Double-barrier Quantum Wells Research Articles

Enhanced intersubband absorption in stepped double barrier quantum wells

Intersubband absorption is measured in the conduction band of GaAs and stepped GaAs/InxGa1−xAs multiple-quantum-wells confined by narrow AlAs barriers. Enhanced absorption from n=1 to n=2 is observed in the stepped wells. This is attributed to relaxation of the intersubband polarisation selection rule.

Photoionization spectrum in double-barrier quantum wells: Exact expansion over resonances and natural line shape

Photoionization spectrum in double-barrier quantum wells: Exact expansion over resonances and natural line shape

Structural and photoelectric studies on double barrier quantum well infrared detectors

GaAs/AlAs/GaAlAs double barrier quantum well (DBQW) structures are employed for making 3–5 μm photovoltaic infrared (IR) detectors with a peak detectivity of 5×10 11 cm Hz 1/2/W at 80 K. Double crystal X-ray diffraction is combined with synchrotron radiation X-ray analysis to determine successfully the exact thickness of GaAs, AlAs and GaAlAs sublayers. The interband photovoltaic (PV) spectra of the linear array of the detectors are measured directly by edge excitation method, providing the information about spatial separation processes of photogenerated carriers in the multiquantum wells and the distribution of built-in field in the active region. The spectral response of the IR photocurrent of the devices is also measured and compared with the temperature dependent IR absorption of the DBQW samples in order to get a better understanding of the bias-controlled optical and transport behavior of the detector photoresponse and thus to optimize the detector performance.

An analytic method for tunneling lifetime in the double-barrier quantum well

An analytic method is presented for solving the time-dependent Schrödinger equation for double-barrier quantum well problems that allows for the presence of incident waves. A simple method is devised for determining the tunneling lifetime based on the analytic method.

Anomalous tunneling transparency of double-barrier quantum wells with transition-metal impurities

A mechanism for the anomalous transparency of a double-barrier quantum well (DBQW) doped with transition-metal impurity is proposed. Apart from the usual resonance peaks, strong features appear in the tunneling transparency, since such a DBQW possesses its own quasi-two-dimensional (2D) continuum together with the impurity level. These strong peaks arise near the edges of 2D bands due to the reconstruction of 2D continuum states in a process of tunneling via a doped DBQW. As a result, the tunneling transparency can have peaks even when the deep impurity level is out of resonance with the chemical potential. Unlike the well-known Kondo singularities, the impurity-induced resonances are temperature independent. The influence of Kondo scattering on these features is also considered.

Transport mechanism of Gamma - and X-band electrons in AlxGa1-xAs/AlAs/GaAs double-barrier quantum-well infrared photodetectors.

The effect of the \ensuremath{\Gamma}- and X-band electrons in the ${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As/AlAs/GaAs double-barrier quantum well (DBQW) is investigated by a microscopic empirical pseudopotential calculation. The DBQW structure used in the calculation is designed as a 3--5-\ensuremath{\mu}m quantum-well infrared photodetector with an associated transition energy of 313 meV. DBQW tunneling transmission via \ensuremath{\Gamma}- and X-like states as a function of electron energy and applied voltage are described and compared to that in a single-barrier AlAs/GaAs quantum well. The dark current is simulated by the confined ground-state electron tunneling out of the well. We find that, at high-bias voltage, tunneling via X-like states increases the current by a few orders of magnitude. We have also varied the additional barrier thickness and found that for a very thin (20 \AA{}) additional barrier DBQW, the excited-state electrons are not blocked by the \ensuremath{\Gamma}-band barrier, and may give a high photocurrent without the assistance of the X band, although the dark current also increases. \textcopyright{} 1996 The American Physical Society.

Theory of semiballistic wave propagation.

