Surface Passivation Of Crystalline Silicon Research Articles

Highly effective passivation of crystalline silicon surfaces achieved through transparent conductive Al-doped ZnO layers

Highly effective passivation of crystalline silicon surfaces achieved through transparent conductive Al-doped ZnO layers

Excellent crystalline silicon surface passivation by transparent conductive Al-doped ZnO/ITO stack

The surface passivation materials commonly used in crystalline silicon (c-Si) solar cells are either electrically insulating or opaque to the solar spectrum, which poses the electrical loss or optical loss. Hence it is of great significance to explore functional films that are simultaneously transparent, conductive and passivating for c-Si. In this study, we successfully tune two most well-known transparent conductive oxides, i.e. Al-doped ZnO (AZO) and indium tin oxide (ITO), as excellent passivation layers for c-Si surface. The AZO film is grown by atomic layer deposition and is capped with a magnetron-sputtered ITO film. Dynamic secondary ion mass spectrometry reveals that such stack design can retain higher hydrogen concentration on c-Si surface than a single AZO film during annealing. In addition, the AZO should be highly doped to reduce the energy barrier height around c-Si surface, thereby increasing the charge carrier asymmetry and thus reducing surface recombination. It is also highly crucial to optimize the annealing temperature and interfacial silicon oxide. As such, the effective minority carrier lifetime reaches 2.0 ms and the implied open-circuit voltage approaches 700 mV. Besides, the AZO/ITO stacked films possess negligible optical absorption, low sheet resistance (∼90 Ω/sq), and outstanding antireflection effect on c-Si surface.

Silicon surface passivation with a-Si:H by PECVD: growth temperature effects on defects and band offset

The surface passivation of crystalline silicon (c-Si) is studied during growth of hydrogenated amorphous silicon (a-Si:H) by means of plasma-enhanced CVD. The surface passivation is characterized by an in situ method of the photocurrent measurement of c-Si during the growth of an a-Si:H passivation layer at various growth temperatures. The passivation is also characterized by an ex situ method of the carrier lifetime measurement performed at RT in air. According to both the in situ and ex situ characterization results, the surface passivation is optimized around a growth temperate of 200 °C, where the defect reduction and the band offset formation at the a-Si:H/c-Si interface play important roles.

Investigation of dual intrinsic a-Si:H films for crystalline silicon surface passivation by spectroscopic ellipsometry: application in silicon heterojunction solar cells

Investigation of dual intrinsic a-Si:H films for crystalline silicon surface passivation by spectroscopic ellipsometry: application in silicon heterojunction solar cells

Investigation of anisotropic textured crystalline silicon surface passivation with intrinsic amorphous silicon layers: role of annealing pressure conditions during passivation recovery

Investigation of anisotropic textured crystalline silicon surface passivation with intrinsic amorphous silicon layers: role of annealing pressure conditions during passivation recovery

Ion bombardment-induced defect reduction at a-Si:H/c-Si interfaces: possible microstructural changes and mobile hydrogen roles

A beneficial effect of argon (Ar) ion bombardment for crystalline silicon (c-Si) surface passivation has been studied. Experiments of an Ar plasma treatment over an hydrogenated amorphous silicon (a-Si:H) layer grown on c-Si are performed, where an a-Si:H layer is prepared at different levels of defect density. Interestingly, the c-Si surface passivation is improved by an Ar plasma treatment for a defect-rich, i.e. low-quality, a-Si:H layer, while it is deteriorated by the treatment for a low-defect, i.e. high-quality, a-Si:H layer. The improvement of passivation is discussed in terms of microstructural changes of a-Si:H, associated with redistribution of hydrogen, where mobile hydrogens play an important role.

Temporal and spatial atomic layer deposition of Al-doped zinc oxide as a passivating conductive contact for silicon solar cells

Recently, stacks consisting of an ultrathin SiO2 coated with atomic-layer-deposited (ALD) Al-doped zinc oxide (ZnO:Al) have been shown to yield state-of-the-art passivation of n-type crystalline silicon surfaces and provide low contact resistivities to n+-doped Si and poly-Si surfaces. Key for achieving good surface passivation are an intentionally-grown SiO2 interlayer, an aluminum oxide (Al2O3) capping layer and a post-deposition anneal, whereas n-type doping of the ZnO is required to achieve a low contact resistivity. In this work, we present the latest results and insights obtained for this contact stack. This includes a study of the minimum required thicknesses of both the ZnO and the Al2O3 capping layer to achieve a high passivation level after post-deposition anneal. Also, we provide details on how to remove the Al2O3 capping layer selectively from the ZnO:Al after the post-deposition anneal using a pH-controlled wet-etch, such that the ZnO:Al can be contacted by a metal. Whereas previous work was based on lab-scale temporal ALD, in this work we highlight the industrialization potential by demonstrating that these layers can be prepared by spatial ALD, yielding good passivation levels on both undiffused n-type and n+-diffused c-Si surfaces. Finally, we demonstrate the capability of ALD to deposit ZnO:Al layers selectively on oxidized regions of an otherwise HF-last treated c-Si surface. Such area-selective deposition opens up potential pathways for local, self-aligned contact formation. Altogether, this work provides valuable insights into the working mechanism and practical aspects of ZnO:Al-based passivating contacts.

