Asymmetric Quantum-well Structures Research Articles

Antenna-enhanced high-resistance photovoltaic infrared detectors based on quantum ratchet architecture

We demonstrate a quantum ratchet detector, which is a high-resistance photovoltaic mid-infrared detector based on an engineered spatial arrangement of subbands. In photovoltaic quantum-well photodetectors, in which unidirectional photocurrent is generated by asymmetric quantum-well structures, maximization of device resistance by suppressing undesired electron transports is crucial for minimizing noise. A semi-quantitative guideline suggests the significance of spatial separation between wavefunctions for reducing the conductance from the ground state. Here, we employ a step quantum well made of a shallow floor and a deep well. Photoexcited electrons are quickly transferred to a separated location from the ground state through fast resonant tunneling and phonon scattering, and then they are allowed to flow in only one direction. This architecture is made possible by the use of a GaAs/AlGaAs material system, and it achieves a resistance as high as 6.0 × 104 Ωcm2 with a single-period structure. Combined with optical patch antennas for responsivity enhancement, we demonstrate a maximum background-limited specific detectivity of 6.8 × 1010 cmHz1/2/W at 6.4 μm, 77 K for normal incidence, and a background-limited-infrared-photodetector temperature of 98 K.

Conduction-band nonparabolicity effect on refractive index and phase match in asymmetric quantum wells pumped by two infrared beams

An asymmetric quantum well (AQW) system that is pumped by two infrared beams is designed to generate terahertz (THz) waves. The refractive index and phase mismatch associated with the intersubband transition of the AQW structure are calculated and analyzed for both present and absent conduction band nonparabolicity. The calculated results reveal that, for increasing wavelengths, the refractive index of the AQW for the short-wavelength pump beam varies more than 0.83 and undergoes a 0.204 [Formula: see text]m redshift, when the conduction band nonparabolicity is considered. The variation of the refractive index of the AQW with the long-wavelength pump beam, changes from 0.225 to 0.316 after considering the conduction-band nonparabolicity. In addition, no redshift is observed. Whether the refractive index of one pump beam with its specific wavelength increases is determined mainly by the linear terms. However, for increasing the other pump wavelengths, the refractive index of one pump beam mainly depends on the nonlinear terms. Subband energy-levels and dipole transition matrix elements show noticeable changes due to conduction-band nonparabolicity, which change the refractive index. Phase matching can be achieved by adjusting the wavelength of the two pump beams. However, both phase mismatch and coverage increase when the conduction band nonparabolicity is considered.

Asymmetric quantum-well structures for AlGaN/GaN/AlGaN resonant tunneling diodes

Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39 A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates that RTD with a 2.8-nm-thick In0.03Ga0.97N sub-well in front of a 2.0-nm-thick GaN main-well can exhibit the I-V characteristic having Ip and PVCR of 0.07 A and 11.6, about 7 times and double the value of RTD without sub-QW, respectively. The purpose of improving the structure of GaN-based QW is to solve apparent contradiction between the device structure and the device manufacturability of new generation RTDs for sub-millimeter and terahertz applications.

Tunneling-induced optical bistability in an asymmetric double quantum well.

Optical bistable behaviors induced by tunneling coupling are demonstrated in an asymmetric double quantum-well structure including a unidirectional ring cavity. We analytically find that optical bistability can be obtained with the existence of resonant tunneling. Furthermore, the threshold of the optical bistability can be modulated by the intensity or the detuning of the driving fields.

Energy Loss Mechanism in Organic and Inorganic Light-Emitting Diodes

We report that there exists a similar energy loss mechanism in fluorescent/phosphorescent organic light-emitting diodes (F/P OLEDs) and inorganic semiconductor optoelectronic devices [1310-nm InGaAsP-InP superluminescent diodes (SLDs)]. The loss of energy in inorganic SLDs based on thickness-altered asymmetric multiple quantum-well (QW) structures occurs depending sensitively on the sequence of QWs, an analogous behavior also observed in F/P OLEDs depending on the sequence of phosphorescent dopants for different colors. It is shown that such an energy (power) loss is evitable by placing long-wavelength QWs near the p-side in inorganic SLDs and similarly long-wavelength phosphors near the hole-transporting layer in F/P OLEDs.

