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

Journal of Crystal Growth 196 (1999) 88—96

High-quality silicon/insulator heteroepitaxial structures formedby molecular beam epitaxy using Al

2O

3and Si

Young-Chul Jung, Hiroyuki Miura, Kentaro Ohtani, Makoto Ishida*Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan

Received 28 April 1998; accepted 12 June 1998

Abstract

Heteroepitaxial growth of c-Al2O

3films on a Si substrate and the growth of Si films on the c-Al

2O

3/Si structures by

molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, c-Al2O

3films with an atomically smooth surface with an RMS values of &3 A_ and high crystalline quality can be grown on Si(1 1 1) substrates at substrate temperatures of 650—750°C. Al

2O

3films grown at higher temperatures above 800°C, did

not show good surface morphology due to etching of a Si surface by N2O gas in the initial growth stage. It has also been

found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatmentprior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si hadsignificantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 A_and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al

2O

3is believed to be due to the Al

2O

3surface modification. ( 1999 Elsevier Science B.V. All rights reserved.

PACS: 68.55.!a; 81.15.Hi

Keywords: SOI; Heteroepitaxial growth; Epitaxial Al2O

3; Al predeposition layer; MBE

1. Introduction

The heterostructure of semiconductor/insula-tor/Si is attracting interest for future applicationssuch as very-high-speed integrated circuits (ICs)and optoelectric ICs (OEICs) [1,2]. We havestudied c-Al

2O

3films as insulator material on Si

substrates by low-pressure chemical vapor depos-ition (LPCVD), metal organic molecular beam

epitaxy (MOMBE) using Al(CH3)3

(TMA) andN

2O gas sources [3,4]. However, carbon contami-

nation cannot be avoided using TMA due to theorganic by-product of its dissociation. We haveproposed the growth of Al

2O

3films on Si (1 1 1)

using an Al solid source instead of TMA, and N2O

gas molecular beam epitaxy (Al-N2O MBE) [5,6]

and reported carbon contamination as seen in thefilms grown with TMA, was not detected from thec-Al

2O

3films grown with this growth method. Epi-

taxially grown Si/Al2O

3/Si stacked structures not

only have the features of silicon-on-sapphire (SOS),

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 0 7 2 5 - 8

but also offer other advantages for sensor devicesusing micromachining technology [7]. A pressuresensor for elevated-temperature operation has beendemonstrated using the stacked SOI structures [8].However, for the Si/Al

2O

3/Si stacked structure to

become useful for device applications such as quan-tum-well structures, it is necessary to improve thecrystalline quality and surface smoothness of thegrown films in the atomic scale. Also, it is preferableto decrease the strain of the heteroepitaxial inter-face which usually occurs in heteroepitaxial growth[3]. Recently, we have reported the growth of highcrystalline quality epitaxial Si films using a thin Allayer deposited onto c-Al

2O

3(1 1 1)/Si (1 1 1) sub-

strates at room temperature, prior to the growth ofSi films by gas source MBE [9]. In order to pro-duce high-quality SOI wafers, it is essential to ob-tain high-quality crystalline and atomically smoothc-Al

2O

3layers and Si layers.

In this article, firstly, a study made on the surfacemorphology of the grown c-Al

2O

3on Si using

atomic force microscopy (AFM) is reported. To thebest of our knowledge, this is the first study whichwas undertaken to establish the clear growth modeof c-Al

2O

3films grown by Al solid and N

2O gas

source molecular beam epitaxy. Secondly, a studycarried out on the effect of the thickness ofpredeposited Al layer and thermal treatment on thesurface morphology and defects of epitaxiallygrown silicon layers is also reported in this article.This study was made in order to have a clearinsight into the role of the predeposited Al layer.The SOI films have been characterized by differentanalysis techniques namely, reflection high-energyelectron diffraction (RHEED), X-ray photoelectronspectroscopy (XPS), atomic force microscopy(AFM), and cross-sectional transmission electronmicroscopy (TEM).

