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The results of using molecular-beam epitaxy for growing piezoelectric AlN films on Si (111) substrates suitable for device applications are reported. The technological conditions for growth of stoichiometric AlN by controlling the surface reconstruction occurring under various thermodynamic conditions on the growth surface are determined. The films of the hexagonal polytype of AlN possess high crystalline perfection and an atomically smooth epitaxial surface. The mechanism for relaxation of the AlN crystal lattice over a distance of one monolayer from the heterojunction is found. It is demonstrated that the AlN film is piezoelectric. Investigations of the temporal characteristics of a SAW attest to a low level of scattering of the wave during propagation. The electromechanical coupling constant is measured in interdigital transducer geometry (λ=16 mm) and is found to be 0.07 % at a frequency f=286 MHz, in good agreement with the theoretical value for a 1.04-µm-thick AlN film.

Epitaxial wurtzite aluminum nitride (AlN) and gallium nitride (GaN) films have been grown on Si(1 1 1) by plasma-assisted molecular beam epitaxy (PA-MBE). Two-dimensional growth (2DG) of single-crystalline AlN films is achieved on Si(1 1 1) near stoichiometric supply of aluminum and atomic nitrogen. Two distinct categories of AlN-surface reconstructions have been observed. GaN films exhibit 1×1 high-energy electron diffraction (RHEED) pattern if grown between 650°C and 770°C substrate temperature on AlN buffers. Stable Ga- and N-adlayer-induced surface reconstructions have been studied below 600°C after the growth. The X-ray diffraction (XRD) pattern show sharp and well separated (0 0 0 l ) reflections of wurtzite GaN and AlN indicating complete texture with GaN[0 0 0 1]‖AlN[0 0 01]‖Si[1 1 1]. From the determined GaN lattice constant complete strain relaxation can be concluded which is further confirmed by the temperature-dependent photoluminescence (PL) investigations.

Epitaxial aluminum nitride AlN(0001) thin films have been grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si(111). The influence of the composition of the nitrogen plasma on the crystal quality, as judged by X-ray diffractometry (XRD) and atomic force microscopy (AFM), has been investigated. Under an Al/N vapor phase ratio close to unity atomically smooth AIN films have been grown at 850°C substrate temperature with maximum growth rates of 2.5 nm min - 1 . A √3 × √3 and a more Al-rich 2 × 6-surface reconstruction have been observed. Transmission electron microscopy (TEM) investigations show that these films are homogeneous 2H-AIN single crystals. Their defect structure consists of threading dislocations mostly. The hetero-interface is abrupt and flat. Processed high-resolution (HR) TEM images demonstrate a 4×d Si(T10) to 5 × d AlN(2110) coincidence between substrate and epilayer. The XRD FWHM of the (0002)-diffraction peak of 0.5 μm AIN is 0.06° in the ω/2 scan and 0.32° in the ω scan. Phonon modes of AIN have been detected by Raman and infra-red spectroscopy.

The effects of substrate cleaning, nitridation time, and substrate temperature in the range 800–1000 °C on the microstructure of AlN/Si(111) films grown by simultaneous plasma-assisted molecular beam epitaxy have been investigated. It has been demonstrated, using a combination of conventional and high-resolution transmission electron microscopy, that the interface structure, the film defect structure, and the film surface roughness are strongly related. The formation of single crystal 2H–AlN films with atomically flat surfaces occurs at 800 °C for conditions of 2.5 nm/min growth rate on very pure, atomically flat Si substrates.

The conditions for the growth of single-crystalline wurtzite aluminum nitride (AlN) films on Si(1 1 1) have been established by plasma-assisted molecular beam epitaxy. The yield of atomic nitrogen from the RF-source has been investigated by optical and by mass-spectrometry. AlN films grown under RF-source conditions with the highest yield of atomic nitrogen, as determined by mass-spectrometry, generally show narrowest AlN(0 0 0 2) rocking curve linewidths in X-ray diffraction. By supplying nitrogen and aluminum close to unity, AlN has been grown two-dimensionally with atomically smooth surfaces. Surface reconstructions have been investigated by reflection of high-energy electron diffraction as a function of the substrate temperature and the Al-overpressure. High-resolution transmission electron micrographs taken at the heterointerface show a coincidence between both materials with a ratio of 5·dAlN(2110) to 4·dSi(110). The misfit is accomodated within the very first monolayers on either side.

Epitaxial wurtzite aluminum nitride (AIN) films have been grown on (111) oriented silicon substrates by plasma-assisted solid source molecular beam epitaxy (MBE). The growth has been performed in the two-dimensional (2D) growth mode. Occuring surface reconstructions have been monitored by reflection of high-energy electron diffraction (RHEED). The films have been characterized by atomic force microscopy (AFM) and transmission electron microscopy (TEM). 2D grown films exhibit atomically smooth surfaces, are singlecrystalline and void of oriented domains. High-resolution (HR) TEM micrographs of the heterointerface show the presence of an interfacial atomic arrangement where (5:4) unit cells coincide along the AIN[2110]/Si[110] direction.Surface acoustic wave (SAW) properties of MBE-grown AIN(0001)/Si(ll1) have been studied for the first time. The AIN film exhibits very good piezoelectricity. The SAW time response indicates low scattering of the waves during their propagation. An electromechanical coupling coefficient (EMC) of 0.07% has been measured for the present interdigital transducer (IDT) geometry.