俄歇复合对应变量子阱激光器阈值电流的影响

通过考虑不同因素对压应变和张应变量子阱激光器阈值电流和特征温度的影响,得到了俄歇复合和非俄歇复合对阈值电流起主要作用的转变温度Tc,小于Tc时,主要是非俄歇复合;大于Tc时,主要是俄歇复合,而且张应变量子阱激光器转变温度要比压应变量子阱激光器的转变温度要高;张应变量子阱激光器与压应变量子阱激光器相比,阈值电流更低,特征温度更高。     

Optical investigations of strained InGaAs quantum wells

InGaAs/GaAs MOCVD-grown quantum wells have been investigated. Photoluminescence (PL) measurements have shown heavy-hole-related excitonic transitions within the temperature range from 10 to 100 K for all samples. In room-temperature photoreflectance (PR), sharp heavy- and light-hole excitonic transitions in the quantum wells have been observed. The transition energies obtained have been compared with values derived from theoretical considerations using the envelope function model including lattice-mismatch-related stress. The heavy- and light-hole transitions have been identified as excitonic transitions of types I and II respectively. © 1997 John Wiley & Sons, Ltd.     

Electroabsorption enhancement in tensile strained quantum wells viaabsorption edge merging

Electroabsorption in quantum wells under biaxial tension is investigated theoretically. It is found that enhanced electroabsorption due to a field-induced merging of the light and heavy hole absorption edges can be achieved in these structures at moderate operating fields. Calculations showing this merging and electroabsorption enhancement for In_xGa_1-xAs-InP and GaAs_xP_1-x-Al_0.35Ga_0.65As quantum well structures are described. Trade-offs involving the advantages of merged absorption edges are identified through comparisons of tensile strained modulators utilizing the merging effect of analogous lattice matched structures. Optimal structures for operation at 1.55 μm in In_xGa_1-xAs-InP and 0.77 μm in GaAs_xP_1-x-Al_0.35Ga_0.65As are identified, and the sensitivities of their electroabsorption characteristics to material and structural parameters are examined     

Calculation of the size-quantization levels in strained ZnCdSe/ZnSe quantum wells

The ZnSe and CdSe parameters required to calculate levels in ZnCdSe/ZnSe quantum wells are determined by fitting to published data. The model is shown to be adequate for the example of structures with a collection of quantum wells whose thickness and composition were determined by independent methods. Fiz. Tekh. Poluprovodn. 31, 939–943 (August 1997)     

Band structure of strained quantum wells grown on novel index surfaces

We present a theoretical study of the in-plane valence subband structure of unstrained GaAs---AlGaAs, compressively strained InGaAs---AlGaAs and tensile strained GaAsP---AlGaAs quantum wells grown along the (001), (111), (011) and (113) directions. From numerical solution of the 6 × 6 Luttinger k · p hamiltonian we find that confinement energies, warping and effective in-plane masses strongly depend on the direction of confinement and on strain. Piezoelectric effects further affect the dispersion for the (111) and (113) directions. Besides, we give analytic expressions for the heavy and light hole in-plane and perpendicular effective masses for any (hkl) growth direction in the limit of uncoupled subbands.     

Germanium islands embedded in strained silicon quantum wells grown on patterned substrates

Germanium islands were embedded in strained silicon quantum wells in order to provide an improved electron confinement in vicinity of the islands. Growth was performed on relaxed SiGe layers. Patterned substrates were used, favouring lattice relaxation as well permitting the fabrication of small Ge islands at deposition temperatures above 500 °C. Photoluminescence analysis reveals a strongly reduced dislocation related signal. The low temperature spectra are dominated by intense signals from the germanium islands. The origin of these signals were investigated by removing the islands by etching, analysing reference samples without a silicon quantum well, varying the germanium deposition and the growth temperature.     

Theoretical study of auger recombination processes in deep quantum wells

The basic processes and mechanisms of Auger recombination of nonequilibrium carriers in a semiconductor heterostructure with deep InAs_0.84Sb_0.16/AlSb quantum wells (QWs) are analyzed. It is shown that a zero-threshold Auger recombination process involving two heavy holes predominates in sufficiently narrow QWs, and a resonant process involving two electrons is dominant in wide QWs. The range of QW widths at which the Auger recombination is suppressed in a given structure to the greatest extent (suppression region) is determined. In this case, the threshold process involving two electrons remains the basic nonradiative recombination process, with its probability being several orders of magnitude lower than those for the zero-threshold and resonant mechanisms. In turn, the zero-threshold mechanism involving two electrons is totally impossible in the heterostructure under study because of the large conduction-band offset (which markedly exceeds the energy gap). Also, the range of emission wavelengths that corresponds to the suppression region is estimated. It is shown that the interval calculated belongs to the mid-IR range.     

