Doped Silicon Films Research Articles

Development of high conducting phosphorous doped nanocrystalline thin silicon films for silicon heterojunction solar cells application

We have investigated the plasma-enhanced chemical vapor deposition growth of the phosphorus-doped hydrogenated nanocrystalline silicon (n-nc-Si:H) film as an electron-selective layer in silicon heterojunction (SHJ) solar cells. The effect of power densities on the precursor gas dissociation are investigated using optical emission spectra and the crystalline fraction in n-nc-Si:H films are correlated with the dark conductivity. With the P d of 122 mW cm−2 and ∼2% phosphorus doping, we observed Raman crystallinity of 53%, high dark conductivity of 43 S cm−1, and activation energy of ∼23 meV from the ∼30 nm n-nc-Si:H film. The n-nc-Si:H layer improves the textured c-Si surface passivation by two-fold to ∼2 ms compared to the phosphorus-doped hydrogenated amorphous silicon (n-a-Si:H) layers. An enhancement in the open-circuit voltage and external quantum efficiency (from >650 nm) due to the better passivation at the rear side of the cell after integrating the n-nc-Si:H layer compared to its n-a-Si:H counterpart. An improvement in the charge carrier transport is also observed with an increase in fill factor from ∼71% to ∼75%, mainly due to a reduction in electron-selective contact resistivity from ∼271 to ∼61 mΩ-cm2. Finally, with the relatively better c-Si surface passivation and carrier selectivity, a power conversion efficiency of ∼19.90% and pseudo-efficiency of ∼21.90% have been realized from the SHJ cells.

Oxygen vacancy modulation of nanolayer TiOx to improve hole-selective passivating contacts for crystalline silicon solar cells

Carrier-selective passivating contacts in crystalline silicon (c-Si) solar cells have expanded from doped silicon films to non-silicon wide-bandgap materials to reduce parasitic absorption and production costs. Titanium oxide (TiOx) has...

Interfacial fixed charges and dipoles to boost carrier selectivity for crystalline silicon solar cells

<p>With the rapid evolution of passivating contact, the carrier transport layer in crystalline silicon (<i>c</i>-Si) solar cells has expanded beyond the doped silicon films to non-silicon films with lower parasitic absorption and manufacturing cost. However, using the emerging carrier transport layers in direct contact with <i>c</i>-Si suffers from the insufficient carrier selectivity, which limits the efficiency of the device. Thus, to make such new strategies practical, it becomes particularly important to enhance the passivation quality and thus minimize the recombination losses at <i>c</i>-Si surfaces. By introducing interfacial fixed charges (<i>Q</i><sub>f</sub>) or dipoles on the <i>c</i>-Si surface, the local electric field can be tuned to affect the recombination and charge carrier transport dynamics. This perspective highlights the significance of interfacial <i>Q</i><sub>f</sub> and dipoles for carrier selectivity, and points out the challenges in a comprehensive understanding of the mechanism and precisely controlling the carrier behavior as well as the solar cell stability.</p>

Enhancing the Selectivity and Transparency of the Electron Contact in Silicon Heterojunction Solar Cells by Phosphorus Catalytic Doping

AbstractAn intrinsic hydrogenated amorphous silicon (a‐Si:H(i)) film and a doped silicon film are usually combined in the heterojunction contacts of silicon heterojunction (SHJ) solar cells. In this work, a post‐doping process called catalytic doping (Cat‐doping) on a‐Si:H(i) is performed on the electron selective side of SHJ solar cells, which enables a device architecture that eliminates the additional deposition of the doped silicon layer. Thus, a single phosphorus Cat‐doping layer combines the functions of two other layers by enabling excellent interface passivation and high carrier selectivity. The overall thinner layer on the window side results in higher spectral response at short wavelengths, leading to an improved short‐circuit current density of 40.31 mA cm−2 and an efficiency of 23.65% (certified). The cell efficiency is currently limited by sputter damage from the subsequent transparent conductive oxide fabrication and low carrier activation in the a‐Si:H(i) with Cat‐doping. Numerical device simulations show that the a‐Si:H(i) with Cat‐doping can provide sufficient field effect passivation even at lower active carrier concentrations compared to the as‐deposited doped layer, due to the lower defect density.

