Hydrogen-induced platelets in disordered silicon

Solid State Communications, Vol. 99, No. 6, pp. 427-431, 1996 Copyright 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved

003%1098/96 $12.00 + .OO PI1 SOO38-1098(96)00283-9

HYDROGEN-INDUCED

PLATELETS

IN DISORDERED

SILICON

N. H. Nickel, G. B. Anderson, and J. Walker Xerox Palo AltoResearch Center, 3333 CoyoteHill Road, Palo Alto, California94304 (Received

15 April

1996; accepted

22 April 1996 by A. lZfros)

It is demonstrated that hydrogen passivation of polycrystalline silicon (poly-Si) causes the formation of hydrogen stabilized platelets. These extended defects appear within 1000

A of the sample surfaoe and are predominantly

oriented

along ( 1111crystallographic planes. Nominally undoped and phosphorous doped poly-Si (Ip]=lO” cn?) show platelet concentrations of =5x10” c&and

1.5~10’~ cni”, respectively. An estimate of the number of H atoms accommoda&d in

platelets by firming Si-H bonds is consistent with the hydrogen concentration in the surface layer measured by SIMS. Platelets were not observed in the grain boundary regions. This is due to two effects: (I) The presence of a depletion layer at graiu boundaries causing hydrogen to migrate in the positive charge state which is unfavorable for the platelet formation; (ii) Platelet nucleation is surpressed due to the presence of a high concentration of hydrogen trapping sites at grain boundaries.

Copyright

0 1996 Published by Elsevier Science Ltd

Keywords: A. disordered systems, A. semiconductors, C. grain boundaries, C. scanning and transmission electron microscopy

are unrelated to plasma or radiation damage because they

The role of hydrogen in semiconductors has attracted a great deal of interest in the past years. The ability of

can be introduced with a remote hydrogen plasma system.3’5

hydrogen to passivate both shallow-level dopants and deep-

These hydrogen-induced defects have localized states in the

level defects is utilized to improve the properties of

band gap and hence, they are of immediate scientific and

semiconductors

technological interest.

such as polycrystalline

silicon (poly-Si)

which contains a high density of localized states in the grain

In this paper, we demonstrate for the first time that

boundary region.’ Hydrogen is most commonly introduced

hydrogen passivation of fine-grain poly-Si can result in the

into semiconductors by exposure to a plasma discharge. As

generation of extended structural defects, in spite of a high

a consequence of hydrogen chemical bonding in the host

concentration

of H traps at the grain boundaries.6 In

semiconductor, energy levels in the band gap are removed.

specimens with similar phosphorous concentrations

A prominent example of this phenomenon is polycrystalline

platelet densities at the surface of poly-Si and c-Si are

silicon where hydrogen passivation results in a decrease of

comparable,

the

concentration decreases and no experimental evidence was

defect

density

thereby

improving

the

electrical

Towards

grain

boundaries

the

the

platelet

properties of poly-Si films and devices.‘**In addition to the

found that these structural extended defects penetrate or

passivation of localized states it has been shown that

nucleate at grain boundaries. The experiments described in this paper were performed

prolonged exposure of intrinsic poly-Si to monatomic hydrogen at elevated temperatures (350 - 450 “C) causes the formation of acceptor-like states which leads to

on nominally undoped and phosphorous doped finegrain

electrical type-conversion?

crystallization of 0.1 and 0.5 pm thick doped and undoped

poly-Si

Moreover, hydrogen diised

The poly-Si

by laser

amorphous

300 “C) can generate extended structural defects.4 These

performed by exposing specimens to monatomic hydrogen

hydrogen-stabilized

oriented

or deuterium generated in an optically isolated remote-

along ( 111) crystallographic planes. Both, the acceptor-like

plasma system which eliminates surface damage resulting

states in poly-Si and the extended structural defects in c-Si

!?om direct immersion in the plasma. The microwave plasma 427

respectively.

