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Dr. DI Nils Sandersfeld

PhD thesis:

Electrical Characterization and Device Applications of Si/SiGeC Heterostructure

Abstract

In this work, Si/SiGe heterostructures are investigated concerning electrical properties and device applications. A technological process is applied in order to fabricate integrated test structures and devices on Si/SiGe samples. The main goal is the development of MODFETs (modulation-doped field-effect transistors) which require Ohmic and Schottky contacts. These contacts are separately developed and characterized by test structures to obtain a well working transistor. From the electrical characterization, important parameters are extracted concerning the electrical properties and the material quality of the Si/SiGe heterostructure.

The charge carrier density and mobility are measured by Hall bars at various temperatures. Strained Si channels on graded SiGe buffers show electron mobilities of up to 120,000cm2/Vs at low temperatures, while low-temperature buffers still suffer from too many defects leading to mobilities of only 60,000cm2/Vs. At room temperature, parallel conduction degrades Hall mobilities, but the obtained values of about 2000cm2/Vs are still higher than in bulk Si.

High-quality Ohmic contacts are provided by ion implantation and subsequent annealing. Annealing temperatures have been tested from 600°C to 680°C. While low-temperature transport is nearly not affected in this temperature range, Ohmic contact quality and homogeneity is well improved.

Schottky contact quality is estimated by I-V and C-V measurements. Pd has been chosen as gate material on n-type samples, and the expected Schottky barrier height of 0.75eV is well reproduced. C-V measurements on Schottky diodes at 77K show the expected plateaus from which the conduction channel depths can be derived.

Both n-channel and p-channel MODFETs are fabricated with varying gate lengths. Also a transistor with a gate length of below 100nm has been developed. On two samples with different n-type doping concentrations, enhancement-mode and depletion-mode devices are realized. By measuring the transistor characteristics at various gate lengths, the electron saturation velocity is estimated. The obtained value of about 107cm/s corresponds to theoretical predictions.

Electron spin resonance (ESR) investigations show a signal that originates from the 2-dimensional conduction electrons. The ESR signal is shown to be directly proportional to the density of states (DOS) at the Fermi level. By measuring the integrated ESR amplitude as a function of gate voltage or illumination dose, a tail in the 2D DOS is observed instead of the expected step-like behavior. This tail is attributed to potential fluctuations in the doping region which are also quantified by calculations.

Applying Thomas-Fermi screening, it is shown that the ESR integrated amplitude can directly be connected to transport properties via the Thomas-Fermi screening wave vector. Localization of electrons by a breakdown of screening of potential fluctuations is identified by this method.

ESR measurements on SiGe quantum wells with various Ge contents provide a shift in g-factor that is sufficient to define distinct regions of microwave absorption in a Si/SiGe structure. This is an important step toward an ESR quantum computing device proposed in the literature.

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