Investigation of HgCdTe waveguide structures with quantum wells for long-wavelength stimulated emission

The photoluminescence and stimulated emission during interband transitions in quantum wells based on HgCdTe placed in an insulator waveguide based on a wide-gap CdHgTe alloy are studied. Heterostructures with quantum wells based on HgCdTe are of interest for the development of long-wavelength lasers in the range of 25–60 μm, which is currently unattainable for quantum-cascade lasers. Optimal designs of quantum wells for attainment of long-wavelength stimulated emission under optical pumping are discussed. It is shown that narrow quantum wells from pure HgTe appear to be more promising for long-wavelength lasers in comparison with wide (potential) wells from the alloy due to the suppression of Auger recombination. It is demonstrated that molecular-beam epitaxy makes it possible to obtain structures for the localization of radiation with a wavelength of up to 25 μm at a high growth rate. Stimulated emission is obtained for wavelengths of 14–6 μm with a threshold pump intensity in the range of 100–500 W/cm² at 20 K.     

Increased radiative recombination of AlGaN-based deep ultraviolet laser diodes with convex quantum wells

An AlGaN-based deep ultraviolet laser diode with convex quantum wells structure is proposed. The advantage of using a convex quantum wells structure is that the radiation recombination is significantly improved. The improvement is attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes, which results in an increased hole injection efficiency and a decreased electron leakage into the p-type region. Particularly, comparisons with the convex quantum barriers structure and the reference structure show that the convex quantum wells structure has the best performance in all respects.     

Side-mode suppression mechanism of gain-coupled DFB lasers with periodically etched quantum wells

The effect of the gain and index coupling on the side-mode suppression ratio (SMSR) is studied for gain-coupled DFB lasers with periodically etched quantum wells. An accurate expression for the SMSR based on the amplified spontaneous emission model is used with the local-normal-mode transfer-matrix method. The mechanism for the strong single-mode stability of the gain-coupled DFB lasers is explained by the difference between the effective gain and loss of the Bloch waves in the grating structures. This new view clearly shows the advantage of the gain-coupled DFB lasers in terms of single-mode stability.     

Proposed enhancement of side-mode suppression ratio in /spl lambda//4 shifted distributed feedback lasers with nonuniform diffused quantum wells

An enhancement of the side mode suppression ratio, by utilizing interdiffused quantum wells, of a /spl lambda//4 shifted distributed feedback laser is demonstrated theoretically. It is found that by introducing a diffusion step along the longitudinal direction of the quantum-well active region, the suppression ratio can be improved significantly for large /spl kappa/L (>2.6) devices. The maximum power for single longitudinal mode operation is increased by more than 50 mW.     

多量子阱有效折射率偏振相关性研究

分析了多量子阱材料各参数对其TE模和TM模有效折射率的影响。结果表明:阱数增多,多量子阱有效折射率降低,当量子阱数目大于3时,其有效折射率的变化不明显。垒厚增加,有效折射率略有降低。存在合适的张应变量使TE模和TM模有效折射率峰值波长接近的同时,折射率差值整体最小,偏振相关性最小。据此提出多量子阱材料有效折射率低偏振相关设计方法,并设计出C波段内(1530~1565 nm)折射率低偏振相关的InGaAs/InGaAsP多量子阱材料。研究结果有助于设计实用化的有效折射率低偏振相关量子阱材料。     

Ridge waveguide sampled grating DBR lasers with 22-nmquasi-continuous tuning range

We demonstrate a ridge waveguide sampled-grating distributed-feedback laser with continuous wavelength coverage over a 22-nm tuning range, the largest ever reported for a ridge waveguide structure. The design is based on a 400-nm-thick 1.4-μm bandgap waveguide optimized for carrier injection tuning with offset quantum wells used to form the active region. The offset quantum wells enabled the device to be fabricated with only a single metal-organic chemical vapor deposition regrowth step. By tuning both mirror sections and the phase control section we were able to obtain 27 wavelength-division-multiplexed channels spaced at 100 GHz and precisely centered on the ITU grid with equal output power and greater than 40 dB of sidemode suppression ratio     

Simulation of solar cells with quantum wells and comparison with conventional solar cells

The efficiency of photoconversion in solar cells based on GaAs with InGaAs quantum wells under the AM 1.5 conditions for various levels of base doping has been simulated using the software package Sim-Windows. The results obtained are compared with the efficiency of photoconversion in conventional solar cells. It is shown that solar cells with quantum wells can exhibit a fairly high efficiency of photoconversion in comparison with the photoconversion efficiency of conventional solar cells under the following conditions: (i) the lifetimes of for charge carriers in the quantum wells are longer than those in the barrier material and (ii) the level of doping of the base is not very high. It is established that the maximum efficiency of photoconversion in conventional solar cells is higher than the photoconversion efficiency in solar cells with quantum wells. This efficiency is attained at high doping levels in the base (～3 × 10¹⁸ cm^?3 at the parameters used in calculations). This is related to a more intense radiative recombination and also to specific features of screening and charge transport in solar cells with quantum wells at high doping levels. It is shown that, at fairly large values for the degree of concentration of incident radiation, the values for the photoconversion efficiency in solar cells with quantum wells for the low and high levels of doping of the base come closer to each other.