prepared

into single-crystal silicon (c-Si) at moderate temperatures (I platelets are predominantly

silicon,

films were

Hydrogenation

was

428

HYDROGEN-INDUCED PLATELETS IN DISORDERED SILICON

was operated at 70 W and at a pressure of 2 Torr.

curve and the dashed line depicts a least-squares fit to the

Hydrogen concentration profiles, were measured by ~cond~-ion-~ss

~~~0~~

Vol. 99, No. 6

convolution of a complementary error function (erfc) with

(SIMS) using a Cs+ ion

the SIMS depth resohrtion.7 For a depth greater than 0.1

beam. For this purpose deuterium was used as a readily

pm fit and data are in good agreement. However, within the

identifiable isotope which duplicates hydrogen chemistry. In

first 0.1 nm of the depth profile the D concentration

order to obtain a higher depth resolution deuterium was

deviates from the least-squares fit indicating a peak in the D

introduced into 0.5 urn thick poly-Si films. These specimens

concentration.

were comparable to the 0.1 pm thick fdms in terms of spin

Johnson

et

aL4 de~~~at~

that

the enhanced

density (Ns = 2x10” cm3) and average grain size. Typical

hydrogen concentration in the surface region of c-Si, after

deuterium

exposure to a hydrogen plasma, is due to the generation of

depth

profiles obtained

on

undoped

and

phosphorous doped poly-Si lilms are shown in Fig. 1. The

hydrogen-stabilized

samples doped with a P concentration of lOi cmT3were

deuterium concentration protiles measured in poly-Si (Fig.

exposed to mo~to~c

platelets.

The

similarities

of

the

D at I.50 “C for 5 and 30 min,

1) to the D depth profiles reported for c-Si (Ref. 4) suggest

respectively. In the first 300 k of the depth profiles both

that the exposure of poly-Si to monatomic D or H also leads

samples reveal a peak in the deuterium concentration which

to the generation of platelets. On the other hand, in poly-Si

is similar to results reported for n-type single crystal

excess hydrogen could also be accommodated in stacking

silicon.’ For comparison a D depth prolile measured on

faults.

undoped poly-Si is also shown in Fig. 1. The undoped polySi film was exposed to monato~c

In order to clarify the origin of the high D concentration

D at 200 “C for 60 min.

The deuterium depth profile is represented by the solid

undo~d 60 min 200 “C

K. B z

[P]=10'7 cm-3

101’

30 mitt 150 “C

~~

0.0

7

0.1

0.2

0.3

0.4

0.5

Depth (pm) Fig. 2: Cross-sectional TEM micrographs, viewed in a (110) Fig. 1: Depth profiles of deuterium in undoped

and

projection. (a) bright-field image of undoped poly-Si prior

phosphorous doped poly-Si. The P doped specimens were

to the exposure to monatomic hydrogen. (b, c) Undoped

exposed to monatomic D at 150 “C for 5 and 30 mitt,

and P doped poly-Si ([Pl=lxlOzo cni3) after an exposure to

respectively, and the undoped sample was hydrogenated for

~Mto~c

60 min at 250 “C. The dashed line depicts a least-squares fit

hydrogenation at 275 “C for 1 h. (d) High-resolution lattice

H at 150 “C for 20 mitt followed by a

to the convolution of a complementary error function (@ii)

image of a platelet in P doped poly-Si, viewed in a (110)

with the SIMS depth resolution.7 Note the enhanced D

projection; (e) electron diffraction pattern of a grain containmg platelets.

concentration in the surface region.

Vol. 99, No. 6

HYDROGEN-INDUCED PLATELETS IN DISORDERED SILICON

429

in the surface region of poly-Si, transmission electron microscopy (TEM) micrographs were taken on P doped and undoped poly-Si films. The bright-field image in Fig. 2 (a) shows a cross-sectional view of undoped poly-Si prior to the exposure to monatomic H. Besides grain boundaries no defects were detectable in the grains of unhydrogenated poly-Si fihns. Undoped and phosphorous-doped

poly-Si