PO x /Al2O3 Stacks for c-Si Surface Passivation: Material and Interface Properties.

Phosphorus oxide (POx) capped by aluminum oxide (Al2O3) has recently been discovered to provide excellent surface passivation of crystalline silicon (c-Si). In this work, insights into the passivation mechanism of POx/Al2O3 stacks are gained through a systematic study of the influence of deposition temperature (Tdep = 100–300 °C) and annealing temperature (Tann = 200–500 °C) on the material and interface properties. It is found that employing lower deposition temperatures enables an improved passivation quality after annealing. Bulk composition, density, and optical properties vary only slightly with deposition temperature, but bonding configurations are found to be sensitive to temperature and correlated with the interface defect density (Dit), which is reduced at lower deposition temperature. The fixed charge density (Qf) is in the range of + (3–9) × 1012 cm–2 and is not significantly altered by annealing, which indicates that the positively charged entities are generated during deposition. In contrast, Dit decreases by 3 orders of magnitude (∼1013 to ∼1010 eV–1 cm–2) upon annealing. This excellent chemical passivation is found to be related to surface passivation provided by hydrogen, and mixing of aluminum into the POx layer, leading to the formation of AlPO4 upon annealing.

A Comprehensive Fundamental Understanding of Atomic Layer Deposited Titanium Oxide Films for c-Si Solar Cell Applications

Titanium oxide (TiO x ) film shows excellent surface passivation of crystalline silicon (c-Si) as well as carrier selectivity. The coexistence of both these properties along with extraordinary optical properties of TiO x opens new opportunities to design novel high-performance c-Si solar cells. However, many inconsistent findings have also been reported in the literature that limit the application of TiO x . A greater fundamental insight into TiO x films therefore needs to be developed in order to produce very high-efficiency c-Si solar cells. In this article, a comprehensive study of atomic layer deposited (ALD) TiO x films is presented, including the processing as well as material, electrical and optical properties. The effects of deposition temperature from 100 to 400 °C and postdeposition annealing treatment on the properties of TiO x films are examined in terms of the ALD growth process and the composition, stoichiometry, crystallographic structure, bonding environment, elemental, chemical, electrical, and optical properties of the films. The screening of the temperature suggests that it is one of the key parameters that can be used to control the TiO x film properties for different applications, by giving excellent c-Si surface passivation with a surface recombination velocity as low as 3.7 cm/s. Further analysis of the underlying passivation mechanism suggests that TiO x films in amorphous phase combined with an interface oxide (SiO y +TiO x ) and a small amount of Cl are favorable for achieving good c-Si passivation. These findings significantly improve the fundamental understanding of TiO x films from their growth to application, and could enable the control of their properties for future device development.

Investigation of silicon surface passivation by sputtered amorphous silicon and thermally evaporated molybdenum oxide films using temperature- and injection-dependent lifetime spectroscopy

Crystalline silicon (c-Si) surface passivation has been investigated by sputtered hydrogenated intrinsic amorphous silicon (S-i-a-Si:H) and thermally evaporated molybdenum oxide (MoOx) thin films. The temperature- and injection-dependent lifetime spectroscopy technique has been adopted for analyzing the passivation quality of the c-Si surface, using parameters such as the minority carrier effective lifetime (τeff), the activation energy of surface/interface defect states (ΔE), and the electron to hole carrier capture cross-section ratio (k) at the interface. With S-i-a-Si:H films, a τeff of ∼70 µs and ΔE of ∼51 meV have been observed in comparison to a τeff of ∼110 µs and ΔE of ∼109 meV from the MoOx films. These entirely different parameters are an indication of the relatively strong carrier recombination with dense interface/surface states from the S-i-a-Si:H passivation layers. The S-i-a-Si:H layers are unable to minimize the c-Si surface trap states with the chemical passivation for reducing carrier recombination due to the generation of additional surface defect states by the sputtering damage. However, the MoOx layers show better c-Si surface passivation due to the reduction of majority carriers by the carrier inversion (field-effect passivation) and chemical passivation. This effect is clearly reflected with the opposite trend in the carrier capture analysis from S-i-a-Si:H and MoOx layers.

Real-time monitoring of surface passivation of crystalline silicon during growth of amorphous and epitaxial silicon layer

Surface passivation of crystalline silicon (c-Si) is experimentally studied during the growth of a hydrogenated amorphous silicon (a-Si:H) and epitaxial silicon (epi-Si) passivation layer at a subnanometer to nanometer scale. The property of surface passivation is monitored in real time via in situ measurement of a photocurrent in c-Si under plasma-enhanced vapor deposition for the passivation layer growth. The measurement results suggest the following. Passivation is improved by the growth of an a-Si:H layer, where a large band offset is formed at the a-Si:H/c-Si interface, and the carrier recombination is suppressed. On the other hand, passivation is deteriorated with the growth of an ultrathin epi-Si layer (d≲2.5±1.0 nm) because the band offset is not formed at the interface, and plasma-induced defects are created in c-Si. However, passivation is improved with a thick epi-Si layer (d≳2.5±1.0 nm), where the band bending is formed near the epi-Si/c-Si interface, which partially suppresses the carrier recombination. The suppression of the plasma-induced defects as well as the formation of the band offset are important for surface passivation.

