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Transparent conducting or semiconducting oxides are important materials for (transparent) optoelectronics and power electronics applications. While most of these oxides can be doped n-type only with room-temperature electron mobilities on the order of 100cm^2/Vs p-type oxides are needed for the realization of pn-junction devices but typically suffer from exessively low (<

The quantum cascade laser (QCL) is a unipolar semiconductor device, in which electronic transitions between the subbands confined in a heterostructure conduction band lead to stimulated light emission. This process stands in contrast with diode lasers, in which photons arise from electronic transitions from the lowest-energy conduction band to the highest-energy valence band. The result is that, while diode lasers emit with photon energy similar to the bandgap energy, QCLs can emit with any photon energy that can be designed within the semiconductor heterostructure. Emission wavelengths for QCLs have been reported from 2.6 to 250 μm. This chapter summarizes some aspects of the fundamental physics of QCLs, an overview of the fabrication process, a review of a new paradigm for power and brightness based on broad-area QCLs, and a description of the physics and optics of external-cavity tunable QCLs

The semiconducting oxide $\ensuremath{\beta}$-gallium oxide ($\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$) possesses a monoclinic unit cell, whose low symmetry generally leads to anisotropic physical properties. For example, its electrical conductivity is generally described by a polar symmetrical tensor of second rank consisting of four independent components. Using van der Pauw measurements in a well-defined square geometry on differently oriented high-quality bulk samples and the comparison to finite-element simulations, we precisely determine the ratio of all elements of the $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ three-dimensional electrical conductivity tensor. Despite the strong structural anisotropy, a weakly anisotropic conductivity at and above room temperature was found. In the ${a}^{*}bc$ coordinate system, the diagonal elements deviate from each other by no more than 6%. Based on these results and the off-diagonal element being $\ensuremath{\approx}5%$ of the diagonal ones, the direction of highest conductivity is rotated ${(59\ifmmode\pm\else\textpm\fi{}15)}^{\ensuremath{\circ}}$ from the $c$ direction towards the ${a}^{*}$ direction with a conductivity of $(1.12\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}$ that in the perpendicular direction of lowest conductivity. Analysis of the temperature dependence of the anisotropy and mobility of differently doped samples allows us to compare the anisotropy for dominant phonon scattering to that for dominant ionized-impurity scattering. For both scattering mechanisms, the conductivities along the $a$ and $b$ direction agree within 2%. In contrast, the conductivity along the $c$ direction amounts to $0.96\ifmmode\times\else\texttimes\fi{}$ that along the $b$ direction for dominant phonon scattering, and up to $1.12\ifmmode\times\else\texttimes\fi{}$ for ionized-impurity scattering. The transport anisotropies are determined to be larger than the theoretically predicted effective mass anisotropy, suggesting slightly anisotropic scattering mechanisms. We demonstrate that significantly higher anisotropies can be caused by oriented, extended structural defects in the form of low-angle grain boundaries, for which we determined energy barriers of up to 93 meV.

Transition metal doped InGaAs layers constitute a distinctive class of ultrafast photoconductors for THz emission and detection, which are expected to avoid disadvantages of low-temperature (LT) grown fast photoconductors, such as process temperature limitations. Using molecular beam epitaxy (MBE), iron-doped InGaAs:Fe and rhodium-doped InGaAs:Rh layers with doping levels above 1020 cm−3 were produced and investigated as THz detectors in time-domain spectroscopy (TDS). Compared to state-of-the-art detectors based on low-temperature (LT) grown InGaAs/InAlAs, transition metal doped InGaAs facilitates larger THz peak amplitudes and an improved dynamic range. In particular, InGaAs:Rh receivers allow for an improvement in dynamic range of 15 dB over the whole spectral range, resulting in a peak dynamic range of 105 dB. This improvement increases the resolution in non-destructive testing applications of thin films with THz TDS.

We describe a long-wavelength external ccavity quantum cascade laser, emitting from 12.3 to $$13.2\ \upmu \text {m}$$ integrated into a compact design to promote portability. For this purpose, a new type of anti-reflection coating was designed as well as a new way of measuring its performance. Finally, a portion of the absorption spectrum of ammonia vapor was recorded by tuning the laser, which demonstrates the spectroscopic capacity of this EC-QCL.

