Room Temperature Electroluminescence Research Articles

InP-based strain engineered InAs(Sb)/InAsPSb multiple quantum wells with tunable emission and high internal quantum efficiency enabled by Sb incorporation

A type I InAs(Sb)/InAsPSb strain engineered multiple quantum wells light emitting diodes system has been demonstrated. Tensile InAsPSb quantum barriers with a high degree of band offset (∆EC = 116–123 meV, ∆EV = 193–250 meV) were used to compensate for the high compressive strain of the InAs(Sb) quantum wells. The structure was grown on the n+-InAsxP1−x metamorphic buffer with a high degree of relaxation (98%), low surface roughness (0.69 nm), and low dislocation density. Through careful strain engineering design, the compressive strain of InAs(Sb) reaches 0.57%–1.52% without strain relaxation. The incorporation of Sb into the multiple quantum wells not only reduces the bandgap but also improves the interface quality by acting as an effective surfactant. Structural analysis reveals superior quality in InAsSb/InAsPSb multiple quantum wells compared to InAs/InAsPSb multiple quantum wells, demonstrating significantly reduced interface roughness and suppression of the Stranski–Krastanov growth mode. Room temperature electroluminescence measurements show a tunable emission wavelength ranging from 2.7 to 3.3 μm, accompanied by a narrow full width at half maximum value of 45 meV. Photoluminescence analysis indicates that the internal quantum efficiency of InAsSb/InAsPSb multiple quantum wells is 5.5%, which is 7 times higher than that of InAs/InAsPSb.

Room-temperature electroluminescence in the InAsSbP/InAs0.95Sb0.05/InAsSbP single quantum well

The electrical and luminescent characteristics of the heterostructure with a 20 nm-wide n-InAsSbP/n-InAs0.95Sb0.05/p-InAsSbP single quantum well, grown on an n-InAs substrate by the MOVPE method, have been studied. An intense room temperature electroluminescence (EL) with a maximum near the photon energy of 0.34 eV and a FWHM of 32 meV was discovered. It was established that the EL in such single quantum well has been emitted due to type I radiative transitions between the ground levels of electrons and holes both at forward and reverse bias. The characteristics of heterostructures with a single quantum well and the active region based on a bulk InAsSb layer of the same composition have been compared.

Light-emitting diodes with Ge(Si) nanoislands embedded in photonic crystals

Room temperature lateral p+–i–n+ light-emitting diodes (LEDs) with photonic crystals embedded in the i-region were fabricated on structures with Ge(Si) self-assembled islands and their optical properties were investigated. The use of preliminary amorphization and solid phase epitaxy of the implanted p+ and n+ contact regions made it possible to reduce the impurity activation temperature from 800 °С–1100 °С to 600 °С, which corresponds to the growth temperature of Ge(Si) islands. This resulted in a significant reduction of the detrimental effect of the high-temperature annealing used for diode formation on the intensity and spectral position of the luminescence signal from the islands. It was shown that significant enhancement (more than an order of magnitude) of room temperature electroluminescence of Ge(Si) islands in the spectral range of 1.3–1.55 μm can be achieved due to their interaction with different modes of the photonic crystals. The measured radiation power of the obtained diodes in the spectral range of 1.3–1.55 μm exceeds 50 pW at a pump current of 8 mA, which is an order of magnitude higher than the previously achieved values for micro-LEDs with Ge(Si) nanoislands. The obtained results open up new possibilities for the realization of silicon-based light emitting devices operating at telecommunication wavelengths.

GeSn/Ge Multiquantum-Well Vertical-Cavity Surface-Emitting p-i-n Structures and Diode Emitters on a 200 mm Ge-on-Insulator Platform

