Diagnostics of the hot-hole distribution function in quantum wells in a strong electric field

Modulation of the fundamental absorption edge by a high lateral electric field in a p-type In_0.21Ga_0.79As/GaAs heterostructure with quantum wells was studied at 4.2 K and electric fields as high as 1.9 kV/cm. The field-induced change in the symmetric part of the hole distribution function was measured.     

The energy structure of quantum dots induced in quantum wells by a nonuniform electric field

The study is focused on a theoretical treatment of electrons in quantum dots produced by a nonuniform electric field in a heterostructure with a single quantum well. A nonuniform electric field can be formed with the use of a mosaic electrode with a regular system of nanometer-sized apertures. A structure involving a one-aperture electrode deposited onto the surface is considered. The potential in the entire region between the electrode plates is calculated numerically. The analytical expression for the depth of the potential well is derived, and the potential profile in the vicinity of the bottom of the well is determined. The energy level structure in the vicinity of the bottom of the potential well is calculated. The geometric characteristics of the mosaic electrode most favorable for conservation of electron spin in the quantum dots of this type are determined. It is shown that in the quantum dots of this type the spin lifetime can be expected to lie within the microsecond region at a liquid-helium temperature.     

Electron transport in semiconductors under a strong high frequency electric field

We propose an analytical approach to study the electron transport in semiconductors when a strong high frequency (HF) electric field is applied together with a weak direct current (DC) electric field. We derive a set of dynamic equations, from which the amplitude and phase of each harmonic component of the electron drift velocity and the electron temperature can be obtained. Our calculation shows that the DC conductivity in an n-GaAs sample decreases dramatically with increase of the first and second harmonic applied electric fields and definitely becomes negative.     

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic I _D (V _D ) and the anomalous dependence of the drain current I _D on the gate voltage V _G are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage V _D that exceeds the V_D1 value, at which the I _D (V _D ) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in V _D , i.e., for V _D >V_D1, the I _D (V _D ) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for V _D =V_D2>V_D1, the current I _D is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance g _m =dI _D /dV _G as a function of V _G .     

Modulation of optical absorption of GaAs/AlGaAs quantum wells in a transverse electric field

Results of experimental investigations of the transformation of intraband light absorption spectra by the quantum-well electrons in a transverse electric field are presented. In addition to the familiar Stark effect, absorption oscillations in the photoionization band are detected. These oscillations are caused by electron transitions from the ground state in a quantum well to quasidiscrete levels arising in the continuum of states above the well due to the linear potential of the external electric field. Fiz. Tekh. Poluprovodn. 32, 849–851 (July 1998)     

On the permanent dipole-induced two-level Raman scattering insemiconductor quantum wells in electric field

The permanent dipole-induced two-level Raman scattering in semiconductor quantum wells in a static electric field is analyzed. Calculations performed for GaAs/Al_xGa_1-xAs multiple quantum wells (MQWs) indicate that reasonable, but not high, Stokes wave amplification could be obtained, and eventually used for IR generation. The gain is limited primarily by reststrahlen absorption and a rather large transition linewidth     

Electron escape time from single quantum wells

A theoretical model for electrons escaping a quantum well under the influence of an applied electric field is developed. Both the thermionic emission and tunneling components of the currents are calculated, taking into account the proper partitioning between the two currents. The group velocity for a nonuniform electron distribution within the quantum well, which is a function of position and energy, and the continuous energy dependence of the quantum well density of states is considered. A comparison between this model and previously reported experimental results are made which demonstrates excellent agreement     

Electron transport in coupled quantum wells with double-Sided doping

The conductivity and Hall mobility have been measured in heterostructures with coupled quantum wells (QW) as functions of temperature and the QW width. If a tunnel-transparent barrier is inserted in the middle of a QW, the mobility increases in narrow wells and decreases in wide wells. The experimental data have been compared with the calculated dependences. It has been shown that the number of filled quantum-well subbands depends on the well width and the presence of a barrier. The magnetoresistance and Hall resistance were measured at a temperature of 4.2 K in the range of magnetic fields of 1–40 T. The filling of subbands was determined from a Fourier analysis of the Shubnikov-de Haas oscillations, and good agreement with the calculated data was obtained.     

Characterizing electric fields in (111)B InGaAs quantum wells using electric field modulated photoluminescence and reflectance techniques

We have performed a series of electroreflectance, photoluminescence, and electric-field-modulated photoluminescence experiments to characterize the strain-induced electric fields in (111)B InGaAs/AlGaAs quantum well p-i-n diode structures. A 180° phase change in the lineshapes of electroreflectance spectra of these samples determines when the quantum well is biased to flatband. Using this bias and a depletion model for the diode, the polarization field in the quantum well can be determined. Contrary to expectations, this polarization field increases significantly with increasing temperature. In addition, at fixed temperature, the quantum well transition energies red-shift with increasing excitation intensity when excited by photons of energy higher than the lowest quantum well transition but lower than the AlGaAs diode's bandgap. When excited with photons of energy greater than the AlGaAs bandgap, the transition energy first red shifts then blue shifts with increasing excitation intensity.     