We describe the use of an angled facet to achieve low reflectance of one facet of a quantum cascade laser. A bent waveguide allows one facet to be angled with respect to the waveguide and still use the natural cleavage plane of the crystal. The application of such structures is external-cavity wavelength tunable quantum cascade lasers. The reflectance is studied as a function of angle with an angle of 11.2° reducing the reflectance from 28% to 3.5%. When combined with a dielectric anti-reflection coating, better external-cavity laser performance is possible.

Mid-infrared hyperspectral imaging has in the past decade emerged as a promising tool for medical diagnostics. In this work, nonlinear frequency upconversion based hyperspectral imaging in the 6 to 8 mu m spectral range is presented for the first time, using both broadband globar and narrowband quantum cascade laser illumination. AgGaS2 is used as the nonlinear medium for sum frequency generation using a 1064 nm mixing laser. Angular scanning of the nonlinear crystal provides broad spectral coverage at every spatial position in the image. This study demonstrates the retrieval of series of monochromatic images acquired by a silicon based CCD camera, using both broadband and narrowband illumination and a comparison is made between the two illumination sources for hyperspectral imaging. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We demonstrate a broadband multilayer anti-reflection (AR) coating applied to a quantum cascade laser (QCL) facet. The bandwidth of the AR coating was optimized for the range of 8.0 to 12.0 μm to cover the entire emission spectrum of a broad-gain QCL. The laser facets are high-reflectance and AR-coated for use in an external cavity QCL. The AR coating is composed of layers of yttrium fluoride, zinc sulfide, and germanium, all deposited using electron-beam evaporation, and the modal reflectance of the laser facet across the wavelength range was reduced to 0.75%. The external cavity laser performance in a Littrow-type configuration is also described.

We describe an external-cavity quantum cascade laser with an intra-cavity out-coupling optical system, tunable from 8.4 μm to 10.8 μm. The new optical configuration allows higher output power compared to a conventional Littrow-type external cavity approach, while keeping the broad tunability of wavelength. We achieved 1 W output power at the maximum in pulse-mode operation, which is more than twice the optical power using the Littrow design with the same gain chip.

We describe a design concept for broadband anti-reflection coatings for the mid-infrared that allows thinner film thickness and characterize its properties in the mid-infrared range from 7 μm to 12 μm. Two demonstrator coatings comprising zinc sulfide, yttrium fluoride, and germanium, all of which are deposited by electron beam evaporation on indium phosphide substrates, are optimized for the 7–12 μm and the 10–12 μm ranges. They exhibit less than 1.0% reflection over the target wavelength ranges with notably thin coating thickness.

We describe an external-cavity quantum cascade laser with an intra-cavity out-coupling optical configuration emitting between 8.5 and 10.5 μm. This new laser design allows higher output power compared to a conventional Littrow-type external cavity approach, while keeping the wide tunability of the wavelength. We demonstrate a compact laser system based on this design and characterize its performance, achieving more than twice the optical power compared to the Littrow design.

We have realized a tandem solar cell design that combines a pin-junction with a photovoltaic intersubband absorber. This concept allows harvesting light in the visible range and the near- and mid-infrared at the same time, and theoretically, energy conversion efficiencies beyond the Shockley–Queisser-limit could be achieved. A test structure was grown, and the operation of this concept could be confirmed, in principal with an optical two-beam experiment. The basic characteristics of the device can be explained with an equivalent circuit design that consists of three individual cells, and we find an obvious analogy to the concept of the intermediate band solar cell with noteworthy advantages at some points. Our results show, that for a working device it is crucial to adjust the properties of the photovoltaic intersubband absorber for optimal charge separating performance at the working point of the solar cell. Copyright © 2015 John Wiley & Sons, Ltd.

Recently, we introduced a scanning light stimulation system with an automated focus correction. The method is however resolution limited by a CMOS sensor used to focus the light beam. Here, to achieve a higher resolution, we enhanced the light beam focusing process by combining it with an edge detection technique. By this modification, the system resolution is improved down to a ${1}/{e^2}$ -diameter of $4.3\,\mu {\rm m}$ , as demonstrated using an example measurement on a multicrystalline solar cell. Furthermore, it is shown that the obtained resolution is mainly limited by height variations of the positioning system, and methods to compensate these limitations are discussed using example measurements on a quantum well solar cell.

