Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

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288

Polarity controlled epitaxy of III-nitrides and

ZnO by molecular beam epitaxy

X. Q. Wang, Peking University, P. R. China and

a. YoshikaWa, Chiba University, Japan

Abstract: This chapter discusses the polarity controlled epitaxy of iii-

nitrides and ZnO by molecular beam epitaxy. The chapter rst explains what

the lattice polarity is and reviews the available detection methods for the

polarity. after a brief introduction of the polarity behavior at heteroepitaxy

and homoepitaxy, the chapter discusses the polarity controlled epitaxy of

gan, aln, inn and Zno, paying special attention to inn.

Key words: polarity control, epitaxy, iii-nitrides, inn, Zno.

12.1 Introduction

III-nitrides and ZnO are very promising materials which have many

applications in for example, solid state lighting, full color display, laser

printers, and high density information storage. These materials all have a

wurtzite lattice structure, with lattice polarity along the c-direction, the most

usual growth direction in their epitaxy. Polarity plays an important role in

the epitaxy since the growth mode/behaviors, surface morphology, crystalline

quality, doping ability, chemical and thermal stability are all inuenced by

the polarity. In this chapter, we introduce the polarity controlled epitaxy

of III-nitrides and ZnO grown by using molecular beam epitaxy, paying

attention to how to control the polarity on non-polar sapphire substrate and

the effect of the polarity on sample properties. Section 12.2 briey introduces

what the lattice polarity is and how to detect it. Section 12.3 illustrates the

polarity control in heteroepitaxy and homoepitaxy, in particular the former.

Section 12.4 briey demonstrates the polarity controlled epitaxy of GaN and

aln. This section is not discussed in detail since gan and aln have been

studied extensively. We discuss in detail polarity controlled epitaxy of inn

in section 12.5 including its p-type doping because research into this has

recently become very topical and inn is the most mysterious material of

the III-nitrides. Section 12.6 shows the epitaxy of ZnO, where the polarity

control issue is more complicated than for III-nitrides. Finally, we present

our conclusions in section 12.7.

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289Polarity controlled epitaxy of III-nitrides and ZnO

12.2 Lattice polarity and detection methods

Crystals with wurtzite lattice structure such as III-nitrides and ZnO are polar

crystal along the c-direction, and lacking inversion symmetry along this

direction. These crystals or lms are either +c polarity or –c polarity. Figure

12.1 shows a lattice structure of a hexagonal crystal, for example, GaN. It

is shown that the lattice polarity can be easily claried, that there is only

one dangling bond along the +c polarity for a Ga atom, while three dangling

bonds are available toward the –c direction. Lattice polarity is an important

issue in the wurtzite semiconductor epitaxy since the growth behavior and

properties of these semiconductors (thermal and chemical stability, optical

properties, surface properties, etc.) are greatly affected by the polarity. For

example, III-nitrides are usually grown on +c-polarity where the growth is

under step-ow mode and atomically at surfaces are obtained. On the other

hand, the –c-polarity usually leads to rough surfaces. Thus, it is important

to perform epitaxy under controlled polarity.

several methods have been established to detect the lattice polarity. They

include chemical etching, convergent beam electron diffraction (CBED),

coaxial impact collision ion scattering spectroscopy (CaiCiss) (Wang and

Yoshikawa, 2004), surface reconstruction revealed from reection high-energy

electron diffraction (RhEED), and circular photogalvanic effect (CPgE).

Chemical etching is a simple method to detect the polarity, but destroys

the sample. Due to the chemical nature of different polarities, the chemical

stability varies. For example, +c-polarity iii-nitrides are more chemically

stable and more difcult to be chemically etched than the –c-polarity ones.

And the step-ow-like morphology usually keeps well for +c-polar samples

except that etching pits appear. on the other hand, hexagonal islands are

–C

12.1 Schematic illustration of wurtzite GaN in different polarities.

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290 Thin ﬁlm growth

usually observed in the –c-polarity case, which is accompanied by roughening

surface. This different behavior after chemical etching is shown in Fig. 12.2

for InN, where the +c and –c-polarity are in- and n-polarity, respectively.

