Monolithic integrated all-GaN-based µLED display by selective area regrowth.

This work demonstrates an all-GaN-based µLED display with monolithic integrated HEMT and µLED pixels using the selective area regrowth method. The monochrome µLED-HEMT display has a resolution of 20 × 20 and a pixel pitch of 80 µm. With the optimized regrowth pattern, the µLED-HEMT achieves a maximum light output power of 36.2 W/cm2 and a peak EQE of 3.36%, mainly due to the improved crystal quality of regrown µLED. TMAH treatment and Al2O3 surface passivation are also performed to minimize the impact of nonradiative recombination caused by the dry etching damage. With a custom-designed driving circuit board, images of "HKUST" are successfully shown on the µLED-HEMT display.

980†nm electrically pumped continuous lasing of QW lasers grown on silicon.

Investigation of high-performance lasers monolithically grown on silicon (Si) could promote the development of silicon photonics in regimes other than the 1.3 -1.5 µm band. 980 nm laser, a widely used pumping source for erbium-doped fiber amplifier (EDFA) in the optical fiber communication system, can be used as a demonstration for shorter wavelength lasers. Here, we report continuous wave (CW) lasing of 980 nm electrically pumped quantum well (QW) lasers directly grown on Si by metalorganic chemical vapor deposition (MOCVD). Utilizing the strain compensated InGaAs/GaAs/GaAsP QW structure as the active medium, the lowest threshold current obtained from the lasers on Si was 40 mA, and the highest total output power was near 100 mW. A statistical comparison of lasers grown on native GaAs and Si substrates was conducted and it reveals a somewhat higher threshold for devices on Si. Internal parameters, including modal gain and optical loss are extracted from experimental results and the variation on different substrates could provide a direction to further laser optimization through further improvement of the GaAs/Si templates and QW design. These results demonstrate a promising step towards optoelectronic integration of QW lasers on Si.

High coupling efficiency waveguide grating couplers on lithium niobate.

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency (CE) grating couplers (GCs) in the lithium niobate on insulator photonic integration platform. Enhanced CE is achieved by increasing the grating strength using a high refractive index polysilicon layer on the GC. Due to the high refractive index of the polysilicon layer, the light in the lithium niobate waveguide is pulled up to the grating region. The optical cavity formed in the vertical direction enhances the CE of the waveguide GC. With this novel structure, simulations predicted the CE to be -1.40 dB, while the experimentally measured CE was -2.20 dB with a 3-dB bandwidth of 81 nm from 1592 nm to 1673 nm. The high CE GC is achieved without using bottom metal reflectors or requiring the etching of the lithium niobate material.

Full-color micro-display by heterogeneous integration of InGaN blue/green dual-wavelength and AlGaInP red LEDs.

A full-color micro-display via bonding of a InGaN blue/green dual-wavelength light-emitting diode (LED) array and a AlGaInP red LED array is demonstrated. The micro-display has a 120 µm pixel pitch, and each pixel consists of 40 µm × 120 µm red/green/blue (R/G/B) subpixels. The red LED array was integrated with the blue/green dual-wavelength LED array by Au/In flip-chip bonding to achieve full-color emission. Full-color images presented by the micro-display have high brightness and a wide color gamut. This heterogeneous integration technology using conventional LED materials shows the feasibility of a cost-effective approach for reliable high-performance full-color LED micro-displays in virtual reality (VR) and augmented reality (AR) devices.

Electrically pumped InP/GaAsP quantum dot lasers grown on (001) Si emitting at 750†nm.

Excellent performance of InAs quantum dot (QD) lasers grown on Si in the datacom and telecom bands has been reported in recent years. InP QD lasers on Si with emission wavelength at 650 nm-750 nm are seldom explored. In this paper, we report the growth and room temperature lasing of electrically pumped InP/GaAsP QD lasers directly grown on (001) Si emitting at 750 nm. The lowest threshold current density obtained is ∼650 A/cm2, measured on a 2 mm × 70 µm device. Moreover, the highest operating temperature of the InP QD laser grown on the GaAs/Si template is above 95°C. This 750 nm near red on-chip light source for the monolithic integration of Si photonics is potentially applicable in display, bio-photonics, and spatial mapping.

