J.Phys.D:Appl.Phys.31(1998)2653–2710.Printed in the UK PII:S0022-3727(98)68952-X
REVIEW ARTICLE
Growth and applications of
Group III-nitrides
O Ambacher
Walter Schottky Institute,Technical University Munich,Am Coulombwall,
D-85748Garching,Germany
Received18February1997,infinal form15June1998
Abstract.Recent research results pertaining to InN,GaN and AlN are reviewed,
focusing on the different growth techniques of Group III-nitride crystals and感应门制作 epitaxialfilms,heterostructures and devices.The chemical and thermal stability of
epitaxial nitridefilms is discussed in relation to the problems of deposition
processes and the advantages for applications in high-power and high-temperature
devices.The development of growth methods like metalorganic chemical vapour
deposition and plasma-induced molecular beam epitaxy has resulted in remarkable
improvements in the structural,optical and electrical properties.New developments
in precursor chemistry,plasma-based nitrogen sources,substrates,the growth of
nucleation layers and selective growth are covered.Deposition conditions and
methods used to grow alloys for optical bandgap and lattice engineering are
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introduced.The review is concluded with a description of recent Group III-nitride
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semiconductor devices such as bright blue and white light-emitting diodes,thefirst
blue-emitting laser,high-power transistors,and a discussion of further applications
in surface acoustic wave devices and sensors.
1.Introduction
Group III-nitrides have been considered a promising system for semiconductor devices applications since1970, especially for the development of blue-and UV-light-emitting diodes.The III–V nitrides,aluminium nitride (AlN),gallium nitride(GaN)and indium nitride(InN), are candidate materials for optoelectrical applications at such photon energies,because they form a continuous alloy system(InGaN,InAlN,and AlGaN)whose direct optical bandgaps for the hexagonal wurtzite phase range from 1.9eV forα-InN and 3.4eV forα-GaN to 6.2eV forα-AlN.The cubic modifications have bandgaps in the range from 1.7eV forβ-InN and 3.2eV for β-GaN to 4.9eV forβ-AlN(figures1and2)[1–6]. Other advantageous properties include high mechanical and thermal stability,large piezoelectric constants and the possibility of passivation by forming thin layers of Ga2O3 or Al2O3with bandgaps of approximately 4.2eV and 9eV.The spontaneous and piezoelectric polarization(in the wurtzite materials)and the high electron drift velocities (2×105m s−1[7])of GaN can be used to fabricate high-power transistors based on AlGaN/GaN heterostructures. In addition,AlN is an important material with a variety of applications such as passive barrier layers,high-frequency acoustic wave devices,high-temperature windows,and dielectric optical enhan
cement layers in magneto-optic multilayer structures[8,9].
Very informative reviews of the growth techniques and structural,optical and electrical properties of Group III-nitrides and their alloys have been presented by Strite et al[10,11].A good overview of applications of Group III-nitride based heterostructures for UV emitters and high-temperature,high-power electronic devices is provided in[12]and[13].This review focuses on the development of the different growth techniques successfully applied to the deposition of Group III-nitride epitaxial films and heterostructures,such as chemical transport and metalorganic chemical vapour deposition(MOCVD), sputtering and molecular beam epitaxy(MBE).The quality of state-of-the-art material and its application for optical and electronic devices are discussed in detail in order to point out possible limitations,promising developments and future trends.
Thefirst systematic effort to grow InN,GaN and AlN by chemical vapour deposition or sputtering processes took place in the1970s in order to characterize the optical and structural properties of thinfilms.At that time, neither metalorganic precursors containing In or Al with electronic grade purity,plasma sources for nitrogen radicals compatible with MBE systems,nor substrate material with reasonably good thermal and lattice matches to the nitrides were available.The InN and GaN material had large concentrations of free electrons,presumed to result from oxygen impurities and int
rinsic defects,and the structural quality of the AlNfilms was not good enough for optical
0022-3727/98/202653+58$19.50c 1998IOP Publishing Ltd2653
O
Ambacher
Figure 1.Bandgap and bowing parameters of hexagonal (α-phase)and cubic (β-phase)InN,GaN,AlN and their alloys versus lattice constant a 0[1–6].
or electronic applications.Primarily,the development of MOCVD and plasma-induced molecular beam epitaxy (PIMBE)over the last eight years has led to a number of recent advances and important improvements in structural properties.
