(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals...

Transport and Electroluminescence Properties of Size-Controlled Silicon

Nanocrystals Embedded in SiO2 Matrix Following the Superlattice Approach

Julià López-Vidrier, Yonder Berencén, Lluís López-Conesa, Oriol Blázquez, Joan Manel Ramírez, Sònia

Estradé, Francesca Peiró, Sergi Hernández, and Blas Garrido

MIND-IN2UB, Departament d’Electrònica, Universitat de Barcelona

Carrer Martí i Franquès 1, E-08028 Barcelona, Spain

We report a thorough analysis of the electrical and optical properties of silicon

nanocrystals (Si-NCs)/SiO2 superlattices (SL), deposited on Si substrate by

means of plasma-enhanced chemical-vapor deposition. A post-deposition

annealing treatment was performed to induce phase separation and, therefore,

precipitate the Si excess in the form of Si NCs. An excellent size control is

reached by the limited thickness of the silicon rich layer of the SL. A discussion

on the NC size effect on the injection and transport (electrical), and on

absorption and electroluminescence (electro-optical) is presented. A strong

correlation between charge transport and electroluminescence is revealed. This

work demonstrates that Si-NCs are a promising photovoltaic and luminescent

material system; especially in the case of a SL-based material.

Introduction

Silicon nanocrystals (Si NCs) embedded in dielectric matrices have aroused great interest on the scientific

community owing to their potential applications as light emitters, within third-generation photovoltaics

and for integrated silicon photonics (1,2). For instance, Si NCs embedded in SiO2 produce a dramatic

increase in the luminescent emission with respect to the bulk material because of the quantum

confinement of the electron-hole pairs in a small region of space, which is associated with a relaxation of

the selection rules of the radiative electronic transitions (3). This way, there is an overlap of the carrier

wave functions allowing its radiative recombination. By controlling the size of the Si NCs where the

carriers are located, it is possible to tune the emission from about 550 nm to the near infrared (about 900

nm). Thus, silicon becomes an efficient light emitting material in the visible. Additionally, when a

material containing Si NC is doped with a rare earth the emission is shifted to the rare earth emission

while the Si NC acts as a media to enhance the absorption of energy from the source (either electrical or

optical) and the transfer of energy. Likewise, it has been proposed to take advantage of the quantum

confinement effect in Si NCs for photovoltaic applications by managing the silicon band gap and

consequently producing efficient tandem solar cells (1).

Different fabrication techniques have been intensively studied for the formation of such Si NCs in various

silicon-based bulk materials. However, most recently, those techniques are being moved towards the

development of amorphous superlattices. In particular, SiO2-based nanometric superlattice (SL) structures

have attracted considerable attention since they allow managing the Si NC size for an optimization of the

carrier transport (4,5,6). For instance, a tight control of size and self-organized array of the Si NCs is

obtained by adjusting the thickness of the SiO2 sublayer barrier in a Si NC/SiO2 superlattice system. This

fact makes also possible the modulation of the carrier transport and thus the luminescence properties or

photovoltaic effects. In other words, the superlattice approach exhibits many interesting transport and

optical properties, which are associated with quantum size effects. Recently, it has also been

demonstrated that such SL structures are suitable as efficient light converters in tandem solar cells and

electroluminescent layers in light emitting devices (7,8).

In this work, we report on the electrical and electroluminescence properties of Si-NCs/SiO2 superlattices,

fabricated on Si substrate by means of plasma-enhanced chemical-vapor deposition (PECVD). In

addition, we demonstrate that an excellent NC size control is possible using the superlattice approach.

10.1149/06105.0133ecst ©The Electrochemical SocietyECS Transactions, 61 (5) 133-139 (2014)

133) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.114.163.7Downloaded on 2014-09-08 to IP

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Moreover, based on the experimental observations, a correlation between carrier transport and

electroluminescence properties is presented.

Experimental Details

Alternating Si-rich silicon oxide (SRON)/SiO2 superlattices were deposited on p-type Si substrate by

means of PECVD. The thickness of the oxide barriers (tSiO2) and the SRON layers (tSRON) were varied

depending on the particular characterization. The SRON layer stoichiometry was held constant at

SiO0.93N0.23, which corresponds to a Si excess of 17 at.%. After the deposition process, an annealing

treatment was carried out at 1150 °C during 1 h under N2 atmosphere. As a consequence, the precipitation

of the Si excess was promoted and, thus, the formation and crystallization of the Si NCs was achieved.

