Phosphorus-doped Amorphous Silicon Research Articles

A study of activated phosphorus distribution within silicon substrate for polysilicon passivating contacts based on an in-line PVD system

An in-line PVD system consisting of plasma oxidation chamber and phosphorus-doped amorphous silicon deposition chamber was used to fabricate highly-doped polysilicon passivating contact structure for commercial TOPCon solar cells, which can help to achieve simple fabrication process and excellent electrical properties. In this paper, we studied how the doping profiles of ionized phosphorus atoms in the passivating contact formed by this method varies with the deposition conditions, especially the in-diffusion profile in the silicon substrate. Increasing the RF power of plasma oxidation can prevent phosphorus from penetrating through SiOx to form a shallow profile, and reducing the power is conducive to the formation of more pinholes on SiOx, resulting in a wide profile following the shallow profile. The shallow profile effectively inhibits the surface recombination rate and thus achieve good passivation quality, while the emergence of the following wide profile helps to reduce the contact resistivity by providing more majority carrier transport channels. The 100 W condition constructed a shallow and gradually widening doping profile, exhibiting a low recombination current density of 16.5 fA/cm2 and a low contact resistivity of 1.87 mΩ·cm2. However, the SiOx thickness and density obtained at 80 W are insufficient for achieving a shallow-first-then-widen doping profile, even with adjustments to thermal activation conditions. Additionally, we found the engineering of surface microstructure on silicon wafers also has an impact on the doping profile.

Effects of tunneling oxide defect density and inter-diffused carrier concentration on carrier selective contact solar cell performance: Illumination and temperature effects

In this study, technology computer-aided design (TCAD) was used to investigate carrier selective contact solar cell performance based on the boundary of tunneling oxide and electron selective contact layer properties. The role of tunneling oxide quality in the passivation and inter-diffusion properties of the plasma-enhanced chemical vapor deposition of phosphorus-doped amorphous silicon as an electron selective contact layer is studied for carrier selective contact solar cells. Tunnel oxide quality varies according to the ratio of Si4+ to Si2+ state. For the Si4+/Si2+ ratio of 4, open-circuit voltage of 730 mV and the lowest interface trap density of 5.3 × 1010 cm−2 eV−1 are achieved. The change in diffusion depth of the doped layer with respect to the annealing temperature is analysed by transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) measurement. Carrier selective contact solar cell parameters are optimized by incorporating the experimental values of interface trap density, inter-diffused impurity concentration, and depth in the Quokka 3 TCAD tool. Solar cell conversion efficiency of 24.31% was obtained. To understand the response of carrier selective solar cell to the environmental changes, the output characteristics of the solar cell were studied by varying the illumination temperature by 10% of the standard test conditions (STC) and temperature from 277 K to 377 K (difference of 100 K).

Thermal Activation of Boron- and Phosphorus-Doped Amorphous Silicon and the Contribution to Improved Efficiency in Hydrogenated Amorphous Silicon Solar Cells

Hydrogenated amorphous silicon (a-Si:H) is an interesting candidate as an absorber material for solar cells. Despite the wealth of research to improve the efficiency of a-Si:H solar cells by improving either the material quality of the absorber layer or by means of light trapping approaches, efforts to improve the efficiency by means of doped layer manipulation are relatively rare. In this work, single-junction a-Si:H solar cells with improved efficiency due to thermal activation of doped layers via thermal annealing will be presented. Temperature-dependent dark conductivity measurements revealed that p- and n-type doped a-Si:H materials show different equilibrium temperatures. External quantum efficiency at different annealing temperatures revealed that front surface collection probability was improved with the activation of a p layer, after which the collection probability of the back surface was improved with the activation of an n layer.

Fully low temperature interdigitated back-contacted c-Si(n) solar cells based on laser-doping from dielectric stacks

This work shows a novel fabrication process of interdigitated back-contacted c-Si(n) solar cells based on laser-doped point contacts. In this approach, all the highly-doped regions have been entirely fabricated through UV laser processing of dielectric layers, avoiding the high temperature steps typically involved in conventional diffusion processes. Additionally, the number of patterning steps has been significantly reduced. Aluminum oxide films deposited by thermal ALD on the front surface passivate and reduce reflection losses, while at the rear surface the same films are also used as aluminum source for p+ emitter contacts. n+ regions are created by laser processing a phosphorus-doped amorphous silicon carbide stack deposited by PECVD. As a proof of concept, solar cells (3×3cm2) have been fabricated with efficiencies beyond 20% with short-circuit current densities and open-circuit voltages up to 40.7mA/cm2 and 654mV respectively.

