Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors
(HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a
grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation
was observed in transmission for both systems in the frequency range 10 - 100 cm-1. The fundamental and harmonic
resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for
MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the
InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.
Pronounced resonant absorption and frequency dispersion associated with an excitation of collective 2D plasmons have
been observed in terahertz (0.5-4THz) transmission spectra of grating-gate 2D electron gas AlGaN/GaN HEMT (high
electron mobility transistor) structures at cryogenic temperatures. The resonance frequencies correspond to plasmons
with wavevectors equal to the reciprocal-lattice vectors of the metal grating, which serves both as a gate electrode for the
HEMT and a coupler between plasmons and incident terahertz radiation. The resonances are tunable by changing the
applied gate voltage, which controls 2D electron gas concentration in the channel. The effect can be used for resonant
detection of terahertz radiation and for "on-chip" terahertz spectroscopy.
The current and frequency dependencies of the low frequency noise have been investigated in 4H-SiC p+-n junctions in the frequency range 100-104 Hz and at current densities from 10-4 to 101 A/cm2. Good quality of the p+-n diode under investigation has been ascertained by high value of the recombination time in the space charge region, &tgr;R ≈ 70 ns, extracted from current voltage characteristic. At small current densities j ⩽ 10-3 A/cm2, the spectral noise density SI ∝1/f3/2. At 10-3 A/cm2 < j < 10-2 A/cm2, the generation-recombination (GR) noise predominates. The amplitude of this GR noise non-monotonically depends on current. At j ⩾ 10-2 A/cm2, the 1/f (flicker noise) is dominant. A new model of GR noise of the recombination current in forward biased
p-n junction has been proposed. The model assumes that a trap level located relatively close to the conduction band is responsible for the observed GR noise. The main contribution to the GR noise comes from the fluctuations of the charge state of the trap. The model describes well both current and frequency dependencies of the observed GR noise.
Far infrared spectra of 14 commonly used explosive samples have been measured by using Fourier Transform Infrared Spectroscopy (FTIR) and THz Time-Domain Spectroscopy (THz TDS). New absorption resonances between 20 cm-1 and 650 cm-1 are reported. Below 20 cm-1, no clear absorption resonances are observed in all the explosives. There is a good consistency of far-IR spectrum measured by Far-FTIR and by THz TDS in explosives 3,5-DNA and 2,4-DNT. Observed far-IR spectrum of TNT is compared with a previously reported theoretical calculation.
Cyclotron resonance of 2D holes in high-mobility undoped multi-quantum-well Ge/GeSi heterostructure has been studied in both 'classical' and quantizing magnetic fields. Effects of hole heating on 2D hole cyclotron resonance has been investigated. The calculations of 2D hole Landau levels in rectangular quantum well have been performed allowing to interpret the evolution of CR spectra in going from 'classical' to 'quantum' range.
Effect of amplification of far-IR radiation on light hole cyclotron resonance in Ge and InSb under the optical pumping by CO2 laser radiation has been calculated using the quantum mechanical model of valence band states in strong magnetic field. The model is based on 6 by 6 Luttinger Hamiltonian for valence band including split-off hole subband. We have found strong resonant dependence of pumping efficiency on magnetic field that is explained by quantum resonance of intersubband absorption of CO2 laser radiation. It was shown that at the optimal magnetic fields the cross-section of the gain can reach 2 X 10-14 cm2 for pumping power density 2 divided by 4 MW/cm2.