M. A. di Forte-Poisson

Surface potential of n- and p-type GaN measured by Kelvin force microscopy

n- and p-type GaN epitaxial layers grown by metal-organic chemical vapor deposition with different doping levels have been characterized by Kelvin probe force microscopy (KFM). To investigate the surface states of GaN beyond instrumental and environmental fluctuations, a KFM calibration procedure using a gold-plated Ohmic contact as a reference has been introduced, and the reproducibility of the KFM measurements has been evaluated. Results show that the Fermi level is pinned for n- and p-type GaN over the available doping ranges, and found 1.34±0.15eV below the conduction band and 1.59±0.18eV above the valence band, respectively.

Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors

We report on our multi–pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3μm)/ AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X–ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson–Schrodinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2–DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.

AlGaN/GaN HEMT High Power Densities on

In this letter, successful operation at 10 GHz of T-gate HEMTs on epitaxial structures grown by metal-organic chemical vapor deposition (MOCVD) or MBE on composite substrates is demonstrated. The used device fabrication process is very similar to the process used on monocrystalline SiC substrate. High power density was measured on both epimaterials at 10 GHz. The best value is an output power density of 5.06 W/mm associated to a power-added efficiency (PAE) of 34.7% and a linear gain of 11.8 dB at VDS = 30 V for the components based on MOCVD-grown material. The output power density is 3.58 W/mm with a maximum PAE of 25% and a linear gain around 15 dB at VDS = 40 V for the MBE-grown material.

\hbox{SiC/} \hbox{SiO}_{2}

This paper reports on the investigation of GaN growth by low-pressure metal organic chemical vapour deposition technique on silicon carbide (SiC) substrates. The critical impact of some growth parameters on the physical properties of the GaN epilayers has been identified and studied, using high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) measurements. The SiC substrate surface preparation (ex situ and in situ surface preparation: deoxydation, annealing temperature, etc.) and the GaN nucleation layer growth conditions (growth temperature, layer thickness, strain) have been found to be the key steps for the GaN/SiC growth. GaN epilayers grown on SiC substrates with a specific surface preparation (Novasic) have shown by HR-XRD full-linewidths at half-maximum of 50 arcsec. TEM measurements have evidenced an improved crystalline quality of the GaN/SiC interfaces at low growth temperature of the first GaN monolayers, showing steps originating from the surface misorientation.

/poly-SiC Substrates

This paper reports some new experimental results concerning InP epitaxy by LP-MOCVD using chemical angle polishing and a sensitive etching technique to characterize the defect morphology of the complete InP layer, interface and substrate. The distribution of defects is associated with the SIMS determined As and Ga content.

LPMOCVD growth of GaN on silicon carbide

InGaN/GaN Multi Quantum Well (MQW) structures were grown by MOCVD. For this study, various thicknesses of InGaN/GaN periods were investigated (from 90 to 130 A) as well as the number of lattice periods (from 1 to 10). Transmission Electron Microscopy (TEM) revealed that the well/barrier interfaces are sharp. The (0002) High Resolution X-ray Diffraction (HRXRD) spectra show multiple satellite peaks due to interferences on well/barrier interfaces. We observed a very accurate correlation between the thickness deduced from X-ray spectra and TEM. All these observations prove that the InGaN/GaN interfaces are of high quality and well defined. The photoluminescence (300 K PL) peak intensity was shown to increase when the number of wells increases. Light Emitting Device (LED) structures have been grown following the above-mentioned model of MQW structures. Electroluminescence (EL) was performed on the devices and the results are in accordance with the material structures involved.

Defect Characterization In InP Epitaxial Layers Grown By LP-MOCVD

This paper shows results obtained on AlGaN/GaN FJEMTs processed on epitaxy grown on composite substrates. The results are very promising for the fabrication of low cost high power microwave transistors for wireless communication systems. The composite substrates constitute a valuable alternative to the silicon since better thermal properties are expected.

Characterization of InGaN/GaN Multiple Quantum Well Structures. Application to LEDs

In this article, we investigate the effect of a rapid thermal annealing (RTA) on electrical properties InAIN/GaN MOSHEMT with Al2O3 insulating film. On samples, we measured input, output and pulse characteristics. Consequently, a threshold voltage and an extrinsic transconductance was determined. A more dramatic influence of RTA was observed after annealing at 700degC. In this case, a reduction of the leakage current was about 7 orders of magnitude compared with HEMT structures without the insulating film.

Characterisation of AlGaN/GaN HEMT epitaxy and devices on composite substrates

GaN based Resonant Cavity Light Emitting Diodes (RCLEDs) have been developed in the frame of the AGETHA project (IST). The device structures, which include GaInN/GaN Multiquantum Wells (MQWs) and GaAlN/GaN Bragg Mirrors, have been grown by MOCVD, and their physical properties were optimised using High Resolution X-Ray Diffraction (HR-XRD) and room temperature photoluminescence mapping measurements. RCLED test heterostructures with a drastic improvement of the photoluminescence efficiency at λ = 480 nm have been obtained at low growth temperature for an appropriate QW/barrier thickness design. However, the growth of the p + cladding layer has been found to be critical for the photoluminescence efficiency of the GaInN/GaN MQWs.