Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5?nm to 10?nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3?meV and decrease to around 3.0?meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.     

Hybrid stable-chaotic states in coupled quantum well stadia

We use tunnel current spectroscopy to investigate the quantum states of two GaAs quantum wells coupled by a low (100 meV) (AlGa)As tunnel barrier. A high tilted magnetic field is used to generate strongly chaotic electron motion in the two wells which act as coupled chaotic ‘stadia'. The effect of the tunnel barrier on the dynamics of the system depends on the magnitude of the applied bias voltage V. For V375 mV, the central potential barrier acts as a perturbation which modifies the trajectories of selected periodic orbits in the quantum well. Scattering off the central barrier also generates new periodic orbits involving multiple collisions on all three barriers. These orbits ‘scar' distinct sets of eigenstates which generate periodic resonant peaks in the current–voltage characteristics of the device. When the device is biased such that the injected electrons just surmount the central barrier, our calculations reveal novel hybrid scarred states with both stable and chaotic characteristics.     

Quantum lifetimes and momentum relaxation of electrons and holes in Ga0.7In0.3N0.015As0.985/GaAs quantum wells

Electronic transport in n- and p-type modulation-doped Ga_0.7In_0.3N_0.015As_0.985/GaAs quantum wells are investigated using magneto-transport measurements in the temperature range between T?=?1.8 and 32?K and at magnetic fields up to B?=?11?T. The momentum relaxation and the quantum lifetimes (τ_q ) of electrons and holes are obtained directly from the temperature and magnetic field dependencies of the SdH oscillation amplitudes, respectively. A detailed analysis of quantum and transport life times indicates that the momentum relaxation of holes is forward displaced in k-space, while a large angle-scattering mechanism is prominent for the electrons. This discrepancy is believed to be due to scattering of electrons with nitrogen complexes and to the lack of such a mechanism for holes.     

Optical properties of a magneto-donor in a quantum dot

The temperature-dependent magnetoresistance effect is investigated in a magnetically modulated two-dimensional (2D) electron gas (2DEG) which can be realized by depositing two parallel ferromagnets on top of a 2DEG electron gas. In the resonant tunnelling regime the transmission for the parallel and antiparallel magnetization configurations shows a quite distinct dependence on the longitudinal wave vector of the incident electrons. This leads to a very large magneto resistance ratio with a strong temperature dependence.     

Optical gain of interdiffused GaInNAs/GaAs quantum wells

A self-consistent model for the band structure and optical gain spectra in interdiffused Ga_xIn_1-xN_0.04As_0.96/GaAs single quantum wells are studied theoretically using Fick’s Law and the Fermi Golden Rule. Due to quantum-well interdiffusion, the peak gain and its peak vary with the annealing time. Our results show that the interdiffusion technique can be used to tune the operation wavelength for multi-wavelength applications without degradation of device performance. Received: 18 April 2001 / Accepted: 19 September 2001 / Published online: 20 December 2001     

Tunneling-induced enhancement of self-Kerr nonlinearity in asymmetric quantum wells

We propose an asymmetric AlGaAs/GaAs double quantum wells (QWs) structure for realizing the enhancement of self-Kerr nonlinearity. It is found, with resonant tunneling, that the self-Kerr nonlinearity can be clearly enhanced, while the absorption of probe field is very small and can be safely neglected. We attribute the enhancement of self-Kerr nonlinearity mainly to the constructive interference induced by resonant tunneling.     

In原子掺入对GaInNAs/GaAs单量子阱光致发光的影响

研究了In掺入GaNAs/GaAs单量子阱对其带间和低于带边发光性质的影响。实验结果显示,随着In浓度的增加,GaInNAs/GaAs量子阱带间发光得到改善,低于带边的发光强度大大地减小。这是由于GaInNAs合金生长在GaAs衬底上,为补偿In和N原子尺度的差异,N原子硬倾向于与In原子形成共价健。GaInNAs/GaAs单量子阱的光调制光谱证实了高能端发光峰来自来征的带边发光。     

Effect of In-segregation on subbands in GaInNAs/GaAs quantum wells emission around 1.3 and 1.55 micron

The effect of In-segregation on optical properties in 7.5-nm GaInNAs/GaAs single quantum well (QW) is studied theoretically. The nominal (In, N) contents in the QW are chosen to be (0.35, 0.015) and (0.39, 0.03) for the emission wavelengths around 1.3 and 1.55 μm, respectively. Muraki’s model is used to model the composition profiles in the QWs. In-plane strain, confinement potential, and subband energy levels of the QW are calculated using multi-band effective mass theory. We show a space-indirect transition between light holes localized in indium deficient region and electrons localized in indium rich region of the quantum well. Our results show that the optical transition energies are approximately constant for the segregation efficiencies smaller than 0.7 in both QWs.     

