A high-quality AlGaN layer with a low density of threading dislocations is realised for the use of ultraviolet (UV) light-emitting diodes (LEDs). The new crystal growth method of using a GaN seed crystal with (1122) facets and lateral growth of Al0.22Ga0.78N through the low-temperature-deposited AlN interlayer enables the overgrown Al0.22Ga0.78N to have a low dislocation density of 2107 cm–2 and be crack-free over the whole wafer. Applying the AlGaN as a base layer in UV-LEDs, high-performance devices with high output powers of more than 0.1 mW under 50 mA drive are demonstrated in a wide range of emission wavelengths from 323 to 363 nm. The highest output power of 1.2 mW at 50 mA driving current is obtained with 363-nm emission wavelength.
Keywords: MOVPE, lattice mismatch, threading dislocation, UV, light-emitting diode.
Inversion behaviour has been demonstrated in gate-controlled
p-GaN diodes using both MgO and Sc2O3 gate dielectrics
and implanted n+ regions to provide a source of inversion charge. The total
surface state density was estimated from capacitance-voltage or charge
pumping measurements to be in the range 3–8 x 1012 cm–2
after the implant activation annealing to form the source and drain regions.
In addition, Mn doping of GaN during growth by molecular beam epitaxy is
found to produce room temperature ferromagnetism under conditions where
the material remains single-phase. The layers can be used as
injectors of spin-polarised carriers into light-emitting
diode structures, with the potential for creating polarised optical output.
Keywords: GaN, magnetic semiconductors, gate dielectrics.
The recent years have seen a continuous transfer of exciting
new technologies from basic research institutions to high yield mass production
and into our everyday lives. Devices made from novel semiconductor compounds
can be found in products ranging from consumer electronics to high speed
backbone communication networks. This includes high power infrared laser
diodes for glass fiber applications, ultra-high brightness light emitting
diodes for display and lighting, high power blue and UV laser diodes for
mass storage as well as all types of transistors made from silicon, III-V
compounds and silicon-carbide. To facilitate the easy and straigtforward
transfer from research scale experimental setups to large area substrates
for mass production AIXTRON offers the whole scale of epitaxy solutions
from single wafer systems to large scale production machines for up to
95 wafers. The easy configurability of the systems in terms of up-scaling
of wafer sizes up to 7x6 inch for phosphides and arsenides and up to 84
inch for nitride materials in concurrence with easy maintenance, high reproducibility
and high uniformity across the wafer and from wafer to wafer make the AIXTRON
systems the ideal solution
for mass production. The growth principle common to all
AIXTRON MOCVD systems allows the easy up-scaling of established processes
to larger configurations, even from single wafer AIX 200 systems to production
type Planetary Reactors®. Add-ons like in-situ monitoring of the growth
process by reflectometry (EpiTune® I and EpiTune® II) or Reflectance
Anisotropy Spectroscopy (Epi-RAS®) help in a considerable reduction
of the development time and costs, hence improving innovation cycles and
the time-to-market of novel devices since the growth of the material can
be monitored in real time.
Keywords: MOCVD technique, doping uniformity, in-situ charactrization, GaN, AlGaAs, AlGaInP.
Codoping in p-type, epitaxial GaN grown by metal-organic vapour phase epitaxy (MOVPE) was investigated. An enhancement of hole concentration was observed experimentally in p-type GaN:Mg codoped with oxygen donors. The hole concentration of GaN:Mg codoped with oxygen increased super-linearly from 8x1016 to 2x1018 cm–3 upon increasing the oxygen dopant partial pressure. A factor of 3–5 enhancement of hole concentration was measured for a fixed oxygen partial pressure during the growth of p-type GaN:Mg. However, when Si was codoped with GaN:Mg, the hole concentration remained constant. Current experimental results are compared with the existing theory of codoping in GaN.
Keywords: GaN, codoping, p-type conductivity.
