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InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

The exploitation of the two-dimensional electron gas (2DEG) channel of an AlN/GaN/AlN double barrier heterostructure, for High Electron Mobility Transistors (HEMTs) with metal–oxidesemiconductor (MOS) tri-gate around a fin-shaped channel (MOS-FinHEMTs), has been investigated by combining fabrication, dc I-V measurements and simulations of single-fin MOS-FinHEMT devices. The dependence of the threshold voltage (Vth) and the maximum drain-source current (Ids,max) on the fin width (Wfin), as well as the effects of ohmic contact resistance, gate-drain and source-gate distance and of the Al2O3 gate dielectric thickness (tox), have been addressed. Fabricated single-fin MOS-FinHEMT devices with tox = 20 nm, exhibited a positive shift of Vth, in comparison to a reference planar-gate device, ranging from +0.8 V for Wfin = 650 nm to +3.4 V for Wfin = 200 nm, due to lateral depletion of the channel by the gate contacts on the fin sidewalls. Simulations reproduced the experimental Vth values and also predicted the Vth of devices with narrower fins, down to Wfin = 10 nm. The boundary for normally-off operation (Vth = 0 V) was determined at Wfin = 17 nm that may increase up to 31 nm if the tensile strain of the top AlN barrier in the fin nanostructure is elastically relaxed. A reduction of maximum drain-source current per top gate width (Ids,max/Wg) with decreasing Wfin in the range of 200–650 nm may result from increased ohmic contact resistance. However, for narrower fins, Ids,max/Wg is predicted to decrease significantly with decreasing Wfin, due to the lateral field of the sidewall gates. The Ids,max/Wg will also decrease with increasing distance between the source, gate and drain contacts for any Wfin. Finally, the Vth and Ids,max/Wg values were calculated for Al2O3 thickness in the range of 5 to 40 nm.

The effects of ultrathin AlN prelayers, with nominal thicknesses between 0 and 1.5 nm, on the spontaneous growth of GaN nanowires (NWs) on Si (1 1 1) substrates were investigated. The morphological and structural characteristics of GaN NWs were analyzed by electron microscopy and X-ray diffraction techniques. The results quantify how the AlN prelayer thickness affects GaN NWs. The increase of AlN thickness gradually limits nitridation of the substrate surface and accelerates 3D GaN nucleation. The formation of amorphous SixNy by Si nitridation is completely avoided for 1.5 nm of AlN that fully covers the Si surface. The dependence of the height, diameter and density of GaN NWs on the AlN thickness was also determined. The 1.5 nm AlN provided the optimum condition for GaN NW nucleation and growth; the NWs exhibited a large homogeneous height with almost no parasitic GaN formation between them. High resolution transmission electron microscopy showed the full relaxation of misfit strain of AlN on Si (1 1 1) and of GaN NWs on AlN. In-situ reflection high energy electron diffraction during AlN nucleation revealed the immediate relaxation of the AlN prelayer before GaN NW nucleation. Formation of β-Si3N4 before AlN nucleation was also observed.

The effects of an amorphous interfacial silicon nitride (SiXNY) layer on the morphology, structure and optoelectronic properties of GaN nanowires (NWs), grown on Si (111) substrates by plasma assisted molecular beam epitaxy, have been investigated. The unintentional Si nitridation, during the first stages of direct GaN NW growth on the bare Si surface, has been compared to intentional Si nitridation prior to GaN growth. The intentional nitridation resulted in a uniform ~1.5 nm amorphous SiXNY interlayer at the GaN/Si interface, while an irregular and non-uninform interface, with partial presence of amorphous SiXNY, appeared for direct GaN on Si growth. The homogeneity of the interfacial structure enhanced the degree of crystallographic alignment of the GaN NWs, concerning both tilt and twist. It also decreased the dispersion of NW heights that is otherwise triggered by different nucleation times on structurally different sites of the substrate. The average height of the NWs was similar for both cases but their average diameter was increased from 25 nm to 40 nm on the uniform amorphous SiXNY interlayer, possibly an effect of weak epitaxial constraints. Reduced overall intensity and increased defect-related emission at 3.417 eV characterized the 20 K photoluminescence spectra for direct GaN growth on Si. The results contribute to a better understanding of how the GaN/Si interfacial structure affects the GaN NW growth and properties.