The vertical device structure was attained by sputtering 70 nm of ITO on top of the SL stack and Al on

the bottom of the Si substrate. Further details on sample preparation and device fabrication can be

consulted elsewhere (5,9).

The correct superlattice deposition and NC growth were monitored by means of energy-filtered

transmission electron microscopy (EFTEM), using a JEOL 2010 instrument (field-emission gun operating

at 200 keV) equipped with a Gatan Imaging Filter (0.8 eV of resolution). The Si contrast was enhanced

by energetically filtering the energy loss spectra around the Si plasmon energy (ESi = 17 eV). Samples

were excited with the 325-nm line of a continuum-pump He-Cd laser, in order to acquire their

photoluminescence (PL) emission spectra with a monochromator coupled to a GaAs photomultiplier tube

(PMT). The electrical characterization was carried out by means of a Cascade Microtech Summit 1100

probe station with a thermal chuck, employing a Faraday cage to avoid undesired electrical noise. The

temperature-dependent electrical measurements were performed on a thermal chuck, varying the

temperature from 300 to 500 K. By means of a 20× objective, the electroluminescence (EL) yielded by

the devices under electrical excitation was collected, and analyzed using a monochromator coupled to a

N2-cooled charge-coupled device. The integrated EL signal was measured with the PMT directly coupled

to the collecting objective.

Si NC Size Control

Before the in-depth study of the optoelectronic properties the Si NC superlattices present, it is almost

mandatory to check their deposition control, i.e. the NC size control. For this, direct observation was

carried out by means of cross-section EFTEM. In Fig. 1(a), an EFTEM image from a sample grown with

different tSRON while fixing tSiO2 = 4 nm is shown. By energy filtering the Si plasmon, Si signal could be

highlighted on the oxide background and, therefore, the presence of Si NCs is clearly distinguished

(bright spots). A statistical analysis on the NC size was performed out of the 50 bilayers per SL, whose

results are plotted in Fig. 1(b) against the measured SRON layer thickness before the annealing process.

As can be observed, both parameters are equivalent up to tSRON = 5 nm, beyond which the NC size no

longer grows, establishing an upper limit for NC nucleation and, in turn, for their size control. In other

words, the superlattice approach gives excellent results up to deposited SRON layer thicknesses of 5 nm.

The luminescence properties of the nanostructures were also investigated by means of the

photoluminescence (PL) technique. The PL spectra are displayed in Fig. 1(c). A Gaussian-like emission is

observed, usually ascribed to the excitonic recombination that takes place within the quantum dot (QD).

This recombination is strongly size-dependent, as shown by the peak blueshift at smaller NCs, attributed

to the quantum confinement of the carriers within the QDs (3). Besides, the PL emission intensity at

medium NC sizes was found to be greater than small or large nanostructures, which has been associated

in the past to the higher quantum yield the medium sizes show (10). Therefore, the effect of the size

control on the structural and optical properties of the Si NCs anticipates the change in the electrical and

electro-optical properties this material may present.

ECS Transactions, 61 (5) 133-139 (2014)



Figure 1. (a) Cross-section EFTEM image performed on a sample prepared with a constant tSiO2 = 4 nm

while varying its tSRON. The NC size statistics out of some of the layers is summarized in (b). A solid line

has been plotted as a trend indicator. (c) Normalized PL spectra corresponding to samples containing

different NC sizes (written down in the figure).

Charge Transport Properties of the NC Superlattices

The electrical properties of devices containing Si NC superlattice stacks as active material were studied,

in order to determine the charge transport mechanisms taking place within the structure. For this study,

the SRON layer thickness was varied with values of 2.5, 3.5, and 4.5 nm, while holding constant tSiO2 = 1

nm. The total number of deposited SRON/SiO2 bilayers was five. In addition, the sputtering of an ITO

layer on top and Al on the bottom completed the device structure [see the device sketch in the inset of

Fig. 2(a)].

The current density versus voltage characteristics, obtained under accumulation (V < 0), are plotted in

Fig. 2(a). As can be observed, the curves present two distinct behaviors, at low and high applied voltages.

At low voltages, the behavior is attributed to displacement current and, therefore, to capacitor-like effects

related to the device geometry (the total active material thickness). Likewise, the high voltage behavior

can be associated to the pure conduction of carriers. Besides, a voltage onset is found for this conduction

to begin. It is worth to realize that both the onset voltage and the conduction regime are strongly tSRON

dependent, which implies that charge transport is tightly related to the NC size. In addition, the strong

dependence of transport on the active layer properties anticipates that the main transport mechanism will

be bulk-limited.