An efficient monolithically integrated light source with complementary metal-oxide-semiconductor (CMOS) compatibility remains the missing component to enable Si photonics for various applications. In particular, vertical-cavity-surface-emitting (VCSE) light sources, such as resonant cavity light-emitting diodes (RCLEDs) and vertical-cavity surface-emitting lasers (VCSELs), are strong contenders due to their compact size, circular emission profile with low beam divergence, wafer-scale fabrication compatibility, high bandwidth, and high coupling efficiency to fiber optic cables. We report the first demonstration of 8-in. wafer-scale GeSn/Ge multiple-quantum-well VCSE p-i-n structures and diodes for laser or light-emitting diode (LED) applications in Si photonics by wafer bonding and layer transfer techniques, which are challenging for all-epitaxy routes. The wafer could be 12 in. (300 mm) since the MQW gain medium is grown on a 300 mm Si wafer, and the bonding is wafer-scale. Alternative dielectric layers (SiO2/SiN/SiO2), introduced by wafer bonding, under the emitting structure, serve as the bottom mirror for the vertical cavity. The Ge0.92Sn0.08/Ge MQW layer is utilized to improve the material quality and to confine injected carriers. As a result, more than 8× enhancement of light emission due to the vertical cavity resonance was demonstrated by photoluminescence spectroscopy. Besides, the spectral purity is enhanced by the single-longitudinal-mode cavity. The intensity of light emission is insensitive to the temperature range from 4 to 300 K and even becomes stronger at higher temperatures since the stronger resonant cavity effect at higher temperatures compensates for the drop in light emission efficiency caused by nonradiative centers. The vertical cavity effect on the light emission is further verified by reflectivity spectroscopy and optical simulations. A positive gain can be achieved as indicated by optical gain calculations. Room-temperature electroluminescence and an excellent carrier injection efficiency of the MQW VCSE diode are observed, showing its potential for electrically injected RCLEDs and VCSELs.

Nondestructive Evaluation of Detuned Wavelength for As-Grown VCSEL Epi-Wafer

We propose a nondestructive method to estimate the peak wavelength of the active quantum well (QW) region of an as-grown vertical-cavity surface-emitting laser (VCSEL) epi-wafer based on room temperature electroluminescence (EL) measurements. Contacts for the electrical excitation are established using flexible conductive materials on the epilayers without the fabrication of metal electrodes. The angle dependence of the EL properties is investigated to analyze the detuned wavelength of the active QW region for the VCSEL structure. The wavelength offset caused by detuning obtained from the as-grown epi-wafer correlates with the laser threshold properties of the VCSEL chips fabricated from the epi-wafer. The results show the possibility of a nondestructive evaluation method without metal electrodes for as-grown VCSEL epi-wafers, particularly the active QW region, which has been challenging to optically analyze from completed VCSEL epi-structures.

Mid-infrared resonant light emission from GeSn resonant-cavity surface-emitting LEDs with a lateral p-i-n structure

We demonstrate room-temperature, mid-infrared resonant electroluminescence from GeSn resonant-cavity LEDs with a lateral p-i-n configuration on a silicon-on-insulator substrate. A vertical cavity to enhance light emission in the GeSn active layer is formed by the low-index buried oxide and deposited SiO 2 layer. A planar lateral p-i-n diode structure favorable for CMOS-compatible, dense integration was designed and fabricated for current injection. Under continuous-wave electrical injection, room-temperature resonant electroluminescence was successfully observed at ∼ 1980 nm with a spectral emission factor of 2.2. These results could pave the way toward efficient electrically injected GeSn light emitters operating at room temperature.

Light sources with bias tunable spectrum based on van der Waals interface transistors

Light-emitting electronic devices are ubiquitous in key areas of current technology, such as data communications, solid-state lighting, displays, and optical interconnects. Controlling the spectrum of the emitted light electrically, by simply acting on the device bias conditions, is an important goal with potential technological repercussions. However, identifying a material platform enabling broad electrical tuning of the spectrum of electroluminescent devices remains challenging. Here, we propose light-emitting field-effect transistors based on van der Waals interfaces of atomically thin semiconductors as a promising class of devices to achieve this goal. We demonstrate that large spectral changes in room-temperature electroluminescence can be controlled both at the device assembly stage –by suitably selecting the material forming the interfaces– and on-chip, by changing the bias to modify the device operation point. Even though the precise relation between device bias and kinetics of the radiative transitions remains to be understood, our experiments show that the physical mechanism responsible for light emission is robust, making these devices compatible with simple large areas device production methods.

Strong room-temperature EL emission from Ge-on-Si (1 1 1) diodes

A Ge(111) p-n structure is epitaxially grown on a Si(111) substrate, where in-situ doping of Phosphorus (P) and Boron (B) is carried out. Also, P δ-doping is performed near the surface to make low-resistivity ohmic contacts. As a result, we obtain strong electroluminescence (EL) emissions at room temperature from fabricated Ge-on-Si(111) vertical light emitting diodes (LEDs) Post-growth annealing is effective to significantly enhance the EL intensity. It is implied that diffusion and re-distribution of the P donor atoms after the annealing play important roles for the EL intensity enhancements.

