David V. Tsu

Effect of hydrogen dilution on the structure of amorphous silicon alloys

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

Intermediate order in tetrahedrally coordinated silicon: evidence for chainlike objects

Abstract In this report, we describe the nature of intermediate order in silicon as determined by recent measurements on thin films using transmission electron microscopy (TEM) and Raman scattering. The TEM images show in addition to the expected continuous random network (CRN), the presence of highly ordered quasi-one-dimensional “chain-like objects” (CLOs) that are 1– 2 nm wide and tens of nm long that meander and show some evidence of cross-linking with each other. The presence of these objects correlate to a Raman feature centered at 490 cm −1 whose width is 35– 40 cm −1 , and is used to quantify the heterogeneity in terms of the CLO and CRN (=475 cm −1 scattering) concentrations. The 490 and 35 cm −1 values are consistent with bond angle deviations approaching 0°, and thus reinforces an association with the CLOs. We find that in reference quality a-Si:H (made using pure SiH4), the CLO concentration is about 5 vol % , while in state-of-the-art material using high H2 levels of dilution during processing, it increases to about 15%. Increased stability of such material to light-soaking is thus not mediated by a direct volumetric replacement of poor with high-quality components. Rather, an important characteristic of intermediate order in silicon is the low-dimensional aspect of its order, which allows it to influence more total volume than which it is itself composed. Consistent with these and other recent findings, we propose a tensegrity model of amorphous silicon.

Development of transparent conductive oxide materials for improved back reflector performance for amorphous silicon based solar cells

A new back reflector comprised of an Al/(multi-layered stack)/ZnO structure is being developed to replace Al/ZnO used in manufacturing and boost conversion efficiencies with improved back reflector performance. Use of the multi-layered stack should lead to improved reflectivity which will in turn improve solar cell currents and efficiencies. The results from studies of different transparent conductive oxides (TCOs) which comprise the multi-layered stack are reported with emphasis on ZnO alloys. Alloying with Si or MgF 2 and using moderately high substrate temperatures, TCOs with low indices of refraction between 1.6 and 1.7 have been fabricated. The Si, Mg and F contents for these alloys were near 14, 12 and 33 at.%. Structural analysis demonstrates that alloys with MgF 2 have smother surfaces and finer morphologies than those for ZnO. The expected high values for multi-layered structures with these alloys have yet to be achieved but this is likely due to properties of layers in the structure other than the ZnO alloys which have yet to be fully optimized.

Photonic crystal nanosecond wavelength switches

We present our design and fabrication methodology of planar photonic crystal wavelength switches and the optical micro-bench surrounding them. The core device is a channel add-drop multiplexer (CADM) whose pass/transfer element can be turned off and on in tens of nanoseconds. The photonic crystal consists of a regular triangular array of SiO2 -filled holes in an amorphous Ge3Si film. The film is sandwiched between two SiO2 cladding layers. The pass and transfer buses consist of linear extended defects in the crystal, with the pass bus and each drop bus separated by a cavity resonator defect tuned to each wavelength. There is a small region where an ECD-designed chalcogenide alloy is incorporated into each resonator. Switching is accomplished by changing the structure of the chalcogenide between amorphous and crystalline, using a short wavelength diode laser. The optical bench consists of photonic wire waveguides formed in the Ge3Si film and deep trenches in an underlying thick SOI film to accommodate bonded access fibers, both features being photolithographically co-aligned to the photonic crystal array. This, along with our impedance-matching interface designs, assures that there is low input-output power loss. The local reconfigurability in effect elevates the CADM to an all-optical router. Sub-100 nanosecond latency enables packet-level discernment. The large difference in optical constants of the two chalcogenide phases provides high on-off contrast (low crosstalk). The stability of the two phases gives complete latching nonvolatility. Our current progress in building and testing prototypes of our switches is also presented.