Most works in the past have been focused on the transport through Si nanostructures embedded in bulk

SiO2. These studies revealed injection-limited transport mechanisms such as Fowler-Nordheim (FN)

tunneling at high fields, and trap-assisted tunneling through defects in the SiO2 matrix acting as trap states

(11). However, the superlattice structure presents different carrier pathways because of the NC ordering.

Marconi et al. proposed a bipolar direct tunneling conduction that well correlated with the high

electroluminescence efficiency (8). Afterwards, Gutsch et al. realized that transport in these structures are

strongly modulated by the presence of defects, probably Si-SiO2 interface related, and also advanced

Poole-Frenkel (PF) as a plausible conduction mechanism (5).

According to the amount of interfaces within our structures, and the bulk-limited origin of our emission,

we initially hypothesized a PF-type as the main transport mechanism taking place through our system. In

the PF conduction mechanism, the current intensity dependence on voltage and temperature is given by:

(12)

−−=

d

qV

Tk

qV

d

ANqTVI

r

t

B

tPF επε

φµ0

·exp),( (1)




with q the elementary charge, µ the carrier mobility, Nt the trap concentration, A the device area, d the

total SL structure thickness, kB the Boltzmann constant, Φt the energetic level of the trap taken from the

SL conduction band edge, ε0 the vacuum permittivity and εr the relative permittivity of the medium.

Taking into account both the voltage and temperature dependences of the PF current intensity, we could

experimentally prove that our devices well fit Eq. (1). As an example, Fig. 2(b) and Fig. 2(c) display the

PF (V-dependent) and Arrhenius (T-dependent) plots for the device containing tSRON = 3.5 nm,

respectively, the former study performed at room temperature (300 K) and the latter at 13 V (well within

the pure conduction regime). From the mathematical adjustments, we estimated εr = 8.7 and an activation

energy for the pure conduction to take place (EA) of 90 meV. Besides, both parameters increased at

thicker SRON layers, which verifies the transport dependence on the NC size.

Figure 2. (a) Current density versus voltage curves (in absolute value) corresponding to devices with

different tSRON. Vertical lines indicate the voltage onset for pure conduction to begin. The inset shows a

sketch of the device structure. (b) Poole-Frenkel fit from the current density versus voltage characteristic

of the device with tSRON = 3.5 nm. (c) Conductivity versus the inverse of temperature (Arrhenius plot)

from the same device and its correspondent fit.

Electroluminescence Properties of the Si NC Superlattices

To assess the electro-optical properties of Si NC superlattices, the electroluminescence emission of the

devices, integrated throughout the whole spectrum, was investigated. The plots are presented in Fig. 3(a).

It can be easily observed that the device with intermediate SRON layer thickness presents the highest EL

output. To quantify the optical output relative to the electrical excitation input, the power efficiency was

calculated as

optPE(%) 100

W

J V= ×

× (2)

where Wopt is the EL optical power density (per unit surface). The resulting PE values, estimated at J =

10-4

A·cm-2

, vary in the range from 10-4

to 10-3

%. The origin of such a low efficiency must be somehow

related to the bulk-limited conduction-type through the SL systems, but its interpretation still requires a

further analysis.

The following experimental step consisted of the acquisition of the EL spectra yielded by the devices,

exciting them with a constant current density of ~10-4

A·cm-2

. Fig. 3(b) displays the results. A strong

emission feature is observed centered between 1.3 and 1.5 eV, which is related to the excitonic

recombination within the nanostructures (10), as anticipated by the PL characterization. In addition, there

is a clear dependence of the EL on the SRON layer thickness, i.e. on the NC size, a peak blueshift

occurring at smaller NCs as expected from the carriers quantum confinement. Besides, the EL intensity is

optimum at medium NC sizes, which is attributed to the higher quantum yield medium-size NCs present

(13).




Further information about the EL emission could be obtained from a temperature-dependent analysis [Fig.

3(c)] carried out on the device with tSRON = 3.5 nm. It is clearly observed that the EL emission intensity

decreases when increasing the temperature. This phenomenon is directly related to the electron-hole

recombination dynamics within the quantum dot. It is well-known that temperature favors fast non-

radiative processes instead of radiative ones (14). Therefore, these results state that the recombination

process that takes place in the NCs under study is radiative or, at least, it dominates over other possible

fast recombination paths.

