S. Valdueza-Felip

Effect of the quantum well thickness on the performance of InGaN photovoltaic cells

We report on the influence of the quantum well thickness on the effective band gap and conversion efficiency of In0.12Ga0.88N/GaN multiple quantum well solar cells. The band-to-band transition can be redshifted from 395 to 474 nm by increasing the well thickness from 1.3 to 5.4 nm, as demonstrated by cathodoluminescence measurements. However, the redshift of the absorption edge is much less pronounced in absorption: in thicker wells, transitions to higher energy levels dominate. Besides, partial strain relaxation in thicker wells leads to the formation of defects, hence degrading the overall solar cell performance.

Nonlinear absorption of InN/InGaN multiple-quantum-well structures at optical telecommunication wavelengths

We report on the nonlinear optical absorption of InN / InxGa1−xN (x=0.8, 0.9) multiple-quantum-well structures characterized at 1.55 µm by the Z-scan method in order to obtain the effective nonlinear absorption coefficient (α2) of the samples at high repetition rate. Saturable absorption is observed for the sample with x=0.9, with an effective α2~−9x103 cm /GW for the studied optical regime. For lower In content in the barrier, reverse saturable absorption is observed, which is attributed to two-photon absorption.

High In-content InGaN layers synthesized by plasma-assisted molecular-beam epitaxy: Growth conditions, strain relaxation, and In incorporation kinetics

We report the interplay between In incorporation and strain relaxation kinetics in high-In-content InxGa1-xN (x = 0.3) layers grown by plasma-assisted molecular-beam epitaxy. For In mole fractions x = 0.13–0.48, best structural and morphological qualities are obtained under In excess conditions, at In accumulation limit, and at a growth temperature where InGaN decomposition is active. Under such conditions, in situ and ex situ analyses of the evolution of the crystalline structure with the layer thickness point to an onset of misfit relaxation after the growth of 40 nm, and a gradual relaxation during more than 200 nm, which results in an inhomogeneous strain distribution along the growth axis. This process is associated with a compositional pulling effect, i.e., indium incorporation is partially inhibited in presence of compressive strain, resulting in a compositional gradient with increasing In mole fraction towards the surface.

Improved conversion efficiency of as-grown InGaN/GaN quantum-well solar cells for hybrid integration

We report on the photovoltaic characteristics of solar cells based on 15 and 30 InxGa1?xN/GaN (x = 0.10 and 0.19) multiquantum wells (MQWs) grown on sapphire. Doubling the number of MQWs increases the peak external quantum efficiency by a factor of 2 for both In contents. Devices with 19% In, with a spectral cutoff at 465 nm, exhibit an open-circuit voltage of 1.7 V and a short-circuit current density of 3.00 mA/cm2 under 1 sun AM1.5G illumination, leading to a conversion efficiency of 2.00%, making them promising for hybrid integration with non-III?nitride photovoltaic devices.

Photovoltaic Response of InGaN/GaN Multiple-Quantum Well Solar Cells

We report on the fabrication and photovoltaic characterization of In0.12Ga0.88N/GaN multi-quantum-well (MQW) solar cells grown by metal–organic vapor phase epitaxy on (0001) sapphire substrates. Increasing the number of MQWs in the active region from 5 to 30 improves a factor of 10 the peak external quantum efficiency of the device at the price of a slight reduction and increase of the shunt and series resistance, respectively. Solar cells with 30 MQWs exhibit an external quantum efficiency of 38% at 380 nm, an open circuit voltage of 2.0 V, a short circuit current density of 0.23 mA/cm2 and a fill factor of 59% under 1 sun of AM1.5G-equivalent solar illumination. Solar cells with the grid spacing of the top p-contact varying from 100 to 200 µm present the same device performance in terms of spectral response and conversion efficiency.

Carrier localization in InN/InGaN multiple-quantum wells with high In-content

We study the carrier localization in InN/In0.9Ga0.1N multiple-quantum-wells (MQWs) and bulk InN by means of temperature-dependent photoluminescence and pump-probe measurements at 1.55 μm. The S-shaped thermal evolution of the emission energy of the InN film is attributed to carrier localization at structural defects with an average localization energy of ∼12 meV. Carrier localization is enhanced in the MQWs due to well/barrier thickness and ternary alloy composition fluctuations, leading to a localization energy above 35 meV and longer carrier relaxation time. As a result, the luminescence efficiency in the MQWs is improved by a factor of five over bulk InN.

Stranski-Krastanow growth of "112 ¯ 2…-oriented GaN/AlN quantum dots

Semipolar GaN(112¯2) deposited on AlN(112¯2) by plasma-assisted molecular-beam epitaxy can follow the Frank–Van der Merwe or the Stranski–Krastanow growth mode as a function of the Ga/N ratio. N-rich grown GaN relaxes elastically at a critical thickness but the resulting GaN islands present multiple crystallographic orientations. In contrast, after deposition of a few two-dimensional GaN monolayers under Ga-rich conditions, a growth interruption in vacuum induces (112¯2)-oriented islanding. Applying this latter procedure, we have synthesized GaN/AlN quantum dot superlattices with reduced internal electric field.

Dependence of the photovoltaic performance of pseudomorphic InGaN/GaN multiple-quantum-well solar cells on the active region thickness

We investigate the photovoltaic performance of pseudomorphic In0.1Ga0.9N/GaN multiple-quantum well (MQW) solar cells as a function of the total active region thickness. An increase in the number of wells from 5 to 40 improves the short-circuit current and the open-circuit voltage, resulting in a 10-fold enhancement of the overall conversion efficiency. Further increasing the number of wells leads to carrier collection losses due to an incomplete depletion of the active region. Capacitance-voltage measurements point to a hole diffusion length of 48 nm in the MQW region.

Waveguide saturable absorbers at 1.55 μm based on intraband transitions in GaN/AlN QDs

We report on the design, fabrication and optical characterization of GaN/AlN quantum-dot-based waveguides for all-optical switching via intraband absorption saturation at 1.55 µm. The transmittance of the TM-polarized light increases with the incident optical power due to the saturation of the s-p(z) intraband absorption in the QDs. Single-mode waveguides with a ridge width of 2 µm and a length of 1.5 mm display 10 dB absorption saturation of the TM-polarized light for an input pulse energy of 8 pJ and 150 fs.

Effect of the barrier thickness on the performance of multiple-quantum-well InGaN photovoltaic cells

The impact of the barrier thickness on the performance of In0.17Ga0.83N multiple-quantum-well (MQW) solar cells is studied. When the barrier thickness is reduced from 9.0 to 3.7 nm, the effect of the internal polarization fields on the MQW band structure results in a blueshift of the cell photoresponse. At the same time, the overlap of the fundamental electron and hole wave-functions in the quantum wells increases and the carrier extraction by field-assisted tunneling is enhanced, impacting the external quantum efficiency and fill-factor of the cells. The experimental results show that the performance of the thinner-barrier cells studied in this work is superior, or at least comparable to the performance of their thicker-barrier counterparts, in spite of the smaller total thickness of their absorbing region. This is due to their higher external quantum efficiency (37% at 370 nm) and improved fill-factor (62%), which result in a conversion efficiency of η = 0.82%.