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AlxInyGa(1-x-y)N materials show promise for use in GaN-based heterojunction devices. The growth of these materials has developed to the point where they are beginning to see implementation in high electron mobility transistors (HEMTs) and light emitting diodes. However, the electrical properties of these materials are still poorly understood, especially as related to the net polarization charge at the AlInGaN/GaN interface (Qπ(net)). All theoretical calculations of Qπ(net) share the same weakness: dependence upon polarization bowing parameters, which describe the deviation in Qπ(net) from Vegard's law. In this study, direct analysis of Qπ(net) for Al0.54In0.12Ga0.34N/ GaN HEMTs is reported as extracted from C-V, I-V, and Hall measurements performed on samples grown by metalorganic chemical vapor deposition. An average value for Qπ(net) is calculated to be 2.015 × 10-6 C/cm2 , with just 6.5% variation between measurement techniques. [ABSTRACT FROM AUTHOR]

Atom probe tomography was used to achieve three-dimensional characterization of in situ Al2O3/GaN structures grown by metal organic chemical vapor deposition (MOCVD). Al2O3 dielectrics grown at three different temperatures of 700, 900, and 1000 °C were analyzed and compared. A low temperature GaN cap layer grown atop Al2O3 enabled a high success rate in the atom probe experiments. The Al2O3/GaN interfaces were found to be intermixed with Ga, N, and O over the distance of a few nm. Impurity measurements data showed that the 1000 °C sample contains higher amounts of C (4×1019/cm3) and lower amounts of H (7×1019/cm3), whereas the 700 °C sample exhibits lower C impurities (<1017/cm3) and higher H incorporation (2.2×1020/cm3). On comparing with Al2O3 grown by atomic layer deposition (ALD), it was found that the MOCVD Al2O3/GaN interface is comparatively abrupt. Scanning transmission electron microscopy data showed that the 900 °C and 1000 °C MOCVD films exhibit polycrystalline nature, while the ALD films were found to be amorphous.VC [ABSTRACT FROM AUTHOR]

The influence of alloy clustering on fluctuations in the ground state energy of the two-dimensional electron gas (2DEG) in AlGaN/GaN and InAlN/GaN heterostructures is studied. We show that because of these fluctuations, alloy clustering degrades the mobility even when the 2DEG wavefunction does not penetrate the alloy barrier unlike alloy disorder scattering. A comparison between the results obtained for AlGaN/GaN and InAlN/GaN heterostructures shows that alloy clustering limits the 2DEG mobility to a greater degree in InAlN/GaN heterostructures. Our study also reveals that the inclusion of an AlN interlayer increases the limiting mobility from alloy clustering. Moreover, Atom probe tomography is used to demonstrate the random nature of the fluctuations in the alloy composition. [ABSTRACT FROM AUTHOR]

We have measured the effect of fast and thermal neutrons on GaN Schottky barriers and ohmic contacts using current-voltage and transmission line method electrical techniques, optical, atomic force and scanning electron microscopy morphological techniques, and X-ray photoemission spectroscopy chemical techniques. These studies reveal a 1015 n/cm2 neutron threshold for Schottky barrier ideality factor increases, a 1015 n/cm2 fast plus thermal neutron threshold for ohmic contact sheet and contact resistance increases, and 1016 n/cm2 neutron fluence threshold for major device degradation identified with thermally driven diffusion of Ga and N into the metal contacts and surface phase changes. These results demonstrate the need for protecting metal-GaN contacts in device applications subject to neutron radiation. [ABSTRACT FROM AUTHOR]

Ultra-small red micro-LEDs (<10 μm) with measurable output power have proved difficult to demonstrate. The smallest state-of-the-art red micro-LEDs (AlInGaP) to have exhibited a decent output power of ∼1.75 mW mm−2 are 20 μm × 20 μm in dimension. InGaN-based red micro-LED development has primarily been impeded due to the large lattice mismatch between the substrate and the quantum wells along with fabrication challenges such as low damage etching. In this work, we demonstrate 6 μm × 6 μm sized InGaN red micro-LEDs with an on-wafer external quantum efficiency of 0.2% and light output power reaching 2.1 mW mm−2. [ABSTRACT FROM AUTHOR]

