Study of the poly-Si/c-Si interface recombination by lifetime spectroscopy

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Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) was used in this work to observe the local microstructure of a-Si:H and poly-Si layers (after deposition and after annealing, respectively) and the local homogeneity of the SiOx layer at the poly-Si/c-Si interface.
TEM is a microscopy technique in which a high-energy beam of electrons is transmitted through a very thin sample (typically 100 nm-thick). From the interactions between the electrons and the atoms, an image of the sample is formed. As the wavelength associated to electrons is much smaller than that associated to photons, the resolution attainable with TEM is higher than the one of a conventional optical microscope.
In this work, the Scanning TEM (STEM) technique was used. STEM enables to scan a very finely focused beam of electrons across the sample in a raster pattern and permits (unlike TEM) the use of other signals e.g. electron energy loss. The Electron Energy Loss Spectroscopy (EELS) coupled to STEM observations enables an additional chemical analysis of the sample. EELS analyzes the energy loss of transmitted electrons that experiment inelastic scattering when passing through the sample. It provides information about the interacting atoms, including elemental identity and chemical bonding. In this work, EELS was used to locate and map the oxygen concentration in the samples observed by STEM. Devoted STEM samples were fabricated from mirror-polished c-Si wafers. They were etched using a Focused Ion Beam (FIB) to obtain 80 nm-thick and 10 μm-large lamellas.

SiOx chemical composition: X-ray Photoelectron Spectroscopy

The chemical composition of SiOx layers was investigated using X-ray Photoelectron Spectroscopy (XPS). XPS enables to analyze the extreme surface of a sample (depth-analysis of ~5 nm) to provide information about the chemical elements and the nature of the chemical bonds that exist between these elements.
The sample is put under ultra-high vacuum and is irradiated with an X-ray source, which results in the emission of electrons from the sample. These electrons are collected through an electrostatic analyzer that measures their kinetic energy Ekin. The kinetic energy of the electrons emitted by the sample depends on the initial X-ray photon energy and on the element they were ejected from as well as its local chemical environment. Knowing the initial photon energy Eph, the binding energy EB of electrons in the sample can be determined from the following equation [111]: 𝐸𝐵=𝐸𝑝ℎ−𝐸𝑘𝑖𝑛, eq.
The resulting XPS spectrum (intensity signal as a function of the binding energy EB) features peaks at specific EB values corresponding to the elements contained in the sample. The intensity of the peaks provides information about the relative concentration of each element in the sample. A process of peak fitting of the XPS spectra can also provide additional information on the local chemical environment of the elements in the sub-surface region of the sample.
In this work, samples analyzed by XPS were made from mirror-polished c-Si wafers. They were cut with a diamond tip into squares of approximately 1×1 cm2 and fixed on the XPS sample holder with copper tape. XPS measurements were performed with a focused monochromatic Al Kα radiation. Spectra were calibrated using the contamination carbon C 1s emission at 285 eV. The XPS spectra were fitted using the CasaXPS software1. In-depth XPS analyses were also performed by alternating sputtering steps to etch the sample surface (using 1 keV Ar ions) and XPS measurements.

Electrochemical Capacitance-Voltage

The Electrochemical Capacitance-Voltage (ECV) technique enables to measure in-depth profiles of electrically active dopants in c-Si materials. The sample surface is put in contact with an NH4F/HF electrolyte contained in an electrochemical cell (see Figure 12). The resulting interface between the sample and the electrolyte is equivalent to a metal/semiconductor Schottky contact. The in-depth profile is obtained by alternating steps of surface etching and C-V measurements.

Electrical properties of the poly-Si layer: Hall effect technique

The free carrier density, conductivity and carrier mobility in poly-Si layers were assessed by an equipment combining the Hall effect and the van der Pauw techniques.
The Hall effect technique enables to estimate the free carrier density in a material. During the measurement, the sample is submitted to a current I and a magnetic field B with a direction perpendicular to the current flow (see Figure 13). In this configuration, the trajectory of free carriers is deviated due to a Lorentz force. It leads to the accumulation of electrons on one side of the sample and of holes on the opposite side. The potential difference resulting from this charge separation (referred to as the Hall voltage) is measured and enables to derive the free carrier density.

