Marco Malinverni

InGaN based micro light emitting diodes featuring a buried GaN tunnel junction

GaN tunnel junctions (TJs) are grown by ammonia molecular beam epitaxy. High doping levels are achieved with a net acceptor concentration close to ∼1020 cm−3, thanks to the low growth temperature. This allows for the realization of p-n junctions with ultrathin depletion width enabling efficient interband tunneling. n-p-n structures featuring such a TJ exhibit low leakage current densities, e.g., <5 × 10−5 A cm−2 at reverse bias of 10 V. Under forward bias, the voltage is 3.3 V and 4.8 V for current densities of 20 A cm−2 and 2000 A cm−2, respectively. The specific series resistance of the whole device is 3.7 × 10−4 Ω cm2. Then micro-light emitting diodes (μ-LEDs) featuring buried TJs are fabricated. Excellent current confinement is demonstrated together with homogeneous electrical injection, as seen on electroluminescence mapping. Finally, the I-V characteristics of μ-LEDs with various diameters point out the role of the access resistance at the current aperture edge.

94-GHz Large-Signal Operation of AlInN/GaN High-Electron-Mobility Transistors on Silicon With Regrown Ohmic Contacts

We report the first 94-GHz (W-band) large-signal performance of AlInN/GaN high-electron-mobility transistors (HEMTs) grown on high-resistivity silicon (111) substrates. A maximum output power density of 1.35 W/mm and peak power-added-efficiency of 12% are measured at 94 GHz. The devices exhibit a dc maximum current drain density of 1.6 A/mm and a peak transconductance of 650 mS/mm. In small-signal operation, cutoff frequencies fT/fMAX = 141/232 GHz are achieved. The large-signal performance of our AlInN/GaN HEMTs on silicon at 94 GHz stills lags the best reported results one on SiC substrates but nevertheless confirms the tremendous interest of GaN-on-Si HEMT technology for low-cost millimeterwave electronic applications.

Low temperature p-type doping of (Al)GaN layers using ammonia molecular beam epitaxy for InGaN laser diodes

We demonstrate state-of-the-art p-type (Al)GaN layers deposited at low temperature (740 °C) by ammonia molecular beam epitaxy (NH3-MBE) to be used as top cladding of laser diodes (LDs) with the aim of further reducing the thermal budget on the InGaN quantum well active region. Typical p-type GaN resistivities and contact resistances are 0.4 Ω cm and 5 × 10−4 Ω cm2, respectively. As a test bed, we fabricated a hybrid laser structure emitting at 400 nm combining n-type AlGaN cladding and InGaN active region grown by metal-organic vapor phase epitaxy, with the p-doped waveguide and cladding layers grown by NH3-MBE. Single-mode ridge-waveguide LD exhibits a threshold voltage as low as 4.3 V for an 800 × 2 μm2 ridge dimension and a threshold current density of ∼5 kA cm−2 in continuous wave operation. The series resistance of the device is 6 Ω and the resistivity is 1.5 Ω cm, confirming thereby the excellent electrical properties of p-type Al0.06Ga0.94N:Mg despite the low growth temperature.

Backward diodes using heavily Mg-doped GaN growth by ammonia molecular-beam epitaxy

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (<3 × 1017 cm−3) responsible for p-type doping compensation. As a result, a net acceptor concentration of 7 × 1019 cm−3 was achieved, and the hole concentration measured by Hall effect was as high as 2 × 1019 cm−3 at room temperature. Using such a high Mg doping level, we fabricated GaN backward diodes without polarization-assisted tunneling. The backward diodes exhibited a tunneling-current density of 225 A/cm2 at a reverse bias of −1 V at room temperature.

High-Mobility AlGaN/GaN Two-Dimensional Electron Gas Heterostructure Grown on (111) Single Crystal Diamond Substrate

High mobility Al0.28Ga0.72N/GaN two-dimensional electron gas (2DEG) is achieved on (111) oriented single crystal diamond substrate. The surface morphology of the epilayer is free of cracks thanks to the use of an AlN interlayer for strain relaxation. The rms roughness of the sample surface deduced from atomic force microscopy is 0.6 nm for a 2 ×2 µm2 scan area, which indicates an excellent surface morphology. Hall effect measurements reveal a 2DEG with room temperature mobility and sheet carrier density of 750 cm2 V-1 s-1 and 1.4 ×1013 cm-2, respectively. These results compare fairly well with AlGaN/GaN 2DEG characteristics obtained on other substrates like silicon and demonstrate that high power electronics can be developed on diamond substrates with high power dissipation capabilities.

