Pn Junction Formation Research Articles

Synthesis of Zn0.87(Fe, Al)0.065O – MO (MO = NiO, Fe2O3, CuO, and CoO) nanocomposites: Structural, vibrational, optical and antibacterial studies

One of the most effective strategies for achieving high-performance antibacterial activity is the synthesis of nanocomposites by combining different nanoparticles. Hence, Zn0.87(Fe, Al)0.065O – MO (NiO, Fe2O3, CuO and CoO) nanocomposites (NCs) with a 7:3 molar ratio were synthesized via the solution combustion method. The produced NCs structural, vibrational, and optical characteristics were investigated using XRD, FTIR, UV–Vis, and PL spectroscopies. The standard disc diffusion method was used to evaluate the antibacterial efficacy of the NCs. The lattice constant, crystallite size, bond length, dislocation density, and other relevant parameters of the NCs were also evaluated. XRD analysis revealed a well-defined mixed-phase structure for all 0.7Zn0.87(Fe, Al)0.065O – 0.3NiO (ZFA-Ni), 0.7Zn0.87(Fe, Al)0.065O – 0.3Fe2O3 (ZFA-Fe), 0.7Zn0.87(Fe, Al)0.065O – 0.3CuO (ZFA-Cu) and 0.7Zn0.87(Fe, Al)0.065O – 0.3CoO (ZFA-Co) NCs, indicating successful mixing of each metal oxide with the ZnO matrix. The average crystallite size within the NCs ranged from 14.21 to 18.10 nm, suggesting a degree of nanocrystallinity nature of all NCs. FTIR confirmed the presence of characteristic functional groups within the NCs. Moreover, all the synthesized NCs exhibited significant optical activation in the ultraviolet radiation range but visible light absorption was also observed when the composite formed with CuO and Fe2O3 nanoparticles. The band gap energy values exhibited variation between 2.72 eV for ZFA-Co NC and 2.90 eV for ZFA-Ni NC. Photoluminescence (PL) emission in the composite formation of ZnO NPs with other metal oxide were significantly influenced by the absorption range of the composite with pn-junction formation in a notable enhancement of antibacterial efficacy against bacterial strains of Escherichia coli and Staphylococcus aureus. Particularly, ZFA-Co NC exhibited an excellent antibacterial performance against both bacterial strains.

Multi‐Dimensional Physically Unclonable Functions: Optoelectronic Variation‐Induced Multi‐Key Generation from Small Molecule PN Heterostructures

AbstractIn this era of demanding invulnerable security systems against the threat of hacking, physically unclonable functions (PUFs), especially in which can spawn multiple security keys within a single device, has gained attention. This investigation explores the multi‐key generable PUF devices employing organic small molecules, specifically C8‐BTBT and PTCDI‐C13. The variation stems from the formation of irregular PN junctions, haphazardly configured grain boundaries of C8‐BTBT. A comprehensive analysis including scanning electron microscopy (SEM), atomic force microscopy (AFM), kelvin probe force microscopy (KPFM), impedance spectroscopy (IS), and optical simulation, has been substantiated the underlying mechanisms. Exploiting the photo‐responsive characteristics within the light wavelength spectrum of 660 and 530 nm, alongside the electrical characteristics, the capability to generate a total of 30 distinct multi‐security keys in a single device is been successfully. These keys, distinguished by variable parameters such as voltage, light wavelength, and the calculated photo‐to‐dark current ratio (PDCR), manifest novel quantitative and qualitative dimensions in security protocol customization. Inter‐Hamming distance and entropy of these cryptographic keys exhibit commendable averages of 51.9–53.1%, and 0.81, respectively. Moreover, a noteworthy average bit‐aliasing mean value of 51.9%, derived from four distinct batches, underscores the pragmatic feasibility of the proposed conceptual framework for PUF devices.

