Room-temperature microwave oscillation in AlAs/GaAs superlattices

Room-temperature microwave (MW) oscillations are observed in GaAs/AlAs (10 nm/2 nm) doped weakly coupled superlattices (SLs) in the first plateau of the I-V curve. Oscillations induced by sequential resonant tunneling are detected in a temperature range from 15 to 300 K by applying DC bias on the Si, diodes. The temperature dependence of current at small fixed bias voltage is also measured. Through analysis, it is found that the dominant transport mechanisms are sequential resonant tunneling and phonon-assisted tunneling when the temperature is below 300 K. The low bias voltage at which oscillations are realized is helpful to restrain thermionic emission through the X valley of AlAs barriers in the room-temperature transport. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Suppression of self-sustained field domain oscillation in GaAs/AlAs superlattice by hydrostatic pressure at room temperature

The behavior of room temperature self-sustained current oscillations resulting from sequential resonance tunneling in a doped weakly-coupled GaAs/AlAs superlattice (SL) is investigated under hydrostatic pressure. From atmosphere pressure to 6.5 kbar, oscillations exist in the whole plateau of the I-V curve and oscillating characteristics are affected by the pressure. When hydrostatic pressure is higher than 6.5 kbar, the current oscillations are completely suppressed although a current plateau still can be seen in the I-V curve. The plateau disappears when the pressure is close to 13.5 kbar. As the main effect of hydrostatic pressure is to lower the X point valley with respect to Gamma point valley, the disappearance of oscillation and the plateau shrinkage before Gamma - X resonance takes place are attributed to the increases of thermoionic emission and nonresonant tunneling components determined by the lowest Gamma - X barrier height in GaAs/AlAs SL structure.

Sequential resonant tunnelling through Landau levels in GaAs/AlAs superlattices

Electron transport in heavily-doped GaAs/AlAs superlattices in parallel electric and magnetic fields is reported. The current-voltage (I-V) characteristic exhibited the feature of negative differential velocity (NDV) and high electric field domain effect at different biases. Under strong magnetic fields, sequential resonant tunnelling through Landau levels in the negative differential velocity regime is observed, which are manifested as oscillations in the conductance-voltage characteristics. (C) 1998 Elsevier Science B.V. All rights reserved.

Electronic structures of GaAs/AlAs lateral superlattices

The electronic energy subbands and minigaps in lateral superlattices (LSLs) have been calculated by the plane-wave expansion method. The effect of the lateral modulation on the critical well width at which an indirect-direct (X-Gamma) optical transition occurs in the LSLs is investigated. Our theoretical results are in agreement with the available experimental data. Totally at variance with the previous variation calculational results, the minigaps between the first two subbands in LSLs, as functions of the modulation period, exhibit a maximum value at a specific length and disappear on decreasing the modulation period further. The modulations of several types of lateral potential are also evaluated; the indication is that the out-of-phase modulation on either side of the wells is the strongest while the in-phase modulation is the weakest. Our calculations also show that the effect of the difference between the effective masses of the electrons in the different materials on the subband structures is significant.

NEW FORMATION MECHANISM OF ELECTRIC-FIELD DOMAIN DUE TO GAMMA-X SEQUENTIAL TUNNELING IN GAAS/ALAS SUPERLATTICES

We have studied the sequential tunneling of doped weakly coupled GaAs/ALAs superlattices (SLs), whose ground state of the X valley in AlAS layers is designed to be located between the ground state (E(GAMMA1)) and the first excited state (E(GAMMA2)) of the GAMMA valley in GaAs wells. The experimental results demonstrate that the high electric field domain in these SLs is attributed to the GAMMA-X sequential tunneling instead of the usual sequential resonant tunneling between subbands in adjacent wells. Within this kind of high field domain, electrons from the ground state in the GaAs well tunnel to the ground state of the X valley in the nearest AlAs layer, then through very rapid real-space transfer relax from the X valley in the AlAs layer to the ground state of the GAMMA valley of the next GaAs well.

