Surface passivant effects on electronic states

of the band edge in Si-nanocrystals

Ying Daia,*, Shenghao Hana, Dadi Daib, Ying Zhanga, Yun Qia

aSchool of Physics and Microelectronics, The State Key Laboratory of Crystal Material, Shandong University,

Jinan 250100, People’s Republic of ChinabDepartment of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA

Received 18 October 2002; accepted 29 January 2003 by H. Eschrig

Abstract

We studied the effect of the surface passivants fluorine (F), chlorine (Cl), oxygen (O) and oxygen-related OH on the energy

band edge states of clusters with the same Si29 and Si47 core by means of the atomic cluster model and density functional theory

(DFT). The results confirm that the electronic states of the band edge in clusters are sensitive to these passivants, and the

passivant O that may form double bonded structure affects the band edge states most strongly. The results may be helpful for

understanding and controlling the electrical and optical properties of nanocrystalline silicon.

q 2003 Elsevier Science Ltd. All rights reserved.

PACS: 71.24, þq; 71.15 2 mb

Keywords: A. Surface; A. Si nanocrystals; C. Point defect

1. Introduction

The silicon nanocrystals have attracted extra attentions

because of their interesting fundamental physical properties,

photoluminescence (PL) properties and promising appli-

cations in advanced electronic devices and optoelectronic

devices [1–5]. It has been believed that reducing the

crystallite size and increasing the surface area to volume

ratio may lead to these unique properties [5–8]. Recent

experimental data also present strong evidence that surface

effects produce a very substantial impact on the electronic

and optical properties of nanocrystalline silicon (nc-Si).

Comparing to the bulk silicon, the surface in Si-nanocrystals

plays a more important role in properties of PL. Accord-

ingly, the study of the surface effect of nc-Si is interesting in

both experiments and theory. Almost all ab initio and

empirical simulations available in literature use silicon dots

passivated with hydrogen [9–11]. Other types of surface

passivation have not been studied. On the other hand,

nanocrystalline silicon studied in experiments is prepared

under a variety of surface conditions determined by the

etching technique and external chemical environment. Only

a fraction of published experimental data refers to ‘pure’

hydrogenated silicon dots [12], whereas some measure-

ments are performed on partially oxidized nanocrystals [13].

In many cases, the precise chemical composition of

nanocrystalline surfaces is unknown [14,15]. In this paper,

we investigate the effects of various surface passivants on

the band edge states and band gap, which directly relates to

the PL properties, in hydrogenated silicon. This may be

helpful to understand and control the electrical and optical

properties of nanocrystalline silicon.

1.1. Computational details

DFT method of determining the electronic properties of

materials has become very popular in recent years. Usually,

most of the calculations carried out on a supercell or

employed a basis of plane waves [8,16]. For many

applications such an approach is not the most efficient.

0038-1098/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0038-1098(03)00086-3

Solid State Communications 126 (2003) 103–106

www.elsevier.com/locate/ssc

* Corresponding author. Tel.: þ86-53-180-60182; fax: þ86-53-

183-67035.

E-mail address: daiy60@sdu.edu.cn (Y. Dai).

Atomic cluster theory has led to significant advances in

understanding local phenomena such as defects in solids and

interaction effect of passivants on solid surface [17]. Our

computation approach is based on the atomic cluster model

and ab initio DFT using a linear combination of atomic

orbitals (LCAO) approach. The program of ADF2.3

package [18] has been used. The STO-3j basis plus

polarization functions are employed with Si (1s, 2s, 2p), F

(1s), Cl (1s, 2p) and O (1s) frozen in core. The VWN

[19] þ Becke88 [20] þ Perdew86 [21] was selected as the

exchange-correction functional. The relative error of the

numerical integration is 1026.

The calculations are performed to a serious of simulating

clusters constructed of the same 29 and 47 silicon cores but

with different passivants. The basic spherical cluster Si29H36

is built as shown in Fig. 1(I). One silicon atom locates on the

center of the cluster, combining with four nearest neighbor

Si atoms. Following, twelve the second and twelve the third-

nearest neighbor sites are occupied by Si atoms in turn.

