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This letter reports on the $W$ -band power performance of N-polar GaN deep recess MIS–high electron mobility transistors (HEMTs) using a new atomic layer deposition (ALD) ruthenium (Ru) gate metallization process. The deep recess structure is utilized to control the DC-RF dispersion and increase the conductivity in the access regions. The ALD Ru effectively fills the narrow T-gate stems aiding realization of shorter gate lengths with lower gate resistance than in prior work. In this work, the gate length was scaled down to 48 nm, resulting in the demonstration of a record high 8.1-dB linear transducer gain measured at 94 GHz by load pull. This increased gain has enabled a record 33.8% power-added efficiency (PAE) with an associated output power density ( $P_{\mathrm {O}}$ ) of 6.2 W/mm.

The influence of the underlying nucleation layer on the properties of semipolar $$(11{\bar{2}}2)$$ GaN grown on m-plane sapphire by metalorganic vapor-phase epitaxy has been investigated. $$(11{\bar{2}}2)$$ GaN epilayers of ~ 1 μm thickness were grown using four different initiating sequences: low-temperature AlN and GaN, and high-temperature AlN buffer layers, and directly (high-temperature GaN). The choice of nucleation layer had a pronounced effect on the surface morphology and crystal quality of the overlying GaN epilayer. In comparison, direct growth of $$(11{\bar{2}}2)$$ GaN without any buffer layer provided the best crystal quality with a rocking-curve $$\omega $$ full-width at half-maximum (FWHM) value of 720 arcsec along the $$[11{\bar{2}}{\bar{3}}]$$ direction and relatively enhanced near-band-edge photoluminescence emission, thus showing this direct growth process to be a simple route for synthesis of semipolar $$(11{\bar{2}}2)$$ GaN layers.

This work presents recent progress in the W-band (94 GHz) power performance of N-polar GaN deep recess HEMTs grown on sapphire substrates. While SiC has been the substrate of choice to achieve the highest level of performance, sapphire substrates are a lower cost alternative. In this work we show that N-polar GaN deep recess HEMTs grown on sapphire match the power performance of a device on SiC up to 14 V with 5.1 W/mm of output power density. At 16 V the device on sapphire starts to suffer from thermal effects but still demonstrated 5.5 W/mm with an associated 20.6% power-added efficiency. This work also examines the impact of encapsulating the device in a low dielectric constant film often used for the implementation of a RF wiring environment.

In this letter, we report the Schottky barrier diode investigation of ruthenium (Ru) deposited by atomic layer deposition on N-polar GaN. The Schottky diodes showed near-ideal thermionic current behavior under forward bias and reverse bias at various temperatures. The barrier height values extracted from both regions agreed well at each temperature and the barrier was extracted to be 0.77 eV at room temperature. The combination of the 0.77 eV barrier and thermionic current characteristic resulted in $\mu \text{A}$ /cm2 reverse current at −5 V, which is a record-low value for N-polar GaN Schottky diodes. As a comparison, the Ru on Ga-polar GaN Schottky barrier diode, which has a barrier height of 1.0 eV, exhibited a ~ two-order of magnitude higher leakage than Ru on N-polar GaN at −5 V due to other parasitic leakage mechanisms.

Though GaN HEMTs have primarily been used for power amplification, they are also well suited for receiver applications. In the front-end of receivers, non-linearities, in particular third-order intermodulation products lead to in-band signal distortion. The intermodulation distortion is primarily dominated by transconductance and its derivatives. In this paper, we report on N-polar GaN MIS-HEMTs able to simultaneously achieve high gain (12.7 dB) and excellent linearity performance (OIP3/ $\text{P}_{\mathbf {DC}}$ of 15 dB) for low-power receiver application at 30 GHz. With a two-tone load-pull input-bias sweep, we demonstrate that the linearity of high performance HEMTs is sensitive to bias, and we present our measurement methodology to accommodate this.