Resonant second harmonic generation by a semiconductor quantum wellin electric field

Both single- and double-resonance operating regimes are analyzed, and the latter is found to enable very high conversion efficiencies of pump to harmonic if accompanied by quasiphase matching by periodic alternation of the field direction. The self-consistent procedure of level energies and wave function calculation is also described     

Electron mobility and electron scattering by polar optical phonons in heterostructure quantum wells

Basic features of confined electron scattering in quantum wells (QWs) by confined polar optical (PO) phonons are analyzed. The dependence of electron mobility in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs on the well width is calculated. It is demonstrated that increases and decreases in electron mobility (relative to the bulk value) as a function of the QW width occur due to resonance intersubband scattering. The dependence of electron mobility limited by PO phonon scattering on the electron density n _s in the QW is calculated. It is shown that anomalous behavior of electrical conductivity, which in certain cases decreases with increasing electron density, can take place in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs for n _s >10¹⁶ m^?2.     

Absorption of a strong electromagnetic wave by electrons in a superlattice in a quantizing electric field

Intraminiband absorption of light by electrons in a quantum superlattice in a quantizing electric field is investigated theoretically taking into account the electron-phonon interaction. It is assumed that the interaction with optical dispersion-free phonons makes the main contribution to electron scattering. It is shown that the point ω=ω ₀ (ω is the light frequency, and ω ₀ is the optical phonon frequency) conditionally divides the ω dependence of the absorption into two parts: ω<ω ₀, the region of exponentially weak absorption and ω>ω ₀, the region of “strong” absorption. An electric field shifts the region of strong absorption in the red direction of the spectrum. Fiz. Tekh. Poluprovodn. 33, 1355–1358 (November 1999)     

The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

Effect of external electric field on the probability of optical transitions in InGaAs/GaAs quantum wells

The effect of external electric field on interband optical transitions in single In_xGa_{1 ? x} As/GaAs quantum wells is studied by electroreflectance spectroscopy. A procedure is suggested for separating the contribution of particular exciton transitions to the complicated modulation spectrum. Nontrivial field dependences of the probability of optical transitions forbidden by the symmetry are observed experimentally. The data are compared with the corresponding theoretical dependences. The strength of the internal electric field in the region of the quantum well is determined from Frantz-Keldysh’s oscillations. Under certain electric fields, the probability of transitions forbidden with no field is higher than the probability of transitions allowed by the symmetry.     

Energy levels of a quantum ring in a lateral electric field

The electronic states of a semiconductor quantum ring (QR) under an applied lateral electric field are theoretically investigated and compared with those of a quantum disk of the same size. The eigenstates and eigenvalues of the Hamiltonian are obtained from a direct matrix diagonalization scheme. Numerical calculations are performed for a hard-wall confinement potential and the electronic states are obtained as a function of the electric field and the ratio r₂/r₁, where r₂ (r₁) is the outer (inner) radius of the ring. The effects of decreasing symmetry and mixing on the energy levels and wave functions due to the applied electric field are also studied. The direct optical absorption are reported as a function of the electric field.     

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.     

Modulation of intersubband absorption in tunnel-coupled quantum wells in electric fields

Modulation of absorption of middle-infrared radiation in double tunneling-coupled quantum wells in longitudinal electric fields is studied. A specific feature of the quantum wells is the small separation in energy between the two lower levels. As a consequence, the levels may exhibit “anticrossing” even in low transverse electric fields. An interpretation of the change in intersubband absorption is suggested. The interpretation is based on the assumption that a transverse electric-field component may appear in the structure. The change in the absorption coefficient is calculated taking into account the redistribution of electrons between size-quantization subbands and the changes in the temperature of electrons in the subbands in the longitudinal electric field, as well as the changes in the optical matrix elements, the energies of transitions, and the concentrations of electrons in the subbands in the transverse electric field. The possibility of using the structure for the efficient modulation of middle-infrared light with the photon energy 136 meV is shown.     

Drift velocity of electrons in quantum wells in high electric fields

It is experimentally found that the maximum drift velocity of electrons in quantum wells of differently arranged AlGaAs/GaAs heterostructures and pseudoamorphous Al_0.36Ga_0.64As/In_0.15Ga_0.85 As heterostructures is higher than the maximum drift velocity of electrons in bulk materials. It is established that no negative differential conductivity is exhibited by the field dependence of the drift velocity of two-dimensional electrons in GaAs and In_0.15Ga_0.85As. The drift velocity in the GaAs quantum well is saturated in fields several times higher than the field corresponding to the Γ-L intervalley transitions of electrons in bulk GaAs.