Wet Chemical Oxidation to Improve Interfacial Properties of Al₂O₃/Si and Interface Analysis of Al₂O₃/SiO_x/Si Structure Using Surface Carrier Lifetime Simulation and Capacitance–Voltage Measurement

Abstract

A thin silicon oxide (SiO_x) layer (thickness: 1.5–2.0 nm) formed at an Al₂O₃/Si interface can enhance the interface properties. However, it is challenging to control the characteristics of thin SiO_x layers because SiO_x forms naturally during Al₂O₃ deposition on Si substrates. In this study, a ~1.5 nm-thick SiO_x layer was inserted between Al₂O₃ and Si substrates by wet chemical oxidation to improve the passivation properties. The acidic solutions used for wet chemical oxidation were HCl:H₂O₂:H₂O, H₂SO₄:H₂O₂:H₂O, and HNO₃. The thicknesses of SiO_x layers formed in the acidic solutions were ~1.48, ~1.32, and ~1.50 nm for SiO_x-HCl, SiO_x-H2SO4, and SiO_x-HNO3, respectively. The leakage current characteristics of SiO_x-HNO3 were better than those of the oxide layers formed in the other acidic solutions. After depositing a ~10 nm-thick Al₂O₃ on an SiO_x-acidic/Si structure, we measured the effective carrier lifetime using quasi steady-state photoconductance and examined the interfacial properties of Al₂O₃/SiO_x-acidic/Si using surface carrier lifetime simulation and capacitance–voltage measurement. The effective carrier lifetime of Al₂O₃/SiO_x-HNO3/Si was relatively high (~400 μs), resulting from the low surface defect density (2.35–2.88 × 10¹⁰ cm⁻²eV⁻¹). The oxide layer inserted between Al₂O₃ and Si substrates by wet chemical oxidation helped improve the Al₂O₃/Si interface properties.

1. Introduction

The surface passivation of crystalline silicon (c-Si) solar cells can be improved using various materials such as SiO₂ [1,2,3,4,5,6], SiN_x [7,8,9], Al₂O₃ [10,11,12,13], TiO_x [14,15,16], MoO_x [17,18], and poly-Si [19,20,21,22]. In particular, Al₂O₃ thin films are most widely used for boron-doped Si surfaces (or p⁺ emitter surfaces) owing to the low surface recombination velocity (SRV) [10,11]. The interface of Al₂O₃ films can be characterized by a low interface trap density and a high negative charge density because of the ultrathin SiO_x and negatively charged interstitial O ions present at the Al₂O₃/Si interface [23,24]. The passivation properties of an Al₂O₃/Si structure can be improved by optimizing the Al₂O₃ thickness and annealing temperature [25,26]. In particular, a thin SiO_x layer is formed at the Al₂O₃/Si interface during the deposition process and can be activated through annealing. However, it is difficult to control the quality of this layer because it forms spontaneously.

In this work, we considered a method to improve the interfacial properties of an Al₂O₃/Si structure by inserting a thin silicon oxide layer between Al₂O₃ and Si substrates. The widely used methods of growing thin silicon oxides are thermal and wet chemical oxidations. In thermal oxidation, although the quality of the silicon oxide formed is excellent, it is difficult to control the thickness (<1.5 nm) at temperatures above 800 °C. If the silicon oxide layer becomes thicker, the field effect passivation of the Al₂O₃/Si interface may deteriorate [27]. Therefore, a thickness of ~1.5 nm or less is required to improve the interfacial properties of the Al₂O₃/Si structure and ensure a good field effect passivation. Wet chemical oxidation is a promising method of growing thin SiO_x, and the quality of thin oxides grown using this method has been verified [28,29,30,31,32]. In this research, a thin oxide layer was utilized to improve the passivation property of an Al₂O₃/Si structure. We applied wet chemical oxidation to grow ~1.5 nm-thick SiO_x and then deposited a ~10 nm-thick Al₂O₃ using plasma-assisted atomic layer deposition (PA-ALD). To analyze the passivation characteristics and interface properties of the Al₂O₃/SiO_x/Si structure, the effective carrier lifetime was measured using quasi steady state photoconductance (QSSPC), and surface carrier lifetime simulation and capacitance–voltage (C–V) measurements were performed.

