Silicon Surface Passivation Research Articles

Optimized nc-Si:H thin films with enhanced optoelectronic properties prepared by micro-waves PECVD used as an effective silicon surface passivation layer

In this work we investigated a novel approach to elaborate hydrogenated nanocrystalline silicon (nc-Si:H) thin films using microwave plasma enhanced chemical vapor deposition (MW-PECVD) system, used as an efficient silicon surface passivation layer. We studied the effect of the hydrogen flow rate on the structural, morphological and optical properties of nc-Si:H thin films. Obtained results confirm its crucial role on the properties of the resulted thin films. Optimized hydrogen flow rate allowed an hydrogenated nanocrystallites (nc-Si:H) with around 80% of a crystallization phase which is a high percentage compared to previous works. The XRD analysis was supported by Raman spectroscopy, indicating the pure phase of cubic structure with a preferred orientation along (111) plane. The average grain size was found to vary between 7 and 16 nm. The surface morphology of the deposited thin films was carried out using atomic force microscopy (AFM), showing a clear dependence of surface roughness with the hydrogen flow rate. The refractive index of the obtained thin films was evaluated by means of the Bruggeman effective medium approximation (BEMA). The optical band gap and the porosity of the films were deduced from spectroscopic ellipsometry measurements and results were found strongly correlated with XRD and AFM analysis respectively. Moreover, the minority carrier lifetime measurements (MCL) prove the effectiveness of the subjected treatment for a high quality nc-Si:H thin film and its crucial role as a passivation layer of crystalline silicon substrate, revealing a significant improvement of their optoelectronic properties.

Annealing Impact on Interface Properties of Sprayed Al2O3-Based MIS Structure for Silicon Surface Passivation

Aluminum oxide (Al2O3) films of different thicknesses were deposited on quartz and silicon (100) substrates by an ultrasonic spray method from a solution of aluminum acetylacetonate dissolved in N,N-dimethylformamide with different molar concentrations. The optical, morphological and electrical properties were investigated. Increasing the molar concentration leads to a refractive index decrease, an increase in the optical band gap from 5.26 eV to 5.52 eV and a change in the surface roughness of the films. The electrical parameters at the Al2O3/Si interface such as the flat band voltage (VFB), effective charge density (Qeff) and interface trap density (Dit) were explored as a function of the molar concentration, film thickness and heat treatment. The latter, done by two annealing processes, namely, the post deposition annealing (PDA) and post metallization annealing (PMA) on the structure, lead to remarkable interface properties. It was found that the positive flat band voltage VFB shift is correlated with the generation of negative effective charge during PMA. A decrease of the Dit distribution in the PMA samples with no significant effect in the case of PDA samples was clearly observed for different molar concentrations. Furthermore, as the Al2O3 film thickness decreases, Dit decreases in both PDA and PMA samples while the relatively high density Qeff and its negative charge polarity were obtained for thinner films. A noticeable passivation effect on the Al2O3/Si interface has been confirmed on samples that underwent the annealing process. These findings related specifically to the interface properties are promising for silicon surface passivation, in particular for solar cells applications.

Silicon Surface Passivation by Laser Processing a Sol–Gel TiOx Thin Film

Silicon surface passivation is highly important for solar cells and other optoelectronic devices. In order to achieve a high-performance, low-cost, room-temperature, and nonvacuum surface passivati...

Enhanced field effect passivation of c-Si surface via introduction of trap centers: Case of hafnium and aluminium oxide bilayer films deposited by thermal ALD

Field effect passivation of crystalline silicon surface is controlled by tailoring the built-in charge in a bilayer dielectric system consisting of hafnium oxide (HfO2) and aluminium oxide (Al2O3). The effective surface recombination velocity (Seff, max) ~ 10 cm/s is achieved at intermediate bulk injection levels with thermal ALD deposited HfO2 (top)/Al2O3 (bottom) bilayer system on n-type silicon with individual layer thickness between 3 and 7 nm. The best realized Seff,max value is lower by a factor of ~ 2.5 with respect to the single Al2O3 layer of similar thickness deposited under the same experimental conditions. The improved field effect passivation is quantified by enhanced effective charge density for the bilayer system in comparison with the corresponding value for single Al2O3 layer. The introduction of extra trap centers at the interface of the bilayer system is primarily responsible for the enriched field effect passivation, however, the sequence of the dielectric layers is important.