High-efficiency TOPCon solar cell with superior P + and P++ layer via one-step processing

The boron diffusion process in the front field of N-type tunnel oxide passivated contact (TOPCon) solar cells is crucial for PN junction formation and the creation of a selective emitter. This study presents a theoretical model of boron diffusion in silicon using molecular dynamics. The research examines the mean square displacement and diffusion coefficient of boron atoms at varying temperatures, confirming their diffusion behavior. The simulations indicate predominant boron diffusion in the z-direction within the silicon matrix, with the diffusion depth being temperature dependent. The optimal temperature range for boron diffusion in silicon is identified as 950 °C to 1050 °C. Using boron-doped silicon paste and boron trichloride as dopants, thermal diffusion experiments were conducted to fabricate the front-field PN junction (p+ layer) and selective emitter (p++ layer) by one step. Subsequent processing and performance evaluation were performed on a production line. Experimental findings reveal a decrease in boron diffusion at higher temperatures, reduced sheet resistance, increased doping concentration, and deeper junction formation. The ideal boron concentration in the p+ layer is 8.68 × 1018 atom/cm3 with a depth of 0.53 μm, while the p++ layer is 2.35 × 1019 atom/cm3 and 0.82 μm. The efficiency of the optimized TOPCon + cell production line reaches up to 25.17 %, marking an improvement of 0.23 % over the standard cell production line. This research contributes to elucidating the mechanism of boron diffusion and offers insights for enhancing the efficiency of TOPCon solar cells.

(Invited) In-Situ MOCVD Etching of GaN Using XeF2 for Selective-Area-Epitaxial-Regrowth of p-Type GaN for High Voltage PN Diodes

PN diodes with multi-kilovolt breakdown voltages have been demonstrated to great than 6 kV in GaN, validating the device potential predicted by the intrinsic material properties of III-nitride semiconductors. However, power control circuits also require switching transistors such as junction field effect transistors (JFETs) and practical diodes such as merged PIN Schottky (MPS) diodes. Such devices usually employ selective areas of p-type semiconductor surrounded by n-type material and require the formation of PN junctions with low reverse leakage current. Selective-area p-type doping in Si and SiC based power devices is typically achieved using implantation and thermal annealing to form the p-type areas. Ion implantation of Mg in GaN has demonstrated p-type conductivity, but this process requires specialized equipment for the high-pressure and high-temperature activation of Mg dopants. We have investigated selective-area-regrowth (SArG) to epitaxially grow p-type GaN as an alternative to dopant implantation. Successful PN diode formation by SArG requires a process to remove residual crystalline damage resulting from the ex-situ inductively coupled plasma (ICP) etch typically used to form the p-well, as well as a method to remove the elevated level of Si found at the regrowth interface of a surface that had been exposed to air. For the case of SArG of p-GaN on air-exposed, blanket ICP etched n-type GaN, we present the novel use of a fluorine-based precursor (XeF2) for in-situ etching of GaN in the MOCVD chamber. Unlike chlorine (TBCl and CCl4) and bromine (CBr4) -based precursors we have studied; we obtained a smooth surface following in-situ fluorine etching of air-exposed GaN. Schottky barrier diodes (SBDs) formed by shadow mask evaporation on in-situ fluorine etched n-GaN that had been previously ICP etched showed reverse leakage currents to -40 V that are equal to those of SBDs formed on as-grown n-GaN layers. The in-situ fluorine/ICP etched Schottky diodes had reverse leakage currents more than 3 orders of magnitude lower than those formed on n-GaN layers that had only experienced ICP etching. This suggests that the in-situ fluorine etch was effective at removing the residual damage from the ICP etch process. Furthermore, we discuss the use of in-situ fluorine etching to reduce the concentration of Si at the regrowth interface required to form PN junctions with low reverse leakage current. This work was supported in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program directed by Dr. Isik Kizilyalli, and in part by the US Department of Energy (DOE) Vehicle Technologies Office (VTO) under the Electric Drive Train Consortium. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

Distinct junction transformations of pixel-arrayed long-wavelength infrared detectors by photocurrent mapping

