Phosphosilicate Glass Layer Research Articles

Structuring Interdigitated Back Contact Solar Cells Using the Enhanced Oxidation Characteristics Under Laser‐Doped Back Surface Field Regions

Interdigitated back contact (IBC) architecture can yield among the highest silicon wafer‐based solar cell conversion efficiencies. Since both polarities are realized on the rear side, there is a definite need for a patterning step. Some of the common patterning techniques involve photolithography, inkjet patterning, and laser ablation. This work introduces a novel patterning technique for structuring the rear side of IBC solar cells using the enhanced oxidation characteristics under the locally laser‐doped n++ back surface field (BSF) regions with high‐phosphorous surface concentrations. Phosphosilicate glass layers deposited via POCl3 diffusion serve as a precursor layer for the formation of local heavily laser‐doped n++ BSF regions. The laser‐doped n++ BSF regions exhibit a 2.6‐fold increase in oxide thickness compared to the nonlaser‐doped n+ BSF regions after undergoing high‐temperature wet thermal oxidation. The utilization of oxide thickness selectivity under laser‐doped and nonlaser‐doped regions serves two purposes in the context of the IBC solar cell, first patterning rear side and second acting as a masking layer for the subsequent boron diffusion. Proof‐of‐concept solar cells are fabricated using this novel patterning technique with a mean conversion efficiency of 20.41%.

A simplified and masking‐free doping process for interdigitated back contact solar cells using an atmospheric pressure chemical vapor deposition borosilicate glass / phosphosilicate glass layer stack for laser doping followed by a high temperature step

AbstractIn this paper a simplified approach for the generation of laterally p‐ and n‐doped structures applicable for cost‐effective production of interdigitated back contact (IBC) solar cells is presented. We use a stack of doping glasses deposited by atmospheric pressure chemical vapor deposition (APCVD), consisting of borosilicate glass (BSG) and phosphosilicate glass (PSG) on Czochralski‐grown (Cz) silicon substrates. A laser process creates the p‐doped regions by local liquid phase diffusion of boron from the BSG layer into the underlying molten Cz‐Si substrate. Simultaneously, the BSG‐PSG stack is removed by laser ablation. In a subsequent high‐temperature step, phosphorus diffuses from the remaining PSG‐BSG layer into the crystalline silicon substrate under inert gas atmosphere, creating complementary to laser doped areas n+‐doped regions. By the use of APCVD, phosphorus and boron contents of the doping glasses can be adjusted freely to vary the resulting p‐ and n‐doped profiles. A higher boron content in the BSG layer enhances the diffusion of phosphorus through the BSG, especially at lower diffusion temperatures. The resulting doping profiles are characterized using electrochemical capacitance‐voltage measurements and the resulting sheet resistances using the four‐point probe method. The amount of minority dopant contamination in n‐ and p‐doped regions is investigated by secondary ion mass spectrometry. Furthermore, transfer length method (TLM)‐measurements indicate contactability of the generated doped regions.

Post annealing to improve the sheet resistance and uniformity of emitter for enhancing conversion efficiency of mass-produced mc-Si solar cells

The influence of annealing on the sheet resistance (Rs) and doping profile of P–N junction emitter and its correlation to the conversion efficiency of mass-produced mc-Si solar cells were investigated. It is found that the Rs of emitter increased reversely after annealing at a low temperature range of 650–750 °C, which differs from the conventional phenomenon that the Rs usually decreases with doping amount increasing during phosphorus (P) thermal diffusion process at a relatively high temperature. The doping profiles show that the shallow surface doping concentration was reduced significantly from 6.6 × 1021 cm−3 to 3.9 × 1021 cm−3 after annealing at 700 °C while the doping profile remained almost constant below the depth of 20 nm. The decreasing of surface doping concentration indicates that the P atoms distributed in the shallow surface of silicon should uphill-diffuse to the region near to phosphosilicate glass (PSG) layer, which reduces the total doping amount in emitter and consequently increases the Rs. For mass production, the average Rs was improved to ∼75 Ω/sq when the diffused silicon wafers with 66 Ω/sq emitter were annealed at 700 °C for 20 min, and moreover the Rs uniformity became better than that of the directly as-diffused silicon wafers with same level Rs value. Additionally, the solar cells exhibited a mean conversion efficiency of 18.63%, which was higher by 0.35% and 0.20% than those fabricated from the as-diffused silicon wafers with ∼66 Ω/sq and ∼75 Ω/sq emitter, respectively.

