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We propose an Fe-doped, InGaAs-based semiconductor heterostructure optimized for the application as an ultrafast photoconductor and investigate its optical as well as THz properties. A sample series with varying doping concentration is grown by molecular-beam epitaxy. After examination of its static optical properties, the pump-induced intraband carrier dynamics are investigated via optical pump-THz probe measurements. Here, we observe conductivity decay times as low as 0.23 ps which are attributed to electron capture into Fe-related defects. These results are corroborated by monitoring the corresponding interband dynamics via all-optical pump-probe measurements. In addition, the competitiveness of THz detectors fabricated from a subset of these samples is demonstrated by integrating them into a standard time-domain spectrometer.

We discuss a solution to the dilemma of high confinement energy and the high interface-scattering rate for the high-barrier InGaAs-InAlAs-AlAs QCLs, operating at wavelengths substantially shorter than 4 µm. The new design includes fewer quantum wells within a single cascade and operates at 3.6 µm. Otherwise, its performance is very similar to the best of recently-shown QCLs at 4.6 µm and 4.0 µm using the same material system.

Currently, the highest power quantum cascade lasers (QCLs) include (i) an efficient active region design, which minimizes the energy fraction transferred into heat, (ii) a relatively large number (40) of cascades, which increases the mode confinement factor, Γ, and (iii) narrow buried-heterostructure waveguides, which efficiently cool the active region through efficient lateral heat flow. This combination allows watt-level continuous wave (cw) emission powers at room temperature. Moreover, narrow waveguides favor the fundamental TM00 lateral mode, resulting in a nearly Gaussian beam. To the extent that power efficiency is already largely optimized, power scaling in narrow-stripe QCLs is only possible through increasing the laser length, which is limited by the needs for robust construction and transparent antireflection coatings. Broad-area (BA) QCLs with a small number of cascades can also deliver watt-level cw powers at room temperature, profiting from the enhanced vertical heat flow. Power can be scaled for these lasers also via the laser width and is nearly unlimited. The usual drawback of the BA lasers is poor beam quality. We discuss the use of tapered QCLs and the first demonstration of room temperature cw operation of the BA QCL in fundamental TM00 mode. A stable and reproducible transverse optical mode is achieved using a double-taper waveguide geometry. The laser emits at 4.6 μm and has a demonstrated room temperature cw power of 100 mW.

We compare the interface roughness scattering of electrons at the In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As heterointerface simultaneously grown on (100) and (411)A oriented InP substrates using gas-source molecular-beam epitaxy (MBE). A modulation-doped double quantum well structure is designed to emphasize the effects of interface scattering. The transport properties for both (100) and (411)A orientations are compared for different MBE growth temperatures. The highest mobilities on (411) oriented structures are 40% higher than those on (100) oriented structures, indicating less electron scattering due to interface roughness scattering.

A THz time-domain spectroscopy (TDS) system is presented, which uses photoconductive antennas made of iron and rhodium doped InGaAs. Due to the unique combination of ultrashort lifetime and high mobility as well as high resistivity, the transition metal doped InGaAs surpasses the performance of state-of-the-art photoconductors when applied as THz antennas: The presented THz TDS system features a spectral bandwidth of 6.5 THz and a dynamic range of up to 105 dB.

Let's talk about your flaws\dots{} The authors present nondestructive examination of the crystallographic properties (including crystal size, facet shape, strain, and defects) of lone InP nanocrystals (NC) grown on Si nanostructures. This sort of three-dimensional structured imaging is of great significance in evaluating the quality of the active nanomaterials in fully processed nanoelectronic and nano-optoelectronic devices, even in an $o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}o$ manner.

Rhodium (Rh)-doped In0.53Ga0.47As grown by gas-source molecular beam epitaxy is investigated as a terahertz (THz) detector antenna for optical excitation at 1550 nm. The 4d transition metal rhodium acts as a deep level and ultrafast trapping center. At a doping concentration around 8 × 1019 cm−3, InGaAs:Rh exhibits ideal properties for application as a THz antenna: an ultrashort carrier lifetime below 200 fs in combination with a mobility of 1010 cm2/Vs. The THz detectors fabricated from this sample show a record peak dynamic range of 105 dB and a bandwidth of up to 6.5 THz.

InP/InGaP surface quantum dots represent an attractive material for optical chemical sensors since they show a remarkable near infra-red emission at room temperature, whose intensity increases rapidly and reversibly depending on the composition of the environmental atmosphere. We show here their emission properties by time resolved photoluminescence spectroscopy investigation. Photoluminescence transients with and without chemical solvent vapours (methanol, clorophorm, acetone and water) were fitted with a 3-exponential decay law with times of about 0.5 ns, 2 ns and 7 ns. The measurements revealed a weak effect on clorophorm, acetone and water, while the initial decay time of InP surface quantum dots increases (up to 15%) upon methanol vapour exposure, indicating that the organic molecules efficiently saturate QD non-radiative surface states. (C) 2015 Elsevier B.V. All rights reserved.

