Microstructure and semiconducting properties of c-BN ®lms using r.f.plasma CVD thermally assisted by a tungsten ®lament

W.L. Wang*, K.J. Liao, S.X. Wang, Y.W. Sun

Department of Applied Physics, Chongqing University, Chongqing 400044, People's Republic of China

Abstract

Cubic boron nitride ®lms were deposited on (100) silicon substrate using r.f. plasma chemical vapor deposition (CVD) thermally by a

tungsten ®lament. The ®lms obtained were characterized by infrared absorption spectra and electron energy loss spectroscopy. Experimental

results showed that the structure and quality of cubic boron nitride (c-BN) ®lms strongly depended on the deposition conditions. The studies

also indicated that the compressive stress in the ®lms increased with h-BN content in the ®lms, while interfacial region corresponding to the

transition from h-BN to c-BN was found to be of very low stress value. P-type conductivity with promising values of the mobility in c-BN

®lms has been found by Be-doped. q 2000 Published by Elsevier Science S.A. All rights reserved.

Keywords: c-BN ®lms; Plasma chemical vapor deposition; Semiconducting properties; Stress

1. Introduction

Cubic boron nitride (c-BN) is of interest owing to the

outstanding material properties as a wide bandgap

(Eg < 6.4 eV) offering many potentially useful characteris-

tics [1±4]. These include high thermal conductivity, high

strength, chemical inertness and optically transparent in a

wide range. The special advantages of c-BN with respect to

diamond are its inertness, even at high temperature, and the

possibility of p-and n-type doping. Thus, there are many

potential applications such as protective coatings, tool coat-

ings, optical windows, heat sinks, and high-temperature

electronic devices. c-BN ®lms have been synthesized by

various methods including boron evaporation with N21

ion-beam bombardment [5], activated reactive evaporation

using the hollow cathode discharge [6], ECR plasma chemi-

cal vapor deposition (CVD) [7], r.f. plasma CVD thermally

assisted by a tungsten ®lament [8], r.f. sputtering [9], MW

plasma CVD [10], ion-induced CVD [11], inductively

coupled plasma (ICP) [12] etc.

In this paper, the microstructure and semiconducting

properties of c-BN ®lms using r.f. plasma CVD thermally

assisted by a tungsten ®lament were investigated by infrared

(IR) absorption spectra, electron energy loss spectra etc. The

experimental results showed that the structure and quality of

c-BN ®lms strongly depended on the deposition conditions.

2. Experimental

The deposition method of cubic boron nitride ®lms by r.f.

plasma CVD thermally assisted by a tungsten ®lament is

similar to that originally reported by Miyamoto and co-

workers [13]. The deposition system was composed of

two main parts, i.e. plasma chamber and a substrate-heating

chamber with a tungsten ®lament situated just above the

substrate. Be material was placed in a ceramic boat in the

plasma chamber for p-type doping. The chamber was evac-

uated to a pressure of 10 mPa using the diffusion pump

system. B6H6 and NH3 diluted with H2 were used as reactive

gases. Substrate material was silicon (100). The reactive

gases were injected into the chamber when the substrate

temperature was heated to 8008C. The reactive gases were

excited into the plasma state by r.f. induction of 13.65 MHz.

The excited plasma was further thermally active by the

heating of the tungsten ®lament, and Be was evaporated

by heating. The Be-doped c-BN ®lms were deposited on

the silicon substrate. The deposition conditions are summar-

ized in Table 1.

The BN ®lms obtained were characterized by infrared

absorption measurement and electron energy loss spectra,

etc. The c-BN and h-BN have strong absorption bands at

1080 and 1380 cm21 (c-plane), respectively. The deposition

ratio of c-BN/h-BN can be obtained by the intensity ratio of

the IR absorption bands.

The stress within the ®lms was evaluated from the

measured curvature radii of the substrate by a sloan

DEKTAK surface pro®lometer, which was also used to

Thin Solid Films 368 (2000) 283±286

0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved.

PII: S0040-6090(00)00783-5

www.elsevier.com/locate/tsf

* Corresponding author.

measure the thin ®lms thickness [14]. The basic formula for

the stress in the ®lms is [14]

s � Ests=6�1 2 vs�tf

� � 1

Rf

21

R0

� �where Es and vs are Young's modulus and Poissons ratio of

the substrate, respectively; ts and tf are the thickness of the

substrate and the ®lms; R0 and Rf are the substrate radii of

curvature without and with the ®lms, respectively.

3. Results and discussion

Fig. 1 shows the IR absorption intensity ratio as a func-

tion of the ®lament and substrate temperature. The IR

absorption intensity ratio means intensity ratio Ic-BN (1080

cm21/Ih-BN (1380 cm21). The r.f. power and gas pressure

were kept at 100 W and 90 Pa, respectively. The ratio Ic-

BN/Ih-BN increases with increasing the ®lament and substrate

temperature, since the deposition rate of c-BN increases

rapidly with the temperature of the ®lament and substrate,

contrary to that of h-BN, which decreases above ®lament

temperature of 16008C and substrate temperature of 6008C.

However, Ic-BN is decreased with the substrate temperature

higher than 8008C. Ic-BN/Ih-BN achieved a maximum when the

®lament and substrate temperature is up to 2000 and 8008C,

respectively.

Fig. 2 shows the dependence of the total stress (thermal

and intrinsic) on the ®lament temperature and the r.f. power.

