Buried Heterostructure Research Articles

Imaging interfacial electrical transport in graphene-MoS2 heterostructures with electron-beam-induced-currents.

Heterostructure devices with specific and extraordinary properties can be fabricated by stacking two-dimensional crystals. Cleanliness at the inter-crystal interfaces within a heterostructure is crucial for maximizing device performance. However, because these interfaces are buried, characterizing their impact on device function is challenging. Here, we show that electron-beam induced current (EBIC) mapping can be used to image interfacial contamination and to characterize the quality of buried heterostructure interfaces with nanometer-scale spatial resolution. We applied EBIC and photocurrent imaging to map photo-sensitive graphene-MoS2 heterostructures. The EBIC maps, together with concurrently acquired scanning transmission electron microscopy images, reveal how a device's photocurrent collection efficiency is adversely affected by nanoscale debris invisible to optical-resolution photocurrent mapping.

Extended tuning of mid-ir quantum cascade lasers using integrated resistive heaters.

We present single mode quantum cascade lasers including a microscopic heater for spectral emission tuning. Through the use of a buried heater element, the active region temperature can be modified without changing the submount one. Emission frequency tuning in continuous-wave as large as 9 cm(-1) at 1270 cm(-1) and 14 cm(-1) at 2040 cm(-1) are observed, corresponding to an increase of the active region temperatures of ∼ 90 K. Due to the proximity of the heaters to the active region, emission can be modulated at several kHz range and the absence of moving parts guarantees the mechanical stability of the system. This method can be successfully applied to all buried heterostructure lasers, becoming an attractive solution for molecular spectroscopy in the IR. Using the presented devices, molecular absorptions of N(2)O have been measured between 1270 cm(-1) and 1280 cm(-1) and are in agreement with data from the HITRAN database.

Photonic Crystal Lasers for Chip-to-Chip and On-Chip Optical Interconnects

A directly modulated photonic crystal (PhC) laser with a wavelength-scale cavity is a key component for chip-to-chip and on-chip optical interconnections because it enables us to achieve ultralow operating energy. A buried heterostructure, where the wavelength-scale active region is buried with an InP layer, is essential for achieving ultralow operating energy because it improves both carrier and photon confinement in the active region and the InP layer improves the thermal conductivity of the device. We call this a lambda-scale embedded active-region PhC, or LEAP, laser. We have already developed an electrically driven LEAP laser with a threshold current of 4.8 μA and operating energy of 4.4 fJ/bit. However, to employ the LEAP laser in chip-to-chip optical interconnects, the output power should be increased. To increase it, we increase the number of multiple quantum wells and reduce the device series resistance. The device exhibits 39.3-μW output power with threshold current of 36 μA. As a result, we measure the bit error rate with 10-Gb/s signal without using an optical amplifier. For an on-chip optical interconnect, the monolithic integration of a laser, waveguide, and photodetector is the main issue because on-chip optical interconnect requires extremely small energy consumption to overcome its electrical counterpart; therefore, unwanted optical loss such as coupling loss should be minimized. We have fabricated a monolithic optical link, in which a LEAP laser exhibits a threshold current of 22 μA. The integrated optical link operates at an ultralow energy of 28.5 fJ/bit. These results indicate that the LEAP laser is highly suitable for use as a transmitter in computercom applications.

1.5-μm and 10-Gb s−1 etched mesa buried heterostructure DFB-LD for datacenter networks

We report a 1.5 μm and 10 Gb s−1 etched mesa buried heterostructure λ/4-shifted distributed feedback laser diode (DFB-LD) for the low-cost application of WDM–based datacenter networks. To reduce the threshold current and improve the modulation bandwidth in a conventional p-/n-/p-InP current blocking structure, a thin undoped-InP (u-InP) layer was inserted between the side walls of the active region and the p-InP layer (i.e., a u-/p-/n-/p-InP structure), and the region containing the active region and the current blocking structures was etched in a mesa form (i.e., an etched mesa). From this work, it was found that a 300 μm long anti-reflection (AR)-AR DFB-LD with a mesa width of 8 μm is reduced by about 25% while a side mode suppression ratio is >50 dB and a 3 dB bandwidth is >10 GHz at a current of 40 mA; in addition, it shows a clear eye-opening with a dynamic extinction ratio of >4.5 dB at 10 Gb s−1, and a power penalty of <1 dB after a 2 km transmission.