2. Experimental

After the Radio Corporation of America (RCA) cleaning developed by Werner Kern to remove ionic and organic impurities in the polished 4~5 Ω∙cm p-type Si(100) substrate [33], we performed wet chemical oxidation using three acidic solutions: (i) HCl:H₂O₂:H₂O = 1:1:5, (ii) H₂SO₄:H₂O₂:H₂O = 1:1:5, and (iii) HNO₃ (68%) [28,29,30]. The wet chemical oxide layers formed in the acidic solutions are hereinafter abbreviated as SiO_x-HCl, SiO_x-H2SO4, and SiO_x-HNO3, respectively. The process temperature was 85 °C for SiO_x-HCl and SiO_x-H2SO4 and 121 °C for SiO_x-HNO3. The immersion time was varied from 10 to 60 min. The thickness and refractive index of the silicon oxide (SiO_x-acidic) layers formed on Si surface was measured using spectroscopy ellipsometry (SE) and transmission electron microscopy (TEM). The quality of SiO_x-acidic was evaluated in terms of the leakage current density, which was measured by conducting a current–voltage (I–V) analysis with mercury probe and Keithley 238 current source meter under dark conditions. To measure the surface passivation characteristics, an approximately 10 nm-thick Al₂O₃ film was deposited on a SiO_x-acidic/Si substrate using PA-ALD. The reaction sources for Al₂O₃ deposition were trimethylaluminum (Al(CH₃)₃, TMA) and O₂ (purity 99.999%) gas; the purge gas was Ar (purity 99.999%). The deposition process was performed at a substrate temperature of 250 °C, a process pressure of 1.0 torr, a plasma power of 200 W, an O₂ exposure time of 0.5 s, and a distance of 20 mm between the showerhead and the substrate. Annealing was then conducted in an electric furnace at 425 °C for 15 min to activate the Al₂O₃ layer. After annealing, the carrier lifetimes of the samples were measured using QSSPC to evaluate the surface passivation characteristics. For a detailed analysis of the interface properties of Al₂O₃/SiO_x-acidic/Si, a surface carrier lifetime simulation and C–V measurement were performed by mercury probe with Agilent E4980A LCR meter.

3. Results and Discussions

Figure 1 shows the thickness of the silicon oxide (SiO_x-acidic) formed in the acidic solutions with various immersion times. The film thickness was measured by SE. The measured wavelength range was 300–100 nm and the incident beam angle was 75° in the SE measurement. With the increase in the immersion time, the thickness increased, saturating in the range of 1.32–1.50 nm. The thicknesses of SiO_x are ~1.48 nm for SiO_x-HCl, ~1.32 nm for SiO_x-H2SO4, and ~1.50 nm for SiO_x-HNO3. These results are similar to those reported previously [28,29,30]. The thickness of SiO_x-HNO3 was also confirmed by TEM measurement, as shown in Figure 2. In the TEM measurements, silicon nitride (SiN_x) was used as the capping layer to avoid additional thin oxide growth in the Al₂O₃ deposition process. The SiO_x-HNO3 thickness was in the range of 1.43–1.54 nm, consistent with SE measurements. As the thicknesses of SiO_x formed in each acidic solution saturated after approximately 15 min, the immersion time was fixed at 15 min (

Table 1. SiO_x layer thickness and refractive index (n) at an immersion time of 15 min. The data are averaged with ten samples.

Solution	Thickness (nm)	Refractive Index (n) at 630 nm
HCl	1.49	1.421
H2SO4	1.32	1.417
HNO3	1.50	1.430

). To evaluate the characteristics of the SiO_x-acidic layer, the leakage current densities were measured by I–V curves under dark conditions. As shown in Figure 3, the leakage current density of SiO_x-HNO3 was the lowest (~9.1 × 10⁻³ A/cm² at 1 V_forward-bias and ~0.92 × 10⁻³ A/cm² at −1 V_reverse-bias), indicating that the quality of SiO_x-HNO3 is relatively better than the other oxides. The leakage current density results are similar to those reported by Kobayashi Asuha et al. [30].

To investigate the surface passivation properties of the SiO_x-acidic, a ~10 nm-thick Al₂O₃ layer was deposited on both sides of the SiO_x-acidic/Si substrate using PA-ALD, and the effective carrier lifetime was measured by QSSPC. The measured effective carrier lifetimes at 1.0 sun injection level [34] of Al₂O₃/SiO_x-HNO3/Si, Al₂O₃/SiO_x-HCl/Si, and Al₂O₃/SiO_x-H2SO4 /Si are ~400, ~317, and ~332 μs (Figure 4); notably, the passivation quality of SiO_x,HNO3 is excellent. Moreover, the reference sample, which did not form an oxide film intentionally, exhibited a significantly lower effective carrier lifetime (~220 μs) than the samples with wet chemical oxides. This indicates that the quality of the thin SiO_x layer formed at the Al₂O₃/Si interface influences the effective carrier lifetime and that the oxides formed in acidic solutions exhibit better passivation properties than oxide films formed naturally during the deposition process. To investigate the interfacial properties of Al₂O₃/SiO_x-acidic/Si, the surface carrier lifetime simulation and C–V measurement were performed. The surface carrier lifetime simulation is based on the extended Shockley–Read–Hall (SRH) recombination equation. This equation was derived from the SRV, expressed in Equations (1) and (2) [35,36].