Monolithic integration of ultraviolet light emitting diodes and photodetectors on a p-GaN/AlGaN/GaN/Si platform.

In this letter, we report the first demonstration of monolithically integrated ultraviolet (UV) light emitting diodes (LEDs) and visible-blind UV photodetectors (PDs) employing the same p-GaN/AlGaN/GaN epi-structures grown on Si. Due to the radiative recombination of holes from the p-GaN layer with electrons from the 2-D electron gas (2DEG) accumulating at the AlGaN/GaN heterointerface, the forward biased LED with p-GaN/AlGaN/GaN junction exhibits uniform light emission at 360 nm. Facilitated by the high-mobility 2DEG channel governed by a p-GaN optical gate, the visible-blind phototransistor-type PDs show a low dark current of ∼10-7 mA/mm and a high responsivity of 3.5×105 A/W. Consequently, high-sensitivity photo response with a large photo-to-dark current ratio of over 106 and a response time less than 0.5 s is achieved in the PD under the UV illumination from the on-chip adjacent LED. The demonstrated simple integration scheme of high-performance UV PDs and LEDs shows great potential for various applications such as compact opto-isolators.

848 ppi high-brightness active-matrix micro-LED micro-display using GaN-on-Si epi-wafers towards mass production.

In this paper, fabrication processes of a 0.55-inch 400 × 240 high-brightness active-matrix micro-light-emitting diode (LED) display using GaN-on-Si epi-wafers are described. The micro-LED array, featuring a pixel size of 20 µm × 20 µm and a pixel density of 848 pixels per inch (ppi), was fabricated and integrated with a custom-designed CMOS driver through Au-Sn flip-chip bonding. Si growth substrate was removed using a crack-free wet etching method. Four-bit grayscale images and videos are clearly rendered. Optical crosstalk is discussed and can be mitigated through micro-LED array design and process modification. This high-performance, high-resolution micro-LED display demonstration provides a promising and cost-effective solution towards mass production of micro-displays for VR/AR applications.

GaSb-based laser diodes grown on MOCVD GaAs-on-Si templates.

We report GaSb-based laser diodes (LDs) grown on on-axis (001) Si substrates and emitting at 2.3 µm. Two series of LDs were studied and compared. For the first series, a GaAs-based buffer layer was first grown by metal organic chemical vapor deposition (MOCVD) before growing the laser heterostructure by molecular-beam epitaxy (MBE). For the second series, a MOCVD GaSb buffer layer was added between the MOCVD GaAs buffer layer and the MBE laser heterostructure. Both series of LDs exhibited threshold currents in the 50-100 mA range and several mW output power at room temperature. They demonstrated continuous wave operation (CW) up to 70°C (set-up limited) without thermal rollover. Broad area LDs exhibited record threshold-current densities in the 250-350 A.cm-2 range for the second series of LDs, in spite of cracks that appeared during device processing. These results show that the design and fabrication steps of the buffer-layer stacks are critical issues in the epitaxial integration of GaSb-based optoelectronic devices on Si substrates and offer room for much performance improvement.

Controlled single-mode emission in quantum dot micro-lasers.

In this paper, we demonstrate an efficient and easy fabrication method for whispering-gallery-mode (WGM) manipulation and report the first electrically driven single-mode quantum dot micro-ring (QDMR) lasers. Using self-assembled InAs/InAlGaAs QD active layers with deeply etched azimuthal gratings, continuous-wave (CW) lasing with controllable single-mode emission wavelengths covering 1300 nm to 1370 nm has been achieved. A record-high side-mode-suppression-ratio (SMSR) value of 49 dB is obtained. These QDMR lasers exhibit excellent single-mode lasing stabilities over the current and temperature tuning range with a thermal tunability of 0.092 nm/°C. The concept is applicable to other wavelength bands depending on the gain spectrum, demonstrating a feasible solution in realizing energy-efficient and densely integrated photonic building blocks.

