Quantum-well Active Region Research Articles

Carrier diffusion in long wavelength InGaN quantum well LEDs after injection through V-defects

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar 101¯1 QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6–1 μm and the hole diffusion coefficient ∼0.6 cm2/s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.

Silicon‐Based 850 nm GaAs/GaAsP‐Strained Quantum Well Lasers with Active Region Dislocation Blocking Layers

A silicon‐based room temperature (RT) continuous wave (CW) operation quantum well (QW) laser emitting at 850 nm is reported in this article. By applying the dislocation filter superlattice, the threading dislocation density of the GaAs pseudosubstrate on Si is reduced to 1.8 × 107 cm−2. The metal‐organic chemical vapor deposition‐grown laser structure with GaAs/GaAsP QW and InAlAs active region dislocation blocking layer are fabricated into broad‐stripe Fabry–Perot laser diodes. A typical threshold current and threshold current density of 286 mA and 715 Acm−2 are obtained with 2 mm cavity length and 20 um stripe width samples. A 94.2 mW single‐facet output power lasing around 854 nm and a 0.314 WA−1 slope efficiency is measured under RT CW operation. After a 10‐min aging process, the tested laser can operate stably under continuous operation conditions at RT and the lifetime can be approximated using an exponential fitting curve, indicating a good life reliability of this QW laser.

Time-division multiplexing wireless light communication network

Owing to emission-detection spectral overlap, a quantum well (QW) diode can detect shorter-wavelength photons emitted from another diode sharing the same QW active region. Therefore, a wireless light communication system can be established by using identical QW diodes that function separately as transmitters and receivers. Here, we investigate the irreversibility between light emission and detection of the QW diode and present a time-division multiplexing (TDM) visible light communication (VLC) network using five identical blue QW diodes that are defined by software as transmitters or receivers to achieve real-time TDM-integrated interconnection via the same optical paths. These results indicate the great potential of realizing an advanced TDM VLC network for diverse applications.

Influence of an Overshoot Layer on the Morphological, Structural, Strain, and Transport Properties of InAs Quantum Wells.

InAs quantum wells (QWs) are promising material systems due to their small effective mass, narrow bandgap, strong spin-orbit coupling, large g-factor, and transparent interface to superconductors. Therefore, they are promising candidates for the implementation of topological superconducting states. Despite this potential, the growth of InAs QWs with high crystal quality and well-controlled morphology remains challenging. Adding an overshoot layer at the end of the metamorphic buffer layer, i.e., a layer with a slightly larger lattice constant than the active region of the device, helps to overcome the residual strain and provides optimally relaxed lattice parameters for the QW. In this work, we systematically investigated the influence of overshoot layer thickness on the morphological, structural, strain, and transport properties of undoped InAs QWs on GaAs(100) substrates. Transmission electron microscopy reveals that the metamorphic buffer layer, which includes the overshoot layer, provides a misfit dislocation-free InAs QW active region. Moreover, the residual strain in the active region is compressive in the sample with a 200 nm-thick overshoot layer but tensile in samples with an overshoot layer thicker than 200 nm, and it saturates to a constant value for overshoot layer thicknesses above 350 nm. We found that electron mobility does not depend on the crystallographic directions. A maximum electron mobility of 6.07 × 105 cm2/Vs at 2.6 K with a carrier concentration of 2.31 × 1011 cm-2 in the sample with a 400 nm-thick overshoot layer has been obtained.

Continuous wave interband cascade lasers near 13 μm

We report the demonstration of continuous-wave interband cascade lasers (ICLs) near 13 μm. The attained lasing wavelength of 13.2 μm at 92 K stands as the longest cw emission wavelength ever reported for III-V interband lasers. This achievement is attributed to the adoption of an innovative quantum well (QW) active region comprising strained InAs0.5P0.5 layers in contrast to the commonly used “W” QW active region, showing the potential of the modified QW active region with InAsP layers in improving device performance and extending wavelength coverage of ICLs.

Multiple data streams over a single optical path

Quantum well (QW) diodes have the capability to function as a light-emitting diode or a photodiode and inherently feature a partial emission-detection spectral overlap. Therefore, QW diode can sense light emission from another diode sharing the same QW active region. In association with distributed Bragg reflection (DBR) technique, we here present a wavelength-division multiplexing (WDM) visible light communication (VLC) over a single channel by using vertical assembly of red, green, and blue (RGB) QW diodes. The identical QW diodes separately functioning as a transmitter and a receiver establish a wireless communications link. The DBRs enable the transmission of longer-wavelength photons or the reflection of shorter-wavelength photons, creating an optical bandpass filter in conjunction with emission-detection spectral overlap, effectively boosting the capacity of an initially single communication channel. Both the transmitter and the receiver can be switched freely by software, forming time-division multiplexing (TDM) wireless light communication system using single optical path. We unite TDM and WDM together to demonstrate real-time TDM multichannel bidirectional communication using the vertical integration of RGB QW diodes, offering the great potential to establish TDM-WDM VLC.

