M. A. Py

Current status of AlInN layers lattice-matched to GaN for photonics and electronics

We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/ AlInN multiple quantum wells ( QWs) suitable for near- infrared intersubband applications. A built- in electric field of 3.64MVcm(-1) solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain- free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack- free highly reflective AlInN/ GaN distributed Bragg reflectors ( R > 99%) and high quality factor microcavities ( Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature ( RT) lasing of a LM AlInN/ GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride- based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/ GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two- dimensional electron gas sheet carrier density ( n(s) similar to 2.6 x 10(13) cm(-2)) combined with a RT mobility mu(e) similar to 1170 cm(2) V-1 s(-1) and a low sheet resistance, R similar to 210 Omega square.

Can InAlN/GaN be an alternative to high power / high temperature AlGaN/GaN devices?

The performance of novel AlInN/GaN HEMTs for high power / high temperature applications is discussed. With 0.25 mum gate length the highest maximum output current density of more than 2 A/mm at room temperature and more than 3 A/mm at 77 K have been obtained even with sapphire substrates. Cut-off frequencies were fT = 50 GHz and fMAX = 60 GHz for 0.15 mum gate length without T-gate. Pulsed measurements reveal a less unstable surface than in the case of AlGaN/GaN structures. Although limited by buffer layer leakage, with field plates a maximum drain bias of 100 V has been reached with these devices. The high chemical stability of this unstrained heterostructure and its surface has been demonstrated with successful operation at 1000 degC in vacuum

Barrier-Layer Scaling of InAlN/GaN HEMTs

We discuss the characteristics of high-electron mobility transistors with barrier thicknesses between 33 and 3 nm, which are grown on sapphire substrates by metal-organic chemical vapor deposition. The maximum drain current (at VG = 2.0 V) decreased with decreasing barrier thickness due to the gate forward drive limitation and residual surface-depletion effect. Full pinchoff and low leakage are observed. Even with 3-nm ultrathin barrier, the heterostructure and contacts are thermally highly stable (up to 1000degC).

Secondary ion mass spectrometry study of oxygen accumulation at GaAs/AlGaAs interfaces grown by molecular beam epitaxy

The accumulation of oxygen at GaAs/AlGaAs interfacesgrown by molecular beam epitaxy has been established by secondary ion mass spectrometry profiling of GaAs/AlGaAs multilayerstructures. An enhanced oxygen peak was observed at the boundary between GaAs and Al x Ga1−x As layers with x=0.35 and x=1 when the binary layer is deposited on top of the ternary layer. The segregation of oxygen may be a contributing factor responsible for the lower luminescence reported in the first GaAs well of multilayerquantum wellstructures and for the difference between normal and inverted interface high electron mobility devices.

Evidence for screening effects on the 1/f current noise in GaAs/AlGaAs modulation doped field effect transistors

Using a delta-doped GaAs/AlGaAs heterostructure with a 10 nm spacer layer, we exploit the metastable nature of the DX centers at low temperatures to control electrostatically their net frozen charge density. The concentration of DX(-) centers at 77 K is only determined by the applied gate voltage V-c during the cool-down of the sample to 77 K, i.e., it is independent, on the time scale of noise experiments, of the subsequent change in the gate bias V-GS. The sheet carrier concentration n(s) of the two-dimensional electron gas is varied through the application of V-GS. Hall experiments performed at 77 K on gated Hall-bar structures show that V-c strongly affects both, the threshold voltage V-t and the exponent k, which enters into the observed power-law dependence of the Hall mobility mu on n(s). These dependencies were also studied directly on modulation doped field effect transistors from the analysis of their transfer characteristics I-DS-V-GS and g(me)-V-GS at low drain bias V-DS. The 1/f drain-current noise was investigated and, after subtraction of the noise arising from the series resistances, quantified by the extracted value of the channel-associated Hooge parameter alpha(ch). This parameter is found to depend on n, and exhibits the same power-law dependence as the reciprocal mobility 1/mu(n(s)). This striking correlation was established for various values of k and reveals screening effects on the 1/f noise. This correlation qualitatively supports the idea that the dominant mechanism of 1/f noise in modulation doped field effect transistors, at 77 K, is due to mobility fluctuations induced by screened fluctuations of Coulomb scattering, generated either by fluctuations of charge-state and/or motion of defects.

Leakage mechanisms in InAlN based heterostructures

We propose a model for leakage currents in Schottky contacts on InAlN/GaN heterostructures based on two distinct tunneling mechanisms. Our modeling relies on structural parameters, in particular, InAlN dielectric constant, interface polarization charges and Schottky barrier height, which are experimentally determined in the first part of our work. The first leakage mechanism is dominant in heterostructures with very thin (≤7 nm) InAlN barriers and consists in tunneling assisted by a deep level located 1.7 eV below the InAlN conduction band edge. We provide experimental evidence for this level through photocapacitance measurements. The second mechanism is on the other hand dominant in thicker InAlN layers and is linked to the appearance of highly doped regions where direct tunneling through the whole InAlN barrier is significantly enhanced. We also show that the two mechanisms may coexist for InAlN layers of intermediate thickness. Our findings confirm a progressive degradation of the InAlN material qualit...

Ultrathin InAlN/GaN heterostructures on sapphire for high on/off current ratio high electron mobility transistors

We report on InAlN/GaN high electron mobility transistors (HEMTs) grown by metal organic vapor phase epitaxy on sapphire with ultrathin buffers. Two dimensional electron gas (2DEG) exhibiting high mobility (1100 cm2/V s) and low sheet resistivity (356 Ω/□) is achieved at room temperature for a buffer thickness as low as ∼0.1 μm. It is shown that despite a huge dislocation density imposed by this thin buffer, surface roughness is the main factor which affects the transport properties. In addition, sapphire surface nitridation is found to drastically affect the properties of the InAlN/GaN 2DEG. Eventually, HEMTs are processed from these heterostructures. Maximum current densities of 0.35 A/mm and current on-off ratios higher than 109 are measured, which make them suitable for high performance GaN based sensing in harsh environments.

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.

Properties of alloyed AuGeNi-contacts on GaAs/Ga/AlAs-heterostructures

The technique of contacting GaAs/AlGaAs heterostructures using an evaporated AuGeNi alloy has been refined with emphasis on optimum contact properties in the quantum Hall regime. The resistance of the electrical contacts to two-dimensional electron gas has been measured in various heterostructures with two different methods. The results of the first method using a transmission line and the second one using the quantum Hall effect as well as the good time and thermal cycling stabilities obtained demonstrate the high quality of the contacts. The contact resistance in the quantum Hall regime is found to be lower than 1 Omega . >

InAlN/GaN heterostructures for microwave power and beyond

InAlN/GaN is indeed an alternative to the common AlGaN/GaN heterostructure in electronics and sensing. It enables operation at extremely high temperature once problems with contact metallization and passivation have been solved. It is the only heterostructure known presently, which allows overgrowth of high quality diamond films to combine two of the most stable semiconductors. Thus, applications reach from high power microwaves systems and high temperature electronics to sensing in harsh environment.