AlGaN Multi-quantum Well Research Articles

Monolithically integrated UVC AlGaN-based multiple quantum wells structure and photonic chips for solar-blind communications

High quality AlGaN material growth and chip fabrication of monolithically integrated solar blind light-emitting diodes (LEDs), waveguides and photodetectors (PDs) on an AlGaN multi-quantum wells (MQWs) wafer are presented. The strong light constraint of waveguides is confirmed, in virtue of predominant ultraviolet transverse magnetic (TM) modes in transverse transmission. Unaffected by the ambient lighting, the PDs demonstrate sufficient photosensitivity to the optical signal traveling along waveguides owing to the emission-responsion spectral overlap. When the LEDs are operated at 20 mA current, the photo-to-dark current ratio (PDCR) in the PDs is up to seven orders of magnitude, which is higher than previously reported values for self-driven nitride PDs. The responsivity, specific detectivity and external quantum efficiency (EQE) of self-driven PDs are 186 A/W, 2.54 × 1014 Jones and 8.4 × 104%, respectively. Benefiting from the effective optical isolation, the light crosstalk between adjacent devices is effectively reduced about 70%. The self-driven PDs exhibit 127/131 ns (rise/decay) rapid response in the on-chip communication. This work opens a pathway to drastically improve the ultraviolet-C (UVC) monolithically integrated systems and extends the fields of application in solar-blind communication.

Large area deep ultraviolet light of Al0.47Ga0.53N/Al0.56Ga0.44N multi quantum well with carbon nanotube electron beam pumping

Large area deep ultraviolet (DUV) light is generated by carbon nanotube (CNT) cold cathode electron beam (C-beam) irradiation on Al0.47Ga0.53N/Al0.56Ga0.44N multi quantum wells (MQWs) anode. We developed areal electron beam (EB) with CNT cold cathode emitters. The CNT emitters on silicon wafer were deposited with an area of 188 mm2, and these were vertically aligned and had conical structures. We optimized the C-beam irradiation conditions to effectively excite AlGaN MQWs. When AlGaN MQWs were excited using an anode voltage of 3 kV and an anode current of 0.8 mA, DUV with a wavelength of 278.7 nm was generated in a large area of 303 mm2. This DUV area is more than 11 times larger than the light emitting area of conventional EB pumped light sources and UV-LEDs.

Emission and material gain spectra of polar compressive strained AlGaN quantum wells grown on virtual AlGaN substrates: Tuning emission wavelength and mixing TE and TM mode of light polarization

It is shown that compressively strained polar AlxGa1−xN/AlyGa1−yN quantum wells (QWs) of various contents grown on virtual AlYGa1−YN substrates (Y = 20, 40, 60, 80, and 100%) are able to cover the whole UV-A, -B, and -C spectral range but their contents and widths have to be carefully optimized if they are to be used as the active region of light emitting diodes and laser diodes. The emission wavelength from AlGaN multi QWs can be tuned by both the QW width and barrier thickness, but the range of QW width for which an efficient luminescence is expected is very small (2–4 nm) due to a very weak electron-hole overlap for wider QWs. The most effective method for wavelength tuning in this QW system is content engineering, i.e., lowering Al concentration in the QW region. The decrease of Al concentration in the QW shifts the emission peak to red, broadens this peak, weakens its intensity, and changes its polarization from transverse magnetic (TM) to TM mixed with transverse electric (TE). For laser diodes the optimal QW design is more rigorous concerning the QW width since this width should be below 3 nm. Moreover it is shown that the TE and TM mode of materials gain overlap and are strongly blueshifted in comparison to emission spectrum.

Localized surface plasmon enhanced deep UV-emitting of AlGaN based multi-quantum wells by Al nanoparticles on SiO2 dielectric interlayer

In this paper, we report a 2.6-fold deep ultraviolet emission enhancement of integrated photoluminescence (PL) intensity in AlGaN-based multi-quantum wells (MQWs) by introducing the coupling of local surface plasmons from Al nanoparticles (NPs) on a SiO2 dielectric interlayer with excitons and photons in MQWs at room temperature. In comparison to bare AlGaN MQWs, a significant 2.3-fold enhancement of the internal quantum efficiency, from 16% to 37%, as well as a 13% enhancement of photon extraction efficiency have been observed in the MQWs decorated with Al NPs on SiO2 dielectric interlayer. Polarization-dependent PL measurement showed that both the transverse electric and transverse magnetic mode were stronger than the original intensity in bare AlGaN MQWs, indicating a strong LSPs coupling process and vigorous scattering ability of the Al/SiO2 composite structure. These results were confirmed by the activation energy of non-radiative recombination from temperature-dependent PL measurement and the theoretical three dimensional finite difference time domain calculations.

