Multiple Quantum Well Active Layer Research Articles

A Route toward High-Detectivity and Low-Cost Short-Wave Infrared Photodetection: GeSn/Ge Multiple-Quantum-Well Photodetectors with a Dielectric Nanohole Array Metasurface.

High-detectivity and low-cost short-wave infrared photodetectors with complementary metal-oxide-semiconductor (CMOS) compatibility are attractive for various applications such as next-generation optical communication, LiDAR, and molecular sensing. Here, GeSn/Ge multiple-quantum-well (MQW) photodetectors with a dielectric nanohole array metasurface were proposed to realize high-detectivity and low-cost SWIR photodetection. The Ge nanohole array metasurface was utilized to enhance the light absorption in the GeSn/Ge MQW active layer. Compared with metallic nanostructures, the dielectric nanohole structure has the advantages of low intrinsic loss and CMOS compatibility. The introduction of metasurface architecture facilitates a 10.5 times enhanced responsivity of 0.232 A/W at 2 μm wavelength while slightly sacrificing the dark current density. Besides, the metasurface GeSn/Ge MQW photodetectors benefit 35% improvement in the 3 dB bandwidth compared to control GeSn/Ge MQW photodetectors, which can be attributed to the reduced RC delay. Due to the high responsivity and low dark current density, the room temperature specific detectivity at 2 μm is as high as 5.34 × 109 cm·Hz1/2/W, which is the highest among GeSn photodetectors and is better than commercial InSb and PbSe photodetectors operating at the similar wavelength. This work offers a promising approach for achieving low-cost and effective photodetection at 2 μm, contributing to the development of the 2 μm communication band.

Modeling the Lamb mode-coupling constant of quantum well semiconductor lasers.

We theoretically compute the coupling constant C between two emission modes of an extended cavity laser with a multiple quantum-well active layer. We use an optimized Monte Carlo model based on the Markov chain that describes the elementary events of carriers and photons over time. This model allows us to evaluate the influence on C of the transition from a class A laser to a class B laser and illustrates that the best stability of dual-mode lasers is obtained with the former. In addition, an extension of the model makes it possible to evaluate the influence of different mode profiles in the cavity as well as the spatial diffusion of the carriers and/or the inhomogeneity of the temperature. These results are in very good agreement with previous experimental results, showing the independence of C with respect to the beating frequency and its evolution versus the spatial mode splitting in the gain medium.

Integrating GeSn photodiode on a 200 mm Ge-on-insulator photonics platform with Ge CMOS devices for advanced OEIC operating at 2 μm band.

High-performance GeSn multiple-quantum-well (MQW) photodiode is demonstrated on a 200 mm Ge-on-insulator (GeOI) photonics platform for the first time. Both GeSn MQW active layer stack and Ge layer (top Ge layer of GeOI after bonding) were grown using a single epitaxy step on a standard (001)-oriented Si substrate (donor wafer) using a reduced pressure chemical vapor deposition (RPCVD). Direct wafer bonding and layer transfer technique were then employed to transfer the GeSn MQW device layers and Ge layer to a 200 mm SiO2-terminated Si handle substrate. The surface illuminated GeSn MQW photodiode realized on this platform exhibits an ultra-low leakage current density of 25 mA/cm2 at room temperature and an enhanced photo sensitivity at 2 μm of 30 mA/W as compared to a GeSn MQW photodiode on Si at 2 μm. The underlying GeOI platform enables monolithic integration of a complete suite of photonics devices operating at 2 μm band, including GeOI strip waveguides, grating couplers, micro-ring modulators, Mach-Zehnder interferometer modulators, etc. In addition, Ge CMOS circuits can also be realized on this common platform using a "photonic-first and electronic-last" processing approach. In this work, as prototype demonstration, both Ge p- and n-channel fin field-effect transistors (FinFETs) were realized on GeOI simultaneously with decent static electrical characteristics. Subthreshold swings of 150 and 99 mV/decade at |VD| = 0.1 V and drive currents of 91 and 10.3 μA/μm at |VG-VTH| = 1 V and |VD| = 0.75 V were achieved for p- and n-FinFETs, respectively. This works illustrates the potential of integrating GeSn (as photo detection material) on GeOI platform for Ge-based optoelectronics integrated circuits (OEICs) targeting communication applications at 2 μm band.

