M. Gonschorek

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

205-GHz (Al,In)N/GaN HEMTs

We report 55-nm gate AlInN/GaN high-electron-mobility transistors (HEMTs) featuring a short-circuit current gain cutoff frequency of fT = 205 GHz at room temperature, a new record for GaN-based HEMTs. The devices source a maximum current density of 2.3 A/mm at VGS = 0 V and show a measured transconductance of 575 mS/mm, which is the highest value reported to date for nonrecessed gate nitride HEMTs. Comparison to state-of-the-art thin-barrier AlGaN/GaN HEMTs suggests that AlInN/GaN devices benefit from an advantageous channel velocity under high-field transport conditions.

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).

Analysis of degradation mechanisms in lattice-matched InAlN/GaN high-electron-mobility transistors

We address degradation aspects of lattice-matched unpassivated InAlN/GaN high-electron-mobility transistors (HEMTs). Stress conditions include an off-state stress, a semi-on stress (with a partially opened channel), and a negative gate bias stress (with source and drain contacts grounded). Degradation is analyzed by measuring the drain current, a threshold voltage, a Schottky contact barrier height, a gate leakage and an ideality factor, an access, and an intrinsic channel resistance, respectively. For the drain-gate bias < 38 V parameters are only reversibly degraded due to charging of the pre-existing surface states. This is in a clear contrast to reported AlGaN/GaN HEMTs where an irreversible damage and a lattice relaxation have been found for similar conditions. For drain-gate biases over 38 V InAlN/GaN HEMTs show again only temporal changes for the negative gate bias stresses; however, irreversible damage was found for the off-state and for the semi-on stresses. Most severe changes, an increase in the intrinsic channel resistance by one order of magnitude and a decrease in the drain current by similar to 70%, are found after the off-state similar to 50 V drain-gate bias stresses. We conclude that in the off-state condition hot electrons may create defects or ionize deep states in the GaN buffer or at the InAlN/GaN interface. If an InAlN/GaN HEMT channel is opened during the stress, lack of the strain in the barrier layer is beneficial for enhancing the device stability.

Gate insulation and drain current saturation mechanism in InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors

The authors investigate 2μm gate-length InAlN∕GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS HEMTs) with 12nm thick Al2O3 gate insulation. Compared to the Schottky barrier (SB) HEMT with similar design, the MOS HEMT exhibits a gate leakage reduction by six to ten orders of magnitude. A maximal drain current density (IDS=0.9A∕mm) and an extrinsic transconductance (gme=115mS∕mm) of the MOS HEMT also show improvements despite the threshold voltage shift. An analytical modeling shows that a higher mobility of electrons in the channel of the MOS HEMT and consequently a higher number of electrons attaining the velocity saturation may explain the observed increase in gme after the gate insulation.

Ultrathin InAlN/AlN Barrier HEMT With High Performance in Normally Off Operation

We present GaN-based high electron mobility transistors (HEMTs) with a 2-nm-thin InAlN/AlN barrier capped with highly doped n++ GaN. Selective etching of the cap layer results in a well-controllable ultrathin barrier enhancement-mode device with a threshold voltage of +0.7 V. The n++ GaN layer provides a 290-Omega/\square sheet resistance in the HEMT access region and eliminates current dispersion measured by pulsed IV without requiring additional surface passivation. Devices with a gate length of 0.5-mum exhibit maximum drain current of 800 mA/mm, maximum transconductance of 400 mS/mm, and current cutoff frequency fT of 33.7 GHz. In addition, we demonstrate depletion-mode devices on the same wafer, opening up perspectives for reproducible high-performance InAlN-based digital integrated circuits.

102-GHz AlInN/GaN HEMTs on Silicon With 2.5-W/mm Output Power at 10 GHz

Grown on a (111) high-resistivity silicon substrate, 0.1-mum gate AlInN/GaN high-electron mobility transistors (HEMTs) achieve a maximum current density of 1.3 A/mm, an extrinsic transconductance of 330 mS/mm, and a peak current gain cutoff frequency as high as fT = 102 GHz, which is the highest value reported so far for nitride-based devices on silicon substrates, as well as for any AlInN/GaN-based HEMT regardless of substrate type. Continuous-wave power measurements in class-A operation at 10 GHz with VDS = 15 V revealed a 19-dB linear gain, a maximum output power density of 2.5 W/mm with an ~23% power-added efficiency (PAE), and a 9-dB large-signal gain. At VDS = 8 V, the output power is 1 W/mm, and the peak PAE reaches 50%. Results demonstrate the interest of AlInN/GaN on silicon HEMT technology for low-cost millimeter-wave and high-power applications.

Ultrahigh-Speed AlInN/GaN High Electron Mobility Transistors Grown on (111) High-Resistivity Silicon with FT = 143 GHz

We report on ultrahigh-speed 80 nm AlInN/GaN high-electron-mobility transistors (HEMTs) grown on (111) high-resistivity silicon substrates. The devices feature a peak measured transconductance gM = 415 mS/mm, a maximum current of 1.43 A/mm with a ratio ION/IOFF > 106, and current gain and maximum oscillation cutoff frequencies of fT = 143 GHz and fMAX = 176 GHz, which are the highest cutoff frequencies ever achieved for any GaN HEMTs on silicon substrates. The results demonstrate the outstanding potential of AlInN/GaN HEMTs grown on silicon for low-cost high-performance millimeter-wave electronics.

Proposal and Performance Analysis of Normally Off

Design considerations and performance of n++ GaN/InAlN/AlN/GaN normally off high-electron mobility transistors (HEMTs) are analyzed. Selective and damage-free dry etching of the gate recess through the GaN cap down to a 1-nm-thick InAlN barrier secures positive threshold voltage, while the thickness and the doping of the GaN cap influence the HEMT direct current and microwave performance. The cap doping density was suggested to be 2 × 1020 cm-3. To screen the channel from the surface traps, the needed cap thickness was estimated to be only 6 nm. Design is proved by an experiment showing a constant value of the HEMT dynamical access resistance, while a single-pulse experiment indicated almost collapse-free performance. On the other hand, it is found that the n++ GaN cap does not contribute to the HEMT drain current conduction, nor does it provide a path for the off-state breakdown. HEMTs with a gate length of 0.25 μm and a 4-μm source-to-drain distance show a drain-to-source current of 0.8 A/mm, a transconductance of 440 mS/mm, a threshold voltage of ~0.4 V, and a cutoff frequency of 50 GHz. A thin and highly doped GaN cap is also found to be suitable for the processing of normally on HEMTs by adopting the nonrecessed gate separated from the cap by insulation.