Search

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 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.

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.

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 ...

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)

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.

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.

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

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).

The nucleation of In-face InN films on GaN(0001) surfaces by plasma-assisted molecular beam epitaxy was investigated. Sustained growth of InN is possible for In/N flux ratio less than one and substrate temperature below 520 ∘C. However, under these conditions, nucleation of InN islands that grow with large height to width aspect ratios is observed. This is attributed to low N adatom mobility on the surface as well as to the significantly higher active nitrogen flux incident on the top surface of the islands as compared to the one incident on their lateral faces. Therefore the substrate temperature dependant nucleation density of the InN islands becomes the key factor determining their coalescence. Suppressed coalescence at high temperatures (450–500 ∘C) allows the growth of InN nano- and micro-columns and porous structures, while fast coalescence at low temperature (300–350 ∘C) results in InN templates suitable for overgrowth of continuous InN films. Strain relaxation by misfit dislocations proceeds faster in the case of continuous films.

The development of high electron mobility transistors (HEMTs) from AlN/GaN heterostructures, grown by plasma assisted molecular beam epitaxy has been investigated. It is shown that AlN can be grown fully strained on GaN, up to a thickness of 5nm, when strain relaxation by introduction of microcracks is initiated. An optimized HEMT structure with 4.5nm AlN barrier and 1nm GaN cap layer exhibited a two-dimensional electron gas (2DEG) density of 3.6x10^1^3cm^-^2 along with room-temperature mobility of ~1200cm^2/Vs, with corresponding sheet resistance of 144@W/sq. The measured 2DEG density is in very good agreement with self-consistent Schrodinger-Poisson calculations for fully strained AlN layer. HEMT transistors with 1@mm gate length exhibited record-high source-drain current density of 1.8A/mm and maximum transconductance of 400mS/mm.

The effects of InN layer thickness (4/7/10 nm) in metal–insulator–semiconductor Ni/SiNx/InN structures have been evaluated. The 7 nm 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 1.1–1.3 eV for all the structures. The output characteristics of the fabricated MISFETs showed modulation of the drain–source current with the highest current density of 0.8 A/mm for the 10 nm InN layer, but the channel could not fully pinch-off.

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...

The current instabilities of high electron mobility transistors (HEMTs), based on thin double AlN/GaN/AlN heterostructures (~0.5 μm total thickness), directly grown on sapphire substrates, have been analyzed and compared for different AlN top barrier thicknesses. The structures were capped by 1 nm GaN and non-passivated 1 μm gate-length devices were processed. Pulsed I–V measurements resulted in a maximum cold pulsed saturation current of 1.4 A mm−1 at a gate-source voltage of +3 V for 3.7 nm AlN thickness. The measured gate and drain lag for 500 ns pulse-width varied between 6%–12% and 10%–18%, respectively. Furthermore, a small increase in the threshold voltage was observed for all the devices, possibly due to the trapping of electrons under the gate contact. The off-state breakdown voltage of V br = 70 V, for gate-drain spacing of 2 μm, was approximately double the value measured for a single AlN/GaN HEMT structure grown on a thick GaN buffer layer. The results suggest that the double AlN/GaN/AlN heterostructures may offer intrinsic advantages for the breakdown and current stability characteristics of high current HEMTs.

Critical aspects of InN channel field-effect transistors (FETs) have been investigated. SiNx dielectric layers were deposited in-situ, in the molecular beam epitaxy system, on the surface of 2 nm InN layers grown on GaN (0001) buffer layers. Metal-insulator-semiconductor Ni/SiNx/InN capacitors were analyzed by capacitance-voltage (C-V) and current-voltage measurements and were used as gates in InN FET transistors (MISFETs). Comparison of the experimental C-V results with self-consistent Schrodinger-Poisson calculations indicates the presence of a positive charge at the SiNx/InN interface of Qif ≈ 4.4 – 4.8 × 1013 cm−2, assuming complete InN strain relaxation. Operation of InN MISFETs was demonstrated, but their performance was limited by a catastrophic breakdown at drain-source voltages above 2.5–3.0 V, the low electron mobility, and high series resistances of the structures.

In the present work the electrical properties of AlN polycrystalline films deposited at low temperatures by plasma-assisted molecular beam epitaxy (PA-MBE) are investigated. The polarization build-up during constant current injection as well as the depolarization process after the current stress have been investigated through monitoring voltage transients in Metal-Insulator-Metal (MIM) capacitors, in temperature range from 300 K to 400 K. Moreover, current-voltage characteristics obtained at different temperatures revealed that charge collection at low fields in these films occurs through variable range hopping.