Wave propagation through waveguides, quantum wires or films with a modest amount of disorder is in the semi-ballistic regime when in the transversal direction(s) almost no scattering occurs, while in the long direction(s) there is so much scattering that the transport is diffusive. For such systems randomness is modelled by an inhomogeneous density of point-like scatterers. These are first considered in the second order Born approximation and then beyond that approximation. In the latter case it is found that attractive point scatterers in a cavity always have geometric resonances, even for Schr\"odinger wave scattering. In the long sample limit the transport equation is solved analytically. Various geometries are considered: waveguides, films, and tunneling geometries such as Fabry-P\'erot interferometers and double barrier quantum wells. The predictions are compared with new and existing numerical data and with experiment. The agreement is quite satisfactory.

A voltage-controlled tunable two-color infrared photodetector using GaAs/AlAs/GaAlAs and GaAs/GaAlAs stacked multiquantum wells

A voltage-controlled tunable two-color infrared detector with photovoltaic (PV) and photoconductive (PC) dual-mode operation at 3–5 μm and 8–14 μm using GaAs/AlAs/AlGaAs double barrier quantum wells (DBQWs) and bound-to-continuum GaAs/AlGaAs quantum wells is demonstrated. The photoresponse peak of the photovoltaic GaAs/AlAs/GaAlAs DBQWs is at 5.3 μm, and that of the photoconductive GaAs/GaAlAs quantum wells is at 9.0 μm. When the two-color detector is under a zero bias, the spectral response at 5.3 μm is close to saturate and the peak detectivity at 80 K can reach 1.0×1011 cmHz1/2/W, while the spectral photoresponsivity at 9.0 μm is absolutely zero completely. When the external voltage of the two-color detector is changed to 2.0 V, the spectral photoresponsivity at 5.3 μm becomes zero while the spectral photoresponsivity at 9.0 μm increases comparable to that at 5.3 μm under zero bias, and the peak detectivity (9.0 μm) at 80 K can reach 1.5×1010 cmHz1/2/W. Strictly speaking, this is a real bias-controlled tunable two-color infrared photodetector. We have proposed a model based on the PV and PC dual-mode operation of stacked two-color QWIPs and the effects of tunneling resonance with narrow energy width of photoexcited electrons in DBQWs, which can explain qualitatively the voltage-controlled tunable behavior of the photoresponse of the two-color infrared photodetector.

Electron capture in AlGaAs/AlAs/GaAs double-barrier quantum well structures: Tunneling versus intervalley scattering

We have investigated electron capture times in n-type GaAs/AlAs/Al 0.3Ga 0.7As double-barrier quantum well (DBQW) structures in an electric field along the growth direction. To this end, the time dependence of the photoluminescence (PL) originating from the GaAs wells and the Al 0.3Ga 0.7As barrier layers is analyzed numerically. Theoretical electron capture times are calculated for the investigated DBQW's for the case of several scattering mechanisms. Electron capture mainly occurs via Γ- X intervalley transfer under participation of X-point subbands located at the AlAs tunnel barriers. Good agreement between experiment and theory is obtained when assuming Γ- X backfolding, while impurity scattering, optical phonon-assisted Γ-tunneling, and deformation potential scattering give rise to much larger electron capture times.

Field dependence of carrier capture in GaAs/AlAs/AlGaAs double-barrier quantum well structures

We report on electron and hole capture times in n-type GaAs/AlAs/Al0.3Ga0.7As double-barrier quantum well (DBQW) structures in an electric field applied perpendicular to the layers. We have measured the time-dependent photoluminescence (PL) originating from the GaAs wells and the Al0.3Ga0.7As barrier layers. The experimental capture times are obtained from least-squares fits of appropriate model functions to the observed PL transients. We have theoretically determined electron capture times for the investigated DBQWs, by calculating the electron wavefunctions and taking into account various scattering processes, including impurity scattering, optical phonon-assisted tunnelling and intervalley scattering. We find evidence that electron capture occurs by Gamma -X intervalley transfer via X-point subbands localized in the AlAs layers.

