Micro-displays with pixel pitch smaller than 10 µm are required for augmented reality. The three primary colors can then be achieved with the same material: the InxGa1-xN alloy. However, when strained on GaN, its InN mole fraction x is limited to 25%. By using a full InGaN structure grown on a relaxed InGaN pseudo-substrate, such as the InGaNOS substrate from Soitec, the In incorporation rate is enhanced. An internal quantum efficiency higher than 10% at 640 nm an In content higher than 25% in InxGa1-xN/InyGa1-yN quantum wells will be shown. 10x10 µm² red micro-light emitting diodes will be also presented.
InGaN-based devices have attracted a lot of attention, thanks to versatile optoelectronic applications. We show that the growth of these layers in an AIXTRON Close Coupled Showerhead is complicated due to the presence of gallium pollution in the chamber after the growth of GaN buffer layers. This pollution reacts with the TMIn precursor in the growth chamber to give thicker InGaN layers with reduced indium incorporation. We overcame this problem by limiting the presence of the metallic gallium in the growth chamber, resulting in more stable and predictable growth of InGaN layers.
InGaN based LEDs are known to be very efficient in the blue range. However, although InGaN can theoretically cover all visible range, quantum efficiency drops when emission wavelength emission is increased due to quantum confined Stark effect. Furthermore, indium incorporation is hindered by compressive strain induced by lattice mismatch between InGaN and GaN. To tackle the lattice mismatch problem, a full InGaN structure on a relaxed InGaN substrate is proposed.
The structure consisting of five InxGa1-xN / InyGa1-yN multi quantum wells on top of an InyGa1-yN buffer layer is grown by MOVPE on an InGaNOs substrate from Soitec company. Three InGaNOs substrates of lattice parameters of 3.190, 3.200 and 3.205 Ångströms were co-loaded in order to compare their ability to incorporate indium for the same growth conditions. For reference, a sample grown on GaN template will allow us to compare the wavelength red-shift resulting of the use of InGaNOs template.
The samples were characterized by photoluminescence at room temperature using 375 nm and 405 nm laser diodes. It is shown that long wavelengths can be reached thanks to the use of InGaNOs substrates. For same active region growth conditions as reference sample, a red-shift up to 65 nm (from 445 to 510 nm) is observed, demonstrating InGaNOs potential for easier In incorporation. Using different growth conditions, wavelengths up to 600 nm have been reached. First internal quantum efficiency measurements demonstrate a good quality material. InGaNOs seems promising for emission in the “green gap”and beyond.
With a long experience in optoelectronics, CEA-LETI has focused on Light Emitting Diode (LED) lighting since 2006. Today, all the technical challenges in the implementation of GaN LED based solid state lighting (SSL) are addressed at CEA-LETI who is now an RandD player throughout the entire value chain of LED lighting. The SSL Line at CEA-LETI first deals with the simulation of the active structures and LED devices. Then the growth is addressed in particular 2D growth on 200 mm silicon substrates. Then, technological steps are developed for the fabrication of LED dies with innovative architectures. For instance, Versatile LED Array Devices are currently being developed with a dedicated μLED technology. The objective in this case is to achieve monolithical LED arrays reported and interconnected through a silicon submount. In addition to the required bonding and 3D integration technologies, new solutions for LED chip packaging, thermal management of LED lamps and luminaires are also addressed. LETI is also active in Smart Lighting concepts which offer the possibility of new application fields for SSL technologies. An example is the recent development at CEA LETI of Visible Light Communication Technology also called LiFi. With this technology, we demonstrated a transmission rate up to 10 Mb/s and real time HD-Video transmission.
GaN/AlN multiple quantum wells (MQWs), designed for intersubband (ISB) absorption in the telecommunication
range, are grown by molecular beam epitaxy. We demonstrate that the use of both AlN template and optimized growth
temperature allows to reach ISB transition energy in the telecom range, i.e. above 0.8 eV (λ = 1.55 μm). Absorption
spectra exhibit narrow linewidth (< 50 meV) with a relative energy broadening of 8%. An electro-optical modulator
based on electron tunnelling in coupled QWs is then fabricated. A modulation bandwidth of 2 GHz at -3 dB cut off
frequency is achieved for 15x15 μm2 mesas. We show that the modulation rate is limited by the device geometry rather
than by the material quality, which makes this technology a good candidate for THz regime.
The photo-response of AlGaN based UV detectors to a 193 nm excimer laser radiation is presented. Two devices have
been tested and compared, a metal-semiconductor-metal (MSM) planar structure and a Schottky diode. These sensors
have already shown good performances in the 240-280 nm region under CW illumination and have been used for the
realization of 2D and linear arrays. Here the capability of these devices to detect the emission of a nanosecond pulsed
excimer laser is proven and the decay time and dependency on the beam's density of energy evaluated. The measured
transient response of the MSM device closely follows the nanosecond laser pulses, with a decay time shorter than 3 ns.
Conversely, the Schottky diodes showed a slower rise and decay kinetics principally limited by the coupling with the
junction capacitance. The decay curve of such a device has been analyzed on the basis of two decay mechanisms: the
second exponential decay has been found to be in the order of 40 ns. This slow kinetic has been attributed to the presence
of trap states localized at a distance from the conduction or valence band larger then the thermal energy of the carriers.
Both the realized devices do follow the Rose's law with a linear response at the lower beam fluxes (density of energy
4×10-5 - 0.2 mJ/mm2) and a transition to a sub-linear regime for higher fluxes.
The fast development of nitrides has given the opportunity to investigate AlGaN as a material for ultraviolet solar blind detection in competition with technologies based on photocathodes, MCP intensifiers, back thinned CCD or hybrid CMOS focal plane arrays. All of the them must be associated to UV blocking filters. These new detectors present both an intrinsic spectral selectivity and an extremely low dark current at room temperature. First we will present the ultimate properties of the AlGaN based devices. These spectral properties are analysed in regards to the sharp cut off required for
solar blind detection around 280nm, and we will quantify how the stringent difficulties to achieve solar blind filters can be reduced. We also investigated the electrical capabilities of Schottky diodes or Metal-Semiconductor-Metal (MSM) technologies to detect extremely low UV signal. We will especially present results from a linear array based on a CCD readout multiplexor.