films are shown in Fig. 2 (b) and (c), respectively, after a two-step hydrogen exposure. The first hydrogenation was performed at 150 “C for 20 min followed by an exposure to monatomic H at 275 “C for 60 min. This procedure is known to produce platelets up to 100 nm long. Platelets nucleate during the low temperature hydrogenation and continue

to grow

during

the second

exposure-step.*

According to the TEM micrographs [Fig. 2 (b) and (c)] the hydrogenation

process produces micro-defects in both

specimens that are identical to platelets observed in single crystal silicon.4 The TEM micrographs show the poly-Si cross-section close to a (110) zone axis. Hence, platelets in

Fig. 3: Cross-sectional

two of the 4 (111) planes appear as linear defects. The

boundary. The high resolution image of a grain boundary

TEM micrographs of a grain

second (111) plane is rotated by 90 with respect to the first

(b) is rotated by 90”. The periodic@ of the silicon lattice

plane. Hence, the H stabilized platelets are rotated by the

does not appear in the lower half of the micrograph due to a

same angle and they appear as two dimensional discs.

different orientation of the grain.

Undoped and phosphorous doped poly-Si samples reveal a difference in platelet concentration.

While bright-tiled

images taken on undoped samples show an average density of =5x10”

cni3 micro defects the platelet density in

phosphorous doped poly-Si

boundaries by platelets (Fig. 3). In fact, no platelets were found within a distance of 240 nm measured from the grain boundaries. In Fig. 4 the average platelet distribution in

is a factor of 3 higher

(=1.5x1016 cme3). A high-resolution

lattice image of a

hydrogen stabilized platelet is shown in Fig. 2 (d). The corresponding electron diiaction-pattern

50 -

shown in Fig. 2

(e) indicates that the platelet is oriented along a { 11 I]

?

crystallographic plane. A Burgers circuit analysis of the

iu ” f: * z 8 z3

platelet in Fig. 2 (d) indicates no net displacement in the lattice which eliminates dislocations

as the origin of

platelets. Similarly, no evidence was found that platelets consist of either interstitial or vacancy loops, since contrast typical of stacking fault8 was not observed. The formation of hydrogen-stabilized platelets or micro cracks induces strain in the silicon lattice that can be

AA A

40-

A

30-

A

20 -

A

;;; z

of lattice strain and disorder in the grain-boundary regions. In these regions the formation

of platelets could be

A A

A

10 -

grain boundary

observed in TEM micrographs (Fig. 2). In contrast to single crystal silicon polycrystalline silicon contains a high degree

A A

0

-/(/.,. A 0

,,,,,., 100

200

300

1 400

500

600

d (rim)

favorable and one would expect to see that platelets nucleate or penetrate grain boundaries. This, however, is

Fig. 4: Platelet concentration in polycrystalline silicon as a

not the case. Cross-sectional TEM micrographs did neither

function of the distance horn the grain boundary (d = 0 nm) into the silicon grain @f> 0 mn).

show evidence for nucleation nor the penetration of grain

430

HYDROGEN-DUCT

FLA~LETS

IN DISO~E~D

Vol. 99, No. 6

SILICON

phosphorous doped silicon grains is plotted as a function of

case of undoped poly-Si a platelet concentration of 5~10’~

the distance &om the grain boundary. The arrow at d = 0

cnX3accommodates roughly a wncentration of 3x10” cm3

~~tes~~~~~d=~~~~tes~

H atoms. This is consistent with hydrogen co~~a~~

center of the grain. The plotted platelet concentrations were averaged over a large number of grains. At the grain

a surface layer 0.1 urn thick obtained

measurements (Fii. 1 and Ref. 11). A similar result was

boundariw (d = 0) the platelet wonton

obtained for p~sphorous

decreases to

in

from SIMS

doped poly-Si samples. This

zero. With increasing distance front the grain boundary the

clearly demonstrates that most of the hydrogen in the first

average platelet wncentration

0.1 pm of the poly-Si samples is accommodated in platelets.

increases monotonically and

reaches a value of =5x10” cni3 in the center of the grains.