Uncooled$C$-Band Wide-Band Gain Lasers With 32-Channel Coverage and$-$20-dBm ASE Injection for WDM-PON

We report a spectrum-sliced amplified spontaneous emission-injected wide-band gain laser covering a temperature range over 110/spl deg/C as well as entire 32 100-GHz-spaced channels (1533-1558 nm) in 155-Mb/s upstream transmissions over 25 km of single-mode fiber in a wavelength-division-multiplexed passive optical network. The wide-band gain laser diode employs asymmetric multiple quantum-well structure and low antireflection coating in order to widen the gain spectrum.

+ 1 dBm average optical output power operation of small-chirp 40-Gbps electroabsorption modulator with tensile-strained asymmetric quantum-well absorption layer

A small-chirp 40-Gbps electroabsorption modulator (EAM) with high optical output power capability has been developed for the first time. An optimized tensile-strained asymmetric quantum-well structure is employed as the absorption layer of the EAM so that small chirp and reduction of the lifetime of the photogenerated holes for high optical output power tolerance is obtained. Deteriorations of frequency response and chirp due to carrier pileup under high optical output power conditions were prevented by enhancing carrier sweepout, which was experimentally confirmed as a hole lifetime as short as 35 ps under high optical output power conditions. As a result, good frequency response (bandwidth > 30 GHz) and small chirp (α<1) were obtained under the condition of the zero bias voltage and +4.5 dBm continuous-wave (CW) optical output power (P/sub out,CW/). Clear eye opening and high dynamic extinction ratio under 40-Gbps non-return-to-zero modulation persisted to a high average output power (P/sub out,ave/) condition of P/sub out,ave/=+1.0 dBm.

Small-chirp 40-Gbps electroabsorption modulator with novel tensile-strained asymmetric quantum-well absorption layer

A small-chirp 40-Gbps electroabsorption modulator (EAM) with a novel tensile-strained asymmetric quantum-well (QW) absorption layer has been demonstrated for the first time. The strain and the band line-up of the asymmetric QW structure were designed in order to obtain a small-chirp operation, a clear eye opening, and a high extinction ratio simultaneously. The chirp measured as α-parameter was reduced without any penalty of extinction ratio and eye opening. The measured α-parameter was smaller than 1.5 at any bias voltage from 0 to -2 V. The measured 3-dB bandwidth of a 75-μm-long EAM exceeded 50 GHz at -1 V bias voltage. Under a 40-Gbps modulation, a clear eye opening was obtained, and the eye diagram showed no violation of the standard STM256/OC768 mask. The measured dynamic extinction ratio was over 11 dB.

Proposal on the suppression of optical absorption by quantum interference via parallel-perpendicular kinetic energy coupling effect

We propose a scheme for suppressing optical absorption in a coupled asymmetric quantum-well (QW) structure via the parallel-perpendicular energy coupling effect. A specific QW structure is suggested and the feasibility of our scheme is proved with the calculation of wavefunctions and absorption coefficients.

Coherent carrier propagation in the continuum of asymmetric quantum-well structures

In specific ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum-well structures with a lack of inversion symmetry, the spectral dependence of the bound-to-continuum photocurrent shows sign reversals as a function of the excitation energy. These sign reversals can be explained in terms of coherently propagating carriers, which experience constructive or destructive interference according to their associated de Broglie wavelength. We show how a coherent control of the carrier motioning the continuum states can be achieved experimentally in appropriate quantum-well structures. A theoretical approach to simulate the observed current sign reversal is presented.

Intersubband electron-electron scattering in asymmetric quantum wells designed for far-infrared emission

Population inversion in inter-subband emitters and lasers depends critically on the lifetimes of the nonradiative inter-subband transitions. We find that the often neglected Auger-type processes and Pauli exclusion effects can dominate the electron-electron scattering contributions to the total scattering rate. Electron exchange is also considered. In a range of three-level asymmetric quantum-well structures designed to produce terahertz radiation, we estimate the potential for laser operation for a range of electron densities and temperatures using the population ratio ${\ensuremath{\tau}}_{32}/{\ensuremath{\tau}}_{21}.$ We find that predicted population ratios have a strong dependence on the electron-electron scattering rates, and that Auger processes and Pauli exclusion are of particular importance. In addition, the temperature dependence of the population ratio is significantly altered by the inclusion of electron-electron scattering processes. Our results indicate that the interplay of electron-phonon and electron-electron scattering rates suggest optimum operating temperatures for different emitter frequencies.