Silicon-Rich Silicon Carbide Hole-Selective Rear Contacts for Crystalline-Silicon-Based Solar Cells.

The use of passivating contacts compatible with typical homojunction thermal processes is one of the most promising approaches to realizing high-efficiency silicon solar cells. In this work, we investigate an alternative rear-passivating contact targeting facile implementation to industrial p-type solar cells. The contact structure consists of a chemically grown thin silicon oxide layer, which is capped with a boron-doped silicon-rich silicon carbide [SiCx(p)] layer and then annealed at 800-900 °C. Transmission electron microscopy reveals that the thin chemical oxide layer disappears upon thermal annealing up to 900 °C, leading to degraded surface passivation. We interpret this in terms of a chemical reaction between carbon atoms in the SiCx(p) layer and the adjacent chemical oxide layer. To prevent this reaction, an intrinsic silicon interlayer was introduced between the chemical oxide and the SiCx(p) layer. We show that this intrinsic silicon interlayer is beneficial for surface passivation. Optimized passivation is obtained with a 10-nm-thick intrinsic silicon interlayer, yielding an emitter saturation current density of 17 fA cm-2 on p-type wafers, which translates into an implied open-circuit voltage of 708 mV. The potential of the developed contact at the rear side is further investigated by realizing a proof-of-concept hybrid solar cell, featuring a heterojunction front-side contact made of intrinsic amorphous silicon and phosphorus-doped amorphous silicon. Even though the presented cells are limited by front-side reflection and front-side parasitic absorption, the obtained cell with a Voc of 694.7 mV, a FF of 79.1%, and an efficiency of 20.44% demonstrates the potential of the p+/p-wafer full-side-passivated rear-side scheme shown here.

Photoelectron yield spectroscopy and inverse photoemission spectroscopy evaluations of p-type amorphous silicon carbide films prepared using liquid materials

Phosphorus-doped amorphous silicon carbide films were prepared using a polymeric precursor solution. Unlike conventional polymeric precursors, this polymer requires neither catalysts nor oxidation for its synthesis and cross-linkage, providing semiconducting properties in the films. The valence and conduction states of resultant films were determined directly through the combination of inverse photoemission spectroscopy and photoelectron yield spectroscopy. The incorporated carbon widened energy gap and optical gap comparably in the films with lower carbon concentrations. In contrast, a large deviation between the energy gap and the optical gap was observed at higher carbon contents because of exponential widening of the band tail.

P/n junction depth control using amorphous silicon as a low temperature dopant source

Phosphorus-doped amorphous silicon thin films, deposited at low temperatures by Plasma Enhanced Chemical Vapour Deposition were used as a dopant source on p-type c-Si substrates. A careful step of dehydrogenation was done in order to maintain the a-Si thin-film integrity. Subsequently, a fine-controlled drive-in of dopant, from the amorphous layer to the crystalline wafer was done, to form the p/n junction, using different time periods and temperatures. Dopant profiling in c-Si wafers as well as dopant concentration in a-Si:H films prior to diffusion, both measured by Secondary Ion Mass Spectrometry, are presented. Junction depths obtained are in the range of 98nm to 2.4μm and surface concentrations are in the range of 1.1×1021 to 4.3×1020at/cm3. A dual diffusion mechanism explains the “kink-and-tail” shape found for dopant profile.

Using Phosphorus-Doped α-Si Gettering Layers to Improve NILC Poly-Si TFT Performance

Ni-metal-induced lateral crystallization (NILC) has been utilized to fabricate polycrystalline silicon (poly-Si) thin-film transistors (TFTs). However, the current crystallization technology often leads to trapped Ni and NiSi2 precipitates, thus degrading device performance. In this study, phosphorus-doped amorphous silicon (p-a-Si) and chemical oxide (chem-SiO2) films were used as Ni-gettering layers. After a gettering process, the Ni impurity within the NILC poly-Si film and the leakage current were both reduced, while the on/off current ratio was increased. This gettering process is compatible with NILC TFT processes and suitable for large-area NILC poly-Si films.

Improved Gettering Efficiency of Ni from Nickel-Mediated Crystallization Silicon Using Phosphorus-Doped Amorphous Silicon

Ni-metal-induced lateral crystallization (NILC) has been utilized to fabricate polycrystalline silicon thin-film transistors. However, the NILC process often leads to Ni and NiSi2 precipitates being trapped. In this study, two kinds of films were used as gettering layers: (1) amorphous Si and (2) phosphorus-doped amorphous Si. After annealing at 550°C for 12 h, it was found that phosphorous dopant did improve the gettering efficiency of amorphous Si.

