MQW Structures Research Articles

III–V Microwires with Reversed Ridge Waveguides Selectively Grown on Pre-Patterned Si Substrates

To construct functional photonic integrated circuits, an efficient and compact laser is expected to be incorporated into the complementary metal–oxide–semiconductor platform. Monolithic integration of III–V lasers on pre-patterned Si substrates by the aspect ratio trapping method is a promising solution. Here, microwires with reversed ridge waveguides (RRWs) on pre-patterned Si substrates were reported. By metal–organic chemical vapor deposition, high-quality InP microwires with RRWs were obtained, and InGaAs/InGaAsP multi-quantum-well structures with InGaAsP separate confinement hetero structure (SCH–MQW) were successfully achieved. The SCH–MQW structure was buried in the InP microwire, which was beneficial for transferring the heat generated in the active region. The micron size also contributes to the efficiency of thermal diffusion. Further, simulation results showed that the metal absorption loss could be less than 4 dB/cm by properly controlling the contact area between metal electrodes and microwires. This proposed structure opens up an alternative pathway for electrically driven III–V lasers seamlessly interfaced with Si-photonics.

(Digital Presentation) Fabrication of Thick SiGe／Ge Multiple Quantum Wells on Ge-on-Si and Their Optical Properties

Recently, silicon photonics has been attracting increased attention toward the realization of on-chip optical interconnection, which requires high-efficiency light-emitting devices to be integrated on Si substrates. Although Ge is an indirect-band-gap semiconductor like Si, it is expected that strong direct-transition luminescence can be obtained from tensile-strain induced Ge-on-Si. We have previously reported that highly efficient room temperature EL emission can be obtained from strained Ge-on-Si p-i-n diodes[1]. Further increase in luminescence intensity and control of emission wavelengths can be made possible by introducing a quantum well structure in the active layer. Although the fabrication of high-quality strained SiGe layers on Ge is essential for the fabrication of quantum well structures, critical thickness limits the total growth thickness and numbers of strained layers, which makes device applications difficult. We have succeeded in fabricating a strained SiGe layer that is much thicker than the critical thickness by using a patterning method, and this result is very promising for the fabrication of SiGe/Ge multi-quantum well structure[2]. It also makes it possible to increase the number of layers of MQW structure. In this work, we fabricate relatively thick strained SiGe/Ge MQW structures on Ge-on-Si by utilizing the patterning method, and evaluate their crystallinity and optical properties.In experiments, the crystal growth was carried out with solid source molecular beam epitaxy. The Ge-on-Si was fabricated using the so-called two-step method. 40 and 500 nm thick Ge layers were subsequently grown on a Si(100) or Si(111) substrate at 350 and 600℃, respectively, followed by annealing at 800℃ for 10 min. Subsequently we carried out the patterning of the Ge-on-Si(111) substrate by photolithography process. 50nm thick Ge buffer layer ware grown on a Ge-on-Si(100) or Ge-on-Si(111), 6nm thick Si0.1Ge0.9 barrier layer and 10nm thick Ge well layer ware grown with 10-20 cycles at 350℃.Laser microscopic measurements showed no clear roughness on the surface. X ray diffraction measurements show periodic peaks originated from the multi layers of SiGe/Ge, which indicates that the sample has high crystallinity and abrupt interface between the Ge and SiGe layers. TEM observation indicated that no defect was found inside the crystal in the MQW structure. Photoluminescence (PL) measurements were carried out at room temperature and a PL peak originated from the quantum-confinement in the SiGe/Ge MQW was obtained. This PL peak wavelength was also found to shift depending on the Ge concentration of the SiGe. PL intensity of the peak was increased compared with Ge-on-Si without the SiGe/Ge MQW overgrowth. From these results, it is demonstrated that high quality SiGe/Ge multiple quantum structures can be fabricated on Ge-on-Si by utilizing the patterning method and strong room temperature PL was obtained from the MQW, indicating that SiGe/Ge MQW is promising for the applications to light-emitting devices integrated on the Si platform.[1] K.Yamada et al. Appl. Phys. Express 14 045504(2021) [2] Y.Wagatsuma et al. Appl. Phys. Express 14 025502(2021) Figure 1

Several processes participating in a decrease and the droop of external quantum efficiency in green InGaN/GaN MQW structures

The obtained experimental results allow us to clarify the nature of mechanisms related to the presence of cations in disordered InGaN alloy and hetero-interfaces. The capture of charge carriers by cations reduces the external quantum efficiency (EQE) in green MQWs at j < 10 A/cm2. The EQE droop phenomenon caused by smoothed out lateral potential fluctuations occurs at j > 10 A/cm2. At j > 40 A/cm2 the droop associated with interactions between charge carriers and dislocations and grain boundaries takes place.

