Mid-wave infrared (3–5μm) AlInSb resonant-cavity LEDs

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Summary form only given. Narrow-gap antimonide-based semiconductors provide great flexibility for band engineering with their bandgap being adjustable in a wide range to achieve emission in the mid-wave infrared spectral region (λ ~3-5μm). Despite the rapid development of light-emitting diode (LED) technology over the last few years, the poor extraction efficiency severely limits the device power performance in the mid-infrared region. The large difference in refractive index between the narrow-gap semiconductor material (n ≥ 3.5) and the surrounding medium (typically air) results in a large portion of the light being trapped within the device through total internal reflection and Fresnel reflection, thus limiting the light extraction efficiency to a few percents [1].Resonant-cavity enhancement, achieved by placing the active region within a Fabry-Pérot cavity, was first demonstrated by Schubert et al. [2] and is now a popular geometry to improve light extraction in the visible and near-infrared wavelength range [3]. The benefits of this geometry were recently demonstrated in the mid-infrared wavelength range on a III-V resonant-cavity LED (RCLED) emitting at 4μm [4]. We report on the enhancement of the photoluminescence spectra at room temperature from an AlInSb RCLED emitting at λ - 4-4.5μm. The cavity is defined by the top air-semiconductor interface and a latticematched AlSb/GaInSb bottom Distributed Bragg Reflector (DBR) mirror. The device is a molecular beam epitaxy (MBE) grown Al(x)In(1-x)Sb p-i-n diode structure, where Be and Te are used as dopants for the pand ncontact layers respectively. The emission wavelength can be tuned in the 3-5μm spectral range by altering the Al fraction in the III-V material. The centre wavelength of the DBR stopband is designed at 4.25μm and, due to the high refractive index contrast between AlSb and GaInSb (-0.6), a 60% reflectivity is achieved with only 5 layer pairs. The cavity thickness, taking into account the penetration depth of the bottom DBR, gives a cavity resonance which is well aligned to the DBR central wavelength. Photoluminescence spectra exhibit a clear emission enhancement and the presence of Fabry-Pérot cavity modes, with a free-spectral range of - 6 THz (Fig. 1(a)). The ratio between the measured PL spectra from the RCLED and a standard non-resonant LED shows a PL enhancement factor of 2.5. The enhancement factor is reported in Fig. 1(b), together with the reflectivity of the device, modelled using the transfer matrix method.In conclusion, we have shown a 2.5-fold increase in the peak PL emission intensity from an AlInSb LED emitting in the mid-wave infrared range. The enhancement has been obtained by including a 5-pairs latticematched AlSb/GaInSb bottom DBR mirror with 60% reflectivity and by designing the cavity thickness in order to achieve resonance at the desired wavelength. A more careful design optimisation of the cavity design is currently being undertaken to further improve the RCLED power performance. This includes reducing the thickness of the n-type layer to minimise optical losses, increasing the number of stacks in the bottom DBR and adding a top DBR. Farfield diffraction pattern measurements and fabrication of electrically biased RCLED are in progress.