Current conduction mechanism, including electron mobility, electron drift velocity (vd) and electrical break-down have been investigated in a 0.5 μm-thick (0001) InN layer grown by molecular-beam epitaxy on a GaN/sapphire template. Electron mobility (μ) of 1040 cm2/Vs and a free electron concentration (n) of 2.1 × 1018 cm−3 were measured at room temperature with only a limited change down to 20 K, suggesting scattering on dislocations and ionized impurities. Photoluminescence spectra and high-resolution X-ray diffraction correlated with the Hall experiment showing an emission peak at 0.69 eV, a full-width half-maximum of 30 meV, and a dislocation density Ndis ∼ 5.6 × 1010 cm−2. Current-voltage (I-V) characterization was done in a pulsed (10 ns-width) mode on InN resistors prepared by plasma processing and Ohmic contacts evaporation. Resistors with a different channel length ranging from 4 to 15.8 μm obeyed the Ohm law up to an electric field intensity Eknee ∼ 22 kV/cm, when vd ≥ 2.5 × 105 m/s. For higher ...

Gallium nitride (GaN) all-around (wrap) gate vertical nanowire (V-NW) field-effect transistors (FETs) are favorable for enhanced electrostatic control of the gate and selectivity for normally on/off operation. In this work, GaN V-NW FETs with a Schottky barrier gate (V-NW MESFETs), were fabricated for the first time. A nanofabrication process with comprehensive description of all processing steps is reported. It was validated with the demonstration of GaN V-NW MESFETs consisting of an array of 900 (30 × 30) GaN NWs with the narrowest until now reported diameter of 100 nm and all-around gate length of 250 nm. The GaN NWs were formed by a top-down approach, which combines conventional nanopatterning techniques and anisotropic wet etching of an initial GaN epilayer, grown by plasma assisted molecular beam epitaxy on a sapphire (0001) substrate. DC I-V characteristics exhibited normally-off operation and threshold voltage of +0.4 V, due to electron depletion region from the all-around Schottky barrier. A maximum drain-source current density (J ds) of 330 A cm-2 and maximum transconductance (g m) of 285 S cm-2 were obtained from I-V measurements. The results and directions for further optimization were discussed.

The mechanism of Si outdiffusion related to the formation of Si-rich GaN clusters during the growth of GaN-on-Si by plasma-assisted molecular beam epitaxy (PAMBE) has been identified. GaN thin films were grown, on high resistivity Si (111) substrates at Ga-rich conditions to favour the 2D step-flow growth mode. Hall-effect measurements revealed very high electron background concentrations, ranging from 2 x 1018 cm-3 to 3 x 1019 cm-3, with degraded carrier mobility in the range of 5-30 cm2/Vs. SEM examination of surfaces and cross-sections of the samples revealed the presence of hollow GaN clusters on the film surface and voids at the Si/III-nitride interface. EDX spot analysis revealed dissolution of Si material in the GaN clusters. Growth of GaN on Si under stoichiometric conditions resulted in films without GaN clusters, which exhibited a serious reduction in carrier concentration, down to 1.4 x 1016 cm-3, with a simultaneous increase of the carrier mobility to 137 cm2/Vs. C-V carrier profile measurements were consistent with Hall-effect measurements and revealed a uniform electron concentration in the GaN film, down to the AlN nucleation layer (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

a-plane () GaN thin films grown under various conditions on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE) were characterized by nanoindentation and transmission electron microscopy (TEM) techniques. It was found that the increase of the built-in threading dislocation (TD) density induces an increase on the values of both the nanohardness (H) and the reduced elastic modulus (E*). Characteristic single ‘pop-in’ discontinuities were detected on the loading segments of the load–unload cycle. The TDs appear to cause a decrease of the load for the onset of the pop-in discontinuity. The dependence of the elastoplastic behaviour on the TD density is attributed to their sessile character, causing them to become obstacles for basal plane slip.