Raman investigation of the electron-phonon interaction in n-type silicon nanocrystals

Silicon nanocrystals prepared in phosphorus-doped (at a concentration of 3.3 × 1020 cm−3) amorphous silicon films under pulsed irradiation with an excimer laser are studied using Raman spectroscopy and electron microscopy. The experimental data can be interpreted in terms of the Fano interference as a manifestation of the electron-phonon interaction effects in n-type silicon nanocrystals. It is assumed that a strong electron-phonon interaction (as compared to similar interactions in n-type bulk silicon) is due to the weakening of the momentum selection rules in nanocrystals.

Very low surface recombination velocity of crystalline silicon passivated by phosphorus‐doped a‐SicxNy:H(n) alloys

AbstractHydrogenated and phosphorus‐doped amorphous silicon carbonitride films (a‐SiCxNy:H(n)) were deposited by plasma‐enhanced chemical vapor deposition (PECVD) on crystalline silicon surface in order to explore surface passivation properties. Very silicon‐rich films yielded effective surface recombination velocities at 1 sun‐illumination as low as 3 cm s−1 and 2 cm s−1 on 1 Ω cm p‐ and n‐type crystalline silicon substrates, respectively. In order to use them as anti‐reflection coating, we increased alternatively either the carbon or nitrogen content of these films. Also, a combination of passivation and antireflective films was analyzed. Finally, we explored the passivation stability under high‐temperature steps. Copyright © 2007 John Wiley & Sons, Ltd.

Investigation of the formation of silicon nanocrystals by annealing of amorphous SiC x/c-Si structures

In this work, we study the changes in the optical properties of 300-nm-thick hydrogenated amorphous silicon carbide layers after an annealing process. Both intrinsic and phosphorus-doped amorphous silicon carbide layers (a-SiC x :H) were deposited on silicon wafers by plasma enhanced chemical vapour deposition (PECVD) at 400 °C and annealed in a quartz furnace at 800 °C. The presence of randomly oriented silicon nanocrystals was confirmed by X-ray diffraction (XRD) measurements after the partial recrystallization process only in the doped layers. The presence or the absence of the nanocrystals clearly changes the Fourier transform infrared (FTIR) spectra. From the fitting of the experimental curves with the model of Lorentz oscillators, the refractive index and the extinction coefficient of the different layers were obtained.

Phosphorus-doped SiC as an excellent p-type Si surface passivation layer

The passivation properties of phosphorus-doped amorphous silicon carbide (a-SixC1−x) layers on monocrystalline Si wafers (floating zone, 1Ωcm) have been investigated. The cleaning and the deposition process were performed in our two source (microwave, radio frequency) plasma-enhanced chemical vapor deposition reactor. In situ plasma etching and deposition at 350°C without any following annealing step led to extraordinary low surface recombination velocities of less than 5cm∕s for injection levels between 1×1014 and 1×1015cm−3. Characterization of the a-SixC1−x layer was done with quasi-steady-state photoconductance, microwave-detected photoconductance, and carrier density imaging techniques. For injection levels of more than 1×1015cm−3 the effective lifetime was limited only by intrinsic Auger recombination.

Alternative phosphorus-doped amorphous silicon using trimethylphosphine diluted in hydrogen

The objective of our work was the development of an alternative doping method of amorphous and microcrystalline silicon for solar cell applications. n-type amorphous silicon films were prepared from gas mixtures containing the dopant vapor trimethylphosphine (TMP, P(CH 3) 3) diluted in hydrogen—instead of the well-known phosphorus source phosphine (PH 3)—silane and hydrogen. Different plasma sources were used—operating at 13.56, 27.12 MHz (parallel plate capacitive type system) and 163 MHz (VHF resonance plasma source). For all the investigated frequencies and plasma sources a dark conductivity maximum level (at room temperature) higher than 3×10 −3 Ω −1 cm −1 could be reached, with dopant levels (TMP/silane concentration) in the range of 10 −3–10 −2. The sharp decrease of the dark conductivity for higher dopant concentrations (which is characteristic for the hydrocarbon-based phosphorus sources due to the formation of silicon carbide within the silicon structure) was not detected up to dopant levels of 4×10 −2.

Infrared Charge-Modulation Spectroscopy of Defects in Phosphorus Doped Amorphous Silicon

AbstractWe present infrared charge-modulation absorption spectra on phosphorus-doped amorphous silicon (a-Si:H:P) with doping levels between 0.17%-5%. At higher doping levels (1% - 5%) we find a sharp spectral line near 0.75 eV with a width of 0.1 eV. We attribute this line to the internal optical transitions of a complex incorporating four fold coordinated phosphorus and a dangling bond. This line is barely detectable in samples with lower doping levels (below 1%). In these samples a much broader line dominates the spectrum that we attribute to uncomplexed dopants. The relative strength of the two spectral features is in rough agreement with a model proposed by Street that has not been previously tested experimentally.

