Ion Beam Mixing for Processing of Nanostructure Materials

Journal of Electronic Materials, May 2006 by Abedrabbo, S, Arafah, D E, Gokce, O, Wielunski, L S, Et al

Ion beam mixing (IBM) has been used to process various nanostructure materials and thin films for applications in microelectronics and optoelectronics. In this paper, a study of alloy formation of Si-Ge, processed at shallow depths followed by oxygen implantation, is presented. The mixture is annealed to form Si-GeO^sub 2^-Si, wherein the germanium oxide may form alone or as a matrix with the source of excitation. Characterization techniques used in this study include investigations of the structural variations due to argon ion-beam irradiation by Rutherford backscattering (RBS) and shallow defects and deep trapping level states by thermoluminescence (TL) measurements. Fourier transform infrared (FTIR) spectroscopy is used to analyze the thin film/islands of GeO^sub 2^ formed in the matrix.

Key words: Ion beam mixing (IBM), oxidation, SiGe

INTRODUCTION

Silicon-germanium (Si-Ge) alloys are already a major player in high-frequency applications such as cellular phones. These alloys are competing with direct bandgap compound semiconductors and are used in the formation of high-mobility strained silicon layers for high-performance applications in silicon microelectronics.1,2 Record electron and hole mobilities have been measured at low temperatures3,4 for these alloys. The enhanced speed of Si-Ge devices surpasses at its peak the bulk of wireless circuit operating frequencies. These devices also have the capability of operating at lower power levels.1 Thus, Si-Ge alloys have enabled designers to merge low-power, high-density digital circuitry of complementary metal oxide semiconductor (CMOS) with the high speed of Si-Ge heterojunction bipolar transistor (HBT). This has led to the development of advanced BiCMOS processes that are ideally suited for highly integrated mixed signal circuits for applications in communications.5 In the area of wired communication applications such as Synchronous Optical Networks [SONET OC-768 laid/dispatched in Wide Area Network (WAN), or 40-Gb/s transport], Si-Ge devices with 0.13-0.18 �m technology nodes are used, in which large-scale integration is feasible and extreme speeds of up to 210 GHz are attainable with Si-Ge HBT.1 Further, Si-Ge devices have proven power-added efficiencies, reaching 70%,6 permitting the use of Si-Ge HBT as power amplifiers. Self-assembled Ge dots in Si are candidates for resonant tunneling devices and memories by using the large valence band offset.7

Another interest in this alloy system is to be able to perform bandgap engineering to obtain efficient and inexpensive photovoltaic cells,8'9 with tailored bandgap that extends the infrared response of the cell and increases the photo-generation and consequent current generation (or collection). Earlier investigations reveal that bandgaps of alloys could be tailored from 1.12 eV to 0.66 eV.9-15

In this work, the interest in enhanced emission efficiency is the motivation to study the creation of thin films/islands of GeO/O2 embedded into bulk silicon and SiO^sub 2^. The next step involves the coupling of the new matrix with rare-earth metals to further enhance the light emission and increase internal quantum efficiency of silicon-based devices. Among other interests in processing Si-Ge via ion beam mixing (IBM) are the following: (a) recrystallization/defect minimization due to multiple fluence/energies;16 and (b) annealing GeO/GeO^sub 2^ in bulk Si or Si-Ge in abundance of oxygen at temperatures in excess of 800�C during which Ge-O bonds break, enhancing the oxidation rate of silicon while maintaining a strained layer above the structure in the active area region.17-21 Thus, silicon on insulator substrates with active device area that possess fast carrier mobility22 can be processed.

To put the role of Si-Ge in perspective, the market for semiconductors with high carrier mobilities is estimated to be ~$30 billion with Si-Ge comprising about 38% of the total.1 Si-Ge offers several advantages over the III-V compound semiconductor materials with the largest single advantage being the complete compatibility with conventional silicon CMOS processes.

PREPARATION METHODS FOR Si-Ge STRUCTURES

Two major preparation techniques have been deployed for Si-Ge alloys.1 These include molecular beam epitaxy (MBE)2,23,24 and chemical vapor deposition (CVD), especially, plasma-enhanced CVD.2 Metal organic CVD28 and laser-assisted physical vapor deposition (PVD) techniques have also been used for processing of multilayer films.29

In the current study, the novel technique of ion beam mixing (IBM) is considered for the formation of Si-Ge alloys. The IBM facilitates achieving nonequilibrium or metastable alloys and intermetallic compounds on surfaces.30-32 The adhesion between two (or multi) layers, the preparation of junctions and electrical contacts, and the low-cost layer deposition by IBM to give requisite interface bond strengths are a few examples of the advantages of this technique. During IBM, the materials processed are bombarded by energetic heavy ions, ballistic collisions, defect production and migration (whether chemically guided or chemically not guided), and radiation-enhanced diffusion (RED).30-34 Subsequently, changes in the spatial distribution of the elemental species accompanied by the formation of defects take place as a result of collision cascades intersecting the interface and produce intimate mixing between the layers. Ion implantation of Ge into Si can yield the desired composition and thickness. However, PVD of Ge followed by IBM using argon implantation offers an attractive alternative method. The latter produces multiple graded layers of Si^sub 1-x^Ge^sub x^ with smoothly distributed bandgap variations allowing maximum light absorption in photovoltaic applications. Further, implantation of Ge into Si may yield dimensional systems of clusters in the locality of the Ge projected range Rp. This system corresponds to nanodimensional Si-Ge structures or solid solutions. Ion mixing (IM) creates similar structures using self-organization phenomena in crystals with defects.'15 Hence, the IBM modification used in this work may add yet another advantage by creating these special nanodimensional structures that may exhibit desired optical/electrical properties.