We investigate photoconductive terahertz (THz) emitters compatible with 1550 nm excitation for THz time-domain spectroscopy (TDS). The emitters are based on rhodium (Rh) doped InGaAs grown by molecular beam epitaxy. InGaAs:Rh exhibits a unique combination of ultrashort trapping time, high electron mobility, and high resistivity. THz emitters made of InGaAs:Rh feature an emitted THz power of 637 μW at 28 mW optical power and 60 kV/cm electrical bias field. In particular for a fiber coupled photoconductive emitter, this is an outstanding result. When these emitters are combined with InGaAs:Rh based receivers in a THz TDS system, 6.5 THz bandwidth and a record peak dynamic range of 111 dB can be achieved for a measurement time of 120 s.

Power and, especially, brightness are important characteristics of quantum-cascade lasers (QCLs) for many applications. Brightness is a measure of the intensity of a laser’s emission that can be delivered by an optical system. Applications based on vibrational-mode gas spectroscopy or mid-infrared illumination benefit from a higher brightness beam providing a better signal-to-noise ratio, external-cavity tunable QCL systems gain efficiency when more of the beam power can be re-focused into the active region, and, of course, infrared countermeasures depend on optical power delivered by a lens system to a distant target. For a given wavelength, brightness requires both optical power and beam quality. The optical power emitted by a QCL depends on its efficiency and its active region volume. The efficiency can be optimized through design features, quality of epitaxy, thermal management, and facet reflectance. The active region volume is governed by the number of cascades and the width and length of the emitter. For a given efficiency – and QCL efficiency is close to optimized today – the route to power is either through a larger number (20–40) of cascades combined with a narrow stripe or a small number (5–15) of cascades using a broad-area emitter. For a given input power – which means for a given laser volume – a broad-area laser will have the same output power and better thermal conductance than a narrow-stripe buried-heterostructure laser. Further, because both emission power and thermal conductance of a broad-area QCL with around 10 cascades scales with area, such broad-area QCLs cascade number can be scaled to higher emission power [1,2,3]. But, wide stripes are notorious for poor beam quality and the reduced cascade number reduces efficiency. The realization of good beam quality from broad-area QCLs has been achieved through several approaches. Angled facets to suppress non-fundamental modes, tapered stripes that combine large area with a section that favors the fundamental mode, photonic crystal facets, constricted stripes to filter high-order modes, and engineered waveguide loss that suppress high-order modes have been demonstrated. Further, the reduced efficiency has been demonstrated to be acceptable down to about 10 cascades. This talk will discuss the paradigm shift to broad-area, reduced-cascade-number QCLs for high brightness applications, including the optimization of efficiency and beam quality. [1] W.T. Masselink, M.P. Semtsiv, A. Aleksandrova, and S. Kurlov, “Power scaling in quantum cascade lasers using broad-area stripes with reduced cascade number,” Optical Engineering 57(1), 011015 (2017); http://dx.doi.org/10.1117/1.OE.57.1.011015. [2] W.T. Masselink, M.P. Semtsiv, Y. V. Flores, A. Aleksandrova, and J. Kischkat, “Design issues and physics for power scaling of quantum-cascade lasers,” Proc. SPIE 9989, 99890B (2016). [3] P. Figueiredo, M. Suttinger, R. Go, A. Todi, H. Shu, E. Tsvid, C.K.N. Patel, and A. Lyakh, “Continuous wave quantum cascade lasers with reduced number of stages,” IEEE Photonics Tech. Lett. 29, 1328–1331 (2017).

Transition metal doped photoconductors are investigated as terahertz (THz) receivers in time-domain spectroscopy (TDS). Compared to state-of-the-art receivers based on low-temperature-grown InGaAs/lnAIAs, rhodium-doped InGaAs receivers allow for an improvement in dynamic range (DR) of 15 dB over the whole spectral range, resulting in a peak DR of 105 dB. This improvement increases the resolution in nondestructive testing applications of thin films with THz TDS.

The growth of Al 0.85 Ga 0.15 P on GaP using gas-source molecular-beam epitaxy is investigated using in situ high-energy electron diffraction, high-resolution x-ray diffraction, atomic-force microscopy, and scanning electron microscopy. Growth temperature and phosphorus flux were varied. The 1.0-μm AlGaP films were grow on a GaP buffer layer and capped with GaP. The investigation indicates that a growth temperature of 490 °C and a cracked PH3 flux of 2.7 sccm resulted in the best AlGaP quality, while maintaining very good GaP quality.

