Optical characteristics of nanocrystalline AlxGa1−xN thin films deposited by hollow cathode plasma-assisted atomic layer deposition

Abstract

Gallium nitride (GaN), aluminum nitride (AlN), and AlxGa1−xN films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and AlxGa1−xN films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and AlxGa1−xN were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (λ = 550 nm) with the increased Al content x (0 ≤ x ≤ 1), while the extinction coefficients (k) were approximately zero in the VIS spectrum (>400 nm). Postdeposition annealing at 900 °C for 2 h considerably lowered the refractive index value of GaN films (2.33–1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 °C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 °C for 2 h. This might be caused by Ga2O3 formation and following phase change. The optical bandgap value of as-deposited AlxGa1−xN films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not affect the bandgap of Al-rich films.

Electronic and optical device applications of hollow cathode plasma assisted atomic layer deposition based GaN thin films

“…Annealing the thin films at temperature levels as high as 800 C is shown not to cause a significant change in the structural properties of HCPA-ALD based GaN layers. 15 Composition as a function of depth is determined by XPS measurements, which revealed 42.19 at. % Ga, 55.18 at.…”

“…The responsivity value decreases significantly at wavelengths >300 nm due to the band edge of HCPA-ALDgrown GaN film as verified by the optical transmission measurements as well. 12,15 Fabricated devices show responsivity values of 0.95, 0.68, and 0.47 mA/W at a wavelength of 300 nm for annealed (in N 2 :H 2 and N 2 ambient) and asdeposited GaN films, respectively, under 7 V reverse bias. Annealing the GaN film in N 2 :H 2 environment significantly increases the responsivity in the UV region due to increased collection efficiency of photo-generated carriers.…”

Metal-semiconductor-metal ultraviolet photodetectors based on gallium nitride grown by atomic layer deposition at low temperatures

“…The responsivity value decreases significantly at wavelengths larger than 300 nm due to the band edge of the HCPA-ALD-grown GaN film as verified by the previously reported optical characterization results. 20,21 Fabricated devices show responsivity values of 1.687 and 0.101 mA∕W at wavelengths of 200 and 390 nm, respectively, under a 13 V reverse bias. Since the GaN films are deposited at temperatures below 200°C, these results signify the first demonstration of functional devices on such layers.…”

“…20 We also reported successful GaN thin film transistors (TFTs) using the same technique. 21 In this work, proof-of-concept ultraviolet (UV) PDs based on such low-temperature ALD-grown GaN layers are demonstrated. Metal-semiconductor-metal (MSM) type PDs are fabricated and characterized.…”

Atomic Layer Deposition of Silicon Nitride Thin Films: A Review of Recent Progress, Challenges, and Outlooks

“…and oxides (Hf x Zr 1−x O 2 , Al-doped ZnO, etc.) have been prepared with this kind of approach [108,109,110,111,112,113]. Based on this concept, Kim et al successfully reduced the dielectric constant of ALD SiN x gate spacer by alloying with boron nitride [114].…”