Figure 3. (a) Integrated electroluminescence optical power as a function of the applied current density, as

well as the obtained power efficiency, for each device under study. The vertical dashed line indicates the

current density at which the power efficiencies were calculated, and the EL spectra acquired. (b) EL

spectra corresponding to the same three devices, acquired at room temperature. (c) EL spectra acquired at

different temperatures from the device with tSRON = 3.5 nm.

The origin of the EL yielded by the devices has already been shown to be NC-related. Nevertheless, a

correlation between the EL excitation mechanism and the carrier transport taking place through our

superlattices is still lacking. Up to now, the experimental results evidenced that (i) carriers follow a

thermally-activated (Poole-Frenkel) transport mechanism and (ii) the electron-hole recombination takes

place within the NCs. Besides, the fact that both conduction and recombination are strongly dependent on

the NC size makes us anticipate that the latter mechanism may be a consequence of the former.

For the recombination to occur, it is required the presence of an electron in the QD conduction band and a

hole in the corresponding valence band. The electron injection through the ITO top electrode is

unquestionable, but the discussion on the hole origin may be tedious. To our understanding, two possible

processes may take place: bipolar injection of electrons from the ITO electrode and holes from the Si

substrate or the QD ionization by direct impact of high-energy electrons in the conduction band. Taking

into account that holes have lower mobility in oxides and deal with higher barrier heights for tunneling

(4.7 eV versus 3.1 eV for electrons) than electrons, it is reasonable to propose impact ionization (II) as the

most plausible excitation mechanism in NCs.

Direct impact ionization of the NCs has been previously ascribed to superlattice systems where unipolar

tunneling of electrons domains (8, 15). Typically, this process takes place under high polarization

conditions, providing injected electrons with high kinetic energy well above the minimum of the

conduction band. In our systems, these high-energy electrons must also be present for II to take place, and

it is possible to link their existence with the actual carrier transport (see the energy band diagram depicted

in Fig. 4). If we suppose an electron transport via thermal hopping through high-energy QD localized

states, then we can understand the activation energy as the required energy for these electrons to jump to

the system (superlattice) conduction band (blue region in the figure), understood as the combination of

the matrix (SiO2) and the QDs conduction bands. Some of the electrons that promote to the system

conduction band will be accelerated by the electric field and, therefore, acquire a high kinetic energy.

These electrons may then impact against the QDs, consequently generating an electron-hole pair (at the

QD fundamental conduction and valence band levels, respectively) that, afterwards, will recombine

delivering a photon with the NC bandgap energy (EEL). In a previous work, we discussed that the electron




energy distribution in a PF-ruled system is lower than in a tunneling one, but a low amount of electrons

may exist with enough energy to induce II (6). Although the probability of this process is low, we believe

it must be the cause of the low efficiency our devices based on Si NC-superlattices yield.

Figure 4. Energy band diagram of the NC-superlattice systems under study. Dashed arrows represent the

different material bandgap energies, whereas dotted and solid arrows indicate electron transport and EL

excitation processes, respectively, as described in the text.

Conclusions

SRON/SiO2 superlattices were deposited on Si substrate by means of PECVD, and a later annealing

treatment induced the Si excess precipitation to form Si nanocrystals. The Si-rich layer thickness and final

NC size control was assessed by means of EFTEM, the nanostructures presenting a PL emission shift

attributed to quantum confinement. Devices containing the SL systems with different SRON layer

thickness were electrically characterized. Voltage- and temperature-dependent measurements make us

consider thermal hopping (Poole-Frenkel) as the main transport mechanism within the structure. A

rigorous study on the EL emission yielded by the devices was carried out, showing similar results than

PL, that could be ascribed to the electron-hole pair radiative recombination within the NCs. All

experimental results support an EL excitation mechanism consisting of electron impact ionization on the

quantum dots. The process probability is low, as it requires high-kinetic energy electrons in a PF transport

model, but the low observed EL efficiency supports our hypothesis.

Acknowledgements

The research leading to these results has received funding from the European Community's Seventh

Framework Programme (FP7/2007-2013) under grant agreement nº: 245977, under the project title

NASCEnT. The present work was supported by the Spanish national project LEOMIS (TEC2012-38540-

C02-01).

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(Invited) Transport and Electroluminescence Properties of Size-Controlled Silicon Nanocrystals...

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