Red-emitting InP quantum dot micro-disk lasers epitaxially grown on (001) silicon.

Direct epitaxy of InP quantum dot (QD) lasers on silicon (Si) provides an on-chip red laser source for integrated Si photonics with different applications. Here, we demonstrate the first, to the best of our knowledge, InP QD lasers directly grown on (001) Si. Combining highly emissive InP QDs and a GaAs/Si template with low defect density, continuous-wave (CW) lasing of micro-disk lasers (MDLs) on Si is achieved at room temperature. The lowest threshold of MDLs on Si is ∼500nW, without considering the micro-disk surface absorption efficiency of the pump power. The MDLs grown on the native GaAs substrate with the same growth and fabrication process are compared using statistical data analysis. Similar material characterization results and device performances on these two substrates further confirm the performance of QD lasers on Si. This demonstration paves the way for future realization of integrated photonic circuits with red and near-infrared (NIR) lasers on Si.

C and L band room-temperature continuous-wave InP-based microdisk lasers grown on silicon.

Quantum-dot (QD) and quantum-dash (QDash) have been shown to be promising gain materials for lasers directly grown on Si due to their better tolerance to crystal defects and thermal stability. Here we report optically pumped InP-based InAs QDash microdisk lasers (MDLs) directly grown on on-axis (001) Si. To the best of our knowledge, this is the first demonstration of room-temperature continuous-wave lasing of a QDash MDL on Si in the C band and L band. To the best of our knowledge, the lowest threshold of around 400 µW and highest operation temperature of 323 K have been achieved. An analysis of experimental results shows that the dominant lasing wavelength of MDLs varies with the thickness and diameter of the MDLs. Our demonstration shows potential application of MDLs for multi-channel operation in densely integrated Si-photonics.

1.55†µm electrically pumped continuous wave lasing of quantum dash lasers grown on silicon.

Realization of fully integrated silicon photonics has been handicapped by the lack of a reliable and efficient III-V light source on Si. Specifically, electrically pumped continuous wave (CW) lasing and operation sustainable at high temperatures are critical for practical applications. Here, we present the first electrically pumped room temperature (RT) CW lasing results of 1.55 µm quantum dash (QDash) lasers directly grown on patterned on-axis (001) Si using metal organic chemical vapor deposition (MOCVD). Adopting a dash-in-well structure as the active medium, the growth of QDash was optimized on an InP on Si template. Incorporating the advantages of the optimized material growth and device fabrication, good laser performance including a low threshold current of 50 mA, a threshold current density of 1.3 kA/cm2 and operation at elevated temperature up to 59 °C in CW mode was achieved. Comparison of lasers grown on Si and native InP substrates in the same growth run was made. Based on the laser characteristics measured at room temperature and elevated temperatures, the QDash quality on the two substrates is comparable. These results suggest that MOCVD is a viable technique for lasers on Si growth and represent an advance towards silicon-based photonic-electronic integration and manufacturing.

Comparison of static and dynamic characteristics of 1550†nm quantum dash and quantum well lasers.

Compared to quantum well (QW) lasers, lower dimensional quantum dot (QD) or quantum dash (QDash) devices demonstrate superior performances, owing to their quantized energy levels and increased carrier confinement. Here, we report the systematic comparison of static and dynamic properties of long wavelength (1550 nm) QDash and QW lasers. For the QDash lasers, a higher maximum operating temperature and lower temperature dependence was achieved for long cavities, although the threshold current densities were larger than the QW reference devices. The lasing characteristics for QDashes are significantly improved following the application of a high reflectance (HR) coating on the rear facets. The QDash lasers also exhibit three orders lower dark current, of 45 µA/cm2 under -1 V reverse bias. Small signal modulation on the 4 × 550 µm2 Fabry-Perot cavities yields a modulation efficiency of 0.48 GHz/√mA and a maximum 3-dB bandwidth of 7.4 GHz for QDashes, slightly larger than that for the QW devices. Meanwhile, a stronger damping effect was observed for the QDash lasers due to their lower differential gain.

