Multiquantum-well Layers Research Articles

Effect of indium pre-flow on wavelength shift and crystal structure of deep green light emitting diodes

To produce a deep green (530 nm–570 nm) LED, the suitable indium (In) composition in the InxGa1-xN/GaN multi-quantum well (MQW) structure is crucial because a lower indium composition will shift the wavelength of emission towards the ultraviolet region. In this paper, we clarify the effects of an indium-rich layer to suppress such blue shifting, especially after the annealing process. According to characterizations by the uses of XRD and TEM, narrowing of the MQW layer was observed by the indium capping, while without the capping, the annealing results in a slight narrowing of MQW on the nearest layer to the p-type layer. By adding an indium capping layer, the blue shift of the photoluminescence was also suppressed and a slight red shift to keep green emission was observed. Such photoluminescence properties were consistent with the tiny change of the MQW as seen in the XRD and TEM characterizations.

Fabrication and chemical lift-off of sub-micron scale III-nitride LED structures.

Nanoscale light emitting diodes (nanoLEDs, diameter < 1 µm), with active and sacrificial multi-quantum well (MQW) layers epitaxially grown via metal organic chemical vapor deposition, were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of the sacrificial MQW layer. PEC etch conditions were optimized to minimize undercut roughness, and thus limit damage to the active MQW layer. NanoLED emission was blue-shifted ∼10 nm from as-grown (unpatterned) LED material, hinting at strain relaxation in the active InGaN MQW layer. X-ray diffraction also suggests that strain relaxation occurs upon nanopatterning, which likely results in less quantum confined Stark effect. Internal quantum efficiency of the lifted nanoLEDs was estimated at 29% by comparing photoluminescence at 292K and 14K. This work suggests that colloidal lithography, combined with chemical release, could be a viable route to produce solution-processable, high efficiency nanoscale light emitters.

Comparison of structural and optical properties of blue emitting In0.15Ga0.85N/GaN multi-quantum-well layers grown on sapphire and silicon substrates

Six periods of 2-nm-thick In0.15Ga0.85N/13-nm-thick GaN blue emitting multi-quantum-well (MQW) layers are grown on sapphire (Al2O3) and silicon (Si) substrates. X-ray diffraction, Raman spectroscopy, atomic force microscopy, temperature-dependent photoluminescence (PL), Micro-PL, and time-resolved PL are used to compare the structural and optical properties, and the carrier dynamics of the blue emitting active layers grown on Al2O3 and Si substrates. Indium clustering in the MQW layers is observed to be more pronounced on Al2O3 than those on Si as revealed through investigating band-filling effects of emission centers, S-shaped peak emission energy shifts with increasing temperature, and PL intensity-peak energy spatial nonuniformity correlations. The smaller indium clustering effects in MQW on Si are attributed to the residual tensile strain in the GaN buffer layer, which decreases the compressive strain and thus the piezoelectric polarization field in the InGaN quantum wells. Despite a 30% thinner total epitaxial thickness of 3.3 µm, MQW on Si exhibits a higher IQE than those on Al2O3 in terms of internal quantum efficiency (IQE) at temperatures below 250 K, and a similar IQE at 300 K (30% vs 33%). These results show that growth of blue emitting MQW layers on Si is a promising approach compared to those conventionally grown on Al2O3.

(Invited) Epitaxy of Direct Bandgap Group IV Si-Ge-Sn Alloys towards Heterostructure Light Emitters

The recent development of optically pumped lasers made from Si-congruent GeSn alloys have strengthened the efforts to fabricate a monolithically integratable light source. Such fully Si-compatible solution may allow merging of state-of-the-art electronics with photonic components on the same chip for on-chip optical interconnects. To advance this prospering field, we investigated the epitaxy of different GeSn/SiGeSn heterostructures, based on GeSn active layers. Double heterostructures allow confinement of carriers in GeSn, though with the deleterious presence of misfit dislocations in the rather thick active regions. This can be overcome, however, in multi quantum well (MQW) structures. Microdisk resonators made from different heterostructures were fabricated, indicating enhanced properties of MQW layers. Epitaxy of in-situ doped p-i-n heterostructure diodes shows paths towards electrical carrier injection into GeSn optoelectronic devices.

