Deposition of silicon nitride thin films by hot-wire CVD at 100 °C and 250 °C

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 ?/s) and at low substrate temperature. Films were deposited using NH₃/SiH₄ flow rate ratios between 1 and 70 and substrate temperatures of 100 °C and 250 °C. For NH₃/SiH₄ ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56-2.74 g/cm³) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 ?/s and < 0.5 ?/s were obtained for films deposited at 100 °C and 250 °C, respectively. Films deposited at both substrate temperatures showed electrical conductivity ~ 10^− 14 Ω^− 1 cm^− 1 and breakdown fields > 10 MV cm^− 1.     

Amorphous silicon thin film solar cells deposited entirely by hot-wire chemical vapour deposition at low temperature (< 150 °C)

Amorphous silicon n-i-p solar cells have been fabricated entirely by Hot-Wire Chemical Vapour Deposition (HW-CVD) at low process temperature < 150 °C. A textured-Ag/ZnO back reflector deposited on Corning 1737F by rf magnetron sputtering was used as the substrate. Doped layers with very good conductivity and a very less defective intrinsic a-Si:H layer were used for the cell fabrication. A double n-layer (µc-Si:H/a-Si:H) and µc-Si:H p-layer were used for the cell. In this paper, we report the characterization of these layers and the integration of these layers in a solar cell fabricated at low temperature. An initial efficiency of 4.62% has been achieved for the n-i-p cell deposited at temperatures below 150 °C over glass/Ag/ZnO textured back reflector.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Low temperature (< 100 °C) fabrication of thin film silicon solar cells by HWCVD

Amorphous silicon films have been made by HWCVD at a very low substrate temperature of ≤ 100 °C (in a dynamic substrate heating mode) without artificial substrate cooling, through a substantial increase of the filament-substrate distance (∼ 80 mm) and using one straight tantalum filament. The material is made at a reasonable deposition rate of 0.11 nm/s. Optimized films made this way have device quality, as confirmed by the photosensitivity of > 10⁵. Furthermore, they possess a low structural disorder, manifested by the small Γ/2 value (half width at half maximum) of the transverse optic (TO) Si-Si vibration peak (at 480 cm^− 1) in the Raman spectrum of ∼ 30.4 cm^− 1, which translates into a bond angle variation of only ∼ 6.4°. The evidence gathered from the studies on the structure of the HWCVD grown film by three different techniques, Raman spectroscopy, spectroscopic ellipsometry and transmission electron microscopy, indicate that we have been able to make a photosensitive material with a structural disorder that is smaller than that expected at such a low deposition temperature.Tested in a p-i-n solar cell on Asahi SnO₂:F coated glass (without ZnO at the back reflector), this i-layer gave an efficiency of 3.4%. To our knowledge, this is the first report of a HWCVD thin film silicon solar cell made at such a low temperature.     

Recent advances in hot-wire CVD R&D at NREL: From 18% silicon heterojunction cells to silicon epitaxy at glass-compatible temperatures

Our research aiming to improve silicon photovoltaic materials and devices extensively utilizes hot-wire chemical vapor deposition (HWCVD). We have recently achieved 18.2% heterojunction silicon solar cells by applying HWCVD a-Si:H front and back contacts to textured p-type silicon wafers. This is the best reported p-wafer heterojunction solar cell by any technique. We have also dramatically improved the quality of HWCVD silicon epitaxy and recently achieved 11 μm of epitaxial growth at a rate of 110 nm/min.     

Fabrication and characteristics of n-Si/c-Si/p-Si heterojunction solar cells using hot-wire CVD

A double-side (bifacial) heterojunction (HJ) Si solar cell was fabricated using hot-wire chemical vapor deposition. The properties of n-type, intrinsic and p-type Si films were investigated. In these devices, the doped microcrystalline Si layers (n-type Si for emitter and p-type Si for back contact) are combined with and without a thin intrinsic amorphous Si buffer layer. The maximum temperature during the whole fabrication process was kept below 150 °C. The influence of hydrogen pre-treatment and n-Si emitter thickness on performance of solar cells have been studied. The best bifacial Si HJ solar cell (1 cm² sample) with an intrinsic layer yielded an active area conversion efficiency of 16.4% with an open circuit voltage of 0.645 V, short circuit current of 34.8 mA/cm² and fill factor of 0.73.     