This letter reports on the improvement of the large-signal W-band power performance of nitrogen-polar gallium nitride deep recess high electron mobility transistors with the addition of a 40-nm-thick ex-situ silicon nitride passivation layer deposited by plasma enhanced chemical vapor deposition. The additional passivation improves the dispersion control allowing the device to be operated at higher voltages. Continuous-wave load pull measurements performed at 94 GHz on a $2\times 37.5\,\,\mu \text{m}$ transistor demonstrated an improvement in the peak power-added efficiency (PAE) to 30.2% with an associated output power density of 7.2 W/mm at 20 V drain bias. Furthermore, at 23 V, a new record-high W-band power density of 8.84 W/mm (663 mW) was achieved with an associated PAE of 27.0%.

In this letter, we report on the demonstration of a Mg-doped GaN/Al0.2Ga0.8N superlattice (SL) based depletion mode p-channel FinFET to improve the on current (ION). A two-step approach involving a dry etch followed by a Tetramethylammonium hydroxide (TMAH) wet etch was employed to obtain fins with minimum width of 50 nm using optical lithography. Normalizing current with fin height, an ION of 52 mA/mm and 110 mA/mm were achieved for 80 and 105 nm wide fins respectively.

The integration of different electronic materials systems together has gained increasing interest in recent years, with the III-nitrides being a favorable choice for a variety of electronic applications. To increase flexibility in integration options, growing nitrides material directly on semi-processed wafers would be advantageous, necessitating low temperature (LT) growth schemes. In this work, the growth of AlN and GaN was conducted via metalorganic chemical vapor deposition (MOCVD) using both NH3 and DMHy as N-precursors. The relationships between growth rate versus temperature were determined within the range of 300 to 550 °C. The growth of AlN/GaN heterostructures was also investigated herein, employing flow modulation epitaxy MOCVD at 550 °C. Subsequent samples were studied via atomic force microscopy, X-ray diffraction, TEM, and Hall measurements. Two-dimensional electron gases were found in samples where the LT AlN layer was grown with NH3, with one sample showing high electron mobility and sheet charge of 540 cm2/V∙s and 3.76 × 1013 cm−2, respectively. Inserting a LT GaN layer under the LT AlN layer caused the mobility and charge to marginally decrease while still maintaining sufficiently high values. This sets the groundwork towards use of LT nitrides MOCVD in future electronic devices integrating III-nitrides with other materials.

N-polar GaN MISHEMTs have recently demonstrated excellent power performance and power-added efficiency at 94 GHz [1] . At mm-wave frequencies and high data rates, the linearity of an RF transistor is an important requisite. Third-order non-linearities lead to undesirable effects such as in-band signal distortion and are therefore important to control. This study presents a novel device concept to enhance the linearity of N-polar GaN MISHEMTs at millimeter wave frequencies (30 GHz and beyond) for low-power receiver application. We have recently reported linearity data on N-polar GaN MISHEMTs with OIP3/P DC of 11.4 dB [2] and 15 dB [3] at 30 GHz. We have observed in [2] , [3] that the peak linearity performance is limited to a narrow input-bias range, resulting in susceptibility to process and temperature variations. Therefore, we explore a novel device structure that can provide its best OIP3/P DC performance over a wide input-bias range.

N-polar GaN based HEMTs have demonstrated superior performance for solid-state millimeter wave power amplifiers [1] , [2] . To further improve the high-frequency and high-power performance in N-polar GaN HEMTs, using a small gate length while preserving a good aspect ratio is critical. Currently, N-polar HEMTs utilize a thin gate dielectric to reduce gate leakage. This reduces the aspect ratio. Therefore, removing the gate dielectrics, i.e. using Schottky-HEMTs in N-polar GaN is very attractive to pursue highly scaled and high-performance devices. Previous studies [3] – [6] have shown that the barrier heights between the gate metals and N-polar GaN are relatively low and the reverse leakages may be too high to be used in practical Schottky-HEMTs. Here, we report the first investigation of ruthenium (Ru) on N-polar GaN Schottky barrier. The device shows near-ideal Schottky characteristic under reverse bias and forward bias . The barrier height values at various temperatures extracted from the forward bias region and the reverse bias region agree well. The extracted barrier height is 0.77 eV at room temperature. The reverse leakage is ultralow with ~10 -6 A/cm 2 at -5 V and follows ideal thermionic behavior .

