Subsurface modification in Si induced by nanosecond laser irradiation

Semiconductor devices are pivotal to modern technology. These devices are batch produced on wafers which must be diced. Conventionally, dicing involves destroying the material between each chip, dictating a minimum spacing between modifications. It follows that eliminating this material loss will allow more chips to be created per wafer. This thesis examines subsurface modifications that form part of a dicing process with no material loss. Subsurface modifications can be induced by focussed pulsed laser irradiation when the photon energy is near or below the material's bandgap. Intensity dependent absorption is used to selectively modify the region near focus. This technique is useful both for technological applications and scientific investigation of material properties and laser-material interaction. This thesis examines columnar modifications created subsurface in Si wafers due to melting induced by nanosecond pulsed laser irradiation. The wafer dicing technique uses rows of closely spaced modifications that can preferentially guide crack propagation over the natural cleavage planes when the sample is cleaved. The modifications themselves are poorly characterised and it is this deficiency that this thesis seeks to address. The work examines both isolated modifications, where the laser-matter interaction can be probed, and closely spaced modifications, which can provide insight into the dicing process. Additionally, modifications created under several different laser conditions are investigated. Transmission electron microscopy is used to examine both the short and long axis of the modifications. Scanning electron microscopy and Raman microspectroscopy are also used. Detailed examination of the short axis of the modifications reveal a range of morphological features. These features can be explained by extending upon the literature of one-dimensional pulsed laser melting experiments into two-dimensions and conditions of higher cooling rates. Rapid, accelerating solidification occurs along multiple crystal orientations and propagates inwards from the melt periphery while simultaneously transitioning through several rate-dependent regimes of solidification behaviour. Examining the long axis allows the solidification process to be understood in three-dimensions. Here, variations observed in the short axis morphology are related to the dependence on the position along the long axis of the modifications. Solidification is also subject to perturbations, often density related. These originate in the densification of Si as it melts, which drives the formation of voids. During solidification the voids are partially, but incompletely refilled. The material displaced by the remanent voids is largely concentrated in the final regions of the modified volume to solidify, creating compressive strain. However, there are also crystallites of high density Si allotropes. These allotropes form if pockets of high density melt lack room to expand during solidification. Interestingly, although these allotropes have been observed in near-static near-equilibrium pressure loading techniques (diamond anvil cell or nanoindentation loading), the pathway by which the Si allotropes form must clearly differ in this work. It is also noteworthy that the manifestation of the density dependant morphological features is highly dependent on the size of the modifications, and duration of the laser pulse used to create them. The modifications were also found to contain many microcracks after solidification. During cleaving this removes the need to nucleate cracks. Instead, many cracks propagate simultaneously before joining together, completing the dicing process. Thus, this thesis has rectified the deficiency in knowledge of the modification morphology. In doing so, new solidification and high pressure related behaviour has been observed. Collectively, this information should serve as the basis to improve the dicing technique in the future

Chalcogen-hyperdoped germanium for short-wavelength infrared photodetection

Obtaining short-wavelength-infrared (SWIR; 1.4 μm–3.0 μm) room-temperature photodetection in a low-cost, group IV semiconductor is desirable for numerous applications. We demonstrate a non-equilibrium method for hyperdoping germanium with selenium or tellurium for dopant-mediated SWIR photodetection. By ion-implanting Se or Te into Ge wafers and restoring crystallinity with pulsed laser melting induced rapid solidification, we obtain single crystalline materials with peak Se and Te concentrations of 1020 cm−3 (104 times the solubility limits). These hyperdoped materials exhibit sub-bandgap absorption of light up to wavelengths of at least 3.0 μm, with their sub-bandgap optical absorption coefficients comparable to those of commercial SWIR photodetection materials. Although previous studies of Ge-based photodetectors have reported a sub-bandgap optoelectronic response only at low temperature, we report room-temperature sub-bandgap SWIR photodetection at wavelengths as long as 3.0 μm from rudimentary hyperdoped Ge:Se and Ge:Te photodetectors

Au-rich filamentary behavior and associated subband gap optical absorption in hyperdoped Si

Au-hyperdoped Si, synthesized by ion implantation and pulsed laser melting, is known to exhibit a strong sub-band gap photoresponse that scales monotonically with the Au concentration. However, there is thought to be a limit to this behavior since ultrahigh Au concentrations (>1 x 10(20) cm(-3)) are expected to induce cellular breakdown during the rapid resolidification of Si, a process that is associated with significant lateral impurity precipitation. This work shows that the cellular morphology observed in Au-hyperdoped Si differs from that in conventional, steady-state cellular breakdown. In particular, Rutherford backscattering spectrometry combined with channeling and transmission electron microscopy revealed an inhomogeneous Au distribution and a subsurface network of Au-rich filaments, within which the Au impurities largely reside on substitutional positions in the crystalline Si lattice, at concentrations as high as similar to 3 at. %. The measured substitutional Au dose, regardless of the presence of Au-rich filaments, correlates strongly with the sub-band gap optical absorptance. Upon subsequent thermal treatment, the supersaturated Au forms precipitates, while the Au substitutionality and the sub-band gap optical absorption both decrease. These results offer insight into a metastable filamentary regime in Au-hyperdoped Si that has important implications for Si-based infrared optoelectronics.The US Army (Contract No. FA5209-16-P-0104) for partial financial support of this project

