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Materials Science and Engineering B73 2000 212 217 www elsevier com locate mseb Large grained polycrystalline Si films obtained by selective nucleation and solid phase epitaxy R A Puglisi 1 H Tanabe C M Chen Harry A Atwater Thomas J Watson Laboratory of Applied Physics California Institute of Technology Pasadena CA 91125 USA Abstract We investigated the formation of large grain polycrystalline silicon films on glass substrates for application in low cost thin film crystalline silicon solar cells Since the use of glass substrates constrains process temperatures our chosen approach to form large grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy SNSPE In this process selective crystallization of an initially amorphous silicon film at lithographically predetermined sites enables grain sizes larger than those observed via random crystallization Selective heterogeneous nucleation centers were created for both P doped B doped and undoped 100 nm thick amorphous silicon films by masked implantation of In or Ni islands followed by annealing at temperatures below 600 C Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy SPE thus obtaining crystallized regions of several tens of square microns The maximum achievable grain size depends on the product of the SPE rate and the incubation time for the spontaneous nucleation We have studied the dependence of the SPE rate and the incubation time on the type of metal In and Ni inducing the nucleation and on the electronic dopant e g P and B concentration in the 1019 1021 cm 3 range 2000 Elsevier Science S A All rights reserved Keywords Solar cells Si Selective nucleation Ni In 1 Introduction Polycrystalline silicon poly Si thin films are currently attracting a great deal of attention for use in solar cells Poly Si has superior carrier mobilities and stability under illumination relative to amorphous silicon a Si Since grain boundaries act as minority carrier recombination centers poly Si films with sufficiently large grain sizes are necessary in photovoltaic applications 1 In particular if the grain size is in the range of 10 100 mm AM1 5 cell efficiency can reach 17 compared to an efficiency of approximately 10 for a Si 2 In order to use low cost glass substrates which have softening temperatures about 600 C it is also necessary to be able to grow poly Si films at low thermal budgets High quality poly Si films can be obtained by either surface energy driven sec Corresponding author Tel 1 626 395 3826 fax 1 626 4495678 E mail addresses rosaria puglisi ct infm it R A Puglisi ming caltech edu C M Chen 1 Present address Universita di Catania and INFM Corso Italia 57 95100 Catania Italy ondary grain growth of as deposited poly Si films by pulsed laser induced crystallization or by selective nucleation and subsequent solid phase epitaxy SNSPE of the a Si For secondary grain growth of as deposited poly Si the temperature necessary to achieve high quality i e large defect free grains poly Si is generally in the 1000 C range 3 Pulsed laser induced crystallization of a Si on the other hand can produce poly Si with the required high quality However laser crystallization is likely to result in a low throughput photovoltaic manufacturing process due to the small area processed per pulse The SNSPE has advantages over other poly Si preparation processes like smoother surfaces better uniformity and when compared to laser crystallized poly Si a higher throughput This process is based on metal induced nucleation of a Si layers Metal rich regions formed by ion implantation through a mask cause selective heterogeneous nucleation to occur at a much earlier time than random nucleation The enhanced nucleation rate has been attributed to the interaction of the free electrons of the metal with the covalent Si bonds near the growing interface 4 Thus crystals which are selectively nucleated can grow to 0921 5107 00 see front matter 2000 Elsevier Science S A All rights reserved PII S 0 9 2 1 5 1 0 7 9 9 0 0 4 6 6 3 R A Puglisi et al Materials Science and Engineering B73 2000 212 217 very large sizes via lateral solid phase epitaxy before their growth is impeded by impingement with randomly nucleated crystal silicon grains Furthermore B and P doping has been shown to enhance SPE rate in Si enabling even larger grains to be obtained in B and P doped films The formation of poly Si in In implanted a Si was first reported by Nygren et al 5 who observed rapid crystallization of a Si implanted with 2 of In and annealed at 550 C for 15 min suggesting a crystallization mechanism via liquid In droplet migration They suggested that the crystallization process is initiated from molten In rich precipitates that form within a Si when it contains high concentrations of In atoms Another system that takes advantage of silicide formation in the transport of Si from the amorphous to crystalline phase is that of Ni on Si Low temperature crystallization of a Si has been reported 6 following Ni deposition on hydrogenated a Si and was believed to result from heterogeneous nucleation of c Si at the interface between NiSi2 and a Si All the above findings refer to metal islands uniformly deposited or implanted in a Si In this paper we propose a new method to selectively induce the nucleation of c Si in a Si by implanting indium or nickel islands into a Si thin films We observed crystallization of the a Si after annealing at temperatures of 550 C The crystallized regions sizes were of the order of several microns Moreover the crystallization process induced by nickel was much faster than the one induced by indium In particular we measured a crystallization velocity of 2 10 3 mm s 1 at 620 C for indium induced crystallization while 8 10 2 mm s 1 at 610 C for nickel induced crystallization 213 intentionally doped On both types of samples photolithography and wet etching with an HF HNO3 and H2O 1 20 40 were used to define rectangular islands of 50 10 mm2 of the a Si film on SiO2 To induce the nucleation In was selectively implanted at 80 keV with a dose of 8 1015 cm 2 into the boron and phosphorus doped samples covered with patterned photo resist The resist defined a window at the edge of the rectangular pattern of area 2 10 mm2 that acted as a nucleation seed The LPCVD samples were implanted in the nucleation seed region with nickel at an energy of 40 keV with doses of 5 1014 5 1015 and 5 1016 cm 2 We will refer to these


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