AlOx Passivation Research Articles

Effect of annealing on MoOx and interface properties for carrier-selective contact solar cells with different passivation layers

The structural, electrical and optical properties of molybdenum oxide (MoOx) thin films prepared by magnetron sputtering are studied with annealing temperature. The interface properties of silicon with the intrinsic amorphous silicon (a-Si:H(i)) and alumina (AlOx) passivation layers are investigated by the minority carrier lifetime and implied-Voc (iVoc). The AlOx shows the better passivation properties and thermal stability for the carrier-selective contact solar cells. The Ce-doped In2O3(ICO) is deposited on the surface of MoOx thin films. It is found that the optimized annealing temperature is 125 °C for the carrier selective contact solar cells with the a-Si:H(i) passivation layer. It could increase to 175 °C for the AlOx passivation layer.

Enhanced stability in InSnO transistors via ultrathin in-situ AlOx passivation

In this study, we present the impact of an ultrathin in-situ AlOx passivation layer on the electrical performance and stability of InSnO (ITO) transistors. Devices incorporating an ultrathin (∼2 nm) AlOx passivation layer demonstrate satisfactory electrical performance and stability, characterized by a decent field-effect mobility, enhancement mode operation, notably reduced subthreshold swing, negligible hysteresis, and, notably, a significantly reduced threshold voltage shift under negative bias stress (NBS) and positive bias stress (PBS), respectively. The total trap density (Ntot), extracted through the sub-threshold slope, and the trap density (Nt), measured using low-frequency noise (LFN) for the AlOx-passivated ITO transistors, are significantly decreased. Those improvements can be attributed to the suppression of oxygen vacancy formation during the reactively-sputtered AlOx and subsequent annealing processes, as demonstrated by comparing the in-depth X-ray photoelectron spectroscopy (XPS) results of the AlOx-passivated and un-passivated ITO films. These results underscore the potential of in-situ reactively-sputtered ultrathin AlOx passivation layers to enhance the stability of thin-film transistors with In-rich channels.

Optical modeling and characterization of bifacial SiNx/AlOx dielectric layers for surface passivation and antireflection in PERC

AbstractIn this research, we analyzed the impact that the optical characteristics of dielectric surface passivation and antireflection coating schemes have on the performance of passivated emitter and rear cell (PERC) silicon solar cells. We employed wafer ray tracer (WRT) and automate for simulation of heterostructure (AFORS‐HET) simulations, as well as experimental characterization of fabricated thin film coatings. We investigated three distinct front surface morphologies: planar surface, upright pyramids, and inverted pyramids. Using WRT, we calculated the photogeneration current densities (JG) for PERC devices with three schemes: (i) SiNx/AlOx as antireflection coating and passivation stacks on both the front and rear sides, (ii) SiNx antireflection coating on the front side and AlOx passivation layer on the rear side, and (iii) SiNx/AlOx as antireflection coating and passivation stacks on the front side with an AlOx passivation layer on the rear side. Following simulation with optimal JG, two schemes are experimentally evaluated: PECVD SiNx (70 nm) and atomic layer deposition (ALD) AlOx (15 and 25 nm). We confirmed the growth effects and optical properties using X‐ray diffraction, Raman spectroscopy, effective lifetime, and refractive index measurements. The most favorable electrical properties were obtained with SiNx (70 nm, front) and AlOx (25 nm, front and rear), where the AlOx can be deposited via ALD bifacially on a single step, minimizing processing while maintaining passivation performance. Finally, we used AFORS‐HET to simulate the maximum performance of PERC bearing such films. The results showed a Voc = 0.688 V, Jsc = 41.42 mA/cm2, FF = 84%, and packing conversion efficiency (PCE) = 24.12% as the optimal solar cell performance values.