Influence of the process environment on the thermal intermixing of GaAs/AlAs/AlGaAs double barrier quantum wells

The influence of the process environment on thermally induced intermixing of GaAs/AlAs/AlGaAs double barrier quantum wells is investigated by effecting different low energy reactive ion etch (RIE) surface treatments and then depositing PECVD/sputtered cap layers prior to annealing. We observe photoluminescence blue shifts, small red shifts and total intermixing, dependent on the actual combination of cap material and RIE treatment. The results indicate that the RIE surface damage is an important factor for the onset of the intermixing process and that PECVD processes may require a few monolayers thick surface disordering in order to trigger off the quantum well intermixing, even using SiO2 cap layers.

Tunneling assisted thermionic emission in double-barrier quantum well structures

We report on experimental and theoretical investigations of the dark current behavior in n-type GaAs/AlAs/Al0.3Ga0.7As double-barrier quantum wells (DBQW). The dark current perpendicular to the 50-period DBQW structures shows evidence of tunneling through the 2-nm-wide AlAs tunnel barriers and thermionic emission across the 25-nm-wide AlGaAs layers. The temperature dependence is in good agreement with our numerical simulations, using an effective escape rate of 2.2×1014 s−1. The dark current is further modified by internal electric fields caused by an asymmetric dopant distribution, which partially arises from a dopant segregation during epitaxial growth. We also present a theoretical simulation showing how the dark current versus voltage dependence is influenced by these space-charge fields.

Excitonic binding energies in diffused-intermixed GaAs/AlAs/AlGaAs double barrier quantum wells

We report a detailed study of the exciton properties in thermally diffused GaAs/AlAs/AlxGa1−xAs double barrier quantum wells (DBQW). The interband transition energies have been calculated with a standard transfer matrix method, while the exciton binding energy is obtained by a variational approach with an elliptic exciton envelope function. It is found that the inserted thin AlAs layer between the GaAs well and the AlGaAs barriers has a substantial effect on the exciton confinement and the intermixing properties. For thin enough AlAs barriers, the exciton binding energy increases for increasing diffusion lengths, reaches a maximum and then decreases gradually. The results show that a DBQW mixes faster than a single QW, due to the additional AlAs layers. Thick AlAs barriers enable the formation of an indirect AlGaAs intermixed well region, and the effects which stem from the indirect band line up are discussed.

Capture of carriers excited by interband and intersubband absorption in GaAs/AlAs/AlGaAs double-barrier quantum wells

We report on the carrier dynamics in n-type double-barrier quantum well structures in an electric field. Both the intersubband and interband photocurrents, excited by long-wavelength (4μm) and short-wavelength (0.5μm) radiation, respectively, show a photovoltaic asymmetry with respect to the applied field. This asymmetry arises from an internal field due to an asymmetric dopant distribution with respect to the well centers. Time-dependent photoluminescence measurements allow us to determine the field dependence of the electron and hole capture times.

A GaAs/AlAs/AlGaAs and GaAs/AlGaAs stacked quantum well infrared photodetector for 3–5 and 8–14 μm detection

A new two-color n-type GaAs/AlAs/AlGaAs double barrier quantum well (DBQW) and bound-to-miniband GaAs/AlGaAs quantum well infrared photodetector (QWIP) with photovoltaic (PV) and photoconductive (PC) dual-mode operation in the 3–5 and 8–14 μm atmospheric spectral windows has been demonstrated in this work. It consists of a stack of the midwavelength infrared (MWIR) QWIP and the long-wavelength infrared (LWIR) QWIP. The PV detection scheme uses transition from the ground bound state to the first quasi-bound excited state for the MWIR-QWIP. The PC detection scheme has two different transitions, one identical to the PV mode detection scheme while the other uses transition from the ground bound state to the miniband state of the superlattice barrier LWIR-QWIP. The peak responsivity for the PV mode was found to be 17 mA/W at λp=4.1 μm and T=50 K with a bandwidth Δλ/λp=15%. The peak responsivities for the PC mode were found to be 25 mA/W at λp=4.1 μm, Vb=1.0 V, and 0.12 A/W at λp=11.6 μm, Vb=3.2 V, and at T=50 K, with a bandwidth Δλ/λp=18%. The PV responsivity was found to be 68% of the PC responsivity at λp=4.1 μm and T=50 K, demonstrating the ability for efficient PV mode operation at 3–5 μm by using the DBQW structure. This is the highest ratio reported for the spectral region of 3–5 μm wavelength with a DBQW structure.