Evaluation of n+a-Si Contact with Indium Tin Oxide by X-Ray Photoelectron Spectroscopy Valence Band Measurement

We investigated the cause of the high resistance in a contact structure of phosphorus-doped amorphous silicon (n+a-Si) with indium tin oxide (ITO) for thin-film transistor liquid crystal displays (TFT-LCDs). X-ray photoelectron spectroscopy (XPS) valence band analysis for this interface was newly developed. TFT-LCD fabrication processes could be simplified by using ITO in pixels as the drain contact electrode. One problem of this drain structure was the increase in electrical resistance associated with annealing. XPS valence band measurements of the n+a-Si/ITO structure revealed that the upper edge of the Si valence band shifted to a lower binding energy after annealing at 250°C. These results suggest that the increase in electrical resistance was mainly caused by the increase in contact resistance. The above-mentioned result was also confirmed by secondary ion mass spectrometry (SIMS) analysis and resistance measurements.

The Gas Combustion of H2 with N2O Used for Rapid Thermal Annealing

The gas combustion of H2 with N2O was investigated to develop a rapid thermal annealing method. Spectra of the light emission caused by the combustion show a gray body irradiation characteristic with a gas temperature of 2200 K with an initial total gas pressure of 500 Torr ([H2]/[N2O]=1). A transient thermometry with a 100-nm-thick Cr film as a temperature sensor formed on a quartz substrate was used to measure temperature changes at the surface during and after combustion. Heating to 800°C was achieved within 2 ms using a substrate preheated to 300°C. The increase of electrical conductivity was achieved from 4×10-7 S/cm to 7×10-4 S/cm by the combustion for 0.5% phosphorus-doped amorphous silicon films.

Interfacial interaction between Al-1%Si and phosphorus-doped hydrogenated amorphous Si alloy at low temperature

The interfacial interaction between phosphorus-doped amorphous silicon hydrogen alloy ((n+)a-Si:H) and thermal evaporated Al-1%Si layer after furnace annealing in the temperature range from 150 to 250 °C has been investigated in detail. The scanning electron microscope photographs show that many dendrites were formed on the original (n+)a-Si:H surface at annealing temperature higher than 170 °C. Raman spectroscopy and transmission electron diffraction show that the original (n+)a-Si:H film has been converted to polycrystalline Si with the crystalline Si dendrites on top. The drastic increase (∼4 orders of magnitude) of electrical conductivity of the 200 °C annealed (n+)a-Si:H films with the Al-1%Si removed is caused by the formation of polycrystalline silicon percolation channel in the background area between dendrites. Auger spectroscopy also provides evidence that no aluminum is incorporated into the converted film during silicon recrystallization and thus no SiAl alloy is formed.

Large grained polycrystalline silicon films by solid phase crystallization of phosphorus-doped amorphous silicon

For the first time, polycrystalline Si films with a grain diameter exceeding 10 μm are prepared by solid phase crystallization of phosphorus-doped, amorphous Si films on glass. Low pressure chemical vapour deposition from Si2H6PH3 serves to form amorphous Si films on glass substrates which are then subsequently solid phase crystallized at a temperature of 600°C. Transmission electron microscopy shows a log-normal grain size distribution in the crystallized films. The average grain size increases with the phosphorus concentration in the films and reaches 3.1 μm at an electron concentration of 1.5 × 1020cm−3. The area weighted average grain size is 5.9 μm. Hall effect measurements yield an electron mobility of about 50 cm2/(V s) for electron concentrations above 1019 cm–3.

N-type polycrystalline silicon films obtained by crystallization of in situ phosphorus-doped amorphous silicon films deposited at low pressure

The low-pressure chemical-vapor deposition of phosphorus-doped silicon film on glass at 550 °C was investigated as a function of silane pressure (1–100 Pa) and phosphine/silane mole ratio ranging between 4×10−6 and 4×10−4. At this low temperature the film is homogeneous in thickness and the silicon is amorphous except for low pressure (1 Pa). Phosphorus concentration varies linearly with mole ratio in amorphous deposited films. The resistivity of films annealed at 600 °C decreases while the incorporation of phosphorus (mole ratio) increases, and varies with phosphorus concentration from 101 to 10−3 Ω cm. For the same phosphorus content, the resistivity is lower if the silicon film is amorphous deposited and subsequently crystallized, than if the film is polycrystalline deposited. Carrier concentration and mobility are measured using the Hall method. Doping efficiency and electrical properties are discussed.