Buried Heterostructure Research Articles

Demonstration of a quick process to achieve buried heterostructure quantum cascade laser leading to high power and wall plug efficiency

Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (>1×108 ohm·cm) semi-insulating InP:Fe in <45 min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15 μm depth, both with and without mask overhang. The fabricated BH-QCLs emitting at ∼4.7 and ∼5.5 μm were characterized. 2-mm-long 5.5-μm lasers with a ridge width of 17 to 22 μm, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-μm BH-QCLs of ridge widths varying from 6 to 14 μm regrown without mask overhang, besides being spatially monomode, TM00, exhibited wall plug efficiency (WPE) of ∼8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE.

Superluminescent Diode With Circular Beam Shape

We report a new design for a superluminescent diode (SLD) that can provide a nearly circular output beam shape and a high output power in a tunable external cavity laser (T-ECL). The proposed SLD consists of an active region and a spot size converter (SSC) region. The active region is realized using a planar buried hetero structure for low electrical and optical leakages and the SSC region is realized using a buried deep ridge waveguide (BD-RWG) for narrow far-field patterns. In particular, we investigate the effects of the structural parameters in the SSC region on the output chip power and field patterns. In our structure, as the width of the passive waveguide core layer decreases, both the lateral and vertical far-field patterns tend to converge at ~ 13° under an etching depth of BD-RWG of ≥ 9 μm. The measured data agree well with the simulated results. Using this design, we can increase the output power of T-ECL from 3.7 to 7.9 mW at an injection current of 60 mA.

Soft-X-ray ARPES facility at the ADRESS beamline of the SLS: concepts, technical realisation and scientific applications

Soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) with photon energies around 1 keV combines the momentum space resolution with increasing probing depth. The concepts and technical realisation of the new soft-X-ray ARPES endstation at the ADRESS beamline of SLS are described. The experimental geometry of the endstation is characterized by grazing X-ray incidence on the sample to increase the photoyield and vertical orientation of the measurement plane. The vacuum chambers adopt a radial layout allowing most efficient sample transfer. High accuracy of the angular resolution is ensured by alignment strategies focused on precise matching of the X-ray beam and optical axis of the analyzer. The high photon flux of up to 10(13) photons s(-1) (0.01% bandwidth)(-1) delivered by the beamline combined with the optimized experimental geometry break through the dramatic loss of the valence band photoexcitation cross section at soft-X-ray energies. ARPES images with energy resolution up to a few tens of meV are typically acquired on the time scale of minutes. A few application examples illustrate the power of our advanced soft-X-ray ARPES instrumentation to explore the electronic structure of bulk crystals with resolution in three-dimensional momentum, access buried heterostructures and study elemental composition of the valence states using resonant excitation.

Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two-, One-, and Zero-Dimensional C60 Nanostructures on Pentacene Substrates

A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures.

Enhanced and suppressed spontaneous emission from a buried heterostructure photonic crystal cavity

We study spontaneous emission control in a buried-heterostructure (BH) photonic crystal (PhC) cavity in which InGaAsP quantum wells (QWs) are embedded in an InP PhC. Spontaneous emission from QW-PhC cavities have been extensively investigated, but they suffer from poor carrier confinement and surface non-radiative recombination. Therefore, the clear spontaneous emission control has been only observed in quantum dots PhC cavities. The present study shows that the BH-QW-PhC cavities can exhibit distinctive spontaneous emission control in QW-PhCs. We observed large emission-rate ratio between the on- and off-resonant conditions of 30:1. This is the highest ratio reported for QW-PhCs.

InGaAs nano-photodetectors based on photonic crystal waveguide including ultracompact buried heterostructure

Ultrasmall InGaAs photodetectors based on a photonic crystal waveguide with a buried heterostructure (BH) were demonstrated for the first time. A sufficiently high DC responsivity of ~1 A/W was achieved for the 3.4-μm-long detector. The dynamic response revealed a 3-dB bandwidth of 6 GHz and a 10-Gb/s eye pattern. These results were thanks to the strong confinement of both photons and carriers in a small BH and will pave the way for unprecedented nano-photodetectors with a high quantum efficiency and small capacitance. Our device potentially has an ultrasmall junction capacitance of much less than 1 fF and may enable us to eliminate electrical amplifiers for future optical receivers and subsequent ultralow-power optical links on a chip.

