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The effects of thermal annealing on the optical properties of Mg-doped cubic zincblende GaN epilayers grown by metalorganic chemical vapor deposition on 3C-SiC/Si (001) substrates are investigated. The photoluminescence spectra show near band edge features and a blue luminescence band that depend on Mg concentration, temperature, and excitation power density. Annealing the sample in a N2 atmosphere causes the intensity of the blue band to increase by a factor of 5. Power dependent photoluminescence measurements show a reduction in the laser excitation density required for saturation of the blue band after annealing, indicating an increase in the recombination lifetime. Time decay measurements confirm this increase, which is attributed to a reduction in the concentration of non-radiative defects after annealing. The results presented here are compared to those reported previously for Mg-doped hexagonal wurtzite GaN. [ABSTRACT FROM AUTHOR]

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite, crystal phase has the potential to improve efficiency. However, optimization of the emission efficiency of these heterostructures is still required to compete with more conventional alternatives. Photoluminescence time decays were used to assess how the quantum well width and number of quantum wells affect the recombination rates, and temperature dependent photoluminescence was used to determine the factors affecting recombination efficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increase weakly with well width, consistent with a change in the exciton binding energy. The relative efficiency at room temperature was found to increase by a factor of five when the number of wells was increased from one to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2.5 to 7.5 nm. These results indicate that thermionic emission is the most important process reducing efficiency at temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombination on well width means that increasing well thickness is an effective way of suppressing thermionic emission and thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in the wurtzite phase. [ABSTRACT FROM AUTHOR]

Current cubic zincblende III-Nitride epilayers grown on 3C-SiC/Si(001) substrates by metal-organic vapor-phase epitaxy contain a high density of stacking faults lying on the {111} planes. A combination of high-resolution scanning transmission electron microscopy and energy dispersive x-ray spectrometry is used to investigate the effects of alloy segregation around stacking faults in a zincblende III-nitride light-emitting structure, incorporating InGaN quantum wells and an AlGaN electron blocking layer. It is found that in the vicinity of the stacking faults, the indium and aluminum contents were a factor of 2.3 ± 1.3 and 1.9 ± 0.5 higher, respectively, than that in the surrounding material. Indium and aluminum are also observed to segregate differently in relation to stacking faults with indium segregating adjacent to the stacking fault while aluminum segregates directly on the stacking fault. [ABSTRACT FROM AUTHOR]

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices. [ABSTRACT FROM AUTHOR]

The photoluminescence spectra of a zincblende GaN epilayer grown via metal-organic chemical vapour deposition upon 3C-SiC/Si (001) substrates were investigated. Of particular interest was a broad emission band centered at 3.4 eV, with a FWHM of 200 meV, which extends above the bandgap of both zincblende and wurtzite GaN. Photoluminescence excitation measurements show that this band is associated with an absorption edge centered at 3.6 eV. Photoluminescence time decays for the band are monoexponential, with lifetimes that reduce from 0.67 ns to 0.15 ns as the recombination energy increases. TEM measurements show no evidence of wurtzite GaN inclusions which are typically used to explain emission in this energy range. However, dense stacking fault bunches are present in the epilayers. A model for the band alignment at the stacking faults was developed to explain this emission band, showing how both electrons and holes can be confined adjacent to stacking faults. Different stacking fault separations can change the carrier confinement energies sufficiently to explain the width of the emission band, and change the carrier wavefunction overlap to account for the variation in decay time. [ABSTRACT FROM AUTHOR]

In a bid to understand the commonly observed hysteresis in the threshold voltage (VTH) in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors during forward gate bias stress, we have analyzed a series of measurements on devices with no surface treatment and with two different plasma treatments before the in-situ Al2O3 deposition. The observed changes between samples were quasi-equilibrium VTH, forward bias related VTH hysteresis, and electrical response to reverse bias stress. To explain these effects, a disorder induced gap state model, combined with a discrete level donor, at the dielectric/semiconductor interface was employed. Technology Computer-Aided Design modeling demonstrated the possible differences in the interface state distributions that could give a consistent explanation for the observations. [ABSTRACT FROM AUTHOR]

LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material. [ABSTRACT FROM AUTHOR]

