123,156 research outputs found
Fabrication and characterisation of novel Ge MOSFETs
No full textAs high-k dielectrics are introduced into commercial Si CMOS (Complimentary Metal Oxide Semiconductor) microelectronics, the 40 year channel/dielectric partnership of Si/SiO2 is ended and the door opened for silicon to be replaced as the active channel material in MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). Germanium is a good candidate as it has higher bulk carrier mobilities than silicon. In addition, Si and Ge form a thermodynamically stable SiGe alloy of any composition, allowing Ge to be implemented as a thin layer on the surface of a standard Si substrate. This thesis is a practical investigation on several aspects of Ge CMOS technology.
High-k dielectric Ge p-MOSFETs are electrically characterised. A large variation in interface state densities is demonstrated to be responsible for a threshold voltage shift and this is proportional to reciprocal peak mobility due to the Coulomb scattering of carriers by charged states. A theoretical mobility is fitted to that measured at 4.2 K and confirms that interface states are the main source of interface charged impurities.
The model demonstrates a reduction in the interface charged impurity density in p-MOSFETs that underwent a PMA (Post Metallisation Anneal) in hydrogen atmosphere and that the anneal also reduces the RMS (Root Mean Square) dielectric/semiconductor interface roughness, from an average of 0.60 nm to 0.48 nm.
High-k strained Ge p-MOSFETs are electrically characterised and have peak mobilities at 300 K (470 cm2 V-1 s-1) and 4.2 K (1780 cm2 V-1 s-1) far in excess of those measured for the unstrained Ge p-MOSFETs (285 cm2 V-1 s-1,785 cm2 V-1 s-1 respectively). Strained Ge n-MOSFETs perform significantly worse than standard Si P, - MOSFETs primarily due to a high source/drain resistance.
A 10 nm thick SiGe-01 (On Insulator) layer with a Ge composition of 58% is obtained from a 55 nm Si0_88Ge1o2. initial layer on 100 nm Si-Ol substrate via the germanium condensation technique. For the first time, germanium is demonstrated to diffuse through the BOX (Buried OXide) during Ge-condensation and into the underlying Si substrate. An order of magnitude increase in the calculated ITOX (Internal Thermal OXidation) rate of the BOX in the final stages of Ge-condensation is hypothesised to be responsible for stopping this diffusion
Electrodeposited Ni/Ge and germanide schottky barriers for nanoelectronics applications
No full textIn recent years metal/semiconductor Schottky barriers have found numerous applications in nanoelectronics. The work presented in this thesis focuses on the improvement of a few of the relevant devices using electrodeposition of metal on Ge for Schottky barrier fabrication. This low energy metallisation technique offers numerous advantages over the physical vapour deposition techniques. Electrical characteristics of the grown diodes show a high quality rectifying behaviour with extremely low leakage currents even on highly doped Ge. A non-Arrhenius behaviour of the temperature dependence is observed for the grown Ni/Ge diodes on lowly doped Ge that is explained by a spatial variation of the barrier heights. The inhomogeneity of the barrier hights is explained in line with an intrinsic surface states model for Ge. The understanding of the intrinsic surface states will help to create ohmic contacts for doped n-MOSFETs. NiGe were formed single phase by annealing. Results reveal that by using these high-quality germanide Schottky barriers as the source/drain, the subthreshold leakage currents of a Schottky barrier MOSFET could be minimised, in particular, due to the very low drain/body junction leakage current exhibited by the electrodeposited diodes. The Ni/Ge diodes on highly doped Ge show negative differential conductance at low temperature. This effect is attributed to the intervalley electron transfer in Ge conduction band to a low mobility valley. The results show experimentally that Schottky junctions could be used for hot electron injection in transferred-electron devices. A vertical Co/Ni/Si structure has been fabricated for spin injection and detection in Si. It is shown that the system functions electrically well although no magnetoresistance indicative of spin injection was observed
Ge-on-Si single-photon avalanche diode detectors: design, modeling, fabrication, and characterization at wavelengths 1310 and 1550 nm
No full textThe design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ~ 6 megacounts/s, resulting in the lowest reported noise-equivalent power for a Ge-on-Si single-photon avalanche diode detector (1×10-14 WHz-1/2). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies
Low-frequency noise characterization of strained germanium pMOSFETs
No full textLow-frequency noise in strained Ge epitaxial layers, which are grown on a reverse-graded relaxed SiGe buffer layer, has been evaluated for different front-end processing conditions. It has been shown that the 1/f noise in strong inversion is governed by trapping in the gate oxide (number fluctuations) and not affected by the presence of compressive strain in the channel. However, some impact has been found from the type of halo implantation used, whereby the lowest noise spectral density and the highest hole mobility are obtained by replacing the standard As halo by P implantation. At the same time, omitting the junction anneal results in poor device characteristics, which can be understood by considering the presence of a high density of nonannealed implantation damage in the channel and the gate stack near the source and the drain
Role of F on the Electrical Activation of As in Ge