High-temperature crystallinity restoring layers and their optimal positions for ultra-thick InGaN/GaN multiple-quantum-well structures

MQW structure with ultra-thick low-temperature barriers that has smooth surface is achieved with the help of crystallinity restoring layers.

Trapezoidal Well Structures for Improving the Light Efficiency in Algainp-Based LEDs

(AlxGa1-x)0.5In0.5P-based LEDs have a small conduction band offset that limits their electron confining potential.1 This weaker electron confinement subsequently leads to electron heterobarrier leakage in AlGaInP heterostructure LEDs, especially in short-wavelength devices, where a fraction of the electrons injected into the active region have a sufficient thermal energy to escape into the cladding layer. To overcome this problem, trapezoidal well structures are introduced in the active region in order to prevent carrier overflow and to gain a higher light output power (Pout). In the field of AlGaInP-based LEDs that emit a short peak wavelength at around 590 nm, however, the effects of the trapezoidal well profile in the active region on LED performance has yet to be systematically studied.In this study, we investigate the behaviors of optical and electrical characteristics according to their trapezoidal well profile in the active region through the analysis of optical behaviors and device performances.To investigate the optical behaviors according to the trapezoidal well shape, the sloped barrier (SB) is inserted between well and flat barrier (FB) and the thickness of SB (dSB) is changed from 2.5 nm to 10 nm. And, the FB thickness is adjusted from 15 to 0 nm to maintain the total barrier thickness 20 nm. The schematic diagram of MQW structures of a trapezoidal-shaped well is depicted in Fig. 1(b).Fig. 2 shows the device performances having different sloped barrier profiles in the MQW LED structures. As the current is increased to 600 mA, the current-voltage (I-V) curves show different behavior according to the SB thickness. In Fig. 1(a) inset, the forward bias voltage at 350 mA decreases from 2.86 V to 2.62 V with an increase in the thickness of SB from 0 to 10 nm. Fig. 1(b) then shows the light output power (Pout)-current characteristics of LEDs on the dSB and the normalized efficacy of the same LEDs at 350 mA operating current is shown in the figure inset. As the dSB between the well and the barrier is increased from 0 to 5.0 nm, the normalized efficacy improves from 1.0 to 1.16 and the maximum Pout increases from 400 to 500 mA, though at a further increase of the dSB to 10 nm, the normalized efficacy degrades to 0.98.AcknowledgmentThis work was supported by a Korea Research Foundation Grant from the super-saving-LED convergence development project (20004475, Development of high power red micro light-emitting diode for full color RGB) funded by the Ministry of Trade, Industry and Energy, Korea.

The impact of the surface morphology on optical features of the green emitting InGaN/GaN multiple quantum wells

AFM study of surface morphology in green LED structures and multifractal analysis allowed us to reveal a relationship between step-meandering morphology quantitatively characterized by a multifractal parameter, the degree of disorder, and features of In incorporation in InGaN/GaN MQW structures. These features manifest themselves in a shape of the distribution of peak external quantum efficiency values with wavelengths (DPEW) in the current range of 0.1–1000 mA. High values of degree of disorder result in spinodal alloy decomposition and/or local In enriched regions in InGaN alloy and in a twofold decrease in EQE values in green LEDs.