     

Calculation of photogenerated carrier escape rates from GaAs/AlGa1-xAs quantum wells

We present a new theory for photogenerated carrier escape rates from single quantum wells, as a function of an applied electric field, that includes thermionic emission, direct tunneling, and tunneling via thermal occupation of upper subbands, and compare the results for GaAs/Al_xGa_1-xAs quantum wells with recent experiments. We account for the two dimensional (2D) density of states below the barrier, assume thermal equilibrium of carriers within the well, allow for the possibility of strain in the well and/or barrier, and include the contribution to electron thermionic emission from indirect conduction band minima. Our expressions for thermionic emission reduce, in the limit of large well width, to those derived by assuming a three-dimensional (3D) density of states. The results for electron emission from GaAs/Al_xGa_1-xAs quantum wells with x=0.2 and x=0.4 barriers at room temperature agree well with experiment. For wells with x=0.2 barriers, thermally assisted tunneling overtakes thermionic emission around 40 kV/cm, while for wells with x=0.4 barriers thermionic emission from the L valley conduction band minima dominates for fields less than 70 kV/cm. For holes we show that the escape rates are very sensitive to the in-plane effective masses, and results using simple expressions for the in-plane masses that do not include light/heavy-hole mixing agree poorly with experiment. The agreement with experiment is improved using in-plane masses that include light/heavy-hole mixing, particularly for wells with high barriers. We suggest that agreement with experiment would be improved by using more accurate in-plane hole masses for all of the subbands     

Influence of segregation effects on the electroluminescence spectra of InGaAs/GaAs quantum well heterostructures grown by H-MOVPE

Surface segregation during epitaxial growth of stressed InGaAs/GaAs quantum-well heterostructures significantly distorts the nominal concentration profile of quantum wells. The consideration of the effect for growth conditions and elastic stresses appearing during epitaxy on segregation made it possible to simulate the concentration profile with a high accuracy and to calculate the electroluminescence wavelength of actual InGaAs/GaAs heterostructures with quantum wells. It was shown that the observed effect of the long-wave-length shift for the interband transition wavelength in the important case of heterostructures with two neighboring quantum wells is caused by the influence of elastic stresses during growth.     

GaInAsP /InP阶梯量子阱中氢施主杂质束缚能

在有效质量近似下,利用变分法对GaxIn1-xAsyP1-y /InP阶梯量子阱中氢施主杂质束缚能进行了理论计算,并研究了外加电场和阶梯阱的高度对阶梯量子阱中氢施主杂质电子态特性的影响。计算结果显示当施主杂质位于阶梯量子阱的中心时,束缚能达到最大值;外加电场使得电子波函数从阱中心偏移,引起束缚能的非对称分布;Ga 与 As组分的变化使得阶梯阱的势能高度发生变化,从而明显的影响阱中氢杂质束缚能。计算结果对一些基于半导体阶梯型量子阱的光电子器件的设计制作有一定的指导意义。     

Studies of physical phenomena in semiconductor nanostructures using samples with laterally nonuniform layers: Photoluminescence of tunneling-coupled quantum wells

The previously suggested spectral-correlative method for studying nanostructures is applied to an analysis of photoluminescence of tunneling-coupled and isolated quantum wells in structures with laterally nonuniform layers. This method made it possible to use a single wafer to study the dependences of intensities of photoluminescence lines and their energy positions on the tunneling-barrier width for a system of tunneling-coupled GaAs-InGaAs-GaAs wells and on the quantum-well widths in a system of isolated AlGaAs-GaAs-AlGaAs quantum wells. Good agreement between the results of calculations and experimental data can be attained if it is assumed that a constant transverse electric field affecting the processes of trapping of charge carriers by quantum wells exists in a structure with tunneling-coupled quantum wells. The dependence of photoluminescence parameters on the width of isolated quantum wells is sensitive to the profile of heterointerfaces and to the processes of trapping the charge carriers by quantum wells.     

1.5 μm λ/4 shifted multiple quantum well distributedfeedback laser diodes

1.5 μm λ/4 shifted multiple quantum well distributed feedback laser diodes have been achieved for the first time. A characteristic temperature value for a threshold current at around room temperature was as high as 88 K. Spectra at 0.9 times the threshold current showed substantial TM mode suppression. The MQW active region consists of four GaInAs wells (75 Å thick) and GaInAsP barriers (λ_g=1.15 μm, 150 Å thick) grown by metalorganic vapour phase epitaxy (MOVPE). 1.3 μm GaInAsP was grown as an optical guide layer     

Refractive index modulation based on excitonic effects inGaInAs-InP coupled asymmetric quantum wells

The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm³/J at a wavelength of 1.55 μm. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials     

On the permanent dipole-induced two-level Raman scattering insemiconductor quantum wells in electric field