The hydrogen conwn~tion

A similar distribution was found in grains of nominally

and optical properties of the material by passivating siliwn

undoped poly-Si samples. These results clearly demonstrate

dangling-bond defects at grain boundaries is less than 1% of

that the presence of lattice strain does not wntribute to the

the surface H concentration.

fo~tion

of hydrogen stabilized platelets.

The

The experimental results presented above have a number of important implications. It is well established that the exposure of single crystal silicon to autos

hydrogen

can cause the formation of platelets. An indirect indiiion

formation

of

necessary to improve electrical

hydrogen

stabii

platelets

introduces a strain field around the micro defects due to a slight displacement of siliwn atoms l?om their substitutional lattice sites. In poly-Si a natural source of lattice strain are the grain boundaries. The diitribution of bond distortions

of this phenomenon is a peak of the hydrogen concentration

has not yet been obtained experimentally and theoretical

in the surface layer (= 0.1 urn) detected by SIMS4

studies have only been carried out for grain boundaries in

Although a near surface hydrogen peak can occur in both,

polycrystalline germanium An ab initio molecular dynamics

n-type and p-type c-Si samples, the formation of platelets

approach for the I;5 and X5’ twist boundaries revealed that

occurs only in n-type silicon.” Hydrogen concentration

the nearest neighbor bond length ranged from 2.2 to 2.8 A.

profiles obtained from boy

The ~~~t~n

undoped and p~sp~rous

doped poly-Si show a similar H accumulation in the near

outnumbering

is asymmetric

with stretched

bonds

compressed bonds. “Jo Stretched bonds

surface region (Fig. 1). In both specimens hydrogenation

produce localized states in the band gap. It is widely

caused the fo~t~n

believed that these states form exponential band tails commonly observed in disordered semiconductors such as

observation

of platelets lFii.

2 (b, c)]. The

of platelets in undoped poly-Si raises an

important question. In contrast to undoped poly-Si, the

poly-Si.

The

introduction

of hydrogen

into

exposure of intrinsic single crystal siliwn to monatomic

decreases the wncentration of strained Si-Si bonds.‘” These

hydrogen does not result in the formation of platelets. In c-

observations

Si the formation of platelets occurs only in n-type samples

favorable for the formation and nucleation of platelets.

suggest that grain boundaries

poly-Si

should be

with a Fermi-level position of EC- EpI 0.45 eV. Moreover,

However, according to the experimental results presented

the platelet w~n~ation

above (Fgs. 2,3 and 4) platelets do neither nucleate at, nor

increases ~~to~y

with temperature

cross grain boundaries. There are several mechanisms that

dependence of the dark conductivity, measured on undoped

can prevent the formation of platelets at grain boundaries.

poly-Si samples prior to the exposure to monatomic H,

These hvo-diiional

revealed activated behavior with an activation energy of EA

of deep and shallow trapping sites for hydrogen. Silicon

= 0.46 eV. This suggests that either the amorphous silicon

dangling bonds, preferentially located at grain bout&ties

was contaminated or some degree of wntamination occurred during laser ~s~tion such as the formation

efficiently capture H atoms to form S&H bonds. These deep

of oxygen-related thermal donors. Hence, the presence of

point for H diffusion. Additional bands of shallow and deep

platelets in nominally undoped poly-Si is consistent with the

defects are located at approximately 0.5 an 1.7 eV below

decreasing

Fermi

energy

(E&?F).

The

defects wntain a high concentration

defects are located ~2.5 eV below the migration saddle

Fermi energy dependency of the platelet formation found in

the migration saddle point, respectively.” In the presence of

single crystal silicon.