Development of Bifacial n‐Type Front‐and‐Back Contact Cells with Phosphorus Back Surface Field via Mask‐Free Approaches

Industrial bifacial n‐type front‐and‐back contact (nFAB) solar cells consist of a boron‐doped p+ emitter and a phosphorus‐doped n+ back surface field (BSF). A conventional BSF formation method with tube‐based POCl3 diffusion typically requires the use of a masking layer to protect the front emitter against cross doping or two wet‐chemical etching steps. Herein, two alternative mask‐free BSF formation approaches are investigated, either via phosphorus ion implantation or atmospheric pressure chemical vapor deposition (APCVD) of phosphosilicate glass (PSG). The fabricated cells indicate comparable efficiencies achievable by mask‐free methods, as compared with conventional tube‐based POCl3 diffusion. In addition, the APCVD process is further improved by optimizing the phosphorus contents in the PSG layer. Cell performances with different phosphorus contents and thus different BSF sheet resistances are studied. A lower phosphorus content (higher BSF's sheet resistance) improves cells' Jsc, whereas a higher phosphorous content results in high fill factor (FF) due to better rear contact resistance and low BSF sheet resistance. The optimized condition results in nFAB cell with a peak efficiency of 21.4% and FF values over 81.0%.

Four-Probe Bridges for In-Line Determination of Thickness and Sidewall Etch Angle of Surface Micromachined Thin Films

Geometrical parameters, such as the thickness and the sidewall etch angle of microelectromechanical systems (MEMS) thin films, are important information for device design and simulation, material property extraction, and quality control in a fabrication process line. This paper presents an in-line test microstructure for measuring geometrical parameters of surface micromachined thin films. The structure consists of four-probe bridges with continuous step structures and deposited at three different angles. The extraction method takes advantage of the resistances of the step structures to determine the thickness and the sidewall etch angle of the phosphosilicate glass (PSG) layer and the thickness of the polysilicon layer. The sheet resistance and the width of the test structure are required for the extraction method and can also be measured by using the test structure. Thicknesses of (2.080 ± 0.011) µm, (2.142 ± 0.012) µm, (1.614 ± 0.014) µm and (2.892 ± 0.012) µm are obtained for the Poly 1 layer, the Oxide 1 layer, the Poly 2 layer and the stacked layer of Oxide 1-Oxide 2, respectively. The sidewall etch angles for the Oxide 1 layer and the stacked layer of Oxide 1-Oxide 2 are obtained as (77.51 ± 0.61)° and (76.17 ± 0.91)°, respectively. In comparison to previously reported thickness measurement approaches, the proposed method is nondestructive, and makes use of four-point probe technique which is featured with electrical input and output configuration, simple operation, low cost, fast response, good repeatability and ease of integration. Therefore, this method is more suited to in-line monitoring the MEMS fabrication process.

Analysis of the electronic and chemical structure in boron and phosphorus passivated 4H-SiC/SiO2 interfaces using HRTEM and STEM-EELS

We report a transmission electron microscopy (TEM) study of the impacts of phosphorus and boron passivation processes at 4H-SiC/SiO2 interfaces. The chemical and electronic structures at these interfaces have been analyzed using high-resolution TEM and spatially resolved electron energy-loss spectroscopy (EELS), uncovering a range of phenomena caused by the presence of B and P within their respective boro- and phosphosilicate glass (BSG/PSG) layers. The phosphorus passivation process was observed to induce roughness at the SiC/PSG interface on the order of 100s of nm. Within the PSG layer, phosphorus was found to segregate into nanometer-scale P-rich clusters, contradicting previous reports that it is distributed uniformly throughout the PSG. Similar to N in nitric oxide annealed devices, boron was determined to accumulate in a thin layer (sub-3 nm in thickness) at the SiC/BSG interface, with a much narrower distribution than previously reported. EELS measurements indicated boron incorporates in a trigonal bonding configuration, supporting the assertion that it softens the oxide and causes significant stress reduction at the interface with 4H-SiC. These results supply further insight into the sources of mobility enhancement in PSG and BSG-gated devices that could be extended into additional improvement in the channel response of SiC MOSFETs.