Pixel-arrayed long-wavelength HgCdTe is of great significance in the development of infrared detection. However, the PN junction formation process for long-wavelength HgCdTe is complex. The damage induced by boron ion implantation and subsequent low-temperature annealing may results in changes in the effective doping concentration and geometry of the actual n-type implantation region. Therefore, an in-situ, non-destructive scanning-photocurrent-microscopy (SPCM) method is needed to test the performance and electroactive defects of HgCdTe devices. Here, the pixel-arrayed long-wavelength infrared detectors with n-on-p structure formed by B+ ion implantation in Hg vacancy-doped Hg0.78Cd0.22Te crystals were fabricated. The SPCM method is used to characterize the PN junction performance of long-wavelength Hg0.78Cd0.22Te array detectors. It is found that the long-wavelength device transformed from n-on-p structure to n-on-n− structure at about 175 K, and further turns into an n−-on-n-on-n− structure with the temperature increasing. In addition, when the temperature increases, the minority carrier diffusion length in the B+ ion implantation region gradually decreases with the activation of the recombination center. Our work provides an effective theoretical and experimental basis for the HgCdTe junction formation process.

Simulation of a Novel Junction-less Solar Cell Structure Using PC-1D Software

The continues demand for Photovoltaic device improvement has led us to search for new sources of photovoltaic action. For that, this paper proves the existence of other sources for photovoltaic action in addition to the main source which is the built-in electrostatic fild generated through controlled composition gradientThese new sources are uncovered through mathematical examination of a specifi material system followed by the construction of four-layer p-type semi-conducting materials (i.e., without pn-junction formation). First three layers of this specifi material system are fabricated with AlxGα1-xAs with diffrent Aluminum fractions, while the fourth layer is fabricated with Ge with a diffsed back surface fild (BSF) layer to reduce surface recombination velocities (SRV).The complete structure is analyzed using the PC-1D simulation softare, and it shows that the junctionless structure possesses the photovoltaic action with low performance.

Etched-And-Regrown GaN P-N Diodes with Low-Defect Interfaces Prepared by In Situ TBCl Etching.

The ability to form pristine interfaces after etching and regrowth of GaN is a prerequisite for epitaxial selective area doping, which in turn is needed for the formation of lateral PN junctions and advanced device architectures. In this work, we report the electrical properties of etched-and-regrown GaN PN diodes using an in situ Cl-based precursor, tertiary butylchloride (TBCl). We demonstrated a regrowth diode with I-V characteristics approaching that from a continuously grown reference diode. The sources of unintentional contamination from the silicon (Si) impurity and the mediating effect of Si during the TBCl etching are also investigated in this study. This work points to the potential of in situ TBCl etching toward the realization of GaN lateral PN junctions.

Selective volatile organic compound gas sensor based on carbon nanotubes functionalized with ZnO nanoparticles

This paper describes the theory and results for a new class of low-cost chemoresistive gas sensors designed for selective hydrocarbon gas detection. The sensors utilize a multiwalled carbon nanotube (MWCNT) backbone functionalized with metal oxide nanocrystals. Specifically, nanoparticles were grown on the surface of the MWCNTs using atomic layer deposition. The crystallinity of the ZnO-MWCNTs’ heterostructure was examined by using a high-resolution transmission electron microscope. The structure of the ZnO/MWCNTs was analyzed using a scanning electron microscope and energy dispersive x ray. The Hall effect measurement shows p-type characteristics of the MWCNTs, supporting the typical PN junction formation with n-type ZnO nanocrystals. The electron-donating ability of ZnO provided a strong response to the ppm levels of toluene at room temperature (25 °C) and showed strong selectivity with other volatile organic compound gases such as benzene, methane, and formaldehyde.

First-principle investigation of hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf)

