Modeling and Simulation of Compositional Engineering in Sige Films Using Patterned Stress Fields

Semiconductor alloys such as silicon-germanium (SiGe) offer attractive environments for engineering quantum-confined structures that are the basis for a host of current and future optoelectronic devices. Although vertical stacking of such structures is routinely achieved via heteroepitaxy, lateral manipulation has proven much more challenging. I describe a new approach that suggests that a patterned elastic stress field generated with an array of nanoscale indenters in an initially compositionally uniform SiGe substrate will drive atomic interdiffusion, leading to compositional patterns in the near-surface region of the substrate. While this approach may offer a potentially efficient and robust pathway to producing laterally ordered arrays of quantum-confined structures, there is a large set of parameters important to the process. Thus, it is difficult to consider this approach using only costly experiments, which necessitates detailed computational analysis. First, I review computational approaches to simulating the long length and time scales required for this process, and I develop and present a mesoscopic model based on coarse-grained lattice kinetic Monte Carlo that quantitatively describes the atomic interdiffusion processes in SiGe alloy film subjected to applied stress. I show that the model provides predictions that are quantitatively consistent with experimental measurements, and I examine the impact of basic indenter geometries on the patterning process. Second, I extend the model to investigate the impact of several process parameters, such as more complicated indenter shapes and pitches. I find that certain indenter configurations produce compositional patterns that are favorable for use as lateral arrays of quantum-confined structures. Finally, I measure a set of important physical parameters, the so-called “activation volumes” that describes the impact of stress on diffusion. The values of these parameters are not well established in the literature. I make quantitative connections to the range of values found in the literature and characterize the effects of different stress states on the overall patterning process. Finally, I conclude with ideas about alternative pathways to quantum confined structure generation and possible extensions of the framework developed

Properties and device applications of silicon and silicon-germanium nanostructures with different dimensions

Defect-free crystalline Si/SiGe(Ge) nanostructures are demonstrated despite the 4% lattice mismatch between Si and Ge. The lattice mismatch-induced strain is sufficiently relaxed through the designed, cluster morphology, or nanowire (NW) structures. Future device applications of these nano structures require complete understanding of their structural, optical, electrical and thermal properties. This study explores these properties in two-dimensional (2D) Si/Si:B delta-doped multilayers, 2D Si/Si1-xGex planar multilayers, three-dimensional (3D) Si/Si1-xGex cluster multilayers, one-dimensional (1D) Si NWs and 1D Si/Ge NW heterojunctions (HJs). Raman scattering and photoluminescence measurements show that by alternating heavily boron-doped layers with layers of undoped Si in Si/Si:B multilayers, dopant segregation and strain can be avoided. Current-voltage and capacitance-voltage measurements show Schottky-barrier-like characteristics in these nano structures. The studied samples exhibit significant dependence of optical reflection on temperature and applied electric field, and hence have a potential to be used as electrically controllable mirrors. High Ge content 2D (planar) and 3D (cluster) Si/SiGe multilayers are studied thoroughly using Raman spectroscopy. Low frequency Raman measurements show formation of strong folded longitudinal acoustic (FLA) phonons in the 2D sample, indicating abrupt interfaces and good superlattice structure. By utilizing the multi-modal feature of Raman scattering, local temperatures are found by comparing the intensities of Stokes and anti-Stokes signals at specific wavenumbers, and the thermal conductivity of each sample is estimated. A strong correlation between FLA and thermal conductivity is found: in samples with high intensity FLA, thermal conductivity is almost twice increases, when compare to samples without FLA. Fabrications of Si NWs and Si/Ge NW HJs are explored, including interference-lithography-based photoresist patterning for Au catalysts. Raman spectroscopy shows significant strain in Si NWs and Si/Ge NW HJs. In the HJs, the temperature dependence in PL peak positions suggested a preferential composition at the hetero-junction. Raman-spectroscopy-based temperature measurements show significant decrease in the thermal conductivity of NW HJs: more than one order of magnitude less than that in Si NWs and two orders of magnitude less than that in c-Si. The high mobility and good carrier transport, combined with the substantially decreased thermal conductivity gives these Si/Ge and Si/SiGe nanostructures great potential in CMOS compatible, integrated thermoelectric device applications

New Material for Si-Based Light Source Application for CMOS Technology

In this chapter, an approach to enhance the radiative recombination of the Ge film grown on the Si substrate is presented. The Ge band gap structure could be modified by applying a tensile strain and high n-doping in the Ge epilayers. It thus becomes a direct band gap material with high photoluminescence efficiency which is compatible with mainstream silicon technology. The interdiffusion effect between Ge film and Si substrate is also mentioned in this section. We proposed a new method to suppress the Si/Ge interdiffusion to reduce the effect of Si atoms on the optical property of Ge film due to Si presence

Effects of phosphorous and antimony doping on thin Ge layers grown on Si

Suppression of threading dislocations (TDs) in thin germanium (Ge) layers grown on silicon (Si) substrates has been critical for realizing high-performance Si-based optoelectronic and electronic devices. An advanced growth strategy is desired to minimize the TD density within a thin Ge buffer layer in Ge-on-Si systems. In this work, we investigate the impact of P dopants in 500-nm thin Ge layers, with doping concentrations from 1 to 50 × 1018 cm−3. The introduction of P dopants has efficiently promoted TD reduction, whose potential mechanism has been explored by comparing it to the well-established Sb-doped Ge-on-Si system. P and Sb dopants reveal different defect-suppression mechanisms in Ge-on-Si samples, inspiring a novel co-doping technique by exploiting the advantages of both dopants. The surface TDD of the Ge buffer has been further reduced by the co-doping technique to the order of 107 cm−2 with a thin Ge layer (of only 500 nm), which could provide a high-quality platform for high-performance Si-based semiconductor devices