For a number of applications, high average power in the mid-infrared (MIR) part of the spectrum is required from a compact laser. The quantum-cascade laser (QCL) emitting in the mid-infrared region of 4 to 8 μm 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. Because the overall efficiency of QCLs is less than 25%, the thermal management in high-power QCLs is critical and generally requires narrow ridges with re-grown InP abutting them. The narrow ridges also result in nearly diffraction-limited emission, allowing the power to result in high brightness. The limited ridge width, however, also results a limited emission power [1]. In this paper, we demonstrate that the most power can be obtained from broad-area QCLs with limited number of cascades. Furthermore, such ultimate-power lasers can deliver good beam quality and excellent brightness. A demonstration at 4.6 μm is presented and discussed.

Compared to the visible or near-infrared (NIR) spectral regions, there is a lack of very high sensitivity detectors in the mid-infrared (MIR) that operate near room temperature. Upconversion of the MIR light to NIR light that is imaged using affordable, fast, and sensitive NIR detectors or cameras represents an attractive possibility and the technique has been successfully demonstrated using a broad-spectrum light source [1]. If, however, high brightness is advantageous, especially using a narrow wavelength band, a laser is a more useful light source. In this paper we present the first MIR imaging system based on upconversion technology with an external cavity quantum cascade laser (ECQCL) used as the illumination source.

In non-destructive testing (NDT) and industrial process monitoring [1], reflection geometry is commonly preferred over transmission, because only one-side access to the sample is possible. State-of-the-art terahertz (THz) time domain spectroscopy (TDS) systems hitherto use different emitter and detector devices [2], such that either an angled THz beam path or a beam splitter is required to enable reflection measurements. This leads to rather bulky and complex setups. An integrated THz device, combining the emitter and detector on a single chip, would significantly facilitate reflection measurements. For competitive integrated devices a photoconductive material, which is both compatible to the excitation with 1550 nm femtosecond pulses and applicable as THz emitter and detector has to be found.

We have developed a bandpass infrared interference filter with sufficiently narrow bandwidth to be potentially suitable for tuning a self-stabilizing external-cavity quantum-cascade laser (ECQCL) in single-mode operation and describe the process parameters for fabrication of such filters with central wavelengths in the 3–12 μm range. The filter has a passband width of 6 nm or 0.14% with peak transmission of 55% and a central wavelength of approximately 4.0 μm. It can be tuned through over 4% by tilting with respect to the incident beam and offers orders of magnitude larger angular dispersion than diffraction gratings. We compare filters with single-cavity and coupled-cavity Fabry–Perot designs.

A computationally efficient temperature-dependent model for scattering processes in mid-infrared quantum cascade lasers is developed. In this approach, the total intersubband scattering rate is described as the product of the overlap integral for the squared moduli of the envelope functions and a form factor that depends on transition energy, temperature, and material. Both inelastic and elastic scattering processes are included in the treatment. The model is used to calculate the temperature-dependent threshold current density $ {J_{{\rm th}}}(T) $Jth(T) in a 10-period strain-compensated InGaAs–InAlAs mid-infrared quantum cascade laser structure and determine the characteristic temperature $ {T_0} $T0 in $ {J_{{\rm th}}}(T) = {J_0}\exp {(T/{T_0})} $Jth(T)=J0exp⁡(T/T0). The effect of doping is also modeled.

We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.

We demonstrate the first lasing emission of a thermo-electrically cooled terahertz quantum cascade laser (THz QCL). A high temperature three-well THz QCL emitting at 3.8 THz is mounted to a novel five-stage thermoelectric cooler reaching a temperature difference of ΔT = 124 K. The temperature and time-dependent laser performance is investigated and shows a peak pulse power of 4.4 mW and a peak average output power of 100 μW for steady-state operation.

