Real-time Spectroscopic Ellipsometry Research Articles

Tracking optical properties of VOx films to optimize polycrystalline VO2 fabrication

In-situ real-time spectroscopic ellipsometry (RTSE) measurements are performed on a growing amorphous vanadium oxide (a-VOx) film to determine the complex dielectric function (ε = ε1 + iε2) spectra, structure, and oxygen content (x) with depth during deposition. VOx with x ∼2 is annealed to produce polycrystalline VO2 and characterized by near-infrared to ultraviolet (0.75–5.9 eV) spectroscopic ellipsometry during heating and cooling from room temperature (RT) to 343 K to RT to track hysteresis effects in the semiconducting-to-metal transition (SMT). Spectra in ε are measured for as-deposited and annealed films prepared with varying deposition parameters to determine the as-deposited film x and verify the SMT. Temperature-dependent IR extended spectroscopic ellipsometry is performed from 0.06 to 0.74 eV to determine the IR ε spectra of polycrystalline VO2 across the SMT. As-deposited amorphous VOx films with x from 1.89 ≤ x ≤ 2.14 transition to polycrystalline VO2 after annealing and exhibit the SMT when heated from RT to 343 K which are verified by substantially increased ε2 magnitude at ≥343 K. By identifying x of a-VOx from ε spectra, a shortened path of qualifying a-VOx films which will crystallize to phase change VO2 is developed.

Real-time spectroscopic ellipsometry of plasmonic nanoparticle growth in polyvinyl alcohol thin films

Real-time spectroscopic ellipsometry of plasmonic nanoparticle growth in polyvinyl alcohol thin films

Spectro-ellipsometric probing of wetting, nucleation, and dot/island formation during photo-excited chemical vapor deposition of Ge on SiO2 substrate

The morphological evolution of Ge layers growing on the SiO2/Si(100) substrate by photo-excited chemical vapor deposition was traced through an analysis of pseudodielectric functions measured by real-time spectroscopic ellipsometry. Simulation and fitting were carried out on multiple samples with various Ge film thicknesses as well as on sequential optical spectra from a sample with an incremental buildup of Ge atoms on one substrate. Single- and two-layer models involving crystalline Ge (c-Ge), amorphous Ge (a-Ge), and void components were employed under the Bruggeman effective medium approximation to represent wetting of the SiO2 surface, nucleation of Ge seeds for the subsequent dot/island formation, and steady-state dot/island growth. A combination of c-Ge and a-Ge represents intermediate crystallinity, and void represents vacant space between dots/islands. A single-layer model with a mixture of c-Ge, a-Ge, and void components was used for crude estimation of the composition from which the time evolution of the volume fraction of the components was derived. However, fitting in the early growth stage resulted in an unrealistic structure, indicating that the dielectric function of the thin hydrogenated Ge network layer was very different from those of c-Ge and a-Ge. The optical spectra of dots/islands at the intermediate growth stage could be reproduced by a two-layer model consisting of a (a-Ge + void) layer overlaid on a (c-Ge + void) base layer. The real-time Ψ–Δ trajectories of ellipsometric angles monitored at a photon energy of 3.4 eV consisted of three branches. They could be reproduced by assuming the growth of an outer layer with an appropriate composition. After wetting on SiO2 (branch 1), the Ge seed layer nucleates while the volume fraction of Ge rapidly decreases from 70% to 25% with proceeding growth (branch 2). Then, the volume fraction of Ge continuously increases up to 65%, eventually reaching steady-state dots/island growth (branch 3)

In situ studies on atomic layer etching of aluminum oxide using sequential reactions with trimethylaluminum and hydrogen fluoride

