Doped Emitter Layer Research Articles

Molybdenum oxide: A superior hole extraction layer for replacing p-type hydrogenated amorphous silicon with high efficiency heterojunction Si solar cells

Transition metal oxides (TMO) are extensively applied as a surface passivation and carrier-selective contact layer through replacing boron/phosphorus doped emitter layers in silicon heterojunction (SHJ) solar cell applications. In this regard, molybdenum oxide (MoO3) has drawn a significant attention as a hole extraction layer owing properties such as wide bandgap (∼3 eV), high work function (>6 eV) and low temperature deposition. Thus, we fabricated SHJ solar cells with a dopant-free MoOx applied at the front surface contact layer. Thermally evaporated MoOx films were exhibited optical characteristics such as high transmittance, high bandgap and low absorption coefficient as compared to a-Si:H(p) and μc-SiOx:H (p) layers. X-ray photoelectron spectroscopy (XPS) analysis confirmed the stoichiometric and oxidation deficiency states of the of the MoOx layers. Whereas, MoOx films undergoing long-term air exposure showed an increase in Mo5+ cations due to the increased oxygen vacancy. The fabricated MoOx/c-Si heterojunction solar cells achieved a significant power conversion efficiency (η) of 20%, best open circuit voltage (Voc) of 695 mV, high short circuit current density (Jsc) of 38.88 mA/cm2 and a fill factor (FF) of 74.0%. These results implying that MoOx is as an excellent dopant-free material for alternate p-doped a-Si:H emitter layers in SHJ solar cell applications.

Boosting spectral response of multi-crystalline Si solar cells with Mn2+ doped CsPbCl3 quantum dots downconverter

Conventional multi-crystalline Si solar cells present limited spectral response in the near ultraviolet region due to the parasitic absorption caused by passivation layer and heavily doped emitter layer. Converting ultraviolet light into visible light is a promising way to improve the performance of the devices. Herein, CsPbCl3: Mn2+ quantum dots are applied onto the front of the multi-crystalline Si solar cells as the luminescent downconverter to improve light harvesting in the short wavelength region. CsPbCl3: Mn2+ quantum dots with large Stokes shift (>1000 meV) and high quantum yield (62%) effectively convert the normally low responding spectral in the ultraviolet region into usable visible light at ∼600 nm for improving photoelectric conversion efficiency of the devices. Combining with the unique light-trapping architecture of multi-crystalline Si solar cells, external quantum efficiency in the ultraviolet region is evidently improved, leading to an increased conversion efficiency of 6.2%. Meanwhile, solar cells coated with CsPbCl3: Mn2+ quantum dots luminescent downconverter layer also exhibit favorable photostability under AM 1.5G illumination and reproducibility. Based on these characteristics, we deem this light-management strategy could be applied on mass product multi-crystalline Si solar cells, and easily extended to enhance the performance of other solar cells.

Nanocrystalline silicon emitter optimization for Si-HJ solar cells: Substrate selectivity and CO2plasma treatment effect

We investigated hydrogenated nanocrystalline silicon (nc-Si:H) films as doped emitter layers for silicon heterojunction solar cells. Firstly, we focused on the effect of the nc-Si:H deposition conditions and film growth on the intrinsic hydrogenated amorphous silicon passivation layer ((i)a-Si:H) underneath. Three different p-doped emitters were compared: nc-Si:H, nc-SiOx:H, and a-Si:H. We found that the nc-Si:H and nc-SiOx:H growth enhances the passivation of the epitaxy-free (i)a-Si:H layer, yielding implied open circuit voltages above 730 mV. Secondly, for (p)nc-Si:H emitters, we observed a trade-off between fill factor (FF) and open circuit voltage (Voc) by using two types of (i)a-Si:H films. A slight epitaxy of the (i)layer seems to promote the rapid nucleation of nc-Si:H, thereby positively affecting the FF (79.5%) and series resistance but reducing Voc (670 mV). Contrarily, on well-passivating (i)a-Si:H the nc-Si:H nucleation is more difficult resulting in S-shaped I–V curves, presumably due to low built-in voltage and a poor emitter/TCO contact. To circumvent this dilemma, a CO2 plasma treatment is used to oxidize the a-Si:H surface before the nc-Si:H emitter deposition thereby enhancing nucleation. Accordingly, a FF of 74.5% with Voc of 727 mV is reached in the best device, yielding a conversion efficiency of 21%. HR-TEM micrograph of the front layer stack of the solar cell. The image shows a region close to the valley between two pyramids. From bottom to top: c-Si substrate, (i)a-Si:H passivation layer showing epitaxially grown regions, (p)nc-Si:H emitter layer, and In2O3:Sn (ITO). Yellow lines highlight layers and individual crystals. Silicon zone axis orientation is .

Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high‐efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture

AbstractA monolithic compound semiconductor phototransducer optimized for narrow‐band light sources was designed for achieving conversion efficiencies exceeding 50%. The III‐V heterostructure was grown by metal‐organic chemical vapor deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p‐type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ~3.4 mm2 were fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%, and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ~58 W/cm2), the multiple tunnel junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers (intensities ~144 W/cm2). Measurements at different temperatures revealed a Voc reduction of −6 mV/°C at ~59 W/cm2. Copyright © 2015 John Wiley & Sons, Ltd.

Silicon heterojunction solar cells: Optimization of emitter and contact properties from analytical calculation and numerical simulation

The key constituent of silicon heterojunction solar cells, the amorphous silicon/crystalline silicon heterojunction (a-Si:H/c-Si), offers a high open-circuit voltage (Voc) potential providing that both the interface defect passivation and the band bending in the c-Si absorber are sufficient. We detail here analytical calculations of the equilibrium band bending in c-Si (ψc-Si) in Transparent Conductive Oxide (TCO)/a-Si:H emitter/c-Si absorber structures. We studied the variation of some electronic parameters (density of states, work function) according to relevant experimental values. This study introduces a discussion on the optimization of the doped emitter layer in relation with the work function of the TCO. In particular, we argue on the advantage of having a highly defective (p)a-Si:H emitter layer that maximizes ψc-Si and reduces the influence of the TCO on Voc.

선택적 에미터 결정질 실리콘 태양전지 제작을 위한 할로겐 램프 장치 개발

Halogen lamp was applied to fabricate the selective emitter crystalline silicon solar cell. In selective emitter structure, the recombination of minority carriers is reduced with heavily doped emitter under metal grid, consequently improving the conversion efficiency. Laser selective emitter process which is recently used the most generally induces the damage on the silicon surface. However the lamp has enough heat to form heavily doped emitter layer by diffusing phosphorus from PSG without surface damage. In this work, we have studied to find the design and the suitable condition for halogen lamp such as power, time, temperature and figured out the possibility to fabricate the selective emitter silicon solar cell by lamp heating. The sheet resistance with was lower to after halogen lamp treatment. Heat transfer to lightly doped emitter region was blocked by using the shadow mask.

A Double-Heterojunction Bipolar Transistor Having a Degenerately Doped Emitter and Backward-Diode Base Contact

A double-heterojunction bipolar transistor having a degenerately doped emitter layer is investigated. The base-emitter Esaki diode introduces very low input impedance in the off- state but does not degrade the current gain at high forward bias. The heavily doped emitter layer makes it possible to scale the emitter-layer thickness considerably. The heavily doped emitter layer also allows contacting the base via the base-emitter backward diode. The high-frequency performance of the device is presented.