2. Experimental procedure

The experiments were carried out in a molecularbeam epitaxy (MBE) system with five-chambers,consisting of a sample exchange chamber, an XPSanalysis chamber, and three growth chambers: Al-N

2O mixed source MBE chamber, Al growth

chamber, and a Si2H

6gas source MBE chamber.

These chambers are separated by gate valves andmagnetically coupled sample transfer arms movethe samples between chambers. A schematic dia-gram of the system is shown in Fig. 1. The basepressures maintained for the three growth cham-bers and the analyzer chamber were better than10~7 and 10~8 Pa, respectively. The Si

2H

6gas

source MBE chamber and the analysis chamber areequipped with in-situ RHEED and XPS, respec-tively. c-Al

2O

3films were grown on Si (1 1 1) sub-

strates in the Al-N2O MBE system, which was

equipped with a pure N2O gas source and an Al

Knudsen cell. The Al Knudsen cell was surroundedby a water-cooled jacket. Al predeposition layerswere grown at room temperature in the Al growthchamber. The Si

2H

6gas source MBE growth

chamber has pure Si2H

6gas for Si growth. Si

substrates used in the present experiments were Si(1 1 1) wafers which were pretreated by the Ishizakaet al. cleaning method [10]. The thin oxide layerwas removed thermally by heating at a temperatureof 900°C for 30 min in the Si

2H

6gas source

MBE growth chamber. After cooling the substrate,observation by RHEED in the growth chamberrevealed a clear 7]7 diffraction pattern. Thiswas followed by the transfer of sample to the Al-N

2O mixed source chamber and layers of c-Al

2O

3were grown by Al solid source and N

2O gas source.

Before the epitaxial growth of Si, Al was evapor-ated from the Knudsen cell at a rate of 0.5 A_ /sonto the c-Al

2O

3(1 1 1)/Si substrate at room

temperature in Al growth chamber, and was sub-jected to thermal annealing at 800°C or 900°Cin Si

2H

6gas source MBE chamber. The growth

of epitaxial Si on the Al/c-Al2O

3(1 1 1)/Si sub-

strate was performed at a substrate temperature of800°C. The substrates were exposed to a Si

2H

6gas

flow of 1 sccm. The estimated growth rate of the Sifilms was 400 A_ /min. The crystalline quality andthe composition of films were studied by in situRHEED at an acceleration voltage of 15 kV, andXPS using a Mg K

aline (1253.6 eV) X-ray source,

respectively. The film thickness was measured byan automatic two-zone null method ellipsometerhaving a 6328 A_ He—Ne laser light source. AFMand TEM were also used to study the surface mor-phology and crystalline quality of the preparedepitaxial films.

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–96 89

Fig. 1. A schematic diagram of the MBE system.

3. Results and discussion

3.1. Formation of epitaxial c-Al2O3 (1 1 1)/Si (1 1 1)structure

In order to grow a high-quality SOI layer, it isessential to identify the best conditions for obtain-ing thin and atomically flat insulator. The crystal-line quality and surface morphology of c-Al

2O

3films were investigated in order to make cleargrowth mode of c-Al

2O

3by Al solid and N

2O gas

source molecular beam epitaxy. The growth tem-perature was varied in the range of 600—950°C andthe growth time was 30 min. Using an Al solidsource and N

2O gas, single crystalline epitaxial

c-Al2O

3layers were successfully grown at growth

temperatures between 650°C and 900°C. Fig. 2shows RHEED patterns observed from the S1 1 0Tazimuth and AFM images of the c-Al

2O

3films

grown at temperature 750°C, 800°C, and 850°C,respectively. c-Al

2O

3films grown at substrate tem-

perature at 750°C and below showed the streakyRHEED patterns and smooth surfaces with RMS

(root mean square of height deviations) values of&3 A_ by AFM. However, at higher temperaturesabove 800°C, the RHEED photograph showed twokinds of patterns: one due to c-Al