In a previous work [Flores et al., J. Cryst. Growth 398 (2014) 40] [3] we demonstrated the advantages of using a thin InAlAs spacer layer in the fabrication of buried-heterostructure quantum-cascade lasers (QCLs), as it improves the morphology of the interface between the laser core and the InP:Fe lateral cladding. In this paper we investigate aspects of InAlAs, which are relevant for its role as insulating lateral cladding of the laser sidewalls: carrier traps, electrical resistivity, and functionality as a sole lateral cladding. We find that a thin InAlAs spacer layer not only improves the regrowth interface morphology, but also eliminates interface-related shallow electronic states, thus improving the electrical resistivity of the interface. We further find that bulk InAlAs grown by gas-source molecular-beam epitaxy as well as InP:Fe are semi-insulating at room temperature, with specific resistivities of 3 × 10 7 Ω cm and 2 × 10 8 Ω cm , respectively. Both materials have also a high thermal activation energy for electrical conductivity (0.79 eV and 0.68 eV, respectively). In order to compare the performance of InP:Fe and InAlAs as a lateral cladding, lasers were fabricated from the same QCL wafer with differing stripe insulation materials. The resulting lasers differ mainly by the lateral insulation material: SiO 2 , InP:Fe (with InAlAs spacer), and pure InAlAs. All devices show a similar performance and similar temperature dependence, indicating insulating properties of InAlAs adequate for application in lateral regrowth of buried-heterostructure QCLs.

The effect of post-growth annealing of epitaxial gallium phosphide grown on silicon substrates using gas-source molecular-beam epitaxy is described. The epitaxial layers were grown at substrate temperatures ranging from 250 to 550 °C. After optimizing the growth temperature, the prepared films were thermally annealed. Two thermal annealing methods are compared: annealing at a constant temperature and step-graded annealing, in which the temperature is raised by a constant rate per unit time. The effect of the thermal annealing on the crystal structure was studied by characterizing the epilayer using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). It was found that after thermal annealing, the epilayer exhibited a reconstructed RHEED pattern, and its quality was much improved.

Molecular vibrational spectroscopy within the mid-infrared part of the spectrum is an attractive analysis technique. The spectral region required for the analysis requires tens of cm−1 tunability centered somewhere typically between 3 to 15 μm, depending on the constituent molecular compound to be analyzed. An important application is medical hyperspectral imaging for early stage cancer diagnosis. For this application, a broadly and continuously tunable light source centered near 11 μm is required [1]. Further, for highly detailed images, the source should be much brighter than traditional glow-bar sources. The external-cavity quantum cascade laser (EC-QCL) meets these requirements and has further advantages in being compact, robust, and relatively inexpensive. We describe an EC-QCL system based on the InGaAs/InAlAs heterosystem with good average power and tunability; the system is implemented as a portable and rugged light source for hyperspectral imaging near 11 μm.

Today, the optimum material systems for photoconductive emitters and receivers are different. In THz reflection measurements, this leads to complicated optics or performance compromises. We present photoconductive emitters and detectors fabricated from molecular beam epitaxy (MBE) grown iron (Fe) doped InGaAs, which are well suited for a THz time-domain spectroscopy as both emitters and detectors. As a photoconductive emitter, 75 mu W +/- 5 mu W of radiated THz power was measured. As a detector, THz pulses with a bandwidth of up to 6 THz and a peak dynamic range of 95 dB could be detected. These results are comparable to state-of-the-art THz photoconductors, which allows for simple reflection measurements without a performance decrease. The incorporation of Fe in InGaAs during MBE growth is investigated by secondary ion mass spectroscopy, Hall, and transient differential transmission measurements. Growth temperatures close to 400 degrees C allow for homogeneous Fe doping concentrations up to 5 x 10(20) cm(-3) and result in a photoconductor with an electron lifetime of 0.3 ps, a resistivity of 2 k Omega cm, and an electron mobility higher than 900 cm(2) V-1 s(-1). We show that iron dopants are incorporated up to a maximum concentration of 1 x 10(17) cm(-3) into substitutional lattice sites. The remaining dopants are electrically inactive and form defects that are anneal-stable up to a temperature of 600 degrees C. The fast recombination center in Fe-doped InGaAs is an unidentified defect, representing approximate to 0.5% of the nominal iron concentration. The electron and hole capture cross section of this defect is determined as sigma(e) = 3.8 x 10(-14) cm(2) and sigma(h) = 5.5 x 10(-15) cm(2), respectively.

