Gilberto Antonio Umana-Membreno

HgCdSe has recently been proposed as a potential alternative material to HgCdTe for fabricating high-performance infrared detectors. This work presents a study on the growth of high-crystalline-quality HgCdSe materials on GaSb (211)B substrates via molecular beam epitaxy and demonstration of the first prototype HgCdSe-based mid-wave infrared detectors. By optimizing the MBE growth parameters, and especially the thermal cleaning process of the GaSb substrate surface prior to epitaxial growth, high-quality HgCdSe material was achieved with a record XRD full width at half maximum of ~65 arcsec. At a temperature of 77 K, the mid-wave infrared HgCdSe n-type material demonstrated a minority carrier lifetime of ~1.19 µs, background electron concentration of ~2.2 × 10¹⁷ cm⁻³, and electron mobility of ~1.6 × 10⁴ cm²/Vs. The fabricated mid-wave infrared HgCdSe photoconductor presented a cut-off wavelength of 4.2 µm, a peak responsivity of ~40 V/W, and a peak detectivity of ~1.2 × 10⁹ cmHz^1/2/W at 77 K. Due to the relatively high background electron concentration, the detector performance is lower than that of state-of-the-art low-doped HgCdTe counterparts. However, these preliminary results indicate the great potential of HgCdSe materials for achieving next-generation IR detectors on large-area substrates with features of lower cost and larger array format size.

The electronic properties of an ultra-narrow-bandgap InAs/InGaSb type II superlattice (T2SL) were investigated for potential terahertz applications using magnetic field-dependent transport measurements and mobility spectrum analysis over a temperature range of 10–300 K. Since the 267.2-nm-thick 40-period T2SL structure was grown on electrically conductive undoped p-type GaSb substrates, detailed transport characterization of all carrier species in representative substrate-only and substrate/buffer reference samples was essential to unambiguously discriminate carriers. The results indicate that a single high-mobility electron species dominates electronic transport in the T2SL structure over the range of 56–300 K. From analysis of the sheet electron density, a temperature-dependent energy bandgap Varshni model was extracted, revealing that the T2SL bandgap can be tuned from 13 meV at 80 K, down to 1.3 meV at 115 K, corresponding to cutoff frequencies in the range of 0.3 to −3.2 THz range. In addition, the results indicate an InAs/GaSb valence band offset (VBO) of 0.605 eV, which is consistent with a T2SL characterized by InSb-like interface bonds.

Recently, InGaAs/InP multiple quantum well nanowires grown by selective area epitaxy have been demonstrated with uniform morphology and high optical quality. The InGaAs quantum wells wrapping around the nanowire core are formed with both axial and radial components. As such the radial quantum well configuration presents a unique advantage for the realization of intraband absorption of normal-incidence light in the nanowires, which cannot be achieved in conventional planar quantum well structures due to polarization selection rules. We report here mid-infrared intraband transitions within the atmospheric window (3–5 μm) in InP nanowire arrays embedded with five InGaAs quantum wells under normal-incidence light. The light absorption coefficient of the quantum wells is modeled, and the absorption peak indicates a bound-to-continuum transition. The intraband photocurrent shows a linear dependence on the incident power, while the interband photoresponse is sublinear due to the surface states of the nanowires. These nanowires with radial quantum wells open up great opportunities for developing next-generation mid- to long-wavelength infrared photodetectors and focal plane arrays.

Mercury cadmium telluride (Hg1−xCdxTe or MCT) is the premier material for infrared detection. However, despite its importance, studies exploring the ultrafast photoresponse in this semiconductor alloy are limited. Here, we use time-resolved terahertz spectroscopy to perform a detailed study of the picosecond charge carrier dynamics in long-wave infrared Hg1−xCdxTe (x ∼ 0.2) films, providing insight into ultrafast carrier cooling and temperature-dependent scattering mechanisms. Due to the multilayer photoexcited sample geometry, an elementary thin-film analysis leads to a negative photoconductivity artifact. We, therefore, derive a modified thin-film photoconductivity formula to accurately extract a Drude photoconductivity spectrum. In our analysis, we include the effects of carrier diffusion and the conduction band non-parabolicity in Hg1−xCdxTe. We extract ultrahigh electron mobilities as large as 6 × 105 cm2 V−1 s−1 at 25 K. At cryogenic temperatures, we find the photoexcited electron mobility is up to four times larger than the dark mobility, which we attribute to suppression of ionized impurity scattering due to hole capture by acceptor-type Hg vacancies. In addition, after photoexcitation, we observe a relatively slow rise in photoconductivity over a 10 ps timescale with a monotonically increasing carrier scattering time and a carrier effective mass that decays exponentially with a time constant of 1.9 ps, which we attribute to hot-carrier cooling dynamics in the non-parabolic conduction band.