Deep Ultraviolet Light Source from Ultrathin GaN/AlN MQW Structures with Output Power Over 2 Watt

AbstractHigh‐output‐power electron‐beam (e‐beam) pumped deep ultraviolet (DUV) light sources, operating at 230–270 nm, are achieved by adjusting the well thickness of binary ultrathin GaN/AlN multiple quantum wells. These structures are fabricated on high‐quality thermally annealed AlN templates by metal‐organic chemical vapor deposition. Owing to the reduced dislocation density, large electron–hole overlap, and efficient carrier injection by e‐beam, the DUV light sources demonstrate high output powers of 24.8, 122.5, and 178.8 mW at central wavelengths of 232, 244, and 267 nm, respectively. Further growth optimization and employing an e‐gun with increased beam current lead to a record output power of ≈2.2 W at emission wavelength of ≈260 nm, the key wavelength for water sterilization. This work manifests the practical levels of high‐output‐power DUV light sources operated by using e‐beam pumping method.

Full-duplex light communication with a monolithic multicomponent system

A monolithic multicomponent system is proposed and implemented on a III-nitride-on-silicon platform, whereby two multiple-quantum-well diodes (MQW-diodes) are interconnected by a suspended waveguide. Both MQW-diodes have an identical low-In-content InGaN/Al0.10Ga0.90N MQW structure and are produced by the same fabrication process flow. When appropriately biased, both MQW-diodes operate under a simultaneous emission-detection mode and function as a transmitter and a receiver at the same time, forming an in-plane full-duplex light communication system. Real-time full-duplex audio communication is experimentally demonstrated using the monolithic multicomponent system in combination with an external circuit.

Fano signatures between intersubband and ponderomotive responses in MQW structures.

We present an in-depth theoretical and numerical discussion on the Fano signatures observed in differential transmission spectra obtained from multiquantum well structures. These signatures stem from ponderomotive and intersubband polarization currents modified by the coupling between the optical responses of different layers. A detailed discussion of this process is provided, evaluating quantitatively the amplitude and shape of the Fano signatures and their dependence on structural parameters, such as carrier concentration and layer width. The theoretical model described here aims to predict quantitatively the weight of the contributions of the ponderomotive currents in relation to the intersubband ones. In order to include the effect of the entire structure in the theoretical spectra, the optical response of each layer is addressed within the density matrix formalism and encompassed in an optical transfer matrix. This method also ensures the correct inclusion of the phase sensitive superposition of optical responses of different layers on which the Fano signatures are based. Numerical simulations obtained from the presented theoretical approach are in excellent agreement with the behavior observed in previous experiments.

(Invited) Si(1-x)Gex/Si Mqw Based Uncooled Microbolometer Development and Integration into 130 nm BiCMOS Technology

The process of epitaxial SiGe layers are getting attractive as they offer a variety of advanced microelectronics devices compatible with silicon processing technology [1-3]. The large research effort of the last decade has focused on the growth of high quality strained SiGe layers. In particular, SiGe superlattice containing high Ge concentration is demanded in order to improve performances of the devices. Recently, IHP has achieved a significant breakthrough in SiGe/Si MQW processing technology [4]. The developed high quality of superlattice of SiGe/Si MQW structure containing 50% Ge has been successfully processed (Fig. 1) and presented as a potential thermistor material with its high performance for uncooled microbolometer applications [5]. The prototype of thermistor device with 50% Ge in SiGe wells exhibited an outstanding TCR of -5.5 %/K and a K1/f value of 3.4 × 10-15 for 200 µm× 200 µm pixel size, respectively, showing a concurrent achievement of a very high TCR and a low 1/f noise performance. One of the main challenge of SiGe/Si MQW structures is the high resistance values including high Ge concentration makes it difficult to integrate as a detector material in a complete microbolometer device. Large pixel resistances cause not only high noises and hence lower sensitivity but also a challenge to design and maximize the performance of the ROICs [6, 7]. A method to optimize and tune the resistance of the detector material in microbolometers is in-situ doping of quantum well layers in order to enhance the combined detector and ROIC performance. In [8], Boron (B) doping level in SiGe quantum wells are optimized in order to keep the pixel resistance of the thermistor in acceptable values for an ROIC design. B doping of SiGe MQWs is used for the resistance tuning of the detector. The main challenge of the method is simultaneous achievement of enough doping in the SiGe region of the MQWs while keeping the barrier intrinsic silicons non-doped. The initial pixel resistance of 3 period of SiGe/Si MQWs with 50% Ge concentration of 21 MΩ is reduced 210 kΩ for 25 × 25 µm2 pixel size by the optimization of in-situ B doping level in SiGe layers of the MQW stack [8]. The optimized B doping density of ~1 × 1018 cm-3 in SiGe wells did not cause any significant change in the TCR value whereas the 1/f noise performance is even enhanced due to the in-situ doping process. All the aforementioned results shows the successful demonstration of intrinsic thermistor based on SiGe/Si MQWs including 50% Ge with an acceptable pixel resistances. Although the successful demonstration of SiGe MQWs has been performed for high performance microbolometer applications, a full integration of the high performance SiGe/Si MQW based detector into a standard CMOS process is still challenging. SiGe/Si MQW process requires higher temperatures than the temperature budget of the post processing of CMOS process. Therefore, the transfer of the optimized detector structure to a CMOS/BiCMOS wafer needs a special layer transfer process (i.e. wafer bonding). More critically, due to the vertical working principle mechanism of the detector device, there is a challenge for full integration of the optimized detector structure into standard BiCMOS process because the suspended detector device needs to be contacted by both sides In this work, the recent progress on SiGe/Si type high performance detector structures will be presented. The process optimization of the detector by means of high TCR, low 1/f noise and appropriate resistance will be detailed. The method of integrating the developed SiGe/Si MQW structures into a 130 nm BiCMOS process will be provided. The integration method is based on the oxide-oxide fusion bonding of ROIC and detector wafer followed by post processing of the bonded wafer stack. The details of thermal conductance optimization of the microbolometer together with the mechanical stress optimization, required for the suspended detector device, will also be given. Finally, the low-pressure hermetic packaging of the microbolometer structure will also be proposed. Figure 1