2O

3epitaxial film

on Si substrate and the other because of Si substra-te. AFM images (Fig. 2b and Fig. 2c) show that thesurface morphology of films grown at temperaturesabove 800°C were very rough with an RMS valueof 50 A_ and these islands of height up to 330 A_ . Thebright uppermost island regions in Fig. 2b andFig. 2c are c-Al

2O

3areas, and the dark contrast regi-

ons correspond to the exposed Si surface. This factis evident from RHEED results shown in Fig. 2band Fig. 2c wherein the appearance of streaky pat-tern due to c-Al

2O

3which are relatively stronger

compared to that of Si. Since the Knudsen celltemperature and N

2O gas flow were maintained

constant during the experiments, the above resultsindicate that the surface morphology of c-Al

2O

3film is a strong function of growth temperature. It isimportant to know the reason for the formationof rough surface morphology, although RHEEDpattern was streaky at higher growth temperatures.

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–9690

Fig. 2. RHEED patterns and AFM images (2 lm]2 lm) of c-Al2O

3films on Si(1 1 1) at the growth temperatures of 700°C, 800°C and

850°C, respectively.

The reasons can be explained in two ways. First,the growth of c-Al

2O

3was a three-dimensional

growth at a higher growth temperature, therefore Sisubstrate was not adequately covered by c-Al

2O

3film, and therefore two kinds of RHEED patternsmight have appeared. Another possibility is thatthe surface roughness of c-Al

2O

3might have been

caused by etching of Si substrates by N2O gas

during the initial growth stage at a higher growthtemperature. Kimura et al. [11] also have reportedthe etching of Si substrates by N

2O gas at an initial

growth stage in ultra-high vacuum CVD process.Further, we have carried out following experi-

ments in order to investigate the cause of the sur-face roughness. The initial layer of c-Al

2O

3film

was grown at 750°C for 30 min, whose thicknesswas 20 A_ , and the succeeding layer of c-Al

2O

3(that

is, the growing of c-Al2O

3onto c-Al

2O

3/Si substra-

te) was grown at 600—900°C for 30 min. The totalthickness of the c-Al

2O

3film grown at 750°C and

then at 600—900°C is shown in Fig. 3a. If the c-Al

2O

3grew three dimensionally above 800°C, the

total thickness of the succeeding c-Al2O

3film

would have been increased. However, the succeed-ing growth of c-Al

2O

3at ¹*800°C neither

showed any noticeable increase of thickness, northe lack of crystalline quality and surface roughnessas can be seen in Fig. 3b and Fig. 3c. These resultscan be considered as follows: (1) No reaction hasoccurred between N

2O gas and Al

2O

3/Si substrate

during the growth of succeeding c-Al2O

3layer,

because no degradation of surface morphology oc-curred. (2) At higher temperatures, growth ratesdecreased due to desorption of AlO causing no in-crease of film thickness [5]. It is believed that the330-A_ -thick c-Al

2O

3islands supposed in Fig. 2 could

not be grown at higher temperatures above 800°C,due to the very low growth rates shown in Fig. 3a.Therefore, the islands were formed by etching of Sisubstrate which was not covered by Al

2O

3layers

(that is, the depth of etched Si was about 300 A_ ).It is clear from the above observation that

a rough surface of c-Al2O

3results due to etching of

Si substrates by N2O gas during the initial growth

stage of c-Al2O

3layer at higher growth temper-

ature above 800°C. Also, it is important to note

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–96 91

Fig. 3. The initial layer of 20-A_ -thick c-Al2O

3films was grown

at 750°C and the succeeding layer of c-Al2O

3films were grown

in the range 600—900°C for 30 min. (a) Growth temperaturedependence of thickness of the c-Al

2O

3films on c-Al

2O

3/Si

substrate. (b) RHEED pattern and (c) AFM image (2 lm]2 lm) of c-Al

2O

3film grown on c-Al

2O

3/Si substrate at 900°C.