20.4: Activation of Doped Silicon Film Using Semiconductor Blue Light Diode Laser Annealing

We have investigated activation annealing of the doped poly silicon film of LTPS TFT by using CW semiconductor blue light diode laser. It has been found that by full melting and re‐crystallization, the sheet resistance can be reduced significantly to 1/3 of which can be realized by using conventional RTA treatment. We will report the experiment results in the paper.

Fabrication of ultra-shallow junction by in situ doped amorphous silicon films and its application in silicon drift detectors

In situ doped amorphous silicon film, deposited by plasma enhanced chemical vapor deposition, is employed as a diffusion source to form an ultra-shallow junction and as voltage dividers in silicon drift detectors. By controlling annealing temperature and time, the junction depth can be adjusted precisely, and the shallowest junction depth of 15 nm is achieved. The I–V characteristics of the ultra-shallow junction, fabricated with different annealing temperature, annealing time and doping concentration, are then analyzed, and the reverse leakage current can be reduced to as low as 2 nA cm−2 under a reverse bias of 200 V at room temperature. The doped silicon film is also used to fabricate voltage dividers in silicon drift detectors. The sheet resistance of the silicon film is adjusted by controlling the thickness and annealing conditions, and a high resistance value and small temperature coefficient of the voltage dividers are achieved. Finally, the in situ doped amorphous silicon film is used to fabricate silicon drift detectors and the final energy resolution of 180 eV at 5.9 keV is achieved.

Boron Detection Technique in Silicon Thin Film Using Dynamic Time of Flight Secondary Ion Mass Spectrometry

The impurity concentration is a crucial parameter for semiconductor thin films. Evaluating the impurity distribution in silicon thin film is another challenge. In this study, we have investigated the doping concentration of boron in silicon thin film using time of flight secondary ion mass spectrometry in dynamic mode of operation. Boron doped silicon film was grown on i) p-type silicon wafer and ii) borosilicate glass using hot wire chemical vapor deposition technique for possible applications in optoelectronic devices. Using well-tuned SIMS measurement recipe, we have detected the boron counts 101~104 along with the silicon matrix element. The secondary ion beam sputtering area, sputtering duration and mass analyser analysing duration were used as key variables for the tuning of the recipe. The quantitative analysis of counts to concentration conversion was done following standard relative sensitivity factor. The concentration of boron in silicon was determined 1017~1021 atoms/㎤. The technique will be useful for evaluating distributions of various dopants (arsenic, phosphorous, bismuth etc.) in silicon thin film efficiently.

Numerical analysis of dopant‐free asymmetric silicon heterostructure solar cell with SiO 2 as passivation layer

Conventional silicon heterojunction solar cells employ defects-prone a-Si:H layers for junction formation and passivation purposes. Substituting these layers with hole-selective MoOx and electron-selective TiOx can reduce parasitic absorption and energy band offsets issues associated with doped silicon films. In this work, dopant-free asymmetric heterostructure Si solar cells are studied with and without SiO2 passivation layer, and their performance has been compared. The inclusion of ultrathin SiO2 insulator as a passivation layer promotes significant band bending that induces interface inversion of crystalline silicon as well as maintains the electric field required to tunnel charge carriers. The energy band diagram studies and variation of oxide thickness show that the IV characteristics of the solar cell critically depend on the insulator thickness; as the carriers tunnelling through the insulator becomes negligible at larger thicknesses. The simulated structure with MoOx as front hole-selective contact and without any passivation exhibited conversion efficiency of 15.73%, which improved to 18.69% by incorporating passivated a-Si:H. However, by employing rear SiO2/TiOx stack with the front SiO2/MoOx, the device performance enhanced to open-circuit voltage of 785 mV, short-circuit current density of 41 mA/cm2, fill factor of 77%, and simulated conversion efficiency of 24.83%, which is ∼10% enhancement in the performance as compared to reference device employing traditional a-Si:H with dopant-free films. Novelty Statement For the first time, a dopant-free asymmetric silicon heterostructure solar cell (DASH) employing silicon oxide (SiO2) as a passivation layer has been physically modelled using Silvaco TCAD. The dopant-free MoOx and TiOx as hole- and electron-selective contacts have been incorporated. An ultrathin SiO2 promotes band bending that induces interface inversion of absorber as well as facilitating carrier tunnelling. An efficiency of 24.83% has been numerically attained which is the best performance among DASH structures designed with oxide passivation. The 2-D cross-section view of the proposed device employing dopant-free layers and oxide passivated film is illustrated in Figure 1 for which we have applied memory intensive numerical method based on the computation of Poisson, charge transport and continuity equations in the Silvaco ATLAS module. In order to undertake in-depth analysis of the proposed structure, a simple device based on MoOx/cSi wafer was first designed and simulated in order to understand the charge transport behaviour. The electrical and optical parameters for various layers have been shown in the Table and the current density due to tunnelling of charge carriers across Silicon oxide (SiO2) passivated layer has been calculated via Fowler-Nordheim approximation. The reference structure with traditional intrinsic a-Si:H has also been simulated.

