Effects of oxygen content on the optical properties of tantalum oxide films deposited by ion-beam sputtering

H. Demiryont; James R. Sites; Kent Geib

doi:10.1364/AO.24.000490

Applied Optics
Vol. 24,
Issue 4,
pp. 490-495
(1985)
•https://doi.org/10.1364/AO.24.000490

Effects of oxygen content on the optical properties of tantalum oxide films deposited by ion-beam sputtering

H. Demiryont, James R. Sites, and Kent Geib

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H. Demiryont, James R. Sites, and Kent Geib, "Effects of oxygen content on the optical properties of tantalum oxide films deposited by ion-beam sputtering," Appl. Opt. 24, 490-495 (1985)

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Abstract

Ion-beam sputter deposition of tantalum oxide films was investigated for possible optical coating applications. Optical properties of such films were found to be a sensitive function of oxygen-to-argon ratio in the ion beam. Refractive index and absorption coefficient were determined in the 250–2000-nm wavelength range by spectrophotometric transmissivity. The different bonding states of the tantalum atoms were revealed by x-ray photoelectron spectroscopy. The visible wavelength refractive index was found to be 2.18 and optical band gap 4.3 eV, so long as the films did not contain inclusions of metallic tantalum. Films with an admixture of oxygen deficient suboxide components had a low-energy tail of increasing magnitude in the absorption spectrum.

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Sample	Oxygen fraction in the beam (%)	Deposition rate (nm/min)	Thickness (nm)	n (λ = 550 nm)	k (λ = 550 nm)	Description
PIIB 11	56	3.75	182	2.18	0
PIIB 10	44	3.7	197	2.18	0	Stoichiometric
PIIB 9	37.5	3.8	415	2.18	0.0001	oxide
PIIB 8	30	5.0	610	2.18	0.0037	Suboxide
PIIB 6	25	5.5	659	2.20	0.018
PIIB 7	17.5	10.0	606	3.42	0.06	Cermet
PIIB 5	12.5	12.5	830	11	0.1
PIIB 2	∼0	14.0	52		2.8	Metal

Binding energy (eV)		Ta⁰ (Ta in Ta)	Ta⁺²^x (Ta in TaO_x)	Ta⁺²^x′ (Ta in TaO_x)	Ta⁺⁵ (Ta in Ta₂O₅)
4f doublet.	4f 5/2	24.0	∼26	∼28	28.5
	4f 7/2	22.2	∼24	∼26	26.7
4d doublet	4d 3/2	238.5	—	—	242.2
	4d 5/2	227.0	—	—	231.0

Type of binding	Type of sample

	Stoichiometric oxide Ta₂O₅	Suboxide TaO_x	Cermet Ta–TaO_x
Surface Bulk	Ta⁺⁵	Ta⁺⁵	Ta⁺⁵
	Ta⁺⁵	Ta⁺⁵, Ta+²^x	Ta⁺⁵, Ta⁺²^x, Ta⁰
	For T ≳ 10 min, sputter reduction occurs, then Ta⁺²^x peak appears.	After ∼10 min, sputter reduction occurs, then Ta⁰ peak appears.	Reduction increased Ta⁰ and Ta⁺²^x peak intensities.

n (λ ≃ 550 nm)	k (λ ≃ 550 nm)	$E_{g}^{0} (eV)$	λ_th(nm)	λ_cf(nm)	Deposition technique	Method of measurement	Ref.
2.18	<10⁻⁴	4.2	252	365	Ion-beam sputtering	Transmission	Present results
2.01	5 × 10⁻³	4.2	290	410	Magnetron reactive sputtering	Reflectance	1
1.92	Very small	4.07–4.5	290	390	Reactive sputtering	Reflectance– transmittance	8
2.16	<3 × 10⁻³	4.15–4.51	—	—	Sputtering	Reflectance– transmittance	10
2.12–2.21	High	—	—	—	Reactive dc sputtering	Reflectance extrema–angle of incidence	11
2.10	≤2 × 10⁻³	—	—	—	Reactive dc sputtering	Transmission	5
2.23	—	—	—	—	Anodic oxidation	Reflectance	18

Sample	Oxygen fraction in the beam (%)	Deposition rate (nm/min)	Thickness (nm)	n (λ = 550 nm)	k (λ = 550 nm)	Description
PIIB 11	56	3.75	182	2.18	0
PIIB 10	44	3.7	197	2.18	0	Stoichiometric
PIIB 9	37.5	3.8	415	2.18	0.0001	oxide
PIIB 8	30	5.0	610	2.18	0.0037	Suboxide
PIIB 6	25	5.5	659	2.20	0.018
PIIB 7	17.5	10.0	606	3.42	0.06	Cermet
PIIB 5	12.5	12.5	830	11	0.1
PIIB 2	∼0	14.0	52		2.8	Metal

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Applied Optics