A gas sensor integrating a pulsed external-cavity quantum-cascade laser and a photoacoustic (PA) cell placed inside the laser cavity is described. The laser design allows a continuously tunable and mode-hop-free operation with an estimated instrument-limited linewidth of 0.063 cm−1 in Littrow or Littman–Metcalf cavity configurations. Due to higher intracavity power levels, the PA signal for the cell located within the laser cavity is 80–320 times stronger than the signal for the same cell placed outside the cavity. The sensor shows a good capability to identify individual absorption lines of isotopic H2O16, H2O8, and H2O17 species within a narrow spectral interval from 1814 to 1821 cm−1 at natural isotope abundances.

We present the growth, fabrication, and characterization of light-emitting diodes based on (Al,Ga)As quantum wells and dots embedded in a p-n GaP structure. Samples were grown on Sulphur-doped GaP (001) substrate using gas-source molecular beam epitaxy. The structures include either GaAs quantum structures with nominal coverage between 1.2 and 3.6 monolayers or Al0.3Ga0.7As quantum wells. For structures with GaAs layer thicker than 1.5 monolayers the 3.6% lattice mismatch in the materials system results in formation of quantum dots via Stranski-Krastanow growth mode with areal density of about 8×1010 cm-2. The atomic-force and transmission-electron microscopy show that with increasing coverage of GaAs from 1.8 to 3.6 monolayers the average lateral size and height of dots change in the range of 17-34 nm and 0.9–2 nm, respectively. The diode structures emit light from the red to the green spectral range up to room temperature. The GaAs/GaP QDs show electroluminescence between 1.8 eV and 2 eV, whereas the Al0.3Ga0.7As quantum wells emit light between 2 eV and 2.2 eV.

Emitting light over a broad spectral range (near infrared to ultra violet) and being photostable, quantum dots as single photon sources have several advantages. Siavalanche photodiodes that are used for photon detection have their highest efficiency in the red spectral window. Ultra low density of InP/InGaP QDs has already been produced with a straightforward technique where the deposition rate of InP is reduced to 0.015 ML/s. With this technique the density of 1 dot per µm2 is realized without the need of lithographic patterning. In the current work we discuss the optical properties of single InP QDs in detail. The low density of QDs allows the investigation of the optical properties of one single dot without using any other apertures. Autocorrelation measurements under cw and pulsed excitation were performed and the values of 0.21 and 0.37 are obtained for g(2)(0), respectively. Electronic structures of the individual QDs are also investigated. The single quantum dots show a distinct perpendicular linearly polarized fine structure splitting up to 320 µeV. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of MBE growth conditions on the low frequency noise features of 1.8 μm thick layers of InSb is examined. Low Hooge factors down to 2 × 10 -5 , which are close to those of pure InSb, show that these material systems are candidates for sensor and other electronic applications. The temperature dependence of the Hooge factors could give further insight in the origins of flicker noise.

Narrow-gap (In,Mn)Sb layers, which offer prospects for infrared spin-photonics, were grown on GaAs (001) substrates using a Riber Compact 21T MBE system and a Veeco valved cracker cell for Sb. Samples consisting 0.4 μm (In,Mn)Sb/GaAs with different Mn content in the range of 1% were examined in this study in respect of their structural, magnetotransport and magnetization properties. It is found that Mn decreases the lattice constant as well as the degree of relaxation of (In,Mn)Sb films. The hole concentration increases with increasing Mn concentration resulting increased acceptor concentration and decreased resistivity. The magnetoresistance is found to be sensitive to the Mn concentration. The temperature-dependent resistivity data at lower than 30 K show an increased resistivity with decreasing temperature, suggesting the insulating behavior of the samples. In addition, magnetization measurements reveal distinct hysteresis loops, indicating the ferromagnetic properties. The measured saturation magnetization values of the films are found to be dependent on Mn concentration. The Curie temperature is estimated to be below 50 K. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The design strategy and performance for short wavelength (λ ⪅ 4 μm) quantum-cascade lasers (QCLs) is discussed. The QCLs are based on strain-compensated AlAs–In0.73Ga0.27As heterostructures grown using gas-source molecular-beam epitaxy on InP substrates. Both composite barriers based on AlAs–In0.55Al0.45As and composite wells based on In0.73Ga0.27As–In0.55Al0.45As are used to achieve laser emission at wavelengths as short as 3.05 µm by avoiding leakage from the upper laser state into either the higher-lying miniband or into indirect states within the heterostructure. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We describe a novel colour-tunable light-emitting diode whose operation is based on direct band-gap emission from coupled configurations of InP quantum dots and quantum wells embedded in GaP. The control of the emission colour stems from a marked difference in the current dependence of intensities of two different emission processes. At lower currents, the emission is dominated by the 720 nm luminescence from the quantum dots and appears red; at higher currents, the emission is dominated by the 550 nm quantum-well luminescence and the perceived colour is green. Thus, we are able to tune the colour of such diodes from red to green by means of drive current. A multi-colour pixel can be realized by a single diode, with rapid switching between colour states to provide a range of colour mix.