Hydrogenation Mechanisms of Poly‐Si/SiOx Passivating Contacts by Different Capping Layers

Hydrogenation Mechanisms Hydrogen can deactivate defects and impurities inside doped polycrystalline silicon fi lms and interfacial oxide layers to improve the passivation qualities of passivating contacts on silicon wafers. In article number 1900476, Thien N. Truong, Hieu T. Nguyen, and co-workers elucidate the hydrogenation mechanisms of different post-treatment techniques inside the fi lms and at the oxide interfaces.

Experimental and simulation investigation of the out-of-equilibrium phenomena on the pseudo-MOSFET configuration under transient linear voltage ramps

The pseudo-MOSFET configuration is an electrical characterization technique developed for silicon-on-insulator (SOI) wafers. The wide variety of experiments that have been performed to date have also extended recently in the study of out-of-equilibrium phenomena for bio-sensing applications. However, the lack of a full understanding of the ohmic contact behaviour between the probes and the low doped silicon film results in simulation inconsistencies. This work proposes a simulated device structure that is capable of reproducing the behaviour of the device and further extends the experiments into large-signal linear ramps which are also reproduced through simulations.

Deposition of Silicon Films Doped with Boron and Phosphorus by the Gas-Jet Plasma-Chemical Method

Doped silicon films are fabricated using diborane and phosphine as doping gases by gas-jet plasma-chemical deposition with the application of an electron beam. The influence of the dopant-gas concentration, the addition of a fluorine-containing gas, and the background pressure on the conductivity and crystalline structure of silicon layers is investigated. Boron-doped amorphous films (a-Si:H) with a conductivity up to 5.2 × 10–3 (Ω cm)–1 are fabricated; when doping with phosphorus, microcrystalline silicon films (mc-Si:H) with a crystallinity up to 70% and conductivity at a level of 1 (Ω cm)–1 are fabricated.

Annealing effect on grain boundary width of polycrystalline silicon for photovoltaic application

Nowadays, LPCVD (Low Pressure Chemical Vapor Deposition) and highly doped polycrystalline silicon films have numerous applications in microelectronic component manufacturing technologies, integrated circuits and solar cells. The complexity of the circuits, and the increasing degree of integration of the components, constantly require improvement and mastery of the properties of this type of material. As part of this work, we are interested in the study of the effect of doping and annealing Effect on grain boundary width of polycrystalline silicon thin films deposited by LPCVD. The results obtained showed, on the one hand, that arsenic doped layers are more resistive than boron doped ones, and on the other hand, and that the diffusion of dopants is generally much greater in grain boundaries than in grains; and considering the importance of the average grain size in the polycrystalline silicon material, which is an important parameter, on which the physical and electrical properties of this material depend.

23% efficient p-type crystalline silicon solar cells with hole-selective passivating contacts based on physical vapor deposition of doped silicon films

Of all the materials available to create carrier-selective passivating contacts for silicon solar cells, those based on thin films of doped silicon have permitted to achieve the highest levels of performance. The commonly used chemical vapour deposition methods use pyrophoric or toxic gases like silane, phosphine and diborane. In this letter, we propose a safer and simpler approach based on physical vapour deposition (PVD) of both the silicon and the dopant. An in-situ doped polycrystalline silicon film is formed, upon annealing, onto an ultrathin SiOx interlayer, thus providing selective conduction and surface passivation simultaneously. These properties are demonstrated here for the case of hole-selective passivating contacts, which present recombination current densities lower than 20 fA/cm2 and contact resistivities below 50 mΩ cm2. To further demonstrate the PVD approach, these contacts have been implemented in complete p-type silicon solar cells, together with a front phosphorus diffusion, achieving an open-circuit voltage of 701 mV and a conversion efficiency of 23.0%. These results show that PVD by sputtering is an attractive and reliable technology for fabricating high performance silicon solar cells.

