Chapter 1 Introduction
1.2 III-Nitride Materials Growth Issues
1.2.1 Crystal Structure of III-Nitride Semiconductors
Like most other semiconductors, the atoms in the nitrides are tetrahedrally
coordinated. The s and p orbitals in the outer electron shells combine to produce hybrid
sp3 orbitals where the probability of finding an electron in a p-state is 3 times as much as finding it in an s-state. As a result, each atomic site has four nearest neighbors
occupying the vertices of a regular tetrahedron, in a manner similar to the
diamond-cubic structure.
There are three common crystal structures in nitride semiconductors: the wurtzite
(Wz), the zincblende (ZB), and the rocksalt structures. At room-temperature, the wutzite
structure is the most stable. The space group for the wurtzite structure is P63mc (C46V )
as shown in Fig. 1.2.1(a). The wurtzite structure has a hexagonal unit cell consisting of
two interpenetrating Hexagonal Close Packed (HCP) sublattices, and two lattice
constants, c and a. The parameters of the hexagonal wutzite structure GaN are a =
3.189Å (Table.1.1). For the Wz structure, the stacking sequence of the (0001) plane is
ABABAB in the <0001> direction as shown in Fig. 1.2.1(b). The space group for the
zincblende structure is F4 3m(T− 2d) as shown in Fig. 1.2.2(a). The zincblende structure has a cubic unit cell, containing four group-III elements and four nitrogen elements. The
lattice parameters of the zincblend structure GaN are a = 4.52 Å (Table.1.1). For the ZB
structure, the stacking sequence of the (111) plane is ABCABC in the <111> direction
as shown in Fig. 1.2.2(b) [2]. These lattices are polar in nature, since the anions and the
cations occupy planes that are displaced from one another along the <0001> and the
<111> directions for the hexagonal and cubic structures, respectively.
1.2.2 Substrate for Nitride Epitaxy
One of the major difficulties, which has hindered GaN research, is the lack of a
suitable substrate material that is lattice matched and thermally compatible with GaN.
There are several substrates such as ZnO, SiC, and sapphire for growing thin films of
GaN. ZnO has the wurtzite structure, and is mismatched by only 1.8% to GaN.
Unfortunately, ZnO crystals are not easy to make, and ZnO is not as thermally stable as
might be desired. Nevertheless, ZnO is important for MBE growth [3]. Another choice
of substrate is SiC because it also has a smaller lattice mismatch (3.3%) with GaN.
However, there are problems in using SiC. For example, the defect densities of the
grown samples both in bulk and surface structure are still quite high [4]. Also, the
p-type contacts and p-type dopant activation are still quite poor [5]. The high price is
another drawback of using SiC as substrate. Nowadays, sapphire is the most commonly
used substrate for epitaxy growth of nitrides. Sapphire (α-Al2O3) has a hexagonal
structure that can be expressed both as a hexagonal as well as a rhombohedral unit cell
as shown in Fig. 1.2.3. As a substrate, sapphire (α-Al2O3) is inferior to others because of
the large lattice mismatch with nitrides (14.8% with GaN and 25.4% with InN) and
significant thermal expansion difference. Despite those disadvantages, sapphire remains
the most frequently used substrate for III-nitride epitaxial growth owing to its low cost,
its stability at high temperatures, its hexagonal symmetry, and a fairly mature
technology for nitride growth on it. The orientation order of GaN films grown on
principal sapphire planes: c(basal)-plane, a-plane (112 0), and r-plane (1− 1 02) was −
studied in great detail by Electron Cyclotron Resonance-MBE (ECR-MBE). The quality
of GaN films grown directly on sapphire was very poor before the advent of buffer
layers. AlN was first used as a buffer layer by Amano and Akasaki. Nakamura et al.,
grew a GaN buffer layer on a sapphire substrate for the first time. They lowered the
substrate temperature to between 450°C and 600°C to grow the buffer layer. Then, the
substrate temperature was elevated to above 1000°C to grow the GaN films [6].
1.2.3 Defects in Nitrides
GaN epitaxial layers are usually grown on sapphire substrates. However, GaN and
sapphire have poor matching in the lattice parameter and thermal expansion coefficient,
resulting in a high density of threading dislocations (TD) (108 ~ 1010 cm-2). There are
three kinds of threading dislocations, edge, screw, and mixed types. A dislocation is a
line defect that is defined by its Burgers vector b and line direction. The Burgers vector
b describes the lattice displacement for the dislocation within the crystal. The
dislocation is an edge type if the b and dislocation are perpendicular, as shown in Fig.
1.2.4, whereas it is a screw type if these vectors are parallel, as shown in Fig. 1.2.5. A
mixed dislocation has both an edge and a screw component (see Fig 1.2.6) [7]. A
threading dislocation usually terminates with a V-shaped defect on the sample surface. It
is well–known that such dislocations act as nonradiative centers in III–V and II–VI
semiconductors [8], [9]. Hino et al. discovered threading dislocations having a
screw-component burgers vector act as strong nonradiative centers in GaN epitaxial
layers, whereas edge dislocations, which are the majority, do not as nonradiative centers
[10]. It was believed that the density of V-shape defects would be reduced if GaN
epitaxial layers were grown on GaN free–standing substrates. Generally speaking,
increasing the quantum well numbers in MQW structures or increasing the indium
contents in the QWs enhances the formation of V-shaped defects. The formation of
V-shaped defect is for strain relaxation and the reduced Ga incorporation on
the{10 11}planes, in comparison with the (0001) surface [11]. The V–shape defect
consists of three important features: (I) the initiation point at a threading dislocation
(TD), which is buried below the bottom of the open apex; (II) buried MQWs on
the{10 11} planes, pyramid walls; and (III) an open hexagonal inverted pyramid as
shown in Fig. 1.2.7. Fig. 1.2.7(a) shows the typical cross-section TEM of V-shape
defects. Fig. 1.2.7(b) shows the perspective view indicating the six sides of the open
hexagonal pyramid (defined by the{10 11}planes) and non-crystallographic plane that
terminates the side-wall quantum wells; then part (c) shows the cross-section view
indicating the open V (defined by the{1011} planes), the side-wall quantum wells, and
the inclined noncrystallographic plane that terminates the side-wall quantum wells [11].