Epitaxial growth of InP to bury directly bonded thin active layer on SiO 2 /Si substrate for fabricating distributed feedback lasers on silicon

The authors have developed a new heterogeneous-integration method for fabricating semiconductor lasers with high modulation efficiency on Si substrates. The method employs the direct bonding of an InP-based active layer to the SiO2 layer of a thermally oxidised Si substrate (SiO2/Si substrate), followed by the epitaxial growth of InP to form a buried heterostructure (BH). By using the InP membrane, the authors realise epitaxial growth of an InP on the InP membrane directly bonded to Si without crystal quality degradation. Both a theoretical estimation and photoluminescence measurements revealed that the total laser thickness must be less than the critical thickness determined by the applied thermal strain. The authors confirmed that the crystal quality of the BH is comparable to that fabricated on an InP substrate when using a 250-nm-thick InP-based membrane. A distributed feedback laser fabricated on a SiO2/Si substrate exhibited continuous-wave operation up to 100°C and was directly modulated by a 40 Gbit/s non-return-to-zero signal with a bias current of 15 mA. These results indicate that epitaxial growth using a directly bonded InP-based active layer on a SiO2/Si substrate allows us to achieve lasers with high modulation efficiency and to use a large-scale Si wafer as a fabrication platform, resulting in low-cost fabrication.

Buried‐heterostructure mid‐infrared quantum cascade lasers fabricated by non‐selective regrowth and chemical polishing

A novel fabrication method of buried-heterostructure (BH) mid-infrared quantum cascade lasers (QCLs) by using non-selective regrowth of iron-doped indium phosphide (InP) (Fe:InP), around deeply etched laser ridges, via metal-organic chemical vapour deposition (MOCVD) and planarisation via chemical polishing is reported. Owing to better heat dissipation, the fabricated 4.75 μm emitting QCLs exhibit about three-fold enhancement in maximum output power under continuous-wave operation at room temperature (T = 20°C) compared with that from lasers without the regrown InP. The demonstrated fabrication method provides a more flexible route to realising BH QCLs by removing strict requirements on the etched-ridge sidewall profile, as well as on the physical dimensions of the dielectric mask for the regrowth via MOCVD. The method can be further employed for making large-emitting aperture, closely packed arrays of QCLs, with planarised geometry, for coherent-power scaling.

Terahertz generation in mid-infrared quantum cascade lasers with a dual-upper-state active region

We report the performance of room temperature terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers with a dual-upper-state (DAU) active region. DAU active region design is theoretically expected to produce larger optical nonlinearity for terahertz difference-frequency generation, compared to the active region designs of the bound-to-continuum type used previously. Fabricated buried heterostructure devices with a two-section buried distributed feedback grating and the waveguide designed for Cherenkov difference-frequency phase-matching scheme operate in two single-mode mid-infrared wavelengths at 10.7 μm and 9.7 μm and produce terahertz output at 2.9 THz with mid-infrared to terahertz conversion efficiency of 0.8 mW/W2 at room temperature.

Concept of a multichannel spin-resolving electron analyzer based on Mott scattering.

The concept of a multichannel electron spin detector based on optical imaging principles and Mott scattering (iMott) is presented. A multichannel electron image produced by a standard angle-resolving (photo) electron analyzer or microscope is re-imaged by an electrostatic lens at an accelerating voltage of 40 kV onto the Au target. Quasi-elastic electrons bearing spin asymmetry of the Mott scattering are imaged by magnetic lenses onto position-sensitive electron CCDs whose differential signals yield the multichannel spin asymmetry image. Fundamental advantages of this concept include acceptance of inherently divergent electron sources from the electron analyzer or microscope focal plane as well as small aberrations achieved by virtue of high accelerating voltages, as demonstrated by extensive ray-tracing analysis. The efficiency gain compared with the single-channel Mott detector can be a factor of more than 10(4) which opens new prospects of spin-resolved spectroscopies in application not only to standard bulk and surface systems (Rashba effect, topological insulators, etc.) but also to buried heterostructures. The simultaneous spin detection combined with fast CCD readout enables efficient use of the iMott detectors at X-ray free-electron laser facilities.

Integrated buried heterostructure DFB laser array for WDM systems based on reconstruction equivalent chirp technique

A distributed-feedback semiconductor laser array with buried heterostructure waveguide based on reconstruction equivalent chirp technique is experimentally demonstrated. The fabricated six-element laser array has channel spacing of 800 GHz and well meets the International Telecommunication Union standard, with relatively low threshold and high efficiency. Good single-longitudinal mode operation has been achieved at high injection current (120 mA) and high temperature (80°C), with side mode suppression ratios not lower than 40 dB, which is promising for the future wavelength division multiplexing systems. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1278–1281, 2015

Surface emitting multi-wavelength array of single frequency quantum cascade lasers

We present a surface emitting laser array design based on distributed Bragg reflectors together with a second order extractor, providing a deterministic mode selection mechanism. The technology is implemented as a buried heterostructure compatible with continuous-wave operation and low dissipation. A proof of principle featured 10 regularly spaced single mode devices with a coverage of 175 cm−1 in the 8–10 μm wavelength range and milliwatt output powers with a far-field pattern full-width half-maximum of 8.3° in the longitudinal direction.