For many applications, optical emission power of many watts in the midinfrared part of the optical spectrum is required. Quantum cascade lasers for these applications typically contain 30 to 40 cascades and have laser ridge widths of about 10 μm. These lasers cannot, however, be readily scaled up in power. This paper demonstrates a path for further power scaling that is based on broad stripes with fewer than 15 cascades. Although the confinement factor Γ for this design approach is lower than for designs with 30 to 40 cascades, the ability to efficiently extract heat from the top and bottom of the lasers more than offsets the Γ disadvantage. We expect this approach to deliver tens of watts of power with high beam quality leading to high brilliance. The paper discusses the design approach, thermal conduction considerations, and confinement factor. Room temperature cw operation of broad-area lasers based on this approach is demonstrated and described.

Thermal behavior and carrier injection of GaAs/GaP quantum-dot light-emitting diodes are investigated. The diodes emit red light between 1.8 eV and 2 eV and operate at room temperature. Adding aluminum to the quantum structures shifts the emission wavelength towards green. Thermal filling and high injection of the carriers in GaAs/GaP quantum dots result in contributions of different radiative recombination channels with higher emission energies and hence a blueshift of the electroluminescence with increasing temperature and very small thermal quenching of the light output.

A passively alignment-stabilized external cavity quantum cascade laser (EC-QCL) employing a “cat’s eye”-type retroreflector and an ultra-narrowband transmissive interference filter for wavelength selection is demonstrated and experimentally investigated. Compared with conventional grating-tuned ECQCLs, the setup is nearly two orders of magnitude more stable against misalignment of the components, and spectral fluctuation is reduced by one order of magnitude, allowing for a simultaneously lightweight and fail-safe construction, suitable for applications outdoors and in space. It also allows for a substantially greater level of miniaturization and cost reduction. These advantages fit in well with the general properties of modern QCLs in the promise to deliver useful and affordable mid-infrared-light sources for a variety of spectroscopic and imaging applications.

XRD techniques determined that 270 nm GaP grown on 400 nm Si 0.85 Ge 0.15 /Si(001) substrates by MOCVD is single crystalline and pseudomorphic, but carry a 0.07% tensile strain after cooling down to room temperature due to the bigger thermal expansion coefficient of GaP with respect to Si (Fig. 2). TEM and AFM examinations indicated a closed but defective GaP layer (Fig. 3(a)) with low root mean square of roughness (rms) of 3.0 nm for 1 μm 2 surface area (Fig. 3(b)). Although TEM studies confirm the absence of misfit dislocations in the pseudomorphic GaP film, growth defects (e.g. stacking faults, microtwins, and anti-phase domains) are detected, concentrating at the GaP/SiGe interface (Fig. 3(c)-(d), Fig. 4). We interpret these growth defects as a residue of the initial 3D island coalescence phase of the GaP film on the Si 0.85 Ge 0.15 buffer. TEM-EDX studies reveal that the observed growth defects are often correlated with stoichiometric inhomogeneities in the GaP film (not shown here). Finally, ToF-SIMS detects sharp heterointerfaces between GaP and SiGe films with a minor level of Ga diffusion into the SiGe buffer (Fig. 5).

