Multiple Quantum Well Layers Research Articles

Well Number Dependence of Internal Quantum Efficiency in AlGaN Quantum Wells on Low‐Dislocation Sputtered AlN Templates

The internal quantum efficiency (IQE) and cathodoluminescence intensity line profile of AlGaN multiple quantum well (MQW) structures on low‐dislocation face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) and on conventional metalorganic vapor‐phase epitaxy‐grown AlN (MOVPE‐AlN) templates are evaluated and the effect of the number of quantum wells (QWs) on the IQE is discussed. The higher IQE in the FFA samples is probably due to the lower threading dislocation (TD) density; however, the IQE also increases with the number of QWs although the TD density remains constant. Effective diffusion length increases with the number of QWs, indicating that the AlGaN MQW layer helps to suppress point defect diffusion, resulting in IQE increase. Furthermore, the segregation of point defects into the TDs and point defect diffusion via the TDs may explain the difference in the IQE improvement rate between the MQWs on the FFA Sp‐AlN and MOVPE‐AlN templates.

Over-67-GHz-Bandwidth Membrane InGaAlAs Electro-Absorption Modulator Integrated With DFB Laser on Si Platform

We fabricate a membrane InGaAlAs electro-absorption modulator (EAM) integrated with a distributed feedback (DFB) laser combining direct wafer bonding and epitaxial regrowth of InP-based layers on a silicon-on-insulator wafer. Heterogeneous integration of an InP-based multiple-quantum-well (MQW) layer into the Si photonics simplifies the integration of the O-band EAMs and laser diodes (LDs). EAMs and LDs can be fabricated using the same MQW layer because of the wide operating range and the direct bandgap of the MQWs. A compact and high-speed lumped-electrode EAM can be made because the membrane lateral p-i-n diode structure has low capacitance and a large optical confinement factor. The effective-refractive-index matching between the membrane InP and Si layers enables the DFB laser to be fabricated with a low-loss supermode waveguide whose MQW core was optically coupled to the Si core. The fabricated 100-μm-long membrane EAM has a high modulation efficiency of about 3 dB/V at 1260 nm and E-O bandwidth of over 67 GHz even without a 50-ohm termination. The EAM-integrated DFB laser has a fiber coupled output power of about −2 dBm and clear eye openings with an extinction ratio of 3.8 and 3.2 dB for 100- and 112-Gbit/s non-return-to-zero signals, respectively.

Highly efficient near ultraviolet LEDs using InGaN/GaN/AlxGa1-xN/GaN multiple quantum wells at high temperature on sapphire substrate

This work demonstrated the highly efficient near-ultraviolet (UV) light-emitting diodes (LEDs) grown on sapphire substrates with InGaN/GaN/AlxGa1-xN/GaN multiple-quantum wells (MQWs), where x is the number of Al composition. The proposed GaN/AlxGa1-xN/GaN quantum barrier and InGaN quantum well of UV-GaN based LED has successfully grown using metal-organic chemical vapor deposition. The result reveals that the GaN/AlxGa1-xN/GaN barrier has higher light output power and lower current leakage than the GaN barrier. A significant increase in wall-plug efficiency was observed at the same wavelength range. The proposed structure in this study serves two functions: it increases the barrier bandgap to enhance carrier confinement. Second, promoting v-pit in the MQW layer may reduce the current resistance while increasing hole mobility. As a result, the internal quantum efficiency (IQE) may increase and lower electrical power consumption while enhancing performance.

Performance improvement of InGaN-based multiple quantum wells embedded nanoporous n-GaN layers

InGaN-based multiple quantum wells (MQWs) embedded nanoporous (NP) GaN layers were fabricated by metal-organic chemical vapor deposition (MOCVD) and followed by electrochemical (EC) etching in oxalic, HF or HNO3 electrolytes. The oxalic-etching and HF-etching do not lead to enhanced luminescence of MQWs, which should be contributed to the damage of InGaN/GaN stacking layers. For the HNO3-etching, the photoluminescence (PL) strength of MQW layer is increased by a factor of 0.52 compared with the as-grown sample, which should be contributed to stress relaxation, light scattering and reflection of quasi vertical nanopore array, and amplify spontaneous emission by resonant cavity. If the nanopore walls in NP-GaN layer are smoothed by the annealing in NH3 atmosphere, the light extraction efficiency of MQWs can be further improved. The study provides a useful method for improving the light extraction efficiency of GaN-based light emitting devices.

