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We present a high-resolution electron tomography study of buried ultra-thin layers and their interfaces from the active region of a (Ga,In)As/(Al,In)As quantum cascade laser (QCL) test structure. Using a high-angle annular dark-field scanning transmission electron microscopy image series, a three-dimensional (3D) reconstruction of the complex layer structure is obtained. From this 3D information, we determine quantitative values for the chemical width and, simultaneously and independently, the root mean square roughness (rms) and lateral correlation length of the individual interfaces of a cascade using topographic height maps. The interfacial widths are comparably small for all interfaces within a cascade and the layer thicknesses show only a small standard deviation of less than one monolayer. The rms roughness is systematically lower at the direct (Ga,In)As-on-(Al,In)As interface compared to its inverse. In addition, using the one-dimensional height–height correlation function, different lateral correlation lengths along the two perpendicular ⟨ 011 ⟩ in-plane directions are detected indicating a distinct anisotropy of the interface morphology. Our accurate and comprehensive results on the QCL test structure will serve as feedback to evaluate the growth process and help to assess the performance of corresponding future laser devices in detail.

In this paper, we give an overview of quantum cascade detector technology for the near- and mid-infrared wavelength range. Thanks to their photovoltaic operating principle, the most advanced quantum cascade detectors offer great opportunities in terms of high detection speed, reliable room temperature operation, and excellent Johnson noise limited detectivity. Besides some important features dealing with their fabrication and their general characteristics, we will also briefly present some possibilities for performance improvement. Elementary theoretical considerations adopted from photoconductive detectors confirm that optimization of such devices always involves various trade-offs.

The versatility of solid-source molecular beam epitaxy for the growth of InP/InGaAs heterojunction bipolar transistors (HBTs) is provided by its excellent control of doping and composition grading profiles in combination with its efficiency for carbon doping. Various designs using doping grading or composition grading in the base are investigated to provide a built-in quasi-electric field that enhances electron transport. All graded-base devices exhibit higher current gains (beta), as compared to uniform-base structures, but the beta improvements are found to be nonproportional to the generated built-in drift fields. The best performances are obtained with a 9% linear composition grading profile. As compared to conventionally grown uniform-base structures, the linearly graded-base HBTs show higher current gains (up to 42%), which is of particular importance particularly in analog and mixed-signal applications.

The near-field intensity distributions of quantum-cascade lasers emitting at λ = 7.7 μm were investigated with a Gaussian telescope, comprising standard optical components together with a microbolometer focal plane array. Although the technique is operated at the diffraction limit, the quality of the near-field patterns obtained allows unambiguous determination of the lateral-mode structure of the laser even for individual laser pulses. The high quality of the recorded near-field intensity distribution allows us to calculate analytically the corresponding far-field intensity distributions, leading to an excellent match with measured far-field patterns. The present experimental technique allows fast and efficient characterization of the beam propagation properties of mid-infrared semiconductor lasers.

In recent years, the technology of the quantum cascade laser (QCL) has drastically improved, as proven by room temperature continuous wave (CW) operation, high optical power, low dissipation devices, and accessible spectral range [1]. As a result, QCLs have drawn a great deal of interest for their various applications in trace-gas sensing, defense countermeasures, spectroscopy and free-space communications [2]. The performance of QCLs is primarily dependent on the design of the active region, usually gown by MBE (molecular beam epitaxy). Nevertheless, the laser performance are also influenced by the thermal properties and optical losses introduced by the fabrication technology, in particular in continuous-wave operation. In that respect, the buried heterostructure (BH) approach, which is a standard and reliable telecommunications laser manufacturing technique, offers unique possibilities to achieve effective thermal dissipation and lateral single modes. In standard BH designs, the process starts by the definition and etching of the ridge mesa using a dielectric mask. The latter (SiO2 or SiN) is also used as the masking material for the selective regrowth by metal-organic vapor phase epitaxy (MOVPE) of a thick insulating InP (Fe) layer over the entire laser structures. Unfortunately, the selective regrowth of InP:Fe is not a trivial process. Indeed, it is extremely difficult to achieve an ideal planarized regrowth, i.e, completely burying the laser ridge, while maintaining a planar surface. In addition to growth conditions, the characteristics of the regrown InP layer strongly depend on the sidewall profile of the ridges, as well as on the physical parameters of the dielectric mask (thickness, width…) [3]. The complexity of the etching and regrowth process further increases for thick layer structures, such as for long wave QCLs, whose ridges can be >10 µm deep. In effect, deep ridge etching generates big undercuts in the structures and consequently, highly non uniform active region widths. Moreover, the thick selective regrowth step, needed for planarization, tends to favor the appearance of crystal growth artifacts (crystallites, voids, “rabbit ears”…) in the vicinity of the ridges, which deteriorate the performance of the regrown structures. In some cases, additional photolithographic and back-etching steps might even be required to improve the planarity of the structure. Another method that allows overcoming the aforementioned limitations is to use a modified buried heterostructure concept, with two MOVPE regrowth steps [4]. In this case, the waveguide pattering and etching is constituted only by the active layers ( In this perspective, we report an optimized BH-QCL approach obtained by adding an InP layer and a sacrificial InGaAs cap layer on top of the MBE grown active region. As shown in the attached figures (FIB/SEM cross-section profiles), the lateral thickness of the insulating and planar regrowth is increased and reduces potential leakage paths. Using this approach, BH QCL regrowths were successfully carried out with reproducible and good epitaxial growth quality (smooth and uniform regrown layers, planar surfaces, no “rabbit-ears” and defect free interfaces) around the etched ridges, permitting the fabrication of BH-QCLs with good beam quality and hundreds of mW of optical power at room temperature. The growth and optimization of the BH process, as well as the fabrication and testing of the laser devices will be presented and compared to conventional double trench devices. References [1] Faist, J., et al. "Quantum cascade lasers," University Press, 2013. [2] Razeghi, M., et al. "Quantum cascade lasers: from tool to product," Optics express 23.7 (2015): 8462-8475. [3] Cheng, L., et al., "Analysis of InP regrowth on deep-etched mesas and structural characterization for buried-heterostructure quantum cascade lasers." J. of electronic materials 41.3 (2012): 506-513. [4] Süess, M. J., et al. "Advanced Fabrication of Single-Mode and Multi-Wavelength MIR-QCLs." Photonics. Vol. 3. No. 2. Multidisciplinary Digital Publishing Institute, 2016. Figure 1