Explorative studies of novel silicon surface passivation materials: Considerations and lessons learned

Despite the existence of several highly effective and well-characterized passivating materials for crystalline silicon surfaces, the topic of surface passivation and the investigation of new passivating materials remain of considerable interest for silicon photovoltaics research. However, the question of whether and under what circumstances a particular material will provide effective surface passivation remains difficult to answer. In this work, we provide an overview of recent insights relating to this question, drawing from our own work on novel passivation materials including MoOx, Nb2O5, TiOx, ZnO, and POx, and illustrated with experimental results. Factors that strongly influence the passivation performance include the use of pre-grown interfacial oxides, the film thickness, the annealing conditions, and the presence of capping layers. The impact of these factors on the surface passivation can vary widely from material to material. Therefore, all of these factors should be taken into account when investigating potential new surface passivation materials.

Graphene oxide films for field effect surface passivation of silicon for solar cells

In recent years it has been shown that graphene oxide (GO) can be used to passivate silicon surfaces resulting in increased photocurrents in metal-insulator-semiconductor (MIS) tunneling diodes, and in improved efficiencies in Schottky-barrier solar cells with either metal or graphene barriers, however, the source of this passivation is still unclear. The suggested mechanisms responsible for the enhanced device performance include the dangling bond saturation at the surface by the diverse functional groups decorating the GO sheets which reduce the recombination sites, or field effect passivation produced by intrinsic negative surface charge of GO. In this work through a series of measurements of minority carrier lifetime with the microwave photo-conductance decay (µPCD) technique, infrared absorption spectra, and surface potential with Kelvin probe force microscopy (KPFM) we show that there is no evidence of significant chemical passivation coming from the GO films but rather negative field effect passivation. We also discuss the stability of GO's passivation and the flexibility of this material for its application as temporary passivation layer for bulk lifetime measurements, or as a potential cheap alternative to current passivation materials used in solar cell fabrication.

Exceptional silicon surface passivation by an ONO dielectric stack

Immeasurably low surface recombination of crystalline-silicon wafers is demonstrated with an oxide-nitride-oxide (ONO) corona charged dielectric stack. We detail experimental variations to each layer of the dielectric stack to establish a procedure which provides outstanding passivation properties on textured and planar silicon wafers. We demonstrate surface recombination velocities of < 1 cm/s and surface recombination prefactors of < 1 fA/cm2, and we show that passivation remains stable over a 2-year period when stored in ambient conditions. The effective carrier lifetimes of n-type silicon are found to exceed the commonly accepted intrinsic lifetime limit, and in one case, a lifetime of 170 ms is attained. These high lifetimes indicate that ONO passivation is amongst the best dielectric passivation, and as such, might find applications in high-efficiency silicon solar cells.

Robust SiNx/GaN MIS-HEMTs With Crystalline Interfacial Layer Using Hollow Cathode PEALD

In this letter, we report gallium nitride-based metal-insulator-semiconductor high-electron-mobility transistors (GaN MIS-HEMTs) with a 16-nm-thick silicon nitride (SiN x ) gate insulator and surface passivation layer grown using low-temperature (300 °C) hollow cathode plasma-enhanced atomic layer deposition. Tris(dimethylamino) silane and a remote N2 plasma were used as the silicon precursor and the nitrogen co-reactant, respectively. Though the growth temperature was as low as 300 °C, we obtained excellent film properties such as a high refractive index (2.00), a high film density (2.9 g/cm3), and a low wet etch rate (0.8 nm/min) in 100:1 hydrofluoric acid. High-resolution transmission electron microscopy images of the gate stacks showed ~1.5 nm $\beta $ -phase Si3N4 crystalline interfacial layer on the GaN surface. The devices showed a negligible hysteresis of ~50 mV, a steep subthreshold slope of 76 mV/dec, and a high ON/OFF drain current ratio of ~109. In addition, the devices showed a small positive/negative bias temperature threshold voltage ( ${V}_{\text {th}}$ ) instability ( $\Delta {V}_{\text {th}}\le 0.2$ V at 25 °C and 150 °C) as well as a mitigated current collapse under an off-state quiescent ${V}_{\text {DS}}$ of 20 V.