Controlled thin film etching is essential for future semiconductor devices, especially with complex high aspect ratio structures. Therefore, self-limiting atomic layer etching processes are of great interest to the semiconductor industry. In this work, a process for atomic layer etching of aluminum oxide (Al2O3) films using sequential and self-limiting thermal reactions with trimethylaluminum and hydrogen fluoride as reactants was demonstrated. The Al2O3 films were grown by atomic layer deposition using trimethylaluminum and water. The cycle-by-cycle etching was monitored throughout the entire atomic layer etching process time using in situ and in real-time spectroscopic ellipsometry. The studies revealed that the sequential surface reactions were self-limiting versus reactant exposure. Spectroscopic ellipsometry analysis also confirmed the linear removal of Al2O3. Various process pressures ranging from 50 to 200 Pa were employed for Al2O3 etching. The Al2O3 etch rates increased with process pressures: Al2O3 etch rates of 0.92, 1.14, 1.22, and 1.31 Å/cycle were obtained at 300 °C for process pressures of 50, 100, 150, and 200 Pa, respectively. The Al2O3 etch rates increased with the temperature from 0.55 Å/cycle at 250 °C to 1.38 Å/cycle at 350 °C. Furthermore, this paper examined the temperature dependence of the rivalry between the removal (Al2O3 etching) and growth (AlF3 deposition) processes using the reactants trimethylaluminum and hydrogen fluoride. The authors determined that 225 °C is the transition temperature between AlF3 atomic layer deposition and Al2O3 atomic layer etching. The high sensitivity of in vacuo x-ray photoelectron spectroscopy allowed the investigation of the interface reactions for a single etching pulse as well as the initial etch mechanism. The x-ray photoelectron spectroscopy measurements indicated that the fluorinated layer is not completely removed after each trimethylaluminum exposure. The Al2O3 atomic layer etching process mechanism may also be applicable to etch other materials such as HfO2.

Impact of Humidity and Temperature on the Stability of the Optical Properties and Structure of MAPbI3, MA0.7FA0.3PbI3 and (FAPbI3)0.95(MAPbBr3)0.05 Perovskite Thin Films.

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.

Mean Free Path of Photoelectronic Excitations in Hydrogenated Amorphous Silicon and Silicon Germanium Alloy Semiconductors

Hydrogenated amorphous silicon germanium alloy (a‐Si1−x Ge x :H) films are prepared by plasma enhanced chemical vapor deposition (PECVD) and characterized by in situ real time spectroscopic ellipsometry (RTSE). From complex dielectric function spectra extracted, the broadening width energy (Γ) of the primary absorption feature centered near 3.7 eV is quantified using the Cody–Lorentz oscillator model. Mean free path length of photoelectronic excitations is calculated from Γ based on reasonable estimates for speed of electron–hole photoexcitations. A model is applied with excited state lifetime assumed to be limited by scattering from network disorder and provides a relative measure of short‐range order. The simple model applied is an extension of that widely used to characterize broadening of optical transitions in polycrystalline semiconductors. Decreases in mean free path of up to ∼30% occur with germanium. Relative increases in mean free path with increased hydrogen during PECVD a‐Si:H, a‐Si0.73Ge0.27:H, and a‐Si0.60Ge0.40:H occur from ∼5% to ∼8% and with hydrogen plasma treatment of a‐Si:H by ∼6%. Mean free path changes are tracked with thickness to provide short range order evolution and the effect of the underlying material. Mean free path of photoexcitations in electronic quality a‐Si1−x Ge x :H is estimated at ∼3.5 Å, on the same order as interatomic spacing.