Analytical and numerical studies of p+-emitters in silicon carbide bipolar devices

In this paper, ways to create silicon carbide p+-emitters with high injection coefficients are considered. Raising the emitter doping level, eliminating the deteriorated layer, and making longer the electron lifetime in the emitter are discussed and analyzed. The efficiency criteria are established for SiC emitters by comparing Si and SiC p+–n junctions. It is shown that the properties of modern SiC p+-emitters are far from being optimal. Analytical models have been developed to describe the properties of SiC p+–n junctions in terms of the well-known ‘saturation current’ (jsn) approach, which takes into account the main nonlinear effects that govern the transport phenomena in highly doped emitter layers: bandgap narrowing, Auger recombination, electron–hole scattering, radiative recombination, and surface recombination. It is demonstrated that the value of jsn for the SiC p+–n junction may be as small as ∼2 × 10−49 A cm−2, i.e. two orders of magnitude smaller than the smallest values of jsn in modern SiC bipolar devices. The dependences of jsn on the electron lifetime and doping level in the emitter are analyzed. It is shown that the electron lifetime in the p+-emitter layer has a vigorous effect on the device forward voltage. The analytical results are confirmed by the data furnished by numerical experiments.

PH-Independent, 520 mV Open-Circuit Voltages of Si/Methyl Viologen2+/+ Contacts Through Use of Radial n+p-Si Junction Microwire Array Photoelectrodes

The effects of introducing an n+-doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (denoted MV2+/+), both planar and wire array p-Si photoelectrodes yielded open-circuit voltages, Voc, that varied with the pH of the solution. The highest Voc values were obtained at pH = 2.9, with Voc = 0.53 V for planar p-Si electrodes and Voc = 0.42 V for vapor−liquid−solid catalyzed p-Si microwire array samples, under 60 mW cm−2 of 808 nm illumination. Increases in the pH of the electrolyte produced a decrease in Voc by approximately −44 mV/pH unit for planar electrodes, with similar trends observed for the Si microwire array electrodes. In contrast, introduction of a highly doped, n+ emitter layer produced Voc = 0.56 V for planar Si electrodes and Voc = 0.52 V for Si microwire array electrodes, with the photoelectrode properties in each system being essentially independent of pH over six pH units (3 < pH < 9). Hence, formation of an n+ emitter layer not only produced nearly identical photovoltages for planar and Si microwire array photoelectrodes, but decoupled the band energetics of the semiconductor (and hence the obtainable photovoltage) from the value of the redox potential of the solution. The formation of radial junctions on Si microwire arrays thus provides an approach to obtaining Si-based photoelectrodes with high-photovoltages that can be used for a variety of photoelectrochemical processes, including potentially the hydrogen evolution reaction, under various pH conditions, regardless of the intrinsic barrier height and flat-band properties of the Si/liquid contact.

Mechanisms of formation of N–S transition in nonisothermal I–V characteristics of a p–i–n diode

On the basis of the self-consistent model of transport processes in the semiconductor p–i–n diode during its self-heating under conditions of limited heat sink, the mechanisms of unusual effect—the formations of N–S transition in nonisothermal I–V characteristics of the device were numerically analyzed. It is established that such an effect is caused by a pronounced temperature reduction of the mobility of carriers in the high-resistivity base and the injection-level saturation at the current densities J > 300–500 A/cm2. The saturation is attained due to the Auger recombination or the leakage of carriers from plasma into heavily doped emitter layers, the integrated current of which, as a rule, exceeds the recombination integrated current in the base under these conditions. The Auger recombination in the anode emitter also starts to play an appreciable role in the injection-level restriction in the base if the impurity concentration becomes higher than 1018 cm−3 in there.

Impact of emitter fabrication on the yield of SiGe HBTs

Epitaxially-grown, in-situ doped emitters used for SiGe HBT fabrication deliver a couple of advantages regarding device performance. Positive effects on emitter resistance, low-frequency noise, emitter-collector breakdown and manufacturability are observed. Here we report, that the removing of the interfacial oxide between the capping Si layer of the base and the emitter layer may have significant impact not only on the As indiffusion but also on the yield of small transistors. Depending on the device geometry, the high-temperature treatment of the heavily As doped emitter layers has proven as critical process step regarding the defect formation.

