Low Threshold Current Density Research Articles

Increased Efficiency of Current-Induced Motion of Chiral Domain Walls by Interface Engineering.

Magnetic racetrack devices are promising candidates for next-generation memories. These spintronic shift-register devices are formed from perpendicularly magnetized ferromagnet/heavy metal thin-film systems. Data are encoded in domain wall magnetic bits that have a chiral Néel structure that is stabilized by an interfacial Dzyaloshinskii-Moriya interaction. The bits are manipulated by spin currents generated from electrical currents that are passed through the heavy metal layers. Increased efficiency of the current-induced domain wall motion is a prerequisite for commercially viable racetrack devices. Here, significantly increased efficiency with substantially lower threshold current densities and enhanced domain wall velocities is demonstrated by the introduction of atomically thin 4d and 5d metal "dusting" layers at the interface between the lower magnetic layer of the racetrack (here cobalt) and platinum. The greatest efficiency is found for dusting layers of palladium and rhodium, just one monolayer thick, for which the domain wall's velocity is increased by up to a factor of 3.5. Remarkably, when the heavy metal layer is formed from the dusting layer material alone, the efficiency is rather reduced by an order of magnitude. The results point to the critical role of interface engineering for the development of efficient racetrack memory devices.

Reduced threshold current density of GaN-based green laser diode by applying polarization doping p-cladding layer

Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.

Гетероструктуры квантово-каскадных лазеров с неселективным заращиванием методом газофазной эпитаксии

The possibility of fabrication of 4.6 µm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA /cm2.

Emergent Topological Hall Effect at a Charge-Transfer Interface.

Exploring exotic interface magnetism due to charge transfer and strong spin-orbit coupling has profound application in the future development of spintronic memory. Here, the emergence and tuning of topological Hall effect (THE) from a CaMnO3 /CaIrO3 /CaMnO3 trilayer structure are studied in detail, which suggests the presence of magnetic Skyrmion-like bubbles. First, by tilting the magnetic field direction, the evolution of the Hall signal suggests a transformation of Skyrmions into topologically-trivial stripe domains, consistent with behaviors predicted by micromagnetic simulations. Second, by varying the thickness of CaMnO3 , the optimal thicknesses for the THE signal emergence are found, which allow identification of the source of Dzyaloshinskii-Moriya interaction (DMI) and its competition with antiferromagnetic superexchange. Employing high-resolution transmission electron microscopy, randomly distributed stacking faults are identified only at the bottom interface and may avoid mutual cancellation of DMI. Last, a spin-transfer torque experiment also reveals a low threshold current density of ≈109 A m-2 for initiating the bubbles' motion. This discovery sheds light on a possible strategy for integrating Skyrmions with antiferromagnetic spintronics.

Continous-wave lasing operation of 1.3-μm wavelength InP-based photonic crystal surface-emitting lasers using MOVPE regrowth.

We report on electrically driven InP-based photonic-crystal surface-emitting lasers (PCSELs), which possess a deep-air-hole photonic crystal (PC) structure underneath an active region formed by metal-organic vapor-phase-epitaxial (MOVPE) regrowth. Single-mode continuous-wave (CW) lasing operation in 1.3-μm wavelength is successfully achieved at a temperature of 15°C. It is shown that the enhancement of lateral growth during the MOVPE regrowth process of air holes enables the formation of deep air holes with an atomically flat and thin overlayer, whose thickness is less than 100 nm. A threshold current of 120 mA (threshold current density = 0.68 kA/cm2) is obtained in a device with a diameter of 150 μm. A doughnut-like far-field pattern with the narrow beam divergence of less than 1° is observed. Strong optical confinement in the PC structure is revealed from measurements of the photonic band structure, and this strong optical confinement leads to the single-mode CW lasing operation with a low threshold current density.

Modification of InGaAs/GaAs heterostructure density of states and optical gain using hybrid quantum well-dots

We show that the density of states and gain spectra of InGaAs/GaAs quantum well-dot (QWD) hybrid nanostructures qualitatively differ from that of quantum wells (QWs) and quantum dots. In QWDs, the density of states does not increase to higher energies and ground-state lasing is maintained up to shorter cavities (higher output loss) as compared to QW lasers emitting in the same optical range. The QWD lasers show lower threshold current densities and better temperature stability than the QW ones.

Heterostructures of Quantum-Cascade Laser for the Spectral Range of 4.6 μm for Obtaining a Continuous-Wave Lasing Mode

The method of molecular-beam epitaxy was used to fabricate an elastically balanced heterostructure of a quantum-cascade laser for the spectral range of 4.6 μm based on a heteropair of solid alloys, In0.67Ga0.33As/In0.36Al0.64As, and indium phosphide layers serving as waveguide cladding layers. An X-ray diffraction analysis demonstrated the high uniformity of the composition and thickness of layers in cascades of the heterostructure over the substrate area. Lasers with four cleaved facets show lasing at room temperature at a wavelength close to 4.6 μm with a comparatively low threshold current density of 1.1 kA/cm2.