Based on bicrystal symmetry and transmission electron microscopy observations, we elaborate on the coexistence of the two orientation variants of semipolar s-plane () InN epilayers grown on r-plane sapphire by plasma-assisted molecular beam epitaxy (PAMBE). It is shown that variant coexistence is favored by a high order of coincident symmetry ensuring significant lattice continuity. The (0002) || () low-energy grain boundary was identified to principally delimit the two InN variants. Aside of the variant coexistence, the InN/sapphire interface was observed to comprise protrusions attributed to the InN buffer layer growth. Rapid thermal annealing was employed in order to improve the epilayer quality and it was found to induce defect reduction attributed to dislocation glide. However, the InN/sapphire interface was adversely affected by this process.

The strain anisotropy of a-plane GaN epilayers is studied by polarization-dependent Raman spectroscopy. The intensity dependence of the A1(TO), E1(TO) and Raman peaks for an almost strain-free epilayer is well explained by using Raman tensors of C6v symmetry. However, the corresponding angular dependence for anisotropically strained GaN with |exx − eyy|∼6 × 10−3 (where x||a and y||m axes) dictates the use of a perturbation term of C2v symmetry that accounts for the orthorhombic distortion of the hexagonal unit cell. The consideration of this term elucidates the appearance of the mode for such epilayer orientations where it is forbidden by the hexagonal symmetry.

The vibrational properties of InN samples grown by molecular beam epitaxy (MBE) technique have been studied using infrared (IR) and Raman scattering spectroscopy at room temperature. In the Raman measurements, the 532 nm (2.33 eV) line of laser was used as the excitation source. Lower branch (L−) of the longitudinal-optic-phonon-plasmon-coupled (LOPC) mode at ∼430 cm−1 was too weak to be observed clearly in Raman measurements. It was however, strong in the IR spectra. A strong A1(LO) mode was also observed in Raman measurements and this mode together with the L− mode were used to calculate the electron effective mass in InN as a function of carrier density. In the theoretical calculation we used both the Drude and Linhard–Mermin models and obtained the electron effective mass in the range between 0.07 and 0.167m0 with increasing electron density from 0.79 to 2.8 × 1019 cm−3.

The structural properties of InN films grown on r -plane (102) sapphire by plasma-assisted molecular beam epitaxy (PAMBE) using low temperature buffer layers are studied. Nonpolar a -plane (120) and semipolar s -plane (101) films were deposited depending on buffer layer and growth conditions. Single crystalline a -plane InN was grown using a GaN buffer layer following sapphire nitridation. Transmission electron microscopy (TEM) observations showed three-dimensional growth and interactions of inclined threading dislocations emanating from the buffer layer. The defect interactions gradually reduced the threading dislocation density. Semipolar s -plane was grown when a thin InN buffer layer was employed. The epilayer was found to comprise two s -plane variants. Semipolar nucleation directly on the sapphire was observed. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

By using a Schrodinger-Poisson equation solver, we calculate band diagrams of potentially record fast III-N high electron mobility transistors (HEMTs), which are based on strained InN channels. Assuming cation polarity, pseudomorphic HEMT devices are proposed with a relaxed InAIN buffer layer having Al mole fraction in the range of 0.10-0.15 and with an InAIN barrier layer. Calculations indicate highly confined electrons in the channel with a density of 1.4-2 × 1013 cm-2 if the 5-10-nm-thick InN channel is separated from the barrier by ≤ 0.8-nm-thick GaN spacer. Alternatively, for a nonpolar structure with an Al mole fraction of 0.3 in the InAIN buffer and for a doping 5 × 1019 cm-3 in the InAIN barrier, we calculate the InN channel carrier density of approximately 1.4 × 1013 cm-2. We propose to use high-k dielectrics to insulate the gate from the barrier for both of the transistor structures.

