Cagliyan Kurdak

Degradation in InAlN/GaN-based heterostructure field effect transistors: Role of hot phonons

We report on high electric field stress measurements at room temperature on InAlN/AlN/GaN heterostructure field effect transistor structures. The degradation rate as a function of the average electron density in the GaN channel (as determined by gated Hall bar measurements for the particular gate biases used), has a minimum for electron densities around 1×1013 cm−2, and tends to follow the hot phonon lifetime dependence on electron density. The observations are consistent with the buildup of hot longitudinal optical phonons and their ultrafast decay at about the same electron density in the GaN channel. In part because they have negligible group velocity, the build up of these hot phonons causes local heating, unless they decay rapidly to longitudinal acoustic phonons, and this is likely to cause defect generation which is expected to be aggravated by existing defects. These findings call for modified approaches in modeling device degradation.

Weak antilocalization and zero-field electron spin splitting in AlxGa1−xN∕AlN∕GaN heterostructures with a polarization-induced two-dimensional electron gas

Spin-orbit coupling is studied using the quantum interference corrections to conductance in AlxGa1�xN / AlN/ GaN two-dimensional electron systems where the carrier density is controlled by the persistent photoconductivity effect. All the samples studied exhibit a weak antilocalization feature with a spin-orbit field of around 1.8 mT. The zero-field electron spin splitting energies extracted from the weak antilocalization measurements are found to scale linearly with the Fermi wave vector ESS =2 kf with an effective linear spin-orbit coupling parameter = 5.5 10 �13 eV m. The spin-orbit times extracted from our measurements varied from 0.74 to 8.24 ps within the carrier density range of this experiment.

1/f noise in gold nanoparticle chemosensors

ing of electrons between neighboring nanoparticles. 7 The reversible partitioning of the chemical species affects the tunnel coupling, and thus the resistance of the Au nanoparticle film typically changes in the presence of chemical species. Because the tunnel coupling is exponentially dependent on the properties of the medium in between the nanoparticles, they are highly sensitive sensors. Even a small amount of swelling associated with the absorption of chemical species into the thiolate coating can lead to a significant increase in film resistance. Such sensors are currently being developed as the detector elements of a microfabricated gas chromatography system which can analyze complex mixtures of gases for a wide range of applications, including monitoring of industrial emissions, detection of explosives, and medical diagnostics. 8,9

Influence of N interstitials on the electronic properties of GaAsN alloys

We have used rapid thermal annealing to investigate the influence of N interstitials on the electronic properties of GaAsN alloys. Nuclear reaction analysis reveals an annealing-induced decrease in the interstitial N concentration, while the total N composition remains constant. Corresponding signatures for the reduced interstitial N concentration are apparent in Raman spectra. Following annealing, both the room-T carrier concentration, n, and the mobility increase. At higher measurement-Ts, a thermally activated increase in n suggests the presence of a trap near GaAsN conduction band edge with activation energy 85±15 meV. The annealing-induced increase in n suggests the association of the trap with interstitial N.

Electric-field-induced heating and energy relaxation in GaN

Electric-field-induced heating is studied using noise measurements in n-type GaN grown on sapphire substrates. The measured electron temperature is found to be an order of magnitude higher than what is expected based on calculations of electron–phonon coupling via acoustic deformation potential scattering processes in GaN. The discrepancy may be explained by a large thermal boundary resistance between the GaN film and the sapphire substrate.

Influence of N on the electronic properties of GaAsN alloy films and heterostructures

We have investigated the effects of N on the electronic properties of Si-doped GaAs1−xNx alloy films and AlGaAs∕GaAsN modulation-doped heterostructures. For bulk-like alloy films, the electron mobility is independent of free carrier concentration and arsenic species, and decreases with increasing N composition. Thus, N-related defects are the main source of scattering in the dilute nitride alloys. For AlGaAs∕GaAsN heterostructures, gated and illuminated magnetoresistance measurements reveal a two-dimensional electron gas mobility which increases with carrier concentration to a constant value. Thus, in contrast to the long-range ionized scattering sources which are dominant in N-free heterostructures, N-induced neutral scattering sources are the dominant source of scattering in AlGaAs∕GaAsN heterostructures. Finally, a decrease in free carrier concentration with increasing N composition is apparent for bulk-like films, while the free carrier concentration is independent of N composition in modulation-doped...