2.Crystal structure,polarity and polarization of InN,GaN and AlN
In contrast to cubic III–V semiconductors like GaAs and InP with the zincblende structure,the thermodynamically stable phase of InN,GaN and AlN,is the hexagonal wurtzite structure (α-phase).Beside the α-phase,a metastable β-phase with zincblende structure exists and a cubic high-pressure modification with NiAs structure was observed for pressures above 25kbar in the case of AlN [14].Because the α-and β-phases of Group III-nitrides only differ in the stacking sequence of nitrogen and metal atoms (polytypes),the coexistence of hexagonal and cubic phases is possible in epitaxial layers,for example due to stacking faults.The hexagonal crystal structure of Group III-nitrides can be described by the edge length a 0of the basal hexagon,the height c 0of the hexagonal prism and an internal parameter u defined as the anion–cation bond length along the (0001)axis.Because of the different
cations and ionic radii (Al 3+:0.39˚A,
Ga 3+:0.47˚A,In 3+:0.79˚A
[15]),InN,GaN and AlN have different lattice constants,bandgaps and binding energies as shown in table 1[16,17].
Both wurtzite and zincblende structures have polar axes (lack of inversion symmetry).In particular,the bonds
in
Figure 2.Experimental results of bandgaps of hexagonal Group III-nitrides versus lattice constant c 0at room temperature [1–6].
Table 1.Lattice constants,bandgaps and binding energies of hexagonal InN,GaN and AlN.
Wurtzite,300K AlN GaN InN a 0(˚A)b 3.112 3.189 3.54c 0(˚A)
b 4.982 5.185 5.705
c 0/a 0(exp.)b 1.6010 1.6259 1.6116c 0/a 0(calc.)a 1.6190 1.6336 1.6270u 0a
0.3800.3760.377a Bohr (˚A)
a 5.814 6.04 6.66E B (M–N)c (eV)b
2.88
2.20
1.98
a From [16].
b From [17].
c
M =In,Ga or Al,N =nitride.
the 0001 direction for wurtzite and 111 direction for zincblende are all faced by nitrogen in the same
direction and by the cation in the opposite direction.Both bulk and surface properties can depend significantly on whether the surface is faced by nitrogen or metal atoms [18,19].The most common growth direction of hexagonal GaN is normal to the {0001}basal plane,where the atoms are arranged in bilayers consisting of two closely spaced hexagonal layers,one with cations and the other with anions,so that the bilayers have polar faces.Thus,in the case of GaN a basal surface should be either Ga-or N-faced.By Ga-faced we mean Ga on the top position of the {0001}bilayer,corresponding to [0001]polarity (figure 3).Ga-faced does not mean Ga-terminated;termination should only be used
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Growth and applications of Group
III-nitrides
Figure3.Different polarities(Ga-and N-faced)of wurtzite GaN.
to describe a surface property.A Ga-face surface might
be N-terminated if it is covered with nitrogen atoms,but
withoutflipping the crystal it will never be N-faced.It
is,however,important to note that the(0001)and(000¯1) surfaces of GaN are inequivalent(by convention,the[0001]
direction is given by a vector pointing from a Ga atom to
a nearest-neighbour N atom).
It has been reported that high-quality epitaxial GaN
films deposited by MOCVD on c-plane sapphire substrates
grow in the(0001)direction with Ga-faced surfaces,while
MBE growth commonly occurs in the(000¯1)direction, yielding an N-facedfilm[20–22].
Polar faces are known to have very marked effects
on growth in binary cubic semiconductors.For example,
growth along the Ga-faced{111}direction of GaAs is
known to be slow and has the tendency to produce planar
surfaces,whereas growth of the As-face is fast and rough
[23].Similarly,Ponce et al found that the smooth side
of bulk single crystal platelets corresponds to the Ga-face
(0001)whereas the N-face(000¯1)is much rougher[20].
In the following we will discuss the influence of
spontaneous and piezoelectric polarization on the physical
properties of Group III-nitrides.This class of polarization-
related properties is obviously important for devices
(section9)because the electricfields influence the shape
of the band edges and the carrier distribution inside
nitride-based heterostructures.Therefore spontaneous
and piezoelectric polarization can influence the radiative
recombination in light-emitting devices as well as the
electrical properties of the transistor structures discussed
in detail later.
