compound and In (Al) GaN alloys with In (Al) content less
than 30% (50%). InGaN alloys with high In contents
(50%), which emit light in the orange to red colour spectral
range, can be replaced by other semiconductors. However,
AlGaN alloys with high Al contents, covering from 350 nm
to 200 nm, cannot be replaced by any other semiconductor
system due to the fact that no other semiconductors possess
such a large direct bandgap (diamond is 5.4 eV with indirect
bandgap) as well as the ability of bandgap engineering
through the use of III-nitride heterostructures.
There is currently a great need of solid-state ultraviolet
(UV) emitters for detection of chemical and biological
agents as well as for general lighting. In such applications
based on III-nitride wide bandgap semiconductors, conductive
n-type and p-type AlGaN or InAlGaN alloys with high
Al contents are indispensable. The use of high Al-content
AlGaN layer is also expected to increase the overall figure
of merit of the AlGaN/GaN HFETs due to the combined
advantages of enhanced band offset, lattice mismatchinduced
piezoelectric effect, and the electron velocity in the
two dimensional electron gas (2DEG) channel. Thus improving
the material quality of high Al content AlGaN alloys
is also of crucial importance for fabricating high performance
AlGaN/GaN HFETs.
This paper summarises some of the recent advances
made by the authors' group on the growth, characterization
and applications of AlGaN and InAlGaN alloys. It was
shown that the effect of carrier localisation in undoped
AlGaN alloys enhances with increased Al contents and is
related to the insulating nature of AlGaN of high Al contents.
It was also shown that AlxGa1-xN alloys could be
made n-type for x up to 1 (pure AlN). Time-resolved
photoluminescence (PL) studies carried out on these materials
have revealed that Si-doping reduces the effect of carrier
localization in AlxGa1–xN alloys and a sharp drop in
carrier localisation energy as well as a sharp increase in
conductivity occurs when the Si doping concentration increases
to above 1 1018 cm–3. For the Mg-doped
AlxGa1–xN alloys, p-type conduction was achieved for x up
to 0.27. From the Mg acceptor activation energy as a function
of Al content, the resistivity of Mg-doped AlxGa1–xN
with high Al contents can be estimated. For example, the
projected resistivity of AlxGa1–xN (x = 0.45) is around
2.2 104 cm. Thus, alternative methods for acceptor activation
in AlGaN or InAlGaN with high Al contents must
be developed before the high performance deep UV emitters
can be realised.
The optical properties of AlGaN/GaN heterostructures
with high Al content were also studied. Due to the strong piezoelectric
polarisation and deep triangular potential notch
in AlxGa1–xN/GaN (x = 0.5) heterointerface, a total of five
emission lines related with the 2DEG in AlxGa1–xN/ GaN (x
= 0.5) heterostructure have been observed, which correspond
to the recombination between the electrons from different
sub-bands (n = 1 to 5) in the conduction band and the
photoexcited holes in the valence band. The 2DEG PL emission
lines were found to be observable at temperatures as
high as 220 K, in sharp contrast to the AlGaAs/GaAs
heterostructures system in which the 2DEG emission lines
were observable only at low temperatures (T <20 K).
Optoelectronic properties of InAlGaN quaternary alloys
were studied. It was observed that the dominant optical
transition at low temperatures in InxAlyGa1–xN quaternary
alloys was due to localized exciton recombination, while
the localisation effects in InxAlyGa1–xN quaternary alloys
were combined from those of InGaN and AlGaN ternary
alloys with comparable In and Al compositions. Our studies
have revealed that InxAlyGa1–xN quaternary alloys with
lattice matched with GaN epilayers (y ~4.8x) have the highest
optical quality. The quantum efficiency of
InxAlyGa1–xN quaternary alloys was also enhanced significantly
over AlGaN alloys with a comparable Al content. It
was also found that the responsivity of the InxAlyGa1–x–yN
quaternary alloy photodetectors exceeded that of AlGaN alloy
of comparable cut-off wavelength by a factor of five.
The AlGaN ternary and InAlGaN quaternary were incorporated
into UV (340 nm) emitter structures. The operation of
340 nm micro-size UV emitters have been demonstrated.
AlN epilayers with high optical qualities have also been
grown on sapphire substrates. Very efficient band-edge PL
emission lines have been observed for the first time with
above bandgap deep UV laser excitation. We have shown
that the thermal quenching of the PL emission intensity is
much less severe in AlN than in GaN and the optical quality