Structure and composition of phosphosilicate glass systems formed by POCl3 diffusion

The phosphosilicate glass (PSG) layer system grown on the silicon surface during diffusion processes with phosphorus oxychloride (POCl3) is a two-layer stack system consisting of a PSG and a silicon dioxide (SiO2) layer. Understanding the stack layers’ structure and composition is essential for further optimizing POCl3 diffusion processes. For diffusion processes with in-situ oxidation (i.e. a high oxygen gas flow during the drive-in step), we find that the intermediate SiO2 layer thickness increases significantly from 8 nm (no in-situ oxidation) to 43 nm, while the PSG layer thickness remains constant at about 8 nm. This thick SiO2 layer seems to hinder the diffusion of phosphorus atoms from the PSG through the SiO2 layer into the silicon. Implementation of a second deposition step with active N2-POCl3 flow at the end of another diffusion process type increases the thickness of the PSG layer from 14 nm to 25 nm, while at the same time the intermediate SiO2 layer thickness decreases by about 40% to 5 nm. The total phosphorus dose within the PSG/SiO2 stack layer thereby increases by a factor of three and is determined to be about 1.7·1015 cm-2. These findings make the approach of a second deposition step very interesting for, e.g., laser doping applications to form selective emitters. For diffusion processes with different N2-POCl3 to oxygen (N2-POCl3:O2) ratios during deposition, we find only small influences on the PSG/SiO2 stack layers present after drive-in. However, the resulting emitter sheet resistances are strongly impacted by the N2-POCl3:O2 ratio; the measured sheet resistances continuously decrease from 343 Ω/sq for a N2-POCl3:O2 ratio of 0.8 to 72 Ω/sq for a ratio of 4.0.

Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivity ρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.

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.

Controllable and air-stable graphene n-type doping on phosphosilicate glass for intrinsic graphene

We proposed and investigated a controllable air-stable graphene n-doping method on phosphosilicate glass (PSG) to achieve intrinsic graphene. Through Raman, XPS, and AFM analyses, it was confirmed that the initially p-type doped graphene was recovered to intrinsic graphene through n-type doping phenomenon. The n-doping control was accomplished by adjusting the concentration of the out-diffused P2O5 molecules from the PSG layer. In particular, a larger amount of P2O5 molecules and a smoother PSG surface were achieved after the higher temperature annealing, consequently yielding a larger doping impact on the graphene layer. Finally, a very small Dirac point shift (1–3V) was observed after 96h of air exposure, compared to the degree of shift by the n-doping effect (17–36V), demonstrating that this n-doping method is fairly stable in air.

Electrical and Structural Properties of P-Type Monocrystalline Silicon Solar Cell with Phosphorus Screen-Printed N+ Emitter

We have fabricated p-type monocrystalline silicon (Si) solar cell with phosphorus (P) screen-printed n+ emitter and investigated its electrical and structural properties. During P screen-printed n+ emitter process, a 16 nm-thick phosphosilicate glass (PSG) layer was formed as a result of interaction between P-dopant paste and Si substrate. Due to the PSG reflow associated with the reduction of viscosity of oxide caused by the amount of P atoms in PSG layer, thinner and thicker PSG film was formed in convex and concave regions of the textured Si surface, respectively, which was quite different from the growth behavior of thermally grown SiO2 layer. Due to a strong dependence of P diffusion on the Si interstitials, deeper and shallower junctions were abnormally formed near the convex and concave regions in the textured Si surface, respectively. The electric field and temperature dependence of the current-voltage characteristics demonstrated that the Poole-Frenkel barrier lowering mechanism along with the generation-recombination mechanism had dominance over the current conduction in the reverse bias region of p-type monocrystalline Si solar cell fabricated using screen printing process.

