Heavy-hole Confinement Research Articles

Influence of Mg on the temperature‐dependent optical properties of CdTe quantum dots embedded in Zn 0.7 Mg 0.3 Te

We compare the temperature-dependent optical properties of CdTe quantum dots with and without Mg in their Zn(Mg)Te barriers. The difficulty of forming CdTe dots on Zn0.7Mg0.3Te barriers (due to the decrease of the lattice mismatch) has been overcome by using a new technique based on an efficient reduction of the surface energy. Mg incorporation in the barriers leads to a better heavy-hole confinement along the growth axis, which is manifested in PL studies by both an extension of the radiative regime temperature range and a strong increase of the activation energy for the non-radiative mechanisms. However, the in-plane confinement is less enhanced, which leads to progressive inter-dot carrier transfers with increasing temperature, as evidenced directly by the analysis of photoluminescence intensities for different single dots. Our temperature-dependent data (time-resolved and micro-photoluminescence) indicate that this transfer consists of a thermally activated process via the two-dimensional wetting-layer states. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-Sensitivity Analysis of 1360-nm Dilute-Nitride Quantum-Well Lasers

The N-content of an InGaAsN quantum-well (QW) laser is found to dramatically affect the temperature sensitivity of the current injection efficiency (/spl eta//sub inj/) and material gain parameter (g/sub oJ/). The increased temperature sensitivity of /spl eta//sub inj/ and g/sub oJ/ of InGaAsN QW lasers with increasing N-content leads to a significant increase in their temperature sensitivity of threshold current and external differential quantum efficiency. Increasing the N-content of the InGaAsN QW potentially results in a reduction of the heavy hole confinement, which may account for the increased temperature sensitivity of the current injection efficiency.

CdTe/Zn 1−x Mg x Te self-assembled quantum dots: Towards room temperature emission

We report the dependence of the growth and the optical properties of self-assembled CdTe/Zn1−xMgxTe quantum dots on the barrier Mg content x (0⩽x⩽0.3). Due to the decrease of the lattice mismatch between CdTe and Zn1−xMgxTe with increasing x, we use a technique for inducing dot formation, based on efficient reduction of the surface energy by deposition of amorphous Te, which is then desorbed. Mg incorporation in the barriers leads to a better heavy-hole confinement along the growth axis, which is manifested in photoluminescence (PL) studies by both an extension of the radiative regime temperature range (up to 150 K for 30% Mg) and a strong increase of the activation energy for the nonradiative recombination. However, the in-plane confinement is less enhanced, which allows observation of interdot carrier transfer with increasing temperature, as evidenced directly by the analysis of PL intensities for different single dots. Our temperature-dependent data (time-resolved and microphotoluminescence) suggest that this transfer consists of a thermally activated process via the two-dimensional wetting-layer states rather than a direct tunneling (hopping) process.

Selectivity control of Stark-ladder transitions in asymmetric double-well GaAs/AlAs superlattices by barrier sequence modulation

We have investigated field-induced features of optical transitions in asymmetric double-well superlattices (ADW-SLs) with and without barrier sequence modulation by low-temperature photocurrent (PC) spectroscopy. The difference between the heavy-hole confinement energies in the wide and narrow wells results in two types of Stark-ladder transitions, so that four ±1st-order spatially indirect transitions are expected to exist in the ADW-SL. The oscillator strength of these indirect transitions is affected by the strength and the direction of coupling between the wide and narrow wells. The introduction of the barrier sequence modulation into the ADW-SL realizes the control of these coupling mechanisms. This technique is a new method to modulate the oscillator strength of the indirect Stark-ladder transitions.

Theoretical Comparison of (111) and (100) GaAs/AlGaAs p-Type Quantum Well Infrared Photodetectors

Intersubband absorptions in p-type GaAs/Al0.3Ga0.7As quantum wells (QWs) grown on (111) substrates are theoretically investigated using the multiband effective mass model. Because of the stronger band mixing of (100) QW, the peak absorption coefficient of the (100) QW is about twice as large as that of (111) QW. Nevertheless, the detectivity of (111) Quantum Well Infrared Photodetector (QWIP) is found to be ∼30% larger than that of (100) QWIP because of the smaller dark current of (111) QWIP. This is due to the smaller heavy-hole confinement energy in (111) QW.

Thermally activated carrier escape mechanisms from InxGa1-xAs/GaAs quantum wells.

The temperature dependence of the photoluminescence (PL) intensity of strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs single-quantum-well (QW) structures grown by metalorganic vapor-phase epitaxy is investigated. We adopt the theoretical model recently proposed by Vening, Dunstan, and Homewood [Phys. Rev. B 48, 2412 (1993)] to describe the variation of the QW PL intensity with temperature. The Arrhenius behavior of the PL intensity at the highest temperatures investigated for each structure is shown to be due to the thermally activated escape of electron-hole pairs from the well. In each case, the deviation of the experimental data at intermediate temperatures is accounted for by assuming the dominant nonradiative recombination process to be the escape of the less-confined carrier species from the QW. The existing model is modified to include this process. The activation energies obtained are in reasonable agreement with the calculated heavy-hole confinement energies assuming a conduction-band offset ratio of ${\mathit{Q}}_{\mathit{c}}$\ensuremath{\approxeq}0.83.

