Biomedical Engineering Reference
In-Depth Information
environment in the growth chamber is changed from As atmosphere to P atmosphere
for the preparation of P-based materials. We choose In0.5Ga0.5P as buffer layer
being grown on GaAs buffer layer. These two epitaxial layers are well lattice
matched. A 200 nm thick In0.5Ga0.5P buffer layer is grown at 470
◦
C with a BEP
V/III ratio of 10 and with a growth rate of 0.5 ML/s. The formations of these bi-
buffer layers (In0.5Ga0.5P/GaAs) are confirmed by the observation of 2
×
4and
2
1 RHEED patterns, respectively.
Droplet epitaxy process starts when substrate temperature is decreased to 250
◦
C
without P beam to minimize the excess P on the surface. The background pressure
in the growth chamber is kept at 10
−
9
Torr before indium droplet deposition
begins. Indium droplets are spread on the InGaP surface at a deposition temperature
between 120 and 290
◦
C and a deposition rate from 0.2 to 1.6 ML/s. Indium
thicknesses are varied from 1.6 to 6.4 ML. These variations of deposition parameters
define the droplet size and droplet density which will be prime condition to control
the configuration and number of dots in each InP ring-shaped QDMs.
Later on, P beam is introduced into the growth chamber again for crystallization
process of InP nanostructures. The crystallization temperature is varied from 150 to
300
◦
C under P BEP of 4
×
10
−
6
Torr for 5 min. At appropriate growth parameters,
InP ring-shaped QDMs are formed.
InP nanostructures are then capped by 100 nm thick In0.5Ga0.5P double layers
grown by the two-step technique. The first capping layer is 10 nm thick InGaP
grown by migration enhanced epitaxy (MEE) at 300
◦
C with 0.5 ML/cycle growth
rate. The second layer of 90 nm thick InGaP is grown by the conventional MBE
process at 470
◦
C with the growth rate of 0.5 ML/s. This capped InP nanostructure
sample is ready for ex situ photoluminescence measurement. The same growth
process of InP ring-shaped QDMs is repeated on the top of sample surface with the
same growth parameters to produce another layer of InP ring-shaped QDMs. Now,
the sample surface morphology is ready to be observed by a tapping mode AFM.
Fractional part of this sample is also the cross section observed by TEM to confirm
the ring-shaped nanostructure. Schematic sample structures at different processing
steps are displayed in Fig.
2.4
.
Major characterization of InP ring-shaped QDMs is conducted by photolumi-
nescence (PL) measurement. The PL intensity reflects the crystal quality of the
nanostructures as well as their density. PL spectra give specific identity of the
nanostructures as well as their uniformity. Ar
×
laser with emission line of 478 nm
is used to excite the sample. The excitation power is varied from 10 to 80 mW.
The laser beam is chopped and focused onto the sample placed in a cryostat with a
cooling sample temperature from room temperature down to 20 K. The PL signal is
collected and resolved by a 1-m monochromator. The resolved PL signal is detected
by a liquid nitrogen cooled InGaAs detector and sent to a lock-in amplifier for PL
data analysis.
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