In this paper we report on low-pressure metalorganic vapour deposition of InAlN/GaN heterostructures grown on different substrates (Sapphire, bulk SiC, composite SiCopSiC) for HEMT applications, and on first device performances obtained with these heterostructures. Optimisation of the crystal growth on each kind of substrate has led to InAlN/GaN HEMT heterostructures grown on bulk SiC and on composite SiCopSiC substrates which are successfully compared, in terms of material quality, to the standard GaAlN/GaN HEMT heterostructures grown on bulk SiC substrates. First devices based on InAlN/GaN heterostructures grown on bulk SiC exhibit very good microwave performances, with output power of 10.3 W/mm at 10 GHz, similar to those obtained with GaAlN/GaN heterostructures, confirming the promising potential of InAlN material. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The mopholological, structural and chemical properties of InxGa1–xN nanopillars directly grown on Si (111) substrates, by molecular beam epitaxy, were investigated employing transmission electron microscopy related techniques. Single crystalline, single phase nanopillars were observed exhibiting a low density of crystal defects, which contribute to good crystal quality. Initial nanostructures merge through subgrain boundaries to form final nanopillars. Energy dispersive X-ray analysis revealed a very low InN mole fraction near the interface with the substrate, owing to high desorption rates from the elevated growth temperature, and gradually higher In incorporation rates near the tips of the nanopillars. This compositional fluctuation is maintained due to poor segregation of indium adatoms along the c-axis of the nanopillars towards the Si interface. A second species of long and narrow nanopillars was found In-free. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InxAl1-xN (0001) wurtzite layers with 0.2 0.45. In the case of the Al rich samples, an additional broad feature appears. The Raman spectra are fitted using the spatial correlation model providing the extent of the phonon confinement imposed by the alloy potential fluctuations is estimated. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper, we report on the catalyst-free growth and the optoelectronic properties of GaN and InN nanowires (NWs) grown on Si(111) substrates by nitrogen RF plasma source molecular beam epitaxy (RF-MBE) without the use of intermediate GaN or AlN buffer layer. The growth conditions were optimized in order to fabricate well-aligned, stress-free single crystalline nanowires. In both cases, the III-nitride NWs were generated from the lattice mismatch strain of the materials on Si and the high surface energy of their nitrogen stabilized surfaces. Both PL and Raman spectroscopy revealed that the NWs were fully relaxed. XRD results further assert the single crystallinity of the nanowires.

The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si(1 1 1) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between a bottom 0.5 μm GaN layer and the top 1.0 μm GaN layer. The interlayer was effective in reducing the tensile stress to the level required for complete avoidance of microcracks, which were present in high densities in the case of GaN-on-Si heterostructures grown without an AlN interlayer. The strain in all the layers of the heterostructure was analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space mapping in XRD indicated that the 15 nm AlN interlayer was on average coherently strained with the GaN. However, TEM observations showed that the 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-shaped surface depressions, GaN was overgrown with a high density of threading dislocations and inversion domain boundaries. A synergistic relaxation mechanism is proposed for the AlN interlayer leading to an elastically strained interlayer interconnected by plastically relaxed patches.

The catalyst-free growth and the optoelectronic properties of GaN nanowires (NWs) grown on (1 1 1) Si substrates by plasma-assisted molecular beam epitaxy have been investigated. At constant N/Ga flux ratio, the NW morphology, density and growth rate are controlled by the substrate temperature, which affects the gallium adatom diffusion length before desorption. An increase in substrate temperature results in lower growth rate and smaller diameter of NWs with lower areal density NWs. Low-temperature photoluminescence spectra at 20 K revealed that PL intensity ratio of donor-bound exciton peak (D°X at 3.470 eV) with defect-related peak (Y 2 at 3.424 eV) increased with increase in substrate temperature. Micro-Raman spectra showed that the GaN NWs are completely stress free irrespective of the growth conditions.

