Arsenide and phosphide based III-V semiconductors
have been extensively researched for many years,
resulting in mature technologies that are used for
many of today's electronics and optoelectronics.
These materials are however limited to medium and
narrow bandgap applications, preventing the fabrication
of wide gap devices such as high power electronics
and short wavelength optoelectronics. The development
of wide-gap material has been slow and problematic,
nevertheless, recent advances in nitrides, such as the
development of blue LEDs and lasers, have seen these
alloys catapulted into the limelight. This has lead to an
explosion of research interest, and the natural progression
to bridge the gap between the wide-gap nitrides and the
medium-gap arsenides. However, instead of the addition
of small amounts of nitrogen to GaAs resulting in the
expected increase of bandgap, it was found to have the
opposite effect, resulting in a rapid reduction of the
bandgap.[1] This unusual behaviour has sparked
considerable interest both from a fundamental physics
point of view as well as for potential narrow-gap
applications. These new hybrid alloys, which have
become known as "dilute nitrides", are the topic of
this current work.
In the late eighties interest in wide-gap materials for
short wavelength optoelectronics (and electronics)
began to grow, most research concentrated on zinc
selenide (ZnSe) although some work was also carried
out on nitrides and silicon carbide (SiC). Despite the
indirect bandgap of SiC, it was used in the late
eighties to produce the first commercial blue LEDs;
unfortunately, the indirect gap meant low efficiencies
(~0.03 % [2]) and no scope for producing lasers. Instead
, ZnSe-based materials were pursued, leading to the
first pulsed blue laser diode in 1991 [3]. Despite
subsequent advances that lead to continuous wave room
temperature lasing, ZnSe was problematic. Unlike SiC
and nitrides that have wide-gaps resulting from strong
chemical bonds, ZnSe's gap is due to its highly ionic
bonds that are weak. As a result, defect growth and
propagation is common, leading to device degradation.
In addition, ZnSe suffers from poor electrical/thermal
properties and Fermi level pinning.
Nitride research dates back to the early seventies [4]
when many physical properties such as refractive index,
[5] bandgap [6] and lattice constant [7] were measured.
However, interest soon diminished, as it was discover
that the strong background n-type doping prevented the
growth of p-type material and no suitable substrate
materials were available. Interest in nitrides was
rekindled in the early nineties when a small research
group led by Nakamura at a virtually unheard of
chemical company in Japan reported blue emission
from InGaN devices. The application of innovative
growth techniques to the nitride system led to rapid
advances in material quality and commercially viable
devices. The problems associated with substrate
mismatch were overcome using buffer layers,[8, 9]
and background doping was reduced via optimised growth.
The key breakthrough of p-doping was finally achieved
using magnesium dopant activated by electron beam
irradiation [10] or thermal annealing.[11] This allowed
control over both n and p-type doping, hence nitride
based p-n junctions could be produced and subsequently
advance heterostructures LEDs and lasers.[12]
The first steps towards bridging the gap between
arsenides and nitrides was made in 1992 by Weyers
et.al.,[1] resulting in the unexpected discovery of a
rapid reduction in bandgap energy with increasing nitrogen.
In most, if not all other, III-V alloys, when a semiconductor
element is replaced by one of smaller ionic radius, the
band gap energy increases. When small amounts of
nitrogen were added to GaAs, instead of the expected blue
shift in emission, a considerable red shift was observed
[1] Furthermore, the decrease in energy per atomic percent
of nitrogen was more than ten times greater than the
typical increase in other semiconductor alloys. While this
behaviour meant that the dilute nitrides could not be used
for visible light emitters, fundamental physics as well as
potential applications were clear incentives to pursue these
materials. Surprisingly, the next few years saw very little
published work on dilute nitrides.
Interest in dilute nitrides really began in the mid nineties
after Kondow et.al. published results on the quaternary
alloy, GaInNAs.[13, 14] This new alloy allowed independent
control over the In:Ga and N:As ratios. Increasing the
In:Ga ratio causes a reduction in bandgap and an increase
in lattice parameter, while increasing the N:As ratio also
causes bandgap reduction, but a decrease in the lattice
parameter. GaInNAs, therefore, gives the flexibility of tailoring
both bandgap and lattice parameter. Such tailoring
potential opens up a wide range of possible applications,
however, the 1.3 mm lasers based on GaAs for optical
communications were identified as a key application.
At this wavelength silica fibre has zero dispersion and
relatively low attenuation, making it an attractive
communications window. Many of today's medium haul
systems operate in the 1.3 mm window, and if transceiver
systems could be made economical then they could be
used in local area networks. The first laser to operate
in the 1.3 mm window was produced in 1976 by J. Hsieh,
using InGaAsP/InP,[15] since then most, if not all, 1.3
mm systems have used InP based lasers. Unfortunately,
this material system is not ideal for producing cheap lasers
as a number of problems ultimately increase costs. While
some innovative attempts have been made to solve these
problems, many are far from ideal, often causing additional
problems or using processing techniques that are not easily
integrated into commercial production. Instead of solving
the problems, considerable research efforts are currently
being put into finding alternative materials that avoid the
problems altogether. InGaAs has been extensively
researched and investigations have been done to see how
far the material can be pushed towards long wavelength
emissions. While highly strained InGaAs QWs lasers
operating out to around 1.2 mm have been produced with
good characteristics using strain-compensated QWs,
[16, 17] increasing the wavelength further becomes very
difficult. Lasers operating close to 1.3 mm based on
GaAsSb QWs have been demonstrated,[18] however,
the conduction band discontinuity between GaAsSb
and GaAs is believed to be small [19] resulting in high
leakage currents and hence poor performance.