Monocrystalline Silicon PERC Solar Cell with Rear-Side AlOx Film Formed by Furnace Oxidation

In this study, AlOx passivation layers on the rear sides of silicon PERC solar cells are formed by thermally oxidizing 3 nm-thick aluminum films deposited in advance by an e-gun evaporator. The oxidation process is conducted in a furnace full of oxygen at 400°C for a duration of 10 minutes, followed by annealing for a duration of 3 minutes at 700°C, and then the stacking of SiNx films on the back surfaces. After that, a second annealing process is done at 400°C for a duration of 10 minutes to repair the defects resulting from the bombardment of ions on the passivation layer. With the thermal oxidation method applied, we confirmed the existence of an AlOx passivation layer with a negative charge density of − 3.21 × 10 12 cm-2 for an annealed sample, in contrast to − 6.17 × 10 11 cm-2 for an unannealed sample.

Enhancing electrical performance and stability of nanometer-thin ITO transistors via thermally oxidized alumina passivation layer

In this study, we investigated the utilization of alumina (AlOx), formed through the oxidation of thermally evaporated aluminum, as a passivation layer for nanometer-thin indium-tin-oxide (ITO) transistors. The ITO transistors passivated with thermally oxidized AlOx passivation exhibited remarkable electrical properties, with an average field-effect mobility of 241 cm2/Vs, significantly higher than the 40 cm2/Vs observed for devices without the AlOx passivation layer. Moreover, the passivated transistors maintained a high on/off current ratio at 108 level. In addition, the passivated transistors demonstrated improved stability, with a decrease in the threshold voltage (Vth) shift under negative bias stress testing conducted over 3600 seconds. The ITO transistors also displayed better air-ambient stability compared to transistors without the AlOx passivation layer. These results demonstrate the potential application of alumina passivation in nanometer-thin ITO field-effect transistors.

Emitter formation with boron diffusion from PECVD deposited boron-doped silicon oxide for high-efficiency TOPCon solar cells

We present a systematic study of emitter formation with dopant diffusion from boron (B)-doped hydrogenated silicon oxide (a-SiOx:H) deposited on textured n-type monocrystalline silicon (n-c-Si) wafers using a 13.56 MHz plasma-enhanced chemical vapor deposition (PECVD) system. The B atoms in the a-SiOx:H are activated and diffused into the n-c-Si wafer during a high-temperature annealing to form a p/n junction for solar cell application. Afterward, the B-doped SiOx layer is removed by hydrofluoric acid selective etching to obtain a clean surface for the deposition of AlOx passivation and SiNx antireflection layers. With the optimization of the PECVD deposition and the following annealing/etching processes, the B diffused region with a sheet resistance of 60–100 Ω/sq and a junction depth of 1.2–1.5 μm is obtained. A champion implied open-circuit voltage (iVoc) of 705 mV and a single-side saturated recombination current density (J0,s) of 17.6 fA/cm2 are obtained from a double-sided B diffused sample after the passivation with an atomic-layer-deposition (ALD) AlOx. In addition, the contact resistivity (ρc) at the metal/emitter interface is lower than 5 mΩcm2, where the metal contact is a Ti/Pd/Ag tri-layer structure. All of the material properties meet the requirements of high-efficiency solar cell. With the optimized emitter in the front, we made a SiOx and heavily phosphorus-doped polycrystalline silicon carbide (n-poly-SiCx) stack on the rear surface to form tunnel oxide passivated contact (TOPCon) solar cells. The results show that the iVoc of semi-finished TOPCon solar cell before metallization is more than 725 mV. Finally, a TOPCon solar cell with an efficiency of 24.24% is obtained, which is comparable with the TOPCon solar cells with the industrial thermally diffused emitter.

Reactively-sputtered AlOx passivation layer for self-aligned top-gate amorphous InGaZnO thin-film transistors

The direct-current reactive sputtering of aluminum oxide (AlOx) films with Al target was investigated to provide a preferable passivation layer (PL) for self-aligned top-gate (SATG) amorphous metal oxide semiconductor (AOS) like amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs). The grown AlOx films exhibited excellent electrical characteristics, with major parameters including a relative dielectric constant around 9, breakdown electric field of 3.7 MV/cm, and leakage current of 6.29 × 10−8 A/cm2 at 2 MV/cm. It is shown that the AlOx PL, together with a PECVD SiO2 interlayer could provide SATG a-IGZO TFTs with trustworthy environmental and electrical stability.

