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Understanding the impact of GaN surface treatment conditions on dielectric/GaN interface chemical properties is critically important for device performance. This point is under intensive research f...

International audience; The impact of several wet etchants commonly encountered in the microelectronic industry on the surface chemistry of GaN on silicon was explored. In order to get closer to fully recessed gate HEMT fabrication processes, we investigated different kinds of GaN surfaces. This study was conducted on as-grown GaN and dry etched GaN, with etching consisting of inductive coupled plasma reactive ion etching (ICP-RIE), followed by atomic layer etching (ALE) and O2 plasma stripping. The impact of each wet treatment was evaluated by parallel Angle Resolved X-ray Photoelectron Spectroscopy (pAR-XPS). Treatment with phosphoric acid (H3PO4) showed a significant modification of the surface and further studies were performed using this treatment. The impact of H3PO4 on GaN surface chemistry and morphology was assessed by pAR-XPS and atomic force microscopy (AFM) respectively. A delayed effect was observed for dry etched samples compared to as-grown samples, with a successful recovery of the surface after 60 minutes of treatment. We also proposed a mechanism explaining the progressive formation on steps on the surface over time. Further research was performed on dry etched samples without ALE which also modified the delay time of the H3PO4 treatment, but still enabled a recovery of the surface morphology. In contrast to other studies, we showed that, with the appropriate choice of parameters for the H3PO4 treatment, it was possible to successfully recover the GaN surface after dry etching without significantly opening dislocation holes. This is therefore a promising treatment to be used during GaN HEMT processing to recover good quality surfaces after etching.

International audience; A new protocol based on Y-function is used for accurate statistical extraction of electrical parameters of High Electron Mobility Transistor (HEMT) devices for GaN technology. This protocol presented here is used for extraction of relevant electrical parameters such as oxide capacitance, threshold voltage, effective mobilities and access resistance. This study has been verified over a large range of channel lengths for two normally-off HEMT GaN wafers having different levels of access resistances.

The fully recessed gate GaN-on-Si MOS-channel HEMTs (MOSc-HEMT) with Back-Barrier (BB) structure gives rise to unexpected threshold voltage (V TH ) behaviors such as V TH increase with decreasing gate length (L G ) (V TH roll-up) and discrepancies between V TH values extracted from I D (V G ) (V TH_IV ) and from C GC (V G ) (V TH_CV ) characteristics. Using TCAD simulations and experimental measurements, we demonstrate that conduction band confinement, especially at gate corners, is responsible for these peculiar V TH behaviors. This band confinement is strengthened by the fully recessed gate configuration coupled with the proximity of a back-barrier.

This paper aims to investigate the interface traps density (Dit) extraction on MOS gate stacks processed on GaN-on-Si substrates. CGV (Capacitance-Conductance) measurements under different frequencies (f = 1kHz-1MHz) and temperatures (T = 20K-500K) on various Al 2 O 3 /UID-GaN MOS capacitors were carried out. Thorough analysis under dark and UV light compared to TCAD/analytical modeling reveal a strong distributed series resistance under the gate related to the high resistivity of UID-GaN layer. This effect leads to an overestimation of the actual Dit value extracted at high frequencies (> 10kHz). Choosing an adequate doping under the gate (n-type) cancels the series resistance effect and unlocks a reliable extraction through {T/f} dependent CGV measurements.

In this study, we investigate the potential of III-V/Si solar cell technology for space applications. We present here the experimental results before and after 1 MeV electrons irradiation at NIRT and LILT conditions.

A new differential method for accurate extraction of electrical parameters of high-electron mobility transistor (HEMT) devices for gallium nitride (GaN) technology is proposed. This method, presented here for the first time, is used to study the mobility degradation with gate length, allowing an analysis of the contribution of the gate edge region to the total metal–insulator semiconductor (MIS)-HEMT device conductance. First, an analytical model is proposed to account for the relative conduction of the edges and the main flat part of the channel HEMTs. Second, the differential method allows the split- CV mobility extraction of each part, allowing a precise analysis of the resistive contribution of each region of the HEMT devices. This study has been performed over a large range of channel lengths for two normally OFF HEMT GaN wafers having different recess depths.

