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All issues Volume 4 (2013) EPJ Photovolt., 4 (2013) 45107 Full HTML
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EPJ Photovolt.
Volume 4, 2013
Topical issue: Photovoltaic Technical Conference (PVTC 2012)
Article Number 45107
Number of page(s) 8
DOI https://doi.org/10.1051/epjpv/2013023
Published online 27 September 2013

© Kotipalli et al., published by EDP Sciences, 2013

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

It is well known that the thermal silicon oxide (SiO2) is a very good surface passivation material for crystalline Si (c-Si). However, the formation of thermal SiO2 requires a high-temperature process (>1000 °C) which does not only increase the processing cost, but may also degrade the quality of the silicon wafer. Therefore, passivation materials that can be deposited at low temperatures are required. Hydrogenated amorphous silicon nitride (a-SiNx:H) obtained by plasma-enhanced chemical vapour deposition (PECVD) is commonly used as front surface emitter passivation and anti-reflection coating (ARC) for both n- and p-type low-resistivity c-Si solar cells. This material can be deposited by PECVD at low temperatures i.e. less than 300 °C. Surface passivation of c-Si solar cell with atomic layer deposited (ALD) aluminum oxide (Al2O3) is a good candidate for both p- and n-type surfaces as well as highly-doped p-type emitters, due to its very high built-in negative fixed charge density (Qf ~ 1012−1013 cm-2), low interface state density (eV-1cm-2) and low surface recombination velocity (cm s-1). Best reported conditions were obtained by plasma-ALD process with film thickness ranging from 5 to 30 nm at deposition temperatures between 150−250 °C followed by an annealing step in nitrogen or forming gas atmosphere at 400 °C on low-resistivity Si wafers [1, 2, 3, 4, 5, 6].

To understand the involved surface passivation mechanism, Girisch et al. [7] introduced an extended Shockley-Read-Hall (SRH) formalism to model the surface recombination mechanism, which included the effects of band bending due to fixed insulator charges (Qf) and charged interface states (Dit = qNit) over the entire band gap. By assuming the case of single defect level at mid-gap, the surface recombination rate (Us) can be written as an energy independent quantity [3, 7, 8, 9, 10]. (1)where:

  • ns and ps are the surface concentrations of electrons and holes respectively.

  • SnSp are the surface recombination velocity parameters given by Sn = σnvthNit,Sp = σpvthNit.

  • σn,σp cross-section of electrons and holes respectively.

  • vth being the thermal velocity.

From the above equation (1) one can estimate the surface recombination velocity (SRV) at a particular injection level given by the excess carrier density(Δn). (2)Surface recombination rate (Us), can be reduced by altering two fundamental mechanisms:

  • (i)

    Reducing the interface state densities Dit (i.e. Chemical passivation) The Dit is dependent on material and chemical processes used in the fabrication of the solar cell. For example its reduction can be realized by diffusing hydrogen into the silicon/dielectric interface to replace the dangling bond defects.

  • (ii)

    Reducing the surface concentration of minority carriers (i.e. Field-effect passivation) The surface recombination rate (Us) can be reduced by decreasing one of two carrier concentrations at the silicon surface typically, the minority carrier concentration. This can be achieved by the fixed charges existing in the dielectric layer. Specifically, these charges creates a built-in electric field which shields the minority carrier to be recombined at the surface by driving the device into accumulation or inversion modes depending on the charge sign (positive or negative) and the chosen substrate type [3, 8].

In this article we study the surface passivation mechanism induced by negative fixed charges in Al2O3 films along with other fixed positive charge dielectrics. After a general introduction about the surface passivation mechanism, we describe the metal insulator semiconductor (MIS) capacitor device fabrication, lifetime sample preparations and electrical characterization techniques we used to extract the main parameters involved in the quality of the passivation, i.e. Qf, Dit and SRV in Section 2. Section 3 reports the experimental results of all the considered dielectrics. A discussion on the influence of films characteristics on the nature of interface passivation will be held. Subsequently, the impact of interfacial SiO2 layer on the field-effect passivation will be addressed in Section 4. Finally, in Section 5, conclusions are drawn from our Al2O3 passivation study of Si surfaces.