Pushing the performance limits of long wavelength interband cascade lasers using innovative quantum well active regions

We report significantly enhanced device performance in long wavelength interband cascade lasers (ICLs) by employing a recently proposed innovative quantum well (QW) active region containing strained InAsP layers. These ICLs were able to operate at wavelengths near 14.4 μm, the longest ever demonstrated for III–V interband lasers, implying great potential of ICLs to cover an even wider wavelength range. Also, by applying the aforesaid QW active region configuration on ICLs at relatively short wavelengths, ICLs were demonstrated at a low threshold current density (e.g., 13 A/cm2 at 80 K) and at temperatures up to 212 K near 12.4 μm, more than 50 K higher than the previously reported ICLs with the standard W-shape QW active region at similar wavelengths. This suggests that the QW active region with InAsP layers can be used to improve device performance at the shorter wavelengths.

Linewidth narrowing in self-injection-locked on-chip lasers

Stable laser emission with narrow linewidth is of critical importance in many applications, including coherent communications, LIDAR, and remote sensing. In this work, the physics underlying spectral narrowing of self-injection-locked on-chip lasers to Hz-level lasing linewidth is investigated using a composite-cavity structure. Heterogeneously integrated III–V/SiN lasers operating with quantum-dot and quantum-well active regions are analyzed with a focus on the effects of carrier quantum confinement. The intrinsic differences are associated with gain saturation and carrier-induced refractive index, which are directly connected with 0- and 2-dimensional carrier densities of states. Results from parametric studies are presented for tradeoffs involved with tailoring the linewidth, output power, and injection current for different device configurations. Though both quantum-well and quantum-dot devices show similar linewidth-narrowing capabilities, the former emits at a higher optical power in the self-injection-locked state, while the latter is more energy-efficient. Lastly, a multi-objective optimization analysis is provided to optimize the operation and design parameters. For the quantum-well laser, minimizing the number of quantum-well layers is found to decrease the threshold current without significantly reducing the output power. For the quantum-dot laser, increasing the quantum-dot layers or density in each layer increases the output power without significantly increasing the threshold current. These findings serve to guide more detailed parametric studies to produce timely results for engineering design.

Low-efficiency-droop c-plane InGaN/GaN light-emitting diodes through the use of thick single quantum wells and doped barriers

Efficiency droop at high current densities is a problem for InGaN-based light-emitting diodes (LEDs), especially for conventional c-plane devices. The large internal electric fields in c-plane quantum wells (QWs) lead to an increase in the active region carrier density (n), causing the electrical efficiency droop onset to occur at low current densities. Here, we present an approach to reduce the internal electric fields (Eint) in c-plane QWs by placing doped p-type and n-type GaN barriers close to the QW. The reduced Eint also allows a thick QW active region design, which helps to lower n to further reduce the droop. The concept of using doped barriers to control Eint is explained using theory and device simulations. Following that, multiple series of thick single QW (SQW) LEDs were grown and characterized. Key parameters in the epitaxial design such as the doping levels and the relative position of the doped barriers were systematically studied and optimized. By using optimized doped barriers and a thick SQW, c-plane LEDs with a low-efficiency droop of 14% at 300 A/cm2 [with respect to the peak external quantum efficiency (EQE)] and a high peak EQE of 49% were demonstrated.

Ultraviolet Electrostatic Field Effect Light-Emitting Diode

The potential of ultraviolet (UV) AlGaN quantum well (QW) light-emitting diodes (LEDs) is stunted due to the lack of efficient p-type doping for this ultra wide bandgap material. In this work, a novel device - UV electrostatic field effect LED (EFELED) is demonstrated to address this issue by significantly improving hole injection into the QW active region which leads to a relative increase of external quantum efficiency (EQE) up to 42%. The UV EFELED is realized by integrating a capacitor layer on top of a conventional 267 nm AlGaN QW LED, which introduces band bending to assist hole injection under a forward bias. A 10x increase in injection current is achieved through the application of a positive bias to the integrated capacitor layer. With 20 V applied to the capacitor, a 42% increase of EQE is achieved. Photon intensity modulation is demonstrated with the application of varying capacitor biases, which presents a novel brightness control at UV wavelength. Thus, the proposed UV EFELED provides significantly improved hole injection through a capacitor integration, which leads to high-efficiency UV LEDs for important applications such as sterilization and purification.