High-quality surface passivation of crystalline silicon with chemical resistance and optical transparency by using catalytic chemical vapor deposition SiNx layers and an ultrathin SiOx film

We have developed high-quality surface passivation films for n-type crystalline silicon (c-Si) with an effective minority carrier lifetime (τeff) of more than 6 ms or a maximum surface recombination velocity (SRVmax) of less than 2 cm/s using two highly optically transparent silicon nitride (SiNx) layers and an ultrathin SiOx film. First, an ultrathin SiOx film is unintentionally formed on the surface of c-Si by annealing c-Si in N2 ambient at 350 °C. Then, on the SiOx film, two SiNx layers are sequentially formed at substrate temperatures (Tsub) of 100 and 250 °C by catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD. With the combination of the ultrathin SiOx film and the two SiNx layers, we can obtain a novel passivation structure with high-quality surface passivation, high optical transparency, and high chemical resistance for subsequent processes.

Efficiency limiting crystal defects in monocrystalline silicon and their characterization in production

Wafers and thicker slices of an entire n-type monocrystalline silicon ingot were studied using production-compatible electrical and optical characterization techniques. We investigated the capability of these techniques to detect efficiency limiting factors in the early phase of solar cell manufacturing. In addition to the standard characterization methods and parameters - carrier lifetime, resistivity, FTIR, photoluminescence (PL) - the OxyMap technique was used to evaluate the thermal history of the wafers, while the distribution of Bulk Micro Defects (BMD) was measured using Light Scattering Tomography (LST). PERT cells and samples with amorphous silicon surface passivation – deposited at low temperature – were produced from neighboring wafers to correlate silicon properties and solar cell efficiency results.An interesting indirect correlation was found between “as-grown” lifetime and cell performance, as both are influenced by the thermal history of the wafer.We also observed a very strong correlation between BMD position, measured by LST on as-grown samples, and defective areas on PERT cells localized by PL measurements. The LST measurements on heat treated samples (simulating the PERT cell process) showed the growth of BMDs in low efficiency areas. It indicates that the detection of harmful defects is possible even in the as-grown material using LST technique.

Impact of c-Si Surface Passivating Layer Thickness on n+ Laser-Doped Contacts Based on Silicon Carbide Films

Laser processing of doped dielectric films has been proposed as a feasible alternative avoiding energy-consuming conventional dopant diffusions. In this approach, silicon surface passivation is usually obtained by means of a thin passivating layer deposited onto the silicon surface following what is done in heterojunction devices. However, laser doping from dielectric films relaxes the tradeoff between surface passivation and carrier transport and, as a consequence, thicker passivating films can be used improving the passivation. In particular, we focus on phosphorus-doped dielectric film stacks with a first thin intrinsic amorphous silicon carbide layer, which is responsible for surface passivation. In this paper, we explore the effect of the thickness of this passivating layer on crystalline silicon (c-Si) surface passivation and on the quality of the n+ contact created by laser processing. Surface recombination velocities for the full dielectric stack of 6 cm/s are measured when the layer thickness is increased to 22 nm while simultaneously keeping specific contact resistivity below 1 mΩ·cm 2 and surface recombination at the contact to values in the 2500–2850 cm/s range. Then, the introduction of thicker passivating films results in a better c-Si surface passivation with similar contact properties that could improve the efficiency potential of solar cells.

Effective passivation of silicon surfaces by ultrathin atomic-layer deposited niobium oxide

This letter reports on effective surface passivation of n-type crystalline silicon by ultrathin niobium oxide (Nb2O5) films prepared by atomic layer deposition (ALD) and subjected to a forming gas anneal at 300 °C. A champion recombination parameter J0 of 20 fA/cm2 and a surface recombination velocity Seff of 4.8 cm/s have been achieved for ultrathin films of 1 nm. The surface pretreatment was found to have a strong impact on the passivation. Good passivation can be achieved on both HF-treated c-Si surfaces and c-Si surfaces with a wet-chemically grown interfacial silicon oxide layer. On HF-treated surfaces, a minimum film thickness of 3 nm is required to achieve a high level of surface passivation, whereas the use of a wet chemically-grown interfacial oxide enables excellent passivation even for Nb2O5 films of only 1 nm. This discrepancy in passivation between both surface types is attributed to differences in the formation and stoichiometry of interfacial silicon oxide, resulting in different levels of chemical passivation. On both surface types, the high level of passivation of ALD Nb2O5 is aided by field-effect passivation originating from a high fixed negative charge density of 1–2 × 1012 cm−3. Furthermore, it is demonstrated that the passivation level provided by 1 nm of Nb2O5 can be further enhanced through light-soaking. Finally, initial explorations show that a low contact resistivity can be obtained using Nb2O5-based contacts. Together, these properties make ALD Nb2O5 a highly interesting building block for high-efficiency c-Si solar cells.