Then, 36 hydrogen (H) atoms terminate the boundary

dangling bonds (DB) along the appropriate tetrahedral

direction. The cluster takes on Td symmetry. The other basic

cluster Si47H60 is constructed as: on the base of Si29, six

fourth-nearest and twelve fifth nearest-neighbor sites are

occupied by silicon atoms in turn, and 60 H atoms saturate

the boundary. The initial bond length of Si–Si and Si–H are

set as 2.35 and 1.48 A, respectively. The other groups of

cluster with the same Si29-core and Si47-core are: (1) Si29

H362kXk; Si47H602kXk (X ¼ F, Cl, and k ¼ 1; 2), the same

as the basic cluster but with k H atoms replaced by F or Cl

(see Fig. 1(II) and (III)); (2) Si29H362kðOHÞk; Si47H602k

ðOHÞk ðk ¼ 1; 2Þ; the same as the basic cluster but with

k OH atoms replacing k H atoms; and (3) Si29H3622kOk;

Si47H6022kOk ðk ¼ 1; 2Þ; the same as the basic cluster but 2k

H atoms are replaced by k O atoms (see Fig. 1(IV)–(VI)).

The initial bond lengths of the Si–F, Si–Cl, Si–O, and

O–H are 1.815, 2.165, 1.85, and 0.96 A respectively. In the

calculations, the bond lengths of Si–H and Si–Si of Si29H36

are first optimized and then all the other structures are

optimized with freezing Si positions.

2. Results and discussions

In order to investigate the electronic properties, we first

performed calculation for the basic cluster Si29H36; the

optimized bond lengths of Si–Si and Si–H are 2.352 and

1.5004 A respectively. The energy gap, which is defined as

the difference between the lowest unoccupied molecular

orbital (LUMO) and the highest occupied molecular orbital

(HOMO), is shown as Fig. 2(a)(1).

To study the effect of surface non-oxygen passivants F

and Cl we calculated the electronic structure of the first

group of clusters Si29H362kXk (X ¼ F, Cl, and k ¼ 1; 2)

and compared them with cluster Si29H36. The optimized

bond lengths of Si–H, Si–F and Si–Cl are 1.500, 1.682

and 2.146 A. The calculated results are shown in

Fig. 2(a)(2)– (5). The comparison indicates that the

Kohn–Sham molecular orbital levels of the band edge of

these clusters are changed in some extent and have some

differences with each other. The shifts of band edge states in

two passivants case are slightly larger than that of one

passivant. The band gaps are also affected by the non-

oxygen passivants F and Cl. The gaps slightly decrease by a

relative value of 1.2–3.9%.

To explain the experiment phenomena that the PL

spectral is sensitive to oxygen [2,4] and evaluate the

influence of oxygen and oxygen-related passivants on the

gap, we have examined the electronic states for the second

group of simulating clusters Si29H362kðOHÞk ðk ¼ 1; 2Þ and

the third group of clusters Si29H3622kOk ðk ¼ 1; 2Þ with

different oxygen-related bonded structures for the surface

passivant O atom. There are two possible bonded structures

for the same cluster Si29H34O, one is SiyO double bonded

structure (see Fig. 1(V)) and the other is Si–O–Si bridged

structure (see Fig. 1(IV)). We firstly performed the bond

energy calculations to the two bonded structures. The results

show that the bond energy of the former is lower than the

latter by about 1.08 eV. This means that the SiyO double

bonded structure is more stable than that of Si–O–Si

bridged. Thus it is reasonable to choose the SiyO double

bonded structure to be investigated. The electronic states of

clusters passvanted by oxygen related atoms are plotted in

Fig. 2(a)(6)–(9). Compared with the result of cluster

Si29H36, two interesting properties appear. First, there is

relatively smaller variation of band edge states and the gap

decreasing by about 4.7 and 7.6%, respectively, in one and

two OH passivants cases [see Fig. 2(a)(6) and (7)]. This

result of the single bonded OH with silicon atom is very

similar to that of the single bonded F and Cl passivants.

Second, in the SiyO case, the band edges shift significantlyFig. 1. Structures of Si29H362kXk (X ¼ F, Cl, OH, k ¼ 1; 2) and

Si29H3622kOk ðk ¼ 1; 2Þ:

Y. Dai et al. / Solid State Communications 126 (2003) 103–106104

and the gap evident closing more than 1 eV as shown in

Fig. 2(a)(8) and (9), which is very different with F, Cl and

OH passivants.

To reach a more general conclusion, we have done same

calculations for the Si47-core. The corresponding bond

lengths adopted are that after optimization in Si29-core. The

structures are not relaxed again due to the changes with

respect to the initial geometries are general small. For

example, in Si29H36 the relaxed Si–Si bond lengths are

0.08% smaller than 2.35 A while the Si–H bonds at the

surface increased from 1.48 A by 1.3%. From the results of

Si47-core shown in Fig. 2(b) we can see the similar effects to

Si29-core.

The above results reveal the fact that though the band

edge states of Si-nanocrystals are sensitive to all of the

studied passivants, all the passivants F, Cl and OH which are

of the single bonded with Si atom have relatively slight

influences on the band edges and gap, while the double

bonded oxygen passivants significantly affects the band

edges and gap. In all cases the band edge shift more

obviously in two passivants than in one passivant, which

means the increase in passivants coverage may cause a

further change of the gap.