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

We present an experimental study of noise in mid-infrared quantum cascade lasers (QCLs) of different designs. By quantifying the high degree of correlation occurring between fluctuations of the optical frequency and voltage between the QCL terminals, we show that electrical noise is a powerful and simple mean to study noise in QCLs. Based on this outcome, we investigated the electrical noise in a large set of 22 QCLs emitting in the range of 7.6–8 μm and consisting of both ridge-waveguide and buried-heterostructure (BH) lasers with different geometrical designs and operation parameters. From a statistical data processing based on an analysis of variance, we assessed that ridge-waveguide lasers have a lower noise than BH lasers. Our physical interpretation is that additional current leakages or spare injection channels occur at the interface between the active region and the lateral insulator in the BH geometry, which induces some extra noise. In addition, Schottky-type contacts occurring at the interface between the n-doped regions and the lateral insulator, i.e., iron-doped InP, are also believed to be a potential source of additional noise in some BH lasers, as observed from the slight reduction in the integrated voltage noise observed at the laser threshold in several BH-QCLs.

Directly Modulated DFB Laser on SiO$_{\bf 2}$ /Si Substrate for Datacenter Networks

Reducing the operating energy of a distributed feedback (DFB) laser is a critical issue if we are to use the device as a directly modulated light source employing wavelength division multiplexing technologies in short-distance datacom networks. A membrane buried heterostructure (BH) DFB laser on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is one candidate for reducing the operating energy because it provides a strong carrier and optical confinement in the active region. For low-cost fabrication, we have proposed and developed a fabrication procedure that employs the buried growth of an InP layer by using a directly bonded InP-based active layer on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate, which enables us to use a large-scale Si wafer. To overcome the problem of the difference between the thermal expansion coefficients of Si, SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and InP, we have used a thin active layer (~250 nm) on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate as a template for the epitaxial growth of a III-V compound semiconductor. A lateral current injection structure is essential for fabricating a device with a 250-nm-thick template. Our fabricated DFB laser with a 73-μm cavity length exhibits a threshold current of 0.9 mA for continuous operation at room temperature and achieves lasing at up to 100 °C. We have also demonstrated 171-fJ/bit operation with a 25.8-Gb/s NRZ signal. These results indicate that the BH DFB laser on a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrate is highly suitable for use as a transmitter for datacom applications.

Flip-chip mounted 25.8-Gb/s directly modulated InGaAsP DFB laser with Ru-doped semi-insulating buried heterostructure

Flip-chip mounted 25.8-Gb/s directly modulated InGaAsP DFB laser with Ru-doped semi-insulating buried heterostructure

Experimental demonstration of the three-phase-shifted DFB semiconductor laser with buried heterostructure using common holographic exposure

In this article, we report the first experimental demonstration of the three-phase-shifted (3PS) DFB semiconductor laser with buried heterostructure based on the Reconstruction-Equivalent-Chirp (REC) technique. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as those of the true 3PS laser. Compared with the quarter-wave-phase shift (QWS) DFB semiconductor laser, the 3PS DFB semiconductor laser shows better single-longitude-mode (SLM) property even at high injection current with high temperature. However, it only changes the μm-level sampling structures but the seed grating is uniform using the REC technique. Therefore, its fabrication cost is low.

Regrowth-free single-mode quantum cascade lasers with power consumption below 1 W

We report on single-mode distributed-feedback quantum cascade lasers emitting at 4.8 μm with continuous-wave threshold power consumption as low as 0.76 W at 20 °C and 0.98 W at 50 °C. Following growth of the laser active region and semiconductor cladding layers by a single molecular beam epitaxy process, devices with 4-μm-wide ridges and vertical sidewall gratings were fabricated using plasma etching and standard dielectric and metal deposition processes. In terms of mode stability, output power, and efficiency, we show that lasers with 1-mm cavity length and high-reflectivity back-facet coatings can match the performance of buried heterostructure devices, but with the advantage of requiring only a single epitaxial growth step.

An Eight-Wavelength BH DFB Laser Array With Equivalent Phase Shifts for WDM Systems

In this letter, we report an eight-wavelength monolithically integrated buried heterostructure distributed feedback laser array with quarter-wave equivalent phase shifts. The laser array operating at 1.3 μm has a chip size of 2 mm × 250 μm. Lasing channel spacing is designed to be 800 GHz (~4.6 nm at 1.3 μm). The laser array exhibits accurate wavelength control and stable single mode lasing operation over a wide range of injection currents and different ambient temperatures. The measured linewidth of these lasers is 278 kHz at 65-mA bias current. In addition, the lasing wavelength of the whole laser array can be tuned continuously over 38.3 nm without mode hopping. The proposed method offers a practical approach of integrated laser array fabrication for wavelength-division multiplexing systems.

