Journal of Crystal Growth 201/202 (1999) 648}651

Improvement of the surface morphology of the epitaxialc-Al

2O

3"lms on Si(1 1 1) grown using template growth with

di!erent temperatures by Al solid and N2O gas source

molecular beam epitaxy (MBE)

Young-Chul Jung, Hiroyuki Miura, Makoto Ishida*

Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, Japan

Abstract

We have investigated the crystalline quality and surface morphology of c-Al2O

3"lms to make clear growth mode of

Al2O

3grown by Al solid and N

2O gas source MBE, and proposed a template growth method with di!erent growth

temperatures. The single-crystalline epitaxial Al2O

3layers were successfully grown at growth temperatures between 650

and 9003C. However, at the higher temperatures above 8003C, the surface morphology of "lms was very rough because ofthe etching of Si substrates by N

2O gas in the initial growth stage of Al

2O

3growth. The Al

2O

3"lms grown at low

temperatures below 6503C were polycrystalline due to SiO2

formed in the initial growth stage. The Al2O

3"lms grown

with a template growth method have a lower values of RMS than that of Al2O

3"lms grown by usual growth method for

same "lm thickness. ( 1999 Elsevier Science B.V. All rights reserved.

PACS: 68.55; 81.15

Keywords: SOI; Heteroepitaxial growth; Epitaxial Al2O

3; Template growth; MBE

1. Introduction

A single-crystal insulator "lm on semiconductorhas potential applications as Si-on-insulator struc-ture and three-dimensional integrated circuit ma-terial. c-Al

2O

3"lms as an insulator material and

growth of the "lms on Si were studied using an Alsolid source and N

2O gas MBE (Al}N

2O MBE)

*Corresponding author. E-mail: ishida@eee.tut.ac.jp.

[1]. However, a clear demonstration of the growthmode of heteroepitaxial Al

2O

3"lms using this

growth method has not yet been reported. We arecurrently advancing research aimed towards devel-oping quantum-well structures with single-crystal-line insulator and silicon. To achieve this, at "rst,a smooth, single-crystal Al

2O

3"lm/Si substrate is

important. Potential applications will depend onbetter understanding and controlling the growthmode of Al

2O

3"lms on Si.

In this article, the e!ects of N2O gas on sur-

face morphology and crystalline quality of Al2O

3

0022-0248/99/$ } see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 3 9 - 0

Fig. 1. RHEED patterns of Al2O

3"lms on Si(1 1 1) the growth temperatures of (a) 9003C, (b) 7503C, (c) 6503C and (d) 6003C. The

electron beam is along the S1 1 0T azimuth.

"lms grown on Si substrates by Al solid andN

2O gas source MBE were investigated. We pro-

pose a template growth with di!erent temper-atures at the initial growth stage and the followingstage.

2. Experimental procedure

The details of the deposition system, substratepreparation technique, and "lm growth conditionsare given in previous publications [1]. Brie#y, theepitaxial growth was carried out for 30 min ata constant N

2O gas pressure of 3]10~2 Pa and the

growth temperature was varied in the range600}9003C. Crystalline quality and surface rough-ness were studied by in situ RHEED and AFM,respectively. The thickness of grown "lms was char-acterized by XPS and ellipsometer.

3. Results and discussion

RHEED patterns of Al2O

3"lms grown for

30 min at di!erent growth temperatures are shownin Fig. 1. The streaky pattern of c-Al

2O

3"lms

grown at the temperatures of 900 and 7503C asshown in Fig. 1a and Fig. 1b. In contrast, thepattern changed to spot patterns and included twinre#ections with decreasing growth temperatures ascan be seen from Fig. 1c. At the growth temper-ature of 6003C, the RHEED pattern shows ringpatterns as shown in Fig. 1d, indicating that theAl

2O

3was not epitaxial "lm. From these results, it

is found that single-crystalline Al2O

3layers can be

epitaxially grown on Si at the growth temperaturesabove 6503C.

The Al2O

3"lms grown for 30 min at di!erent

growth temperatures were analyzed by in situ XPSmeasurements. The Si(2p) spectra obtained from

Y.-C. Jung et al. / Journal of Crystal Growth 201/202 (1999) 648}651 649

Fig. 2. XPS spectra Si(2p) recorded from Al2O

3/Si interfaces

formed at various growth temperatures.

these samples are shown in Fig. 2. Si signalsfrom Si}Si bonds (99.2 eV) were obtained, whichare from Si substrates. Therefore, it is consideredthat the XPS signals were measured from both theepitaxial Al

2O

3layer and the Si substrate because

the thickness of Al2O

3overlayer was very thin. It

can be seen from Fig. 2 that no signal from SiO2

was detected at growth temperatures between 650and 9003C. This indicates that the Si substrateswere not oxidized at the Al

2O

3}Si interface by the

introduction of N2O gas in the initial growth stage.

For the Al2O

3grown at 6003C, however, Si signal

(104 eV) from SiO2

was detected. It is consideredthat the Si substrate was oxidized by N

2O gas in

the initial growth stage at the low growth temper-ature [2], so that the Al

2O

3"lm could not grow

epitaxially.Fig. 3 shows cross-sectional HR-TEM micro-

graphs of the Al2O

3"lm grown at 9003C for 60 min

on Si(1 1 1) substrate. At the interface between theSi substrate and the Al

2O

3"lm, pits are observed.

In the initial growth stage, Si surface not coveredby Al

2O

3is etched by the O radical. The details of

these results are given in other publication [3].Therefore, it is considered that the etching reaction

Fig. 3. Cross-sectional HR-TEM micrograph of an Al2O

3/

Si(1 1 1) structure formed at growth temperature of 9003C for60 min.

and/or the oxidizing reaction of Si substrate byN

2O gas in the initial growth stage also occurred in

MBE system. Due to strong etching, the surface ofAl

2O

3"lms grown at high growth temperatures

became very rough. From the above results, theAl

2O

3"lms must be grown from above 6003C to

below 8003C for the good crystalline quality andsurface morphology. In our experiments, the Al

2O

3"lm grown at 7503C have the best crystalline qual-ity and surface morphology. Even though, how-ever, very smooth surfaces of 30 As -thick Al

2O

3were obtained at the substrate temperature of7503C, the surface of grown "lms became rough asthe growth proceeds. These results suggest thattwo-dimensional growth of Al

2O

3was followed by

three-dimensional island growth.To increase the thickness of Al

2O

3"lm with

atomically #at surface, a template growth methodwas carried out using di!erent temperatures at theinitial growth stage and the following stage: (1) the"rst step is the heteroepitaxial growth of Al

2O

3"lm

on a Si(1 1 1) substrate at 7503C for 30 min, (2) thesecond step is homoepitaxial growth of Al

2O

3"lm

at a lower growth temperatures. From AFM im-ages of Al

2O

3grown with the template growth, the

RMS (root mean square of height deviations)values as functions of thickness of Al

2O

3"lms

grown at several growth conditions, are comparedas shown in Fig. 4. All RMS values reported aredetermined from scans of a 2 lm]2 lm region(512]512 pixels) of the Al

2O

3"lms. For as-grown

samples the roughness increases with the "lm thick-ness. However, Al

2O

3"lms grown using the

650 Y.-C. Jung et al. / Journal of Crystal Growth 201/202 (1999) 648}651

Fig. 4. The RMS values as functions of thickness of Al2O

3"lms

grown at several growth conditions. The scan area was2 lm]2 lm and the AFM was operated in tapping mode.

template growth with lower second growth temper-atures have a lower values of RMS than that ofAl

2O

3"lms grown by usual growth for same "lm

thickness as shown in Fig. 4. In the templategrowth, Al

2O

3"lms at a lower second growth tem-

perature have a lower values of RMS.The reason to be able to attain good epitaxial

Al2O

3"lms with the template growth is not clear,

but it can be considered as follows: (1) by growinga very thin "lm at 7503C on the top of the substratebefore the following low-temperature growth, theetching of Si substrate by N

2O gas was prevented

in the initial growth stage, as the Si substrate wascovered by very thin and smooth Al

2O

3layer; (2) at

the subsequent Al2O

3growth the Al

2O

3layer was

grown at a higher growth rate [3], which results inthick continuous "lms with smooth surface.

4. Conclusions

Heteroepitaxial growth of c-Al2O

3"lms on Si

substrates by Al solid and N2O gas source MBE

has been investigated. At higher growth temper-atures above 8003C, the "lms grown do not havegood surface morphology because of etching of Sisurface by N

2O gas in the initial growth stage. On

the other hand, Al2O

3"lms grown at 6003C was

polycrystalline due to SiO2

formed in the initialgrowth stage. Al

2O

3"lms grown with the template

growth have a lower values of RMS than that ofAl

2O

3"lms grown by usual growth for same "lm

thickness. In the template growth, Al2O

3"lms at

a lower second growth temperature have a lowervalues of RMS.

Acknowledgements

This work was supported by the Murata ScienceFoundation.

References

[1] H. Wado, T. Shimizu, M. Ishida, Appl. Phys. Lett. 67 (1995)2200.

[2] T. Kimura, A. Sengoku, H. Yaginuma, Y. Moriyasu, M.Ishida, Jpn. J. Appl. Phys. 36 (1997) 7126.

[3] Y.-C. Jung, H. Miura, K. Ohtani, M. Ishida, J. CrystalGrowth 196 (1999) 88.

Y.-C. Jung et al. / Journal of Crystal Growth 201/202 (1999) 648}651 651