Investigation of crystalline silicon surface passivation by positively charged POx/Al2O3 stacks

We investigate the passivation of crystalline Si (c-Si) surfaces by phosphorus oxide (POx) thin films deposited in an atomic layer deposition (ALD) reactor and capped in-situ by ALD Al2O3. Passivation is demonstrated on both n- and p-type (100) Si surfaces, and for POx/Al2O3 stacks deposited at both 25 °C and 100 °C. In contrast to Al2O3 alone, POx/Al2O3 passivation is activated already by annealing at temperatures as low as 250 °C in N2 in all cases. Best results were obtained after annealing at 350 °C and 450 °C for films deposited at 25 °C and 100 °C respectively, with similar implied open-circuit voltages of 723 and 724 mV on n-type (100) Si. In the latter case an outstandingly low surface recombination velocity of 1.7 cm/s and saturation current density of 3.3 fA/cm2 were obtained on 1.35 Ω cm material. Passivation of p-type Si appeared somewhat poorer, with surface recombination velocity of 13 cm/s on 2.54 Ω cm substrates. Passivation was found to be independent of POx film thickness for films of 4 nm and above, and was observed to be stable during prolonged annealing up to 500 °C. This excellent passivation performance on n-type Si is attributed partly to an unusually large positive fixed charge in the range of 3–5 × 1012 cm−2 (determined from capacitance–voltage measurements) for stacks deposited at both temperatures, which is significantly larger than that exhibited by existing positively charged passivation materials such as SiNx. Indeed, passivation performance on n-type silicon is shown to compare favourably to state-of-the-art results reported for PECVD SiNx. POx/Al2O3 stacks thus represent a highly effective positively charged passivation scheme for c-Si, with potential for n-type surface passivation and selective doping applications.

POx/Al2O3 stacks: Highly effective surface passivation of crystalline silicon with a large positive fixed charge

Thin-film stacks of phosphorus oxide (POx) and aluminium oxide (Al2O3) are shown to provide highly effective passivation of crystalline silicon (c-Si) surfaces. Surface recombination velocities as low as 1.7 cm s−1 and saturation current densities J0s as low as 3.3 fA cm−2 are obtained on n-type (100) c-Si surfaces passivated by 6 nm/14 nm thick POx/Al2O3 stacks deposited in an atomic layer deposition system and annealed at 450 °C. This excellent passivation can be attributed in part to an unusually large positive fixed charge density of up to 4.7 × 1012 cm−2, which makes such stacks especially suitable for passivation of n-type Si surfaces.

Zirconium oxide surface passivation of crystalline silicon

This letter reports effective passivation of crystalline silicon (c-Si) surfaces by thermal atomic layer deposited zirconium oxide (ZrOx). The optimum layer thickness and activation annealing conditions are determined to be 20 nm and 300 °C for 20 min. Cross-sectional transmission electron microscopy imaging shows an approximately 1.6 nm thick SiOx interfacial layer underneath an 18 nm ZrOx layer, consistent with ellipsometry measurements (∼20 nm). Capacitance–voltage measurements show that the annealed ZrOx film features a low interface defect density of 1.0 × 1011 cm−2 eV−1 and a low negative film charge density of −6 × 1010 cm−2. Effective lifetimes of 673 μs and 1.1 ms are achieved on p-type and n-type 1 Ω cm undiffused c-Si wafers, respectively, corresponding to an implied open circuit voltage above 720 mV in both cases. The results demonstrate that surface passivation quality provided by ALD ZrOx is consistent with the requirements of high efficiency silicon solar cells.

Improving the Silicon Surface Passivation by Aluminum Oxide Grown Using a Non‐Pyrophoric Aluminum Precursor

In this work, an alternative nonpyrophoric Al‐precursor [dimethylaluminum isopropoxide (DMAI)] is used to deposit Al2O3 on p‐type c‐Si by plasma‐enhanced and thermal atomic layer deposition (ALD). Al2O3 films grown by thermal ALD provide a relatively modest level of crystalline silicon surface passivation compared to their plasma‐assisted ALD grown counterparts. This difference is attributed to a significant difference in the interfacial SiOx film as identified by Brewster's angle Fourier transform infrared spectroscopy. The energy barrier for DMAI to react with Si‐H groups on the surface is significantly lower compared to TMA, which impedes the formation of the interfacial SiOx layer. As thermal ALD is the strongly preferred method in the photovoltaic industry, there is a strong incentive to improve the performance of the thermal ALD process to allow for the application of this intrinsically safer Al2O3 deposition process. A chemically grown thin oxide is shown to significantly improve the level of surface passivation provided by the thermal ALD Al2O3 film resulting in an increase in implied open circuit voltage from 653 to 723 mV. This surface pretreatment thus solves a major barrier for the application of this intrinsically safer process in the photovoltaic industry.