We describe an optimized growth sequence for the overgrowth of quantum cascade laser ridge sidewalls with semi-insulating InP:Fe. A thin In0.52Al0.48As spacer layer grown on the laser ridge sidewalls before InP:Fe prevents the formation of void defects at this interface, which appear otherwise. Elimination of these voids at the sidewalls has led to more than a two-fold improvement of the thermal conductivity in a 7 μ m wide buried-heterostructure quantum-cascade laser overgrown with InP:Fe by gas-source molecular-beam epitaxy, and has allowed the continuous-wave operation of the laser up to 210 K. The measured thermal conductance is G th = 500 W / K cm 2 at 210 K and G th = 1020 W / K cm 2 at 127 K, comparable to the state of the art literature values obtained with regrowth by metal-organic vapor-phase epitaxy.

We studied the effect of solvent vapours on the photoluminescent emission of self-assembled InP surface quantum dots (SQDs). Their room temperature near infrared emission undergoes a fully reversible intensity enhancement when the dots were exposed to vapours of polar solvents since polar molecules are likely to be adsorbed onto intrinsic surface states and thus reducing non radiative surface recombination. The shape and position of the emission band does not change. The observed effect is dependent on solvent type and concentration with linear law over a limited concentration range.

A recent experiment [Hortelano et al., Semicond. Sci. Technol. 32, 125005 (2017)] reported a rectification effect that appeared in curved narrow ballistic channels of a two-dimensional electron gas when the strength of a magnetic field applied to the channels was tuned. The phenomenon was reproduced by classical billiard simulations as resulting from a transmission asymmetry caused by diffuse boundary scattering. However, this manifests breakdown of a commonly used simple model for diffuse boundary since the magnetic-field dependence of the transmission in two-terminal geometries has to be symmetric in equilibrium. We demonstrate here that this tendency of the system predicted by the billiard simulations is a real transmission asymmetry effect that emerges in the nonequilibrium transport. We perform nonequilibrium quantum-mechanical simulations with taking into account Coulomb repulsion. Experimental observations are presented to demonstrate consistencies with the numerical results.

We investigate the effect of morphology and coverage of self-assembled epitaxial InP quantum dot on the photoluminescence sensitivity to methanol vapour. The photoluminescence of quantum dots is in the near infrared (750–900 nm) for quantum dot height and lateral size of 2–3 nm and 20–30 nm, respectively. Photoluminescence intensity undergoes an enhancement when the samples were exposed to methanol vapours. The dimension, density and effective surface of quantum dots have been estimated by statistical analysis of the dot morphology based on atomic force micrographs. We observed that it is possible to increase the magnitude of the intensity change due to methanol vapour by tailoring quantum dot size and density to increase the sample coverage. We observe a linear dependence on the coverage of the luminescence intensity changes induced by exposition to methanol vapours.

We describe the design, gas-source MBE growth, and performance of a highly strained photovoltaic intersubband detector with peak responsivity between 3 and 4 μ m . The absorbing region is composed of a highly strained InGaAs–InAs–InAlAs–AlAs heterostructure with its average lattice constant matched to InP. The detector design is based on a combination of bound-to-bound and bound-to-continuum transitions, which together allow a broad-band photoresponse, comparable to that of detectors with heterogeneous absorption region.

We describe the realization of buried-heterostructure strain-compensated quantum-cascade lasers that incorporate a very high degree of internal strain and are grown on InP substrates using gas-source molecular-beam epitaxy (GSMBE). The active region of the lasers contains AlAs layers up to 1.6 nm thick with 3.7% tensile strain; restricting any post-growth processing to temperatures below 600 °C to avoid relaxation. We demonstrate that buried-heterostructure devices can be realized by using GSMBE to over-grow the etched laser ridge with insulating InP:Fe at temperatures low enough to preserve the crystal quality of the strain-compensated active region. Two distinct growth techniques are described, both leading to successful device realization: selective regrowth at 550 °C and non-selective regrowth at 470 °C. The resulting buried-heterostructure lasers are compared to a reference laser from the same wafer, but with SiO 2 insulation; all three have very similar threshold current densities, operational thermal stability, and waveguide losses.

The influence of substrate temperature during the growth of quantum-cascade lasers using gas-source molecular-beam epitaxy on performance and crystal quality of quantum-cascade laser is investigated. Lower substrate temperature is consistently resulting in narrower X-ray diffraction satellites, and lower laser threshold current. This correlation is attributed to increased interface roughness at higher growth temperature.

Surface InP quantum dots grown by gas source molecular beam epitaxy on In0.48Ga0.52P buffer layer lattice matched to GaAs substrate shows a broad-band near-infrared photoluminescence ranging from 750 to 865 nm dependent on their dimensions. A reversible luminescence intensity enhancement has been observed when the quantum dots were exposed to vapours of different chemical solvents with the highest sensitivity for alcohol (methanol and ethanol) vapours. The luminescent behaviour is dependent on the solvent type and concentration. The peak energy and band shape of the luminescence are not affected by the solvent vapour.