The electron beam is along the [1 1 0] azimuth.

that, in order to avoid etching of Si substrate byN

2O gas, c-Al

2O

3film growth must be performed

at temperatures below 800°C. Resulting from theabove facts, all subsequent c-Al

2O

3films were

grown at 750°C because it had the best surfacemorphology among several samples.

3.2. Growth of Si films on c-Al2O3 (1 1 1)/Si (1 1 1)structure using Al predeposition layer

As mentioned earlier, epitaxial Si layers on ac-Al

2O

3(1 1 1)/Si (1 1 1) structure were grown by

Si2H

6gas source MBE method with an Al

predeposition layer, and were examined using insitu RHEED to investigate the change of crystal-linity of epitaxial Si layer as well as thepredeposited Al layer. Fig. 4 shows the RHEEDpatterns of a sample for different stages of epitaxialgrowth, observed S1 1 0T direction. Fig. 4a showsthe RHEED pattern from a c-Al

2O

3layer on a Si

substrate grown at 750°C for 60 min. The RHEEDpattern after the growth is streaky indicating a verysmooth surface with high crystalline quality. Afterthe deposition of 10-A_ -thick Al layer at room tem-perature, a spotty pattern of Al layer appears inaddition to the streaky pattern of the c-Al

2O

3layer, as shown in Fig. 4b. This RHEED resultindicates that the Al layer on c-Al

2O

3was grown

epitaxially. The growth of Al layer on c-Al2O

3is

still under investigation and the results will bepublished separately. The RHEED pattern from anAl/c-Al

2O

3layer changed to that of a c-Al

2O

3layer after the wafer was heated to 800°C for 5 min.This observation indicates the possible desorptionof Al predeposited layer from c-Al

2O

3surface dur-

ing the process of thermal treatment. The details ofthese results are discussed below in the light of XPSmeasurements carried out. Fig. 4d shows theRHEED pattern from an epitaxial Si layer on c-Al

2O

3using Al predeposition layer. The 7]7

RHEED pattern indicates the grown Si films havea very smooth surface and high crystalline quality.The growth of epitaxial Si films on c-Al

2O

3layer

using the Al predeposition layer is effective in im-proving the quality of the grown film. However, theincrease of thickness of the predeposited Al layerresults in poor crystalline quality of Si films, that isan optimum thickness for the Al predepositionlayer is necessary as reported in the literature [9].

In order to investigate the effect of an Alpredeposition layer thickness on the crystallinequality of epitaxially grown silicon layers, XPSmeasurements were performed. Photoelectronspectra (Al 2p) of the samples with 10- and 25-A_ -thick Al predeposition layer were taken after eachstep in the formation of an Al predeposition layeras shown in Fig. 5; (a) before Al deposition (as-grown c-Al

2O

3(1 1 1)/Si), (b) after Al deposition on

c-Al2O

3(1 1 1)/Si substrates at room temperature,

(c) after thermal annealing of Al/c-Al2O

3/Si sub-

strates at 800°C for 5 min. The binding energy of Alin Fig. 5a corresponds to that of Al—O bonds ofAl

2O

3(74.7 eV). Al—Al bonds (72.7 eV) were not

observed. The ratio of signal intensities of Al 2p andO 1s spectra was similar to that of sapphire(O 1s/Al 2p"5.7). After the thin Al deposition onthe c-Al

2O

3(1 1 1)/Si substrate at room temper-

ature, both Al—O bonds and Al—Al bonds were

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–9692

Fig. 4. A series of RHEED patterns with S1 1 0T azimuth taken during the Si MBE at 800°C on the c-Al2O

3/Si (1 1 1) structure;

(a) after growth of c-Al2O

3, (b) after deposition of Al predeposition layer, (c) after thermal annealing at 800°C, (d) after epitaxial

growth of 4000 A_ Si with 10-A_ -thick Al predeposition layer.