At present, the main research drive for alternative
1.3 mm lasers is split between self-organised quantum
dots (QD) and dilute nitrides. QD lasers have attracted
substantial interest, as the physics of QDs could
potentially improve laser performance considerably.
They are expected to have reduced temperature
dependence,[20] reduced thresholds and higher efficiencies.
[21] However, this can only be realised if methods are found
to control dot density, distribution and, most importantly,
size. Even small size fluctuations can 'smear' the density
of states, producing bulk-like behaviour. While considerable
progress has been made towards dot uniformity, there
appears to be some trade off with dot density, hence
gain.[22] In addition, the expected temperature
performance of QD lasers has not yet been demonstrated,
so far devices have exhibited poor characteristic
temperatures no better than InP lasers.[23]
Considerable interest in dilute nitrides for long wavelength
lasers was stimulated by the first reported GaInNAs
lasers.[13] The materials were expected to have very
good temperature characteristics resulting from the
predicted large electron confinement. In addition, being
based on GaAs, dilute nitrides would be easily
integrated with the established GaAs technology,
including AlGaAs based Bragg mirrors. However, producing
high quality dilute nitride has proved difficult, with
increasing nitrogen fraction resulted in reduced optical
quality.[13, 24-28] Post-growth annealing was soon
found to improve the optical quality, however, the increase
in optical quality was usually accompanied by a blue
shift,[29] which partly offset the red shift induced by nitrogen
incorporation. Despite these problems, early research
lead to the demonstration of a range of devices and a
gradual improvement in material quality.[30] Progress in
dilute nitride devices has been rapid, 1.3 mm laser
emission was realised in 1998,[31] and good characteristic
temperatures have been reported.[32, 33] The emission
wavelength has even been pushed out to 1.515 mm
using GaInNAs with high indium and nitrogen fractions
.[34, 35] Unfortunately, many early lasers had high
thresholds and low slope efficiencies compared to
nitrogen-free lasers. These problems have largely
been attributed to poor crystal quality, which is hardly
surprising in a new material system. However,
considerable improvements have been reported recently,
with some 1.3 mm single and triple quantum well lasers
having thresholds as low as 400 and 680 A/cm2
respectively.[36] A number of dilute nitride VCSELs
have also been successfully fabricated, the first in 1997
operated at 1.22 mm under optical pumping,[37] however,
it was not long before an electrically pumped version was
demonstrated.[38] Subsequently, VCSELs operating close
to 1.3 mm were demonstrated, first optically pumped
[39, 40] and then electrically pumped.[41] In addition to
lasers, dilute nitrides have been used in a number of other
devices including:
- Resonant cavity enhanced (RCE) photodetectors,[42, 43]
- Electro-absorption (e/a) modulators,[44]
- Solar cells,[45-47]
- Heterojunction bipolar transistors (HBTs).[41, 48-52]
References
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10. H. Amano, et.al., Jpn. J. Appl. Phys., 28 , L2112 (1989).
11. S. Nakamura, et.al., Jpn. J. Appl. Phys., 31 , L139 (1992).
12. S. Nakamura, et.al., Jpn. J. Appl. Phys., 35 , L74 (1996).
17. S. Sato et.al., Jpn. J. Appl. Phys., 38 , L990 (1999).
18. M. Yamada, et.al., IEEE Photon. Tech. Lett., 12 , 774 (2000)
21. M. Asada, et.al., IEEE J. Quantum Electron., 22 , 1915 (1986)
23. V.M. Ustinov, et.al., Semicond. Sci. Tech., 15 , R41 (2000).
27. S. Sato, et.al., Jpn. J. Appl. Phys., 36 , 2671 (1997).
28. I.A. Buyanova, et.al., MRS Internet J. Nitride Semi. Research, 6 (2) (2001).
29. R.J. Potter, et.al., Superlattice Microst., 29 , (2) 169 (2001).
32. T. Kitatani, et.al., Jpn. J. Appl. Phys., 39 , 86 (2000).
37. M.C. Larson, et.al., Elect. Lett, 33 , 959 (1997).
41. K.D. Choquette, et.al., Electron. Lett., 36 , 1388 (2000).
44. Y.S. Jalili, et.al., Electron. Lett., 38 , 343 (2002).
46. D.F. Friedman, et.al., 2nd World conference and exhibition on photovoltaic solar energy conversion, July (1998).
51. N.Y. Li, et.al., Electron. Lett., 36 , 81 (2000).
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