Nanoscale thermal analysis of InGaAs quantum well based semiconductor disk laser with different pump geometry

Using the finite element method, thermal effects including the maximum temperature, the heat flux and the temperature gradient in the active region of semiconductor disk lasers with front and end pumped geometry are numerically analyzed at the first time. Nanoscale thermal conductivities of the multiple quantum wells and the distributed Bragg reflector are used to overcome the underestimate of the temperature rise which comes from the use of the weighted average of the bulk thermal conductivities in the previous works, and the calculated results are compared with the corresponding experiments. The maximum temperature of quantum wells in active region with end pump is always higher than that with front pump under same pump power. Because of its better mode matching, output powers of the end pumped laser are bigger than that of the front pumped laser when the pump power is relatively lower and the thermal rollover of laser has not happened. In comparison, the front pumped laser can tolerate much bigger pump power and produce much higher output power thanks to its better heat dissipation.

M-shaped quantum wells for active region of interband cascade laser

This work proposes an M-shaped design of quantum wells (QWs) based on a combination of InAs and GaInSb materials forming a broken gap type II system, which can be used as an active region of interband cascade lasers emitting in the mid infrared. Results of band structure modelling revealed that compared to more common W-shaped QWs, the M-design can offer some advantages. For instance, there can be obtained a significant electrical tunability of optical transitions oscillator strength and hence also characteristic lifetimes, which would make such structures beneficial in construction of passively mode-locked laser devices.

Diode Lasers with Near-Surface Active Region

Edge-emitting diode lasers with shallow location of a quantum-well active region at a depth of 0.92 μm were developed and their thermal resistance was evaluated from the analysis of true spontaneous emission spectra through the top contact window. Owing to a close spacing between the active region and laser heatsink, effective heat removal is achieved under continuous wave operation that is characterized by a low value of specific thermal resistance of 6 × 10–3 K/(W cm2).

Time of carrier escape and recombination coefficients in InGaN quantum-well active regions of blue, cyan, and green light-emitting diodes

Addressing the recently observed unexpected low-current saturation of differential carrier lifetime (DLT) in the active region of InGaN-based light-emitting diodes (LEDs), a novel procedure is suggested for the evaluation of both escape time of carriers from the InGaN quantum wells and their recombination coefficients. The procedure uses coupled measurements of external quantum efficiency and DLT as a function of LED continuous-wave operating current. The procedure has been applied to three LEDs with InGaN multiple quantum well (QW) active regions emitting blue, cyan, and green light. Laser diodes operating at different wavelengths were employed for small-signal resonant pumping of the LED active regions in order to understand a possible dependence of the escape time and recombination coefficients on the excitation wavelength. The revealed wavelength dependence of the Shockley-Read-Hall recombination coefficient in the green LED is attributed to lateral non-uniformity of the InGaN QW with high Indium content. The radiative and Auger recombination coefficients evaluated by the above procedures in all three LEDs turned out to be close to those reported earlier for the LEDs with single QW active regions. The obtained recombination coefficients are used for further performance comparison of the studied devices. The suggested procedure is helpful for the investigation of carrier escape and the optimization of III-nitride devices, such as photo-detectors, solar cells, and quantum-cascade lasers.

GaN surface as the source of non-radiative defects in InGaN/GaN quantum wells

Blue light-emitting diodes based on III-nitride semiconductors are nowadays widely used for solid-state lighting. They exhibit impressive figures of merit like an internal quantum efficiency close to 100%. This value is intriguing when considering the high dislocation density running throughout the InGaN/GaN quantum well (QW) active region. This striking feature is currently ascribed to carrier localization occurring in the InGaN alloy, which hinders their diffusion toward dislocations. However, it was recently reported that another source of defects, disconnected from dislocations, dramatically decreases the radiative efficiency of InGaN/GaN QWs. Those defects, present at the surface, are usually trapped in an InGaN underlayer (UL), which is grown before the QW active region. To get insight into the trapping mechanism, we varied the UL thickness, In content, and materials system (InGaN or InAlN) and studied the photoluminescence decay time at 300 K of a single InGaN/GaN QW. Our data demonstrate that defects are incorporated proportionally to the indium content in the UL. In addition, we show that those defects are created during the high-temperature growth of GaN and that they segregate at the surface even at low-temperature. Eventually, we propose an intrinsic origin for these surface defects.