2 Samples fabrication and characterization techniques

It clearly appears from equations (1)−(2) that insight knowledge about the density of interface states (Dit) and surface concentrations of carrier ns, ps (depends on Qf) gives in-depth information about the passivation quality of dielectric films. To extract these values we have considered MIS capacitors as test vehicle.

2.1 Dielectric film deposition

Thermal SiO2 was grown up to a thickness of 20 nm at 1050 °C in an ultra-dry oxygen atmosphere using vertical furnace from Koyo Thermo Systems for a duration of 10 min.

For PECVD SiO2 20 nm-thick layers were deposited in a parallel plate reactor from Oxford Plasmalab system 100. The deposition parameters used during the film growth were: chamber pressure 0.8 Torr, deposition temperature 300 °C, gas flow: SiH4-500 sccm, N2O-20 sccm and O2-5 sccm. PECVD a-SiNx:H 20 nm-thick layers were also deposited using the same Plasmalab system 100. In this case, Silane (SiH4) and ammonia (NH3) were used as reactive gases in the chamber. During the deposition, SiH4 was diluted to 5% in pure nitrogen. The deposition parameters used for the film growth were: chamber pressure 0.8 Torr, deposition temperature 300 °C, radio frequency (RF) power 20 W, plasma frequency 13.56 MHz, gas flow: NH3-1.8 sccm, SiH4-10 sccm, and N2-700 sccm.

In the case of Al2O3, 15 nm-thick layers were deposited in a Fiji F200 ALD system from Cambridge NanoTech by thermal atomic layer deposition T-ALD and plasma enhanced atomic layer deposition PE-ALD. In both cases, trimethylaluminum precursor (TMA) from Sigma-Aldrich was used as aluminum source. Depositions were performed at 250 °C for both T-ALD and PE-ALD with argon as a carrier gas. Purge after precursor pulse is mandatory to avoid chemical vapor deposition. Before the deposition, all the samples were kept in the deposition chamber for 1800 s for pumping away H2O or O2. Each precursor flows separately through the deposition chamber. TMA pulse duration and purge time were 0.06 s and 10 s respectively for T-ALD and PE-ALD depositions. For T-ALD the precursor was de-ionized water while for PE-ALD, oxygen flow is used instead of water pulse. The flow was 30 sccm and plasma power was 300 W. The pulse duration and purge time of the plasma were 20 s and 5 s, respectively. The growth rates were observed to be 1 Å per cycle for both PE-ALD and T-ALD [11, 12, 13, 14, 15].

2.2 Device fabrication and C-V-G measurements

MIS capacitors were fabricated on p-type, Cz ⟨ 100 ⟩ silicon wafers with a resistivity of (1−3) Ωcm as illustrated in (Fig. 1).

thumbnail Fig. 1

Schematic of the metal insulator semiconductor (MIS) capacitor.

Before the deposition of the dielectric layers, all the wafers were cleaned using Piranha solution (3:1:H2SO4:H2O2, for a duration of 20 min at 120 °C). After the Piranha cleaning the wafers were dipped in dilute HF solution (1:50:HF:DI water) at room temperature to remove the native oxide. The etching of native oxide layer is confirmed by the appearance of hydrophobic Si surface. Next, dielectric layers described in the above Section 2.1 were grown or deposited. Then, gate electrodes with an active area of 1 mm2 were patterned on the front side of the samples using an image reversal lithography step (i.e. resist coating, pre-bake, image exposure with mask, post- exposure bake, flood exposure and development). A 300 nm Al layer was evaporated on the front side of the samples followed by a lift-off in acetone. After front side device fabrication, full-area aluminum back contact (300 nm) is evaporated on the backside of the wafers. Finally, all the samples were annealed in forming gas (N2/H2: 90/10%) ambient at 432 °C for 30 min.

