E. Iliopoulos

Heteroepitaxial growth of In-face InN on GaN (0001) by plasma-assisted molecular-beam epitaxy

The thermodynamic aspects of indium-face InN growth by radio frequency plasma-assisted molecular-beam epitaxy (rf-MBE) and the nucleation of InN on gallium-face GaN (0001) surface were investigated. The rates of InN decomposition and indium desorption from the surface were measured in situ using reflected high-energy electron diffraction and the rf-MBE “growth window” of In-face InN (0001) was identified. It is shown that sustainable growth can be achieved only when the arrival rate of active nitrogen species on the surface is higher than the arrival rate of indium atoms. The maximum substrate temperature permitting InN growth as a function of the active nitrogen flux was determined. The growth mode of InN on Ga-face GaN (0001) surface was investigated by reflected high-energy electron diffraction and atomic force microscopy. It was found to be of the Volmer–Weber-type for substrate temperatures less than 350°C and of the Stranski–Krastanov for substrate temperatures between 350 and 520°C. The number of m...

Broadening of near-band-gap photoluminescence in n-GaN films

This letter addresses the broadening mechanism of the near-band-gap photoluminescence in GaN films doped n type with silicon. The films were produced by plasma assisted molecular beam epitaxy and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He–Cd laser at 77 K. At low carrier concentration ( 1018 cm−3) the full width at half maximum increases monotonically with carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to tailing of the density of states caused by potential fluctuations due to randomly distributed impurities. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [Phys. Rev. 139, A343 (1965)], and the agreement between model and experim...

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

Growth kinetics of AlGaN films by plasma-assisted molecular-beam epitaxy

In this letter, we report on the kinetics of growth of aluminum gallium nitride films by rf plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates at the temperature range of 750–800 °C. The determination of the incorporation probabilities of aluminum and gallium atoms was arrived at by measurements of the growth rate and composition of the films. We find that, in both the nitrogen-rich and group-III-rich growth regimes, the incorporation probability of aluminum is unity for the entire investigated temperature range. On the other hand, the incorporation probability of gallium is constant and equals 0.75 at a substrate temperature of 750 °C only in the nitrogen-rich growth regime. The temperature dependence of the incorporation probability in this regime has an activation energy of 2.88 eV which is consistent with gallium desorption from the surface. In the group-III-rich growth regime, the incorporation probability of gallium decreases monotonically with group-III fluxes due to the competiti...

Energy bandgap bowing of InAlN alloys studied by spectroscopic ellipsometry

InxAl1−xN films were heteroepitaxially grown on AlN∕Al2O3 (0001) templates by molecular beam epitaxy. The compositions studied spanned the whole ternary range. The complex dielectric function of the films was investigated by variable angle spectroscopic ellipsometry in the energy range from 0.55to6eV. The energy bandgap bowing parameter was found to strongly depend on composition, monotonically increasing with decreasing InN mole fraction. This behavior is in agreement with theoretical predictions of large charge transfer contributions to bandgap bowing.

Epitaxial growth of gallium nitride thin films on A-Plane sapphire by molecular beam epitaxy

In this article, we propose a crystallographic model to describe epitaxy of GaN on (1120) sapphire (A plane). The (1102) cleavage plane in sapphire is shown to extend to the GaN lattice as the (1120) plane, facilitating the formation of cleaved facets. It is shown that, although the lattice mismatch is much smaller than in the case of epitaxy on (0001), the difference in the planar symmetry in this case results in high-strained bonds near the interface. The use of nitridation and a low temperature buffer is therefore necessary. A systematic study of GaN growth on the A-plane sapphire by plasma-assisted molecular beam epitaxy was carried out to study the effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films. Transmission electron microscopy (TEM) studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientation...

Biaxial strain and lattice constants of InN (0001) films grown by plasma-assisted molecular beam epitaxy

We present a systematic study, using high resolution x-ray diffraction, of the in-plane a and out-of-plane c lattice parameters of high quality InN films grown by molecular beam epitaxy on GaN∕Al2O3 (0001) substrates. It is found that their values are dependent on the nucleation and growth conditions. Films nucleated in a two- or three-dimensional growth mode exhibit biaxial compressive or tensile strain, respectively. The linear dependence of c on a is consistent with biaxial strain being present in the films. A biaxial strain relaxation coefficient of 0.43±0.04 is deduced. The values of the lattice constants for the case of strain-free InN are estimated to be in the ranges c=5.699±0.004A and a=3.535±0.005A.

Chemical ordering in AlGaN alloys grown by molecular beam epitaxy

Aluminum gallium nitride alloys were grown by molecular beam epitaxy and their film composition, structure, and microstructure were investigated by Rutherford backscattering spectroscopy, atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It was found that the ratio of group-III to group-V fluxes influences the relative incorporation of gallium and aluminum in the films. The transmission electron microscopy and x-ray diffraction studies revealed the existence of three types of spontaneously formed superlattice structures with periodicities of 2, 7, and 12 ML. While the 2 ML ordering is preferred under group-V rich conditions of growth, the 7 and 12 ML orderings were observed under group-III rich conditions of growth.

Physical model of InN growth on Ga-face GaN (0001) by molecular-beam epitaxy

A consistent physical model of the growth of InN on GaN (0001) by radio-frequency plasma-assisted molecular-beam epitaxy is presented. Four distinct regimes of InN growth are observed due to the temperature dependence of indium adatoms’ mobility and of the InN decomposition rate. At substrate temperatures higher than 450°C, indium adatoms are highly mobile and a self-regulating mechanism of InN islands’ diameter takes place, so that a stoichiometric N:In atomic ratio on the top face of the islands is established. As a result, two-dimensional growth is possible only with In∕N atomic ratio on the substrate surface equal to unity. The self-regulating mechanism could be exploited to engineer self-organized nanostructures.

Mechanism of compositional modulations in epitaxial InAlN films grown by molecular beam epitaxy

A mechanism for compositional modulations in InxAl1−xN films is described which considers growth kinetics during molecular beam epitaxy. InAlN crystalline films with various indium contents, grown on GaN or AlN buffer layers to create a variation in lattice mismatch conditions, were studied by transmission electron microscopy. Films comprise of columnar domains which are observed regardless of mismatch, with increasing indium concentration toward domain edges. We propose that indium is incorporated preferentially between adjacent dynamical InAlN platelets, owing to tensile strain generated upon platelet coalescence. The resulting In-rich boundaries are potential minima for further indium adatoms, creating a permanent indium composition gradient.