Emitter Profiles Research Articles

Field emission dependence on the tilted angle of palladium nanogaps for surface conduction electron-emitter displays

The emission efficiency of novel surface conduction electron-emitters corresponding to tilted angles (thetas) of a nanogap-typed driving cathode is optimised. Among the parameters of the emitter profile, the apex angle is the most significant because the smaller angles induce a higher electric field. The higher electric field then attracts more particles into vacuum and then increases the emission current. However, the structure of the driving cathode limits the electron trajectory while the angle decreases, and it reflects that the portion of collected current by the anode decreases and results in a drop in emission efficiency. The electric field with larger tilted angles will be weakened, but most of the emitted electrons could be collected by the anode, which increases emission efficiency. This shows there is highest emission efficiency (about 37%) under an optimum angle (thetas) of 60° owing to a trade-off between emission efficiency and emitter apex.

Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere

Net thermal radiation cooling, from surfaces at sub-ambient temperatures, to the night sky is amplified if the aperture to the sky is partially blocked with heat mirrors. The temperature at which radiation loss stagnates (the effective sky temperature) falls continuously as the aperture closes and is derived in terms of the aperture size and the spectral properties and temperatures of the atmosphere and of the emitting surface. Cooling surfaces must have high absorptance between 7.9 μm and 13 μm where the atmosphere is most transparent. The best response for the remainder of the Planck radiation spectrum surprisingly switches between two spectral extremes at a temperature which falls as the aperture gets smaller. A perfect absorber is best above this switch, while surfaces which reflect all of this radiation are best below it. A simple formula is presented for the cross-over temperature as a function of aperture size. With known material properties plus representative non-radiative heat gains a high emittance surface is generally better except when heat mirrors are not used. A known high emittance roof paint at 10° C below ambient, under a 45° aperture lined with shiny aluminium, can achieve a net output power near 135 W m −2 under a clear sky.

Phosphorus emitter and metal - grid optimization for homogeneous (n+p) and double-diffused (n++n+p) emitter silicon solar cells

This work focuses on studying two types of structure: homogeneous and double-diffused emitter silicon solar cells. The emitter collection efficiencies and the recombination current densities were studied for a wide range of surface dopant concentrations and thicknesses. The frontal metal-grid was optimized for each emitter, considering the dependence on the metal-semiconductor contact resistivity and on the emitter sheet resistance. The best efficiency for n+p structures, η≈ 25.5%, is found for emitters with thicknesses between (0.5-3) µm and surface doping concentrations in the range 2 x 10(19) cm-3- 4 x 10(18) cm-3; while the n++n+p structure a maximum efficiency of η≈ 26.0% was identified for an even wider range of emitter profiles.

Fine line printed silicon solar cells exceeding 20% efficiency

AbstractSilicon solar cells with passivated rear side and laser‐fired contacts were produced on float zone material. The front side contacts are built up in two steps, seed and plate. The seed layer is printed using an aerosol jet printer and a silver ink. After firing this seed layer through the silicon nitride layer, the conductive layer is grown by light induced plating. The contact formation is studied on different emitter sheet resistances, 55 Ω/sq, 70 Ω/sq, and on 110 Ω/sq. These emitters are passivated with a PECVD silicon nitride layer which also acts as an anti‐reflection coating. Even on the 110 Ω/sq emitters it was possible to reach a fill factor of 80·1%. The electrical properties i.e., the contact resistance of the front side contacts are studied by transfer length model (TLM) measurements. On a cell area of 4 cm2 and emitter sheet resistance of 110 Ω/sq, a record efficiency of 20·3% was achieved. Excellent open‐circuit voltage (Voc) and short‐circuit current (jsc) values of 661 mV and 38·4 mA/cm2 were obtained due to the low recombination in the 110 Ω/sq emitter and at the passivated rear surface. These results show impressively that it is possible to contact emitter profiles with a very high efficiency potential using optimized printing technologies. Copyright © 2008 John Wiley & Sons, Ltd.