Formation of Epitaxial Heavy-doped Silicon Films by PECVD Method

Plasma-enhanced CVD (PECVD) epitaxy at 200 °C was used to deposit heavy doped n-type silicon films. Post-annealing by rapid thermal processing was applied to improve the properties of the epitaxial layer. By analyzing the Raman spectra and the imaginary part of the dielectric constant spectra of the samples, it was found that high-quality heavy-doped epitaxial n-type silicon layer can be obtained by optimizing the parameters of the PECVD depositing process. Reducing the electrodes distance of the PECVD had a great effect on the crystallzation of the epitaxialed n-type silicon films. Sillicon films with high-crystallization were obtained with the electrodes distance of 18 mm. Post-annealing process can improve the crystallization and reduce the resistance of the epitaxial films. In our research, it was found that the sheet resistance (R□) of the post-annealed films with thickness of about 50 nm has a simple relationship with RPH3/SiH4 (ratio of the flow rate of PH3 and SiH4) of the PECVD processing: R□=-184-125 lg(RPH3/SiH4). In the end, high-quality epitaxial n-type silicon film was obtained with R□ of 15 Ω/□ and thickness of ~50 nm.

The influence of the two-dimensional sinusoidal gratings on the near-field radiative heat flux between two doped silicon films

The near-field radiative heat flux between two two-dimensional sinusoidal grating surfaces made of doped-silicon is investigated by the finite difference time domain (FDTD) method. The parameter Λ, which is defined as the ratio of the amplitude to the period of the two-dimensional sinusoidal grating surface, is used to characterize the surface feature. When the amplitude is much smaller than the period, the surface effect is not significant to modify the near-field radiative heat flux between two smooth films. However, when the amplitude is comparable with the period, the near-field radiative heat flux between two smooth films can be enhanced remarkably. The coupling of the localized surface modes between the may be responsible for the variation of the near-field radiative heat flux when the parameter Λ is changed. Moreover, the effect of the lateral displacement of the films on the near-field radiative heat flux is also studied. The results indicate that the near-field radiative heat flux will achieve the maximum value when the peaks of the sinusoidal gratings are aligned. This work will be valuable to research on contactless thermal management in nanoscale.

Chip-integrated plasmonic Schottky photodetection based on hybrid silicon waveguides

We numerically and theoretically investigate the plasmonic Schottky photodetection in a novel hybrid silicon-on-insulator waveguide system, which consists of the silicon waveguides and detection area with the metal stripes and doped silicon film on the silicon dioxide substrate. The results illustrate that the fundamental TE mode in the silicon waveguide can be effectively coupled into the metal/silicon waveguide with the excitation of surface plasmon polaritons (SPPs). The coupling is suppressed for the TM mode due to the mismatch between the electric field distributions of the TM and SPP modes. It is found that the coupling efficiency from the TE to SPP mode is dependent on the width and height of the silicon waveguide and can significantly approach ~36.1%. The ultracompact configuration yields a high responsivity of ~21.7 mA/W and low dark current of ~0.45 μA for the photodetection at the communication wavelength. The plasmonic Schottky photodetector could find favorable applications in the chip-integrated optical interconnects and signal processing.

Large area and broadband ultra-black absorber using microstructured aluminum doped silicon films

A large area and broadband ultra-black absorber based on microstructured aluminum (Al) doped silicon (Si) films prepared by a low-cost but very effective approach is presented. The average absorption of the absorber is greater than 99% within the wide range from 350 nm to 2000 nm, and its size reaches to 6 inches. We investigate the fabrication mechanism of the absorber and find that the Al atom doped in silicon improves the formation of the nanocone-like microstructures on the film surface, resulting in a significant decrease in the reflection of incident light. The absorption mechanism is further discussed by experiments and simulated calculations in detail. The results show that the doped Al atoms and Mie resonance formed in the microstructures contribute the broadband super-high absorption.