The quantum-cascade laser (QCL) emitting in the mid-infrared region of 4 to 8 m has been refined to the point that its internal quantum efficiency is approaching fundamental limits. QCLs designed for power typically contain 30-40 cascades, are less than two wavelengths in width, and laser ridge lengths are typically between 3 and 6 mm. Even with state-of-the-art efficiency and thermal management, room temperature operation of such lasers is fundamentally limited to several watts. This paper describes a path to power scaling that is not fundamentally limited. Power requires volume and thermal conductance. We propose that this combination is best achieved using fewer than 15 cascades combined with broad areas. We demonstrate the first room temperature continuous-wave emission of broad-area QCLs and discuss how this scaling concept can deliver MIR emission of 10's of watts at room temperature with beam quality required for high brilliance.

The epitaxial integration of highly heterogeneous material systems with silicon (Si) is a central topic in (opto-)electronics owing to device applications. InP could open new avenues for the realization of novel devices such as high-mobility transistors in next-generation CMOS or efficient lasers in Si photonics circuitry. However, the InP/Si heteroepitaxy is highly challenging due to the lattice (∼8%), thermal expansion mismatch (∼84%), and the different lattice symmetries. Here, we demonstrate the growth of InP nanocrystals showing high structural quality and excellent optoelectronic properties on Si. Our CMOS-compatible innovative approach exploits the selective epitaxy of InP nanocrystals on Si nanometric seeds obtained by the opening of lattice-arranged Si nanotips embedded in a SiO2 matrix. A graphene/InP/Si-tip heterostructure was realized on obtained materials, revealing rectifying behavior and promising photodetection. This work presents a significant advance toward the monolithic integration of graphene/III-V based hybrid devices onto the mainstream Si technology platform.

We characterize structural and magnetic properties of the dilute magnetic semiconductor (In,Mn)Sb grown on GaAs (0 0 1) by molecular beam epitaxy. The films have surface features consisting of dense orthogonally oriented strain-driven hillocks. In addition, triangularly shaped hillocks, presumed to be MnSb clusters, are observed with diameters in the range of 200 nm. The density and size of these triangular hillocks depend strongly on the Mn content. X-ray scattering shows that the presence of Mn in the InSb films decreases the average lattice constant as well as the degree of relaxation of the (In,Mn)Sb films. The distribution of Mn is also investigated by cross-sectional transmission electron microscopy. Two regions are observed: a (In,Mn)Sb film with small defect density and MnSb clusters on the surface. Magnetization measurements indicate that both the (In,Mn)Sb alloy as well as the MnSb inclusions are ferromagnetic.

Catalyst-free self-assembled InP nanoneedles are grown on (0 0 1) InP-substrates using gas-source molecular beam epitaxy. The nanoneedles are investigated using SEM and TEM imaging, as well as micro-Raman measurements. Two nanoneedle orientations were observed: vertical (0 0 1)-oriented with the zincblende crystal structure and tilted needles with mixed zincblende and wurtzite structure.

A series of quantum-cascade lasers emitting at λ ≈11 μm with different numbers of cascades in the active region – between 25 and 200 –grown by gas-source molecular beam epitaxy, is compared in terms of their room-temperature performance in pulsed mode as a function of the number of cascades. Within the given range of cascade number we observe a superlinear increase in both slope efficiency and differential quantum efficiency as functions of cascade number. The differential quantum efficiency scaled per number of cascades, however, shows a sublinear increase. A differential quantum efficiency of 17% per cascade and pulse power exceeding 7 W per facet, both at room temperature, was demonstrated for quantum-cascade lasers with 200 cascades. All lasers with stripe width below 30 μm are operated in the fundamental TEM 00 spatial mode. This combination of high pulse power and TEM 00 mode operation for room-temperature mid-infrared laser emission can be attractive for a number of applications.

Double perovskites with the structural formula A2BBO6±δ, especially the strontium ferromolybdate Sr2FeMoO6-δ, have attracted a lot of attention due to their unique magnetic and electrical properties and a possible application in spintronic devices. However, a strict correlation between the functional characteristics of Sr2FeMoO6-δ and its synthesis technology has been lacking up to date. Thus, we have studied in the present work the crystallization kinetics of Sr2FeMoO6-δ using reagents with different pre-history as well as the structural and magnetic properties of the obtained compounds. Differences in the crystallization kinetics as well as higher magnetic inhomogeneity of Sr2FeMoO6-δ synthesised from a mixture of MoO3, Fe2O3, SrCO3 in comparison with the compound synthesised from the SrFeO2.5 and SrMoO4-у precursors have been found and interpreted.