The permanent dipole-induced two-level Raman scattering in semiconductor quantum wells in a static electric field is analyzed. Calculations performed for GaAs/Al_xGa_1-xAs multiple quantum wells (MQWs) indicate that reasonable, but not high, Stokes wave amplification could be obtained, and eventually used for IR generation. The gain is limited primarily by reststrahlen absorption and a rather large transition linewidth     

Effect of the number of quantum wells in the active region on the linearity of the light-current characteristic of a semiconductor laser

The light-current characteristic of a semiconductor laser with multiple quantum wells (QWs) is calculated, with the delayed capture of charge carriers from the waveguide region into the wells taken into account. It is shown that increasing the number of QWs is a more effective way to improve the power characteristics of a laser, compared with an increase in the velocity of carrier capture into each of the wells. For example, using two QWs as the active region leads to a substantial increase in the internal quantum efficiency of stimulated emission and to a significantly better linearity of the light-current characteristic of the laser, compared with a single-well structure. At the same time, using three or more QWs only slightly improves the power characteristics of the laser, compared with the double-well structure. Thus, a double-well structure is the optimal as regards high output power and simplicity of growth.     

Enhancement of the Stark effect in coupled quantum wells foroptical switching devices

Significant enhancement of the Stark effect on the electronic state and the optical dipole moments of coupled quantum wells is shown theoretically. The multiband effective mass theory (k&oarr;-p&oarr;), which takes into account coupling between heavy- and light-hole states of the coupled quantum wells is used. Mixing of states in the coupled quantum wells leads to the splitting of subband energy levels. An applied electric field causes repulsion between the split levels as well as the spin-splitting of the valence-subband structure. Comparison with the single quantum well shows that the optical dipole moment is substantially more reduced for the coupled quantum wells at the same electric field because of enhanced charge separation in this structure. A variational method is used to solve the exciton problem in coupled quantum wells. Calculated exciton peak positions versus electric field show very good agreement with recent experiments. Calculated exciton absorption spectra for the ground state show the quenching of the exciton peak at F=30 kV/cm at 5 K. These results may have interesting applications to low-voltage optoelectronic switching devices based on the quantum-confined Stark effect     

Enhanced interband-resonant light modulation byintersubband-resonant light in selectively n-doped quantum wells

The interband-resonant light modulation by the intersubband-resonant light in selectively n-doped quantum wells is investigated. The modulation efficiency depends greatly on the degree of nonlinear optical coupling between the interband and intersubband-resonant lights. It is shown theoretically and experimentally that the selective n-doping in the barrier layers of the quantum wells is very effective to increase the nonlinear coupling degree and thus the modulation efficiency. The thermal and the hot carrier effects on the modulation are also discussed     

Thermoelectric properties of symmetric and asymmetric double quantum well structures

The electronic states and carrier transport in (100)PbTe/Pb _{1 ? x} Eu _x Te double quantum wells are theoretically analyzed. The dependences of the mobility and Seebeck coefficient on the thickness of the internal barrier in symmetric and asymmetric structures are investigated. It was found that at great distance between the wells even small violation of the structure symmetry and essential reconstruction of electron wave functions results in suppression of intersubband scattering with carriers transfer between the wells and provides the correct limit to isolated quantum well in kinetic coefficients. Some possibilities of increasing the thermoelectric power factor are found, and a suitable set of structure parameters is calculated within the proposed model.     

Experimental Study of Spontaneous Emission in Bragg Multiple- Quantum-Well Structures with InAs Single-Layer Quantum Wells

The time-resolved photoluminescence of a Bragg structure formed by InAs single-layer quantum wells in a GaAs matrix is investigated experimentally. The comparison of photoluminescence spectra recorded from the edge and the surface of the sample indicates that Bragg ordering of the quantum wells leads to substantial modification of the spectra, in particular, to the appearance of additional modes. The spectrum recorded at the edge of the sample features a single line corresponding to the exciton ground state. In the spectrum recorded at the surface, an additional line whose frequency and propagation angle correspond to the Bragg condition for quantum wells, appears at high excitation levels. The calculation of the modal Purcell factor explains the fact that spontaneous emission is enhanced only for specific propagation angles and frequencies, rather than for all angles and frequencies satisfying the Bragg condition.     

Effects of well number, cavity length, and facet reflectivity onthe reduction of threshold current of GaAs/AlGaAs multiquantum welllasers

The optimum design for reducing the threshold current of GaAs/AlGaAs multiquantum well lasers is determined experimentally. The lowest threshold current density is realized by using single and multiquantum wells at long and short cavity lengths, respectively. The threshold current has a minimum at the optimum cavity length: the minimum threshold current is smaller for a larger number of quantum wells, and the optimum cavity length is inversely proportional to the number of wells. Experiments are compared to the theory developed by P.W.A. McIlroy, et al. (ibid., vol.21, no.12, p.1958-63, 1985) and limiting performances of quantum well lasers with various numbers of wells are presented. The reduction of the threshold current by high reflectivity coatings is also demonstrated, and a threshold current as low as 1.86 mA at 15°C is reported