the furnace using a glass rod. In the present work, oxidize 10 Si substrates at one time.</li>
		<li>Start to blow dry N2 gas into the furnace by opening the gas valve. Then, set the gas flow rate to 0.5 L/m by controlling the valve.</li>
		<li>Set the annealing temperature by changing the program. In the present work, use &#34;pattern step (mode 2)&#34; and set the process temperature to 900 &#176;C. Then, run the program by pushing function&#160;&#8594; run.</li>
		<li>As the temperature reaches 900 &#176;C, close the dry N2 valve, open the dry O2 valve (O2 flow = 1 L/m), and keep for 2 h. 
		NOTE: Perform steps 2.1.9-2.1.16 in a yellow room.</li>
		<li>Coat the oxidized Si substrates with a surfactant (OAP) using a spin coater and then bake at 110 &#176;C for 90 s on a hotplate.</li>
		<li>After the surfactant coating, coat the Si substrates with a photoresist (e.g., ZEP520A) using a spin coater and then bake at 180 &#176;C for 5 min on a hotplate.</li>
		<li>Load the Si substrates with the surfactant and photoresist into an electron beam (EB) writer.</li>
		<li>Read the design file (prepared in step 2.1.2) in the EB writer and make an operation file (WEC file). Set dose quantity as 120 &#181;C/cm2 in the WEC file. As the substrate loading finishes, perform EB exposure by clicking the single exposure button.</li>
		<li>Unload the substrate from the EB writer by clicking wafer carry &#8594;&#160;unload as the exposure finishes.</li>
		<li>Prepare a photoresist developer (ZED) and a rinse for the developer (ZMD) in a draft chamber. Dip the exposed Si substrates into the developer for 60 s at room temperature.</li>
		<li>Remove the Si substrates from the developer, and then dry the substrate with N2 gas.</li>
		<li>Put the developed Si substrates on a hotplate to bake at 110 &#176;C for 90 s.</li>
		<li>Dip the Si substrates into a buffered hydrofluoric acid (BHF-63SE) for 1 min in order to remove part of the SiO2 layers exposed to the air as the result of EB exposure and development.</li>
		<li>Remove the photoresist from the Si substrates by dipping into an organic photoresist remover (e.g., Hakuri-104) for 15 min.</li>
		<li>Dip the Si substrates into 0.5% diluted hydrofluoric acid for 4 min to remove the thin native oxide in the window regions but to retain the SiO2 masks. Then load onto an ultrahigh-vacuum chemical vapor deposition (UHV-CVD) chamber to grow Ge. Figure 2 shows the UHV-CVD system used in the present work.</li>
	</ol>
	</li>
	<li>Epitaxial Ge growth
	<ol>
		<li>Load the Si substrate with SEG masks (fabricated as in step 2.1) into a load lock chamber.</li>
		<li>Set the buffer/main growth temperature in the Recipe tab shown on the operation computer. Determine the durations for the main growth of Ge so that SEG Ge layers coalesce with adjacent ones. To decide the durations for the main growth, consider the growth rate of Ge on the {113} planes, which determines the growth in the in-plane/lateral direction26. In the present work, set the durations for the main growth as 270 min and 150 min for 650 &#176;C and 700 &#176;C, respectively.</li>
		<li>Click start in main window, and then the Si substrate is automatically transferred into the growth chamber. 
		NOTE: Protocol on epitaxial Ge growth (steps 2.2.4-2.2.7) is automatically processed.</li>
		<li>Grow Ge buffer on the loaded Si substrate at low-temperature (&#8776;380 &#176;C). Use GeH4 diluted at 9% in Ar as the source gas and keep the partial pressure of GeH4 for 0.5 Pa during the buffer growth.</li>
		<li>Grow Ge main layer at an elevated temperature. Keep the partial pressure of GeH4 for 0.8 Pa during the main growth. In the present work, use two different temperatures of 650 and 700 &#176;C for the main growth temperature in order to compare SEG Ge with a round-shaped cross-section and with a {113}-facetted cross-section25. 
		NOTE: The growth rate of Ge on the (001) plane was 11.7 nm/min independent of the temperature.</li>
		<li>In order to visualize the evolution of SEG Ge and their coalescence, perform Ge growth with periodic insertion of 10-nm-thick Si0.3Ge0.7 demarcation layers on another Si substrate. Si0.3Ge0.7 layers were formed using Si2H6 and GeH4 gases. During the Si0.3Ge0.7-layer growth, set the partial pressure of Si2H6 gas at 0.02 Pa and the partial pressure of GeH4 gas at 0.8 Pa.</li>
		<li>As the Si substrate is automatically transferred from the growth chamber to the load lock chamber, vent the load lock chamber and unload the Si substrate manually.</li>
	</ol>
	</li>
	<li>Etch pit density (EPD) measurements
	<ol>
		<li>Dissolve 32 mg of I2 in 67 mL of CH3COOH using an ultrasonic cleaning machine.</li>
		<li>Mix the I2-dissolved CH3COOH, 20 mL of HNO3, and 10 mL of HF.</li>
		<li>Dip the Ge-grown Si substrates into the CH3COOH/HNO3/HF/I2 solution for 5-7 s in order to form etched pits.</li>
		<li>Observe the etched Ge surfaces with an optical microscope (typically 100x) to ensure that etched pits are successfully formed.</li>
		<li>Employ an atomic force microscope (AFM) to count the etched pits. Put the etched Ge sample on an AFM stage, and then approach the probe by clicking auto approach.</li>
		<li>Decide the observation area using an optical microscope integrated with an AFM, and scan five different 10 &#181;m x 10 &#181;m areas. The amplitude damping factor is automatically determined.</li>
	</ol>
	</li>
	<li>TEM observations
	<ol>
		<li>Pick up TEM specimens from the coalesced/blanket Ge layers by using a focused Ge ion beam (FIB micro sampling method)27.</li>
		<li>Polish the TEM specimens in an ion milling system using Ar ions. In the present work, thin down TEM specimens for cross-sectional observations to be 150-500 nm in the [110] direction, and for plan-view observations to be 200 nm in the [001] direction.</li>
		<li>For plan-view TEM specimens, protect the top surfaces of the Ge layers with amorphous layers, and then thin down from the bottom (substrate) side of the Ge layers.</li>
		<li>Perform TEM observations under an acceleration voltage of 200 kV. Perform cross-sectional bright-field scanning TEM (STEM) observations in order to observe thick (500 nm) TEM specimens.</li>
		<li>For a coalesced Ge with Si0.3Ge0.7 demarcation layers, perform cross-sectional high-angle annular dark field (HAADF) STEM observations under an acceleration voltage of 200 kV.</li>
	</ol>
	</li>
</ol>

