Multi‐crystalline Si solar cells with very fast deposited (180 nm/min) passivating hot‐wire CVD silicon nitride as antireflection coating

Hot‐wire chemical vapor deposition (HWCVD) is a promising technique for very fast deposition of high quality thin films. We developed processing conditions for device‐ quality silicon nitride (a‐SiN_x:H) anti‐reflection coating (ARC) at high deposition rates of 3 nm/s. The HWCVD SiN_x layers were deposited on multicrystalline silicon (mc‐Si) solar cells provided by IMEC and ECN Solar Energy. Reference cells were provided with optimized parallel plate PECVD SiN_x and microwave PECVD SiN_x respectively. The application of HWCVD SiN_x on IMEC mc‐Si solar cells led to effective passivation, evidenced by a V_oc of 606 mV and consistent IQE curves. For further optimization, series were made with HW SiN_x (with different x) on mc‐Si solar cells from ECN Solar Energy. The best cell efficiencies were obtained for samples with a N/Si ratio of 1·2 and a high mass density of >2·9 g/cm³. The best solar cells reached an efficiency of 15·7%, which is similar to the best reference cell, made from neighboring wafers, with microwave PECVD SiN_x. The IQE measurements and high V_oc values for these cells with HW SiN_x demonstrate good bulk passivation. PC1D simulations confirm the excellent bulk‐ and surface‐passivation for HW SiN_x coatings. Interesting is the significantly higher blue response for the cells with HWCVD SiN_x when compared to the PECVD SiN_x reference cells. This difference in blue response is caused by lower light absorption of the HWCVD layers (compared to microwave CVD; ECN) and better surface passivation (compared to parallel plate PECVD; IMEC). The application of HW SiN_x as a passivating antireflection layer on mc‐Si solar cells leads to efficiencies comparable to those with optimized PECVD SiN_x coatings, although HWCVD is performed at a much higher deposition rate. Copyright © 2007 John Wiley & Sons, Ltd.     

Dielectric thin films for MEMS-based optical sensors

Nanoindentation and optical measurements have been employed in order to investigate the mechanical properties of low-temperature (50–330 °C) plasma-enhanced chemical vapour deposited (PECVD) SiN_x, as well as thermally evaporated SiO_x and Ge thin films for applications in micro-electro-mechanical systems (MEMS) fabricated on temperature sensitive, non-standard substrates. The temperature of the SiN_x deposition process is found to strongly influence Young’s modulus, hardness, and stress, with a critical deposition temperature in the 100 °C to 150 °C range which depends on the details of other deposition conditions such as chamber pressure and RF-power. The properties of PECVD SiN_x films deposited above this critical temperature are found to be suitable for MEMS applications, whereas films deposited at lower temperatures exhibit low Young’s modulus and hardness, as well as environment-induced stress instabilities. The investigated thin films have been incorporated into a monolithic integrated technology comprising low-temperature (125 °C) MEMS and HgCdTe IR detectors, in order to realize successful prototypes of tuneable IR microspectrometers.     

Front and rear silicon‐nitride‐passivated multicrystalline silicon solar cells with an efficiency of 18.1%

A solar cell process designed to utilise low‐temperature plasma‐enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) films as front and rear surface passivation was applied to fabricate multicrystalline silicon (mc‐Si) solar cells. Despite the simple photolithography‐free processing sequence, an independently confirmed efficiency of 18.1% (cell area 2 × 2 cm²) was achieved. This excellent efficiency can be predominantly attributed to the superior quality of the rear surface passivation scheme consisting of an SiN_x film in combination with a local aluminium back‐surface field (LBSF). Thus, it is demonstrated that low‐temperature PECVD SiN_x films are well suited to achieve excellent rear surface passivation on mc‐Si. Copyright © 2002 John Wiley & Sons, Ltd.     

Extremely low surface recombination velocities on low‐resistivity n‐type and p‐type crystalline silicon using dynamically deposited remote plasma silicon nitride films

Extremely low upper‐limit effective surface recombination velocities (S_eff.max) of 5.6 and 7.4 cm/s, respectively, are obtained on ~1.5 Ω cm n‐type and p‐type silicon wafers, using silicon nitride (SiN_x) films dynamically deposited in an industrial inline plasma‐enhanced chemical vapour deposition (PECVD) reactor. SiN_x films with optimised antireflective properties in air provide an excellent S_eff.max of 9.5 cm/s after high‐temperature (>800 °C) industrial firing. Such low S_eff.max values were previously only attainable for SiN_x films deposited statically in laboratory reactors or after optimised annealing; however, in our case, the SiN_x films were dynamically deposited onto large‐area c‐Si wafers using a fully industrial reactor and provide excellent surface passivation results both in the as‐deposited condition and after industrial‐firing, which is a widely used process in the photovoltaic industry. Contactless corona‐voltage measurements reveal that these SiN_x films contain a relatively high positive charge of (4–8) × 10¹² cm⁻² combined with a relatively low interface defect density of ~5 × 10¹¹ eV⁻¹ cm⁻². Copyright © 2012 John Wiley & Sons, Ltd.     