Relationships between Diffusion Parameters and Phosphorus Precipitation during the POCl3 Diffusion Process

The POCl3 diffusion process is still a common way to create the pn-junction of Si solar cells. Concerning the screen- printing process, it is necessary to find a compromise between low emitter recombination, low contact resistance and high lateral conductivity. The formation of a homogeneous emitter during the POCl3 diffusion process depends on several diffusion parameters, including duration, temperature and gas flow. This primarily controls the growth of the highly doped phosphosilicate glass (PSG) layer, which acts as a dopant source during the diffusion process. Detailed investigations of the PSG layer have shown a distinct correlation between the process gas flows and the composition of the PSG layer. Specifically, in this research we examine the influence of phosphorus precipitation at the PSG/Si interface. Furthermore, we show the influence of phosphorus precipitation during the pre-deposition phase on the passivation quality of the corresponding emitter. In a second step, we use the results to create emitters with a reduced density of phosphorus precipitates. In a last step, the optimized emitter structure was transferred to screen-printed solar cell processes, whereby efficiencies up to 19.4%abs. were achieved on monocrystalline p-type Cz material with full area Al-BSF rear side.

A model for phosphosilicate glass deposition via POCl3 for control of phosphorus dose in Si

Effective control of the dose of diffused phosphorus emitter profiles is crucial for optimization of crystalline silicon solar cells, but it requires detailed understanding of the POCl3 doping process. We measure concentration profiles within the deposited phosphosilicate glass (PSG) layer for a range of POCl3 doping conditions and find that (i) its composition is nearly independent of process conditions and (ii) it is separated from Si by a thin SiO2 layer. We also find strong accumulation of P at the SiO2-Si interface. As common linear-parabolic models cannot fully explain the observed kinetics of PSG thickness and phosphorus dose in Si, we present an improved model including oxygen depletion and dose saturation, giving a better explanation of the experimental data. In contrast to previous models that adjust the peak phosphorus concentration at the Si surface to match the measured profiles, our models accurately predict the time-dependent dose behavior under different experimental conditions. We further couple this growth model with previously reported phosphorus diffusion and deactivation models in silicon, providing full modeling of the POCl3 doping process.

Ohmic Contact Effect of Ag-Nanodots on Quantum Efficiency of Si Solar Cell

The authors investigated Si solar cell with the inclusion of nano-Ag dots using the ink-jet printer. These nano-Ag dots were used for the Ohmic contact layer on phospho-silicate glass layer, which was not removed after the formation of Si emitter layer by phosphorus diffusion process. The SiNx layer deposited on the nano-Ag dots shows the catalyst selective growth and so the layer formed beneath of nano-Ag dots. The photoreflectances show that the long wavelength from 360 nm to 1200 nm tends to be increased as the density of the nano-Ag is increased. In case of short wavelength from 294 nm to 367 nm, it shows the opposite trend, indicating the plasmon effect of the nano-Ag. As embedding the nano-Ag dots on the phospho-silicate glass layer, the blocked Ohmic contact was opened and the quantum efficiency of 14.4% was achieved, which is higher than the reference sample of 12.72% without the glass layer. The nano-Ag dots form the good Ohmic contact and also enhance the light conversion efficiency with the formation of surface plasmon.

Inclusion of CdSe Quantum Dots on the P-Doped Emitter of Si Solar Cells

We investigated Cadium Selenide quantum dots embedded in the Si solar cell in order to improve the efficiency of conventional Si solar cell. CdSe quantum dots with 3 to approximately 4 nm size were printed on the phospho-silicate glass layer grown over the emitter surface of p-n junction Si solar cells during the phosphorous diffusion process. Ohmic contact was formed by the contribution of nanoparticles at the Si emitter in spite of the existance of phospho-silicate glass layer. The enhanced light absorption due to the quantum dots was ranged from 500 to 600 nm where the CdSe nanodots have the corresponding emission wavelength of 560 nm. The efficiency of reference solar cell with the glass layer was measured to be 1.0% and it was increased to 12.72% for the reference sample without the glass layer. Furthermore, the efficiency of CdSe quantum dot sample was measured to be 13.6%. This indicates that the quantum dots play the roles of both the formation of tunneling channel and the enhancement of the light conversion efficiency in the visible spectral range.