The authors have nothing to disclose.

In the present work, TDD of 4 x 107/cm2 were experimentally shown. For further TDD reduction, there are mainly 2 critical steps within the protocol: SEG mask preparation and epitaxial Ge growth.
Our model shown in Figure 4 indicates that TDD can be reduced lower than 107/cm2 in coalesced Ge when APR, Wwindow/(Wwindow + Wmask), is as small as 0.1. Toward further TDD reduction, SEG masks wi...

Epitaxial Ge on Si has attracted substantial interests as an active photonic device platform since Ge can detect/emit light in the optical communication range (1.3-1.6 &#181;m) and is compatible with Si CMOS (complementary metal oxide semiconductor) processing techniques. However, since the lattice mismatch between Ge and Si is as large as 4.2%, threading dislocations (TDs) are formed in Ge epitaxial layers on Si at a density of ~109/cm2. The performances of Ge photonic devices are deteriorated by TDs because TDs works as carrier generation centers in Ge photodetectors (PDs) and modulators (MODs), and as carrier recombination centers in laser diodes (LDs). In turn, they would increase reverse leakage current (Jleak) in PDs and MODs1,2,3, and threshold current (Jth) in LDs4,5,6.
Various attempts have been reported to reduce TD density (TDD) in Ge on Si (Supplemental Figure 1). Thermal annealing stimulates movement of TDs leading to the reduction of TDD, typically to 2 x 107/cm2. The drawback is the possible intermixing of Si and Ge and out-diffusion of dopants in Ge such as phosphorus7,8,9 (Supplemental Figure 1a). The SiGe graded buffer layer10,11,12 increases the critical thicknesses and suppresses the generation of TDs leading to the reduction of TDD, typically to 2 x 106/cm2. The drawback here is that the thick buffer reduces light coupling efficiency between Ge devices and Si waveguides underneath (Supplemental Figure 1b). Aspect ratio trapping (ART)13,14,15 is a selective epitaxial growth (SEG) method and reduces TDs by trapping TDs at the sidewalls of thick SiO2 trenches, typically to &#60;1 x 106/cm2. The ART method uses a thick SiO2 mask to reduce TDD in Ge over the SiO2 masks, which locates far above Si and have the same drawback (Supplemental Figure 1b,1c). Ge growth on Si pillar seeds and annealing16,17,18 are similar to the ART method, enabling TD trapping by the high aspect ratio Ge growth, to &#60;1 x 105/cm2. However, high temperature annealing for Ge coalescence has the same drawbacks in Supplemental Figure 1a-c (Supplemental Figure 1d).
To achieve low-TDD Ge epitaxial growth on Si that is free from the drawbacks of the above-mentioned methods, we have proposed coalescence-induced TDD reduction19,20 based on the following two key observations reported so far in SEG Ge growth7,15,21,22,23: 1) TDs are bent to be normal to the growth surfaces (observed by the cross-sectional transmission electron microscope (TEM)), and 2) coalescence of SEG Ge layers results in the formation of semicylindrical voids over the SiO2 masks.
We have assumed that the TDs are bent owing to the image force from the growth surface. In the case of Ge on Si, the image force generates 1.38 GPa and 1.86 GPa shear stresses for screw dislocations and edge dislocations at distances 1 nm away from the free surfaces, respectively19. The calculated shear stresses are significantly larger than the Peierls stress of 0.5 GPa reported for 60&#176; dislocations in Ge24. The calculation predicts TDD reduction in Ge SEG layers on a quantitative basis and is in good agreement with the SEG Ge growth19. TEM observations of TDs are carried out to understand TD behaviors in the presented SEG Ge growth on Si20. The image-force-induced TDD reduction is free from any thermal annealing or thick buffer layers, and thus is more suitable for photonic device application.
In this article, we describe specific methods for the theoretical calculation and experimental verification employed in the proposing TDD reduction method.