The interfacial structure and extended defects in a-plane (1120)-oriented GaN layers are investigated using conventional and high resolution transmission electron microscopy (HRTEM). The a-plane GaN epilayers have been grown by nitrogen rf plasma source molecular beam epitaxy (RFMBE) on r-plane sapphire substrates, which have been nitrided either at 200 °C or at 800 °C for one hour. A thin interfacial layer of strained AlN, with an average thickness of about 1 nm, was formed during nitridation of the sapphire surface at both temperatures. The extended defects observed in the GaN layers are predominantly threading dislocations and basal stacking faults (BSFs), which originate from the GaN/sapphire interface. The threading dislocation density is slightly reduced in the film nitrided at 800 °C, as is the density of inversion domains, although stacking fault density does not appear to change with nitridation conditions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of temperature on the bonding environment of indium in an InN epilayer is studied using X-ray absorption fine structure (EXAFS) spectroscopy. Shell-by-shell fitting of the EXAFS spectra reveals that, in the temperature range 80-245 K, the change in the nearest neighbour distances is smaller than the uncertainty of the measurement. The Debye-Waller factor of the second neighbouring shell (In-In pair) shows strong temperature dependence. Fitting using the Einstein model yields an Einstein temperature equal to 193 K + 5%. An alternative way of extracting information on the vibrational properties of the lattice is the simulation using the equation of motion method for the determination of the f Debye-Waller factors. The used values of 120 N/m and 50 N/m of the force constants for In-N and In-In bond stretching, respectively, simulate satisfactory the spectra measured at five different temperatures in the range 80-245 K.

The strain distribution and defects in a graded AlN/GaN heterostructure comprising AlN layers from 3 nm up to 100 nm grown by plasma-assisted MBE were studied using transmission electron microscopy techniques. Gradual strain relaxation was observed as well as strain partitioning between the GaN spacers and the thicker AlN layers. Elastic strain is retained even in the thicker layers of the heterostructure. Extensive introduction of threading and misfit dislocations is observed at and above the 7 nm AlN layer. The threading dislocations adopt inclined zig-zag line directions thus contributing to the relief of alternating compressive-tensile elastic strain across the the layers of the heterostructure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the bonding environment of In in In X Al i - x N (0.07 < x < 0.25) and InyGai-yN (0.7

We investigate the use of the AlGaN/GaN high electron mobility transistor (HEMT) as a novel transducer for the development of ion-selective chemically modified HEMT sensors (ChemHEMTs). For this, polyvinyl chloride (PVC) membrane doped with ion-selective ionophores is deposited onto the area of the gate for the chemical recognition step, while the AlGaN/GaN HEMT is used as the transducer. In particular, the use of a valinocycin doped membrane with thickness of 50 microm generates a sensor with excellent analytical characteristics for the monitoring of K(+). The K(+)-ChemHEMT has sensitivity of 52.4 mV/pK(+)in the linear range of 10(-5) to 10(-2)M, while the detection limit is in the order of 3.1 x 10(-6)M. Also, the sensor shows selectivity similar to valinomycin-based ISEs, while the signal stability over time and the measurement to measurement reproducibility are very good.

The in-plane lattice parameters of InN, GaN and Al 2 O 3 in a InN/GaN/Al 2 O 3 (0001) heterostructure have been measured as a function of temperature in the range of 25-350 °C, using high resolution X-ray diffraction. The results reveal that both the GaN and InN crystals follow the in-plane thermal expansion of the Al 2 O 3 substrate's lattice and there is no rearrangement of misfit dislocations at the InN/GaN and GaN/Al 2 O 3 interfaces. It was also found that either compressive or tensile character of residual biaxial strain is possible for the InN films, depending on the two-dimensional (2D) or three-dimensional (3D) growth mode of InN on the GaN(0001) buffer layer. The tensile strain is inherent to the nucleation and coalescence of 3D islands.

Chemically modified Field Effect Transistor (ChemFET) sensor for ammonium (NF 4+ ) ions was developed from AlGaN/GaN High Electron Mobility Transistor (HEMT) structures (NH + 4 -ChemHEMT). The sensor consists of a HEMT device with non-metallized gate, which was coated with a polyvinyl chloride (PVC) membrane enriched with ammonium ionophore (Nonactin). The AlGaN/GaN -CNH + 4 -ChemHEMT with gate width of 100 μm and gate length of 100 μm exhibited a constant sensitivity of 55.5 mV/pNH + 4 for NH + 4 concentration in the range from 10 -5 M to 10 -2 M. The NH + 4 concentration detection limit was 5.4 x 10 -6 M, which is approximately one order of magnitude lower compared to Si based insulator gate ChemFETs. The sensor exhibited also sufficiently long storage lifetime, indicating the strong adhesion of the PVC membrane to the GaN(0001) surface.