Lifetime measurements: Photo Conductance Decay

The Photo Conductance Decay (PCD) technique enables the determination of the effective minority carrier lifetime τeff for a wide range of excess charge carrier density Δn [114]. Under certain conditions, it can also provide information about the surface passivation quality of symmetrical samples (i.e. with similar passivation layers on both sides) through the determination of the implied open circuit voltage (iVoc) and the recombination current density at the surface (J0).
In this work, a WCT-120 Sinton tool was used (see Figure 14). During the measurement, a flash illumination is applied to generate extra electron-hole pairs in the sample, increasing its conductance. The light intensity and the conductance are both recorded over time through a reference cell and through a coil (part of an RF-bridge), respectively. The excess carrier density Δn is derived from the photo conductance Δσ (obtained from the subtraction of the conductance in the dark to the conductance under illumination) according to: Δ𝑛=Δ𝜎𝑞(𝜇𝑛+𝜇𝑝) ,

Transport of charge carriers in the poly-Si/SiOx contact: Conductive Atomic Force Microscopy

Conductive atomic force microscopy (C-AFM) is a mode of Atomic Force Microscopy (AFM) that enables to measure simultaneously the topography of the sample surface and a current flow in the sample. The AFM measurement performed in contact mode consists of scanning the sample surface with a tip attached to a cantilever. The force applied between the tip and the sample is kept constant while the deflection of the cantilever is measured with an optical system consisting of a laser and a photodiode (see Figure 17), resulting in a topography map of the sample surface. In C-AFM mode, an additional voltage is applied between the tip and a second contact placed on the sample. It enables to generate a current map associated to the topography map obtained by AFM.

Effect of the silane dilution in dihydrogen

The second way to reduce the blistering was to increase the SiH4 dilution in H2 (i.e. increase of the gas flow ratio R = H2/SiH4) during the deposition step, as reported by Nemeth et al in ref. [21]. The increase of R led to a reduction of the blister density after deposition and after annealing, from 1.5 mm-2 to 0.6 mm-2 for R from 6 to 50, respectively (see Figure 18, ②). The further increase of R to 65 enabled to obtain blister-free poly-Si(B) layers. However, the blistering mitigation was obtained at the expense of the deposition rate (see Figure 19a). Indeed, the decrease of the deposition rate likely enables the H atoms to diffuse toward the layer top surface during deposition, thus reducing the blistering phenomenon. Figure 19a also depicts iVoc as a function of the gas ratio R. For R = 25 to R = 50, an increase of iVoc of 90 mV was observed, likely resulting from blistering reduction. For higher R values (> 50), iVoc dropped by 20 mV. This drop was attributed to the excessive H2 concentration in the plasma that damaged the thin SiOx layer (either by etching [128] or by causing radiation induced charges in the SiOx layer [129,130]).
In conclusion, an optimal gas ratio of R = 50 was found to reduce the blister density (0.6 mm-2 for a 30 nm-thick poly-Si layer) and preserve decent iVoc values after annealing. The remaining blistering was avoided by decreasing the thickness of the poly-Si(B) from 30 nm down to 25 nm, which resulted in blister-free layers. Therefore, the optimal deposition conditions retained for the following studies on the in-situ doped poly-Si/SiOx contact are the followings: {Tdep = 300°C; R = 50; poly-Si thickness of 20-30 nm}.