MOCVD growth of InP-based 1.3 μ m quantum dash lasers on (001) Si

Quantum dot and quantum dash (QDash) lasers exhibit lower threshold, less temperature sensitivity, and larger modulation bandwidths than the conventional quantum well lasers. For III–V lasers monolithically grown on Si, the stronger carrier confinement and the discrete distribution of these three-dimensional (3D) quantum structures add to their immunity to material defects resulted from hetero-epitaxy. In this study, we report InAs/InAlGaAs/InP QDash lasers emitting at 1.3 μm directly grown on compliant InP/Si substrates by metalorganic chemical vapor deposition. Room-temperature lasing has been demonstrated on both nano-V-groove patterned and unpatterned planar (001) Si under pulsed electrical pumping, with a low threshold current density of 1.05 kA/cm2. A comparison of lasers grown on these two categories of InP/Si templates in terms of material quality and device performance is presented. Results presented in this work demonstrate the possibility of integrating both datacom and telecom lasers on Si, using the same InAs/InP quantum dash material system on a developed InP-on-Si virtual substrate.

InAs-based quantum cascade lasers grown on on-axis (001) silicon substrate

We present InAs/AlSb quantum cascade lasers (QCLs) monolithically integrated on an on-axis (001) Si substrate. The lasers emit near 8 μm with threshold current densities of 0.92–0.95 kA/cm2 at 300 K for 3.6-mm-long devices and operate in pulsed mode up to 410 K. QCLs of the same design grown for comparison on a native InAs substrate demonstrated a threshold current density of 0.75 kA/cm2 and the same maximum operating temperature. The low threshold current density of the QCLs grown on Si makes them suitable for photonic integrated sensor implementation.

Mid-infrared laser diodes epitaxially grown on on-axis (001) silicon

The direct epitaxial growth of III-V semiconductor lasers on standard, CMOS-compatible, on-axis (001) Si substrates is actively sought for the realization of active photonic integrated circuits. Here we report on the first mid-infrared semiconductor laser epitaxially grown on on-axis Si substrates, i.e., compatible with industry standards. Furthermore, these GaSb-based laser diodes demonstrate low threshold current density, low optical losses, high temperature operation, and high characteristic temperatures. These results represent a breakthrough toward the integration of semiconductor laser sources on Si for smart sensors.

Performance deterioration of GaN-based laser diode by V-pits in the upper waveguide layer

Abstract The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In0.017Ga0.983N/u-GaN multiple UWG or u-In0.017Ga0.983N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm2, and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In0.017Ga0.983N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

Гетероструктуры квантово-каскадных лазеров спектрального диапазона 4.6 μm для реализации непрерывного режима генерации

An elastically balanced heterostructure of a 4.6-μm wavelength range quantum-cascade laser was grown by using molecular beam epitaxy. Heterostruсture was based on a heteropair of In0.67Ga0.33As / In0.36Al0.64As solid alloys and indium phosphide layers, which served as waveguide claddings. A high homogeneity of the layers composition and thicknesses in cascades over the wafer was proved by using X-ray diffraction for the grown heterostructure. Lasers with four cleaved facets operate at room temperature at a wavelength near 4.6 μm with a low threshold current density of 1.1 kA / cm2

Superior optical (λ ∼ 1550 nm) emission and detection characteristics of Ge microdisks grown on virtual Si0.5Ge0.5/Si substrates using molecular beam epitaxy

We report the optical characteristics of relatively large sized (∼7.0–8.0 μm) but low aspect ratio Ge microdisks grown on a virtual Si0.5Ge0.5 substrate using molecular beam epitaxy following the Stranski–Krastanov growth mechanism. Grown microdisks with very low aspect ratio Ge islands exhibit direct band gap (∼0.8 eV) photoluminescence emission sustainable up to room temperature, enabled by the confinement of carriers into the microdisks. p–i–n diodes with an intrinsic layer containing Ge microdisks have been fabricated to study their emission and photoresponse characteristics at an optical communication wavelength of ∼1550 nm. A strong electroluminescence at 1550 nm has been achieved at low temperatures in the device for a very low threshold current density of 2.56 μA cm−2 due to the strong confinement of injected holes. The emission characteristics of the fabricated device with respect to the injected current density and temperature have been studied. Novel emission and optical modulation characteristics at 1550 nm of the fabricated p–i–n device containing Ge microdisks grown on a virtual SiGe substrate indicate its potential for Si CMOS compatible on-chip optical communications.

Record Low Threshold Current Density in Quantum Dot Microdisk Laser

We demonstrate a record low threshold current density of 250 A/cm2 in a quantum dot microdisk laser with a 31-μm diameter operating at room temperature in continuous wave regime without temperature stabilization. This low threshold current density is very close to the transparency current density estimated in broad-area edge-emitting lasers made of the same epitaxial wafer.

