April 10, 2009 / Vol. 7, No. 4 / CHINESE OPTICS LETTERS 319
Silicon nanocrystals to enable silicon photonics
Invited Paper
Min Xie
∗
, Zhizhong Yuan, Bo Qian, and Lorenzo Pavesi
Nanoscience Laboratory, Department of Physics, University of Trento, Via Sommarive 14, I-38100 Povo (Trento), Italy
∗
E-mail: nllab@science.unitn.it
Received January 7, 2009
Low dimensional silicon, where quantum size effects play significant roles, enables silicon with new photonic
functionalities. In this short review, we discuss the way that silicon nanocrystals are prod uced, their
optoelectronic properties and a few device applications. We demonstrate that low dimensional silicon is
an optimum material for developing silicon photonics.
OCIS codes: 160.4236, 190.4400, 200.4650, 250.3140.
doi: 10.3788/COL20090704.0319.
1. Introduction
Silicon photonics is the technology where photonic de-
vices are pr oduced by standard microe le c tronics pro-
cesses by using the same paradigm of electronics: in-
tegration of a large number of devices to yield a high
circuit complexity which allows high per fo rmances and
low costs
[1]
. Thus, the real truth is to develop photonic
devices that can be easily integrated to improve the
single device performance and to allow high volume pro-
duction: integration and mass-manufacturing ar e where
silicon photonic can outperform other photonic plat-
forms, such as InP-based or glass-based ones. To this
aim, one further add-on is making silicon do something
where it is not able to do in its standard (bulk) form.
Low dimensional s ilicon, where small silicon nanocrys-
tals or na noclusters (Si-ncs ) are developed, is one way to
compel silicon to act as an active optical material
[2]
. In
this short review, we will go through a few applications
where Si- nc enables silicon doing photonic functions oth-
erwise not possible by using bulk silicon.
Figure 1 reports the number of results one gets if looks
for Si-nc in Google
TM
Scholar. It is obse rved a steady
rise in the numb e r of publications witnessing the ris-
ing interest in this material. The first results found in
this search refer to four papers reporting about differ-
ent properties of Si-nc: the first is our paper on the
observation of optical gain
[3]
, the second is the pa per
by Tiwari et al. on the use of Si-nc for memor ie s
[4]
, the
third is the paper by Wilson et al. on the demonstration
of quantum size effects in Si-nc
[5]
, and the fourth is the
paper by Mutti et al. on the observation o f room tem-
perature luminescence in Si-nc
[6]
. While if one makes the
same search on Google
TM
, the first result concerns the
use of Si-nc for photovoltaics
[7]
. All the key ingredients
that make Si-nc appealing for photo nics are discussed in
these papers: quantum s iz e effects make new phenom-
ena appear in silicon, such as r oom temperature visible
photoluminescence, optical gain, Coulomb blockade, and
multiexciton generation. In this short review, we will
discuss these topics and see how they can be exploited in
working devices. For more detailed information, we refer
readers to o ther reviews tha t we wrote in the past
[8−11]
.
2. Silicon nanocrystals: production
Silicon nanocrystals are produced by a wealth of dif-
ferent techniques
[12]
. They can be essentially distin-
guished into three types: direct synthesis, phase separ a-
tion in a silicon-rich dielectric, and top-down production
from bulk silicon. Examples of the first type are cluster
deposition
[13]
or chemical synthesis in a solution
[14]
. The
second class of techniques is mor e widely diffused and
essentially is bas e d on the production of a silicon-rich
dielectric (e.g., silicon oxide or silicon nitride) and on the
phase separation of the constituent phases (silicon and
the dielectric) by an annealing treatment. The duration
and temp e rature of the annealing treatment determine
the size and crystallinity of the nanoparticles. Various
methods can be use d to produce the starting silicon-rich
dielectric: ion implantation
[6]
, sputtering
[15]
, chemical
vapor deposition (CVD)
[16]
, so l-gel synthesis
[17]
, etc.
The third clas s of techniques is the one us ed in the pro-
duction of porous silicon (electrochemical etching) where
silicon is partially dissolved and the remaining skeleton
is composed of interconnected Si-ncs
[18]
. Alternatively,
lithographic technique followed by rep e ated oxidations on
a silicon wafer can define small silico n islands or columns
where Si-ncs are formed
[19]
. This is useful for single Si-
nc production, or when the geo metr ical arrangement of
Si-nc has to b e controlled.
The preparation method strongly influences electrical
Fig. 1. Number of articles versus year as reported by
Google
TM
Scholar in a search performed on 31/12/2008.
The search keys were “low dimensional silicon” OR “silicon
nanocrystal/s” OR “silicon nanocluster/s”. The total num-
ber of articles integrated over the years is 6220, while the
results for “silicon photonics” is 1740 and for “porous sili-
con” is 28800. The single point referred to year 2008 is low
due to the time delay between the search date and the actual
database construction.
1671-7694/2009/040319-06
c
2009 Chinese Optics Letters