S. A. Dvoretskii

Molecular-beam epitaxy of mercury-cadmium-telluride solid solutions on alternative substrates

Growth processes were considered for heteroepitaxial structures based on a mercury-cadmium-telluride (MCT) solid solution deposited on GaAs and Si alternative substrates by molecular-beam epitaxy. Physical and chemical processes of growth and defect-generation mechanisms were studied for CdZnTe epitaxy on atomically clean singular and vicinal surfaces of gallium-arsenide substrates and CdHgTe films on CdZnTe/GaAs surfaces. ZnTe single-crystalline films were grown on silicon substrates. Methods for reducing the content of defects in CdZnTe/GaAs and CdHgTe films were developed. Equipment for molecular-beam epitaxy was designed for growing the heteroepitaxial structures on large-diameter substrates with a highly uniform composition over the area and their control in situ. Heteroepitaxial MCT layers with excellent electrical parameters were grown on GaAs by molecular-beam epitaxy.

Efficient long wavelength interband photoluminescence from HgCdTe epitaxial films at wavelengths up to 26 μm

Photoluminescence (PL) and photoconductivity (PC) studies of Hg1−xCdxTe (0.19 ≤ x ≤ 0.23) epitaxial films are presented. Interband PL is observed at wavelengths from 26 to 6 μm and in the temperature range 18 K–200 K. The PL line full width at half maximum is about 6 meV (4kT) at 18 K and approaches its theoretical limit of 1.8kT at higher temperatures. Carrier recombination process is also investigated by time resolved studies of PL and PC at pulsed excitation. Radiative transitions are shown to be the dominating mechanism of carrier recombination at high excitation levels.

Time resolved photoluminescence spectroscopy of narrow gap Hg1−xCdxTe/CdyHg1−yTe quantum well heterostructures

Photoluminescence (PL) spectra and kinetics of narrow gap Hg1−xCdxTe/CdyHg1−yTe quantum well (QW) heterostructures grown by molecular beam epitaxy technique are studied. Interband PL spectra are observed from 18 K up to the room temperature. Time resolved studies reveal an additional PL line with slow kinetics (7 μs at 18 K) related to deep defect states in barrier layers. These states act as traps counteracting carrier injection into QWs. The decay time of PL signal from QW layers is about 5 μs showing that gain can be achieved at wavelengths 10–20 μm by placing such QWs in HgCdTe structures with waveguides.

Temperature-driven massless Kane fermions in HgCdTe crystals

Bulk films and heterostructures based on HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D [1], 2D [2] and even 3D [3] massless particles can be observed in topological phase transitions driven by intrinsic (quantum well thickness, Cd content) and external (magnetic field, temperature or pressure) physical parameters. So far, the phases of 2D [1] and 3D [4] topological insulator have already been experimentally demonstrated in HgCdTe-based heterostructures. More recently, clear experimental evidence of the existence of 3D electronic states with conical-like spectrum was obtained in HgCdTe bulk films at specific Cd content [3]. These 3D massless particles, called Kane fermions, have unique symmetry properties, which are not equivalent to any well-known case of massless particles in the ultrarelativistic limit of the quantum electrodynamics.

Temperature-driven single-valley Dirac fermions in HgTe quantum wells

We report on the temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness dc. Our results, obtained in magnetic fields up to 16 T and s temperature range from 2 to 150 K, clearly indicate a change in the band-gap energy with temperature. A quantum well wider than dc evidences a temperature-driven transition from topological insulator to semiconductor phases. At a critical temperature of 90 K, the merging of inter- and intraband transitions in weak magnetic fields clearly specifies the formation of a gapless state, revealing the appearance of single-valley massless Dirac fermions with a velocity of 5.6×105ms−1. For both quantum wells, the energies extracted from the experimental data are in good agreement with calculations on the basis of the eight-band Kane Hamiltonian with temperature-dependent parameters.

Long wavelength stimulated emission up to 9.5 μm from HgCdTe quantum well heterostructures

Stimulated emission from waveguide HgCdTe structures with several quantum wells inside waveguide core is demonstrated at wavelengths up to 9.5 μm. Photoluminescence line narrowing down to kT energy, as well as superlinear rise in its intensity evidence the onset of the stimulated emission, which takes place under optical pumping with intensity as small as ∼0.1 kW/cm2 at 18 K and 1 kW/cm2 at 80 K. One can conclude that HgCdTe structures potential for long-wavelength lasers is not exhausted.

Specific features of the spectra and relaxation kinetics of long-wavelength photoconductivity in narrow-gap HgCdTe epitaxial films and heterostructures with quantum wells

The spectra and relaxation kinetics of interband photoconductivity are investigated in narrow-gap Hg1 − xCdxTe epitaxial films with x = 0.19–0.23 and in structures with HgCdTe-based quantum wells (QWs), having an interband-transition energy in the range of 30–90 meV, grown by molecular-beam epitaxy on GaAs (013) substrates. A long-wavelength sensitivity band caused by impurities or defects is found in the spectra of the structures with quantum wells in addition to the interband photoconductivity. It is shown that the lifetimes of nonequilibrium carriers in the structures with QWs is less than in bulk samples at the same optical-transition energy. From the measured carrier lifetimes, the ampere-watt responsivity and the equivalent noise power for a film with x = 0.19 at a wavelength of 19 μm are estimated. When investigating the relaxation kinetics of the photoconductivity at 4.2 K in high excitation regime, it is revealed that radiative recombination is dominant over other mechanisms of nonequilibrium-carrier recombination.

HgCdTe Heterostructures on Si (310) Substrates for Midinfrared Focal Plane Arrays

Results of studies of the molecular beam epitaxial growth of HgCdTe alloys on Si substrates as large as 100 mm in diameter are presented. Optimum conditions for obtaining HgCdTe/Si(310) heterostructures of the device quality for the spectral range of 3–5 μm are determined. The results of measurements and discussion of photoelectric parameters of an infrared photodetector of a format of 320 × 256 elements with a step of 30 μm based on a hybrid assembly of a matrix photosensitive cell with a Si multiplexer are presented. A high stability of photodetector parameters to thermocycling from room temperature to liquid-nitrogen temperature is shown.

Properties of MIS structures based on graded-gap HgCdTe grown by molecular beam epitaxy

The effect of near-surface graded-gap layers on the electrical characteristics of MIS structures fabricated based on heteroepitaxial Hg1 − x CdxTe films grown by molecular beam epitaxy with a two-layer SiO2/Si3N4 insulator and anodic oxide film is studied experimentally. It is shown that a larger modulation of capacitance (depth and width of the valley) is observed compared with the structures without the graded-gap layer. The field dependences of photovoltage of MIS structures with the graded-gap layers had a classical form and were characterized by a drop only in the enrichment region. For the structures without the graded-gap layer with x = 0.22, a drop in the voltage dependence of the photocurrent is observed in the region of pronounced inversion. This drop is governed by limitation of the space charge region by processes of tunneling generation via deep levels. The properties of the HgCdTe-insulator interfaces are studied.

Long wavelength superluminescence from narrow gap HgCdTe epilayer at 100 K

Experimental evidence of long wavelength superluminescence (SL), i.e., amplification of spontaneous emission, in narrow gap HgCdTe bulk epitaxial film at 100 K is reported. Photoluminescence line narrowing is observed at 8.4 μm as pump power increases. However, plasmonic contribution to dielectric function is shown to be detrimental for light confinement at high pumping intensities, limiting the SL line intensity growth. The design of the structures optimal for obtaining stimulated emission in 10–36 μm range is further discussed.