CBED is a powerful method to detect polarity in sample, which works

in the transmission electron microscope (TEM) (Ponce et al., 1996). in the

measurement, the CBED pattern observed from the sample is compared

with the simulated CBED pattern by considering the lattice structure and

sample thickness as well. The CBED measurement is a unique method to

detect polarity at the atomic level, and demonstrates signicant advantages in

analyzing domain structures at the micro- to nano-scale. The disadvantage is

that the measurement is complicated and the sample preparation is also rather

difcult. The surface reconstruction revealed from RHEED has been widely

used to determine the polarity of iii-nitrides and Zno (smith et al., 1998).

This measurement can be taken in-situ in molecular beam epitaxy (MBE).

For example, the N-polar GaN surface shows the reconstruction of 1 ¥ 1, 3

¥ 3, 6 ¥ 6, and 6 ¥ 12 while that of Ga-polar shows the reconstruction of 2 ¥

2, 5 ¥ 5, 6 ¥ 4 and pseudo-1 ¥ 1. as for the aln, it has been suggested that the

reconstructions of 1 ¥ 1 and 2 ¥ 6 showed Al-polarity while that of 1 ¥ 1, 1 ¥

3, 3 ¥ 3, and 6 ¥ 6 indicated n-polarity (Lebedev et al., 1999). one problem

concerning this method is that it is difcult to nd surface reconstruction for

semiconductor layers since it requires rather at and metal-rich surfaces.

In-polarityN-polarity

0 [nm] 1.13

0 [nm] 6.88

As grown

0 [nm] 16.02

0 [nm] 157.26

Etched 13h

12.2 Atomic force microscopy images of as-grown and 13 h-etched

InN layers with different polarities. The scanned area for each image

is 1 ¥ 1 mm.

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291Polarity controlled epitaxy of III-nitrides and ZnO

CaiCiss is a derivative of the generic technique of ion scattering

spectroscopy, in particular of low energy ion scattering spectroscopy. In

CAICISS measurement, a simple 180° backscattering geometry enables us

to determine the atomic positions at both an outermost surface and slightly

deeper layers.

CaiCiss spectra, including information on the surface structure

and lattice polarity, is obtained by analyzing the intensity of scattered signals

as a function of incident angle. The polarity is determined by comparing the

experimental CAICISS spectra to the simulated ones shown in Fig. 12.3, for

example in the case of InN. As shown in the gure, three peaks at 23°, 47°

and 72° are observed in simulated CAICISS spectra for +c-polarity while

six peaks at 15°, 23°, 32°, 51°, 67° and 74° are seen for –c-polarity.

CPGE is a newly reported method to detect the lattice polarity (Zhang

et al., 2009). In CPGE, a net current was generated without adding any

external bias under the irradiation of a circularly polarized light. The origin

of the CPgE can be attributed to the lack of spatial inversion symmetry.

The former degeneration band splits into two bands with opposite spin

indices as symmetry reduces. When the semiconductor layer is radiated by

circularly polarized light, an asymmetrical distribution of excited electrons

in momentum space is generated due to the optical selection rules, which

leads to a net current in the sample.

As a wurtzite semiconductor, III-nitrides and ZnO are lacking in inversion

symmetry. along the c-direction, the lattice of +c-polarity can be regarded

as a complete inversion of that of –c-polarity. Thus the spin-orbit interaction

in the samples with contrary polarities should be opposite, resulting in an

opposite spin splitting. Therefore, the sign of photocurrent due to CPgE

should be opposite for samples with different polarities because of the opposite

Signal intensity of ln (a.u.)