The results may be explained as follows. (1) The

principal effect of the passivants is to break the Td

symmetry structure of Si-core. At atomic level, the

electronic properties of solids should be dominated by

local chemical combination valance according to the

molecular chemistry theory [22]. Thus the different Si–X

bond orbitals according to the different passivants result in

the shift of the band edge states or band gap in the electronic

structure of clusters. The valence electrons of silicon atoms

in the cluster are driven energetically to form sp3 hybrid

orbital and covalent bond in a local tetrahedral structure.

Because the clusters Si29H602kXk (X ¼ F, Cl, OH) have the

same Si29-core as Si29H36, all Si–Si bond orbitals are

equivalent or quite similar in the core. The silicon remains

favorable sp3 network in the fully hydrogenated case. When

one or two hydrogen atoms are replaced by the single

bonded passivants, the Si29 core states have only slightly

altered and the Si’s sp3 network changed little, the band

edge states are determined mainly by the Si29-core structure.

Accordingly, it has relatively weak effect on the gap, though

the band edges slightly change comparing to Si29H36.

When the double bonded passivant oxygen is added to the

cluster, the considerable distortion of the sp3 network of

Si29-core states leads to the significant change of band edge

states and the great decrease of gap. Consequently, the band

edge states are dominated greatly by the SiyO bonded

structure. (2) In another point of view, the closing HOMO

and LUMO levels, for example, in cluster Si29H34O [see

Fig. 2(8)], can also be regarded as a pair of bonding and anti-

bonding states caused by a relatively weak bond in SiyO,

similar to the SiySi double bond. (3) The molecule orbital

compositions also show that for the single bonded surface

impurities, their levels lies inside the valence bands and

accordingly are indirectly related to the nature of the band

gap. However the nature of the HOMO and LUMO are

changed significantly in the case of SiyO. For example, for

the Si29H32O2 cluster the HOMO state localize on the Py and

Px orbitals of two silicon atoms near the silicon double

bonds and on the Py and Pz orbitals of two oxygen atoms,

which means they locate on the passivants. Thus the SiyO

structure directly affects the band edge states and the gap.

These results agree well with both the experimental results

(Fig. 3 in Ref. [5]) and the theoretical results (Fig. 4 in Ref.

[5]) in quantity within the diameter range of Si-nanocrystals

from 1.0 to 1.34 nm.

No evidence shows that the principal mechanism of gap

Fig. 2. Electronic structures and passivants structures of clusters. (a)

(1) Si29H36, (2) Si29H35F1, (3) Si29H34F2, (4) Si29H35Cl1 (5)

Si29H34Cl2 (6) Si29H35(OH)1, (7) Si29H34(OH)2, (8) Si29H34O1,

(9) Si29H32O2. (b) (1)Si47H60, (2) Si47H59F1, (3) Si47H58F2, (4)

Si47H59Cl1 (5) Si47H59Cl2 (6) Si47H59(OH)1, (7) Si47H58(OH)2, (8)

Si47H58O1, (9) Si47H56O2. The bottom of the conduction band EC is

defined as LUMO and the top of valence band EV is defined as

HOMO.

Y. Dai et al. / Solid State Communications 126 (2003) 103–106 105

formation for two passivants existing in the surface is

different with that for one passivants. Therefore we think the

fact that the band edges shift more obviously in two

passivants than one could be owed to the interactions among

the passivants-induced electronic states.

3. Conclusions

Our calculations confirm that band edge states and gap of

Si-nanocrystals are influenced sensitively by the passivants

on the surface. There are relatively weak and rare changes of

the band gap and band edge states when different passivants

such as F, Cl and OH are single-bonded structure on the

surface. However, the double bonded oxygen passivant have

lager impact on the band gap and band edge states.

We interpret the results as follows: with the same Si core,

the single bonded elements almost keep the sp3 network

nature so that the Kohn–Sham HOMO and LUMO levels

shift little; contrarily, the double bonded SiyO passivant

destroys the sp3 network nature of Si core significantly, and

because the HOMO and LUMO locate on the passivants, the

bonding state and anti-bonding state levels for the weak

bond SiyO are separated from the conduction and valence

band edges. The interactions among passivants-induced

electronic states result in a further change of gap with

increasing impurity concentration.

Acknowledgements

The financial support comes from the ministry of science

and technology of China (973, 001CB610504). The

Advanced Visiting Scholar Foundation of Key Lab of

China in Peking University supported the work. The

calculations performed in the computer of the network

center of Chinese science academy and Oringe 2100

workstation of Physics department, Shandong University.

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