Experimental and analytical study of saturation current density of laser-doped phosphorus emitters for silicon solar cells

Heavily doped emitters with low saturation current density are of particular interest for selective emitter solar cells. These emitters can be obtained by laser doping through the phosphosilicate glass layer formed after thermal diffusion from POCl 3 gas. The experimental results show that in contrast to purely POCl 3 furnace-diffused emitters, the saturation current density of laser-doped emitters does not increase linearly as sheet resistance decreases, but rather features two distinct regimes. In one of these regimes, the saturation current density is found to decrease as the sheet resistance decreases, reaching values lower than those of furnace emitters. This peculiar behaviour was explained by both qualitative analysis and numerical simulations.

Finite Element Simulation Of Laser-Induced Diffusion In Silicon

Abstract Laser-assisted diffusion of dopants is a promising way to realize selective emitter solar cells with a reduced number of technological steps. This paper discusses the simulation by finite element method of laser doping in order to optimise the fabrication process. A finite element method is used to solve the heat-transfer equation which describes the thermal effects and Fick's second law which describes the diffusion of dopants. The phosphosilicate glass (PSG) layer that is produced during the emitter formation on p-type silicon solar cells is used as the doping source during the laser-assisted diffusion process. The influence of laser parameters and material properties are studied. Modelling results are compared to SIMS measurements of the phosphorous doping profile. A structure is discussed in the perspective of a self-aligned process for selective emitter fabrication, where the PSG layer is present underneath the silicon nitride (SiNx) passivation layer.

Influence of rapid thermal vacuum annealing and high temperature treatment on the properties of PSG films

The effect is presented of rapid thermal annealing (RTA) in vacuum and thermal annealing in water vapor at 850 °C on the properties of phosphosilicate glass (PSG) films deposited in PECVD and μPCVD reactors. The films were characterized by etch rates and XPS and AES analyses. The RTA was carried out at 800 - 1400 °C at annealing times varying from 15 to 180 sec. The RTA caused a significant decrease in the etch rate, which is indicative of structural changes. The XPS and AES analyses showed that the PECVD PSG films contain excess Si due to the lower oxidation activity of N2O. The excess Si can be oxidized in water vapor at high temperatures. The excess Si leads to a decrease in the etching rate of the PECVD PSG layers as compared to that of the μPCVD films.

Phosphorous doped Ru film for advanced Cu diffusion barriers

Copper diffusion barrier properties of phosphorous doped Ru film are studied. Phosphorous out-diffusion to Ru from underneath phosphosilicate glass (PSG) layer results in P doped Ru film. The doped Ru film improves copper barrier properties and has excellent thermal stability. XRD graph indicates that there is no copper silicide and ruthenium silicide formations after annealing at 550 °C for 30 min in vacuum. This result is consistant with AES depth profiles which show no Cu, Ru, O and Si inter-diffusion. The phosphorous doped Ru barrier also blocks oxygen's diffusion to copper from the PSG layer. The phosphorous doped Ru film could be an alternative Cu diffusion barrier for advanced Cu interconnects.

The etching behavior of APCVD PSG thin films used as sacrificial layers for surface micromachined resonant microstructures

Abstract The deposition and the configuration of the sacrificial (phosphosilicate glass (PSG)) and active layers (polysilicon) are one of the most sensitive processing steps in surface micromachining technique for resonant microstructures. Starting from the technological steps for the resonant microstructures manufacturing, a new solution in order to optimise these technological steps are proposed. During the doping processes of polysilicon (∼1000 °C) a significant densification of the PSG layer appears. This leads to a dramatical decrease of the PSG etch rate in buffered HF solution and a strong limitation in surface micromachining technique (with unwanted effects on resist mask and metallic layer integrity) is resulting. The decrease of the PSG etch rate is mainly due to the ex-diffusion of the phosphorus from PSG film. An original solution is proposed in order to minimize the decrease of the PSG etch rate: the replacement of the PSG film with a Si 3 N 4 /PSG/SiO 2 /Si 3 N 4 multilayer. The using of a thin Si 3 N 4 layer has the advantage of suppressing the ex-diffusion of P atoms and avoids the attack of HF on the polysilicon layer.