A Study of Second‐Order Susceptibility in Digital Alloy‐Grown InAs/AlSb Multiple Quantum Wells

AbstractA preliminary measurement of the second‐order nonlinear optical susceptibility of symmetric, coupled, InAs/AlSb multiple quantum well (MQW) structures is acquired through optical second‐harmonic generation (SHG) at fundamental wavelength 1.55 µm. High quality crystalline MQW structures of variable thickness and corresponding bulk AlSb control samples are achieved using a digital alloy epitaxial growth technique that avoids cluster formation and phase segregation. All samples are grown in between a GaSb cap and substrate layer. To isolate SHG from the MQW (or control) layers of interest from cap and substrate contributions, a multilayer optical response matrix model is built and independently tested by accurately reproducing linear reflectivity spectra. While a simplified response matrix analysis of SHG based solely on bulk χ(2)s does not reproduce the distinct SHG responses of the two sets of samples, the inclusion of an additional interface SHG contribution leads to a successful fit of the data and implies . The results demonstrate a proof‐of‐concept quantification of χ(2) in symmetric MQWs and suggest the possibility of engineering χ(2) in these structures, particularly with the introduction of well asymmetries.

Influence of pulsed Al deposition on quality of Al-rich Al(Ga)N structures grown by molecular beam epitaxy

Surfaces of AlN epitaxial layers grown by molecular beam epitaxy (MBE) in continuous mode were analyzed by means of X-ray photoelectron spectroscopy, atomic force microscopy and reflection high-energy electron diffraction without breaking the ultra-high vacuum. Our studies show the presence of the Al-Al and N-N bonds on the AlN samples, as well as a rather rough 3D-like structure. Significant improvement of the surface quality was achieved by employing a low flux Al deposition in pulsed mode at the end of the MBE growth (still under a nitrogen background pressure). Subsequently, on the obtained AlN buffer layers, Al0.85Ga0.15N/GaN multiple quantum wells (MQWs) were grown to show the influence of the buffer layers quality on the entire structure properties. The photoluminescence studies indicate that the introduction of pulsed low flux Al deposition improves optical properties of the grown MQWs layers, i.e., the relative intensity between the QW emission and the low energy emission is clearly in favor of pulsed Al deposition.

50-GHz-Bandwidth Membrane InGaAsP Electro-Absorption Modulator on Si Platform

We fabricate a membrane InP-based electro-absorption modulator (EAM), in which an InGaAsP-based multiple-quantum-well (MQW) absorption region is buried with an InP layer, on Si-waveguide circuits. By optical coupling between the MQW absorption region and Si core, a low-loss and large-absorption-length (300-μm-long) supermode waveguide is designed to suppress electric-field screening at high optical input power. The EAM is fabricated by combining direct bonding of the MQW layer and regrowth of the InP layer on a thin InP template bonded on a silicon-on-insulator wafer. The fabricated membrane EAM shows an on-chip loss of less than 4 dB at wavelengths over 1590 nm and temperatures from 25 to 50 °C. Since the membrane lateral p-i-n diode structure is beneficial for reducing the RC time constant of a lumped-electrode InP-based EAM, the EO bandwidth of the EAM is around 50 GHz without a 50-ohm termination up to fiber-input power of 10 dBm. Using the device, we demonstrate clear eye openings for 56-Gbit/s NRZ and 112-Gbit/s PAM4 signals at temperatures from 25 to 50 °C.