N-polarity

In-polarity

0 20 40 60 80

Incident angle q (deg)

12.3 Simulated CAICISS spectra of InN.

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292 Thin ﬁlm growth

spin splitting. It has been conrmed by experimental observation that both

CPgE photocurrents and their dependences on the incident phase angle are

opposite for InN layers with different polarities, as shown in Fig. 12.4. It

is found that the above phenomenon is independent of the conduction type,

i.e. both n- and p-type InN shows the same sign of CPGE current and the

same dependence on the incident phase angle as well. Therefore, polarity can

be easily determined by investigating CPgE photocurrents in the measured

samples. It is worth noting that the CPGE measurement system is very easy

to construct at low cost and the measurement itself is also very simple and

non-destructive to samples. The disadvantage is that measurement is at the

macro-scale and it is very difcult to detect samples at the micro- or nano-

scale.

12.3 Polarity issues at heteroepitaxy and

homoepitaxy

although free-standing gan, aln and Zno substrates are commercially

available now, they are seldom used in epitaxy yet because they are several

tens to hundreds times more expensive than sapphire, which is the most popular

substrate up to now. It means that most epitaxy is heteroepitaxy. Sapphire

itself is non-polar along the c-direction. its atomic structure consists of an

al atom bilayer and an o single layer. Thus, it is very important to control

the surface of sapphire just in front of epitaxy. next, the effect of surface

treatment on rotation domains/polarity is shown, using as an example ZnO

epitaxy on sapphire by MBE.

Table 12.1 shows the surface treatment methods for sapphire substrate

before epitaxy of ZnO buffer layer, where etching, TC, H*, O*, Ga and N*

means chemical etching by a mixture of h

= 3:1 at 110°C, thermal

cleaning (TC) under ultra-high vacuum (UhV), atomic hydrogen treatment,

oxygen radical pre-treatment, ga atomic layer pre-deposition and nitridation

by using nitrogen radical, respectively (Yoshikawa et al., 2004). Figure

12.5 shows the XRD f-scans for (102) plane of ZnO epilayers grown with

the different sapphire surface pre-treatments listed in Table 12.1. The XRD

f-scan for (113) Al

is also shown for comparison. The dominant domain

in sample A is 30°-rotated compared to the ‘main’ domains observed in the

other samples B–I whose peaks are located at the same j angles as those

of (113) plane of Al

. These two domains have the epitaxy relationship

[112

0]ZnO//[1120]Al

and [1010]ZnO//[1120]Al

, respectively. The

latter is often called the main domain and the former is called the 30°-rotated

domain. There are three kinds of possible surface structures on clean sapphire:

terminated by o layer, single al layer, and al bi-layer. The oxygen layer is

sixfold symmetric while the Al layer is threefold symmetric. On the other

hand, both Zn and o are sixfold symmetric in Zno (the same is also true for

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Photocurrent (nA)

–4

–8

0.8

0.4

0.0

–0.4

–0.8

0.8

0.4

0.0

–0.4

–0.8

–4

–8

0 60 120 180 240 300 360

(a)

0 60 120 180 240 300 360

(c)

0 60 120 180 240 300 360

(d)

0 60 120 180 240 300 360

(b)

n-type ln-polarity

n-type N-polarity

Quarter wave plate angle (deg)

p-type ln-polarity

p-type N-polarity

12.4 Photocurrents

observed in

four kinds of

InN layers with

different polarities

as a function of

quarter-wave

plate angle j.

Samples in (a)–(d)

are n-type In-

polarity, n-type

N-polarity, p-type

In-polarity and

p-type N-polarity,

respectively.

Incidence angles

are +25° in all

measurements.

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294 Thin ﬁlm growth

III-nitrides). The main domain is usually formed when the Zn atom forms

bonds with O atoms on the sapphire surface with epitaxial relationship of

[101

0]ZnO//[1120]Al

and lattice mismatch of about 18.2%, as shown in

Fig. 12.6. On the other hand, the 30° domain is formed due to the bonding

of O atoms in ZnO with Al atoms resulting in an epitaxial relationship of

[112

0]ZnO//[1120]Al

with a mismatch of about 31.8%. It is clear that

Intensity (logarithmic scale)

0 50 100 150 200 250 300 350

f–scan (deg)

30°

102

(113)

ZnO

21.8°

12.5 XRd f-scans for (102) ZnO epilayers grown with several different

procedures of sapphire surface treatments. Three kinds of rotation

domains were observed after several surface treatments, but single

domain was obtained after Ga pre-exposure and sapphire nitridation.