Thermopile Infrared Detector with Detectivity Greater Than 10<Superscript>8</Superscript> cmHz<Superscript>(1/2)</Superscript>/W

In this paper, the design, fabrication and experimental results of the thermopile infrared detector, with a single layer of low-stress SiN_x membrane, instead of thin sandwich layer membrane of SiO₂–Si₃N₄ are presented. Thermal isolation is achieved by using back etching of bulk silicon. Thermopiles are consisted of serially interconnected p-poly-Si/Al thermocouples supported by the single layer of SiN_x membrane with low stress. Au/Ti reflective coating was evaporated on the surface of cold junctions of the thermopile to block incident radiation. In the measurement, we find that infrared absorbance of SiN_x membrane to different wavelength is diverse and less than 100%, which has great influence on calculating the actual absorbing power of the detector, so the infrared (IR) transmission spectrum is measured to calibrate the actual infrared absorbing amount of the detector. The analysis result shows that only 43.72% infrared radiation is absorbed by the detector. Based on the measurement of IR transmission spectrum and output voltage of the detector, the response sensitivity of the detector is calculated as 31.65 V/W, detectivity of the detector is 1.16 × 10⁸ cmHz^(1/2)W⁻¹, and response time of the detector is 126 ms.     

Highly reliable hybrid nano-stratified moisture barrier for encapsulating flexible OLEDs

Herein, a novel thin-film encapsulation for flexible organic light-emitting diodes (FOLEDs) is proposed, and its long-term reliability in tensile stress conditions was tested. The hybrid nano-stratified moisture barrier consists of 2.5 dyads of an Al₂O₃/ZnO nano-stratified structure and a S-H nanocomposite organic layer. The nano-stratified structure is prepared by low-temperature atomic layer deposition and the S-H nanocomposite by spin-coating at a thickness of 30 and 120 nm, respectively. An optical transmittance of 89.05% was measured with the 2.5-dyad hybrid nano-stratified moisture barrier with a total thickness of 330 nm. A low water vapor transmission rate (WVTR) of 1.91 × 10⁻⁵ g/m²day was recorded based on an electrical Ca test at 30 °C and 90% R.H. without losing its properties after a bending test. With this highly reliable hybrid nano-stratified moisture barrier, FOLEDs were successfully encapsulated. After 30 days under conditions of 30 °C and 90% R.H. with tensile stress, the J-V-L performances of the FOLEDs were comparable to those of the initial state without dark spots. These results suggest that this hybrid nano-stratified moisture barrier is an excellent method for encapsulating FOLEDs.     

Ultra Thin Film Encapsulation of Organic Light Emitting Diode on a Plastic Substrate

We have carried out the fabrications of a barrier layer on a polyethersulfon (PES) film and organic light emitting diode (OLED) based on a plastic substrate by means of atomic layer deposition (ALD). Simultaneous deposition of 30 nm AlO_x film on both sides of the PES film gave a water vapor transition rate (WVTR) of 0.062 g/m²/day (@38°C, 100% R.H.). Further, the double layer of 200 nm SiNx film deposited by plasma enhanced chemical vapor deposition (PECVD) and 20 nm AlOx film by ALD resulted in a WVTR value lower than the detection limit of MOCON. We have investigated the OLED encapsulation performance of the double layer using the OLED structure of ITO / MTDATA (20 nm) / NPD (40 nm) / AlQ (60 nm) / LiF (1 nm) / Al (75 nm) on a plastic substrate. The preliminary life time to reach 91% of the initial luminance (1300 cd/m²) was 260 hours for the OLED encapsulated with 100 nm of PECVD‐deposited SiNx and 30 nm of ALD‐deposited AlO_x.     

