AlGaN and InAlGaN epitaxial growth, optical y electrical propierties, and applications

III-nitride wide bandgap semiconductors, with energy band

gap varying from 0.8 eV (InN) to 3.4 eV (GaN) to about

6.2 eV (AlN), have been recognised as technologically important

materials [1–10]. Photonic devices based on

III-nitrides offer benefits including UV/blue emission,

large band offsets of GaN/AlN or InN/AlN heterostructures

allowing novel quantum well (QW) device design, and inherently

high emission efficiencies. Furthermore, electronic

devices based on III-nitride heterostructures, including

heterojunction field effect transistors (HFETs) and bipolar

transistors (HBTs), have great promises in microwave and

millimeter-wave electronic device applications, due to the

high peak electron velocity, high saturation velocity, high

breakdown voltage, low noise, and thermal stability of the

system. III-nitride based optoelectronic and electronic devices

may operate at much higher temperatures and voltages/

power levels for any dimensional configuration and in

harsher environments than other semiconductor devices

and are expected to provide much lower temperature sensitivities,

which are crucial advantages for many applications.

Although tremendous progress has been made for

III-nitrides research and development in terms of both fundamental

understanding as well as devices applications, the

materials we understand relatively well today is just GaN

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

of AlN can be as good as GaN.

Jorge Polentino
CI 19769972
EES
http://www.wat.edu.pl/review/optor/10(4)271.pdf

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