III-nitride materials are used in a variety of optoelectronic devices due to their high carrier mobility and high current densities. Recently optoelectronic waveguides and field effect transistor junctions are being fabricated with a new type of heterostructure based on In0.18Al0.82N/GaN, as this provides a large band gap difference at the heterojunction coupled with lattice-matching to GaN, inhibiting the formation of straininduced defects known to reduce carrier recombination and mobility [1]. Despite its attractive properties, InAlN growth is challenging owing to the very different optimum growth temperatures for InN and AlN films, requiring InAlN to be grown at a temperature low enough to prevent dissociation of InN, yet high enough to allow sufficient aluminium adatom surface diffusion to deposit high crystalline quality film. There are to date very few studies on the microstructure and morphology of InAlN films, yet such observations are vital in order to fabricate InAlN-based devices with the greatest performance, and to understand the basic structural properties of InAlN.

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.

0.5–1 μm thick InN {0001} films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is most readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the GaN/InN hetero-interface of the N-polar sample, in addition to the passivation effect. These effects raised the band diagram up by about 0.65 eV, reaching a flat-band profile.

The full performance of GaN devices for high power applications is not exploited due to their self-heating. Possible solutions are the integration of materials with high heat conductivity i.e., single crystalline diamond and graphene layers. We report the growth of single crystalline (0001)-oriented GaN thin films on (100), (110) and (111) diamond single crystals studied by transmission electron microscopy (TEM) in cross-sections. As for graphene, we show a high quality GaN layer that was deposited on patterned graphene layers and 6H-SiC. The atomic structures of the interfaces in the heterostructure are studied using aberration-corrected scanning TEM combined with energy dispersive x-ray and electron energy-loss spectroscopy. Funding Agencies|OTKA project (Hungary) [K 108869]; European Union under the Seventh Framework Program [312483 ESTEEM2]

The spontaneous growth of GaN nanopillars (NPs) by direct plasma-assisted molecular beam epitaxy on nitridated r-plane sapphire substrates has been studied. The emanation of metal-polarity NPs from inside an a-plane nonpolar GaN film was found to depend on both the substrate nitridation and the growth conditions. The density of NPs increased with increasing the duration of the nitridation process and the power applied on the radio-frequency plasma source, as well as the III/V flux ratio, while variation of the first two parameters enhanced the roughness of the substrate's surface. Transmission electron microscopy (TEM) techniques were employed to reveal the structural characteristics of the NPs and their nucleation mechanism from steps on the sapphire surface and/or interfacial semipolar GaN nanocrystals. Lattice strain measurements showed a possible Al enrichment of the first 5–6 monolayers of the NPs. By combining cross-sectional and plan-view TEM observations, the three-dimensional model of the NPs was constructed. The orientation relationship and interfacial accommodation between the NPs and the nonpolar a-plane GaN film were also elucidated. The NPs exhibited strong and narrow excitonic emission, suggesting an excellent structural quality.

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-resisitivity 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.

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 [300 nm GaN/ 200 nm 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 6.8 × 1012 to 2.1 × 1013 cm−2 as the AlN barrier thickness increased from 2.2 to 4.5 nm, while a 4.5 nm AlN barrier would result to 3.1 × 1013 cm−2 on a GaN buffer layer. The 3.0 nm AlN barrier structure exhibited the highest 2DEG mobility of 900 cm2/Vs for a density of 1.3 × 1013 cm−2. The results were also confirmed by the performance of 1 μm gate-length transistors. The scaling of AlN barrier thickness from 1.5 nm to 4.5 nm could modify the drain-source saturation current, for zero gate-source voltage, from zero (normally off condition) to 0.63 A/mm. The maximum drain-source current was 1.1 A/mm for AlN barrier thickness of 3.0 nm and 3.7 nm, and the maximum extrinsic transconductance was 320 mS/mm for 3.0 nm AlN barrier.

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 400–450 °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 of InN, reduces significantly the crystal mosaicity and the density of threading dislocations is about an order of magnitude less compared to InN films grown using an AlN/GaN intermediate nucleation/buffer layer on Si. The InN films exhibit the In-face polarity and very smooth atomically stepped surfaces.

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.

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.

The variation in surface electronic properties of undoped c-plane InxAl1−xN alloys has been investigated across the composition range using a combination of high-resolution x-ray photoemission spectroscopy and single-field Hall effect measurements. For the In-rich alloys, electron accumulation layers, accompanied by a downward band bending, are present at the surface, with a decrease to approximately flatband conditions with increasing Al composition. However, for the Al-rich alloys, the undoped samples were found to be insulating with approximate midgap pinning of the surface Fermi level observed.

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.

The improvement potential for the structural, electrical and opto-electronic properties of heteroepitaxial InN-on-GaN (0001) films by using optimal conditions (substrate temperature, In and N fluxes) of plasma-assisted molecular beam epitaxy and increasing the epilayer thickness to few micrometres has been investigated. The increase of InN thickness to 3.7 μm resulted to a-type component threading dislocation density of 6x109 cm−2 and directly measured electron mobility of 2330 cm2/Vs and concentration of 4.5x1017 cm−3. The optical bandgap of this film at 300K was 0.637 eV. However, a degradation in the integrity of the interfacial InN/GaN region was observed in films thicker than 1 μm, with the formation of voids and the nucleation of microcracks, which may be related to strain relaxation or thermal decomposition.