XRd f-scan for (113) Al

is also shown here for comparison. The

surface pre-treatment conditions are shown in Table 12.1.

Table 12.1 The different pre-treatment methods of sapphire substrate before the

growth of ZnO buffer layer

Samples Etching time

(mins)

TC time

(mins)

h* time

(mins)

O* time

(mins)

Ga thickness

(ML)

N* time

(mins)

A 30

b 30 30

C 30 30 10

d 30 30 30

E 30 30 30 30

f 30 30 2

G 30 30 10 2

h 30 30 30 30 2

I 30 30 10 60

ML = monolayer.

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295Polarity controlled epitaxy of III-nitrides and ZnO

the main domain is dominant in samples B–I in Fig. 12.5, though the 30°-

rotated domain is dominant in sample A. The third rotation domain, which

is 21.8°-rotated compared to the main domain, is observed in samples B,

C, and D, where it is stronger in longer H*-treated sample D. But if the

surface treatment for sample D is followed by O* pre-treatment, those peaks

become broad and nally the 21.8° domain cannot be identied though the

integrated intensity around both 21.8°- and 30° -rotated domains becomes

stronger. The 21.8°-rotated domain is most likely due to the Al-rich surface,

which has been conrmed by CAICISS and scanning force microscopy

(SFM) measurements (Barth and Reichling, 2001). The sapphire surface is

Al-rich after TC and it becomes Al-richer after H* pre-treatment due to the

loss of O atoms. Figure 12.5 shows that the intensity of the diffraction peak

of the 21.8° domain becomes stronger with increasing exposure period of

H*. It shows evidence that this rotation domain is ascribed to the Al layer

of sapphire. When ZnO buffer layer growth begins, O atoms form bonds

with Al atoms rst due to the Al-rich surface and relatively stronger bonds

of al–o in comparison to Zn–o. hence, the rotation domains of al layer

inuence the following ZnO layer and results in the 21.8° domain. On the

other hand, the surface is terminated at the O-plane after O* pre-treatment

and the Al metallic plane cannot be formed on the surface, which leads to

the elimination of the 21.8° domain as shown in Fig. 12.5.

Al + 3

O–2

12.6 Atomic structure of sapphire (perpendicular to c-axis) and

schematic illustration of the epitaxial relationship between ZnO and

sapphire. The dotted and solid lines show the bonding conﬁgurations

of two rotation domains, respectively.

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296 Thin ﬁlm growth

Ga pre-exposure on the O*-treated surface affects the ZnO epitaxy

dramatically as shown in Fig. 12.5, and the rotation domains were completely

suppressed resulting in a single domain, which is independent of treatment

condition, i.e., TC, TC+H* and TC+H*+O*. This is because Ga acts as the

template following ZnO layer deposition since it can migrate more easily

than the Al onto the surface and can uniformly cover it. When the growth

of the ZnO buffer layer begins, O atoms will bond with Ga atoms, resulting

in a single rotation domain. Furthermore, sapphire nitridation is also very

effective in eliminating the rotation domains as shown in Fig. 12.5. Following

sapphire nitridation, a very thin n-polarity aln layer is formed and this layer

acts as the template following ZnO layer growth. The AlN layer is a single

domain with hexagonal structure, which leads to the single domain ZnO.

In addition, the ZnO shows O-polairty on the Ga-treated surface while Zn-

polarity is formed on the nitrided sapphire surface. This will be discussed

in detail in section 12.6.

The important point to be emphasized here is that the samples with

multiple domains usually show mixed polarity and the treatment of surfaces

has great inuence on the epitaxy of ZnO and nitrides. Thus, it is important

to control the surface to eliminate the multiple domains and thus to be able

to control the polarity in heteroepitaxy.

