Hot electron light-emitting and lasing semiconductor heterostructures--type 1

A new light-emitting device has been proposed based on the incorporation of a GaAs quantum well on the n-side, and in the depletion region, of a Ga_1-xAl_xAs p-n junction, where the bias is applied parallel to the layers. Light is emitted when electrons and holes on the n- and p-side of the structure, respectively, heated by the applied longitudinal electric field, transfer to the quantum well, by resonant tunnelling and thermionic emission (electrons) and diffusion (holes), where they recombine radiatively. The intensity of the light emitted is independent of the polarity of the applied bias. A demonstration of the device is presented and it is shown that the quantum well needs to be in the depletion region for light emission to occur. The device is modelled theoretically by solving the Schrödinger's and Poisson's equations self-consistently by including the carrier dynamics for hot electrons and holes.     

Photo-luminescence studies of hot electrons and real space transfer effect in a double quantum well superlattice

A GaAsAlGaAs superlattice of which unit cell has two GaAs quantum wells shows a negative differential resistance under electric fields applied parallel to the superlattice plane at E ≳ 1 kV/cm (300 K) or at E ≳ 0.5 kV/cm (77 K). The photo-luminescence at 77 K shows a dominant peak I_A followed by a small peak I_B. Under the electric field the high energy tail of the dominant peak is enhanced and the intensity of the second peak I_B is increased, which confirms the occurence of the real space transfer of hot electrons from the wide quantum well to the narrow one.     

Super-radiant surface emission from a quasi-cavity hot electron light emitter

The Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure (HELLISH-1) device is a novel surface emitter which utilises hot carrier transport parallel to the layers of a Ga1–xAlxAs p-n junction incorporating a single GaAs quantum well on the n-side of the junction plane. Non-equilibrium electrons are injected into the quantum well via tunnelling from the n-layer. In order to preserve the charge neutrality in the depletion region, the junction undergoes a self-induced internal biasing. As a result the built-in potential on the p-side is reduced and hence the injection of non- equilibrium holes into the quantum well in the active region is enhanced. The work presented here shows that a distributed Bragg reflector grown below the active region of the HELLISH device increases the emitted light intensity by two orders of magnitude and reduces the emission line-width by about a factor of 3 in comparison with the original HELLISH-1 structure. Therefore, the device can be operated as an ultrabright emitter with higher spectral purity.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

Ambipolar tunneling in near-surface quantum wells

We study the photoluminescence from a near-surface quantum well in the regime of ambipolar tunneling to the surface states. Under steady-state excitation an electric field develops self-consistently due to the condition of equal tunneling currents for electrons and holes. The field induces a Stark shift of the photoluminescence signal which compares well with experimental data from near-surface GaAs/AlGaAs single quantum wells.     

Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al_0.37Ga_0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.     

Polarised hot electron luminescence in p-GaAs: Electric field effects

Electrons photo-excited to high-energy conduction band states of GaAs exhibit complex energy and momentum distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs a fraction of these hot electrons combine with localised acceptor states, producing a hot electron luminescence (HEL) spectrum with a cascade of peaks corresponding to discrete energy losses resulting from LO-phonon emission. The highest peak involves unscattered electrons, and their energy distribution is due to warping of the initial heavy-hole (HH) bands. We report measurements of the line shape of this 0-HH peak, and its polarisation profile which identifies emission from electrons along particular directions. An applied electric field of 1 kV cm⁻¹ distorts the hot electron momentum distribution, and this is reflected in the polarisation profile. These line shapes and profiles, with and without field, are calculated using a computer model incorporating a band structure and optical matrix elements, the effect of electric field being included using a k-broadening model. The data and model are in good quantitative agreement assuming an electron lifetime of 100 fs, and confirm the expected differences in the profiles for different excitation polarisation states and applied field directions.     

