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The techniques of the photoreflectance (PR) and
electroreflectance (ER) were used to study the built-in
electric fields and the surface Fermi levels of InP surface-
intrinsic-n+(SIN+) structures. The substrates of SIN+ structures
are either Fe-doped semi-insulated InP or Sn-doped N+ InP with
same doping concentrations as its buffer layer. The built-in
electric field and the Fermi level were calculated from the
Franz-Keldysh oscillations (FKOs) of the photoreflectance
spectra. Our studies found that for the samples with the same
doping concentration in buffer layer and substrate, the built-in
electric field increases as their top layer thickness
decreases. The surface Fermi level, on the other hand, remains
approximately constant. For samples with a semi-insulated
substrate, the photoreflectance spectra indicate the
simultaneous existence of two built-in electric fields, one in
the top layer and the other at the interface region between the
buffer layer and substrate. ER spectra were measured with the
application of a modulation electric field across the top layer.
The built- in electric field across the top layer obtained from
the ER spectra increases as the top layer thickness decreases
while the surface Fermi level, again, remains approximately
constant. Next, we have studied the band gaps and the
surface Fermi level positions of a series of In1-xAlxAs SIN+
structures at room temperature by PR. Experiments demonstrated
that over the aluminum mole concentrations x from 0.42 to 0.57,
the surface Fermi level is not pinned at midgap, as commonly
believed, but instead varies, respectively, form 0.50 to 0.81
eV below the conduction band edge. The samples were grown by
molecular beam epitaxy with a undoped layer thickness of 1000
A. The undoped layer was subsequently etched to 800, 600, 400,
and 200 A. Different chemical solutions were used in the
etching process and the built-in electric field is found
independent of the etching process. While the surface Fermi
level, in general, varies with the undoped layer thickness,
there exists, for each Al concentration, a certain range of
thickness within which the surface Fermi level is weakly
pinned. From the dependence of electric field and surface Fermi
level on the undoped layer thickness, we conclude that the
surface states distribute over two separate regions within the
energy band gap and the densities of surface states are as low
as 1.02*10^11 cm^(-2) for the distribution near the conduction
band and 2.91*10^11 cm^(-2) for the distribution near valence
band. Finally, we measured the surface Schottky barrier of
In(0.53)Al(0.47)As SIN+ structure as a function of temperature.
Based on diffusion theory, thermionic- emission theory, and
the relation between the image force effect and current/
voltage, a theoretical relation of the surface Schottky barrier
and temperature, pinning positions of the surface Fermi level,
surface state densities etc. was obtained. From the least
squares fits of the experiments data to the theoretical
relation, we obtained the pinning positions, 0.47 eV and 1.00 eV
from the conduction band edge which correspond to the
densities of surface state, 2.61*10^11 and 2.96*10^11 cm^(-2),
respectively. The results are in good agreement with those
obtained from the study by changing the thickness of the undoped
layer. This study has further approved the model of double
distributions of the surface states. Furthermore, the surface
state dependent parameters, the geometric factors (r1=4.50*10
^(-4) and r2=4.70*10^(-4)) and the ideal factors (h1=3.60 and
h2=2.90) were also obtained from the fitting results.
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