The successful development of both discrete UV photodiodes and large-format UV imaging arrays consisting of 128x128 and 320x256 AlGaN p-i-n photodiodes is reported. Detectors and detector arrays have been successfully developed to sense radiation in the 300-365 nm visible-blind and 240-285 nm solar-blind portions of the UV spectral region. Details of the device synthesis and processing are discussed, including flip-chip bonding procedures employed to mate the UV photodiode arrays to silicon read-out integrated circuits (ROICs). A selection of UV images obtained using these new large-format UV digital imagers is included.
Keywords: UV detector, UV focal plane array, UV digital camera.
Maskless pendeo-epitaxy (PE) involves the lateral and, commonly, the vertical growth of cantilevered “wings” of material from the side-walls of unmasked etched forms. Cross-sectional SEM micrographs revealed that films grown at 1020oC exhibited similar vertical [0001] and lateral [1120] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(1120). The (1120) surface was atomically smooth under all growth conditions with an RMS = 0.17 nm. High resolution X-ray diffraction (HRXRD) and atomic force microscopy of the PE films confirmed transmission electron microscopy results regarding the reduction in dislocation density in the wings. Measurement of strain indicated that the wing material is crystallographically relaxed as evidenced by the increase in the c-axis lattice parameter and the upward shift of the E2 Raman line frequency. However, tilting of the wings of <= 0.15o occurred due to the tensile stresses in the stripes induced by the mismatch in the coefficients of thermal expansion between the GaN and the underlying substrate.
Keywords: pendeo-epitaxy, gallium nitride, metalorganic vapour phase epitaxy, lateral growth rate, vertical growth rate, dislocations, strain, relaxation, wing tilting, atomic force microscopy, X-ray diffraction, photoluminescence, Raman.
The paper summarises some of recent advances made by the
author’s group on the growth, characterization and applications of AlGaN
and InAlGaN alloys. Electrical, optical as well as optoelectronic properties
have been studied. It is 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. Si-doping reduces
the effect of carrier localization in AlGaN alloys and a sharp increase
in conductivity occurs when the Si doping concentration
increases to above 1018 cm–3. For the Mg-doped AlxGa1-xN
alloys, p-type conduction was achieved for x up to 0.27. Due to the strong
piezoelectric polarization and deep triangular potential notch in AlxGa1-xN
(x = 0.5) heterointerface, a total of five emission lines related with
the two-dimensional electron gas in heterostructure have been observed,
which correspond to the recombination between the electrons from different
sub-bands. The dominant optical transition at low temperatures in AlxGa1-xN
quaternary alloys is due to localized exciton recombination, while the
localisation effects were combined from those of InGaN and AlGaN ternary
alloys with comparable In and Al compositions. Finally micro-size ultraviolet
light emitters are demonstrated.
Keywords: AlGaN alloys, InAlGaN alloys, AlN epilayers, light emitters, time-resolved photoluminescence.
We report on the enhancement of optical and electrical properties of 280 nm UV LEDs using low dislocation density HVPE-grown GaN substrate. Compared with the same structure grown on sapphire, these LEDs show ~30% reduction in current-voltage differential resistance, ~15% reduction in turn-on voltage, more than 200% increase in output power slope efficiency and saturation at higher currents. Lower density of defects due to higher material quality and better heat dissipation are believed to be the reason behind these improvements
Keywords: HVPE growth, GaN LEDs.
We report on development of MSM and Schottky barrier visible blind detectors on gallium nitride which exhibit responsivities of 0.5 A/W and 0.1 A/W respectively. GaN band edge absorption occurs at 365 nm and naturally provides “visible blindness” of devices. The fabricated Schottky barrier devices exhibit flat spectral response for the UV light. Typical dark current of detector is 1 nA per square millimetre. The estimated detectivity and noise equivalent power of our devices are close to the best reported elsewhere.
Keywords: GaN, visible blind photodetector.