Concentrating PhotoVoltaics (CPV) can increase the efficiency of power systems in space while reducing their cost. This paper presents the first concentrator prototype using III-V/Si cells for space applications. This new approach will significantly reduce cost by concentrating sunlight then decreasing cell material, and in particular using III-V/Si triple-junction (3J) cells will allow the usage of Silicon, which is an abundant, twice less dense and low-cost material than the typically used Germanium. The efficiency of III-V/Si solar cells used is 25.7% @AM0. In our refractive module design, Compound Parabolic Concentrator (CPC), twelve 1.5x13mm2 cells are embedded in a Dow Corning® 93-500 silicone optics that provides an ∼8x concentration factor with an optical efficiency of 68%. Additionally, this linear-focus lens enables the use of single-axis sun tracking.

In this study, we review the potential of III-V/Si solar cell technology for space applications. We present here the experimental results of wafer bonded 2-terminal III-V/Si solar cells electron irradiation. The Begin-Of-Life (BOL) and End-Of-Life (EOL) electrical performances, after 1-MeV electron irradiations, are characterized under AM0 spectrum, and the radiation hardness is compared in Normal Irradiance Room Temperature (NIRT) and Low Irradiance Low Temperature (LILT) conditions.

Monolithic two‐terminal III‐V on Si dual‐junction solar cells, designed for low concentration applications, were fabricated by means of surface‐activated direct wafer bonding. The III‐V top cell is a heterojunction formed by an n‐Ga₀.₅In₀.₅P emitter and a p‐Al₀.₂Ga₀.₈As base. An efficiency of 21.1 ± 1.5% at one sun and 23.7 ± 1.7% at 10 suns is demonstrated, which to our knowledge is the best dual‐junction two‐terminal III‐V on Si tandem cell efficiency reported to date under verified reference conditions. The I‐V characterization of these 1‐cm² tandem cells under concentration required the development of a new method using a single‐source multiflash solar simulator and not perfectly matched component cells, also known as pseudo‐isotypes, formed by Si single‐junction cells and optical filters. In addition, the spectrum of the pulsed solar simulator was measured using a high‐speed CMOS spectrometer, allowing the calculation of the spectral mismatch correction factor. Merging these two techniques results in the hybrid corrected pseudo‐isotype (HCPI) characterization method, which shows a fast and accurate performance with a simplified procedure based on a single‐source solar simulator. Pseudo‐isotypes are easily adaptable to new cell designs by simply using a different filter, hence allowing the characterization of new multijunction solar cell architectures.

— In this study, we evaluate the potential of III-V//Si solar cell technology for space applications. We present experimental results on wafer bonded 2-terminal III-V//Si solar cells degradation under 1 MeV electrons irradiation. Beginning-Of-Life (BOL) and End Of Life (EOL) electrical performances were measured under Normal Irradiance (100% AM0) and Room Temperature (300K) conditions (NIRT). No degradation of the wafer bonding interface was detected. The impact of electron irradiation on effective diffusion length in the silicon bottom cell was calculated using the Internal Quantum Efficiency model for different solar cells architectures. Improvement paths towards higher EOL III-V//Si solar cells are discussed.

International audience; Hybrid III-V/Si multi-junction solar cells, which combine efficient III-V top cells with a silicon bottom cell, the most wide-spread photovoltaic material, offer great opportunities. Recent advances in this field have shown that mechanically stacked or wafer bonded III-V/Si can reach similar AM1.5g efficiencies than the standard GaInP/GaAs/Ge space triplejunction solar cells. In this study, we review the potential of this new hybrid III-V/Si cell technology for space applications. The first experimental results of wafer bonded 2-terminal GaInP/GaAs/Si solar cell electron irradiation will be presented. The BOL and EOL (1 MeV electrons) performances will be characterized under AM0 spectrum, and the radiation tolerance will be compared with single-junction silicon and standard Ge based triple-junction.

