G Gijs Dingemans

Status and prospects of Al2O3-based surface passivation schemes for silicon solar cells

The reduction in electronic recombination losses by the passivation of silicon surfaces is a critical enabler for high-efficiency solar cells. In 2006, aluminum oxide (Al2O3) nanolayers synthesized by atomic layer deposition (ALD) emerged as a novel solution for the passivation of p- and n-type crystalline Si (c-Si) surfaces. Today, high efficiencies have been realized by the implementation of ultrathin Al2O3 films in laboratory-type and industrial solar cells. This article reviews and summarizes recent work concerning Al2O3 thin films in the context of Si photovoltaics. Topics range from fundamental aspects related to material, interface, and passivation properties to synthesis methods and the implementation of the films in solar cells. Al2O3 uniquely features a combination of field-effect passivation by negative fixed charges, a low interface defect density, an adequate stability during processing, and the ability to use ultrathin films down to a few nanometers in thickness. Although various methods can...

Influence of the Deposition Temperature on the c-Si Surface Passivation by Al2O3 Films Synthesized by ALD and PECVD

The material properties and c-Si surface passivation have been investigated for Al 2 O 3 films deposited using thermal and plasma atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD) for temperatures (T dep ) between 25 and 400°C. Optimal surface passivation by ALD Al 2 O 3 was achieved at T dep = 150-250°C with S eff < 3 cm/s for ∼2 Ω cm p-type c-Si. PECVD Al 2 O 3 provided a comparable high level of passivation for T dep = 150-300°C and contained a high fixed negative charge density of ∼6 x 10 12 cm -2 . Outstanding surface passivation performance was therefore obtained for thermal ALD, plasma ALD, and PECVD for a relatively wide range of Al 2 O 3 material properties.

Low Temperature Plasma-Enhanced Atomic Layer Deposition of Metal Oxide Thin Films

Many reported atomic layer deposition (ALD) processes are carried out at elevated temperatures (>150°C), which can be problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the plasma-enhanced ALD of Al 2 O 3 , TiO 2 , and Ta 2 O 5 has been investigated at 25-400°C using [Al(CH 3 ) 3 ], [Ti(O i Pr) 4 ], [Ti(Cp Me )(O i Pr) 3 ], [TiCp*(OMe) 3 ], and [Ta(NMe 2 ) 5 ] as precursors. An O 2 plasma was employed as the oxygen source in each case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al 2 O 3 ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.

Hydrogen induced passivation of Si interfaces by Al2O3 films and SiO2/Al2O3 stacks

The role of hydrogen in Si surface passivation is experimentally identified for Al2O3 (capping) films synthesized by atomic layer deposition. By using stacks of SiO2 and deuterated Al2O3, we demonstrate that hydrogen is transported from Al2O3 to the underlying SiO2 already at relatively low annealing temperatures of 400 °C. This leads to a high level of chemical passivation of the interface. Moreover, the thermal stability of the passivation up to 800 °C was significantly improved by applying a thin Al2O3 capping film on the SiO2. The hydrogen released from the Al2O3 film favorably influences the passivation of Si interface defects.

Stability of Al2O3 and Al2O3/a-SiNx:H stacks for surface passivation of crystalline silicon

The thermal and ultraviolet (UV) stability of crystalline silicon (c-Si) surface passivation provided by atomic layer deposited Al2O3 was compared with results for thermal SiO2. For Al2O3 and Al2O3/a-SiNx:H stacks on 2 Ω cm n-type c-Si, ultralow surface recombination velocities of Seff 800 °C) used for screen printed c-Si solar cells. Effusion measurements revealed the loss of hydrogen and oxygen during firing through the detection of H2 and H2O. Al2O3 also demonstrated UV stability with the surface passivation improving during UV irradiation.

Influence of the Oxidant on the Chemical and Field-Effect Passivation of Si by ALD Al2O3

Differences in Si surface passivation by aluminum oxide (Al2O3) films synthesized using H2O and O-3-based thermal atomic layer deposition (ALD) and plasma ALD have been revealed. A low interface defect density of D-it = similar to 1011 eV(-1) cm(-2) was obtained after annealing, independent of the oxidant. This low D-it was found to be vital for the passivation performance. Field-effect passivation was less prominent for H2O-based ALD Al2O3 before and after annealing, whereas for as-deposited ALD films with an O-2 plasma or O-3 as the oxidants, the field-effect passivation was impaired by a very high Dit

Role of field-effect on c-Si surface passivation by ultrathin (2–20 nm) atomic layer deposited Al2O3

Al2O3 synthesized by plasma-assisted atomic layer deposition yields excellent surface passivation of crystalline silicon (c-Si) for films down to ∼5 nm in thickness. Optical second-harmonic generation was employed to distinguish between the influence of field-effect passivation and chemical passivation through the measurement of the electric field in the c-Si space-charge region. It is demonstrated that this electric field—and hence the negative fixed charge density—is virtually unaffected by the Al2O3 thickness between 2 and 20 nm indicating that a decrease in chemical passivation causes the reduced passivation performance for <5 nm thick Al2O3 films.

Controlling the fixed charge and passivation properties of Si(100)Al2O3 interfaces using ultrathin SiO2 interlayers synthesized by atomic layer deposition

Al2O3 synthesized by atomic layer deposition (ALD) on H-terminated Si(100) exhibits a very thin (∼1 nm) interfacial SiOx layer. At this interface, a high fixed negative charge density, Qf, is present after annealing which contributes to ultralow surface recombination velocities  ∼5 nm), the polarity of the effective charge density changed from negative to positive. The observed changes in Qf and the associated field-effect passivation had a significant influence on the injection-level-dependent minority carrier lifetime of Si.

Influence of annealing and Al2O3 properties on the hydrogen-induced passivation of the Si/SiO2 interface

Annealing at moderate temperatures is required to activate the silicon surface passivation by Al2O3 thin films while also the thermal stability at higher temperatures is important when Al2O3 is implemented in solar cells with screenprinted metallization. In this paper, the relationship between the microstructure of the Al2O3 film, hydrogen diffusion, and defect passivation is explored in detail for a wide range of annealing temperatures. The chemical passivation was studied using stacks of thermally-grown SiO2 and Al2O3 synthesized by atomic layer deposition. Thermal effusion measurements of hydrogen and implanted He and Ne atoms were used to elucidate the role of hydrogen during annealing. We show that the passivation properties were strongly dependent on the annealing temperature and time and were significantly influenced by the Al2O3 microstructure. The latter was tailored by variation of the deposition temperature (Tdep = 50 °C–400 °C) with hydrogen concentration [H] between 1 and 13 at.% and mass den...

Effective passivation of Si surfaces by plasma deposited SiOx/a-SiNx:H stacks

Very low surface recombination velocities 6 and 11 cm / s were obtained for SiO x / a -SiN x : H stacks synthesized by plasma-enhanced chemical vapor deposition on low resistivity n - and p -type c -Si , respectively. The stacks induced a constant effective lifetime under low illumination, comparable to Al 2 O 3 on p -type Si. Compared to single layer a -SiN x : H , a lower positive fixed charge density was revealed by second-harmonic generation measurements, while field-effect passivation was absent for a reference stack comprising thermally grown SiO 2 . The results indicate that hydrogenation of interface states played a key role in the passivation and remained effective up to annealing temperatures > 800 ° C .