In this work, we have compared SiNx passivation, hydrogen peroxide, and sulfuric acid treatment on μlGaN/GaN HEMTs surface after full device fabrication on Si substrate. Both the chemical treatments resulted in the suppression of device pinch-off gate leakage current below 1 lμ/mm, which is much lower than that for SiNx passivation. The greatest suppression over the range of devices is observed with the sulfuric acid treatment. The device on/off current ratio is improved (from 104-105 to 107) and a reduction in the device sub-threshold (S.S.) slope (from ∼215 to 90 mV/decade) is achieved. The sulfuric acid is believed to work by oxidizing the surface which has a strong passivating effect on the gate leakage current. The interface trap charge density (Dit) is reduced (from 4.86 to 0.90×1012cm-2 eV-1), calculated from the change in the device S.S. The gate surface leakage current mechanism is explained by combined Mott hopping conduction and Poole Frenkel models for both untreated and sulfuric acid treated devices. Combining the sulfuric acid treatment underneath the gate with the SiNx passivation after full device fabrication results in the reduction of Dit and improves the surface related current collapse. [ABSTRACT FROM AUTHOR]

Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one-dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments, these structures emit visible light, which is optically polarized up to 86% at 10 K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2 nm to 8 nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarized along the length of these nanostructures. [ABSTRACT FROM AUTHOR]

Incorporating GaN capping layers in conjunction with recessing has been identified as a means to maximize the high frequency performance of AlGaN/GaN high electron mobility transistors (HEMTs). Doping the cap heavily n-type is required in order to ensure minimal loss of carriers from the channel. Using a SiCl4/SF6 dry etch plasma recipe, 250 nm gate length HEMTs with recess lengths varying from 300 nm to 5 μm are fabricated. Heavily doped n+GaN caps enabled contact resistances of 0.3 Ω mm to be achieved. Recessing using a SiCl4/SF6 recipe does not introduce significant numbers of bulk traps. Gate recessing in conjunction with Si3N4 passivation reduces rf dispersion to negligible levels. [ABSTRACT FROM AUTHOR]

The optical properties of AlxGa1-xN epilayers with x ranging from 0.08 to 0.52 have been studied by photoluminescence (PL). The temperature dependent PL of the AlxGa1-xN epilayers shows a classical “S-shape” behavior. This behavior is attributed to exciton localization due to compositional fluctuations in the AlxGa1-xN layers. The localization parameter σ extracted from temperature dependent PL, which gives an estimate of degree of localization, is found to increase with Al composition, up to a value of 52 meV at the highest Al composition studied. Several phonon replicas are observed at the lower energy side of the main excitonic emission peak in these epilayers at low temperature. In all cases, the Huang-Rhys parameter has been estimated. The Huang-Rhys parameter is found to increase with x indicating that the degree of localization again increases with x. In addition, the Huang-Rhys parameter is found to increase with higher order phonon replicas. [ABSTRACT FROM AUTHOR]

SiGe/Si multiple quantum wells, nominally 4 nm thick, were grown by low pressure chemical vapor deposition and the Ge distribution within the wells was studied using a variety of transmission electron microscope-based techniques. Energy-dispersive x-ray spectroscopy and electron energy-loss imaging were used to directly measure the Ge compositional profile across the SiGe wells. In addition, the average Ge concentration was deduced indirectly from measurement of the strain-induced lattice displacements in high resolution images, obtained from the relative phase shift of the Si lattice planes on either side of a SiGe well. The results from both the direct and indirect measurement techniques were compared and found to be in good agreement with one another. The Ge profiles exhibited an asymmetric shape consistent with the occurrence of Ge segregation during growth. However, the amplitude of the asymmetry indicated that an additional factor, in particular gas dwell times within the reactor, also needed to be taken into account. Based upon this approach, a successful theoretical model of the growth process was derived. [ABSTRACT FROM AUTHOR]

The luminescence properties of cubic GaN films grown upon 3C-SiC/Si (001) substrates by MOCVD were investigated. The spectra show luminescence peaks which are associated with donor bound exciton recombination and donor acceptor pair recombination. A reduced peak energy for the D0X emission compared with values reported in the literature suggests a tensile-strain-reduced bandgap of approximately 3.27 eV, which is consistent with the absorption edge in photoluminescence-excitation spectroscopy. The presence of hexagonal material introduces a broad emission band at 3.40 eV with a FWHM of 190 meV, extending to energies up to 3.60 eV. The intensity of this emission scales linearly with excitation power, its peak energy and width remaining unchanged. This band is associated with an absorption edge below 3.70 eV and therefore is not caused by absorption into phase-pure cubic or hexagonal GaN. The photoluminescence lifetimes measured across this band reduce from 0.40 to 0.20 ns with increasing emission energy. All these observations can be explained by considering a type-II-band alignment adjacent to stacking faults within the cubic GaN. [ABSTRACT FROM AUTHOR]