Impact ionization and electroluminescence in single barrier tunnelling structures

At high electric fields, hot electrons injected into the undoped regions of n-i-n structures can give rise to impact ionization of the host lattice and electron-hole pair production. The holes created by impact ionization recombine with majority carrier electrons, leading to electroluminescence (EL) from the device at fields in excess of 10⁵V/cm. In the present work, we show that the study of such EL in GaAs/AlGaAs/GaAs single barrier tunnelling structures provides a simple and effective quantitative method to determine impact ionization coefficients in GaAs. Structures with undoped regions of length 100, 150 and 200nm are shown to give very similar results for the impact ionization coefficient. The values obtained for the impact ionization coefficient are shown to be consistent with those obtained by carrier multiplication methods.     

Terahertz emission from square wells in a longitudinal electric field

Terahertz emission accompanying heating of two-dimensional electrons by a strong electric field applied along size-quantized GaAs/AlGaAs layers is observed and investigated. The emission is due to indirect optical transitions of hot electrons in the bottom size-quantization band. The experimentally obtained emission spectra are compared with the spectra calculated taking into account scattering of electrons by optical phonons, impurities, and interfacial roughness and electron-electron scattering. Satisfactory agreement is obtained. The temperature of the hot electrons is determined from a comparison of the spectra. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 507–511 (10 April 1998)     

加偏置电场的抛物量子阱中的电光效应

本文利用密度矩阵方法得到了加偏置电场的抛物量子阱中电光效应的解析表达式,并以典型的GaAs抛物量子阱为例进行了数值计算研究结果表明,电光效应随偏置电场和抛物势频率的增大而增强,同时也表明GaAs量子阱中的电光效应比体GaAs中的要强一个数量级以上。     

共轭聚合物MEH-PPV的固态阴极射线发光

在有机／无机异质结ZnS／MEH－PPV／ZnS器件中，交流驱动条件下，实现了共轭聚合物MEH－PPV的固态阴极射线发光，器件的发光光谱与MEH－PPV的光致发光谱相同。电子经过ZnS层加速后，成为过热电子，这些过热电子直接碰撞激发MEH－PPV而发光。在日光灯照明下，可以看到器件的均匀发光。     

Hot electron energy and momentum relaxation in GaAs/AlAs and GaAs/Ga1-xAlxAs multiple quantum wells

Experimental results on high electric field longitudinal transport in GaAs/AlAs and GaAs/Ga_1-xAl_xAs multiple quantum wells (MQW) are presented and compared with the prediction of a dielectric continuum model. We draw from our experiments the following four conclusions.(i) In GaAs/Ga_1-xAl_xAs systems the dominant energy and momentum relaxation mechanism is through scattering with GaAs -modes.(ii) However, in GaAs/AlAs systems the AlAs interface mode is dominant in relaxing the energy and momentum of the quantum well electrons.(iii) The hot electron momentum relaxation as obtained from the high-field drift velocity experiments is strongly affected by the production of hot phonons as expected from a model involving a non-drifting hot phonon distribution.(iv) The importance of the AlAs interface mode in GaAs/Ga_1-xAl_xAs MQW is not the result of the intrinsic scattering rate but related to its shorter lifetime, compared to GaAs modes.     

Two-wave-mixing dynamics and nonlinear hot-electron transport in transverse-geometry photorefractive quantum wells studied by moving gratings

The photorefractive response to an applied electric field is measured in a photorefractive quantum well, providing evidence in favor of the nonlinear transport in the device due to the hot electrons. The reduced mobility of the hot electrons limits the drift length, and thereby limits fringe overshoot. Thus the nonlinear transport prevents the slowing down of the grating writing rate for increasing fields which is common in bulk photorefractives. The photorefractive phase shift in transverse-field photorefractive quantum wells is measured as a function of the frequency offset between two laser writing beams that generate moving gratings. The two-wave mixing passes through a maximum at an optimum frequency which depends on the magnitude and the sign of the applied dc electric field. The phase shift associated with the moving grating adds or subtracts from the static phase shift induced by hot-electron transport in the semiconductor quantum wells, depending on the sign of the field and the sign of the dominant photocarriers. We observe a linear relationship between the roll-off frequency and the power of the writing beams. Received: 26 November 1998 / Revised version: 22 January 1999 / Published online: 12 April 1999     