Hybrid InGaAsP-Si Evanescent Laser by Selective-Area Metal-Bonding Method

A 1.55-μm InGaAsP-Si hybrid laser operating at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C under continuous-wave operation is fabricated using a selective-area metal-bonding method with AuGeNi/Au on InGaAsP gain structure as both cathode and bonding metal and AuSn on silicon-on-insulator (SOI) as bonding metal. The maximum single-facet output power is 9 mW. The slope efficiency of the hybrid laser is 0.04 W/A, four times that of the laser before bonding. A semi-insulating InP:Fe buried heterostructure laser is flip-chip bonded onto an SOI waveguide. The light generated in the active area is evanescently coupled into the silicon waveguide. The simplicity and flexibility of the fabrication process and high yield make the hybrid laser a promising light source.

40-Gb/s Colorless Reflective Amplified Modulator

In this letter, we demonstrate a colorless reflective amplified modulator operating within the C- and L-band spectral ranges with the modulation data rate up to 40 Gb/s. We obtain a stable, open eye performance of the device at the temperature until 85°C. The presented device is fabricated using an indium phosphide (InP) monolithic integration platform, which relies on an AlGaInAs quantum well active material, gap engineering by selective area growth, and low-parasitic RC semi-insulating buried heterostructures.

Contact vs bulk effects in N-semi-insulating-N and P-semi-insulating-P diodes

We analyse the charge transport mechanism in semi-insulating (SI) materials for N-SI-N and P-SI-P structures. The SI layers are large band gap semiconductors obtained by deep levels compensation of residual shallow donors or acceptors. A preceding theory is extended to the case of two, donor or acceptor, deep compensating levels [1]. The conduction mechanism is complex: ambipolar transport, heavy recombination, space charge effect and we show that the current is controlled by both bulk and interface effects at the reverse biased N-SI or P-SI junction. We develop a simple model, without the usual assumption of space charge neutrality, valid up to the beginning of one carrier space charge current. We show that a linear relation exists between the bulk excess free carrier densities and that depending on the value of a quantity M, given as a function of the deep levels electrical parameters, energy position, concentration and capture cross sections, as well as the dopants concentrations, the conduction mechanism is either contact controlled showing a pronounced current saturation effect or bulk controlled with one and for long sample, two, quasi linear J–Va relationship. Numerical modelisations of the drift–diffusion transport model confirm these analytical results for the case of one or two compensating deep levels. GaAs (SI) or InP (SI) layers are used for their high resistivity and insulating properties in FET technology, in buried heterostructures, diode laser and in radiation detectors technologies. These results are of importance for the interpretation of conductivity and Hall Effect measurements and explain the parasitic side-gating effect in GaAs or InP MESFET’s.

Fully automatized quantum cascade laser design by genetic optimization

Using a transport model based on the density matrix formalism, a fully automatized technique to design quantum cascade structures in the mid-infrared is presented that implements a genetic algorithm where the wallplug efficiency has been used as merit factor. Starting from a reference design, the model converges after few generations on an optimized design that presents a better carrier injection in the upper lasing state. Both the designs have been fabricated using buried heterostructure process and the optimized design shows a pronounced increase in the laser operation range and higher output powers. In good agreement with the simulations, the laser efficiency increases from 5% to 12%.

Monolithic Integration of a Semiconductor Optical Amplifier and a High-Speed Photodiode With Low Polarization Dependence Loss

We demonstrate the monolithic integration of a buried heterostructure semiconductor optical amplifier (SOA) and a deep ridge PIN photodiode for high-speed on-off keying links at 1.55 μm. The structure allows separate optimization of the SOA and the photodiode. The integrated receiver presents simultaneously a peak responsivity of 88 A/W with a low polarization dependence loss (<; 1 dB), a low noise figure (8.5 dB), and a wide 3-dB electrical bandwidth (≈ 50 GHz). This corresponds to a very large gain-bandwidth product of 3.5 THz. To our knowledge, this is the first time that a monolithically integrated SOA-PIN receiver has achieved such performances.

Buried-heterostructure quantum-cascade laser overgrown by gas-source molecular-beam epitaxy

We describe the realization of buried-heterostructure quantum-cascade lasers (QCLs) using gas-source molecular beam epitaxy both for the growth of the active region as well as for the regrowth of InP:Fe. The regrowth of the semi-insulating InP:Fe layer was carried out at 470 °C, which is more than 100 °C below the standard growth temperature during metal-organic vapor-phase epitaxy, the standard method for laser overgrowth. The electrical resistivity of the InP:Fe insulation layer, measured in test samples grown on (001) InP, is as large as 2×108Ωcm. High-resistivity InP:Fe is overgrown non-selectively over the etched laser ridge, followed by the top contact alloyed through it to the active region. The processed quantum-cascade lasers show no evidence of parallel leakage current and exhibit lower threshold current density than lasers using SiO2 as an insulator. The ability to fabricate buried heterostructure lasers without exceeding 600 °C is important for strain-compensated AlAs-InGaAs quantum cascade lasers with large internal strain because these devices do not typically withstand temperatures used to grow InP:Fe using vapor-phase epitaxy.

Thermal characteristics of InP-based mid-infrared quantum cascade lasers at λ ∼ 8.8 µm

We have theoretically investigated the thermal characteristics of double-channel ridge-waveguide InGaAs/AlInAs quantum cascade lasers (QCLs) emitting at λ ∼ 8.8 µm in comparison with the experimental results. Based on the measured data, the thermal parameters were obtained with heat source densities using a two-dimensional anisotropic heat dissipation model. For 21 µm × 4 mm QCLs, the thermal conductance (G th ) value was 215.5W/K-cm2 at 298 K in continuous-wave (CW) operation. The G th value was significantly increased by using a buried heterostructure (BH) via the regrowth of undoped InP layer around the laser ridge. Also, an additional improvement in the G th was made by the use of a diamond submount, and the temperature dependency of the devices was largely decreased. The G th value was improved to 306.7 W/K-cm2 at 298 K in the CW mode for epilayer-down bonded BH QCLs with an InP embedded layer on a diamond submount.

Long-Term Degradation Behavior of 2.3-$\mu\hbox{m}$ Wavelength Highly Strained InAs/InP MQW-DFB Lasers With a p-/n-InP Buried Heterostructure

We investigate the long-term degradation of 2.3-μm wavelength InAs/InP highly strained multiquantum-well distributed-feedback lasers with a p- and n-type InP buried heterostructure during constant power aging. Stable operation beyond 20 000 h is confirmed at an ambient temperature of 45 °C and with a constant output power of 3 mW, and we successfully fabricate a practical optical source for trace gas monitoring applications. A conventional diffusion process dominates the main degradation mechanism as found with conventional telecommunication lasers. Furthermore, we show that some defects are located in the regrown p-type InP layer above the active layer and that downward diffused defects degrade the guide, upper separate confinement heterostructure (SCH), and active layers.

Sb-free quantum cascade lasers in the 3–4 μm spectral range

In this work, the design and implementation of Sb-free short wavelength strain-compensated quantum cascade lasers in the 3–4 μm spectral range is presented. Due to the presence of highly strained AlAs-barrier layers, the optimization of the epitaxial growth process is firstly discussed. The used active region design is then presented together with the observed laser performance. Watt-level room temperature emission at 3.3 μm is shown for Fabry–Perot devices and laser operation in pulsed mode is observed above 350 K. The laser performance is comparable with Sb-containing quantum cascade lasers. Spectral tuning of the lasers in an external cavity configuration over more than 275 cm−1 is achieved with an emission wavelength as short as 3.15 μm. For the first time in this spectral range, results on single-mode buried heterostructure distributed feedback lasers are shown.

