Growth diagram and morphologies of AlN thin films grown by molecular beam epitaxy

Abstract

A growth diagram for molecular beam epitaxy of AlN on sapphire and 6H–SiC was established using reflection high energy electron diffraction, atomic force microscopy, and Rutherford backscattering spectrometry. In varying the Al/N ratio and growth temperature, distinctive surface morphologies emerge, which are assigned to three regimes of growth, one N-rich (Al/N<1) and two Al-rich (Al/N>1) regimes. Under N-rich conditions, AlN films exhibit rough surface morphologies. In contrast, Al-rich conditions produce excellent smooth surface morphologies, but with the constraint of Al droplet formation at very high Al/N ratios and low temperatures. The differentiation between N-rich and Al-rich regimes is given only by the Al/N ratio, while the two Al-rich regimes (intermediate self-regulated and droplet regime) are separated by the boundary line of Al droplet formation. For this boundary an Arrhenius dependence of growth temperature was found, yielding an activation energy of 3.4±0.1 eV. The observed morphology transitions are attributed to varying surface adatom mobilities present under the different Al/N ratios.

Growth and Surface Reconstructions of AlN(0001) Films

“…This result is in contrast to the situation for Ga on GaN(0001), in which deposition of Ga at temperature of 20°C or above does result in the formation of Ga droplets. For our AlN(0001) surfaces we find that the excess Al forms droplets for deposition (or annealing) at a temperature of 750°C, in agreement with prior reports [12,13]. We have not studied in detail the temperature dependence of the droplet formation (i.e.…”

A comprehensive diagram to grow InAlN alloys by plasma-assisted molecular beam epitaxy

“…Surprisingly, as shown in Fig. 4͑c͒, for those samples grown in the N-rich regime smooth surfaces with atomic steps and rms values below 0.8 nm were obtained instead of the rough surfaces normally observed for III-N layers grown under N-rich conditions [5][6][7][8] In summary, In incorporation into InAlN layers grown by PA-MBE was analyzed as a function of the growth temperature and the impinging In flux. The In incorporation into the layers was found to decrease with the substrate temperature due to thermal InN decomposition and to increase with the impinging In flux until stoichiometry was reached at the growth front.…”

“…[1][2][3][4] Growth diagrams, useful to identify optimum conditions, have been established for binaries ͑AlN, GaN, InN͒, grown by plasma-assisted molecular beam epitaxy ͑PA-MBE͒, from the surface morphology dependence on growth temperature and impinging fluxes. [5][6][7][8] However, the growth of InAl͑Ga͒N alloys poses much more difficulties due to InN thermal decomposition [9][10][11][12] and strong differences between InN and AlN.…”

Composition and luminescence of AlInGaN layers grown by plasma-assisted molecular beam epitaxy

“…Since throughout the range of growth temperatures used the desorption rate of Al is completely negligible 8 and metal was not accumulated on the growing surface ͑such as in the form of droplets͒, all impinging Al atoms should be incorporated into the layer. The Ga desorption rate ͑in vacuum͒ has been reported to be approximately 0.02 ML/s at 630°C and as low as 0.005 ML/s at 600°C.…”

“…However the growth of AlInGaN layers over a wide compositional range is difficult due to the very different properties and optimum growth conditions of the constituent binaries. [3][4][5][6][7][8] The growth temperature is the most critical parameter to achieve high quality AlInGaN layers, since a balance is required between the high temperatures suited for Al-and Ga-containing layers and the low temperatures needed for efficient In incorporation. 3,5,8 The strong dependence of In incorporation with substrate temperature may lead to compositional nonuniformities due to temperature gradients along the wafer during growth.…”