CMOS Monolithic Nanoparticle-Coated Chemiresistor Array for Micro-Scale Gas Chromatography

Miniaturized detector arrays are critical to reducing size and maintaining measurement quality of integrated micro-gas chromatographs (μGC) used for the analysis of complex vapor mixtures. This paper presents an array of chemiresistors (CRs) with monolayer-protected gold nanoparticle films formed on the surface of a complementary-metal-oxide semiconductor (CMOS) readout chip, featuring high-resolution resistance measurement with adaptive cancellation of baseline resistance. The 8-channel readout circuit occupies 2.2 × 2.2 mm2 in 0.5 μm CMOS and consumes 66 μW per channel from a 3.3-V power supply. It achieves a worst-case resolution of 125 ppm over a broad baseline resistance range of 60 kΩ to 10 MΩ, equivalent to 122 dB dynamic range. Implementation of the CMOS monolithic detector array is discussed, and preliminary measurement results using chamber exposures to several vapors are presented. Eventual integration into a μGC is discussed.

Illumination and annealing characteristics of two-dimensional electron gas systems in metal-organic vapor-phase epitaxy grown AlxGa1−xN∕AlN∕GaN heterostructures

We studied the persistent photoconductivity (PPC) effect in AlxGa1−xN∕AlN∕GaN heterostructures with two different Al compositions (x=0.15 and x=0.25). The two-dimensional electron gas formed at the AlN∕GaN heterointerface was characterized by Shubnikov-de Haas and Hall measurements. Using optical illumination, we were able to increase the carrier density of the Al0.15Ga0.85N∕AlN∕GaN sample from 1.6×1012to5.9×1012cm−2, while the electron mobility was enhanced from 9540to21400cm2∕Vs at T=1.6K. The persistent photocurrent in both samples exhibited a strong dependence on illumination wavelength, being highest close to the band gap and decreasing at longer wavelengths. The PPC effect became fairly weak for illumination wavelengths longer than ∼530nm and showed a more complex response with an initial negative photoconductivity in the infrared region of the spectrum (λ>700nm). The maximum PPC efficiency for 390nm illumination was 0.011% and 0.005% for Al0.25Ga0.75N∕AlN∕GaN and Al0.15Ga0.85N∕AlN∕GaN samples, resp...

Influence of Si–N complexes on the electronic properties of GaAsN alloys

We have investigated the influence of Si–N complexes on the electronic properties of GaAsN alloys. The presence of Si–N complexes is suggested by a decrease in carrier concentration, n, with increasing N-composition, observed in GaAsN:Si films but not in modulation-doped heterostructures. In addition, for GaAsN:Te (GaAsN:Si), n increases substantially (minimally) with annealing-T, suggesting a competition between annealing-induced Si–N complex formation and a reduced concentration of N-related traps. Since Si–N complex formation is enhanced for GaAsN:Si growth with the (2×4) reconstruction, which has limited group V sites for As–N exchange, the (Si–N)As interstitial pair is identified as the dominant Si–N complex.

Magnetotransport properties of AlxGa1-xN/AlN/ GaN heterostructures grown on epitaxial lateral overgrown GaN templates

We studied the low-temperature magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures with a two-dimensional electron gas (2DEG). Structures with different Al compositions were grown by metal-organic vapor-phase epitaxy on three types of templates: conventional undoped GaN, in situ epitaxial lateral overgrown GaN using a SiNx nanomask layer, and ex situ epitaxial lateral overgrown GaN (ELO-GaN) using a stripe-patterned SiO2 mask. All of the samples display Shubnikov–de Haas (SdH) oscillations that confirm the existence of 2DEGs. Field-dependent magnetoresistance and Hall measurements further indicate that the overgrown heterostructures have a parallel conducting layer in addition to the 2DEG. To characterize the parallel channel, we repeated the measurements after the 2DEG was etched away. 2DEG carrier density values were then extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total. The quantitative mob...