Efficiency Improvement of Deep‐Ultraviolet Light Emitting Diodes with Gradient Electron Blocking Layers

In this paper, a gradient electron blocking layer (GEBL) is introduced to improve efficiency of deep‐ultraviolet light‐emitting diodes (DUV‐LEDs). Various structures of DUV‐LEDs are simulated to determine the energy band diagram variation and carrier injection mechanism resulting from the insertion of the GEBL. The simulation results show the improved electron and hole transport behavior in AlGaN multi quantum wells (MQWs). Especially, the injection efficiency of holes is improved with the increasing number of GEBL steps, which lead to the enhancement of internal quantum efficiency (IQE). The DUV‐LED structures with GEBL are grown in a high‐temperature metal‐organic chemical vapor deposition (HT‐MOCVD) system. Electroluminescence (EL) spectra show that the emission intensity at a wavelength of 280 nm from a DUV‐LED with a 12‐steps EBL is about 2.3 times that from a DUV‐LED with a single EBL.

Structural and optical investigations of AlGaN MQWs grown on a relaxed AlGaN buffer on AlN templates for emission at 280 nm

10-period Al0.57Ga0.43N/Al0.38Ga0.62N multi-quantum wells (MQWs) were grown on a relaxed Al0.58Ga0.42N buffer on AlN templates on sapphire. The threading dislocations and V-pits were characterized and their origin is discussed. The influence of V-pits on the structural quality of the MQWs and on optical emission at 280nm was analyzed. It was observed that near-surface V-pits were always associated with grain boundaries consisting of edge threading dislocations originating from the AlN/Al2O3 interface. Although the high density of V-pits disrupted MQWs growth, it did not affect the internal quantum efficiency which was measured to be ~1% at room temperature even when V-pit density was increased from 7×107cm−2 to 2×109cm−2. The results help to understand the origin, propagation and influences of the typical defects in AlGaN MQWs grown on AlN/Al2O3 templates which may lead to further improvement of the performance of DUV devices.

Experimental and simulation study on ultraviolet light emission from quaternary InAlGaN quantum wells with localized carriers

Different-indium-content quaternary InAlGaN multi-quantum-well (MQW) structures with emission wavelength around 290–310 nm were grown by metalorganic chemical vapor deposition, and their emission characteristics including the indium-segregation effect were investigated by using not only experimental evaluation but also applying simulation technique, on the basis of the weakly localized exciton model. The value of an effective localized level in the quaternary InAlGaN MQWs was estimated to be around 70 meV from the relationship between photoluminescence lifetimes and photon energies. The simulation study, which was conducted by fitting the emission spectra, also derived the value of around 50 meV. The present study also indicated that the quaternary InAlGaN MQW structures with the indium segregation have clear advantages over ternary AlGaN MQW ones especially in the range of dislocation density larger than approximately 1 × 108 cm−2.

Optical detection of nonradiative recombination centers in AlGaN quantum wells for deep UV region

AbstractBy superposing a chopped below‐gap excitation (BGE) light on a cw above‐gap excitation and observing the intensity change of photoluminescence (PL) due to the BGE, we clarified a distribution of nonradiative recombination (NRR) centers in AlGaN multi‐quantum well (MQW) structures for the wavelength region of deep ultraviolet (UV). The decrease in band‐edge PL peak intensity at 10 K exemplified the presence of a pair of NRR centers in AlGaN well layers whose transition energy corresponds to that of the BGE. The BGE power dependence of the normalized PL intensity showed a quenching saturation due to trap‐filling effect of electrons in one of the centers which were activated by the BGE. The BGE energy dependence of the normalized PL intensity revealed that the optical activation energy of the dominant NRR process via these trap states in the AlGaN MQW is around 1.17 eV. The purely optical scheme of detection enables us to obtain clues for identifying defect levels in DUV region and eliminating them during growth process without any preparation of electrodes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low dislocation density AlGaN epilayers by epitaxial overgrowth of patterned templates