1550-nm InGaAsP multi-quantum-well structures selectively grown on v-groove-patterned SOI substrates

We report direct growth of 1550-nm InGaAsP multi-quantum-well (MQW) structures in densely packed, smooth, highly crystalline, and millimeter-long InP nanoridges grown by metalorganic chemical vapor deposition on silicon-on-insulator (SOI) substrates. Aspect-ratio-trapping and selective area growth techniques were combined with a two-step growth process to obtain good material quality as revealed by photoluminescence, scanning electronic microscopy, and high-resolution X-ray diffraction characterization. Transmission electron microscopy images revealed sharp MQW/InP interfaces as well as thickness variation of the MQW layer. This was confirmed by atom probe tomography analysis, which also suggests homogenous incorporation of the various III-V elements of the MQW structure. This approach is suitable for the integration of InP-based nanoridges in the SOI platform for new classes of ultra-compact, low-power, nano-electronic, and photonic devices for future tele- and data-communications applications.

Formation properties of an InGaN active layer for high-efficiency InGaN/GaN multi-quantum-well-nanowire light-emitting diodes

Nitride-based nanowires (NWs) have several advantages, such as flexibility in choosing a substrate, easy fabrication, large light-emitting area, no internal electric field, enhanced light extraction, and reduced defects by strain relief, that are useful for enhancing the efficiency of light-emitting diodes (LEDs). Here, we report how crucial the formation properties of the InGaN active layer are for enhancing the efficiency of core-shell InGaN/GaN multi-quantum-well (MQW)-NW LEDs that are selectively grown on oxide templates with perfectly-circular hole patterns. The nanostructures are analyzed for two types of LEDs, one containing defect-free MQW active layer and the other containing MQW layer with defects by using high-resolution transmission electron microscopy. The I-V curve of the defect-free LED shows a rectifying behavior with an on/off ratio of ~109, typical of a diode, and the off-state leakage current of the LED with defects is much larger than that of the defect-free LED, resulting in brighter electroluminescence from the latter device. These results suggest that well-defined nonpolar InGaN/GaN MQW-NWs can be utilized for the realization of high-performance LEDs.

Multichannel DFB Laser Arrays Fabricated by Upper SCH Layer SAG Technique

Multiwavelength laser arrays (MWLAs) fabricated by a novel upper separate confinement heterostructure (SCH) layer selective area growth (SAG) technique are studied in comparison with laser arrays fabricated by the conventional multiquantum well (MQW) SAG technique. For MWLAs by the MQW SAG, the range of the laser emission of the array is smaller than the span of the PL wavelengths of the MQW active layers. In addition, the material quality of the MQW layer degrades greatly as the width of the SAG strips is increased. Thus, the MWLAs fabricated by the method have scattered L-I characters and large fluctuation of wavelength spacings. In contrast, the MWLAs fabricated by the novel SAG method have much better wavelength spacing uniformity than those by MQW SAG, which is comparable to or better than the MWLAs fabricated by E-beam direct writing. Furthermore, the upper SCH layer SAG technique is suitable for the fabrication of MWLAs with wavelength interval, which is a challenge even for E-beam technique. In the SCH layer SAG, the MQW layer is untouched, and the same is true for all the laser elements in an array. The metal organic vapor phase epitaxies ability to control the material properties with high accuracy can be fully utilized for the fabrication of MWLAs. Combined with partially gain-coupled distributed feedback structures, the technique is promising for the fabrication of low-cost, high-quality MWLAs.

InGaN-based multi-quantum well light-emitting diode structure with the insertion of superlattices under-layer

This paper reports on the improved crystalline quality of In0.11Ga0.89N/In0.02Ga0.98N multi-quantum well (MQW) light-emitting diode (LED) structure with the insertion of n-AlN/n-GaN multilayer (ML) and n-Al0.06Ga0.94N/n-GaN strained-layer superlattices (SLSs) cladding under-layer. The LED sample structure was epitaxially grown on Si (111) substrate by metal organic chemical vapor deposition (MOCVD) system with laminar-flow horizontal growth reactor. From X-ray rocking curves (XRCs) omega-scan (ω-scan), the n-Al0.06Ga0.94N/n-GaN SLS under-layer facilitate in improving the crystalline quality of the LED sample structures by reducing the threading dislocation density (TDD). Reciprocal space mapping (RSM) confirms the strain state of the GaN epitaxial layers. Triple axis (TA) X-ray diffraction (XRD) omega-2Theta (ω-2θ) scan reveals a higher-order fringes indicate the ML and MQW layers with sharp layer interfaces. Photoluminescence (PL) measurement of the sample with n-Al0.06Ga0.94N/n-GaN SLS under-layer shows band-to-band photon energy of 2.97eV with narrower full-width at half-maximum (FWHM) as compared to the sample with n-Al0.03Ga0.97N under-layer. Surface morphology examination by atomic force microscopy (AFM) demonstrate smoother sample surface for the sample with n-Al0.06Ga0.94N/n-GaN SLS as compared to conventional sample without the SLS layer.

