We discuss state-of-the-art mid-infrared light emitters and detectors based on the so-called 6.1 Å family of semiconductors, i.e. InAs, GaSb, and AlSb. Via epitaxial design routines, heterostructures composed of their binary, ternary or quaternary alloys allow unique features such as optically active type-II superlattices enabling light emitters and detectors suitable for the mid-infrared wavelength region. Here we compare and discuss the design differences between interband cascade infrared detectors employing Ga free Type II superlattices and resonant tunneling diodes (RTD) employing the quaternary alloy GaInAsSb. We show that by substituting the standard InAs/GaSb superlattice for a Ga-free superlattice, i.e. InAs/InAsSb, one requires an inverted carrier extraction path. Here it is needed to form a hole-ladder in the electron-barrier, instead of an electron-ladder in the hole-barrier. At elevated temperatures, we observe seven negative-differential-conductance (NDC) regions due to electrons tunnelling through the electron barriers of the seven cascade stages. The detector operates in photovoltaic mode with a cut-off wavelength of 8.5 μm. The RTD photodetector on the other hand utilizes GaInAsSb absorbers that allow efficient operation in the 2-4 μm range with significant electrical responsivity of 0.97 A/W at 2 μm. Contrary to interband cascade infrared detectors, RTD PD operate only at finite voltages and hence these devices are Shot noise limited.
We present recent work on III-V semiconductor mid-infrared light emitters and detectors. The employed type-II broken bandgap alignment between InAs and GaxIn1-xSb allows for widely tunable emission and absorption wavelengths with energies below the individual material bandgaps. We demonstrate room temperature operation of GaSb-based interband cascade lasers (ICLs) emitting between 6.1 and 6.9 μm. Furthermore, we investigate ideal growth conditions for InAs/GaSb type-II superlattices (T2SL) for the implementation in interband cascade detectors (ICDs) with cut-off wavelengths up to 7.5 μm at room temperature. We focus on strain balancing different SL compositions for different cutoff wavelengths via Sb-soak and sub-monolayer (SML) growth of InSb. An ideal growth temperature of TSub=430 °C is found by comparing the quality of different sets of samples by means of high-resolution X-ray diffractometry (HRXRD) and room temperature photoluminescence (PL) measurements.
Molecule and gas sensing is a key technology that is applied in multiple environmental, industrial and medical fields. In particular optical detection technologies enable contactless, nondestructive, highly sensitive and fast detection of even smallest concentrations of trace gases and molecules. During the past years, an increasing demand for mid-infrared (MIR) light sources suitable for, e.g. molecule or gas sensing applications, has driven the development and optimization of novel MIR lasers and light sources, such as quantum cascade lasers (QCL) or interband cascade lasers (ICL). Despite the progress on MIR light sources, there is still a lack in appropriate MIR detectors. Here, we present and discuss two promising and novel GaSb/InAs-based detector concepts. First, resonant tunneling diode (RTD) photodetectors as an alternative to avalanche photodetectors. In RTDs, amplification of photogenerated minority charge carriers is based on modulation of a majority charge carrier resonant tunneling current. Second, interband cascade photodetectors (ICD), in which a cascading scheme allows for fast carrier extraction and a compensation of the diffusion length limitation.
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