Low-Temperature,Strong SiO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> Covalent Wafer Bonding for III–V Compound Semiconductors-to-Silicon Photonic Integrated Circuits

We report a low-temperature process for covalent bonding of thermal SiO₂ to plasma-enhanced chemical vapor deposited (PECVD) SiO₂ for Si-compound semiconductor integration. A record-thin interfacial oxide layer of 60 nm demonstrates sufficient capability for gas byproduct diffusion and absorption, leading to a high surface energy of 2.65 J/m² after a 2-h 300°C anneal. O₂ plasma treatment and surface chemistry optimization in dilute hydrofluoric (HF) solution and NH₄OH vapor efficiently suppress the small-size interfacial void density down to 2 voids/cm², dramatically increasing the wafer-bonded device yield. Bonding-induced strain, as determined by x-ray diffraction measurements, is negligible. The demonstration of a 50 mm InP epitaxial layer transferred to a silicon-on-insulator (SOI) substrate shows the promise of the method for wafer-scale applications.     

采用AlN/SiNx钝化的高性能AlGaN/GaN HEMTs

两种不同的钝化层结构被应用到势垒层厚度为12 nm的AlGa/GaN 高电子迁移率场效应晶体管中。首先采用等离子增强原子层沉积(PEALD)技术生长5 nm的AlN薄膜,然后再覆盖50 nm的等离子增强化学气相淀积(PECVD)生长的SiNx。相比于传统的SiNx钝化,AlN钝化层的插入更有效地抑制了电流崩塌效应,同时获得了小的亚阈值斜率(SS)。AlN钝化层的插入增大了器件的射频跨导从而获得了较高的截止频率。另外,通过变温直流特性测试发现,AlN/SiNx钝化的器件在高温时饱和电流和最大跨导的衰退相对于仅采用SiNx钝化的器件都要小,表明AlN钝化层的插入改善了器件的高温稳定性。     

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlO_x layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlO_x film growth and device longevity is presented. First, AlO_x film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7 nm by 500 cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100 °C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150 °C range. Ultra-thin AlO_x encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000 h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlO_x interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.     

Enhancement of the Stability of Ti and Ni Ohmic Contacts to 4H-SiC with a Stable Protective Coating for Harsh Environment Applications

We report on wafer-level measurements of the long-term stability of Ti and Ni ohmic contacts to n-4H-SiC during thermal treatments in air or air/moisture environments up to 500°C. Contact metallizations with and without a sputtered Ti (20 nm)/TaSi _x (200 nm)/Pt (150 nm) diffusion barrier stack and Ti (20 nm)/TiN (10 nm)/Pt (150 nm)/Ti (20 nm) interconnects were compared. A protective coating consisting of a SiO _x (250 nm)/SiN _y (250 nm) stack deposited by plasma-enhanced chemical vapor deposition (PECVD) was used. The stability of the contact metallizations during long-term thermal treatments in air and air/moisture was studied. The best performance was achieved with Ti ohmic contacts without the Ti/TaSi _x /Pt stack. This system successfully withstood 1000 h thermal treatment at 500°C in air followed by 1000 h at 500°C in air/10% moisture. After the aging, the contact failure ratio was below 1% and the specific contact resistivity amounted to (2.5 ± 1.1) × 10⁻⁴ Ω cm². Scanning electron microscopy (SEM) cross-sectional analysis indicated no degradation in the contact metallization, demonstrating the effectiveness of the SiO _x /SiN _y protective coating in preventing oxidation of the contacts. These results are very promising for applications in harsh environments, where the stability of ohmic contacts is crucial.     

Evaluation of encapsulation and passivation of InGaAs/InP DHBT devices for long-term reliability

Device encapsulation and passivation are critical for long-term reliability and stability. Several encapsulation techniques were evaluated in terms of degradation of electrical characteristics, gap filling under the mesa structures, and adhesion to the semiconductor and metal surfaces. These included plasma enhanced chemical vapor deposited (PECVD) SiO₂, electron cyclotron resonance CVD SiN_x, spin-on glass, benzocyclobutene, and polyimide. Damage from plasma exposure caused gain degradation in the devices. Spin-on coatings cause little to no gain degradation, provided that there is minimal stress in the cured film. SOG and BCB films have acceptable adhesion properties and were excellent for gap filling. Polyimide films have excellent adhesion properties, however, they were poor at gap filling and had a great deal of shrinkage during curing. Device passivation was evaluated using double heterojunction bipolar transistor structures with either an abrupt or graded emitter-base junction. Abrupt junction devices had the self-aligned base metal directly on the p⁺ InGaAs base. Graded junction devices had the base metal on top of graded InGaAsP layers, which the metal was diffused through, to make contact to the base region. Abrupt junction devices stressed at an initial J_E of 90 kA/cm² at a V_CE of 2V at 25°C degraded 20% within 70 h of operation, whereas, the graded junction devices show no degradation in dc characteristics after operation for over 500 h. Typical common emitter current gain was 50. An f_t of 80 and f_max of 155 GHz were achieved for 2×4 μm² emitter size devices.     