Quantum structures for multiband photon detection

AbstractThe work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Tunneling throughX-valley-related impurity states in GaAs/AlAs resonant-tunneling diodes

We have investigated resonant tunneling through impurity states with large binding energy of the GaAs/AlAs double-barrier resonant tunneling heterostructure. These states originate due to the penetration of Si impurity atoms from the heavily doped emitter layer that is adjacent to one of the two AlAs layers. Magnetotunneling transport results demonstrate that resonant peaks and steps arise due to tunneling through the X-valley-related donor states localized in an AlAs layer. The strong asymmetry of the doping profile in our structures provides substantial accumulation of the resonant electrons at one bias polarity, which gives rise to the intrinsic bistability observed for the impurity-assisted resonant tunneling. Using perturbation theory, we estimated the value of the current and tunneling rates through the X-valley-related donor states, which is in good agreement with the measured current value and the registered bistability effect. Observed Zeeman splitting of the current peak allowed us to determine the value of the g factor of the confined impurity states.

Evidence for the co-existence of two- and three-dimensional electron gases in the emitter of double barrier devices

We have analysed the dimensionality of the emitter electron gas of double barrier devices (DBD) under bias and found evidence for the coexistence of both two- and three-dimensional (2D and 3D) electron gases. Magnetotunneling measurements performed on a series of n-type GaAs/(AlGa)As DBD with varying emitter doping profiles have shown that the electron gas may have a 2D and/or 3D character depending on the emitter doping level and on the applied bias. In devices with heavily doped emitter layers, electrons tunnel from 3D-states, whereas for lightly doped emitters a 2D-electron gas is formed in the emitter contact. For intermediary doping levels both 2D- and 3D-electron gases may coexist, depending on the applied bias.

A study of base contact formation in epitaxial [formula omitted] HBT structures

An HBT structure was grown by LP-VPE and comprises a 45 nm thick Si 0.88Ge 0.12 base layer, of which the central 25 nm is doped with boron (2 × 10 19 cm −3). The base was capped by a 150 nm thick low doped emitter (LDE) layer (10 18 As cm −3). Three different implantation conditions were employed in order to make contact to the base through the LDE, two of which involved preamorphisation with Ge +. The redistribution of epitaxial and implanted boron after low temperature (600°C) furnace annealing and rapid thermal processing (RTP) at 1000°C was studied by SIMS. The information from SIMS is correlated with observations of the defect structures (TEM and RBS/C) and amorphous layer thicknesses before and after annealing. This study shows that preamorphisation leads to a significant reduction in the defect density of the LDE after RTP, reduces the amount of implant channelling and increases control over the boron profile. Care must be taken to ensure that the residual damage from the implants, does not intersect the base layer.

Proposal and demonstration of multi-emitter HBTs

The authors have designed and tested multi-emitter HBTs (ME-HBTs). The emitter electrodes of the transistor work as both an emitter and a base depending on their potential. A highly doped emitter layer was used for a low turn-on voltage for the collector current. Using an ME-HBT, the room-temperature operation of AND/NOR gates was tested.

Transit time and charge storage measurements in heavily doped emitters

We report results of a first direct measurement of the minority-carrier transit time in a transparent heavily doped emitter layer. The value was obtained by a high-frequency conductance method recently developed and used for low-doped Si. The transit time coupled with the steady-state current enables the determination of the quasi-static charge stored in the emitter and the quasi-static emitter capacitance. Using a transport model, we estimated, from the measured transit time, the value for the minority-carrier diffusion coefficient and mobility. The measurements were done using a heavily doped emitter of the Si p+-n-p bipolar transistor. The new result indicates that the position-averaged minority-carrier diffusion coefficients may be much smaller than the corresponding majority-carrier values for emitters having a concentration ranging from about 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> .