The growth rate of GaN on the GaAs (110) substrate as deduced from reflectivity measurements exhibits a peculiar behavior. A transient regime with a relatively high initial growth rate progressively decreases to a limit value during the early stage of high temperature GaN growth. An optical model, incorporating time-dependent profiles of the growth rate and surface roughness, was used to simulate the corresponding in situ reflectivity recorded during the growth of GaN layers on the GaAs (110) substrate. The growth rate transient, characterized by a time constant (τ) and a diffusion length (L), is proposed to originate from Ga self-diffusion in the GaN layer. Both the time constant (τ) and the diffusion length (L) increase with the rising growth temperature. This allows for the estimation of the Ga self-diffusion coefficient in GaN, which, in the growth temperature range of 750°C to 900°C, was found to be equal to D=1.35 10−11 exp(−(0.28∓0.04)/kBT) cm2 s−1.
Hexagonal boron nitride (h-BN), a two-dimensional (2D) ultrawide-bandgap semiconductor, has seen fast progress in the last decade and attracted tremendous attention and intensive investigation for its association with III-Nitride heterostructures. More specifically, the weak van der Waals interactions between the III-Nitrides heterostructures and 2D h-BN layers is a major enabler because it allows a mechanical release and transfer of freestanding membranes of III-Nitrides heterostructures to foreign substrates which open new pathways for III-Nitride heterogeneous integration and flexible devices. In this talk we will present the results of our work on LEDs grown on h-BN and their transfer to foreign substrates.
Giant room temperature persistent photoconductivity in hexagonal boron nitride under UVC irradiation has been demonstrated. Scanning lasers dots on sample surface at two different wavelengths (213 and 266 nm) have been used to induce such effect, the former being more efficient. Conductivity has been increased by 6 orders of magnitude upon illumination. Such increase persists significantly for at least 6 months. Decrease of photoinduced current has been shown to be 10% more important for samples irradiated under vacuum than for those irradiated under atmospheric pressure. Type of photoinduced carriers were investigated using p-hBN/n-AlGaN junctions through C/V measurements. P-type carriers are believed to be generated by 213 nm illumination whereas 266 nm laser is supposed to trigger n-type carriers.
The size and density of nanopits, generated at the surface of their top layer, strongly affect the electrical and optical properties of AlGaN-based structures. Therefore, the control of the layer quality evolution as a function of the nanopits size/density is a crucial issue to enhance the device performance. In this paper, the effects of the nanopits diameter observed at the surface of AlGaN on carrier dynamics are systematically investigated. The variation of nanopits diam-eter is achieved through thermal annealing of a set of AlGaN/GaN heterostructures at different temperatures. The samples are characterized using the scanning electron microscope (SEM), energy-dispersive x-ray, high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL spectroscopies. SEM images have revealed an increase in the nanopits diameter with increasing annealing temperature. In addition, we observed a linear development in the yellow luminescence intensity, accompanied by a deterioration in the PL decay times due to an increase in the density of point-defect complexes that act as nonradiative recombination centers. We also performed temperature-dependent PL measurements to study the impact of the nanopits diameter on electron–phonon scattering processes. Both electron-acoustic- and electron-longitudinal optical phonon interactions enhance with increasing nanopits diameter.
Combined photonic and electronic systems require diverse devices to be co-integrated on a common platform. This heterogeneous integration is made possible through several separation and transfer methods where the functioning epilayers are essentially released from their growth substrate. The use of 2D layered h-BN as a mechanical release layer has been demonstrated to be a promising technique for the hybrid integration of III-nitride devices. In this talk we will give an overview of our results on wafer-scale van der Waals epitaxy by MOVPE of different III-N heterostructure devices such as LEDs, HEMTs, solar cells, sensors and photodetectors. Furthermore, mechanical release and transfer techniques of crack-free III-N devices on foreign substrates will be presented along with a comparison between the device performances before and after transfer.
We present a critical study of LEDs on h-BN compared to the conventional LEDs on sapphire from materials characterizations, device fabrication to the device performances measurements performed before and after liftoff and transfer with and without intermediary adhesion layer to arbitrary substrates
Metal-semiconductor-metal solar blind ultraviolet photodetectors have been fabricated using both BGaN-GaN
and BGaN-AlN superlattices as active layers. A high internal gain (up to 3 × 104 for optical power in the nW
range) is obtained with a highly reduced dark current thanks to the boron incorporation. In the high optical
power regime (W range), the time response is in the nanosecond range, which is much smaller than that of GaNand
ZnO-based ultraviolet photodetectors. Moreover, the boron incorporation in GaN material allows the tuning
of the cutoff wavelength.
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