We investigate electrical properties of Ni/Al2O3/GaN metal–oxide–semiconductor (MOS) structures having different pre-treatment of GaN surface by O2, Ar and NH3, combined with various temperature of annealing. MOS and reference Ni/GaN Schottky contact are characterized using current–voltage and capacitance–voltage methods. MOS structures compared with the Schottky contact ones show leakage current reduction for all types of processing, from 3 to 5 orders of magnitude in reverse direct. We observed substantial influence of the pre-treatment on electrical parameters of MOS structures.

The molecular beam epitaxy of In-face InN (0001) epilayers with optimized surface morphology, structural quality, and electrical properties was investigated. Namely, compact InN epilayers with atomically flat surfaces, grown in a step-flow mode, were obtained using stoichiometric fluxes of In and N and substrate temperatures in the range from 400°C to 435°C. Typical values for the electron concentration and the Hall mobility at 300 K were 4.3 × 1018 cm−3 and 1210 cm2/Vs, respectively. The growth mode of InN during the very first stage of the nucleation was investigated analytically, and it was found that the growth proceeds through nucleation and fast coalescence of two-dimensional (2-D)–like InN islands. The preceding conditions were used to grow an InN/GaN quantum well (QW) heterostructure, which exhibited well-defined interfaces. Schottky contacts were successfully fabricated using a 15-nm GaN barrier enhancement cap layer. Capacitance-voltage measurements revealed the confinement of electrons within the InN QW and demonstrated the capability to modulate the electron density within an InN channel. The sheet concentration of the confined electrons (1.5 × 1013 cm−2) is similar to the calculated sheet polarization charge concentration (1.3 × 1013 cm−2) at the InN/GaN interface. However, electrons may also originate from ionized donors with a density of 8 × 1018 cm−3 within the InN layer.

The growth properties of InN quantum dots on GaN (0001) surfaces by molecular beam epitaxy are being investigated. The dependence of the dimensions and density of the dots on the nucleation temperature and their evolution during growth at a constant substrate temperature are described. It is shown that both dimensions and density can be accurately controlled through nucleation temperature and deposition time. In the range from 400 °C to 450 °C, the formation of InN quantum dot structures of small dimensions and high density is feasible. InN dots with less than 3nm height, less than 22 nm diameter and with density higher than 1.7×1011 cm–2 have been obtained. Finally, vertical alignment of dots was observed for a multi-period InN quantum dot structure in GaN matrix, grown at 430 °C. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structural properties of a 10 μm thick In-face InN film, grown on Al 2 O 3 (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10 10 cm −2 , 4.82×10 8 cm −2 and 1.69×10 9 cm −2 for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10 9 cm −2 and 1.20×10 9 cm −2 , respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10 9 cm −2 , in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.

This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 〈111〉-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer. Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the (10 over(1, ̄) 5) reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer. GaN layers, n-doped with silicon in the range from 1015 to 1019 cm-3 exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm2/V s at 300 K and 10 220 cm2/V s at 77 K for a sheet carrier density of 9.6E12 cm-2. The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger-Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band. © 2006 Elsevier Ltd. All rights reserved. 40 313 319 313-319

Quaternary In 0.085 Al 0.285 Ga 0.63 N/GaN multiple quantum wells (MQW) grown by plasma-assisted molecular beam epitaxy are characterized by high resolution transmission electron microscopy (HRTEM), geometric phase analysis, and energy dispersive X-ray (EDX) nano-analysis. The MQW exhibit sharp well-defined InAlGaN/GaN interfaces while the GaN/InAlGaN interfaces are more smeared. The InAlGaN quantum wells and the GaN barriers are lattice-matched. Chemical distribution profiles are extracted from EDX line scans, obtained with a nanoprobe, which are compared to convoluted theoretical profiles, and are in good agreement with the HRTEM observations. Indium clustering occurs after prolonged observation under the electron beam.