Phosphorous and boron doping of nc-Si:H thin films deposited on plastic substrates at 150 °C by Hot-Wire Chemical Vapor Deposition

Gas-phase phosphorous and boron doping of hydrogenated nanocrystalline thin films deposited by HWCVD at a substrate temperature of 150 °C on flexible-plastic (polyethylene naphthalate, polyimide) and rigid-glass substrates is reported. The influence of the substrate, hydrogen dilution, dopant concentration and film thickness on the structural and electrical properties of the films was investigated. The dark conductivity of B- and P-doped films (σ_d = 2.8 S/cm and 4.7 S/cm, respectively) deposited on plastic was found to be somewhat higher than that found in similar films deposited on glass. n- and p-type films with thickness below ∼ 50 nm have values of crystalline fraction, activation energy and dark conductivity typical of doped hydrogenated amorphous silicon. This effect is observed both on glass and on plastic substrates.     

Physical aspects of a-Si:H/c-Si hetero-junction solar cells

We report on the basic properties of amorphous/crystalline hetero-junctions (a-Si:H/c-Si), their effects on the recombination of excess carriers and its influence on the a-Si:H/c-Si hetero-junction solar cells. For that purpose we measured the gap state density distribution of thin a-Si:H layers and determined its dependence on deposition temperature and doping by an improved version of near-UV-photoelectron spectroscopy. Furthermore, the Fermi level position in the a-Si:H and the valence band offset were directly measured. In combination with interface sensitive methods such as surface photovoltage analysis and our numerical simulation program AFORS-HET, we found an optimum in wafer pretreatment, doping and deposition temperature for efficient a-Si:H/c-Si solar cells without an i-type a-Si:H buffer layer. We reached at maximum 19.8% certified efficiency by a deposition at 210 °C with an emitter doping of 2000 ppm of B₂H₆ on a well cleaned pyramidally structured c-Si(n) wafer.     

Amorphous/crystalline silicon heterojunction solar cells with varying i-layer thickness

We study the effect on various properties of varying the intrinsic layer (i-layer) thickness of amorphous/crystalline silicon heterojunction (SHJ) solar cells. Double-side monocrystalline silicon (c-Si) heterojunction solar cells are made using hot-wire chemical vapor deposition on high-lifetime n-type Czochralski wafers. We fabricate a series of SHJ solar cells with the amorphous silicon (a-Si:H) i-layer thickness at the front emitter varying from 3.2 nm (0.8xi) to ~ 96 nm (24xi). Our optimized i-layer thickness is about 4 nm (1xi). Our reference cell (1xi) performance has an efficiency of 17.1% with open-circuit voltage (V_oc) of 684 mV, fill factor (FF) of 76%, and short-circuit current density (J_sc) of 33.1 mA/cm². With an increase of i-layer thickness, V_oc changes little, whereas the FF falls significantly after 12 nm (3xi) of i-layer. Transient capacitance measurements are used to probe the effect of the potential barrier at the n-type c-Si/a-Si interface on minority-carrier collection. We show that hole transport through the i-layer is field-driven transport rather than tunneling.     

Hot-wire deposition of a-Si:H thin films on wafer substrates studied by real-time spectroscopic ellipsometry and infrared spectroscopy

Film growth of hydrogenated amorphous silicon (a-Si:H) by hot-wire chemical vapor deposition was studied simultaneously and in real-time by spectroscopic ellipsometry and attenuated total reflection infrared spectroscopy. The a-Si:H films were deposited on native oxide-covered GaAs(100) and Si(100) substrates at temperatures ranging from 70 to 350 °C. A temperature dependent initial growth phase is revealed by the evolution of the surface roughness and the surface and bulk SiH_x absorption peaks. It is discussed that the films show a distinct nucleation behavior by the formation of islands on the surface that subsequently coalesce followed by bulk a-Si:H growth. Insight into a temperature-activated smoothening mechanism and the creation of a hydrogen-rich interface layer is presented.     