This study experimentally shows the existence of acceptor traps at positive polarization interfaces (PPIs) acting as the source of holes in N-polar p-type modulation doped GaN/(AlN/AlGaN) superlattices (SLs) with low Mg doping. The observation of measured hole concentration higher than the dopants (here, Mg) put in the samples during growth can be explained by the ionization of acceptor traps, which are placed 0.8 eV above the valence band of GaN at the PPI. All samples were epitaxially grown using metal organic chemical vapor deposition and were characterized using x-ray diffraction and room-temperature Hall measurements. The measured hole concentrations are compared against calculated values from STR FETIS®, and the measured mobility trends are explained using the separation of the positive polarization interfaces from the two-dimensional hole gas in the systems, strengthening the hypothesis. A second study is also presented where acceptor traps were ionized in a p-type modulation doped GaN/AlGaN SL without an AlN layer—by keeping the superlattice period thickness constant and increasing the AlGaN composition. Following the hypothesis of the existence of an acceptor trap, these experimental studies demonstrate the charge-balance in systems that show p-type behavior without sufficient doping.

Gate dielectric plays an integral role in advancing the performance and reliability of GaN-based transistors. Si-alloying of aluminum oxide (Al2O3) dielectrics have been shown to provide a promising route to improve gate dielectric properties in GaN. In this letter, we report on the first demonstration of a GaN FET with aluminum silicon oxide (AlSiO) as the gate dielectric. Vertical normally-off GaN MOSFETs were fabricated on bulk GaN substrate. Excellent dc performance was achieved with a breakdown voltage of 1.2 kV, an ON-resistance of 2 $\text{m}\Omega $ .cm2, and a threshold voltage of 1.5 V (defined at $I_{\textsf {DS}} =1\,\,\mu \text{A}$ /mm). A high breakdown electric-field of 2.3 MV/cm was calculated in these devices. In addition to vertical GaN MOSFET results, a comparative study of Al2O3 and AlSiO-based in situ GaN MOS capacitors, including time-dependent dielectric breakdown characteristics is also presented.

Nitrogen-polar (N-polar) Gallium Nitride (GaN) deep-recess high-electron mobility transistors (HEMTs) have demonstrated exceptional power density (8 W/mm) and high efficiency at 30 and 94 GHz. Unlike the conventional Galliumpolar GaN which have typical Silicon Nitride passivation, the Npolar deep-recess structure has a GaN cap layer added in the access region of the transistor to enhance the conductivity while reducing the DC-to-RF dispersion. Previously, the MIT virtual source (VS) model has been proposed as a physics-based approach to modeling Ga-polar devices. This work investigates the application of the VS model to N-polar GaN HEMTs. We present a comparison of DC-IV between the developed model and a fabricated device to demonstrate good agreement.

In this study the MOCVD growth and electrical properties of N-polar modulation doped p-AlGaN/GaN superlattices (SLs) were investigated. Hole sheet charge density and mobility were studied as a function of the concentration of the p-type dopant Mg in the SL and the number of SL periods. Room temperature Hall measurements were carried out to determine the hole mobility and the sheet charge density. While the hole density increased with increasing number of SL periods, the hole mobility was largely unaffected.Hole mobilities as high as 18cm2/Vs at a simultaneous high hole density of 6.5e13 cm-2 were observed for N-polar SLs with a Mg modulation doping of 7.5e18 cm-3. For comparable uniformly doped Ga-polar SL samples, a mobility of 11cm2/Vs was measured. Lowest sheet resistance in the GaN/AlGaN materials system of 5kOhm/sq is also reported. Test-structure transistors were also fabricated to investigate the applicability of these SL structures, with planar device resulting in a current of 5mA/mm, and a FinFET structure resulting in a current of over 100mA/mm., 8 figures