Lifetime analysis of quasibound states in coupled quantum wells

A simple time-dependent model is presented to investigate lifetimes of the quasibound states in coupled quantum wells (CQWs). The transfer matrix approach is employed to discretize the conduction-band profile of the heterostructure and form a dispersion equation whose zeros correspond to the complex eigenenergies. Both the bound and quasibound states are extracted numerically in the complex plane by Newton's method. The lower and higher well subbands are found to have negative and positive energy shift, respectively, as following the no level crossing theorem. Besides, the decay rate of the quasibound state is approximately proportional to the absolute energy shift. The quasibound states, which have larger energy shift, have shorter lifetime and decay more quickly. Furthermore, the differences in lifetime between the quasibound states in CQWs can be easily realized as all the wave functions are specially adjusted to form the relative probability density distributions.     

Spatial imaging of two-dimensional electronic states in semiconductor quantum wells

We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.     

Electric field modulation of valence band mixing in semiconductor quantum wells

Eigenstates and transition probabilities in a GaAs---Ga_1−xAl_xAs. single quantum well in the presence of an external electric field are computed by means of a tight-binding approach. The field changes the energy levels allowing for the mixing between different states. Special attention is paid to second and third valence band levels at k = 0 where mixing effects become crucial.     

Dynamics of excitonic states in GaAs/AlGaAs quantum wells

The influence of photoexcited carriers on the dynamics of the absorption spectra of GaAs/Al_xGa_1−2x As multilayer quantum wells is investigated experimentally. It is found that at quasiparticle densities all the way up to 10¹¹ cm⁻² the saturation of the excitonic absorption is due to both a decrease of oscillator strength and broadening of the excitonic lines. It is shown that in the case of femtosecond resonance laser exci-tation the decrease of oscillator strength is due to free electron-hole pairs, while the broadening and energy shift of the excitonic lines are due to the exciton-exciton interaction. The lifetimes of free electron-hole pairs and excitons (≈65 ps and ≈410 ps, respectively) are determined from the exponential decrease of the change in the oscillator strength and in the width and energy position of the excitonic lines. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 139–144 (10 August 1997)     

Hydrostatic pressure effects on impurity states in GaAs/AlAs quantum wells

Within the framework of effective-mass approximation, we have studied the effects of hydrostatic pressure on the binding energy of a shallow donor impurity in an infinite quantum well by means of a variational method. It is found that the first derivative of the binding energy and energy shift is reliable parameter for describing the structure.     

Influence of nonparabolicity on boundary conditions in semiconductor quantum wells

In this Letter we present precise derivation of the second boundary condition for the wavefunctions (the first boundary condition is the continuity of the wavefunctions) at the interface of two III-V compound semiconductors, starting from an accurate expression for the bulk conduction-band structure expanded up to fourth order in wavevector. The obtained boundary condition is valid for all states (both bound and continuous) of the quantum well, and follows directly from constantness of the probability current along the quantum well and does not conflict with the double integrating of the Schrödinger equation around the interface.     

GaAs量子阱太阳能电池量子效率的研究

将量子阱结构引入到单结GaAs太阳能电池中能够有效扩展吸收光谱.为了研究量子阱结构在GaAs太阳能电池中的作用机理,本文采用实验和理论的方法研究了InGaAs/GaAsP量子阱结构对电池量子效率的影响.实验结果表明,量子阱结构的窄带隙阱层材料将电池的吸收光谱从890 nm扩展到1000 nm.同时,量子阱结构的引入提高了680—890 nm波长范围内的量子效率,降低了波长在680 nm以下的量子效率.通过计算得到的量子阱结构和GaAs材料的光吸收系数,可以用来解释量子阱结构对太阳能电池量子效率的影响.     

Screening of exciton states by quasi-two-dimensional electron gas in quantum wells

Changes in the binding energy and oscillator strength of an exciton state due to screening by a quasi-two-dimensional electron gas were calculated self-consistently in a nonlinear approximation. It was shown that the collapse of the bound state proceeds at very small concentrations N _s ?5×10⁹ cm^?2, which is a consequence of taking into account the nonlinearity of the system response to the Coulomb perturbation.     

Investigation of band non-parabolicities in strain-balanced GaInAs/GaAlInAs coupled quantum wells

Magneto-optical investigations of a series of strain-balanced InGaAs/GaAlInAs coupled quantum wells are described, showing how changes in the strain within the InGaAs wells modify the conduction and valence-band dispersion curves. Large non-parabolicities in both bands are demonstrated by comparison of measured and calculated excitonic Landau levels. Increasing tensile strain within the wells results in the heavy-hole in-plane mass changing from a very high value to less than 0.2m₀as the first light-hole confined level shifts to an energy above that of the first heavy-hole state.