Determination of the Complex Dielectric Function of Ion-Implanted Amorphous Germanium by Spectroscopic Ellipsometry

Tivadar Lohner,Endre Kótai,Peter Petrik,Levente Illés,Edit Szilágyi,Zsolt Fogarassy,Attila Németh,Zsolt Zolnai,Miklós Fried

doi:10.3390/coatings10050480

Abstract

Accurate reference dielectric functions play an important role in the research and development of optical materials. Libraries of such data are required in many applications in which amorphous semiconductors are gaining increasing interest, such as in integrated optics, optoelectronics or photovoltaics. The preparation of materials of high optical quality in a reproducible way is crucial in device fabrication. In this work, amorphous Ge (a-Ge) was created in single-crystalline Ge by ion implantation. It was shown that high optical density is available when implanting low-mass Al ions using a dual-energy approach. The optical properties were measured by multiple angle of incidence spectroscopic ellipsometry identifying the Cody-Lorentz dispersion model as the most suitable, that was capable of describing the dielectric function by a few parameters in the wavelength range from 210 to 1690 nm. The results of the optical measurements were consistent with the high material quality revealed by complementary Rutherford backscattering spectrometry and cross-sectional electron microscopy measurements, including the agreement of the layer thickness within experimental uncertainty.

Highlights

The amorphous Ge layer was formed on top of the crystalline Ge (c-Ge) substrate
The thickness of the ion bombardment-amorphized Ge (ia-Ge) layer agrees with the Stopping and Range of Ions in Matter (SRIM) simulation within the experimental uncertainty of RBS
The complex dielectric function of ia-Ge produced by ion implantation was determined by spectroscopic ellipsometry (SE)

Summary

Introduction

Accurate and reliable optical data of materials are scarce in the literature, they are of key importance for the modeling of coatings, as well as optical or structural materials [1,2]. Ge (a-Ge) films, papers dealing with the optical and structural characterization of evaporated Ge layers can be found in the literature [12,13,14,15,16], and only a few papers discuss the optical and structural characterization of a-Ge layers obtained by low energy (0.5–1.0 keV) ion bombardment [17,18]. They performed SE measurements and determined the dielectric function of the ion bombardment-amorphized Ge (ia-Ge) layers. They obtained a 9 nm thick ia-Ge layer for 1-keV Ne bombardment and determined its dielectric function [17] This layer thickness can be considered as ultra-thin and even an atomically thin transition “layer” between the c-Ge and ia-Ge region can cause more than 10% uncertainty. To describe the optical properties on an amorphous material as a function of photon energy or wavelength the Tauc-Lorentz (TL) or Cody-Lorentz (CL) dispersion models are frequently used. The results of the optical characterizations were verified by complementary methods including Rutherford backscattering spectrometry combined with channeling (RBS/C) and cross-sectional transmission electron microscopy (XTEM)

Experimental Details

Results and Discussion

Conclusions

Methods