H. Cheng

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.

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.

Photoconductive sampling with an integrated source follower/amplifier

A hybrid, optoelectronic sampling circuit based on a photoconductive switch and a junction‐field‐effect‐transistor (JFET) source follower/amplifier has been demonstrated to have picosecond response, high‐sensitivity, absolute‐voltage capability, and a very high impedance. The distributed capacitance of the electrical measurement system is reduced to the gate input capacitance of the JFET, and the conventional photoconductive current measurement is transferred into an absolute voltage measurement. Gating measurements with an improvement of 150 times in output voltage over unamplified photoconductive gates have been made using only 10 μW of average optical power. The effective on‐state resistance of the photogate has also been increased by more than 150 times, indicating that a photoconductive probe with very low invasiveness may be produced.

Bias dependent two-channel conduction in InAlN/AlN/GaN structures

Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that although the additional unintended layer may have beneficial aspects, we discuss a strategy to prevent its occurrence.

Two-subband conduction in a gated high density InAlN/AlN/GaN heterostructure

Magnetotransport measurements on an In0.16Al0.84N/AlN/GaN gated Hall bar sample have been performed at 0.28 K. By the application of a gate voltage we were able to vary the total two-dimensional electron gas density from 1.83×1013 to 2.32×1013 cm−2. Two frequency Shubnikov–de Haas oscillations indicate occupation of two subbands by electrons. The density of electrons in the first and second sublevels are found to increase linearly with gate voltage with a slope of 2.01×1012 cm−2/V and 0.47×1012 cm−2/V, respectively. And the quantum lifetimes for the first and second subbands ranged from 0.55 to 0.95×10−13 s and from 1.2 to 2.1×10−13 s.

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.

Modulation bandwidth and noise limit of photoconductive gates

The modulation bandwidth and noise limit of a photoconductive sampling gate are studied by reducing the parasitic capacitance and leakage current of the sampling circuit using an integrated junction field-effect transistor (JFET) source follower. The modulation bandwidth of the photoconductive sampling gate is limited by the external parasitic capacitance, and its efficiency is found to saturate at a laser gating power of about 1 mW. It is determined that the noise of the photoconductive sampling gate is dominated by the photovoltaic current due to the gating laser amplitude fluctuation. A minimum noise level of 4 nV Hz−1/2 has been measured, and an enhancement in signal-to-noise ratio by a factor of >45 has been achieved after the integration of the source follower with the photoconductive sampling gate. The JFET source follower serves to increase the modulation bandwidth of the photoconductive sampling gate by about 15 times and buffer the charge of the measured signal using its extremely high gate input impedance. The performance of the photoconductive sampling gate in regard to invasiveness and gating efficiency has been optimized, while a picosecond temporal resolution has been maintained and the signal-to-noise performance has been enhanced using a gating laser power as low as 10 μW.

Spin-orbit coupling in AlGaN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas

Spin-orbit coupling is investigated by magnetoconductivity measurements in wurtzite AlxGa1-xN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas with different Al concentrations ranging from x = 0.1 to 0.35. By employing the persistent photoconductivity effect and by gating we are able to vary the carrier density of the samples in a controllable manner from 0.8 ×1012 cm-2 to 10.6 ×1012 cm-2. The samples are characterized using magnetoresistance measurements. To characterize the spin-orbit interaction we measured quantum corrections to conductance at low magnetic fields. All the samples we studied exhibit a weak antilocalization feature at liquid He temperatures. The zero-field electron spin-splitting energies extracted from the weak antilocalization measurements are found to scale with the Fermi wavevector kF as 2( ακF + γκF 3) with effective linear and cubic spin-orbit parameters of -α= 5.01×10−13 eV • m and γ= 1.6 ×10−31 eV •m3, respectively. The linear spin-orbit coupling arises from both the bulk inversion asymmetry of the crystal and the structural inversion asymmetry of the heterostructure whereas the cubic spinorbit coupling parameter is purely due to the bulk inversion asymmetry of the wurtzite crystal. We also extracted phase coherence times from the amplitude of the weak antilocalization feature. The measured phase coherence times ranged from 3-40 ps and were in agreement with the theory of decoherence based on electron-electron interactions.

Magneto-transport properties of MOVPE-grown AlxGa1-xN/AlN/GaN heterostructures with high-mobility two-dimensional electron gas

We study AlxGa1-xN/AlN/GaN heterostructures with a two-dimensional-electron-gas (2DEG) grown on different GaN templates using low-temperature magneto-transport measurements. Heterostructures with different Al compositions are grown by metal-organic vapor phase epitaxy (MOVPE) on three different templates; conventional undoped GaN (u- GaN), epitaxial lateral overgrown GaN (ELO-GaN), and in situ ELO-GaN using a SixNy nanomask layer (SiN-GaN). Field-dependent magneto-resistance and Hall measurements indicated that in addition to 2DEG, the overgrown heterostructures had a parallel conducting layer. The contact resistance for the parallel channels was large so that it introduced errors in the quantitative mobility spectrum analysis (QMSA) of the data. Notwithstanding complexities introduced by parallel conducting channels in mobility analysis in SiN-GaN and ELO-GaN samples, we were able to observe Shubnikov-de Haas (SdH) oscillations in all samples, which confirmed the existence of 2DEGs. To characterize the parallel channel, we repeated the transport measurements after the removal of the 2DEG by etching the heterostructure. The 2DEG carrier density values were extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total conductivity. The resulting 2DEG mobility was significantly higher (about a factor of 2) in the ELO-GaN and SiN-GaN samples as compared to the standard control sample. The mobility enhancement is attributed to the threading dislocation reduction by both ELO techniques.