We present an epitaxial overgrowth process for reducing threading dislocations in AlxGa1−xN over the entire compositional range. This process avoids the use of UV-absorbing GaN layers and results in a spatially uniform defect reduction which eliminates the need for precise alignment of devices to low-defect areas of the wafer. Using the described overgrowth process, we demonstrate Al0.3Ga0.7N and Al0.6Ga0.4N epilayers with a dislocation density of 2×108cm−2 and 5×108cm−2 respectively, rendering them suitable as templates for deep-UV bottom-emitting LEDs and laser diodes. The process involves patterning of submicron-wide-stripes, less than 1μm in height, into an AlGaN/AlN/sapphire template and subsequent regrowth of a 5–10μm thick AlGaN epilayer. The sub-micron width of the mesa allows for bending of threading dislocations that would continue to thread vertically through wider mesas. Utilizing 1.3mm-thick sapphire substrates (3× thicker than commonly used), epilayer cracking from regrowth is eliminated and wafer bow over a 2-in. diameter substrate is reduced to less than 15μm. We observed a 7× increase in photoluminescence intensity from GaN–AlGaN multi-quantum well structures emitting at 340nm and a 15× increase in electroluminescence from laser diode heterostructures when grown on patterned Al0.3Ga0.7N templates.

Surface-plasmon-enhanced deep-UV light emitting diodes based on AlGaN multi-quantum wells

We report the development of complete structural AlGaN-based deep-ultraviolet light-emitting diodes with an aluminum thin layer for increasing light extraction efficiency. A 217% enhancement in peak photoluminescence intensity at 294 nm is observed. Cathodoluminescence measurement demonstrates that the internal quantum efficiency of the deep-UV LEDs coated with Al layer is not enhanced. The emission enhancement of deep-UV LEDs is attributed to the higher LEE by the surface plasmon-transverse magnetic wave coupling. When the proportion of the TM wave to the Al layer increases with the Al content in the AlxGa1-xN multiple quantum wells, i.e., the band edge emission energy, the enhancement ratio of the Al-coated deep-UV LEDs increases.

High quality AlGaN epilayers grown on sapphire using SiN x interlayers

We have investigated the optimization of Al 0.2Ga 0.8N layers directly grown on sapphire by metalorganic vapor phase epitaxy (MOVPE). The quality of the AlGaN epilayers was improved by in situ nano-masking employing ultra-thin SiN x interlayers. Transmission electron microscopy (TEM) investigations reveal an enormous reduction of edge-type dislocations by SiN x nano-masking. Furthermore, formation of bundles of dislocations converging to the surface is visible, leading to large areas up to 1 μ m in size on the surface which are almost defect free. The SiN x surface coverage was carefully optimized resulting in narrower (1 0 2)-reflections of high resolution X-ray diffraction (HRXRD), down to full width at half maximum (FWHM) values of 570 arcsec, in addition to narrow symmetric HRXRD reflections down to FWHM of 150 arcsec. Structures with double SiN x interlayer revealed even higher quality of the epilayers with a FWHM of 440 arcsec for the (1 0 2)-reflection. A series of GaN–AlGaN multi-quantum wells were grown on such high quality templates. The MQWs show a significant decrease in peak widths and also an increase in the luminescence intensity. Furthermore, these templates have been used as buffer layers for ultraviolet light-emitting diodes emitting at 350 nm.

