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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]

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]

AlGaN/GaN high electron mobility transistors with different Fe-doping density were studied using electrical and optical analysis to gain insight into the nature of traps responsible for the kink effect in electrical characteristics. Kink effect has been previously suggested to result from direct trapping of carriers in defects related to yellow luminescence (YL) centers. However, the results demonstrate that YL is suppressed by Fe doping, whereas the kink effect is not affected to the same extent. YL related defect states are therefore not exclusively responsible for the kink effect, suggesting a more complex trapping mechanism to affect device output characteristics. [ABSTRACT FROM AUTHOR]