Wurtzite is the structure with highest symmetry
compatible with the existence of spontaneous polarization
[16,24,25]and the piezoelectric tensor of wurtzite has
three independent nonvanishing components.Therefore,
polarization in these material systems will have both a
spontaneous and a piezoelectric component.Because
of the sensitive dependence of spontaneous polarization
on the structural parameters,there are some quantitative
differences in polarization of the three nitrides studied here.
The increasing nonideality of the crystal structure going Table2.Spontaneous polarization,piezoelectric and dielectric constants of AlN,GaN and InN.
Wurtzite AlN GaN InN
P SP(C m−2)−0.081−0.029−0.032
e33(C m−2) 1.46a0.73a0.97a
1.55b1c
0.65d
0.44e
e31(C m−2)−0.60a−0.49a−0.57a
−0.58b−0.36c
−0.33d
−0.22e
e15(C m−2)−0.48b−0.3c
−0.33d
−0.22e
ε119.0b9.5f
ε3310.7b10.4f
a From[16].
b From[26].
c From[27].
d From[28].
e From[29].
f From[30].
from GaN to InN to AlN(u0increases and c0/a0decreases (table1)),corresponds to an increase in spontaneous polarization.In the absence of external electricfields,the total macroscopic polarization P of a solid is the sum of the spontaneous polarization P SP in the equilibrium lattice and the strain-induced or piezoelectric polarization P P E.
Here we consider polarizations along the(0001)axis, because this is the direction along which standa
rd bulk materials,epitaxialfilms and heterostructures are grown. Spontaneous polarization along the c-axis is P SP=P SP z (the direction of spontaneous polarization is determined by the polarity;the direction of the piezoelectric polarization depends on the polarity and whether the material is under tensile or compressive stress)and piezoelectric polarization can be calculated by using the piezoelectric coefficients e33 and e13(table2)as
P P E=e33εz+e31(εx+εy)(1) where a0and c0are the equilibrium values of the lattice parameters,εz=(c−c0)/c0is the strain along the c-axis, and the in-plane strainεx=εy=(a−a0)/a0is assumed to be isotropic.The third independent component of the piezoelectric tensor,e15,is related to the polarization induced by shear strain and will not be discussed.
To give an example of the possible influence of polarization on the physical properties of nitride-based heterostructures,we calculate the electricfield caused by polarization inside a Ga-faced Al x Ga1−x N/GaN/Al x Ga1−x N quantum well.We assume that the GaN is grown pseudomorphically on the AlGaN(a(GaN)=a(AlGaN))
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O Ambacher
Table3.Experimental and calculated values of the piezoelectric constants and bulk modulus for wurtzite and zincblende Group III-nitrides.
AlN GaN InN
GPa
wurtzite exp.a calc.b exp.c calc.b exp.d calc.b
C11345396374367190223
C12125137106135104115
C131201087010312192
C33395373379405182224
C44118116101951048
B201207180202139141
zincblende calc.e calc.b calc.e calc.b calc.e calc.b
C11304304296293184187
C12152160154159116125
C4419919320615517786
a From[31].
b From[32].
c From[33].
d From[34].
e From[35].
碎花刀刀
and that screening effects due to free carriers and surface states can be neglected. The lattice constants a and c of the GaN layer are decreased and increased respectively,due to the biaxial compressive stress which becomes larger with increasing Al content of the AlGaNfilm.The relation between the lattice constants of the hexagonal GaN is given by
c−c0 c0=−2C13
C33
a−a0
cd12a0
(2)
where C13and C33are elastic constants(table3). Using equations(1)and(2)the amount of piezoelectric polarization in the direction of the c-axis can be determined by
P P E=2a−a0
a0
e31−e33
C13
C33
.(3)
The strain of the pseudomorphically grown GaN can be calculated using Vegard’s law(linear interpolation of the lattice constants of relaxed Al x Ga1−x N from the values for GaN and AlN:a(x)=(−0.077x+3.189)˚A(table1)), leading to
P P E(GaN)=0.0163x C m−2(4) and a total polarization of
P(GaN)=P SP(GaN)+P P E(GaN)
=(−0.029+0.0163x)C m−2.(5) The polarization generates an electricfield E(GaN)inside the GaN layer:
E(GaN)=−
P(GaN)ε(GaN)ε0
=(3.6×106−2.1×106x)V cm−1(6) whereε(GaN)(table2)andε0are the dielectric constants of GaN and
vacuum.Figure4.Polarization(spontaneous,piezoelectric and total polarization)of a relaxed Al x Ga1−x N and a pseudomorphic on top of Al x Ga1−x N grown GaN layer versus Al content x. The interface chargeσis caused by the different total polarizations of the GaN and the AlGaNfilm.