InAlGaN/GaN heterostructures and thin films with In composition ranging from 0.03 to 0.1 are characterized by means of Raman scattering excited at various energies in the ultra violet range, tuning the laser excitation energy through the band gap of In x Al y Ga 1-x-y N. It is shown that the addition of In to the Al y Ga 1-y N alloy diminishes considerably the vibration energy of the A 1 (LO) phonon mode. The phonon line is asymmetric on the low energy side, and the asymmetry increases with In content, while the main peak shifts to lower energies. A shift of the phonon energy has also been observed when the excitation energy is close to the absorption edge of the In x Al y Ga 1-x-y N layer. The nature of this shift is discussed in relation with intrinsic and extrinsic inhomogeneities in the quaternary alloy.

Raman spectroscopy and transmission electron microscopy were employed to study the vibrational properties and the microstructure of epitaxially grown InN films on GaN/Al 2 O 3 templates. The variations of the InN lattice constants, as deduced by electron diffraction analysis, along with the red-shifted E 2 2 mode frequency reveal that InN films exhibit residual tensile stress, strongly dependent on the epilayer growth temperature. Threading dislocations are the dominant structural defects in the films, having a density in the order of 10 9 -10 10 cm -2 . Profile analysis of the E 2 2 Raman peak by means of the Spatial Correlation Model provides useful information concerning the effective mean length for free phonon propagation (L), which is a measure of the structural quality of the samples. In all the studied samples, L monotonically increases with decreasing threading dislocation density of pure screw and mixed type character.

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.

High resolution transmission electron microscopy (HRTEM), Raman spectroscopy and temperature dependent photoluminescence (PL) are employed to compare the structural and optical properties of compact InN films grown by molecular beam epitaxy (MBE) on GaN/Al 2 O 3 (0001) with a single-step (S) or a two-step (T) growth process. The S-type film was grown at 300 °C, whilst the T-type film was nucleated at the same temperature and overgrown at 410 °C. The T-type film exhibits better structural and optical properties as indicated by a sharper InN/GaN interface, the lower threading dislocations density, sharper Raman peaks and higher intensity PL peaks. Quantitative comparison of the red-shifted Raman E 2 2 peak and the lattice constants of S and T samples suggests that the InN epilayers are under different values of stress due to thermal reasons and point defects present in them. The PL emission peak difference of 32 meV between S and T provides further evidence of the unequal residual strain present in InN.

Indium gallium nitride alloy (0001) films in the entire composition range were grown heteroepitaxially by radio-frequency plasma assisted molecular beam epitaxy on Ga-polarity GaN(0001)/Al 2 O 3 substrates. A growth approach based on low substrate temperatures and near-stoichiometric growth conditions was followed. Under these conditions incorporation efficiency of indium atoms was equal to one. X-ray diffraction data reveal that phase separation phenomena were effectively suppressed. As the indium mole fraction increased, a tendency of the strain state of the films to change from compressive to tensile was observed. This was attributed to growth through nucleation and coalescence of three-dimensional InGaN islands with high lattice mismatch on GaN(0001).

We report on time-resolved photoluminescence (PL) experiments on ‘polarization-matched’ GaN/InAlGaN heterostructures, to get unique information on the recombination kinetics of nitride quantum wells (QWs) without the obscuring influence of polarization-induced internal fields. The samples have been grown by MBE choosing an In content of 14% and an Al content of 24% in order to compensate the internal fields. We have measured PL time decay as a function of several experimental parameters on a selected number of InAlGaN/GaN QWs. Careful analysis of time resolved spectra allows to isolate the intrinsic contribution from extrinsic bands. Very fast recombination times have been observed, demonstrating the enhancement of the exciton oscillator strength in these zero-field GaN based QWs, with recombination times down to about a hundred ps. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the temperature dependence of I-V curves of GaN Schottky diodes with two crystal polarities. Relatively large barrier height differences have been observed between Ga- and N-face GaN polarities which we ascribed to the possible influence of the surface fixed charge caused by the opposite spontaneous polarization in GaN with Ga- and N-polarities. Large differences between barrier height calculated from I-V and C-V curves may also be caused by the fixed surface polarization charge. Barrier heights decrease with decreasing temperature for both polarities and ideality factors increase. Low ideality factors and no excess leakage currents indicate thermionic transport mechanism for Ga-face diodes at temperatures over 200 K. On the contrary large ideality factors and reverse leakage currents for N-face polarity diodes are probably connected with other form of current transport-tunneling.