A Quick and a Flexible Hydride Vapor Phase Epitaxy Process to Achieve Buried Heterostructure Quantum Cascade Lasers

Quantum cascade lasers (QCL) first demonstrated by Faist and co-workers1 are attractive sources for spectroscopy, medical and biosensing, remote gas sensing, free-space communication and applications in defense security countermeasures as these cover a wide range of spectrum from near-infrared to mid THz2-3. Several of these application areas necessitate high power, good beam quality, efficient thermal dissipation and high wall plug efficiency (WPE). Together with the optimal basic design, buried heterostructure QCL (BH-QCL) with semi-insulating InP:Fe (SI-InP) regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode with the above desired performance. So far only MOVPE (metal organic vapour phase epitaxy)4 and molecular beam epitaxy (MBE)5 have been used to realize BH-QCLs and not hydride vapour phase epitaxy (HVPE), an equilibrium process. The growth rate in HVPE is one order of magnitude higher than that in MOVPE and MBE. We demonstrate here buried heterostructure QCLs emitting at 4.7 µm with a single step regrowth of highly resistive (>1x108 ohm·cm) semi-insulating InP:Fe in less than 45 minutes in a HVPE reactor for burying 6-15 µm deep etched ridges. This has to be compared with the normally used growth time of several hours, e.g., in an MOVPE reactor. Maintenance of the QCL ridges at the regrowth temperatures for longer time to realize BH-QCL can cause severe inter well-barrier mixing6 leading to the loss of the original laser design characteristics and hence HVPE regrowth is advantageous. In addition, its planarizing capacity, total selectivity during the growth on patterned (planar or non-planar) surfaces, non-appearance of “rabbit ears” near the ridge (or mesa) edge, flexibility in growing around [110] and [-110] oriented ridges, both with or without mask overhang, and insensitivity to the profile of the etched ridge are its unique flexible features7. The HR-coated 5 mm long BH-QCLs of ridge widths 6-15 µm were characterized. Besides being spatially monomode, TM00, all the characterized lasers exhibited WPE of ~8-9% with an output power of 1.5 – 2.5 W at room temperature and under CW operation. The performance can be further improved by proper mounting and facet coating. As-cleaved BH-QCLs were also characterized and compared with the HR-coated ridge-alone QCLs; the former outperform the latter in their output power more than twice even though the latter were HR-coated. This indicates the good thermal dissipation in the BH-QCL with HVPE regrowth. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE. Acknowledgements This work was supported by EU FP7 MIRIFISENS Integrated Project 317884. Linné Center of Excellence, ADOPT is also acknowledged.

Directly modulated buried heterostructure DFB laser on SiO₂/Si substrate fabricated by regrowth of InP using bonded active layer.

We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO₂/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO₂/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

Analysis of optical re-modulation by multistage modeling of RSOA

The time-resolved multistage reservoir model well-known for semiconductor optical amplifier (SOA) is extended to analyze the behavior of a bulk homogeneous InP-InGaAsP buried heterostructure reflective semiconductor optical amplifier (RSOA). Parameters for simulation have been extracted from the experimental RSOA characteristics. We have employed the model to explain the steady-state and re-modulation dynamics in the RSOA. Electrical modulation bandwidth and intermodulation distortion in the RSOA have been derived from the model and close agreement is obtained with the reported data. It is found out that the ripples in the upstream output from the RSOA for incomplete modulation erase of downstream modulated data follow Gaussian distribution, which simplifies the calculation of upstream SNR and bit error rate. It is explained in detail that amplitude ripples in the upstream data can be reduced by judicious choice of optical and electrical parameters of the RSOA. In particular, for an average low downstream power level (<−20dBm) a good downstream modulation erase factor about 89% and 23dB extinction ratio in the upstream modulated signal can be achieved.

Continuous wave operation of buried heterostructure 46µm quantum cascade laser Y-junctions and tree arrays

Room-temperature continuous-wave operation for buried heterostructure 4.6 µm quantum cascade laser Y-junctions and tree arrays, overgrown using hydride vapor phase epitaxy, has been demonstrated. Pulsed wall plug efficiency for the Y-junctions with bending radius of 5mm was measured to be very similar to that of single-emitter lasers from the same material, indicating low coupling losses. Comparison between model and experimental data showed that the in-phase mode was dominating for 10mm-long Y-junctions with 5 µm-wide 1mm-long stem and 5 µm-wide branches. Total optical power over 1.5 W was demonstrated for four-branch QCL tree array.