High barrier properties of transparent thin-film encapsulations for top emission organic light-emitting diodes

This paper reported a low-temperature thin film encapsulation (TFE) process based on atomic layer deposition Al₂O₃ layer for top-emission organic light-emitting devices (TE-OLEDs). The barrier characteristics of both H₂O-based and O₃-based Al₂O₃ films were investigated. O₃-based Al₂O₃ TFE showed lower water vapor transmission rate (WVTR) of 8.7 × 10⁻⁶ g/m² day and longer continuous operation lifetime of 5 folds compared to the device with H₂O-based Al₂O₃ TFE under identical environmental and driving conditions. Furthermore, the extraction of emitting light of the devices with barrier layer was enhanced compared to the bared one. The theory simulation data were consistent with our experimental results and showed the potential for the design of TFE structures optimized for enhancing light transmission.     

A flexible moisture barrier comprised of a SiO2-embedded organic–inorganic hybrid nanocomposite and Al2O3 for thin-film encapsulation of OLEDs

We demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded organic–inorganic hybrid (S–H) nanocomposite and Al₂O₃. The multi-barrier was prepared by low-temperature Al₂O₃ atomic layer deposition and with a spin-coated S–H nanocomposite. The moisture barrier properties were investigated with a water vapor transmission rate (WVTR), estimated by a Ca test at 30 °C, 90% R.H.. Moisture diffusion was effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed silica nanoparticles in the organic layer. A low WVTR of 1.14 × 10^?5 g/m² day and average transmittance of 85.8% in the visible region were obtained for the multi-barrier. After bending under tensile stress mode, the moisture barrier property of the multi-barriers was retained. The multi-barrier was successfully applied to thin-film encapsulation of OLEDs. The thin-film encapsulated OLEDs showed practicable current–voltage–luminance (I–V–L) characteristics and stable real operation over 700 h under ambient conditions.     

Investigation of optical and chemical bond properties of hydrogenated amorphous silicon nitride for optoelectronics applications

The aim of this work is to determine optimal deposition parameters of silicon nitride for optical applications. The authors present the investigation of hydrogenated amorphous silicon nitride SiN_x:H deposited by the low temperature PECVD method in high frequency reactors. The study of hydrogen bonds in the SiN_x:H thin films were detailed. The impact of NH₃, SiH₄ and N₂ flow ratio and radio frequency power on optical coefficients in relation to chemical composition and roughness of the film is studied. The correlation between chemical bonds (N–H, Si–H) and refractive index and extinction coefficients is systematically verified. The experimental results show that the films with high refractive indexes superior to 2.05 and low roughness of about 0.35 nm can be achieved for optoelectronics applications by tuning the flow ratio or decreasing the RF power. A variety of processes have been suggested as compatible with low thermal budget (under 350 °C) in order to integrate optical waveguides with lower loss. In particular, the incorporation of N₂ as dilution gas is suited to the fabrication of SiN_x:H films optical waveguide requiring low N–H bonds, low concentration of hydrogen [H] and high refractive index.     

All ink-jet printed low-voltage organic field-effect transistors on flexible substrate

In this work, all ink-jet printed (IJP) low-voltage organic field-effect transistors (OFETs) on flexible substrate are reported. The OFETs use IJP silver (Ag) for source/drain/gate electrodes, poly(4-vinylphenol) (PVP) for gate dielectric, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) blended with polystyrene (PS) as the semiconducting layer and CYTOP for encapsulation layer. All the printing processes were carried out in ambient air environment using a single laboratory ink-jet printer Dimatix DMP-2831. The all IJP device presents state-of-the-art performance with low operation voltage down to 3 V, small subthreshold swing (SS) of 0.155 V/decade, mobility of 0.26 cm² V⁻¹s⁻¹, threshold voltage (V_th) of −0.17 V and on/off ratio of 3.1 × 10⁵, along with a yield of 62.5%. Through interface engineering and proper process optimization, this work demonstrates a promising low-voltage all IJP device platform for low-cost flexible printed electronics.     

