Different Numbers Of Quantum Wells Research Articles

Carrier density distribution in AlGaAs/GaAs superlattices with different numbers of quantum wells determined by capacitance-voltage profiling

The charge carrier density distribution in uniformly doped AlGaAs/GaAs superlattices with layer thicknesses of 1.5/10 nm and different numbers of quantum wells was studied. Both energy band theory and capacitance–voltage profiling were used to determine the carrier concentration profiles of the structures. During the analysis of the experimental and simulated capacitance–voltage characteristics, it was found that the maximum electron concentrations increase with an increase in the number of quantum wells from ∼ 7.1⋅1016cm−3 for three wells to ∼ 9.3⋅1016cm−3 for 25 wells with an overall superlattice doping level of ∼ 1017cm−3.

Effect of dislocations on the performance of GaSb-based diode lasers grown on silicon

Silicon photonics is a promising technology for the fabrication of dense photonic chips, thanks to the very mature silicon industry. The direct epitaxial growth of III–V lasers on silicon is one of the main challenges for the realization of compact and robust mid-infrared sensors based on photonic integrated circuits. The crystal defects arising from this heteroepitaxial growth affect the laser performance and, therefore, need to be mitigated but also studied to better understand their impact on the laser operation. Here, we studied the effect of threading dislocations on laser performance by comparing the series of GaSb-based diode lasers grown on native GaSb and Si substrates with different numbers of quantum wells (nQW) in their active zones. As expected, the laser threshold currents are higher in the case of diode lasers on Si, and they rapidly vary with nQW. Still, the lowest threshold current densities are achieved with nQW = 1 for both substrates. With the help of a theoretical gain model, we attribute these results to the fact that dislocations create non-radiative recombination but do not introduce additional optical losses. This work allows a better understanding of the origin of performance degradation and the decision to be made regarding the heterostructure design.

Balancing the Number of Quantum Wells in HgCdTe/CdHgTe Heterostructures for Mid-Infrared Lasing.

HgCdTe-based heterostructures with quantum wells (QWs) are a promising material for semiconductor lasers in the atmospheric transparency window (3-5 μm) thanks to the possibility of suppressing Auger recombination due to the no-parabolic law of carrier dispersion. In this work, we analyze the thresholds of stimulated emission (SE) under optical pumping from heterostructures with a different number of QWs in the active region of the structure. Total losses in structures are determined from the comparison of thresholds for the different number of QWs in the active region. It is shown that, thanks to the increased modal gain, a higher number of QWs results in lower threshold pumping intensity and, consequently, higher temperature of SE. These results indicate that improvements to the modal gain can result in a moderate uplift in the temperature of SE from mid-infrared HgCdTe-based heterostructures. On the other hand, at a high enough QW count threshold, the intensity no longer depends on the number of the QWs and is determined by the transparency concentration of a single QW.

Determination of the electron distribution in thin barrier AlGaAs/GaAs superlattices by capacitance-voltage profiling

Electron density distribution in uniformly doped AlGaAs/GaAs superlattices with respective layer thicknesses 1.5/10 nm and a different number of quantum wells was investigated. Experimental samples containing 3, 5 and 25 periods with the same layer parameters were grown by molecular beam epitaxy. Capacitance-voltage profiling was used to determine the carrier concentration profiles in the structures both numerically and experimentally. During the analysis of experimental capacitance-voltage characteristics it was found that the maximum electron concentration increases with an increase in the number of quantum wells starting from 7,1∙1016 сm–3 for 3 wells up to 9,2∙1016 сm–3 for 25 wells with overall superlattice doping level of 1017 сm–3. In some samples saturation areas are observed on the concentration profiles, that are associated with the region of superlattice. Concentration values, obtained from computer modeling, correspond to the experimental data with an error of less than 10 %. Capacitance-voltage profiling is a suitable technique for determining the carrier concentration profiles in thin barrier superlattices. Despite the fact that the method provides distribution of the “apparent” carrier concentration profile, it can be used to estimate the dopant atoms distribution in the strongly coupled quantum well heterostructures.