Fig. 5. XPS spectra (Al 2p) of the sample with 10- and 25-A_ -thick Al predeposition layer at the different stages in forming theAl predeposition layer, (a) before Al deposition (as-grown c-Al

2O

3(1 1 1)/Si), (b) after Al deposition on c-Al

2O

3(1 1 1)/Si

substrates at room temperature, and (c) after thermal annealingof Al/c- Al

2O

3(1 1 1)/Si substrate at 800°C for 5 min.

observed as shown in Fig. 5b. This indicates thata thin metallic Al layer was formed on the c-Al

2O

3/Si substrate. After the sample with 10-A_ -

thick the Al predeposition layer was heated to

800°C, however, the signals of Al—Al bonds werenot observed, and only the Al—O bonds remained.The ratio of signal intensities of Al 2p and O 1sspectra was the same as that obtained in the case ofFig. 5a. This confirms that a metallic Al layer didnot exist on the surface of the c-Al

2O

3(1 1 1)/Si

substrate before Si epitaxial growth. The metallicAl layer is no longer present because of desorptionfrom the c-Al

2O

3layer, not from diffusion into the

c-Al2O

3. On the other hand, in the case of a 25-A_ -

thick Al predeposition layer, the peak of Al—Albonds remained after thermal annealing at 800°Cfor 5 min. It is believed that the surface ofthe c-Al

2O

3has a Al rich layer because of

insufficient desorption, which causes the Si growthon this Al-rich surface resulting in rough filmmorphology [12].

The effect of thermal treatment after Al depo-sition on the growth of Si using an Al predepositionlayer was studied. The investigation was carried outfor three different samples A, B and C. SampleA was grown without an Al predeposition layer.

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–96 93

Fig. 6. AFM surface images of three samples (2 lm]2 lm): (a) sample A (without Al predeposition layer), (b) sample B (annealed at800°C), (c) sample C (annealed at 900°C).

Fig. 7. Cross-sectional TEM images of three samples:(a) sample A (without Al predeposition layer), (b) sample B (an-nealed at 800°C), (c) sample C (annealed at 900°C).

Sample B was grown with an Al predepositionlayer and annealed at 800°C for 5 min. SamplesA and B were grown on the same substrate consist-ing of two different regions. In one region, an Alpredeposition layer was deposited onto the c-Al

2O

3/Si structure, and then a Si film was depos-

ited at 800°C (sample B). In the other region, the Sifilm was directly deposited onto the c-Al

2O

3sur-

face (sample A). Sample C was grown with an Alpredeposition layer and annealed at 900°C for5 min. The thickness of the Al predeposition layerwas 10 A_ . Fig. 6 shows the AFM images of thethree samples. It was found that the use of an Alpredeposition layer improved the surface morphol-ogy of the grown epitaxial Si film and the surfacemorphology of the film annealed 900°C (Fig. 6c) isbetter than that of those (Fig. 6b) annealed at800°C. The surface morphology of sample C wasa very smooth surface with the atomically smoothterraces of 1 nm-high-steps. This fact suggests thatthe thermal treatment process of c-Al

2O

3/Si sub-

strates with an Al predeposited layer is very impor-tant for epitaxial Si growth. It is considered that theannealing process desorbs the metallic Al layerfrom the surface of c-Al

2O

3layer and then the

surface of c-Al2O

3retains only Al—O bonds. At

a annealing temperature of 900°C, the surface ofc-Al

2O

3layers do not have any Al rich regions

(Al—Al bonds) because of sufficient desorption,which causes the grown Si film to have a verysmooth surface morphology.

Photographs of the cross-sectional TEM imagesof the three samples are shown in Fig. 7. Defectssuch as stacking faults and misfit dislocations wereobserved in epitaxial Si layer near the interfacebetween the c-Al

2O

3and Si layer in samples A and

B. However, in sample C, density of defects wasmuch lower than those observed in the other two

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–9694

Fig. 8. Schematic illustration of the growth model of Si onAl

2O

3using an Al predeposition layer.

samples. These results clearly show the effect of Alpredeposited layer and indicate that the epitaxial Sifilms grown using the Al predeposition layer notonly have a higher crystalline nature, but alsoa very smooth surface. We can clearly see that thethickness of grown Si film using an Al predeposi-tion layer (sample B) is thicker than that of those Sifilms without an Al predeposition layer (sample A).This increase in the Si film thickness is causedbecause of a reduction in the incubation time asreported elsewhere [9].