Interrelation Between the Internal Quantum Efficiency and Forward Voltage of Blue LEDs

In InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs), carrier accumulation in MQWs due to the saturation of the radiative recombination rate affects the internal-quantum-efficiency (IQE) and forward-voltage ( ${V} _{\text {F}}$ ) characteristics simultaneously. In this letter, we investigate the interrelation between the IQE and ${V} _{\text {F}}$ at an operating current density, using 31 blue LEDs with MQW active layers grown under slightly different conditions. The general trend observed demonstrates that ${V} _{\text {F}}$ decreases as the IQE increases. We analyze this interrelation between the IQE and ${V} _{\text {F}}$ through separation of radiative and nonradiative current densities, and propose to use the active efficiency (AE) to quantify the performance of the active layer more precisely. We examine the two cases where only the radiative (nonradiative) current density changes and establish that the increase of the radiative current density is more desirable than the decrease of the nonradiative current density in improving the AE of the device.

Fabrication and properties of wafer-scale nanoporous GaN distributed Bragg reflectors with strong phase-separated InGaN/GaN layers

High photoluminescence (PL) efficiency InGaN/GaN multiple quantum well (MQW) active layers have been fabricated by metalorganic chemical vapor deposition (MOCVD) and followed by electrochemical (EC) etching. The MQW active layers were embedded nanoporous GaN (NP-GaN) distributed Bragg reflectors (DBRs). The PL intensity of MQW structure on the NP GaN mirror was several times higher than for as-grown sample, which should be attributable to improved light extraction efficiency (LEE) due to the increased light-extracting surface area and buried DBR mirror, and improved internal quantum efficiency (IQE) due to the relaxation of strain in the MQWs embedded in the NP-GaN DBRs.

Optimized Ge1−<italic>x</italic>Sn<italic>x</italic>/Ge Multiple-Quantum-Well Heterojunction Phototransistors for High-Performance SWIR Photodetection

We present the design and analysis of three-terminal Ge1− x Sn x /Ge multiple-quantum-well (MQW) heterojunction phototransistors (HPTs) for high-performance short-wave infrared (SWIR) photodetection. The active layer incorporates Ge1− x Sn x /Ge MQW structures between base–collector junctions pseudomorphically grown on Ge-on-Si substrates, thereby providing compatibility with CMOS platforms for low cost and scalable manufacturing technology. Introducing Sn into the MQW structures can effectively reduce the direct bandgap, thereby considerably extending the photodetection range in the SWIR spectral range with improved absorption efficiencies. In addition, the use of MQW structures as the active layer can enhance the absorption coefficient via quantum-confinement effects, thus further enhancing the photoresponses. We develop theoretical models to calculate the strained electronic band structures and optical absorption coefficients for the Ge1− x Sn x /Ge MQW structures, and the current gain is evaluated as a function of the barrier/well thicknesses, number of periods of the MQW structures, and doping concentrations of the layers for the HPT structures. The calculation results show that the absorption coefficient can be significantly enhanced by increasing the Sn content in the Ge1− x Sn x /Ge MQW active layer, and the current gain can be considerably enhanced by optimizing the HPT structures. With the enhanced absorption coefficient and current gain, high-responsivity photodetection can be achieved in the SWIR spectral range, thus making this device very promising for a wide range of SWIR photodetection applications.

Carrier accumulation in the active region and its impact on the device performance of InGaN-based light-emitting diodes

Carrier recombination and transport processes play key roles in determining optoelectronic performance characteristics such as the efficiency droop and forward voltage in InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs). In this work, we investigate the dominant carrier transport and recombination processes inside and outside the MQW region as functions of injection current from the viewpoint of carrier-energy-loss mechanisms. It is experimentally shown that carrier accumulation and subsequent spill-over at MQW active layers due to the insufficient carrier recombination rate, mainly the radiative recombination rate, explain the dependences of both the efficiency droop and the forward voltage on the injection current.

A new interpretation for performance improvement of high-efficiency vertical blue light-emitting diodes by InGaN/GaN superlattices

The effect of InGaN/GaN superlattices (SLs) on quantum efficiency and forward voltage of vertical blue InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LED) grown on Si substrate has been experimentally and theoretically investigated. We have prepared two LED samples, in which the 30 and 45 periods of SLs are inserted between MQW active layers and n-GaN layer, respectively. Electroluminescence measurement shows that the LED with 45 periods of SLs has higher quantum efficiency but lower forward voltage. It is observed that V-shaped pits grow up in size with an increase in SLs period number by means of scan transmission electron microscope and secondary ion mass spectrometry. Further numerical simulations confirm that the performance improvement of LED by SLs is mainly ascribed to enhancing hole injection from the V-shaped pits.