Enhancement in light-emission efficiency of InGaN/GaN multiple quantum well layer by a porous-GaN mirror

Mixed electrolyte of oxalic acid and NaNO3, which can be used to fabricate an InGaN/GaN multiple quantum well (MQW) structure with an embedded porous-GaN distributed Bragg reflector (DBR), is more effective than oxalic acid or NaNO3 solutions. Compared to the as-grown GaN-based film, the etched sample was significantly enhanced and narrowed in the intensity and line-width of photoluminescence (PL), respectively, which are contributable to strain relaxation of MQW layer and light-interference effect induced by forming the embedded porous-DBR mirror. Due to the presence of nanopores in the MQW layer, the mass-transport cause the crystalline quality of the MQW layer to deteriorate significantly during the regrowth of GaN-based light emitting diode (LED). Therefore, the performance enhancement of the PL in the LED with the embedded DBR structure should be contributable to improved external and internal quantum efficiencies.

Direct observation of charge accumulation in quantum well solar cells by cross-sectional Kelvin probe force microscopy

We report here the direct observation of charge accumulation in GaAs/AlGaAs multiple quantum well (MQW) solar cells by employing cross-sectional Kelvin probe force microscopy (KPFM). This sample is characterized by thin barrier layers that enable miniband formation. The contact potential difference, or potential between the tip and the semiconductor sample, was measured along the p–i–n junction. We observed, under illuminated conditions, a change in the potential gradient, or bending, at a position of the MQW layer, but not in the reference sample without quantum well. This clearly shows that charge is accumulated in the MQW region. We also found that electron accumulation in the MQW layer and the density measured on the surface is about 1 × 1011 cm−2. Our experimental results show that KPFM is a powerful way of understanding the device physics of nanostructure-based solar cells.

Ge/SiGe multiple quantum well fabrication by reduced-pressure chemical vapor deposition

In this paper we deposit structures comprising a stack of 10 periods made of 15-nm-thick Ge multiple quantum wells (MQWs) enclosed in a 15-nm-thick Si0.2Ge0.8 barrier on SiGe virtual substrates (VSs) featuring different Ge content in the 85%–100% range to investigate the influence of heteroepitaxial strain on Si0.2Ge0.8 and Ge growth. With increasing Ge concentration of the VS, the growth rate of Si0.2Ge0.8 in the MQWs increases. Si incorporation into the Si0.2Ge0.8 layer also becomes slightly higher. However, almost no influence of the growth rate is observed for Ge growth in the MQWs. We argue that increased tensile strain promotes the Si reaction at the surface. In the case of Si0.2Ge0.8 growth on Ge, we observe a smeared interface due to Ge segregation during the growth. Furthermore, we observe that the interface width increases with increasing Ge concentration of the VS. We attribute this observation to the increased segregation of Ge driven by increased strain energy accumulated in the Si0.2Ge0.8 layers. We also observe that the MQW layer “filters out” threading dislocations formed in the VS.

Effect of dual-temperature-grown InGaN/GaN multiple quantum wells on luminescence characteristics

Room-temperature electroluminescence spectra of InGaN/GaN multiple quantum well (MQW) structures grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) were investigated. It is found that, in comparison with the same-temperature growth of MQW layers, GaN barrier layers grown at a temperature higher than the well layer growth of 790 °C are beneficial to improve the emission intensity of InGaN/GaN MQWs. The XRD and AFM measurements show a better interface quality and less non-radiative recombination centers (trench defects) exist in the dual-temperature-grown InGaN MQWs between InGaN well and GaN barrier layers. In addition, increasing barriers growth temperature will induce less concentration of deeper localization states which may contribute to a blue shift of EL emission wavelength.

Light-emitting diodes with surface gallium nitride p\u2013n homojunction structure formed by selective area regrowth

In this study, the blue light-emitting diode (LED) structures based on gallium nitride (GaN) were presented. Each structure possessed a surface GaN p–n junction, which was formed through selective area regrowth on an InGaN/GaN multiple quantum well (MQW) structure and served as the carrier injector. The LEDs that showed efficient hole injection and current spreading were configured to form a p-type GaN layer between the MQW and regrown n-type GaN top layer. These LEDs exhibited higher luminous efficiency and lower operation voltage than the LEDs with regrown p-type GaN top layers. The LEDs with n-type GaN top layers emitted single-peak spectra at approximately 450 nm under a forward bias. The UV peak at 365 nm (i.e., the GaN band-edge emission) was absent because the regrown surface GaN p–n junctions behaved as carrier injectors rather than photon injectors. In other words, the single-peak blue emission was not generated by the optical pumping of UV light emitted from the surface p–n GaN homojunction.