The polarity control epitaxy on polar substrate is much easier than on

non-polar substrate. Due to the stronger bonds between metal atoms and N

or O atoms, the polarity usually follows that of the substrate. For example,

+c-polar nitride is grown on +c-polarity SiC while –c-polarity nitride is

obtained on –c-polaritysiC. in the case of homoepitaxy of iii-nitrides and

Zno, it is quite simple that the polarity usually remains the same during

epitaxy. The only exception happens when we do doping, for example,

p-type doping of III-nitrides by using Mg as a dopant. Figure 12.7 shows the

TEM image and the polarity of inn layers. This multiple-inn layer structure

sample consists of four 390 nm thick InN layers grown at different [Mg]

levels, three 110 nm thick undoped spacer layers, a non-doped InN layer and

cap layer, respectively. [Mg]s in four Mg:InN layers from bottom to top are

1.0 ¥ 10

(1st layer), 5.6 ¥ 10

(2nd), 2.9 ¥ 10

(3rd) and 1.8 ¥ 10

–3

(4th). It is shown that the polarity was inverted from In- to N-polarity above

the V-shaped domains, which happens at [Mg] ~ 2.9 ¥ 10

–3

. Further

detailed investigation shows that the polarity inversion happens at [Mg] ~1

¥ 10

–3

(Wang et al., 2007b). The observed N-polarity at position E in

Fig. 12.7 indicates that polarity inversion does not happen with further Mg

doping in the n-polarity case. investigation of the n-polarity sample does

not show any polarity inversion and thus this kind of polarity inversion only

happens in the in-polarity case. Very similar phenomena have also been

observed in gan (green et al., 2003).

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297Polarity controlled epitaxy of III-nitrides and ZnO

12.4 Polarity controlled epitaxy of GaN and AlN

The most popular epitaxy methods for gan and aln are metal–organic vapor

phase epitaxy (MOVPE/also called MOCVD) and MBE. The polarity of

GaN and AlN can be controlled in both methods. In MOCVD growth, due

to the near quasi-equilibrium, the growth window to get +c-polar nitrides

is rather broad, while it is difcult to get –c-polar samples, in particular to

get a at surface for –c-polarity, since the growth window for –c-polarity

is rather narrow and the sample often includes +c-polarity domains, which

lead to rough surface with hexagonal grains with size up to hundreds of

micrometers. In MBE growth, it is relatively easy to control the polarity by

controlling the buffer layer growth.

If GaN is grown directly on sapphire substrate, it is mainly Ga-polarity,

which is due to the Al-rich surface of sapphire substrate after thermal cleaning

under UhV (Xu et al., 2002). However, sapphire nitridation is commonly

used in order to improve the crystal quality of GaN and/or AlN. During the

nitridation, n atoms replace o atoms and a thin aln or alno layer is formed

with an epitaxial relationship of [101

0]AlN//[1120]Al

(namkoong et al.,

2002). The nitridation temperature has a large effect on the properties of

the formed AlN thin layer and inuences the properties of the grown GaN.

Low temperature nitridation (200–400°C) is often used since the AlN layer

formed is much purer and includes less no. But this process usually takes

a much longer time than high temperature nitridation though the aln layer

formed by the lattice process often includes no in a higher atom ratio. as far

g = [0002]

Cap

Spacer

buffer

GaN

Spacer

Mg:InN

(4th)

Mg:InN

(3rd)

Mg:InN

(2nd)

Mg:InN

(1st)

500 nm

Idbs

Ga-polarity

N-polarity

In-polarity

[Mg]:1.8 ¥ 10

–3

[Mg]:2.9 ¥ 10

–3

[Mg]:5.6 ¥ 10

–3

[Mg]:1.0 ¥ 10

–3

12.7 Cross-sectional dark ﬁeld TEM images of a multiple-layer-

structure InN ﬁlm recorded with g = [0002]. The sample structure is

shown on the left.

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