type-II Quantum Wells Research Articles

Electron tunneling effects on radiative recombination in modulation n-doped ZnSe/BeTe type-II quantum wells

We have studied the cyclotron-resonance absorption and photoluminescence properties of the modulation n-doped ZnSe/BeTe/ZnSe type-II quantum wells. It is shown that only the doped sample shows electron cyclotron-resonance absorption. Also, the undoped sample shows two distinctive peaks in the spatially indirect photoluminescence spectra, and the doped one shows only one peak. The results reveal that the high concentration electrons accumulated in ZnSe quantum well layers from n-doped layers can tunnel through BeTe barrier from one well layer to the other. The electron concentration difference between these two well layers originating from the tunneling results in a new additional electric field, and can cancel out a built-in electric field as observed in the undoped structures.

Temperature dependence of the photoluminescence from InP/GaAs type-II ultrathin quantum wells

Temperature dependence of the photoluminescence (PL) spectra has been investigated for InP/GaAs type-II ultrathin quantum wells (QWs). Room temperature PL has been observed for 1.43 monolayer thick ultrathin QW. Fitting parameters of the Bose–Einstein empirical relation for ultrathin QWs show that the temperature dependence of PL peak energy is similar to the temperature dependence of the band gap for InP and GaAs materials. In addition, we have also determined the PL quenching mechanism from the Arrhenius-like plot of integrated PL intensity. Thermal escape of carriers from these ultrathin QWs into the GaAs barrier is mainly responsible for the PL quenching with temperature. This is also supported by the observation that the PL intensity related to the GaAs barrier increases with increasing temperature.

Observation of electron confinement in InP/GaAs type-II ultrathin quantum wells

The issue of type-II band alignment for InP/GaAs heterostructure is addressed by means of simple layer architecture of ultrathin quantum wells (QWs). From specific signatures of the radiative recombination in type-II QWs especially the cube root dependence of blueshift in the lowest excitonic transition energy on excitation power in photoluminescence measurements indicates that the observed luminescence is originating from spatially separated electrons and holes. Such a blueshift is seen to increase with the QW thickness again confirming a type-II band alignment. A direct evidence of electron confinement in the conduction band of InP is provided by the capacitance voltage measurements.

Dynamical control of Mn spin-system cooling by photogenerated carriers in a (Zn,Mn)Se/BeTe heterostructure

The magnetization dynamics of the Mn spin system in an undoped (Zn,Mn)Se/BeTe type-II quantum well was studied by a time-resolved pump-probe photoluminescence technique. The Mn spin temperature was evaluated from the giant Zeeman shift of the exciton line in an external magnetic field of 3 T. The relaxation dynamics of the Mn spin temperature to the equilibrium temperature of the phonon bath after the pump-laser-pulse heating can be accelerated by the presence of free electrons. These electrons, generated by a control laser pulse, mediate the spin and energy transfer from the Mn spin system to the lattice and bypass the relatively slow direct spin-lattice relaxation of the Mn ions.

Optical gain improvement in type-II InGaN/GaNSb/GaN quantum well structures composed of InGaN/and GaNSb layers

Optical gain characteristics of type-II InGaN/GaNSb quantum well (QW) structure are investigated by using the multiband effective mass theory. These results are compared with those of conventional InGaN/GaN QW structures. The transition wavelength rapidly increases with increasing the Sb composition in GaNSb layer while it is less sensitive to the In composition in InGaN layer. Hence, longer wavelength QW structures with a relatively lower In composition can be easily obtained by controlling Sb composition, compared to the conventional type-I InGaN/GaN QW structures. The optical gain and the differential gain (dg/dn) of a type-II QW structure are shown to be much larger than that of a conventional QW structure in an investigated range of carrier densities. This is due to the reduction in the effective well width, in addition to the increase in the optical matrix element.

Interface structure effects on optical property of undoped ZnSe/BeTe type-Ⅱ quantum wells

The results are reported of the spatially indirect photoluminescence (PL) spectrum measurements performed on undoped ZnSe/BeTe type-Ⅱ quantum wells with special interface structures at low temperatures (5—10 K). The PL spectra have two main peaks that show a weak PL intensity and a low linear polarization degree and that their linear polarizations are contrary to each other, And the PL spectra are strikingly dependent on an applied external electric field perpendicular to the layers. The results show that the special interface structures reduce spatially indirect radiative recombination efficiency and linear polarization degree, and that a weak built-in electric field exists in the heterostructure. With the increase of excitation intensity, the PL peak on high energy side shows a rapid increase. This is explained by the formation of high charge density on both sides of the high energy side interface.

Anisotropic exciton and charged exciton dichroic photoluminescence in undoped ZnSe/BeTe type-II quantum wells in magnetic fields

The in-plane anisotropy of the spatially indirect (SID) photoluminescence (PL) in an undoped ZnSe/BeTe type-II quantum structure has been systematically investigated and analyzed. With a new method we have succeeded in extracting intrinsic circular polarization degree of this SID PL by a special combination of a 1 4 wave-plate and a linear polarizer with respect to the in-plane crystal axis, without the influence of the in-plane anisotropy.

