P. Lorenzini

High-electron-mobility AlGaN/GaN heterostructures grown on Si(111) by molecular-beam epitaxy

We report on the growth of high-electron-mobility AlGaN/GaN heterostructures on silicon (111) substrates by molecular-beam epitaxy using ammonia as the nitrogen source. Crack-free GaN layers up to 3 μm are obtained. Their optical properties are similar to those commonly obtained for films grown on sapphire, but photoluminescence spectra indicate that GaN on Si(111) is in a tensile strain state which increases with the epitaxial layer thickness. Such uncracked GaN buffer layers grown on Si(111) have been used to achieve undoped AlGaN/GaN heterostructures having electron mobilities exceeding 1600 cm2/V s at room temperature and 7500 cm2/V s at 20 K.

Luminescence and reflectivity studies of undoped, n- and p-doped GaN on (0001) sapphire

GaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states

ULTRAVIOLET GAN LIGHT-EMITTING DIODES GROWN BY MOLECULAR BEAM EPITAXY USING NH3

Si- and Mg-doped GaN layers were grown on c-plane sapphire substrates by molecular beam epitaxy with NH3 as the nitrogen precursor. Their optical and electrical properties were investigated by photoluminescence experiments and Hall measurements, respectively. P-type conductivity, with a net acceptor concentration of 3×1017 cm−3 and a mobility of 8 cm2/V s, was obtained. Mesa-etched light-emitting diodes were processed from p–n junctions. The turn-on voltage is 3 V and the forward current reaches 20 mA at 3.5 V. The room-temperature electroluminescence exhibits a strong emission at 390 nm.

Acoustic phonon scattering of two-dimensional electrons in GaN/AlGaN heterostructures

We have measured the temperature dependence of the mobility of the two-dimensional electron gas in AlGaN/GaN heterostructures grown on bulk GaN substrates. The linear dependence of the inverse mobility on temperature at temperatures below 50 K indicates the importance of acoustic phonon scattering in these high mobility heterostructures. Using the temperature dependence of the mobility at a range of carrier densities, we determined the GaN conduction band deformation potential to be a(c)=9.1+/-0.7 eV. This result provides a crucial parameter for accurate calculations of intrinsic mobility limits in AlGaN/GaN heterostructures

MBE growth of AlGaN/GaN HEMTS on resistive Si(111) substrate with RF small signal and power performances

In this paper, we report on the properties of GaN films and AlGaN/GaN HEMT structures grown by molecular beam epitaxy on resistive Si(1 1 1) substrates. The properties of the GaN buffer layer and the AlGaN/GaN HEMTs are presented. Finally, both static and high-frequency performances of sub-micron gate length devices are analyzed demonstrating their RF power capability

Influence of MBE growth conditions on the quality of InAlAs/InGaAs metamorphic HEMTs on GaAs

Abstract In this work, we investigate the influence of the growth temperature and the arsenic flux on the quality of InAlAs/InGaAs metamorphic HEMTs on GaAs with inverse step based InAlAs buffer layers. We show besides the temperature, how the composition modulations make the arsenic flux a critical parameter for the HEMT quality and lead to the generation of threading dislocations in the inverse step layers when an excessive arsenic flux is involved.

Quantum and transport lifetimes of two-dimensional electrons gas in AlGaN/GaN heterostructures

The transport and quantum lifetimes were respectively deduced from low-temperature mobility and Shubnikov–de Haas measurements as a function of carrier density in metal organic vapor phase epitaxy-grown AlGaN∕GaN∕sapphire heterostructures. We show experimentally that the lifetime ratio varies as a bell curve, qualitatively confirming a recent theoretical prediction. However the experimental ratio varied much less than was theoretically predicted: From 9 to 19 for carrier densities in 1–9×1012cm−2 range. Moreover, we show the variation of quantum time with carrier density presents some discrepancy with the theoretical study. We also show that transport to quantum lifetime ratio cannot be used alone as a clear figure of merit from AlGaN∕GaN heterojunctions.

Low electron mobility of field-effect transistor determined by modulated magnetoresistance

Room temperature magnetotransport experiments were carried out on field-effect transistors in magnetic fields up to 10 T. It is shown that measurements of the transistor magnetoresistance and its first derivative with respect to the gate voltage allow the derivation of the electron mobility in the gated part of the transistor channel, while the access/contact resistances and the transistor gate length need not be known. We demonstrate the potential of this method using GaN and Si field-effect transistors and discuss its importance for mobility measurements in transistors with nanometer gate length.

Low temperature electron mobility and concentration under the gate of AlGaN/GaN field effect transistors

High electron mobility field effect transistors were fabricated on AlGaN∕GaN heterostructures and their magnetoresistance was measured at 4.2K up to 10T with simultaneous modulation of the gate potential. Low and high magnetic field data were used to determine the electron mobility (μ) and concentration (n), respectively, in the gated part of the transistor channel. With these measurements we present a method to determine μ and n under the gate of a transistor, which does not require knowledge of the transistor gate length, access resistance, threshold voltage, or capacitance. We discuss applications of this method for nanometer and ballistic transistors.

DX Centers in AlGaAs:Si under Pressure: Electron Statistics for Negative-U and Multilevel Model

It has been established that the DX center presents several configurations in the GaAlAs alloy, depending on the local environment of the impurity atom in the relaxed ground state. A statistics including these 4 configurations is developed in the frame of the negative-U model and is used to explain transport experiments under low magnetic field and high hydrostatic pressure. We show that a combination of emission transients experiment and Hall measurements at thermal equilibrium under pressure gives access to a quantitative determination of the thermal energies for the different configurations of the DX center.