The validity of the Brodsky, Cardona and Cuomo (BCC) [1] and the Connell and Lewis (CL) [2] methods to analyze infrared transmission data of hydrogenated amorphous silicon (a-Si:H) was examined using computer simulations. Transmission spectra for a-Si:H films 0-5μm thick and containing up to 30 atomic% hydrogen were simulated assuming coherent reflections in the film and incoherent reflections in the c-Si substrate. Analysis of the simulated data for the 640cm−1 Si-H wagging mode shows that the BCC and CL techniques systematically overestimate the absorption coefficeint, α, and hence hydrogen content, CH, when the film thickness, d, is less than ~1μm. The error is nearly independent of CH and is as large as 80% in the limit d→0. On this basis, previously reported experimental evidence for the dependence of CH on d is shown to be an analysis artifact. A simple method to correct the hydrogen content determined by the BCC or CL analysis using only the film thickness is presented.

We have prepared device quality hydrogenated amorphous silicon (a-Si:H) films by DC magnetron reactive sputtering in an Ar+H2 atmosphere. The hydrogen content (CH) of these films ranges from ∼10 to ∼28 at.% (determined from IR absorption) and is controlled mainly by the hydrogen partial pressure in the discharge. The electron and hole transport, and the density of defect states show that these films are suitable for solar cell applications. In this report we will emphasize the carrier transport properties of the films. The electron drift mobilities, determined by time-of-flight experiments, are between 0.5 and 2 cm2/V-sec. The minority carrier (hole) mobility-recombination lifetime product, (μτrec)p, has been determined on Schottky barrier structures from a fit of the voltage dependence of the photocurrent to the Hecht expression. This measurement gives results of (μτrec)p ∼ 0.9- 3×10−8 cm2/V. The density of deep-level defect states for these films (10≤CH≤28), measured with CPM, is between ∼1×1015 and ∼1×1016 cm−3. The AM-1 photoconductivity is in the range from 0.8−3.5×10−5 (Ω-cm)−1 and the dark conductivity decreases from ∼1×10−9 to ∼1×10−12 (Ω-cm)−l as the hydrogen content (CH) increases. These results show that device quality a-Si:H films can be grown by the DC magnetron reactive sputtering technique.

We demonstrate a photonic integrated circuit using a novel monolithic integration platform combining InGaAsP gain elements and index matched amorphous silicon waveguide devices. The AWG based multi-frequency laser emits eight 100-GHz-spaced wavelengths near 1550 nm.

Low loss, single mode rib waveguides, based on PECVD deposited multi-layer amorphous silicon are fabricated. These waveguide are refractive index and mode-matched to III/V laser waveguides. Methods for monolithic integration of these passive amorphous silicon waveguides with InGaAsP/InP gain sections are demonstrated. Results of a multi-wavelength laser based on an amorphous silicon arrayed waveguide grating integrated on a single chip with InGaAsP gain sections are presented.

Electro-refractive InP/InGaAsP ring-resonators with quality factors of up to 20000 are demonstrated. A novel modified MMI-coupler is used to provide reproducible low coupling to the access waveguide. We demonstrate modulation and the first link experiments.

The design and operation of an advanced bimorph microcantilever based infrared imaging detector are presented. This technology has the potential to achieve very high sensitivities due to its inherent high responsivity and low noise sensor and detection electronics. The sensor array is composed of bimaterial, thermally sensitive microcantilever structures that are the moving elements of variable plate capacitors. The heat sensing microcantilever structures are integrated with CMOS control and amplification electronics to produce a low cost imager that is compatible with standard silicon IC foundry processing and materials. The bimorph sensor structure is fabricated using low thermal expansion, high thermal isolation silicon oxide and oxynitride materials, and a high thermal expansion aluminum alloy bimetal. The microcantilever paddle is designed to move away from the substrate at elevated imaging temperatures, leading to large modeled sensor dynamic ranges (~16 bits). A temperature coefficient of capacitance, ▵C/C, (equivalent to TCR for microbolometers) above 30% has been modeled and measured for these structures, leading to modeled NEDT < 20 mK and thermal time constants in the 5-10 msec range giving a figure-of-merit [1] NEDT.Tau = 100-200 mK.msec. The development efforts to date have focused on the fabrication of 160x120 pixel arrays with 50 micron pitch pixels. Results from detailed thermo-electro-opto-mechanical modeling of the operation of these sensors are compared with experimental measurements from various test and integrated sensor structures and arrays.

Solarex, a business unit of Amoco/Enron Solar, is scaling up its a‐Si:H/a‐SiGe:H tandem device technology for the production of 8 ft2 modules. The current R&D effort is focused on improving the performance, reliability and cost‐effectiveness of the tandem junction technology by systematically optimizing the materials and interfaces in small‐area single‐ and tandem junction cells. Average initial conversion efficiencies of 8.8% at 85% yield have been obtained in pilot production runs with 4 ft2 tandem modules.

