III-nitride intersubband photonics

ABSTRACT This paper reviews the recent progress towards III-nitride in tersubband devices based on quantum wells. We first present recent achievements in terms of GaN-based quantum cascade detectors operating at near-infrared wavelengths. We show that these devices are intrinsically extremely fast based on femtosecond time-resolved measurements of the photocurrent. The design of III-nitride quantum cascade detectors, which relies on the engineering of the internal electric field, is flexible enough to allow for two-color detection. We finally discuss the potential of III-nitride intersubband devices in the THz frequency domain a nd present the recent observ ation of THz absorption using low aluminium content AlGaN/GaN step quantum wells. Keywords: III-nitride semiconductors, intersubband transitions, optoelectronic devices, quantum cascade detectors, quantum wells. INTRODUCTION Intersubband photonics (ISB) have emerged within the last 25 years and led to a number of fascinating control-by–design devices relying on the quantum engineering of electron confinement in semiconductor quantum wells (QW) or quantum dots (QD) [1]. One famous example is the quantum cascade laser (QCL), which was invented in the mid-90’s at Bell Laboratories [2]. This device relies on the repetition of multiple quantum well (MQW) active regions where ISB light emission takes place and injector/extractor regions to transfer electrons from one period to the other. The ISB emission wavelength can be tuned in a wide spectral range simply by adjusting the layer thickness. Using materials such as GaAs/AlGaAs, InGaAs/AlInAs or antimonides, the operation wavelength of ISB devices such as the QCLs can be tuned from the mid-infrared to the THz spectral range. Operation at short wavelengths is limited by the available conduction band offset and by the material transparency. Thanks to the progress achi eved in terms of epitaxial growth of ultrathin layers, III-nitride semiconductors (GaN, AlN, InN and their alloys) have emerged within the last ten years as excellent materials for ISB photonics devices in the near-infrared spectral range and in particular in the 1.3-1.55 µm wavelength window used for fibre optics telecommunications. Not only they are transparent in a wide spectral region (360 nm to 13 µm for GaN) but the conduction band offset prov ided by their heterostructures is quite large, of the order of 1.75 eV for GaN/AlN [3]. In contrast to InAs/AlSb materials, which also exhibit a large conduction band discontinuity, the remote valleys of GaN lie very high in energy (>2 eV above the + point [4]), offering the potential for ISB light emitting devices at record short near-infrared wavele ngths. Another specificity of nitride materials is the large longitudinal-optical (LO) phonon energy (92 meV for GaN) as well as the presence of huge internal fields induced by the spontaneous and piezoelectric polarizatio ns along the c axis inherent to their wu rtzite structure. The presence of these