InGaN/GaN Multiquantum-Well Metal-Semiconductor-Metal Photodetectors With Beta-${\rm Ga}_{2}{\rm O}_{3}$ Cap Layers

InGaN/GaN multiquantum-well (MQW) metal-semiconductor-metal photodetectors with a beta-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> cap layer formed by the furnace oxidation of a GaN epitaxial layer are fabricated and characterized. The beta-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> cap layer is found to suppress the reverse leakage current by at least about two orders of magnitude with a 5-V applied bias because it creates a thicker and higher potential barrier. The reverse leakage current can be further reduced and a 90-fold larger ultraviolet-to-visible rejection ratio can be achieved by using InGaN/GaN MQW layers, which confine the electron-hole pairs generated by lower-energy photons. In addition, the noise level is reduced and detectivity is increased. With a 5-V applied bias, the noise-equivalent power and normalized detectivity are 8.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> W and 7.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively.

Electroluminescence emission from light-emitting diode of p-ZnO/(InGaN/GaN) multiquantum well/n-GaN

We report on the fabrication and characteristics of light-emitting diodes (LEDs) which consist of antimony (Sb) doped p-ZnO, (InGaN/GaN) multiquantum well (MQW), and n-GaN. An electroluminescence (EL) emission at a wavelength of 468 nm is observed from the hybrid LEDs after thermal annealing at 750 °C, showing that Sb-doped p-ZnO can be used as a hole supplying layer in hybrid LEDs. Furthermore, the EL peaks are redshifted as the injection current is increased, indicating that the compressive strain in MQW layers is relaxed due to Sb-doped p-ZnO layer.

A SiGe/Si multiple quantum well avalanche photodetector

The present work investigates the performance of APDs with a SiGe/Si multi-quantum well (MQW) structure, which was fabricated using ultrahigh-vacuum chemical vapor deposition (UHV/CVD). Absorption of radiation and avalanche multiplication occur in both SiGe/Si MQW and the i-SiGe layer. Intense photoluminescence (PL) from strained, epitaxial SiGe alloys grown using UHV/CVD was reported with multiple SiGe/Si MQW and i-SiGe layer. It was found that the avalanche multiplication occurred at about 7 V, when exceeding 7 V, the responsiveness and quantum efficiency rapidly increased. An APD consisting of an epitaxial SiGe/Si MQW as the active absorption layer with intense response in the 800–1500 nm wavelength range is also demonstrated.

Characterization of GaSb/AlGaSb Multi-Quantum-Well Structures Grown on Si(001) Substrates

We prepared GaSb/AlGaSb multi-quantum-well (MQW) structures on n-type silicon (001) substrates by molecular beam epitaxy (MBE). To minimize dislocations in MQW layers and decrease the total thickness of the epitaxial layer, we employed not only an AlSb initiation layer but also a superlattice buffer layer (SL-BL) in a moderately thick GaSb buffer layer. For comparison we also fabricated other MQW structures on a considerably thick GaSb buffer layer without SL-BL. The obtained atomic force microscopy (AFM), transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated that the definite MQW structures for both the samples and the quality of MQW layers prepared using SL-BL were generally better than those of the reference sample. The PL emission of these samples at about 1.30–1.55 µm was observed at room and low temperatures. The dependence of PL emission energy on GaSb well width was well explained by the finite square well potential model.

Design and Fabrication of Low-Driving-Voltage Electroabsorption Modulators Operating at 40 Gb/s

In this paper, a design for low-driving-voltage InGaAlAs/InAlAs electroabsorption modulators (EAMs) operating at 40 Gb/s is described. The theoretical calculation clarified that the tensile-strained InGaAlAs/InAlAs multiquantum-well layers with thin wells provide large and steep extinction characteristics. This was experimentally confirmed. We modeled an EAM with a low-loss coplanar waveguide for both the input and output ports and designed an optimized core structure that assures a sufficient extinction ratio and electrical-to-optical bandwidth for 40-Gb/s operation, in terms of well number and core length. A fabricated device driven by a peak-to-peak voltage as low as 1.1 V shows a 3-dB bandwidth of over 50 GHz and an RF extinction ratio of 10 dB. Error-free operation at 40 Gb/s is confirmed.