A photo-assisted capacitance voltage (C-V) characterization technique for interfaces between positive valence band offset dielectrics (Al2O3, SiO2) and wide bandgap semiconductors is presented. It is shown that the valence band barrier for holes at the interface affects the measurement and a method to extract border trap and interface state density values from the measured C-V curves is suggested. Dielectric-semiconductor interface characterization has been well studied for silicon but the characterization techniques are not transferable to wide bandgap semiconductors, such as GaN and SiC, due to the low minority carrier generation rate. Multiple dielectrics deposited by various techniques have been employed in these devices; but in order to ascertain the most suitable dielectric, an effective characterization technique that works well with dielectrics on wide-bandgap semiconductors is required. [ABSTRACT FROM AUTHOR]

To realize the full spectrum of advantages that the III‐nitride materials system offers, the demonstration of p‐channel III‐nitride‐based devices is valuable. The first p‐type field‐effect transistor (pFET) based on an AlGaN/GaN superlattice (SL), grown using metal–organic chemical vapor deposition (MOCVD), is reported. Magnesium is used as the p‐type dopant. A sheet resistance of 11.6 kΩ □−1 and a contact resistance of 14.9 Ω mm are determined using transmission line measurements (TLMs) for Mg doping of 1.5 × 1019 cm−3. Mobilities in the range of 7–10 cm2 (V s)−1 and a total sheet charge density in the range of 1 × 1013–6 × 1013 cm−2 are measured using room temperature Hall effect measurements. Without tetramethylammonium hydroxide (TMAH) treatment, the fabricated pFETs have a maximum drain‐source current (IDS) of 3 mA mm−1 and an on‐resistance (RON) of 3.48 kΩ mm and do not turn off completely. With TMAH treatment during fabrication, a maximum IDS of 4.5 mA mm−1, RON of 2.2 kΩ mm, and five orders of current modulation are demonstrated. [ABSTRACT FROM AUTHOR]

This paper reports on the hot-carrier effects and semi-on-state behavior of nitrogen-polar GaN MIS-HEMTs at cryogenic temperatures (from 300 K down to 100 K). In the semi-on-state $(\text {V}_{\text {G}} \approx -{2}\,\,\text {V})$ , holes are generated by impact ionization in the high field region at the drain-side of the gate-edge. At room temperature, holes overcome the SiN/AlGaN stack and are collected at the gate-terminal, resulting in measurable hole gate-current (~11 nA/mm). Conversely, at cryogenic temperatures, the top SiN/AlGaN stack confines the holes within the GaN channel, thus inducing a negative threshold voltage shift (−0.4 V) and a sharp increase in drain current (0.18 A/mm). This behavior, referred to as “kink,” is readily observable on the $\text {I}_{\text {D}} - \text {V}_{\text {D}}$ characteristics. We demonstrated that the kink is related to impact ionization and follows a non-monotonic behavior maximized in the semi-on-state. Our interpretation is supported by a quantitative analysis based on the latest experimental impact-ionization coefficients available in the literature. [ABSTRACT FROM AUTHOR]

It has been long recognized that the capacitance-voltage (C-V) profiling technique only yields an apparent carrier concentration n⁁, which, in general, differs from the majority carrier concentration. However, it is still possible to extract the conduction band offset ΔEc and interface charge σi from n⁁, provided the doping profile and the exact hetero-interface position are known. We will show that in the case of σi∼1012cm-2, which is typical for the polar III-nitrides, an interface misplacement as small as 1 nm leads to a large error in the extracted ΔEc, making the technique challenging to use, even with the help of a computer-assisted reconstruction. We used one-sided p+-n junctions to experimentally investigate σi and ΔEc for Ga-face n-N GaN/AlGaN heterostructures through C-V profiling. The extracted σi matched the polarization charge reported by others. [ABSTRACT FROM AUTHOR]