1×1 mm2대면적 녹색 LED의 전기 광학적 특성 분석

본 논문은 InGaN/GaN 다중양자우물 구조를 가지는 녹색 발광다이오드의 활성층 내 인듐(In) 조성비와 piezoelectric field에 대한 전계 흡수 현상을 연구하였다. 활성층 내 결정학적 성질과 In 조성비는 double crystal X-ray diffraction 측정으로 분석하였으며, 1×1 ㎟ 대면적 칩을 제작하여 발광특성을 조사하였다. 또한, 활성층 내 piezoelectric field는 electro-reflectance spectroscopy로부터 측정한 compensation voltage를 이용해 계산하였고, 인가전압에 따른 photocurrent의 변화를 측정함으로써 녹색 발광 소자의 전기 광학적 특성을 분석하였다.

Excitonic ultraviolet lasing in ZnO-based light emitting devices

The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films. The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers. The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively. The exciton binding energy in the MQW region is exceptionally large (263meV). Exciton-related lasing was observed by optically pumping the MQWs. ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature. The lasing mechanism is inelastic exciton-exciton collision.

Investigation of Zn diffusion by SIMS and its effects on the performance of AlGaInP-based red lasers

The behaviour of Zn diffusion in a GaInP/(Al0.5Ga0.5)0.5In0.5P multiple quantum-well (MQW) layer was investigated as a function of the growth temperature and Zn/III ratio by secondary ion mass spectroscopy (SIMS). Then the diffusion length of Zn in the undoped (Al0.7Ga0.3)0.5In0.5P layer from the Zn-doped (Al0.7Ga0.3)0.5In0.5P (Zn: 1.0–1.2 × 1018 cm−3) cladding layer was evaluated to design a barrier layer for Zn diffusion into the GaInP/(Al0.5Ga0.5)0.5In0.5P MQW active layer. As a result of incorporating a 130 nm thick diffusion barrier on top of the MQW layers of the AlGaInP red laser, the full width at half maximum (FWHM) of the photoluminescence (PL) spectrum for the GaInP/(Al0.5Ga0.5)0.5In0.5P MQW layers was reduced from 60 meV to 30 meV at room temperature (RT), and the threshold current was also greatly reduced from 110 mA to 75 mA for a standard AlGaInP-based ridge laser as compared to a AlGaInP laser without a diffusion barrier.

Microscopic simulation of the temperature dependence of static and dynamic 1.3-μm multi-quantum-well laser performance

The temperature dependence of the performance of 1.3-/spl mu/m Fabry-Perot (FP) multiple-quantum-well (MQW) lasers is analyzed using detailed microscopic simulations. Both static and dynamic properties are extracted and compared to measurements. Devices with different profiles of acceptor doping in the active region are studied. The simulation takes into account microscopic carrier transport, quantum mechanical calculation of the optical and electronic quantum well properties, and the solution of the optical mode. The temperature dependence of the Auger coefficients is found to be important and is represented by an activated form. Excellent agreement between measurement and simulation is achieved as a function of both temperature and doping profile for static and dynamic properties of the lasers, threshold current density, and effective differential gain. The simulations show that the static carrier density, and hence the contribution to the optical gain, varies significantly from the quantum wells on the p-side of the active layer to those on the n-side. Furthermore, the modal differential gain and the carrier density modulation also vary. Both effects are a consequence of the carrier dynamics involved in transport through the MQW active layer. Despite the complexity of the dynamic response of the MQW laser, the resonance frequency is determined by an effective differential gain, which we show can be estimated by a gain-weighted average of the local differential gain in each well.

Single transverse-mode filtering utilizing ion implantation: application to 1.48-μm unstable-cavity lasers

We demonstrate the relevance of ion implantation of the multiple quantum-well active layer in unstable-cavity lasers as a means of efficiently filtering the parasitic higher order waves by introducing additional propagation loss within the cavity. Several H/sup +/ implantation schemes are proposed and a comparison is successfully made of experiment to a beam propagation method (BPM) model on the basis of modal behavior. The work finally resulted in improved single transverse-mode behavior of those lasers: more than 1.3 W CW of diffraction-limited power at 1.48 /spl mu/m was then obtained utilizing a two-step implantation process.

Metalorganic vapor-phase epitaxy (MOVPE) growth of InGaAsP multiple-quantum-well distributed feedback lasers on InP corrugated substrate

We describe metalorganic vapor-phase epitaxy (MOVPE) growth of InGaAsP multiple-quantum-well (MQW) distributed feedback (DFB) lasers. The MQWs and optical guide layers were grown directly on a corrugated InP substrate, whose corrugation was preserved by a layer pregrown in a mixed arsine and triethylgallium flow at 350°C. Before the pregrowth, only hydrogen flowed over the corrugated InP substrate. The corrugated InP substrate with the pregrowth layer was heated to 650°C in an arsine ambient, then an InGaAsP waveguide layer and MQW layers were grown. The pregrowth layer was confirmed to be InGaAsP, enabling us to grow a thin InGaAsP waveguide layer to realize small optical confinement inside the MQW active layers. A fabricated laser diode operating at 1.62 μm showed a threshold current of 10.2 mA and an external quantum efficiency of 0.28 mW/mA at an ambient temperature of 25°C. A side longitudinal mode suppression ratio of 40 dB at an optical output power of 10 mW was obtained, in the range from −20 to 85°C.

