arXiv subjects

Pietro Pampili

Publications and source records attributed to Pietro Pampili.

5 recordsLinked to original sources

arXiv2026

Barrier-channel intermixing and 2-dimensional electron gas degradation in Al-rich Al(Ga)N/AlGaN high electron mobility transistor heterostructures

In this work, we report on recent results in understanding and addressing the issue of interface smearing in high-aluminium content AlGaN/AlGaN heterostructures. On the one hand, the growth of high-crystal quality AlGaN by metal-organic vapour phase epitaxy (MOVPE) requires the use of high temperatures, but on the other hand this may lead to alloy intermixing between barrier and channel layers, which smoothens out the polarization contrast and severely degrades or even completely destroys the 2-dimensional electron gas (2DEG). We show that X-Ray Diffraction (XRD) analysis can be used as a non-destructive way to assess the sharpness of the interface, and that improved growth schemes can be successfully used to achieve high-quality 2DEG, as confirmed by contactless resistivity measurements. In particular, sheet resistivities around 2,500 $Ω/\Box$ were demonstrated for AlN/Al$_{0.75}$Ga$_{0.25}$N, consistent with the best-reported values in the literature.

cond-mat.mtrl-sci↗

arXiv2025

Interplay of carrier density and mobility in Al-Rich (Al,Ga)N-Channel HEMTs: Impact on high-power device performance potential

Despite considerable advancements, high electron mobility transistors (HEMTs) based on gallium nitride (GaN) channels remain largely limited to power applications below 650 V. For higher power demands, the ultra-wide bandgap semiconductor alloy aluminium gallium nitride, (Al,Ga)N, has emerged as a key contender for next-generation HEMTs. In this theoretical study, we show that Al-rich Al$_x$Ga$_{1-x}$N-channel HEMTs (with $x \geq 0.5$) outperform the GaN-channel counterparts at and above room temperature, across all Al compositions, $x$. This contrasts with recent theory reports which suggest that only Al$_x$Ga$_{1-x}$N HEMTs with high Al content ($x \geq 0.85$) offer comparable performance to GaN-channel devices. Unlike previous assumptions of a constant two-dimensional electron gas (2DEG) density across the entire composition range $x$, we show that the 2DEG density is highly sensitive to both the Al content and thickness of the individual layers in a HEMT structure. We demonstrate that the superior performance of Al-rich (Al,Ga)N-channel HEMTs is driven by a competing effect between 2DEG density and electron mobility. This work challenges the assumptions of prior studies, which can result in a significant under or overestimation of the potential of high Al content HEMTs. The insights gained from our work provide a comprehensive understanding of the trade-offs between device and material parameters, thus help to guide the design of future Al-rich ($x = 0.5-1.0$) Al$_x$Ga$_{1-x}$N-channel HEMTs for high-power applications.

cond-mat.mtrl-sci↗

arXiv2025

Temperature-dependent investigation of polarisation doping in 330 nm ultraviolet light-emitting diodes

Polarisation doping of Al$_x$Ga$_{1-x}$N, through grading of $x$, has realised major improvements in $p$-type conductivity in ultraviolet (UV) light-emitting diodes (LEDs) compared to conventional impurity doping. However, the exact balance between the two doping regimes to achieve the best device performance is not clear, especially as a function of operating wavelength. In this work, 330 nm LEDs with varied $p$-doping approaches were characterised as a function of temperature: Mg doped only (reference); polarisation doped and Mg doped (co-doped); and polarisation doped only. At room temperature, the co-doped LED showed the highest electroluminescence (EL) intensity, with a similar operating voltage to the reference LED. The highest hole concentration, confirmed by Hall effect measurements, as well as improved injection efficiency revealed by simulations, are credited as the main reasons for EL improvement. A parasitic near-UV luminescence tail, analogous to the "blue luminescence" in $p$-GaN, was observed in both the reference and co-doped LEDs, but was absent in the polarisation doped LED. The reference LED demonstrated the highest increase in operating voltage with decreasing temperature, while the LED with only polarisation doping showed a temperature-independent behaviour, demonstrating the benefits of a polarisation field-induced carrier concentration. Further optimisation of the compositional grading with concurrent Mg doping can potentially produce higher performance LEDs with cleaner spectra.

physics.app-ph↗

arXiv2024

Geometrically-Controlled Microscale Patterning and Epitaxial Lateral Overgrowth of Nitrogen-Polar GaN

In this study we report on a novel two-step epitaxial growth technique that enables a significant improvement of the crystal quality of nitrogen-polar GaN. The starting material is grown on 4° vicinal sapphire substrates by metal organic vapour phase epitaxy, with an initial high-temperature sapphire nitridation to control polarity. The material is then converted into a regular array of hexagonal pyramids by wet-etch in a KOH solution, and subsequently regrown to coalesce the pyramids back into a smooth layer of improved crystal quality. The key points that enable this technique are the control of the array geometry, obtained by exploiting the anisotropic behaviour of the wet-etch step, and the use of regrowth conditions that preserve the orientation of the pyramids' sidewalls. In contrast, growth conditions that cause an excessive expansion of the residual (000-1) facets on the pyramids' tops cause the onset of a very rough surface morphology upon full coalescence. An X-ray diffraction study confirms the reduction of the threading dislocation density as the regrowth step develops. The analysis of the relative position of the 000-2 GaN peak with respect to the 0006 sapphire peak reveals a macroscopic tilt of the pyramids, probably induced by the large off-axis substrate orientation. This tilt correlates very well with an anomalous broadening of the 000-2 diffraction peaks at the beginning of the regrowth step.

cond-mat.mtrl-sci↗

arXiv2024

Nitrogen-polar growth of AlN on vicinal (0001) sapphire by MOVPE

We report about metalorganic vapour phase epitaxy of smooth nitrogen-polar AlN templates on vicinal (0001) sapphire substrates. The influence of V/III ratio, growth temperature, growth rate, as well as sapphire-nitridation time and temperature were studied. With 4° offcut sapphire, step-flow growth was possible only with V/III ratios below 2. However, optimal V/III ratio required precise adjustment, possibly dependent on reactor history and geometry. A rather narrow temperature window of less than 40°C existed for smooth surface morphology. Reducing the temperature affected adatom mobility, eventually disrupting step-flow growth; increasing the temperature favoured the formation of high-aspect-ratio defects on the epilayer. A low thermal-budget nitridation step with a short nitridation time of 15 s proved to be effective in controlling polarity without inducing excessive surface damage on the sapphire substrate. Growth rate also influenced surface morphology, with an increase in RMS roughness and step-bouncing for faster growths; however, at growth rates of 1.4 $μ$m/h or higher step-flow growth could no longer form. Finally, we developed a V/III ratio fine-tuning procedure, whereby the reactor-specific value that induces optimal growth is inferred by growth-rate variations. With this method, N polar AlN templates with sub-nanometre RMS roughness were demonstrated for both 4° and 2° offcut sapphire substrates.

cond-mat.mtrl-sci↗