Effect of number of quantum wells on modulation and distortion characteristics of transistor laser

Numerical analysis on the effect of number of quantum wells (QW) on the modulation and distortion characteristics of 1300 nm Transistor Laser (TL) is reported. The terminal currents are found by using triangular charge model in the base region. The parameters such as threshold current and optical power are analyzed from the DC characteristics for different number of quantum wells. Modulation depth, frequency response are determined by providing an ac input signal with dc bias offset. It is found that the modulation depth decreases for increasing the number of quantum wells and input bias current under CE and CB configuration respectively. From this analysis, the Transistor Laser bandwidth, resonance frequency are estimated for different number of quantum wells and a maximum bandwidth of 27.5 GHz and 44.3 GHz for 25Ith are observed under CE and CB configurations respectively. Further, the distortion characteristics such as second harmonic and intermodulation products are analyzed at 2.4 GHz band for Radio over Fiber applications. The magnitude of 2HD and IMD3 components are found to decrease for increase in number of quantum wells and bias current under both CE and CB configurations. A minimum value of magnitude of 2HD components is obtained as −27.58 dBc at 25IBth and −17.82 dBc at 1.8IEth under CE and CB configuration respectively for six quantum wells in the transistor laser. Similarly, minimum value of IMD3 magnitude is obtained as − 40.47 dBc at 25IBth and − 36.91 dBc at 1.8IEth under CE and CB configuration respectively. A maximum Spurious Free Dynamic Range of 58.85 dB-Hz2/3 is predicted at 10IEth for four number of quantum wells under CE configuration.

Photoreversible Current in InGaN/GaN-Based LED Heterostructures with Different Numbers of QWs

InGaN/GaN-based LED heterostructures with different numbers of quantum wells are investigated by photocurrent spectroscopy in the wavelength range of 350–500 nm. As a result of the analysis of a series of spectra obtained at various p–n-junction biases, the effect of changing photocurrent direction when varying the excitation wavelength (photoreversible effect) is discovered. The range of p–n-junction biases for which this effect is observed in structures with different numbers of quantum wells in the active region is established.

Фотореверсивный ток в светодиодных гетероструктурах на основе InGaN/GaN c разным количеством квантовых ям

Abstract InGaN/GaN-based LED heterostructures with different numbers of quantum wells are investigated by photocurrent spectroscopy in the wavelength range of 350–500 nm. As a result of the analysis of a series of spectra obtained at various p – n -junction biases, the effect of changing photocurrent direction when varying the excitation wavelength (photoreversible effect) is discovered. The range of p – n -junction biases for which this effect is observed in structures with different numbers of quantum wells in the active region is established.

Effects of a Si-doped InGaN Underlayer on the Optical Properties of InGaN/GaN Quantum Well Structures with Different Numbers of Quantum Wells.

In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. When the number of quantum wells is low then the room temperature internal quantum efficiency can be dominated by thermionic emission from the wells. This can occur because the radiative recombination rate in InGaN polar quantum wells can be low due to the built-in electric field across the quantum well which allows the thermionic emission process to compete effectively at room temperature limiting the internal quantum efficiency. In the structures that we discuss here, the radiative recombination rate is increased due to the effects of the Si-doped underlayer which reduces the electric field across the quantum wells. This results in the effect of thermionic emission being largely eliminated to such an extent that the internal quantum efficiency at room temperature is independent of the number of quantum wells.

Simulation and fabrication of N-polar GaN-based blue-green light-emitting diodes with p-type AlGaN electron blocking layer

N-polar GaN-based blue-green light-emitting diodes (LEDs) with p-AlGaN electron blocking layer (EBL) were numerically investigated by simulation and experimentally grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. By numerical simulation, we can find that p-AlGaN EBL in N-polar LEDs is able to play a more important role in blocking electron overflow than that in Ga-polar LEDs due to the reversed polarization, which leads to a high output power and internal quantum efficiency. Besides, the holes injection efficiency is enhanced in N-polar LED, resulting in a lower turn-on voltage. In experimental studies, N-polar LEDs based on different numbers of quantum wells were grown on vicinal C-face n-SiC substrates. When a forward bias is applied to the epitaxial N-polar LED with two quantum wells, a strong blue-green emission located at 480 nm can be observed. This work indicates that N-polar group-III nitrides have great potential in the application of optoelectronic devices.

The ABC model of recombination reinterpreted: Impact on understanding carrier transport and efficiency droop in InGaN/GaN light emitting diodes

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region.