Growth and optical characterization of strained CdZnTe/ CdTe quantum wells

We have grown strained Cd_1-xZn_xTe(x ≈ 0.2)/CdTe single and multiple quantum wells by molecular beam epitaxy. GaAs was used as a substrate. The well widths were systematically increased until the critical thickness was exceeded. Low-temperature (liquid helium) photoluminescence (PL) spectroscopy was used to characterize the films. Two prominent PL peaks were observed: one arising from the quantum well and the other from the barrier material. The energy of the quantum well luminescence is consistent with theory when strain is included. The critical layer thickness for the CdTe quantum wells was found to be between 150 and 175 å, in agreement with the model of Matthews and Blakeslee.     

Auger recombination effect on threshold current of InGaAsP quantum well lasers

The Auger recombination effect on the threshold current of the InGaAsP quantum well (QW) laser is studied theoretically. All possible transitions between the quantized subbands of two-dimensional carriers are taken into account in evaluating the radiative process with thek-selection rule and the Auger process. The calculated threshold current agrees well with the reported experimental results for 1.07 μm InGaAsP QW lasers. The Auger component of the threshold current and its temperature dependence strongly depend on the QW structure, resulting in the necessity for an elaborate QW structure design, although both cannot be optimized at the same time. A design procedure is elucidated for a structure which gives the lowest threshold current density for the 1.07, 1.3, and 1.55 μm InGaAsP QW lasers.     

Field screening in (111)B InAsP/lnP strained quantum wells

Low-temperature photoluminescence measurements were performed on InAsP/InP strained quantum wells grown on InP (lll)B substrates by gas-source molecular beam epitaxy. The emission energy was observed to increase as the pump-power density increased. This was attributed to the screening of the internal piezoelectric field by photo-generated carriers. The energy shift was as large as 35 meV for an InAs_0.28P_0.72/InP quantum well with a lattice mismatch of ~0.9%. A similar structure with a smaller strain showed saturation of the energy shift with increasing pump-power density. We performed a model calculation which includes the quantum confined Stark effect, and this saturation was correlated with a flat-band structure of the quantum well due to the nearly complete screening of the built-in electric field.     

Enhanced radiative recombination in AlGaN quantum wells grown by molecular-beam epitaxy

Dependences of the intensity of cathodoluminescence in multiple Al_0.55Ga_0.45N/Al_0.45Ga_0.55N quantum wells grown by molecular-beam epitaxy on the growth conditions are studied. An increase (by almost two orders of magnitude) in the intensity of the cathodoluminescence peak with an energy of 4.45 eV is observed as the quantum-well layer grows in the conditions of deep depletion with respect to ammonia. In this case, a tendency towards the mode of three-dimensional growth can be inferred from the pattern of diffraction of high-energy electrons; this effect is interpreted using a model of formation of AlGaN quantum dots.     

Interband Auger recombination in InGaAsP

The interband Auger recombination lifetimes of two Auger processes have been calculated to correlate measured threshold current densities and carrier lifetimes for InGaAsP and InGaAsSb lasers. Good aggreement with experimental data was obtained for lasers with low nominal threshold current densities. These results demonstrate the importance of Auger recombination in the threshold characteristics of InGaAsP/InP lasers.     

Direct observation of piezoelectric fields in GaN/ InGaN/GaN strained quantum wells

Off-axis electron holography is used to examine a single thin InGaN quantum well in GaN viewed in cross-section. The results show a phase offset across the well, which, under weakly diffracting conditions, is an approximately linear function of specimen thickness. This phase offset is ascribed to a change AV0 in the specimen mean inner potential V0 caused by a piezoelectric field induced by misfit strains in the InGaN layer. This paper examines the dependence of the phase offset on the diffracting conditions and on thin foil relaxation effects. It is shown that relaxation is negligible for the film thicknesses involved. Using a range of weakly diffracting conditions, the phase offset is measured as deltaV0/V0 = 0.042+/-0.012. Zone axis convergent beam electron diffraction patterns were taken and compared to simulations to determine the crystal polarity, showing the magnitude of the inner potential increased in the [0001] direction. By using dark-field displacement fringes to measure the InGaN layer thickness, and recent estimates of V0, the magnitude of the piezoelectric field is determined. This paper assesses the accuracy and limitations of electron holography for the studies of electric fields in other GaN structures.     