On the effect of copper as wetting agent during growth of thin silver films on silicon dioxide substrates

We study the effect of Cu incorporation on the morphological evolution and the optoelectronic properties of thin Ag films deposited by magnetron sputtering on weakly-interacting SiO2 substrates. In situ and real time spectroscopic ellipsometry data show that by adding up to 4at.% Cu throughout the entire film deposition process, wetting of the substrate by the metal layer is promoted, as evidenced by a decrease of the thickness at which the film becomes continuous from 19.5nm (pure Ag) to 15nm (Ag96Cu4). The in situ data are consistent with ex situ x-ray reflectometry analyses which show that Cu-containing films exhibit a root mean square roughness of 1.3nm compared to the value 1.8nm for pure Ag films, i.e., Cu leads to smoother film surfaces. These morphological changes are coupled with an increase in continuous-layer electrical resistivity from 1.0×10-5Ωcm (Ag) to 1.25×10-5Ωcm (Ag96Cu4). Scanning electron microscopic studies of discontinuous layers reveal that the presence of Cu at the film growth front promotes smooth surfaces (as compared to pure Ag films) by hindering the rate of island coalescence. To further understand the effect of Cu on film growth and electrical properties, in a second set of experiments, we deploy Cu with high temporal precision to target specific film-formation stages. The results show that longer presence of Cu in the vapor flux and the film growth front promote flat morphology. However, both a flat surface and a continuous-layer electrical resistivity that is equal to that of pure Ag films can only be achieved when Cu is deployed during the first 2.4nm of film deposition, during which morphological evolution is, primarily, governed by island coalescence. Our overall results highlight potential pathways for fabricating high-quality multifunctional metal contacts in a wide range of optoelectronic devices based on weakly-interacting oxides and van der Waals materials.

Real-Time Optimization of Anti-Reflective Coatings for CIGS Solar Cells.

A new method combining in-situ real-time spectroscopic ellipsometry and optical modeling to optimize the thickness of an anti-reflective (AR) coating for Cu(In,Ga)Se2 (CIGS) solar cells is described and applied directly to fabricate devices. The model is based on transfer matrix theory with input from the accurate measurement of complex dielectric function spectra and thickness of each layer in the solar cell by spectroscopic ellipsometry. The AR coating thickness is optimized in real time to optically enhance device performance with varying thickness and properties of the constituent layers. Among the parameters studied, we notably demonstrate how changes in thickness of the CIGS absorber layer, buffer layers, and transparent contact layer of higher performance solar cells affect the optimized AR coating thickness. An increase in the device performance of up to 6% with the optimized AR layer is demonstrated, emphasizing the importance of designing the AR coating based on the properties of the device structure.

Growth of Bi2O3 Films by Thermal- and Plasma-Enhanced Atomic Layer Deposition Monitored with Real-Time Spectroscopic Ellipsometry for Photocatalytic Water Splitting

Ultrathin Bi2O3 films were grown by thermal- and plasma-enhanced ALD by using [Bi(tmhd)3] as bismuth precursor. The use of an O2 plasma instead of H2O as oxidant increases the growth per cycle (GPC...

Understanding Electromagnetic Interactions and Electron Transfer in Ga Nanoparticle–Graphene–Metal Substrate Sandwich Systems

Plasmonic metal nanoparticle (NP)–graphene (G) systems are of great interest due their potential role in applications as surface-enhanced spectroscopies, enhanced photodetection, and photocatalysis. Most of these studies have been performed using noble metal NPs of silver and gold. However, recent studies have demonstrated that the noble metal–graphene interaction leads to strong distortions of the graphene sheet. In order to overcome this issue, we propose the use of Ga NPs that, due to their weak interaction with graphene, do not produce any deformation of the graphene layers. Here, we analyze systems consisting of Ga NP/G/metal sandwich coupling structures, with the metal substrate being, specifically, copper (Cu) and nickel (Ni), i.e., Ga NP/G/Cu and Ga NPs/G/Ni. We experimentally show through real-time plasmonic spectroscopic ellipsometry and Raman spectroscopy measurements of the quenching of the Ga NP localized surface plasmon resonance (LSPR) depending on the wetting of the graphene by the Ga NPs and on the electron transfer through graphene. Theoretical finite-difference time-domain (FDTD) simulations supportively demonstrate that the LSPR in such sandwich structures strongly depends on the contact angle of the NP with graphene. Finally, we also provide evidence of the electron transfer from the Ga NPs into the graphene and into the metal substrate according to the work function alignments. These considerations about the contact angle and, consequently, geometry and wetting of the metal NPs on graphene, are useful to guide the design of those plasmonic systems to maximize electromagnetic enhancement.