The integration of III-nitride and non-III-nitride materials for tandem solar cell applications can improve the efficiency of the photovoltaic device due to the added power contributed by the III-nitride top cell to that of high-efficiency multi-junction non-III-nitride solar cells if the device components are properly designed and optimized. The proposed tandem solar cell is comprised of a III-nitride top cell bonded to a non-III-nitride, series-constrained, multi-junction subcell. The top cell is electrically isolated, but optically coupled to the underlying subcell. The use of a III-nitride top cell is potentially beneficial when the top junction of a stand-alone non-III-nitride subcell generates more photocurrent than the limiting current of the non-III-nitride subcell. Light producing this excess current can either be redirected to the III-nitride top cell through high energy photon absorption, redirected to the lower junctions through layer thickness optimization, or a combination of both, resulting in improved total efficiency. When the non-III-nitride cell's top junction is the limiting junction, the minimum power conversion efficiency that the III-nitride top cell must contribute should compensate for the spectrum filtered from the multi-junction subcell for this design to be useful. As the III-nitride absorption edge wavelength, λN, increases, the performance of the multi-junction subcell decreases due to spectral filtering. In the most common spectra of interest (AM1.5 G, AM1.5 D, and AM0), the technology to grow InGaN cells with λN < 520 nm is found to be sufficient for III-nitride top cell applications. The external quantum efficiency performance, however, of state-of-the-art InGaN solar cells still needs to be improved. The effects of surface/interface reflections are also presented. The management of these reflection issues determines the feasibility of the integrated III-nitride/non-III-nitride design to improve overall cell efficiency. [ABSTRACT FROM AUTHOR]

One-dimensional electron gas based devices are of great interest due to their promise in high-performance electronics and future device applications. However, synthesis and patterning of arrays of nanowires is a challenge in all material systems. Here we demonstrate a novel system based on vicinal AlGaN/GaN heterostructures that enables direct electrostatic tuning of the dimensionality of electrons from 1 D to 2 D. Our approach, based on polarization engineering, enables top-down fabrication of dense arrays of pure 1-dimensional electron channels with carrier confinement equivalent to 90 meV, that are capable of carrying technologically relevant current densities up to 130 mA/mm. A direction-dependent small-signal capacitance-voltage profiling to probe the Fermi occupation function of electron gas was used to demonstrate distinct signatures of 1-dimensional density of states and transport in these structures at room temperature. The system discussed here is based on polarization-induced anisotropic charge in vicinal AlGaN/GaN heterostructures. We developed a 2-sub-band model consisting of 1-D and 2-D sub-bands to describe the behavior of these wires. We find excellent agreement between our model and experimental data, confirming the channels are indeed 1-dimensional. Our demonstration of 1-dimensional electron channel arrays in this system could enable optical, electronic and magnetic devices with added functionalities and performance. [ABSTRACT FROM AUTHOR]

