High-quality double-side-polished 1 Ω·cm–5 Ω·cm FZ p-type silicon wafers were used as substrates. The wafers experienced a standard RCA cleaning and a 1-min oxide removal process in a 1-wt% HF solution prior to PECVD processing.

The SiO_yN_x films were deposited on silicon wafers in a PECVD system (PlasmaLab System133) from Oxford Instruments. Silane (SiH₄) and nitrous oxide (N₂O) were used as precursor gases. The microwave power and the pressure were maintained at a constant value of 47 mW/cm² and 500 mTorr (1 Torr = 1.33322 × 10² Pa), respectively. The deposition temperature varied from 35 °C to 400 °C. The R of SiH₄ and N₂O flow ratio, R, was controlled from 0.44 to 77.

After the deposition of SiO_yN_x film, the capping layer, a 70-nm SiN_x film, was deposited in the same PECVD chamber. Ammonia (NH₃) and SiH₄ were used as precursor gases. The microwave power, the pressure, and the deposition temperature were kept at 58 mW/cm², 800 mTorr, and 400 °C, respectively. The refractive index of the SiN_x layer, which was measured via spectroscopic ellipsometry (JobinYvon UVISEL), was 2.04 at wavelength λ = 630 nm.

After the deposition of the SiO_yN_x/SiN_x stack structure, the rapid thermal annealing (RTA) was performed on each of the samples at approximately 800 °C for 5 s. This process imitates the co-firing screen printing of Ag and Al at the end of the production of the silicon solar cells.

Metal–insulator–semiconductor (MIS) structures were prepared to perform the C–G–V characterization. For all kinds of samples, the aluminum contact of 100 nm was thermally evaporated to produce Al/Si/SiO_yN_x/SiN_x/Al device and Al/Si/SiN_x/Al as references. The diameter of the circular gate electrode was 3 mm. Multifrequency (10 kHz to 2 MHz) C–V and G–V measurements were performed with a Keithley 4200 SCS using the parallel model.^[13] All measurements were compensated for by a series resistance. The interface state density (D_it) in half the band gap could be extracted by combining the C–V and G–V tests. A noncontact C–V metrology (PV2000, Semilab) was used to investigate the energy distribution of the D_it in the band gap for post-RTA sample.^[14]

The effective minority carrier lifetime (τ_eff) of p-type silicon wafer that was double-side coated with the SiO_yN_x/SiN_x stacks was observed. A silicon wafer that was double-side coated with the single 100-nm-thick SiN_x film was used as reference. QSSPC measurements (Sinton Instruments lifetime tester) were performed with the optical constant as “1- reflection index” at an injection level of Δn = 7 × 10¹⁴ cm^-3. The effective surface recombination velocity (S_eff) can be calculated from^[15]

The multifrequency C–G–V characteristics of one of the post-RTA SiO_xN_y/SiN_x stack samples are shown in Fig. 1. The regions of accumulation, depletion, and inversion are clearly shown, thereby verifying a typical metal–oxide–semiconductor (MOS) behavior. No dependence on the frequency of C–V curve is observed in the accumulation because the the device structure is automatically compensated for by a series resistance during the measurement. However, the C–V curve is stretched out at the low frequency in depletion possibly because of the capacitive response of the interface states of dielectrics/Si to the measurement frequency.^[16] The distinctive peaks of the G/ω–V curves are shown in this region, thereby indicating the presence of interface-trapped charges.^[17] These distinctive peaks are correspondingly helpful to analyze the electrical responses of interface defect states to different frequencies. Moreover, at 40 kHz the G/ω–V curve reaches a highest G/ω–V peak, which is used for C–G–V measurement to obtain D_it information.

Fig. 1.

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	Fig. 1. Multifrequency (10 kHz to 1 MHz) C–G–V characteristics of an MIS capacitor utilizing post-RTA SiOyNx/SiNx stacks. (a) The C–V curves were measured in inversion–accumulation direction. (b) The peaks of G/ω–V curves lie in the range of depletion.

The varying N₂O/SiH₄ gas flow ratio (R) changes the Si, O, and N concentrations in the SiO_yN_x layer. The hysteresis formation of the SiO_yN_x (30 nm)/SiN_x (70 nm) stacks is measured using inversion–accumulation–inversion complete circles as shown in Fig. 2. From Figs. 2(a) to 2(e), the R varies from 77 to 0.44; R = 77 means that the film is basically the SiO_y layer measured via the Fourier transform infrared (FTIR) spectroscopy (not shown in this paper). The result of the single SiN_x (100 nm) layer (f) is also shown for reference.

Fig. 2.