Analyses and Excess Oxygen Investigations by Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy at AlOx/Si Interfaces in Passivated Emitter and Rear Solar Cells

Progressive optimization of the surface passivation of Si solar cells requires access to the elemental composition and chemical bonding characteristics of ultrathin layers and buried interfaces on textured or structured surfaces. The passivation of the rear side of passivated emitter and rear solar cells, for instance, can be realized through AlOx/SiNx stacks, which are known to provide an additional field effect passivation on top of the chemical surface passivation. Herein, transmission electron microscopy is used at locally prepared cross‐sections of such stacks. Electron energy loss spectroscopy is used to extract depth‐resolved information of elemental composition and chemical bonding at nanometer scale. With this, the interfacial excess O fraction, which is crucial for the field effect passivation, is successfully determined for the first time for AlOx passivation layer systems on industrially produced Si solar cells.

Performance Enhancement of Transparent Amorphous IGZO Thin-Film Transistor Realized by Sputtered Amorphous AlOx Passivation Layer

We report a high-mobility transparent Indium-Gallium-Zinc-Oxide (IGZO) thin-film transistor (TFT) with sputtered AlOx passivation layer. The interfacial region between the IGZO layer and the AlOx layer played a crucial role in improving the field-effect mobility (the maximum field-effect mobility increased from 6.292 cm2 Vs−1 for the TFT without the AlOx layer to 69.01 cm2 Vs−1 for the TFT with the passivation layer) and the on/off current ratio (from ∼107 without the layer to ∼108 with the layer). The driving current of IGZO TFT was also significantly enhanced. The formation of the interfacial layer has been investigated and verified. The ion bombardment during the AlOx deposition broke the In-O bond in IGZO, generating oxygen ions (O2−). The segregation of the O2− was facilitated by the sputtered amorphous AlOx. A metallic In-rich layer with high oxygen vacancy concentration was formed at the interface, leading to an increase in the carrier concentration in the interfacial layer. Besides the electrical performance, the reliability tests, including long-term exposure in the ambient environment and positive bias illumination stress (PBIS), showed improved results as well.

Anisotropic thermal conductivity of layered indium selenide

Layered indium selenide (InSe) has emerged as a promising two-dimensional semiconductor due to its high electron mobility and direct optical bandgap in the few-layer limit. As InSe is integrated into high-performance electronic and optoelectronic systems, thermal management will become critical, thus motivating detailed characterization of intrinsic thermal properties. Here, we report the room-temperature thermal conductivity of exfoliated crystals of InSe along the through-plane and in-plane directions using conventional and beam offset time-domain thermoreflectance (TDTR), respectively. InSe crystals with varying thicknesses were prepared by mechanical exfoliation onto Si(100) wafers followed by immediate encapsulation with a 3-nm-thick AlOx passivation layer to prevent ambient degradation prior to coating with metal films for TDTR measurements. The measured thermal conductivity in the in-plane direction, Λin ≈ 8.5 ± 2 W/m K, is an order of magnitude higher than that in the through-plane direction, Λthrough ≈ 0.76±0.15 W/m K, which implies a high thermal anisotropy ≈11 ± 3. These relatively high anisotropy and low thermal conductivity compared to other layered semiconductors imply that InSe will require unique thermal management considerations when implemented in electronic, optoelectronic, and thermoelectric applications.

Crystalline Silicon PERC Solar Cell with Ozonized AlOx Passivation Layer on the Rear Side

We present a method of ozonation to form the rear-side passivation layers of crystalline silicon PERC cells. In the method, a thin aluminum film was deposited on the back surface of a silicon wafer and then was oxidized into an aluminum oxide layer by gaseous ozone. Lifetimes of the wafers with such passivation layers proved to be increased with respect to those untreated, and the resultant PERC cells showed a performance improvement compared with standard cells with full back surface fields.