BNL test electron beam ion source operation on a 100kV platform (abstract)

At BNL, we are preparing to build an electron beam ion source (EBIS) that will satisfy the requirements of the relativistic heavy ion collider (RHIC), e.g., Au32+ ions of about 3×109 particles/pulse in 10–40μs pulses at 5Hz. In addition, the EBIS is required to deliver beams such as He2+, C6+, O8+, Si14+, Ti18+, Fe21+, and Cu22+ in ∼2–3mA, 10μs pulses at 5Hz to meet the demands of the NASA Space Radiation Laboratory (NSRL) experimental program. Development studies continue to be carried out on the BNL test EBIS, a half-length full electron beam power prototype of the RHIC EBIS which is in the construction phase. In preparation for acceleration of EBIS ions by a radio frequency quadrupole, we have upgraded the test EBIS for operation on a nominal 100kV pulsed platform to allow seed ion injection at 10–20kV and ion extraction up to 100qkV. The installation includes a pulsed high voltage power supply, a 200kVA 100kV isolation transformer, and a capacitive electron collector supply and electron gun bias supply which allows operation in a similar mode to the RHIC EBIS design. Initial testing has been made using ion beams of He2+ and Ar10+ formed from gas injection and ions have been extracted with energies up to 88qkeV. Total ion current measurement has been made and charge state distributions have been obtained using a Mamyrin time of flight. Results of emittance measurements and profile distributions, obtained at various energies using both pepperpot and slit-foil emittance meters, will be presented.

Beam characteristics of the new DESY H− source and investigations of the plasma load

The new DESY H(-) source with a completely insulated plasma has been thoroughly investigated with emittance and beam profile measurements. The beam profile was measured with a multi-Faraday cup, and for the emittance measurements a slit and grid device was used. More than 200 beam profile and emittance measurements for x as well as y were done. Their dependences on rf power, filter field strength, and gas pressure were measured. The plasma load was investigated with a substitution method. It was possible to determine the real and imaginary parts which is seen by the rf antenna.

N‐Type emitter epitaxy for crystalline silicon thin‐film solar cells

AbstractCrystalline silicon thin‐film (cSiTF) solar cells are an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of the epitaxial wafer equivalent (EpiWE), where 20 µm of silicon are deposited epitaxially by high temperature atmospheric pressure CVD (APCVD) on a cheap silicon substrate. The EpiWE can then be processed using standard industrial cell production. Furthermore, it is possible to simplify the solar cell process when depositing the emitter by epitaxy in‐situ after the growth of the base. The epitaxial emitter could be an alternative method of n‐type emitter processing with adjustable emitter profile and short deposition time. This paper presents results of cSiTF solar cells with epitaxial emitters and photolithographic contacts, which prove the good performance of the emitter and the applicability of the EpiWE to conventional solar cell processes. Included in the discussion is a description of the CVD deposition principle developed at Fraunhofer ISE and the applied clean room solar cell process. The emitter doping profiles and concentrations, as well as the structural quality, are discussed in detail using SIMS and SEM data. The phosphorus out‐diffusion during cooling is prevented by cooling the samples in a PH3/H2 atmosphere. Moreover, a double‐layer emitter is formed having blue sensitive properties. The internal quantum efficiencies (IQEs) show that the emitter can be passivated as well as a ‘standard’ emitter formed by POCl3 diffusion. Efficiencies up to 14·9 and 13·6% for large area cSiTF solar cells on highly doped Cz and mc, respectively, are presented. Copyright © 2007 John Wiley & Sons, Ltd.

Ge quantum-dot polysilicon emitter heterojunction phototransistors for 1.31–1.55 μm light detection

A Si/SiGe HBT-type phototransistor with 10 Ge-dot layers (8ML in each layer, separated by 30 nm Si spacer) incorporated in the base-collector junction was grown by UHV-CVD. To achieve low dark current density and high photo-responsivity, a high-low emitter profile consisting of a heavily doped polysilicon contact on top of a thin epitaxial emitter cap has been fabricated. At room temperature, I– V measurement showed a low dark current density of 3 × 10 −5 A/cm 2 at 3 V. The measured breakdown voltage BV ceo was about 12 V. A photo-responsivity of 1.94 and 0.028 mA/W was achieved at 1.31 and 1.55 μm for normal incidence, respectively. Compared to a p–i–n reference detector with the same quantum-dot layer, the responsivity is improved by a factor of 45 and 20 at 1.31 and 1.55 μm, respectively.