Multi-heterojunction InAs/GaSb nano-ridges directly grown on (001) Si.

We report on multi-stacked InAs/GaSb nano-ridges directly grown on (001) patterned Si substrates by metal-organic chemical vapor deposition (MOCVD). Uniform GaSb and InAs nano-ridges were demonstrated with optimized growth parameters. By adjusting the switching sequences, we also obtained defect-free InAs/GaSb and GaSb/InAs interfaces. Based on these fine-tuned growth conditions, multi-stacked InAs/GaSb nano-ridges were developed and characterized. The nano-ridges showed uniform morphology from scanning electron microscopy (SEM), and no observable crystalline defects were detected at the hetero-interfaces by transmission electron microscopy (TEM). These InAs/GaSb nano-ridges show great potential for applications in nano-scale tunneling devices and long wavelength light emitters and detectors. The demonstrated growth techniques provide helpful insights for the growth process control of 6.1 Å family compound semiconductors directly on Si by MOCVD.

Electrically pumped 1.5 µm InP-based quantum dot microring lasers directly grown on (001) Si.

Directly grown quantum dot (QD) lasers on silicon are appealing for monolithic integration of photonic circuits from a technoeconomic perspective. In this Letter, we report miniaturization of these Si-based lasers employing high-quality whispering-gallery mode microresonators. Based on previously developed InAs/InAlGaAs QDs on the complementary metal-oxide-semiconductor-standard (001) Si platform and optimized device implementation techniques, on-chip electrically pumped InP-based QD microring lasers (MRLs) on Si are successfully realized for the first time. Room-temperature pulsed lasing in the 1.5 µm wavelength band, with a threshold of 50 mA, is measured for 50-µm-diameter MRLs. Lasing up to 70°C is achieved with a characteristic temperature of 51.5 K.

Telecom InP/InGaAs nanolaser array directly grown on (001) silicon-on-insulator.

A compact, efficient, and monolithically grown III-V laser source provides an attractive alternative to bonding off-chip lasers for Si photonics research. Although recent demonstrations of microlasers on (001) Si wafers using thick metamorphic buffers are encouraging, scaling down the laser footprint to nanoscale and operating the nanolasers at telecom wavelengths remain significant challenges. Here, we report a monolithically integrated in-plane InP/InGaAs nanolaser array on (001) silicon-on-insulator (SOI) platforms with emission wavelengths covering the entire C band (1.55 µm). Multiple InGaAs quantum wells are embedded in high-quality InP nanoridges by selective-area growth on patterned (001) SOI. Combined with air-cladded InP/Si optical cavities, room-temperature operation at multiple telecom bands is obtained by defining different cavity lengths with lithography. The demonstration of telecom-wavelength monolithic nanolasers on (001) SOI platforms presents an important step towards fully integrated Si photonics circuits.

Interfacially Bound Exciton State in a Hybrid Structure of Monolayer WS2 and InGaN Quantum Dots.

van der Waals heterostructures that are usually formed using atomically thin transition-metal dichalcogenides (TMDCs) with a direct band gap in the near-infrared to the visible range are promising candidates for low-dimension optoelectronic applications. The interlayer interaction or coupling between two-dimensional (2D) layer and the substrate or between adjacent 2D layers plays an important role in modifying the properties of the individual 2D material or device performances through Coulomb interaction or forming interlayer excitons. Here, we report the realization of quasi-zero-dimensional (0D) photon emission of WS2 in a coupled hybrid structure of monolayer WS2 and InGaN quantum dots (QDs). An interfacially bound exciton, i.e., the coupling between the excitons in WS2 and the electrons in QDs, has been identified. The emission of this interfacially bound exciton inherits the 0D confinement of QDs as well as the spin-valley physics of excitons in monolayer WS2. The effective coupling between 2D materials and conventional semiconductors observed in this work provides an effective way to realize the 0D emission of 2D materials and opens the potential of compact on-chip integration of valleytronics and conventional electronics and optoelectronics.