Integration of nitrides with other material systems has recently become of interest due to the high performance of GaN-based high-electron mobility transistors. However, the elevated growth temperatures often used to grow high quality AlN pose challenges toward metalorganic chemical vapor deposition (MOCVD) on temperature sensitive substrates such as processed wafers. In this work, the growth of AlN was conducted at temperatures below 550 °C via MOCVD using a flow-modulated epitaxy scheme, and their morphological, compositional, and electronic properties of these films were investigated. Sheet charges up to 2.1 × 1013 cm−2 and mobilities on the order of 400 cm2/V s were measured for two dimensional electron gases, which formed at the interface between the low temperature grown AlN layers and the semi-insulating GaN base layers deposited at high temperatures. Despite their low growth temperatures, nominally pure AlN barrier layers exhibited measurable unintentional gallium incorporation adjacent to the GaN interface. The result sets the stage for the integration of nitride-based electronics via epitaxy-based schemes on temperature sensitive substrates.

Hillock-free thick InGaN layers were grown on N-polar GaN on sapphire by metal organic vapor phase epitaxy using a digital growth scheme and H2 as surfactant. Introducing Mg to act as an additional surfactant and optimizing the H2 pulse time, In compositions up to 17% were obtained in 100 nm thick epilayers. Although Mg adversely affected the In incorporation, it enabled maintenance of a good surface morphology while decreasing the InGaN growth temperature, resulting in a net increase in In composition. The parameter space of growth temperature and Mg precursor flow to obtain hillock-free epilayers was mapped out.

We report a simple direct route for the synthesis of high-quality semipolar (112¯2) AlGaN alloys across the composition range by metalorganic vapour phase epitaxy. We show that a single high-temperature growth step without the use of a low-temperature nucleation or buffer layer is a convenient way to grow high-quality (112¯2) AlGaN layers on m-plane sapphire substrate. We also report a detailed investigation of the microstructure and optical properties of these layers including phase-purity, surface morphology, epilayer tilt, and polarization-resolved optical transmission measurements.

Crack-free 1.3 μm thick elastically relaxed AlxGa1−xN layers were demonstrated on compliant high fill-factor 10 × 10 μm2 tile patterned GaN-on-porous-GaN pseudo-substrates (PSs). Porous GaN was utilized as a semi-flexible underlayer. The AlxGa1−xN layer was grown in steps of 200, 300, 400, and 400 nm. While the AlxGa1−xN layer regrown on a co-loaded GaN-on-sapphire template cracked after deposition of only 200 nm AlGaN, on the GaN-on-porous GaN patterned substrates, no cracks were observed, and smooth films were achieved. In addition, an enhanced aluminum uptake was observed as the AlGaN growth progressed on the GaN-on-porous-GaN PS, owing to the composition pulling effect, until the point when the tiles started to coalesce. Upon further regrowth on these coalesced tiles, the aluminum uptake saturated, while the surface remained crack-free and smooth.