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	Fig. 2. Compositions of the SiOyNx layer of the SiOyNx (30 nm) / SiNx (70 nm) stacks influence the hysteresis in C–V curves. The complete measurement circle is inversion–accumulation–inversion. In the SiOy/SiNx stacks, (a) R = 77. In the SiOyNx/SiNxstacks, (b) R = 11.11, (c) R = 1.54, (d) R = 0.83, and (e) R = 0.54. The reference is (f) the single SiNx (100 nm) layer.

The largest hysteresis is observed in the sample with only a single SiN_x layer at ΔV_FB = 13 V. For PECVD-prepared SiN_x film, the hysteresis of the Si-rich film with numerous Si dangling bonds is larger than that of the N-rich film.^[18] The SiO_yN_x films deposited with low R values of 0.44 and 0.83 each show a counter-clockwise hysteresis formation, which can be interpreted by the following hole-trapping mechanism:^[19] High-density hole trapping sites possibly exist at the interface. These trapping sites can be filled with majority carrier holes at the accumulation state when voltage sweeping is applied from inversion to accumulation. Therefore, the excess positive charge trapped at the interface can make the flat voltage (V_FB) shift to a negative applied bias voltage when voltage sweeping is applied from accumulation back to inversion. The V_FB is affected by the applied bias sweep range and by its history, and the injected charges generate hysteresis during the C–V measurements, thereby leading to the shift of the V_FB. For instance, the hysteresis effect of the single SiN_x layer is reduced by inserting a SiO_yN_x layer at R = 0.83, thereby making the ΔV_FB decrease to 2.5 V. This observation indicates the reduction of the trap state density existing in the stacks.

The hysteresis effect changes the feature from counter-clockwise to clockwise direction when R is increased up to a value of 1.54 (ΔV_FB = 1 V). The hysteresis effect leads to a counter-clockwise feature in SiN_x^[20] and a clockwise feature in SiO₂.^[21] The former indicates the hole trapping at the negative bias, whereas the latter shows the electron trapping. This observation implies that the increase of R results in the change of the charge-trapping center in stacks from the nitrogen-related (similar to SiN_x films) to the oxygen-related silicon dangling bonds (similar SiO₂ films).

The R also affects the energy distribution of the D_it in the band gap. The variations of measured D_it with energy level in the band gap of post-RTA SiO_yN_x (10 nm)/SiN_x (70 nm) stacks for different N₂O:SiH₄ ratios are shown in Fig. 3. The D_it distribution curves each have a nearly symmetrical U shape, and the minimum value is reached near the mid-gap. For stacks, the minimum value of D_it in the mid-gap rises as the R increases from 6.5 × 10¹¹ eV⁻¹·cm⁻² at R = 0.44 to 7.9 × 10¹¹ eV⁻¹·cm⁻² at R = 77. The D_it of the single SiN_x layer in the mid-gap is 2.4 × 10¹² eV⁻¹·cm⁻², which is one order of magnitude higher than that of SiN_x layer in the stacks. The N–H bonds in the SiO_yN_x films are considerably less than those in the SiN_x films. However, the increase of R (i.e., more oxygen content is introduced into the SiO_yN_x film) results in the increase of the N–H bonds with the decrease of Si–H bonds.^[20] Previous results show that the N–H bonds in films are inferior to Si–H bonds for silicon surface passivation.^[22] Consequently, the more oxygen content incorporated in the SiO_yN_x layer significantly affects the passivation quality of the interface. If a small oxygen content is introduced into the dielectric layer, low N–H bond density results in decreases of D_it and Q_f compared with the case of less oxygen content incorporated into the single SiN_x layer.^[22] However, an increased oxygen content is incorporated into the dielectric layer; thus, the decrease of the Si–H bond density and the increase of the N-H bond density result in forming numerous oxygen-related silicon dangling bonds, such as N≡Si·, N₂O≡Si·, NO₂≡Si·, and O≡Si·.^[23]

Fig. 3.

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	Fig. 3. Energy distribution of Dit in the band gap for the post-RTA SiOyNx (10 nm)/SiNx (70 nm) stacks with the single SiNx (100 nm) layer as reference. The SiOyNx layers are deposited at different R values with the deposition temperature of 130 °C.