Formation of electronic defects in crystalline silicon during hydrogen plasma treatment

Electronic defects in crystalline silicon induced by hydrogen plasma treatments are studied, based on in-situ photocurrent measurements and real-time spectroscopic ellipsometry. The electronic defects are generated by the plasma treatments, and annihilated partially by postannealing. The generation and annihilation of defects strongly depends on both the treatment time and the annealing temperature. A long-time plasma treatment results in the formation of the residual defects in the silicon bulk. The density of these defects is estimated to be of the order of 1013 cm−2. Interestingly, the electronic defects are formed even before a strong modification of the surface structure, i.e., the formation of a nanometer-scale disordered surface layer.

Optically addressing interaction of Mg/MgO plasmonic systems with hydrogen

Magnesium-based films and nanostructures are being studied in order to improve hydrogen reversibility, storage capacity, and kinetics, because of their potential in the hydrogen economy. Some challenges with magnesium (Mg) samples are their unavoidable oxidation by air exposure and lack of direct in situ real time measurements of hydrogen interaction with Mg and MgO surfaces and Mg plasmonic nanoparticles. Given these challenges, the present article investigates direct interaction of Mg with hydrogen, as well as implications of its inevitable oxidation by real-time spectroscopic ellipsometry for exploiting the optical properties of Mg, MgH2 and MgO. The direct hydrogenation measurements have been performed in a reactor that combines a remote hydrogen plasma source with an in situ spectroscopic ellipsometer, which allows optical monitoring of the hydrogen interaction and results in optical property modification. The hydrogen plasma dual use is to provide the hydrogen-atoms and to reduce barriers to heterogeneous hydrogen reactions.

In-operando spectroscopic ellipsometry studies of IrO2 dynamic instabilities: Guide to in-situ growth of pyrochlore iridate thin films

The instability of iridium oxide at high temperature has long been a bottleneck for in growing pyrochlore iridate thin films in a vacuum chamber. To overcome this problem, we investigated the chemical instability of IrO2 thin films, which are the simplest form of iridate, via in-operando spectroscopic ellipsometry (SE). We observed that IrO2 thin films undergo IrO2 dissociation and IrO3 gas formation depending on the thermodynamic conditions. The chemical kinetics observations of IrO2 were confirmed by ex-situ X-ray diffraction and atomic force microscopy. SE experimental data were compared with models used to describe the evolution of the two chemical reactions. Real-time in-operando SE analysis based on the Maxwell Garnett theory yielded a precise IrO2 dissociation speed for the given thermodynamic conditions. Moreover, the real-time in-operando SE technique allowed us to observe the phase transition from solid IrO2 to gaseous IrO3. This study on the chemical instability of IrO2 at high temperature affords insights into a new method for in-situ pyrochlore iridate and other iridates thin-film growth.

Effect of deposition rate on the growth mechanism of microcrystalline silicon thin films using very high frequency PECVD

The intrinsic microcrystalline silicon thin films were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Two series of films with different deposition rate 0.30 nm/s and 1.94 nm/s were prepared. The film surface and gas phase reaction growth processes were monitored with real-time spectroscopic ellipsometry and optical emission spectroscopy. The effect of deposition rate on the microcrystalline silicon thin film growth mechanism has been studied. The microcrystalline silicon surface growth was analyzed with KPZ model. The results show that the growth exponent of β is 0.448 for the films with low deposition rate, and the growth exponent of β is 0.302 for the films with high deposition rate. The growth exponent does not increase with deposition rate, but declines. And the reasons for this phenomenon were explained.