Development of 230–270 nm AlGaN‐based deep‐UV LEDs

AbstractWe demonstrated AlGaN multi‐quantum well (MQW) deep‐ultraviolet (UV) light‐emitting diodes (LEDs) with wavelengths in the range of 227.5 to 273 nm fabricated on high‐quality AlN buffers on sapphire substrates grown by metal‐organic chemical vapor deposition (MOCVD). We realized crack‐free, thick AlN buffers on sapphire with a low threading dislocation density (TDD) and an atomically flat surface by using the ammonia (NH3) pulse‐flow multilayer (ML) growth technique. We obtained single‐peaked operation of an AlGaN‐MQW LED with a wavelength of 227.5 nm, which is the shortest wavelength of AlGaN‐based LED on sapphire. The maximum output power and the external quantum efficiency (EQE) of the 261‐ and 227.5‐nm LEDs were 1.65 mW and 0.23% in room‐temperature (RT) continuous‐wave (CW) operation, and 0.15 mW and 0.2% in RT pulsed operation, respectively. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(3): 24–33, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10197

222 nm single‐peaked deep‐UV LED with thin AlGaN quantum well layers

AbstractWe fabricated 222 nm AlGaN multi‐quantum well (MQW) deep‐ultraviolet (DUV) light emitting diode (LED) on high quality AlN buffers grown by ammonia (NH3) pulse‐flow multilayer growth method on sapphire substrate. We obtained remarkable enhancement of emission intensity of 220 nm‐band DUV LED by introducing thin (<1.6 nm) quantum well. We realized a single peaked operation of a 222 nm DUV AlGaN MQW LED under pulsed current injection, which is the shortest wavelength of AlGaN LED on sapphire substrate. The maximum output power and external quantum efficiency of 222 nm AlGaN MQW LED were 14 μW and 0.003%, respectively. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Milliwatt power 270 nm‐band AlGaN deep‐UV LEDs fabricated on ELO‐AlN templates

AbstractWe demonstrated CW milliwatt power operations of 270 nm‐band AlGaN multi‐quantum well (MQW) deep‐ultraviolet (DUV) light‐emitting diodes (LEDs) fabricated on epitaxial lateral overgrowth (ELO) AlN templates on sapphire. An initial AlN stripe layer was grown directly on sapphire by using ammonia (NH3) pulse‐flow multilayer (ML) growth method. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was completely coalesced after the growth of an approximately 15 μm‐thick ELO‐AlN layer grown by low‐pressure metal‐organic chemical vapor deposition (LP‐MOCVD). The edge‐type threading dislocation density (TDD) of the wing region of the ELO‐AlN layer was 3×108 cm‐2, as observed by cross‐sectional transmission electron microscope (TEM) image. The maximum output power of 2.7 mW was obtained from an AlGaN MQW LED with emission wavelength at 273 nm fabricated on ELO‐AlN template under room temperature (RT) CW operation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Remarkable enhancement of 254‐280 nm deep ultraviolet emission from AlGaN quantum wells by using high‐quality AlN buffer on sapphire

AbstractWe observed remarkable enhancement of 254‐280 nm deep ultraviolet (UV) emission from AlGaN multi quantum wells (MQWs) by using low threading dislocation density (TDD) AlN templates on sapphire substrates. High‐quality AlN templates were fabricated using ammonia pulse‐flow multi‐layer (ML) AlN growth technique by metal‐organic chemical vapor deposition (MOCVD). The photoluminescence (PL) intensity of AlGaN‐QW with emission wavelength of 280 nm was increased by approximately 30 times by reducing the full width at half‐maximum (FWHM) of X‐ray diffraction (102) ω‐scan rocking curve (XRC) of AlGaN buffer on ML‐AlN from 1214 to 488 arcsec. We obtained similar emission enhancement for AlGaN QWs with emission wavelengths of 254, 260 and 270 nm. We found that PL intensity of the QWs were rapidly increased when FWHM of XRC (102) is reduced as narrow as 800‐1000 arcsec. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

230-270nm深紫外AlGaN系LEDの進展

We demonstrated AlGaN multi-quantum well (MQW) deep-ultraviolet (UV) light-emitting diodes (LEDs) with wavelengths in the range of 227.5 to 273 nm fabricated on high-quality AlN buffers on sapphire substrates grown by metal-organic chemical vapor deposition (MOCVD). We realized crack-free, thick AlN buffers on sapphire with a low threading dislocation density (TDD) and an atomically flat surface by using the ammonia (NH3) pulse-flow multilayer (ML) growth technique. We obtained single-peaked operation of an AlGaN-MQW LED with a wavelength of 227.5 nm, which is the shortest wavelength of AlGaN-based LED on sapphire. The maximum output power and the external quantum efficiency (EQE) of the 261- and 227.5-nm LEDs were 1.65 mW and 0.23% in room-temperature (RT) continuous-wave (CW) operation, and 0.15 mW and 0.2% in RT pulsed operation, respectively. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(3): 24–33, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10197