Room-temperature electrically-pumped 1.5 µm InGaAs/InAlGaAs laser monolithically grown on on-axis (001) Si.

Hetero-epitaxial growth of high quality InP on a complementary metal-oxide-semiconductor (CMOS)-compatible Si platform is compelling for monolithic integration of optoelectronics. It will provide the combined strength of mainstream mature InP-based photonic integrated circuits (PIC) technologies and large-volume, low-cost silicon-based manufacturing foundries. Direct monolithic integration of InP-based laser diodes (LDs) on silicon helps fully exploit the potential of silicon photonics and benefits the application of dense wavelength division multiplexing (DWDM) for telecommunications. Here, we report the first InGaAs/InAlGaAs multi-quantum-well (MQW) lasers directly grown on on-axis V-grooved (001) Si by metalorganic chemical vapor deposition (MOCVD). Lasing near 1.5 µm was achieved for the first time with a threshold current density Jth = 3.3 kA/cm2 under pulsed current injection at room temperature. A high characteristic temperature T0 of 133 K in the range of 20°C-40°C was measured. These results demonstrate the potential of adopting this large-area InP-on-Si substrate for integrating diverse III-V laser diodes, photodetectors, and high-frequency and high-speed transistors.

Monolithic integration of III-nitride voltage-controlled light emitters with dual-wavelength photodiodes by selective-area epitaxy.

We report for the first time on-chip integration of III-nitride voltage-controlled light emitters with visible and ultraviolet (UV) photodiodes (PDs). InGaN/GaN and AlGaN/GaN heterostructures were grown in specific regions by selective-area epitaxy, allowing monolithic integration of versatile devices including visible light emitting diodes (LEDs), visible-light PDs, AlGaN/GaN high electron mobility transistors (HEMTs), and UV-light Schottky barrier (SB) PDs. A serial connection between the LED and HEMT through the epitaxial layers enables a three-terminal voltage-controlled light emitter (HEMT-LED), efficiently converting voltage-controlled signals into visible-light signals that can be coupled into an adjacent visible-light PD generating electrical signals. While the integrated blue HEMT-LED and PD transmits signals carried by visible light, the visible-blind SB-PD on a chip receives external UV light control signals with negligible interference from the on-chip visible-light source. This integration scheme can be extended to open an avenue for developing a variety of applications, such as smart lighting, on-chip optical interconnect, optical wireless communication, and opto-isolators.

InGaAs/InP quantum wires grown on silicon with adjustable emission wavelength at telecom bands.

We report the growth of vertically stacked InGaAs/InP quantum wires on (001) Si substrates with adjustable room-temperature emission at telecom bands. Based on a self-limiting growth mode in selective area metal-organic chemical vapor deposition, crescent-shaped InGaAs quantum wires with variable dimensions are embedded within InP nano-ridges. With extensive transmission electron microscopy studies, the growth transition and morphology change from quantum wires to ridge quantum wells (QWs) have been revealed. As a result, we are able to decouple the quantum wires from ridge QWs and manipulate their dimensions by scaling the growth time. With minimized lateral dimension and their unique positioning, the InGaAs/InP quantum wires are more immune to dislocations and more efficient in radiative processes, as evidenced by their excellent optical quality at telecom-bands. These promising results thus highlight the potential of combining low-dimensional quantum wire structures with the aspect ratio trapping process for integrating III-V nano-light emitters on mainstream (001) Si substrates.