Refractive index and extinction coefficient of doped polycrystalline silicon films in infrared spectrum

The refractive index and extinction coefficient in infrared spectrum of the polycrystalline silicon films with different doped dosages, base on the inverse calculation, are obtained by means of utilizing the measured reflectance and transmittance of a layer of material and multilayer films, and the equations derived from photonics and electromagnetic theory. The calculation results demonstrate that the refractive index of the doped polycrystalline silicon films decreases with the doped dosages increasing and the extinction coefficient increases with the doped dosages increasing for a given wavelength. This method used for determining the refractive index and extinction coefficient of the polycrystalline silicon films is effective and has the advantage of that the measured samples are fabricated simply.

Silicon nitride/silicon oxide interlayers for solar cell passivating contacts based on PECVD amorphous silicon

This Letter demonstrates improved passivating contacts for silicon solar cells consisting of doped silicon films together with tunnelling dielectric layers. An improvement is demonstrated by replacing the commonly used silicon oxide interfacial layer with a silicon nitride/silicon oxide double interfacial layer. The paper describes the optimization of such contacts, including doping of a PECVD intrinsic a‐Si:H film by means of a thermal POCl3 diffusion process and an exploration of the effect of the refractive index of the SiNx. The n+ silicon passivating contact with SiNx /SiOx double layer achieves a better result than a single SiNx or SiOx layer, giving a recombination current parameter of ∼7 fA/cm2 and a contact resistivity of ∼0.005 Ω cm2, respectively. These self‐passivating electron‐selective contacts open the way to high efficiency silicon solar cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Selective Silicon Nitride Etch with Hot Diluted HF– an Alternative to Orthophosphoric Acid

Orthophosphoric acid is the worldwide standard chemistry to selectively etch silicon nitride towards silicon oxide [1]. Several alternatives have arisen since few years. Among them, a hot very diluted HF process has been developed in the early 2000’s [2]. This paper deals with its applications and adoption in production. Until the 45nm node, CMOS transistors manufacturing only one or two silicon nitride etch operations were use, namely to define the Shallow Trench Isolation [3]. Latest nodes require more and more SiN etch operations either to define hardmasks or to generate localized stress: SPT (Stress Proximity Technic) [4] [5] or SMT (Stress Memory Technic) [6]. H3PO4 benefits from tremendous etch selectivity towards silicon oxide. Nonetheless it still remains in reclaim immersion batch tool with drawbacks: particles and metals intra or inter batch cross contamination. Various alternatives are under development: chemical formulation [7], single wafer approach [8] or plasma solutions [9]. There’s still a long way to overpass cross contamination concerns, and compete with high productive, low cost of ownership H3PO4 wet benches. Last alternative relies on hot very diluted HF. It has never really been adopted by the industry due to relatively low SiN etch rates. It perfectly suits etching application of small SiN removal amount. Two applications are hereby described. First, a SPT technic in a 28nm node is considered. A comparison between two chemicals is made to remove the PECVD SiN spacer (figure 1). Its high etch rate of 9nm/min for the 85% H3PO4 wet bench at 165°C leads to short process (110s), poorly uniform. Moreover the SiN / SiO2 selectivity is only 30:1, and phosphoric acid is incompatible with highly As doped silicon films (figures 1 b) & 2). Si Gate and active areas are both corroded. Indeed, the transistor architecture makes the doped gates quite at risk with thin silicon oxide protection. Whereas H3PO4 has very poor process window, the hot very diluted HF in a spray batch tool enables better selectivity towards silicon oxide (60:1) and reduced corrosion of doped silicon. Moreover, excellent defectivity is obtained on silicon oxide surfaces (figure 3) making this process industrial. Second described application relies on SiN patterning by wet etching. Phosphoric acid isn’t used for such application due to reclaim issues, and swelling resist with high temperature. Applications exist with low temperature H3PO4 formulations [10] but decreases significantly the etch rate. Fresh, hot very diluted HF enables LPCVD SiN patterning without any resist lift off. The process window is large and without any resist optimization (248nm DUV resist 235nm thick). Last optimization would be the within wafer uniformity (15% 3 sigma) (figure 5). conclusion: Hot diluted HF is proposed as an alternative to H3PO4 for thin SiN film etching with or without resist presence. Its benefits are: fresh chemical use, low cross contamination risks, good defectivity, high compatibility with resist.