Novel MMIC spiral inductors on GaN-on-low-resistivity silicon (LR-Si) substrates ( \sigma < 40~\Omega \cdot \text cm ) are demonstrated with enhanced self-resonance frequency ( f\mathrm{ SRF} ) and Q -factor. The developed technology improves inductor performance by suppressing substrate coupling effects using air-bridge technology above benzocyclobutene dielectric as an interface layer on the lossy substrate. A 0.83-nH spiral inductor with peak Q -factor enhancement of 57% ( Q = 22\mu \textm elevation in air. An accurate broad-band model for the fabricated inductors has been developed and verified for further performance analysis up to 40 GHz. The proposed inductors utilize cost-effective, reliable, and MMIC-compatible technology for the realization of high-performance RF GaN-on-LR Si MMIC circuits for millimeter-wave applications. [ABSTRACT FROM PUBLISHER]

The use of III-nitride-based light-emitting diodes (LEDs) is now widespread in applications such as indicator lamps, display panels, backlighting for liquid-crystal display TVs and computer screens, traffic lights, etc. To meet the huge market demand and lower the manufacturing cost, the LED industry is moving fast from 2 inch to 4 inch and recently to 6 inch wafer sizes. Although Al2O3 (sapphire) and SiC remain the dominant substrate materials for the epitaxy of nitride LEDs, the use of large Si substrates attracts great interest because Si wafers are readily available in large diameters at low cost. In addition, such wafers are compatible with existing processing lines for 6 inch and larger wafers commonly used in the electronics industry. During the last decade, much exciting progress has been achieved in improving the performance of GaN-on-Si devices. In this contribution, the status and prospects of III-nitride optoelectronics grown on Si substrates are reviewed. The issues involved in the growth of GaN-based LED structures on Si and possible solutions are outlined, together with a brief introduction to some novel in situ and ex situ monitoring/characterization tools, which are especially useful for the growth of GaN-on-Si structures. [ABSTRACT FROM AUTHOR]

A critical element for the successful growth of GaN device layers on Si is accurate control of the AlGaN buffer layers used to manage strain. Here we present a method for measuring the composition of the AlGaN buffer layers in device structures which makes use of a one-dimensional x-ray detector to provide efficient measurement of a reciprocal space map which covers the full compositional range from AlN to GaN. Combining this with a suitable x-ray reflection with low strain sensitivity it is possible to accurately determine the Al fraction of the buffer layers independent of their relaxation state. [ABSTRACT FROM AUTHOR]

In this work a novel ultra-low loss transmission media for RF GaN-on-low-resistivity silicon (LR-Si) substrates ( $\sigma < 40~\Omega \cdot$ cm) has been successfully demonstrated. The developed shielded-microstrip lines achieve comparable performance to those on semi-insulating (SI) GaAs substrates with transmission loss of 0.9 dB/mm for frequencies up to 67 GHz. Line performance was further enhanced by additional elevation of the shielded-microstrip lines using air-bridge technology above a 5~\mu \text {m} layer of benzocyclobutene (BCB) on shielded metalized ground planes. Transmission loss of 0.6 dB/mm for frequencies up to 67 GHz was obtained as a result of the extra elevation. Structure parameters were designed and optimized based on EM simulation for best performance. The work shows that the RF energy coupled into the substrate was eliminated, indicating the suitability of III-V-on-LR Si technology for millimeter-wave applications. [ABSTRACT FROM PUBLISHER]

The influence of a thermal boundary resistance (TBR) on temperature distribution in ungated AlGaN/GaN field-effect devices was investigated using 3-D micro-Raman thermography. The temperature distribution in operating AlGaN/GaN devices on SiC, sapphire, and Si substrates was used to deter- mine values for the TBR by comparing experimental results to finite-difference thermal simulations. While the measured TBR of about 3.3 × 10-8 W-1 · m2 · K for devices on SiC and Si substrates has a sizeable effect on the self-heating in devices, the TBR of up to 1.2 × 10-7 W-1 · m2 · K plays an insignificant role in devices on sapphire substrates due to the low thermal conductivity of the substrate. The determined effective TBR was found to increase with temperature at the GaN/SiC interface from 3.3 × 10-8 W-1 · m2 · K at 150 °C to 6.5 × 10-8 W-1 · m2 · K at 275 °C, respectively. The contribution of a low-thermal-conductivity GaN layer at the GaN/substrate interface toward the effective TBR in devices and its temperature dependence are also discussed. [ABSTRACT FROM AUTHOR]

We report on the development of time-resolved Raman thermography to measure transient temperatures in semi-conductor devices with submicrometer spatial resolution. This new technique is illustrated for AIGaN/GaN HFETs and ungated devices grown on SiC and sapphire substrates. A temporal resolution of 200 ns is demonstrated. Temperature changes rapidly within sub-200 ns after switching the devices on or off, followed by a slower change in device temperature with a time constant of ~10 and ~140 µs for AIGaN/GaN devices grown on SiC and sapphire substrates, respectively. Heat diffusion into the device substrate is also demonstrated. [ABSTRACT FROM AUTHOR]

