Passivation mechanism of thermal atomic layer-deposited Al2O3 films on silicon at different annealing temperatures
© Zhao et al.; licensee Springer. 2013
Received: 14 November 2012
Accepted: 20 February 2013
Published: 2 March 2013
Thermal atomic layer-deposited (ALD) aluminum oxide (Al2O3) acquires high negative fixed charge density (Qf) and sufficiently low interface trap density after annealing, which enables excellent surface passivation for crystalline silicon. Qf can be controlled by varying the annealing temperatures. In this study, the effect of the annealing temperature of thermal ALD Al2O3 films on p-type Czochralski silicon wafers was investigated. Corona charging measurements revealed that the Qf obtained at 300°C did not significantly affect passivation. The interface-trapping density markedly increased at high annealing temperature (>600°C) and degraded the surface passivation even at a high Qf. Negatively charged or neutral vacancies were found in the samples annealed at 300°C, 500°C, and 750°C using positron annihilation techniques. The Al defect density in the bulk film and the vacancy density near the SiO x /Si interface region decreased with increased temperature. Measurement results of Qf proved that the Al vacancy of the bulk film may not be related to Qf. The defect density in the SiO x region affected the chemical passivation, but other factors may dominantly influence chemical passivation at 750°C.
Excellent surface passivation is required to realize the next-generation industrial silicon solar cells with high efficiencies (>20%). Silicon oxide films thermally grown at very high temperatures (>900°C) are generally used to suppress the surface recombination velocities (SRVs) to as low as 10 cm/s and applied in front- and rear-passivated solar cells. In recent years, atomic layer-deposited (ALD) aluminum oxide (Al2O3) thin films have been investigated as candidate surface passivation materials [1–3]. ALD Al2O3 thin films enable perfect passivation similar to high-quality thermally grown silicon oxide and can be prepared at low temperatures (<300°C). Given that the silicon bulk lifetime is sensitive to high temperatures, ALD Al2O3 has a natural advantage over thermal SiO2 in terms of integration into industrial cell processes. Extensive experiments on Al2O3 film applications in photovoltaics have demonstrated that Al2O3 can passivate both low-doped n- and p-type silicons. ALD Al2O3 also exerts a better passivation effect on p+-type emitters than other dielectric layers. Very recently, Hoex et al.  found that Al2O3 can also enable high-surface passivation for n+-type emitters within the range of 10 to 100 Ω/sq.
Low SRVs for dielectric passivation are attributed to two passivation mechanisms: chemical passivation and field-effect passivation [5, 6]. Chemical passivation (e.g., thermal SiO2 films) decreases the interface defect density (Dit). In dielectric layers such as SiN x and Al2O3, a high fixed charge density (Qf) near the silicon surface generates an electric field, repelling electrons or holes to reduce carrier recombination on the surface. Thermal ALD Al2O3 reportedly acquires a negative Qf as high as 1013 cm-2 with sufficiently low Dit (about 1011 eV-1 cm-2) after annealing [7, 8]. Experiments have shown that the fixed charge located near the Al2O3/Si interface is related to some types of defect proposed as Al vacancies, interstitial O, and interstitial H in Al2O3 film or at the interface . Positron annihilation is a useful technique for vacancy-type defect investigation. Edwardson et al.  performed Doppler broadening of annihilation radiation (DBAR) studies and found an interface that traps positrons in an ALD Al2O3 sample, which significantly differed from the S-W result of DBAR in the current work. The discrepancy can be attributed to the different annealing conditions.
In the present study, the effect of annealing temperature on the surface passivation characteristics of Al2O3 films was investigated. Corona charging experiments were performed to distinguish between chemical and field-effect passivation mechanisms. Slow positron beam DBAR measurements were performed to probe the defects in Al2O3 films annealed at 300°C, 500°C, and 750°C.
Aluminum oxide films were deposited onto a 1 to 10 Ωcm p-type Czochralski Si (100) substrate using the thermal ALD method. The 420-μm-thick double-sided polished wafers were cleaned using the RCA standard method and dipped in 1% hydrofluoric acid for 1 min before deposition to remove the native oxide layer on the surface. Thermal ALD Al2O3 films about 23 nm thick were prepared with Al(CH3)3 and H2O as reactants at 250°C. The optimum deposition temperature that led to the highest as-deposited effective lifetime was determined to be 250°C. Double faces were deposited to prepare symmetrical Al2O3/Si/Al2O3. After deposition, the samples were annealed at different temperatures (300°C to 750°C) for 10 min in air. Annealing in air was performed because it closely resembles the firing condition in the manufacturing process of solar cells. The effective lifetimes of these samples were measured before and after annealing, and a negative Qf of the Al2O3 films was obtained using corona charging measurements using Semilab WT2000 (Semilab Semiconductor Physics Laboratory Co. Ltd., Budapest, Hungary). DBAR measurements of the three annealed samples (300°C, 500°C, and 750°C) were performed to investigate the defects in the films. A slow beam of positrons that had variable energies (<10 keV) was used to obtain information from the thin films.
