Hydrogen-Related Defects in Silicon, Germanium, and Silicon–Germanium Alloys

CONTENTS 2.1 Introduction ........................................................................................................................ 27 2.2 Incorporation, Diffusion, and Solubility of Hydrogen................................................. 29

2.2.1 Incorporation ........................................................................................................... 29 2.2.2 Diffusion................................................................................................................... 30 2.2.3 Solubility .................................................................................................................. 33

2.3 Isolated Hydrogen ............................................................................................................. 34 2.4 Passivation .......................................................................................................................... 37 2.5 Electrically Active Hydrogen Complexes ...................................................................... 38

2.5.1 Vacancy-Hydrogen Complexes............................................................................ 38 2.5.2 VOH.......................................................................................................................... 39 2.5.3 CH ............................................................................................................................. 40 2.5.4 Transition Metal-Hydrogen Complexes ............................................................. 41 2.5.5 Au-Hn ....................................................................................................................... 41 2.5.6 Pt-H2......................................................................................................................... 42

2.6 Hydrogen Molecules ......................................................................................................... 43 2.7 Hydrogen Clusters and Platelets..................................................................................... 44 2.8 Hydrogen in Germanium................................................................................................. 44

2.8.1 Hydrogen Incorporation, Diffusivity, and Solubility in Ge ............................. 45 2.8.2 Isolated Hydrogen Species and Complexes with Intrinsic Defects in Ge ...... 45 2.8.3 Hydrogen Passivation of Deep Centers and Activation

of Neutral Impurities in Ge................................................................................... 46 2.8.4 Device-Related Aspects of Hydrogen in Ge ....................................................... 46

2.9 Hydrogen in Silicon-Germanium Alloys....................................................................... 47 2.10 Summary............................................................................................................................. 49 References...................................................................................................................................... 49

Hydrogen is a very common impurity in semiconductors and has been investigated in great detail in silicon. There is also a small amount of work on hydrogen in silicon-germanium and germanium. Czochralski-grown ingots of silicon and germanium contain almost no

near surface regions and in some cases deep in the bulk of the materials. How hydrogen is incorporated into silicon and germanium is discussed in detail in Section 2.2, so here it is sufficient to say that processes such as chemical etching, polishing, and plasma processing release considerable quantities of atomic hydrogen, which is free to diffuse and react with the semiconductor. In some technological processes hydrogen is deliberately incorporated, a classic case is where silicon metal-oxide semiconductor (MOS) devices are heated in forming gas at about 4508C to passivate interface traps or surface states, a process that was patented by Fowler in 1974 [1]. This passivation process is discussed in Chapter 7 of this book, but quite obviously there is the potential for hydrogen to diffuse into the semiconductor itself. The incorporation of hydrogen in silicon and germanium is a rather complex process

that depends on the form of the hydrogen and the doping and impurity content of the semiconductor. Hydrogen molecules diffuse in silicon, but more importantly, so do atomic defects of hydrogen. These can exist in positively or negatively charged states or, exceptionally, in neutral states. The charged single atomic species are extremely reactive and are commonly observed to passivate shallow donors and acceptors as well as deep-level defects. Much work has been devoted to studying the reaction products of various defect species with hydrogen. This reactivity makes the diffusion behavior of atomic hydrogen at lower temperatures somewhat complex; a key issue is its trapping at various impurity sites. The diffusion depends on the detail of the silicon doping, the saturation of capture sites, and the subsequent thermal release of the hydrogen from these trapping sites. In consequence, the solubility and diffusion of hydrogen in highly doped silicon is dramatically different than that of its behavior in undoped material. Isolated hydrogen atoms are known to exist in silicon, silicon-germanium, and germa-

nium at low temperatures; various techniques have been used to determine their precise position in the semiconductor lattice. Similarly detailed work has been conducted on hydrogen passivation and on electrically active hydrogen complexes. Although this chapter is about hydrogen, it is important to say that deuterium is often used independently or in combination in studies of hydrogen-related defects. The reasons for this are that (1) secondary ion mass spectrometry (SIMS) has a much higher sensitivity to deuterium than hydrogen and (2) that some techniques such as local mode spectroscopy can distinguish between hydrogen and deuterium, thereby making the identities of complexes (especially those containing more than one hydrogen atom) easier to resolve. In general, the total concentration of hydrogen in silicon determined by SIMS is much

higher than the concentration of hydrogen and hydrogen complexes determined their electrical or optical measurements. Combinations of theoretical studies and optical work have revealed that this is due to the presence of hydrogen molecules. These are neutral and, in their usual siting within the silicon lattice, are largely electrically and optically inactive. However, much work has now been done to resolve the weak optical activity of molecular species using Raman studies and optical absorption techniques. In recent years, this work has been given a particular stimulus by the development of the SmartCut process for producing silicon on insulator wafers [2]. In this technique, light ions (predominantly hydrogen) are implanted at high concentrations to produce gas bubbles just below the surface. Subsequent heat treating can then be used to defoliate a thin uniform layer. This behavior is now used extensively to produce silicon on insulator wafers for high performance, extremely scaled complementary MOS (CMOS), and indeed for many other applications. The technique can be applied to silicon, silicon-germanium, and germanium, and is now of considerable commercial importance. All these issues are discussed in detail in the subsequent sections of this chapter, but attention should be drawn to a number of previous reviews of hydrogen in silicon and related materials and also to conference proceedings focused specifically to hydrogen in silicon [3-8].