Self-consistent multicomponent envelope function calculation of normal incidence second-harmonic generation in p-type doped symmetric quantum wells

In a self-consistent multicomponent envelope function approach, we showed that χxxx(2) component of the second-harmonic susceptibility tensor of a symmetric p-type quantum well (QW) exists, because of the valence-band mixing at nonzero values of the in-plane wave vector. For a highly doped narrow GaAs/Al0.3Ga0.7As QW, our calculation indicates that at low temperatures a large value (∼10−7 m/V) of χxxx(2) can be achieved in the far-infrared frequency range without satisfying double resonance conditions. This value is comparable to the experimentally observed value of χzzz(2) in n-type asymmetric QW structures.

Polaron effects in asymmetric semiconductor quantum-well structures

In this paper, polaron effects in asymmetric quantum-well structures (QW's) are investigated by using second-order perturbation theory and the modified Lee-Low-Pines (LLP) variational method. By applying the Greens-function method, explicit analytical expressions for the electron extended-state wave functions and the density of states in a general step QW's are given. Within the framework of second-order perturbation theory, the ground-state polaron binding energy and effective mass in step and asymmetric single QWs are studied as due to the interface optical phonons, confined bulklike LO and half-space LO phonons. The full energy spectrum is included in our calculations. The effects of the finite electronic confinement potential and the subband nonparabolicity are also considered. The relative importance of the different phonon modes is analyzed. By means of the modified LLP variational method, the binding energy of a polaron confined to asymmetric single QW's is also investigated. Our results show that in ordinary asymmetric QWs, the asymmetry of the QW's has a significant influence on the polaron effect, which has a close relationship to the interface phonon dispersion. When the well width and one side barrier height of asymmetric single QWs are fixed and identical with those of symmetric QW's, the polaron binding energy in asymmetric QWs is always smaller than that in symmetric QW's. We have also found that it is necessary to include the continuum energy spectrum as intermediate states in the perturbation calculations in order to obtain the correct results; the subband nonparabolicity has a small influence on the polaron effect. Comparing our results obtained by using two different methods, good agreement is found.

Creation and annihilation of positively and negatively charged excitions in GaAs quantum wells

The electro- and photoluminescence of p-i-n and n-i-n GaAs double-barrier resonant tunnelling diodes have been investigated for various concentrations of free carriers (either holes or electrons) in the quantum well (QW). For these structure it is possible to change the relative electron or hole density quasi-continuously with applied bias voltage. At electron densities well below that for exciton unbinding, the formation of negatively charged excitons (X −) is demonstrated. In asymmetric QW structures the electron densities can be increased above the critical density and excition unbinding is observed in the luminescence spectra. We also report the observation of the positively charged exciton (X + 2) in the luminescence spectra of a GaAs quantum well. The conditions for forming X + 2 are compared with those needed to see the negatively charged exciton (X −), the heavy-hole free exciton (X) and free carrier recombination.

MIRRORLESS OPTICAL PARAMETRIC OSCILLATORS

We have considered two novel configurations for optical parametric oscillators (OPO’s): transversely-pumped counter-propagating and nondegenerate backward OPO’s due to the distributed feedback provided through the opposite propagation directions of the signal and idler. In both configurations, by changing the incident angle of the pump beam, one can tune the output frequency in a large range. The threshold pump powers for the oscillation can be as low as ~10 W for the transversely-pumped counter-propagating OPO’s and 44 W for the nondegenerate backward OPO’s. The quasi-phase matching is achieved by spatially modulating second-order susceptibility along the growth direction based on semiconductor alternating thin layers or asymmetric quantum-well domain structures or by electric-field poling in conventional second-order nonlinear materials. The nondegenerate backward OPO’s offer the most efficient conversion among all the configurations for the OPO’s having the same threshold pump power. The transversely-pumped counter-propagating OPO’s have the optimal pump power.