Recently, perovskite ferroelectric photovoltaic materials have been studied extensively. Traditional photovoltaic device usually uses the internal electric field formed by PN junction to realize the separation of photogenerated carriers to form the photovoltaic effect, while ferroelectric material, due to the existence of spontaneous polarization, can spontaneously realize the separation of photogenerated electrons and holes without the formation of PN junction, presenting the ferroelectric photovoltaic effect. Chalcogenide perovskite with suitable band gap and visible light absorption is expected to be a new generation of ferroelectric photovoltaic materials. However, its application is limited due to the lack of ferroelectric properties. Hybrid improper ferroelectricity (HIF) in layered perovskites has opened a new way for developing the new ferroelectrics. In contrast to the proper ferroelectricity in which the polarization is the main order parameter as the driving force, the improper ferroelectricity possesses the ferroelectric polarization that becomes a secondary order parameter induced by other orders. In this work, we study the ground state, electronic structure and hybrid improper ferroelectricity of <i>n</i> = 2 Ruddlesden-Popper (RP) Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) based on the first principles. The total energy calculations and phonon spectrum analysis show that the ground state of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (B = Zr, Hf ) is of <i>A</i>2<sub>1</sub><i>am</i> polar phase. The hybrid improper ferroelectricity originates from the coupling between two rotation modes of <i>B</i>Se<sub>6</sub> octahedron. Electronic structure calculations show that Sr<sub>3</sub>Zr<sub>2</sub>Se<sub>7</sub> and Sr<sub>3</sub>Hf<sub>2</sub>Se<sub>7</sub> are semiconductors with direct band-gaps, which are around 1.56 eV and 1.84 eV, respectively. The ferroelectric polarization values calculated by the Berry phase method are around 12.75 μC/cm<sup>2</sup> and 9.69 μC/cm<sup>2</sup>, respectively. The contribution of each atomic layer to the ferroelectric polarization is investigated when the Born effective charge method is used. The results show that the polarization of Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) mainly comes from the Sr-Se atomic layers. To sum up, Sr<sub>3</sub><i>B</i><sub>2</sub>Se<sub>7</sub> (<i>B</i> = Zr, Hf ) show strong ferroelectric polarization and good visible light absorption characteristics and they are expected to be candidates of a new generation of ferroelectric photovoltaic materials.

Admittance studies of modification of HgCdTe surface properties with ion implantation and thermal annealing

Metal–insulator–semiconductor (MIS) structures based on HgCdTe were fabricated after various stages of pn junction formation using As+ implantation and activation annealing. The energy of As+ ions was 200 keV with the fluence of 1014 cm−2. Heteroepitaxial HgCdTe films with near-surface graded-gap layers were grown by molecular beam epitaxy (MBE) on silicon substrates. It was shown that the electron concentration in the near-surface semiconductor layer increases after implantation to values of about 1017 cm−3, and after implantation and annealing in the near-surface semiconductor layer, a p+ layer appears with a hole concentration of more than 1.5 × 1018 cm−3. The generation rate of minority charge carriers in the space charge region after implantation is low, which indicates a low defectiveness of the thin near-surface MBE HgCdTe layer. After implantation and annealing, the generation rate increases significantly, which may be due to high defectiveness near the interface between Al2O3 and MBE HgCdTe. Dopant profiles were constructed in the near-surface HgCdTe layers after various technological procedures. It was shown that after implantation in films, the dopant concentration increases with distance from the interface to the depth of 0.1 μm.

Production of sensitive gas sensors using CuO/SnO2 nanoparticles

Metal oxide nanoparticles, such as CuO and SnO2, are outstanding systems for H2S gas sensing in air. In this work, those nanoparticles were deposited with different mixing percentages on substrates to form percolating networks of nanoparticles. Electrical electrodes were deposited on the nanoparticles’ films to investigate their gas sensing response against H2 and H2S, and their electrical characteristics. The sensor devices based on CuO–SnO2 nanoparticles revealed enhanced sensing characteristics against H2S with a sensitivity of 10 ppm. The enhanced sensing characteristics could be attributed to the formation of PN-junctions among CuO and SnO2 nanoparticles. The reasonable production cost (due to simple structure and cheap used materials), low power consumption ( ~ 1 µW for H2S at room temperature), high sensitivity, high response, and reasonable response time of the present sensors qualify them for practical implementation in portable gas sensing devices with enhanced characteristics.

Pn junction formation by Si paste coated on metal substrates

In this work, p- and n-type Si pastes were prepared in a way by utilizing a planetary ball miller to pulverize Si source material. pn homojunction was formed by coating Si pastes onto metal substrates, followed by annealing under Ar or Ar + H2 atmosphere. The optimal annealing temperature was found to be around 1050 °C that exhibits the lowest resistivity. A typical rectifying property of diode with small photovoltaic was observed for the device fabricated. It is anticipated that Si pastes could have potential for low cost device fabrication such as solar cells. All Cz Si ingot growth procedures could be skipped, consequently it is expected that the manufacturing cost for solar cell can be greatly reduced.