Titanium oxynitride films for surface passivation of crystalline silicon deposited by plasma-enhanced atomic layer deposition to improve electrical conductivity

For achieving improved electrical conductivity through surface passivation of crystalline silicon, we investigated TiON films by combining TiO2 and TiN deposition cycles in plasma-enhanced atomic layer deposition. To control the composition of the TiON films, a super-cycle—composed of one cycle of TiO2 and x-cycles of TiN—was adopted. The thickness of the films could be precisely controlled on the nanometer and sub-nanometer scale, regardless of the TiO2:TiN sub-cycle ratio. The chemical state, crystalline phase, and interface characteristics of the TiON films were examined. For the TiO2:TiN = 1:20 film, the carrier lifetime was increased from 30 to 243 μs, while the resistivity decreased from 3.1 × 108 to 7.1×10−1 Ω•cm compared to the TiO2 film.

Plasma-induced damage and annealing repairing in ALD-Al2O3/PECVD-SiNx stacks

We study the effect of plasma-enhanced chemical vapor deposition (PECVD) SiNx process to atomic layer deposited (ALD) Al2O3 films on crystalline silicon surface passivation. The plasma-induced damage on Al2O3 films is affected by the methods of PECVD process (direct or microwave), and the states (as-deposited or annealed) and the thickness of ALD-Al2O3 films. The passivation degradation may be related to the doping of Si, N and H atoms into Al2O3 films during PECVD process, and is mainly manifested in the form of affecting interface defect density rather than negative fixed charge density. In particular, the annealed thick Al2O3 films with direct-PECVD SiNx process show more severe passivation degradation. Anyhow, to the great degree, the passivation quality can be repaired and further improved by post-annealing. Low temperature long time annealing can effectively increase the lifetime of the ALD-Al2O3/PECVD-SiNx stacks, while it cannot be simply replaced by a rapid high temperature firing. By post-annealing repairing, an absolute 0.13% increase in efficiency can be achieved for p-PERC solar cells.

Surface passivation of crystalline silicon by intrinsic a-Si:H films deposited in remote low frequency inductively coupled plasma

Intrinsic hydrogenated amorphous silicon (a-Si:H) thin films was prepared in remote low-frequency inductively coupled plasma chemical vapour deposition (ICP-CVD) system for surface passivation on single crystalline silicon (c-Si). The influences of deposition temperature and precursor gas flow rate on the physical properties of a-Si:H thin films and the passivation effect of a-Si:H/c-Si interfaces were investigated. High quality a-Si:H thin films with less vacancies and defects were fabricated in remote ICP-CVD by eliminating the direct impacting of energetic plasma radicals on deposition surface. The passivation performance on c-Si were also improved in remote ICP-CVD, a minority carrier lifetime ~2.32 × 10−3 s was achieved by fabricating 10 nm a-Si:H passivation layer on c-Si substrate.

Roles of hydrogen atoms in p-type Poly-Si/SiOx passivation layer for crystalline silicon solar cell applications

Cyclic dehydrogenation-rehydrogenation experiments have been performed on p-type poly-Si/SiOx stack to study the roles of hydrogen (H) atoms for crystalline silicon (c-Si) surface passivation. The effective lifetime of minority carriers is enhanced to 1.4 ms by the hydrogenation of hydrogen plasma treatment (HPT) at 300 °C. The dehydrogenation by thermal annealing at 450 °C decreases the lifetime to ∼0.6 ms, which is recovered by a consecutive HPT, underlining the c-Si surface passivation by H atoms diffused into the SiOx/c-Si interface. The lifetime recovery follows a stretched exponential function, indicating the dispersive nature of the p-type poly-Si/SiOx stack.

In situ Photoluminescence Study of Plasma Effects on Passivation of Crystalline Silicon Coated with Aluminum Oxide

A degradation of crystalline silicon surface passivation provided by aluminum oxide (Al2O3) is generally observed after plasma processes, e.g., deposition of amorphous silicon nitride. To minimize such detrimental effect, a better understanding of the interaction between plasma species and the Al2O3 layer is required. Using in situ photoluminescence, the passivation quality of as‐deposited and annealed crystalline silicon wafers coated with Al2O3 grown by atomic layer deposition is characterized in real time during argon‐hydrogen plasma exposure. The photoluminescence intensity of as‐deposited samples instantly steps up after the plasma ignition, and then it gradually decreases as a function of plasma exposure time. However, only degradation of photoluminescence signal can be found if the samples are annealed prior to the plasma treatment. The interaction between vacuum UV light from plasma and different types of chemical bonds in the Al2O3 layer is proposed to explain the obtained results. Understanding the mechanisms and root cause leading to different behavior between as‐deposited and annealed samples under plasma exposure is a first step toward redesigning the process flow for better surface passivation.

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.

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.