Silicon CVD deposition for low cost applications in photovoltaics

This paper presents a low cost application of high-temperature silicon CVD depositions for photovoltaics. Crystalline silicon thin-film (cSiTF) solar cells could be an attractive alternative to bulk silicon solar cells. At Fraunhofer ISE we follow the concept of an epitaxial wafer equivalent (EpiWE), where 20 μm of silicon are deposited epitaxially by atmospheric pressure CVD on a cheap silicon substrate. The EpiWE can then be processed in a standard industrial cell production. Furthermore, it is possible to simplify the solar cell process when depositing the emitter by epitaxy in-situ after the growth of the base. The emitter formation made by epitaxy could provide an alternative to the conventional POCl 3 emitter-diffusion for cSiTF solar cells. In this paper the concept of the EpiWE is introduced and the RTCVD deposition reactor is described in detail. The lab-type horizontal CVD reactor was constructed at Fraunhofer ISE to obtain a cheap deposition tool with a sufficient layer quality for solar cells. The deposition process as well as the resulting emitter profiles is discussed with regard to their effect on the solar cells. Solar cell parameters of emitters on n-type Fz and on p-type cSiTF are presented with efficiencies up to 14.2% and 14.8%, respectively. The high open circuit voltages of more than 645 mV show the good performance of the epitaxial emitters.

Analytical modelling and minority current measurements for the determination of the emitter surface recombination velocity in silicon solar cells

A new analytical model is used to describe the emitter of silicon solar cells in order to gain information on the surface recombination velocity S. The procedure takes advantage of the combined use of experimental measurements, done to determine the emitter saturation current J oe, and analytical modelling to relate J oe to S. Several experiments have been carried out on silicon solar cells having different emitter profiles subjected to various surface treatments. The influence of the surface on significant cell parameters has been analysed.

Influence of temperature during phosphorus emitter diffusion from a spray‐on source in multicrystalline silicon solar cell processing

AbstractWe have investigated the influence of diffusion temperature during phosphorus emitter diffusion from a spray‐on source on the performance of screen‐printed multicrystalline silicon solar cells. Because of the dual diffusion mechanism present at high concentration in‐diffusion of phosphorus in silicon, applying lower diffusion temperatures for a longer duration results in significantly enhanced penetration of the low concentration tail relative to the high concentration region. Moreover, we show that the sheet resistance of in‐diffused emitters from a high concentration source depends primarily on the extension of the high concentration region, thus significantly different emitter profiles can be manufactured without altering the sheet resistance considerably. Because of the enhanced tail penetration, emitters of a specified sheet resistance diffused at reduced temperatures can result in higher fill factors of screen‐printed solar cells due to diminution of Schottky type shunts. Furthermore, emitters diffused at lower temperatures for longer durations can yield a higher gettering efficiency, resulting in increased bulk recombination lifetime, and thus a higher internal quantum efficiency at long wavelengths. The deeper tail extension of low temperature emitters, however, causes increased absorption within the highly recombinative emitter, resulting in current losses due to a lower internal quantum efficiency at short wavelengths. Copyright © 2006 John Wiley & Sons, Ltd.

Effects of oxide layer thickness on Si-nanocrystal photoluminescence intensity in Si+-implanted SiO2∕Si systems

Photoluminescence measurements are used to investigate the light emission of silicon nanocrystals prepared by Si+ implantation into SiO2∕Si layered structures. Strong variations of the luminescence intensity are observed as a function of the SiO2 thickness, for laser excitations at 405 and 488nm. These changes are associated with optical interference which produce spatial modulation of the pump laser intensity and spectral modulation of the PL emission. Model calculations using Fresnel equation solvers where the depth distribution of emitters is adjusted to minimize the difference to the measured PL intensities indicate a wider emitter profile for excitation at 405nm.

NUMERICAL INVESTIGATION OF THE EFFECT OF DOPING PROFILES ON THE HIGH FREQUENCY PERFORMANCE OF InP/InGaAs SUPER SCALED HBTs

Our simulations show that the emitter and collector doping profile engineering is very important for the transistor optimization, in particular, adjusting the low doped emitter section to the depletion length resulted in the decrease of the emitter series resistance and increased ft and fmax; decreasing of the collector doping concentration and shrinking the collector thickness reduced the collector transit time. Accounting for the lateral diffusion of hot electrons in the device with submicron emitter was found to be important in the transistor optimization process. This effect determines the effective thickness of the emitter finger and the value of the push-out current.