Effect of organic/inorganic coating on moisture diffusion in a chip-on-board package with globtop

The moisture diffusion in globtop material for a chip-on-board (COB) package coated with SiN_x and silicone, respectively, or coated with SiN_x plus silicone were measured by embedding a humidity sensor in the globtop and recording the capacitive change in three different temperature/humidity environments. The experimental results were simulated by Fick’s diffusion law with finite-element method modeling. The moisture diffusion coefficients and activation energies were calculated to quantitatively compare the moisture-resistance effects of different coatings. For example, at 85°C/85% RH, the moisture diffusion coefficients for the uncoated reference, SiN_x-coated, silicone-coated, and silicone/SiN_x double-layered coated samples are 1E-5, 0.8E-5, 0.7E-5, and 0.2E-5 mm²/s, respectively. The experimental and simulation results show that double-layered coating with silicone/SiN_x has excellent moisture-resistance properties because it not only smoothes the steps on a printed circuit board (PCB) but also keeps the good moisture resistance of the inorganic films.     

Electrical and Optical Characterization of AlGaN/GaN HEMTs with <Emphasis Type="Italic">In Situ</Emphasis> and <Emphasis Type="Italic">Ex Situ</Emphasis> Deposited SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Layers

A comparative study of AlGaN/GaN high-electron-mobility transistor (HEMT) surface passivation using ex situ and in situ deposited SiN _x is presented. Performing ex situ SiN _x passivation increased the reverse gate leakage and off-state channel leakage by about three orders of magnitude. The in situ SiN _x layer was characterized using transmission electron microscopy (TEM) and capacitance–voltage (CV) measurements. Photoluminescence (PL) spectra indicated a reduction of nonradiative recombination centers in in situ SiN _x -passivated samples, indicating improved crystal quality. CV measurements indicated a reduction of surface state density as well, and thus better overall passivation using in situ SiN _x . Electroluminescence (EL) images of the channel regions in AlGaN/GaN HEMT devices operating in forward blocking mode with up to 400 V drain bias demonstrated reduced channel emission profiles of in situ-passivated devices. Compared with a nonpassivated reference sample, the reduced EL emission profiles correlated with a reduced channel temperature on ex situ SiN _x -passivated devices.     

Al-enhanced PECVD SiNx induced hydrogen passivation in string ribbon silicon

The effectiveness of manufacturable gettering and passivation technologies is investigated for their ability to improve the quality of a promising Si photovoltaic material. The results of this study indicate that a lifetime enhancement of 30 μs is attained when a backside screen-printed aluminum layer and a thin film of SiN_x, applied by plasma-enhanced chemical vapor deposition (PECVD), are simultaneously annealed at 850°C in a lamp-heated belt furnace. Based on the results of this study, a model is proposed to describe the Al-enhanced SiN_x induced hydrogen defect passivation in String Ribbon silicon due to the simultaneous anneal. According to this model, three factors play an important role: i) the release of hydrogen from the SiN_x film into the substrate; ii) the retention of hydrogen at defect sites in silicon; and iii) the generation of vacancies at the Al−Si interface due to the alloying process which increases the incorporation of hydrogen and creates a chemical potential gradient which enhances the migration of hydrogen in the substrate. A PC1D device simulation indicates that screen-printed cell efficiencies approaching 16% can be achieved if the gettering and passivation treatments examined in this study are employed, the substrate thickness is reduced, and a high-quality surface passivation scheme is applied.     

Approach for Al2O3 rear surface passivation of industrial p‐type Si PERC above 19%

Atomic layer deposition (ALD) of thin Al₂O₃ (≤10 nm) films is used to improve the rear surface passivation of large‐area screen‐printed p‐type Si passivated emitter and rear cells (PERC). A blister‐free stack of Al₂O₃/SiO_x/SiN_x is developed, leading to an improved back reflection and a rear recombination current (J_0,rear) of 92 ± 6 fA/cm². The Al₂O₃/SiO_x/SiN_x stack is blister‐free if a 700°C anneal in N₂ is performed after the Al₂O₃ deposition and prior to the SiO_x/SiN_x capping. A clear relationship between blistering density and lower open‐circuit voltage (V_OC) due to increased rear contacting area is shown. In case of the blister‐free Al₂O₃/SiO_x/SiN_x rear surface passivation stack, an average cell efficiency of 19.0% is reached and independently confirmed by FhG‐ISE CalLab. Compared with SiO_x/SiN_x‐passivated PERC, there is an obvious gain in V_OC and short‐circuit current (J_SC) of 5 mV and 0.2 mA/cm², respectively, thanks to improved rear surface passivation and rear internal reflection. Copyright © 2012 John Wiley & Sons, Ltd.