Ultralow-power all-optical RAM based on nanocavities

Using photonic crystal cavities with a buried heterostructure design, scientists demonstrate all-optical RAM at power levels 300 times lower than previous attempts. The 30 nW devices could enable low-power large-scale optical RAM systems for handling high-bit-rate optical signals.

Tapered 47 μm quantum cascade lasers with highly strained active region composition delivering over 45 watts of continuous wave optical power

A strain-balanced, Al0.7In0.22As/In0.72Ga0.28As/InP quantum cascade laser structure, designed for light emission at 4.7µm using the non-resonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in tapered buried heterostructure geometry and then mounted on AlN/SiC composite submounts using hard solder. A 10 mm long laser with 7.5µm-wide central section tapered up to 20µm at laser facets generated over 4.5W of single-ended CW/RT optical power at 283K. Maximum wallplug efficiency of 16.3% for this laser was reached at 4W level. Reliability of over 2,000h has been demonstrated for an air-cooled system delivering optical power of 3W in a collimated beam with overall system efficiency exceeding 10%.

Room-temperature operation of npn- AlGaInAs/InP multiple quantum well transistor laser emitting at 13-µm wavelength

Room-temperature pulsed operation of a 1.3-µm wavelength transistor laser (TL), consisting of a buried heterostructure (BH) with an npn configuration and an AlGaInAs/InP multiple-quantum-well (MQW) active region, was successfully attained. A threshold base current of 18 mA (threshold emitter current of 150 mA) was obtained with a stripe width of 1.3 µm and a cavity length of 500 µm. The transistor activity as well as the lasing operation were achieved at the same time, which is essential for the high-speed operation of TLs.

High-Temperature Operation of Photonic-Crystal Lasers for On-Chip Optical Interconnection

SUMMARY To meet the demand for light sources for on-chip optical interconnections, we demonstrate the continuous-wave (CW) operation of photonic-crystal (PhC) nanocavity lasers at up to 89.8 ◦ Cb y using InP buried heterostructures (BH). The wavelength of a PhC laser can be precisely designed over a wide range exceeding 100 nm by controlling the lattice constant of the PhC. The dynamic responses of the PhC laser are also demonstrated with a 3-dB bandwidth of over 7.0 GHz at 66.2 ◦ C. These results reveal the laser’s availability for application to wavelength division multiplexed (WDM) optical interconnection on CMOS chips. We discuss the total bandwidths of future on-chip optical interconnections, and report the capabilities of PhC lasers.

1.3-μm InGaAsP planar buried heterostructure laser diodes with AlInAs electron stopper layer

This study reports on the realization of 1.3-μm InGaAsP buried-heterostructure (BH) laser diodes (LDs) via an Fe-doped semi-insulating InP layer and an AlInAs electron stopper layer (ESL). Experimentally, the as-cleaved BH LD with an AlInAs ESL exhibited improved characteristics in terms of threshold current, slope efficiency, and maximum light output power at 90 °C as compared to those of the normal BH LD without an AlInAs ESL. In addition, high internal quantum efficiency or reduced threshold current density was observed, indicating increased modal gain in BH LDs fabricated with an AlInAs epilayer on top of the active region. It was also found that the temperature sensitivity of the BH LDs with an AlInAs ESL is more stable than that of the normal BH LDs. These results could be attributed to the suppression of thermal carrier leakage out of strain-compensated multiple-quantum-well by a large conduction-band offset of the AlInAs/InGaAsP heterojunction. Otherwise, without consideration of damping factor or coupling loss, the 3-dB bandwidth of the proposed BH LDs reaches a high value of 15.3 GHz. Finally, this TO-can packaged BH LD shows an eye-opening feature with the extinction ratio of 7.49 dB while operating at 10 Gbit/s at 50 mA.