Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics

Parag Doshi; Gerald E. Jellison; Ajeet Rohatgi

doi:10.1364/AO.36.007826

Applied Optics
Vol. 36,
Issue 30,
pp. 7826-7837
(1997)
•https://doi.org/10.1364/AO.36.007826

Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics

Parag Doshi, Gerald E. Jellison, and Ajeet Rohatgi

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Parag Doshi, Gerald E. Jellison, and Ajeet Rohatgi, "Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics," Appl. Opt. 36, 7826-7837 (1997)

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Abstract

We have optimized plasma-enhanced chemical vapor deposition (PECVD) of SiN-based antireflection (AR) coatings with special consideration for the short-wavelength (<600 nm) parasitic absorption in SiN. Spectroscopic ellipsometry was used to measure the dispersion relation for both the refractive index n and the extinction coefficient k, allowing a precise analysis of the trade-off between reflection and absorption in SiN-based AR coatings. Although we focus on photovoltaic applications, this study may be useful for photodetectors, IR optics, and any device for which it is essential to maximize the transmission of light into silicon. We designed and optimized various AR coatings for minimal average (spectrally) weighted reflectance (〈R_w〉) and average weighted absorptance (〈A_w〉), using the air mass 1.5 global solar spectrum. In most situations 〈R_w〉 decreased with higher n, but 〈A_w〉 increased because k increased with n. For the practical case of a single-layer AR coating for silicon under glass, an optimum refractive index of ∼2.23 (at 632.8 nm) was determined. Further simulations revealed that a double-layer SiN stack with an n = 2.42 film underneath an n = 2.03 film gives the minimum total photocurrent loss. Similar optimization of double-layer SiN/SiO₂ coatings for silicon in air revealed an optimum of n = 2.28 for SiN. To determine the allowable tolerance in index and film thickness, we generated isotransmittance plots, which revealed more leeway for n values below the optimum than above because absorption begins to reduce photocurrent for high n values.

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n(632.8 nm)	d rough (Å)	d (SiN) (Å)	d (SiN) Correlated Error (Å)	E_g (eV)	A (eV)	E₀ (eV)	C (eV)	∊₁ (∞)	χ₂
2.03	24	491	3	2.76	70	8.15	6.12	1.40	0.77
2.12	22	589	2	2.50	93.3	7.75	9.47	1.10	0.16
2.20	22	606	1	2.37	93.7	6.28	8.03	1.41	0.19
2.23	33	568	2	2.34	105	6.90	9.50	1.11	0.72
2.25	27	563	2	2.32	98	6.39	8.16	1.31	0.84
2.28	34	595	1	2.30	106	6.36	8.96	1.31	0.61
2.33a	35	516	2	2.14	108	6.33	9.50	1.16	0.90
2.35a	32	549	2	2.14	113	6.12	9.50	1.12	2.14
2.42	30	739	1	2.18	117.1	5.23	7.09	1.30	0.45
2.55	33	668	1	2.12	126.4	4.78	6.22	1.36	0.51

	Δn		Δk					Δ[exp(-αd)]
λ (nm)	Absorptance	% Error	Absorptance	% Error	αda	Δ(αd)	exp(-αd)	Absorptance	% Error
270	0.036	1.4	0.017	1.8	0.05	0.072	0.004	0.072	4.6
350	0.035	1.3	0.010	2.1	0.02	0.352	0.006	0.352	2.1
500	0.030	1.2	0.002	5.7	0.003	0.949	0.003	0.949	0.30
800	0.029	1.2	0	0	0	0	0	0	0

					Photocurrent Loss (mA/cm²
Index (632.8 nm)	Optimized Thickness (Å)	〈R_w〉 (%)	〈A_w〉 (%)	〈T_w〉 (%)	Reflected	Absorbed	Total
Uncoated	—	19.02	0.00	80.98	7.89	0.00	7.89
2.03	760	6.00	0.03	93.97	2.49	0.01	2.50
2.12	720	5.14	0.38	94.48	2.13	0.16	2.29
2.20	690	4.52	0.94	94.54	1.87	0.39	2.26
2.23a	680a	4.37a	1.03a	94.60a	1.81a	0.43a	2.24a
2.25	680	4.30	1.15	94.55	1.78	0.48	2.26
2.28	670	4.16	1.34	94.50	1.72	0.56	2.28
2.33	650	3.98	2.42	93.60	1.65	1.00	2.65
2.35	650	3.97	2.62	93.41	1.65	1.09	2.73
2.42	630	4.09	2.66	93.25	1.70	1.10	2.80
2.55	590	4.45	3.53	92.02	1.85	1.46	3.31
2.40, k = 0b	645	3.68	0.00	96.32	1.53	0.00	1.53
DLAR 2.42/2.03c	400/480	2.96	1.74	95.30	1.23	0.72	1.95

