M. P. Semtsiv

Mid-infrared optical properties of thin films of aluminum oxide, titanium dioxide, silicon dioxide, aluminum nitride, and silicon nitride

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.

Short-wavelength (λ≈3.05μm) InP-based strain-compensated quantum-cascade laser

The design and implementation of a short-wavelength quantum-cascade laser based on the strain-compensated In0.73Ga0.27As–In0.55Al0.45As–AlAs heterosystem on InP is described. Lasers with a reduced level of doping in the active region require a larger bias voltage and emit at shorter wavelength; the emission wavelength is 3.05μm at T≈80K. The lasers operate up to T≈150K and electroluminescence persists up to room temperature, where the peak position is close to 3.3μm. The short-wavelength limit of such lasers is evaluated based on the dependence of their maximum operation temperatures and on the probable energies of the indirect valleys in the active region.

Above room temperature operation of short wavelength (λ=3.8μm) strain-compensated In0.73Ga0.27As–AlAs quantum-cascade lasers

We demonstrate the design and implementation of a broad-gain and low-threshold (Jth=860A∕cm2 at 8K) quantum-cascade laser emitting between 3.7 and 4.2μm. The active region design is based on strain-compensated In0.73Ga0.27As–AlAs on InP. Laser operation in pulsed mode is achieved up to a temperature of 330K with maximum single-facet output peak powers of 6W at 8K and 240mW at 296K. The temperature coefficient T0 is 119K.

Low divergence single-mode surface emitting quantum cascade ring lasers

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95μm with a side mode suppression ratio of 25dB. A linear tuning coefficient of 0.13cm−1∕K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications.

Influence of tip-induced band bending on tunnelling spectra of semiconductor surfaces

A theory based on the Bardeen formalism is developed for computing the tunnel current between a metal tip and a semiconductor surface. Tip-induced band bending in the semiconductor is included, with the electrostatic potential computed in a fully three-dimensional model whereas the tunnel current is computed in the limit of large tip radii. Localized states forming at the semiconductor surface as well as wavefunction tailing through the semiconductor depletion region are fully accounted for. Numerical results are provided and compared with data obtained from p-type GaAs surfaces, and generalization of the method to semiconductor heterojunctions is discussed.

Low-threshold intersubband laser based on interface-scattering-rate engineering

The dependence of the scattering rate between different electronic states in semiconductor heterostructures due to interface roughness on the barrier height is exploited to enhance the population inversion in intersubband lasers. Barriers with differing heights are used within a strain-compensated InGaAs-InAlAs heterostructure to either increase or decrease the interface-roughness scattering component for specific confined states. In particular, low barriers are used where the upper laser state has its highest probability, thus maximizing the lifetime of the upper laser state; the higher barriers are used where the lower laser state and the few subsequent confined states have their highest probabilities, thus minimizing the lifetime of the lower laser state. By combining differing barrier heights in this way, the lifetime of the upper laser state is increased, while simultaneously the lifetime of the lower laser state is decreased; thus, the population inversion is significantly enhanced. This design appr...

Impact of doping on the performance of short-wavelength InP-based quantum-cascade lasers

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm−3 (sheet densities between 1.6 and 20.9×1011 cm−2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2−3×1017 cm−3, corresponding to a sheet density of about 1.5×1012 cm−2.

Thermally activated leakage current in high-performance short-wavelength quantum cascade lasers

The threshold condition for a 4-level quantum cascade laser (QCL)-active region is formulated to include thermally activated leakage of charge carriers from active region confined states into states with higher energy. A method is described and demonstrated to extract the associated thermal escape current density from measurements at laser threshold. This current is modeled by including both the temperature dependent subband-distribution of charge carriers and longitudinal optical-phonon probability. The method is used to analyze the thermally activated leakage of charge carriers in two short-wavelength strain-compensated InGaAs/InAlAs QCL-structures. The energies of the higher-lying states extracted from the model are in good agreement with the values calculated numerically within the effective-mass approximation. The estimated scattering time for the thermal activation process agrees with the expected value as well. Our approach offers a straightforward and accurate method to analyze and troubleshoot th...

Grating-coupled surface emitting quantum cascade ring lasers

We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Perot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging.

Short-wavelength intersubband absorption in strain compensated InGaAs/AlAs quantum well structures grown on InP

We have studied intersubband absorption in strain compensated InxGa1−xAs/AlAs/InyAl1−yAs multiple quantum wells and superlattices grown on InP. X-ray diffraction shows that the layers are pseudomorphically strained and exhibit slight compositional grading of the interfaces. Owing to the high AlAs barriers, the intersubband absorption can be tailored to wavelengths shorter than 2 μm. In some samples, a small, but non-negligible absorption is also observed with s-polarized light.