n(+)-GaN grown by ammonia molecular beam epitaxy: Application to regrown contacts

We report on the low-temperature growth of heavily Si-doped (>1020 cm−3) n+-type GaN by N-rich ammonia molecular beam epitaxy (MBE) with very low bulk resistivity (<4 × 10−4 Ω·cm). This is applied to the realization of regrown ohmic contacts on InAlN/GaN high electron mobility transistors. A low n+-GaN/2 dimensional electron gas contact resistivity of 0.11 Ω·mm is measured, provided an optimized surface preparation procedure, which is shown to be critical. This proves the great potentials of ammonia MBE for the realization of high performance electronic devices.

InGaN laser diode with metal-free laser ridge using n+-GaN contact layers

We report on InGaN edge emitting laser diodes with a top metal electrode located beside the laser ridge. Current spreading over the ridge is achieved via a highly doped n(+)-type GaN layer deposited on top of the structure. The low sheet resistance of the n(+)-GaN layer ensures excellent lateral current spreading, while carrier injection is confined all along the ridge thanks to current tunneling at the interface between the n(+)-GaN top layer and the p(++)-GaN layer. Continuous-wave lasing at 400nm with an output power of 100mW is demonstrated on uncoated facet devices with a threshold current density of 2.4 kA.cm(-2)

InGaN laser diodes emitting at 500 nm with p-layers grown by molecular beam epitaxy

We demonstrate hybrid laser diodes by combining n-type layers and an active region grown by metal organic vapor phase epitaxy with p-type layers grown by molecular beam epitaxy. These p-doped layers, grown at 740 degrees C, exhibit state-of-the-art electrical characteristics and prevent the indium-rich quantum wells from thermal degradation. Narrow ridge-waveguide lasers with high-reflectivity coatings show a threshold current density of 9.7 kA.cm(-2), a threshold voltage of 5.4V, and a lasing wavelength of 501 nm. The internal optical loss and material gain of the epitaxial structures are also measured and discussed

Vacancy-type defects in Mg-doped GaN grown by ammonia-based molecular beam epitaxy probed using a monoenergetic positron beam

Vacancy-type defects in Mg-doped GaN were probed using a monoenergetic positron beam. GaN films with a thickness of 0.5–0.7 μm were grown on GaN/sapphire templates using ammonia-based molecular beam epitaxy and characterized by measuring Doppler broadening spectra. Although no vacancies were detected in samples with a Mg concentration [Mg] below 7 × 1019 cm−3, vacancy-type defects were introduced starting at above [Mg] = 1 × 1020 cm−3. The major defect species was identified as a complex between Ga vacancy (VGa) and multiple nitrogen vacancies (VNs). The introduction of vacancy complexes was found to correlate with a decrease in the net acceptor concentration, suggesting that the defect introduction is closely related to the carrier compensation. We also investigated Mg-doped GaN layers grown using In as the surfactant. The formation of vacancy complexes was suppressed in the subsurface region (≤80 nm). The observed depth distribution of defects was attributed to the thermal instability of the defects, wh...

Recent progress on GaN-based superluminescent light-emitting diodes in the visible range

Superluminescent light emitting diodes (SLEDs) have beam-like optical output similar to laser diodes (LDs) while offering a broader emission wavelength spectrum. They represent, therefore, an interesting alternative to conventional LDs for applications where a short coherence length or low speckle noise are required. Visible SLEDs emitting in the red, blue, and green are ideal candidates for the manufacturing of speckle-free light sources in portable or wearable compact projection systems. In this paper, we review the current status of EXALOS’ GaN-based SLED technology in the violet-blue spectral range and report on our recent progress in terms of performance for devices with 440-460 nm emission. Furthermore, we discuss the challenges in achieving light output at even longer wavelengths. As a matter of fact, lower refractive index contrast between the waveguiding and cladding layers, decreased p-type doping efficiency when growing at low temperatures, low crystal quality and thermal stability of the active region have to be addressed and solved in order to achieve green emission. The epitaxial structures were grown by metalorganic vapor phase epitaxy (MOVPE) on c-plane freestanding GaN substrates. Growth was followed by standard fabrication of SLEDs with a ridge waveguide design. A record CW output power of 150 mW (at an operating current of 330 mA) and a wall-plug efficiency (WPE) of 8% have been obtained at an emission wavelength >440 nm.