<table border="0" cellpadding="0" cellspacing="0" style="width:655px;" width="656">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:222px;">AFM</td>
			<td style="width:140px;">SII NanoTechnology</td>
			<td style="width:123px;">SPI-3800N</td>
			<td style="width:171px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">BHF</td>
			<td>DAIKIN</td>
			<td>BHF-63U</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">CAD design</td>
			<td>AUTODESK</td>
			<td>AutoCAD 2013</td>
			<td>Software</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">CH3COOH</td>
			<td>Kanto-Kagaku</td>
			<td>Acetic Acid</td>
			<td>for Electronics</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">CVD</td>
			<td>Canon ANELVA</td>
			<td>I-2100 SRE</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Developer</td>
			<td>ZEON</td>
			<td>ZED</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Developer rinse</td>
			<td>ZEON</td>
			<td style="width:123px;">ZMD</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">EB writer</td>
			<td>ADVANTEST</td>
			<td>F5112+VD01</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Furnace</td>
			<td>Koyo Thermo System</td>
			<td>KTF-050N-PA</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">HF, 0.5 %</td>
			<td>Kanto-Kagaku</td>
			<td>0.5 % HF</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">HF, 50 %</td>
			<td>Kanto-Kagaku</td>
			<td>50 % HF</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">HNO3, 61 %</td>
			<td>Kanto-Kagaku</td>
			<td>HNO3 1.38</td>
			<td>for Electronics</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">I2</td>
			<td>Kanto-Kagaku</td>
			<td>Iodine 100g</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Photoresist</td>
			<td>ZEON</td>
			<td>ZEP520A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Photoresist remover</td>
			<td>Tokyo Ohka</td>
			<td>Hakuri-104</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Surfactant</td>
			<td>Tokyo Ohka</td>
			<td>OAP</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">TEM</td>
			<td>JEOL</td>
			<td>JEM-2010HC</td>
			<td></td>
		</tr>
	</tbody>
</table>

theoretical calculation and experimental verification for dislocation reduction in germanium epitaxial layers with semicylindrical voids on silicon

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the realization of monolithically integrated photonics circuits. The present paper describes methods of theoretical calculation and experimental verification of a novel model for the reduction of TDD. The method of theoretical calculation describes the bending of threading dislocations (TDs) based on the interaction of TDs and non-planar growth surfaces of selective epitaxial growth (SEG) in terms of dislocation image force. The calculation reveals that the presence of voids on SiO2 masks help to reduce TDD. Experimental verification is described by germanium (Ge) SEG, using an ultra-high vacuum chemical vapor deposition method and TD observations of the grown Ge via etching and cross-sectional transmission electron microscope (TEM). It is strongly suggested that the TDD reduction would be due to the presence of semicylindrical voids over the SiO2 SEG masks and growth temperature. For experimental verification, epitaxial Ge layers with semicylindrical voids are formed as the result of SEG of Ge layers and their coalescence. The experimentally obtained TDDs reproduce the calculated TDDs based on the theoretical model. Cross-sectional TEM observations reveal that both the termination and generation of TDs occur at semicylindrical voids. Plan-view TEM observations reveal a unique behavior of TDs in Ge with semicylindrical voids (i.e., TDs are bent to be parallel to the SEG masks and the Si substrate).

