GaN-Based Materials for Blue Emitting Device Structures Grown in Multiwafer Planetary® Reactors
As
device structures of GaN based materials are produced for commercial
application in LEDs and Lasers [1], tools for industrial mass production become
necessary. These machines are required to provide material with
state-of-the-art characteristics while maintaining high throughput, high
reproducibility, high growth efficiency and good uniformity of individual
layers for maximal yield of the production line. The AIXTRON multiwafer MOVPE
systems with the Planetary® Reactor design are uniquely suited
to meet these specifications. The growth processes are continuously optimised
regarding total pressure, growth temperature and precursor flows to meet the
material quality requirements while the reactor design ensures high yield.
All
GaN based structures discussed here were grown with a GaN nucleation layer on
sapphire substrates in the (0001) surface orientation. The layers were produced
in a AIX 2000 HT Planetary® Reactor where the uniformity of the
layer characteristics is ensured by a two fold rotation of the substrates: up
to 7 sattelite disks carrying 2" substrates are rotated by the Gas Foil
Rotation® principle while the main disk is turned by mechanical
drive. The precursors NH3, TEGa, TMGa, TMAl, TMIn, SiH4
and Cp2Mg are injected in the center of the main disk together with
N2 or H2 as carrier gas, seperated in an upper and a
lower flow for MO and hydride precursors respectively. Reactor temperatures up
to 1200°C and total pressures between 50 and 1000 mbar were used for the
growth processes.
Non
intentionally doped GaN layers are semi-insulating or lightly n-type with
background electron concentrations below 5·1016 cm-3.
Intentional n-type doping is obtained by introducing
SiH4 into the gas phase. Free electron concentrations of up to
1·1020 cm-3 have been obtained.
Figure 1 shows a
topology of the sheet resistance of a highly n-doped 0.5 µm thick GaN
layer mapped by a Lehighton inductive meassurement setup. The average sheet
resistance is 15.75 
/square with a standard deviation of
0.86 %. This indicates excellent doping uniformities taking
into account that both thickness distribution and doping homogeneity contribute
to this value (see Section 4).
Intentional p-type doping has been achieved by using Cp2Mg as
precursor for the acceptor. With Mg concentrations of more than
1·1020 cm-3 hole concentrations up to
1·1018 cm-3 are obtained. Uniformities for p-type
carrier density with a standard deviation around 10 % have been
found, revealing incorporation uniformity as well as influences of the
activation process.
The
layer thickness of single layer GaN on sapphire structures are routinely
examined by white light interference evaluation on a Waterloo PLM 100 wafer
mapper system. Figure 2 shows a mapping of a single layer GaN on sapphire wafer -
the average thickness is 2.44 µm with a standard deviation of
0.75 %. Usual variations for all nitride based materials are in
the range of 2 % standard deviation.
The
Waterloo PLM 100 photoluminescence (PL) mapper was used to study the
distribution of the peak wavelength of the bandgap related emission line of
ternary layers at room temperature. Structures of GaInN and AlGaN layers on a
GaN buffer on sapphire have been thus examined. Figure 3a shows the wafer mapping
(left) and wavelength distribution (right) for a GaInN layer with an estimated
average of 6 to 8 % InN in the lattice - the average wavelength
is 382.44 nm with a standard deviation of 0.94 nm. For a AlGaN layer with
roughly 8 to 12 % AlN (S) an average peak wavelength of 340.31
nm and a standard deviation of 0.26 nm were found - see Figure 3b. The standard
deviation for emitted intensity is typically less than 10 % for
both ternary systems.
A
simple double-hetero (DH) structure consisting of a 10 nm GaN cap, 50 nm GaInN
with ca. 16 % InN (S) and 1.6 µm GaN buffer was cleaved
into roughly 1 mm wide pieces and exposed to high intensity optical pumping
with a pulsed N2 laser. [2] At excitation densities up to 1
MW/cm2 the emission from that sample was found to concentrate into a
single lasing mode at 78 K, as shown in Figure 4. This lasing action without a
Fabry-Perot cavity is evidence of the high optical quality and high gain
coefficient of the sample.
Electrical
and optical measurements prove our GaN material and its ternary alloys to be of
high quality. Doping and ternary compositions have been shown to have
state-of-the-art homogeneity, except for p-type GaN where the growth process
and activation of the acceptors require further optimisation. These
characterization results combined with the high yield due to the unique design
of the AIXTRON Planetary® Reactors prove these systems to be an
optimal tool for GaN based LED and - in future - laser mass production.
We
hereby gratefully acknowledge the work of G. Yablonskii et al., Institute of
Physics, Minsk, on the optical excitation experiments quoted above.
last updated Monday, October 19, 1998 6:03:31 PM.© 1998 The Materials Research Society