We propose the existence of an acceptor-like trap at positive polarization interfaces in p-type III-nitride semiconductor heterostructures, using N-polar p-type GaN/AlN/AlGaN superlattices as a demonstration platform. Metal Organic Vapor Phase Epitaxy was used to grow all samples, with a p-type modulation doping scheme using Mg as the dopant. The samples were characterized using x-ray diffraction and room-temperature Hall measurements, and energy band-diagram simulations were carried out using STR FETIS® and Silvaco packages. For higher doped samples (Mg > 1.5 × 1019 cm−3) with thinner AlN interlayers (≤0.7 nm), the total sheet charge measured using Hall measurements agreed with the value observed in standard simulations without invoking any traps, whereas for lower doped samples (Mg < 1.5 × 1019 cm−3) and those with thicker AlN interlayers (≥ 0.7 nm), the measured charge was very high compared to the value obtained from simulations and higher than the Mg doping in the films. The higher charge was attributed to the existence of an acceptor trap at positive polarization interfaces, which became ionized at lower doping and/or at higher AlN thicknesses. A consistent ionization energy of the trap was obtained by comparing the energy band diagram with and without acceptor traps with the experimental results. This work also elucidates the source of charge balance in p-type samples with insufficient or no Mg doping.

We report thin high quality n+ type doped N-polar GaN contact layers grown using metal-organic chemical vapor deposition with carrier concentration as high as 3.5×1020 cm-3 and an electron mobility of 80 cm2V-1s-1 at room temperature resulting in a low sheet resistance of 57.3 Ω/ and specific contact resistance of 1.7×10-7 Ωcm2. These results were obtained via silicon doping of (000-1) N-polar GaN grown on 4o miscut sapphire substrates using a flow modulation growth scheme at a deposition temperature of 850 oC.

In this paper, we demonstrate the first GaN/InGaN HEMT with a partially relaxed InGaN channel, realized by utilizing a porous GaN buffer obtained by the selective and controlled electrochemical etch of GaN. According to reciprocal space map analysis, ~70% of InGaN relaxation relative to GaN was achieved by this process. With the transfer length method (TLM) and extracted charge profile, the relaxed InGaN channel demonstrates ~9.6% 2DEG mobility enhancement with respect to the strained InGaN channel which is consistent with a reduced effective mass predicted by theoretical analysis based on Vegard's law. On comparing the output characteristics of devices with relaxed and strained InGaN channel, the relaxed channel devices show a 10% improvement of on-resistance while maintaining similar saturation current. A non-uniform charge distribution was found in the relaxed InGaN channel, which can be attributed to the strain relaxation fabrication process.

'There is an interest in growing N-polar GaN on bulk GaN for high electron mobility transistors (HEMTs). Current N-polar HEMT technology is dominated by devices grown on non-native substrates which possess a high density of dislocations due to lattice mismatch. N-polar GaN films grown directly on non-miscut GaN substrates have displayed a high density of pits and depressions on the surface. This work demonstrates that surface impurities play a major role in the formation of surface-pits in epitaxially grown film. By subjecting the GaN substrate to an ultra-violet (UV) O3 (ozone) clean prior to growth and initiating growth with a 2 nm coherently strained AlN layer, grown under metal-rich conditions, we have demonstrated N-polar GaN films with a nearly pit-free surface. This AlN initiation layer (AIL) likely captures impurities on the substrate surface thus decoupling the substrate surface from the epitaxially grown film. It is demonstrated that utilizing thicker AILs, up to 8 nm, further improves film quality and surface morphology. The methods employed in this study to produce high-quality N-polar GaN grown on bulk GaN will pave the way for future GaN devices with an order of magnitude or more lower threading dislocation density (TDD).

We report the synthesis and characterization of semipolar ( 11 2 ¯ 2 ) oriented Al 1− x In x N alloys over a wide composition range ( 0 ≤ x ≤ 0.55 ) using metalorganic vapour phase epitaxy (MOVPE) in a close-coupled showerhead reactor system. AlInN layers were deposited on AlN buffer layers on m -plane ( 10 1 ¯ 0 ) sapphire substrates using trimethylaluminium (TMAl), trimethylindium (TMIn), and ammonia (NH 3 ) precursors and the alloy composition tuned by changing the growth temperature from 860°C to 625°C, corresponding to an indium content from 2.9% to 54.6%. High resolution X-ray diffraction (HRXRD) measurements were used to determine the microstructure and anisotropic biaxial strain arising from the intrinsic tricilinic distortion of the unit cell in semipolar nitrides. The epilayers exhibit an overall tilt about the [ 1 1 ¯ 00 ] direction and phase separation was observed for samples with indium content over 40%. The ternary alloy compositions evaluated from HRXRD were compared with those obtained from optical transmission measurements.