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Evaluation of the poly-Si electrical properties through deposition optimizations

In this paragraph, we focus on the conductivity and carrier mobility of the poly-Si layer over Tdep and R = H2/SiH4 optimizations. The lateral conductivity and carrier mobility of 30 nm-thick poly-Si(B) layers were evaluated by Hall effect measurements as functions of Tdep and R, after annealing at Ta = 700°C (see Figure 19b). For the different Tdep and R conditions investigated Tdep= 300 CR = 25Tdep= 300 CR = 6Tdep= 220 CR = 6Tdep= 300 CR = 50Tdep= 300 CR ≥ 65100 μm100 μm100 μm100 μm100 μm100 μm100 μm100 μm100 μm100 μm12 in this work, we measured constant Hall carrier density of 1.1±0.2×1020 cm-3. Consequently, the carrier mobility dominated the poly-Si(B) conductivity trend (see Figure 19b). The poly-Si(B) average conductivity increased from 160 S∙cm-1 for {Tdep = 220°C; R = 6} to 220 S∙cm-1 for {Tdep = 300°C; R = 50}. This increase could result from the reduction of the poly-Si(B) blistering leading to a better lateral transport of charge carriers through the layer.

Effect of the blistering on the stability of surface passivation

To study the impact of blistering on the surface passivation stability over time, symmetrical samples were made from mirror-polished 4’’ c-Si wafers using two values of the PECVD gas ratio R = SiH4/H2 (= 25 or 50) and targeting two different poly-Si thicknesses (15 and 30 nm). It resulted in poly-Si(B) layers with different blister densities ranging from 0 to 1.1 mm-2 after annealing at Ta = 700°C. Figure 20 depicts optical microscope images of the resulting layers (see insets) and plots the iVoc measured right after samples processing, after 24 hours, and after 2 months of storage in the dark, under air. On Figure 20, one can observe the following trends:
▪ Samples featuring 30 nm-thick poly-Si layers deposited with R = 25 exhibited the highest blister density. As a result, they showed the lowest initial passivation level and poorest stability (the initial iVoc value of 600 mV decreased down to 540 mV after 2 months of storage).
▪ For R = 25, the reduction of the poly-Si thickness down to 15 nm led to a decrease of the blister density, enhancing the initial iVoc value (660 mV), but it did not solve the lack of stability previously observed as iVoc dropped to 620 mV after 2 months.
▪ For R = 50, samples with a 30 nm-thick poly-Si exhibited an initial iVoc value of 665 mV, which slightly decreased to 660 mV after 2 months.
▪ For R = 50, the reduction of the poly-Si thickness to 15 nm enabled to obtain not only the highest initial iVoc value (670 mV) but also an excellent stability of the carrier effective lifetime and iVoc after 2 months.

Table of contents :