Determination of band offsets in a-Si:H/c-Si heterojunctions from capacitance-voltage measurements: Capabilities and limits

The capabilities and limitations of the well-known C-V technique for the determination of the conduction band offsets in (n)a-Si:H/(p)c-Si heterojunctions are presented. In particular, the effects due to the presence of an inversion layer in c-Si and a non-negligible defect density at the a-Si:H/c-Si interface on the reliability of the C-V intercept method are discussed. The influence of the Fermi level positions in (p)c-Si and (n)a-Si:H on the inversion layer formation and the influence of the interface defect density have been analysed using numerical simulations and experimental measurements.     

Studying early time HWCVD growth of a-Si:H by real time spectroscopic ellipsometry

We have applied real time spectroscopic ellipsometry and secondary ion mass spectrometry to study the growth of amorphous silicon by hot-wire chemical vapor deposition. Differences in temperature and hydrogen content affect the optical properties of the film. These effects provide valuable insight into the growth process. We have compared a-Si:H films grown at two different temperatures to better understand these effects. Our studies reveal the presence of a distinct 100–200-thick layer at the top of the growing film. The properties of this layer are primarily determined by the ambient conditions in the growth chamber and appear relatively independent of substrate temperature. In contrast, the properties of the bulk of the film are strongly influenced by substrate temperature. These results imply that differences in film properties associated with substrate temperature are the result of subsurface reconstruction and diffusion processes.     

Doping of high-quality epitaxial silicon grown by hot-wire chemical vapor deposition near 700 °C

We demonstrate that epitaxial layers with a wide range of controllable dopant densities (7 × 10¹⁵-3 × 10¹⁸/cm³ and 10¹⁷-10¹⁸/cm³ for n-type and p-type, respectively) can be grown on wafer substrates at 700 ± 25 °C by hot-wire chemical vapor deposition. Phosphorus from PH₃ is incorporated into the film more efficiently than silicon from SiH₄, leading to efficient doping. Comparison of Hall carrier concentrations to secondary ion mass spectrometry atomic dopant concentration shows that all incorporated dopants are electrically active. The Hall measurements also reveal that the electron mobility in the P-doped films is close to the impurity-scattering limit for crystal Si wafers at room temperature, indicating that our deposited epitaxial materials are high quality.     

Deposition of device quality silicon nitride with ultra high deposition rate (> 7 nm/s) using hot-wire CVD

The application of hot-wire (HW) CVD deposited silicon nitride (SiN_x) as passivating anti-reflection coating on multicrystalline silicon (mc-Si) solar cells is investigated. The highest efficiency reached is 15.7% for SiN_x layers with an N/Si ratio of 1.20 and a high mass density of 2.9 g/cm³. These cell efficiencies are comparable to the reference cells with optimized plasma enhanced (PE) CVD SiN_x even though a very high deposition rate of 3 nm/s is used. Layer characterization showed that the N/Si ratio in the layers determines the structure of the deposited films. And since the volume concentration of Si-atoms in the deposited films is found to be independent of the N/Si ratio the structure of the films is determined by the quantity of incorporated nitrogen. It is found that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high 7 nm/s for device quality SiN_x films, much faster than commercial deposition techniques offer [S. von Aichberger, Photon Int. 3 (2004) 40].     

Amorphous Si1−xCx:H films prepared by hot-wire CVD using SiH3CH3 and SiH4 mixture gas and its application to window layer for silicon thin film solar cells

B-doped hydrogenated amorphous silicon carbon (a-Si_1−xC_x:H) films have been prepared by hot-wire CVD (HWCVD) using SiH₃CH₃ as the carbon source gas. The optical bandgap energy and dark conductivity of the film are about 1.94 eV and 2 × 10^− 9 S/cm, respectively. Using this film as a window layer, we have demonstrated the fabrication of solar cells having a structure of the textured SnO₂(Asahi-U)/a-Si_1−xC_x:H(p)/a-Si_1−xC_x:H(buffer)/a-Si:H(i)/μc-Si:H(n)/Al. The conversion efficiency of the cell is found to be 7.0%.     

