Douglas J. Paul

Si/SiGe heterostructures: from material and physics to devices and circuits

Silicon germanium (SiGe) has moved from being a research material to accounting for a small but significant percentage of manufactured semiconductor devices. This percentage is predicted to increase substantially as SiGe begins to be used in complementary metal oxide semiconductor (CMOS) technology in the future to substantially improve performance. It is the development of Si/SiGe heterostructures which has enabled band structure and strain engineering allowing Si/SiGe to be used in many different ways to improve conventional microelectronic device performance along with allowing new concepts to be explored. This paper presents a review of the material properties, growth techniques, band structure and the main electronic devices of the Si/SiGe heterostructure system. In particular, the important device technologies in mainstream microelectronics of the SiGe heterostructure bipolar transistor (HBT) and strained-Si CMOS will be reviewed before future device and optoelectronics concepts are explored.

Design and fabrication of memory devices based on nanoscale polyoxometalate clusters

Flash memory devices—that is, non-volatile computer storage media that can be electrically erased and reprogrammed—are vital for portable electronics, but the scaling down of metal–oxide–semiconductor (MOS) flash memory to sizes of below ten nanometres per data cell presents challenges. Molecules have been proposed to replace MOS flash memory, but they suffer from low electrical conductivity, high resistance, low device yield, and finite thermal stability, limiting their integration into current MOS technologies. Although great advances have been made in the pursuit of molecule-based flash memory, there are a number of significant barriers to the realization of devices using conventional MOS technologies. Here we show that core–shell polyoxometalate (POM) molecules can act as candidate storage nodes for MOS flash memory. Realistic, industry-standard device simulations validate our approach at the nanometre scale, where the device performance is determined mainly by the number of molecules in the storage media and not by their position. To exploit the nature of the core–shell POM clusters, we show, at both the molecular and device level, that embedding [(Se(iv)O3)2]4− as an oxidizable dopant in the cluster core allows the oxidation of the molecule to a [Se(v)2O6]2− moiety containing a {Se(v)–Se(v)} bond (where curly brackets indicate a moiety, not a molecule) and reveals a new 5+ oxidation state for selenium. This new oxidation state can be observed at the device level, resulting in a new type of memory, which we call ‘write-once-erase’. Taken together, these results show that POMs have the potential to be used as a realistic nanoscale flash memory. Also, the configuration of the doped POM core may lead to new types of electrical behaviour. This work suggests a route to the practical integration of configurable molecules in MOS technologies as the lithographic scales approach the molecular limit.

Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.

High-performance nMOSFETs using a novel strained Si/SiGe CMOS architecture

Performance enhancements of up to 170% in drain current, maximum transconductance, and field-effect mobility are presented for nMOSFETs fabricated with strained-Si channels compared with identically processed bulk Si MOSFETs. A novel layer structure comprising Si/Si/sub 0.7/Ge/sub 0.3/ on an Si/sub 0.85/Ge/sub 0.15/ virtual substrate (VS) offers improved performance advantages and a strain-compensated structure. A high thermal budget process produces devices having excellent on/off-state drain-current characteristics, transconductance, and subthreshold characteristics. The virtual substrate does not require chemical-mechanical polishing and the same performance enhancement is achieved with and without a titanium salicide process.

Midinfrared Plasmon-Enhanced Spectroscopy with Germanium Antennas on Silicon Substrates

Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.

Silicon germanium heterostructures in electronics: the present and the future

A review is presented of Si/Si1−xGex heterostructures which may be used for electronic transistors along with more exotic designs of electrical devices including quantum effect devices. Important issues including integration of the devices with present technology along with defect problems will be presented. The final section will explore exotic quantum devices and review the potential for different SiGe devices to reach the market place.

Silicon as a model ion trap: Time domain measurements of donor Rydberg states

One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr–Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus- and arsenic-doped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times—key to many well known schemes for quantum computing qubits—for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.

Nanofabrication of high aspect ratio (∼50:1) sub-10 nm silicon nanowires using inductively coupled plasma etching

The development of nanofabrication techniques for creating high aspect ratio (∼50:1) sub-10 nm silicon nanowires (SiNWs) with smooth, uniform, and straight vertical sidewalls using an inductively coupled plasma (ICP) etching process at 20 °C is reported. In particular, to improve the quality and flexibility of the pattern transfer process for high aspect ratio SiNWs, hydrogen silsesquioxane, a high-resolution, inorganic, negative-tone resist for electron-beam lithography has been used as both the resist for defining sub-10 nm patterns and the hard mask for etching the underneath silicon material. The effects of SF6/C4F8 gas flow rates, chamber pressure, platen power and ICP power on the etch rate, selectivity, and sidewall profile are investigated. To minimize plasma-induced sidewall damage, moderate plasma excitation power (ICP power of 600 W) and low ion energy (platen power of 6–12 W) were used. Using the optimized etch process at room temperature (20 °C), the authors have successfully fabricated sub-1...

Ohmic contacts to n-type germanium with low specific contact resistivity

A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) × 10−7 Ω-cm2 for anneal temperatures of 340 °C. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.

Measurement of the Earth tides with a MEMS gravimeter

The ability to measure tiny variations in the local gravitational acceleration allows, besides other applications, the detection of hidden hydrocarbon reserves, magma build-up before volcanic eruptions, and subterranean tunnels. Several technologies are available that achieve the sensitivities required for such applications (tens of microgal per hertz1/2): free-fall gravimeters, spring-based gravimeters, superconducting gravimeters, and atom interferometers. All of these devices can observe the Earth tides: the elastic deformation of the Earth’s crust as a result of tidal forces. This is a universally predictable gravitational signal that requires both high sensitivity and high stability over timescales of several days to measure. All present gravimeters, however, have limitations of high cost (more than 100,000 US dollars) and high mass (more than 8 kilograms). Here we present a microelectromechanical system (MEMS) device with a sensitivity of 40 microgal per hertz1/2 only a few cubic centimetres in size. We use it to measure the Earth tides, revealing the long-term stability of our instrument compared to any other MEMS device. MEMS accelerometers—found in most smart phones—can be mass-produced remarkably cheaply, but none are stable enough to be called a gravimeter. Our device has thus made the transition from accelerometer to gravimeter. The small size and low cost of this MEMS gravimeter suggests many applications in gravity mapping. For example, it could be mounted on a drone instead of low-flying aircraft for distributed land surveying and exploration, deployed to monitor volcanoes, or built into multi-pixel density-contrast imaging arrays.