3D system integration on 300 mm wafer level: High-aspect-ratio TSVs with ruthenium seed layer by thermal ALD and subsequent copper electroplating

The copper electrochemical deposition (Cu-ECD) filling capability of high aspect ratio through silicon vias (HAR-TSVs) and homogeneity over 300 mm wafers were investigated on a film stack of thermal ALD (thALD) TaxNy barrier with thermal ALD Ru seed in comparison to TixNy barrier with a standard Cu i-PVD seed layer using a commercial 300 mm plating tool. As a first step, Cu-ECD was conducted on wafers with TSV blind holes with aspect ratios (AR) of 10 to 12. To achieve this, a thermal ALD film stack of approximately 6 nm TaxNy and 9 nm Ru (with a sheet resistance of [25.6 ± 1.4] Ω/ϒ) were deposited at 250 °C. The reactants for the barrier layer were (tert-butylimido)tris(diethylamino)tantalum(V) (TBTDET) and ammonia (NH3) as co-reactant. For the Ru seed layer deposition (ethylcyclopentadienyl)(pyrrolyl)ruthenium(II) (ECPR) and molecular oxygen as co-reactant were used supplemented by a hydrogen purge step after every third ALD cycle. The corresponding ALD growth was observed during the entire process by in-situ real-time spectroscopic ellipsometry (irtSE). Blister-free deposition and satisfactory film stack adhesion with no delamination was verified ex situ by scanning electron microscopy (SEM). The deposited copper inside the TSVs was analyzed by focused ion beam (FIB) imaging and X-ray tomography.The Cu ECD filling capability in HAR-TSVs was shown on a film stack of thALD TaxNy thALD Ru seed using a commercial industry standard 300 mm plating tool. A novel blister-free ultra-thin Ru ALD film having good adhesion properties and unique advantages, e. g. high conformity in high-aspect-ratio through-silicon vias large-scale film uniformity over 300 mm wafers, as well as good reproducibility was developed.

Initial nucleation stage in photo-CVD of GeH4 on SiO2 substrate monitored by real-time spectroscopic ellipsometry and photo reflectance: Accurate determination of incubation time

The initial stage of photoexcited chemical vapor deposition (CVD) of Ge layers on SiO2 substrate was studied through a combined analysis of real-time spectroscopic ellipsometry and photoreflectance (PR). Just after GeH4 gas was admitted into the growth chamber, there was a certain period during which both the ellipsometric (Ψ, Δ) angles and PR intensity remained unchanged from their initial values. This phenomenon prior to steady-state growth is known as incubation, and it is due to the very slow reactive sticking of GeHx species on the dangling-bond free SiO2 surfaces. The incubation time was accurately determined from the time derivatives of (Ψ, Δ) and the PR intensity. In most cases, the incubation times independently derived from (Ψ, Δ) and the PR intensity coincided, but for other cases, those derived from the PR intensity were slightly longer than those derived from (Ψ, Δ). The incubation time decreased as thermally activated decomposition of GeHx species became prevalent above 300 °C. An activation energy of 18 kJ/mol was obtained by assuming that the rate of creating nucleation seeds on SiO2 is proportional to the inverse of the incubation time. Such a low activation energy, much lower than that of thermal CVD, is due to partial decomposition of GeH4 by photoexcitation to create reactive GeHx fragments. At the end of incubation, the Ge layer wetting the SiO2 surface changed into self-assembled Ge dots when more Ge atoms were deposited. The dot density was maximum at the beginning of dot formation and gradually decreased as the dot size became larger. The activation energy of continuous Ge growth on Ge-covered SiO2 surface was derived from the time evolution of Δ to be 13 kJ/mol. The minimum PR intensity for three-dimensional (3D) dot formation was lower than that of two-dimensional (2D) layer growth, reflecting greater surface roughness. Contrasting behaviors between 2D and 3D growth were also observed in terms of the recovery level of the PR intensity after prolonged growth; continued 2D growth recovered up to 90% of the initial level, while 3D dot formation recovered only 50 −60%.

