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2. Experimental
All the samples were grown on (001) orient semi-insulting
Cr-doped GaAs substrate in a VG V80H MBE system with
a valved Arsenic cracker cell. Prior to growth, surface oxide
was desorbed under the suitable As2/As4 ratio from the GaAs
substrate until the reflective high-energy electron diffraction
(RHEED) pattern showed a clear 2×4 surface reconstruction.
This surface reconstruction transformation was adopted as a
means for calibration of the substrate temperature which was
set to 580 ◦
C measured with an infrared pyrometer of suitable
wavelength sensitivity. A 300 nm GaAs buffer layer was
grown on the substrate firstly to obtain a smooth GaAs surface,
followed by a step-graded buffer consisting of undoped InAlAs
layers,whose detailed structure is showed in figure 1. The step-
graded indium content was obtained by increasing the indium
cell temperature. Samples A, B and C were grown at 360 ,
380 and 400 ◦
C under a 5.0×10−6
Torr arsenic flux. Samples
D and E were grown at 380 ◦
C under an As overpressure of
3.0×10−6
Torr and 7.6×10−6
Torr respectively. RHEED was
used to monitor InAlAs growth.
The effect of growth conditions on the quality of the
top InAlAs layer was characterized by HRXRD, RHEED
patterns,AFM and PL. HRXRD measurements were carried
out using Bede D1 triple-axis diffractometer with a parabolic
graded multiplayer Gutman mirror collimator, followed by a
four-bounce channel-cut Si (220) monochromator, delivering
aCuKα1 line of wavelength λ = 0.154056 nm. The
asymmetric two-bounce Si (220) analyser crystal was placed
in front of the detector. The surface morphology of the
samples was characterized by contact mode AFM. The PL
measurements were performed with the samples mounted in
a variable temperature He gas cryostat using the 632.8 nm
line from a He–Ne laser. The resulting luminescence signal
is analyzed with a grating monochromator and detected by a
photon counting system. The carrier concentrations and Hall
mobilities of the samples were measured using the Van der
Pauw–Hall method at room temperature (RT) and 77K.
3. Results and discussion
All the sample surfaces were monitored directly during layer
deposition by means of RHEED. Except sample E, all the
samples of step-graded InAlAs buffer layers show streaky
patterns of the reconstructed surface during growth, which
indicate the two-dimensional growth mode. Due to large
differences in Al and In sticking coefficients on stepped
surfaces, the InAlAs epilayer is very sensitive to growth
conditions.