Antoon Theuwis

Experimental and theoretical evidence for vacancy-clustering-induced large voids in Czochralski-grown germanium crystals

Optical inspection of polished Czochralski-grown Ge wafers typically reveals the presence of surface pits similar to the Crystal Originated Particles (COP’s) observed in silicon but in a wider variety of sizes and shapes. Computer simulation of vacancyclustering during the Cz germaniumcrystal growth suggests that the surface pits originate from large voids formed by the diffusion-limited clustering of an excess of vacancies.

Recent progress in understanding of lattice defects in Czochralski-grown germanium: catching-up with silicon

Recent progress is presented in the understanding of grown-in defects in Czochralskigrown germanium crystals with special emphasis on intrinsic point defects, on vacancy clustering and on interstitial oxygen. Whenever useful the results are compared with those obtained for silicon.

Particle Deposition and Removal from Ge Wafers

The past 40 years, a lot of research has been done on Si transistors. In many ways, Si has been nature’s gift to engineers. However, Moore’s law is pushing scientists to the edge of the possibilities of downscaling. As a result, Ge is reappearing in semiconductor research because of its high low field mobility [1]. However, the cleaning of Ge surfaces has not been studied very extensively in the past. In this work, we want to focus on the cleaning of Ge surfaces. Removing organic, particle and metallic contamination is a prerequisite for device manufacturing [2]. The cleaning of metallic contamination has been discussed elsewhere [3]. In this work, we present the results on particle removal tests.

Chapter 1 – Germanium Materials

Publisher Summary This chapter discusses germanium (Ge) fabrication techniques. Other than the manufacturing of Czochralski Ge substrates, this chapter also discusses the possible approaches for making germanium-on-insulator (GOI) materials. The chapter reviews the present status and future outlook for 200 and 300 mm wafers. Although at the early transistor technology development, the quality of Ge crystals was far better than thos of Silicon (Si), Si has been dominating the semiconductor market for the past 40 years. Ge material improvements have been focusing on other market segments–– such as detectors and solar cells. However, because of the potential revival of Ge for deep submicron complementary-metal-oxide-semiconductor (CMOS) applications, much effort has been devoted in recent years to fabricate high-quality 200 and 300 mm Ge wafers.

On the Impact of Metal Impurities on the Carrier Lifetime in N-type Germanium

ABSTRACT The impact of metallic impurities on the carrier lifetime in n-Ge is studied using microwave reflection and absorption techniques. Co, Fe, Ti, Ni and Cr are introduced by ion implantation followed by a thermal anneal and quenching to room temperature. Excess carrier decay transients are examined by microwave reflection and absorption probing after pulsed light excitation. A detailed analysis allows to evaluate the ratio of the capture cross-sections for minority and majority carriers revealing an acceptor-like character of the metal induced traps. Cross-sectional lifetime measurements show an U-shaped depth distribution with the lowest lifetimes in the bulk of the wafer. The lifetime results are correlated with those of deep level transient spectroscopy in order to clarify the properties of the dominant metal related recombination centres. Fe and Co are the most effective lifetime killers in n-Ge while Cr has the least influence. INTRODUCTION There is a strong interest to use Ge layers as active device layers in deep submicron CMOS technology for the production of high frequency devices. In advanced silicon technology, metal films are used in various processing steps and also the use of germanides for contacting purposes is actively pursued. As metal impurities can be detrimental for carrier lifetime, there is a revival of the investigation of metal related recombination centers in Ge. Transition metals are fast diffusing contaminants in Ge and may distribute inhomogeneously within device structures and can lead to U-shaped depth profiles throughout the wafer thickness [1,2]. Several shallow and deep trap levels can be attributed to the presence of metal impurities [3]. The aim of this work is a comparative analysis of recombination characteristics in n-Ge after ion implantation with Co, Fe, Ti, Ni, or Cr, using different fluences and thermal anneals. The impact of metallic impurities on the carrier lifetime is studied by using microwave absorption and reflection transient techniques. It is shown that high densities of implantation related defects can cause a redistribution of carrier capture and recombination flows by trap filling [4-6] and can cause charge sign inversion effects when recombination processes are more complicated than the Shockley-Read-Hall mechanism. The electrical activity of the metal implantation induced recombination defects depends on the thermal anneal after metal implantation. The anneal temperature and time determine not only the annealing of the implantation related defects but also the in- and out-diffusion of the metals as well as their state (interstitial/substitutional) within the lattice [3]. The

Grown-in defects in germanium

In contrast to silicon, little is known about the possible gettering approaches that can be applied to Ge. The type of harmful defects is depending on the application that is envisaged. Different vacancy and interstitial related defects are studied in function of the crystal growth parameters. In this chapter, a review is given of the main grown-in lattice defect issues that can be encountered during germanium crystal growth and their relation with the application, which an individual has in mind. Most of the available data on material properties of germanium are several decades old.