Cavity-enhaced and quasi-phase-matched optical frequency doublers in surface-emitting geometry

We show that we can efficiently achieve surface-emitting second-harmonic generation in vertical and horizontal cavities. The fundamental beam is coupled into the waveguide, which consists of III–V or II–VI semiconductor multilayers or asymmetric quantum-well domain structures. The generated second-harmonic radiation propagates along the growth direction of these layers (which is normal to the propagation direction of the fundamental beam). The quasi-phase matching is achieved when second-order susceptibility is modulated along the growth direction in these structures. By the proper design of these structures, the frequency doublers based on these structures together can cover the spectrum of 0.8–2.0 μm. If the pump power density is sufficiently large, the conversion efficiency approaches saturation. The saturation power per unit waveguide width is between ~0.9 and ~435 mW/μm. At such a power density, 72% conversion efficiency can be achieved. In addition, the proposed frequency doublers are, in principle, broadband.

Electron-phonon scattering in asymmetric semiconductor quantum-well structures

We calculate scattering rates of intrasubband and intersubband electronic transitions in asymmetric single quantum wells (QW's) and step QW's due to interface phonons, confined bulk-like LO phonons, and half-space LO phonons. The relative importance of the different phonon modes is analyzed. The results show that the electron-phonon scattering rates have intimate relation to the QW parameters.

Traversal time in an asymmetric double-barrier quantum-well structure

A study of traversal time for asymmetric double-barrier quantum-well structure (DBQWS) is presented. The calculations are self consistent in nature. The analysis shows that traversal time is affected more strongly by the width of the emitter barrier than that of the collector barrier of the structure in the presence of space charge. Bistability in traversal time is observed. Bistability becomes greater if the emitter barrier is made wider than the collector barrier and is explained in terms of space charge developed in the quantum well. However, a wider collector will dominate traversal time at zero bias, especially at eigen energy. >

Structure dependence of second harmonic generation in asymmetric quantum-well structures

Second harmonic generation in stepped asymmetric quantum wells (AQW) is discussed and shown to depend strongly on several controllable AQW parameters. The influence of inter-subband absorption is also investigated. It is demonstrated that with appropriate choice of semiconductor materials and precise bandgap engineering, optimized devices may be developed for a given input laser frequency. >

Linear and quadratic electrooptic effects in symmetric and asymmetric quantum-well structures

In the first part of the paper, the changes in the complex index of refraction of quantum-well structures proportional to the applied dc electric field (F) and its square are obtained from stationary perturbation theory. Expressions are obtained for the linear and quadratic changes in index and absorption coefficients of asymmetric and symmetric quantum well structures. The triple resonance term does not contribute to the quadratic Kerr effect for a symmetrical well in an expansion about F=0. In the second part of the paper, these results are applied to several types of modulators. First we consider two kinds of silicon/silicon-germanium high-barrier asymmetric quantum well structures at 1.55 /spl mu/m. For the step well, the dominant triple resonant term of the quadratic Kerr effect adds to the linear electrooptic effect (LEG) to yield an intensity extinction ratio of 15 dB in switching from F=+8 V//spl mu/m to -8 V//spl mu/m and a very small insertion loss of 0.04 dB, for a modulator length of 200 pm and for narrow linewidths (FWHM=7.6 meV). For the case of asymmetric barriers confining silicon wells, a comparable insertion loss is obtained but with an extinction ratio of 44 dB for FWHM=20 meV, a clearly superior performance. For symmetrical silicon-based wells, the double-resonance modulation, though small at /spl lambda/=1.55 /spl mu/m, is appreciable at /spl lambda/=10.6 /spl mu/m. For the Kerr effect in symmetrical gallium arsenide wells at /spl lambda/=10.6 /spl mu/m reported by Sa'ar et al., we show that it is the double-resonance term which yields the six order-of-magnitude enhancement with respect to bulk and that an even larger enhancement of the Kerr effect can be obtained in an asymmetric GaAs structure together with the LEO. >