Crystal structure of laser-induced subsurface modifications in Si

Laser-induced subsurface modification of dielectric materials is a well-known technology. Applications include the production of optical components and selective etching. In addition to dielectric materials, the subsurface modification technology can be applied to silicon, by employing near to mid-infrared radiation. An application of subsurface modifications in silicon is laser-induced subsurface separation, which is a method to separate wafers into individual dies. Other applications for which proofs of concept exist are the formation of waveguides and resistivity tuning. However, limited knowledge is available about the crystal structure of subsurface modifications in silicon. In this work, we investigate the geometry and crystal structure of laser-induced subsurface modifications in monocrystalline silicon wafers. In addition to the generation of lattice defects, we found that transformations to amorphous silicon and Si-iii/Si-xii occur as a result of the laser irradiation

Exotic silicon phases synthesized through ultrashort laser-induced microexplosion: Characterization with Raman microspectroscopy

Exotic metastable phases of silicon formed under high pressure are expected to have attractive semiconducting properties including narrow band gaps that open up novel technological applications. Confined microexplosions induced by powerful ultrashort laser pulses have been demonstrated as an advanced tool for the creation of new high-pressure phases that cannot be synthesized by other means. Tightly focused laser pulses are used to generate localized modifications inside the material structure, providing the possibility for precise controlled band-gap engineering. In this study, noninvasive Raman spectroscopy was used for analysis of laser-modified zones in silicon and to determine the metastable high-pressure phases contained. Low laser energies induced the formation of amorphous-only silicon, while higher energies led to crystalline silicon polymorphs within the modifications, albeit under considerable residual stress up to 4.5 GPa. The presence of the structurally similar r8-Si, bc8-Si, and bt8-Si phases is revealed, as well as other yet to be identified phases, and the stacking-related 9R Si polytype is evidenced, presumably stress-induced by the highly compressed laser-modified zone. The ab initio random structure searching approach is used to calculate the Raman signatures and to help identify different Si polymorphs. These findings by Raman spectroscopy from ultrashort laser-induced microexplosion sites may yield insights into the local structure and properties of new silicon metastable phases and the prospect of utilizing exotic phases for extending current applications.The authors acknowledge the support by the Australian Government through the Australian Research Council’s Discovery scheme, Project No. DP170100131

Ion beam synthesis and photoluminescence study of supersaturated fully-relaxed Ge-Sn alloys

Direct-bandgap germanium-tin (Ge-Sn) alloys are highly sought-after materials for applications in silicon photonic integrated circuits. Other than crystal quality, two main factors determine the transition from the indirect to direct bandgap: the high Sn concentration and the strain relaxation in the materials. Using ion implantation and pulsed laser melting, we demonstrate a fully-relaxed Ge-Sn alloy with a Sn concentration of 6at.%. This concentration is at least 10 times higher than the equilibrium solubility of Sn in Ge. Cross-sectional transmission electron microscopy shows unconventional threading-like defects in the film as the mechanism for the strain relaxation. Due to the high degree of strain relaxation and the good crystal quality, photoluminescence could be obtained from the samples to examine the indirect-direct bandgap transition in the alloys.The authors would like to acknowledge the Australian Research Council for the funding support, the National Collaborative Research Infrastructure Strategy for the access to the Australian National Fabrication Facility and the Heavy Ion Accelerator Facility. J. Mathews would also like to acknowledge funding support from an Air Force Office of Scientific Research Young Investigator Award, Grant number FA9550-17-1-014

Synthesis of Ge1−xSnx alloys by ion implantation and pulsed laser melting: Towards a group IV direct bandgap material

The germanium-tin (Ge₁ˍₓSnₓ) material system is expected to be a direct bandgap group IV semiconductor at a Sn content of 6.5−11 at. %. Such Sn concentrations can be realized by non-equilibrium deposition techniques such as molecular beam epitaxy or chemical vapour deposition. In this report, the combination of ion implantation and pulsed laser melting is demonstrated to be an alternative promising method to produce a highly Sn concentrated alloy with a good crystal quality. The structural properties of the alloys such as soluble Sn concentration, strain distribution, and crystal quality have been characterized by Rutherford backscattering spectrometry, Raman spectroscopy, x raydiffraction, and transmission electron microscopy. It is shown that it is possible to produce a high quality alloy with up to 6.2 at. %Sn6.2. The optical properties and electronic band structure have been studied by spectroscopic ellipsometry. The introduction of substitutional Sn into Ge is shown to either induce a splitting between light and heavy hole subbands or lower the conduction band at the Γ valley. Limitations and possible solutions to introducing higher Sn content into Ge that is sufficient for a direct bandgap transition are also discussed.The authors would like to acknowledge the Australian Research Council for the funding support, the National Collaborative Research Infrastructure Strategy for the access to the Australian National Fabrication Facility, and the Heavy Ion Accelerator Facilit