Understanding near infrared absorption in tin doped indium oxide thin films

Tin doped indium oxide (In2O3:Sn (ITO)) thin films RF sputtered under similar conditions on soda lime glass and single crystal silicon wafers (c-Si) are used to study near infrared optical absorption variations during film growth. The presence of strong free carrier absorption and phonon absorption in the low photon energy spectral range provides an opportunity to disentangle tailing effects extending in the near infrared to visible spectral range. Toward that end, a model describing ITO film optical properties in the form of the complex dielectric function (ε = ε1 + iε2) from ex situ ellipsometry collected over 0.4 to 4.1 meV, 0.035 to 0.4 eV, and 0.75 to 5.89 eV spectral range is developed to describe these features along with other higher energy electronic transitions. In situ real time spectroscopic ellipsometry (RTSE) from 0.75 to 5.89 eV is used to track changes film structure and optical response during thin film growth. Optical emission spectroscopy indicates the plasma is very stable during deposition implying that film property changes with thickness are not correlated with changes in plasma chemistry. Film resistivity (ρ), mobility (µ), scattering time (τ), and carrier concentration (n) are determined from the free carrier absorption component of ε. Increased near infrared absorption manifested in ε obtained from RTSE data analysis is hypothesized to originate from enhancement of OH-group phonon absorption due to the presence of water molecules in the chamber at the beginning of growth.

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe₂ Growth: Indium-Gallium Selenide Co-Evaporation.

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control.

Characterization and Analysis of Ultrathin CIGS Films and Solar Cells Deposited by 3-Stage Process

In view of the large-scale utilization of Cu(In,Ga)Se2 (CIGS) solar cells for photovoltaic application, it is of interest not only to enhance the conversion efficiency but also to reduce the thickness of the CIGS absorber layer in order to reduce the cost and improve the solar cell manufacturing throughput. In situ and real-time spectroscopic ellipsometry (RTSE) has been used conjointly with ex situ characterizations to understand the properties of ultrathin CIGS films. This enables monitoring the growth process, analyzing the optical properties of the CIGS films during deposition, and extracting composition, film thickness, grain size, and surface roughness which can be corroborated with ex situ measurements. The fabricated devices were characterized using current voltage and quantum efficiency measurements and modeled using a 1-dimensional solar cell device simulator. An analysis of the diode parameters indicates that the efficiency of the thinnest cells was restricted not only by limited light absorption, as expected, but also by a low fill factor and open-circuit voltage, explained by an increased series resistance, reverse saturation current, and diode quality factor, associated with an increased trap density.

Optical gradients in a-Si:H thin films detected using real-time spectroscopic ellipsometry with virtual interface analysis

Virtual interface analysis (VIA) is applied to real time spectroscopic ellipsometry measurements taken during the growth of hydrogenated amorphous silicon (a-Si:H) thin films using various hydrogen dilutions of precursor gases and on different substrates during plasma enhanced chemical vapor deposition. A procedure is developed for optimizing VIA model configurations by adjusting sampling depth into the film and the analyzed spectral range such that model fits with the lowest possible error function are achieved. The optimal VIA configurations are found to be different depending on hydrogen dilution, substrate composition, and instantaneous film thickness. A depth profile in the optical properties of the films is then extracted that results from a variation in an optical absorption broadening parameter in a parametric a-Si:H model as a function of film thickness during deposition. Previously identified relationships are used linking this broadening parameter to the overall shape of the optical properties. This parameter is observed to converge after about 2000–3000 Å of accumulated thickness in all layers, implying that similar order in the a-Si:H network can be reached after sufficient thicknesses. In the early stages of growth, however, significant variations in broadening resulting from substrate- and processing-induced order are detected and tracked as a function of bulk layer thickness yielding an optical property depth profile in the final film. The best results are achieved with the simplest film-on-substrate structures while limitations are identified in cases where films have been deposited on more complex substrate structures.