Capacitance-voltage-conductance (C-V-G) measurements were performed with Agilent B1500A Semiconductor device analyzer at different frequencies ranging from 1 kHz to 1 MHz. To confidently extract the interface trap charge densities (Dit) at the silicon/dielectric interface we used different available extraction methods namely: High-Low frequency method, Terman method and Conductance method. The fixed charge density (Qf) in the dielectric was estimated from the flat-band voltage of the low-frequency C-V curve [16, 17, 18, 19, 20, 21].

2.3 Sample preparation for lifetime measurements

In our experiments, to emphasize the electronic properties of the dielectrics, we have chosen p-type, Boron doped, 200 μm thick double-side polished, ⟨ 111 ⟩, Float zone (FZ) silicon wafers in order to neglect the bulk lifetime in SRV extractions. A choose of high resistivity >5000 Ωcm wafers in particular is to avoid the band-gap defects induced by impurity dopants which acts like an effective recombination centers for SRH. Since the dopant impurity concentration sets the Fermi level, low-resistivity materials are more sensitive to these defects than high-resistivity material. So the dominant recombination in these samples is only due to Auger and radiative mechanisms. Lifetime measurements were performed using Sinton WCT-120 lifetime tester in both quasi steady state and transient modes. The different dielectric layers under consideration were symmetrically deposited on both sides of the wafers, followed by a forming gas annealing at 432 °C for 30 min to activate the passivation mechanism [22, 23].

3 Passivation analysis using different dielectrics

The C-V characteristics of MOS capacitors measured at 10 kHz for the different considered dielectrics are shown in Figure 2. The flat-band voltage (Vfb) of the low-frequency C-V curve allows calculation of the density and the polarity of charges present in the dielectric film from the following equation: (3)where Φms = −0.96V is the difference between the aluminum and silicon work functions.

The flat-band voltage (Vfb) of the MOS capacitors with the PECVD Si3N4, PECVD SiO2 and thermal SiO2 films is negative (i.e. lower than Φms), meaning these films contain fixed positive charges (Qf). For the T-ALD and PE-ALD Al2O3 films, the Vfb is positive and Qf is negative [16, 17, 18].

thumbnail Fig. 2

Normalized C-V Characteristics of different dielectrics at 10 kHz to extract the fixed charge density (Qf).

3.1 Interface states densities (Dit) extraction methodology

The extraction of interface state density (Dit) using only one method may suffer from parasitic effects such as high leakage currents through the dielectrics (thin) films, series resistance from the measurement setup and frequency-dispersion related problems in accumulation region. These parasitic effects will definitively alter the measured capacitance-conductance (C-G) values, which will in turn affect the interpreted Dit by up to an order of magnitude. To minimize the influence of these effects on the extracted interface states densities (Dit), all the measured C-V-G curves were first corrected for parasitic free C-V-G curves using “dual-frequency five-element small-signal circuit model” as shown in (Fig. 3) [19, 20, 21, 24].

thumbnail Fig. 3

Typical quasi-static (QSCV) and high-frequency (HF) C-V curves for a MIS capacitor with SiO2 dielectric. The HF curve is also represented after parasitic free (frequency-dispersion related problems in accumulation gate voltages) correction using “dual-frequency five-element small-signal circuit model”.

To assure that the extracted Dit values are correctly estimated over the entire band gap and not affected by parasitic effects, we extract the Dit using three different methods described below.

Figure 4 exemplifies the measurement results for the MIS capacitor with PECVD SiO2 dielectric as an example using the different extraction methods as follows [16, 17, 18, 19, 20, 21].

  • Figure 4b shows the Dit extraction using high-low frequency method, which compares the quasi-static C-V (QSCV) curve with a high frequency (1 MHz) C-V curve. In the QSCV measurement the interface traps are assumed to follow the slowly varying dc bias, contributing to interface trap charge capacitance (Cit). On the contrary, in the high-frequency C-V measurement the interface traps cannot follow the applied high-frequency ac signal, making the interface trap charge capacitance be zero (Cit = 0). The value of Cit can then be estimated by comparing the difference in capacitance between quasi-static and high frequency C-V curves from depletion – inversion regions (i.e. yielding Dit for mid-gap potentials) [20, 21].