Numerical analysis of SiGeSn/GeSn interband quantum well infrared photodetector

In this paper, detailed theoretical investigation on the frequency response and responsivity of a strain balanced SiGeSn/GeSn quantum well infrared photodetector (QWIP) is made. Rate equation and continuity equation in the well are solved simultaneously to obtain photo generated current. Quantum mechanical carrier transport like carrier capture in QW, escape of carrier from the well due to thermionic emission and tunneling are considered in this calculation. Impact of Sn composition in the GeSn well on the frequency response, bandwidth and responsivity are studied. Results show that Sn concentration in the GeSn active layer and applied bias have important role on the performance of the device. Significant bandwidth is obtained at low reverse bias voltage, e.g., 200 GHz is obtained at 0.28 V bias for a single Ge 0.83 Sn 0.17 layer. Whereas, the maximum responsivity is of 8.6 mA/W at 0.5 V bias for the same structure. However, this can be enhanced by using MQW structure.

Transferrable monolithic III-nitride photonic circuit for multifunctional optoelectronics

A monolithic III-nitride photonic circuit with integrated functionalities was implemented by integrating multiple components with different functions into a single chip. In particular, the III-nitride-on-silicon platform is used as it integrates a transmitter, a waveguide, and a receiver into a suspended III-nitride membrane via a wafer-level procedure. Here, a 0.8-mm-diameter suspended device architecture is directly transferred from silicon to a foreign substrate by mechanically breaking the support beams. The transferred InGaN/GaN multiple-quantum-well diode (MQW-diode) exhibits a turn-on voltage of 2.8 V with a dominant electroluminescence peak at 453 nm. The transmitter and receiver share an identical InGaN/GaN MQW structure, and the integrated photonic circuit inherently works for on-chip power monitoring and in-plane visible light communication. The wire-bonded monolithic photonic circuit on glass experimentally demonstrates in-plane data transmission at 120 Mb/s, paving the way for diverse applications in intelligent displays, in-plane light communication, flexible optical sensors, and wearable III-nitride optoelectronics.

Dynamics of carrier tunneling and recombination in asymmetric coupled InGaN multiple quantum wells.