All samples were deposited onto Si (100) at the substrate

temperature of 8008C for gas ¯ow rate of 100 sccm. The gas

pressure and the substrate temperature were at 90 Pa and

8008C, respectively. The thickness of the ®lms was about

0.6 mm. From Fig. 2, it can be seen that the stress is

compressive, and reduces as the ®lament temperature and

r.f. power are increased. The stress of the ®lms was

decreased from 5 £ 108 to 0.81 £ 108 N/m2 when the ®la-

ment temperature and the increased from 14008C and 20 W

to 20008C and 100 W, respectively.

Infrared absorption alone is not a completely sound diag-

nostic to verify the presence of c-BN due to the fact that

oxides of the silicon substrate have a coincidental overlap in

absorption in the vicinity of the c-BN absorption at 1080

cm21. So, the electron energy loss spectrum can provide the

most reliable determination for c-BN. Fig. 3 shows the elec-

tron energy loss spectra (EELS) of BN ®lms at different

®lament temperature. The p* peak is characteristics of h-

BN in Fig. 3. The p*characteristic peak of h-BN became

very strong with decreasing ®lament temperature.

For in-depth investigation, the layered structure of BN

®lms was obtained by reactive ion etching. The reactive

ion etching was performed in a r.f. chamber in a SF6:O2

(50:50) mixture under a pressure of 2.5 £ 1022 mbar with

a r.f. power of 20 W. At each etching step, IR and stress

were measured. The thickness of the ®lms is about 100 nm.

Fig. 4 shows the changes of ratio Ic-BN/Ih-BN with the thick-

W.L. Wang et al. / Thin Solid Films 368 (2000) 283±286284

Table 1

Deposition conditions for c-BN ®lms

Gas source B6H6 1 NH3 1 H2

B6H6/NH3 1/3

B6H6/H2(%) 1

NH3/H2(%) 1

Gas pressure (Pa) 20±100

Filament temperature (8C) 1500±2000

r.f. power (W) 100±200

Substrate temperature 800

Gas ¯ow rate (sccm) 100

Fig. 1. IR absorption intensity ratio as a function of the ®lament temperature

for BN ®lms: (a) Ic-BN/Ih-BN ratio as a function of ®lament temperature, (b) Ic-

BN/Ih-BN ratio as a function of substrate temperature.

ness of the ®lms, and Fig. 5 shows the relationship between

the stress and the thickness. The samples were deposited at

the same conditions. From Figs. 4 and 5, it is apparent that

the ratio Ic-BN/Ih-BN is about 2.75 when the thickness of the

®lms from the surface is about 100 nm, then abruptly drops

to zero when the thickness is within 7 nm at the interface

between the ®lms and the substrate. The changes of the

stress with the thickness of the ®lms are similar to that of

ratio Ic-BN/Ih-BN. However, the stress is gradually increased

from 0.8 £ 108 to 1.8 £ 108 N/m2, ranged from 100 to 7 nm,

then drastically increased up to 7.5 £ 108 N/m2.

As mentioned results above, the internal stress in the ®lms

depends upon the c-BN content in the ®lms. The compres-

sive stress was increased when the h-BN content in the ®lms

increased. On the other hand, the presence of this high

compressive stress in the pure h-BN sublayer implies that

mechanism of stress-induced nucleation proposed by

McKenzie [15] is to the be validated. A high compressive

stress can induce phase transformation from h-BN phase

with volumes mass of 2.28 g/cm3 to a much denser c-BN

phase (3.5 g/cm3). The high compressive stress in h-BN

layers was caused by a large lattice mismatch between the

®lms and the substrate and ion bombardment in the plasma.

We have investigated the transport properties of c-BN

®lms grown using Be-doped described above as well. Be

doping of c-BN ®lms can be achieved by introducing Be

powder in the CVD deposition chamber. During the growth

process Be is etched by hydrogen plasma to form Be

W.L. Wang et al. / Thin Solid Films 368 (2000) 283±286 285

Fig. 3. EELS of BN ®lms at different ®lament temperature.

Fig. 4. Changes of ratio Ic-BN/Ih-BN with ®lm thickness.

Fig. 2. Dependence of the total stress in c-BN ®lms on the ®lament tempera-

ture and r.f. power:(a) stress as a function of ®lament temperature, (b) stress

as a function of r.f. power.

hydrides. The Be hydrides enter the plasma and Be is incor-

porated into the c-BN ®lms during the growth process. Hall-

effect measurement yields carrier-type concentration and

mobility. The hole mobility in Be doped samples decreases

with increasing carrier concentration. The Hall mobility

with a room temperature carrier concentration of 4 £ 1018

cm23 was 410 cm2/V s.

4. Conclusions

We have investigated microstructure and semiconducting

properties of c-BN ®lms using r.f. plasma CVD thermally

assisted by a tungsten ®lament. We have shown that the

deposition conditions have a signi®cant effect on the quality

of the ®lms. The internal stress in the ®lms depends on the

content of h-BN in the ®lms, and a high stress in the h-BN

layers can induce phase transformation from h-BN phase to

c-BN phase. The experimental results also indicated that the

P-type semiconductor c-BN ®lms could be achieved by

beryllium doping.

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W.L. Wang et al. / Thin Solid Films 368 (2000) 283±286286

Fig. 5. Relationship between the stress and the thickness.