We report the growth and characterization of group III-nitride semiconductor layers on graphene grown by chemical vapour deposition. GaN, AlGaN alloys, and InN layers are grown using an AlN nucleation and a buffer layer. We investigate the effect of varying growth temperature and V/III ratio and show that under optimized growth conditions preferentially semipolar ( 10 1 ¯ 1 ) oriented nitride layers can be obtained. These layers, though polycrystalline, are highly oriented and show strong room temperature photoluminescence, thus showing graphene to be a novel substrate for the growth of III-nitride materials.

Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs–Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni–Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be Ni2Ga3. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We henc...

We report a comparative study of the microstructure of a-plane (1 1 2 0) InN epilayers grown on different buffer layers via metalorganic vapour phase epitaxy. Under optimized growth conditions, the crystalline quality of the InN epilayer is found to be best on an AlN buffer layer compared to that on GaN buffer layers or InN nucleation layers. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity with the in-plane tilt value being different along the c- and m-directions. We also observe a Nagai-like tilt in all the epilayers. The magnitude and nature of this anisotropy and the Nagai-like tilt is also strongly influenced by the buffer layer. Our investigations show that InN grown on an AlN buffer has smaller in-plane tilt, basal plane stacking fault density, and background carrier concentration resulting in a higher mobility and a low band gap energy of ∼ 0.70eV.

We have performed a comprehensive investigation of the growth parameter space for the MOVPE of a- plane (11 20) InN on a-plane GaN buffer layers deposited on r-plane (1 102) sapphire substrates. About 0:2 m thick a-plane InN epilayers were grown on 1 m thick a-plane GaN buffer layers in a close-coupled showerhead reactor. The growth parameters - substrate temperature, reactor pressure, V/III ratio - were systematically varied and their effect on structural, electrical, optical and morphological properties of a- plane InN films were studied. All a-plane InN epilayers show an anisotropy in the in-plane mosaicity. The (11 20) !-fwhm varies depending on the scattering vector being parallel to the c-direction or the m- direction. The magnitude and nature of this anisotropy is strongly influenced by the growth parameters. In general compared to c-plane InN, we observed a higher growth rate and a slightly higher optimum growth temperature for the a-plane InN epilayers. The optimum growth conditions are found at T = 550 o C, P = 500 Torr, V/III = 11; 000, where the !-fwhm for symmetric (11 20) reflection are 0:83 degree and 1:04 degree along (0001) and (1 100) direction respectively and for the skew-symmetric (10 11) plane is 1:47 degree. The optimized a-plane InN has a photoluminescence peak emission at 1750 nm ( 0:71 eV) at low temperature (11 K) and a mobility of 234cm 2 =V:s, carrier concentration 1:4 10 19 cm 3 at room temperature.

Gallium nitride nanowires were grown on c-plane, r-plane and m-plane sapphire substrates in a showerhead metalorganic chemical vapor deposition system using nickel catalyst with trimethylgallium and ammonia as precursors. We studied the influence of carrier gas, growth temperature, reactor pressure, reactant flow rates and substrate orientation in order to obtain thin nanowires. The nanowires grew along the and axes depending on the substrate orientation. These nanowires were further characterized using x-ray diffraction, electron microscopy, photoluminescence and Raman spectroscopy., Replaced the "various" in the title with the specific planes used

We report a comprehensive characterization of the microstructure of non-polar ( 1 1 2 ¯ 0 ) a -plane Al x Ga 1− x N epilayers on ( 1 1 ¯ 0 2 ) r -plane sapphire substrates over the entire Al composition range using high-resolution X-ray diffraction. All a -plane AlGaN epilayers show an anisotropic in-plane mosaicity which is strongly influenced by Al incorporation and growth conditions. We use data from symmetric and skew-symmetric reflections combined with Williamson–Hall analysis to estimate the mosaic parameters and follow trends across the Al composition. We also analyze the dependence of the microstructure on thickness for a set of Al 0.5 Ga 0.5 N epilayers.