  • Figure 4c illustrates the Dit extraction using Terman method, which is based on the stretch-out phenomenon in the experimental high-frequency C-V curve compared to theoretically simulated high-frequency C-V curve (i.e. ideal) with no interface traps. From the ideal C-V curve finding the surface potential (ϕs) for a given capacitance value in the depletion regime, and interpolating it on the experimental gate voltage (VG) curve gives us the (ϕs − VG) relation. Repeating this for other points from accumulation to inversion regimes results in a (ϕs − VG) curve. This ϕs versus VG curve is stretched-out when compared to theoretical curve without Dit, this stretch-out yields the information about the interface state densities [17, 19].

  • Figure 4d represents the Dit extraction using conductance method. This method is based on measuring the equivalent parallel conductance per unit area (Gp) as a function of bias voltage and frequency (ω). This equivalent parallel conductance represents the energy loss caused by capture and emission of carriers from the interface traps when gate bias is swept from accumulation to inversion regimes. Plotting with respect to frequencies in the depletion range of gate voltages yields the maximum of energy loss mechanism due to interface states. This peak (maximum energy loss) value of gives direct information on Dit [16, 17, 18].

thumbnail Fig. 4

PECVD-deposited SiO2 MOS Capacitor (C-V-G) curves after parasitic effects correction using dual-frequency five-element circuit model: (a) C-V curves at different frequencies, (b) Dit extraction using high-low frequency method, (c) Dit extraction using Terman method, (d) Dit extraction using conductance method.

Tables 1 and 2 summarize extracted results from C-V measurements on MIS capacitors: oxide capacitance in accumulation (Cox), flat band voltage (Vfb), fixed charge density (Qf) and interface trap charge density (Dit) using different methods. The range of Qf and Dit values are estimated considering variations of oxide thickness ±(0.1−2)nm and substrate resistivity (1−3) Ωcm.

Table 1

Summary of extracted values for fixed charge density (Qf).

Table 2

Extracted interface state density (Dit) from C-V measurements.

Table 1 highlights that the PECVD Si3N4 layer has a high density of positive charge ~4.2 × 1012cm-2 compared to other dielectrics. Al2O3 dielectric films exhibit negative fixed charge densities as high as −5.3 × 1012cm-2 and −2.3 × 1012cm-2 when deposited by PE-ALD and T-ALD respectively. In our experiments, Al2O3 deposited by PE-ALD exhibits more negative charges than T-ALD process. Table 2 summarizes the extracted Dit values using different methods. The relative differences can be related to the various specificities and sensitivity ranges of the different extraction methodologies, but also to the fact that the Dit values extracted from each method may not be extracted at the same gate voltage (i.e. depletion point). In addition, other errors could be due to non-uniform doping of the substrate, failure to obtain accurate 1 MHz high-frequency curves or inaccurate band bending during low-frequency measurements, which may affect the estimations. The Dit orders of magnitudes are correctly estimated and can be used as figures of merit to compare the different dielectrics. However, the Dit values extracted using conductance method on Al2O3 PE-ALD, T-ALD samples exhibit slightly higher values due to the asymmetry of the capture cross-sections σn/σp in these dielectrics, influencing the extractions using this method [16, 25, 26].

3.2 Carrier lifetime measurements

To enable carrier lifetime testing, the respective dielectric layers were deposited on both sides of a wafer to maintain symmetrical structures. Supporting the above extracted C-V-G parameters (Qf, Dit), minority carrier lifetime measurements (Fig. 5) show that thermally grown SiO2 leads to a good passivation quality independent of injection level, mainly due to very low Dit[8, 9]. The quality of surface passivation is indeed also examined by effective surface recombination velocity (Seff) calculated from the lifetime measurements. A common typical injection point of Δn = 5 × 1015 cm-3 has been chosen for Seff extraction for the comparison of the different dielectrics [8, 11, 12]. (4)where W is the thickness of the substrate.