Nonresonant electron capture in GaAs/AlAs/AlGaAs double-barrier quantum well infrared detectors

We have investigated electron and hole capture times in n-type double-barrier quantum well (DBQW) infrared (3–5 μm) detector structures. Photoluminescence upconversion allows us to observe the relaxation of photoexcited carriers from AlAs/(AlGa)As/AlAs barriers into GaAs quantum wells. For 2 nm AlAs, 25 nm Al0.3Ga0.7As and 5 nm GaAs, the electron capture process is non-resonant, with a capture time of 47 ps at a temperature of 5 K and 28 ps at 77 K. Theoretical calculations show that electron tunneling across the 2 nm AlAs layers mainly occurs via the Γ-minimum, and that the Γ-X contribution is negligible.

Electrostatic correction to the square-potential model of nanostructures significantly affects current resonances

Depletion of carriers in nanostructures generates an electrostatic correction to the square potential model. This correction changes the tunneling probability and consequently affects current resonances. We analyze this effect numerically for the double-barrier quantum well (DBQW) at various temperatures and carrier densities, and conclude that the electrostatic correction significantly affects the resonant voltage, the current at resonance, the peak-to-valley ratio, and may even cause disappearance of the lowest resonance.

Type II narrow double barrier quantum well structures : Γ-X coupling and interface effects

Photoluminescence (PL), PL excitation and time resolved PL experiments have been performed on Al 0.42 Ga 0.58 As/AlAs/GaAs symmetric double barrier quantum wells (DBQW) with only one or two AlAs monolayers constituting the intermediate barriers. In agreement with the envelope function predictions we show that such DBQW's undergo a type I-type II transition when the GaAs thickness is reduced below 7 and 5 monolayers for 2 and 1 AlAs molecular planes respectively. In type II configuration the PL decay time is found to be strongly dependent on the energy difference between AlAs X z - and gaAs Γ-electron confined states and the coupling parameter of the Γ and X z valleys can be reduced (4.2 meV on average). Similar experiments on DBQW's grown on 2 o off (001) vicinal surfaces show that the excition lifetime remains close to that of nominal samples. It is concluded that the Γ-X mixing potential in quantum structures is intrinsically linked to the ALAs/GaAs interface

Resonant tunneling of plane, cylindrical, and spherical double-barrier quantum wells

Resonant tunneling of double-barrier quantum well (DBQW) in plane, cylindrical, and spherical geometry has been studied by a simple model for the first time. Transmission spectrum, current-voltage characteristic, and bias dependent conductance have been calculated for these structures and are found oscillating in the cylindrical and spherical DBQW. Those oscillations are explained by the different coherence of tunneling in these geometries. Dependence of the oscillation on the structures, the materials, and the external fields is discussed.

Growth of ultra-thin AlAs layers on GaAs (001) vicinal surfaces: a search for lateral confinement

Type II double barrier Al xGa 1- x As/AlAs/GaAs quantum wells with ultra-thin AlAs layers (1 or 2 monolayers (ML) wide) have been studied by photoluminescence as a function of the substrate misorientation. A blue shift of the recombination lines is observed when double barrier quantum wells are grown on vicinal surfaces. This is interpreted as an additional lateral confinement induced by the step array on the electron states in the AlAs layers. For structures with 1 ML of AlAs, evidence of the influence of the step array disorder on the “QWire” confinement energy is given.