From the detailed experimental results presentedabove, a grown mechanism as shown in Fig. 8 canbe proposed to explain the silicon growth on Al

2O

3film with Al predeposition layer. The surfaces ofas-grown Al

2O

3have not only Al atoms but also

O atoms. These top O atoms may be the cause ofthe surface reactions between Al

2O

3surface and

Si2H

6gas in the initial growth stage, which have

a bad influence on the epitaxial growth of Si onAl

2O

3layers. However, it is necessary to character-

ize these top atoms by other methods such ascoaxial impact-collision ion scattering spectro-meter (CAISISS). On the other hand, in the samples

with the sequence of predeposition of Al layer andannealing, the predeposited Al atoms are desorbedfrom the surface during the thermal treatment pro-cess. As a result of the process, the Al predepositionlayer is transformed into a monolayer of Al whichincorporates oxygen atoms on the Al

2O

3surface

(that is, surface bonds of Al2O

3are terminated by

Al atoms) just prior to epitaxial Si growth, so thatthe top surface layer affects the epitaxial Si during itsinitial growth stage, but existence of Al rich layer onc-Al

2O

3layer (Al—Al bond layer) because of insuffi-

cient desorption as shown in Fig. 5 is undesirablefor obtaining a high crystalline quality Si film.

4. Conclusions

Heteroepitaxial growth of c-Al2O

3films on a Si

substrate and the growth of Si films on the c-Al

2O

3/Si structures by molecular beam epitaxy

have been investigated. Epitaxial c-Al2O

3layers

were successfully grown on Si (1 1 1) substrates atgrowth temperatures between 650°C and 900°Cusing Al-N

2O MBE method. It was shown that

c-Al2O

3films with an atomically smooth surface

with an RMS value of &3 A_ and high crystallinequality can be grown on Si (1 1 1) substrate atsubstrate temperatures below 750°C. At highergrowth temperatures above 800°C, c-Al

2O

3films

grown do not have good surface morphology be-cause of etching of Si surface by N

2O gas in the

initial growth stage.By using the thin Al layer predeposited in situ at

room temperature, we have been able to obtainsignificantly uniform Si films of better crystallinequality than the Si films grown by conventionaldirect growth of Si onto c-Al

2O

3/Si substrates.

Analysis of sample by XPS technique revealed thatthe metallic Al bonds did not exist before Si epi-taxial growth for 10-A_ -thick Al predeposition layer.However, results indicated the existence of Albonds for 25-A_ -thick Al predeposition layer. It hasbeen found from AFM and cross-sectional TEMobservations that the thickness of the Al predeposi-tion layer and the thermal treatment temperatureof the layer before Si growth is important to growepitaxial silicon layer with high crystalline qualityand very smooth surface. When the Al predeposition

Y.-C. Jung et al. / Journal of Crystal Growth 196 (1999) 88–96 95

layer thickness was 10 A_ and the thermal treatmenttemperature was 900°C, the epitaxial Si layer exhib-ited a very smooth surface with atomically flat terra-ces of 1 nm-high-steps and much lower defects thanthose observed in the other samples. The presentreported growth method is very useful for Si growthon c-Al

2O

3/Si substrates, and is applicable to fabri-

cation of quantum devices with thin and highcrystalline quality epitaxial Si and insulator layers.

Acknowledgements

This work was supported by Grants-in-Aidfor Scientific Research (B) (08455149) from theMinistry of Education, Science and Culture. Theauthors would like to thank Mr. K. Muramoto ofthe Technology Development Center of ToyohashiUniversity of Technology for his help in TEMmeasurements.

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