Recent contributions of the Kaiserslautern research group to thin silicon solar cell R&D applying the HW(Cat)CVD

This article reviews the results obtained in Kaiserslautern for research and development on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon based thin film solar cells as well as heterojunction solar cells applying entirely or mainly the HWCVD. The activities of the group cover the development of appropriate intrinsic and doped a-Si:H and μc-Si:H films for the different solar cell structures, the realization of many types of such structures with different deposition sequences and the detailed study of their stability behavior. Also the preparation of an HW solar cell on medium size area is demonstrated. Initial and stabilized conversion efficiencies are presented and discussed for the different cell structures realized within about ten years of activity. Main focus will be on the recent activities dealing with the integration of μc-Si:H films into solar cell structures and the extensive study of their stability behavior. In addition the degradation of the applied Ta catalyzer was intensively investigated. Finally advantages and disadvantages will be discussed concerning the commercial use of the HWCVD for solar cell fabrication.     

Revisiting the B-factor variation in a-SiC:H deposited by HWCVD

In order to understand material properties in a better way, it is always desirable to come up with new variables that might be related to the film properties. The B-parameter is such a variable, which relates to the quality of a-SiC:H films both in terms of electronic and optical properties. B (scaling factor) is essentially the slope of the straight-line part of the (E)^1/2–E (Tauc plot). Due to dependence on a large number of parameters and no detailed research, many previous authors have surmised that B has an ambiguous correlation with carbon content. We have made an attempt to establish the relation between the B-parameter as a quality-indicating factor of a-SiC:H films in both carbon- and silicon-rich material. For this we studied a-SiC:H films deposited by the HWCVD method with broad deposition parameters of substrate temperature (T_s), filament temperature (T_F) and C₂H₂ fraction. Our results indicate that the B-parameter varies considerably with process conditions such as T_F, total gas pressure and carbon content. An attempt is made to correlate the B-parameter with an opto-electronic parameter, such as the mobility edge, which has relevance to the device-quality aspects of a-SiC:H films prepared by HWCVD.     

Growth kinetics of nc-Si:H deposited at 200 °C by hot-wire chemical vapour deposition

We report on the growth kinetics of hydrogenated nanocrystalline silicon, with specific focus on the effects of the deposition time and hydrogen dilution on the nano-structural properties. The growth in the crystallite size, attributed to the agglomeration of smaller nano-crystallites, is accompanied by a reduction in the compressive strain within the crystalline region and an improved ordering and reduction in the tensile stress in the amorphous network. These changes are intimately related to the absorption characteristics of the material. Surface diffusion determines the growth in the amorphous regime, whereas competing reactions between silicon etching by atomic hydrogen and precursor deposition govern the film growth at the high-dilution regime. The diffusion of hydrogen within the film controls the growth during the transition from amorphous to nanocrystalline silicon.     

Preparation of SnO2 thin films at low temperatures with H2 gas by the hot-wire CVD method

H₂ additional effect for crystallization of SnO₂ films prepared by the hot-wire CVD method was investigated. The crystallization of SnO₂ films starts at 170 °C. The selectivity enhancement of the solar cell substrate will contribute to reduce the cost of silicon thin film solar cells. The atomic hydrogen assisted nano-crystallization exists for the depositions of SnO₂ films by the hot-wire CVD method. Furthermore, the addition of H₂ gas improved the electrical conductivity up to 5.3 × 10⁰ S/cm. However, these effects are limited in the deposition condition of a small amount of hydrogen. Addition of much higher hydrogen concentration starts an etching effect of oxygen atoms.     

ZnO:Al films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells

Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are usually achieved by RF sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, on the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented.