inn thin-film growth using an ecr plasma source

E L S E V I E R Materials Science and Engineering B35 (1995) 171 175

MATERIALS SCIENCE &

ENGINEERING

B

InN thin-film growth using an ECR plasma source

Yuichi Sato, Susumu Sato Department of Electrical and Electronic Engineering, Mining College, Akita University, 1-1 Tegatagakuen-cho, Akita OlO, Japan

Abstract

An electron-cyclotron-resonance (ECR) plasma source is used in an InN thin-film growth process by reactive evaporation. Some properties of prepared films are measured and compared with those of InN films grown using radio frequency (RF) plasma. Deteriorations of surface morphology and crystallinity of the films are not observed even when their growth rate is extremely low. Dependences of the film properties on other parameters, such as distance between the ECR source and substrates, or orifice size of the ECR source, are also investigated. Moreover, some plasma parameters of the ECR source are compared with those of the RF plasma by optical emission spectroscopy and Langmuir probe measurements.

Keywords: Thin films; Indium nitride; Nitrides; Plasma processing

I. Introduction

Indium nitride (INN), gallium nitride (GaN) and aluminum nitride (A1N) have direct energy band gaps of 1.9, 3.4 and 6.2 eV, respectively. The band gaps of their alloys are tunable in the wavelength region from visible to ultraviolet, and various applications of these materials to optoelectronic devices are expected.

Among these III-V nitrides, the growth of InN is most difficult because of its high dissociation pressure [1,2]. In growth processesof InN thin films, decomposi- tion of the films tend~ to occur when the growth temperature is above about 500 ° C [3]. Therefore, active nitrogen is needed to be supplied effectively at low growth temperatures to satisfy the stoichiometry of InN films. In previous work [4], we prepared InN films by reactive evaporation u:~ing N2 radio frequency RF plasma. Surface morphology o f the films became rougher and the crystallinity deteriorated with decreasing the growth rate. Tile degree of the deteriorations was improved by lowering the RF power, however, properties of the films were not improved sufficiently.

These deteriorations were probably owing to damage by plasma bombardment because they were most noticeable when their film growth rate was extremely low. Therefore, active nitrogen has to be supplied by less damaging methods. Generally, in N2 discharges, the ratio of excited neutral N2 molecules increases relatively to that of NJ- ions when the pressure of N2 gas is

0921-5107/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

SSDI 0921-5107(95)01344-X

raised. Inner gas pressure of a discharge cavity of a plasma ceil having an orifice plate can be varied by changing the orifice diameter and is independent of inner gas pressure of a growth chamber. In addition, electron-cyclotron-resonance (ECR) discharge acti- vates gases more effectively than RF discharge does. Therefore, an ECR plasma source having an orifice plate is considered to satisfy the above requirements. In this work, we attempted InN thin-film growth using the ECR plasma source in reactive evaporation, and some qualities of the films were compared with those of the films grown using RF plasma.

2. Experimental details

A growth chamber (ULVAC JAPAN Ltd.) 'shown in Fig. l(a) was evacuated by a turbo molecular pumi9 to an order of 10 -s Pa, then purified N2 gas was~intro - duced to a pressure of 0.13 Pa through an ECR plasma source (IRIE KOKEN Co., Ltd.). The flow rate of N2 gas was controlled fo r4 SCCM by a mass flow con- troller. Some plates with various orifice 'diameters were set to the top of the ECR source to vary inner pressure of a discharge cavity of the ECR source. Pyrolitic: boron nitride (P-BN) plates of 1 mm thickness with orifice diameters of 2.7 mm (one point) or '0.5 mm i3 points) were used, and then the inner pressures o f the discharge cavity were, respectively, about two or three

172 Y. Sato, s. Sato / Materials Science and Engineering B35 (1995) 171-175

orders raised from the inner gas pressure of the growth chamber. Microwave power of 80 W (2.45 GHz) was supplied to the ECR source to generate N2 plasma. A P - B N liner tube was inserted into the discharge cavity to avoid the emission of contaminants by sputtering. Indium was supplied from an effusion cell using a P -B N crucible. The distance between the indium cell and substrates was 20 cm, while the distance between the ECR source and the substrates was set for 5 cm or 10 cm.

Substrates used for InN film growth were a-AlzO 3 (0001) of 0.4 mm thickness, and the films were grown to the thickness of about 0.2 gm. For comparison, InN films were also grown by the system shown in Fig. l(b) using RF (13.56 MHz) N2 plasma, where same values of the N 2 gas pressure and the exciting power in the case of the ECR source, that is, 0.13 Pa and 80 W were used, respectively.

Thicknesses of prepared films were measured by dou- ble-beam interference microscopy. Surface morphologies of the films were observed by scanning electron microscopy (SEM), and the crystallinites were analyzed by X-ray diffraction measurements. Furthermore, optical emission spectra of the ECR and RF plasmas were measured through an optical fiber and by a grating monochromator (NIKON G250) and a photomultiplier (HAMAMATSU R928). A Langmuir probe (0.8 mm ~b x 4 mm, tungsten) was inserted into the growth chamber and was set at the point of 1 cm below the substrates. Voltage-current characteristics of the ECR and RF plasmas were measured by this probe, and some plasma parameters were estimated from the characteristics.

3. Results and discussion

3.1. Dependencies o f the fi lm properties on the growth temperature and the growth rate

InN thin films were grown at various growth temperatures with a constant growth rate of about 0.25 pm h -1. In this case, the orifice diameter of the ECR plasma source was 2.7mm, and the distance between the ECR source and the substrates was 5 cm. Surface morphologies of the films grown using each plasma are shown in Fig. 2, and they are relatively smooth when the growth temperatures are under 300 °C. The morphologies became rougher when the growth temperature was 450 °C, however, the degree of roughness was relatively low on the film grown using the ECR source. However, in the X-ray diffraction patterns shown in Fig. 3, the degree of c-axis orientation (wurtz- ite-type structure) became higher with increasing the growth temperature for each case. Crystallinity of the films grown using the ECR source was relatively poor as

I I

Ion izat i~ ¢ Gauge =1 ,[

] rof I ' ' I

EI fus i on Cell

N2 =

(a) ECR

I I , ,

Ionizat i ~ TIP Gauge ~ ~]

(b) RF Fig. 1. Schematic diagram of the reactive evaporation apparatus.

compared with that of the films grown using the RF plasma. The full-width at half-maximum (FWHM) of the diffraction peak of the film grown at 450 °C was larger than that of the film grown using the RF plasma.

nonheated

300 *(3

450 "C

m m m

(a) ECR l~m (b) RF

Fig. 2. Surface morphologies of the films prepared at various growth temperatures.

Y. Sate, S. Sate / Materials Science and Engineering B35 (1995) 171-175 173

! £

6 nonheated e~

300 °C

450 "C

£ <, i i

dO 40 50 60

~-, ~ nonheated

3o0 "c

~" 450 "0

L ° ° - - - " - " t ' - ' o°° 30 40 50 60

2 e (deg) (a) ECR (b) RF

Fig. 3. X-ray diffraction patterns (Cu K~) of the films prepared at various growth temperatures.

The growth temperature was fixed at 450 °C, and the growth rate was varied fi'om about 0.25 to 0.05 pm h -~ by varying an indium supply rate from the indium effusion cell. Surface morphologies and X-ray diffraction patterns of the prepared films are shown in Figs. 4 and 5, respectively. The ,;urface morphology of the film grown using RF plasma deteriorated and the FWHM of the X-ray diffraction peak became larger with decreasing the growth rate. However, the surface morphology did not become poor and the crystallinity was improved in the case of the ECR source.

Differences of the species existing in the plasmas used were investigated by optical emission spectroscopy and Langmuir probe measurements. Optical emission spectra of the ECR plasma source and the RF plasma are shown in Figs. 6(a) and (b), respectively. The N2 pressure of the chamber inside was 0.13 Pa (4 SCCM) and the exciting power was 80 W on each plasma. In these spectra, optical emission,; concerning the NJ- first negative system, the N2 first positive system and the N2 second positive system were observed mainly. In the case of the ECR source, the pressure of the discharge cavity inside was raised by the orifice plate. Therefore, the ratio of the optical emission intensity concerning the neutral N2 molecules to that concerning the N~-

i O. 25 p m/h

m O. I)5 p m/h

(a) ECR lpm (h') RF

Fig. 4. Surface morphologies of the films prepared at various growth rates.

,~[ ~ O. 25 la m/h

~ - i

'~ ~ ~ d 0. 05 P m/h.~

_ ,.,

~ J~__ , ~-- = i i

30 40 50 60

0. 25 pm/h Is s

c

O. 05 p m/h O 4 C ~

3'0 4'0 5'0 6'0 ' 2 0 (deg)

(a) ECR (b) RF

Fig. 5. X-ray diffraction patterns (Cu K~) of the films prepared at various growth rates.

ions is relatively high as compared with that of the RF plasma. Furthermore, the total relative intensity of the optical emission of the ECR source was stronger than that of the RF plasma.

Some plasma parameters obtained by the Langmuir probe measurements are shown in Table 1. The ion energy and the ion density supplied from the ECR source were smaller than those of the RF plasma, respectively. Therefore, the differences in their film properties are probably owing to the difference of the bombardment effect by these species.

3.2. Dependencies of the film properties on the orifice diameter and the distance between the ECR source and the substrates

The influences of the distance between the ECR source and the substrates, or the orifice diameter of the

N2 + First Negative System O . . . . . . . . . . , • , , , ,

Nz Second Positive System N2 First Positive System r l , J J , , , J ~ r i l l i i ~ i , H , , , , , , , , , , , , , , , , , ,

o X (a) ECR 2.7 r~¢

g o

O

>, 4 - 1

! o

46o

(b) RF

x5

O O

500 600 700 Wavelength (nm)

Fig. 6. Optical emission spectra of (a) the ECR plasma (2.7 mm orifice) and (b) the RF plasma.

174 Y. Sato, S. Sato / Materials Science and Engineering B35 (1995) 171-175

Table 1 Plasma parameters of the ECR plasma (2.7 mm orifice) and the RF plasma

Ion energy (eV) Ion density (cm -3)

ECR 8 1.7 x 10 s RF 39 7.4 x 108

ECR source, were also investigated. The distance was expanded to l0 cm, and InN films were grown at this condition. Their surface morphology and X-ray diffraction pattern are shown in Figs 7(a) and (b), respectively. The diffraction peak other than that concerning with InN c-faces was observed in this case, while such peaks were not observed when the distance was 5 cm. In addition, some areas in which unreacted indium separated were observed in this film. The ion density decreased abruptly when the distance was over about 3 cm as shown in Fig. 8. However, the lifetime of the N2 3 + A Eu state is about 2 s [5] without any collisions, therefore, the neutral species can reach the substrates within the lifetime. However, the mean free path at this chamber pressure (0.13 Pa) is several centimetres, then their lifetime becomes shorter by collisions among the species when the distance between the ECR source and the substrates is long. Because of these two decreasing factors, the reaction between indium and nitrogen becomes insufficient.

03

i E 0

O0

x

.m

c-

t ~

c- O

m

10

8

6

4

2 0 0

I I !

2 4 I~ 8 D i s t a n c e (cm)

0

Fig. 8. Dependence of the ion density on the distance between the ECR source and the substrates (2.7 mm orifice).

The distance was fixed at 5 cm and the orifice diameter was changed to 0.5 mm (3 points). The pressure of the growth chamber inside and the N 2 flow rate were set at the same values as the case using the 2.7 mm orifice plate, then the pressure of the cavity inside of the ECR source became about one order higher than the case of the 2.7 mm orifice. The surface morphology and the X-ray diffraction pattern are shown in Figs. 9(a) and (b), respectively, and the separation of unreacted metallic indium is noticeable in this film. The intensity ratio of neutral N 2 molecules to N f ions still increased

(a )

i

e -

>-.d JI =, z = , -

x 30 4'0 5(} 6(] 2e (deg)

(b) Fig. 7. (a) Surface morphology and (b) X-ray diffraction pattern (Cu Ke) of the film grown using the ECR source. The distance between the ECR source and the substrate was 10 cm.

¢"- O~

c- l r ~ . ~ ~

(a )

, ~ j , m

m

I - e -

l

ao 50 60 2e (deg)

(b) Fig. 9. (a) Surface morphology and (b) X-ray diffraction pattern (Cu Ke) of the film grown using the ECR source. The orifice diameter was 0.5 mm (3 points).

Y. Sato, S. Sato / Materials Science and Engineering B35 (1995) 171-175 175

N2 + F i r s t Negative System O

N2 Second Posi t ive System , , , . . . . . . . . . , , , , , , , , ,

% . .

, . j

>.

c -

c-

Nz F i r s t Pos i t i ve System , , , 1 i l l = , J ~ i , , i i i J

ECR 0.5 mm@X3

400 500 600 700 Wave length (nm)

Fig. 10. Optical emission spectrum of the ECR plasma (0.5 mm x 3 points orifice).

more as compared with the other plasmas as shown in Fig. 10. However, the total relative intensity of the plasma decreased greatly probably owing to the decrease of the mean free ]paths of moving species by the increase of N2 pressurel In this case, plasma parameters could not be measured by the Langmuir probe tech- nique because their ion density was extremely low. On the assumption that the reactivity of neutral excited N2 molecules is lower than that of N~- ions, the relative decrease of N~- ions seems to be one of the causes for the insufficient reaction.

4. Conclusions

InN thin films were grown by reactive evaporation using an ECR plasma source, and optical emission spectra, ion energies and ion densities of the plasmas were measured. Excited nitrogen species of low energy were produced and supplied from the ECR source in their growth process. The deterioration tendency of the film qualities observed in the films grown using RF plasmas at extremely low growth rates was not observed in the films grown using the ECR source.

Acknowledgement

We would like to thank Professor I. Takashima for his help in making X-ray diffraction measurements possible.

References

[1] J.B. MacChesney, P.M. Bridenbaugh and P.B. O'Connor, Mater. Res. Bull., 5 (1970) 783.

[2] R.D. Jones and K. Rose, J. Phys. Chem. Solids, 48 (1987) 587. [3] Y. Sato and S. Sato, J. Cryst. Growth, 144 (1994) 15. [4] Y. Sato and S. Sato, J. Cryst. Growth, 146 (1994) 262. [5] G. Bekefi (ed.), Principles of Laser Plasmas, John Wiley, New

York, 1976; p. 171.

inn thin-film growth using an ecr plasma source

Documents