In this work, dynamics of carrier tunneling and recombination in InGaN-based asymmetric coupled multiple quantum wells (AC-MQWs) are systematically studied by excitation power-dependent and temperature-dependent photoluminescence (PL) measurements. With different pumping wavelengths of 405 and 325 nm, distinctly different PL spectral evolutions are observed, which could be well explained by the proposed anomalous carrier "reverse tunneling" based on the forbidden 1h→2e transitions in the AC-MQWs. The forbidden transitions are identified through the well agreement between the measured photo-modulated reflectance (PR) spectrum and the calculated interband transition energies. Our results indicate that, by ingeniously designing the MQW structure of the InGaN-based optoelectronic devices, it is possible to realize a specific interband optical transition which is even not allowed by the selection rule, and thereby effectively improve the carrier distribution across the QWs through the conventional and/or anomalous "reverse" carrier tunneling.

On-chip optical interconnect using visible light

We propose and fabricate a monolithic optical interconnect on a GaN-on-silicon platform using a wafer-level technique. Because the InGaN/GaN multiple-quantum-well diodes (MQWDs) can achieve light emission and detection simultaneously, the emitter and collector sharing identical MQW structure are produced using the same process. Suspended waveguides interconnect the emitter with the collector to form in-plane light coupling. Monolithic optical interconnect chip integrates the emitter, waveguide, base, and collector into a multi-component system with a common base. Output states superposition and 1×2 in-plane light communication are experimentally demonstrated. The proposed monolithic optical interconnect opens a promising way toward the diverse applications from in-plane visible light communication to light-induced artificial synaptic devices, intelligent display, on-chip imaging, and optical sensing.

Sn influence on MBE growth of GeSiSn/Si MQW

Temperature and composition dependencies of the critical thickness of transition from two-dimensional to three-dimensional growth for GeSiSn films on Si(100) with a lattice mismatch of 1 – 5% were experimentally determined. To understand the Sn influence on growth of SiGeSn/Si multi-quantum wells, the phase diagram of surface superstructures during the growth of pure Sn on Si(100) was created. A possibility of synthesizing multilayer structures by molecular beam epitaxy was shown, and the crystal lattice constants were determined using high-resolution transmission electron microscopy. We obtained GeSiSn/Si MQW structures which demonstrated photoluminescence for the Sn content in GeSiSn layers of up to 6%.

Three-wavelength on-line infrared thermometry for MOCVD

According to the development needs of on-line infrared thermometry of metal organic chemical vapor deposition(MOCVD), a three-wavelength infrared thermometry is proposed, which can avoid the calibration of the effective area of detector hole and the modification of reflectivity. The design scheme of the three-wavelength (1 300 nm, 1 150 nm, 940 nm) on-line infrared thermometry probe and the light path are presented. The probe is used in the 5.08 cm(2 inch) silicon (111) substrate growing 10 μm GaN epitaxial wafer in THOMAS SWAN CCS MOCVD system.Result shows that the accuracy is within 1℃ in the range from 950℃ to 1 100℃, referring to the EpiTT infrared thermometry.The repeatability is within 1.0℃ from 700℃ to 1 100℃.The distance tolerance is 2 mm.Moreover, the probe is used in the 5.08 cm silicon (111) substrate growing blue light LED epitaxial wafer with InGaN/GaN MQW structure in homemade MOCVD system. Result shows that the lowest range of the probe is equally 435℃.The measurement noise of n-GaN layer during growing is at 0.75℃. The analysis of the measured results indicate that the method for single layer thin film epitaxial growth has the certain practicability, however, further improvements are needed for the multi-layer complex structure epitaxial growth.

Si-Based Ge and GeSn Material Epitaxy and Thermal Stability Characterization

_In the past decade, Si photonics has become one of the hottest research topics in the world since it holds great promise for maintaining the performance roadmap known as Moore’s law. The application of silicon photonics technologies to requires the development of high quality Si-based materials for photonics devices monolithically integrated to a Si platform. Germanium (Ge), due to its large absorption coefficient at near-infrared frequencies, and its lower cost and compatibility of parallel processing with silicon compared to III-V semiconductors, is perceived as the best candidate for on-chip photodetectors. Meanwhile, Ge is a pseudo-direct bandgap material because its direct Γ-valley CB is just 136.5 meV above the lowest L-valley CB at room temperature . This energy difference can be effectively reduced by incorporating Sn, another group-IV element, into Ge to modulate the band structure. GeSn alloy has been proven to be another promising material for the Si-based laser and PDs covering all telecommunication windows. In this presentation, Ge 、GeSn and GeSn/Ge MQW have been grown on Si, Ge/Si and Ge. The crystallinity and growth quality of the Ge and GeSn films were investigated through material characterization methods including x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. In order to grow GeSn on Si and Ge subtrate,the growth is performed at low temperature, but the devices fabrication process will be carried out at high temperature, so the stability study is important. The effect of rapid thermal annealing on the conversion of metastable elastically strained GeSn layers into a plastically relaxed state is also examined. the results show that GeSn alloy have good thermal stability at 500℃if Sn contents are as low as ~3%. The stability of the GeSn/Ge MQW structure is temperature-dependentand and the temperature over 400℃ should be avoided to guarantee the structural integrity of the MQWs.