Dielectric passivation is important to improve the stability and reliability of gallium nitride based semiconductor devices. We need to characterize various dielectrics and their interfaces to nitrides accurately to be able to exploit the benefits efficiently. Earlier, B. L. Swenson and U. K. Mishra [J. Appl. Phys. 106, 064902 (2009)] have detailed a photo-assisted high frequency CV characterization technique for the Ga-polar SiN/GaN interface that gives an accurate value of interface state density (Dit) across the bandgap of the dielectric. In this work, we extend the technique to study the interface states at the SiN/AlGaN interface on GaN for Ga and N polar material. This simulates the AlGaN/GaN HEMT structure. A MIS-type structure comprised of a metal on SiN on an AlGaN/GaN heterojunction was used for the study. For a structure with 1 nm AlGaN interlayer, a peak interface state density of 2.8 × 1012 cm-2 eV-1 was measured. For Ga polar devices, the measured Dit decreases with increasing AlGaN thickness. In the N-polar case, the measured Dit increases with increasing AlGaN thickness. The variations of measured Dit with AlGaN thickness, in both cases, can be explained by screening from the accumulation charge at the AlGaN/GaN interface. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by inserting a composite AlN/aluminum oxide (AlOx) interlayer structure at the inversion interface. The change in polarity was verified in situ by reflection high energy electron diffraction via intensity transients and postgrowth surface reconstructions, and ex situ by convergent beam electron diffraction and etch studies in an aqueous potassium hydroxide solution. The inverted materials showed smooth surfaces and good electrical properties. AlGaN/GaN high electron mobility transistors fabricated on the inverted epilayers showed good dc and high frequency performance. A current-gain cutoff frequency (fT) of 21 GHz and maximum oscillation frequency (fmax) of 61 GHz were measured in devices with a gate length of 0.7 μm. These data compare favorably to those of Ga-face AlGaN/GaN devices with a similar structure grown on Si-face SiC substrates. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the influence of an AlN interlayer on the electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown by metal-organic chemical vapor deposition on miscut sapphire substrates. Use of miscut substrates leads to the formation of multiatomic steps at the AlGaN/GaN interface and anisotropy in electron transport properties. A combination of van der Pauw Hall, gated transfer length measurements, and capacitance-voltage measurements has been used to study the desired properties in directions parallel and perpendicular to the multiatomic steps and qualitative explanations were provided for the observed trends. Similar to the Ga-polar devices, the introduction of AlN interlayer improved the device performance by increasing both the electron mobility and the two-dimensional electron gas charge density in the devices. Orienting the devices such that the conduction occurred parallel to the multiatomic steps was beneficial for better electron transport and device performance. [ABSTRACT FROM AUTHOR]

The dependence of electron density (ns) on AlGaN barrier thickness (dAlGaN) was studied for AlGaN/GaN single heterostructures whose dAlGaN was controlled by low-power Cl-based reactive ion etching (RIE) instead of growth. The samples showed a constant increase not only in ns but also in AlGaN surface barrier height (e[lowercase_phi_synonym]B) with dAlGaN, indicating the existence of low-density and distributed donor states on the AlGaN surface. Such a distribution of donor states differs from the commonly accepted model based on high-density and single-level surface donor states as the source of electrons in the two-dimensional electron gas (2DEG). The presence of a distribution of donor states is confirmed by first-principles calculations for a variety of surface structures for oxidized AlGaN surfaces. Donor states arise from areas of the surface that deviate from the electron-counting rule, leading to occupied surface states in the upper half of the band gap. The oxide formed on the surface after RIE results in a low-density distribution of surface donor states in which the highest occupied levels span the range from 1-2 eV below the AlGaN conduction-band minimum. The density of these states is comparable to the ns in the 2DEG and insufficient to pin the Fermi level, leading to a constant increase in e[lowercase_phi_synonym]B with dAlGaN. [ABSTRACT FROM AUTHOR]

InxAl1-xN layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward <1010>GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by simultaneously exposing the surface to Mg and activated N fluxes during a growth interruption at a reduced substrate temperature. Growth studies suggested that a MgxNy compound was responsible for inverting the crystal. The change in polarity was verified in situ by reflection high energy electron diffraction via GaN surface reconstructions, and ex situ by convergent beam electron diffraction and KOH etch studies. The surface of the inverted material showed smooth step flow features. Ga-face high electron mobility transistors with good dc and small signal performance were fabricated on the inverted epilayers. A drain-source current of 0.84 A/mm was measured at a gate-source voltage of +1 V. Current-gain cutoff and maximum oscillation frequencies of 22 and 53 GHz, respectively, were measured in these devices. The device performance is similar to that of Ga-face transistors with comparable dimensions. [ABSTRACT FROM AUTHOR]

Smooth, high-quality N-polar GaN films were grown on C-face SiC substrates by metal organic chemical vapor deposition (MOCVD). Growth on substrates misoriented at 4° toward the m-plane suppressed the formation of hexagonal hillocks commonly observed for the growth of N-polar GaN by MOCVD. Aside from the misorientation, the growth temperature of the initial GaN was observed to have a strong impact on the structural and morphological properties of films grown on vicinal substrates as characterized by x-ray diffraction, atomic force microscopy, photoluminescence, and transmission electron microscopy. This strong temperature dependence was discovered to be a consequence of the island growth mode of the initial GaN. A two-step process was developed, which resulted in GaN films that exhibited XRD rocking curves with a full width at half maximum of 203 arc sec for a (0002) scan and 497 arc sec for a (2021) scan. [ABSTRACT FROM AUTHOR]

