The authors report electrical and optical characterization of zinc oxide (ZnO) and Al-doped zinc oxide (AZO) films grown by atomic layer deposition (ALD). A detailed analysis of ZnO growth morphology is presented with the help of atomic force microscopy imaging, roughness analysis, and x-ray photoelectron spectroscopy surface chemistry information. Initially the film grew as islands, which coalesced to complete the substrate coverage at 50 ALD cycles. The AZO films to be used as transparent conducting oxides for solar cell applications were grown on single crystalline Si (100) and float-glass substrates at temperatures from 150–325 °C. The amount of aluminum doping was varied from 2 to 8 %. The AZO film with 5% Al exhibited the highest conductivity in the film, which increased as the growth temperature increased. Hall effect measurements of an AZO film of thickness 575 nm doped at 5% on silicon and glass substrates showed a sheet resistance (Rs) of 100 Ω/□, which improved further to 25 Ω/□ after annealing at higher temperatures in an argon-hydrogen environment. The transmittance and reflectance of the films grown on glass substrates were used to calculate the band gap. The band gap of AZO increased with Al-doping level. The transmittance of the films in the entire 0 to 8 % doping range, was found to be 80 to 90 % in the visible region. In addition, the growth rates of ZnO, Al2O3, and AZO films were also studied. The growth rate of the AZO films was 1.95 Å per ALD cycle with a cycle time of 1 s. This growth rate is relatively large for an ALD process. The interface of the AZO-Al2O3-Si(100) imaged using high resolution transmission electron microscopy showed a random texture and a continuous interface.
Aluminum doped zinc oxide (AZO) thin film is gaining momentum as a transparent conducting oxide because its optical and electrical properties are optimal for a photovoltaic device. We report growth of AZO thin film using an Atomic Layer Deposition (ALD) system, which is known to deposit layers on a substrate with atomic layer precision. The precursors used for aluminum and zinc were Trimethylaluminum (TMA) and Dimethylzinc (DMZ) respectively. Alternating cycles of TMA and DMZ were introduced to the heated deposition chamber such that a desired aluminum doping was obtained on the AZO films grown. The films were grown on flexible substrates like PET and also on single crystalline Si(100) and float-glass substrates. An optimal aluminum doping (3 at.%) on the AZO film of thickness 575 nm gave a sheet resistance (Rs) of 97 Ω/□ with corresponding resistivity of 5.6 x 10-3 Ω.cm. The value of the carrier concentration and hall mobility were 1.86 x 1020 cm-3 and 6.5 cm2V-1s-1 respectively. Optical measurement showed 90% transmission in most of the visible spectrum. X-ray diffraction spectra of the film showed all characteristic ZnO hexagonal lattice peaks.
Aluminum doped zinc oxide (AZO) thin film is gaining momentum as a transparent conducting oxide (TCO) because its optical and electrical properties are optimal for a photovoltaic device [1]. We report a growth of AZO thin film using an Atomic Layer Deposition (ALD) system, which is known to deposit layers on a substrate with atomic precision [2]. The precursors used for aluminum and zinc oxides were Trimethylaluminum (TMA) and Dimethylzinc (DMZ) respectively. Alternative cycles of TMA and DMZ were introduced to the heated deposition chamber such that a desired aluminum doping was obtained on the AZO films grown. The films were grown on 4" diameter single crystalline Si (100) and float-glass substrates heated at 325°C in nitrogen ambient. On silicon, a buffer layer of Al 2 O 3 is also grown before the AZO layer. An optimal aluminum doping on the AZO film of thickness 575 nm gave a sheet resistance (R s ) of 100 utes annealing at 400°C in an argon environment, reduced to 25 ivity of 1.4 x 10 -3. The carrier concentration and hall mobility were 2.39 x 10 20 cm -3 and 17.76 cm 2 V -1 s -1 respectively. Optical measurement showed 90% transmission in most of the visible spectrum.Surface morphology of the thin films was imaged by Scanning Electron Microscopy (SEM) [3]. The as-grown films are relatively smooth with uniform distribution of grains which are around 60-100 nm long and 10-20 nm wide as shown in Figure 1. The stoichiometry of the film was estimated by Energy Dispersive Spectrometry (EDS) analysis, which showed that aluminum doping was around 3 atomic percent. The X-ray Diffraction spectra of the film showed all characteristic ZnO hexagonal lattice with space group P63mc (186). The film texture is dominantly (100) oriented. Roughness analysis was performed on the images taken by atomic force microscopy (AFM) on as-grown and annealed samples as shown in Figure 2. The annealed sample surface was found to be relatively smoother as compared to the as-grown films. The growth morphology at the interface was examined using a JEOL JEM-2100F field emission transmission electron microscope (TEM). HRTEM images across the interface are taken to characterize the growth orientation. Nanobeam electron diffraction (NBED) technique is used to further verify the crystal structure.In conclusion, AZO films with very smooth surface and uniform texture are successfully grown by ALD system and the micro-structure is studied using imaging techniques such as SEM, AFM and TEM. This transparent conducting oxide has a potential to replace high cost TCOs such as tin-doped indium oxide (ITO).
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