Stacked quantum cascade laser and detector structure for a monolithic mid-infrared sensing device

We report the stacked quantum cascade laser and detector structures for a monolithic midinfrared sensing device. In the fabricated quantum cascade device, the layer structures for the detector are grown directly on the quantum cascade laser structure. The laser and detector structures thereby are flexibly designed to operate at the same wavelength, without any compensation needed in the way that the active region is designed in the laser and detector. The fabricated monolithic quantum cascade devices exhibit a low threshold current density at the midinfrared wavelength of 5.7 μm. As a result, light emission and detection on the same chip are demonstrated in continuous wave operation at room temperature.

Type-II AlInN/ZnGeN2 quantum wells for ultraviolet laser diodes

We propose a type-II AlInN/ZnGeN2 quantum well (QW) structure serving as the active region for ultraviolet (UV) laser diodes. A remarkably low threshold current density can be achieved using the type-II AlInN/ZnGeN2 QW structure, providing a pathway for the realization of electrically-driven nitride-based semiconductor UV laser diodes. ZnGeN2 has both a very similar lattice constant and bandgap to GaN. Its large band offsets with GaN enable the potential of serving as a hole confinement layer to increase the electron-hole wavefunction overlap in the active region. In this study, we investigate the spontaneous emission and gain properties of type-II AlInN/ZnGeN2 QWs with different ZnGeN2 layer thicknesses. Our findings show that the use of ZnGeN2 layers in the active region provides a significant improvement in hole carrier confinement, which results in ∼5 times enhancement of the electron-hole wave function overlap. Such an enhancement provides the ability to achieve a significant increase (∼6 times) in the spontaneous emission rate and material gain, along with a remarkable reduction in threshold carrier density compared to the conventional AlGaN-based QW design, which is essential for practical UV laser diodes.

Origin of Defect Tolerance in InAs/GaAs Quantum Dot Lasers Grown on Silicon

High-performance III–V quantum-dot lasers monolithically grown on Si substrates have been demonstrated as a promising solution to realize Si-based laser sources with very low threshold current density, high output power, and long lifetime, even with relatively high density of defects due to the material dissimilarities between III–Vs and Si. On the other hand, although conventional III–V quantum-well lasers grown on Si have been demonstrated after great efforts worldwide for more than 40 years, their practicality is still a great challenge because of their very high threshold current density and very short lifetime. However, the physical mechanisms behind the superior performance of silicon-based III–V quantum-dot lasers remain unclear. In this paper, we directly compare the performance of a quantum-well and a quantum-dot laser monolithically grown on on-axis Si (001) substrates, both experimentally and theoretically, under the impact of the same density of threading dislocations. A quantum-dot laser grown on a Si substrate with a high operating temperature (105 °C) has been demonstrated with a low threshold current density of 173 A/cm2 and a high single facet output power >100 mW at room temperature, while there is no lasing operation for the quantum-well device at room temperature even at high injection levels. By using a rate equation travelling-wave model, the quantum-dot laser's superior performance compared with its quantum well-based counterpart on Si is theoretically explained in terms of the unique properties of quantum dots, i.e., efficient carrier capture and high thermal energy barriers preventing the carriers from migrating into defect states.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

4–λ hybrid InGaAsP-Si evanescent laser array with low power consumption for on-chip optical interconnects

A 4–λ hybrid InGaAsP-Si evanescent laser array is obtained by bonding III–V distributed feedback lasers to a silicon on insulator (SOI) substrate using a selective area metal bonding technique. Multiple wavelengths are realized by varying the width of the III–V ridge waveguide. The threshold current is less than 10 mA for all wavelength channels under continuous-wave (CW) operation at room temperature, and the lowest threshold current density is 0.76 kA/cm2. The side mode suppression ratio (SMSR) is higher than 40 dB for all wavelength channels when the injection current is between 20 mA and 70 mA at room temperature, and the highest SMSR is up to 51 dB. A characteristic temperature of 51 K and thermal impedance of 144°C/W are achieved on average. The 4–λ hybrid InGaAsP-Si evanescent laser array exhibits a low threshold and high SMSR under CW operation at room temperature. The low power consumption of this device makes it very attractive for on-chip optical interconnects.

450 nm GaInN ridge stripe laser diodes with AlInN/AlGaN multiple cladding layers

We investigated and improved optical waveguides along the vertical and horizontal directions in 450 nm GaInN laser diodes. As a result, we demonstrated a low threshold current density (1.15 kA cm−2) of a GaInN ridge stripe laser diode containing a 3-pair 40 nm Al0.82In0.18N/25 nm Al0.03Ga0.97N multiple bottom cladding layer at room temperature under pulsed condition. This threshold current density is smaller than our typical value with a 1 μm Al0.03Ga0.97N bottom cladding layer. AlInN/AlGaN multiple layers are useful as n-type cladding layers in visible laser diodes to achieve higher optical confinement factors while smooth surfaces were obtained.