General introduction
Part 1. State-of-the-art
I. Towards higher passivation level for c-Si solar cells
1. Working principle of conventional c-Si solar cells
2. Main loss mechanisms of a c-Si solar cell
3. Concept of full-area passivating contacts
II. Poly-Si contact fabrication and integration in solar cells
1. Brief historical review
2. Working principle of the poly-Si contact
3. Fabrication process of the poly-Si contact
a. Growth of the thin SiOx layer
b. Si-based layer deposition and doping
c. Annealing step
d. Hydrogenation step
4. Integration of the poly-Si contact in c-Si solar cells
a. Record efficiencies on small area solar cells
b. Towards industrial solar cells integrating the poly-Si contact
i. The metallization challenge
ii. Large area c-Si solar cells integrating the poly-Si contact
III. Current understanding of the passivation and transport mechanisms
1. Control of the final surface passivation
a. Growth of the SiOx layer
b. Role of doping
c. Optimization of the annealing temperature
d. Role of hydrogen
2. Transport of charge carriers through the SiOx layer
Part 2. Experimental
I. Fabrication process of B-doped poly-Si/SiOx contacts by PECVD
1. Si-based layers by Plasma-Enhanced Chemical Vapor Deposition (PECVD)
2. c-Si substrates and samples configuration
3. Fabrication of in-situ doped poly-Si/SiOx contacts
4. SiOxNy:B layers for ex-situ doping of the poly-Si/SiOx contacts
II. Characterization techniques
1. Structural and chemical characterization
a. Thickness and microstructure of the layers
i. Spectroscopic ellipsometry
ii. Transmission Electron Microscopy
b. SiOx chemical composition: X-ray Photoelectron Spectroscopy
c. Doping profiles in a-Si:H and poly-Si layers
i. Electrochemical Capacitance-Voltage
ii. Secondary Ions Mass Spectroscopy
2. Electronic characterizations
a. Electrical properties of the poly-Si layer: Hall effect technique
b. Lifetime measurements: Photo Conductance Decay
c. Defect characterization in c-Si: lifetime spectroscopy
i. SRH description of an electrically active defect center
ii. Lifetime spectroscopy methods
iii. Case of several SRH defect centers
iv. Bulk and surface contributions to the effective lifetime
v. SRH formalism at the surface
d. Transport of charge carriers in the poly-Si/SiOx contact: Conductive Atomic Force Microscopy
Part 3: Fabrication of the poly-Si/SiOx contact
I. Optimization of the in-situ doped poly-Si/SiOx contact
1. Optimization of the deposition conditions for blister-free poly-Si(B) layers
a. Effect of the deposition temperature increase
b. Effect of the silane dilution in dihydrogen
c. Evaluation of the poly-Si electrical properties through deposition optimizations
d. Effect of the blistering on the stability of surface passivation
2. Crystallization and doping of the poly-Si layer
a. Crystallization of the poly-Si layer
b. Doping of the poly-Si layer
3. Surface passivation properties of the in-situ doped poly-Si(B)/SiOx contact
a. Impact of the annealing temperature
b. Impact of the hydrogenation step
II. Ex-situ doping of the poly-Si/SiOx contact
1. Deposition of the intrinsic Si layer by PECVD
a. Decrease of the in-situ B-doping
b. Optimization of the intrinsic Si layer
2. Ex-situ doping
a. Si layer microstructure
b. Comparison of the two ex-situ doping processes
i. Electrical properties
ii. Passivation properties
c. Doping profile of the ex-situ doped poly-Si/SiOx contact
3. Comparison of in-situ and ex-situ doped poly-Si/SiOx contacts
a. Si-layer microstructure
i. After deposition
ii. After annealing
b. Electrical properties
c. Surface passivation properties
4. Effect of the doping density and hydrogen diffusion on surface passivation
a. Passivation stability upon annealing and SiN:H deposition
b. H diffusion profile in poly-Si(i) and poly-Si(B) contacts
c. Effect of the firing temperature
Part 4. Investigation of the poly-Si/SiOx interface
I. Chemical and structural evolution of the SiOx layer
1. SiOx grown at the c-Si surface
a. Ex-situ annealing in the tube furnace
b. In-situ annealing in the XPS chamber
c. Quantitative comparison of the XPS spectra after annealing
2. XPS study of the poly-Si/SiOx/c-Si stack
a. Qualitative study of the oxides upon annealing
b. Quantitative analysis
c. Structural evolution of the SiOx at the poly-Si/c-Si interface
3. Conclusions on XPS and TEM analysis of the interface
II. Investigation of the transport mechanism within the SiOx layer
1. Impact of the surface preparation of the sample on the C-AFM measurement
2. C-AFM investigation of the pinholes formation within the SiOx layer
a. Transversal measurements
i. With and without interfacial SiOx
ii. Comparison of chemical and thermal interfacial SiOx layers
b. Lateral C-AFM measurements
3. KPFM investigation of the pinholes formation within the SiOx layer
III. Study of the poly-Si/c-Si interface recombination by lifetime spectroscopy
1. Sample preparation
2. Extraction of the surface recombination velocity
3. Linearization of τsurf using Murphy’s approach
4. Lifetime spectroscopy for the determination of the defect parameters
General conclusion and perspectives
Appendix A: Correction of ECV profiles measured on ex-situ doped poly-Si contacts
Appendix B: Fitting procedure of the XPS Si 2p spectral range of a SiOx layer
List of publications
Résumé des travaux


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