Assuming a very high lifetime, thanks to the use of FZ wafers equation (4) can be simplified and the maximum Seffmax can be calculated by (5)while the lower limit is the case where no recombination occurs. In reality the value of S lies in-between (0 < S < Seffmax) depending on the chosen injection level (Δn)[8, 11, 12, 27, 28].

thumbnail Fig. 5

Injection-level-dependent effective lifetime measurements on different dielectrics layers.

Table 3 presents the surface recombination velocities (SRV) extracted from the lifetime measurements using equation (5) for 200 μm thick wafers covered with the different dielectrics. Samples with PE-ALD show the lowest SRV among all other dielectrics considered in this experiment with a value of less than 5 cm s-1 though it presented the highest Dit.

Table 3

Extracted τeff, Seff,max values from lifetime measurements at an injection point Δn = 5 × 1015 cm-3.

Our experimental results have shown that a good silicon surface passivation is achieved with ALD Al2O3 as dielectric film. This is attributed to a high density of negative fixed charges presented in the film, compensating the detrimental role of higher Dit.

The source of these negative fixed charges is attributed to the trapped hydroxyl groups in the film due to the deposition process [10, 14, 15, 29]. Another possible reason could be the presence of Al vacancies in the Al2O3 film or oxygen interstitials located at the Si/SiOX/Al2O3 interfaces [3, 8, 9, 11, 12, 30]. Al2O3 surface passivation can be described as a combination of both field-effect passivation and chemical passivation. Similar to other dielectrics, Al2O3 also chemically passivates the surface by releasing hydrogen atoms, which diffuse to the Si/dielectric interface to passivate the dangling bond defects. Apart from this, Al2O3 also serves the field-effect passivation mechanism due to its high fixed negative charge density, which is one-two orders of magnitude higher when compared to SiO2 deposited by PECVD and two orders magnitude higher than thermally-grown SiO2 (Tab. 1). This high amount of negative fixed charges in the overlying dielectric film drives the silicon surface into accumulation mode in case of p-type substrate creating a built-in E-field (electric field) at the surface shielding the minority carriers (here electrons) to recombine at the surface This difference in field passivation significantly relaxes the requirements on the interface defect density (Dit) at the c-Si/Al2O3 interface. Figure 5 clearly shows two separate groups of curves, two lower curves corresponding to SiO2-PECVD and Si3N4-PECVD and three others. The field effect is more effective in the low injection regime, whereas at high injection, while photo-generated excess charges compensate the fixed charges that induced the field effect, and mainly the “chemical passivation” is dominant [3, 4, 8, 9, 11, 12, 13, 24, 30, 31, 32, 33].

The shape of the τ(Δn) curves (Fig. 5) can reveal information regarding the involved interface passivation mechanism [34].

  • From Tables 1 and 2, PE-ALD and T-ALD Al2O3 layers have almost same level of chemical passivation i.e. interface defect density Dit ~(1−3) × 1011 cm-2 eV-1. Thus the difference in lifetimes behavior especially at lower injection range can be solely attributed due to the field passivation. This is mainly induced by fixed charges in dielectrics. Comparing Qf for the two Al2O3 deposition processes, PE-ALD (Qf ~ −5 × 1012 cm-2) is more efficient compared to T-ALD (Qf ~ −2 × 1012 cm-2).

  • PECVD-Si3N4 layers exhibit poor chemical passivation (Dit ~ 2 × 1011 cm-2 eV-1) in combination with a high density of positive Qf ~ 4 × 1012 cm-2. Even if the net charges is at the same level as Al2O3, the field effect passivation due to positively charged layers is less effective on p-type substrates resulting in a lower passivation quality. Accumulation mode caused by negative charges is more efficient than inversion caused by positive charges on p-type surfaces. This difference is probably due to the depletion layer beneath the inversion layer where ns and ps concentrations are at the same level.