No full textFluorine is known to strongly influence the dopant diffusion and electrical activation in Si. Similar effects might be exploited in Ge for optimizing its application in microelectronics. The role of F on the electrical activation of As in Ge after thermal treatments is elucidated. We have found that F, enriching the Ge matrix with vacancies strongly affects the electrical response of As-doped junctions. We also demonstrated that the F-interstitials clusters, formed next to the end-of-range region, have an acceptor-like behavior. These phenomena are characterized by chemical and electrical profiling analyses and by positron annihilation lifetime spectrometry. (C) 2012 The Electrochemical Society. All rights reserved
Laser-vibrometric ultrasonic characterization of resonant modes and quality factors of Ge membranes
No full textThe vibrations of a single-crystal germanium (Ge) membrane are studied in air and vacuum using laser vibrometry, in order to determine mechanical properties such as Q-factors, tensile stress, anisotropy, and robustness to shock. Resonance modes up to 3:2 are identified, giving a residual stress measurement of 0.22 GPa, consistent with the value obtained from x-ray relaxation studies. The membrane is found to be isotropic, with Q-factors ranging from around 40 at atmospheric pressure to over 3200 at 5 x 10-4 mbar, significantly lower than those found in polycrystalline Ge micromechanical devices. The robustness to shock is explained through the high resonance mode frequencies and the dissipation mechanism into the substrate, which is a direct consequence of having a high quality film with low residual tensile stress, giving the potential for such films to be used in optoelectronic devices
Fluorine in Ge: Segregation and EOR-defects stabilization
No full textIn this paper we investigate the F behavior in Ge during solid phase epitaxy (SPE) and post-SPE annealing. Fluorine implanted with a fluence of 1 x 10(15) F/cm(2) and an energy of 35 key induced the formation of an amorphous Ge layer. Detailed chemical and structural characterizations of the as implanted and annealed samples evidenced a strong segregation of F at the moving amorphous/crystalline interface, leading to a remarkable SPE rate retardation. In addition, we observed that F accumulates in correspondence of the end of range (EOR) defects. The comparison between the thermal evolution of damage produced by self-implantation and F implantation in Ge suggests that F increases significantly the stability of EOR. Such behavior clarifies the role of F in modifying the As diffusion in Ge recently reported in literature. (C) 2011 Elsevier B.V. All rights reserved
Mechanism of vertical Ge nanowire nucleation on Si (111) during subeutectic annealing and growth
No full textThe direct integration of Ge nanowires with silicon is of interest in multiple applications. In this work, we describe the growth of high-quality, vertically oriented Ge nanowires on Si (111) substrates utilizing a completely sub-Au-Si-eutectic annealing and growth procedure. With all other conditions remaining identical, annealing below the Au-Si eutectic results in successful heteroepitaxial nucleation and growth of Ge nanowires on Si substrate while annealing above the Au-Si eutectic leads to randomly oriented growth. A model is presented to elucidate the effect of the annealing temperature, in which we hypothesized that sub-Au-Si-eutectic annealing leads to the formation of a single and well-oriented interface, essential to template heteroepitaxial nucleation. These results are critically dependent on substrate preparation and lead to the creation of integrated nanowire systems with a low thermal budget process
Fluorine effect on As diffusion in Ge
No full textThe enhanced diffusion of donor atoms, via a vacancy (V)-mechanism, severely affects the realization of ultrahigh doped regions in miniaturized germanium (Ge) based devices. In this work, we report a study about the effect of fluorine (F) on the diffusion of arsenic (As) in Ge and give insights on the physical mechanisms involved. With these aims we employed experiments in Ge co-implanted with F and As and density functional theory calculations. We demonstrate that the implantation of F enriches the Ge matrix in V, causing an enhanced diffusion of As within the layer amorphized by F and As implantation and subsequently regrown by solid phase epitaxy. Next to the end-of-range damaged region F forms complexes with Ge interstitials, that act as sinks for V and induce an abrupt suppression of As diffusion. The interaction of Ge interstitials with fluorine interstitials is confirmed by theoretical calculations. Finally, we prove that a possible F-As chemical interaction does not play any significant role on dopant diffusion. These results can be applied to realize abrupt ultra-shallow n-type doped regions in future generation of Ge-based devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3592962
Strain-relaxed, high Ge content, SiGe layers grown on Si (100) substrate by reduced pressure - chemical vapour deposition (RP-CVD)
No full textA different approach was taken to relieve strain from a high Germanium (Ge) content,
Silicon-Germanium (SiGe) layers on a Silicon (Si) (100) substrate by growing a thin Ge
under-layer between substrate and layer. The Ge under-layer acts as a strain reliving platform
for further growth of a high Ge content SiGe layer to improve the structural quality of the
sample by reducing the Root Mean Squared Roughness (RRMS) and threading dislocation density
(TDD).
The proposed structure involves the growth of thin Si0.3Ge0.7 and Si0.5Ge0.95 buffer layers of
an average thickness of 350 nm grown on a Si (100) substrate and their structural qualities
assessed. Experimental techniques include High Resolution X-Ray Diffraction, Atomic Force
Microscopy, Transmission Electron Microscopy, and Defect Etching. All samples were
shown to be fully relaxed and have a surface roughness between 1-8 nm. However, a
threading dislocation density of 109 cm-2 was witnessed. Although these results are the first of
their kind, further research into improving structural qualities is to be investigated in the
future
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