Cu2ZnSnSe4 solar cell absorbers are synthesized by large-area electrodeposition of metal stack precursors followed by selenization. A champion solar cell exhibits 8.2% power conversion efficiency, a new record for Cu2ZnSnSe4 solar cells prepared from electrodeposited metallic precursors. Significant improvements of device performance are achieved by the application of two etching procedures and buffer layer optimization. These results validate electrodeposition as a credible alternative to vacuum processes (sputtering, co-evaporation) for earth-abundant thin-film solar cell fabrication at low cost. Copyright (C) 2015 John Wiley & Sons, Ltd.

In this study, fabrication of monolithic triple junction (3J) GaInP/AlGaAs/Si solar cells is reported by different wafer bonding approaches: (i) the most straightforward, direct GaAs/Si wafer bonding; (ii) an innovative 2-step approach combining epitaxy of GaAs bonding layer on Si cell followed by GaAs/GaAs direct wafer bonding; and (iii) Surface-Activated GaAs/Si wafer Bonding (SAB). Bonding interfaces properties are comparatively studied, and the performance of resulting III-V on Si triple junction devices is examined. Open-circuit voltages close to 2.9V are obtained in all cases, but large differences in fill factors between the bonding methods are observed, probably due to the presence of an oxide layer.

Different GaAs//Si wafer bonding approaches are compared in order to fabricate III-V tandem solar cells. Bonding interface characteristics and electrical properties will be discussed for standard and improved direct wafer bonding sequences as well as for covalent bonding technique using SAB.

This paper aims at providing an updated overview of the main achievements in the development of solar cells based on Cu2ZnSn(S,Se)4 (CZTS(Se)) kesterite absorbers obtained by electrodeposition. Although undoubtedly challenging, the ultimate goal is to learn from the past works and build a solid framework for future advances in this field. What is the reason for the lower efficiency of electrodeposited CZTS(Se)-based devices (8%) compared to the world record efficiency achieved with a hydrazine-based solution approach (12.6%)? Can this gap be filled, or there are intrinsic limitations for this achievement? The review is divided into the three main electrodeposition approaches: sequential elemental layer, alloy co-deposition, and chalcogenide co-deposition. It is argued that considerable technical challenges must be overcome for the latter approach to be successfully applied. Plot of the record power conversion efficiencies of kesterite sulfide-based solar cells obtained by electrodeposition (hollow dots), and world record efficiency of CZTS(Se)-based devices (full dots). The dashed line shows the 15% minimum efficiency threshold considered relevant for potential industrial application.

Electrodeposition followed by rapid thermal annealing is a favorable process for industrial solar thin film fabrication. In this regard, we attempt to develop a process for earth-abundant pure sulfide kesterite solar cells fabrication of Cu2ZnSnS4 (CZTS) at pre-industrial scale (15 × 15 cm2). Synthesis at a large scale is a challenging issue that we attempt to address discussing the choice of annealing parameters. We found out that in our system a low background pressure is needed to ensure a good vapors distribution. However, S, SnS, Zn volatilities are enhanced, making difficult the control of the composition. Annealing temperature profile has a strong influence on the final absorber composition. The introduction of an intermediate reactive pre-alloying step previously to the high temperature annealing is shown to help to obtain more compact absorbers, leading to a maximum power conversion efficiency of 2.4% for a 0.435 cm2 CZTS device.

Cu2ZnSnSe4 solar cells were synthesized by electrodeposition of metal stack precursors followed by selenization, a high potential process for industry, leading to conversion efficiencies above 5%. An additional selenium-capping layer deposited on the precursor before annealing showed improved uniformity and morphology of CZTSe layers compared to other selenization routes. Two different atmospheric annealing systems were used: a closed graphite box in a tubular furnace and a three-chamber dynamic rapid thermal processing furnace. The RTP system gave larger grains and more compact layers, whereas CZTSe selenized in tube furnace had smaller grains and a higher series resistance. Both annealing systems gave best cells power conversion efficiencies over 5%. We will discuss the device photoelectric properties and their relation to material structures and processing.