All optical broadband steering by phase angle controlled stationary element (PACSE) mirrors

We describe a method to achieve phased array steering at the near infrared (i.e., optical) frequencies used in telecommunication (1550 nm) as an alternative to physical movement of standard mirrors. A stationary and planar multi-layer device utilizes a chalcogenide phase change material1,2 (PCM) as its active element whose refractive index changes by very large amounts (> 1.7X) between its amorphous and crystalline states. The optical phase angle upon reflection off this surface can change by more than 180° depending on physical state of the PCM. Phasor analysis is used to explain how such large phase angle shifts can be accomplished for a PCM layer only 20 nm thick. Not only can this be used to make rewritable diffractive elements, but since the phase taper can be made nearly continuous, the surface can also steer the beam in non-specular directions with no diffractive distortions. To date, we have steered a telecom beam 2° in one direction, and expect deflections by more than 10°. The steering is broad-banded, self latching, and potential switching speeds are expected to be less than 100 ns.

Low thermal budget optical recording: a method for higher recording densities

The recording of amorphous marks in crystalline Chalcogenide tracks of rewritable optical storage media depends critically upon management of the total thermal budget. This management depends not only directly on the physical make-up of the optical device, but also importantly depends on the recording strategy. We show through analytical and numerical calculations that the heating during a laser pulse or cooling after the end of the pulse can be described in terms of two mechanisms having very different time constants. The short and long term mechanisms have time constants of about 1 ns and more than 6 ns, and are governed by thermal capacitance and thermal conductance, respectively. A low thermal budget (LTB) control over the shape may find use in a multi-level recording strategy. Moreover, even in binary recording, LTB offers greater control over the amorphicity of the marks compared to standard write strategies by preventing back crystallization during middle pulses. This should lead to lower jitter values, which would allow the incremental mark size to be reduced and thus yield greater storage capacity.

Germanium: the good, the bad, and the ugly, how d-orbitals can ruin materials or create new opportunities

We examine from a high-level perspective the chemical basis that has limited use of Ge in modern technology to niche applications. From the measured optical properties [n,k] of c-Ge, GeTe, and Ge2Sb2Te5 (GST-225), we examine their sum-rule and superconvergence (SC) properties (to effectively measure the total Ne/atom) and thereby demonstrate the significance of d-orbitals. We find that while the crystalline form (fcc) of GST-225 only makes use of s- and p-orbitals, the amorphous form has a greater overall Ne/atom ratio, which can only occur if d-orbitals also participate. Similar results are found for GeTe. We interpret these Ne/atom densities by a simple valence model, which places rather strict limitations on the type and concentrations of different local atomic environments. Moreover, we show that since the SC is closely related to electric susceptibility, quantitative changes in the SC between the crystalline and amorphous forms imply removal (suppression) of Te’s lone pair orbitals (existing in the crystalline form) by the d-orbitals (existing in the amorphous form). With this, it now appears that we have a more comprehensive understanding of how d-orbitals play important roles in the switching behaviors in both optical and electronic devices. We propose that this understanding can eventually be extended to the ultimate “single-atom” man-made electronic devices.

Development of multilayer back reflectors for improved a-Si based solar cell performance

A new back reflector comprised of an Al/(multi-layered stack)/ZnO structure is being developed to replace Al/ZnO used in manufacturing and boost conversion efficiencies with improved back reflector performance. Use of the multi-layered stack should lead to improved reflectivity that will in turn improve solar cell currents and efficiencies. Using TCOs with low indices of refraction between 1.6 and 1.7, AI(specular)/ML/ZnO back reflectors have been fabricated with reflectance values in the red portion of the light spectrum (600-1000 nm) that are close to those obtained with the AI/MgF/sub 2//Si/MgF/sub 2/ optical stacks and Ag/ZnO back reflectors. With the AI(specular)/ML/ZnO back reflector stacks, a 1.7 mA/cm/sup 2/ improvement in the red light short circuit current has been obtained for a-SiGe cells. However, the gain in red light efficiency is not as large as expected with textured back reflectors. Improvements should come through re-optimization of the multi-layer stack or use of different texturing schemes.