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Application note

New insights into the electronic structure of the c-Si interface by
optical second-harmonic generation spectroscopy at temperatures
ranging from 4 K to 300 K
Vincent Vandalon, Erwin Kessels and Ageeth Bol
Eindhoven University of Technology, Department of Applied Physics, The Netherlands

Introduction
This application note describes the characterisation of the c-Si/Al2O3 interface with the nonlinear optical techniqueof second-harmonic generation (SHG) spectroscopy at temperatures ranging from 4 K up to 300 K using theOxford Instruments’ OptistatTMDry Cryofree® cryostat. The buried silicon/dielectric interface plays a key role in theperformance of most semiconductor devices and therefore its characterisation is essential. Valuable information on
the structure, electronic properties, and built-in charge near this interface has been obtained by performing SHG
spectroscopy over a wide temperature range.

Figure 1: The SHG spectroscopy setup in which the Oxford Instruments’OptistatDry Cryofree cryostat was integrated.

Equipment used
Oxford Instruments’ OptistatDry Cryofree cryostat
Oxford Instruments’ FlexAL atomic layer deposition (ALD) system
Oxford Instruments’ MercuryiTC intelligent temperature controller

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Application note

Background: SHG of c-Si
In SHG, two photons from an ultrashort laser pulse mix to form a new photon at twice the photon energy, seealso Fig. 2 (a). It can be shown that SHG is inherently interface selective in centrosymmetric materials such as c-Si,
amorphous SiO2, and amorphous Al2O3 due to its nonlinear nature. This property makes the SHG process excellently
suited to study interface properties and phenomena. Typically, the SHG signal of an interface is very weak - it is an
second-order phenomenon with a low cross section and it only takes place at an interface - but the SHG signal isresonantly enhanced by electronic transitions of interface states. Therefore, the electronic structure of the interface
is reflected in the spectral shape of a SHG spectra. Moreover, it has been demonstrated that SHG is very sensitive tobuilt-in charge near the interface due to the electric-field induced SHG (EFISH) effect in c-Si.

Figure 2: (a) Schematic representation of the SHG process in the geometry used in the experiments. Alaser beam impinges on a sample consisting, in this case, of a substrate (c-Si) with a thin-film on top(SiO2 or Al2O3). At the interface between the two materials a SHG signal is generated due to lower local
symmetry; in the bulk of both materials no SHG signals are generated. This SHG signal propagates inthe same direction as the reflected laser beam. (b) A schematic of the SHG setup into which the Oxford
Instruments’ OptistatDry Cryofree cryostat was integrated at the location of the sample.

The capability of SHG to probe built-in charge without contacting the sample using the EFISH effect is ofpractical interest and it has been utilized to study the passivation properties of dielectrics such as Al2O3 forphotovoltaic (PV) applications. To employ SHG as a metrology technique, however, a better fundamental
understanding of the SHG response of silicon is required. The SHG response of the c-Si interface at room
temperature is fairly well understood and typically shows an electronic transition at 3.3 eV. This transition isrelated to a surface state and it is similar in position to the bulk E1 transition although shifted by 0.1 eV tolower energies. Moreover, the aforementioned EFISH contribution is situated at 3.4 eV with its amplitude
proportional to the built-in charge density. For virtually charge free samples such as Si(100) with a dry
thermally grown SiO2 film on top, the EFISH contribution is small. On the other hand,annealed Al2O3 films are known for their large amount of built-in charges resulting inan EFISH contribution dominating the SHG response. How the spectral SHG response ofc-Si varies with temperature has not been studied extensively and might reveal if the E1like resonance is composed of multiple contributions like its bulk counterpart. Therefore,
the spectral SHG response of the prototypical c-Si/Al2O3 interface wasstudied using the OptistatDry Cryofree cryostat at temperatures ranging between
4 K and 300 K.