  • Low fixed positive charges in PECVD-SiO2 and thermal dielectric layers leads to weak inversion or depletion mode for which the resulting field effect is not efficient. SiO2-thermal lifetime curves exhibit almost injection independent behavior meaning that the dominant passivation mechanism involved at the interface is the chemical passivation, i.e. lowest Dit ~ 2 × 1010 cm-2 eV-1.

4 Field effect passivation dependency on interfacial (SiO2) layer thickness

Kessels et al. [13] reported that a thin interfacial SiO2 (~1−2nm) layer is formed naturally between Si surface and Al2O3 layer. This thermal SiO2 (~1−2 nm) is the only means of chemical passivation at the interface. The thickness of this SiO2 is too thin to completely passivate the interface states. Another disadvantage of this interfacial oxide is that it may not have the same quality as SiO2 produced by thermal oxidation of Si. In some experiments we have introduced a stack of SiO2/Al2O3 on the Si surface. The main goal of this experiment is to chemically passivate the Si surface by thermally growing SiO2 and maintains field-effect passivation with negative charges present in Al2O3. To perform this and investigate trade-offs between concurrent Qf and Dit reductions, we have thermally-grown SiO2 layers with two different thicknesses: 8 and 20 nm [11, 13].

An Al2O3 film of 15 nm was deposited on SiO2 samples using PE-ALD. The deposition was also performed directly on the Si as a reference sample [3, 11, 12, 13, 24, 30, 31, 32, 33]. All these samples were treated in the same way as in the earlier experiments for both C-V-G and lifetime measurements except that the Al2O3 layer deposition was performed at 200 °C.

thumbnail Fig. 6

C-V characteristics of SiO2/Al2O3 stacks.

From SRV results, we observe that the SiO2 (8 nm)/Al2O3 (15 nm) stack exhibits the lowest SRV values compared to other samples. This can be correlated to negative Qf and low Dit extracted (Tab. 4) from C-V-G curves (Fig. 6).

Table 4

Parameters extraction on SiO2/Al2O3 stacks.

As reported by other authors [4, 13], the chemical passivation at the interface of Al2O3/Si occurs during the annealing step, when a very thin interfacial AlxSiOy layer is created in between the two materials. The formation of this layer is not well understood at this time as oxygen and hydrogen seem to play an important role, but consequences have been demonstrated and shown that electrically active interface traps were reduced. For the reference Al2O3 sample, the flat-band (Vfb) is positive and the density of negative fixed charges (Qf ~ 8.5 × 1012 cm-3) and the interface defect density (Dit ~ 2.7 × 1011eV-1 cm-2) are higher than any other sample in this experiment. However, an SRV of 9 cm s-1 is obtained from lifetime measurements meaning that the field effect passivation is predominant in this sample and slightly relaxes the requirement for lower Dit values.

For the sample with SiO2 (8 nm)/Al2O3 (15 nm) stack, the Vfb is negative but still exhibits negative fixed charges, however, twenty times lower than in the reference sample (only Al2O3). It is important to notice that the Dit also reduces by four times due to the presence of thermal-SiO2 layer, which reduces the defects at the interface and also the field induced by Al2O3 layer. This reduction of the effective field when using an SiO2 layer can be explained by the fact that the charge centroid is driven away from the silicon surface with increasing SiO2 thickness, as well as by the contribution of fixed positive charges in SiO2 layer resulting in overall reduction of net effective negative charge density. However, the trade-off between Qf and Dit obtained in these conditions leads to a much better effective lifetime at all injection levels. The difference is larger at low injection level where the field effect is the more efficient showing that 8 nm-SiO2 does not shield too much the Al2O3 charges [3, 4, 8, 9, 11, 12, 13, 24, 27, 28, 30, 31, 32, 33].