We present progress on bandpass infrared interference filters with very narrow passbands to be used for sensitive trace gas and volatile compound imaging and detection and are suitable for mode selection and tuning in singlemode External Cavity Quantum Cascade Lasers. The process parameters for fabrication of such filters with central wavelengths in the 3-12 μm range are described. One representative fillter has a passband width of 6 nm or 0.14% with peak transmission of 62% and a central wavelength of 4.4μm. Theoretically, it can be tuned through about 4% by tilting with respect to the incident beam and offers orders of magnitude larger angular dispersion than diffraction gratings. We compare filters with single-cavity and coupled-cavity Fabry-Perot designs. The filters pass the tests for adhesion and abrasion as stated in MIL-C-48497.

The leakage current in two quantum-cascade (QCL) structures is measured and analyzed. The structures illustrate a new design feature, exploiting the interface roughness scattering at the well/barrier interfaces to intentionally shorten the lifetime of the lower laser state while increasing that of the upper laser state. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in J th = 1.7 kA/cm 2 at 300 K, slope efficiency η = 1.4 W/A, T 0 = 175 K, and T 1 = 550 K for lasers emitting at 3.9 μm. A further analysis allows the extraction of the leakage current into higher minibands from the temperature dependence of the threshold current density and to reconstruct the energies of the higher-lying states from this current. The modeling includes the thermal population of LO phonons that drive the leakage.

High and low field transport properties of electrons confined to quasi-two-dimensional sheets in semiconductor heterostructures are discussed within the context of their importance in the operation of field effect transistors. Modulation-doped heterostructures, in which the two-dimensional electron gas is in undoped material, have extremely large mobilities; these mobilities are larger than what can actually be fully used by the transistor. Doped-channel quantum well structures have lower mobilities and velocities, but field effect transistors fabricated from such structures operate very well in part because of the larger electron concentration which is possible.

This paper describes the growth and optical characteristics of InyGa1-yP with 0.3

We demonstrate second harmonic generation in photonic crystal nanocavities fabricated in the semiconductor gallium phosphide. We observe second harmonic radiation at 750 nm with input powers of only nanowatts coupled to the cavity and conversion efficiency $P_{\rm out}/P_{\rm in, coupled}^2 = 430%/{\rm W}$. The large electronic band gap of GaP minimizes absorption loss, allowing efficient conversion. Our results are promising for integrated, low-power light sources and on-chip reduction of input power in other nonlinear processes.

A comprehensive study, including growth, optical characterization and anisotropic transport in quantum well (QW), quantum wire (QWr), and quantum dot (QD) systems, is carried out for the InAs/InP nanostructures grown by gas-source molecular beam epitaxy on InP (001) substrates. Role of substrate misorientation in the self-organized formation, shape, and alignment of InAs nanostructures is investigated. The emission and absorption properties related to interband transitions of the InAs/InP nanostructures are studied by means of polarization-dependent photoluminescence (PL) and transmission spectroscopy. It is demonstrated that the emission wavelength of grown nanostructures extends up to 2 μm, including the technologically important 1.3 μm and 1.55 μm, at room temperature. Polarization-dependent PL and transmission measurements for all QWrs and QDs reveal much similarity in temperature behavior in spite of a qualitatively different character of the one (1D)- and zero (0D)-dimensional density-of-states functions. The in-plane transport of electrons and holes in QWrs, QDs, and QWs is investigated and interpreted in terms of anisotropic two-dimensional carrier systems, and in terms of coupled 1D or 0D systems. Peculiar band structure and carrier relaxation in the InAs/InP nanostructures suggest currently the large application potential for the optical devices – mainly for the telecommunication wavelength of 1.55 μm.

A design strategy for short wavelength (3μm ≤ λ ≤ 5μm) quantum-cascade lasers (QCLs) on InP substrates is discussed and the performance of these lasers evaluated. The QCLs are based on strain-compensated AlAs-In 0.73 Ga 0.27 As heterostructures grown using gas-source molecular-beam epitaxy on InP substrates. Both composite barriers based on AlAs-In 0.55 Al 0.45 As and composite wells based on In 0.73 Ga 0.27 As-In 0.55 Al 0.45 As are used to achieve laser emission at wavelengths as short as 3.05 μm by avoiding leakage from the upper laser state into either the higher-lying miniband or into indirect states within the heterostructure.