Improved performance of near UV-blue n-ZnO/p-GaN heterostructure LED with an AlN electron blocking layer

High quality single crystalline n-type ZnO films, with a thickness of 200 nm, were grown on top of AlN/GaN layers under 25 W, 50 W and 100 W sputtering powers. The AlN and p-type GaN layers were deposited by metalorganic chemical vapor deposition (MOCVD). From X-ray diffraction measurements, the samples exhibited the (000l) peaks corresponding to both ZnO, GaN and AlN monocrystalline layers. As the sputtering power was increased, the ZnO grain size increased and the dislocation density decreased. This result is supported by a reduction of the rms values obtained on the ZnO layers from Atomic Force Microscopy (AFM) results. In addition, photoluminescence peaks of ZnO at 372 nm, 375 nm and 380 nm were seen as dependent on the sputtering power. We have used an AlN electron blocking layer between ZnO and GaN films to improve the electroluminescence from the n-ZnO side. Room temperature electroluminescence (EL) of the LEDs demonstrated near UV-blue emission consisting of predominating peaks centred at 405 nm, 390 nm and 380 nm for the device with ZnO deposited at 25 W, 50 W and 100 W sputtering powers, respectively. Moreover, the I-V curves of the LEDs showed a rectifying behavior with 6.8 V, 6.4 V, 5.2 V threshold voltages for 25 W, 50 W and 100 W values.

Mid-infrared HgTe Colloidal Quantum Dot LEDs

Mid-infrared HgTe colloidal quantum dot electroluminescent devices are demonstrated. With emission at 4 μm, devices achieved an external quantum efficiency of ∼10-3 and power conversion efficiency of ∼10-4 under biases of a few volts. The power conversion efficiency benefited from lowering the transparent electrode resistance through the incorporation of a metal conductive grid. The average power emitted was about 16 μW at 2 V bias with 50% duty cycle and a 1 mm2 device. The room-temperature electroluminescence efficiency at low current was limited by the photoluminescence efficiency of the quantum dots, while the diode structure provided efficient electron-hole recombination.

Electroluminescence characterization of mid-infrared InAsSb/AlInAs multi-quantum well light emitting diodes heteroepitaxially integrated on GaAs and silicon wafers

Heteroepitaxy of mid-infrared Sb-based III-V semiconductor devices on highly mismatched wafers such as GaAs and silicon are a promising route towards high-density integration benefiting from the mature fabrication technology of these substrates. This work reports on the electrical performance of heteroepitaxially grown mid-infrared InAs0.915Sb0.085/Al0.12In0.88As multi-quantum wells light emitting diodes on GaAs and offcut Si substrates using molecular beam epitaxy. Both devices exhibited a strong room temperature electroluminescence signal peaking at around 3.4 µm. Analysis of the output power results obtained from both devices revealed that the Si-based LED exhibited higher external quantum efficiency despite the higher defect density which is attributed to the superior thermal properties of the Si wafer.

Electroluminescence study of InGaN/GaN QW based p-i-n and inverted p-i-n junction based short-wavelength LED device using laser MBE technique

In the present work, the Laser Molecular Beam Epitaxy (Laser MBE) technique has been used for the fabrication of InGaN/GaN quantum well LEDs. A comparative study was performed for analysing the performance of LEDs with two different device structures i.e conventional (p-GaN/QW/i-GaN/n-GaN/Substrate) and inverted (n-GaN/QW/i-GaN/p-GaN/Substrate). The fabricated QW based Inverted p-i-n junction and conventional (p-i-n) LEDs exhibited non-linear rectifying current-voltage characteristics with a forward threshold voltage of about 0.7 V and 0.5 V, respectively. Room temperature electroluminescence was observed with increasing injection current in both conventional and inverted junction LED, where LEDs with 5 QW periods showed carrier localization effect and higher external quantum efficiency (EQE) at fixed injection current. The emission in fabricated QW LEDs was found to exhibit a spectral blue shift with increasing injection current. Convention LEDs (topmost layer: p-type GaN) exhibited higher luminous efficiency (i.e., 2.83 lm/W) than the Inverted LEDs (top most layer: n-type GaN) luminous efficiency (i.e., 1.56 lm/W). The efficiency droop observed is 14% in the conventional and 8% in the inverted LED device, respectively. This study focuses on the demonstration of room temperature electroluminescence in InGaN/GaN based LEDs grown using Laser MBE technique. The successful utilization of Laser MBE technique for growth of InGaN/GaN quantum wells can be a revolutionary step towards the establishment of a reliable, cost-effective, and ammonia-free process for fabrication of LED.