Characterization of Ge Implanted with Ni and Hf Ions

Since the early days of Ge research, the impact of transition metals on the electrical characteristics (resistivity, carrier lifetime, ...) has been well documented [1]. The development of advanced CMOS on Ge substrates has renewed the interest of metal contamination during processing on the properties of the material. Besides the traditional fast diffusing transition metals (Ni, Cu and Fe) other less common metals are nowadays also of interest: the implementation of so-called high-k gate dielectrics introduces metals like Hf, La,... in the process flow [2]. However, not much is known about the properties (diffusivity, deep levels) of these metals in Ge. In an attempt to fill the gap in our knowledge, two doses of Ni and Hf have been introduced in 100mm diameter nand p-type Ge substrates by ion implantation, placing the peak concentration at 45 and 43nm deep, respectively. Annealing has been performed at 350C for 1min (Ni) and 500°C for 5min (Hf) and at 700C for 1hr. The latter anneal serves to activate the metals and remove the implantation damage. A combination of chemical (SIMS), structural (TEM) and electrical techniques (Microwave Absorption lifetime measurements, DLTS) has been applied. As will be shown, both Ni and Hf reduce significantly the carrier lifetime, which can be ascribed to the deep levels found by DLTS and the extended defects observed in TEM. Both SIMS (Secondary Ion Mass Spectroscopy) and x-TEM (cross section Transmission Electron Microscopy) will probe the upper layer of the substrate. Hf was detected by TofSIMS. The as-implanted profile corresponds to a typical implantation profile. Ni was detected by SIMS and as expected the peak position was ~50nm for the as-implanted sample. When annealing at 350°C, a segregation towards the substrate takes place as well as a loss of the original dose. The sample annealed at 700°C shows an unexpected profile: the dose that can be extracted from the profile is higher than the dose for the as-implanted sample. By further analysis using other techniques (TEM and optical microscopy), we could detect some extended defects at the surface of this sample. These defects can play a role in the SIMS measurement but do not clarify the origin of the profile. Further investigation is needed. TEM images were obtained for the Ni implanted samples on n-type Ge. A peak concentration of 5.10 at/cm Ni atoms at a position of 45nm, results in a 200nm amorphous Ge layer. When annealing these samples at 350°C (1min), a 30 nm amorphous layer remains and a high density of defects is still present at the amorphous/crystalline interface. When annealing at 700°C, the Ge crystal is fully regrown but large defects are observed at the surface. The formation of NiGe precipitates could lay at the origin of these defects. However, further investigation is needed to support this assumption (XRD measurements). TEM images were also made for the Hf implanted in n-type Ge. Here, 2 series of samples were studied: samples with a peak concentration at 5.10at/cm and at 5.10at/cm. Some differences are observed. The lower concentration gives rise to a partially amorphous layer of 50nm. While the higher concentration shows a fully amorphous layer of 85nm. When annealing the low-dose material, the Ge lattice is recrystallized but small defects are still present in the surface layer. The higher dose, gives rise to fully recrystallized Ge. Both series are fully recrystallized at 700°C. These physical observations can now be linked to some electrical measurements. MCLT (Minority Carrier Lifetime) was measured by microwave photoconductivity. A passivation layer was applied to reduce surface recombination effects. Ge was implanted with Ge to study the influence of the damage on the LT. A Ge sample that was not implanted nor annealed acts as a reference. After implantation, the LT is determined mainly by the amorphous top layer. The LT is significantly reduced due to this damage. When annealing these samples at intermediate temperature, the damage is significantly reduced and this explains the increase in LT. Annealing at 700°C restores the LT back to the original level. The same trends are observed for the Ni and Hf implanted samples. However, annealing at 700°C restores the lattice but is also activating the metal impurities and the life time drops significantly. This indicates that both Hf and Ni impurities introduce trap levels in Ge and are able to influence the MCLT when activated. A more direct way to study these levels introduced by metals in Ge, is by DLTS (Deep Level Transient Spectroscopy). Measurements are planned in the future and will contribute to a deeper insight in the position of these trap levels. To conclude a study was performed on the influence of metals on the physical characteristics of Ge. It can be seen that damage introduced by implantation is determining the lifetime of the carriers. The damage can be restored by annealing but the temperature on which full recrystallization is seen, depends on the metal implanted and on the implantation dose. When restoring the Ge lattice, MCLT values are increasing. For all implantation conditions, full recrystallization is achieved by annealing at 700°C (1h). Only for the Ni implantation, extended defects were seen at the surface. The origin of these defects is yet unclear but the formation of NiGe precipitates could explain this observation. Both Ni and Hf are introducing trap levels in Ge. This can be seen from the fact that after full recrystallization, traps are activated and MCLT values drop significantly for both metals. A further study by DLTS on the position of these trap levels is currently been undertaken.