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Application note

Spectral SHG response of Si/Al2O3 between 4 K and 300 K
The Si/Al2O3 sample was prepared by growing a 30 nm Al2O3 film on a Si(100) substrate by plasma-assisted atomiclayer deposition (ALD) using an Oxford Instruments’ FlexAL® system. To perform temperature dependent SHG
studies, the Oxford Instruments’ OptistatDry Cryofree cryostat was integrated into our SHG setup. In Figure 2(b),the cryostat takes the place of the sample where the alignment of the sample is now done using the adjustable
mount of the cryostat. For each measurement, the sample was mounted in the cryostat and the sample chamber wasevacuated to a pressure < 1· 10-6 mbar. The temperature of the sample was controlled between 4 K and 300 K using
the He compressor and the Oxford Instruments’ MercuryiTC temperature controller. The SHG measurements wereperformed using a 90 fs tunable laser system by stepping through the desired wavelength range while detecting theSHG signal using a single photon detector.
The temperature of the sample was stepped from 4 K up to 300 K and at each temperature a SHG spectrum wasrecorded. Figure 3 shows the SHG spectra acquired using this procedure. The spectrum recorded at 300 K is in-linewith results obtained in a separate experiment without cryostat. The overall signal strength increased with decreasing
temperature and a new feature appears at ~3.48 eV, shown in Figure 3. At 4 K, the 3.48 eV contribution is dominant
but an inflection point can be seen at 3.4 eV. This is similar to what is observed for the linear optical response of bulk
c-Si and indicates that the E1 peak is actually a superposition of two contributions.

Figure 3: The SHG spectra acquired at selected temperatures between 4 K and 300 Kof the c-Si/Al2O3 sample using the Oxford Instruments’ OptistatDry Cryofree cryostat.

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Application note

Conclusions and outlook
The temperature dependence of the spectral SHG response of the c-Si/
Al2O3 interface was studied using the OptistatDry Cryofree cryostat attemperatures ranging between 4 K and 300 K. The layout and design ofthe cryostat are such that it allows for easy (optical) access and alignmentof the cryostat. As a result, it was easily integrated into the already existing
SHG setup. The OptistatDry Cryofree cryostat has extended our capabilities
significantly and has led to new insights into the origin of the EFISH
contribution. Since the sample can also be contacted electronically, uniqueopto-electronic experiments are possible in the future.
We also expect that the OptistatDry Cryofree cryostat integrates well withour broadband sum-frequency generation (BB-SFG) setup. This nonlinear
optical techniques probes the vibrational fingerprint of surface species
such as -CH3 and -OH groups using 90 fs laser pulses. Performing BBSFG spectroscopy at low temperatures should allow the determination of
fundamental parameters such the coupling between vibrational modes andvibrational dephasing times.

OptistatDry
Cryofree cryostat

About the OptistatDry Cryofree cryostat
The OptistatDry provides a temperature controlled sample in vacuum
measurement environment within a Cryofree cryostat. The OptistatDrycomprises a range of compact cryostats with optical access cooled by aclosed cycle refrigerator. The system is capable of cooling samples to heliumtemperatures without the need for liquid cryogens. This provides significant
benefits in terms of ease of use and running costs. The system enables
optical and electrical measurements to be carried out on your samples, as
shown in this application note.

FlexAL atomic layer
deposition (ALD) system

Contact us at:About the FlexAL ALD system

Oxford Instruments

Oxford Instruments’ FlexAL systems provide a new range of flexibility andcapability in the engineering of nanoscale structures and devices by offering
remote plasma atomic layer deposition (ALD) processes and thermal ALD
within a single ALD system. The Si/Al2O3 sample was prepared by growing
a 30 nm Al2O3 film on a Si(100) substrate by plasma-assisted atomic-layer
deposition (ALD) using FlexAL system, as mentioned in this application note.

Tubney Woods,

NanoScience
Abingdon
OX13 5QX
United Kingdom
Tel: +44 (0)1865 393200
Fax: +44 (0)1865 393333
Email: nanoscience@oxinst.com

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September 2017, Issue 2