$$S\equiv \frac{U_s}{\Delta n_s}$$

(1)

$$U_s=\frac{n_s{p}_s-n_i^2}{\frac{n_s+n_1}{S_{p0}}+\frac{p_s+p_1}{S_{n0}}} \mathrm{with} S_{p0}={\sigma }_p{V}_{th}{D}_{it}, S_{n0}={\sigma }_n{V}_{th}{D}_{it}$$

(2)

where S is the surface recombination velocity, U_s is the surface recombination rate, and Δn_s is the excess carrier density at the surface, S_n0 and S_p0 are the surface recombination velocities of electrons and holes, n_s and p_s are the electron and hole concentrations at the surface, σ_n and σ_p are the capture cross-sections for electrons and holes, n₁ and p₁ are parameter in the SRH recombination equation and D_it is the number of surface states per unit area. However, if band bending exists on the Si surface by fixed charges (such as negative or positive charges), it is difficult to evaluate Δn_s because the carrier lifetime is determined by the carrier density in the bulk. To this end, we considered the surface of the semiconductor region (z = 0) and the space charge region (z < d_sc), as shown in Figure 5.

Therefore, the effective surface recombination velocity (S_eff) is the sum of the interface recombination velocity and the space charge recombination velocity, as shown in Equations (3)–(9) [35,36,37,38,39].

$$S_{eff}=\frac{1}{\Delta n_{d_{sc}}}\left[U_s+{{\displaystyle \int }}_0^{d_{sc}}U\left(z\right)dz\right]=S_{it}+S_{sc}$$

(3)

S_it can be expressed in Equation (4) based on Equations (1) and (2).

$$S_{it}=\frac{1}{\Delta n_{d_{sc}}}\left[\frac{(n_s{p}_s-n_i^2)D_{it}{V}_{th}{E}_g}{\frac{n_s+n_1+\Delta n_s}{{\sigma }_p}+\frac{p_s+p_1+\Delta n_s}{{\sigma }_n}}\right]$$

(4)

S_sc can be described by Equation (5) as follows by using Equations (6)–(9) for surface potential (Ψ_s) and surface electron and hole concentrations by effective charge density (Q_f) [34,35].

$$S_{sc}=\frac{1}{\Delta n_{d_{sc}}}{{\displaystyle \int }}_0^{d_{sc}}\frac{(n_s\left(z\right)p_s\left(z\right)-n_i^2)N_{st}{V}_{th}{E}_g}{\frac{n_s\left(z\right)+n_1+\Delta n_{sc}}{{\sigma }_p}+\frac{p_s\left(z\right)+p_1+\Delta n_{sc}}{{\sigma }_n}}dz$$

(5)

$${{\displaystyle \int }}_0^{d_{sc}}{n}_s\left(z\right)dz={\beta \lambda }_D{{\displaystyle \int }}_{{\Psi }_s}^0\frac{n\left(\Psi \right)}{F}d\Psi ,{{\displaystyle \int }}_0^{d_{sc}}{p}_s\left(z\right)dz={\beta \lambda }_D{{\displaystyle \int }}_{{\Psi }_s}^0\frac{p\left(\Psi \right)}{F}d\Psi$$

(6)

$$Q_f=\mp {\epsilon }_s\frac{F\left({\Psi }_s,{\Phi }_p,{\Phi }_n\right)}{{q\beta \lambda }_D}$$

(7)

$$F\left({\Psi }_s,{\Phi }_p,{\Phi }_n\right)=\sqrt{\frac{2}{p_b+n_b}\left[p_b\left(e^{-{\beta \Psi }_s}+{\beta \Psi }_s-1\right)+n_b\left(e^{{\beta \Psi }_s}-{\beta \Psi }_s-1\right)\right]}$$

(8)

$$p_b=n_i{e}^{{\beta \Phi }_p},n_b=n_i{e}^{{\beta \Phi }_n}$$

(9)

Here, S_it is the interface recombination velocity between the Si surface and the dielectric layer (z = 0), S_sc is the recombination velocity in the space charge region (z < d_sc), d_sc is the distance of the space charge region, and z is the coordinate perpendicular to the semiconductor surface and increases toward the bulk of Si, V_th is the thermal carrier velocity, Eg is the energy band gap of Si, Ψ is the surface potential, Q_f is the fixed charge density, Φ_n and Φ_p are quasi-fermi level for electrons and holes, p_b and n_b are the carrier concentration for electrons and holes, n_i is intrinsic carrier concentration. With Equation (3), we can calculate the theoretical effective surface carrier lifetime using Equation (10).

