There are many applications for light-emitting diodes (LEDs) that can operate in the mid-infrared spectral range. However, the efficiency of these devices at room temperature is limited by competing nonradiative recombination mechanisms, inadequate carrier confinement, and insufficient optical extraction. Earlier devices based on bulk materials and heterojunctions have been quite successful to date, leading to some commercialization, but several new designs containing quantum structures for the active region have since been proposed and are being studied. Similarly, there is growing interest in using more cost-effective substrates requiring the development of metamorphic buffer layers as well as resonant cavity structures to increase optical extraction. An overview of the current status of mid-infrared LED technology is given here together with a brief summary of some recent developments.

The practical realization of thermophotovoltaic (TPV) cells, which can directly convert heat into electric power, is of considerable technological interest. However, most existing TPV cells require heat sources at temperatures of ∼1800°C. Here we report a low bandgap mid-infrared cell based on InAs and demonstrate TPV operation with heat sources at temperatures in the range 500–950°C. The maximum open circuit voltage ( V oc ) and short circuit current density ( J sc ) were measured as 0.06 V and 0.89 A cm −2 for a blackbody temperature of 950°C and an incident power density of 720 mW cm −2 without antireflection coating or electrode optimisation. TPV operation was obtained with heat sources at temperatures as low as 500°C, which represents progress towards energy scavenging and waste heat recovery applications.

We demonstrate the first low bandgap thermophotovoltaic (TPV) arrays capable of operating with heat sources at temperatures as low as 345 °C, which is the lowest ever reported. The individual array elements are based on narrow band gap InAs/InAs0.61Sb0.13P0.26 photodiode structures. External power conversion efficiency was measured to be ∼3% from a single element at room temperature, using a black body at 950 °C. Both 25-element and 65-element arrays were fabricated and exhibited a TPV response at different source temperatures in the range 345–950 °C suitable for electricity generation from waste heat and other applications.

InAsSb/InAsSbP double heterojunction lasers have been grown by liquid phase epitaxy in which free carrier absorption loss was investigated and minimized by the introduction of two undoped quaternary layers on either side of the active region. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 μm with a threshold current density as low as 118 A/cm2 at 85 K. Compared to the conventional 3-layer DH laser, reducing the optical loss increases the maximum lasing temperature by 95 K to ∼210 K in the optimized 5-layer structure.

The liquid phase epitaxial growth of InAs0.91Sb0.09 lattice-matched onto GaSb is reported for use in the active region of a mid-infrared light-emitting diode. Epitaxy was carried out from Sb-rich solution using Gd gettering to purify the material. The photoluminescence and electroluminescence emission spectra were studied over the temperature range 4–300 K using different excitation intensities. Interpretation of the resulting spectra revealed the existence of two acceptor levels with activation energies of 8 meV and 16 meV. Room temperature electroluminescence at 4.2 µm was readily obtained from homo-junction p–i–n diodes fabricated from this material. The temperature dependence of the electroluminescence was studied and the decrease in the radiative recombination coefficient was found to be primarily responsible for the luminescence quenching.

We report on the liquid phase epitaxy (LPE) growth of an optimized double heterostructure (DH) 3-4 μm laser and the use of linear rapid slider boat technology for the production of quantum well (QW) structures based on InAsSb/InAsSbP. Typical characteristics of some of these prototype sources are presented and analyzed, including the results of SEM, X-ray diffraction, photo- and electro-luminescence characteristics of prototype DH & QW devices. The optimized 5 epi-layer diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 μm at 170 K with a threshold current density as low as 118 A/cm2 at 85 K. Coherent emission was obtained up to 210 K. LPE growth of InAsSb QW has been successfully obtained experimentally. The QW structure has been confirmed by SEM and electroluminescence measurements at different temperatures.

We report on the liquid phase epitaxy (LPE) growth of an optimized double heterostructure (DH) 3-4 mu m laser and the use of linear rapid slider boat technology for the production of quantum well (QW) structures based on InAsSb/InAsSbP. Typical characteristics of some of these prototype sources are presented and analyzed, including the results of SEM, X-ray diffraction, photo- and electro- luminescence characteristics of prototype DH & QW devices. The optimized 5 epi-layer diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 mu m at 170 K with a threshold current density as low as 118 A/cm(2) at 85 K. Coherent emission was obtained up to 210 K. LPE growth of InAsSb QW has been successfully obtained experimentally. The QW structure has been confirmed by SEM and electroluminescence measurements at different temperatures.

In this work we report on a specially optimized type-I InAsSb/InAsSbP double heterostructure (DH) ridge laser grown by liquid phase epitaxy (LPE). To remove residual impurities and reduce Shockley-Read recombination, the active region was purified using a Gd gettering technique. In addition free carrier absorption loss was minimized by the introduction of two undoped quaternary layers with the same composition of the cladding layers either side of the active region. The inserted layers also helped alleviate inter-diffusion of unwanted dopants towards the active region during or after growth and reduced current leakage of the device. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 mu m at 170 K with a threshold current density as low as 118 A/cm(2) at 85 K. Compared to the conventional 3-layer DH laser, the optimized 5-layer structure with reduced optical loss can raise the maximum lasing temperature by 95K to similar to 210K.

. InAs quantum wells have been grown using a novel rapid-slider liquid-phase epitaxial growth technique. Lattice-matched quantum wells down to 2.5 nm in thickness have been produced with InAs 0.36 Sb 0.20 P 0.44 barriers. Bright photoluminescence was observed from 33-nm InAs quantum wells consistent with emission from a type-II InAsSbP/InAs system.

Light emitting diodes (LEDs) and lasers operating in the 2 to 3 mu m spectral region at room temperature are been demonstrated. The devices were fabricated from InxGa1-xAs/InAsyP1-y double heterostructures grown on n-type InP (100) substrates by molecular beam epitaxy. A strain relaxed buffer layer which incorporates composition reversals was used to reduce the threading dislocation density and to accommodate the large lattice mismatch (up to 2.7%) between the InP substrate and the device active region. Efficient electroluminescence emission at wavelengths between 2 and 3 mu m was obtained from the LEDs at room temperature, while diode lasers exhibited coherent emission in the range 2-2.6 mu m at temperatures up to 130 K. For one of the LEDs a characteristic absorption was readily observed at 2.7 mu m in the diode electroluminescence emission spectrum, corresponding to strong water vapour absorption in the atmosphere. These devices could easily form the key component of an infrared gas sensor for water vapour detection and monitoring at 2.7 mu m in a variety of different applications.