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GROWTH MODE AT THE Ge/(li02) SAPPHIRE INTERFACE

GEOFFREY P. MALAFSKY Naval Research Laboratory,

Wash, DC 20375-5000

ABSTRACT

The growth mode of Ge on the (1i02) X-ray

photoelectron

spectroscopy.

Ge

explored with

surface of sapphire is

exists

in

two

bonding

states

at

the

Ge forms islands at submonolayer coverage for

interface, Ge-Ge and Ge-sapphire.

The formation of the islands is

deposition temperatures of 25°C and 625°C. revealed by the rapid increase in

the relative fraction of the Ge-Ge bonding

state for Ge coverage less than 1 ML.

The shift in the Ge-Ge peak binding energy

to the bulk Ge value at less than i

ML suggests

that the islands

are three

dimensional for deposition at 625°C.

I.

Introduction

The

heteroepitaxial

growth

of

Ge

on

sapphire

technological benefits as silicon on sapphire (SOS).

may

offer

the

same

These include lower cross-

talk, latch-up, and parasitic capacitance compared to a device built on a noninsulating substrate

[1].

Single crystal Ge(ll0)

(1102) crystal face of sapphire,

films can be grown on the

but the crystallinity of the epilayer depends

upon the substrate temperature during deposition [2]. Both Ge

[2]

and Si

[3] form islands on the

thicknesses as low as 50 A. growth mode, Typically,

one

must

However,

probe

heteroepitaxy

is

the

in

nucleation

Conversely,

in

of

sapphire surface

deposition

of

the

first

three dimensional

at

the microscopic few

monolayers.

described by the Frank-van der Merwe,

Krastanov, or the Volmer-Weber growth modes [4]. by the

(1i02)

order to determine

Stranski-

The Volmer-Weber mode proceeds

islands

directly

on the substrate.

the Frank-van der Merwe mode the epilayer grows in

a layer-by-

layer fashion. The Stranski-Krastanov model is a combination of the two previous models

with the

first

few

layers

growing

in

a

layer-by-layer

fashion

and

subsequent layers growing via three dimensional islands. This paper describes the interfacial growth of Ge on (1102)

sapphire. X-

ray photoelectron spectroscopy (XPS) is used to monitor the interfacial chemistry

Mat. Res. Soc. Symp. Proc. Vol. 159. ©1990 Materials Research Society

128

for

submonolayer

coverage

of Ge.

The

Ge

deposition

is

performed

at room

temperature (25 - 50°C) and at 625°C.

II. Experimental

The

Ge

films

were

deposited

in

a

UHV molecular

beam

epitaxy

(MBE)

instrument with a base pressure of 5 x 10-11 Torr. The Ge was deposited at a rate of 2-4 A/min with a monolayer of Ge defined to be 1.415 A. After deposition, the sample

was

transferred

under

UHV

to

the

VG

ESCALAB

analysis

chamber

which

contained the XPS system. The photon energy used was 1486.6 eV (Al K ) and the pass energy of the hemispherical analyzer was set to 10 eV. At this pass energy, the FWHM of the Ag 3d 5 / 2 peak from Ag foil

is

collected

sapphire wafers

in

0.1

eV steps.

The 3"

diameter

0.88 eV.

The

XPS

scans were

were

chemically

cleaned [2] and then annealed in vacuum at 1400°C for 30 minutes.

T