This letter reports the RF performance of a 0.3- \mu \textm gate length AlGaN/AlN/GaN HEMT realized on a 150-mm diameter low-resistivity (LR) ( \sigma <10~\Omega \cdot \textrm cm ) silicon substrate. Short circuit current gain ( fT ) and maximum frequency of oscillation ( f\textrm {MAX} ) of 55 and 121 GHz, respectively, were obtained. To our knowledge, these are the highest fT/f{\textrm {MAX}} values reported to date for GaN HEMTs on LR silicon substrates. [ABSTRACT FROM AUTHOR]

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (\sigma<40\ \Omega.cm). Transmission losses (S21) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si. [ABSTRACT FROM PUBLISHER]

Time-resolved Raman thermography, with a temporal resolution of -10 ns, was used to study the thermal dynamics of AIGaN/GaN electronic devices (high-electron mobility transistors and ungated devices). Heat diffusion from the device active region into the substrate and within the devices was studied. Delays in the thermal response with respect to the electrical pulse were determined at different locations in the devicés Quasi-adiabatic heating of the AIGaN/GaN devices is illustrated within the first ~7O ns of device operation. The temperature of devices on SiC was found to reach `~25% of the dc temperature when operated with 200-ns-long electrical pulses. Index Terms-AIGaN, communication, FETs, GaN, heat dif- fusion, high-electron mobility transistors (HEMT5), nanosecond, pulsed, radar, Raman spectroscopy, reliability, temperature, thermography. [ABSTRACT FROM AUTHOR]

The heterogeneous integration of InSb quantum well transistors onto silicon substrates is investigated for the first time. 85 nm gate length FETs with fT=305 GHz at Vds=0.5 V and DC performance suitable for digital logic are demonstrated on material with a buffer just 1.8 µm thick. An initial step towards integrating InSb FETs with mainstream Si CMOS for high-speed, energy-efficient logic applications has been achieved. [ABSTRACT FROM AUTHOR]

We illustrate the use of micro-Raman mapping to study the local effect of defects on device temperature in active AlGaN/GaN heterostructure field-effect transistors. Significant temperature rises in active devices, 50–100% above average device temperatures, were identified in the vicinity of defects. Measured temperature distributions were compared to finite difference simulations. Reduced thermal conductivity in the defect vicinity was found to be responsible for the local temperature rises in these devices, combined with possible changes in the current flow distribution. [ABSTRACT FROM AUTHOR]

The structural characteristics of AlN exclusion layers used to enhance the electron mobility in GaN-based field-effect transistor structures are investigated using scanning transmission electron microscopy. It is shown that a peak in electron mobility is achieved for an AlN exclusion layer with a nominal thickness of 2.3 nm, although significant compositional grading at the interfaces appears to be present. For longer growth times (i.e., 30 s), a transition to three-dimensional growth occurs, roughening the GaN/AlN/AlGaN interfaces. This roughening is likely to be associated with scattering of carriers in the two-dimensional electron gas (2DEG) consistent with an observed increase in 2DEG sheet resistivity. [ABSTRACT FROM AUTHOR]

An x-ray diffraction technique is described which, by careful choice of the x-ray reflection used, minimizes errors in composition measurements resulting from strain and uncertainties in the elastic constants of a material. The method is applied to the AlGaN system, which shows a wide range of values for Poisson’s ratio in the literature and significant variation in strain state due to the high dislocation content and large thermal expansion mismatch with the substrate. It is demonstrated that accurate composition measurements of partially relaxed AlxGa1-xN layers (x<0.3) with thickness >20 nm can be made from a single measurement. [ABSTRACT FROM AUTHOR]

We present a study of the effect of the growth of a thin AlN exclusion layer between the AlGaN barrier layer and GaN buffer layer in microwave heterojunction field-effect transistor structures. A dramatic improvement in carrier drift mobility is observed and we present evidence from electronic structure calculations and capacitance–voltage experiments that this improvement is associated with reduced alloy scattering. However, no significant benefit is seen at low carrier concentrations. Reduced electron trapping in the AlGaN is an additional benefit. [ABSTRACT FROM AUTHOR]

The impact of the stress in room temperature inductively coupled plasma chemical vapour deposited (ICP-CVD) SiNx surface passivation layers on off-state drain (IDS-off) and gate leakage currents (IGS) in AlGaN/GaN high electron mobility transistors (HEMTs) is reported. IDS-off and IGS in 2 μm gate length devices were reduced by up to four orders of magnitude to ~10 pA/mm using a compressively stressed bilayer SiNx passivation scheme. In addition, Ion/Ioff of ~1011 and subthreshold slope of 68 mV/dec were obtained using this strain engineered surface passivation approach. [ABSTRACT FROM AUTHOR]