Design and test of a beam profile monitoring device for low intensity radioactive beams

Development efforts have gone into the construction and performance testing of a device that can be used to provide snap shot images of the beam profile. It is intended to function even at very low ion intensities, such as those expected from a rare-isotope accelerator. Intensity profiles and emittance analysis are among the most critical tools used for optimizing beam transport through accelerators. This article describes the design and performance of a beam image monitor. The device is sensitive at a wide dynamic range which spans from ∼102 to ∼1012 pps. With the advent of double-plane slits or a pepper pot plate, this system can be used to scan transverse emittance profiles in both the x–x′ and y–y′ phase space planes, simultaneously. Conventional diagnostic devices used for heavy ion accelerators generally require at least 109 pps intensity to carry out similar diagnostics, which is not practical when considering beams with very low intensities, such as rare isotope beams. Furthermore, the detection system used here can be used for a wide range of incident ion velocities. Compared with solid-state detectors and scintillators that are inserted directly into the beam, this type of detection system is less susceptible to beam induced damage resulting in longer lifetimes and less maintenance. The test was done using single charge state Kr beams at energies ranging from 3.6 to 18 keV/u. The device’s sensitivity was monitored for intensities below 1012 pps and an emittance scan was recorded and analyzed. The spatial resolution was characterized by comparing the emittance profile with that obtained by a wire scanning device which had better resolution but was sensitive only to intensities above 1011 pps. Recently, the device has been used to aid in the transport of ∼6 MeV/u radioactive beams, such as F17, produced by pickup reactions with a gas cell target at the ATLAS accelerator facility.

Bulk resistivity optimization for low‐bulk‐lifetime silicon solar cells

AbstractGuidelines are presented which are designed to achieve planar solar cell efficiencies as high as 17.5% using existing fabrication technologies and silicon substrates with lifetimes as low as 20 μs. Device simulations are performed to elucidate the need and impact of base doping optimization for different back‐surface passivation schemes, cell thicknesses, emitter profiles, and degrees of dopant–defect interaction. Results indicate that optimal resistivity is a function of back‐surface passivation, with the aluminum back‐surface field (BSF) requiring the highest resistivity, the oxide/nitride stack passivation excelling at an intermediate resistivity, and the ohmic contact needing the lowest resistivity. A comparison of simulated 300 and 100 μm cells shows that thinner cells magnify the differences in optimal resistivity for the three back‐surface passivation schemes. A lifetime model is used to account for dopant–defect interaction that can lower bulk lifetime at higher doping levels. It is demonstrated that cell efficiency decreases and optimal resistivity increases at higher levels of dopant–defect interaction. Simulated devices with an optimized base doping showed an efficiency improvement of as much as 2% (absolute) compared with identical devices with a typical base doping level (1.6 or 1.8 Ω cm) and bulk lifetime of 20 μs. Copyright © 2001 John Wiley & Sons, Ltd.

Study of porous silicon, silicon carbide and DLC coated field emitters for pressure sensor application

This paper is a revue of our experimental data regarding field emitter array fabrication, various field emission materials and application in pressure sensors domain. Silicon emitter's arrays of different sizes and geometrical shapes were realised using micromachining technologies. Some important aspects as control in etch rate, emitter profile, selectivity and surface morphology were investigated. The emitter surface was modified or was covered by different materials in order to improve the emission properties. The most usual materials investigated for FED applications were: Si, diamond-like carbon layers, silicon carbide, and porous silicon. The main application which is present in our attention is the field emission pressure sensor.

Dry etching of Si field emitters and high aspect ratio resonators using an inductively coupled plasma source

An inductively coupled plasma (ICP) source has been used to generate a Cl2 plasma for the etching of Si field emitters and resonators. An optimized etch condition was used to etch Si field emission devices with controllable sidewall angles as well as good uniformity for large arrays of emitters. A mask erosion technique was used to control the Si emitter profile. The sidewall angle of Si emitters increased from 55° to 75° as plasma pressure was increased from 0.1 to 5.0 mTorr. Sharp tips with good uniformity and a high packing density of 6.25×106 tips/cm2 were fabricated using 200 W ICP source power, 100 W stage power at 1 mTorr with 20 sccm of Cl2 flowing and an ICP source to sample distance of 8 cm. Released Si mechanical resonators were also fabricated with a vertical etch profile and smooth surfaces. An optimized etch condition of 250 W ICP source power and 70 W stage power at 5 mTorr with 20 sccm of Cl2 flow and an ICP source to sample distance of 6 cm provided an etch rate of 219 nm/min and a selectivity to a Ni–Ti mask of 26. Due to the high selectivity and etch rate, resonators and cantilevered beams as thick as 40 μm were fabricated giving improved device performance compared to thinner devices. Submicrometer Si resonators were also fabricated that were 3.1 μm thick with 1.7 μm wide comb drive fingers and 0.2 μm gaps between them.