Effect of post-deposition annealing on electrical properties and structures of aluminum oxide passivation film on a crystalline silicon substrate

We studied the effect of post-deposition annealing (PDA) on the electrical property and the structure of aluminum oxide (AlOx) passivation films prepared by atomic layer deposition with various temperatures. Surface recombination velocity (Smax), interface trap density (Dit), and fixed charge density (Qeff/q) before and after the PDA were evaluated employing lifetime and capacitance–voltage measurements. The structural change by the PDA was investigated by a Stokes ellipsometer and X-ray reflectivity (XRR) measurements at SPring-8. The Smax of all samples was improved by the PDA. The improvement of Smax in the samples deposited at the temperature of 200 °C and 300 °C was due to the Dit decrease, but that in the sample deposited at the temperature of 25 °C was due to both the Dit decrease and the Qeff/q increase. From the XRR measurements, it revealed that the formation of the interfacial thin layer is essential for the appearance of negative fixed charges.

H2O/O2 Vapor Annealing Effect on Spin Coating Alumina Thin Films for Passivation of Silicon Solar Cells

Aluminum acetylacetonate-based AlOx thin films were introduced as a low-cost, high-quality passivation layers for crystalline silicon solar cells. Films were formed by a spin coating method on p-type silicon substrates at 450°C in ambient air, O2, or water vapor (H2O/O2) for 15 or 120 min. XPS analysis confirms the AlOx formation and reveals a high intensity of interfacial SiOx at the AlOx/Si interface of processed wafers. Ambient H2O/O2 was found to be more beneficial for the activation of introduced AlOx passivation films which offers high lifetime improvements with a low thermal budget. Carrier lifetime measurements provides that symmetrically coated wafers reach 119.3 μs and 248.3 μs after annealing in ambient H2O/O2 for 15 min and 120 min, respectively.

Simple method for significant improvement of minority-carrier lifetime of evaporated BaSi2 thin film by sputtered-AlOx passivation

A novel surface passivation of AlOx on BaSi2 thin films fabricated by vacuum evaporation for solar cell application has been developed. The minority-carrier lifetime of evaporated BaSi2 film is shorter than that of epitaxial film. This lifetime has been improved by forming AlOx passivation layer on the BaSi2 surface by RF sputtering of Al followed by oxidation in air. A drastic improvement of the lifetime upon the passivation of AlOx was observed up to 15μs with 3-nm of AlOx layer after rapid thermal annealing at 475°C for 15min. The dependence of lifetime on passivation layer thickness disappeared for thickness higher than 9nm, especially after annealing, showing the chemical passivation effect of AlOx. The result confirmed that AlOx is a promising passivation-layer for improving properties of BaSi2 film.

Interdigitated back contact solar cells with polycrystalline silicon on oxide passivating contacts for both polarities

We demonstrate an independently confirmed 25.0%-efficient interdigitated back contact silicon solar cell with passivating polycrystalline silicon (poly-Si) on oxide (POLO) contacts that enable a high open circuit voltage of 723 mV. We use n-type POLO contacts with a measured saturation current density of J0n = 4 fA cm−2 and p-type POLO contacts with J0p = 10 fA cm−2. The textured front side and the gaps between the POLO contacts on the rear are passivated by aluminum oxide (AlOx) with J0AlOx = 6 fA cm−2 as measured after deposition. We analyze the recombination characteristics of our solar cells at different process steps using spatially resolved injection-dependent carrier lifetimes measured by infrared lifetime mapping. The implied pseudo-efficiency of the unmasked cell, i.e., cell and perimeter region are illuminated during measurement, is 26.2% before contact opening, 26.0% after contact opening and 25.7% for the finished cell. This reduction is due to an increase in the saturation current density of the AlOx passivation during chemical etching of the contact openings and of the rear side metallization. The difference between the implied pseudo-efficiency and the actual efficiency of 25.0% as determined by designated-area light current–voltage (I–V) measurements is due to series resistance and diffusion of excess carriers into the non-illuminated perimeter region.