The Properties of Zn-Doped AlSb Thin Films Prepared by Pulsed Laser Deposition

Aluminum antimony (AlSb) is a promising photovoltaic material with a band gap of about 1.62 eV. However, AlSb is highly deliquescent and not stable, which has brought great difficulties to the applications. Based on the above situation, there are two purposes for preparing our Zn-doped AlSb (AlSb:Zn) thin films: One is to make P-type AlSb and the other is to find a way to suppress the deliquescence of AlSb. The AlSb:Zn thin films were prepared on glass substrates at different substrate temperatures by using the pulsed laser deposition (PLD) method. The structural, surface morphological, optical, and electrical properties of AlSb:Zn films were investigated. The crystallization of AlSb:Zn thin films was enhanced and the electrical resistivity decreased as the substrate temperature increased. The scanning electron microscopy (SEM) images indicated that the grain sizes became bigger as the substrate temperatures increased. The Raman vibration mode AlSb:Zn films were located at ~107 and ~142 cm−1 and the intensity of Raman peaks was stronger at higher substrate temperatures. In the experiment, a reduced band gap (1.4 eV) of the AlSb:Zn thin film was observed compared to the undoped AlSb films, which were more suitable for thin-film solar cells. Zn doping could reduce the deliquescent speed of AlSb thin films. The fabricated heterojunction device showed the good rectification behavior, which indicated the PN junction formation. The obvious photovoltaic effect has been observed in an FTO/ZnS/AlSb:Zn/Au device.

Formation of pn-junction with stable n-doping in graphene field effect transistors using e-beam irradiation

Graphene an allotrope of carbon having outstanding electronic properties can be used as an alternative material for silicon based technology. We report the influence of electron beam irradiations by exposing graphene device to different doses ranges from 1500 μC/cm2–4500 μC/cm2. The intensity of D peaks obtained from the Raman spectra increases gradually with the increase of e-beam dosage, which indicates that e-beam introduced localized defects in graphene. The effect of irradiations on graphene FET is also performed by studying electrical transport measurements of the device, indicating n-type doping. A decrease in charge carrier mobility upon exposure to e-beam irradiation reveals the strong effect of induced defects. We further investigated that by exposing selected region of graphene to e-beam irradiations results in the formation of pn-junction, which enable us to fabricate graphene based pn-junctions that could be beneficial for future electronics.

Deposition of ZnO based thin films by atmospheric pressure spatial atomic layer deposition for application in solar cells

The use of Atmospheric Pressure Spatial Atomic Layer Deposition (AP-SALD) has gained popularity in the last decade. The success of this technique relies on the possibility to deposit thin films in a fast, vacuum-free, low-cost, low-damage, and high throughput way. In this work, we present ZnO and Aluminium doped ZnO (AZO) films deposited by AP-SALD at low temperature (<220 °C) with high uniformity and conformity. The ZnO films present a high transparency of 80%–90% in the visible range, with a tuneable band-gap, between 3.30 eV and 3.55 eV, controlled by the deposition temperature. The carrier density reaches values greater than 3 × 1019 cm−3, while the electron mobility of the films is as high as 5.5 cm2 V−1 s−1, resulting in an optimum resistivity of 5 × 10−2 Ω cm. By doping ZnO with aluminium, the resistivity decreases down to 5.57 × 10−3 Ω cm, as a result of a significant increase in the carrier density up to 4.25 × 1020 cm−3. The combination of ZnO thin films with p-type cuprous oxide (Cu2O), deposited by aerosol assisted metal organic chemical vapor deposition, allowed the formation of oxide-based pn junctions. The dark I-V characteristic curve confirms a rectifying behaviour, opening the window for the production of all-oxide solar cells completely by chemical vapour deposition methods. We also show the potential of AP-SALD to deposit AZO as a transparent conductive oxide layer for silicon heterojunction solar cells.

Rapid thermal annealing treated spin-on doped antireflective radial junction Si nanopillar solar cell.