In the case of thicker SiO2 layer (20 nm), the “chemical interface” between Si and SiO2 is the same as thinner SiO2 (8 nm) layer. We can observe from Figure 7 that the effective lifetime is affected over the complete range of injection level. C-V measurements on this sample confirm that the silicon oxide layer has reduced the interface trap charge density (Dit ~ 2.1 × 1010) and also the fixed charges density (Qf ~ −1.9 × 1011). The effective lifetime is even lower than with Al2O3 layer alone and has returned to the values previously obtained for SiO2 only, meaning that field-effect passivation has been completely lost and the only means of passivation is due to chemical-passivation [10].

thumbnail Fig. 7

Lifetime measurements on SiO2/Al2O3 stacks.

5 Conclusion

Electronic properties (Qf, Dit) of different dielectrics were extracted. In addition, parasitic C-V-G corrections were applied to accurately estimate the interface trap charge density. Extracted parameters were discussed and compared with lifetime measurements to understand the passivation mechanisms involved at the interface.

In case of the PE-ALD Al2O3 layer, the extracted fixed charge density is negative and about −5.2 × 1012 cm-2, which provides an effective field-effect passivation for impeding the surface recombination of minority carriers. Interface trap charge density Dit has been calculated and is found to be about 3 × 1011 cm-2eV-1 as a mean value in the depletion gate voltage range. Such high negative fixed charge density resulted in surface recombination velocity less than 3 cm/s due to formation of accumulation regime at the silicon surface.

We have demonstrated the dependency of field-effect passivation on the thickness of SiO2 interfacial layer. From the C-V-G parameter extractions and lifetime measurements we concluded that an optimal thickness of SiO2 (here 8 nm) reduces the interface state densities while still maintaining field-effect passivation. Thick SiO2 layer reduces the net negative charge effect in the overall dielectric and may lead to a loss of the field-effect passivation.

We have observed that accumulation mode leads to better passivation than inversion mode. In our experiments, high negative fixed charges dielectrics were less sensitive to interface trap defects and exhibits better passivation behavior over all injection range. More generally, for all dielectrics, field-effect passivation mainly drives surface recombination at low injection, as chemical passivation is more predominant at higher level.

Acknowledgments

The authors thank the teams of the nanofabrication shared facility WINFAB and electrical characterization WELCOME platforms at UCL for their technical support. This work is supported by FRS-FNRS Belgium.

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Cite this article as: R. Kotipalli, R. Delamare, O. Poncelet, X. Tang, L.A. Francis, D. Flandre, Passivation effects of atomic-layer-deposited aluminum oxide, EPJ Photovoltaics 4, 45107 (2013).

All Tables

Table 1

Summary of extracted values for fixed charge density (Qf).

In the text
Table 2

Extracted interface state density (Dit) from C-V measurements.

In the text
Table 3

Extracted τeff, Seff,max values from lifetime measurements at an injection point Δn = 5 × 1015 cm-3.

In the text
Table 4

Parameters extraction on SiO2/Al2O3 stacks.

In the text

All Figures

thumbnail Fig. 1

Schematic of the metal insulator semiconductor (MIS) capacitor.
In the text
thumbnail Fig. 2

Normalized C-V Characteristics of different dielectrics at 10 kHz to extract the fixed charge density (Qf).
In the text
thumbnail Fig. 3

Typical quasi-static (QSCV) and high-frequency (HF) C-V curves for a MIS capacitor with SiO2 dielectric. The HF curve is also represented after parasitic free (frequency-dispersion related problems in accumulation gate voltages) correction using “dual-frequency five-element small-signal circuit model”.
In the text
thumbnail Fig. 4

PECVD-deposited SiO2 MOS Capacitor (C-V-G) curves after parasitic effects correction using dual-frequency five-element circuit model: (a) C-V curves at different frequencies, (b) Dit extraction using high-low frequency method, (c) Dit extraction using Terman method, (d) Dit extraction using conductance method.

In the text
thumbnail Fig. 5

Injection-level-dependent effective lifetime measurements on different dielectrics layers.
In the text
thumbnail Fig. 6

C-V characteristics of SiO2/Al2O3 stacks.
In the text
thumbnail Fig. 7

Lifetime measurements on SiO2/Al2O3 stacks.
In the text

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