A. Adikimenakis

Growth mechanism and microstructure of low defect density InN (0001) In-face thin films on Si (111) substrates

Transmission electron microscopy has been employed to analyze the direct nucleation and growth, by plasma-assisted molecular beam epitaxy, of high quality InN (0001) In-face thin films on (111) Si substrates. Critical steps of the heteroepitaxial growth process are InN nucleation at low substrate temperature under excessively high N-flux conditions and subsequent growth of the main InN epilayer at the optimum conditions, namely, substrate temperature 400–450u2009°C and In/N flux ratio close to 1. InN nucleation occurs in the form of a very high density of three dimensional (3D) islands, which coalesce very fast into a low surface roughness InN film. The reduced reactivity of Si at low temperature and its fast coverage by InN limit the amount of unintentional Si nitridation by the excessively high nitrogen flux and good bonding/adhesion of the InN film directly on the Si substrate is achieved. The subsequent overgrowth of the main InN epilayer, in a layer-by-layer growth mode that enhances the lateral growth o...

Electron density and currents of AlN/GaN high electron mobility transistors with thin GaN/AlN buffer layer

AlN/GaN high electron mobility transistor (HEMT) structures with thin GaN/AlN buffer layer have been analyzed theoretically and experimentally, and the effects of the AlN barrier and GaN buffer layer thicknesses on two-dimensional electron gas (2DEG) density and transport properties have been evaluated. HEMT structures consisting of [300u2009nm GaN/ 200u2009nm AlN] buffer layer on sapphire were grown by plasma-assisted molecular beam epitaxy and exhibited a remarkable agreement with the theoretical calculations, suggesting a negligible influence of the crystalline defects that increase near the heteroepitaxial interface. The 2DEG density varied from 6.8u2009×u20091012 to 2.1 × 1013 cm−2 as the AlN barrier thickness increased from 2.2 to 4.5u2009nm, while a 4.5u2009nm AlN barrier would result to 3.1u2009×u20091013 cm−2 on a GaN buffer layer. The 3.0u2009nm AlN barrier structure exhibited the highest 2DEG mobility of 900u2009cm2/Vs for a density of 1.3u2009×u20091013 cm−2. The results were also confirmed by the performance of 1u2009μm gate-length transistors...

Selective-area growth of GaN nanowires on SiO2-masked Si (111) substrates by molecular beam epitaxy

We analyze a method to selectively grow straight, vertical gallium nitride nanowires by plasma-assisted molecular beam epitaxy (MBE) at sites specified by a silicon oxide mask, which is thermally grown on silicon (111) substrates and patterned by electron-beam lithography and reactive-ion etching. The investigated method requires only one single molecular beam epitaxy MBE growth process, i.e., the SiO2 mask is formed on silicon instead of on a previously grown GaN or AlN buffer layer. We present a systematic and analytical study involving various mask patterns, characterization by scanning electron microscopy, transmission electron microscopy, and photoluminescence spectroscopy, as well as numerical simulations, to evaluate how the dimensions (window diameter and spacing) of the mask affect the distribution of the nanowires, their morphology, and alignment, as well as their photonic properties. Capabilities and limitations for this method of selective-area growth of nanowires have been identified. A windo...

Micromachined GaN-based FBAR structures for microwave applications

This paper reports on microwave characteristics of micromachined GaN-based thin-film bulk acoustic resonator devices. The 2.2 micron active piezoelectric layer was epitaxially grown on (111)-oriented high-resistivity silicon substrate. Bulk micromachining techniques were used for the release of the resonating GaN membrane structure. S-parameter measurements have shown a fundamental mode resonance around 1.2 GHz. Extracted material parameters such as acoustic velocity and effective coupling coefficient are in good agreement with those reported in the literature using other methods. These are, to our knowledge, the first FBAR results with GaN-based active layers.

Investigation of thin InN/GaN heterostructures with in situ SiNx dielectric grown by plasma-assisted molecular beam epitaxy

The effects of InN layer thickness (4/7/10u2009nm) in metal–insulator–semiconductor Ni/SiNx/InN structures have been evaluated. The 7u2009nm thick SiNx layer is deposited in situ, by plasma assisted molecular beam epitaxy, on the surface of InN grown on GaN (0001) buffer layers. Metal–insulator–semiconductor capacitors (MISCAPs) and InN channel field-effect transistors (MISFETs) were fabricated and the electrical characteristics of the devices were studied and discussed. Room temperature current versus voltage analysis of the MISCAPs suggested ohmic conduction by hopping at low electric fields, while field emission was prevailed for high electric fields with an extracted trap barrier height in the range of 1.1–1.3u2009eV for all the structures. The output characteristics of the fabricated MISFETs showed modulation of the drain–source current with the highest current density of 0.8u2009A/mm for the 10u2009nm InN layer, but the channel could not fully pinch-off.

Different polarities of InN (0001) heterostructures on Si (111) substrates

The growth processes of plasma assisted molecular beam epitaxy (PAMBE) that result to heteroepitaxial growth of high crystal quality single polarity {0001} InN thin films on Si (111) substrates, with either N-face or In-face polarity, have been determined. N-face InN (000-1) films can be grown using high temperature AlN nucleation layer on Si (111) and In-face In (0001) films can be grown by direct InN nucleation on Si at low temperature and N-rich conditions.

Material and electrical properties of N-polar (GaN)/InN surfaces

Angle-resolved XPS-analysis of the bare InN surface has revealed surface electron accumulation and negative band bending by about 0.65 eV at a distance of 1.1 nm. On the other hand this effect was substantially reduced by capping the InN surface by 2 to 4 monolayer of GaN. Negative polarisation charge at the GaN/InN interface of (partially) strained cap layer together with elimination of dangling bonds at the InN surface were consequently suggested. RIE in CCl2F2 provided highly selective etching of GaN over InN. Al2O3 deposited by ALD on (GaN)/InN surface created structures with metal-insulator-metal-like behaviour.

AlN/GaN/AlN double heterostructures with thin AlN top barriers

AlN/GaN/AlN double heterostructures grown by PAMBE on sapphire (0001) substrates with different AlN top barrier thicknesses, ranging between 1.5 and 4 nm, were investigated. The two-dimensional electron gas (2DEG) density increased with AlN barrier thickness in agreement with the theoretical values. The highest value of 2.2×1013 cm-2 for the HEMT structure with 4.5 nm AlN barrier thickness and the highest mobility of 900 cm2/Vs for the structure with 3 nm AlN barrier thickness were obtained. HEMT devices, processed with a gate length of 1μm, exhibited a maximum drain current of 1.1 A/mm for 3 and 3.7 nm AlN barrier thickness. The transistor threshold voltage scaled linearly from +0.2V to-2.7V by varying the AlN barrier thickness from 1.5 nm to 4.5 nm respectively showing a normally-off potential use.

Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy

The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si (111) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between the bottom 0.5 micron GaN layer and the top 1.0 micron GaN layer. The interlayer was very effective to reduce the tensile stress in the overall 1.5 micron GaN/Si film to the level required for complete avoidance of microcracks, which were present in high densities in GaN/Si heterostructures grown without an AlN interlayer. The strain of the AlN interlayer, as well as the strain in all the layers of the entire GaN/Si heterostructure was analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space map in XRD indicated that the 15 nm AlN interlayer was coherently strained with the GaN films. However TEM observations revealed that the AlN interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as V-defects, GaN was overgrown with a large density of threading dislocations and inversion domain boundaries.