Lattice-matched InGaAs-InAlAs quantum cascade lasers were prepared to compare differences between using a (411)A InP substrate and a (100) InP substrate. The lasers grown on the (411)A substrate showed higher gain, lower threshold current density, higher slope efficiency, and twice the power efficiency compared to the otherwise-identical structure on the (100) substrate. This performance improvement is attributed to less interface roughness scattering at the heterointerfaces in the (411)A structure. The lower interface roughness scattering appears to result from the high-spatial frequency steps on the (411)A surface.

The transport properties of n- and p-doped AlP layers grown by gas-source molecular beam epitaxy are investigated. n- and p-types of conductivities are achieved using Si and Be with peak room-temperature mobilities of 59.6 cm2/Vs and 65.0 cm2/Vs for electrons and holes, respectively. Si-doping results are then used for the design of n-doped AlP/GaP distributed Bragg reflectors (DBRs) with an ohmic resistance of about 7.5 ± 0.1 Ω. The DBRs are integrated as bottom mirrors in GaP-based light-emitting diodes (LEDs) containing InGaP/GaP quantum dots. The functionality of the LED structure and the influence of the DBRs on the InGaP/GaP electroluminescence spectra are demonstrated.

Nanometer-sized channels are fabricated in (In,Ga)As-InP heterostructures using Ar ion milling. The ion milling causes spontaneous creation of nanowires, and moreover, electrical conduction of the surface as carriers is generated by sputtering-induced defects. We demonstrate a method to restore electrical isolation in the etched area that is compatible with the presence of the nanochannels. We remove the heavily damaged surface layer using a diluted HCl solution and subsequently recover the crystalline order in the moderately damaged part by annealing. We optimize the HCl concentration to make the removal stop on its own before reaching the conduction channel part. The lateral depletion in the channels is shown to be almost absent.

The properties of rhodium (Rh) as a deep-level dopant in InGaAs lattice matched to InP grown by molecular beam epitaxy are investigated. When InGaAs:Rh is used as an ultrafast photoconductor, carrier lifetimes as short as 100 fs for optically excited electrons are measured. Rh doping compensates free carriers so that a near intrinsic carrier concentration can be achieved. At the same time, InGaAs:Rh exhibits a large electron mobility of 1000 cm2/V s. Therefore, this material is a very promising candidate for application as a semi-insulating layer, THz antenna, or semiconductor saturable absorber mirror.

In this work, the tunability properties of short-wavelength (λ ~ 3.9 μm) quantum-cascade lasers (QCLs) were studied, which is a first in the world at such short wavelengths. The experimental setup of an external cavity (EC) QCL was arranged in a Littrow configuration. A tuning range over 75 cm−1 has been achieved by using an uncoated 23 μm stripe-width QCL at room temperature. A single-mode operation could be obtained at 2527 and 2544 cm−1 at different grating angles by using an anti-reflection (AR) coated 23 μm ridge. A 5 μm stripe-width QCL without an AR coating could be tuned over 160 cm−1.

Magnetic sensitivity and low-frequency noise in micro-Hall sensors based on pseudomorphic InAs quantum well channels are investigated as functions of InAs thickness. The strained InAs quantum wells with thickness between 0 and 20 Aring are added into lattice-matched doped channel In0.53Ga0.47As/In0.47Al0.48As/InP heterostructures and enhance the absolute magnetic sensitivity by increasing the electron mobility. We show that the increase of the InAs channel thickness also leads to a significant reduction of the low-frequency noise. The result is an improved magnetic field detection over a broad frequency range. A minimum magnetic field detection limit of nT is resolved by a 40-m wide doped channel micro-Hall device at frequencies around kHz.

The generation-recombination noise in doped-channel In0.53Ga0.47As/In0.52Al0.48As/InP micro-Hall devices is characterized using deep level noise spectroscopy. Three noise sources are detected with trap energies of 140, 170 and 40 meV respectively. The levels are likely to be associated with arsenic vacancies.

Micro-Hall sensors with high sensitivity, low noise, and high thermal stability, 5 μm square, are fabricated using pseudomorphic Al0.3Ga0.7As/GaAs/In y Ga1-y As (0.2 ≤ y ≤ 0.3) heterostructures with Si-doped channels. The structures were optimized for thermal stability using a calculation of the self-consistent solution of Schrodinger-Poisson equations and Fermi-Dirac statistics in Hartree approximation. The optimized structure based on a Si-δ-doped 144 A In0.2Ga0.8As quantum well embedded into uniformly doped GaAs channel showed thermal drifts of only 90 ppm·K−1 in current drive mode and 192 ppm K−1 in voltage drive mode. The measurements of the absolute magnetic sensitivity and the low frequency noise were done. The micro-Hall sensor, optimized for thermal drift, is able to resolve the magnetic field of 438 nT.

