Igor Romandic

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.

First principles calculations of the formation energy and deep levels associated with the neutral and charged vacancy in germanium

Density functional theory with local density approximation including on-site Coulomb interaction has been used to calculate the formation energy of the neutral and charged vacancy in germanium as a function of the Fermi level. The calculations suggest that vacancies in germanium are multiple-level acceptors with a first level at 0.02eV and a second level at 0.26eV above the valence band maximum in agreement with published experimental data. The formation energies of the neutral and charged vacancies line up well with the experimental values estimated from quenching experiments.

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.

First Principles Calculations of the Formation Energy of the Neutral Vacancy in Germanium

Density functional theory (DFT) with local density approximation has been used to calculate the formation energy (EF) of the neutral vacancy in germanium single crystal. It was shown that careful checking of convergence with respect to the number of k-points is necessary when calculating the formation energy of the intrinsic point defects in Ge. The formation energy of the single neutral vacancy was estimated at 2.35 eV which is in excellent agreement with published experimental data.

Formation of germanates on germanium by chemical vapor treatment

Optical properties of dislocations free state-of-the-art Germanium(Ge) and Germanium-oninsulator(GeOI) wafers have been characterized using Fourier transformed infrared spectroscopy at oblique incidence, attenuated total reflectance, laser Raman scattering, linear and nonlinear optical transmission. In n-type Ge, in addition to vibrational modes observed in intrinsic(i) Ge, a band at 535cm-1 which is likely due to carbon and a strong peak at 668 cm-1 were observed at non-normal incidence. Despite the strong heavy hole to light hole absorption band at low energies, the 668 cm-1 peak was also observed in p-Ge. The appearance of new bands and the enhancement in band strength are in general observed in both type of wafers at oblique incidence. GeOI exhibits a strong disorder induced LO-TO coupling mode which can only be observed at non-normal incidence. Optical absorption at the near bang edge reveals the presence of doping related disorder and band shrinkage, which is supported also by Ge-Ge one-phonon line broadening at 301 cm-1. Different nonlinear optical absorption behavior was observed in n-Ge, p-Ge and GeOI wafers. The p-Ge becomes transparent to CO2 laser line at 10.6 micrometer, while transmitted power decreases in n-Ge with increasing UV-VIS pump power.The surface of a single crystal Germanium wafer was transformed to fluoride and oxide crystals upon exposure to a vapor of HF and HNO3 chemical mixture. Ellipsometry. X-ray, SEM and photoluminescence were used to investigate the physical properties of the resultant surface structure The analysis indicates that the transformation results in a polycrystalline hexagonal ammonium fluogermanates and a hexagonal alpha-Germanium oxide Clusters with a preferential crystal growth orientation in direction The fluogermanates grow particularly around the germanium oxide Clusters as evidenced by electron dispersive spectroscopy profiling Local vibrational mode analysis confirm the presence of N-H and Ge-F vibrational modes of NH4+ and GeF6- ions. The vibrational modes at around 840 cm(-1) is significative of GeOx stretching bands originating from the partial coverage surface oxide formed together with the fluogermanates and clusters oil the Germanium Electronic band structure as probed by ellipsometry is typical of Ge and any discrepancy was associated with disorder induced band tailing effects originating possibly from the effect of oxide clustering.

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.

Characterization of bulk microdefects in Ge single crystals

The work reported here concerns the characterization of bulk microdefects in germanium single-crystal wafers. From comparison with the case of silicon, it is expected that bulk microdefects are formed due to diffusion and interaction of self-interstitials and vacancies during Czochralski growth, solely dependent on the growth parameters. Unfortunately, several of the defect characterization methods, which can be used for silicon, fail in the case of germanium due to its lower band gap. Therefore, the possibilities of two alternative techniques, x-ray topography and small-angle x-ray scattering, respectively, were investigated. The results, combined with a characterization of the material surface using scanning laser reflectometry, indicate that in Ge, the same kind of intrinsic growth-related defects exist, as in Si.

A Comparison of Intrinsic Point Defect Properties in Si and Ge

Intrinsic point defects determine to a large extent the semiconductor crystal quality both mechanically and electrically not only during crystal growth or when tuning polished wafer properties by thermal treatments, but also and not the least during device processing. Point defects play e.g. a crucial role in dopant diffusion and activation, in gettering processes and in extended lattice defect formation. Available experimental data and results of numerical calculation of the formation energy and diffusivity of the intrinsic point defects in Si and Ge are compared and discussed. Intrinsic point defect clustering is illustrated by defect formation during Czochralski crystal growth.

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.

Large diameter germanium wafers for CPV applications

Umicore has a long tradition in supplying germanium wafers to the space solar cell market. The main product for this application up to today is a substrate with a diameter of 100 mm and a thickness in the range of 140 – 180 ¼m. With the new emerging market of high-efficiency concentrator photovoltaics (CPV) the need has arisen to investigate the use of germanium wafers with a larger diameter. Especially for small size CPV cells, increasing the wafer diameter will significantly reduce the processing cost per die, which will also contribute to lowering the overall cost/kWh of CPV technology. In this paper the development of dislocation free 150 mm germanium wafers for CPV applications is presented. Different wafer thicknesses down to 200 ¼m have been realized. The process steps starting from crystal pulling up to final epi-cleaning and drying will be addressed. First measurement results on wafer level are shown as well. Finally, the future challenges in optimizing the 150 mm wafer specifications will be reviewed.