The AlGaN is assumed free of strain and therefore the piezoelectric polarization equals zero.The total polarization of the AlGaN can be described by a linear approximation between the spontaneous polarization of GaN and AlN:
P(AlGaN)=P SP(AlGaN)=(−0.029−0.052x)C m−2.
(7)
A charge density at the GaN/AlGaN interfaces,σ(GaN/ AlGaN),is caused by the different polarizations of GaN and AlGaN:
±σ(GaN/AlGaN)=P(GaN)−P(AlGaN)
=±0.068x C m−2.(8) The spontaneous polarization,piezoelectric polarization and interface charge density of GaN embedded in two Al0.15Ga0.85N layers are determined to be−0.029,0.0025 and±0.0025C m−2respectively.(For AlGaN/GaN/AlGaN heterostructures with different Al content x,se
efigure4.) The electricfield caused by polarization effects can reach a strength of3×106V cm−1.
The modification of the band edges due to spontaneous polarization and piezoelectricfields inside the GaN layer can have a significant influence on the optical properties (figure5).Due to the Stark and Franz–Keldysh effects, the effective bandgap of GaN will be red-shifted and the recombination probability of electron hole pairs will be decreased because of the spatial separation of electrons and holes[36,37].These physical effects thus change the energy of the electroluminescence out of GaN or InGaN
2656
Growth and applications of Group
III-nitrides
Figure5.Conduction and valence band edges of a pseudomorphic grown AlGaN/GaN/AlGaN(x=0.15)and
GaN/InGaN/GaN(x=0.06)heterostructure.The arrows indicate schematically the radiative recombination of an electron and a hole,which is red-shifted in comparison to the bandgap energy due to the Stark effect.
quantum wells and the recombination rates of carriers inside a Group III-nitride based laser structure(section9.5).The strong electricfields can also enhance electron or hole accumulation at AlGaN/GaN interfaces(figure5).This effect can be used in heterostructurefield effect transistors, as discussed later in section9.3.At which interface (lower or upper)of a AlGaN/GaN/AlGaN heterostructure electrons or holes are confined depends on the polarity of the material.
麦弗逊式
独立悬架
In respect of polarization effects,the Group III-nitrides exhibit unusual properties.The piezoelectric constants have the same sign as in II–VI compounds,and opposite to those of III–V compounds.The absolute values of the piezoelectric constants are up to ten times larger than in conventional III–V and II–VI compounds.In particular the constants e33and e31of AlN are larger than those of ZnO and
BeO[38],and are therefore the largest known so far among the tetrahedrally bonded semiconductors.The spontaneous polarization(the polarization at zero strain) is also very large in the nitrides.That of AlN is only about three tofive times smaller than in typical ferroelectric perovskites[39].For these reasons,the spontaneous and piezoelectric polarization of hexagonal Group III-nitrides can have a much larger influence on the electrical and optical properties of devices than in other III–V compounds. Finally it should be mentioned that free carriers with a concentration above1018cm−3,charged defects or compensation of surface charges by adsorbates can reduce the polarization-induced electricfields and have to be considered in a detailed analysis of polarization-related effects.
3.Thermal properties and stability
The primary methods of obtaining crystal material rely on growing epitaxial layers on different substrates at high temperatures.Unfortunately,the different coefficients of thermal expansion between substrate and nitride introduce residual stress upon cooling.These induced stresses can cause additional structural defects and piezoelectricfields and will influence the optical and electrical properties of films and devices.
The determination of thermal expansivity is not only related to other thermal properties(thermal conductivity, specific heat)but can also yield parameters pertinent to
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