Changes of the absorption cross section of Si nanocrystals with temperature and distance.

Michael Greben,Sebastian Gutsch,Daniel Hiller,Petro Khoroshyy,Jan Valenta,Margit Zacharias

doi:10.3762/bjnano.8.231

Abstract

The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We clearly demonstrate that roughly doubling the barrier thickness (from ca. 1 to 2.2 nm) induces a decrease of the ACS by a factor of 1.5. An optimum separation barrier thickness of ca. 1.6 nm is calculated to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by smaller occupation number of phonons and expansion of the band gap of Si NCs at low temperatures. This study clearly shows that the ACS of Si NCs cannot be considered as independent on experimental conditions and sample parameters.

Highlights

Silicon – an abundant, nontoxic material with high attainable purity – has been a dominant material for microelectronics and photovoltaics
The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping
All higher excited states are not taken into account supposing that Auger recombination efficiently quenches the population of double excited NCs. (2) The ground state and single-excited state roughly have the same ACS, σ, as we assume that the presence of one e–h pair does not influence the absorption of the second one (because of the relatively high [1] density of optical states (DOS) in Si NCs)

Summary

Introduction

Silicon – an abundant, nontoxic material with high attainable purity – has been a dominant material for microelectronics and photovoltaics. The constantly increasing energy consumption and environmental issues challenge researchers to develop fundamentally new concepts to overcome the limitations of current technologies. The nanocrystalline form of silicon, which reveals all advantages of the quantum confinement effect [1], is a promising candidate for the development of a new generation of Si photovoltaic and photonic devices [2]. SiO2-embedded silicon nanocrystals (Si NCs) can be relatively easy integrated into current CMOS technology. Nanocrystalline Si is a promising material for the Beilstein J.

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