Electrical and Optical Doping of Silicon by Pulsed-Laser Melting

Shao Qi Lim,James S Williams

doi:10.3390/micro2010001

Abstract

Over four decades ago, pulsed-laser melting, or pulsed-laser annealing as it was termed at that time, was the subject of intense study as a potential advance in silicon device processing. In particular, it was found that nanosecond laser melting of the near-surface of silicon and subsequent liquid phase epitaxy could not only very effectively remove lattice disorder following ion implantation, but could achieve dopant electrical activities exceeding equilibrium solubility limits. However, when it was realised that solid phase annealing at longer time scales could achieve similar results, interest in pulsed-laser melting waned for over two decades as a processing method for silicon devices. With the emergence of flat panel displays in the 1990s, pulsed-laser melting was found to offer an attractive solution for large area crystallisation of amorphous silicon and dopant activation. This method gave improved thin film transistors used in the panel backplane to define the pixelation of displays. For this application, ultra-rapid pulsed laser melting remains the crystallisation method of choice since the heating is confined to the silicon thin film and the underlying glass or plastic substrates are protected from thermal degradation. This article will be organised chronologically, but treatment naturally divides into the two main topics: (1) an electrical doping research focus up until around 2000, and (2) optical doping as the research focus after that time. In the first part of this article, the early pulsed-laser annealing studies for electrical doping of silicon are reviewed, followed by the more recent use of pulsed-lasers for flat panel display fabrication. In terms of the second topic of this review, optical doping of silicon for efficient infrared light detection, this process requires deep level impurities to be introduced into the silicon lattice at high concentrations to form an intermediate band within the silicon bandgap. The chalcogen elements and then transition metals were investigated from the early 2000s since they can provide the required deep levels in silicon. However, their low solid solubilities necessitated ultra-rapid pulsed-laser melting to achieve supersaturation in silicon many orders of magnitude beyond the equilibrium solid solubility. Although infrared light absorption has been demonstrated using this approach, significant challenges were encountered in attempting to achieve efficient optical doping in such cases, or hyperdoping as it has been termed. Issues that limit this approach include: lateral and surface impurity segregation during solidification from the melt, leading to defective filaments throughout the doped layer; and poor efficiency of collection of photo-induced carriers necessary for the fabrication of photodetectors. The history and current status of optical hyperdoping of silicon with deep level impurities is reviewed in the second part of this article.

Highlights

Silicon (Si) is one of the most technologically important materials in the modern era and has found widespread applications across the microelectronics, telecommunications and photovoltaics industry, and more recently, for quantum communication and computing technologies
When activated, the shallow dopants used for electrical doping introduce donor/acceptor levels in the bandgap that are relatively shallow with respect to the Si band edges (
In terms of electrical doping of Si, pulsed laser annealing/melting (PLM) was viewed over four decades ago as a potentially attractive technique for electrically doping Si above the solid solubility limit

Summary

Introduction

Silicon (Si) is one of the most technologically important materials in the modern era and has found widespread applications across the microelectronics, telecommunications and photovoltaics industry, and more recently, for quantum communication and computing technologies. Micro 2022, 2 of Si CMOS manufacturing infrastructure, further enhancing Si’s electrical and optical properties is very attractive in terms of the potential for extending its applications space. Over the last half of the last century, electrical doping of the near-surface of Si with impurities such as P, As, and B has been the basis for fabricating a range of electronic devices (e.g., microprocessors, CCD and CMOS cameras, solar cells, flat panel displays), and is the basis of the current CMOS manufacturing process. Optical doping of Si with less conventional impurities such as the chalcogens (S, Se, Te) and transition metals (e.g., Au, Ti, Ag) has received significant research interest over the last two decades as a result of the potential for Si-based near-to-mid infrared light detection and intermediate-band photovoltaics [1,2]

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