Growth of thin, crystalline oxide, nitride, and oxynitride films on NiAl and CoGa surfaces

Abstract

The growth of thin, well-ordered oxide, nitride, and oxynitride films on NiAl and CoGa surfaces was investigated by means of high-resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). At 300K, an amorphous Al-oxide film is formed on NiAl(001) upon oxygen adsorption. Annealing of the oxygen-saturated NiAl(001) surface to 1200 K leads to the formation of thin well-ordered θ-Al 2 O 3 films. At 300K, and low-exposure oxygen atoms are chemisorbed on CoGa(001) on defects and on step edges of the terraces. For higher exposure up to saturation, the adsorption of oxygen leads to the formation of an amorphous Ga-oxide film. The EEL spectrum of the amorphous film exhibits two losses at 400 and 690 cm -1 . After annealing the amorphous Ga-oxide films to 550 K thin, well-ordered β-Ga 2 O 3 films are formed on top of the CoGa(001) surface. The EEL spectrum of the β-Ga 2 O 3 films show strong Fuchs-Kliewer (FK) modes at 305, 455, 645, and 785 cm -1 . The β-Ga 2 O 3 films are well ordered and show (2 x 1) LEED pattern with two domains, oriented perpendicular to each other. The STM study confirms the two domains structure and allows the determination of the two-dimensional lattice parameters of β-Ga 2 O 3 . The vibrational properties and the structure of β-Ga 2 O 3 on CoGa(001) and θ-Al 2 O 3 on NiAl(001) are very similar. Ammonia adsorption at 80 K on NiAl(111) and NiAl(001) and subsequent thermal decomposition at elevated temperatures leads to the formation of AIN. Well-ordered and homogeneous AIN thin films can be prepared by several cycles of ammonia adsorption and annealing to 1250 K. The films render a distinct LEED pattern with hexagonal [AIN/NiAl(111)] or pseudo-twelve-fold [AIN/NiAl(001)] symmetry. The lattice constant of the grown AIN film is determined to be a AIN = 3.11 A. EEL spectra of AIN films show a FK phonon at 865 cm -1 . The electronic gap is determined to be Eg = 6.1 ± 0.2 eV. GaN films are prepared by using the same procedure on the (001) and (111) surfaces of CoGa. The films are characterized by a FK phonon at 695 cm -1 and an electronic band gap Eg = 3.5 ± 0.2 eV. NO adsorption at 75 K on NiAl(001) and subsequent annealing to 1200 K leads to the formation of aluminium oxynitride (AION). An oxygen to nitrogen atomic ratio of 2:1 was estimated from the analysis of AES spectra. The AION films shows a distinct (2 x 1) LEED pattern and the EEL spectrum exhibits characteristic Fuchs-Kliewer modes. The energy gap is determined to be Eg = 6.6 ± 0.2 eV. The structure of the AION film is derived from that of θ-Al 2 O 3 formed on NiAl(001).