LARGE-AREA, WAFER-SCALE EPITAXIAL GROWTH OF GERMANIUM ON SILICON AND INTEGRATION OF HIGH-PERFORMANCE TRANSISTORS

Building on a unique two-step, simple MBE growth technique, we have investigated possible dislocation locking mechanisms by dopant impurities, coupled with artificially introduced oxygen. In the case of n-type Ge grown on Si, our materials characterization indicates that the dislocation density (DD) can reach the \uf07e105 cm-2 level, compared to p-type and undoped Ge on Si (GoS). We note that our Ge film covers the entire underlying Si substrate at the wafer scale without mesas or limited-area growth. In this presentation, we will focus on the use of n-type impurity (phosphorus) diffusing from the Si substrate and the introduction of O at the Ge-Si interface. The O is introduced by growing a thin chemical SiO2 layer on top of the Si substrate before Ge epitaxy begins. Z-contrast cross-sectional TEM images suggest the presence of oxygen precipitates in n-type Ge, whereas these precipitates appear absent in p-type Ge. These oxygen precipitates are known to lock the dislocations. Supporting the argument of precipitate formation, the TEM shows fringes due to various phase boundaries that exist at the precipitate/Ge-crystal interface. We speculate that the formation of phosphorus (P) segregation resulting from slow diffusion of P through precipitates at the precipitate/Ge-crystal interface facilitates dislocation locking. Impurity segregations in turn suppress O concentration in n-type Ge indicating reduced magnitude of DD that appears on the top surface of n-Ge compared to p-Ge film. The O concentrations (1017 to 1018 cm-3) in the n- and p-type GoS films are measured using secondary ionization mass spectroscopy. We also demonstrate the technique to improve the Ge epitaxial quality by inserting air-gapped, SiO2-based nanoscale templates within epitaxially grown Ge on Si. We have shown that the template simultaneously filters threading dislocations propagating from Ge-Si interface and relieves the film stress caused by the TEC mismatch. The finite element modeling stress simulation shows that the oval air gaps around the SiO2 template can reduce the thermal stress by 50% and help reduce the DD. We have then compared the structural and electrical characteristics of n-type Ge films with its p-type counter parts. In n-type Ge, the DD decreases from \uf07e109cm-2 near the Ge-Si interface to \uf07e105 cm-2 at the film surface. In contrast, we observe 5\uf0b4107 cm-2 TDD at the film surface in p-type Ge. The full width at half-maximum for our n-type Ge(004) XRD peak is ~70% narrower than that of p-type Ge. As a stringent test of the dislocation reduction, we have also fabricated and characterized high-carrier-mobility MOSFETs on GoS substrates. We also report p- and n-MOSFETs with μeff of 401 and 940 cm2/V-s and a subthreshold slope of 100 and 200 mV/decade, respectively. These effective mobilities show an exceptional 82 and 30% improvement over that of conventional Si channel MOSFETs. We also investigate the optical quality of ultra-low DD GoS film by measuring photoluminescence (PL). The n-type Ge PL main peak shows pronounced tensile-strain (x0.8%) than that of p-type which is an indicator of direct BG shrinking at the \u0413 band-edge. Going beyond epitaxial engineering and device fabrication, we have also recently demonstrated a scalable path to create a 2D array of Ge quantum dots (QDs) on responsive SiGe substrates based on elastic mechanical deformation and subsequent SiGe compositional redistribution, coupled with MBE growth. For large-scale manufacturing of single-electron transistors, we have also demonstrated that a spatially structured elastic compressive stress to the SiGe substrate with thermally annealing leads to a compositional redistribution of Si and Ge in the near-surface region of SiGe substrates, forming a 2D array of Ge-depleted nanoscale regions. Based on these latest findings, we have also begun to chart a future direction for my research group, where one can explore new advanced device architectures, such as Si-compatible, optically actuated, Ge-quantum dot-based field effect transistors

The development and optimization of potential germanium on silicon single photon avalanche diodes

The work presented in this thesis explores a potential single photon detection technology using Silicon and Germanium, and a possible direction for the future. Instead of the more commonly used III-V materials, the desirable characteristics of each of the Group IV materials is implemented in designing a separate absorption and multiplication region device. Key structural features of the device are investigated and optimised, so that single photon detection in the near infrared is made possible. Growth of these layers is performed using an RP-CVD system, the ultimate industry tool in this field of research. Doping profiles and smooth crystalline growth is implemented using a range of techniques, to produce suitable epitaxial structures which are ideal for further fabrication. Several techniques are used to ensure that the quality of these layers are fully optimised. This optimisation work has resulted in the first single photon detection at a wavelength of 1550 nm, and has also brought the Silicon and Germanium device onto a comparable level to their III-V counterparts at 1330 nm. The superior repetition rate of these Group IV devices also holds an advantage over those designed using InGaAs/InP. The boron doping of Silicon has also been investigated. It has been shown that fully crystalline Silicon boron layers can be produced with boron concentrations (4.5x1020 cm-3) that are higher than their solubility limit at 700oC. The reproducibility of these layers, along with quick turnaround, offers an excellent possibility for industrial use, with a significant advantage over other competing growth techniques. In relation to the work on the single photon detection devices, these B-doped layers offer an interesting etch resistant capability. Suspended structures (wires and membranes) have been produced and characterized using synchrotron measurements. Layers have shown small levels of strain, similar to structures made using Germanium, but overall exhibit a flat platform for further growth. This has led to the idea that suspending a single photon detector that incorporates a reflective mirror could enhance detection efficiency