Room-temperature 2 μm luminescence from Tm doped silicon light emitting diodes and SOI substrates

Room temperature electroluminescence in the eye safe region of the spectrum over the range 1.7–2.1 μm is demonstrated from a thulium doped silicon diode. The same room temperature photoluminescence can be attained on a silicon-on-insulator substrate. The emission lines are from the first excited to ground state of the Tm3+ ion, 3F4→3H6. A detailed study has been made to establish the optimum implant and processing conditions for efficient room temperature luminescence. The importance of the correct placement of the thulium ions with respect to the depletion region edge and dislocation loops formed upon boron implantation has been established. Tm3+ has been demonstrated to lase in other systems and is the basis of widely applied, commercial, optically pumped 2 μm lasers. The demonstration of electroluminescence in silicon and luminescence on an SOI platform are necessary prerequisites for the potential development of Tm injection lasers and optical amplifiers.

Enhanced electroluminescence from Ge-on-Si by precise in-situ doping and post-annealing

We obtain strong room-temperature electroluminescence (EL) from a Ge epitaxially grown on a Si. The epitaxial Ge is in situ doped with Boron and Phosphorous by low-temperature growth, allowing for precisely controlled p-i-n structures. Also Phosphorus delta-doping is performed at the surface, resulting in low-resistivity Ohmic contacts. Vertical-type mesa-defined diodes are fabricated and an excellent rectifying property with an on/off ratio over 105 is obtained, leading to the strong EL. It is remarkable that the post-growth-annealing drastically enhances the EL intensity, indicating that the Ge-on-Si is a promising high-efficiency light source on the Si platform.

Enhanced room-temperature electroluminescence from a germanium waveguide on a silicon-on-insulator diode with a silicon nitride stressor.

Germanium (Ge) is an attractive material for monolithic light sources on a silicon chip. Introduction of tensile strain using a silicon nitride (SiNx) stressor is a promising means for Ge-based light sources due to the enhancement of direct band gap recombination. We propose a device structure that enables current injection from a silicon-on-insulator (SOI) diode to a Ge waveguide with a SiNx stressor formed by a simple fabrication process. Direct-band-gap electroluminescence and direct-band-gap shrinkage due to the applied SiNx stressor was confirmed. Intensity of electroluminescence from the Ge waveguide with the SiNx stressor was about three times higher than that corresponding to the device without the SiNx stressor.

Strong Room-Temperature Electroluminescence from Ge-on-Si by Precise in-situ Doping Control

Germanium (Ge) has been considered as a promising material for optoelectronic integrated circuit (OEIC) on a silicon (Si) platform [1-3]. Although Ge is an indirect-band-gap semiconductor, the energy difference between its direct and indirect band gaps is rather small and it has been shown that introduction of tensile strain can reduce this difference and significantly enhance the direct-gap light emission [4]. For realization of high efficiency electroluminescence from the tensile strained Ge, high quality diode structures with low leakage current should be formed. In this study, a p-i-n junction is formed within the epitaxial Ge by the precise in-situ doping method and additionally high density P delta doping is performed in the vicinity of the surface to create a low resistivity Ohmic contact. After the growth, a mesa-defined vertical diode is fabricated to reduce the leakage current. As a result, an excellent diode property with very large on-off ratio is observed and strong room temperature electroluminescence is observed owing to the very low leakage current.Solid-source molecular beam epitaxy was used for the growth of the Ge-on-Si (GOS) structure. First, an undoped Ge buffer layer of 40 nm was grown on a p-type Si (100) substrate at a temperature of 350°C. Subsequently, a B-doped (5×1018 cm-3) Ge layer of 500 nm, an undoped Ge layer of 90 nm and a P-doped (1×1019 cm-3) Ge layer of 500 nm were grown at 300°C. For p and n-type in-situ doping, B and GaP effusion cells were used, respectively. After the growth of the p-i-n structure, a growth of an ultra-thin (UT) Si layer (2 monolayers), P delta doping (2×1014 cm-2) and 7 nm Ge capping were performed to obtain a low resistivity Ohmic contact [5]. As top and back metal contacts, Au and AuGa were deposited, respectively, where the top metal contact area (90μm×40μm) was defined by lift-off process. An optical active area (125μm×75μm) were mesa-defined by the photo-lithography and following reactive ion etching (RIE) process.SIMS depth profiles of B and P in the Ge epilayer are shown in Fig. 1(a). It is found that uniform and abrupt doping can be realized for both B and P. Current-voltage characteristic shown in Fig. 1(b) reveal a good diode rectifying property with an on/off ratio of over 105. We can say that the leak current is highly suppressed by the mesa etching. A current injected light emission was measured by micro-photoluminescence (PL) system equipped with a cooled InGaAs detector, a detection limit of which was 1600 nm. Room temperature EL spectra at various continuous injected currents are shown in Fig. 1(c). The EL spectrum appears at around 200 mA injected current, and the EL intensity drastically increases with the injection current increase. As shown in the inset of Fig.1 (c), the EL intensity against the injected current shows a behavior with a threshold around 260 mA.In summary, a GOS mesa structure p-i-n vertical diode was fabricated by the in-situ doping method. We observed excellent I-V characteristic with an on/off ratio of over 105. As a result, strong EL spectra were observed with injection currents over 260 mA, indicating that the Ge-on-Si p-i-n structure has high potential to realize high efficiency Ge lasers monolithically integrated on the Si platform.[1] J. Liu et al, Semicond. Sci. Technol. 27. 094006 (2012)[2] P. Boucaud et al, Photo. Res. 1, 102 (2013)[3] J. Liu et al, Opt. Express. 15. 11272 (2007)[4] X. Sun et al, Appl. Phys. Lett. 95. 011911 (2009)[5] M.Yamada et al, Appl. Phys. Lett. 107. 132101 (2015) Figure 1