Hybrid Organic-Inorganic Perovskite Memory with Long-Term Stability in Air

Organic-inorganic perovskite materials have attracted extensive attention for wide range of applications such as solar cells, photo detectors, and memory devices. However, the lack of stability in ambient condition prevented the perovskite materials from applying to practical applications. Here, we demonstrate resistive switching memory devices based on organic-inorganic perovskite (CH3NH3PbI3) that have been passivated using thin metal-oxide-layers. CH3NH3PbI3-based memory devices with a solution-processed ZnO passivation layer retain low-voltage operation and, on/off current ratio for more than 30 days in air. Passivation with atomic-layer-deposited (ALD) AlOx is also demonstrated. The resistive switching memory devices with an ALD AlOx passivation layer maintained reliable resistive switching for 30 d in ambient condition, but devices without the passivation layer degraded rapidly and did not show memory properties after 3 d. These results suggest that encapsulation with thin metal-oxide layers is easy and commercially-viable methods to fabricate practical memory devices, and has potential to realize memory devices with long-term stability and reliable, reproducible programmable memory characteristics.

Proposal of leak path passivation for InGaN solar cells to reduce the leakage current

We propose some general ways to passivate the leak paths in InGaN solar cells and report some experimental evidences of its effectiveness. By adopting an AlOx passivation process, the photovoltaic performances of GaN pn-junctions and InGaN solar cells, grown by molecular beam epitaxy, have been significantly improved. The open circuit voltage under 1 sun illumination increases from 1.46 to 2.26 V for a GaN pn junction, and from 0.95 to 1.27 V for an InGaN solar cell, demonstrating evidence of leak path passivation (LPP) by AlOx. The proposed LPP is expected to be a realistic way to exploit the potential of thick and relaxed but defective InGaN for solar cell applications.

Light-induced Degradation of Silicon Solar Cells with Aluminiumoxide Passivated Rear Side

Light-induced degradation (LID) has been shown to significantly affect the performance of multicrystalline silicon (mc-Si) solar cells with aluminium oxide (AlOx) passivated rear side. Within this work, the impact of LID on the conversion efficiency of different silicon solar cell architectures with and without AlOx passivation is investigated. Under conditions representing realistic module operation, significant light-induced degradation of up to Δη = -2.9 %relin conversion efficiency has been observed for multicrystalline silicon (mc-Si)solar cells with AlOxpassivation. This degradation has been found to be higher than the degradation of, both,a mc-Si aluminium back surface field(Al-BSF) solar cell and, remarkably, a Czochralski-grown silicon (Cz-Si) solar cell with AlOx passivation. For a more detailed investigation of the interaction of mc-Si and AlOx passivation during degradation,a photoluminescence-based“effective defect”imaging has been performed on AlOx-passivated mc-Si lifetime samples. The local effective defect lifetimerelated to recombination due toLID-induced defects is found to vary strongly in the range of τeff,defect = 5 to 75 μs and, furthermore, areas with low effective lifetime could be identified as areas with relatively high dislocation density.

Light-induced degradation of PECVD aluminium oxide passivated silicon solar cells

Light-induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron-doped silicon substrates. Typically, Czochralski-grown silicon (Cz-Si) has been reported to suffer from stronger LID than block-cast multicrystalline silicon (mc-Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc-Si solar cells degrade more than a reference aluminium back surface field mc-Si cell and, remarkably, an AlOx passivated Cz-Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re-evaluation of the suitability of boron-doped Cz- and mc-Si for solar cells with very high efficiencies. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Light illumination stability of amorphous InGaZnO thin film transistors with sputtered AlOx passivation in various thicknesses

Light illumination stability of amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) passivated by sputtered AlOx thin films were investigated in this paper. Light wavelength and passivation-layer thickness were intentionally controlled to ascertain the related two physical mechanisms, i.e., electron–hole pair (EHP) generation and oxygen vacancy (VO) formation. A qualitative model was proposed to effectively compare and distinguish the above two mechanisms in thermal stability of a-IGZO TFTs with passivation layers. With light wavelength decreasing EHP generation became evident where the “threshold wavelength” was between 420 and 400 nm for the a-IGZO TFTs used in this study. Meanwhile, passivation-layer could significantly improve the stability of a-IGZO TFTs under long-wavelength light illumination by suppressing the escape of oxygen atoms to form VO in a-IGZO films. The “threshold thickness” of AlOx passivation-layer in our devices was estimated to be about 90 nm.