The replacement of native oxides with deposited oxides in CMOS technology opens the door to replacing the Si with semiconductors without high-quality native oxides. For example, the use of InSb in logic applications could allow much lower operating voltages and power dissipation due to the InSb channels reaching saturation at significantly lower electric fields. Epitaxy of InSb onto Si could be done directly or using an intermediate layer such as GaP, GaAs, or InP. In the current work we describe the growth of InSb on Si (001) and discuss the structural and electrical properties of the resulting InSb films. The samples were characterized in terms of background electron concentration, mobility, deep level traps, Hall sensitivity, and x-ray rocking curve width.Samples were grown using molecular-beam epitaxy in a Riber-Compact 21T system. Antimony was supplied with a Veeco valved cracker cell. Vicinal Si(001) substrates offcut by 4º toward [110] were prepared by repeated oxidation and oxide-removal and then loaded into the MBE system. After the substrate temperature had been increased to about 820ºC, the surface shows a clear 24 reconstruction and appears to be free of oxide. This reconstruction remains until the substrate temperature reaches 1015ºC, at which temperature a 21 appears, indicating a dominance of double-height steps. After allowing the substrate to cool to the intended growth temperature for InSb, it is exposed to cracked Sb, resulting in the surface going from 21 to 11. This 11 reconstruction remains throughout the subsequent InSb deposition. InSb was deposited with a Sb/In flux ratio of about 5 and a growth rate of 0.2 nm/s. We have investigated growth temperatures between 300 and 420ºC for growth. To prevent the formation of the defects we introduced in some samples GaSb/AlSb supperlattice buffer layer. The best structural quality has been achieved at a growth temperature of 420ºC using GaSb/AlSb supperlattice buffer layer, resulting in our best electron mobility of 2.6104 cm2/Vs for a 2m film at room temperature. The samples grown at 420°C have the narrowest x-ray rocking curve width (FWHM of about 950 arcsec). Deep level noise spectra indicate the existence of the deep levels. The sample with the best crystal quality and highest mobility has the lowest traps. The deep levels have a temperature dependent behavior.

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 Xt valleys transverse to the growth direction, that the valley degeneracy of this state is gν=2, and that the cyclotron effective mass mtml = (0.52 ± 0.01) × m0. These results indicate that the biaxial strain resulting from the lattice mismatch of AlP quantum well with respect to the GaP substrate and barrier layers causes the longitudinal Xl valley to be lifted above the transverse Xt valley. Further, the two‐fold degeneracy of the Xt valley indicates that the conduction band minimum in AlP is located exactly at the X‐point of the Brillouin zone.

Measurements of the electron velocity as a function of electric field in bulk InGaAs and in modulation‐doped InGaAs/InAlAs (lattice‐matched to InP) indicate that hot electrons transfer out of the InGaAs channel into the InAlAs. This added scattering mechanism results in a lower peak velocity for electrons in the heterosystem than in bulk InGaAs.

We have studied electron transport in a variety of doped and modulation‐doped InGaAs/AlInAs quantum wells within the context of field‐effect transistor performance. Both quantum‐well width and doping profile were varied with all layers lattice matched to the InP substrate. Electron transport properties in the structures were characterized using Hall, geometric magnetoresistance, and microwave velocity‐field measurements, and transistor performance in terms of transconductance and channel current in 1.8‐ and 0.5‐μm gate‐length devices. Transconductances in 0.5‐μm gate‐length transistors were as high as 590 mS/mm and were more directly correlated to the peak electron velocity than to the low‐field mobility.

We report a detailed structure and defect characterization study on gallium phosphide (GaP) layers integrated on silicon (Si) (001) via silicon-germanium (SiGe) buffer layers. The presented approach uses an almost fully relaxed SiGe buffer heterostructure of only 400 nm thickness whose in-plane lattice constant is matched to GaP—not at room but at GaP deposition temperature. Single crystalline, pseudomorphic 270 nm thick GaP is successfully grown by metalorganic chemical vapour deposition on a 400 nm Si0.85Ge0.15/Si(001) heterosystem, but carries a 0.08% tensile strain after cooling down to room temperature due to the bigger thermal expansion coefficient of GaP with respect to Si. Transmission electron microscopy (TEM) studies confirm the absence of misfit dislocations in the pseudomorphic GaP film but growth defects (e.g., stacking faults, microtwins, etc.) especially at the GaP/SiGe interface region are detected. We interpret these growth defects as a residue of the initial 3D island coalescence phase o...