A dual bandgap, dual junction a-Si:H/a-SiGe:H solar cell has been selected as the structure to be used in Solarex's new 10 MW solar module plant. This structure offers high initial efficiency, good stability, low material usage, and short deposition time. Thousands of one square foot and four square foot tandem modules have been produced on the Solarex pilot line, and an average of 8% stabilized efficiency on 4 square foot modules in the trial production, and 8.6% on 1 square foot modules in R&D were achieved with this structure. Good agreement was found in the stability of modules tested outdoors and indoors, and also with an array of test modules set up at NREL. An improved tandem structure with better stability is being developed by optimizing the i-layer thickness and back junction bandgap. The current loss associated with thinner i-layers was well compensated by improvements in fill factor, open-circuit voltage, and stability. Two environmental degradation modes for modules other than Staebler-Wronski effect were identified and solutions implemented

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation R. R. Arya, M. Bennett, L. Yang, J. Newton, Y. M. Li, N. Maley, B. Fieselmann, L. F. Chen, K. Rajan, A. Wilczynski, G. Wood; Recent improvements in amorphous silicon‐based multijunction modules. AIP Conf. Proc. 30 June 1994; 306 (1): 120–126. https://doi.org/10.1063/1.45750 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

This report describes work performed dudng the period 1 July 1992 through 30 June 1993. Dudng this period, major improvements were achieved in the stabilized conversion efficiency of triple-junction modules. These resulted in the demonstration of triple-junction initial conversion efficiency of 11.3% and stabilized conversion efficiency of approximately 9%. Significant advances were made in the deposition of a-Si:H intrinsic layers that led to higher open-circuit voltage and improved stability. Thin microcrystalline n-layers were developed and scaled up for the recombination junctions in triple-junction modules that resulted in higher open-circuit voltage and fill factors. These improvements resulted in the demonstration of a-Si/a-Si/a-SiGe triple-junction modules with initial conversion efficiencies as high as 11.35% and ``stabilized`` efficiencies of about 9%.

We report the electronic properties, stability, and microstructure of a-Si:H films grown at high substrate temperature (320–440 °C) by dc reactive magnetron sputtering. The initial defect state density, determined by the constant photocurrent method, varies from 2–5×1015 cm−3 with H content changing from 15–10 at. % as Ts increases from 320–375 °C. For 100 h of white light exposure at 1 W/cm2, the midgap state density reached an apparent saturation at 2–3×1016 cm−3 over this temperature range. By contrast, films grown at 230–300 °C saturate at 9×1016 cm−3.

We use in situ spectroscopic ellipsometry to analyze the microstructure of the μc-Si:H/a-Si:H interface deposited by reactive magnetron sputtering of a Si target in (Ar+H2). We increase the hydrogen pressure to promote μc-Si:H formation and observe several effects. Initially, H penetrates ∼45 Å into the a-Si:H substrate and increases its hydrogen content. Then ∼55 Å of hydrogen-rich a-Si:H deposits. Finally, μc-Si:H nucleates on top of this ∼100 Å thick, high H-content a-Si:H interface layer. As μc-Si:H grows, the thickness of the amorphous interface layer decreases by ∼40 Å; the void fraction in the μc-Si:H layer is always ≤15 vol %, ruling out the possibility that the a-Si:H is etched away. These results suggest that a-Si:H can be transformed into μc-Si:H in a subsurface region under appropriate conditions.

Advances in amorphous silicon based alloys and device structure have led to the demonstration of 10.5% initial efficiency and 9.1% stabilized efficiency on 1 ft2 a-Si/a-SiGe tandem junction module. Scale-up to 4 ft2 in pilot production has resulted in the best initial efficiency of 9.75% as measured by NREL, which should result in a stabilized efficiency over 8%. The 4 ft2 modules have an average initial efficiency of 8.75% and a stabilized average efficiency of about 7.5%, as measured by Solarex

Very thin layers of hydrogenated and unhydrogenated amorphous silicon have been deposited on tin oxide substrates under different temperatures and H2 partial pressures by dc magnetron sputtering. The deposition processes are monitored by real time in situ ellipsometry. We observe the reduction reaction of the SnO2 exposed to a H- or Si-containing deposition flux in the early stages of film growth by real time ellipsometry. The chemical states of Sn at the a-Si:H (or a-Si)/SnO2 interface have been studied by x-ray photoelectron spectroscopy (XPS). XPS confirms that a Si flux alone can reduce SnO2 to elemental Sn, and that this reaction is temperature dependent. However, the contribution of Si is secondary to that of H atoms in the reduction reaction.

Substrate temperature and doping gas ratios are used to vary the boron and hydrogen content of p-type hydrogenated amorphous silicon (a-Si:H). The effects of the doping gas ratio (B2H6:SiH4) and substrate temperature have been studied and conditions were used such as to give optical gaps and dark conductivity activation energies which changed by as much as 0.3 eV while the difference between them remained unchanged. The effects of changes in the absorption coefficient and activation energy on p-i-n solar cell device characteristics are quantified by comparing the measured thin film properties with the collection efficiencies and dark and light current-voltage characteristics

We use real time, high sensitivity infrared reflectance spectroscopy to quantitatively study hydrogen incorporation during the growth of hydrogenated amorphous silicon by magnetron sputtering. For the range of deposition conditions studied (substrate temperature between 120 and 270 °C and H2 partial pressure between 0.1 and 1.0 mTorr, which result in film hydrogen content between 10 and 30 at. %), hydrogen incorporation reaches steady state for film thickness, d≳25 Å. Deviations from uniform growth for d<25 Å are attributed to plasma-substrate interactions due to energetic hydrogen implantation tens of angstroms beneath the growing surface and changes in the surface area during nucleation and coalescence.

We report the evolution of Si-H bonding, measured using in situ real time infrared reflectance spectroscopy, for hydrogenated amorphous silicon growth by reactive magnetron sputtering. To distinguish H incorporation and release on the film surface and sub-surface, we compare the growth of a-Si:H on SiO2 vs. a-Si, and of a-Si:D on a-Si:H. These reveal the magnitude of the H flux, and the existence of hydrogen interactions to a depth of 40 Å in the film.