Nonlinear optical absorption and optical rectification in near-surface double quantum wells: combined effects of electric, magnetic fields and hydrostatic pressure

The theoretical study of the combined effects of electric and magnetic fields and hydrostatic pressure on the nonlinear optical absorption and rectification is presented for electrons confined within an asymmetrical GaAs?Ga_1-x Alx As double quantum well. The effective mass, parabolic band, and envelope function approaches are used as tools for the investigation. The electric field is taken to be oriented along the growth direction of the heterostructure and the magnetic field is applied parallel to the interfaces of the quantum wells. The pressure-induced mixing between the two lowest conduction bands is considered both in the low and high pressure regimes. According to the results obtained it can be concluded that the nonlinear optical absorption and rectification coefficients depend in a non-trivial way on some internal and external parameters such as the size of the quantum wells, the direction of applied electric field, the magnitude of hydrostatic pressure, the stoichiometry of the wells and barriers, and the intensity of the applied magnetic field.     

Electronic structure of GaAs/AlGaAs quantum double rings in lateral electric field

A three-dimensional model of GaAs/A1GaAs quantum double rings in the lateral static electric field is investigated theoretically.The eigenvalue problem with the effective-mass approximation is solved by means of the finite-element method.The energy levels and wave functions of quantum-confined electrons and heavy holes are obtained and show an agreement with our previous theoretical and experimental studies.It is shown in the approximation of neglecting the Coulomb attraction between the electron and heavy hole that a relatively large Stark shift of exciton emission of 4 meV is attainable with an applied electric field of 0.7 kV/cm.     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

DX<Superscript>−</Superscript> center formation in planar-doped GaAs:Si in strong electric fields

The kinetics of current decay and partial restoration in planar doped GaAs:Si due to the formation of DX^? centers in strong electric fields has been experimentally studied. The existence of thresholds with respect to the field strength and donor concentration is explained. A model of the DX^? center formation is proposed, which is based on the notions about variation of the depth and width of a potential well created by planar doping, caused by the redistribution of hot electrons between quantum confinement subbands. As a result, the energy level of DX^? centers, which is situated above the potential well depth in the absence of strong field, decreases and falls within the potential well. This makes possible the DX^? center formation, provided that hot electrons, occupying the resonance electron levels in the conduction band, simultaneously excite local vibrational modes.     

Birefringence and absorption of light during intersubband transitions of hot electrons in quantum wells

The changes in the birefringence and absorption of infrared radiation accompanying direct electron transitions induced between size-quantization subbands in a GaAs/AlGaAs system of doped quantum wells by heating of the electrons by an electric field applied parallel to the layers of the heterostructure are observed and investigated. An explanation of the observed effects is offered whereing the change in the absorption spectrum and the contribution of free electrons to the permittivity are related to the exchange interaction and nonparabolicity of the energy spectrum of the hot electrons. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 525–530 (10 April 1997)     

Hot electron photoluminescence in GaAs crystals

The spectrum and the linear polarization of photoluminescence of hot electrons in GaAs crystals were investigated. Oscillations in the hot photoluminescence (HPL) spectrum due to the subsequent emission of LO-phonons were observed. The study of HPL depolarization in an external magnetic field yielded the scattering time due to the emission of a LO-phonon by a hot electron in the Γ-valley (τ_?0 = 1 × 10^?13 sec) as well as the Γ?L intervalley scattering time. The radiative recombination of hot electrons created in the central Γ-valley via the subsidiary L-valley was observed. The distribution function of hot electrons in a wide energy range was evaluated from the spectra.     

Electric field control of exciton states in quantum dot molecules

Semiconductor nanostructures have attracted considerable interest during the recent years in view of the potential application in quantum information processing. In particular, quantum dots have been suggested to fulfill an essential requirement for quantum computation: controllable interaction that couples two quantum dot qubits. Previous experiments on two vertically aligned quantum dots have demonstrated the formation of coupled exciton states. We show that this coupling between two In_0.60Ga_0.40As/GaAs quantum dots can be tuned by an electric field applied along the molecule axis. This controllable coupling in such a relatively simple configuration could be implemented in a solid-state-based quantum device.