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Register a new userMicrostructures Diagram Of Magnetron Sputtered Aln Deposits : Amorphous And Nanostructured Films
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Valerie Brien
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In order to get homogeneous nanostructured Aluminum Nitride deposits, thin films were grown at room temperature on [001] Si substrates by radio frequency magnetron reactive sputtering. The deposits were analysed by Transmission Electron Microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy. Their microstructure and chemical composition were studied versus the plasma working pressure and the radio frequency power. Systematic analysis of cross views of the films allowed the authors to draw a microstructure/process parameters map. Four microstructural types were distinguished according to the decrease of the deposition rate. One is the well-known columnar microstructure. The second one is made of interrupted columns or fibrous grains. The third one is made of nano-sized particles (size of the particles ranges from 1.7 to 8 nm). The fourth and last microstructure is amorphous. The deposit morphology-process parameters correlation is commented on.
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Er-doped aluminum nitride films, containing different Er concentrations, were obtained at room temperature by reactive radio frequency magnetron sputtering. The prepared samples show a nano-columnar microstructure and the size of the columns is dependent on the magnetron power. The Er-related photoluminescence (PL) was studied in relation with the temperature, the Er content and the microstructure. Steady-state PL, PL excitation spectroscopy and time-resolved PL were performed. Both visible and near infrared PL were obtained at room temperature for the as-deposited samples. It is demonstrated that the PL intensity reaches a maximum for an Er concentration equal to 1 at. % and that the PL efficiency is an increasing function of the magnetron power. Decay time measurements show the important role of defect related non radiative recombination, assumed to be correlated to the presence of grain boundaries. Moreover PL excitation results demonstrate that an indirect excitation of Er 3+ ions occurs for excitation wavelengths lower than 600 nm. It is also suggested that Er ions occupy at least two different sites in the AlN host matrix.
We demonstrate the self-assembled formation of AlN nanowires by molecular beam epitaxy on sputtered TiN films on sapphire. This choice of substrate allows growth at an exceptionally high temperature of 1180 {deg}C. In contrast to previous reports, the nanowires are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the nanowire sidewalls. The high crystalline quality of the nanowires is evidenced by the observation of near band edge emission in the cathodoluminescence spectrum. The key factor for the low nanowire coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices employing these nanowires as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range.
The effect of magnetron power on the room temperature 1.54 $mu$m infra-red photoluminescence intensity of erbium doped AlN films grown by r. f. magnetron sputtering, has been studied. The AlN:Er thin films were deposited on (001) Silicon substrates. The study presents relative photoluminescence intensities of nanocrystallized samples prepared with identical sputtering parameters for two erbium doping levels (0.5 and 1.5 atomic %). The structural evolution of the crystallites as a function of the power is followed by transmission electron microscopy. Copyright line will be provided by the publisher 1 Introduction For some time now, rare-earth (RE)-doped semiconductors represent significant potential applications in the field of opto-electronic technology. Part of this technological interest relies on the shielded 4f levels of the RE ions as they give rise to sharp and strong luminescence peaks [1-5]. Among the RE elements, Er is preferred to its counterparts since the Er ions can produce both visible light at 558 nm (green, one of the primary colours) and IR light at 1.54 $mu$m whose spectrum region coincides with the main low-loss region in the absorption spectrum of silica-based optical fibres, combining so potential applications towards photonic devices and towards optical communication devices operating in the infrared domain. These interesting emissions can however only be exploited when placed into host matrixes. On one side, the shielding of the intra 4f levels prevents the shifting of the RE 3+ energy levels and ensures the frequency emission stability. Moreover the intra 4f transitions are parity forbidden for the isolated ions. Matrixes can render the Er 3+ ions optically active, via a relaxation of selection rules due to crystal field effects. As silicon based materials were tested in the 1960s to the 90s with no clear industrial success it was found that the
In the prospect of understanding the photoluminescence mechanisms of AlN films doped with erbium and targeting photonic applications we have synthesized non doped and Er-doped AlN films with different crystallized nanostructures by using PVD magnetron sputtering. Their crystalline morphology and their visible photoluminescence properties were precisely measured.Due to the weak cross-section absorption of rare earths like erbium, it is necessary to obtain an efficient energy transfer mechanism between the host matrix and the rare earth to obtain high luminescence efficiency. Our strategy is then to elaborate some nanostructures that could introduce additional intermediate electronic levels within the gap thanks to the presence of structural defects (point defects, grain boundaries{ldots}) and could lead to energy transfer from the AlN matrix to the rare earth.Doped and non-doped AlN films were prepared by radio frequency magnetron sputtering by using different experimental conditions that will be detailed. It will notably be shown how a negative polarization of samples during deposition allows obtaining crystalline morphologies ranging from the classical columnar structure to a highly disordered polycrystalline structure with grains of several nanometers (nearly amorphous). The nanostructures of the films could be categorized in three types: 1) type 1 was nanocolumnar (width of column ~ 15 nm), 2) type 2 was made of short columns (width of column ~ 10 nm) and 3) the last type was made of equiaxed nanocrystallites (size of grains ~3-4 nm).High-resolution photoluminescence spectroscopy was performed to characterize their optical behaviour. The samples were excited by the laser wavelengths at 458, 488 or 514 nm. A broad photoluminescence band was observed centred around 520 nm in columnar samples. In the same energy range, the highly resolved spectra also showed several sharp emission peaks. This fine structure could be attributed to erbium transitions. This fine structure tended to disappear going from type 1 to type 3 samples. Indeed, the relative intensity of the peaks decreased and their full width at half maximum increased. This change could be related to the density of defects that increased when the size of the grains decreased. The photoluminescence properties of the films in the visible range will be discussed in relation with their structure.
Nanocrystalline n-AlN:Er thin films were deposited on (001) Silicon substrates by r. f. magnetron sputtering at room temperature to study the correlation between 1.54 $mu$m IR photoluminescence (PL) intensity, AlN crystalline structure and Er concentration rate. This study first presents how Energy-Dispersive Spectroscopy of X-rays (EDSX) Er Cliff Lorimer sensitivity factor alpha = 5 is obtained by combining EDSX and electron probe micro analysis (EPMA) results on reference samples. It secondly presents the relative PL intensities of nanocrystallized samples prepared with identical sputtering parameters as a function of the Er concentration. The structure of crystallites in AlN films is observed by transmission electron microscopy.
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