Strong Room-Temperature Electroluminescence from Ge-on-Si by Precise in-situ Doping Control

We observe strong room-temperature electroluminescence from Ge-on-Si. Ge p-i-n junction is formed within the epitaxial Ge by the precise in-situ doping method and additionally high density P delta doping is performed in the vicinity of the surface to create a low resistivity Ohmic contact. An EL device was mesa-defined by the photo-lithography and following reactive ion etching process in order to suppress leak current. Current-voltage characteristic reveal a good diode rectifying property with an on/off ratio of over 105. The EL spectrum appears at around 2.1 kA/cm2 injected current, and the EL intensity drastically increases with the injection current increase. These results indicate that the Ge-on-Si p-i-n structure has high potential to realize high efficiency Ge lasers monolithically integrated on the Si platform.

Room temperature electroluminescence from Laser MBE grown Gallium nitride LEDs

Gallium Nitride (GaN) based inverted p-i-n junction and InGaN/GaN quantum well LEDs have been demonstrated using Laser Molecular Beam Epitaxy technique. The fabricated p-i-n junction and QW LEDs exhibited rectifying I-V characteristics with a forward threshold voltage of about 3.4 V and 4.5 V, respectively. The electrically injected LEDs showed room temperature electroluminescence, wherein inverted p-i-n junction LED exhibited single peaked UV emission around 365 nm wavelength and the InGaN/GaN QW LED demonstrated both UV (368 nm) and visible (430 nm) spectral components. The emission in QW LED was found to exhibit a spectral blue shift with increasing injection current.

Room-temperature photoluminescence and electroluminescence of 1.3-μm-range BGaInAs quantum wells on GaAs substrates

We report the room temperature photoluminescence and electroluminescence properties of boron incorporated into highly strained InGaAs, forming BGaInAs, grown on GaAs substrates. X-ray diffraction was used to determine the alloy composition and strain of BGaInAs quantum wells on GaAs. As expected, the addition of boron reduced the quantum well compressive strain, preventing strain-relaxation and enabling extension of the peak emission wavelength of InGaAs quantum wells to 1.3 μm on GaAs. We also report both the longest wavelength emission observed from BGaInAs (1.4 μm) and electrically injected photoemission from a dilute-boride active region. We observed a blueshift in electroluminescence, due to unintentional in situ annealing of the active region, which we mitigated to demonstrate a path to realize true 1.3 μm emitters in the presence of in situ annealing.

Growth of strain-relaxed InGaN on micrometer-sized patterned compliant GaN pseudo-substrates

The compliant behavior of high fill-factor 10 × 10 μm2 square patterned 60–140 nm thick GaN-on-porous-GaN tiles was demonstrated by utilizing porous GaN as a semi-flexible underlayer. High resolution x-ray diffraction measurements showed a larger a-lattice constant of InGaN layers deposited on these patterned GaN-on-porous GaN pseudo-substrates in comparison to those deposited on co-loaded planar GaN-on-sapphire templates. Additionally, InGaN based light emitting diode (LED) structures deposited on these GaN pseudo-substrates exhibited room temperature electroluminescence at 547 nm compared to 506 nm for the LED structures grown on co-loaded planar GaN on sapphire templates, corresponding to a redshift of around 40 nm. The longer emission wavelength was associated with the higher indium incorporation into the InGaN quantum wells deposited on the compliant GaN pseudo-substrates, owing to a reduced lattice mismatch between the quantum well and the n-InGaN base layers grown on the compliant pseudo-substrates, due to the composition pulling effect.