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

Abstract

Phosphorus oxide (PO _x ) 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 PO _x /Al₂O₃ stacks are gained through a systematic study of the influence of deposition temperature (T _dep = 100-300 °C) and annealing temperature (T _ann = 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 (D _it), which is reduced at lower deposition temperature. The fixed charge density (Q _f) 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, D _it 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 PO _x layer, leading to the formation of AlPO₄ upon annealing.

Figure 1

An overview of c-Si surface passivation materials and stacks in terms of interface defect density (D_it) and fixed charge density (Q_f). D_it with corresponding Q_f are given for Al₂O₃,^,,− PO_x/Al₂O₃, SiN_x,⁻ SiO_x,⁻ SiO_x/SiN_x,^, Ga₂O₃,^, AlN, ZrO₂, and HfO_2.⁻ The data are divided into materials with negative (left) and positive (right) fixed charge on silicon. Other passivating materials, which are not shown because the magnitudes of D_it or Q_f have not been reported in the literature, include: Ta₂O₅ (negative Q_f), TiO₂ (negative Q_f), Nb₂O₅ (negative Q_f), and a-Si:H (very low D_it).

Figure 2

Maximum effective surface recombination velocity (S_eff,max), as a function of annealing temperature for PO_x/Al₂O₃ stacks on 280 Ωm-thick double-side-polished 3.3–3.7 Ω cm n-type FZ Si wafers with deposition temperatures ranging from 100 to 300 °C. Annealing was performed at consecutively higher annealing temperatures starting at 300 °C in steps of 50 °C up to 500 °C on the same samples for 10 min in N₂.

Figure 3

(a) Overall refractive index (λ = 589 nm), (b) overall mass density, (c) atomic hydrogen content, and (d) atomic concentrations of O, Al, and P in PO_x/Al₂O₃ stacks, as a function of deposition temperature, together with values for Al₂O₃ films from ref (52) (Refractive index, mass density, and hydrogen content only). Open symbols represent annealed films (400 °C in N₂ for 10 min) and closed symbols represent as-deposited films. The refractive index was obtained by spectroscopic ellipsometry, while the mass density and the compositional data were extracted from RBS and ERD.

Figure 4

Maximum effective surface recombination velocity S_eff,max as a function of deposited corona charge density for PO_x/Al₂O₃ stacks prepared at temperatures ranging from 100 to 300 °C. The peaks in S_eff,max represent the point at which the deposited corona charge has fully compensated the fixed charge (Q_f) in the layer. The fixed charge has the same magnitude as the deposited corona charge at the S_eff,max peak position, but has the opposite polarity.

Figure 5

Interface properties of PO_x/Al₂O₃ stacks deposited at various deposition temperatures annealed at 400 °C and an as-deposited stack deposited at 100 °C. (a) Fixed charge density (Q_f) as determined by CV measurements and corona-lifetime experiments. (b) Interface defect density (D_it) as determined by CV measurements and the peak value of the maximum surface recombination velocity S_{eff,max peak} determined from the data in Figure 4. This value is a measure for the surface defect density.

Figure 6

Cross-sectional bright-field TEM image of a PO_x/Al₂O₃ stack deposited at 100 °C on c-Si. The sample was annealed at 400 °C. SiO_x was deposited on top of the stack to protect the layers during the TEM lamella preparation by FIB.

Figure 7

Infrared spectra revealing oxygen-related bonding configurations of PO_x/Al₂O₃ stacks deposited at (a) 100 °C, (b) 200 °C, and (c) 300 °C on 280 Ωm-thick double-side polished 1–5 Ω cm n-type float zone (FZ) Si (100) wafers. The stacks were measured in the as-deposited state and after subsequent annealing (10 min in N₂) starting at 200 °C with 50 °C increments up to 500 °C, as indicated by the color bar.

Figure 8

ToF-SIMS measurements of PO_x/Al₂O₃ stacks deposited at 100 °C, in the as-deposited state and annealed at 400 °C. The intensity of the negative ions originating from the sample is plotted as a function of depth. The depth is calculated by assuming a constant sputter rate throughout all layers and a total stack thickness of 15 nm. The vertical dashed lines indicate the different layers in the stack, which consist of around 10 nm Al₂O₃, 5 nm PO_x, 1 nm SiO_x, and finally the Si substrate. A set of ions is shown in each panel: (a) hydrogen- and oxygen-related, (b) aluminum-related, (c) phosphorus-related, and (d) silicon-related ions. The intensities are normalized to the Si bulk signal (set to 1000).

Figure 9

Hydrogen effusion rate dN/dt as a function of temperature (heating rate 20 °C/min) for PO_x/Al₂O₃ stacks deposited at different temperatures. The measurements were done in vacuum, and the apparatus was baked out prior to the measurements to eliminate influences of background water on the measured hydrogen effusion spectra.