K. Tsagaraki

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.

6.3-GHz Film Bulk Acoustic Resonator Structures Based on a Gallium Nitride/Silicon Thin Membrane

This letter describes the fabrication and the morphological and microwave characterization of film bulk acoustic resonator structures, supported on very thin GaN membranes. We have demonstrated, by employing both white-light profilometry as well as X-ray diffraction, the low deflection and low stress of the GaN membranes supporting the resonator metallization. Using as test structure two FBAR structures connected in series, by a floating backside metallization, we have obtained resonance frequencies of 6.3 GHz for a 0.5-mum-thick membrane. The quality factor, at 6.3 GHz, was higher than 1100.

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.

Plasma-assisted MBE growth of quaternary InAlGaN quantum well heterostructures with room temperature luminescence

We report on the growth by rf plasma-assisted molecular beam epitaxy (RF-MBE) of high quality quatemary In_xAl_1-xGa_1-x-yN/GaN quantum well (QW) heterostructures, showing room temperature photoluminescence and lasing under optical pumping.

InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy

Single crystalline and single phase In(x)Ga(1-x)N nanopillars were grown spontaneously on (111) silicon substrate by plasma assisted molecular beam epitaxy. The surface morphology, structural quality, and optoelectronic properties of InGaN nanopillars were analyzed using scanning electron microscopy (SEM), energy dispersive x-ray (EDXA) analysis, high resolution x-ray diffraction (HR-XRD), and both room and low temperature photoluminescence spectra. The EDXA results showed that these nanopillars were composed of InGaN and the amount of indium incorporation in In(x)Ga(1-x)N NPs could be controlled by changing the growth temperature. The room temperature and low temperature PL spectra revealed that the emission wavelength could be tuned from a blue to green luminescent region depending on the growth temperature. The wavelength tuning was attributed to a higher amount of In incorporation at a lower growth temperature which was consistent with the EDXA and HR-XRD results.

Epitaxial growth, electrical and optical properties of a-plane InN on r-plane sapphire

The heteroepitaxy of a-plane (112¯0) InN films on r-plane (11¯02) sapphire substrates, by nitrogen radio frequency plasma-assisted molecular beam epitaxy, has been investigated and compared to that of c-plane (0001) InN. The epitaxial growth of a-plane InN proceeded through the nucleation, growth, and coalescence of three-dimensional islands, resulting in surface roughness that increased monotonically with epilayer thickness. The full width at half maximum of (112¯0) x-ray diffraction rocking curves decreased significantly with increasing InN thickness, characteristic of structural improvement, and it reached the value of 24 arcmin for a 1 μm thick film. Hall-effect measurements exhibited a similar dependence of electron concentration and mobility on thickness for both the a- and c-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with Ns>1014 cm−2 at the films’ surface/interfa...

Single-crystal hexagonal and cubic GaN growth directly on vicinal (001) GaAs substrates by molecular-beam epitaxy

Single-crystal hexagonal and cubic GaN thin films have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy directly on vicinal (001) GaAs substrates, misoriented by 2° toward [100], without using an incident As beam during oxide desorption or the following stages of growth. Both the GaAs nitridation and GaN growth conditions were found to control the structure of the layers. Cubic layers could be grown only without nitridation and under stoichiometric N/Ga flux ratio conditions. N-rich conditions favored the growth of hexagonal layers, which exhibited significantly higher photoluminescence intensities compared to cubic ones. Hexagonal single crystalline GaN films were grown with (1012) planes and presented characteristic surface roughness striations along a 〈110〉 substrate direction. On the contrary, a stepped surface morphology was observed for cubic GaN.

Structural anisotropic properties of a-plane GaN epilayers grown on r-plane sapphire by molecular beam epitaxy

Heteroepitaxial non-polar III-Nitride layers may exhibit extensive anisotropy in the surface morphology and the epilayer microstructure along distinct in-plane directions. The structural anisotropy, evidenced by the “M”-shape dependence of the (112¯0) x-ray rocking curve widths on the beam azimuth angle, was studied by combining transmission electron microscopy observations, Raman spectroscopy, high resolution x-ray diffraction, and atomic force microscopy in a-plane GaN epilayers grown on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). The structural anisotropic behavior was attributed quantitatively to the high dislocation densities, particularly the Frank-Shockley partial dislocations that delimit the I1 intrinsic basal stacking faults, and to the concomitant plastic strain relaxation. On the other hand, isotropic samples exhibited lower dislocation densities and a biaxial residual stress state. For PAMBE growth, the anisotropy was correlated to N-rich (or Ga-poor) condit...

The effect of arsenic overpressure on the structural properties GaAs grown at low temperature

The structural properties of GaAs grown by molecular‐beam epitaxy at low temperatures have been investigated by scanning electron microscopy, transmission electron microscopy, and high‐resolution x‐ray double‐crystal rocking curves as a function of arsenic overpressure during growth. It was found that surface smoothness and excess arsenic incorporation both depend strongly on growth temperature and on As/Ga flux ratio. For each growth temperature there is a ‘‘window’’ in the flux ratio which results in smooth surfaces. As‐grown layers have an increased lattice constant in the growth direction. This relative lattice expansion increases with flux ratio at a constant growth temperature and eventually saturates. Transmission electron micrographs have revealed the presence of arsenic precipitates in material annealed at 600 °C. Increasing the As4 pressure during growth results in increases in precipitate diameter by almost 50% while their density and shape remain constant. Based on these observations a model h...

Evaluation of reactive ion etching processes for fabrication of integrated GaAs/AlGaAs optoelectronic devices

Abstract Reactive ion etching (RIE) was used for the fabrication of GaAs/AlGaAs optoelectronic devices (laser diodes and photodetectors) for optical interconnect applications. Smooth, vertical sidewalls with a smooth surface at the field were obtained after optimizing RIE conditions in BCl 3 -formed plasma. Accurate in-situ monitoring of the etching process was realized by laser interferometry end-point detection. This led to good process control and reproducibility of the demanding fabrication of the optoelectronic devices. The RIE etching process did not affect the electrical properties of the device by increasing the surface recombination currents. Lasers with etched mirrors exhibited a threshold current density of 970 A cm −2 , which is one of the best values ever reported. The feasibility of a simple technology for the fabrication of optoelectronic circuits, based on a BCl 3 RIE process for laser mirror etching, has been demonstrated.