Spatially separated indirect photoemission in undoped ZnSe/BeTe type-II quantum wells studied in pulse magnetic fields

Spatially separated indirect photoemission in an undoped ZnSe/BeTe type-II quantum structure is investigated by applying pulse magnetic field up to 50 T. A method was applied to extract the intrinsic circular polarization degree from photoluminescence (PL) with strong in-plane anisotropy. The magnetic field dependences of the circularly dichroic PL are explained by an optical transition model associated with a positively charged exciton composed of an electron and two light holes. The positively charged exciton PL is strongly suppressed by the magnetic field and also by temperature.

GaAs 0.7 Sb 0.3 / GaAs type-II quantum well with an adjacent InAs quantum-dot stressor layer

We report the optical properties of a composite structure comprising a type-II GaAs0.7Sb0.3/GaAs quantum well (QW) and an InAs quantum-dot (QD) layer adjacent to the QW. The low-temperature photoluminescence (PL) of the composite structure with a 5-nm-thick GaAs spacer demonstrates a redshift of 44 meV, as compared with that of GaAs0.7Sb0.3/GaAs single QW at low excitation level. The redshift reveals the existence of local potential minimums, induced by the stress exerted by the adjacent QDs, in the type-II QW. At higher temperature, the composite structure shows stronger PL intensity than the GaAs0.7Sb0.3 QW, indicating the potential of the applications to laser diodes.

8H Stacking Faults in a 4H-SiC Matrix: Simple Unit Cell or Double 3C Quantum Well?

The electronic structure of in-grown 8H stacking faults in 4H-SiC matrix has been investigated in detail. After assessment of the structural properties by high resolution transmission electron microscopy, we focus on the electronic structure. We show that one unit cell of 8H does not behave like a single type-II quantum well but, rather, like two type-II quantum wells of 3C coupled by a thin hexagonal barrier. Using a transfer matrix method, we compute the corresponding transition energies, taking into account the effect of the valence band offset and built-in electric field. A good agreement is found with the experimental data collected from low temperature photoluminescence spectroscopy.

Effect of type-II quantum well of m-MTDATA/α-NPD on the performance of green organic light-emitting diodes

The type-II multiple quantum well (MQW) structure is prepared and introduced into green organic green light-emitting diodes consisting of 4,4′-bis-[N-(naphthyl)-N-phenyl-amino]biphenyl (α-NPD) and tris-(8-hydroxyquinolinato)-aluminum (Alq3). The quantum well (QW) and wall are fabricated by 4,4′,4″-tris-(3-methylphenylphenylamino)triphenylamine (m-MTDATA) and α-NPD, respectively. The device performance of MQW organic light-emitting diodes (OLEDs) has been improved; the luminous efficiency by 25% and power efficiency by 17% compared with the reference device. The performance improvement can be explained by the increased electron–hole balance in the device due to the hole confinement in the QW structure.

Optically Pumped Midinfrared Laser With Simultaneous Dual-Wavelength Emission

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We demonstrate optically pumped semiconductor lasers that are capable of simultaneously emitting at two different midinfrared wavelengths. The wavelengths can be independently chosen and designed into the heterostructure. The epitaxial III–V antimonide structure employs two sets of type-II quantum wells in a waveguide that is partitioned by a thin electrical barrier that is transparent to the pump radiation. Two-color devices emitting at wavelengths as far apart as <emphasis><formula><tex>$\sim$</tex></formula></emphasis>4.0 and <emphasis><formula> <tex>$\sim$</tex></formula></emphasis>5.4 <emphasis><formula formulatype="inline"> <tex>$\mu$</tex></formula></emphasis>m are reported. </para>

Self-consistent gain analysis of type-II ‘W’ InGaN–GaNAs quantum well lasers

Type-II InGaN–GaNAs quantum wells (QWs) with thin dilute-As (∼3%) GaNAs layer are analyzed self-consistently as improved III-nitride gain media for diode lasers. The band structure is calculated by using a six-band k⋅p formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The type-II InGaN–GaNAs QW structure allows large electron-hole wave function overlap by confining the hole wave function in the GaNAs layer of the QW. The findings based on self-consistent analysis indicate that type-II InGaN-GaNAs QW active region results in superior performance for laser diodes, in comparison to that of conventional InGaN QW. Both the spontaneous emission radiative recombination rate and optical gain of type-II InGaN–GaNAs QW structure are significantly enhanced. Reduction in the threshold current density of InGaN–GaNAs QW lasers is also predicted.