Multiple Angle Analysis of 30-MeV Silicon Ion Beam Radiation Effects on InGaN/GaN Multiple Quantum Wells Blue Light-Emitting Diodes

High-resolution X-ray diffraction, temperature-dependent photoluminescence (PL), time-resolved PL, and positron annihilation spectroscopy are employed to investigate the degradation mechanism of InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) under silicon ion irradiation. Reduction of the quantum-confined Stark effect due to crystalline strain relaxation, enhancement of indium localization due to thermal spike, generation of nonradiative recombination centers (NRCs), and carrier removal effect due to atom displacement are revealed to be critical factors in LEDs postirradiation performance. New NRCs are proven to be the main reason for the degradation of the internal quantum efficiency of MQWs. The increase of the threshold voltage and leakage current in LEDs are caused by the carrier removal effect and new defects in bandgap induced by radiation. In addition, new NRCs are found to appear earlier than indium localization and carrier removal effect with increasing silicon ion fluence. Atom displacement defects are revealed to be located mainly in p-type GaN and MQWs layers. Radiation-induced nitrogen vacancies are considered compensation donors in p-type GaN, whereas all other nitrogen and gallium-related defects are NRCs in MQWs.

Photoluminescence properties of etched GaN-based LEDs via UV-assisted electrochemical etching

A GaN-based light emitting diode (LED) with an InGaN/GaN multiple quantum well (MQW) structure was electrochemically etched in an oxalic acid solution under ultraviolet (UV) light and fabricated into nanoporous structure for the first time. Compared to that of the as-grown GaN-based LED, photoluminescence (PL) emission of the etched LEDs shows (i) a marked blue-shift which is ascribed to a stress relaxation and a relative decrease of indium (In) component in the MQW layer, (ii) a two-fold enhancement of PL intensity which results from the increase of light-extracting surface area, light-guiding effect, and internal quantum efficiency.

GaAsBi/GaAs MQWs grown by MBE using a two-substrate-temperature technique

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) on (100) GaAs substrates were grown by molecular beam epitaxy (MBE) using the two-substrate-temperatures (TST) technique. In the TST technique, the substrate temperature was set at TGaAsBi = 350 °C for GaAsBi layers and TGaAs = 550 °C for GaAs layers for the growth of MQWs. The structural and optical characterization of GaAsBi/GaAs MQWs using TST technique was carried out for growth optimization by changing As4/Ga beam equivalent pressure (BEP) ratio. Atomic force microscope observation shows high As4/Ga BEP ratio supply can prevents roughening of the GaAsBi layer surface of MQWs. High resolution x-ray diffraction analysis results reveal Bi incorporation enhancement with increasing As4/Ga BEP ratio. The reciprocal space mapping shows highly strained MQWs layers can be grown without relaxation with respect to the GaAs substrate at optimized growth conditions. At room temperature 1.22 μm photoluminescence emission was obtained from GaAs0.962Bi0.038/GaAs MQWs grown using TST technique.

Studies of the light output properties for a GaN based blue LED using an electro-optical simulation method

In this study, an electro-optical simulation method is developed to predict the light intensity distribution and luminous flux of an in-house fabricated GaN based blue LED chip. The entire modeling process links an electrical simulation with ANSYS and optical simulation with LightTools, by assuming a proportional relation between the distributed current density and light emission energy on the multiple quantum well (MQW) layer. Experimental results show that the proposed simulation method can give a good prediction on the light intensity distribution for a semi-packaged GaN based blue LED chip. Further analysis on the simulation results reveals that an increase of at most 8% of the luminous flux can be achieved when the current density is controlled to evenly distribute on the MQW layer whereas the chip structure and electro pattern remains the same.