DMS based quantum wells with magnetically tuned heavy hole confinement

We discuss the results of a magneto-optical study of ZnFeSe/ZnSe/ZnFeSe and ZnMnSe/ZnSe/ZnMnSe quantum well structures grown by molecular beam epitaxy on GaAs. In both systems the zero field heavy hole offset is near zero and the heavy hole confinement is controlled by an externally applied magnetic field which produces large spin splittings in the diluted magnetic semiconductor (DMS) layers. The changes in the band structure induced by the magnetic field result in a pronounced asymmetry of the heavy hole exciton spin components as the barrier and well hole populations become spin-polarized. Both the iron-and the mangenese-based structures show similar behavior even though ZnFeSe is a Van Vleck paramagnet while ZnMnSe is a Brillouin paramagnet.

Resonant reflectivity study of exciton oscillator strength in CdTe/(Cd,Mn)Te quantum wells and superlattices

The exciton oscillator strength in CdTe/Cd 0.75Mn 0.25Te quantum wells and superlattices as a function of well width or period is studied by resonance exciton reflection spectroscopy. A strong increase of the oscillator strength by a factor of 30 is found when the quantum well width decreased down to 15 Å due to exciton confinement. The oscillator strength decreases in thinner wells as a result of substantial penetration of the exciton wave function into the barrier layers. A similar nonmonotonic behaviour is demonstrated for superlattices where at short periods the electron wave function changes its character from a 2D subband to a 3D miniband. The role of an additional heavy-hole confinement caused by the electron-hole Coulomb interaction is shown.

Aluminum ion-implantation enhanced intermixing of GaAs-AlGaAs quantum-well structures

We have studied aluminum-implantation enhanced intermixing of GaAs-AlGaAs quantum-well structures using low-temperature photoluminescence. The energy shift of the heavy-hole exciton was determined for Al doses varying from 2×1013 cm−2 to 1×1015 cm−2 after either furnace annealing at 800 °C or optical rapid thermal annealing at 925 °C. A variational calculation yields the diffusion length from the energy shift of the exciton. This shift is due both to the increase of Al in the center of the well and to the change in electron and heavy-hole confinement energies. The ion-implantation enhancement of the diffusion length depends on Al-ion dose but not significantly on annealing time or temperature. This work indicates that Al-ion implantation should be useful for the fabrication of structures of reduced dimensionality by patterned implantation of AlGaAs-GaAs quantum wells.

Band structure engineering for maximal light-hole behaviour in strained quantum well systems

Abstract We show that the highest valence subband can be light-hole-like over a significant energy range (50–100 meV) in the plane of a strained layer superlattice where (i) the light hole and heavy hole confinement wells are in different regions of the superlattice and (ii) the heavy hole confinement well is deeper than the light hole confinement well. The separate confinement minimises the interaction between light hole and heavy hole zone centre states, thus reducing subband non-parabolicity due to state mixing, while the different well depths enhances the splitting between the highest heavy hole and light hole states. (InAlGa)As-(GaAl)As sould be an ideal material system to demonstrate maximal light-hole behaviour, behaviour of significant benefit for high hole mobility and optoelectronic applications.

Manganese luminescence in GaAs/GaAlAs superlattices

Luminescence transitions associated with manganese have been studied in the GaAlAs layers and GaAs-GaAlAs superlattices. In GaAlAs, manganese shows a behaviour characteristic of a centre that is partly deep and partly hydrogenic. In a superlattice, the ionisation energy of manganese increases as a function of the well width: it is 110 meV in GaAs, 132 meV in a 67 AA well and 192 meV in a 13 AA well. The change in ionisation energy can be explained by taking into account both the 'deep' and the hydrogenic character of manganese. Changes in ionisation energies are compared with theoretical calculations of the heavy hole confinement energy.

Axial-strain effects on superlattice band structures

The author has used Harrison's (1980, 1983) universal tight-binding method to calculate the valence band (001) axial deformation potential b in group IV and III-V semiconductors. It is found that b= alpha cp(27.4)/d2- alpha csp(16.95)/d2 where d is the bond length in angstroms and alpha cp and alpha csp are the covalency of p and sp3 bonds respectively. This is in reasonable agreement with experiment. A 1% net axial strain then splits the valence band maximum by about 25 meV in most group IV and III-V semiconductors. The resultant bands are anisotropic, with different effective masses parallel and perpendicular to the strain axis. In a strained-layer superlattice, such as GaAs-InAs, significant strain (to about 5%) can be incorporated in each layer, and the highest hole band can then be a light-hole band. The author discusses how it should be possible to achieve hole-based 'effective-mass superlattices' where the light- and heavy-hole confinement well maxima are in different layers of a strained-layer superlattice.

Manganese Luminescence in GaAs/GaAlAs Superlattices

Luminescence transitions associated with manganese have been studied in the GaAlAs layers and GaAs-GaAlAs superlattices. In GaAlAs, manganese shows a behaviour characteristic of a centre that is partly deep and partly hydrogenic. In a superlattice, the ionisation energy of manganese increases as a function of the well width: it is 110 meV in GaAs, 132 meV in a 67 AA well and 192 meV in a 13 AA well. The change in ionisation energy can be explained by taking into account both the 'deep' and the hydrogenic character of manganese. Changes in ionisation energies are compared with theoretical calculations of the heavy hole confinement energy.