Amplifying characteristics of 1.55-/spl mu/m polarization-insensitive SOAs with MQW and strained-bulk active Layers for device application

Fundamental amplifying characteristics of 1.55-mum semiconductor optical amplifiers (SOAs) that contain multiquantum well (MQW) active layers with an optimized tensile strain or bulk active layers with various tensile strains were investigated. Both types of SOAs have their own merits and demerits. For device application in networks, driving current and wavelength in the low-polarization-sensitivity condition are very important, but they have not been clarified yet. The results of the present investigation clarify the driving current and wavelength characteristics of low-polarization-sensitivity SOAs with MQW and strained-bulk active layers. The low-polarization-sensitivity condition of SOAs with a strained-bulk active layer is shown to have a very wide range of driving current and wavelength for network-device application. These results, with other requirements taken into account, provide guidelines for choosing the type of SOA most suitable for a given application

Milliwatt Power 350 nm-band Quaternary InAlGaN UV-LEDs on GaN Substrates

We have demonstrated milliwatt output power from quaternary InAlGaN-based multi-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) with an emission wavelengths of 350–360 nm. GaN substrates were used in order to reduce the density of threading dislocations. By examining a cathode luminescence (CL) image, we confirmed that the threading dislocation density of the quaternary InAlGaN MQW layer was reduced to as little as 1 × 107 cm–2 by using GaN substrate. A significant increase in the UV output power was achieved by increasing the Al content of the p-AlxGa1–xN electron blocking layer up to 28%. As a result, the maximum UV output power obtained was as high as 3.8 mW at 351 nm and 6.3 mW at 358 nm under room temperature (RT) CW operation. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of critical thickness on structural and optical properties of InxGa1−xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kühne, and Richter.

Radiation hard multi-quantum well InP/InAsP. Solar cells for space applications

The effect of proton irradiation on the photovoltaic response of p-i-n InP solar cells that include an InP/InAsP multi-quantum well (MQW) layer in the intrinsic region is studied. Data from cells with three different quantum well structures are presented along with data from a control cell grown without MQW layer. The results show how the addition of the MQW's improves the solar energy conversion efficiency thereby allowing the base to be as thin a 1 μm while still maintaining a good photocurrent. In addition, the. radiation response of the MQW cells, is shown to be equal to or better than the InP control cell.

Electron Overflow to the AlGaN p-Cladding Layer in InGaN/GaN/AlGaN MQW Laser Diodes

Current flow through an InGaN/GaN/AlGaN multi-quantum well (MQW) laser diode is simulated. We found that electron overflow to the AlGaN p-cladding layer is very large, which prevents the current injection into the MQW layers. We clarified that the electron overflow occurs easily in nitride lasers because of three intrinsic reasons; poor hole injection due to the small hole mobility and thermal velocity, the small conduction band offset for InGaN/GaN, and the high threshold carrier density. We show that the Al composition and the p-doping of the AlGaN p-cladding layer is of critical importance to obtain laser oscillation by current injection.

1.3 µm InAsP/InAlGaAs MQW lasers for high-temperatureoperation

InAsP/InAlGaAs strained multiquantum-well (MQW) 1.3 µm lasers with a large conduction band discontinuity are proposed and demonstrated for the first time. An InAsP/InAlGaAs MQW layer with high structural and optical quality was obtained by gas-source molecular beam epitaxy growth combined with post-growth 650°C rapid thermal annealing. The lasers exhibited a low threshold current density of 1.1 kA/cm2 with an excellent characteristic temperature as high as 116 K.

Selective growth of InAlAs by low pressure metalorganic vapor phase epitaxy

InAlAs selective growth on a masked planar InP substrate with patterned SiO 2 has been studied with low-pressure metalorganic vapor phase epitaxy. No polycrystals were observed on the 10 × 200 μm square mask stripes at a growth temperature of 650°C and a growth rate of 0.6 μm/h. A bandgap energy shift and growth-rate enhancement were confirmed in growth regions between the pair of SiO 2 masks. In a double hetero-structure consisting of selectively grown In 0.52 Al 0.48 As In 0.53 Ga 0.47 As multi-quantum well (MQW) active layers and In 0.52Al 0.48As cladding layers, a bright photoluminescence (PL) spectrum from the selectively grown MQW layer was obtained for the first time.