We report the synthesis and optical characterization of GaN/AlGaN and InGaN/GaN quantum well structures grown on CVD graphene layers. We demonstrate a novel process allowing facile transfer of QW multilayers to other (cheap, flexible) substrates.

Unlike c-plane nitrides, “nonpolar” nitrides, e.g., those grown in the a-plane or m-plane orientation encounter anisotropic in-plane strain due to the anisotropy in the lattice and thermal mismatch with the substrate or buffer layer. Such anisotropic strain results in a distortion of the wurtzite unit cell and creates difficulty in accurate determination of lattice parameters and solid phase group-III content (xsolid) in ternary alloys. In this paper we show that the lattice distortion is orthorhombic, and outline a relatively simple procedure for measurement of lattice parameters of nonpolar group III-nitrides epilayers from high resolution x-ray diffraction measurements. We derive an approximate expression for xsolid taking into account the anisotropic strain. We illustrate this using data for a-plane AlGaN, where we measure the lattice parameters and estimate the solid phase Al content, and also show that this method is applicable for m-plane structures as well.

Gallium nitride (GaN) nanowires (NWs) have been intensely researched as building blocks for nanoscale electronic and photonic device applications; however, the mechanical properties of GaN nanostructures have not been explored in detail. The rigidity, thermal stability, and piezoelectric properties of GaN make it an interesting candidate for nano-electromechanical systems. We have fabricated doubly clamped GaN NW electromechanical resonators on sapphire using electron beam lithography and estimated the Young's modulus of GaN from resonance frequency measurements. For wires of triangular cross section with side ∼90 nm, we obtained values for the Young's modulus to be about 218 and 691 GPa, which are of the same order of magnitude as the values reported for bulk GaN. We also discuss the role of residual strain in the nanowire on the resonant frequency and the orientation dependence of the Young's modulus in wurtzite crystals.

In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2ϑ-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and co...

We report the synthesis and optical characterization of semipolar-oriented III-nitride quantum well (QW) structures obtained by growth on chemical vapor deposited graphene layers using metalorganic vapor phase epitaxy. Various multi-quantum well stacks of GaN(QW)/AlGaN(barrier) and InGaN (QW)/GaN (barrier) were grown. Growth on graphene not only helps achieve a semipolar orientation but also allows facile transfer of the QW multilayer stack to other cheap, flexible substrates. We demonstrate room-temperature photoluminescence from layers transferred to flexible Kapton films.

Exciton Lande g-factors in wurtzite GaN epitaxial films with (0001) C-plane and (112¯0) A-plane orientations have been measured in magnetic fields B up to 1.8 T, using polar magneto-optical Kerr effect (MOKE) spectroscopy. A procedure is developed for extracting the Zeeman splitting and thereby the g-factor, from Kerr ellipticity and rotation spectra of A-plane films, which have in-plane polarization anisotropy. In the C-plane film the measured g-factors for the A, B, and C exciton transitions were gA=0.09±0.02, gB=0.74±0.05, and gC=3.9±0.2, respectively, with B∥c-axis and comparable to earlier reports. The MOKE spectra of the A-plane film have one dominant exciton feature each for analyzer axis ⊥ and ∥ to the c-axis of GaN, and they arise at different energies. The measured g-factors for these were much larger, with values g⊥c=4.7±1 and g||c=7.1±1.2 with B⊥c-axis. Comparison with a k·p perturbation theory based calculation, which included the influence of strain, indicates that the features in the A-plan...