Strong luminescent coupling (LC) effect reduces current mismatch in III-V-based multijunction solar cells and consequently improves their power conversion efficiency. The LC current production in the Si bottom cell of InGaP/AlGaAs//Si triple junction solar cells having different sizes was investigated to determine the cell area dependence of LC effect. This was probed through laser beam-induced current (LBIC) mapping. The areal mapping was then assessed by obtaining the absolute difference between the quantum efficiencies calculated from the fitted LBIC map and simulated LC current considering uniform distribution using a quasi-two-dimensional electro-optical prediction model. At 9.17 suns concentration difference between the AlGaAs middle cell and the current-limiting Si bottom cell, the absolute LC quantum efficiency differences ranged between 0.16% and 1.68% in various cell sizes, which indicate potential current production increase if the LC current is made uniform.

Strong luminescent coupling (LC) effect in a multijunction solar cell (MJSC) allows better current balance among its subcells. The temperature dependence of wafer-bonded III-V on silicon (Si) MJSCs with LC effect was investigated. Experimentally, this was explored through light current density–voltage (J – V) characteristic measurements, external quantum efficiency measurements, and laser beam-induced current mapping of InGaP/AlGaAs//Si triple junction solar cells at different cell temperatures. Measurement results were analyzed using a quasi-two-dimensional electro-optical prediction model for LC current production and bandgap temperature dependence models of various semiconductors. It was revealed that at lower temperatures, LC current production in the limiting Si bottom cell resulted in a larger absolute LC quantum efficiency difference. At 15°C, the absolute LC quantum efficiency difference calculated was 0.17%, which indicates potential current production improvement if vertical LC current is made uniform.

session poster (I43); International audience; Monolithic two-terminal III-V on Si dual-junction (2J) solar cells were fabricated by means of Surface-Activated direct wafer Bonding (SAB). Al 0.2 Ga 0.8 As single-junction cells are grown on GaAs substrate by Metal-Organic Vapor Phase Epitaxy (MOVPE) and bonded at room temperature to independently fabricated Si solar cells. The n+-GaAs//n+-Si bonding interface is characterized by Transmission Electron Microscopy (TEM) revealing a 2-3 nm thick amorphous interlayer. The performance of the 1 cm 2 tandem cells, designed for low concentration applications, was studied by External Quantum Efficiency (EQE) and J-V measurements showing a power conversion efficiency of 17% under one-sun AM1.5G spectrum. To our knowledge, this is the highest efficiency ever reported for a wafer-bonded 2J III-V on Si solar cell. Limitations to performance have been identified and therefore higher efficiencies are expected.

Photovoltaic Technical Conference, Aix en Provence, FRANCE, 2015; International audience; Earth-abundant kesterite Cu2ZnSnSe4 material is a promising candidate for the mass production of low-cost thin film solar cells. However, the synthesis of single-phase kesterite films is especially challenging, since the kesterite single-phase region in the equilibrium phase diagram is very narrow. In this study, the metal composition was varied within the Cu-poor composition range in order to study the presence of Sn-Se secondary phases. Both SnSe and SnSe2 are found in copper-poor CZTSe absorbers where Zn/Sn1 because the studied composition range is actually copper-poor zinc-rich and tin-rich. The Sn-Se secondary phases can be detected using X-ray diffraction, a bulk detection method. They are also detected at the absorber's surface by SEM and Raman spectroscopy. Therefore, when the Sn-Se phases are present, at least a part of them is located at the absorber's surface, which is highly detrimental to device performance. Acting as shunting paths, they reduce the device power conversion efficiency and demonstrate an apparent quantum efficiency effect under reverse bias. Removal of these phases from the surface by chemical etching greatly reduces their detrimental influence. Published by AIP Publishing.