The magnetic-field dependence of the mid-infrared quantum-cascade laser emission spectra is used to identify the particular Wannier-Stark states responsible for the laser action in two different laser designs. The active regions in both QCLs are based on the bound-to-continuum design, but have differing degrees of coupling between the injector miniband and the bound state. The effects of the magnetic field and the injection-barrier width on the emission wavelength indicates that the laser emission in the QCL with less coupling between the injector and the bound state originates from a transition between the injector and extractor minibands. The transition from injector miniband to extractor miniband has both a lower energy and a lower oscillator strength than the transition originating from the bound state, but dominates because of the low population of the upper bound state. This result has important implications for further miniband engineering of QCL active regions for laser action at the shortest possible wavelengths. New quantum-cascade laser sample, designed regarding the high-magnetic-field spectroscopy results mentioned above, indeed operates at shorter wavelength and demonstrates outstanding performance up to room temperature.

We investigate the properties of quasi-two-dimensional electrons in AlP quantum wells by measuring cyclotron resonance, quantum Hall effect, and Shubnikov de Haas oscillations in modulation-doped AlP - GaP type-II quantum wells. We find that in wide AlP wells, the lowest conduction band states are in the Xx and Xy valleys transverse to the growth direction, that the valley degeneracy of this state is gv=2, and that the cyclotron effective mass [Formula: see text]. These results indicate that the biaxial compressive strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal Xz valley to be lifted above the transverse Xx and Xy valleys. Further, the two-fold degeneracy of the Xx and Xy valleys indicates that the conduction band minimum in AlP is located exactly at the X-point of the Brillouin zone.

We describe the design and implementation of a short-wavelength quantum-cascade laser emitting in the range of 3.6-3.7 mum at 77 K and in the range of 3.8-3.9 mum at 300 K. The shortest wavelength laser emission observed at 77 K is lambda=3.62 mum. The active region is based on the strain-compensated In0.73Ga0.27As-AlAs heterosystem on InP. The laser operates in pulsed mode up to 300 K, and in continuous-wave mode at 77 K. Laser threshold current densities are as low as 600 A/cm2 at 10 K, 800 A/cm2 at 77 K, and 3.8 kA/cm2 at 300 K in pulsed mode. The temperature coefficient T0 is as high as 143 K. Driven with 50% duty cycle at 77 K, lasers deliver up to 70 mW average power per facet and the differential quantum efficiency is as high as 14% per cascade

We demonstrate a shallow-ridge quantum-cascade laser (QCL) with performance comparable or better than that of deep-ridge QCLs fabricated from the same wafer. The shallow-ridge QCL emits at /spl ap/4 /spl mu/m with a 4.6-4.8 kA/cm/sup 2/ threshold current density at room temperature which is similar to the deep-ridge QCL. At the same time the shallow-ridge QCL shows a better temperature stability, T/sub 0/=160 K, than the deep-ridge QCL, with T/sub 0/=120 K. The increase in the characteristic temperature of the shallow-ridge laser compared to the deep-ridge laser results from the improved heat dissipation out of the laser ridge through the lateral heat flow. Lateral spreading of the injection current-usually a drawback of shallow-ridge lasers-is suppressed by proton implantation into the strain-compensated InGaAs-AlAs active region layers on either side of the ridge. In contrast to the case of In/sub 0.53/Ga/sub 0.47/As layers and of In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As heterostructures lattice matched to InP, the proton implantation of strain-compensated In/sub 0.73/Ga/sub 0.27/As-AlAs heterostructure on InP creates deep (180 meV) carrier traps, resulting in this material being electrically insulating even at room temperature.

The noise spectrum in micro-Hall devices based on pseudomorphic Al/sub 0.2/Ga/sub 0.8/As/In/sub 0.1/Ga/sub 0.9/As/GaAs modulation-doped heterostructures was measured between 4 Hz and 65 kHz, allowing components due to thermal, 1/f, and generation-recombination to be characterized. Applying deep level noise spectroscopy (DLNS) in the temperature range of 77-300 K to analyze the generation-recombination part of the spectrum, two electron traps contributing to noise density were identified. An emission activation energy of 474 meV was measured for the dominant trap, corresponding to the well-known DX center originating from the AlGaAs barrier. The other deep level, with an emission activation energy of 242 meV, is probably related to defects in the InGaAs layer. The structures under investigation resulted in high-performance micro-Hall devices: a supply-current-related sensitivity up to 725 V/spl middot/A/sup -1//spl middot/T/sup -1/ at 77 K independent of bias current, a signal-to-noise sensitivity of 155 dB/spl middot/T/sup -1/ and a detection limit of 340 pT/spl middot/mm/spl middot/Hz/sup -1/2/ at 77 K were measured.