Spectral dynamical behavior in two-section, quantum well, mode-locked laser at 1.064 μm**Project supported by the National Basic Research Program of China (Grant Nos.~2013CB933304 and 2012CB932701), the National Natural Science Foundation of China (Grant Nos. 61274125 and 61435012), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB01010200).

In this study, two-section mode-locked semiconductor lasers with different numbers of quantum wells and different types of waveguide structures are made. Their ultrashort pulse features are presented. The spectral dynamical behaviors in these lasers are studied in detail. In the simulation part, a two-band compressive-strained quantum well (QW) model is used to study thermally induced band-edge detuning in the amplifier and saturable absorber (SA). A sudden blue shift in laser spectrum is expected by calculating the peak of the net gain. In the experiment part, the sudden blue shift in the emission spectrum is observed in triple QW samples under certain operating conditions but remains absent in single QW samples. Experimental results reveal that blue shift phenomenon is connected with the difference between currents in two sections. Additionally, a threshold current ratio for blue-shift is also demonstrated.

Impact of strain on periodic gain structures in vertical external cavity surface-emitting lasers

In this article, the impact of strain relaxation on the emission properties of InGaAs/GaAs multiple quantum wells without strain compensation was examined. Structures consisting of different numbers of quantum wells, namely 4, 8, 12 and 16, on top of distributed Bragg reflectors were grown by molecular beam epitaxy as a typical vertical external cavity surface-emitting laser (VECSEL). The relation between emission parameters in the lasing regime and strain relaxation were investigated. A two-step control of the growth rate allowed for obtaining fixed spectral detuning in all structures regardless of the number of quantum wells. The heterostructures varied in its strain and the microcavity length. The other parameters remained unchanged. In consequence, for the first time a unique set of VECSEL-like heterostructures was investigated. The strain was analyzed by reciprocal space mapping using high-resolution X-ray diffractometry. It was found that the degree of structure relaxation caused by misfit dislocation generation depends linearly on the number of quantum wells. By fitting numerical simulations to the experimental results, we have quantitatively determined the extent to which output power was suppressed by increase in non-radiative recombination arising from misfit dislocations. The non-radiative coefficients were determined. Taking output power as a criterion, we determined the optimal number of QWs to be 12 and the maximum tolerable relaxation value of 0.27 for InGaAs/GaAs VECSEL structures with uniformly distributed quantum wells in microcavity. The dependence of the monomolecular recombination coefficient on structure relaxation has been determined.

Effects of the number of quantum wells on the performance of near-ultraviolet light-emitting diodes

Near-ultraviolet light-emitting diodes (NUV LEDs) with different numbers of quantum wells (QWs) were grown by using metal-organic chemical vapor deposition (MOCVD) to investigate the effects of the number of QWs on the performance of NUV LEDs. With increasing number of QWs from 5 to 7 when using the same quantum well growth process, the normalized external quantum efficiencies (EQE) of 6- and 7-QW NUV LEDs are increased by 19.7% and 30.4%, respectively at 35 A/cm2 compared with that of the 5-QW NUV LED. As the number of QWs is increased from 5 to 7, the forward voltage at 350 mA is decreased from 3.94 V to 3.78 V and 3.77 V. The red shift of the peak wavelength is also decreased with increasing number of QWs. These data suggest that the improved EQE, the reduced peak wavelength shift, and the improved electrical characteristics are due to an increase in effective active volume with increasing number of QWs in NUV LEDs.

Optimal number of quantum wells for blue InGaN/GaN light-emitting diodes

Blue InGaN/GaN light-emitting diodes with different numbers of quantum wells (QWs) are numerically investigated. With increase of QW number, the output power and forward voltage are both increased consistently. In terms of power conversion efficiency, the optimal number of QWs increases with the increase of injection currents and is twelve at experimental current of 42 A/cm2. Simulation results show that the bias voltage in QW plane is reduced with increase of QW number. This effect improves the current spreading and uniformity of carrier distribution in QW plane which results in the reduction of electron leakage and enhancement of emission efficiency.