Electro-optic low-voltage InGaAs/GaAs multiple quantum well modulator with organic–inorganic distributed Bragg reflector

We have realized a reflection-type electro-optic InGaAs/GaAs multiple quantum well (MQW) modulator using an organic–inorganic distributed Bragg reflector (DBR). The MQW active layer is embedded in the intrinsic region of a p-i-n diode. The DBR consists of few pairs of CFx/TiOxlayers, fabricated by room-temperature ion beam sputtering on the rear side of the device. The reflectivity of the mirror approaches 98% in the infrared spectral region and is centered at then=1 exciton resonance of the MQW. ON–OFF driving reverse voltages of 0.5 and 1.8 V are measured at room temperature. In this range the static response of the device is linear so that it can be used for analog electro-optic modulation.

Monolithic integration of a quantum-well laser and an optical amplifier using an asymmetric twin-waveguide structure

We demonstrate the monolithic integration of a 1.55 μm wavelength InGaAsP-InP multiple-quantum-well (MQW) laser and a traveling-wave optical amplifier using an asymmetric, vertical twin-waveguide structure. The laser and amplifier share the same strained InGaAsP MQW active layer grown by gas-source molecular beam epitaxy, while the underlying passive waveguide layer is used for on-chip optical interconnections between the active devices. The asymmetric twin-waveguide structure uses the difference in modal gains to discriminate between the even and odd modes.

Polarization switching in a tensile-strained InGaAs/InGaAsP multiple quantum well distributed feedback laser diode

An inhomogeneously biased distributed feedback (DFB) laser diode (LD) with two electrodes switched its polarization mode by 3 mA change of the bias current, maintaining single longitudinal mode oscillation. In the active layer of the LD, 13 nm thick and 0.6% tensile-strained InGaAs multiple quantum well (MQW) equalized the transverse electric and the transverse magnetic modal optical gain at 1.55 μm. With various grating pitches on the same MQW active layer, polarization switching DFB LDs were realized in the wavelength range as wide as 18 nm. The linewidth characteristics during the polarization switching were confirmed to be narrow due to the small switching current.

Double-wavelength laser array with InGaAsP/InGaAsP multiple quantum well grown by Ar ion laser-assisted metalorganic molecular beam epitaxy

This letter reports a laser diode array of InGaAsP/InGaAsP multiple quantum well (MQW) lasers, emitting at different wavelengths, grown by Ar ion laser assisted metalorganic molecular beam epitaxy. A separate confinement heterostructure MQW laser film was locally irradiated during well growth with an Ar ion laser and processed into a laser diode array having a 6 μm stripe width and 300 μm pitch. The laser diode with an irradiated MQW active layer operated at 1.40 μm; the laser diodes with nonirradiated MQW active layers operated at 1.28 μm. The threshold currents of the 300 μm-long laser diodes were 23 mA for the irradiated diode and 55 mA for the nonirradiated diodes.

Stable cw operation at room temperature of a 1.5-μm wavelength multiple quantum well laser on a Si substrate

Room-temperature cw operation of an InGaAs/InGaAsP multiple quantum well (MQW) laser diode on a Si substrate is reported. The MQW laser emits at a 1.54 μm wavelength and exhibits no degradation after over 2000 h of operation. Employing a hybrid organometallic vapor phase epitaxy/vapor mixing epitaxy method and a layer structure for improving crystalline quality, high-quality MQW layers are obtained. A stable longitudinal mode spectrum demonstrates the effectiveness of the MQW active layer.

1.5 µm-Long-Wavelength Multiple Quantum Well Laser on a Si Substrate

Room-temperature cw operation of an InGaAs/InGaAsP multiple quantum well (MQW) laser diode on a Si substrate is reported. The MQW laser emits at 1.54-µm wavelength and exhibits no degradation after over 1,000 hours of operation. Employing a hybrid organometallic vapor-phase epitaxy/vapor mixing epitaxy method and a layer structure for improving crystalline quality, high-quality MQW were obtained. The stable longitudinal mode spectrum demonstrates the effectiveness of the MQW active layer and results in low-intensity noise. Annealing phenomena under cw operation are also discussed.