GaAsBi/GaAs MQWs MBE growth on (411) GaAs substrate

Eleven periods of GaAsBi/GaAs multiple quantum wells (MQWs) grown on (411)A and (411)B GaAs substrates at TGaAsBi= 350 °C and TGaAs= 550 °C by molecular beam epitaxy (MBE) and their structural and optical properties were investigated. The high resolution X-ray diffraction (HRXRD) analysis shows increment in Bi composition with the increasing As4 beam equivalent pressure (BEP). The reciprocal space mapping (RSM) reveals highly strained GaAsBi/GaAs MQWs layers can be grown without relaxation with respect to the (411) GaAs substrate on both A and B sides at an optimized growth condition of Bi and As4 BEP; while (411)A sample with higher bismuth content (5.5 %) shows lattice dislocation of MQWs due to lateral strain. The temperature-dependent photoluminescence (PL) shows 1.26 μm emission at room temperature from both the (411)A and (411)B samples for GaAs0.97Bi0.03/GaAs MQWs. It is perceived that GaAsBi/GaAs MQWS can be successfully grown on both sides of (411) GaAs substrate.

The effect of composite GaN/InGaN last barrier layer on electron leakage current and modal gain of InGaN‐based multiple quantum well laser diodes

In order to enhance the optical confinement and to reduce the vertical electron leakage current of InGaN‐based multiple quantum well (MQW) laser diodes, a 2 nm GaN/5 nm In0.05Ga0.95N composite layer is designed as the last barrier layer between the MQW and AlGaN blocking layer in the p‐region. The device simulation shows that the laser diode then has a lower threshold current and a higher output light power under same injection current than the conventional laser diodes without such a composite barrier layer. It is found that the thickness of InGaN has little influence on the P–I characteristic, while the In composition significantly changes the threshold current and output light power properties of laser diodes, which is attributed to the enhanced optical confinement and reduced vertical electron leakage current escaping from active region to p‐doped region. The influences of the two parameters to modal gain are analyzed, too.

AlGaInAs MOVPE selective area growth for photonic integrated circuits

Abstract We developed a generic photonic integration platform based on selective area growth (SAG) by metal organic vapor-phase epitaxy (MOVPE) of AlGaInAs/InP multiple quantum well (MQW) material. For efficient and predictive band gap engineering of photonic integrated circuits, different SAG zones of active and passive function heterostructures are precisely modeled and characterized. With the vapor-phase diffusion model, using numerical simulations of finite volumes, we extracted the three effective diffusion lengths of Al, Ga, and In species. In our growth conditions, these lengths were 32, 65, and 14 μm, respectively. The Kardar-Parisi-Zhang (KPZ) equation, a classic approach to describe the growing interface profile, is used. AlGaInAs MQW properties are then simulated in terms of thickness, composition, band gap, and biaxial strain variations. Highly resolved μ-photoluminescence and optical interferometer microscopy measurements confirm the validity of the band gap and thickness variations for both bulk and MQW layers. A new diffractometer, with a submillimeter X-ray spot, was used to study the structural properties of the MQW in the center of the SAG area. As an application, we present the realization and operation of full-monolithic high-speed advanced modulation format transmitters based on novel prefixed optical phase switching by fast electro-absorption modulators.

Correlation between the structural and cathodoluminescence properties in InGaN/GaN multiple quantum wells with large number of quantum wells

Cathodoluminescence (CL) characteristics on 30-period InGaN/GaN multiple quantum well (MQW) solar cell structures are investigated, revealing the relationship between optical and structural properties of the MQW structures with a large number of quantum wells. In the bottom MQW layers, a blueshift of CL peak along the growth direction is found and attributed to the decrease of indium content due to the compositional pulling effect. An obvious split of emission peak and a redshift of the main emission energy are found in the top MQW layers when the MQW grows above the critical layer thickness. They are attributed to the segregation of In-rich InGaN clusters rather than the increase of indium content in quantum well layer. The MQW structure is identified to consist of two regions: a strained one in the bottom, where the indium content is gradually decreased, and a partly relaxed one in the top with segregated In-rich InGaN clusters.