We use terahertz time-domain spectroscopy to measure the complex dielectric function of long-wave infrared Hg _1-x Cd _x Te films (x = 0.18, 0.20, 0.22) as a function of temperature in a non-contact manner. Using a Drude-Lorentz model fit to the measured complex transmission function combined with a Kane model description of the band structure, we obtain the temperature-dependent conduction band carrier density, effective mass, scattering time and carrier mobility for all three Hg _1-x Cd _x Te films. The optical properties of a bare substrate of Cd _0.96 Zn _0.04 Te were also measured in the terahertz region. The high quality of the Hg _1-x Cd _x Te films is demonstrated by ultrahigh mobilities exceeding 10 ⁵ cm ² V ^-1 s ^-1 and ionized donor densities less than 3 × 10 ¹⁵ cm ^-3 at temperatures below 100 K.

The sensitivity of III-V-based infrared detectors is critically dependent upon the carrier concentration and mobility of the absorber layer, and thus, accurate knowledge of each is required to design structures for maximum detector performance. Here, measurements of the bulk in-plane resistivity, in-plane mobility, and carrier concentration as a function of temperature are reported for non-intentionally doped and Si-doped mid-wave infrared InAs0.91Sb0.09 alloy and InAs/InAs0.65Sb0.35 type-II superlattice materials grown on GaSb substrates. Standard temperature- and magnetic-field-dependent resistivity and the Hall measurements on mesa samples in the van der Pauw configuration are performed, and multi-carrier fitting and modelling are used to isolate transport of each carrier species. The results show that up to 5 carrier species of the surface, interface and bulk variety contribute to conduction, with bulk electron and hole mobility up to 2·105 cm2/V s and 8·103 cm2/V s, respectively and background dopant concentration levels were between 1014 and 1015 cm−3. The in-plane mobility temperatures dependence is determined and trends of each carrier species with temperature and dose are analysed.

Careful selection of the physical model of the material for a specific doping and selected operating temperatures is a non-trivial task. In numerical simulations that optimize practical devices such as detectors or lasers architecture, this challenge can be very difficult. However, even for such a well-known material as a 5 µm thick layer of indium arsenide on a semi-insulating gallium arsenide substrate, choosing a realistic set of band structure parameters for valence bands is remarkable. Here, the authors test the applicability range of various models of the valence band geometry, using a series of InAs samples with varying levels of p-type doping. Carefully prepared and pretested the van der Pauw geometry samples have been used for magneto-transport data acquisition in the 20–300 K temperature range and magnetic fields up to ±15 T, combined with a mobility spectra analysis. It was shown that in a degenerate statistic regime, temperature trends of mobility for heavy- and light-holes are uncorrelated. It has also been shown that parameters of the valence band effective masses with warping effect inclusion should be used for selected acceptor dopant levels and range of temperatures.

Long-term operation of chemical sensors under dynamic pressure/temperature conditions for industrial or environmental monitoring is desirable and yet rarely achieved. We have investigated long-term (>20 hours) operation of a novel ion sensor technology under varying temperature and pressure conditions whilst controlling for chemical effects by use of chemically inert hydrocarbon oil as the sensor analyte. The gallium nitride-based transistor-like sensor technology under study has previously been demonstrated for sensing a wide-range of heavy metal and nutrient ions and offers potential improvements regarding chemical stability but until now only short-term testing (1 to 3 hours) in water has been reported, and not under multi-variate environmental conditions. We investigated the response under varying. We show that for measurements under continuously cycling pressure (between 10.3 and 172.3 bar) and temperature (between 30 and 60 °C) conditions, within any two-hour observation window, the sensor response is linear and repeatable with a high coefficient of determination. Furthermore the temperature dependence strongly dominates the pressure dependence. However, when the observation window during pressure and/or temperature variation is extended to approximately 20 hours, anomalous variations in response that are uncorrelated to any environmental conditions (temperature and pressure which were measured, and light and ions, which were not present) become evident. Yet this anomalous behaviour is not present when the pressure and temperature are invariant. This finding has significant implications for implementing these sensors in dynamic environments.