Self-aligned selective-emitter plasma-etchback and passivation process for screen-printed silicon solar cells

We studied whether plasma-etching techniques can use standard screen-printed gridlines as etch masks to form self-aligned, patterned-emitter profiles on multicrystalline-silicon (mc-Si) cells from Solarex. We conducted an investigation of plasma deposition and etching processes on full-size mc-Si cells processed in commercial production lines, so that any improvements obtained would be immediately relevant to the PV industry. This investigation determined that reactive ion etching (RIE) is compatible with using standard, commercial, screen-printed gridlines as etch masks to form self-aligned, selectively doped emitter profiles. This process results in reduced gridline contact resistance when followed by plasma-enhanced chemical vapor deposition (PECVD) treatments, an undamaged emitter surface easily passivated by plasma-nitride, and a less heavily doped emitter between gridlines for reduced emitter recombination. This allows for heavier doping beneath the gridlines for even lower contact resistance, reduced contact recombination, and better bulk defect gettering. Our initial results found a statistically significant improvement of about half an absolute percentage point in cell efficiency when the self-aligned emitter etchback was combined with a PECVD-nitride surface passivation treatment. Some additional improvement in bulk diffusion length was observed when a hydrogen passivation treatment was used in the process. We attempted to gain additional benefits from using an extra-heavy phosphorus emitter diffusion before the gridlines were deposited. However, this required a higher plasma-etch power to etch back the deeper diffusion and keep the etch time reasonably short. The higher power etch may have damaged the surface and the gridlines so that improvement due to surface passivation and reduced gridline contact resistance was inhibited.

Simultaneous dopant diffusion and surface passivation in a single rapid thermal cycle

In this work, we present results on simultaneous formation of emitter/back-surface field or emitter/surface passivation in a single rapid thermal cycle. We have investigated the diffusion kinetics of dopant elements like phosphorus, boron (from a doped spin-on glass (SOD) film), aluminium (from evaporated films) or aluminium-boron (from an A1-B SOD film). In particular, we have shown that rapid thermal co-diffusion of P and A1 (or A1-B) leads to low sheet resistances, optical emitter profiles and a hig h gettering effect. Furthermore, the possibility of using the remaining SOD films as a surface passivation layer was investigated. Dark saturation current measurements as deduced from the photoconductivity decay technique demonstrate the passivation effec t of the remaining SOD film. The highest efficiency of 12.8% obtained was achieved on SOD oxide-coated solar cells.

Simulation and dry etching of field emitter tips in Si

To increase the efficiency of field emission devices, the design of the emitters should be optimized. A simulation program using the finite element method was developed to investigate the effects of emitter geometry on the emission current and voltage for ungated emitters. It was found that the tip-anode spacing and tip radius have the most significant effects on the emitted current. The simulation predicted that a field emitter consisting of a vertical base and a cone shaped tip on the top has a higher enhancement factor and therefore a lower operating voltage. Emitters were fabricated in Si based on the optimized design. Dry etching in CF4 was used to control the slope of the SiO2 mask. The SiO2 sidewall angle decreased from 80° to 51° as pressure was increased from 40 to 300 mTorr. The SiO2 mask profile was also changed substantially by adding CHF3 or O2 to CF4 during etching. An electron cyclotron resonance source was used to generate a Cl2 plasma to etch Si field emitters. SiO2 mask was eroded during etching to control the Si emitter profile. The slope of the Si etch profile increased from 52° to 78° as the SiO2 slope increased from 17° to 45°. Radio frequency and microwave power had little influence on the Si etch profile, but they were adjusted to achieve the desired etch rate. The lateral etch rate of SiO2 decreased at higher pressure, resulting in more vertical Si profiles. Oxygen was also added to the Cl2 plasma and the Si sidewall angle was changed between 47° and 80°. Using the above results, the Si etch profile was controlled and field emitter tips and arrays with different geometries were etched in Si. To comply with the optimum design, emitter tips with two different sidewall angles of 85° and 70° were fabricated by changing the pressure from 5 to 0.5 mTorr during the etch. Arrays of emitter tips with sidewall angle of 80°, 11 μm height, and 2.2 μm basewidth were also fabricated at 10 mTorr with a packing density of 4×106 tip/cm2.