Corona charging measurement
Positron annihilation is used to analyze defects in oxides and semiconductors [11–13]. When a positron is implanted into a matter, it annihilates an electron and emits two γ rays. The energy of γ rays varies around 511 keV because of the energy and momentum conservation of the positron-electron system given by the relation Eγ = 511 ± ΔEγ keV, where ΔEγ is the Doppler shift. Even a slight change in momentum can lead to a large shift of energy. The S and W parameters were calculated to characterize Doppler broadening. The S parameter is defined as the ratio of the mid-portion area to the entire spectrum area. The W parameter is the ratio of the wing portion to the entire area. With increased concentration of vacancy in solid, the positron is mostly trapped and annihilates low-momentum electrons, leading to a narrow Doppler peak with a high S parameter. W parameters are higher and S parameters are lower when annihilation of the core electrons of atoms occurs. Given that the momentum distribution of electrons varies in different types of defect, changes in S-W plots can also characterize the types and distributions of defects in the films .
Results and discussion
Influence of annealing temperature on surface passivation
Notably, Qf reached 1012 cm-2 after annealing at 750°C, and this value was almost one magnitude higher than that of the as-deposited sample. However, the effective lifetime was low (Figure 2) because of the poor chemical passivation at 750°C in Figure 3b of the minimum lifetime change value. Therefore, chemical passivation was a prerequisite in achieving excellent surface passivation.
DBAR analysis at different annealing temperatures
DBAR analysis was performed at the Beijing Slow Positron Beam (Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China). A positron beam generated from a Na22 radioactive source was used, and the energy of the positrons was modulated between 0 and 10 keV to obtain the incident energy profile of positron annihilation. The energy region of the S parameter ranged from 510.24 to 511.76 keV, whereas the W parameter ranged from 504.2 to 508.4 and from 513.6 to 517.8 keV. Thus, the total energy region of the peak ranged from 504.2 to 517.8 keV.
The S and W parameters of the same incident energy were plotted in one graph, as shown in Figure 5c. The S vs. W diagrams of monolithic materials present clusters of points because all S or W parameters are almost the same . For example, in one type of defect, the S and W parameters may vary with the positron incident energy, and the S-W plot extends to the line passing the data point of the bulk region without defect [13, 14]. The slope of the line changes with the layers of different compositions and defect types. Thus, the annealed sample consisted of a three-layered structure in which each curve consisted of three extended line segments (Figure 5c). This finding corresponded with the S-E curve analysis result, which also suggested that the film contained a layer different from Al2O3 and Si near the interface. No significant interface response in the S-W result has been previously observed , and the discrepancy may be a result of the different annealing environments (air vs. N2). Annealing in air may lead to a thicker interface oxide (SiO x ) resulting in more evident responses in the DBRA result.
The different slopes of the Al2O3 segment of the three samples indicated that the defect types or chemical environments of these samples were different. The three lines crossed one another to avoid passing through a single point of bulk sample without defects, indicating that each of the samples had more than two types of defect. As mentioned in the section ‘DBAR analysis at different annealing temperatures,’ the S parameter was mainly influenced by Al and neutral O vacancies. Thus, residual C during deposition and O-H bond content also possibly influenced the S-W line slope. Residual C varied with the annealing temperature and may have thus influenced the environment of Al vacancies, although further investigations are needed.
Al vacancies, O interstitials, and H interstitials are proposed as the reasons for the negative Qf of Al2O3[23, 24]. The measured Qf in Figure 3 and information on Al vacancies in Figure 7 were considered in analyzing the effect of Al vacancy density on the negative fixed charge Qf. With increased annealing temperature from 300°C to 500°C, the increase in Qf was opposite to the decrease in Al vacancy in the bulk film. Thus, Qf may not be related with Al vacancies in the Al2O3 films. The measured minimum effective lifetime in Figure 3 and S parameters of SiO x interface in Figure 7 were correlated, and the decrease in vacancy of SiO x was coincident with the enhanced chemical passivation at annealing temperatures lower than 500°C. However, the chemical passivation breakdown at 750°C cannot be explained: among the samples annealed at 300°C and 750°C, the chemical passivation at 750°C was the poorest, but the defect density at the interface region still decreased. The functions of interstitial atoms (O or H) near the interface require further investigation.
Qf did not significantly affect the passivation at a low annealing temperature (300°C). The interface trap density markedly increased at a high annealing temperature (750°C) and failed at surface passivation even at a high Qf. Positron annihilation techniques were used to probe the vacancy-type defects. A three-layered microstructure of thermal ALD Al2O3 films on Si substrate was found. The Al defect density in the bulk film and the vacancy density near the interface decreased with increased temperature based on the fitted S parameter at different positions in the Al2O3 films. The Al vacancy of the bulk film was not related to Qf based on the Qf measurement results. The effects of interstitial atoms on Qf need further investigation. The defect density in the SiO x region may affect chemical passivation, but other factors may also influence chemical passivation particularly beyond 500°C.
This study was supported by the National High Technology Research and Development Program of China (grant no. 2011AA050515) and the National Basic Research Program of China (grant no. 2012CB934204). The authors are grateful to Dr. Cao for the DBAR measurements at the Beijing Slow Positron Beam, Institute of High Energy Physics, Chinese Academy of Sciences.
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