The heteroepitaxy of In-face InN on Ga-face GaN (0001) by nitrogen RF plasma source molecular beam epitaxy, using a two-step growth process, has been investigated. InN nucleation is enhanced at low substrate temperatures and smooth continuous nucleation layers can be grown at 300-350°C, which provide the necessary template for overgrowing films at a higher temperature near 500 °C. Porous columnar InN structures are grown without the low-temperature nucleation layer, exhibiting a multiplied growth rate along the c-axis. The continuous InN/GaN (0001) films are under tensile residual stress at room temperature attributed to the different thermal expansion of InN and GaN. Typical lattice constants are c = 0.5691 nm and a = 0.3544nm for a continuous 0.9μm film and c = 0.5700nm and a = 0.3533 nm for a columnar 1.6 μm epilayer. A difference in the density of misfit dislocations at the InN/GaN (0001) interface between columnar and continuous films was observed, in agreement with measurements of the films' lattice constants.

Radiofrequency (RF) nitrogen plasma sources are commonly used for the growth of III-nitrides by plasma-assisted molecular beam epitaxy. Their output mainly consists of a mixture of atomic nitrogen and metastable excited molecules. The relative concentration of these species as a function of RF power and nitrogen gas flow rate has been investigated by plasma optical emission spectroscopy. It is shown that the concentration of excited molecular nitrogen species is monotonically increasing with both parameters while the concentration of nitrogen atoms is almost independent of the nitrogen gas flow rate and increases monotonically with the coupled RF power. As a result of this functional dependence, the plasma source can be tuned to produce mainly excited nitrogen molecules or a mixture of excited molecules and atomic nitrogen. Comparison of the growth rates of gallium nitride films grown under different operating conditions of the RF source with the optical spectroscopy data shows that the metastable excited nitrogen molecules component of the source is the predominant factor contributing in the III-nitrides growth.

The structural properties of the interfacial area of InN thin films, grown by rf-plasma MBE on top of GaN/Al 2 O 3 substrates have been investigated by TEM and HRTEM. Continuous or columnar film growth highly depends on the pre-growth of an InN nucleation layer and low substrate temperatures. The lattice constants in both growth cases were determined by electron diffraction analysis. A network of 60° misfit dislocations was introduced in the InN/GaN interface to accommodate the lattice mismatch between the two crystals. The degree of residual compressive strain in the continuous film is also calculated, considering the columnar film closer to the relaxed configuration. The good agreement between the experimental and the calculated value for the misfit dislocation spacing provides evidence that the residual compressive strain is minimized. The in-plane lattice expansion, observed in the continuous InN film, is attributed to thermal tensile strain originating from the difference in the thermal expansion coefficients of InN and GaN.

We investigate the properties of high-k dielectric insulators on GaN. 22 nm thick ZrO2 is deposited on N- or Ga-polarity GaN. Al/ZrO2/GaN metal–oxide–semiconductor and reference Au/Ni/GaN Schottky contact structures are characterized using current–voltage and capacitance–voltage methods. If compared with Schottky contacts, metal–oxide–semiconductor structures show leakage current substantially reduced for the N-polarity GaN while comparable values were obtained for the Ga-polarity GaN. The oxide relative permittivity is found to be in the range of 20–30. Light-assisted capacitance–voltage method shows density of interface states Dit ~ 1 × 1012 cm−2 eV−1 for Ga-polarity and ~2 × 1012 cm−2 eV−1 for N-polarity GaN/ZrO2. Finally, we prepared AlGaN/GaN metal–oxide–semiconductor heterostructure field-effect transistor proving minimal transconductance deterioration and small threshold voltage shift. It is suggested that ZrO2 deposition optimization may further reduce the leakage current.