these H trapping

From the concentration and size of the platelets we can

sites the formation of platelets is

energetically unfavorable. A second effect preventing the

estimate the amount of hydrogen accommodated in these

nucleation of platelets at grain boundaries can be attributed

~o-~~~~

defects. With an average diameter of 80

to the band bending. In n-type and undoped poly-Si charge

nm each platelet contains roughly 6x10’ Si-H bonds. In

at the grain boundaries causes the formation of depletion

Vol. 99, No. 6

HYDROGEN-~UCED

PLATELETS IN DISORDERED SILICON

431

layers. Hence, the Fermi level resides deeper in the band gap

number of H atoms accommodated in the platelets by

with respect to the center of the grains. Platelet formation

forming Si-H

strongly depends on the Fermi level position and thus, on

concentration

the charge state of the bats

H atoms. If the Fermi level

Although platelets introduce lattice strain, the presence of

is pinned around mid gap, H atoms diffise in the positive

lattice strain in form of grain boundaries does not promote

bonds is consistent with the hydrogen in the surface layer measured by SIMS.

charge state and platelet nucleation does not occur. The

the nucleation or growth process of platelets. Platelet

formation of platelets is only observed when hydrogen

formation is surpressed due to the presence of deep

migrates in the negative charge state.”

Both effects

hydrogen traps and due to depletion layers that cause H to

effectively prevent the nucleation and growth of platelets in

diffuse in the positive charge state. This is portent

the grain boundary regions.

recent results obtained on doped c-Si samples!’

with

In summary, we have presented experimental evidence for the formation of platelets in poly-Si during exposure to a remote hydrogen plasma. The platelets are observed in the { 111) ~s~~ap~

planes. Platelet co~n~~~

of

The authors would like to thank N. M. Johnson and C. Herring for many s~u~t~g

diissions.

One of the

=5x10if cni” and 1.5~10’~ cni’ were measured in nominally

authors (N.H.N.) also likes to acknowledge partial support

undoped and phosphorous doped poly-Si, respectively. The

from the Alexander-von-Humboklt

micro defects are stabiied

Republic of Germany.

by H atoms. An estimate of the

foundation,

Federal

References

’ T. I. Kamins and P. J. Marcoux, IEEE Electron Device Lett. 1,159 (1980). ’ F. Boulitrop, A. Chenevas-Paule, and D. J. Dunstand, Solid St. Commun. 48,181( 1983). 3 N. H. Nickel, N. M. Johnson, and J. Walker, Phys. Rev. Lett. 75,372O (1995). ’ N. M. Johnson, F. A. Ponce, R. A. Street, and R. J. Nemanich, Phys. Rev. B35,4166 (1987). ’ N. M, Johnson, in Hydrogen in Semiconductors,edited by J. I. Pankove and N. M. Johnson, Semiconductors and Semimetals VoL 34 (Academic Press, Boston, 1991), Chap. 7. 6 W. B. Jackson, N. M. Johnson, C. C. Tsai, I.-W. Wu, A. Chiang, and D. Smith, AppL Phys. Len. 61, 1670 (1992). ’ P. V. Santos and W. B. Jackson, Phys. Rev. B 46, 4595 (1992). a N. M. Johnson, C. Herring, C. Doland, J. Walker, G. Anderson, and F. Ponce, Mat. Science Forum, %3-87,33

(1992). ’ F. A. Ponce, T. Yamashita, R. H. Bube, and R. Sinclair, in Defecrs in ~emic5~ctors,

edited by J. Narayan and

T. Y. Tan, Mat. Res. Sot. Symp. Proc., 2 (NorthHolland, Amsterdam, 198 1) p. 503. lo N. H. Nickel, G. A. Anderson, C. Herring, N. M. Johnson, and J. Walker, to be published. l1 N. H. Nickel, W. B. Jackson, and J. Walker, Phys. Rev. B 53,775O (1996). l2 E. Tamow, P. D. Bristowe, J. D. Joannopoulos, and M. C. Payne, in Atomic Scale Calculations in A4aterials Science, edited by J. Tersoff, D. Vanderbilt, and V. Viiek, MRS Symposia Proceedings No. 141 (Materials Research Society, Pittsburgh, 1989). p. 333. I3 E. Tamow, P. Dallot, P. D. Bristow, J. D. Joannopoulos, G. P. Francis, and M. C. Payne, Phys. Rev. B 42, 3644 (1990). I4 W. B. Jackson, N. M. Johnson and D. K Biegelsen, Appl. Phys. Lett. 43,195 (1983).