Theoretical Calculation
Figure 3&#160;shows&#160;calculated trajectories of TDs in 6 types of coalesced Ge layers: here, we define the aperture ratio (APR) to be Wwindow/(Wwindow + Wmask). Figure 3a shows a round-shaped SEG origin coalesced Ge of APR = 0.8. Here, 2/6 TDs are trapped. Figure 3b</stron...

Watch this Scientific Journal Video about Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon at JoVE.co

Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Theoretical calculation and experimental verification are proposed for a reduction of threading dislocation (TD) density in germanium epitaxial layers with semicylindrical voids on silicon. Calculations based on the interaction of TDs and surface via image force, TD measurements, and transmission electron microscope observations of TDs are presented.

Reduction of threading dislocation density (TDD) in epitaxial germanium (Ge) on silicon (Si) has been one of the most important challenges for the ...

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2.1K Views.  The University of Tokyo. Theoretical calculation and experimental verification are proposed for a reduction of threading dislocation (TD) density in germanium epitaxial layers with semicylindrical voids on silicon. Calculations based on the interaction of TDs and surface via image force, TD measurements, and transmission electron microscope observations of TDs are presented. 

Video: Theoretical Calculation and Experimental Verification for Dislocation Reduction in Germanium Epitaxial Layers with Semicylindrical Voids on Silicon

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 &#956;m from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,&#945;&#947;)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

We illustrate the application of 1H(15N,&#945;&#947;)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

SSVEP-based Experimental Procedure for Brain-Robot Interaction with Humanoid Robots

Brain-Robot Interaction (BRI), which provides an innovative communication pathway between human and a robotic device via brain signals, is prospective in helping the disabled in their daily lives. The overall goal of our method is to establish an SSVEP-based experimental procedure by integrating multiple software programs, such as OpenViBE, Choregraph, and Central software as well as user developed programs written in C++ and MATLAB, to enable the study of brain-robot interaction with humanoid robots.
This is achieved by first placing EEG electrodes on a human subject to measure the brain responses through an EEG data acquisition system. A user interface is used to elicit SSVEP responses and to display video feedback in the closed-loop control experiments. The second step is to record the EEG signals of first-time subjects, to analyze their SSVEP features offline, and to train the classifier for each subject. Next, the Online Signal Processor and the Robot Controller are configured for the online control of a humanoid robot. As the final step, the subject completes three specific closed-loop control experiments within different environments to evaluate the brain-robot interaction performance.
The advantage of this approach is its reliability and flexibility because it is developed by integrating multiple software programs. The results show that using this approach, the subject is capable of interacting with the humanoid robot via brain signals. This allows the mind-controlled humanoid robot to perform typical tasks that are popular in robotic research and are helpful in assisting the disabled.

The overall goal of this method is to establish an SSVEP-based experimental procedure by integrating multiple software programs to enable the study of brain-robot interaction with humanoid robots, which is prospective in assisting the sick and elderly as well as performing unsanitary or dangerous jobs.

Brain-Robot Interaction (BRI), which provides an innovative communication pathway between human and a robotic device via brain signals, is prospective in ...

Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.

Here, we combine polarization-variable 7-eV laser with spin- and angle-resolved photoemission technique to visualize the spin-orbital coupling effect in solid states.

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser ...

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic glasses (MGs). Despite their amorphous nature, the identification of hyperfine interactions unveils faint structural modifications. For this purpose, we have employed two techniques that utilize nuclear resonance among nuclear levels of a stable 57Fe isotope, namely M&#246;ssbauer spectrometry and nuclear forward scattering (NFS) of synchrotron radiation. The effects of heat treatment upon (Fe2.85Co1)77Mo8Cu1B14 MG are discussed using the results of ex situ and in situ experiments, respectively. As both methods are sensitive to hyperfine interactions, information on structural arrangement as well as on magnetic microstructure is readily available. M&#246;ssbauer spectrometry performed ex situ describes how the structural arrangement and magnetic microstructure appears at room temperature after the annealing under certain conditions (temperature, time), and thus this technique inspects steady states. On the other hand, NFS data are recorded in situ during dynamically changing temperature and NFS examines transient states. The use of both techniques provides complementary information. In general, they can be applied to any suitable system in which it is important to know its steady state but also transient states.

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Here, we present a protocol to describe ex situ and in situ investigations of structural transformations in metallic glasses. We employed nuclear-based analytical methods which inspect hyperfine interactions. We demonstrate the applicability of M&#246;ssbauer spectrometry and nuclear forward scattering of synchrotron radiation during temperature-driven experiments.

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic ...

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...