227-261nm AlGaN-based Deep Ultraviolet Light-emitting Diodes Fabricated on High-quality AlN Buffer on Sapphire

We demonstrated AlGaN multi-quantum well (MQW) deep ultraviolet (UV) light-emitting diodes (LEDs) with wavelength in the range of 227-261 nm fabricated on high-quality AlN buffers on sapphire substrates. We achieved crack-free, thick AlN buffer on sapphire with low threading dislocation density (TDD) and atomically flat surface by introducing an ammonia (NH3) pulse-flow multi-layer (ML) growth method through metal-organic chemical vapor deposition (MOCVD). The edge- and screw-type dislocation densities of AlGaN layer on AlN buffer were reduced to 7.5×108 and 3.8×107 cm-2, respectively, by using a ML-AlN buffer. We achieved single-peaked high-brightness operations of AlGaN deep-UV LEDs by fabricating them on the ML-AlN buffers on sapphire substrates. The maximum output power and external quantum efficiency (EQE) of the 261 nm and 227.5 nm LEDs were 1.65 mW and 0.23% under room-temperature (RT) CW operation, and 0.15 mW and 0.2%, under RT pulsed operation, respectively.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxial AlN/Sapphire Templates

AlGaN films and deep ultraviolet light-emitting diode (UV-LED) structures with AlGaN multi-quantum wells (MQWs) were grown directly on 2-in.-diameter epitaxial AlN/sapphire template (AlN template) by metalorganic chemical vapor deposition (MOCVD). It was confirmed that crack-free and good-crystal-quality AlGaN films and UV-LED structures can be grown on AlN template without the use of any buffer layer technique such as superlattice structures or pulsed atomic layer deposition. This seemed to be mainly due to the in-plane compressive stress caused by using AlN template as underlying substrates. Deep UV-LED devices were successfully fabricated using MOCVD-grown wafers. Electroluminescence spectra for those LEDs exhibited sharp near-band emissions ranging from 265 to 277 nm, depending on their Al compositions in MQWs. It was also confirmed that the insertion of a thin AlN layer between the active region and the p-type cladding layer can drastically suppress a parasitic sub-band emission around 320 nm.

Avalanche breakdown and breakdown luminescence of AlGaN multiquantum wells

AlGaN p-i-n diodes were fabricated on 6H-SiC substrate using Al0.1Ga0.9N∕Al0.15Ga0.85N multiquantum wells (MQWs) for the active region. Avalanche breakdown and breakdown luminescence of these AlGaN MQW diodes were experimentally investigated. Breakdown electroluminescence from the MQW active layers was observed for unusually low bias values of 9 V. A continuous red shift of the interband luminescence resulting from the Stark effect was observed with increasing reverse bias. The breakdown threshold was found to be as low as 9 V. Polarization-induced electric fields in the Al0.1Ga0.9N well layers were found to have the same direction as the applied field. These polarization fields greatly enhance the ionization coefficient of electrons and help lower the threshold for avalanche breakdown. Substantial enhancement of the ionization coefficient produced by the polarization fields is quantitatively demonstrated with GaN∕AlGaN quantum wells by considering a polarization field with the same direction as the applied field within the GaN well layers.

Optical evidence for lack of polarization in (112¯0) oriented GaN∕(AlGa)N quantum structures

We apply continuous and time resolved photoluminescence spectroscopy for studying GaN∕AlGaN multiquantum wells structures grown on nonpolar a-plane GaN templates. We found that (a) the energy of the emission from the nonpolar samples decreases slightly with the quantum well width, in a manner explained by the quantum size effect only; (b) the energy differences between the absorption and the emission peaks are independent of the well width; and (c) the decay time of the photoluminescence is only slightly dependent on the quantum well width and is quite similar to that of bulk GaN. These observations are markedly different from measurements obtained from conventional polar [0001] oriented quantum well samples. They clearly demonstrate the absence of an electric field in the nonpolar samples. Our observations are favorably compared with an eight bands k∙P model calculations.