234 nm and 246 nm AlN-Delta-GaN quantum well deep ultraviolet light-emitting diodes

Deep ultraviolet (DUV) AlN-delta-GaN quantum well (QW) light-emitting diodes (LEDs) with emission wavelengths of 234 nm and 246 nm are proposed and demonstrated in this work. Our results reveal that the use of AlN-delta-GaN QW with ∼1–3 monolayer GaN delta-layer can achieve a large transverse electric (TE)-polarized spontaneous emission rate instead of transverse magnetic-polarized emission, contrary to what is observed in conventional AlGaN QW in the 230–250 nm wavelength regime. The switching of light polarization in the proposed AlN-delta-GaN QW active region is attributed to the rearrangement of the valence subbands near the Γ-point. The light radiation patterns obtained from angle-dependent electroluminescence measurements for the Molecular Beam Epitaxy (MBE)-grown 234 nm and 246 nm AlN-delta-GaN QW LEDs show that the photons are mainly emitted towards the surface rather than the edge, consistent with the simulated patterns achieved by the finite-difference time-domain modeling. The results demonstrate that the proposed AlN-delta-GaN QWs would potentially lead to high-efficiency TE-polarized surface-emitting DUV LEDs.

Electrical and optical properties of convex-type metamorphic In0.75Ga0.25As/In0.7Al0.3As quantum well structures grown by MBE on GaAs

In0.75Ga0.25As/In0.7Al0.3As quantum well (QW) structures were grown by molecular beam epitaxy on GaAs(0 0 1) substrates by using linear- and convex-graded InxAl1−xAs metamorphic buffer layers (MBLs). Influence of a type of the InxAl1−xAs MBL, as well as design and growth conditions of the QW active region (for the convex-type MBL) on electrical and optical properties of the QW structures was studied. The convex-graded InxAl1−xAs MBL was found to result in obtaining the In0.75Ga0.25As/In0.7Al0.3As QWs with the higher electron mobility in comparison with the linear MBL. Effect of persistent photoconductivity was observed and explained by localization of charge carriers on potential fluctuations caused by the QW interface roughness and In-rich regions formed in the InAlAs barriers and InGaAs QW. Two conductivity channels in the vicinity of the bottom and top interfaces of the In0.75Ga0.25As/In0.7Al0.3As QW were found to exist even for optimized structures grown on convex-graded InxAl1−xAs MBL.

Electrical tuning of the oscillator strength in type II InAs/GaInSb quantum wells for active region of passively mode-locked interband cascade lasers

Two designs of active region for an interband cascade laser, based on double or triple GaInSb/InAs type II quantum wells (QWs), were compared with respect to passive mode-locked operation in the mid-infrared range around 4 µm. The layer structure and electron and hole wavefunctions under external electric field were engineered to allow controlling the optical transition oscillator strength and the resulting lifetimes. As a result, the investigated structures can mimic absorber-like and gain-like sections of a mode-locked device when properly polarized with opposite bias. A significantly larger oscillator strength tuning range for triple QWs was experimentally verified by Fourier-transform photoreflectance.

Performance enhancement of AlGaN-based ultraviolet light-emitting diodes by inserting the last quantum well into electron blocking layer

The characteristics of AlGaN-based ultraviolet light-emitting diodes (UV LEDs) with the last quantum well inserted into electron blocking layer(EBL) have been investigated numerically in this study. The simulation results indicate that the light-emitting EBL(LEEBL) can suppress electron leakage better than traditional EBL due to the superior electron confinement and hole injection of the quantum well in LEEBL. Besides, the LEEBL with a quantum well closer to active region can further improve the performance of the UV LED owning to better electron blocking and hole injection for the quantum wells in active region. As a result, the output power of the UV LED with the quantum well located in the middle of LEEBL increases by 17.83% and the utilization of LEEBL with a quantum well located closer to the active region can further enhance the output power by 54.11% compared with traditional UV LED.

Study of intersubband transitions in GaN-ZnGeN2 coupled quantum wells

In this work, we design and analyze a closely lattice-matched wide bandgap GaN-ZnGeN2 coupled quantum well (QW) structure targeting for near-infrared (IR) (λ ≤ 3 um) intersubband transition for quantum cascade laser applications. The coupled quantum well structure comprised two GaN wells separated by a thin ZnGeN2 barrier layer. The QW active region is surrounded by thick ZnGeN2 layers as barriers. The computations of the electron-phonon and electron-photon scattering rates are carried out by employing the Fermi's golden rule for transitions. The calculation takes into consideration the conservation of energy and momentum in scattering processes. The coupled QW structure is optimized through tuning the confined subband energy levels in the conduction band to achieve (1) electron-LO phonon resonant scattering when the energy separation between the first and second conduction subband levels matches the phonon energy of GaN (92 meV); and (2) dominant electron-photon transition in near-IR between the third and second conduction subband levels.