The effect of the deposition temperature on the electrical properties of the interface between the SiO_yN_x/SiN_x stacks and the Si substrate is studied. The D_it and Q_f of the stacks and the single SiN_x layer-coated silicon wafers are summarized in Fig. 4. The D_it of the SiO_yN_x/SiN_x stacks increases with the increase of deposition temperature: it increases from 2.3 × 10¹⁰ eV⁻¹·cm⁻² at 100 °C to 2.9 × 10¹¹ eV⁻¹·cm⁻² at 400 °C. The D_it of the single SiN_x layer is 4.7 × 10¹¹ eV⁻¹·cm⁻² in a typical range from 1 × 10¹¹ eV⁻¹·cm⁻² to 5 × 10¹² eV⁻¹·cm⁻², which was reported by Aberle.^[24] The Q_f of the SiO_yN_x/SiN_x stacks rises with deposition temperature increasing from 1 × 10¹² cm⁻² at 100 °C to 2.1 × 10¹² cm⁻² at 400 °C. The Q_f of the single SiN_x layer is 2.5 × 10¹² cm⁻², which is larger than that in the SiO_yN_x/SiN_x stacks. In their stable form N₃≡Si⁺ (K⁺ centers), the K centers contribute primarily to the fixed positive charges in the SiN_x films.^[25] These fixed positive charges are situated in a 20-nm interface layer between the dielectric and the Si substrate.^[23] The stacks present lower Q_f and D_it than the reference sample (100-nm SiN_x-coated silicon). This observation illustrates that the reduction of the structure defect at the SiO_yN_x/Si interface or in film results from the effect of O atom incorporation during deposition and annealing.

Fig. 4.

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	Fig. 4. Summary of the electrical parameters of the SiOyNx (30 nm)/SiNx (70 nm) stacks, including Qf and Dit, with the single SiNx (100 nm) film as reference. The SiOyNx layers are deposited at different deposition temperatures of 100 °C–400 °C, with R = 1.54.

The S_eff of the SiO_yN_x (30 nm)/SiN_x (70 nm) stacks is obtained through the QSSPC measurement at an injection level of Δn = 7 × 10¹⁴ cm⁻³. The effect of the N₂O/SiH₄ gas flow ratio (R) is presented in Fig. 5 using the single SiN_x (80 nm) layer as reference. The S_eff of the single SiN_x layer is 71 cm/s. For SiO_yN_x/SiN_x stacks, the S_eff increases from 6 cm/s to 700 cm/s as R increases from 0.44 to 77. In addition, the S_eff is approximately 1000 cm/s on the entire surface of an “O-rich” sample, for instance, when R is above 10. The highest S_eff = 2360 cm/s is obtained at R = 15.56. In this condition, the film is basically the SiO_x layer measured by FTIR spectroscopy (not shown in this paper). In a range of R = 0.44–1.54 (refractive index: 2.1–1.6), the S_eff increases from 6 cm/s at R = 0.44 to 35 cm/s at R = 1.54. All values are lower than those in the cases of single SiN_x coating. Therefore, good passivation properties are clearly obtained when the SiO_yN_x layer, not the SiO_x, is introduced as the passivation layer.

Fig. 5.

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	Fig. 5. Seff of the post-RTA SiOyNx (30 nm)/SiNx (70 nm) stacks at an injection level of Δn = 7 × 1014 cm−3 through QSSPC with the single SiNx (80 nm) layer as reference. The SiOyNx layer is deposited at different R values with the deposition temperature of 300 °C.

The effect of the deposition temperature is presented in Fig. 6. The S_eff increases from 6 cm/s to 70 cm/s as deposition temperature increases from 100 °C to 400 °C for sample deposited with R = 1.54. Good passivation properties are clearly obtained when the SiO_yN_x deposition is conducted at a low temperature. Numerous Si–O bonds are introduced into the films as the deposition temperature increases,^[26] thereby creating many oxygen-related defects at the interface. The total H content decreases when the deposition temperature increases, thereby resulting in many non-passivated defects in the film and at the interface, and leading to the increases of Q_f and D_it (as shown in Fig. 4). Figure 4 shows that the D_it significantly increases as deposition temperature increases; however, Q_f exhibits a slight change: it increases from 1 × 10¹² cm⁻² when the deposition temperature is 100 °C to 2.1 × 10¹² cm⁻² when the deposition temperature is 400 °C. The combination of the relationship among S_eff, D_it, and Q_f with the deposition temperature shows that the surface passivation is influenced mainly by the interface state (chemical passivation). High Q_f provides a certain degree of field-effect passivation; however, high D_it induces the increase of S_eff with deposition temperature rising because of poor chemical passivation. According to the preceding result, the optimized deposition parameter for the SiO_yN_x film is obtained at a low deposition temperature and a low ratio of N₂O/SiH₄ flow rate.

Fig. 6.

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	Fig. 6. Seff of the post-RTA SiOyNx (30 nm)/SiNx (70 nm) stacks at a corona charge injection level of Δn = 7 × 1014 cm−3 through QSSPC. The SiOyNx layer is deposited at different deposition temperatures, with R = 1.54.