We have investigated intersubband transitions in strain-compensated AlAs/(In,Ga)As heterostructures, demonstrating both absorption and quantum-cascade laser emission at short wavelengths. Short-wavelength optical transitions in such structures are associated with a number of challenges in both the growth and design, including managing the internal strain and designing around indirect valleys. We achieve absorption peaks at wavelengths as short as 1.7 μ m in fully strain-compensated AlAs/(In,Ga)As structures. Quantum cascade lasers based on similar heterojunctions exhibit laser emission as short as 3.7 μ m . These lasers exhibit low-temperature threshold current densities of 860 A / cm 2 in pulsed mode and output power as high as 6 W per facet (12 W total). At room temperature, the threshold current density is 4.5 kA / cm 2 and the maximum power 240 mW per facet.

We have investigated the recombination dynamics in self-assembled InP/GaP quantum dots by means of time-resolved photoluminescence measured at low temperatures between 2 and 100 K and high hydrostatic pressure up to 2 GPa. Due to the high-power levels for pulsed excitation, the quantum dot emission exhibits two components with typical decay times of 5 and 30 ns, corresponding to direct Γ-Γ and X- Γ interband recombination processes, respectively. These decay times appear to be independent of pressure. At a very low pressure of about 0.1 GPa the intensity of the dot emission drops abruptly relative to that of the wetting layer indicating a switching off of a carrier relaxation channel.

The vibration states of diluted magnetic narrow-gap Hg1−x−yCdxMnyTe (x=0–0.34, y=0–0.04) semiconductors are studied by means of far infrared reflection. The comparison of spectral features common for Hg1−x−yCdxMnyTe and Hg1−xCdxTe single crystals demonstrates the substantial effect of magnetic doping on the optical spectra, providing indirect evidence of improved crystal structure in quaternary compounds. The inherent fundamental phonon modes of Hg1−xCdxTe become significantly narrower in Hg1−x−yCdxMnyTe whereas the defect and some cluster modes are of much lower intensity. These modifications of the lattice dynamics are assumed to be caused by a manganese concentration effect on the relaxation of crystalline defects. For Hg1−x−yCdxMnyTe single crystals this concentration is found to be within 0.01

The electronic subband structure of self-assembled InP/GaP quantum dots (QDs) has been investigated by means of photoluminescence (PL) measurements as a function of hydrostatic pressure up to 8 GPa and temperature. At ambient pressure the PL emission of the sample arises from direct optical transitions between the lowest electron and hole Γ-point states confined in the QDs. At a very low pressure of about 0.15 GPa, the Γ-X conduction band crossover occurs, after which the PL emission of the dots becomes roughly 20 times weaker in intensity and its energy exhibits the slight redshift typical of indirect recombination processes from the conduction band X valleys. Our results indicate a type-I band alignment for the strained InP/GaP dot structure at low pressure and yield a value of 300 ± 30 meV for the valence-band offset.

Quantum well micro-Hall devices based on uniformly Si-doped Al 0.3 Ga 0.7 As/GaAs and Si-δ-doped Al 0.3 Ga 0.7 As/GaAs/In 0.3 Ga 0.7 As heterostructures are investigated as function of electric field and compared in terms of sensitivity and noise properties. The data show that at high electric fields, doped-channel quantum well devices are advantageous over high-mobility structures and that the use of pseudomorphic InGaAs results in better performance than does GaAs. A maximal signal-to-noise sensitivity (SNS) of 138 dB T −1 is achieved in a 10 μm ×10 μm device at 300 K, at frequency of 100 kHz and bandwidth of 1 Hz. This performance corresponds to a lowest detection limit of 127 nT Hz −1/2 , with no degradation for electric fields up to 2.4 kV cm −1 ; these values represent the best reported at such high electric fields. Furthermore, our results suggest that a signal-to-noise sensitivity of 160 dB T −1 and a lowest detection limit of 10 nT is achievable in doped-channel structures.

We present new results concerning the carrier transfer between quantum dots (QDs) in dense InAs/GaAs QD arrays with a bimodal size distribution explored by means of steady-state and time-resolved photoluminescence (PL). Interdot carrier transfer involves transitions from the ground state of small QDs into lower lying states of larger QDs, a relaxation channel that saturates at high excitation densities. A staircase-like spectral dependence of the PL decay time is observed and analyzed by taking into account the bimodal QD size distribution. We show that monitoring the spectral behavior of the ground-state PL time constants offers a new and independent tool for distinguishing between PL contributions from different dot size families.