Development of AlInN photoconductors deposited by sputtering

In this work, we have developed photoconductor devices based on Al0.39In0.61N layers grown on sapphire by reac-tive radio-frequency magnetron sputtering. The fabricat-ed devices show a sublinear dependence of the photocur-rent as a function of the incident optical power. The above-the-band-gap responsivity reaches 7 W/A for an ir-radiance of 10 W/m2 (405 nm wavelength). The response decreases smoothly for below-the-bandgap excitation, dropping by more than an order of magnitude at 633 nm. The devices present persistent photoconductivity effects associated to carrier trapping at grain boundaries.Ministerio de Economía y CompetitividadComunidad de MadridUniversidad de Alcal

AlxIn1−xN on Si (100) Solar Cells (x = 0–0.56) Deposited by RF Sputtering

We investigate the photovoltaic performance of solar cells based on n-AlxIn1−xN (x = 0–0.56) on p-Si (100) hetero-junctions deposited by radio frequency sputtering. The AlxIn1−xN layers own an optical bandgap absorption edge tuneable from 1.73 eV to 2.56 eV within the Al content range. This increase of Al content results in more resistive layers (≈10−4–1 Ω·cm) while the residual carrier concentration drops from ~1021 to ~1019 cm−3 . As a result, the top n-contact resistance varies from ≈10−1 to 1 MΩ for InN to Al0.56In0.44N-based devices, respectively. Best results are obtained for devices with 28% Al that exhibit a broad external quantum efficiency covering the full solar spectrum with a maximum of 80% at 750 nm, an open-circuit voltage of 0.39 V, a short-circuit current density of 17.1 mA/cm2 and a conversion efficiency of 2.12% under air mass 1.5 global (AM1.5G) illumination (1 sun), rendering them promising for novel low-cost III-nitride on Si photovoltaic devices. For Al contents above 28%, the electrical performance of the structures lessens due to the high top-contact resistivityThis research was funded by the national projects from the Ministry of Research and Innovation TEC2017-84378-R and NERA (RTI2018-101037-B-I00); the projects from the Comunidad de Madrid SINFOTON2-CM (P2018/NMT-4326), MADRID-PV2 (P-2018/EMT-4308) and SOLA (CM/JIN/2019-013); the projects from the University of Alcalá ANIS (CCG2018/EXP-042) and PISA (CCG19/IA-005); and by the FEDER program. R. Blasco acknowledges the financial support of his contract associated with the Ramon y Cajal Fellowship RYC-2013-1408

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 quality is 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 analysis of the evolution of the crystalline structure with the growth thickness points 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

Two-step method for the deposition of AlN by radio frequency sputtering

This paper presents a detailed study of the influence of deposition conditions on structural and morphological properties of AlN thin films synthesized on c-sapphire substrates by radio frequency (RF) reactive sputtering. After the optimization of deposition parameters such as RF power and substrate temperature, the substrate bias has been identified as a critical variable to improve the structural properties of the AlN layers. The use of negative bias leads to a decrease of the full-width at half-maximum (FWHM) of the rocking curve of the AlN( 0002) x-ray reflection and an increase of the grain size. However, 2θ/ω x-ray scans of layers grown under negative bias reveal lattice disorder at the AlN/sapphire interface, which is attributed to the highly accelerated positive ions (Al+, N+, N2 +) arriving to the substrate at the initial stages of the deposition process. In order to prevent this interface degradation, we propose a twostep deposition methodwhich consists of starting the growth with an unbiased AlN buffer layer, at least 30 nm thick, followed by AlN deposition under negative bias. This procedure results in high-quality AlN layers with FWHM of the rocking curve of the (0002) reflection of 1.63°, grain size of ~40 nm and root-mean-square surface roughness of 0.4 nm

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 lm. 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.European CommissionMinisterio de Ciencia e InnovaciónComunidad de Madri

Comparison of the material quality of AlxIn1-xN (x ~ 0-0.50) films deposited on Si (100) and (111) by reactive RF sputtering

Here, we compare the material quality of AlxIn1-xN layers deposited on Si with different crystallographic orientations, (100) and (111), via radio-frequency (RF) sputtering. To modulate their Al content, the Al RF power was varied from 0 to 225 W, whereas the In RF power and deposition temperature were fixed at 30 W and 300oC, respectively. X-ray diffraction measurements reveal a c-axis-oriented wurtzite structure with no phase separation regardless of the Al content (x = 0-0.50), which increases with the Al power supply. The surface morphology of the AlxIn1-xN layers improves with increasing Al content and it is similar for samples grown on both Si substrates (the root-mean-square roughness decreases from 12 nm to 2.5 nm). Furthermore, from TEM images we notice a similar grain-like columnar morphology and defect density on samples deposited on both Si substrates under the same conditions. Simultaneously grown AlxIn1-xN-on-sapphire samples point to a residual n-type carrier concentration in the 1020-1021 cm-3 range. The optical band gap energy of these layers evolves from 1.75 eV to 2.56 eV with increasing Al content, consistent with the blue shift of their low-temperature photoluminescence. In general, the material quality of the AlxIn1-xN films on Si is similar for both crystallographic orientations. Nonetheless, samples deposited on sapphire show an improved structural and morphological characteristic likely due to the lower difference in lattice constants between the nitride and the sapphire substrate

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 lm. 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.European CommissionMinisterio de Ciencia e InnovaciónComunidad de Madri

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

International audienceWe 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. InGaN alloys are considered as promising candidates for high-efficiency photovoltaic devices [1-4] since their band gap spans almost the whole solar spectrum from 0.7 eV (InN) to 3.4 eV (GaN). This makes theoretically possible the development of all-InGaN multijunction solar cells with a freely customizable number of junctions to enhance the overall efficiency. However, the large lattice mismatch between GaN and InN has led several groups to study the possibility of hybrid integration, combining an InGaN cell in a tandem device with silicon [5,6] or other non-III-nitride [7] photovoltaic cells. The difficulty of growing high-quality InGaN layers increases with the In content. Reports of InGaN-based junctions with an In mole fraction exceeding 0.3 are rare [1]; the best external quantum efficiencies (EQEs) exceeding 0.7 are obtained at around 400 nm and quickly drop for longer wavelengths [8-10]. The main challenges are the large dislocation density and In-clustering, caused by the strong tendency to phase separation during growth. Absorbing layers in the form of a multiple quantum well (MQW) structure are often used to delay strain relaxation. Furthermore, the quantum confined Stark effect (QCSE) associated to the strong piezoelectric fields in the InGaN/GaN system [11] offers the possibility to tune the effective band gap of the structure by adjusting the quantum well (QW) and barrier thickness (tQW and tB, respectively). The effect of tuning tB in InGaN/GaN MQW photovoltaic devices has been studied by Wierer et al. [12] and Watanabe et al. [13]. According to their results, the absorption cutoff of the solar cells redshifts with decreasing tB. However, this does not always translate in enhanced overall cell efficiency, since the short circuit current density (Jsc) and open circuit voltage (Voc) also depend on tB. In this paper, we focus on the influence of the QW thickness on the effective band gap of the junction and its impact on the overall cell efficiency. We experimentally demonstrate that the band-to-band transition in InGaN QWs can be significantly redshifted in larger QWs. However, this redshift appears linked to a dramatic enlargement of the Stokes shift, so that increasing the tQW above a few nm is n