Insulating and passivating plasma-enhanced atomic layer deposited aluminum oxide thin films for silicon solar cells

Plasma-enhanced atomic layer deposited (PEALD) aluminum oxide (Al2O3) thin film were investigated with respect to their application in silicon solar cells where not only excellent surface passivation but also excellent electrical insulation properties are required. In this study, the insulation properties of about 10 nm to almost 250 nm Al2O3 thin films, deposited at temperatures in the range of 100 °C to 350 °C and annealed in the range of 250 °C to 450 °C, were correlated to various thin film properties, most notably the thin film defect density, i. e. pinhole density, and the thin film stress. Al2O3 thin films provide sufficient electrical insulation after a post-deposition annealing step at 450 °C when deposited at 250 °C with thickness of at least 80 nm which can be attributed to a low defect density and low stress in the thin films, allowing a low leakage current density and a high breakdown voltage that was independent of the type of metals used for the electrode. Al2O3 thin films deposited and annealed at these temperatures enabled a very good surface passivation of silicon. These results indicate that Al2O3 thin films realized by PEALD are a promising candidate for silicon solar cells that rely on passivating as well as insulating thin films.

A simultaneous deep micromachining and surface passivation method suitable for silicon-based devices

Three novel methods for simultaneous micromachining and surface passivation of silicon are reported. A thin passivation layer is achieved using continuous and sequential plasma processes based on SF6, H2 and O2 gases. Reducing the recombination by surface passivation is crucial for the realization of high-performance nanosized optoelectronic devices. The passivation of the surface as an important step, is feasible by plasma processing based on hydrogen pulses in proper time-slots or using a mixture of H2 and O2, and SF6 gases. The passivation layer which is formed in situ during the micromachining process obviates a separate passivation step needed in conventional methods. By adjusting the plasma parameters such as power, duration, and flows of gases, the process can be controlled for the best results and acceptable under-etching at the same time. Moreover, the pseudo-oxide layer which is formed during the micromachining processes will also improve the electrical characteristics of the surface, which can be used as an add-on for micro and nanowire applications. To quantify the effect of surface passivation in our method, ellipsometry, lifetime measurements, x-ray photoelectron spectroscopy, current–voltage and capacitance–voltage measurements and solar cell testing have been employed.

Surface passivation of silicon photonic devices with high surface-to-volume-ratio nanostructures

Recently, a high-speed and high-efficiency silicon photodiode (PD) was demonstrated by enabling light trapping micro/nanostructured holes. While attractive for light manipulation, these high surface-to-volume-ratio nanostructures, which are created by top-down dry etching processes, can also bring other challenges, such as creating silicon surface damage and crystalline defects. To reduce the dark current level and minimize surface recombination, successful passivation is a vital step to achieving the ultimate performance of the silicon PD based on photon-trapping structures. In this paper, we present and compare several effective silicon surface passivation schemes including plasma-enhanced chemical vapor deposited SiO2 and Si3N4, hydrofluoric acid (HF) treatment, thermal oxidation, and low ion energy dry etch. These passivation techniques can reduce the dark current by more than 4 orders of magnitude, bringing it down to the nanoampere level. Among these passivation schemes, HF treatment is relatively simple and cost effective, while other techniques may alter the light trapping characteristics of silicon PDs to some extent, thus affecting their external quantum efficiency. The passivation techniques discussed in this paper are CMOS compatible and can also be applied to other silicon photonic devices, such as photovoltaics.

Passivation properties of alumina for multicrystalline silicon nanostructure prepared by spin-coating method

In this paper, we report passivation properties of inverted pyramidal nanostructure based multi-crystalline silicon (mc-Si) by Al2O3 films with spin-coating method. Precursors AlCl3 and Al(acac)3 for Al2O3 films were chosen for comparison. Al2O3/SiOx stacks were found to be able to passivate the nanostructured surface well. With the number of spin-coating up to five, the Al2O3 films could conformally attach the nanostructure. The weighted average reflectance values (ranging from 400–900 nm) of the passivated silicon surface could be reduced to 10.74% (AlCl3) and 11.12% (Al(acac)3), and the effective carrier lifetime could reach 7.84 and 16.98 μs, respectively. This work presented a potential process to fabricate low cost high efficiency mc-Si solar cells.