Comparison of Auger recombination in GaInAs-AlInAs multiple quantum well structure and in bulk GaInAs

Carrier lifetime has been measured in GaInAs-AlInAs multiple quantum well structures and in thick GaInAs samples for local carrier densities between2 times 10^{17}and5 times 10^{19}cm^-3. Carrier lifetime and Auger recombination are found to be very close (±20 percent) in the two structures. The Auger limited T0values calculated for DH and MQW lasers are found to be, respectively, 93 and 170 K. Optimization of the quantum well laser as a function of the number of wells in the active region is discussed.     

Photoluminescence of localized exitons in coherently strained ZnS-ZnSe/GaAs(001) quantum wells

The low-temperature photoluminescence (PL) of ZnS-ZnSe heterostructures grown in the form of single quantum wells (QW) by the non-conventional technology of photo-assisted vapor phase epitaxy has been investigated. It is shown that the inhomogeneity of the quantum wells can be explained in terms of a model based on disordering of the heterointerfaces. It is found that the mobility edge which separates the localized states from the delocalized states is 6 meV below the heavy-exciton ground state in the quantum wells with a nominal width L _z=11 Å.     

Auger recombination in long-wavelength strained-layer quantum-wellstructures

A model calculation of Auger recombination in strained-layer InGaAs-InGaAlAs and InGaAs-InGaAsP quantum-well structures is presented as an extension of an empirical Auger theory based on the effective mass approximation. The valence band effective masses around k_∥=0 are calculated by using a six-band Luttinger-Kohn Hamiltonian and the quasi-Fermi levels are determined with a self-consistent Poisson-Schrodinger solver under the effective mass approximation. Three basic Auger processes are considered with the excited carrier being in a bound state of the quantum well, as well as an unbound state. The empirical model includes Fermi statistics as well as a revaluation of the Coulomb interaction overlap integral in the Auger recombination rate. Bound-unbound Auger transitions are proved to be an important nonradiative recombination mechanism in strained-layer quantum-well systems. Our calculations of Auger coefficient are in reasonable agreement with the experimental data     

Auger recombination rate in InGaAsP lasers

For the Auger recombination rate in InGaAsP there is an order of magnitude discrepancy between the measurements by Su et al. and the calculations by Dutta and Nelson, and Sugimura. It is suggested that a major source of this discrepancy is the method used to calculate the overlap integrals. A calculation that supports this suggestion is described.     

Photoluminescence excitation spectroscopy of InAs0.67P0.33/InP strained single quantum wells

The optical emission characteristics of biaxially compressed InAs _x P_{1− x} /InP strained single quantum well (QW) structures, with nominal compositionx=0.67, have been investigated using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. The highly strained QWs exhibit intense and narrow PL in the 0.9–1.5 μm wavelength range, similar to the lattice-matched InGaAs(P)/InP system. The 20 K PLE spectra exhibit well-resolved features attributed ton=1 heavy hole (E1H1) and light hole (E1L1) transitions in the 1.0–1.5 μm wavelength range. In addition, features attributed to transitions betweenn=2 electrons and heavy holes (E2H2), and betweenn=1 electrons and unconfined holes (E1Hf), were observed. The energy splitting between the heavy-hole and light-hole bands was found to be a sensitive measure of the band offsets in the system. The best prediction of this splitting was obtained for a valence band offset of δE _V ∼0.25δE _G . This value of band offset was in agreement with the energy position of the E1Hf transition. The observed transition energies were also compared with the results of a finite square well model, taking into account the effects of strain, and the results offer further support for the band offset assignment. This study indicates that the InAsP system may be advantageous for application in strained-layer optoelectronic devices operating in the 1.3–1.6 μm wavelength range.     

Enhancement of third-order nonlinear optical susceptibilities incompressively strained quantum wells under the population inversioncondition

Third-order optical nonlinear susceptibilities χ⁽³⁾ in compressively strained and nonstrained InGaAs-InGaAsP quantum wells (QW's) under the population inversion condition are discussed. The small effective mass of compressively strained QW's increases the contribution of the carrier density pulsation effect and the carrier heating effect of χ⁽³⁾. The hole burning effect is also increased due to the decrease of the carrier-carrier scattering rate. The calculation including these effects shows an enhancement of factor 3 due to 0.8% compressive strain. The values of χ⁽³⁾ are experimentally estimated from the data of nondegenerate four-wave mixing in λ/4-shifted distributed feedback lasers. χ⁽³⁾ in 0.8% compressively strained QW's is three times larger than that in nonstrained QW's with the same linear gain