$$\frac{1}{{\tau }_{eff}}=\frac{1}{{\tau }_{it}}+\frac{1}{{\tau }_{sc}}=\frac{W}{2}\left(\frac{1}{s_{it}}+\frac{1}{s_{sc}}\right)$$

(10)

where τ_eff is the effective surface carrier lifetime, τ_it is the carrier lifetime in the interface region between the Si surface and the dielectric layer, τ_sc is the carrier lifetime in the space charge region, and W is the thickness of the wafer. The surface carrier lifetime simulation was performed by comparing the effective carrier lifetime (τ_measured) measured using Equation (10). Assuming that the capture cross-section of the electron and hole is σ_n = σ_p = 1 × 10¹⁵ cm², we analyzed the interface trap density (D_it) and the fixed charge density (Q_f) as variable parameters so that the curves of τ_simulated and τ_measured matched well. Figure 6 shows the τ_simulated and τ_measured curves.

The surface carrier lifetime analysis results show that the Al₂O₃/SiO_x-HNO3/SiO_x sample exhibits a lower D_it (2.35 × 10¹⁰ cm⁻²eV⁻¹) than the other samples (

Table 2. Results of capacitance–voltage (C–V) and surface carrier lifetime analyses.

Sample (Average of Five Samples)	Capacitance–Voltage (C–V) Analysis			Surface Carrier Lifetime Analysis
	Qf (1012 cm−2)	Dit (1010 cm−2eV−1)	VFB (V)	Qf (1012 cm−2)	Dit (1010 cm−2eV−1)
Al2O3/Si	−1.53	7.01	1.20	−1.00	7.50
Al2O3/SiOx-HCl/Si	−3.03	4.94	2.97	−3.30	5.70
Al2O3/SiOx-H2SO4/Si	−3.12	4.60	3.12	−3.50	3.90
Al2O3/SiOx-HNO3/Si	−3.24	2.88	3.28	−3.52	2.30

). The effective charge densities are similar regardless of the acidic solution used. Considering the lifetime of the SiO_x-HNO3 layer and values of Q_f and D_it extracted from the surface carrier lifetime analysis, the passivation properties would be more influenced by D_it than Q_f. C–V measurements were also performed to compare the values of D_it and Q_f with those obtained from the surface carrier lifetime analysis. The C–V measurement results show that V_FB (flat band voltage) of the silicon substrates with wet chemical oxide layers shifts by 1.77–2.08 V toward the positive bias, as shown in Figure 7.

Q_f was calculated using V_FB extracted from the measured C–V graph, and D_it was obtained using the Terman method (Table 2) [40,41]. The negative Q_f values of the silicon oxides formed in the different acidic solutions did not change significantly in the range of 3.03–3.24 × 10¹² cm⁻². However, D_it was markedly different depending on the acid solution used (Table 2). Therefore, we confirm that the quality of SiO_x at the Al₂O₃/Si interface affects the passivation characteristics, and the insertion of a wet chemical oxide layer improved the passivation quality, compared with the SiO_x layer simultaneously formed during the deposition of Al₂O₃.

4. Conclusions

In this study, we improved the interfacial properties of an Al₂O₃/Si structure by performing wet chemical oxidation on an Si substrate. The thickness of the wet chemical oxides grown on the Si surface in different acidic solutions was in the range of 1.32–1.5 nm, which was similar to that of SiO_x naturally formed at the interface of the Al₂O₃/Si sample during PA-ALD deposition. In particular, SiO_x-HNO3 showed a relatively lower leakage current than SiO_x-HCl and SiO_x-H2SO4. The carrier lifetime of Al₂O₃/SiO_x-HNO3/Si measured by QSSPC to evaluate the passivation characteristics was ~ 400 µs, which was higher than those of the other SiO_x-acidic layers (~317 µs for SiO_x-HCl and ~332 µs for SiO_x-H2SO4). C–V measurement and surface carrier lifetime analysis were performed to analyze the interface characteristics of the Al₂O₃/SiO_x/Si samples. The fixed charges showed little change with the oxides, and the D_it values changed significantly. The wet chemical oxide formed in the HNO₃ solution showed better passivation characteristics than those formed in other acidic solutions, resulting from the low interface trap density of SiO_x-HNO3. We confirmed the improvement in the passivation characteristics of the Al₂O₃/Si sample by inserting a wet chemical oxide between Al₂O₃ and Si substrates. In addition, the values of the parameters associated with the interface properties, such as Q_f and D_it, obtained from the C–V measurement and surface carrier lifetime simulation were similar. Therefore, this simulation can be a useful tool to analyze interfacial characteristics. Moreover, this study lays a foundation for analyzing the interfacial properties of samples, such as poly-Si/SiO_x/Si structures with ultra-thin SiO_x, that cannot be analyzed by C–V measurements.