Excitons in rolled up nanotubes of type-II semiconductor quantum wells: Theoretical study of a quasi-one-dimensional bosonic gas

The excitonic states and the quantum degenerate excitonic gas of spatially separated electron hole in an ideal rolled up nanotube of type-II semiconductor quantum wells with reverse bias configuration are studied theoretically. To illustrate the situation using the material constants appropriate to GaAs/AlAs type-II semiconductor, the exciton binding energy, its ground-state wave function, and the energies of the two low lying rotational states are calculated by numerical diagonalization of the single electron-hole Hamiltonian. By exploiting the ground-state wave function, the exciton-exciton interaction potential in the low-density limit is evaluated and found to be repulsive. It is then shown that a gas of such excitons at low temperature below the energies of the low lying rotational states behaves as a quasi-one-dimensional bosonic gas, which, at low density, is in the strong-coupling regime, and the excitons become fermions and the orthoexcitonic gas is paramagnetic.

Quantum Spin Hall Effect in Inverted Type-II Semiconductors

The quantum spin Hall (QSH) state is a topologically nontrivial state of quantum matter which preserves time-reversal symmetry; it has an energy gap in the bulk, but topologically robust gapless states at the edge. Recently, this novel effect has been predicted and observed in HgTe quantum wells and in this Letter we predict a similar effect arising in Type-II semiconductor quantum wells made from InAs/GaSb/AlSb. The quantum well exhibits an "inverted" phase similar to HgTe/CdTe quantum wells, which is a QSH state when the Fermi level lies inside the gap. Due to the asymmetric structure of this quantum well, the effects of inversion symmetry breaking are essential. Remarkably, the topological quantum phase transition between the conventional insulating state and the quantum spin Hall state can be continuously tuned by the gate voltage, enabling quantitative investigation of this novel phase transition.

High-Power Continuous-Wave Single-Longitudinal-Mode Operation of an Optically Pumped DFB Laser at $\lambda \sim 3.64~ \mu$m

High-power continuous-wave (CW) single-longitudinal-mode emission at 3.64 mum is obtained from an optically pumped distributed-feedback (DFB) laser. The Bragg stopband and two degenerate DFB modes are observed at certain pump powers. The laser incorporates 14 InAs-InGaSb-InAs type-II quantum wells imbedded in an InGaAsSb waveguide. The index-coupled 1-D grating is fabricated in the top clad using interference lithography and plasma etching. A 110-mum-wide stripe from a 1.9-mum CW laser provides both optical pumping and gain guiding. Record high output power of more than 560 mW per side is obtained at 80 K. The wavelength is tunable over a 6.8-nm range by varying the pump power from 1 to 8.1 W.

Optically induced long-lived electron spin coherence in ZnSe∕BeTe type-II quantum wells

The spin coherence of photoexcited electrons in ZnSe∕BeTe type-II quantum wells has been investigated by the time-resolved Kerr rotation technique. Fast and efficient escape of photoexcited holes from the ZnSe layers to the BeTe layers suppresses the electron-hole recombination and their exchange interaction. This effect leads to the formation of dense electrons in ZnSe layers and long electron spin dephasing time reaching a value of 6.1ns at 1.4K.

Spatially direct charged exciton photoluminescence in undoped ZnSe∕BeTe type-II quantum wells

Photoluminescence (PL) spectra occurred as a spatially direct optical transition inside of the ZnSe layer in undoped ZnSe∕BeTe∕ZnSe type-II quantum structures have been studied. We have found that the charged exciton transition was observed at the lower energy side of the exciton transition in the spatially direct PL. The formation of the charged exciton was attributed to the accumulated electrons in the ZnSe layer after the photoexcitation accompanied by the holes being escaped from this well and injected into the BeTe layer.

Type-II InGaN-GaNAs quantum wells for lasers applications

We present a visible III-nitride gain medium based on type-II InGaN-GaNAs quantum well (QW), employing thin dilute-As (∼3%) GaNAs layer. The utilization of GaNAs layer shifts the hole confinement to the center of the type-II QW, which significantly reduces the charge separation effect. The optical gain and spontaneous recombination rate of the type-II InGaN-GaNAs QW are analyzed and compared with those of conventional InGaN QW emitting in the blue regime (λ∼450nm), using six-band k.p formalism for energy dispersion of the III-nitride wurtzite semiconductor. The use of type-II QW leads to significant improvement in the optical gain and spontaneous recombination rate.

Built-in electric field and a new type of charged excitons observed in modulation-doped ZnSe/BeTe type-Ⅱ quantum well

We report results of the photoluminescence, photoluminescence excitation, and magneto-photoluminescence spectra measurements performed on modulation doped ZnSe/BeTe/ZnSe type-Ⅱ quantum well structures at low temperature (2—5 K). The non-doped sample showed two distinct peaks, while the doped one showed one peak only. The linear polarization degree measured in both photoluminescence and the photoluminescence excitation spectroscopy indicates that photoluminescence transitions of an indirect type takes place at respective heterointerfaces. The peak splitting in the non-doped sample was caused by a built-in electric field resulting from the spatial separation of charges along the growth axis. We found optical Shubinkov-de Hass oscillations in both the photoluminescence intensity and the transition energy under a high magnetic field perpendicular to the well. These features and the additional absorption (or reflection) spectra measurements demonstrated that two-dimensional electron gas of a high concentration was formed in the doped samples. All these present the signature of charged excitons and also of a correlated excitonic phase of type-Ⅱ quantum wells.