Radial junction nanopillar Si solar cells are interesting for cost effective efficiency improvement. Here, we address a convenient top-down fabrication of Si nanopillar solar cells using spin-on doping and rapid thermal annealing (RTA) for conformal PN junction formation. Broadband suppressed reflection as low as an average of 5% in the 300-1100 nm wavelength range and un-optimized cell efficiency of 7.3% are achieved. The solar cell performance can be improved by optimization of spin-on-doping and suitable surface passivation. Overall, the all RTA processed, spin-on doped nanopillar radial junction solar cell shows a very promising route for low cost and high efficiency thin film solar cell perspectives.

Nanostructured metal-oxide thin film towards the electronic device applications

Today, most of the low voltage electronics and opto-electronic devices are based on silicon technology. Metal oxide nanostructured semiconducting materials could be one of the alternatives for such devices. Metal oxide semiconductors are economical and can be easily synthesised by wet chemical processes. The formation of pn-junction is an important task for converting any two semiconducting materials interface to suitable application like rectifiers, photodetectors, light emitting diodes (LED), photovoltaic devices and lasers etc. The low cost semiconductor metal oxide materials like NiO, CuO, ZnO, TiO2, SnO2 are useful to develop pn-junction for such application. These metal oxides are easily synthesised by wet chemical process like chemical bath deposition (CBD), hydrothermal deposition (HTD), and spin coating techniques. In this present work, the ITO/NiO and NiO/ZnO two different junctions have been prepared using wet chemical processes. The morphological studies are performed by using scanning electron microscopy (SEM) and I-V characterisation studied using Keithley source meter. I-V characterisation confirmed the formation of pn-junction with rectification ratio of 22.4 and 15.7 at 2 V for NiO/ITO and NiO/ZnO junction respectively. The high ideality factor 2.06 and 2.2 depicts the high recombination of hole-electron carrier at diffusion region.

Light trapping of crystalline Si solar cells by use of nanocrystalline Si layer plus pyramidal texture

The surface structure chemical transfer (SSCT) method has been applied to fabrication of single crystalline Si solar cells with 170μm thickness. The SSCT method, which simply involves immersion of Si wafers in H2O2 plus HF solutions and contact of Pt catalyst with Si taking only ∼30s for 6 in. wafers, can decrease the reflectivity to less than 3% by the formation of a nanocrystalline Si layer. However, the reflectivity of the nanocrystalline Si layer/flat Si surface/rear Ag electrode structure in the wavelength region longer than 1000nm is high because of insufficient absorption of incident light. The reflectivity in the long wavelength region is greatly decreased by the formation of the nanocrystalline Si layer on pyramidal textured Si surfaces due to an increase in the optical path length. Deposition of phosphosilicate glass (PSG) on the nanocrystalline Si layer for formation of pn-junction does not change the ultralow reflectivity because the surface region of the nanocrystalline Si layer possesses a refractive index of 1.4 which is nearly the same as that of PSG of 1.4–1.5. The PSG layer is found to passivate the nanocrystalline Si layer, which is evident from an increase in the minority carrier lifetime from 12 to 44μs. Hydrogen treatment at 450°C further increases the minority carrier lifetime approximately to a doubled value. The solar cells with the <front Ag electrode/nanocrystalline Si layer/pyramidal Si substrate/boron-diffused back surface field/Ag rear electrode> structure show a high conversion efficiency of 18.5% in spite of the simple cell structure without antireflection coating. In this case, the short circuit photocurrent density is 40.1mA/cm2 under AM1.5 100mW/cm2 irradiation.

Low thermal budget n-type doping into Ge(001) surface using ultraviolet laser irradiation in phosphoric acid solution

We have investigated phosphorus (P) doping into Ge(001) surfaces by using ultraviolet laser irradiation in phosphoric acid solution at room temperature. We demonstrated that the diffusion depth of P in Ge and the concentration of electrically activated P can be controlled by the number of laser shots. Indeed, a high concentration of electrically activated P of 2.4 × 1019 cm−3 was realized by 1000-times laser shots at a laser energy of 1.0 J/cm2, which is comparable or better than the counterparts of conventional n-type doping using a high thermal budget over 600 °C. The generation current is dominant in the reverse bias condition for the laser-doped pn-junction diodes independent on the number of laser shots, thus indicating low-damage during the pn-junction formation. These results open up the possibility for applicable low thermal budget doping process for Ge-based devices fabricated on flexible substrates as well as Si electronics.