The SiNx/GaN interface barrier height for N-polar GaN based metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) was investigated. N-polar SiNx/GaN/AlGaN/GaN MISHEMT structures with different GaN cap thicknesses were grown by metal-organic chemical vapor deposition. The properties of the SiNx/GaN interface are of critical importance to device operation and modeling in these devices. An analytical expression for the pinch-off voltage of the HEMT was obtained, and capacitance-voltage (C-V) measurements with different Schottky metals were used to extract the barrier height. The Fermi level at the interface was found to be pinned at approximately 1 eV with respect to GaN conduction band edge, irrespective of the work function of the gate metal. Hall measurements of the two-dimensional electron gas density were found to corroborate the predicted interface barrier height. An approximate value for interface charge causing this pinning was calculated to be 4.5×1012 cm-2. [ABSTRACT FROM AUTHOR]

We report on the electrical and structural characterization of Mg-doped Al-rich p-type Al0.69Ga0.31N alloys on SiC substrate grown by metal organic chemical vapor deposition. The impact of growth conditions and dopant activation annealing conditions on the electrical conductivity was investigated. The most efficient activation was achieved by annealing in N2 gas at 1000 °C for 3 min in the presence of a SiO2 cap. The impact of p-GaN contact layer thickness was also studied. Higher growth temperature resulted in lower Mg incorporation, and for constant Mg concentration the resistivity increased with the increase in the growth temperature. Mg incorporation increased linearly with Mg effective flow. Transmission electron microscopy revealed the generation of vertical spike-like defects with increasing Mg doping. The resistivity also increased with the increase in Mg doping. A minimum resistivity of 10 Ω cm was achieved at 670 K. [ABSTRACT FROM AUTHOR]

Radiative and nonradiative processes in nearly strain-free Al[sub x]Ga[sub 1-x]N alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately -0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x<=0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (V[sub III]) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with V[sub III] where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (η[sub int]) with increasing x for x<=0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the V[sub III]-related trap density. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

AlGaN/GaN current aperture vertical electron transistors with regrown aperture and source regions have been fabricated and tested. A 2 μm thick GaN:Si drain region followed by a 0.4 μm GaN:Mg current-blocking layer were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. Channel apertures were etched, and a maskless regrowth was performed to grow unintentionally doped GaN inside the aperture as well as above the insulating layer, and to add an AlGaN cap layer. Cl[sub 2] reactive ion etching was used to pattern the device mesa, and source, drain, and gate metals were then deposited. Devices were achieved with a maximum source-drain current of 750 mA/mm, an extrinsic transconductance of 120 mS/mm, and a 2-terminal gate breakdown of 65 V while exhibiting almost no DC-RF dispersion for 80 μs pulsed I–V curves. The suppression of DC-RF dispersion was shown to result from the absence of the large electric fields at the surface on the drain-side edge of the gate that are present in high electron mobility transistors. Parasitic leakage currents, which were present in all devices, have been studied in detail. Three leakage paths have been identified, and methods to eliminate them are discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A methodology for the design of modulation doped AlGaN/GaN multiple channel heterostructures is presented. Doping is utilized in conjunction with polarization effects to achieve high carrier mobility and high sheet carrier density in each channel, while maintaining a low energy barrier for majority carrier transfer between channels. Several eight-period Si-doped Al[sub 0.22]Ga[sub 0.78]N/GaN heterostructures were grown by metalorganic chemical vapor deposition, according to the methodology. Sheet electron densities around 7.7×10[sup 13] cm[sup -2], and room temperature electron mobilities as high as 1200 cm2/V s were measured. Applications for the structure include lateral current spreading layers in III-nitride visible light and UV emitters and detectors, and high conductance source and drain access regions in AlGaN/GaN high electron mobility transistors. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]