Large-Signal Analysis of a Transistor Laser

Small- and large-signal analyses of transistor lasers (TLs) are demonstrated for 0.98-μm wavelength GaInAs/GaAs and 1.3-μm AlGaInAs/InP systems. The modulation bandwidth of the TL was larger than that of a laser diode due to the lower damping effect in the former. Comparisons between TLs with different numbers of quantum wells indicated that a large signal response and high modulation bandwidth could be realized simultaneously. However, in the case of large-signal analysis, the calculated eye diagrams were degraded by a resonance oscillation peak. By changing structural parameters such as the facet reflectivity and by controlling the damping effect, the resonance frequency peak was suppressed and clear eye diagrams of >;40 Gb/s were obtained.

Effect of low energy electron irradiation on optical properties of InGaN/GaN light emitting structures

AbstractThe effect of low energy e‐beam irradiation on the InGaN related cathodoluminescence in the MQW InGaN/GaN LED structures with different number of quantum wells has been studied. It is shown that the e‐beam exposure leads to an increase of emission intensity and to a formation of new blue‐shifted emission bands. It is shown that the changes in the optical properties are determined by the excess carrier injection. The changes observed were associated with the local changes of InGaN content due to In and/or Ga diffusion stimulated by the excess carrier recombination (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study of the effect of changes in the number of quantum wells of dual-wavelength LED

A two-dimensional simulation of electrical and optical characteristics of the dual-wavelength LED with different numbers of quantum wells is conducted with APSYS software. The results show that the increase of the number of quantum wells will cause uneven distribution of hole concentrations. Therefore, the increase in the number of quantum wells cannot effectively enhance carrier recombination rate, internal quantum efficiency and luminous intensity. Furthermore, it will lead to the rising of threshold voltage and affect the energy conversion efficiency.

Temperature-dependent excitonic absorption in long-period multiple In x Ga1 − x As/GaAs quantum well structures

Temperature variations in the fundamental absorption edge of long-period InxGa1 − xAs/GaAs structures are studied for samples with different numbers of quantum wells and similar periods. The quantum wells were close in composition and width. Experimental data are interpreted in the model of exciton-polariton light transfer involving localized excitons in confined structures with a finite number of quantum wells. The experimentally observed low-temperature anomaly of the integrated absorption coefficient is attributed to reemission of resonance localized excitons along a finite chain of quantum wells, with no excitonic transfer. The radiative decay time of an exciton in a single quantum well is estimated from the experimental data. It is demonstrated that, at low temperatures, the major contribution to the width of the experimentally observed absorption line corresponding to the ground heavy-hole exciton state is made by inhomogeneous broadening of the line by the field of potential fluctuations associated with the compositional disorder of the alloy. At low temperatures, the inhomogeneous broadening is much more pronounced than the broadening governed by the true radiative and nonradiative dissipative decay.

Effect of normal and reversed polarizations on optical characteristics of ultraviolet-violet InGaN laser diodes

The optical characteristics of ultraviolet-violet InGaN laser diodes with different numbers of quantum wells under normal and reversed polarizations are numerically investigated. For the laser structures under normal polarization, the lowest threshold current is obtained when the number of quantum wells is two in the spectral range of 380–408 nm. For the laser structures under reversed polarization, the single quantum-well laser structure possesses the lowest threshold current. The simulation results suggest that the physical origin for these phenomena is caused by the sufficiently suppressed electron and hole leakage currents when the laser diode is under reversed polarization.

MOVPE growth of InGaAs/GaAsP-MQWs for high-power laser diodes studied by reflectance anisotropy spectroscopy

We have intensively studied the growth and strain behaviour of In x Ga 1− x As multi-quantum wells (MQWs) for application in high-power laser diodes. Their growth in metal-organic vapour-phase epitaxy (MOVPE) was monitored by reflectance anisotropy spectroscopy (RAS). The influence of indium content variation in the InGaAs quantum wells and strain compensation using GaAsP layers on the reflectance anisotropy for single and multi-quantum wells has been studied. A clear dependence of the RAS signal on the indium and phosphorus content has been found. Furthermore, the beginning of strain relaxation during growth can be monitored by a difference in the RAS signals of the GaAs layers below and above the MQW structure. With optimised growth conditions complete laser structures have been grown. Laser diodes with very small far-field angles and different numbers of quantum wells were processed into broad-area lasers, which show output powers up to 60 W in pulse mode.