					Photocurrent Loss (mA/cm²
Index (632.8 nm)	Optimized Thickness (Å)	〈R_w〉 (%)	〈A_w〉 (%)	〈T_w〉 (%)	Reflected	Absorbed	Total
Uncoated	—	34.17	0.00	65.83	14.17	0.00	14.17
2.03a	780a	7.98a	0.02a	92.00a	3.31a	0.01a	3.32a
2.12	740	8.54	0.33	91.13	3.54	0.14	3.68
2.20	720	9.25	0.80	89.95	3.84	0.33	4.17
2.23	710	9.46	0.89	89.65	3.92	0.37	4.29
2.25	700	9.64	0.99	89.37	4.00	0.41	4.41
2.28	690	9.97	1.15	88.88	4.13	0.48	4.61
2.33	680	10.43	2.12	87.45	4.32	0.88	5.20
2.35	680	10.60	2.28	87.12	4.39	0.95	5.34
2.42	660	11.64	2.29	86.07	4.83	0.95	5.78
2.55	630	13.31	3.05	83.64	5.52	1.26	6.78
2.0, k = 0b	790	7.38	0.00	92.62	3.06	0.00	3.06

						Photocurrent Loss (mA/cm²)
SiN Index (632.8 nm)	Optimum SiN Thickness (Å)	Optimum SiO₂ Thickness (Å)	〈R_w〉 (%)	〈A_w〉 (%)	〈T_w〉 (%)	Reflected	Absorbed	Total
2.03	670	890	5.41	0.04	94.55	2.24	0.02	2.26
2.12	660	950	4.12	0.41	95.47	1.71	0.17	1.88
2.20	630	970	3.10	0.99	95.91	1.29	0.41	1.70
2.23	630	980	2.85	1.08	96.07	1.18	0.45	1.63
2.25	620	980	2.71	1.20	96.09	1.12	0.50	1.62
2.28a	610a	980a	2.46a	1.38a	96.16a	1.02a	0.57a	1.59a
2.33	590	990	2.12	2.42	95.46	0.88	1.00	1.88
2.35	590	990	2.05	2.61	95.34	0.85	1.08	1.93
2.42	570	1000	1.86	2.66	95.48	0.77	1.10	1.87
2.55	540	1020	1.84	3.49	94.67	0.76	1.45	2.21
2.55, k = 0b	575	1100	1.54	0.00	98.46	0.64	0.00	0.64

						Photocurrent Loss (mA/cm²)
SiN Index (632.8 nm)	Optimum SiN Thickness (Å)	Optimum MgF₂ Thickness (Å)	〈R_w〉 (%)	〈A_w〉 (%)	〈T_w〉 (%)	Reflected	Absorbed	Total
2.03	710	1000	4.66	0.03	95.31	1.93	0.01	1.94
2.12	680	1030	3.56	0.41	96.03	1.48	0.17	1.65
2.20	650	1050	2.76	0.98	96.26	1.14	0.41	1.55
2.23a	640a	1050a	2.57a	1.07a	96.36a	1.07a	0.44a	1.51a
2.25	630	1050	2.47	1.19	96.34	1.02	0.49	1.52
2.28	620	1050	2.29	1.37	96.34	0.95	0.57	1.52
2.33	610	1050	2.04	2.45	95.51	0.85	1.02	1.86
2.35	600	1050	2.03	2.62	95.35	0.84	1.09	1.93
2.42	590	1080	1.98	2.67	95.35	0.82	1.11	1.93
2.55	560	1100	2.20	3.50	94.30	0.91	1.45	2.36

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Applied Optics