GIANT CAPACITANCE OSCILLATIONS RELATED TO THE QUANTUM CAPACITANCE IN GAAS ALAS SUPERLATTICES

We have observed periodic current and capacitance oscillations with increasing bias on doped GaAs/AlAs superlattices at a temperature of 77 K. The maximum of the observed capacitance is larger than usual geometric capacitances in superlattices, being comparable to the quantum capacitance of the two-dimensional (2D) electron system proposed by Luryi. A model based on well-to-well sequential resonant tunneling due to the movement of the boundary between the electric field domains in superlattice was proposed to explain the origin of the giant capacitance oscillations. It was demonstrated that the capacitance at the peaks of capacitance-voltage (C-V) characteristics reflects the quantum capacitance of the space-charge region at the boundary between the domains (a novel 2D electron system).

COMPARISON BETWEEN THE INFLUENCES OF LAYER THICKNESSES AND ALAS MOLE FRACTION-X ON VERTICAL-CAVITY SURFACE-EMITTING LASERS

Based on the n(x, lambda), the calculation of the reflection spectrum for vertical cavity surface emitting lasers shows that the deviation of the central wavelength caused by the change of layer thickness is much more than that caused by the change of AlAs mole fractions. Therefore the control of the MBE growth rate is very important.

NEAR-RESONANT RAMAN-SCATTERING FROM GAAS/ALAS SUPERLATTICES

This article presents the results of near-resonant Raman scattering measurements on GaAs/AlAs superlattices at room temperature. A strong enhancement of GaAs LO phonon-even modes resulted owing to a dipole-allowed Frohlich interaction in superlattices. Similar to the previous results, the LO phonon-even modes in a polarized configuration are observed. In contrast to previous work, however, what we observed in depolarized configurations is the LO phonon-odd modes instead of even modes. It is confirmed that the selection rules for near-resonant Raman scattering from LO phonons in this kind of superlattices are the same as those for off-resonant scattering. From the second-order Raman scattering, it is confirmed that polarized second-order Raman scattering spectra consist of overtones and combinations of two even modes, and depolarized second-order Raman scattering spectra consist of combinations of an even mode and an odd mode. Our experimental results coincide with the predictions using the recently developed Huang-Zhu model. A brief discussion on interface modes and their combination with confined modes is also presented.

SOME ASPECTS ON INTERFACE MODES AND LO EIGENMODES IN GAAS/ALAS QUANTUM-WELLS

The interface modes and LO phonon modes in GaAs/AlAs quantum wells is investigated within the isotropic dispersionless dielectric continuum with nodes in displacement u at the interfaces as boundary condition. The interface modes are found to be purely interface polarization charge effect while LO eigenmodes induce only bulk polarization charges. Analytical expression is determined for LO eigenmodes and is found in good agreement with realistic model calculation, and its labeling index is interpreted as the helicity of electric field as it travels from one side to the other side of the slab.

NEAR-RESONANCE RAMAN-SCATTERING OF LONGITUDINAL OPTICAL PHONON MODES AND INTERFACE MODES IN GAAS/ALAS SUPERLATTICES

The near-resonance Raman scattering of GaAs/AlAs superlattices is investigated at room temperature. Owing to the resonance enhancement of Frohlich interaction, the scattering intensity of even LO confined modes with A1 symmetry becomes much stronger than that of odd modes with B2 symmetry. The even modes were observed in the polarized spectra, while the odd modes appear in the depolarized spectra as in the off-resonance case. The second-order Raman spectra show that the polarized spectra are composed of the overtone and combinations of even modes, while the depolarized spectra are composed of the combinations of one odd mode and one even mode. The results agree well with the selection rules predicted by the microscopic theory of Raman scattering in superlattices, developed recently by Huang and co-workers. In addition, the interface modes and the combinations of interface modes and confined modes are also observed in the two configurations.

RAMAN-SCATTERING BY LO PHONONS IN (GAAS)N1/(ALAS)N2 ULTRATHIN-LAYER SUPERLATTICES

Raman spectra of (GaAs)n1/(AlAs)n2 ultrathin-layer superlattices were measured at room temperature and under off-resonance conditions. The experimental results show that there are two effects in ultrathin-layer superlattices: the confinement effect of LO phonons and the alloy effect. It is found that the relative intensity of the disorder-activated TO mode can give a measure of the alloy effect. The Raman spectra of one-monolayer superlattices measured in various scattering configurations are very similar to those of the Al0.5Ga0.5As alloy, and thus the alloy effect is prominent. However, in the case of monolayer number n greater-than-or-equal-to 4, the confined effect is prominent, while the alloy effect is only shown as an interface effect.