Although electrons in center delta‐doped AlGaAs/GaAs quantum wells have lower low‐field mobilities than do electrons in uniformly doped quantum wells, experimental results presented here show that at electric fields between 2 and 4 kV/cm the differential mobility in delta‐doped quantum wells rises dramatically. This large increase in differential mobility may be a result of the heating of the electrons out of the symmetric ground state into the antisymmetric first excited state. Because the excited state has a node at the delta doping, these hot electrons have a much smaller overlap with the ionized impurities of the doping spike in the well centers and therefore higher mobility.

Distributed Bragg reflectors with high reflectivity bands centered at wavelengths from 530 to 690 nm (green to red) based on AlP/GaP quarter-wave stacks are prepared on (001)GaP using gas-source molecular-beam epitaxy. Additionally, the complex refractive index of AlP is measured using spectroscopic ellipsometry within the range of 330–850 nm in order to facilitate an accurate reflector design. Structures consisting of 15 quarter-wave stacks reach a peak reflectance between 95% and 98%, depending on the spectral position of the maximum.

Monte-Carlo analysis [1] indicates that as FET gate lengths shrink to the sub-100 nm regime, the electric fields in a field-effect transistor become large enough that the electrons occupy most of the Brillouin zone. Because carriers outside of the Γ valley behave similarly in most semiconductors, transistor performance in this small-gate-length limit is predicted to be rather material-independent.

The complex refractive index components, n and k, have been studied for thin films of several common dielectric materials with a low to medium refractive index as functions of wavelength and stoichiometry for mid-infrared (MIR) wavelengths within the range 1.54-14.29 μm (700-6500 cm(-1)). The materials silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and titanium oxide are prepared using room temperature reactive sputter deposition and are characterized using MIR variable angle spectroscopic ellipsometry. The investigation shows how sensitive the refractive index functions are to the O2 and N2 flow rates, and for which growth conditions the materials deposit homogeneously. It also allows conclusions to be drawn on the degree of amorphousness and roughness. To facilitate comparison of the materials deposited in this work with others, the index of refraction was also determined and provided for the near-IR and visible ranges of the spectrum. The results presented here should serve as a useful information base for designing optical coatings for the MIR part of the electromagnetic spectrum. The results are parameterized to allow them to be easily used for coating design.

Quantum dots as single-photon sources have several advantages, such as emitting light over a broad spectral range and being photostable. Quantum dots with densities as low as 1 dot/μm2 have been achieved using ultra-low-rate epitaxy and single-dot emission measured without apertures or post-growth processing. Both excitionic and biexcitonic emissions are observed from single dots created in this way, appearing as doublets with a fine-structure splitting of 320 μeV. The polarization of the split states is also investigated. Hanbury Brown-Twiss correlation measurements for the excitonic emission under cw excitation show anti-bunching behavior with an autocorrelation value of g(2)(0) = 0.2.

Hall effect measurements of the mobility of electrons confined in doped GaAs/AlGaAs quantum wells indicate that the ionized impurity scattering of a quasi-two-dimensional electron gas immersed in the identical concentration of ionized impurities is greater than that of a bulk electron gas of the same density. This enhancement arises from fundamental differences between 2-D and 3-D scattering of charged carriers with ionized impurities. The enhanced scattering rate may be further increased by confining the dopant ions to a delta-like doping profile in the center of the well because of the further overlap of the electronic wavefunction with the impurities. Although electrons in center delta-doped AlGaAs/GaAs quantum wells have lower low-field mobilities than do electrons in uniformly doped quantum wells, at electric fields between 2 and 4 kV/cm the differential mobility in delta-doped quantum wells rises dramatically. This large increase in differential mobility may be a result of the heating of the electrons out of the symmetric ground state into the anti-symmetric first excited state.