Influence of GaInNAs/GaAs QWs composition profile on the transitions selection rules

In this work authors present the results of structural and optical characterization of the GaInNAs/GaAs MQW structures grown by AP-MOVPE. The growth conditions of quantum well layers were modified in order to counteract the phase segregation within GaInNAs QWs. Profiles of indium and nitrogen contents across the QWs layers were investigated by using HRXRD. The optical properties were investigated by applying photoluminescence spectroscopy. The inhomogeneous composition of the GaInNAs QW layers is considered as a factor suppressing the PL intensity due to change in the electron and holes wave functions distributions and tuning the transition's selection rules.

Investigation of Trapezoidal Well for Improving the Light Efficiency in Algainp-Based LEDs

(AlxGa1-x)0.5In0.5P-based LEDs have a small conduction band offset that limits their electron confining potential.1 This weaker electron confinement subsequently leads to electron heterobarrier leakage in AlGaInP heterostructure LEDs, especially in short-wavelength devices, where a fraction of the electrons injected into the active region have a sufficient thermal energy to escape into the cladding layer. To overcome this problem, trapezoidal well structures are introduced in the active region in order to prevent carrier overflow and to gain a higher light output power (Pout). In the field of AlGaInP-based LEDs that emit a short peak wavelength at around 590 nm, however, the effects of the trapezoidal well profile in the active region on LED performance has yet to be systematically studied. In this study, we investigate the behaviors of optical and electrical characteristics according to their trapezoidal well profile in the active region through the analysis of optical behaviors and device performances. To investigate the optical behaviors according to the trapezoidal well shape, the sloped barrier (SB) is inserted between well and flat barrier (FB) and the thickness of SB (dSB) is changed from 2.5 nm to 10 nm. And, the FB thickness is adjusted from 15 to 0 nm to maintain the total barrier thickness 20 nm. The schematic diagram of MQW structures of a trapezoidal-shaped well is depicted in Fig. 1(b). Fig. 2 shows the device performances having different sloped barrier profiles in the MQW LED structures. As the current is increased to 600 mA, the current-voltage (I-V) curves show different behavior according to the SB thickness. In Fig. 1(a) inset, the forward bias voltage at 350 mA decreases from 2.86 V to 2.62 V with an increase in the thickness of SB from 0 to 10 nm. Fig. 1(b) then shows the light output power (Pout)-current characteristics of LEDs on the dSB and the normalized efficacy of the same LEDs at 350 mA operating current is shown in the figure inset. As the dSB between the well and the barrier is increased from 0 to 5.0 nm, the normalized efficacy improves from 1.0 to 1.16 and the maximum Pout increases from 400 to 500 mA, though at a further increase of the dSB to 10 nm, the normalized efficacy degrades to 0.98.

High internal quantum efficiency in AlGaN multiple quantum wells grown on bulk AlN substrates

The internal quantum efficiency (IQE) of Al0.55Ga0.45N/AlN and Al0.55Ga0.45N/Al0.85Ga0.15N UVC MQW structures was analyzed. The use of bulk AlN substrates enabled us to undoubtedly distinguish the effect of growth conditions, such as V/III ratio, on the optical quality of AlGaN based MQWs from the influence of dislocations. At a high V/III ratio, a record high IQE of ∼80% at a carrier density of 1018 cm−3 was achieved at ∼258 nm. The high IQE was correlated with the decrease of the non-radiative coefficient A and a reduction of midgap defect luminescence, all suggesting that, in addition to dislocations, point defects are another major factor that strongly influences optical quality of AlGaN MQW structures.