Ion Beam Mixing of Silicon-Germanium Thin Films

Journal of Electronic Materials, May 2005 by Abedrabbo, Sufian, Arafah, D-E, Salem, S

An overview of processing silicon-germanium (Si-Ge) alloys for various applications is presented here. Several methods of formation are briefly summarized. In particular, results of preliminary experiments on ion-beam mixing of Si-Ge layered structures deposited by physical vapor deposition and subsequently ion implanted with varying doses of argon are presented. Different layered structures have been designed and mixed to obtain optimal process conditions. The ion beam mixing process yields films with a gradual band-gap variation from 1.12 eV to 0.85 eV, thus allowing quite a wider spectrum of wavelengths to be absorbed. Rutherford backscattering spectrometry (RBS) has been used to characterize the nature and extent of the mixing of as-deposited and irradiated films.

Key words: Ion beam mixing, silicon-germanium (Si-Ge), thin films, Rutherford backscattering spectrometry (RBS), layered structures

INTRODUCTION

Silicon-germanium (Si-Ge) alloys are becoming major players in high-frequency applications such as cellular phones and are competing with direct band-gap compound semiconductors such as GaAs.1 The alloys are also used in the formation of high-mobility strained silicon layers for high-performance applications in silicon microelectronics. Our interest in this alloy system, however, is the ability to perform band-gap engineering to obtain efficient and inexpensive photovoltaic cells.2

To put the role of Si-Ge in perspective, the market for compound semiconductors is estimated to be ~$7.2 billion with Si-Ge comprising 38% of the total.3 Si-Ge offers several advantages over the III-V compound semiconductor materials with the largest single advantage being the complete compatibility with conventional silicon complementary metal oxide semiconductor (CMOS) processes. In addition, Si-Ge creates the ability to merge low-power, high-density digital circuitry of CMOS with the high speed of a Si-Ge heterojunction bipolar transistor, making advanced Bi-CMOS processes ideally suited for highly integrated mixed signal circuits for applications in communications.4

Si-Ge Structures for Photovoltaic Applications

Thin film solar cells have largely been recognized to be potentially important for energy conversion and production of large area photovoltaic cells.5,6 Current state of the art small-scale cells deliver conversion efficiencies of more than 16%, while larger size modules yield efficiencies around 10 pet.7,8 Recent work performed by various groups yielded improved efficiencies surpassing 19 pet.9,10 Significant efforts are being made in solar cell research to either increase the efficiency or reduce the cost of silicon solar cells for commercial production.11

Previous investigations include data on deep and shallow defect states in Si-Ge and reveal that the efficiency is markedly influenced by many factors. These include the method of preparation, processing conditions, and structural alterations within the film components and incorporation of light impurities followed by the formation of an interfacial layer possibly by interdiffusion processes.12-14 More specifically, the efficiency of silicon solar cells can be enhanced if a lower band-gap material is introduced into the device. In fact, Si^sub 1-x^Ge^sub x^ alloys are miscible for most values of Ge concentration, x. The increased lattice constant of the alloys relative to silicon is accommodated by lattice strain in thin layers or by misfit dislocations, such as in graded multiple layers. The commensuration of the interfaces, however, depends upon the film thickness, x, growth method, and thermal treatment.

These alloys have a lower band gap than that of silicon and act to extend the infrared response of the cell and increase the photo-generation and consequently current generation (or collection). Earlier investigations reveal that band gaps of alloys could be tailored from 1.12 eV to 0.66 eV.7,15-19 In particular, for solar cells, the publications in the area of Si^sub 1-x^Ge^sub x^ alloys have been numerous and a relatively few favorable conclusions for the use of Si^sub 1-x^Ge^sub x^ alloys have been established.5,6,15-18,20 In this work, the authors envision a Si matrix of solar cells adjacent to SiGe cells. Typical Si cells will capture the light in the visible and short infrared (IR) region, while the SiGe cells will enhance the absorption process to the longer IR region.

Methods of Preparation for Si-Ge Structures

Two major preparation techniques have been deployed for Si-Ge alloys.1 These include molecular beam epitaxy (MBE)1,21,22 and chemical vapor deposition (CVD), especially plasma-enhanced CVD.23-25 Metal organic CVD[26] and laser-assisted physical vapor deposition techniques have also been used for processing multilayers of the films.27

In addition to the above methods, the novel technique of ion beam mixing (IM) by which nonequilibrium or metastable alloys and intermetallic compounds on surfaces can be formed is of value. The rapid development in materials science and technology based on the surface and near-surface properties of materials and their modifications under ion beam irradiation28-30 has gained much attention in a wide variety of scientific and industrial fields (for example, the references in Reference 29). Important applications can be cited and include the adhesion between two (or multi-) layers, the preparation of junctions and electrical contacts, and the low cost layer deposition by IM to give requisite interface bond strengths. Ballistic collisions, defect production, migration (whether chemically guided or chemically not guided), radiation-enhanced diffusion (RED), and other phenomena are, however, initiated when thin films are bombarded by energetic heavy ions.28-33 Subsequently, changes in the spatial distribution of the elemental species accompanied by the formation of defects take place as a result of collisional cascades intersecting the interface and produce intimate mixing between the layers. Limitations in the maximum energy of the bombarding ions will limit the thickness of the active device. While ion implantation of Ge into Si can yield the desired composition and thickness, physical vapor deposition (PVD) of Ge followed by ion beam mixing 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. Further, implantation of Ge into Si may yield a dimensional system of clusters in the local of the Ge-projected range R^sub p^. This system corresponds to nanodimensional Si-Ge structures or solid solutions. Also, modifications of materials by IM create similar structures using self-organization phenomena in crystals with defects.34 Hence, the IM modification used in this work may add yet another advantage by creating those special nanodimensional structures that may exhibit wanted optical/ electrical properties.