This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 μm to THz radiation at 87 μm using different semiconductor material systems.

The influence of injector doping concentration Ninj on the performance of InP-based quantum-cascade (QC) lasers is investigated for devices emitting around 9.2-mum wavelength and injector doping concentrations between 1times1017 cm-3 and 3times1017 cm-3. The threshold current density, the dynamic range, the maximum emitted output power as well as the maximum operating temperature are found to increase with increasing N inj. All in all, there exists no optimal value of Ninj per se. In fact, the injector doping concentration has to be adjusted individually depending whether emphasis is placed on obtaining low threshold current densities or on high-power operation

An overview of quantum cascade detector technology for the near- and mid-infrared wavelength range will be given. Thanks to photovoltaic instead of photoconductive operation, quantum cascade detectors offer great opportunities in terms of detection speed, room temperature operation, and detectivity. Besides some crucial issues dealing with fabrication and general characteristics, some possibilities for performance improvement will also be briefly presented. In a theory section, some basic considerations adopted from photoconductive detectors confirm the necessity of various trade-offs for the optimization of such devices. Nevertheless, we will show several possible measures to push the key performance figures of these detectors closer to their physical and technological limits.

A midinfrared quantum cascade detector with a spectrally broad (ΔE/E = 27.3%) response is designed, fabricated, and tested. This detector consists of 26 differently designed active region stages in order to cover a wavelength region from 4.7 to 7.4 μm. The device could be operated above room temperature and showed peak responsivities of 13 mA/W at 10 K and 1.25 mA/W at room temperature. A background limited detectivity of 1.55×1010 Jones was seen up to a temperature TBLIP of 110 K.

The authors report on short-wavelength In0.53Ga0.47As/AlAs0.56Sb0.44 quantum cascade detectors (QCDs). At room temperature, one device detects at 505 meV (2.46 mu m) with a responsivity of 2.57 mA/W, while a second QCD is sensitive at 580 meV (2.14 mu m) with a responsivity of 0.32 mA/W.

The authors extracted the thermal resistance (R-L=9.6 K/W) and the electrical power dependence of the electronic temperature (R-e=12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx quantum-cascade lasers (QCLs) operating at 4.9 mu m, in the lattice temperature range of 60-90 K. The low electron-lattice coupling constant alpha=10.4 K cm(2)/kA can be related to the beneficial effect of the high conduction band offset, peculiar to the GaInAs/AlGaAsSb material system, on the electron leakage. The authors found an active region cross-plane thermal conductivity value k(perpendicular to)=1.8 +/- 0.1 W/(K m), which is approximately three times larger than that measured in QCLs with GaInAs/AlInAs heterostructures. (c) 2007 American Institute of Physics.

Room-temperature (300 K) intersubband electroluminescence has been obtained from a quantum cascade structure adopting triple-well vertical-transition active regions, based on Ga0.47In0.53As/AlAs0.56Sb0.44 heterostructures grown lattice-matched on InP substrate by molecular beam epitaxy. The emission peak wavelength varies from 4.3 µm at 77 K to 4.5 µm at 300 K.

A report is presented on the continuous-wave operation of λ≃5 µm GaInAs/AlInAs/InP-based quantum cascade lasers up to a heatsink temperature of 222K (−51°C). The devices are mounted epi-side down with high-reflection coated facets.